24th Aachen Colloquium Automobile and Engine Technology 2015
1
New 3-Cylinder Engine and Automatic
Transmission 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 innovations include the use of a dual clutch transmission and a supercharged threecylinder 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 car 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 engine 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 suspension 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 absorber; 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 improved significantly by increasing the track width by roughly ten centimeters.
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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 devoted to oil/water cooling and the cooling-air flow when the vehicle was designed. To
this end, space for an additional electric fan 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 intercooler 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 significantly 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 requirements of urban areas.
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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 installation 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 manual transmission 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 tripleshaft architecture with electromechanical actuation, constitutes a long-term step
toward the optimization of comfort while preserving the efficiency of manual transmissions. 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 additional reverse gear lock was incorporated into the design. As the only vehicle in this
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vehicle category, the smart is now also using a six-speed dual clutch transmission
with a dry clutch and dual-mass flywheel (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 result 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 crankcase, the two drive assemblies 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 bore 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 intake manifold. In order to fulfill
the emissions regulations on the US market, Daimler AG undertook important technical modifications to the basic EU engine.
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These changes are oriented toward fulfilling the specific emissions legislation of the
US market, ULEV70/LEV3, and involved adaptation of the entire exhaust aftertreatment. To this end, the three-cylinder engine concept was equipped with additional
secondary air injection, and the catalytic converter 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 camshaft positioners on the intake side.
This required a secondary air system to be installed, which was a particularly complex task due to the limited installation space and the resulting installation location of
the secondary air injection pump. 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.
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2.2
24th Aachen Colloquium Automobile and Engine Technology 2015
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
cylinder head and the corresponding layout of the components:
- Cylinder head with secondary air bores
- Turbocharger with an air duct
- US-specific catalytic converter & sensors
- Secondary pump with piping, an OBD sensor and a valve
- Wide open throttle 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 exhaust 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.
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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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24th Aachen Colloquium Automobile and Engine Technology 2015
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 distribution (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 manifold area. This enables the catalytic converter to heat up extremely quickly and en-
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sures a low light-off temperature, which is a mandatory design factor to be considered 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.
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24th Aachen Colloquium Automobile and Engine Technology 2015
The specified torque of 115 Nm at 1,500 rpm requires a specific valve overlap in order 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 lambda 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 camshaft 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 closing.
24th Aachen Colloquium Automobile and Engine Technology 2015
2.4
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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 requirements placed on the drive under all boundary conditions need to be developed
specially for small vehicles with small downsizing turbocharged engines and a consequentially 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 requirements:
- 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 permitting 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 torque, the ancillary equipment is decoupled in a cascading fashion (here: air conditioner 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.
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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 crankshaft and the dual clutch
unit.
Huge torsional vibration decoupling is achieved by integrating the sensor rotor/
starter ring gear, the primary flywheel mass and a secondary mass that is decoupled
by means of multiple coil springs (Figure 11).
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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 ensuring 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-
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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 rejuvenating 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 installation 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
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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.
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Figure 14: Gear set and interior gearshift with control unit
Since the positions of the shift actuators are specified by the electromechanical control 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 subtransmissions (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.
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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 downshifting from gear to gear. It thus provides the comfort of an automatic transmission combined 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 upshifting and a low speed level, which enables maximum NVH decoupling thanks in particular 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 performance-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-segment, enable driving agility to be maximized even at low rotational speeds; this, paired with a dual-mass flywheel, also guarantees an extremely high level of quiet running. Rounded off by the use of a dual clutch transmission, the new smart has succeeded 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.