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1995 Hybrid-electric vehicle design and applications Eric Adams
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Recommended Citation Adams, Eric, "Hybrid-electric vehicle design and applications" (1995). Thesis. Rochester Institute of Technology. Accessed from
This Thesis is brought to you for free and open access by the Thesis/Dissertation Collections at RIT Scholar Works. It has been accepted for inclusion in Theses by an authorized administrator of RIT Scholar Works. For more information, please contact [email protected]. Hybrid-Electric Vehicle Design and Applications
by Eric E. Adams
A Thesis Submitted in Partial Fulfillment of the Requirements for the Master of Science
in Mechanical Engineering
Approved by: Professor Alan Nige Thesis Advisor Professor _
Professor _
Professor _
Department Head
Department of Mechanical Engineering College of Engineering Rochester Institute of Technology December 1995 I, Eric Adams, hereby grant permission to the Wallace Memorial Library of the Rochester Institute. of Technology in Rochester, NY, to reproduce my thesis entitled, Hybrid Electric Vehicle Design and Applications, in whole or in part. Any and all reproductions will not be for commercial use or profit.
Eric E. Adams Abstract:
This paper discusses the considerations involved in hybrid-electric vehicle design. The tradeoffs between issues such as drive scheme/arrangement, motor choice, batteries, and temperature control are investigated. The technologies and components which are currently available, and those which are likely in the near future, are described. A sport utility vehicle is taken as a specific case study because they are very popular and relatively inefficient. Calculations indicate that a hybrid sport utility vehicle with all wheel drive is feasible using existing components. The next generation vehicle using new technologies is also predicted. Preface:
The past decades have made the internal combustion engine the standard power source for vehicles. However, at the turn of the twentieth century there was much more variety with steam, electric, and gasoline power competing nearly equally. Electric vehicles were popular because they were quiet and clean, but they had short range and were only common in cities. Steam power was used for heavy duty jobs, but both steam and gas powered vehicles were dirty and difficult to operate. Ironically, it was the development of the electric starter motor which allowed easy operation of gasoline powered vehicles, and thus sealed the fate of early electric vehicles.
One hundred years later, the transportation industry is again facing change. The petroleum reserves which have kept gasoline inexpensive are dwindling, and the air pollution which has been a constant side effect of internal combustion is now affecting the health of the population.
This has brought electric vehicles back to the forefront of vehicle development, despite the fact that they still suffer from some of the same energy storage problems as their ancestors. The hybrid-electric vehicle has been developed as an method of addressing energy storage issues and improving the
usefulness of the electric drivetrain.
The delay in electric vehicle development is partly technical (batteries), but is mostly political. The petroleum industry and the Big 3 automakers have invested enormous amounts of money developing the current system, and they are loathe to change the business they have dominated for so long. Many small companies are rushing to fill the demand that the major automakers are stubbornly ignoring, and the next few years will bring many unique vehicles from a variety of sources. Table of Contents:
Abstract: i
Preface: ii
Table of Contents: iii
List of Figures: vi
List of Tables: vii
List of Abbreviations: viii
Introduction: 1
Hybrid Design Overview: 5
Fundamental Question: 5
APU: 6
Drive Schemes: 9
Series: 9
Parallel: 13
EV Comparison: 16
Regeneration: 17
Battery Configuration: 19
Motors: 21
Climate Control: 24
Chassis: 27
Summary: 28
Technology Overview: 29
Batteries: 29
Electrochemical: 31 Flywheels: 41
Ultracapacitors: 42
Others: 44
Battery Comparison: 45
Motors: 46
DC Motors: 46
AC Motors: 49
Special Motors: 52
Alternative Fuels: 53
Reformulated Gasoline: 53
Natural Gas: 54
Propane: 55
Alcohol Fuels: 56
Hydrogen: 57
Summary: 58
Auxiliary Power Units: 60 Combustion Engines: 60
Fuel Cells: 66
Direct Conversion: 69
Summary: 70
Case Study: 72
All Wheel Drive: 74
Sport Utility Vehicle Definition: 76
Hybrid SUV: 78
Hybrid All Wheel Drive: 81
Series Scheme #1 : 82
Series Scheme #2; 85
Series Scheme #3: 89
IV Series Scheme #4: 91
Parallel Scheme #1 93
Parallel Scheme #2. 95
Parallel Scheme #3. 98
Battery Enclosure: 100
Calculations: 102
Road Loads: 103
Aerodynamics: 103
Rolling Resistance: 106 Roadway Grades: 109
Total Road Load: 111
Acceleration: 113
Other Losses: 114
Towing: 115
Battery Storage: 116
ZEV Range: 116
AAWD: 117
Fuel Consumption: 119
Results: 120
Design Solutions: 122
Near-Term Hybrid: 122
Mid-Term Hybrid: 124
Long-term Solution: 127
Conclusions: 129
Bibliography: 131
References: 135
Appendix: 154 List of Figures:
Figure 1 APU Load-Leveling 7
Figure 2 Schematic of Series Hybrid 9
Figure 3 Schematic of Engine-Electric Hybrid 12
Figure 4 Schematic of Parallel Hybrid 13
Figure 5 Light Truck Sales Distribution 72
Figure 6 Series Scheme #1 82
Figure 7 Series Scheme #2 85
Figure 8 Series Scheme #3 89
Figure 9 Series Scheme #4 91
Figure 10 Parallel Scheme #1 93
Figure 1 1 Parallel Scheme #2 95
Figure 12 Parallel Scheme #3 98
Figure 13 Road Load vs. Temperature 105
Figure 14 Road Load vs. Wind Speed 106
Figure 15 Rolling Resistance vs. Speed 109
Figure 16 Vehicle on Grade 110
Figure 17 Road Load vs. Highway Grade 111
Figure 18 Road Load vs. Speed 112
VI List of Tables:
Table 1 Selected UQM Motor Data 23
Table 2 USABC Goals 31
Table 3 Summary of Electrochemical Batteries 40
Table 4 Summary of Battery Technologies 45 Table 5 Sport Utility Comparison 77
Table 6 HE-SUV Specifications 79
Table 7 Series Scheme #1 83
Table 8 Series Scheme #2 86
Table 9 Series Scheme #3 90
Table 10 UQM Motor Data 90
Table 1 1 Series Scheme #4 92
Table 12 Parallel Scheme #1 94
Table 13 Parallel Scheme #2 96
Table 14 Parallel Scheme #3 99
Table 15 Battery Enclosure Issues 100 Table 16 1995 Sport Utility Data 102 Table 17 Rolling Resistance Coefficients 108
Table 18 Fuel Consumption 120
Table 19 Near-term Hybrid Components 123
Table 20 Mid-term Hybrid Components 125
VII List of Abbreviations:
ABS Anti-lock Brakes
APU Auxiliary Power Unit
AAWD Adaptive All Wheel Drive
AWD All Wheel Drive
CARB California Air Resources Board
CNG Compressed Natural Gas
DOE United States Department of Energy EV Electric Vehicle
EPA Environmental Protection Agency FFV Flexible Fuel Vehicle
HEV Hybrid-Electric Vehicle
HEVC Hybrid Electric Vehicle Challenge, student competition
ICE Internal Combustion Engine
LEV Low Emission Vehicle, defined by CARB
LNG Liquid Natural Gas
MCFC Molten Carbonate Fuel Cell
PAFC Phosphoric Acid Fuel Cell
PEM Proton Exchange Membrane
PNGV Partnership for a New Generation Vehicle
RFG Reformulated Gasoline
SOC State of Charge
ULEV Ultra-Low Emission Vehicle, defined by CARB USABC United States Advanced Battery Consortium
ZEV Zero-Emission Vehicle
VIII Introduction:
As the remaining stocks of petroleum are steadily depleted, and smog and air pollution become extremely serious concerns, pressure is growing to evaluate our transportation systems. Two key objectives have been identified in
response to these issues: reduction of oil consumption, and reduction of air
pollution levels.
The problem of air pollution has received serious investigation, and the automobile industry is among the first to be regulated. The California Air
Resources Board (CARB) is leading the way by setting strict standards for
allowable vehicle emissions in the state of California. The emissions standards
progressively toughen from low emission vehicle (LEV) down to zero emission
vehicle (ZEV), and a timeline has been established which dictates how many
vehicles of each level may be sold each year. Many other governing bodies
have adopted the California regulations or passed similar legislation to control
the environmental performance of new vehicles (exhaust emissions and fuel
consumption). The implications of these regulations and their effectiveness in
controlling the environmental issues which they address is still the subject of
active debate. The questions raised by these debates and the arguments set
forth by all involved parties are well documented in other sources, and will not
be addressed in this paper. Regardless of the debate, a new generation of
vehicles is coming, and they promise to be very different from current
automobiles.
Attaining the improvements which are mandated in these regulations
represents a challenging task. Much of the research and subsequent
development of the diverse technologies which will be required is beyond the
means of any single company. Many groups and consortiums have been formed to help distribute the work load. Nearly all of these groups represent a cooperative effort between government and industry. Since the government is mandating the speed with which these changes must occur1, it is assuming a key role in their development. The Hybrid Propulsion Plan is one such cooperative effort between the U.S. Department of Energy (DOE) and the major U.S. automakers. The efforts of this group are aimed at the short term development of vehicles which can be available at the turn of the century. The main goal of all the projects is to increase the fuel economy and lower the exhaust emissions of new vehicles. Secondary goals involve the appropriate development of new technologies and alternative fuels to support future improvements in vehicle
performance.
Current research is laying the groundwork for the Partnership for a
New Generation Vehicle (PNGV). This program is another joint venture between
government and industry, and has ambitious goals such as tripling average fuel
economy and improving safety and performance. The PNGV represents the long term plan for automobile development in the United States.
College students make up another group involved in new vehicle
development. The Hybrid Electric Vehicle Challenge (HEVC) was begun in 1993
as a design competition requiring college teams to develop a hybrid vehicle in
one of several classifications, and then present their vehicles to the DOE and its
industry partners. The students have a fresh outlook on vehicle design which
produces a very diverse group of vehicles, and provides a valuable look at many
more vehicle formats than any of the consortiums could produce. In 1996, the
HEV Challenge will evolve into the FutureCar Challenge which is targeted
directly at meeting the goals of the Partnership for a New Generation Vehicle.
1 The Department of Energy defines their efforts as near-term (until 2000), mid-term (until definitions will be used 2010), and long term (beyond 2010). These throughout this paper. One of the most apparent solutions to the air pollution and fuel consumption problems is the use of pure electric vehicles. Electric vehicles are very attractive for urban use because they do not emit any pollutants. The generation of electricity still carries pollution problems, but the number of sources is reduced to a handful and kept away from the urban centers.
Electricity can be generated in many ways, including renewable and non- polluting methods, but even if fossil fuels continue to be used for power generation the emissions of a few powerplants can be more effectively
monitored than millions of vehicles.
Consumers have come to expect the remarkable power and range that
modern internal combustion powered automobiles have been able to offer.
Although the electric vehicle industry has also been able to produce equally
usable vehicles, all of them are handcuffed by their lack of travel range. As an
excellent and widely publicized example, take the General Motors Impact. This
is a very attractive and powerful two passenger vehicle with all the expected
comforts: ABS, heat and air conditioning, air bags, sound system, etc.. The
tradeoff for this power and comfort shows clearly in its range of travel. The
vehicle is rated by the EPA at 70 miles in the city and 90 miles on the highway
range2 as its maximum (under good conditions). While this is sufficient for daily
does not allow for longer trips or for commuting and around town shopping, it
transport of people and cargo. While state of the art vehicles such the Impact
expensive for the average consumer to are theoretically practical, they are too
actually purchase.
The requirements for the automobile market in the next decade need to
because it is important to and solve the be carefully investigated, very identify
correct problem. Certain regions, such as California, have imminent health
; Impact Specifications, General Motors Literature. concerns due to air pollution, but most of the rest of the United States does not yet face threats which are as severe. However, steps must be taken immediately to prevent them from developing.
Contrary to some sources, the internal combustion engine does not need to be eliminated right away. It is important to recognize that reducing air pollution and fuel dependence are the two key goals. A hybrid-electric drivetrain can effectively meet these goals for the near to middle future. Eliminating the internal combustion engine is not a stated goal, and should not be the sole focus of research. Improving the efficiency and performance of the complete vehicle as a system is the critical goal that should be closely investigated. Hybrid drivetrains hold important possibilities which merit further discussion. Hybrid Design Overview:
A hybrid vehicle uses two sources of motive power, which usually means both an electric motor and a fuel powered engine. (By convention, the fuel powered engine is called an auxiliary power unit or APU). It can travel as a pure electric without the APU, and still achieve excellent long distance travel range by operating with the APU. This allows the vehicle to operate as a zero- emission vehicle in order to meet regulations for various geographic areas, but overcomes the main shortcoming of the electric vehicle, its short travel range.
At first glance, the hybrid drivetrain seems inherently inefficient because two different, and presumably capable systems are being carried in the vehicle. The redundancy increases weight, complexity, and cost. The advantages lie in the fact that neither of the drive systems is completely acceptable for the entire range of vehicle power needs. While the electric drivetrain is extremely flexible and allows regenerative braking, it cannot store sufficient energy for extended operation. On the other hand, the APU can be quickly refueled, but emits pollution and is likely to perform best when operated
"hybrid" at a constant speed. Combining both systems into a vehicle allows each to be optimized to take full advantage of its strengths.
Fundamental Question:
The hybrid vehicle has important advantages over conventional vehicles, but faces a fundamental question: What are the performance requirements for the vehicle?
The performance of this new generation of efficient vehicles is particularly sensitive to design. If, for instance, the vehicle will operate on a city cycle and is required to have good acceleration characteristics, then total mass becomes very important. A low mass vehicle is easier to accelerate and decelerate during the frequent starts and stops of urban driving. Aerodynamic factors would become a lesser issue due to the low average speed of the vehicle.
On the other hand, if the vehicle is required to have extended travel
range during high speed expressway conditions, then aerodynamics become critical and mass becomes a secondary consideration. In this case, the vehicle does not accelerate often, and once the vehicle has been accelerated to cruising
speed, the only work it must expend is to overcome drag. Designing the vehicle to have good aerodynamic characteristics such as low frontal area and low drag
coefficient will allow the vehicle a longer range because it can slip through the
air without expending as much energy.
APU:
It may not be immediately clear why the hybrid-electric drivetrain is an
improvement, since it will continue to use a fuel engine of roughly the same
types used today. The advantages lie in the way that the engine (APU) is
operated. Two of the benefits are steady state operation, and the elimination of
idling.
Allowing the engine to operate at steady state has two important
advantages: fuel usage can be optimized, and exhaust emissions can be tightly
controlled. Tuning any engine to perform well throughout its entire range of
speeds and loadings is difficult. Operating the APU at a single point on this
"map" allows very accurate tuning of many parameters, including fuel usage and
exhaust emissions. Steady state operation is one of the main benefits of using a "buffering" hybrid drivetrain, and is made possible by the APU from the peak power loads that the vehicle experiences.
Buffering, or load-leveling the APU involves smoothing out the power demands. Acceleration requires a large amount of power, yet occurs for a short
period of time. Cruising Figure 1: APU Load-Leveling requires a small amount of power, but occurs for long
periods of time. Therefore,
the average power needed
by the vehicle is low.
Figure 1 shows an
illustration of a theoretical CD
o power cycle. The vehicle Q_ initially accelerates, and
then cruises at constant
speed. It then decelerates
to a stop. The next event is
a larger acceleration, followed by cruising at a higher speed, and a hard deceleration. After a short stop, the vehicle again accelerates. It is clear that this could represent any number of driving situations. Despite the fact that the peak power is high, the average area under the power curve is much lower. The object of load-leveling the APU is to allow the engine to operate consistently at this lower power level.
The surge power for acceleration is supplied by the electric portions of the drivetrain. This example is greatly simplified because it does not take into account an operating strategy for the APU, such as shutting down the APU during the slow travel and deceleration periods. Even though this illustration is idealized, it illustrates the concept behind load-leveling: decoupling the APU from the peak power loads.
Current automobiles use their combustion engines to meet all the power needs of the vehicle. The engine must be large in order to meet peak demands and provide acceptable acceleration despite the fact that the full capacity of the engine is only occasionally used. This leads to a difficult question: should the vehicle perform well by using a large engine, or should it
return good fuel economy and emissions performance by using a small engine?
It is very difficult for current automobiles to do both.
Decoupling the APU from the peak power demands allows it to operate
at constant load and steady speed while providing the relatively low average
power of the vehicle. This allows the displacement of the APU to be smaller than
current engines, and therefore return better fuel economy.
The APU also benefits from the elimination of engine idling. Current
vehicles operate at engine idle about 19% of the time3. Their large engines are
running and polluting, but are producing virtually no useable work. This is
extremely inefficient and leads to poor environmental performance. In a hybrid
vehicle, the APU is always producing useful work during the time that it is
running. If the APU is not needed to move the vehicle or to store electrical
energy against future demand, then it is shut down. This keeps the overall
efficiency of the vehicle higher, and plays a large part in the fuel economy
improvements of hybrid vehicles over conventional vehicles.
3 U.S. Department of Energy, Office of Transportation Technologies, Hybrid Propulsion Plan. October 1994. Drive Schemes:
The layout of components in a hybrid vehicle has a significant impact
on its performance. Each drive scheme has different characteristics and
advantages which arise from the
Figure 2: Schematic of Series Hybrid arrangement of its components.
Series: APU
The series hybrid gets its
name from the fact that energy
Generator conversion and torque transmission
are constrained to occur serially
- Battery through the components. Figure 2
shows a schematic of a series drive Motor arrangement. As shown, the primary
drive of the vehicle is accomplished
by the electric motor. The APU is
used to drive a generator to
supplement the power available
from the battery pack.
The battery pack would be charged from the power grid, and the vehicle would normally operate as an electric. If the state of charge of the battery pack fell below a certain level, the APU/generator would be activated until the state of charge was restored. The APU would then continue to cycle on and off as required. The sustained power demands of high speed cruising would require the APU to operate continuously in order to avoid rapid depletion of the battery.
"range-extender" The series hybrid can also be called a hybrid, because it can be considered as a pure electric vehicle which carries a generator to extend its travel range.
This is a simplified description, but it highlights a disadvantage of the series hybrid: the energy passes through several conversions before being used by the vehicle. The APU first converts the fuel into mechanical power which is then converted to electricity by the generator. The electricity flows into the battery with charging losses, and then back out with discharge losses. The motor and driveline also have an efficiency penalty.
As an example, assume the following efficiencies: 30% for the APU,
85% for the generator, 95% into and out of the batteries, 85% for the motor and controller, and 90% for the driveline. Efficiency during cruising is therefore:
= * * * . Series APU APU Generator Motor Driveline
* * * Series APU =0.30 0.85 0.85 0.90
Series APU =0.19
The energy conversion rate during APU operation in a series hybrid is only
about 19%, which is even worse than current vehicles.
If the APU is used to charge the batteries, then the efficiency is:
* * Series APU to Battery = APU Generator Battery
Series APU to Battery =0.24
This low conversion efficiency is the reason that the APU/generator is not
intended to charge the battery pack. The power plants which maintain the power
grid operate at efficiencies much higher than 24% and must be the primary
source of charge for the batteries. The APU/generator is meant as a source of
supplemental power to improve vehicle travel range.
The advantage of the series drivetrain is that the APU is not
mechanically connected to the drive wheels, which simplifies optimization of
performance attributes. The physical location of components is also flexible
10 because the APU/generator can be located anywhere in the vehicle. A disadvantage is that the electric motor must be large because it provides all the propulsion power for the vehicle.
Series hybrids will not be truly practical until a better APU becomes available. One option is the gas turbine. Small turbines which have been integrated into generator sets have shown conversion efficiencies of 30%, yet produce fewer emissions than the utilities supplying the power grid. These turbine generator sets also promise to cost less than a conventional internal combustion engine, and run on the same gasoline4. Although the series arrangement has a somewhat low efficiency, actual fuel consumption could be very low due to the ability to tightly optimize for steady state5. Improvements such as these turbines are still a few years away, and their arrival will be critical to the development of series hybrids.
Due to its resemblance to pure electric vehicles, the series powered
hybrid is likely to target the same driving requirements. It is well suited to urban
and short-trip driving and will have substantial ZEV range6. The APU will provide
reserve power for the occasional long trip, as well as consumer security in case
of emergency. The series hybrid can have all of the advantages of an electric vehicle, and still have excellent range with little fear of being stranded. Since the
buffered APU can be tuned to generate low levels of exhaust emissions, the overall performance of the vehicle can be excellent.
4 Mackay, R., Development of a 24 kW Gas Turbine-Driven Generator Set for Hybrid Vehicles. SAE Technical Paper Series #940510. 5 Burke, A. F., Hybrid/Electric Vehicle Design Options and Evaluations. SAE Technical Paper Series #920447. 6 Wouk, V., Hybrids: Then and Now. IEEE Spectrum, July 1995, pl8.
11 Engine-Electric:
Another proposed hybrid scheme is called an engine-electric hybrid.
This type of hybrid uses ultracapacitors instead of batteries. While it is conceivable that both series and parallel arrangements could be used for an engine-electric, the most likely arrangement is series7.
Figure 3 shows a schematic of a
Figure 3: Schematic of "series" engine-electric arrangement. The Engine-Electric Hybrid APU turns a generator to make electricity,
which is then used directly to drive the
vehicle. Ultracapacitors store enough
energy to provide the high current pulses
needed for good acceleration, and also
accept regenerated energy. Although very Ultracapacitors similar in layout to a series hybrid, there is
no significant energy storage on board the
vehicle. The advantage of this system is
that the engine is buffered from the peak
loads, with all the benefits discussed
earlier. The complete system is simple and
light because no heavy or bulky batteries
are needed. Fuel economy is improved and exhaust emissions are lowered because the APU is smaller and can be operated near steady state. The disadvantage is that the vehicle must have the engine running in order to move, and therefore has little useful ZEV range.
7 Burke, A. F., Hybrid/Electric Vehicle Design Options and Evaluations. SAE Technical Paper Series #920447.
12 This arrangement has been proposed to increase the fuel economy of liquid fueled vehicles. Current steel body vehicles could benefit by as much as o
50% , but purpose built composite chassis vehicles would benefit even more.
The vehicle would generate all of its energy from the liquid fuel, and would operate independent of the power grid. This is an interesting scheme because it does not require special batteries, yet promises significant benefits due to engine buffering. The unavailability of appropriate capacitors is the main
problem.
Parallel:
The parallel hybrid allows
Figure 4: of Schematic Parallel its energy paths to occur in parallel to
Hybrid each other, as shown in Figure 4.
Either the APU or the electric motor
can be connected to the drive APU wheels. A separate generator is not
required because the drive motor can
=!= Clutch be used as a generator by powering it
from the main driveshaft.
The parallel hybrid allows Motor the APU and electric motor to work
together, effectively summing their
power capabilities. Since the sum of
the APU and electric motor powers
must equal the peak power, both can
Burke, A. F., On-Off Engine Operation for Hybrid/Electric Vehicles. SAE Technical Paper Series #930042.
13 be smaller. As an example, let the peak power demand be P. In a series hybrid, the electric motor must be large enough to provide all of P by itself. The parallel hybrid, on the other hand, can have the APU provide P/2 and the electric motor provide P/2. The full power demands can still be met because they work together, yet the individual components are smaller.
The parallel hybrid is more complicated than a series hybrid, but can yield better efficiency. Assume again that the APU is 30% efficient and the driveline is 90%. The energy conversion during cruising is now:
* Parallel APU = APU Driveline
Parallel APU =0.30*0.90
Parallel APU =0.27
The energy conversion rate during APU operation for the parallel hybrid is therefore nearly 27%. This better efficiency allows the APU to be smaller than the APU from a similar performing series hybrid.
This driveline arrangement is very similar to current vehicles, and this efficiency represents the maximum that current vehicles could achieve. During transient operation they exhibit much lower efficiencies, but steady state cruising approaches this calculation. The hybrid vehicle is able to maintain its APU near this optimum condition at all times.
The biggest difference between the series and parallel hybrids is the connection of the APU to the vehicle. The parallel hybrid connects the APU directly to the drive wheels without any intervening components. This allows
much better conversion efficiency as just seen, but reduces design flexibility.
The APU and the electric drivelines must be integrated together while being constrained by their connections to the drive wheels. The complete drive system becomes more complicated.
14 Parallel drivetrains can be classified into three groups: single shaft, dual shaft, and two axle. The single shaft arrangement places the motor on the same shaft as the APU. The inertia of the motor can then replace the flywheel from the APU. The dual shaft setup has the electric motor and APU on separate shafts as shown in Figure 4, and uses clutches to regulate their interaction. The dual axle hybrid places the APU on one axle of the vehicle, and the electric motor on the other axle. These classifications are general, and the variations and adaptations of them are many. The specific application will dictate which setup, or combination of them, will meet the required performance guidelines.
