24th Aachen Colloquium Automobile and Engine Technology 2015 1

New 3- Engine and for the Series smart Dr. Ralf Wörner, Dr. Axel Heuberger, Mr. Bernd Wagner, Mr. Joachim Luick Daimler AG, Stuttgart, Germany

Summary

The vehicle concept for the new generation of smart model has been completely revised with regard to the powertrain for the upcoming market launch. The major in- novations include the use of a dual clutch transmission and a supercharged three- cylinder turbocharged engine with electric wastegate control, as well as special exhaust aftertreatment use for all global markets, in particular in due consideration of the strict exhaust emissions standards.

The expectations of the young clientele are focused on the attributes of driving pleasure and ease of shifting. Adapted application strategies for the engine and transmission, which allowed these objectives to be fulfilled with equal status, have been provided in order to implement these attributes.

1 Introduction

1.1 A city concept with rear installation of powertrain

It has now been 17 years since the first generation of the two-seater smart was launched back in 1998. The vehicle has been sold in the USA since 2008 and in China since 2009. The new generation was also designed to meet the requirements of urban areas in particular.

The aim was to develop a vehicle with an extremely compact length of 2.69 meters while providing the maximum amount of space in the vehicle interior at the same time. The only way to achieve these objectives was to maintain the rear-mounted en- gine concept. The fact that the tank is located in front of the rear axle and thus below the interior compartment results in an elevated seat position, which allows for a better 360 degree view (Figure 1). In addition, the newly designed MacPherson front sus- pension enabled the steering angle to be further increased and the turning circle of the two-seater to be reduced to a minimum. The engine and the transmission are positioned above the DeDion rear axle, with a coil spring and a twin-tube shock ab- sorber; this rear axle concept is particularly conducive to driving stability during a load change. Driving stability in terms of lateral dynamics and crosswind impact was im- proved significantly by increasing the track width by roughly ten centimeters. 2 24th Aachen Colloquium Automobile and Engine Technology 2015

Figure 1: Rear-mounted engine concept at a vehicle length of 2.69 m With the engine unit being located in the rear area, special attention had to be de- voted to oil/water cooling and the cooling-air flow when the vehicle was designed. To this end, space for an additional electric that enables the variable activation of forced ventilation was taken into account in the design of the engine compartment. Furthermore, Figure 1 displays the compact three-part radiator module unit that is located in the front end. This consists of an engine radiator, an air-conditioning radiator, and an air-to-water that is installed in the front-most section. These components are connected to the engine via lines in the vehicle underbody. The cooling circuit is additionally equipped with a zero-mass valve to improve heating behavior.

One further advantage of a rear-mounted engine is that the driven axle is decoupled from the steering axle. While the operating angle of the drive shaft and the distance between the longitudinal members are decisive factors in the case of a front-mounted engine concept with front wheel drive, a rear-mounted engine concept allows signifi- cantly larger steering angles to be implemented (Figure 2). Thanks to its maximum steer angle of 51° on the inner wheel (38° on the outer wheel) and a resulting turning circle of 6.95 m, the two-seater is maneuverable and perfectly suited for the require- ments of urban areas. 24th Aachen Colloquium Automobile and Engine Technology 2015 3

Figure 2: Comparison of a front-mounted and a rear-mounted engine concept 1.2 Engines

The new smart vehicle relies on the use of efficient and compact three-cylinder gasoline engines that cater specifically to the requirements of a 2.70 m to 3.40 m vehicle concept. The engines are integrated into the rear of the vehicle, in an instal- lation position pivoted by 49°. The concept is based on a modular system consisting of a 1.0 liter naturally aspirated engine with a rated output of 52 kW and 92 Nm of torque, plus a 0.9 liter turbocharged engine with a rated output of 66 kW and 135 Nm of torque. Furthermore, these engine variants are designed for use all over the world, and have been adapted to include specific components for improved exhaust aftertreatment based on the applicable regulations and regional boundary conditions (China, USA, etc.). In combination, both manual and automatic transmissions have been taken into account. The is now included in the portfolio of the smart vehicle as a five-speed transmission. Moreover, the first use of dual clutch technology with a six-speed transmission system, designed on the basis of triple- shaft architecture with electromechanical actuation, constitutes a long-term step toward the optimization of comfort while preserving the efficiency of manual transmis- sions. All powertrain components are manufactured in a network concept within Europe.

