Hybrid vehicle drivetrain Hybrid vehicles are vehicles with two or more power duced at present, have both an internal combustion engine sources in the drivetrain. There are many different types (ICE) and an electric motor coupled. If they are joined at of hybrid vehicles, although only the gasoline-electric hy- an axis in parallel, the speeds at this axis must be identical brid is currently commercially available. and the supplied torques add together. Most electric bi- cycles are of this type. When only one of the two sources Hybrids are classified by the division of power between sources; both sources may operate in parallel to simul- is being used, the other must either also rotate in an idling manner, be connected by a one-way clutch, or freewheel. taneously provide acceleration, or they may operate in series with one source exclusively providing the accel- With cars, the two sources may be applied to the same eration and the second being used to augment the first’s shaft- for example with the electric motor lying between power reserve. The sources can also be used in both se- the engine and transmission. The speeds are thus equal ries and parallel as needed, the vehicle being primarily and the torques add up, with the electric motor adding or driven by one source but the second capable of providing subtracting torque to the system as necessary. The Honda direct additional acceleration if required. Insight uses this system. Current hybrids use both an internal combustion en- Parallel hybrids can be further categorized depending gine (ICE) and a battery/electric drive system (using upon how balanced the different portions are at provid- ultracapacitors) to improve fuel consumption, emission, ing motive power. In some cases, the combustion en- and performance. Electrically assisted pedal bicycles are gine is dominant (the electric motor turns on only when a a form of hybrid drive. Other combinations of energy boost is needed) and vice versa. Others can run with just storage and conversion are possible, although not yet in the electric system operating. But because current paral- commercial production. lel hybrids are unable to provide all-electric (ICE=OFF) propulsion, they are often categorized as mild hybrids Combustion-electric hybrids have larger battery sets (see below). than what a normal combustion engine only vehicle would have. Battery and supercapacitor technology is Because parallel hybrids can use a smaller battery pack advancing.[1] A potential advantage is that when these as they rely more on regenerative braking and the internal battery sets require renewing in the future, the newer bat- combustion engine can also act as a generator for supple- tery sets will be potentially superior having higher energy mental recharging, they are more efficient on urban stop storage giving greater range enhancing a vehicle. and go conditions or city driving compared to highway driving.[2] Honda's Insight, Civic, and Accord hybrids are examples of production parallel hybrids.[2] General Mo- tors Parallel Hybrid Truck (PHT) and BAS Hybrids such 1 Types by drive train structure as the Saturn VUE and Aura Greenline and Chevrolet Malibu hybrids are also considered as utilizing a paral- 1.1 Parallel hybrid lel architecture. 1.1.1 TTR hybrid Electric Battery Converter motor An alternative parallel hybrid layout is the 'through the road' type.[3][4] Here a conventional drivetrain powers one axle, with an electric motor or motors driving the other. The batteries can be recharged through regenerative brak- ing, or by loading the electrically driven wheels during cruise. Power is thus transferred from the engine to the Reservoir Engine batteries through the road surface. This layout also has the advantage of providing four-wheel-drive in some con- ditions. An example of this principle is a bicycle fitted Structure of a parallel hybrid electric vehicle. The gray squares with a front hub motor, which assists the cyclist’s pedal represent differential gears. power at the rear wheel. Other examples include the Audi 100 Duo II and Subaru VIZIV concept cars, the Peu- Parallel hybrid systems, which are most commonly pro- geot 3008 HYbrid4, the Volvo V60 plug-in hybrid and 1 2 1 TYPES BY DRIVE TRAIN STRUCTURE the BMW i8. quite independently of each other. This has advantages. This arrangement allows for a smaller sized generator en- gine to be used compared to the size of a conventional 1.2 Series hybrid direct drive engine. The traction motors receive electric- ity from the battery or generator or both. The traction motors, depending on the size of the battery bank, will in Reservoir Battery many cases of operation have most of the power provided only by the electric battery, which may be charged from Electric Engine Generator Charger Converter external sources such as the electricity grid. The vehicle motor can move primarily using energy from only the battery Flywheel bank with the generator operating as a second stage only or Capacitor