The APU drivetrain efficiency is much better than the series hybrid, but
once again this does necessarily indicate actual fuel consumption. Similarly to the series, the parallel hybrid uses the APU to achieve long travel range. Since
cruising requirements are predictable and relatively small the displacement of
the APU can be small, but because the APU is coupled directly to drive wheels
during high speed travel, its speed must vary during operation. The variations
are small, only a few percent, so the operation of the APU can still be optimized
for quasi-steady state operation.
The improved APU conversion efficiency indicates that the parallel
hybrid will have good high speed performance. Recall that the series hybrid can
be considered an electric urban vehicle with acceptable highway performance.
zero- The parallel hybrid can then be considered a highway vehicle with good
emission capabilities for urban operation9. As such, the parallel hybrid could
would reduce its ZEV range but would make the carry fewer batteries, which
vehicle lighter and allow a smaller APU. It is equally possible for a parallel hybrid
to carry enough batteries to have as much zero-emission range as any series
hybrid.
9 Wouk, V., Hybrids: Then and Now. IEEE Spectrum, July 1995, p 18.
15 One specific operating strategy for a parallel hybrid is called a "power
assist" hybrid. In this case the APU is sized to provide the average power requirements of the vehicle, and is always connected to the drive wheels. When the engine cannot provide enough power, the electric motor is activated to provide supplemental power. Once the desired speed has been reached and the engine is capable of propelling the vehicle, the electric motor shuts down. This allows the battery pack to be small and light because of the limited operation of the electric motor. The power assist hybrid is a near term method of increasing fuel economy. It allows a conventional vehicle to achieve improvements in fuel economy and exhaust emissions by using a smaller engine, as well as take advantage of regenerative braking, but has limited opportunity for zero emission
propulsion. The engine operates over the full range of vehicle speeds, and cannot take advantage of steady state benefits.
EV Comparison:
Using the same assumptions which were outlined earlier, a vehicle
traveling using electric power operates at approximately: Electric Battery * Motor * Driveline Electric= 0.95* 0.85* 0.90
Electric=0.126
About 73% of the electrical energy stored on board gets converted into useful
work. Assuming that charging is 80% efficient shows that approximately 60% of
the energy input to the vehicle is used for propulsion. This high efficiency is an
attractive benefit of electric drivetrains, but insufficient energy storage capability
prevents it from being widely exploited.
16 Regeneration:
Regenerative braking allows the energy which is usually lost during deceleration of the vehicle to be recovered. A conventional vehicle uses
mechanical brakes to convert the excess kinetic energy of the vehicle into heat
energy which dissipates into the surrounding air and is lost. An electric motor which usually takes electric energy and converts it into mechanical energy, can
be used as a generator to absorb mechanical energy and convert it into
electrical energy. This allows the excess kinetic energy of the vehicle to be
saved in the battery pack and made available for future use. Regenerative
braking allows significant energy recovery with corresponding improvements in
travel range. The amount of energy recovered depends strongly on the travel
profile of the vehicle, and could represent as much as 30% additional travel
range10. This is a significant increase that must be explored for both hybrid and
pure electric vehicles.
The effectiveness of regenerative braking depends on several factors
relating to the design of the vehicle. External losses such as aerodynamic drag
and rolling resistance dissipate energy which could otherwise be recovered.
Reduction of these major losses, as well as seemingly minor losses like
mechanical brake drag and wheel bearing drag, will allow the vehicle to have a
longer travel range by reducing its energy demand and improving its ability to
regenerate. An efficient vehicle will be able to take best advantage of its
regenerative potential.
In addition to these energy losses, the physical layout of the vehicle
and drivetrain are also important factors. All vehicles experience a phenomenon
10 Miiller, R., Niklas, J., Scheurer, K., The Braking System Layout of Electric Vehicles Example BMW El. SAE Technical Paper Series #930508.
17 known as weight transfer during acceleration and deceleration. During braking, a fraction of the vehicle weight effectively moves to the front wheels. The front tires now have more weight on them and can generate more traction and provide more braking force. By the same token, the unloaded rear tires can provide less braking force11. This weight transfer effect can have a large impact on
regenerative braking potential.
If the drivetrain is planned as rear wheel drive, it must be expected that
less braking traction will be available to use for regeneration. The regeneration
potential will be limited by the weight transfer away from the rear axle during deceleration. The inefficient mechanical brakes at the front of the vehicle will
need to provide the remaining braking force. Placing the drivetrain in the front of the vehicle avoids these losses and allows more efficient regeneration.
However, both front wheel and rear wheel regeneration act on only one
axle of the vehicle. During normal deceleration on good road conditions, single
axle braking should be sufficient with mechanical brakes available during panic stops. On bad roads or in low traction situations, it would not be desirable to
brake only one axle.
Regenerative braking of all the wheels of the vehicle represents the
best solution to this problem. Energy which would otherwise be lost at one axle can now be recovered, and control of the vehicle is enhanced. This solution faces several drawbacks in a practical system. All wheel drive (AWD) would be
more complex, need more components, weigh more, cost more, and be more difficult to package into the vehicle.
A last consideration for regeneration is the characteristics of the motor/generator. The efficiency of the motor/generator is not constant and it
11 A more complete discussion of weight transfer and vehicle dynamics can be found in Fundamentals of Vehicle Dynamics, by Thomas D. Gillespie.
18 varies with both rotational speed and load. In general it is most efficient at higher speeds. If the motor is directly connected to the drive wheels without any sort of transmission, then its speed will always be directly proportional to the vehicle speed. The motor speed will slow at the same rate as the vehicle speed. At low speeds the motor can not regenerate as effectively as at higher speeds, so the effectiveness of regenerative braking will decrease with vehicle speed. This is not desirable because significant energy recovery is possible at low speeds.
A multi-speed transmission can keep the motor operating near its highest efficiency point. A discrete transmission could accomplish this, but a continuously variable transmission (CVT) may be the best option. Studies have demonstrated that using a CVT to control motor speed allows 58% more energy to be recovered below 30 mph, and requires 60% less current at the motor
during acceleration12. These improvements are especially significant for urban vehicles which frequently operate at low speeds. The transmission would add
complexity, weight, and losses13, but these are offset by lower current draw
during acceleration and the additional energy available from regeneration.
Battery Configuration:
In order to provide good acceleration, a large amount of current must
be supplied to the motor in a short amount of time14. Good travel range on the
other hand, requires that a smaller amount of current be delivered for a long
period of time. Currently available batteries cannot provide for both of these
12 Fitz, F. A., Pires, P. B., A High Torque. High Efficiency CVT for Electric Vehicles.
SAE Technical Paper Series #9 1025 1 . 13 SAE Paper #910251 discusses a CVT which exhibits 90% efficiency. 14 SAE Technical Paper #93 1007 notes that ratios of peak power to average power can be
as high as 20: 1 in good performing vehicles.
19 situations. Batteries tend to have either the fast discharge capability, or the high energy storage, but rarely both.
"hybrid" One solution to the limitations of current batteries is a pack using more than one type of battery. The main pack would consist of storage batteries which are chosen to provide good travel range. They would deliver the moderate current needed to cruise for long distances. The other section of the pack would be made up of batteries which can accept and provide high current.
This smaller pack would be used to supplement the main pack during acceleration and regenerative braking.
The secondary pack would be charged by the main battery during times of low power demand, and would dump its charge very quickly to provide good acceleration. Regenerative braking generates very high currents which can not be accepted by the main storage batteries without damage. The secondary pack with fast charge/discharge characteristics would again be desirable.
Using two types of batteries in a setup such as this allows the advantages of each type of battery to be fully realized. It also increases the complexity of the energy storage system. The origin and destination of current in the vehicle are now dependent on a number of factors. A controller, almost certainly a computer, will be required to keep the system functioning correctly15.
This is not a significant problem because a hybrid (or electric vehicle) would already be carrying a sophisticated microprocessor to control the drive system
(motor and APU). The duties of controlling the battery pack can be added to the main vehicle controller.
15 Cheiky, M. C, Danczyk, L. G., Wehrey, M. C, A Power Coupler for Use in Zinc- Air/NiCd EV's and Other Hybrid Configurations. SAE Technical Paper Series #931007.
20 Motors:
The electric motor is key to many of the advantages that have been discussed, and has many implications in the overall design of the vehicle.
First, the electric motor does not have any reciprocating parts which leads directly to several advantages over a combustion engine. On a simplified scale the energy conversion sequence is much more direct for the electric motor.
A combustion engine produces linear motion of pistons, which is then converted
into rotation by the machinery of the crankshaft and the rest of the engine. An electric motor produces rotation directly and much more efficiently, and so eliminates most of the parts required in the combustion engine. Reciprocation
also generates huge stresses, vibrations and noise. The immediate consequence of using an electric motor is the elimination of many parts, vibration, noise, and nearly all maintenance.
Aside from minimal cooling requirements, an electric motor does not demand any special accommodations. The aerodynamics of the vehicle can be
improved because no major ductwork or intake considerations are required. The design of the vehicle will also benefit from the fact that the electric motor is smaller than its combustion counterparts, and can be packaged more effectively.
The packaging issue is an important concern because even though the electric
motor itself is small, space must be provided for the batteries and auxiliary
power unit.
The electric motor will also be appealing to the consumer. No
maintenance is required during normal operation, the motor will always start
regardless of the weather, doesn't require any messy lubricants, does not produce any offensive exhaust smells, and makes very little noise. Many of the basic themes of vehicle design are likely to change as new ideas are developed to take full advantage of new possibilities.
21 Electric motors do generate heat which must be removed in order to avoid damage to windings and magnets. Most motors can dissipate heat through convection with the surrounding air, sometimes assisted by fans to improve air flow. (This would require a small cooling air duct in a vehicle). As motors become larger, their bulk begins to interfere with effective heat transmission by convection. Large motors are usually fitted with a liquid cooling system. A liquid cooling system adds complexity and a certain amount of pumping losses, but allows more powerful motors to be packaged in a small amount of space.
The selection of a drive motor is not as straightforward as expected.
Coupling a group of smaller motors together can yield the same peak power as a single large motor, but weigh less. Using smaller motors also allows more flexibility in packaging the drivetrain, but increases the number of parts which
must be packaged.
Inc.16 As an example, consider the motor data from Unique Mobility,
shown in Table 1. Using this sample data, notice the following comparisons.
Using two DR156s motors together yields 42.4 horsepower and 37 pounds.
Compare this to a single SR180P which has 42.9 horsepower and weighs 52
pounds. The same amount of horsepower is available from motors weighing 15
pounds less. Proper design work should allow the two small motors to be
arranged in a desirable package that fits the intended vehicle.
16 Data was selected from information obtained from Unique Mobility, Inc., of Golden, Colorado.
22 As another example, consider using four DR156s motors in an all wheel drive design. The motors yield 84.8 horsepower and weigh 74 pounds. An alternative design would be to use a single SR218P and a mechanical driveline to distribute the power. In this case the array of small motors shows several advantages: they weigh 32 pounds less than the single motor alternative, they
Table 1 : Selected UQM Motor Data
Model Number Power Weight Cooling DR156S 21.2hp 18.5 lbs Forced Air (15.8 kW) (8.4 kg) SR180P 42.9 hp 52 lbs Forced Air (32 kW) (23.6 kg) SR218P 84.4 hp 106 lbs Liquid (63 kW) (48.1 kg) Coolant
eliminate most of the mechanical driveline, and their placement can be flexible to
fit the specific vehicle. The disadvantages are that the controller needed to
manage and synchronize four motors at once is more complicated than a
controller for a single motor, and costs will be higher because of the increased
number of motors.
Another possibility is integrating the motor directly into the wheel of the
vehicle, allowing the elimination of aN driveline components. The propulsion
power is generated directly at the wheels without need for gear boxes or
differentials.
However, there are possibilities for problems. The motor becomes
unsprung weight and causes problems for suspension performance. Vehicle
operation also produces high loads in the wheels which could cause them to
23 deflect and misalign the motor. Protecting the motor from environmental elements such as water and road salt is also difficult because of its location.
The motor-in-the-wheel design is feasible and hugely attractive.
Prototypes have proven that the concept is feasible17, and the New Generation
Motors Corporation of Vienna, Virginia, is currently selling an integrated wheel motor for solar car applications. This could potentially be a very important development for vehicle propulsion.
Climate Control:
One problem facing hybrid and electric vehicles is climate control of
both the passenger cabin and the battery pack. It is assumed throughout this
paper that the temperature of the battery pack is controlled and that it is not
subject to ambient conditions. The following comments assume that the same
systems will be used to maintain both cabin and battery enclosure temperatures.
In contrast to conventional vehicles which produce large amounts of waste heat,
the components of the electric drivetrain produce little waste heat for use in
cabin heating. An electrically powered vehicle will need to generate heat either
electrically or through chemical action.
Heat generation using electricity from the main battery pack is a
straightforward solution, but has an adverse effect on vehicle performance and
range. The standard approach has been to use electrical resistors to generate
heat. This can pose safety problems because the resistors get extremely hot and
can meltdown.
A new group of devices known as Positive Temperature Coefficient
(PTC) ceramics are being applied to vehicle heating. These materials exhibit increasing electrical resistance of as much as 10,000% over their designed
17 Motorized Wheels, Discover, June 1995, p58.
24 operating range. Since their resistance changes with temperature, they automatically regulate their power demands and operate at a nearly constant temperature regardless of ambient conditions18. This offers much better safety characteristics because the ceramic core self-regulates its temperature and cannot overheat. Using PTC ceramics in a hybrid or electric vehicle would still compromise the limited amount of energy storage on board, but would be more effective then simply using resistors.
A fuel-fired heater avoids the energy losses of electric heat, but
imposes weight and packaging constraints, as well as safety issues involved with the reaction. Despite this the current ZEV definition allows the use of a fueled heater as long as the fuel does not produce any evaporative emissions19.
This makes diesel fuel, propane or natural gas the likely fuels. A fuel fired heater
is attractive because it does not consume precious energy from the battery pack,
but it adds some small amount of emissions and adds another system to the vehicle. A hybrid vehicle has the unique opportunity of using the fuel which is
already onboard for the APU to generate heat. This simplifies many issues and
avoids the requirement of a separate fuel storage area.
A hybrid vehicle can also use waste heat from the APU for temperature
control. This is complicated by the fact that the APU cycles on and off during
operation. One approach for dealing with the fluctuating heat source is the use
"charged" of a latent heat device20. The device is with heat energy which it stores
and releases at a regulated rate. The excess heat from the APU could be saved
for a later time when it is not available. Other proposals such as imbedding
18 Improved Vehicle Heating Using PTC Ceramic, Automotive Engineering, March 1990, pp. 55-59. Zero-Emission Vehicle Technology Assessment, New York State Energy Research and Development Authority, Report #95-1 1, August 1995. 20 Latent Heat Storage, Automotive Engineering, February 1992, pp. 58-60.
25 heating wires in the windshield for improved defrosting, and preheating of the vehicle prior to use, will also be important considerations21. Since the hybrid does not rely completely on its battery pack for propulsion, it can tolerate a small current drain for heating purposes.
Cooling the cabin is an especially tricky task in a vehicle with limited energy reserves. Many improvements have been outlined which will make new vehicles easier to cool, including better insulation, high-tech window glazings, and careful examination of the ventilation system. Small heat pumps and new compressors have also been developed to provide more efficient cooling capabilities22. These systems will need their own motors because neither the electric motor or the APU operate continuously in a hybrid vehicle. This poses
an undesirable power drain, but is unavoidable with current technologies.
Another device which deserves investigation for vehicle applications is the Peltier, or thermoelectric module. These are currently used for cooling
electronics, lasers, and other sensitive equipment. They are described as solid
state heat pumps which draw heat from one face and reject it at the other23.
Existing modules are small and require a moderate amount of power, but have
no moving parts. When the current is reversed, the direction of heat flow is also
reversed, so the modules also have heating capabilities.
Pre-heating and cooling the vehicle would have a significant impact on
vehicle power requirements. If the vehicle is connected to the power grid while it
is parked, then a very aggressive cooling strategy can be used to condition the
cabin while utility grid power is available. This is important because a large
21 Dauvergne, J. L., Thermal Comfort of Electric Vehicles, SAE Technical Paper Series #940295. 22 Dieckmann, J., Mallory, D., Climate Control for Electric Vehicles. SAE Technical Paper Series #910250. 23 Thermoelectrics" Tellurex 1995. "An Introduction to , Corporation,
26 amount of energy is required to initially condition a heat or cold soaked vehicle.
The vehicle would then need only to maintain the desired temperature.
Cooling requirements are likely to pose a more serious power drain than heating requirements, but both require more research. Luckily, the hybrid vehicle can tolerate the extra load from immature vehicle temperature control technologies.
Chassis:
A final issue in hybrid vehicle production is their similarity to current vehicles. They will be able to take advantage of evolving production techniques
and facilities, but have their own special considerations. Most current electric vehicles are conversions of existing vehicles. This is a cost effective method, but
is badly constrained. All existing chassis are designed to meet the packaging
requirements of the internal combustion drivetrain. The unique packaging of a
hybrid or electric drivetrain cannot be satisfactorily addressed. The components
must be fit into whatever space is available, not into the optimum configuration.
A dedicated chassis will be required to best take advantage of all potential
benefits of the different drivetrains. This is not a problem because little new
development work is required, simply a redefinition of the constraints and some
new tooling. Fully optimizing all available systems will be very important for the
success of alternative propulsion vehicles.
In the longer term, new materials will play a major role in vehicle
development. Composite materials are rapidly developing and the methods for
producing, shaping, repairing, and recycling them are maturing. The key benefit
of composite materials is that extremely high strength to weight ratios can be
achieved. This will make the structure of vehicles much lighter without
lighter chassis will then allow smaller compromising their strength. The
27 components and higher efficiency. The benefits of reducing mass compound each other, as each reduction in mass allows smaller and lighter components which use less energy. Composite materials can effectively decouple size from mass, so that a large vehicle no longer needs to suffer from high weight. The effects are all good, and will bring exciting changes to the way in which vehicles are designed.
Summary:
The hybrid vehicle can meet the requirements of improving fuel economy
and reducing exhaust emissions while maintaining expected performance levels.
It represents an interesting compromise between today's automobiles and pure
electric vehicles. Since hybrids are broadly similar to today's automobiles, most
of the components and manufacturing infrastructure already exist. Their
production would require changes, but not a completely new start. They could
therefore be the stepping stone into the new generation of vehicles.
28 Technology Overview:
This section will summarize the major classes of components which are expected to see use in future vehicles.
The technologies will be divided into four groups:
Batteries
Motors
Alternative Fuels
Auxiliary Power Units (APU)
The descriptions of each group will include a short discussion of the technology
involved, with advantages and disadvantages, and an indication of the status of current and future development.
Batteries:
The word battery is immediately assumed to mean an electrochemical cell like those which are so universally used in our appliances. However, the dictionary definition of a battery is: a cell or group of cells storing an electrical
charge and able to furnish a current. Nowhere in this definition is it specified that the electricity must stored using chemicals. The cells can therefore be
mechanical or chemical, or any other arrangement which will allows electrical charge to be stored. This extended definition widens the field of available batteries considerably. For this paper the definition of a battery will be extended to include any devices which can store energy. It will be implied that if this energy is not stored as electricity, it can be readily converted to electricity.
Battery: a device capable of storing energy
29 In light of this definition, the following technologies will be discussed: Electrochemical
Flywheels
Capacitors
Others
The conventions which are used to rate battery technologies deserve some explanation, because they may not be familiar. The first rating is the
depth-of-discharge (DOD). The depth-of-discharge is given as percent of rated capacity. Therefore, an 80% DOD would indicate that 80% of the initial energy in the battery has been expended. The specific energy of a battery is the total
amount of energy that it can store per kilogram of its mass for a specified rate of discharge. A high specific energy is desired, because more energy can then be stored with less weight. The next rating is specific power. The specific power of a
battery is the maximum number of watts per kilogram that it can deliver at a specific depth of discharge. This indicates how quickly the battery can deliver current, and is very important for acceleration and hill-climbing. Another rating is the cycle life of the battery. A cycle is taken to be a discharge and subsequent
recharge. This indicates how many times the battery can be cycled before its
performance degrades. The rated cycle life will be closely tied to the DOD of those cycles. These are the most common ratings which are used for batteries.
While the conventions themselves are universal, the test procedures
used to test battery technologies are not. Each independent industry group seems to have its own slightly different procedure for rating the capabilities of batteries. Therefore, most the of the available data is not comparable. Ratings such as specific power are very closely tied to the depth of discharge of the cell.
As the cell discharges, its power capacity often diminishes. The DOD at which the power measurements are taken is rarely the same, leading to conflicting
30 data. Specific energy is closely tied to the rate of current draw. Some technologies yield greatest energy returns from slow discharges at moderate
while current, some work best at higher loadings. Once again, the absence of a
standard test procedure makes comparison very difficult. In addition to different
test procedures, different cells are also used. As new technologies are
developed in the labs, preliminary results from small scale cells are often
extrapolated to predict the performance of full size cells. This may or may not be
accurate, but it most certainly leads to conflicting information.
Electrochemical:
Electrochemical battery cells are in widespread use in nearly all
aspects of society. Our toys, tools and cars all require electricity, which is
provided by batteries. Many different types of batteries are being investigated
for use in hybrid and electric vehicles. Some are familiar and in current use, but
some are new and more exotic. The U.S. Department of Energy has joined an industry group called the United States Advanced Battery Consortium (USABC) to help develop these advanced batteries. The USABC has set ambitious goals for acceptable battery capabilities and is sponsoring development through
research grants. Table 2 shows some of the Consortium's goals. Other
manufacturers not involved with this effort are also pursuing their own research.
The following sections will describe various electrochemical batteries.
Table 2: USABC Goals
USABC Goals Specific Energy Specific Power Lifetime Cost
Watthour/kg Watts/kg years kW'hour
Mid-Term 80-100 150-200 5 <$150
Long-Term 200 400 10 <$100
31 will be They classified by availability as near, mid, or long term, and any companies which are actively pursuing development of the technology will be named. Hard numbers will not be given for two reasons. The first is that available data is inconsistent and not comparable. The second reason is that the industry is changing rapidly. Research programs are proceeding at breakneck speed, and the arrival of the USABC has further intensified the work.
Competition has made information into proprietary secrets, and any values given would be immediately out of date. The names of key companies will be given so that more information can be obtained.
Lead Acid:
Lead acid storage batteries are the most common large battery in use today, and are a near term battery option. They are cheap and rugged, easily recyclable, and adaptable to nearly any application. However, when compared to other batteries, they tend to be heavy, have lower energy density, and poor low temperature performance. Lead-acid batteries also have a limited cycle life which depends on discharge levels and maintenance.
Lead-acid batteries have been widely used in hybrid and electric vehicle efforts. They were the only affordable option until recently when more exotic types of batteries have been scaled up and taken out of the laboratory.
The resulting vehicles are very useable, but suffer from the predictable shortcomings: excessive weight which leads to poor performance, and short range due to the low energy density. The new generations of lead-acid battery technologies have shown steady improvements which have kept them a viable technology, and they will continue to be used, perhaps extensively, because they cheap, durable, and available.
The lead-acid battery industry is huge, and only a few companies can be named here. Electrosource, Inc., of Austin, Texas, has developed a high
32 performance lead acid battery called the Horizon, which it is supplying to major auto-makers24. Other cells are available from: Johnson Controls, Inc., EXIDE,
Delco, Chloride Motive Power, and Sonnenschein, among many, many others25.
Nickel-Cadmium:
Nickel-cadmium batteries are another near term technology that is currently in widespread use. (They are usually known as Nicads). Nicads have excellent power characteristics with acceptable energy storage. They have a
long cycle life, are tolerant of deep discharges, and can be recharged very
rapidly. However, they can not be recycled easily, generate a great deal of heat during discharge, and are expensive because of the nickel electrode.
Nicads have also been used in test vehicles with very good results.
The vehicles outperform others which are powered by lead-acid batteries, but at
many times the cost. Nissan has demonstrated that Nicad battery packs can be
recharged very quickly, and that overall vehicle performance is good26. The cost
of the battery is still a problem to be addressed.
Several companies market Nicad batteries, including: Saft America,
Inc., of Cockeysville, Maryland, and Marathon Power Technologies of Waco,
Texas.
