The document describes efforts by Jaguar Land Rover, ZF, and ITI GmbH to develop a complete powertrain torsional system model using SimulationX to increase efficiency in powertrain development. A multi-body dynamics model of the engine is coupled to a torque converter damper and 8-speed automatic transmission model. Analysis of the model shows good correlation to test data and allows investigation of noise and vibration issues and optimization of modal placement early in the design process.

1. 2 Automotive
Application of CAE Methods to Increase Efficiency in
Powertrain Development
Sean Biggs
Technical Specialist, Jaguar Land Rover Ltd
Dr Antonio Turturro
MBS Engineer, Jaguar Land Rover Ltd
Salim Chaker
Engineering, ITI GmbH, Dresden
Kurzfassung
Die Konzipierung eines raffinierten Fahrzeugs erfordert detaillierte Kenntnisse der
Antriebsstrangeigenfrequenzen und des vom Verbrennungsmotor geleisteten Erregungsgrads. Um einen Kompromiss bei der Beseitigung von Geräusch hörbaren bzw.
Vibration spürbaren Schwingungen (NVH: Noise, Vibration and Harshness) im Kraftfahrzeug und bei der Verbesserung von Leistungseigenschaften vermeiden zu können, soll in früherer Designphase eine tiefgründige Planung der Modenplazierung und
der Steuerung der Erregerkräfte erfolgen.
Da der Antriebsstrang eine Reihe von Wechselwirkungen mit den restlichen Fahrzeugaggregaten aufweist, ist eine fundierte Systemanalyse unabdingbar. Der vorliegende Beitrag beschreibt den durch Jaguar Land Rover (JLR), ZF und ITI GmbH entwickelten Prozess zur Vermeidung von Iterationsschleifen in der Entwicklungsphase.
Abstract
The delivery of a refined vehicle requires a detailed knowledge of the powertrain natural frequencies and the level of forcing applied by the engine. Modal placement and
the management of the forcing levels have to be planned and executed early in the
design phase, if compromises in the NVH (Noise, Vibration and Harshness) and other
attribute performance are to be avoided.
The Powertrain as installed has a number of interactions with the vehicle which requires a system level analysis to be carried out. This paper will describe the process
jointly being developed by Jaguar Land Rover (JLR), ZF and ITI GmbH to avoid late
changes in the development process.
Introduction
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2. CAE Methods to Increase Efficiency in Powertrain Development
Engineering a luxury vehicle to have acceptable levels of refinement, delivered in an
efficient manner is a technical challenge. To do this requires a knowledge of the natural frequency distribution in a mechanical system and how these frequencies may
change during operation of the vehicle. The potential for modal interaction, where
one mode magnifies the forcing levels for a second mode (with an insufficient frequency separation) must be avoided and thus requires the development of a modal
placement strategy.
The forced response of the system and the levels of forcing change during the operation of the vehicle are important factors. The torsional forcing from an internal
combustion engine is comprised of two significant components in opposition to each
other. The first is the torque component generated by the cylinder pressure acting
through the crank slider mechanism. This can be very non-linear as shown in Figure
1 (l) below.
Figure 1: (l): Full Load Cylinder Pressure Map vs. Crank Speed; (r): Single Cylinder Inertia
Torque vs. Crank Angle
The second torsional forcing component is the torque required to overcome the inertia of the reciprocating pistons. This is known as the inertia torque and it changes
magnitude and direction as the crankshaft rotates. Figure 1 (r) shows the torque for
a single cylinder engine being motored at various speeds. Note from Figure 1 (r)
that the instantaneous torque values are comparable to the mean torque values of
a typical automotive engine and that the total inertia torque for the engine would
require the contribution from each cylinder to be added with the correct phase relationship. Taking the instantaneous peak values of Inertia torque from Figure 1 (r)
for each speed shows that the peak torques increase at the square of the crankshaft
speed. Piston mass is also a very important parameter especially for high speed diesel
engines which require a lightweight design piston in order to generate the minimum
inertia force but is robust enough to withstand the resultant forces during the engine
work-cycle. The combustion and inertia torques continually interact with the relative
influence of the components dependant on the speed and load on the engine. The
torsional forcing during a full load acceleration of the vehicle will initially be dominated by the combustion torque but as the engine speed builds the inertia torque
increases until it becomes dominant as the combustion torque reduces. If the driver
lets the vehicle decelerate using the engine to brake the vehicle then the inertia tor73

3. 2 Automotive
que will be dominate as the engine fuelling will only ensure that it continues to run,
significantly reducing the combustion torque.