1.3 Transmission and drive programs

The basic version of the new smart is available with a manual transmission for the first time. It is designed as a five-speed manual transmission with a gear ratio spread of approx. 5.0. It was developed for rear installation in east-west orientation, weighs a total of 35 kg, has an integrated differential, and gear packages/ sets/ trains with friction optimization. The gears are selected via cables in the transmission; an ad- ditional reverse gear lock was incorporated into the design. As the only vehicle in this 4 24th Aachen Colloquium Automobile and Engine Technology 2015 vehicle category, the smart is now also using a six-speed dual clutch transmission with a dry clutch and dual-mass (DMF). The compact design is achieved thanks to the triple-shaft arrangement of the two sub-transmissions. The driver also has the option to use shift paddles on the steering wheel to shift gears manually.

Figure 3: Powertrain/drivetrain system and shift paddles

2 Engine/Drivetrain

2.1 Three-cylinder engine concept

The three-cylinder turbocharged and naturally aspirated engines installed in the new smart feature four-valve technology and variable inlet valve control which are the re- sult of a further development based on the cooperation partner's existing engines for a front wheel driven powertrain variant.

In addition to the uniform main dimensions of the , the two drive assemb- lies also have the same installation angle with a swivel angle of 49 degrees, which was developed specially for this purpose, in the rear end of the vehicle. The main dimensions, which are distinguished by the diameter of approx. 72 mm and the distance between cylinders of 85 mm, are the same for both engines.

In contrast to its predecessor, the US version is designed as a turbocharged engine, with an air-to-water heat exchanger integrated in the manifold. In order to fulfill the emissions regulations on the US market, Daimler AG undertook important tech- nical modifications to the basic EU engine. 24th Aachen Colloquium Automobile and Engine Technology 2015 5

These changes are oriented toward fulfilling the specific emissions legislation of the US market, ULEV70/LEV3, and involved adaptation of the entire exhaust aftertreat- ment. To this end, the three-cylinder engine concept was equipped with additional secondary air injection, and the coating as well as the combustion control were completely redesigned to satisfy the aforementioned emissions goals. This was achieved, for example, by installing an electric wastegate actuator and - shaft positioners on the intake side.

This required a secondary air system to be installed, which was a particularly com- plex task due to the limited installation space and the resulting installation location of the secondary air . This endeavor required complex line and hose routing in the rear end of the vehicle.

The definition and design of the catalytic converter (incl. the protection of adjacent peripheral components against thermal radiation), as well as further adaptations that were necessary due to the use of wide-band oxygen sensors before the catalytic converter, ensure that the heat-up time of the catalytic converter is short and that the exhaust aftertreatment is stable in broad operating ranges.

Figure 4 highlights the changed parts necessary for the exhaust aftertreatment of this three-cylinder engine.

Figure 4: Assemblies relevant for exhaust aftertreatment The following section addresses selected special features of this three-cylinder engine system in more detail. 6 24th Aachen Colloquium Automobile and Engine Technology 2015

2.2 Secondary Air System

The engine was developed such that it would fulfill even SULEV requirements for later applications; this required the developers to place a particular focus on the and the corresponding layout of the components:

- Cylinder head with secondary air bores

- with an air duct

- US-specific catalytic converter & sensors

- Secondary pump with piping, an OBD sensor and a valve

- Wide open and part throttle blow-by system

- Fuel canister scavenging system with increased performance and venture system in the WOT path

The biggest challenge with this engine was to implement the secondary air bores and the air duct.

The aim was to create air injection bores that are as close to one another as possible and located behind the exhaust valves in order to ensure the best mixing behavior of exhaust and air. Preliminary examinations on the test bench confirmed that the secondary air injection concept was feasible. A second aim was to prevent the ex- haust flow and the cooling output from being affected negatively. These aims were achieved through the CFD-based optimization of the details of the bore design, the exhaust opening, and the water jacket design. 24th Aachen Colloquium Automobile and Engine Technology 2015 7

Figure 5: Comparison of the cylinder head for EU & NAFTA applications The secondary air duct was integrated in the flange of the turbocharger, since there was not enough installation space available to integrate it in the cylinder head or on the cylinder block. The block for the US engine was designed to be retained as a shared component part with the EU variant.

The turbocharger is sealed with special seals on the cylinder head, which enable a calibrated air flow from the duct (see Figure 5 and Figure 6).