Nickel-Metal Hydride:
Nickel-Metal Hydride batteries are identified by the USABC as a mid
term technology. They have excellent specific energy and peak power, and can
24 Chrysler Volume Number 1 Horizon Battery Selected for Minivan, Evolution, III, , May/June 1995, p6. (Published by the Electric Vehicle Association of the Americas). 25 Madeka, F. C, Davis, G. W., Hodges, G. L., The Selection of Lead-Acid Batteries for Use in Hybrid Electric Vehicles, SAE Technical Paper Series #940338. 26 Fukino, M., Irie, N., Ito, H., Development of an Electric Concept Vehicle with a Super Quick Charging System, SAE Technical Paper Series #920442.
33 tolerate many charge/discharge cycles. Nickel-metal hydride batteries suffer from high self discharge rates, meaning they do not hold a charge very well.
They generate heat during charging, and are expensive because of the nickel electrode.
One unique feature which neither lead-acid or Nicads can duplicate is that the capacity of the nickel-metal hydride battery is nearly independent of discharge rate. With both lead-acid and Nicad, a slow discharge will yield more energy than a fast discharge. With the nickel-metal hydride cell, no matter how quickly or slowly the current is drawn, roughly the same amount of energy can be extracted. This is an important feature because hybrid and electric vehicles experience a wide range of power requirements during travel. Rapid acceleration will draw a lot of current from the battery, and it is important that this not incur any penalties.
Nickel-metal hydride cells have many favorable characteristics which make them well suited to vehicle use. Problems still exist in developing manufacturing methods, and cost remains an issue. The USABC has awarded research contracts to: Ovonic Battery Corp., of Troy, Michigan, and to Saft
America of Cockeysville, Maryland, to further develop this technology27.
Sodium-Sulfur:
The sodium-sulfur battery is another USABC mid-term battery technology. In contrast to the solid electrodes of other battery types, the sodium electrode in this battery is molten, and is pumped through the cells of the battery. The characteristics of this battery are almost universally good, except for one. The molten electrode must remain molten in order to generate electricity.
This requires a high temperature system, and despite the best insulation
27 USABC Update, United States Advanced Battery Consortium, October 1994.
34 methods available, heat losses are inevitable. During operation this is not a big problem, but when the vehicle is not being used the electrode must be kept molten. A significant part of the energy stored in the battery is used to maintain it at operating temperature. In addition, the battery has a very short calendar life of about 2 years. Both of these issues compromise vehicle range and performance.
Despite the high temperature drawback, this type of battery is being actively developed. Ford has prototyped several vehicles using sodium-sulfur battery packs28. The USABC has awarded a research contract to Silent Power
GmbH, of Essen, Germany, for further development29.
Zinc-Bromide:
Zinc-bromide represents another flowing electrode design, but this technology operates at room temperature. This type of battery has good characteristics, and its design is very flexible. The size of the electrode stack determines the power available, while the quantity of liquid electrode determines the energy content. This indicates that some interesting features may be available, such as adding more electrolyte tanks when going on a long trip30. A sophisticated maintenance and control scheme is required for correct operation, but the batteries promise low manufacturing cost and long life31.
The zinc-bromide battery has interesting advantages, but uses largely unproven methods. Proving the reliability and safety of the required systems during the daily abuse of normal automotive usage will be a very important task.
28 Life With an Electric Ford, Car and Driver, September 1994, p 147. 29 USABC Update, USABC, October 1994. 30 Burke, A. F., Herriksen, G. L., Electric Vehicle Design and Performance Using Advanced Batteries. SAE Technical Paper Series #891663. 31 Zagrodnik, J. P., Bolstad, J. J., Miles, R. C, The Zinc/Bromine Battery: Recent Advances for Electric Vehicle Applications, SAE Technical Paper Series #891692.
35 Zinc-bromide batteries are being researched by companies such as: S.E.A. of Austria, and Johnson Controls, Inc., and are reasonably well but it is developed, unclear if they will be scaled up to production.
Aluminum-Air:
This battery represents another intriguing departure from conventional
and is thinking, actually closer to being a fuel cell than a battery. The aluminum
anode of the can be battery physically removed when it is discharged, and a new
anode inserted in its place. The battery is instantly, and fully recharged. The is energy capacity excellent, but issues such as power delivery and size are still
being investigated. The cell also produces a significant amount of heat and
requires a cooling system32. These cells continue to be tested with varying
results33, and seem to function best when hybridized with another type of
battery34 which has high power capabilities.
Alcan International Limited of Kingston, Ontario, Canada, is one
company which is working on the aluminum-air battery.
Zinc-Air:
Zinc-air batteries use a similar scheme as aluminum-air batteries, and
offer even higher energy density. Both zinc-air and aluminum-air have very long
shelf lives, good environmental characteristics, are cheap, and their capacity is
independent of load. The zinc-air battery requires a cooling system, as well as a
system to humidify the air coming into the cell. In the same way as aluminum-air
32 Rudd, E. J., The Development of Aluminum-Air Batteries for Electric Vehicles. SAE Technical Paper Series #891660. 33 Parish, D. W., O'Callaghan, W. B., Fitzpatrick, N. P., Anderson, W. M., Demonstration of Aluminum-Air Fuel Cells in a Road Vehicle. SAE Technical Paper Series #891690. 34 Cheiky, M. C, Danczyk, L. G., Hobbs, R. S., A Technical Overview of an Electric Saturn at the 1993 Solar and Electric 500, SAE Technical Paper Series #931791.
36 cells, the zinc anode can be mechanically replaced to provide a full recharge of the device. Like the aluminum cells, the zinc-air battery works best when used together with a high specific power battery35.
Zinc-air batteries are being developed by: Westinghouse Electric
Corp., and Zinc Air Power Corp., of Canton, Mich., among many others.
Sodium-Nickel Chloride:
This is yet another type of battery which being investigated. These
cells will tolerate hundreds of cycles in which they are completely discharged,
without being damaged. They have good capacity retention throughout their life
span, and also maintain their internal resistance36. Rapid recharging is feasible,
but reliability remains dubious. These batteries are still in the laboratory, but are
yielding promising results.
These cells are being developed by: Hughes Aircraft Company of
Torrance, CA, and Eagle Picher Industries of Joplin, MO, among others.
Others:
This list is by no means complete. Many other technologies such as
Nickel-Iron from Eagle-Picher, and Zinc-Nickel Oxide from Acme Electric Corp.37,
are being investigated. Independent manufacturers are certain to be developing
other possibilities as well.
35 Zinc- Cheiky, M. C, Danczyk, L. G., Scheffler, R. L., Air Powered Electric Vehicle Systems Integration Issues. SAE Technical Paper Series #910249. 36 Sudworth, J. L., Bbhm, H., Performance Data from an Improved Sodium/Nickel Chloride Cell. SAE Technical Paper Series #911915. 37 Electric and Hybrid Vehicles Program, 17th Annual Report to Congress for Fiscal Year 1993, August 1994, DOE/EE - 0026.
37 Long-term Possibilities:
The USABC is investigating lithium batteries, and believes that they
have the potential to meet all of the long-term goals. Lithium is the lightest and
most reactive of all metals and holds great promise for batteries. One drawback
is that lithium is a very toxic material.
One technology is lithium-ion cells. These cells use a lithium-carbon
compound as the negative electrode. (No lithium metal is present in the cell).
technology" They utilize so-called "rocking chair (RCT) during charge and discharge. This nickname describes the motion of lithium ions as they move
back and forth between the plates of the cell. This is a key advantage because
no material is plated and the plates therefore last much longer. The cells
promise very good fast charge capabilities, but would require a thermal
management system to maintain performance during cold weather. Yardney
Technical Products Inc., of Pawcatuck, CN is researching these cells, and the
USABC has awarded a research contract to the team of Duracell Inc., of Bethel,
CN, and Varta Batterie AG of Hanover, Germany38.
Another lithium cell uses lithium metal for its electrodes. The extreme
reactivity of the metallic lithium requires special consideration. The electrodes deteriorate quickly during plate/replate cycles, and lose effectiveness. To compensate for this, solid polymer electrolytes are bonded to the electrodes.
This makes a lithium-polymer cell. These cells have the potential for flexible shape and production, good safety because they use no liquid, very low self- discharge, and exceptional specific energy39. The USABC has awarded research
38 USABC Update, United States Advanced Battery Consortium, Winter 1995. 39 The Searchfor Better Batteries, IEEE Spectrum, May 1995, pp. 51-56.
38 contracts to W.R. Grace of Boca Raton, FL, and 3M of St. Paul, MN to further develop this technology40.
The USABC has also awarded a contract to Saft America to develop lithium-iron disulfide batteries which use liquid electrolyte with solid lithium metal.
40 USABC Update, USABC, October 1994.
39 Electrochemical Comparison:
The following table shows a quick comparison of the electrochemical technologies. It indicates the likely timetable of their availability, and mentions noteworthy characteristics of each type.
Table 3: Summary of Electrochemical Batteries
Electrochemical Availability Features Technology
Lead-Acid Immediate cheap, durable, and available
Nickel-Cadmium Immediate expensive, quick recharge, excellent power density
Nickel-Metal USABC sponsored expensive, excellent performance * Hydride Mid-term characteristics
Sodium-Sulfur USABC sponsored high-temp, molten electrode, * Mid-term excellent performance characteristics
* Zinc-Bromide Mid-term room-temp, flowing electrode, good performance characteristics
* Aluminum-Air Mid-term mechanical recharge, excellent energy density
* Zinc-Air Mid-term mechanical recharge, excellent energy density
** Sodium-Nickel Long-term good charge/discharge Chloride characteristics, unknown reliability
Lithium-Ion USABC sponsored outstanding characteristics, requires ** Long-term thermal management system
Lithium Metal USABC sponsored outstanding characteristics, flexible ** Long-term production
before the year 2010
"after the year 2010
40 Flywheels:
At the beginning of this section, the definition of a battery was
extended to include any devices capable of storing electricity. Flywheels are
mechanical batteries which store energy in a rotating disk. This arrangement has
several advantages: long life, complete imperviousness to all weather
conditions, and the ability to recharge quickly. This is an important feature for
regenerative braking systems, and is possible because the flywheel is not
restrained by time consuming chemical reactions. The charge/discharge rates
are limited only by overheating of the electrical drive mechanisms41.
The rotational speed of the disk is a problem which has only recently
been addressed satisfactorily. The allowable speed of the flywheel is controlled
by the strength of its components. As rotational speed increases, centripetal
forces eventually tear the flywheel apart. Flywheels intended for vehicle use
need to be small and light. To compensate for their lack of mass they spin very fast, because energy stored is proportional to the square of the rotational speed.
Modern materials, such as composite fibers have finally allowed construction of flywheels which are acceptable for in-vehicle use.
Modern flywheels feature composite disks with integrated motor/generators that spin on magnetic bearings inside vacuum chambers.
Motor windings are included on the main shaft of the flywheels to allow energy input and recovery. When electricity is put into the motor windings, motor action accelerates the flywheel. When electricity is needed in the vehicle, the windings act as a generator and the kinetic energy of the rotating wheel is used to generate electricity. The magnetic bearings and evacuated housing are both crucial to the success of the flywheel. The drag on the wheel, both aerodynamic
41 Compositeflywheel rotorsfor hybrid Ev's, Automotive Engineering, October 1995, pp. 25-26.
41 and frictional, is minimized allowing the flywheel assembly to maintain its spin for a useful length of time.
Several companies are actively developing flywheel technology, including Unique Mobility, Inc., of Denver, and SatCon Technology Corp., of Cambridge, MA, as well as Lawrence Livermore National Laboratory, Oak Ridge
National Laboratory, and the University of Ottawa. Another company, American
Flywheel Systems (AFS) of Bellevue, WA, has built a prototype vehicle, and claims that their flywheel batteries can power the entire vehicle for 350 miles42.
The AFS concept vehicle highlights a couple of potential drawbacks including high cost and relative fragility. AFS has gimbaled the flywheel batteries to isolate them from minor road shocks, but durability, especially during a collision will be very important for development43.
A related consideration is that each flywheel battery has its own
motor/generator setup with corresponding inefficiencies. Having twenty separate motors in a battery system, for example, increases the likelihood of
maintenance problems, as well as cost, but increases the redundancy/safety
characteristics of the overall battery pack. A higher capacity battery would
lessen this dilemma, but has yet to be developed.
Ultracapacitors:
A capacitor consists of two or more electrodes which are separated by
a dielectric material. This arrangement allows energy to be stored through the
displacement of charge between the electrodes. Capacitors have been widely
used for building circuits in nearly all electronic devices. Recent advances have
42 A Glimpse at the Supercar. Design News, October 10 1994, pp. 1 10-1 18. 43 The Flywheel Battery Car, Car and Driver, June 1994, pp. 90-91.
42 indicated that capacitors can now be built which are able to store significant amounts of energy.
The new generation of capacitors are being called ultracapacitors because they represent a large improvement from current devices, yet operate on the same principles. Charging a capacitor involves applying an electric potential and inducing a charge on the electrodes. No chemical reactions or motions are involved, so this process occurs nearly instantaneously. The high current capabilities combined with the storage capacity of the new ultracapacitors have made them an intriguing prospect for vehicle propulsion.
"hybrid" They would be nearly ideal for use in a battery as discussed earlier. The main storage battery cannot cope with high power demands, so another small battery is needed. This secondary battery needs characteristics remarkably similar to those of the ultracapacitor: the ability to accept and provide electric
power" current at high rates. This makes them so called "pulsed devices44.
Ultracapacitors are particularly attractive because they have extremely high
round trip (into and out of) efficiency. If these new capacitors can store enough energy to be practical, then they have immense potential for future vehicle use45.
One of the new technologies is double layer capacitors, which immerse the electrodes in an electrolyte. Energy is stored in a polarized layer which forms in the liquid electrolyte between the electrodes. Much of the current research is focused on finding better electrodes. Carbon/metal fiber composite, aerogel carbon, mixed metal oxides, and new doped polymers are some of the materials
made47 being researched46. Encouraging progress has been and further
44 "pulses" This refers to their ability to handle large of energy in short periods of time. 45 Burke, A. F.( Electric/Hybrid Super Car Designs Using Ultracapacitors. Institute of Transportation Studies, University of California at Davis. 46 Surging Ahead With Ultracapacitors, Mechanical Engineering, Volume 117, No. 2, February 1995, pp. 76-79.
43 improvements are being tested. Federal research laboratories and companies
such as Pinnacle Research Institute of Los Gatos, California, are developing various aspects of ultracapacitors, and the USABC is also considering further development48.
Others:
A huge of variety energy accumulators have been developed for many applications. One of them, the hydraulic accumulator has experienced renewed
interest for use in vehicles. New hydraulic motors have been developed with
good efficiencies and benign operating characteristics which make them
acceptable for vehicle propulsion49. A hydraulic accumulator stores hydraulic
fluid under pressure to be used at a later time. This allows many of the benefits
of a hybrid drivetrain which have been discussed in this paper: the storage
buffer means that the engine need not operate continuously, and that
regenerative braking is possible. This represents a very unconventional
approach to energy storage, together with a drive system which has not been
used for passenger vehicles. widely Consumer acceptance, as well as safety
certifications may be difficult to achieve.
Another technology which has been considered is superconducting
magnetic storage, but it is not considered promising for light-duty vehicle
applications due to cost, size, and development difficulties.
Other methods such as mechanical storage of energy in a spring element (mechanical or gas) are available, but are not being developed.
47 Trippe, A. P., Burke, A. F., Blank, E., Improved Electric Vehicle Performance with Pulsed Power Capacitors. SAE Technical Paper Series #931010. 48 Electric and Hybrid Vehicles Program, 18th Annual Report to Congress for Fiscal Year 1994, April 1995, DOE/EE - 0059. 49 Hydraulic Drive, Popular Science, June 1995, p 34.
44 Battery Comparison:
Solid, comparable numbers are difficult to obtain, but predictions based on current development have been made to indicate what should be feasible.
Table 4 shows what various technologies are expected to achieve in the next few years. These data are given for reference and should not be taken as absolute.
50 Table 4: Summary of Battery Technologies
Technology Specific Energy Specific Power Life Watthour/kg Watt/kg
Electrochemical 80 150 5 years USABC mid-term goals
Flywheel 20 - 200 >1000 nearly infinite
Ultracapacitor 15 >1000 nearly infinite
Notice that specific energy remains low compared to gasoline (400
Whr/kg). This indicates that pure electric vehicles will probably still suffer from
worse range than their gasoline counterparts. Hybrid vehicles do not require
extremely long electric operating range, but instead need high power output from
the electric driveline to achieve good acceleration. Based on the information in
Table 4, hybrid vehicles are likely to become an even more attractive
compromise because they can make good use of available components.
50 Hybrid Propulsion Plan, U.S.D.O.E. Office of Transportation Technologies, October 1994.M51-A214901.
45 Motors:
Electric motors can be divided into two major groups: direct current and alternating current. A number of special motors also exist which blur the lines between the categories. This section will provide an overview of the most common motors which have been used for vehicle propulsion in the past, and the most likely candidates for future use. There is a no perfect vehicle motor; each of them has its own strong points which will help determine its application.
DC Motors:
The basic types of DC motors are named by how the field windings are connected: series, shunt, and compound. New technologies have yielded two
important variations on these: the permanent magnet motor, and the brushless
motor.
DC motors have always been relatively easy to control with
inexpensive controllers. This has made them common in nearly all applications,
but they also have some minor drawbacks. Conventional DC motors use brushes
and commutators to transfer current to the rotor. This restricts their performance
and adds a tiny amount of maintenance due to brush wear. They also tend to be
somewhat larger, heavier, and less efficient than AC motors.
The widespread use of solid state electronic controls has greatly
extended the capabilities of the traditional DC motor, as well as improving
efficiency. DC motor systems still fall short of the performance of AC systems,
but this is offset by much lower prices. It is very likely that they will remain the
most economical choice for near term vehicles, without incurring substantial
penalties to the functionality of the vehicle.
46 Series:
The series DC is the most common motor which has been used for vehicle traction applications. It gets its name from having the field winding connected in series with the armature. This yields the following characteristics:
very high startup torque
speed tending to infinity at zero load
reversing requires transposing the field winding and armature
connections
difficult regeneration with tendency to go unstable, but this is easily
countered by solid state electronic controls
The series DC motor is well suited for vehicle propulsion because of its excellent low speed torque characteristics. The shortcoming of the series motor is its
"runaway" tendency to at low loads and destroy itself. This has historically been a problem, but modern electronic controllers can easily compensate for this and for the difficult regenerative properties.
Shunt:
The shunt motor has its field winding connected in parallel with the armature, yielding the following characteristics:
moderate startup torque
high loads can cause excessive current flow through the armature
and damaging heat generation
reversing requires transposing the field winding and armature
connections
excellent, very stable regenerative capabilities
Shunt motors run at nearly constant speed regardless of load. As load increases the current through the motor increases correspondingly, and the high currents
47 produce excessive heat which can damage the motor. The shunt motor cannot produce as much startup torque as a series wound motor, but has excellent regenerative properties and can perform as well as a series motor when used with an electronic controller.
Compound:
The compound motor represents a combination of both series and shunt motor windings. The cumulative compound motor connects the windings together so that they assist (rather than oppose) each other, and is the most
useful for vehicle applications. The cumulative compound motor exhibits these characteristics:
slightly higher torque than the shunt motor
eliminates runaway speeds associated with series
the series winding can be designed to give good torque, and the
shunt winding to give lower current draw and better regeneration at
high speeds
reversing depends on the connection of the windings, but is not
difficult
excellent regeneration characteristics from shunt winding
The compound motor can be the best of both worlds when compared to series
and shunt motors, and modern controllers allow these characteristics to be
further improved.
Permanent Magnet:
New technologies have allowed the manufacture of very powerful
permanent magnets which are suitable for use in motors. These magnets
allows the motors to be smaller and replace the field winding of the motor. This
because the field is not lighter, as well as simpler and more efficient winding
48 required. They continue to use a commutator and brushes, and resemble a shunt motor in characteristics. However, they are able to produce several times more torque and have more predictable responses to loading.
Brushless:
Modern technology has also allowed the commutator and brushes to be eliminated, and with them almost all maintenance concerns. This is accomplished through the use of high power semiconductors. In these motors, permanent magnets are used on the armature and the main windings are moved into the motor case. With signals from position sensors, the electronics alter the fields generated by the main windings in such a way as to generate motion of the rotor.
Brushless motors exhibit performance similar to permanent magnet motors, and are even more efficient because they do not require a commutator.
Since the operation of the motor is software controlled, significant alterations in characteristics can easily be obtained. In practice, a brushless DC motor very closely resembles an AC motor, and the distinctions between them are few.
AC Motors:
AC motors are small, light, and efficient, and are becoming the motor of choice for vehicle propulsion. The widespread use of solid state electronic controls is a key factor in making AC motors appropriate for vehicles. Without these controls, they are basically constant speed machines. AC motors exhibit
many improvements over DC motors, including:
49 elimination of commutator and brushes
higher efficiency
better reliability
smaller size and lighter weight
lower cost
To a certain extent, these are the potential benefits. Although the motors are
relatively simple and inexpensive, the control electronics are not. AC drive systems are very expensive, but further development will bring prices down as
production quantities increase.
Synchronous Motors:
Synchronous motors are the simplest type of AC motor, and are
essentially alternators which are being fed current. The motor operates in step
with the excitation fields (hence the name synchronous), but has serious
limitations for vehicle traction applications. A synchronous motor does not
generate any torque at zero rpm, and therefore will not start on its own. A
different type of winding is required to provide the initial motion. In addition to
this, the design is best suited to single speed applications, although variable
speed can be obtained by manipulating the frequency of the input current. Both
the torque and speed problems can be overcome, but the work and expense
cannot be justified due to the ready applicability of induction motors.
Induction Motors:
There are two basic types of induction motors, the squirrel cage and the
wound rotor.
The rotor of a squirrel cage motor consists of conductor bars which are
placed parallel to the shaft of the motor close to the rotor surface. The bars are
short circuited together by end caps. The windings in the stator, or "case", of the
50 motor are positioned such that a rotating magnetic field can be generated. When this field interacts with the rotor, a current is induced in the conductor bars. The induced current creates magnetic fields in the rotor, which in turn interact with the fields from the stator, and the rotor turns. The rotor always turns slightly
"slip" slower than the rotating field of the stator. This is called and occurs
because the fields in the rotor are induced by the motion of the stator fields through the conducting bars. The relative motion is required to maintain the field.
If the rotor turns at the same speed as the stator field, then no current is
induced, and no field or torque is generated. Even at full load, slip is always
small, usually from 1-4%51, and does not affect the operation of the motor.
A wound rotor motor uses a rotor which is wound with wire. The ends of
the wire are brought out of the motor using slip rings, allowing the resistance of
the rotor to be varied. Using a high rotor resistance allows high starting torque
with low currents. The resistance can then be gradually lowered as speeds rise,
until the rotor is short circuited and the motor behaves like the squirrel cage
motor. The arrangement allows control over the torque generated by the motor.
The simplest form of AC motor is the single phase induction motor.
Adding additional phases increases the power density and improves torque
characteristics. The standard multi-phase arrangement is three phase. A three
phase motor has the following advantages over a single phase motor: generates
torque at zero rpm, has insignificant rotor current and losses at no load, and
exhibits lower overall power losses52. This is not a new discovery, but only
recently have controllers become sophisticated enough, and magnets strong
enough53, to make these motors practical. Approximately three times as many
51 Electric Motors. Anderson, Edwin P., Theodore Audel & Co., Indiana, 1968, p. 78. 52 Energy Efficient Motors, Andreas, John C, Marcel Dekker, 1992, pp. 17-18. 53 Drive Systems with Permanent Magnet Synchronous Motors, Automotive Engineering, February 1995, pp. 75-81.
51 components are required to generate the extra phases, and this extra bulk has always limited three phase AC motors to stationary applications where size and weight were not critical. Electronics have drastically reduced in size and weight and allow three phase motors to be used in vehicles. They are nearly universally
prototypes54 used in advanced because of their many desirable features.
Special Motors:
Many kinds of special purpose motors exist, and some have
applicability to vehicles. One of these is the switched reluctance drive55. This is a
brushless DC machine, but does not use magnets in the rotor. Instead, the rotor
consists of laminated iron bars which spin to orient themselves with the stator
field. The stator field is generated by power electronics together with real time
sensors. The motor is high strength, low inertia, ultra high speed (up to 100,000
rpm), and runs nearly without heat generation. The computer control is key to its
operation, and allows the speed and torque characteristics to be tailored Using
software. This type of motor seems well suited to vehicle propulsion, but is a
new idea which will need much testing.
54 the GM with vehicles from AC Propulsion A quick survey reveals that Impact, along , US Electricar, and Solectria all use three phase AC motors. 55 Light- Lovins, A. B., Barnett, J. W., Lovins, L. H., Supercars: The Coming Vehicle Revolution, 1993, Research Report of the Rocky Mountain Institute, Snowmass, Colorado.