The torsional output from the engine acts on the torque converter and damper system
before acting on the automatic transmission. The torque converter (TC) is a fluid coupling used to connect an internal combustion engine to an automatic transmission.
One of these features is the introduction of damping into the torsional system, but
with this comes a significant loss of efficiency. Running the TC in unlocked mode
is minimised and an additional damper system using springs is added to attenuate
the engine cyclic input to the transmission. The attenuation of the torque converter
damper under varying speed and load conditions is very non-linear and when combined with the combustion engine forcing the system is complex. A typical single
stage damper system with the drive plate (see Figure 2 (l)) consists of a number of
springs installed at a fixed radius of the damper which are compressed by the cyclic
torque generated by the engine. The spring coils are also subjected to centrifugal
forces generated from the speed of rotation and significant friction forces results at
the contact points with the housing. If these friction forces are higher than the cyclic
forces generated by the engine then the individual coils will stop moving and change
the attenuation performance of the damper. TC Damper designs can also consist of
more than one set of springs which act in series to give an overall rate. The stiffness
characteristics for a two stage damper are shown in Figure 2 (r). The stiffness and travel characteristics are different for the drive and over run condition further increasing
the complexity of the system.
Figure 2: TC Damper: (l) Typical single stage TC Damper; (r) Dual Stage TC Damper
Characteristics
The TC damper is connected to the input shaft of an 8 speed automatic transmission
supplied by ZF as detailed in reference [1]. The output from the transmission is coupled to a conventional rear wheel drive automotive driveline.
Model Description
In an attempt to accelerate the development process, JLR, ZF and ITI are collaborating
in the development of a complete Powertrain torsional system modelled using SimulationX. JLR provide the engine and driveline elements of the model with ZF providing
the damper and transmission elements. For the purpose of profound system analysis
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4. CAE Methods to Increase Efficiency in Powertrain Development
and for determining the system vibration behavior all these components were modeled by ITI in the modeling and simulation environment SimulationX. The engine model
(Figure 3) consists of a rigid crankshaft coupled to number of crank mechanisms. Each
crank mechanism acts on a mass element representing the reciprocating mass of each
cylinder. The inertia torque contribution is adequately captured using this method.
The combustion torque is captured using maps of cylinder pressure for the load cases
of interest.
Figure 3: Four cylinder engine SimulationX-model generated by JLR
The cylinder pressure maps are generated using a combustion simulation code and
then as prototypes are developed and the engine calibration is refined. These are replaced with test cylinder pressure maps. The SimulationX model has been compared
with a similar model generated in the MBS code MSC/ADAMS with no significant
differences. To perform a whole system analysis this component will be integrated
into complete drive line model.
The next element in the system is the torque converter damper. JLR uses various
torsional damper models. One of the torsional damper versions is given as FMU
(“Black-Box”), which was provided by the manufacturer ZF and adjusted according
to the provided torsional damper component. The modelling principle is to capture
the response of each active coil in the damper stage, under the influence of the cyclic
torque input and the friction forces acting on it. The modelling method is similar to
that detailed in Ref [2].