when needed to drive the vehicle or charge the batteries. A great advantage is that the vehicle can be driven in ur- Structure of a series-hybrid vehicle. The grey square represents ban areas with zero emissions with the generator cutting a differential gear. An alternative arrangement (not shown) is to in for longer journey’s when the battery bank is depleted. have electric motors at two or four wheels. In short, a series-hybrid is simple, the vehicle is driven only by electric traction motors with a generator set pro- Series hybrids have also been referred to as extended- [5] viding the electric power when needed. An electric bat- range electric vehicles (EREV) or range-extended elec- tery acts as buffer evening out demand and the stored en- tric vehicles (REEV). ergy may be use only as the prime source to propel the vehicle. When using the generator set to propel the ve- 1.2.1 Overview hicle the batteries reserve energy can be used to assist in acceleration or pulling heavy loads. Firstly, look at an electric transmission, which dates The choice of electric traction motors has great advan- from 1903. Conventional mechanical transmissions add tages. Unlike piston internal combustion engines, electric weight, bulk and saps power from the engine with auto- motors are highly efficient with exceptionally high power- matic shifting being complex when used. Unlike combus- to-weight ratios providing adequate torque when running tion engines, with electric motors matched to the vehicle over a wide speed range. Internal combustion engines run a multiple-speed transmission is not essential. The me- at their most efficient when turning at a constant speed. chanical transmission between the engine and the wheels An engine turning a generator can be designed to run at is removed and replaced by electric driving traction mo- its maximum efficiency constant speed, or a series of con- tors which are powered by an electric generator turned stant speeds. Combining the two, which can operate quite by an internal combustion engine. The driving electric independently of each other, gives maximum efficiency traction motors are the only form of propulsion. and performance overall. In certain operational situations it is economical and flex- The arrangement was difficult for early production cars ible as the driving internal combustion engine is discon- as synchronization of power generation and demand was nected from the demand. This is a series-hybrid arrange- inefficient, resulting in higher fuel consumption. This is ment and is common in diesel-electric locomotives and no longer an issue with modern computer engine man- ships. The river ship the Russian Vandal launched in agement systems optimizing when the generator runs to 1903, was the world’s first diesel-powered and diesel- match the power needed. Since Ferdinand Porsche’s electric powered ship. Ferdinand Porsche successfully series-hybrid car, electric motors have become substan- used this arrangement in the early 20th century in rac- tially smaller, lighter and efficient over the years. One of ing cars. Porsche named the system, System Mixt. A the advantages of a series-hybrid system is the smoother wheel hub motor arrangement, with a motor in each of progressive ride with no stepped gear ratio changes. the two front wheels, setting speed records. Ferdinand The electric transmission is currently viable in replac- Porsche’s system was even used, with much less success, ing the mechanical transmission. However, the modern to power his competitor for the “Tiger” tank contract, and series-hybrid vehicles takes the electric transmission to a the heaviest armored fighting vehicle design ever built, higher plane adding greater value. Modern series-hybrids both during World War II. incorporate: However greater flexibility, higher efficiencies and less emissions are achieved in a series-hybrid system for road • Electric traction only - using only one or more elec- vehicles when an intermediate electric battery, acting as tric motors to turn the wheels. an energy buffer, is between the electric generator and the • Combustion engine - that turns only a generator. electric traction motors. The internal combustion engine is mechanically disconnected from the driving wheels, in • A generator - turned by the combustion engine to effect disconnecting the engine from the demand. The make up a generator set that also acts as an engine electric traction motors and the generator may operate starter. 1.2 Series hybrid 3 • A battery bank - which acts as an energy buffer. more efficient or alternative engine designs, such as a microturbine,[8] rotary Atkinson cycle engine or a linear • Regenerative braking - The driving motor be- combustion engine.[9] comes a generator and recovers potential and kinetic (inertial) energies through its conversion to electri- General Motors in 1999 made the experimental EV1 se- cal energy, a process which in turn is able to slow the ries hybrid using a turbine generator set.