52 Alternative Fuels:
Alternative fuels are attractive because they have the potential to burn cleaner than gasoline, and they can usually be produced from renewable sources. Hybrid vehicles offer exciting possibilities for alternate fuel usage.
A number of different fuels are available and research is continuing, but all must be evaluated against the following issues56:
capacity for production
infrastructure to distribute to users
on vehicle storage capability
availability of suitable engines
exhaust emissions
COST
These issues are important because alternate fuels will need to display significant benefits in order to prompt a large scale change. Incremental
improvements of a few percent are unlikely to be seriously considered. The current industry is well entrenched and unlikely to change, and building a completely new infrastructure is not practical.
This section will outline the major alternate transportation fuels which are
available, and indicate the implications of their use.
Reformulated Gasoline:
Reformulated gasoline (RFG) represents the easiest way to change fuels, because it consists of traditional gasoline which has additives to reduce exhaust emissions. It is already in use in several states due to federal mandates.
56 Hydrogen as Alternative Automotive Fuel, Automotive Engineering, October 1994, pp. 25-29.
53 The additives are intended to reduce emissions without compromising vehicle performance.
Currently, oxygen is added to gasoline in order to reduce the amounts of carbon monoxide in the exhaust. This is accomplished by adding methyl tertiary butyl ether (MTBE) or tertiary amyl methyl ether (TAME) to gasoline. It is expected that this will shift to using ethanol as a winter additive and ethyl ethers as a summertime additive57.
RFG is expected to provide the same driving performance as current
gasoline with improvements in exhaust emissions, a small decrease in economy,
and cost of a few cents more per gallon. It will be distributed through the same
network and use the same infrastructure which is currently in use. RFG is a short
term attempt to reduce air pollution, but does not improve energy security
because it can not be made from renewable sources.
Natural Gas:
Natural gas is a fossil fuel that is currently used for a wide variety of
heating needs in buildings and homes. The infrastructure for its production and
distribution is already in place, and could possibly be utilized by the vehicle
industry. Compressed natural gas (CNG) is the favored storage method, but
liquefied natural gas (LNG) can provide longer ranges. LNG is dependent on
cryogenic storage methods that are not yet adapted to vehicle use. CNG, on the
sources through the use of compressor other hand, is available from current
stations. These stations would take the low pressure gas from the distribution
rate vehicles. network and provide it at high pressure and high flow to
57 Gasoline Composition Trends Predicted for the Post-2000 Era, The Clean Fuels June p!72. Report, J. E. Sinor Consultants Inc., Volume 6, No. 3, 1994,
54 Natural gas has higher octane than gasoline, but is more difficult to store. The compressed gas requires heavy, bulky containment cylinders which compromise vehicle performance, require the special fueling equipment mentioned above, and increase costs. The high pressure system can also cause consumers to perceive a risk, despite stringent safety regulations. The size of the storage containers makes natural gas well suited to heavy duty vehicles which can better accommodate them.
Many different types of vehicles have been built which use natural gas as a fuel58. They have proven to be capable of excellent exhaust emission characteristics, with a couple of them meeting California's Ultra Low Emission
Vehicle standards, only one step away from being ZEV's59. Studies have also shown that when the complete fuel cycle emissions are considered60, natural gas vehicles can outperform even pure electric vehicles for certain emissions. New
designs promise to extend these results farther into the vehicle market61.
Propane:
Propane is another fuel which is commonly used for a variety of
purposes. It has a moderately well developed infrastructure and delivery
network, but could not handle the volume required for general vehicle
applications. Substantial construction would be required to meet increases in
demand.
58 Auto Manufacturers Offer Variety ofAFV's in MY 1995, AFDC Update, published by the Alternative Fuels Data Center, Volume 3, Issue 3, February 1995, p6. 59 CARB Updates Cost Estimates for Ultra-Low-Emission Vehicles, The Clean Fuels Report, Volume 6, No. 3, June 1994, p6-9. 60 The complete fuel cycle includes all the emissions attributable to the production,
conversion, transportation, and marketing of the fuel. 61 DOE Unveils Advanced Natural Gas Vehicle, AFDC Update, Volume 3, Issue 4, April 1995, pi.
55 Although the containment cylinders for propane are not as heavy or safety critical as those for natural gas, there is still perceived risk. Propane is a high octane fuel, and it has been predicted that for heavy duty applications it could compete effectively with diesel fuel62. A key benefit of propane is extremely low exhaust emission levels. Propane powered industrial equipment has been safely used indoors for many years.
Alcohol Fuels:
Methanol and Ethanol are widely known, but not widely available, alternate fuels.
Methanol is a colorless, odorless, toxic liquid fuel. It can be produced from natural gas, wood, coal, crop residues, and some municipal wastes63. The
industry for production is already in place and supplying significant quantities.
The technologies must be made more efficient and scaled-up to meet increased demand. Methanol has higher octane than gasoline, but also has a lower vapor
pressure which makes cold starting difficult. In addition to this, methanol
produces different emissions from gasoline. New catalyst systems are required to provide adequate control of exhaust emissions. Methanol is currently mixed with gasoline to reduce cold-start problems, and is available as blends such as
M85 (85% alcohol, 15% gasoline).
Ethanol is another non-fossil liquid alcohol fuel. It can be made from
grains and sugars, as well as many other bio-mass substances which are
essentially limitless. It has roughly the same characteristics as methanol: high
octane and poor cold starting, but is not nearly as toxic. Ethanol is water soluble
62 Role Seen for Propane in Heavy-Duty Applications, The Clean Fuels Report, Volume 6, No. 3, June 1994, p98. 63 Cantoni, U., Alternative Fuels Utilization in Fuel Cells for Transportation. SAE Technical Paper Series #931816.
56 and biodegradable, which is good for spills, but may be bad for corrosion.
Ethanol is also blended with gasoline to reduce starting problems using blends such as E85.
The advantages of alcohol fuels are high octane, and the fact that they can be synthesized from renewable sources. Their lower energy density causes storage problems, and in terms of emissions performance one study concluded that methanol had no advantage over RFG64. Alcohol fuels have attractive possibilities, and await further development work. They are certain to be used in some form as a transportation fuel.
Hydrogen:
Hydrogen is the holy grail of fuel research. When burned it produces only water vapor and trace amounts of nitrogen oxides. However, it is expensive to produce, difficult to store, and virtually no infrastructure exists for it. Using
hydrogen as a transportation fuel would require starting from scratch. When
used in an internal combustion engine, hydrogen produces about 20% less
power than gasoline. Although it is attractive as a combustion fuel, hydrogen can
70- be more efficiently used in a fuel cell to generate electricity. The theoretical
90% efficiency of the hydrogen fuel cell is far better than combustion could
achieve65.
Pure hydrogen is notoriously difficult to store, especially on board a vehicle. The most conventional method is to use a simple alloy tank to store
hydrogen in its liquid form. However, it must be stored cryogenically in order to
remain a liquid. The support systems such as coolers and insulators make this a
64 Texaco Questions Cost-Effectiveness of Alternate Fuels, The Clean Fuels Report, Volume 6, No. 3, June 1994, pl4. 65 Fuel Updated , The Clean Fuels Use ofHydrogen as an Automotive Report, Volume 6, No. 3, June 1994, pl43.
57 bulky and inefficient method of storing a small amount of liquid. Although it is being investigated66, the liquid storage tank does not seem ideally suited to vehicle use.
Another solution is being developed in the laboratory. Materials known as metal hydrides have the unique ability to absorb hydrogen into interstitial spaces in their structure. Hydrogen is absorbed under temperature and pressure, and then safely stored until the conditions are re-established. Similar research is being performed with activated carbon as the base material67. Both activated carbon and metal hydrides store pure hydrogen more effectively than liquid storage, and promise to be a much needed solution.
Summary:
Another approach available on the market is flexible fuel (FFV), or bi- fuel vehicles. These are setup to accept more than one type of fuel, such as methanol and gasoline or compressed natural gas and gasoline. This allows more flexibility for the user to find fuel since some alternate fuels are scarce. The price for fuel flexibility is reduced efficiency. Since the engine must accept different fuels, it cannot be optimized for any one of them and is always compromised. This means that exhaust emissions and fuel consumption are higher than they could conceivably be. This may still be better than conventional vehicles, but is not optimum. The flexible fuel approach is as attractive for hybrids as it is for any vehicle, because it allows the benefits of alternate fuels when they are available, and the convenience of standard gasoline.
66 Advanced Research at BMW, Automotive Engineering, October 1994, pp. 12-15. 67 Hydrogen Storage on Activated Carbon, New York State Energy Research and Development Authority, Report #94-20, November 1994.
58 Although it is not clear when they will enter widespread use, alternate fuels are likely to be a critical part of future vehicles, and hybrids are positioned to take advantage of them. The combination of high efficiency and smaller APUs mean that hybrids do not need to store as much fuel as current vehicles. Near steady state operation will also allow very good control of combustion and avoid
some of the power problems encountered in conventional vehicles.
While it is certain that alternative fuels will be more widely used in the
future, it is equally clear that gasoline is not going to be eliminated any time
soon. The infrastructure and vehicle base are simply too large for rapid changes
to occur.
59 Auxiliary Power Units:
The APU is a crucial part of the hybrid vehicle because it allows the vehicle to have a useful traveling range, and also because it has a large effect on environmental performance.
This section will describe available APU technologies and indicate how they would perform in a hybrid vehicle. The technologies fall into three major groups: combustion engines, fuel cells, and direct conversion.
Combustion Engines:
The combustion engine, in various forms, represents the mainstay of current transportation. Virtually all vehicles, from diesel powered trains to turbine
powered aircraft, use combustion of a fuel as their power source. Hybrid vehicles
offer the unique opportunity of accommodating many different types of engines.
Internal Combustion:
The internal combustion engine is by far the most common type used
for land vehicle propulsion. Internal combustion can be achieved in several
different ways:
4 stroke:
The four stroke engine uses reciprocating pistons and spark ignition in
order to operate on the Otto cycle. It requires four piston strokes to complete a
power cycle and uses mechanically driven valves in the cylinder head to control
airflow. It is relatively simple and has a good power to weight ratio. Extensive
research has developed effective controls for its otherwise high exhaust
emissions. However, the four stroke suffers from poor partial-load efficiency and
is dependent on its emission controls at all times.
60 The four stroke engine is the most mature and refined of current engine
technologies. It has proven to be versatile, and a huge variety of cylinder and
valve configurations have been used for many diverse applications. The
manufacturing and repair infrastructure already in place is very comprehensive,
and will be important for future use of four strokes. It can be optimized for near
steady state operation, and will function most efficiently at high loads. These
characteristics, together with good power to weight ratios and effective emission
controls, make the four stroke engine a viable candidate for hybrid vehicles. Perhaps the most important factor concerning four stroke engines, however, is their familiarity. Like the lead-acid battery, the four stroke is well understood,
effective, and available. These factors alone assure that it will be a front runner
for the first hybrid vehicles.
2 stroke:
The two stroke also uses reciprocating pistons and spark ignition to
operate on the Otto cycle. However, it requires only two piston strokes to
complete a power cycle, and can either have its valves in the head, or built into
the cylinder walls. This type of engine is well developed and is used for nearly
all marine outboard engines, as well as recreational vehicles such as
motorcycles and snowmobiles. It is simpler and lighter than a four stroke, has a
high power to weight ratio, and is easy to manufacture. The lubrication for the
engine is carried with the incoming fuel charge, so it does not require lubricating
oil in the crankcase. This eliminates some of the oil leaks and the required oil changes which are so common with four strokes. Countering these attractive advantages is the fact that until recently, little work has been done to control exhaust emissions. Two stroke emissions are slightly different than four stroke emissions, so some new development is required.
61 The high power to weight, simplicity, and small size make two strokes very attractive for use in hybrid vehicles. The emission problems are mitigated by near steady state operation, reducing a key problem with current applications.
One company, Orbital Engine Corporation Limited of Western Australia, is particularly active in two stroke development. They claim to have produced engines which can meet the Ultra Low Emission standards while delivering fuel economy improvements of up to 30% compared to similar four stroke engines68.
These improvements, together with the overall performance of two stroke
engines make them very likely to be used in future vehicles.
Rotary:
The Wankel, or rotary, engine is a relative newcomer. In contrast to the
reciprocating pistons of two and four stroke engines, the rotary uses a triangular
piston which rotates on an eccentric shaft inside an epitrochoidal housing. It is
fired by spark ignition, and uses auxiliary equipment (such as fuel and ignition
systems) similar to those used on two and four stroke engines. The absence of
reciprocating parts means that these engines are inherently smooth running,
small, and lightweight. They boast high power to weight ratios, and can use the
same emission control technologies which have been developed for four stroke
engines. However, rotary engines demonstrate relatively poor efficiency, and
require good emission controls for acceptable operation.
Another variation on the rotary engine theme is the Rand Cam engine
which is being developed by Reg Technologies Inc., of Columbus, Indiana. This
engine has a cam shaped housing, and uses axial vanes to define the
combustion regions. It is a four-stroke, has only nine moving parts, and operates
at low speeds (about 2000 rpm). It is said to be lightweight, compact, quiet, and
68 Orbital Engine Corporation Limited, public literature.
62 have good multifuel capabilities with reasonable exhaust emissions. The engine is being developed in both spark ignition Otto cycle, and compression ignition versions. The rotary engine format has demonstrated important advantages, and the Rand Cam engine has the potential to be a useful device. Further
applicability.69 development will determine availability and
The rotary engine is well suited to hybrid vehicle use because of its high power to weight ratio, small size, and its ability to use available four stroke auxiliary technologies. Currently, only Mazda is using a Wankel style rotary engine for vehicle propulsion, but many others ranging from NASA to John
Deere are developing the Wankel70. Reg Technologies is continuing development of the Rand Cam engine. This active research is likely to further improve all aspects of performance and make rotary engines extremely attractive for future vehicles.
Newbold:
The Newbold engine is a very intriguing combination of both linear piston and rotary motion engines. Three pistons are spaced equally around the engine block, and are connected to an offset wheel/cam mechanism. The geometry of the system generates lateral forces in the pistons which causes the entire block containing the pistons to rotate. A supercharger forces air into the
"crankcase" area of the engine. The fuel is also introduced into the crankcase, and lubricates the moving parts as it is pushed out to the pistons. The pistons operate on a very short stroke with compression ignition, and the engine operates at speeds of up to 10,000 rpm. Extremely high compression ratios are
69 A New Spin on the Rotary Engine, Mechanical Engineering, Volume 1 17, No. 4, April 1995, pp. 80-82. 70 Gelman, D. J., Perrot, T. L., Advanced Heat Engines for Range Extender Hybrid Vehicles. SAE Technical Paper Series #930041.
63 achieved, and the fuel mixtures run very lean (16:1). The fuel is thoroughly combusted at relatively low temperatures, and therefore produces low levels of pollution as well as good fuel usage figures. The initial production run of engines was optimized to use diesel fuel, and produced 50 horsepower from a 25 pound package.
This engine has few parts, which promises low maintenance, and delivers excellent performance. It can use many fuels, and produces few exhaust emissions. The list of characteristics indicate that it is a good candidate for use as a hybrid vehicle APU. Reliability needs to be proven, and availability is
possibilities.71 questionable, but the Newbold engine offers superb
Diesel:
The diesel engine uses reciprocating pistons which are fired by compression ignition. They have low fuel consumption, high part-load efficiency, and excellent durability. They produce relatively low emissions, but generate significant particulate matter and objectionable noise. Diesels also tend to be large and heavy which gives them low power to weight ratios, but are very well suited to steady speed operation. They have traditionally been widely used for heavy duty applications, and are well suited to that role.
New development by big names such as Audi, is showing that the small diesel engine may have important contributions to offer. Direct injection of fuel shows increases in efficiency of up to 20%, and new particulate traps are reducing soot72. Diesel cycle engines are inherently efficient and have low emissions. As such they would be well suited to use in a hybrid vehicle.
71 A Reciprocating Rotating-Block Engine, Mechanical Engineering, Volume 117, No. 6, June 1995, p. 70. 72 The Greening ofthe Diesel, Popular Science, July 1994, pp. 46.
64 Problems such as weight and refinement are being addressed, and ultimate use
of diesel engines will depend on the results.
External Combustion:
External combustion engines have not been widely used for land vehicle propulsion, but they have meaningful benefits for hybrid vehicles.
Stirling Cycle:
The Stirling cycle engine heats a working fluid which is sealed into a closed loop. The heated fluid passes through a piston arrangement where it generates work, is cooled in a heat exchanger and then returned to the heater.
Since this engine can use any source of heat it is quite flexible and has been developed for use in space using solar heat. For land vehicle applications, a fuel
burner is used to generate heat. A Stirling engine is a complicated device, but it
state73 offers several advantages. The external burner operates at steady and low pressure, which allows very precise control of the combustion process. This yields excellent fuel economy and very low emissions. The drawbacks of this device are complicated design and high cost. It also exhibits low power to weight ratios, and requires large surface areas for cooling.
past74 The Stirling cycle engine has been used in vehicles in the with good success, but was hobbled by cost and complexity. New developments and modern technologies and materials have improved Stirling engine performance75, but they remain complicated and expensive. They are very well
73 Schreiber, J. G., Shaltens, R. K., Beremand, D. G., Evaluation of a Free-Piston Stirling Power Converter for the Ultra-Low Emission Hybrid Vehicle Application. SAE Technical Paper Series #930047. 74 Agarwal, P. D., Mooney, R. J., Toepel, R. R., Stir-Lee. A Stirling Electric Hybrid Car. SAE Technical Paper Series #690074. 75 Gelman, D. J., Perrot, T. L., Advanced Heat Engines for Range Extender Hybrid Vehicles. SAE Technical Paper Series #930041.
65 suited to hybrids due to their steady state efficiency, clean operation, and but flexibility, cost and weight are likely to decide the future of this engine for
vehicle applications.
Turbines:
Gas turbines have long been used to power aircraft, and have been
used successfully for land vehicles76. Acceptable operation of a conventional vehicle was never achieved, but hybrid vehicles have different requirements.
A gas turbine burns fuel in a combustor area, and uses the hot gases
produced during combustion to turn turbine blades. It runs very smoothly, is small and light with excellent power to weight ratios, and produces low emissions. Turbines are quite expensive however, have poor part-load efficiency and poor transient response.
"drawbacks" These actually fit very well with the requirements of hybrid vehicles. In a hybrid vehicle, it would be desirable to operate the turbine at steady state with high loads. In short, the turbine is nearly perfect. New developments such as the purpose built, low cost generator set from NoMac
Energy Systems, Inc., promise that the technology is feasible and that costs can be reasonable77.
Fuel Cells:
Fuel cells are actually close relatives to electrochemical storage batteries. A fuel cell is an electrochemical cell having two electrodes and an intervening electrolyte. It does not produce any electric potential on its own. This
7fi Chrysler's Turbine Cars: Promise Unkept, Cars That Never Were, Publications International, Ltd., 1994 (by the Auto Editors of Consumer Guide). 77 Mackay, R., Development of a 24 kW Gas Turbine-Driven Generator Set for Hybrid Vehicles. SAE Technical Paper Series #940510.
66 "fuel" is where the plays its role. When hydrogen gas is introduced at the anode, and oxygen is introduced at the cathode, a chemical reaction occurs which produces electricity. The oxygen needed for this reaction is conveniently available from the air, which simplifies operation. The key to this reaction is the makeup of the electrodes. The electrodes must contain a catalyst, such as platinum, to trigger the reaction. In the presence of the catalyst, hydrogen breaks down into ions. The resulting protons migrate to the cathode, and the electrons pass through the external circuit. When everything meets back up at the cathode, the easiest recombination of these constituents is H20, or water. This water, along with the unused nitrogen from the air, become the only waste products of the cell. Neither of them is toxic, so they can be vented directly without further control. The fact that it does not burn the fuel is a very big advantage of fuel cells. The more controlled reactions that occur produce many fewer emissions.
Reformers:
The most promising solution for fuel storage in the near to mid-term
uses reformer technology. A reformer takes a gas which is rich in hydrogen, such as methanol, ethanol, or natural gas, and extracts the hydrogen for use in the fuel cell78. Three major processes are used for reforming: Steam Reforming
(SR), Partial Oxidation(POX), and Autothermal Reforming (ATR). Steam
reforming has been the predominant candidate, but the other methods are being
pursued. The hydrogen output from the reformer depends on the fuel and the
process79.
78 Cantoni, U., Alternative Fuels Utilization in Fuel Cells for Transportation. SAE Technical Paper Series #931816. 79 Cantoni, U., Alternative Fuels Utilization in Fuel Cells for Transportation. SAE Technical Paper Series #931816.
67 The advantage of the reformer is that alternate fuels which are readily available and easily stored can be used to power a fuel cell. The disadvantage is that more equipment is required, leading to higher costs, weight, and complexity. To improve vehicle performance, the reformer can be kept at the refueling station with the vehicle storing only the hydrogen. This is an attractive prospect except that very large equipment and infrastructure changes would be needed to provide good fuel availability, and hydrogen is a difficult fuel to store.
The current infrastructure could most easily support an alternate fuel such as methanol which would be reformed on board the vehicle.
Argonne National Laboratory is apparently close to producing a
reformer which is appropriate for vehicle use. Their latest device occupies only
27-L of volume and would fit under the hood of a compact car alongside their own fuel cell design80.
Cells:
There are currently four types of fuel cells which are actively being developed.
The phosphoric acid fuel cell (PAFC) operates at 200C, and is the only type which currently available for stationary applications. About 200 stationary installations are currently producing power around the world.
The proton exchange membrane (PEM) is the prime candidate for vehicle uses. It operates at temperatures from 80C to 200C using a solid polymer membrane electrolyte. Ballard Power Systems of Vancouver, B.C.,
Canada, is supplying a PEM fuel cell unit with a built in reformer to the Chicago
Transit Authority. The fuel cell power plants are being installed in transit busses for evaluation. If the results are encouraging, the 2000 bus fleet may be
80 Fuel Reformer, Automotive Engineering, October 1995, pp. 26.
68 retrofitted81. The current devices fit into the same compartment as existing bus engines, but are still too large for general vehicle use.
Molten Carbonate Fuel Cells (MCFC) remain in development. They operate at 650C and benefit from the higher temperature. The high temperature eliminates the need for a precious metal catalyst, and allows reforming to occur directly within the cell without special equipment. Commercial sized cells are being evaluated, and the initial applications are planned to be stationary power plants.
Solid Oxide Fuel Cells (SOFC) are also still under development. The electrolyte in this cell is a solid ceramic which can take a variety of shapes. The cell operates at 1000C and does not require any front end reforming of the fuel.
The high temperature has benefits, but also causes problems with corrosion, thermal expansion and sealing. Cells are being tested and are planned for
stationary applications.
The fuel cell is a promising technology which blurs the definition of a
battery somewhat by continuously converting chemical energy to electricity. They have been used for many years in space vehicles, but remain very
expensive, and quite large. However, intense research has yielded many
developments in the last few years, and fuel cells are quickly becoming viable
for the automobile market.
Direct Conversion:
Direct conversion of thermal energy into electricity is a novel approach
Metal Thermal to which has the potential for many benefits. Alkali Electric
Conversion (AMTEC) is a process that was conceived in 1968, but has not been
actively developed until recently.
81 October pp. 84-88. The Coming Age ofFuel Cells, Mechanical Engineering, 1995,
69 The AMTEC device is called a sodium heat engine, and relies on the properties of a special ceramic material. When one side of the cell is heated to
600-850C, direct current is produced through the establishment of an ion gradient in the ceramic. There are no moving parts which promises excellent
reliability, and almost any heat source can be used. Current test cells are operating at 19% efficiency, and numerical models indicate that 40% efficiency should be possible. If this can be achieved, an AMTEC device would produce approximately 500 W/kg. Power densities currently remain much lower than theoretically possible.
The AMTEC device is being planned as a power plant for space vehicles because it can operate for decades without interruption or maintenance.
The possibilities for hybrid vehicle applications are outstanding if power
densities can be raised and costs lowered. Current research is answering many
questions, and devices which are practical for space based applications are
nearing production82.
Summary:
A vital part of APU operation is the ability to use alternate fuels.
Alternate fuels can offer better pollution characteristics, and can relieve national
dependence on foreign oil. All of these APU technologies can operate using an
alternate fuel. In the piston engines, it is a matter of design and choosing
appropriate compression ratios and fuel delivery systems. The external
combustion engines and the fuel cells are inherently able to use many different
fuels.
82 Alkali Metal Thermal to Electric Conversion, Mechanical Engineering, October 1995,
pp. 70-75.