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5. 2 Automotive
Figure 4: Torque Converter Damper FMU Black Box generated by ZF
In order to carry out fundamental investigations and parametric studies regarding
attenuation power, several models (with different spring mass configurations) of the
torsional damper have been created by ITI as open structure. The Figure 5 shows
the hierarchical structure of these models. The internal structure consists primarily of
BowSpring elements. In accordance with Ref [3], each set of bowed springs is modelled as a purely rotary multi-mass system with a two-stages elastic characteristic
and discretized in 8 spring segments and 7 masses. Their friction interaction with the
bearing shells is modelled in discrete points considering the dependency on the radial
component of the springs’ elastic reaction, and the centrifugal force and including
constructional end stops and gaps. The BowSpring element represents a set of bowed
springs acting in a torsional damper and disposed at the same mean radius.
Figure 5: BowSpring element – icon (connectors) and internal structure
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6. CAE Methods to Increase Efficiency in Powertrain Development
Analogous to the 9HP Gearbox-model presented in Ref [4] an 8-speed automatic
transmission model was created by ITI (see Figure 6). This model captures the change
in power flow in each of the gears. The desired engaged gear or sequence of gears can
be assigned to a controller model, which implements all the information needed for
the 8HP gearbox shift logic and sends proper actuation signals to all clutches, brakes
and claws of the gearbox structure. The element type discClutch models Clutch and
brakes. It has a signal input to get the actuation signal generated by the controller and
two 1D rotational connectors for exchanging kinetic and kinematic information with
the gearbox model structure.
Figure 6: 8 Speed Automatic Transmission generated by ZF
The driveline system (Figure 7) consists of the propeller shaft connected to a final
drive unit, which drives through two half shafts to a simple dynamometer to control
the system. The flexible shafts are discretised to ensure that the modal content of
the system is captured and potentially excited by the cyclic torques from the engine
model. The complete system can be run at a fixed speed or in a transient condition
for either run up or over run conditions.
Figure 7: Driveline (RWD) and Dynamometer System generated by JLR
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7. 2 Automotive
Analysis Results
Using the presented models both transitory and steady state simulations can
be performed. Figure 8 shows the speeds reached during a complete gearshift
sequence at different components along a driveline model.
Figure 8: engine speed (no markers), gearbox input shaft speed (squared markers) and
gearbox output shaft speed (triangular markers) over a run up
The consideration of the rotary backlash in the gear meshings allow to highlight possible undesired vibrational phenomena in the transmission like the rattling or the clonk.
Running a transient simulation the forces generated at the tooth contact represent the
main physical quantities to be observed: The model makes this information available
at both sides of the engaged teeth of a meshing. The clonk can be highlighted by normal forces arising alternatively at opposite flanks of the teeth when the transmitted
torque changes its sign.
JLR have tested a 2WD Jaguar powertrain with a ZF 8-speed gearbox (see Figure 6)
with a number of TC dampers of various constructions. A sample of the correlation
being achieved can be seen in Figure 9 (l) which details the crankshaft velocity fluctuations for an engine running at a steady state condition. Every mechanical system
has a number of natural frequencies which magnify any applied forcing at resonance.
One important mode of interest is that of the propshaft torsional mode. When this
system is at resonance it results in an increase in the cyclic torque fluctuations reacted
by the final drive assembly, mounted to a very sensitive part of the vehicle body via
rubber bushings. If these torque fluctuations are excessive this can cause a booming
noise in the vehicle cabin. The vehicle under test showed a very significant difference
in the propshaft boom response between full load and no load conditions, at the
speed at which the engine forcing was close to the propshaft resonance frequency.
The interior noise increased when the load on the engine was suddenly reduced. The
change from full load to no load condition resulted in a reduction in the peak cylinder
pressure by 75% together with a change in shape as can be seen in Figure 9 (r).
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8. CAE Methods to Increase Efficiency in Powertrain Development
Figure 9: (l): Engine Primary Side Correlation (Steady State Condition); (r): Full and No
Load Cylinder Pressures
Another factor that influenced the difference in the boom response was that the engine speed at propshaft resonance also corresponded to the minimum cyclic torque at
full load. As stated previously this is the speed at which the inertia torque starts to
dominate the combustion torque as seen in Figure 10 (l) and labelled as the torque
transition point.
