New planetary based hybrid automatic transmission
with true on-demand actuation
Dipl.-Ing. Gereon Hellenbroich1, Dipl.-Ing. (FH) Thomas Huth2
1: FEV Motorentechnik GmbH, Neuenhofstrasse 181, 52078 Aachen, Germany
2: Institute for Combustion Engines (VKA), RWTH Aachen University, Schinkelstrasse 8, 52062 Aachen
major role in reducing the CO2 emissions of
Abstract: Within the scope of work of the
tomorrow s vehicles.
HICEPS project funded by the European Union,
FEV has developed a new hybrid transmission for
transverse installation. This transmission is based
on the technology of planetary automatic
transmissions and realizes seven forward speeds
with only three planetary gear sets, three clutches
and two brakes. Another innovative feature is the
on-demand actuation system. Both an electro-
hydraulic and electro-mechanical version have
been developed, which both significantly decrease
the required actuation energy compared to
conventional automatic transmissions. The
Figure 1: Transmission market share worldwide
component test results of the electro-mechanical
2010 vs. 2014 [1]
actuator including durability, controllability and
achievable dynamics are very promising.
The efficiency of current state-of-the-art ATs has
Additional benefits are achieved with an on-
been greatly improved by increasing ratio spread,
demand cooling and passive lubrication, again a
number of gears and by lots of optimization in
first for planetary-based automatic transmissions.
detail. However, two major sources of losses still
The passive lubrication for all gears has been
persist even in the most modern ATs: The
successfully established on a functional test rig. In
hydrodynamic torque converter and more
the next step, the transmission will be put on a
importantly, the need for a permanent, high
three-dyno-test bench for efficiency
pressure oil flow to feed clutches and brakes.
measurements, mechanical durability testing and
Within the HICEPS project (Highly Integrated
to continue the development of cooling and
Combustion Electric Propulsion System) funded
shifting strategies.
by the European Union, FEV has developed a
hybridized automatic transmission which
Keywords: AT, actuation, hybrid, on-demand
eliminates these two major sources of losses
while retaining the full powershift capability of
1. Introduction
conventional ATs. This paper describes the new
The role of the transmission within automotive
concept and its key features to achieve superior
powertrains is becoming increasingly important,
efficiency.
with the modern automatic transmission being a
key element in the vehicle s drivability. After the
2. Transmission Concept
combustion engine, the transmission also shows
the greatest potential to improve the fuel economy
The new transmission concept is based on three
of a new vehicle. Because of this, transmission
planetary gear sets with no more than three
optimization has become a major focus in the
clutches and two brakes. Despite this low
automotive industry.
mechanical complexity, the concept features
seven forward gears and one reverse gear for the
During the last decade, the introduction of dual
internal combustion engine (ICE). Four of the
clutch transmissions (DCT) has triggered an
seven forward gears can also used by the electric
unforeseen competition between conventional
motor (EM) of the hybrid system, which, together
automatic transmissions (AT) and the dual clutch
with the first planetary gear set and a first lockup
transmissions. However, the forecast in Figure 1
clutch (C1), is installed into the transmission s bell
still suggests that the prevalent automatic
housing. The three members of the first planetary
transmission type worldwide is going to remain
gear set (PGS1) are connected as follows:
the conventional automatic. Therefore, the
optimization of this transmission type will play a
Sun gear: Internal combustion engine which are defined by the speeds of both carrier
and ring gear as a result of active shift elements
Ring gear: PGS2
at PGS2 and PGS3. 1st gear is one of the power
Carrier: PGS3 and electric motor
split gears. In case launch is not performed purely
electrically, brake B1 can be used as launch
clutch. Using a brake for launch has the major
advantage that a lot of thermal inertia can easily
be packaged without increasing rotating inertias.
Together with the electric machine s support, this
greatly reduces the required cooling flow during
launch.
It is visible from the lever diagram that the ratio of
the reverse gear is very tall, being comparable to
the ratio of 6th gear. Therefore, reverse driving is
performed by turning the electric motor backwards
while using 3rd gear. For future versions, it would
Figure 2: Simplified transmission layout
also be possible to turn the reverse gear into an
8th forward gear.