70 In the near future, hybrid APU's will almost certainly be derived from the internal combustion engines which are so common. Their availability, mature designs, and broad manufacturing base make them the only cost effective solution. It is also likely that they will operate using an alternate fuel, since hybrid vehicles have less stringent power demands.
The specific engine technology which will be used is an open question.
Two-stroke piston engines and rotary engines hold great promise because of their high power to weight ratios, but intense development work could make any current engine into a reasonably attractive choice. The large number of
independent engine developers will almost certainly bring many options to the
market, so it is conceivable that all the discussed engines could be used in small
quantities.
Alternative fuels fall into the same situation. No single fuel has an
overwhelming advantage that could not be overcome by diligent work.
"favorite" Companies committing to the market will bring their fuel, and it is again
likely that all the possibilities will tried. The mid-term APU technologies will be
diverse, and much valuable experience will be gained to apply to future
development work.
The long term solutions will be turbines and fuel cells. They have many
favorable characteristics, and are almost ideally suited to the hybrid vehicle
application. Current equipment suggests that both of these technologies are
remain high. The turbine and fuel cell are close to being acceptable, but costs them back. good solutions and only development time is holding
development of the It will be interesting to watch the AMTEC devices
because they have no moving parts, and will generate power from any heat
are attractive for mainstream source. The zero-maintenance possibilities very
are applications such as passenger vehicles. AMTEC devices likely to be a very
long term option.
71 Case Study:
The light truck market which includes sport-utilities, minivans, and pickup trucks, is one of the most popular and fastest growing segments of the automobile industry. North American sales of this group of vehicles have
increased by more than 40% in the last few years, from 48 million in 1991 up to
68 million in 199483. The sales distribution within the light truck segment is
shown in Figure 5. As seen, the minivan, compact Sport Utility Vehicle (SUV)
and compact pickup truck groups make up 62% of all the light trucks sold in
North America, which is a very significant number of vehicles. Light trucks are
currently exempt from many standards, but starting as early as 1997 they will be
required to meet impact standards, and more importantly will face fuel economy
Figure 5: Light Truck Sales Distribution
Full-sized SUV 3.6% Full-sized Van Minivan 6.8% 20.
Compact SUV 21.9%
Compact Pickup Source: Market DataBook, SUV=Sport 19.6% Vehicle Automotive News, May 24, 1995. Utility
83 Market DataBook, Automotive News, May 24, 1995.
72 requirements. The next sections will attempt to explain the growing popularity of light trucks, and will discuss how the environmental performance of this class of vehicle can be improved by the application of a hybrid drivetrain.
Light trucks (minivan, SUV, and pickup) now rival many passenger cars in terms of ride, comfort, and performance, and this has allowed new markets to be penetrated. Most of today's compact trucks are used primarily as commuting vehicles, and only rarely are used off road. This market shift has required
redesign of new compact trucks to provide satisfactory on-road traits, while maintaining the ruggedness of their ancestors.
Compact trucks have many attractive characteristics which are no longer
"feels" offset by poor ride and comfort. One is the way that the vehicle to its occupants. They are taller and larger than cars, which leaves more interior space for passengers to stretch out. The seating position is also higher, which translates to a better view of the road around the vehicle, often over the top of
nearby cars.
Another impression is of solidity and safety. They are built to withstand
heavier duty than passenger cars, and also tend to weigh more. An interesting counterpoint to this trend is the Jeep Cherokee, which has long been a very
popular vehicle. The Cherokee achieves the same utility as its competitors, yet weighs hundreds of pounds less. The trimmer weight allows better performance and a competitive advantage. The sizable bulk of most compact trucks is a drawback to efficiency, but the Jeep shows that high weight is not a requirement for success.
Trucks have always been known for two key strengths: the ability to carry and tow cargo, and the ability to handle adverse terrain. These remain very
important features of modern compact trucks. The suburban demands for towing are not nearly as severe as rural demands, but the ability to tow safely and
73 effectively is an important consideration because even though the towing demands are comparatively light, they would overwhelm a smaller vehicle.
The ability to handle bad road conditions is also a very important feature affecting the popularity of compact trucks. Mobility is critical in today's society, and long daily commutes are becoming common, despite the fact that Mother
Nature has not lessened the severity of the weather. The availability of all-wheel drive (AWD), or four wheel drive (4WD)84, can be a crucial advantage on treacherous winter roads and greatly improves vehicle stability. SUV's and pickups are commonly available with all wheel drive systems, and so are many
minivans. The ability to drive all four wheels allows these vehicles to travel
safely in conditions which would be dangerous for a two wheel drive passenger
car.
Bad weather capability explains their popularity in the northern regions,
but compact trucks are popular nearly everywhere. This might be explained by
the rugged, go-anywhere image that they project without imposing any actual
discomfort. Compact trucks have also proven that they can be modified and
personalied fairly easily. Many people buy compact trucks because they
style and image also prompt sales. genuinely need their special capabilities, but
All Wheel Drive:
All wheel drive has been identified as one of the most important features
travel on bad road of compact trucks, because it allows the vehicle to safely
advantages gained in vehicle conditions while maintaining better control. The
dynamics also come with drawbacks. Driving all the wheels of the vehicle
compromises the design of requires more components, causes higher weight,
84 mean a full time while four wheel drive All wheel drive is usually considered to system,
is a part time system.
74 the vehicle and chassis, and consumes more power to operate. Controlling the additional equipment can get complicated and requires careful design which further increases the financial cost of the vehicle.
all Full time wheel drive systems benefit from improved traction at all times, without needing input from the driver. However, the losses are also always present, regardless of road conditions. This is addressed in part-time systems, which require the driver to engage them. Part-time systems operate as two wheel drive with reduced driveline losses until the driver deems four wheel
to needed. drive be The driver engages the four wheel drive system, and disengages it when it is no longer needed. This arrangement improves vehicle efficiency when operating on good roads, but suffers by requiring the driver to manually engage the system. If the conditions change suddenly, or are questionable, the driver may not have time to engage the system before losing control of the vehicle. Full time all wheel drive systems do not suffer from this problem.
A new technique which has recently become available on general purpose compact trucks promises to address these problems. The difference in speed between the driven and undriven wheels is detected by sensors as soon as the drive wheels begin spinning. This prompts the system to transfer torque to all the wheels. A system of clutches or viscous couplings operate the vehicle as all wheel drive until the conditions improve. The system then automatically transfers back to two wheel drive for better efficiency85. This is not truly full-time or part-time all wheel drive. It takes advantage of the efficiency gains of part-time all wheel drive while maintaining the constant vigilance of full time all wheel drive.
85 Masters ofMobility, Popular Science, October 1995, pp. 82-88.
75 For the purpose of this paper, a new name will be coined. A drive system
"actively" which monitors wheel slip and distributes driving torque accordingly
Drive" will be called "Adaptive All Wheel (AAWD). Adaptive all wheel drive
represents the best of both worlds, and is very attractive for vehicle propulsion.
Implementing an adaptive all wheel drive system with a hybrid-electric drivetrain holds some exciting possibilities. One of the key issues of AAWD is the rapid activation of the undriven wheels. Conventional mechanical drivetrains
require a complex system of clutches or couplings, which have engagement and
disengagement problems which limit reaction time. The application of an electric
motor to this problem seems particularly appropriate because the torque and
engagement characteristics of the motor can easily be modified. In addition to
good response times and tune-able torque, the elimination of the transfer case
simplifies design and improves efficiency.
Sport Utility Vehicle Definition:
In order to quantify the competition, information was gathered on a small
group of vehicles which have similar markets. Table 5 shows some statistics on
1995 model year sport utility vehicles. The first seven entries show a
representative sample of what is available. Notice that they are almost all the
same size based on wheelbase, and that they all show remarkably similar
acceleration times of about 10 seconds to 60 mph. Fuel economy is universally
poor, and prices and curb weights vary widely.
76 Table 5: Sport Utility Comparison
Vehicle Price Wheelbase Curb 0-60 Observed inches Weight Acceleration MPG lbs sec
Chevrolet Blazer LT $26,000 107 4146 9.1 19
Ford Explorer XLT $25,000 111.5 4442 10.7 18
Jeep Grand $26,000 105.9 3762 9.7 18 Cherokee Laredo
Range Rover 4.0 SE $55,000 108 4836 10.5 14
Honda Passport $26,000 108.5 4091 10.4 13
Mitsubishi Montero $32,000 107.3 4742 9.7 16 SR
Toyota Land Cruiser $40,000 112.2 5153 10.7 15
Subaru Legacy $21,000 103.5 3089 10.6 22 Outback Wagon
Suzuki X90 Mini-ute $15,000 87 2500 10.5 25
Toyota RAV4 $15,000 87 2600 10.1 28 Mini-ute
The last three table entries are included to show some unconventional vehicles which target roughly the same market as traditional sport utility vehicles. The Subaru Outback wagon is a strengthened and raised version of
wheel drive. It represents a the standard Legacy wagon, featuring full time all serious consideration for some consumers because it offers the security and performance advantages of all wheel drive in a smaller, lighter package which delivers better fuel economy. The reduced cargo and towing capacity is not a
commuters. The Subaru Outback is a drawback for many users, especially daily
86 periodicals which had tested 1995 model These statistics were taken from a variety of year vehicles.
77 its' new vehicle so popularity is still unknown, but it seems to have the right features for at least part of the market.
The last two entries of Table 5 show concept vehicles which are being considered. They are very small imitations of sport-utility vehicles, and are a
"Mini-utes" fairly dramatic departure from tradition. These are not yet available on the American market, but they indicate the growing popularity of all wheel drive vehicles.
The application of hybrid-electric drivetrains to this class of vehicles will
be extremely important because of the evolving attitude toward environmental
protection. Due to their size and weight sport utility vehicles do not achieve good
fuel mileage, and this is perhaps their single most important drawback. Despite
this, the popularity of SUV's is growing rapidly. In order to meet both the
consumer demands and the regulatory requirements, a notable shift in
propulsion is needed. Power requirements are high, so a pure electric SUV will
not be feasible until much better battery technologies are available. That is likely
to be years away, so the hybrid drivetrain will be an effective intermediate
measure. A hybrid SUV will be able to maintain reasonable performance while
delivering serious improvements in economy when compared to current vehicles.
It is important to recognize that the improvements are going to be relative. A
hybrid SUV will still use more energy than a hybrid passenger car, but will use
substantially less energy than current vehicles.
Hybrid SUV:
Based on the information in Table 5 about current vehicles and some
educated assumptions about future needs, a general vehicle specification has
been determined. It is shown in Table 6, and lays out some of the minimum
standards which will be acceptable. The specifications are targeted at hybrid
78 powered sport utility vehicles, but are sufficiently general to be applied to a wider market.
Table 6: HE-SUV Specifications
Requirements
maintain reasonable performance for SUV:
? 0-60 mph in 10-12 seconds
? 4-5 passengers and cargo
? all wheel drive capability, AAWD preferred
? ability to meet Class 1 towing standards
able to sustain AWD mode for reasonable trip length
unlimited HEV range
significant ZEV range (about 30 miles)
competitive price
meaningful environmental/financial advantage
The first and foremost of the requirements of a hybrid SUV is to maintain
approximately the same level of performance as current vehicles. Consumers
will not accept a significant decrease in acceleration or towing abilities. The first
models will be especially important because the hybrid SUV will be new, and it
will need to demonstrate similar performance with superior environmental
characteristics.
Another important part of SUV popularity is interior volume. They must
retain the ability to carry 4 or 5 people and their luggage. Other vehicles can
few can both the people and carry as many people, or as much cargo, but carry
their luggage. An SUV can do this through almost any weather and road
condition.
79 Two of the most critical features of current SUV's, all wheel drive and towing capacity, must also be preserved. All wheel drive is particularly important because it is the universal requirement of nearly all users. Not everyone needs to tow cargo, and not everyone likes the way an SUV looks or feels, but the security of all wheel is drive is always desirable.
The required towing capacity can be reduced somewhat from current vehicles because of the suburban lifestyle that so many light duty SUV's end up
in. Class 1 towing standards require towing up to 2000 pounds. This is the standard capability of most light trucks; extra capacity must be specifically ordered. Class 1 should be sufficient for early hybrid sport utilities. Once the vehicle design has matured and proven itself, larger capacity and heavier duty vehicles can follow.
As noted earlier, Adaptive All Wheel Drive is specified as standard for a
hybrid SUV. An important part of AAWD is improved efficiency because it is only
activated on demand.
A key advantage of hybrid vehicles is that they have many of the benefits
of pure electric vehicles, yet do not suffer from reduced range. It will be critical that the hybrid vehicle be able to sustain high speed travel indefinitely in HEV
mode. This will ease acceptance, and provide good general purpose
capabilities. It will also be critical that the vehicle have a significant zero-
emission capability. It seems likely that some regions, especially urban areas, will be designated as zero-emission zones. In order to provide satisfactory utility, the hybrid SUV must be able to enter these areas and perform satisfactorily.
Allowing 30 miles at reduced speed (limited to a top speed of perhaps 40 mph) to accomplish this should be generous enough to account for most possibilities.
It is also understood that the vehicle should be able to maintain highway speeds even with a depleted battery, so that if the vehicle leaves a zero-emission zone with low battery reserves it can travel effectively in HEV mode.
80 The last requirements are financial. These are included because consumers will need a reason to purchase HEV's. Resistance to change is a common human trait, so if there is not a clear advantage to owning a different type of vehicle then few will be purchased. Therefore, purchase prices need to be in the same range as those listed in Table 5 in order to be competitive. A part of competitiveness is a meaningful increase in environmental performance. A gain of 5 miles per gallon with a $15,000 price hike will not be an incentive. The
advantages need to be distinct, and desirable.
Meeting these requirements does not mean that a vehicle will be
successful, but they do outline the minimum which is likely to be accepted. As
experience is gained, the requirements will evolve along with the market.
Hybrid AH Wheel Drive:
With these requirements and definitions in mind, the following sections
will discuss various ways that all wheel drive can be implemented using a hybrid
drivetrain. They will be divided into two groups which will discuss series
arrangements, and then parallel arrangements. For each solution the
advantages and disadvantages will be listed and the issues pertaining to them
will be investigated.
81 Series Scheme #1:
This arrangement is very similar to existing vehicles. It uses an existing mechanical drivetrain powered by an electric motor. The APU/generator set can
be located wherever is convenient in the vehicle.
Figure 6: Series Scheme #1
f > ( N
Transfer Case
The most distinct advantage of this setup is the ease with which it could
be implemented. The vehicle and driveline are essentially identical to current vehicles except that the motive power comes from an electric motor. This drive scheme could be accomplished quite simply as a conversion of a current
82 chassis, but would be badly compromised because the considerations for the overall vehicle are very different. Locating the APU and the batteries in order to maintain weight distributions and satisfactory performance would require a dedicated chassis. However, even though these issues can easily be addressed, this drive scheme is not feasible for other reasons.
The electric drive motor would be required to produce approximately the same power as the internal combustion engine that it replaces. Electric motors of this size are expensive, and can be heavy. In any case, simply substituting an electric motor for an ICE ignores many advantages that electric motors can provide.
The mechanical driveline and transfer case have the difficult task of distributing torque to the drive wheels. The complexity leads to power losses and to added weight. Both of these are serious problems for electric propulsion.
In addition to the transfer case, the propeller shafts which transmit the
Table 7: Series Scheme #1
Advantaqes Disadvantaqes
very easy to implement, no new the single motor would be very hardware needed large and therefore expensive and heavy
similar construction and losses and weight attributable to maintenance to conventional the transfer case
vehicles
APU not tied to driveline, placement long propeller shafts increase of both is flexible weight and reduce response time due to inertia
transfer case and propeller shafts constrain chassis design
traction control and regenerative braking would be limited by mechanical systems
83 power to each axle are heavy and bulky. The inertia of the propeller shafts saps power and reduces the response time of the drivetrain. Both the transfer case and propeller shafts represent constraints to the vehicle design. The chassis must reflect appropriate space to accommodate them, and the drive components
must be placed to provide the appropriate connections.
A last consideration is the implementation of the adaptive all wheel drive
system. This drive scheme relies entirely on existing mechanical drivelines. The
ability to achieve AAWD, traction control, or effective regenerative braking are
all limited to the capabilities of the purely mechanical mechanisms. As was
mentioned earlier, the mechanical systems have drawbacks. Electric drive has
the potential for excellent solutions to these problems, but those possibilities are
ignored in this scheme.
A summary of this scheme indicates that it is basically a conversion of an
existing vehicle. As such it ignores many potential advantages, and is seriously
compromised by current mechanical systems.
84 Series Scheme #2:
This arrangement eliminates the transfer case with its associated losses,
but retains mechanical differentials at each axle. The APU/generator set can be
located wherever is convenient in the vehicle.
Figure 7: Series Scheme #2
~\
J L. O
-P o 2:
i. o
-p o A r \
J
This solution still relies heavily on current mechanical systems, but also begins to take advantage of the characteristics of electric motors. A dedicated chassis would be needed to attain the best design, but little new hardware would needed. The biggest change in this setup is the elimination of the transfer case
85 and the propeller shafts. This not only reduces weight and inertia, but also increases design flexibility. The differentials at each axle are retained, and the motors drive them directly with a minimum of intervening equipment. Retaining the differentials allows a single motor to drive each axle and simplifies design.
The potential for AAWD is very apparent in this setup. If each axle has approximately the same power, then either one could serve as the primary drive axle. It is conceivable that the primary drive axle could change depending on conditions or driver preference. The vehicle could operate as either front, or rear wheel drive with no change needed to the vehicle. Regardless of which axle is being driven, the other axle serves as the AAWD system. When a predetermined slip condition occurs between the driven and undriven axles, the undriven axle would be activated to maintain control of the vehicle. Once the
Table 8: Series Scheme #2
Advantaqes Disadvantaqes
transfer case and propeller shafts mechanical drag and losses from are eliminated differentials
flexible location of components, mechanical gear train has inertia, constrained only by axles drag, and wear
differentials simplify design by differentials limit controllability of allowing single motor to drive axle drive wheels
good design flexibility for placement redundant drive systems increase of batteries and APU cost, weight, complexity
capable of independent control of
each axle for traction control and regenerative braking
either axle could be driven depending on conditions
redundant drive systems improve safety in adverse weather
86 conditions stabilize, the secondary axle would be deactivated. The flexibility of such a setup is intriguing, because the vehicle can be either front or rear wheel drive while maintaining all wheel drive capabilities in both cases.
The potential for traction control and regenerative braking is also very good for this scheme. When the conditions of slip are detected by the system, the motor controllers can reduce drive torque to the axle which is slipping in order to regain traction. This would occur almost instantly with electric motors, and acting in concert between both axles would provide levels of traction control which would be difficult to attain with current systems.
The vehicle would also benefit during braking. The potential to achieve
slip conditions under braking is increased dramatically on poor road conditions.
When deceleration is requested, both axles would be activated as regenerative
brakes. This avoids the losses and problems with vehicle control which were
discussed earlier. The same sensors that monitor wheel slip for the AAWD
system would be used during braking. In this case, when a speed difference is
detected, the level of regenerative torque would be reduced at the slipping axle
until traction is regained. This is a direct application of existing anti-lock braking
technologies, and the sensors required to monitor wheel slip for the AAWD and
anti-lock regenerative braking have already been developed and are in use on
current vehicles. Applying them to this new application should be relatively
straightforward.
An issue relating to the differentials is their effect on the traction and
braking control systems. Whether the differential is open or is a limited slip
design of some type, it effects the axle as a whole. The motor drives the wheels
through the differential and the ability of the motor to modulate the individual
wheels is therefore limited. If slip is detected at only one wheel, the entire axle
must be retarded to regain traction. It is not likely that this presents a real
problem, because ultimate traction is rarely needed for street use vehicles.
87 Redundancy also becomes an issue with this scheme. The two separate drive setups are highly desirable from a performance standpoint, but they increase the weight, complexity, and cost of the vehicle. The extra cost can be partially justified for adverse climate areas because redundancy decreases the likelihood of complete failure. Even if a one drive system should fail, the other setup would provide some motive power and prevent the vehicle from being stranded.
88 Series Scheme #3:
This arrangement eliminates both the transfer case and the front to differential reduce losses and maintenance. The APU/generator set can again be located wherever is convenient in the vehicle.
Figure 8: Series Scheme #3
^ f
o o
-f- O o
J ^
O
-P o C \
\ J v )
This scheme is nearly identical to scheme number two, except that it eliminates one of the differentials. Removing a differential eliminates some parasitic losses and weight, and also allows independent control of the front wheels. When slip is detected during AAWD or regeneration, the individual
89 wheels can be retarded separately to regain traction. This theoretically should improve performance by some small amount, but it may not be significant enough to justify the added complexity.
Table 9: Series Scheme #3
Advantaqes Disadvantages
front differential are eliminated rear differential
independent control of front wheels for traction control and regenerative braking
There is also some question about whether or not the two smaller motors
would weigh less than the differential based setup. Using the same motor data
as before from Unique Mobility, Inc., shown in Table 10, the following sample
calculation can be made while considering only the motors, and disregarding
any joints or axles which would be required. Using two DR156s motors yields the
same power as a single SR180P. Even assuming that the differential weighs
lighter. In the single motor only 15 lbs, the two motor setup is noticeably fact,
with differential would weigh 67 pounds, while the two motors weigh 37 pounds,
meaningful and further nearly 45% lighter. This is a very improvement,
exploration is deserved. If this sample calculation were to hold up with real
justified the significant equipment, then the extra complications could be by
weight savings.
Table 10: UQM Motor Data
Model Number Power Weight
DR156s 21.1 hp 18.5 lbs SR180P 42.9 hp 52 lbs
90 Series Scheme #4:
This arrangement eliminates the transfer case, both differentials and all of the gear train. Each wheel can now be controlled independently. The
APU/generator set can be located wherever is convenient in the vehicle.
Figure 9: Series Scheme #4
/ \ f \
^ i. o O
-H> -t-> O o SL 2:
\ ) \ J
r "\ ! \
i_ i_ o o +> -\-> o o s: .s_ L J 1 )
further and eliminates This scheme takes scheme number three a step
all mechanical transmission the other differential. This removes nearly
and losses. The vehicle can now components and the possibilities for wear
91 independently control all of its wheels to maintain optimum acceleration and deceleration traction. Any slipping wheel can be dealt with separately from any of the others, which should allow the best possible control of the vehicle. Using smaller motors at each wheel also allows more options for packaging.
This scheme represents the ultimate of flexible drive options. Regardless of which axle is being driven, the benefits of independent control are available.
Driving each wheel independently opens up options including the use of wheel motors. (Scheme number three could also make use of the same setup). Using motors which are entirely integrated into the wheels would remove all drive components from the vehicle and greatly increase the flexibility of the remaining sections of the vehicle. This is likely to be the end that vehicles will evolve to because it represents the ultimate simplification of the drivetrain.
The drawback of this degree of design flexibility is complexity and redundancy. Controlling four separate motors requires a more complex control system. The number of parts also increases together with redundancy and cost.
Table 1 1 : Series Scheme #4
Advantaqes Disadvantages
no mechanical gear train, zero-lash, complicated control strategy less inertia
independent control of all wheels for traction control and regenerative braking
smaller motors are easier to
package
92 Parallel Scheme #1:
This arrangement is also very similar to conventional vehicles. It retains
the mechanical driveline with the APU and electric motor connected in parallel at
the transfer case.
Figure 10: Parallel Scheme #1
r a f N
c 0) c U
Transfer Case
The advantage of this scheme is that it continues to use a conventional driveline almost intact. It would therefore be easy to implement would require very few new parts. The similarity to current vehicles again brings up the
93 of a possibility direct conversion of an existing chassis, but this would still not be wise because the considerations involved in connecting the diverse parts of the hybrid require that they not be constrained to fit into available space. A dedicated chassis is required for proper exploitation of potential benefits. The APU and electric motor interact with each other directly, so their physical locations are constrained by mechanical connections. This alone is a huge disadvantage when compared to the series hybrid schemes. As a result of these required interactions, the transfer case becomes even more complicated.
The additional complexity leads to weight, losses and cost.
Propeller shafts are still used to transmit power to the axles, and they have weight and inertia as discussed earlier. The chassis is also constrained to allow room for the revised transfer case, as well as the propeller shafts.
In the same way as series scheme number one, the possibilities for traction and regenerative control are limited by the mechanical systems. Many of the potential benefits of electric drive are ignored.
Table 12: Parallel Scheme #1
Advantaqes Disadvantages
uses conventional driveline APU/electric motor relationship constrains design possibilities
easy to implement, little new transfer case becomes more hardware needed complicated with associated losses and weight
similar construction and long propeller shafts increase maintenance to conventional weight and reduce response time
vehicles due to inertia
chassis design is constrained
traction control and regenerative braking is limited by mechanical systems
94 Parallel Scheme #2:
This arrangement eliminates the transfer case and the propeller shafts..