Figure 6: Cylinder head – position of the secondary air injection in the outlet duct

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The special packaging of the drivetrain did not allow the secondary air inlet to be positioned in the center of the engine. Due to the proximity to the first cylinder, the distribution of the air flow was very uneven at first. The secondary air system was optimized by means of static and dynamic CFD. The aims of the optimization were:

- To ensure an overall air flow of 10 kg/min.

- To achieve homogeneous air flow distribution in all exhaust openings.

- To reduce the reverse flow of exhaust in the secondary air system.

The fine-tuning for the design of the air duct, the bore diameters, and the calibration bore for the seal required multiple iterations before the final and satisfactory overall concept was identified.

Despite the challenging boundary conditions, a largely homogeneous air flow distri- bution (with deviations of just 3% from the mean value) in idle state was achieved in the end.

The illustration below (Figure 7) shows a comparison of the situations before and after the optimization of the flow conditions. After extensive engine test field and vehicle tests, a homogeneous secondary air-mass flow rate of approx. 3.3 kg/h per cylinder was achieved.

Figure 7: Optimization of secondary air injection

It shall be noted that the use of an electromechanical wastegate actuator allows the generated energy to be guided directly to the catalytic converter, without a further reduction of temperature at the turbocharger, by means of afterburning in the mani- fold area. This enables the catalytic converter to heat up extremely quickly and en- 24th Aachen Colloquium Automobile and Engine Technology 2015 9 sures a low light-off temperature, which is a mandatory design factor to be consider- ed when it comes to critical emissions requirements such as those of LEV III.

2.3 Optimization of the Turbocharging and Exhaust Aftertreatment Concepts

One of the objectives of the smart powertrain was to achieve the highest possible torque even at low rotational speeds. However, the associated fill levels can be realized only at a higher air mass flow rate. Three-cylinder turbocharged engines generally allow a very high scavenging rate, especially at low rotational speeds when only the exhaust back pressure in the area of the valve overlap returns to the ambient level. When performing the scavenging process in an MPI system, it must also be considered that the usable injection area is fairly small (as compared to a DI system).

Figure 8 below explains the process of the injection strategy for MPI systems during scavenging, using the smart three-cylinder engine as an example.

Figure 8: Valve timing diagram during scavenging

As with direct injection engines, the scavenging area (on the right-hand side of the diagram) is limited by the pressure ratio in the engine and the available time (dashed red line). This area cannot be fully exploited with an MPI system, since the injection period already exceeds the entire intake period in °CA in the case of very high loads and engine speeds, and each valve overlap leads to an increase of the load and a reduction of the available injection time. This, in turn, is necessary for scavenging the fuel in order to create a stoichiometric exhaust mixture. 10 24th Aachen Colloquium Automobile and Engine Technology 2015

The specified torque of 115 Nm at 1,500 rpm requires a specific valve overlap in or- der for the exhaust gas mass flow and the associated increase in charge-air pressure to be achieved. In order to achieve minimized emission levels, the exhaust gas lamb- da is adjusted to the stoichiometric operating point over the entire scavenging range.

This causes the catalytic converter temperatures to rise considerably with the increasing scavenging rate as unburned fuel from the fuel-rich combustion reacts with the scavenged air.

The approach that is taken here is to set the scavenging rate to as high a level as is required to reach the requested torques, while keeping it just low enough to reduce the temperature of the catalytic converter and the load. Figure 9 illustrates the usable engine-map ranges in the conflict of objectives between the maximum temperature of the catalytic converter and the achievable torque.

Figure 9: ISO diagram of the torque and catalytic converter temperature during scavenging

In the case of lower engine speeds and smaller loads, the positioner can be adjusted such that scavenging does not take place. Nevertheless, the maximum amount of charging efficiency is limited. The complete valve overlap and, as a result, scavenging can be used to its full potential only at very low mass flows, since a stoichiometric exhaust gas lambda can be achieved only if the injection period is shorter than the period between the exhaust valve closing and the intake valve clos- ing.

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2.4 Control strategies for optimizing handling characteristics

The energy required by the ancillary equipment in current vehicles is continuously increasing due to the growing demands in terms of comfort; air conditioning, for example, is now a standard feature. The required torque depends on the vehicle category only to a small extent, which means that strategies that fulfill the require- ments placed on the drive under all boundary conditions need to be developed specially for small vehicles with small downsizing turbocharged engines and a con- sequentially small basic torque. The conditions for the US market are exacting, since driving situations at absolute heights of > 4,000 m above sea level and ambient temperatures of > 50°C must also be covered.