Figure 2 shows a simplified layout of the new
transmission. PGS2 and PGS3 serve as two-
speed-transmissions with one brake and one
3. Technical Specification
clutch each (B1/C2 and C3/B2 respectively).
PGS1 has two different functions: with the clutch
The first transmission prototype will be used with
C1 closed, the combustion engine and the electric
FEV s three-cylinder Extreme Downsized Engine
motor are locked together and can use four direct
(EDE). This turbocharged engine has a
gears which are selected by engaging one of the
displacement of 698 cm3 and uses direct gasoline
shift elements B1, C2, C3 or B2. With the clutch
injection to provide 74 kW of power and 130 Nm
C1 open, PGS1 acts as a mechanical power-split
of torque. The transmission itself is able to handle
device distributing the combustion engine torque
a combustion engine torque of 200 Nm. Figure 4
to PGS2 and PGS3, where one shift element each
specifies the prototype transmission in more
has to be closed (combinations B1/C3, B1/B2,
detail.
C2/C3 and C2/B2). This adds four power-split
gears. Figure 3 shows the speed relations for
PGS1 in the lever diagram and a shift element
table.
Figure 4: Technical specification of prototype
Figure 3: Lever diagram PGS1 and shift element
table
4. Actuation System
The four horizontal lines represent the direct
One key feature of the new transmission is the on-
gears in which PGS1 is locked up by clutch C1. In
demand actuation system for all clutches and
these direct gears, all members of PGS1 have the
brakes. In conventional ATs, all clutches are
same speed which is defined by the selected shift
actuated by rotating actuators (hydraulic pistons)
element at PGS2 or PGS3 respectively. The
fed with oil through shafts and leaking seal rings.
angular lines represent the power-split gears
Because of the leakage, a permanent high
pressure oil flow is required. For FEV s new shifting element has always to be kept under
concept, all three planetary gear sets are axially pressure. The resulting leakage at the control
accessible. This allows an engagement of all valves requires frequent recharging of the power
clutches via release bearings and non-rotating pack s accumulator, causing an average power
actuators. These actuators can be leakage-free consumption of 30 W in NEDC including the valve
hydraulic pistons or even electro-mechanical currents. In order to minimize bearing loads, the
actuators. The design of the prototype hydraulic pressure and thus the axial forces on
transmission is modular in order to be able to test clutches and brakes are dynamically controlled
both variants. The alternatives are shown in based on the torque to be transmitted.
Figure 5.
4.2. Electro-Mechanical System
Main targets of the actuator development were
minimum required actuation energy and shifting
performance comparable to state-of-the-art
automatic transmissions. The main challenge in
obtaining the required shifting performance was
the trade-off between high dynamics and high
maximum actuation force. The development was
started with an evaluation of different basic
actuation principles, e.g. magneto-rheological.
The outcome of the study was that an electro-
mechanical system can fulfill all requirements with
regard to dynamics, force and package.
Figure 5: Electro-mechanical and electro-hydraulic
actuation systems
4.2.1. Electro-Mechanical System: Design
For the electro-mechanical system, each clutch
In the following chapter, the two actuation
and brake has its own actuator consisting of an
systems will be explained in more detail.
electric stepper motor with permanent excitation,
a reduction gear and a rotation/translation
transformer. The stepper motors are placed
4.1. Electro-Hydraulic System
around the planetary gear sets in order to ensure
The actuation principle is based on an electro-
a compact transmission package. Stepper motors
hydraulic power pack and leakage-free, non-
have been chosen because of their high torque
rotating hydraulic pistons. Figure 6 shows a cross-
and simple control mechanism, which even allows
section of PGS3 including the electro-hydraulic
sensorless control. As rotation/translation
actuation of the first prototype.
transformer, a cam disc is used. Figure 7 explains
the actuator design in more detail.
Figure 6: Cross-section of PGS3 with electro-
hydraulic actuation
Torque is routed through PGS1 into the carrier of
PGS3. The clutch C3 is used to lock PGS3 by
connecting carrier and sun, providing a 1:1 ratio.