One axle is driven by a parallel hybrid drive, and the other axle is driven by an electric motor. It is assumed that the hybrid axle is the primary drive with the electric axle activated on demand.
Figure 1 1 : Parallel Scheme #2
/ \ Engine
Motor
O
-P o A f
J V
conventional type of all wheel This scheme moves away from the
driveline. The transfer case and propeller shafts are eliminated to save weight
vehicle therefore becomes more and reduce losses. The design of the flexible,
95 which allows it to accommodate the other changes which the hybrid drive requires.
This scheme places the APU on one axle of the vehicle, and uses electric
to provide drive supplementary power and all wheel drive. Two axle hybrids have been but built, they have exclusively used only one type of propulsion per axle.
This leads to problems because the vehicle must change drive axles in order to change propulsion methods, which can lead to stability problems for the vehicle.
The methods which will be explained here avoid most of these problems.
axle" The of "hybrid the vehicle is intended to be the primary drive axle.
The secondary axle which is powered by an electric motor provides AAWD capabilities, as well as supplemental power. It is expected that the APU and the electric motor can power the hybrid axle cooperatively or independently. This provides consistent drive to maintain the stability of the vehicle. The design is such that the hybrid axle alone could power the vehicle satisfactorily. This may be important for future production concerns. It is conceivable that the secondary
Table 13: Parallel Scheme #2
Advantages Disadvantages
transfer case and propeller shafts mechanical gear train has inertia, are eliminated drag, and wear
hybrid primary drive can be placed differential limits controllability of on either axle to suit design of drive wheels
vehicle
independent control of axles for redundant drive systems increase traction control and regenerative cost, weight, complexity braking
differential simplifies design by allowing single motor to drive axle
redundant drive systems improve safety in adverse weather
96 axle could be a purchase option, so that after the base vehicle was purchased,
AAWD capabilities could be obtained later by adding the secondary drive axle.
The disadvantages of this scheme consist mainly of the difficulties
involved in interfacing the APU and electric motor. Their relationship constrains the design of the vehicle, but is a critical piece of the design. As with all the
parallel hybrid drive schemes, a fair amount of mechanical mechanism and
transmission pieces are required. This means that maintenance and lubrication
will be issues, but these areas are well understood and are unlikely to be a
problem.
97 Parallel Scheme #3:
This arrangement eliminates transfer case and the propeller shafts, as well as the rear differential. One axle is driven by a parallel hybrid drive, and the other axle is driven by an electric motor. It is assumed that the hybrid axle is the
primary drive.
Figure 12: Parallel Scheme #3
r \ / \ Engine
Motor
Motor Motor
V ) K J
This drive scheme improves on parallel scheme number two by
axle. In addition to lower losses and eliminating the differential on the secondary
control of each of the wheels at improved packaging, this allows independent
98 secondary axle. The performance improvements which can be expected from this have been discussed earlier.
The disadvantage of this setup is added complication. The control strategy and equipment will need to be able to handle the additional motors. The added parts increase cost and redundancy, although redundancy has been shown to be a mixed blessing.
Table 14: Parallel Scheme #3
Advantages Disadvantages
elimination of differential complicated control requirements
independent control of secondary wheels for traction control and regenerative braking improved design flexibility
99 Battery Enclosure:
The battery pack is another reason that a dedicated chassis design is desirable, as opposed to a conversion of an existing chassis. A dedicated chassis will allow the following issues to be addressed.
Table 15: Battery Enclosure Issues
Safety Performance
Weight Distribution
Temperature Control
It is advantageous to keep the battery in a single chamber in order to
simplify the process of isolating it from the vehicle and occupants. The amount of
heavy gauge, high voltage wiring is minimized, as are the losses associated with
the resistance of the wire. The single chamber can also be treated as a
vessel" "containment and designed to contain all the battery components in the
event of a crash. Significant effort will be required to create an appropriate
battery enclosure, and these efforts can be simplified by concentrating on a
single region.
The placement of the battery enclosure will have a large effect on vehicle
dynamics, and should be kept low and near the center of the vehicle. Both the
weight distribution, and polar moment of inertia of the vehicle are favorably
impacted by central placement of the battery pack. Placing the enclosure low
and in the center of the vehicle also allows it to be well protected. The
surrounding vehicle provides protection against intrusions, and provides a
certain amount of crush distance. The enclosure will need to be an integral part
of the chassis and maintain its integrity during any sort of crash.
100 Another pertinent reason for locating batteries in a single chamber near the center of the vehicle has to do with temperature control. Both the passenger cabin and the battery enclosure will require temperature controls, and keeping them close together will simplify control of both areas.
101 Calculations:
Comprehensive calculations are not possible without detailed information about torque curves and gear ratios, but preliminary design work can be completed using some educated assumptions. All assumptions will be based on a light duty sport utility vehicle application. A spreadsheet will be used to evaluate the design equations.
In order to maintain functionality and appeal, the overall size of the vehicle should not change significantly. Approximate frontal areas of three
American made sport utility vehicles are shown in Table 16. Based on this data, the frontal area used for all subsequent calculations will be 30 ft2. This is slightly smaller than current vehicles, but reducing the frontal area even slightly will
reduce aerodynamic losses.
87 Table 1 6: 1 995 Sport Utility Data
ft2 Vehicle Frontal Area
Chevrolet Blazer LT 31
Ford Explorer XLT 32
Jeep Grand Cherokee Laredo 31
The aerodynamic drag coefficient will be taken as 0.30. This figure is
typical of passengers cars, but is a very aggressive assumption for a light truck.
This value was chosen to emphasize the importance of reducing drag. Much
"boxy" lower coefficients have been achieved for cars, but the sport utility styling
is a tougher problem.
87 Car and Driver, 1995.
102 The weight of the HESUV will be optimistically taken as 3000 pounds.
This is a another difficult choice and was made for the following reasons. The large iron engine and heavy driveline components are eliminated. They are replaced with a small aluminum block engine and electric motors. These gains are partially offset by the weight of battery pack. However, the reduced size and weight of the drive components will allow a more ideal frame structure to be created, which can also integrate the battery enclosure. Special attention must be paid to weight savings throughout the vehicle, but no special problems exist which would prevent this goal from being met.
Road Loads:
The steady state loads which are encountered by a vehicle consist of three parts: aerodynamics, rolling resistance, and roadway grades. In order to
maintain a steady speed, the vehicle must overcome these road loads which can
be represented as:
Road Load = DAero + DRolling + DGrade ( 1 )
These drag forces are converted to required horsepower using the following equation:
(Drag Force'Velocity) Horsepower= 550
Aerodynamics:
The aerodynamic drag force acting on a vehicle is given by the following
equation:
VWind)2 DAero=\pCDAf(V + (2)
103 where p is the density of air, CD is the drag coefficient, Af is the frontal area, V is the speed of the vehicle, and VWind is the speed of the wind. Since the frontal area and the required velocity are not variable, the drag coefficient becomes the main method of reducing the drag for the vehicle. The drag coefficient is strongly dependent on streamlining issues such as an undercarriage cover, body panel gaps, flush mounting of windows, rear view mirrors, and the overall shape of the vehicle. A teardrop is the optimum shape from the aerodynamic point of view, but practical issues prevent this from being achieved. The light truck market has not historically placed much emphasis on streamlining issues, but the growing
use of light trucks for high speed travel has prompted development. The smooth shapes of new minivans are a clear sign that these issues are now being considered.
519 = 0.00236l pK ^ nn nn (3) V 29.92 A 460+ TF
Air density is a function of temperature and pressure as shown in
Equation 3, where P is ambient pressure in inches of mercury, and TF is the
ambient temperature in Fahrenheit. For this paper the altitude will be assumed
to be sea level, which simplifies the density equation to:
1 ^ 519 p =0.00236 (4) v460+7>,
While changes in air density would not seem to be a large factor affecting
noticeable Consider a vehicle performance, they do have a impact. temperature
drop from summer temperatures near 100F to below 0F during the winter
months. The effect of temperature on road load is shown in Figure 13. As seen,
the isolated change in temperature increases road loads by nearly two
horsepower. Two horsepower is a significant loss, because it is fully 10% of the
horsepower required. The APU of a hybrid vehicle is sized to operate near
power significant. maximum power at high speeds, so a 10% increase in is
104 Figure 13: Road Load vs. Temperature
Road Load vs. Temperature
at 60 mph
Road Load @30psi
-20 20 40 60 100 Temp. F
In addition to cold temperatures, winter usually brings strong winds. Wind has a strong effect on aerodynamic drag depending on the direction which it comes from. A headwind effectively increases the apparent speed of the vehicle and therefore its losses, while a tailwind decreases losses. The effect of wind speed on road load is shown in Figure 14. The figure of 50 mph for wind was
winds at this speed are but arbitrarily chosen based on experience. Steady rare,
common. Sustained winds of 30 mph or more gusty conditions to this speed are
will be considered a storm for the purpose of this paper, and performance need
not be maintained under those conditions.
The presence of wind is completely unpredictable, so worst case
provide conditions must be assumed. The APU must be sized to acceptable
because there is a chance performance when driving into a strong head wind,
travels. that it could face these conditions every time it
105 Figure 14: Road Load vs. Wind Speed
Road Load vs. Wind Speed
at OF and 60 mph
-20 0 20 40 60
Wind mph
As seen in Figure 14, a 30 mph headwind increases the power
requirements by about 20 horsepower. The ability to cope with strong winds must be included into the power level calculations for the APU.
Notice in Figure 14 that even a very strong tailwind does not reduce
power requirements to zero. This is due to rolling resistance which is dependent on vehicle speed and weight, and will be discussed next.
Rolling Resistance:
Rolling resistance is dependent on many factors including vehicle weight, tire temperature, and road surface. Quantitative characterization of rolling
resistance is difficult because the construction of the tire has a large influence its operation. Empirical relationships have been developed through experimental studies, and these are used to estimate the losses which can be expected.
106 It has been found that a tire usually needs to roll for nearly 40 miles before it reaches a steady operating temperature. During this time, the drag induced by the tire decreases to a minimum at operating temperature. This dependency on trip length is another reason why rolling resistance can only be estimated.
The road surface also has a huge impact on rolling resistance. As expected, a hard surface such as concrete induces less drag than a soft surface such as sand or snow. The coefficient of rolling resistance can vary from as low as 0.012 for concrete to .25 for sand88, a 5000% increase. Rolling resistance is estimated by the equation:
DR = fRW (5) where W is vehicle weight and fR is the coefficient. Therefore, a 3000 pound vehicle on concrete could face 36 pounds of rolling drag. If the surface changes to sand, the rolling drag increases to 750 pounds. This estimate clearly shows the difficulty in predicting and accommodating rolling resistance.
The inflation of the tire also has a large effect on the coefficient of rolling
resistance. Higher inflation levels produce lower coefficients, but lead to other problems such as reduced traction and reduction in ride quality. Research into reducing rolling resistance without compromising other performance factor is underway.
Considering the rolling resistance coefficient to be a constant value as was done earlier is a very crude approximation. A better relationship is one which increases linearly with speed, such as:
fR = 0.01 (1+V/100) (6)
88 Gillespie, Thomas D., Fundamentals of Vehicle Dynamics. SAE 1992.
107 However, Equation 6 is not accurate at high speeds.
For the specific case of rolling on concrete, an empirical equation has been developed which is good for a wide range of speeds89.
3.24fs(V/100)25 fR = f0 + (7)
The coefficients used in this relationship are given in Table 17.
Table 17: Rolling Resistance Coefficients
30psi 40 psi
fo 0.010 0.0085
fs 0.005 0.0025
Substituting these values into Equation 7 gives rolling coefficients which are then used in Equation 5 to determine the drag forces. A plot of the power required to overcome rolling drag at both levels of tire inflation is shown in
Figure 15. At low speeds, there is not a large difference between the low pressure tire and the high pressure tire. As speeds rise, the high pressure tire begins to show its advantage, and at 60 mph requires nearly two fewer horsepower.
89 Gillespie, Thomas D., Fundamentals of Vehicle Dynamics. SAE 1992.
108 Figure 15: Rolling Resistance vs. Speed
Rolling Resistance vs. Speed
20 30 40 50 60
Vehicle Speed mph
These empirical relationships were developed with a distinct bias toward passenger cars and passenger tires. It is not clear how well truck tires are modeled by this relationship. General purpose truck tires are designed for high speed travel, so the model should be acceptable. To accommodate this uncertainty, the values for the 30 psi tires were used for all subsequent calculations. It was assumed that this would represent an efficient truck tire, and that the 40 psi values are not attainable for truck tires without significant research.
Roadway Grades:
The last load faced by vehicles comes from gravity. When a vehicle climbs a hill, a component of its weight vector acts against the motion of the vehicle. A simple trigonometry calculation shows that this component is equal to
109 WsinO as shown in Figure 16. This force is completely unavoidable and is encountered by all vehicles under all conditions.
Figure 16: Vehicle on Grade
Wsin&
It is clear that this drag force depends on the slope of the hill and on the weight of the vehicle. The only reduction available is the weight of the vehicle. A light vehicle will require less power to climb hills than a heavy vehicle.
Highway grades are given as a percentage which expresses rise per 100 feet. Therefore a 5% grade will rise 5 feet per 100 feet horizontal distance. The steepest grade which can be expected on highways is about 10%. In order not to impede the flow of traffic it is important to be able to maintain highway speeds up the grades. Secondary roads contain steeper grades, but it is not as important to maintain high speeds on these hills.
The effect of changing highway grades on total road load is shown in
Figure 17. Notice that at 60 mph a 10% grade requires an additional 40 horsepower. This is a very significant increase, but peak power which cannot be provided by the APU can be provided by the electric portions of the hybrid drivetrain.
110 Figure 17: Road Load vs. Highway Grade
Road Load vs. Roadway Grade at 60 mph
120.00 j
100.00 -
25
Road Load @30psi
Grade %
Notice also that a 5% downhill is required before the drag due to
shallow hills aerodynamic and rolling losses is overcome. This indicates that
available for regenerative gains. A which are frequently encountered will not be
expect to gain significant relatively steep hill is needed before the vehicle can
control. At lower aerodynamic and energy from regenerative speed speeds,
and can be supplemented regenerative braking. The rolling drag are lower by
will therefore be better at lower opportunities for gravity assisted regeneration
speeds.
Total Road Load:
the road load faced a vehicle at a As stated in Equation 1 , by traveling
equal the sum of the from aerodynamic, rolling and steady speed is to drag
a vehicle through still air at grade losses. The total road load faced by traveling
shown in Figure 18. Aerodynamic 60 degrees Fahrenheit along level ground is
111 losses become dominant at high speeds which emphasizes the importance of reducing air drag. Notice that rolling resistance is the dominant loss at low speeds. This is the reason that slow moving vehicles can disregard aerodynamic considerations. Vehicles which never exceed 30 mph, such as a strictly urban vehicle, function may take precedence over streamlining.
Figure 18: Road Load vs. Speed
Road Load vs. Vehicle Speed
45.00 at 60F and 0 mph wind 40.00 +
35.00 - Total Road Load
- ._ 30.00 Aero Drag
- | 25.00 - Rolling Drag @30psi $ 20.00
X 15.00
10.00
5.00 -
0.00 A 10 20 30 40 50 Speed mph
Based on the preceding discussions and the results shown in Figure 18, a
total of 20 horsepower are required to maintain 60 mph on level ground. As
much as 2 additional horsepower may be needed during cold weather, as many
as 20 additional horsepower may be required to fight high winds, and as many
as 40 additional horsepower may be required to maintain speed up a grade. This
does not mean the APU must be capable of producing 82 horsepower, because
these are worst case scenarios. Most of the extra surge power, especially for hill
components of the drivetrain. The climbing, can be provided by the electric APU
needs to able to counter the wind and cold weather for extended cruising
horsepower are required. periods, so approximately 40 continuous
112 Acceleration:
Acceleration calculations require detailed information about torque curves and gear ratios. Since this information is not available at this stage of the design process, only rough approximations can be obtained. The simplest way to approximate the power required is to use Newton's Second Law of motion.
The following approach assumes a constant acceleration during the entire acceleration event. This is a poor assumption because acceleration is higher than average early in the event and tapers off to zero as the final speed is
reached.
Acceleration is equal to the change in velocity over the change in time.
The time required to accelerate from zero to 60 mph (88 feet per second) has
been specified at no less than 1 2 seconds.
Av 88-0 ft a=^_=^_^=13^_= =7.3^-.
sec2 (8) At 12
From Newton's law:
1 550 Horsepower g ._. a = F* = - (9) M Velocity W
Solving for the horsepower term yields:
V aW Horsepower - 550 g
88 7.3- (3000/fo) secyJ^y v Horsepower^ ^^ \09Hp 550*32.2- F
sec2
Approximately 110 horsepower are required to accelerate the vehicle from 0
mph to 60 mph in less than 12 seconds. This is an approximation because of the
113 constant acceleration assumption, neglecting of the inertia of wheels, tires, and drive components, neglecting traction issues, and neglecting the effects of gear ratios and torque characteristics. Another important consideration is road loads.
As seen in Figure 18, the road loads increase with vehicle speed. Therefore, the load which must be overcome to accelerate also increases, an effect which is not considered in this simple model.
Despite the inaccuracies involved in obtaining this figure, it does give a feel for how much power is required to meet the stated acceleration goal. The combined power of the APU and electric motors needs to about 110 horsepower.
Since it has been established that the APU will provide about 40 horsepower continuously, the electric motors must provide the additional 70 horsepower.
Other Losses:
The above calculations have neglected two other important considerations. The first is gearbox and machinery losses. The calculated power
levels are required at the wheels of the vehicle, so they must be increased appropriately to account for transmission losses. Transmissions are usually
modeled as being 80-90% efficient. It is reasonable to assume that special attention would be paid to losses in a hybrid vehicle, so efficiency will assumed to be at least 90%. Therefore, the 40 continuous horsepower may need to be 44, and the 110 peak horsepower may need to be 121. These figures will vary depending on the presence and arrangement of transmission machinery.
The second consideration is accessory losses. Accessories such as
power steering, power brakes, or air conditioning would also require power. The steady state power levels must be increased to account for whatever accessory
losses are present in the power system. It will be important to minimize all of these other losses.
114 Towing:
Towing adds another complication to the power calculations. As an example, the effects of towing a medium size boat will be calculated. The following assumptions have been made:
Weight = 1000 pounds
ft2 Frontal Area = 36
Drag Coefficient = 0.5
The aerodynamic interaction between the tow vehicle and trailer will be
neglected. Issues such as axle weights due to the addition of the trailer, traction
problems, and other vehicle attachment issues will also be neglected.
The immediate effect is of increasing the weight of the vehicle by 33%.
This will increase acceleration times and also increase rolling resistance.
Solving Equation 9 for acceleration and substituting known values yields:
550* g* 550* Horsepower 32.2*110 __ ft a= = =55- V'Weight 88 4000 sec2
Referring again to Equation 8 gives:
AV 88 A 1* At= = = 16sec a 5.5
Therefore the addition of the weight of the trailer increased acceleration time by
at least four seconds. While this is only an approximation, it does not seem to be
an unacceptable change in performance.
Updating the road load calculations shows that 48 hp is now needed to
maintain 60 mph on level ground in still air. The APU power level which was
chosen earlier is not sufficient to maintain highway speeds with this trailer.
Increasing the power of the APU to 50 horsepower will allow satisfactory
performance on level ground, but will not leave much margin for wind or grades.
115 The electric portion of the drivetrain will provide this supplemental power, and will operate more frequently when towing.
Battery Storage:
The storage capacity of the battery pack is the other important consideration for HEV design. The hybrid electric sport utility vehicle has two electric power requirements: zero emission travel range, and Adaptive All Wheel
Drive.
ZEV Range:
The capacity of the battery pack is dictated by the ZEV requirements which were stated in Table 6. A worst case scenario will chosen to be 30 miles of continuous zero emission range at 40 mph. It is unlikely that this scenario would arise because most low speed urban travel involves frequent starts and stops at lower speeds. The lower speed increases range, and the frequent stops
allow recovery of energy through regeneration.
Referring again to Figure 18, a constant speed of 40 mph on level ground
requires 7.5 horsepower, or 5592 Watts. Since this represents the power
required at the wheels, the efficiencies of intervening components will modify this value. As an example, assume the stored electrical energy is released from the battery with 90% efficiency, and passes through a motor and controller which are also 90% efficient. Neglecting any other transmission losses yields:
PoweratWheels 5592W ,-__. Power fromBattery= = =6903W TWTl^ (0.9)(0.9)
The battery must therefore supply 6903 Watts to sustain 40 mph.
116 The required range at this speed is 30 miles. At 40 mph, this distance would be covered in 45 minutes. The energy storage capacity of the battery pack must therefore be:
Battery Capacity = {6903W){015hr)= 5177 Whr= 5.2kWhr
In order to meet the specified range requirements, the battery pack must be able to provide 5.2 kWhr of useable energy.
To provide perspective and emphasize the feasibility of this approach, the
GM Impact carries 16.5 kWhr of energy in its battery pack using simple lead acid batteries. This makes it clear that many of the batteries which are already available can easily meet the requirements of this design, making it especially attractive for near term vehicles.
AAWD:
The capabilities of the AAWD system will be investigated at 40 mph using the 5.2 kWhr battery capacity which was just determined.
The AAWD system activates the secondary axle to balance torque distribution and maintain the stability of the vehicle. As noted in the previous section, 7.5 horsepower or 5592 Watts are required to maintain 40 mph.
Theoretically, the 7.5 Hp should be distributed across all the driving wheels. The limitations of practical systems and the added drag from poor road conditions make this unlikely. For this reason, it will be assumed that in order to balance the vehicle, the AAWD system requires an additional 7.5 horsepower. This is an
the power requirements of the vehicle extremely conservative approach because
conditions. Another assumption is are unlikely to double except in very bad that all the electric power for the secondary axle will be drawn from the battery pack, and that the APU will not provide any recharging power.
117 Using the assumption that the electric drivetrain provides 7.5 Hp to
complement 7.5 Hp from the APU, and that the secondary axle operates 100% of the time yields the following:
Energy = Power Time
5200Whr . Energy nMt rr Time = Si= =0.92 hr=55mm Power 5592 W
Dist =Vel Time= 40mph 0.92 hr= 36.8 miles
Full time all wheel drive during hybrid operation can be sustained for 36 miles.
The advantage of AAWD is that it does not operate continuously.
Assuming that the secondary axle operates 50% of the time, and using the same
7.5 Hp assumption as before, the AAWD range doubles to 110 minutes and 73
miles.
05 Time= -=UMhr= 11 0.4min 5592W
Dist=40mph 1.84 hr = 13.6miles
This is a more representative figure because the secondary axle is not likely to
operate continuously during normal driving.
AAWD operation during ZEV travel would require a larger power draw
from the battery pack because both axles must be powered by electric motors.
ZEV travel is likely to occur at lower speeds and require less power. Assuming that 10 horsepower (7456 Watts) are required during low speed ZEV AAWD
operation indicates that it could be continuously maintained for 42 minutes. This
assumes that the secondary axle operates 100% of the time, and is therefore
conservative enough to indicate that sufficient ZEV range can be maintained
during AAWD operation.
A last case to be considered is that the 7.5 Hp is truly distributed across all the drive wheels. This requires only 3-4 horsepower from the electric drivetrain and increases AAWD operation to 2 hours and 80 miles.
118 These calculations confirm that even in very bad conditions the HESUV will be able to perform satisfactorily. The AAWD system will operate using the
5.2 kWhr of energy which is available from the battery pack.
Thermal:
An issue closely related to battery capacity is thermal control of the battery pack. It will be essential that the batteries be maintained at moderate temperatures to avoid serious performance reductions. (Lead-acid batteries can lose as much as 50% of their storage capacity at low temperatures.) A hybrid vehicle can use waste heat from the APU to help maintain battery temperatures, so the impact on vehicle performance will not be as severe as a pure electric vehicle will face.
Fuel Consumption:
Fuel consumption during APU operation will be strongly dependent on the type of APU which is used. A rough approximation can be obtained using the following method. The North East Sustainable Energy Association (NESEA) uses a value of
32.2 kWhr of energy per gallon of gasoline. Assuming that the APU operates at
25% efficiency, this means that 8.05 kWhr of energy reach the vehicle from the
APU.