The challenge that arose in the course of adopting the ancillary equipment of the European variant for the current US smart vehicle was to satisfy the following require- ments:

- Stability at idle speed

- Creeping in the case of the DCT variant

- Optimized starting and restarting

- Acceleration characteristics

- Minimum DCT downshift times in due consideration of the fact that the mass inertia of the drivetrain is large in comparison to the swept volume of the engine.

In addition to engine-related measures such as increasing the idle speed and per- mitting turbocharging while the vehicle is idle, it is necessary to influence the ancillary equipment load in a targeted way. To this end, cascading decoupling strategies for the ancillary equipment were implemented for the engine timing.

This particular decoupling strategy is explained in the further course of this paper using the example of an air conditioner compressor. If the torque required by the ancillary equipment and the creeping torque approach the available WOT engine tor- que, the ancillary equipment is decoupled in a cascading fashion (here: air con- ditioner compressor = AC). Since the air conditioning system cannot be regulated, it is deactivated as required (acceleration and uphill gradient in the example) until the surplus torque of the engine is sufficient for operating the compressor again. Figure 10 illustrates this control strategy. 12 24th Aachen Colloquium Automobile and Engine Technology 2015

Figure 10: Control strategy for consumers during acceleration

Evaluating the available surplus torque for the current non-derated WOT of the engine system also allows special decoupling strategies in driving mode, e. g. in the event of acceleration, downshifting by the automatic transmission, or in similar driving situations that are restricted with regard to the available tractive force.

2.5 Dual-Mass Flywheel (DMF)

The three-cylinder engine in combination with the DCT is equipped with a dual-mass flywheel. This component part is located between the and the dual clutch unit.

Huge torsional vibration decoupling is achieved by integrating the sensor rotor/ ring gear, the primary flywheel mass and a secondary mass that is decoupled by means of multiple coil springs (Figure 11). 24th Aachen Colloquium Automobile and Engine Technology 2015 13

Figure 11: Illustration of the dual-mass flywheel

This active damping enables the three-cylinder turbocharged engine to be operated even at comparatively low rotational speeds by implementing special NVH comfort characteristics, a feature that is unique in this vehicle category.

In contrast, it was possible to avoid additional damping measures on the drivetrain side (e. g. side shaft as a solid shaft), which made an additional contribution to ensur- ing quick torque build-up at the drive wheel without interconnected flexibility.

3 Transmission/ Drivetrain

3.1 Transition of the evolution from AMT to DCT

Now the 3rd generation of the smart vehicle was being designed, it was time to place even more significance on the ease of shifting. The uncomfortable pitch motions resulting from the interruption of tractive power that occurs with automated manual transmissions (AMT) put a strain on the customers. The development department replaced the AMT with a load-directed transmission. The aim was to have no more interruptions of tractive power and pitching of the vehicle body. In the previous model series, the AMT was equipped with five gears and a single-disc dry clutch, which is typical for this design type. The evolutionary step toward the DCT is a transition to a dry dual clutch system and a gear set with six gears, as well as a mechanical park pawl (Figure 12). A system with electric actuating elements for the shift mechanism and the clutch and extremely low electrical consumption was chosen. The series production breakpoint of a mass-produced A-segment vehicle with dual clutch trans- 14 24th Aachen Colloquium Automobile and Engine Technology 2015 mission (DCT) was a world premiere when it was launched in April 2015. This innovative system is called Twinamic.

An established mass-produced platform transmission, the PS250 from Getrag in Untergruppenbach, was used as the basis and subjected to a typical smart rejuven- ating cure. The main focus of the design was to reduce the weight by 10% and adapt the transmission to the geometric boundary conditions of a rear installation position in the smart vehicle.

The development of the transmission software for the combination with the small three-cylinder engines with between 90 Nm and 135 Nm of torque constituted a further challenge.

Figure 12: Sectional view of the DCT and layout of the gear set

3.2 Specific DCT features

3.2.1 Housing/ actuating elements/ gearshift

3.2.2 Housing and gear set

The PS250 platform transmission had to be adapted for the rear and underfloor in- stallation that is unique to the smart (Figure 13). This adaptation essentially involved rotating the transmission around the drive shaft axle in combination with attaining the specified ground clearance and adapting the transmission to the smart engine family.