A second gear ratio is realized with brake B2,
Figure 7: Design of electro-mechanical actuator
which fixes the ring gear to the housing. Because
[2]
both the clutch and the brake are designed
normally open for safety reasons, the active
The cam contour was optimized to produce amplifiers for the sensor signals is shown in
minimum jerk during engagement. This is Figure 8.
necessary for durability and simplifies the control
strategy for the stepper motor. The contour also
provides locked end positions, which means that
the end positions are held without any actuation
energy. Power is only required during
engagement/disengagement, but not to keep a
gear engaged. Because the cam contour
prescribes the end positions of the clutch
engagement, an additional mechanism for the
compensation of wear and thermal extension is
required. The solution is a preloaded, non-linear
spring element between the cam disc and the
Figure 8: Component test bench
clutch. The preload of the spring element reduces
the required travel for clutch engagement. In the
area of maximum clutch force, the gradient of the The first durability test of 15.000 cycles resulted in
spring characteristic is close to zero, thereby local damage of the cam disc. The damage was
ensuring a constant clutch force independent of caused by a deformation of the cam bolts, which
created high local surface pressures. This issue
wear and thermal expansion.
was solved by an improved cam contour and a
cam bolt of bigger diameter. The motion
To optimize the design of the electro-mechanical
transformer development steps can be seen in
actuator, the whole mechanism including the
Figure 9.
stepper motor was modeled in Matlab/Simulink.
This allows a variation of different stepper motor
types, reduction gear ratios and cam contours. All
parts are described with parameters for easy and
automated parameter variation. Also friction is
considered in order to achieve realistic dynamic
results. Based on these results, the best
compromise between maximum axial force,
minimum engagement time and required package
space was chosen for further development. The
simulation model was also used to calculate
maximum and average power consumption of the
electro-mechanical actuation system based on the
prototype transmission in NEDC. The calculated
average power consumption is only 6 W or 20% of
the on-demand electro-hydraulic version. For
Figure 9: Motion transformer development
comparison, a conventional hydraulic actuation
system with a mechanically driven oil pump would
For the control of the stepper motor, a model
require an average power of 120 W to 240 W.
based physical approach is used. The software
development tool chain is dSPACE with the rapid
4.2.2. Electro-Mechanical System: Test Results control prototyping system MicroAutoBox and
RapidPro. This allows a flexible and fast
For the hardware and control development of the
development of control strategies. In order to
new electro-mechanical actuator, a component
reduce the number of sensors required on the
test bench was built. This test bench is used for
prototype transmission, a model based sensor
durability testing, software validation and control
replacement is included. Based on the required
strategy development. All parts of the mechanism
clutch force, a corresponding stepper motor
including the clutch have the final design which is
torque can be calculated. The difference between
also used in the transmission prototype. This
required torque for the engagement and maximum
ensures that a realistic dynamic behavior is
available stepper motor torque minus a safety
measured. The test bench is equipped with a
margin can be used for a highly dynamic
speed sensor for the stepper motor, a clutch force
acceleration. The safety margin is required in
sensor, a clutch position sensor and a digital
order to avoid unrecognized step losses which
switch which determines a reference position. The
could otherwise occur because of the sensorless
test bench setup including the control unit and the
stepper motor. The results after validation are
shown in Figure 10.
Figure 11: Results of dynamic clutch engagement
4.3 Control strategy
Figure 10: Validation of model based sensor
replacement The new transmission concept has high control
requirements, due to the missing torque converter
The left curves in Figure 10 show the measured
and complex shift 4 5. Without torque converter,
clutch position compared to the simulated one.
damping is reduced and shift shocks are directly
The small deviation is caused by the clearance
transmitted to vehicle and engine. For the shift
between cam bolts and cam contour which had
4 5, four shift elements have to handled
not been modeled. As even a small error has a
simultaneously. Therefore, a new control strategy
significant influence on the calculation of the
with a potential for increased shift quality was
clutch force, the machining tolerances were
developed. The control topology is shown in
reduced and the clearance was included in the
Figure 12.
model of the force calculation. This optimization
was necessary to ensure good controllability with
reduced hysteresis in order to ultimately achieve
good shift quality. After optimization, the model-
based calculation showed a deviation of less than
1% from the measured values. On the prototype
transmission, the adaptation of the model will be
performed using the existing shaft speed sensors.
A low level routine performs reference point and
kiss point detection, both required for the main
control strategy of the actuators.