Travel at 40 mph requires 5.592 kW of energy. Neglecting transmission
losses yields:
5592kW.^ll.=o.695^- 8.05kWhr hr
0.017^U575^ 0.695^.-^ = hr 40miles mile gal
119 Continuing these calculations to include other vehicle speeds gives the information shown in Table 18. The actual efficiency of the APU and transmission at each vehicle speed will have a large effect on the results.
Table 18: Fuel Consumption
Speed mph Hp required Watts gal/hr gal/mile mpg
40 7.5 5592 0.69 0.017 57.5
50 12.5 9320 1.15 0.023 43.2
60 20.0 14912 1.85 0.031 32.4
70 30.0 22368 2.78 0.039 25.2
These results indicate the approximate fuel consumption which can be expected for this design. Remember that the APU will always operate at these conditions.
Results:
As a result of these calculations, preliminary requirements for the hybrid
electric sport utility vehicle have been defined.
In order to maintain highway speeds while towing a nominal trailer, the
APU must be capable of sustaining 50 horsepower.
Peak vehicle power of approximately 110 horsepower will provide
unloaded acceleration times of 12 seconds from 0 to 60 mph.
The battery pack must store 5.2 kWhr of available energy to meet the
specified zero emission travel goal and to provide acceptable AAWD
capabilities.
These results will allow selection of components, but they must be further
modified to include climate considerations such as increased rolling resistance
120 due to slush and low temperatures, increased use of auxiliary loads such as blowers, heaters, wipers, and headlights, and increased viscosity of lubrication fluids. Once the specific characteristics of the components and environment are known, another iteration of these calculations can be performed to increase accuracy.
121 Design Solutions:
The previous sections of this paper have detailed many considerations which face the design of a hybrid vehicle, with special consideration of the Sport
Utility Vehicle application. Based on all of these, components have been selected which would allow the construction of a hybrid SUV. Two time frames were considered, immediate production and turn of the century. The near-term solution is viable for immediate production without major technological problems.
The mid-term solution predicts a solution which will be viable in 5 years (at the turn of the century). Both solutions make certain assumptions which will be discussed next.
Near-Term Hybrid:
Components and materials which are currently available can be used to
produce a satisfactory hybrid vehicle. The choices for immediate use are
outlined in Table 19.
Parallel drive scheme number two is the most appropriate for immediate
application. It is the best compromise for currently available components
because it allows existing engines to be combined with available motor
technologies without placing unachievable demands on either one. The electric
in the vehicle design. AAWD methodology represents the most drastic step
As just mentioned, this solution does not place extreme demands on the
electric portions of the drivetrain. As a result, standard brush commutated DC
somewhat motors which are available and inexpensive can be used despite low
wide application in all types efficiency levels. These motors have seen extremely
are a well proven of electric vehicles which have been built to date. They
developed. Production motors are available solution and have been very highly
of with efficiencies of better than 80%. Advanced D.C. Motors, Inc., Syracuse
122 Table 19: Near-term Hybrid Components
Drive Scheme Parallel Scheme #2
Motor DC
Battery Advanced Lead-Acid / Nicad
APU Internal Combustion
Fuel Gasoline
Chassis Steel
NY, is one of many companies which have engineering expertise and custom production capacity for the electric vehicle market.
A closely related component is the motor controller. Solid-state controllers for DC motors are also relatively inexpensive and proven technologies which are in production. Curtis PMC of Dublin, CA, has long been a leading supplier of vehicle controllers, and they are now being joined by other companies such as
Auburn Scientific Company of Carmichael, CA, and the ZAPI company of Italy.
This hybrid design does not require the battery pack to store a large amount of energy, but does require decent power delivery. The new generation of advanced lead-acid batteries exhibit favorable power characteristics, but still
Nickel-Cadmium batteries have better have relatively poor energy densities.
power characteristics than lead-acid, and worse energy storage. The Nicad cells
are also are somewhat better suited because they are lightweight, but they
expensive. Lead-acid batteries are significantly more cheap, durable, available,
excellent choice and recyclable. These benefits make them an despite their
and the factor lower performance. Either type of battery is acceptable, deciding
is likely to be economic.
123 It will be important that any new vehicle be readily usable, and this dictates that they be able to use existing infrastructure. The immediate choice for
APU and fuel is therefore constrained to gasoline and an existing engine type.
The most likely engine is a traditional four-stroke which has been aggressively tuned to provide the best efficiency and lowest emissions. This will allow current production capabilities to be utilized to reduce costs and ease changeover, and will also allow the vehicle to take advantage of the widespread availability of gasoline.
The chassis of the vehicle will need to be designed specifically for this application. Since design methods and production tools cannot be changed quickly, it will be necessary to continue to use a traditional steel chassis. Current production methods are well understood and highly developed and can be effectively exploited to make a new type of vehicle.
The critical factor in designing and building a hybrid vehicle for immediate
production is compromise. Available components can be combined to make a vehicle which demonstrates significant improvements over current vehicles, but
it will not be optimized. However, in the very short term it does not need to be the optimum solution, it simply needs to be better.
124 Mid-Term Hybrid:
Production of the near-term vehicle which was just outlined will refine the actual requirements and performance considerations involved with hybrid vehicles. The increased knowledge base combined with newly matured technologies will allow the next generation of hybrid vehicles to show meaningful improvements. The components which are likely to be available for the turn of the century hybrid vehicle are outlined in Table 20.
The first noticeable change is the drive scheme. New technologies will allow a series hybrid to be successfully implemented. The series hybrid, as shown in Figure on 2 page 9, is a more flexible scheme both in terms of design and in terms of operating strategy. The APU is no longer mechanically tied to the
and can be drivetrain, operated to take best advantage of all steady state considerations. Series scheme number four (Figure 9 on page 91) represents the most advantageous arrangement and was chosen as the goal.
the turn of the By century AC or brushless DC motors should be widely
Table 20: Mid-term Hybrid Components
Drive Scheme Series Scheme #4
Motor AC
Battery Nickel-Metal Hydride/
Flywheel
APU Turbine
Fuel Flexible
Chassis Aluminum
125 available. These motors can be obtained now, but together with their controllers are prohibitively expensive. A few years of development will bring costs down and further improve performance. The growing market for electrically powered vehicles will provide a powerful incentive to stimulate advancement. The application of AC motors should create a very robust drivetrain because the motors will not require any maintenance during the entire life of the vehicle.
Integrated wheel motors are also likely to be available, but it is unlikely that they will be feasible for general production in this time frame.
The pace of current work also indicates that a new battery technology will be in production by this time. The Nickel-Metal hydride battery represents a significant improvement over current batteries. It is currently being tested and showing excellent results, and should be widely available and affordable in a few years.
The ever-present lead-acid battery will still be a player in the market.
Reasonable performance combined with unbeatable price will keep the lead-acid industry alive. Both lead acid and Nickel-Metal hydride batteries will benefit from integration with ultracapacitors. Newly available ultracapacitors will greatly expand the power capabilities of conventional battery technologies.
A completely new battery technology will also be available. Flywheels are attractive because they have excellent power characteristics, have a nearly infinite life and are impervious to weather conditions. Application of flywheels would eliminate many problems with electrochemical technologies.
The APU is the key factor which allows the implementation of a series hybrid scheme. Small turbines will be available for vehicle use, and they will revolutionize the way in which vehicles operate. Inherent to their design are flexible fuel capabilities and outstanding steady state performance. As discussed earlier, turbines have the potential to be cost competitive with current engines, and produce lower emissions than the main power grid facilities. The flexible fuel capabilities will allow alternative fuels to be phased in safely, and guarantee that whatever fuel is nearby will be usable. Small turbines are currently undergoing
126 evaluation and testing, and a few years of development will make them widely available and cost competitive.
While composite chassis materials are attractive for many reasons, it not likely that they will be widely used. The change in infrastructure which is necessary to produce composite vehicles is fundamental and very extensive. It will take time to implement these changes.
It reasonable to expect that chassis materials will change slightly.
Research into techniques for using aluminum and magnesium in vehicles is ongoing and achieving good results90. A key consideration in this research is compatibility with existing equipment and production techniques. These studies are showing that the lighter metals can be successfully used with appropriate considerations. Using lighter materials brings many benefits, and the ability to use existing machinery is a strong incentive.
A few years of development will allow many ongoing programs to yield mature technologies, which will produce further improvements in hybrid vehicle performance. There is still significant compromise in the interest of cost, but production can now be scaled up drastically because the performance of the vehicles has improved to the point where they show distinct and desirable advantages.
Long-term Solution:
At the beginning of this paper it was stated that hybrids were an intermediate step toward pure electric vehicles. Curiously, this may not be completely true.
Energy storage is the biggest stumbling block which is preventing widespread use of electric vehicles. Despite the fact that advanced batteries are promising to vastly improve traveling range, it does not seem likely that any of
90 Aluminum Space Frame Technology, Automotive Engineering, January 1994, pp. 70- 73.
127 the storage technologies will prove completely satisfactory. Problems such as weight, volume, and recharge time will always remain.
Pure electric vehicles will certainly be common, but will always lack the energy storage capacity for unlimited use. Based on all the preceding discussions, the fuel cell is the most likely power source for future vehicles. A fuel cell produces low emissions, can use many different fuels, and can be started and shutdown as needed. The fuel supply can be rapidly replenished, and the vehicle can sit idle for long periods of time without degradation. These capabilities will allow the vehicle to complete all general duties without
compromise.
The curious part of this scenario is that a fuel cell is an APU, and
therefore a vehicle using one is a hybrid-electric vehicle. The initial prediction
that all vehicles would be pure electric is only partially correct. Pure electric
hybrid- vehicles will meet many requirements, but a very refined version of the
electric vehicle will continue to see widespread use.
128 Conclusions:
The purpose of this paper was to explore the design of hybrid-electric vehicles, and to investigate a specific application. The preceding discussions have made several points clear:
Pure EV's lack sufficient energy storage for general purpose use. Although
they have proven to be extremely competitive in every other regard, the over
riding problem with current EV's is their lack of range. Electric vehicles will
enter large scale production within two years, and will be intended for niche
applications. There is no doubt that many vehicles travel short predictable
distances and are ideally suited for replacement with EV's. However, the
battery which will allow the general purpose family car to be electric is not yet
available.
The technologies used in electric vehicles are evolving rapidly. The belated
development push is beginning to yield results, and components are
improving almost daily. This means that the early electric drivetrains will be
outdated shortly after they reach the market. This is unavoidable, and should
be an impediment to releasing vehicles. It must be understood that early
vehicles will require upgrading on a regular basis and this may make it
desirable to use some form of rental program. The consumer could buy the
rolling chassis and rent a drivetrain and battery pack for a fixed period of
time. At the end of the rental period any available upgrades would be
installed and the old components recycled. A system like this would enable
automakers to get their vehicles on the street, and would ensure that the
vehicles remain competitive. A completely different car ownership experience
could result, with the consumer buying a rolling chassis, and then upgrading
obvious scenario is the baseline it as they are able to. The most replacing
battery pack with state of the art technologies to yield better range or power,
as well as using more efficient motors and controllers.
129 The hybrid-electric vehicle is both attractive and feasible as a counterpart to
pure electric vehicles. As was discussed throughout this paper, a HEV can
yield great improvements over current vehicles without appreciable loss of
performance. The HEV also has excellent potential for utilizing alternate fuels
and achieving excellent environmental characteristics. Combining the
benefits of the electric drivetrain with the unlimited range of the combustion
powered APU gives the hybrid vehicle a very wide range of applications.
The hybrid drivetrain can be applied to light trucks. The light truck market,
especially sport utility vehicles, is growing rapidly despite their low
efficiencies. Many light trucks are used for high speed, light duty commuting
which makes them well suited to hybrid power.
The hybrid-electric drivetrain has many applications, and has the distinct advantage of being suitable for immediate use. It is very important that hybrids be considered as a viable contributor to improving the environmental performance of vehicles.
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134 References:
The following pages list books, magazine and journal articles, and Society of Automotive Engineers technical papers. Many of these listings were consulted for this paper, but not all. The list cannot be considered complete or comprehensive, and no claims are made concerning the quality of the information contained in them.
The World Wide Web also has much information to offer, with more being added every day. Many sites exist, and a good place to start is the Greenwheels
Electric Vehicle Resource page at http://www.shore.net/~kester.
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"cars" Electric star in clean room. handlinq 1-Apr-93
Mother Earth News Jun/Jul 93 1993: Dave Arthur's amazinq hybrid electric car Matt Scanlon 40(6) Daniel Charles v42 GM to make Impact? Ross Motor Trend 1-Jun-90 42(1)
1-Auq-91 Electric cars: the silence of the cams Ron Coqan Motor Trend v43 71(7)
139 References: Periodical Articles
Blown away in an electric Storm Ron Coqan Motor Trend v44 1-Jan-92 113(1)
Defininq Zero Emmisions Vehicle Ron Coqan Motor Trend v44 1-Jun-92 35(1)
Speed Picnic Don Sherman Motor Trend 1-Dec-92
Impact of EV battery devlopment Ron Coqan Motor Trend v45 1-Jan-93 42(1)
Sudden Impact 3 Ron Coqan Motor Trend v45 1-Apr-93 35(1)
Unique approach to EV powertrains Ron Coqan Motor Trend v45 1-May-93 35(1)
Electric vehicles:powerplay on the auto circuit Ron Coqan Motor Trend v45 1-Oct-93 99(7)
Emerqence of an environmental car company Ron Coqan Motor Trend v45 1-Dec-93 35(1)
GM Impact Ron Coqan Motor Trend v46 1-Jan-94 83(3)
GM Impact: redefininq electric vehicle state-of-the-art Motor Trend 46 1-Jan-94 80
the for Pavinq way electric vehicles Motor Trend 46 1-Jan-94 22
Future electrics of dims Ted Orme Motor Trend v46 1-Feb-94 24(1)
Hiqh Hopes for Hydroqen Motor Trend 46 1 -May-94 35
Parlez-vous electrique? Motor Trend 46 1-Nov-94 27
Buildinq the EV infrastructure Motor Trend 47 1-Jan-95 31
EVS: from conception to reality Motor Trend 47 1-Mar-95 28
Propellinq EVs to the Fore j Motor Trend 47 1-Apr-95 36
Awinqinq both ways with Hyundai's HCD-III Gila Motor Trend 47 1-May-95 22
Farewell to imtemal combustion? National Review v45 18-Jan-93 12(2)
New Scientist V134 13-Jun-92 21
New Scientist v138 26-Jun-93 20
Electric Cars? Cut the Cord James Graham New York Times v141 26-Sep-91 A19
Really cool cars Lesley Hazleton New York Times V141 29-Mar-92 34
A Quicker Charqe for Electric Cars New York Times V141 21-Jul-92 C3
Expectinq a fizzle, GM puts electric car to test Wald, M. L New York Times 28-Jan-94 A1
Pentaqon turninq plowshares into swords Wald, M. L. New York Times 16-Mar-94 D1
The truth about electric cars Noel Perrin New York Times v143 2-Auq-94 19,21
Electric car: GM wary New York Times v143 8-Auq-94 col6 Lawrence Plant to make electric cars is to open in northern NY VanGelder New York Times v143 26-Auq-94 16 Andrew H. Card
Electric car tradeoff Jr. New York Times v144 25-Sep-94 F40
Contractors put on show to save their subsidies New York Times 10-May-95 D20 The power of a voltswagon:you can't hear them, but electric cars are cominq Sharon Beqley Newsweek v117 1-Apr-91 62(1) California electric vehicle mandate reaffirmed amid controversy Oil & Gas Journal 92 30-May-94 25
Do fuel cell vehicles just shift emissions elsewhere? Hardinq, G. Physics Today 48 1-Mar-95 11
140 References: Periodical Articles
A nation on wheels John Naisbitt Popular Mechanics v136 1-Jul-86 172(11) Batteries are included: GM Impact is a bright spot in the development of practical electric car Rick Titus Popular Mechanics v167 1-Apr-90 134(2)
Home automation: the future at your finqertips Popular Mechanics v168 1-Nov-91 79(6)
New aqe of the electric car Cliff Gromer Popular Mechanics v171 1-Feb-94 38(4)
Gasoline - electric sports car Dan McCosh Popular Science v229 1-Auq-86 76(4)
Battery Motoren Werke Dan McCosh Popular Science v235 1-Auq-89 16(2)
Diesel-electric VW Popular Science 237 1-Dec-90 30
"qreen" Goinq For the Brian Woodward Popular Science V238 1-May-91 33(2)
Electric vehicles only Len Frank Popular Science v238 1-May-91 76(7)
Electric car showdown in Phoenix: Zinc-air battery wins Rick Cook Popular Science v239 1-Jul-91 64(3)
Germany pluqs In David Scott Popular Science v239 1-Jul-91 37(2)
Electriccar startups Mark Fischetti Popular Science v239 1-Oct-91 39(1)
Ambitious hybrid Dawn Stover Popular Science v239 1-Nov-91 30(1)
BMW's electric debut Popular Science v239 1-Dec-91 27(1)
Electric startup Popular Science v240 1-Jan-92 41(2)
A ride in a hybrid Stewart F. Brown Popular Science v240 1-Mar-92 28(3)
California Dreaminq Stewart F. Brown Popular Science v240 1-Apr-92 39(1)
Electric Record Dan McCosh Popular Science 1-Jul-92
Fuel Cells on wheels Dennis Normile Popular Science v241 1-Auq-92 29(1)
Battle of the batteries Rick Cook Popular Science v241 1 -Nov-92 66(1)
Popular Science v242 1-Feb-93 35
Popular v244 1-Jan-94 Concept cars: Volvo environmental turbine car Len Frank Science 48(1)
v244 1-Jan-94 We drive the worlds best electric car Dan McCosh Popular Science 52(7)
244 1-Jun-94 Formula hybrid at LeMans Popular Science 101
Popular Science 244 1-Jun-94 99 The promise and perils of flywheels
246 1-Feb-95 29 Burinq the Midniqht Sun Popular Science
Popular Science 246 1-Feb-95 62 It's the battery, stupidl
Popular Sciencs 244 1-Jan-94 52 We drive the world's best electric car
Proceedings 1-Sep-93 An Overview of Electric Vehicle Technoloqy. Dennis and Track v43 1-May-92 Electric vehicles Simanaitis Road 126(7)
Road and Track v44 1-Oct-92 AC proplusion CRX: harbinqer of thinqs electric Kim Reynolds 126(3) Dennis Track v44 1-Jun-93 Volvo ECC: A brief drive into the turbine/electric future Simanaitis Road and 120(4) Dennis Track v45 1-Sep-93 An electrityinq lap of Monaco Simanaitis Road and 112(1)
Road and Track v45 1-Jan-94 42(1) Driving GM Impact electric car: greater impact elsewhere? Ken Zino
141 References: Periodical Articles
Watts new in racinq Joe Rusz Road and Track v45 1-Ann-94 142(2) ^ 1 _ a_=
About the Sport-if you thouqht the competition- most Road and track 1-Aua-94
Batteries included Gary Drevitch Scholastic Update v126 15-Apr-94 22(2)
A nickel metal hydrate battery for electric vehicles S R. Ovshinsky Science v260 9-Apr-93 176(6)
Fuel Cells Firinq up Science News 144 1 3-Nov-93 314
On the qo Tom Waters Science World v47 5-Apr-91 16(3) Howard G Lessons of Sunraycer Wilson Scientific America v260 1-Mar-89 90(8)
Electric car pool Gary Stix Scientific American v266 1-May-92 126(2)
Pipe Dream Scientific American 270 1-Feb-94 113
The 21 st Century's First Fleet: Developmq an Electr.. Pain, Geoff Search 1-Jun-94
Here comes the electric car, sporty, aqqressive and clean Mark Fischetti Smithsonian v23 1-Apr-92 34(10)
PV Electric Vehicle Recharqinq Station Stefanakos, E Solar today 1-Jan-93 Hackleman, The Electric Vehicle: A Silent Revolution Michael Sunworld I989 Gill Andrews EV's: on road aqain the Pratt Technoloqy Review v95 1-Sep-92 50(10)
Turbine cars: major contender, bumpy road Wilson, D. G. Technoloqy Review 98 feb/mar 95 50
Voltinq alonq the freeway The Economist v321 16-Nov-91 80(1)
Sudden Impact: drivinq the electric car The Economist v322 7-Mar-92 90(1)
Wattever next The Economist V325 17-Oct-92 13(3)
To the swift, the prize The Economist V325 26-Dec-92 111(2)
Charqel The Economist v329 23-Oct-93 105(2)
Electric Cars: the drive toward fresh air Andre Bacard The Humanist v54 May/June 94 43(2) The Natural IRS Asks for Comments on Developinq Rules for Clean-... resources tax review 1-Sep-93 The Natural Company Offers Suqqestions for Electric Vehicle Cred... resources tax review 1-Sep-93 The Natural Electric Vehicle Group Would Expand Definition of Qu... resources tax review 1-Nov-93 The Natural
California Institute Voices Concern With Proposed Re resources tax review 1 -Nov-93 The Natural converted and Hybrid Vehicles Should Qualify for Cre... resources tax review l-Nov-93 The Natural
Electric Vehicle Credit Should Apply to Lease Transa... resources tax review 1 -Nov-93 The Natural
Coalition Suqqests Chanqes To Maximize Stimulus Effe... resources tax review 1 -Nov-93 Rep. Woolsey Predicts Proposed Regs Will Hamper The Natural
Dome... resources tax review 1 -Nov-93 The SAE-
Electric vehicle show for Melbourne Ausfralasia : journal 1-Mar-93 Circle games: the flywheel battery could run rings around its chemical cousin Zimmerman, R. The Sciences 34 sep/oct 94 13
Look Ma, No fumesl Time v135 15-Jan-90 49(1) LA's high watt highway: electric cars get a boost in the Phillip Elmer- capital of smoq Dewitt Time v135 30-Apr-90 96(1) Michael D. i The biq qreen payoff Lemonick Time v139 1-Jun-92 I 62(2)
142 References: Periodical Articles
Batteries with bounce Time v141 22-Mar-93 27(1) William Off and humminq McWhirter Time v141 26-Apr-93 53(1) US News and World GM's electrifyinq 2-seater Report v108 15-Jan-90 13(1) US News and World Paul MacCready: Drivinq ahead Report v108 23-Apr-90 69(1) Jump Start to the future: GM wants to do good and do well US News and WOrld by marketinq an electric car William J. Cook Report v108 30-Apr-90 48(1) US News and World The soul of a new machine:risk and innovation William J Cook Report V111 26-Auq-91 80(3) USA Today New battery for electric cars (Maqazine) v122 1-Auq-93 13(1) 1990 National Personal Transportation Survey USDOT 24 The Automobile Industry: health care, air pollution and the electric car Vital Speeches v60 1-Jun-94 492(3) GM's Team Impact: Get Me To The Chruch On Time. WAW Lowell, Jon Ward's auto world 1-Oct-91 Matthew A. International Battery Technology Overview Dziechiuch 1-Oct-92
143 References: Books
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Automotive Handbook - 2nd Adler, U. Robert Bosch Gmbh Edition 1986
Alternatives to the internal Baltimore, Published for Enlronmentai policy, motor vehicles - Ayres, Robert U. Richard P. combustion McKenna, engine; impacts on REsources for the Future by the 1972 motors: IC engines, automobiles, environmental quality Johns Hopkins University Press electric
GM: The First 75 Years of Bailey, L Scott General Motors 1983 Transportation Products
vehicles - drawings Baynes, Ken Pugh, Francis The art of the engineer Woodstock. NY : Overlook Press 1981 engineering drawings
Brown, Lester Flavin, Christopher The future of the automobile in Washington : WorldWatch 1979 Automobiles Russell Norman, Colin an oil short world Institue
on empty. The future of Brown, Lester Running the automobile in an oil short New York : Norton 1979 Russell world
Christian, Jeffery World Guide to Battery Powered M.