The two gear set variants are adapted for the characteristics of naturally aspirated engines and turbocharged engines. The reduction in weight was achieved mainly by 24th Aachen Colloquium Automobile and Engine Technology 2015 15 reducing the thickness of the housing wall and equipping the transmission with a compact differential with an aluminum housing.

3.2.3 Actuating Elements

The electromechanical actuation of the dry clutch system and the shifting system in this compact DCT deserve some attention. The actuators that operate the clutches are positioned on the outside of the clutch housing of the transmission. The motors that actuate the shift mechanism are located in the controller unit of the transmission. All four actuators are controlled by the electronics of the transmission control unit. The illustration of the electrical system (Figure 13) provides a complete overview. The actuators, the sensors and the transmission control unit are connected through the transmission wiring harness.

Figure 13: Illustration of the electrical system; sensor/actuator cluster incl. control unit

3.2.4 Gearshift

Figure 14 illustrates the electromechanical operation of the DCT's interior gearshift. The shift forks, which are commonly used in this type of transmission, are operated by electromechanically actuated shift drums. The two shift drums are allocated to the two sub-transmissions for the 1st/ 3rd/ 5th gears and the 2nd/ 4th/ 6th/ reverse gears, respectively. 16 24th Aachen Colloquium Automobile and Engine Technology 2015

Figure 14: Gear set and interior gearshift with control unit

Since the positions of the shift actuators are specified by the electromechanical con- trol unit, it was necessary to include a gear reduction between the two shift drum axles and the associated electromechanical actuators of the two sub-transmissions.

Figure 15 shows that a gear is engaged (1st and 2nd gear in this case) in both sub- transmissions (red and blue) while driving. In the left half of the figure, clutch one is closed and clutch two is open. The next gear, 2nd gear in this case, has already been preselected. The right half of the figure shows how clutch one now opens and clutch two closes. The overlap of the two clutches prevents a sharp decrease of the tractive force. 24th Aachen Colloquium Automobile and Engine Technology 2015 17

Figure 15: Power flow in the sub-transmissions

Conclusion: The dual clutch transmission works without an interruption of tractive power. If necessary, the electronic control skips individual gears instead of downshift- ing from gear to gear. It thus provides the comfort of an automatic transmission com- bined with the efficiency of a manual transmission.

3.3 Driving comfort and ease of shifting (diagrams)

Figure 16 shows model diagrams as examples of acceleration/deceleration upshifts/ downshifts. The handover and speed synchronization phases are shown together with the engine speed curve and the torque curves.

The aim of the control strategy is to achieve phase-shifted torque transfer between the two clutches, and to synchronize the gear speeds in order to optimize comfort while shifting gears.

The driver can choose between two shift programs: In "E" mode, shift operation is designed to be hardly noticeable and comfortable. It is characterized by early upshift- ing and a low speed level, which enables maximum NVH decoupling thanks in parti- cular to the additional vibration damper (DMF).

In "S" mode, shift operation is designed to be more adaptive. It is characterized by an increased willingness to downshift combined with a higher speed level in the perfor- mance-oriented engine-map range of the gasoline engine.

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Figure 16: Shift curves for upshifts and downshifts

Conclusion: The most important features of the Twinamic transmission system are the ease of shifting and the vehicle dynamics. The torque handover and speed synchronization phases enable the transmission software to be applied in an ideal way in every driving situation.

4 Summary & Outlook

The entirely new powertrain for the smart vehicle, which consists of a three-cylinder turbocharged engine with secondary air technology and an electric wastegate, allows the smart to be driven all over the world, in compliance with the various legislative and regional boundary conditions.

At the same time, the technological modules such as scavenging for MPI systems with camshaft adjustment on the intake side, which are unprecedented in the A-seg- ment, enable driving agility to be maximized even at low rotational speeds; this, pair- ed with a dual-mass flywheel, also guarantees an extremely high level of quiet run- ning. Rounded off by the use of a dual clutch transmission, the new smart has suc- ceeded in optimizing the starting performance and shift operation to reach a level previously undreamed of in the A-segment, thus emphasizing the premium standard of the smart vehicle.

The future tightening of emissions and CO2 regulations in the core markets of the EU, the USA and China will cause the existing technologies to be questioned in the near future. Thanks to its architecture and the technological modules mentioned above, the smart powertrain presented here is well prepared.