Figure 10: TC Damper Full Load Torque vs Crankshaft RPM; (r): TC Damper Torque for Full
and No Load
The forcing for the no load condition was investigated by changing the cylinder pressure maps to the no load condition, spinning the powertrain system at high speed and
allowing the vehicle dyno to decelerate against the engine resistance. The resulting
torque was recorded at the TC Damper and can be seen in Figure 10 (r). Note that
without the influence of the combustion torque the inertia torque is significantly greater than the full load torque seen at propshaft resonance/full load torque transition.
When analysing the influence of the different forcing mechanisms on the crankshaft
cyclic velocity (at the propshaft resonance speed) we can see that the amplitude for
the no load (over run) condition is higher together with a change in the shape and
hence the frequency content. It should be noted that some powertrain systems exhibit a change in propshaft resonance frequency with selected gear and with modern
transmissions offering 8 to 9 gears with potentially the addition of a selectable 2/4
wheel drive system. This increases the opportunity to miss an error states during te79

9. 2 Automotive
sting. There are other torsional modes of the system influenced by engine and TC
damper parameters which also need to be considered during development. The use of
SimulationX allows JLR and ZF engineers to test the robustness of the system design
and to identify potential issues early in the development phase.
Summary and outlook
We presented a new vibration model for the dynamic simulation of complete drive
line including a combustion engine, torsional damper and an 8-speed automatic
transmission. In addition to the detailed nonlinear damper model consisting of multiple masses and nonlinear elastic and friction properties, the transmission model takes
into account the gear contact stiffnesses depending on rotational angles, the profile
shift and the contact ratio. The model is valid for all gears and can simulate gearshifts.
It is suitable for the analysis and prevention of combined forced and parametrically
excited vibrations, which are typical for gears. The physical modeling enables a simulation which takes into account the interactions with the neighboring oscillatory
components of the drive system. Thus the used modeling approaches promote the
essential understanding of the inner relationships of the system and differ significantly
from traditional models with regard to adaptability, flexibility and adjustability of the
properties to be considered.
The presented simulation model can be used for both, dimensioning tasks as well as
diagnosis. It is applicable to a vast variety of tasks and applications in the transmission
development process ranging from concept design to virtual testing, such as NVH
assessment of the powertrain. This will be increasingly important as the automotive
industry strive for more efficiency. The trend to downsize the capacity of the engine is leading to boosted cylinder pressures in fewer cylinders, this is also leading to
stratagies which de-activate cylinders when the engine load levels allow. All of the
previously mentioned factors will significantly impact the system performance. The
application of electric motors will also continue with the challenge of combining the
two power sources in a refined manner. The opportunity to examine the system performance virtually using SimulationX will be vital to deliver a refined and robust system.
References
[1]
ZF 8 Speed Transmission Product information. http://www.zf.com/corporate/en/products/innovations/8hp_automatic_transmissions/8hp_automatic_transmission.html
[2]
Converter Damper Attenuation Performance using Multi-Body Systems Analysis. Sean Biggs, NAFEMS World Congress. Salzburg 2013
[3]
Meingaßner, G. J.: Entwicklung von Simulationsmodellen zur Abbildung der Hystereseeffekte in der Dämpfereinheit und des Drehzahlschlupfes in der Wandlerüberbrückungskupplung von Pkw-Drehmomentwandlern. Diplomarbeit. Landshut, 2010
[4]
V. Langella, U. Schreiber, H. Bertels: A New Physical Simulation Model for the 9HP
Gearbox Incorporating a Nonlinear Twin Torsional Damper and Backlashes. In:
VDI-Berichte Nr.2158: Getriebe in Fahrzeugen 2012, VDI-Verlag. Düsseldorf, 2012
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