Datta, S. K Power Electronics and Controls Reston Publishing Inc. 1985
Emissions of Greenhouse Gases DeLuchi, Mark A. from the Use of Transportation Argonne National Laboratory 1991 amended by 4/22/92 memo Fuels and Electricity
DeWaard, E. Electric Cars John
DiLavore, Philip Energy: Insights from Physics John Wiley & Sons 1984
Power Semiconductor Controlled Dubey, Gopal K. Prentice Hall Inc. 1989 Drives
Energy Information Annual Energy Review 1991 Department of Energy Administration
Energy 1992 Annual Energy Outlook, Information Department of Energy with Projections to 2010 Administration
Fundamentals of Vehicle Gillespie, SAE 1992 Thomas D. Dynamics
Earth in the Balance - Ecology Gore, VPAI Houghton Mifflin 1992 page 40 and the Human Spirit
Electric Motors and Control M. Tab Books Inc. 1982 Gottlieb, Irving Techniques
Choose' Gross, Darwin A Look at the Sun SOS Publishing 1986 "Your Right to
Electric Automobiles: energy, Hamilton, William environmental, and economic Bogota : New York : McGraw Hill 1980 Automobiles. Electric prospects for the future
International Electric vehicle council Proceedings of the ... New York, NY : Electric Vehicle Electric Vehicle electric vehicle International Electric Vehicle 1969 ev - congresses Council Symposium associations of Canada Symposium
144 References: Books
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Automobiles, Racing -Design and A study of design parameters of Kords, Donald N. Thesis 1990 construction Automobiles. a road racing endurance car Racing -Aerodynamics
Lacy, Robert Ford, The Men and the Machine Little, Brown and Company 1986
Handbook of Batteries and Fuel Linden, David McGraw-Hill Co. 1984 Cells Publishing
Daniel D Traficante Maciel, E. D. C. Heath and 1978 Gary David Lacalee Chemistry Company
Moffatt, Ronald A microprocessor based control Thesis 1980 C. system for an electric vehicle
Ford: The Times, The Man, The Nevins, A Charles Scribner's Sons 1954 Company
Nicholson, T. R. Passenger Cars 1863-1904 The Manmillan Company 1970
Perez, Richard A. The Complete Battery Book Tab Books Inc. 1985
Sachs, Leslie R. Bennett, James R. Cheap Wheels Pocket Books 1989
Vehicle aerodynamics : recent Automobiles -Aerodynamics - SAE Warrendale, PA : SAE 1991 progress Congresses
Tire and Rim 1991 Yearbook Copely, Ohio 1991 Association
Blue Ridge Summit, PA : Tab All about electric and hybrid cars 1982 Automobiles, Electric Traister, Robert J. Books
1982 EV Unnewehr, L E. Nasar, S. A. Electric vehicle technology New York: Wiley
DC Motor Control for Electric Valentine, Motorola 1993 Richard Vehicles
Ernest Ann Arbor, Mich. : Ann Arbor Wakefield, The consumer's electric car 1977 Automobiles, Electric Science Publishers Henry
Whitener, The Electric Car Book Barbara
The Prize: The Epic Quest for Simon and Schuster 1991 Daniel Yergin, Oil, Money and Power
Electric Vehicles: International Energy EV technology,performance and Paris : OECD 1993 Agency potential
Park Ridge, NJ : Noyes hybrid vehicles 1979 Electric vehicles Collie, M. J. Electric and DataCorp.
- design Electric vehicle design and electric vehicles and Warrendale. PA . SAE 1991 SAE - development construction congresses
- SAE Electric vehicle technology Warrendale, PA : SAE 1990 ev congresses
145 References: Books
AuthoM Author2 Title Publisher Year Subject
Barone, Martin R., Kamal, Automobiles -Design and Modem automotive structural New York : Van Nostrand Mounir M., Wolf, J. A. 1982 construction. Structural analysis Reinhold Co. (Joseph Allen) analysis (Engineering)
Crashworthiness and occupant Motor vehicles -
protection in transportation New York, NY : ASME 1991 Crashworthiness -Simulation systems methods -- Congresses
Motor Vehicle Manufacturer's Facts & Figures '91 1991 Association
Draft 1988 Air Quality South Coast Air Quality 1989 Management Plan Management District
National Air Pollutant Emission EPA 1989 Estimates, 1940-1987
General Electric SCR Manual, 1967 4th Edition
146 References: SAE Papers
Paper Title Authors- Number
670175 Electrovair - A Battery Electric Car Rishavy Bond Zechin
670178 A High Performance AC Electric Drive System Agarwal Levy
680453 Review ot Battery Systems tor Electrically Powered Vehicles Ragone
690072 Development of Electric Vehicles in Japan Miyake
690074 Stir-Lee I, A Stirling Electric Hybrid Car Agarwal Mooney Toepel
690461 Special Purpose Urban Cars Gumbleton Frank Genslak Lucas
An Electromechanical Transmission for Hybrid Vehicle 710235 Gelb Richardson Berman - Wang Power Trains Design and Dynamometer Testing
710238 Power Requirements of Electric Vehicles Herbert Anderson
The Dynamic Characterization of Lead-Acid Batteries for 730252 Taylor Siwek Vehicle Applications
Nickel-Zinc Storage Batteries as Energy Sources for Electric 750147 Kucera Plust Schneider Vehicles
750192 Silent Raider - A Project for City Center Transport Morris
Battery- A Study of the Energy Utilization of Gasoline and 760119 Sheridan Bush Kuziak, Jr. Electric Powered Special Purpose Vehicles
760121 Hyblrd Vehicle for Fuel Economy Unnewehr Auller Foote Moyer Stadler
Clean Transportation for New Towns DAIHATSU Electric 770385 Honda Sugitani Yumoto Kawakatsu Vehicles in the Senboku Area
780087 Electric Vehicles in Germany - Present and Future Bader Stephan
Applicability of Safety Standards to Electric and Hybrid 780156 MacLaughlin Vehicles
780157. Crashworthiness Tests on Two Electric Vehicles Enserink Hackney MacLaughlin
780215 Electric! Component Modeling and Sizing for EV Simulation Unnewehr Knoop
Digital Computer Program for Electric Vehicle A Simulating White 780216 Performance
On the Relationship Between Gross Vehicle Weight, 780220 Altendorf Kalberiah Saridakis Payload, Effective Range, and the Cost of Electric Vehicles
Stale-of-the- Test and Evaluation of 23 Electric Vehicles for Dustin Denington 780290 Art Assessment
147 References: SAE Papers
A High Energy Tubular Batten/ for a 1800 kg Payload Electric i 790162 Whitehead Delivery Van
800057 System Design of the Electric Test Vehicle One (ETV-1) Rowland Schwarze
The Drive System of the DOE Near-Term Electric Vehicle 800058 Wilson (ETV-1)
8001 1 1 Impact of Electric Cars on National Energy Consumption Agarwal
891658 Fuel Cell Power Plants for Public Transport Vehicles Kevala Marinetti
The Technological Constraints of Mass, Volume, Dynamic 891659 Power Range and Energy Capacity on the Viability of Hybrid Bullock and Electric Vehicles
The Development of Aluminum-Air Batteries for Electric 891660 Rudd Vehicles
891661 Future Vehicles will Run with Solar Energy Fujinaka
Electric Vehicle Design Considerations for Cold Weather 891662 Adams Operation Song
Electric Vehicle Design and Performance Using Advanced 891663 Burke Henriksen Batteries
A Generic SFUDS Battery Test Cycle for Electric Road 891664 Cole Vehicle Batteries
891690 Demonstration of Aluminum-Air Fuel Cells in a Road Vehicle Parish Fitzpatrick O'Callaghan Anderson
Characterization of Electric Vehicle Velocity and Power 891691 Burke Fink Richardson Dowgiallo Profiles Using Road Test Data
The Zinc/Bromine Batten/: Recent Advances for Electric 891692 Zagrodnik Bolstad Miles Vehicle Applications
Safety Considerations for Sodium-Sulfur Batteries for Electric 891693 Vehicles Stodolsky
A New Compact Drive-System with a Microcomputer 891694 Kahlen Drumm Controlled Dual DC-Chopper
The Concept of a Future Electric Electric Vehicle and the 891695 Shimizu likura Naitoh Ono Development of an Electric Motorcycle
891696 Application of Solar Cells to the Automobile Kumagai Tatemoto
891724 Fuel Cell Powered Electric Vehicles Swan
Row-by Lead-acid - Improving the Performance Standard for 900135 Budney Andrew EV Battery Systems
900136 NaS Batteries for Electric Vehicles Shemmans Sedgwick Pekarsky
900137 On-Road Test and Evaluation of the GM Griffon Electric Van Tripp
Comparative Evaluation of Acoustical Noise Levels of Soleq 900138 MacDowall Escort EV and ICE Counterpart
148 References: SAE Papers
900139 Electric Vehicle Program in Hawaii Neill Bac Yano Xue
TOWNOBILE Purpose-Built Electric 900177 Commuter Cars, Vans and Mini-Buses Leembruggen
900178 a Fuel Cell a Installing in Transit Bus Romano Price
Design of an E-bus for Crosstown Operation on 42nd Street 900179 Woulk in New York City
Determining Component Specifications for Conventional On- I 900180 Ohbe road Electric Vehicles
900181 An Electric Van with Extended Range Anderson !
i -. Design Considerations and Component Selection for Volume 900578 Produced EV Controllers Morris Adams
900579 Crashworthiness of the Electric G-Van Palvoelgyi Stangi
The Future of Electric Vehicles in Meeting the Air Quality 900580 Wuebben Challenges in Southern California Lloyd Leonard
910242 Performance Testing of the Vehma G Van Electric Vehicle Whitehead Keller
Track and Dynamometer Testing of the Eaton DSEP Minivan 910243 Burke MacDowall and Comparisons with Other Electric Minivans
910244 Electric Vehicle Development in Fiat Brusaglino
Electrically Propelled Vehicles at BMW - Experience to Date 910245 Braess Regar and Development Trends
910246 Integrated Electric Vehicle Drive Anderson Cambier
910247 Electric Hybrid Drive Systems for Passenger Cars and Taxis Kalberlah
910248 Natural Gas Hybrid Electric Bus Gilbert Gunn
910249 Zinc-Air Powered Electric Vehicle Systems Integration Issues Chieky Danczyk Scheffler Frank
910250 Climate Control for Electric Vehicles Dieckmann Mallory
910251, A High Torque, High Efficiency CVT for Electric Vehicles Fitz Pires
TTie Significance of Automated Opportunity Charging to the 911911 Bolger Viability of General Purpose Electric Vehicles
911912 Zinc-Air Batteries for Electric Vehicles Merry
for Near-Term Electric Vehicles 911914 Battery Availability (1998) Burke !
Performance Data from an Improved Sodium/Nickel Chloride 911915 Sudworth Bohm Cell I
149 References: SAE Papers
911916 Thermal Characteristics of Electric Vehicle Batteries Keller Whitehead
Electric Vehicle and Batten/ Testing at the Electric Vehicle 911917 Barnett Tataria Test Facility
Ananthakrishn 911919 A Commercially-Viable Electric Car a
Real World Electric Vehicle Activities at the Sacramento 911920 Freeman Municipal Utility District
Performance Testing of the Extended-Range (Hybrid) 920439 Keller Electric G Van Whitehead
920440 XA-100 Hybrid Electric Vehicle Reuyl
920441 Gas Turbine Generator Sets for Hybrid Vehicles Mackay
Development of an Electric Concept Vehicle with a Super 920442 Fukino I tie Ito Quick Charging System
920443 Introduction to the BMW - E1 Faust Goubeau Scheuerer
Variable Speed Compressor, HFC-134a Based Air 920444 Dieckmann Conditioning System for Electric Vehicles Mallory
ETX-II 70 Hp MCT Inverter Electric Drive System 920445 Park Clock Watrous Performance Tests King
920446 Electric Drivetrain for Hybrid Electric Bus Gilbert Rehn
920447 Hybrid/Electric Vehicle Design Options and Evaluations Burke
920448 Second-Generation Zinc-Air Powered Electric Minivans Cheiky Danczyk Wehrey
930041 Advanced Heat Engines for Range Extender Hybrid Vehicles Gelman Perrot
930042 On-Off Engine Operation for Hybrid/Electric Vehicles Burke
930044 Hybrid Vehicle Gas Turbines Mackay
Can Hybrid Vehicles Reduce the Pollutant Emission in Urban 930046 Petris Giglio Police Environments?
Evaluation of a Free-Piston Stirling Power Converter for the 930047. Schreiber Shaltens Beremand Ultra-Low Emission Hybrid Vehicle Application
The Braking System Layout of Electric Vehicles - Example 930508 Muller Niklas Scheuerer BMWE1
Qualitative and Quantitative Influence of a Fully Electronically 930668 Hendriks Controlled CVT on Fuel Economy and Vehicle Performance
A Power Coupler for Use in Zinc-Air/NiCd EV's and Other 931007 Danczyk Wehrey Hybrid Configurations Cheiky
A Novel Stator Construction for High Power Density and 931008 Debruzzi Riso High Efficiency Permanent Magnet Brushless DC Motors Huang
150 References: SAE Papers
Battery Modeling for Electric Vehicle Applications Using 931009 Swan Arikara Patton Neural Networks
Improved Electric Vehicle Performance with Pulsed Power
931010 Trippe . Burke Blank Capacitors
Global Opportunities and Risks for Electric and Hybrid Low 931011 Grren McGrath Emission Vehicles Murray
Comparison of Advanced Battery Technologies for Electric 931789 Dickinson Swan Lalk Vehicles
931790 Electric Vehicle Magnetic Field Measurement Karady Berisha Hobbs Demcko
A Technology Overview of an Electric Saturn at the 1993 931791 Danczyk Hobbs Solar and Electric 500 Cheiky
931792 Vehicle Design Optimization for Minimizing Operating Costs Baer Frank
931793 Hybrid Vehicle Engine Size Optimization Triger Paterson Drozdz
Energy Storage Options for Electric Vehicle Recharging 931794 Bieri Meier Stations Schoenung
931795 Urban Electric Vehicle for Public Transportation Romero Chicurel Soto
931796 Analysis of a Diesel-Electric Hybrid Urban Bus System Marr Sekar Ahlheim
Tonatiuh, the Mexican Solar Race Can A Vehicle for 931797 de Silva Svenson Technology Transfer
931798 A Smart Control System for Electric Vehicle Batteries Arikana Dickinson Branum
931799 Switched Linear Induction Motor Freeway System Data Ulbrich Jr.
931815 A Spreadsheet Model tor Air Fuel Cell Stacks Lee Swan Lalk
931816 Alternative Fuels Utilization in Fuel Cells for Transportation Cantoni
931817 Ballard PEM Fuel Cell Powered ZEV Bus Howard Greenhill
Characterization of a Fuel Cell/Battery Hybrid System for 931818 Dickinson Lalk Hervey Electric Vehicle Applications
Chiappini 931883, Some Considerations on Solarcar Performance
Battery Electric Passenger Vehicles - Comparative 931884 Wyczalek Assessment
Characteristics on Traction Drive Influence of Battery Winter Brandes 940293 Performance
Michalitsis Panteliou Dimarogonas 940294 Chassis Design for a Small Electric City Car Chondros
940295 Thermal Comfort of Electric Vehicles Dauvergne
151 References: SAE Papers
Proton Exchange Membrane Fuel Cell Characterization for 940296 Swan Dickinson Arikara Electric Vehicle Applications
Power Quality Problems at Electric Vehicle's Charging 940297 Berisha Blake Hobbs Station Karady
Low Frequency Magnetic Field Generated at Electric 940298 Berisha Muralidhar Hobbs Vehicle's Charging Station Karady
Specifcc Analysis on Electric Vehicle Performance 940336 Frantzeskakis Krepec Sankar Characteristics with the Aid of Optimization Techniques
The Development and Performance of the AMPhibian Hybrid 940337 Davis Hodges Madeka Electric Vehicle
The Selection of Lead-Add Batteries for Use in Hybrid 940338 Madeka Davis Hodges Electric Vehicles
Development of the University of Alberta Entry in the 1 993 940339 Checkel Duckworth Collie Workun HEV Challenge
Hybrid Electric Vehicle Development at the University of 940340 Duvall Cobene II Eng Kruetzfeldt California, Davis: The Design of Ground FX Riley
Development of a 24 kW Gas Turbine-Driven Generator Set 940510 for Hybrid Vehicles Mackay
The NGV Challenge - Student Participation, Faculty 940556 Lueptow Involvement, and Costs
Analysis of Data from Electric and Hybrid Electric Vehicle 940557 Wipke Hill Larsen Student Competitions
950176 Technial Analysis of the 1994 HEV Challenge LeBlanc Duoba Quong Larsen Stithim
Testing Hybrid Electric Vehicle Emission and Fuel Economy 950177 Duoba Quong LeBlanc Larsen at the 1994 DOE/SAE Hybrid Electric Vehicle Challenge
950178 Electro Vehicle Performance in 1994 DOE Competitions Quong Duoba Larsen LeBlanc Gonzales
950179 Design and Analysis of a Hybrid Electric Vehicle Chassis Aerni Radcliffe Martin
A Hybrid Vehicle Evaluation Code and its Application to 950491 Aceves Smith Vehicle Design
950492 Controlling a CVT-Equipped Hybrid Car Schmid Dietrich Ginsburg Geering
The Effects of APU Characteristics on the Design of Hybrid 950493 Anderson Pettit Control Strategies for Hybrid Electric Vehicles
ECTAM, A Continuous Combustion Enine for Hybrid Electric 950495 Palmer Allen Vehicles
950955 Computerized Speed Control of Electric Vehicles Fan Koren Wehe
The Effect of Regenerative Braking on the Performance and 950957 Davis Madeka Range of the AMPhibian II Hybrid Electric Vehicle
Fuel Economy Analysis for a Hybrid Concept Car Based on a 950958 Ross Wu Buffered Fuel-Engine Operating at an Optimal Point
A Comparison of Modeled and Measured Energy Use in 950959 Cuddy Hybrid Electric Vehicles
152 References: SAE Papers
Comparison Betwenn On-Road and Simulated Performance 951069 Kim Koh of the KEV Electric Vehicle NamGoong
153 Appendix A: Temperature
' ' T3 I : n ! i ! ! CO 1 i o ^ ; | _l a
o^ ,_ O) CJ) o> o CM CD o m ,_ CO CD * f CO s o CO CO CO m CO o CO in CO 1 ' ' d d d oS oS ai CO CO CO CO r^ t>^ 1 C\J C\J CM 1 !
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en a G ^ o Q in m in in in in m in in in m LO in 8 00 LO in m in m in m m in tn in in m > <9 a, CO CO CO CO CO CO CO CO CO CO CO CO CO 2 ,_ in Ol CM CM ,_ r^ O) co CO ** ,_ CM w h- * 1 Q w o CO CO O) CO CD o in o CO CM a + o S l< TT CM d CO iri CO CM d oi CD a. 1 u. CD - 1 E h- wo / c 4 .E Q o O O o o o o O o o o o o > to r g E - "O / - *b o o o o o o o o o o o o o CO CO CO CO CO CO CO CO CO CO CO CO CO < j o CM oooooooo' U_ uioinpinoirip o o ci o o o o o o o o o o PJOJCMPJOJOJ-" CM o r^ o CM CO *r m CD CO O) , CD J3M0d35J0H H o: 1 X a in in I CO CM W O o o CD w I'll ' : ! ^ i . Z3 ' c V) CO ' 0) o c '! o CO 3= CO "O ' CO (D CO CO n n it a - (D g o < o a o CD 1l'ii o 00 : : O o c CO c: DC o E QJ ul Q 5 < > ; t i i 154 Appendix A: Wind xs i 'I III CO | I ! | 1 o j j _i a ^ -o CO h- , h- CO r-~ o CD LO CO CD CO 1 i = I-*. I , I , in CO CO Tt o co LO LO T O m 1 o 1 g. * LO T in in CO CO o m CO CO LO Tf I CM CM CO CO tT LO CO 1 ' rt O 1 ; 1 o H 1 1 ! ! 1 . 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(0 Ifl * <*) CM *- Q. jsmocImsjoh o o o o o o o o E o o o o o CD 1- Ol - I a in in CO CM CA o o o CO ^r o o .c a. o (A d d % E c -O o CO o o o CO d o o CM o a o o CD CO o in o o cri CO o o CM d d 1 \ a> in c 1 c cu Cfl x: a D (0 "u "D OJ OJ CO cu CD n n CO D_ O 1 CO 5 *p ~? < o a. O 1 CO c CD To O cu a "a c a E !c ' o co ! LL O 5 1 > 155 Appendix A: Rolling Resistance CO LO ! o_ *T OO CO CO 8 S 8 c a X S 3 a> S O d O CM CM CM CO CO "T LO i j en (0 i ^ ^ 1 o o CO CO CO O CO o o in CO LO in Lf 1^ r*. O) w a C\ CN 8 CO CO CD CM S" If in IT m in CO CO r^ I-- CO d .E d CM CM CM CM CM CM CM CM CM CM CM CM CO CO O g CC 1 " Q. 1 ^ O * IT m LO CO CO CO CD .- CO CD CO ir tp CO CC o o 1 a C e e C CO s 8 s 8 s s o o 'o d d O d d d O d d d d d d B ^ CC CD o o 0_ o O LO o O) CO CM CD in 8 CO CO LO CO X 81 8 CD j d d d CM CM CO tT LO CO CO ! 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CM CO d CD CM in d d in CO CM CO d o CO CD I-- CO 1 i 15 O i I o H ' j j "O ro o 1 . i -1 9 o CD CM Tf o LO o o in CO CD i O CD CO LO CO CM CO o5 S CM CO CO d d o a: g in CO CM op CM CO CO CD ; n 8 o \- (0 CM 00 CO CM m CO CO CO o CD CO o CO cd in Q. CD CD q r^ o a CO d o r^ r^ d d d d tT m CO d r^ CO in co 2 CO d CO CO tT CO CM CM CO in to Q 3? o > m CM CM in CO -o CD to CO o 5 CO CO CD CM T5 6 3 CD w i_ XJ Q - CO CO CO CO CO CO CO CO CO 00 CO CO CO CM CM CM CM CM CM CM CM CM CM CM CM CM CD "g in 1" CM CM CM CM CM CM CM CM CM CM CM CM CM T = CO CO CO CO CO CO CO CO CO CO CO CO CO O fl CC CD to a- q in in in m to m in m m in m m m in in in in in in in in in in m m in S co CO CO CO CO CO CO CO d d d d Si ^j- T* *j- *j - tj- ^r Tf ^r ^t Tf ^r 3 in O S \ 3 en tr c CO CO CO CO CO CO ao CO CO CO CO CO CO \ "~ 1 Q OT o o o o o o o o o o o o O | o CM CM CM CM CM CM CM CM CM CM CM CM CM 1 CO CO CO 00 CO CO 00 CO CO CO CO CO CO *- a! < a. \ C3 .E ex o O o O o o o o o o O O O 5 E "O o O O O O O P\ > o o -ECl o O o O o o o o o o O O O V 5 o o o o o\ g to CO CO to CO CO CO CO CO Q- CO CO CO CO d d o d o o V i d o o E CM O CO CO * CM * CO Q *~ *" * O CO CO CO CO CO CO CO CO CO CO CO CO CO a CC O d d d d d d d d d d d d d fir o o o o o o o o o o o o o \ CO CO CO CO CO CO CO CO CO CO CO CO CO < LL w V ci o o o o o o o o o o o o o CO CO CO CO CO to CO CO CD CO CD CO CO E jeModesjoH l- en - X a. in in CO CM OT o o o >* o o x: x: CL u CO <3> d d % E c X} o CO o o o CO d o o CM o Q o o CD CO o in o o CD CO o o CM d d 1 ! OT i ! c c CO OT x: o ai (0 o "O SE CO i u il a o S ^o 2 ! < o CL c o "to o CO o o CD c X) x: o (0 CC Q 5 1 > 157 Appendix A: Total Road Load 4 5 6 6 8 9 I | l | rr' ~* i id n 0 0 3 5 7 9 I " '"\ j ill ' -000000000000 ^'O'rz O'O g-^ooooooooooooooc 0 0 ' jr. radooooooooooooo.c .0 0 '' Q 1 ' > 1 i | n ^oooooooooo 0010 0 c 0 0 ffl T3 I 5 3 cc r cm > CO CO 40 30 3P 30 34 36 38 ) a> cnc0oooooooooooooc ^qOOOOOOOOOOOOOOC 2 0 0 in co \ ii -- 64 T- O tO \ \ \ > >' *~ V 0 0 0 0 1 2 5 \\ * L \l CM r) -Q = 00 33 h ) 36 68 96 T3 0 5 9 S >0 ? i Mt m c Q.00000000000000 S E -1 <0 *i o J \ E CO ! in U mmnnmnnmcimmmam inn 0 1 1 T \] O odddoddddo'oooo fe 88888888888888 388 , , , t oooooooooo > < MMOdttJOfl co H X tin cvj 8 O 30 0 O 1 IO 30 8 j 8 d 1 - n f. 1 1 ! 158 Appendix A: Towing TD T3 ^ tj en o co g, en 1 cc g j O o "O r: 173 rt, ! _ i 1 *S 1 15 - _ i 8 c75 |C0 2 li Q 1 Q 1 i ! 0- ^ CD l 1 o c5 2 cc cc fa ofi co V, o o?5 * C3> fa ^ $ ft 3 Q CL to O (/> CO la o o o d o O CC g o o CD CO to o -^ co co < < CD at s Is CN Is DO 3 co CD < < it o if o 8 s CO CO o d o d o CO s < < Ll_ o o 1? 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