Figure 12: Control topology
Figure 11 shows measurement results for a highly
dynamic clutch engagement. This measurement
represents a worst case, because the maximum
The control strategy is based on a cascade of
actuator force of 11 kN is applied. For this worst
several sub-controllers which are:
case, the maximum engagement time of the
system is below 140 ms. The constant gradient of
the position is a result of the open loop control of Driver identification:
the engagement, which is used in the current
The driver identification module categorizes the
version of the controller. The clutch force has a
driver continuously between zero and one. The
resolution of 50 N in full-step mode that can be
extremely sporty driver is described with one and
doubled to 25 N in half-step mode.
the economical driver with zero. All driver types
vary between these extremes. The driver type will
be calculated from the driver input the
acceleration pedal and will be stored in a FIFO
(first input first output) buffer which allows
calculating the driver type for the last
600 seconds.
Acceleration pedal prediction:
The acceleration pedal prediction is based on a
Taylor series expansion which allows a realistic
prediction horizon of 500 ms. A larger prediction
horizon shows too high deviations and is not
needed for the following cascade sub-controllers.
Shift strategy:
The shift strategy is based on a model predictive
controller (MPC) of the vehicle. With the driver
type information, the shift points are optimized for
optimum NVH, fuel consumption and available
power. The optimum gear is selected based on a
cost function for all gears. This approach ensures
Figure 13: Prototype transmission with external oil
a driver type dependant shift-strategy with
pump
minimum calibration work.
As already explained in chapter 4 the three
planetary gear sets of the transmission (PGS1,
Shift action:
PGS2 and PGS3) are all axially accessible. This
The shift action is based on a state machine
allows carrying over traditional lubrication
which defines the necessary procedure for
techniques from transversally installed layshaft
changing a gear. Depending on the current gear,
transmissions like oil catchers, oil baffles and oil
a predefined shift event is chosen. A simple up-
slingers. No pressurized oil is needed to feed oil
shift is performed in the following steps:
versus centrifugal forces into rotating shafts. This
- Open/Close clutches/brakes up to kiss-point
is a major difference and big advantage compared
- Torque handover between clutches/brakes
to most conventional automatic transmissions.
- Synchronize engine speed
For clutch cooling during and after shifting, an
- Drive to end position/force
external BLDC-motor drives a G-rotor-pump with
a small suction filter which delivers approximately
Torque controller: 6 l/min at 2 bar into the shafts. In case of non-
sufficient cooling performance, the electric oil
The torque controller is the most important
pump can also be used together with an injector
controller for ensuring an optimized
pump in order to increase the short-term volume
synchronization. In this application an optimal
flow. The cooling oil enters the shafts from the
control strategy with a square cost function is
actuator side ( active path ), while the lubrication
optimized to ensure a no-lurch condition.
oil is fed into the shafts from the differential side
Additionally, this control strategy considers the
( passive path ). Figure 14 shows the two different
actuator-specific dynamic behavior. To perform an
oil paths.
optimal synchronization, the acceleration pedal
prediction is necessary and delivers the future
change of the torque request.
Together with the described sub-controllers, the
cascade controller enables a high level of comfort
with minimal fuel consumption.
5. Lubrication and Cooling System
Because of the on-demand actuation system and
Figure 14: Active and passive oil paths
the absence of a mechanical, constantly driven oil
pump, an excellent passive lubrication is essential
in order to minimize the runtime of the external
cooling pump which is driven by a brushless direct
current (BLDC) motor. The prototype transmission
including the external oil pump is shown in Figure
13.
The passive lubrication for all gears has been
successfully established in a first test series on a
functional test rig. In the next step, the
transmission will be put on a three-dyno-test
bench for efficiency measurements, mechanical
durability testing and to continue the development
of cooling and shifting strategies.
6. Acknowledgement
The presented transmission is being developed
within the HICEPS (Highly Integrated
Combustion Electric Propulsion System) project
funded by the European Union. This project s goal
is to take the new transmission from concept to a
working prototype. The authors would like to thank
the European Union for their kind support of this
ambitious research project.
7. References
[1] Gumpoltsberger, G.; et al.:
The optimal automatic transmission for front-
transverse applications
VDI report No. 2029, VDI Verlag Düsseldorf,
2008
[2] Janssen, P.; Speckens, F.-W.; Huth, T.;
Hellenbroich, G.:
FEV new hybrid transmissions
CTI Berlin, 2009
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