PREDICTIVE EVALUATION OF THE FUEL ECONOMY VS NVH TRADE-OFF USING SIMULATION

Mario Felice, Jack Liu, Imad Khan

(Ford Motor Company, USA)

Jonathan Zeman, Llorenç Forasté Gómez (Gamma Technologies, USA)

Wulong Sun (MSC Software, USA)

Dr. Michael Platten (Romax Technology, UK)

1. Introduction

The design of a refined automotive powertrain requires a careful balance of multiple attribute objectives, such as: fuel economy, durability, drivability, and NVH (Noise, Vibration and Harshness). Much of the design today is carried out via simulation using a collection of CAE tools, together with hardware testing, to deliver the optimum attribute balance performance.

Many new technological developments aimed at reducing fuel consumption, such as engine down-sizing, turbocharger boosting and cylinder deactivation, to mention a few, have created new challenges for the NVH engineer looking to isolate powertrain vibration and noise from the passenger compartment. To compound this technological difficulty, manufacturers are continuously seeking ways to reduce the amount of testing required during the development process in order to shorten the time taken to bring a new vehicle to the market.

Hence, to meet the above mentioned challenges, further advancement in the state-of-the-art simulation process is needed. To accomplish this, Ford has combined its efforts with Gamma Technologies (makers of GT-SUITE), Romax Technology (makers of RomaxDESIGNER), and MSC Software (makers of Adams) to develop and apply a new system simulation methodology that will allow a more effective way to evaluate the effects of drivetrain component selection on overall NVH performance and at the same time determine its impact on fuel economy.

The new process combines detailed, predictive engine, transmission and driveline models in a complete vehicle system model. Specifically, the engine and torque converter system, including lockup clutch slip controller and vibration damper are modeled using GT-SUITE. The driveline, suspension, and vehicle system are modeled using Adams. The simulation of the automatic transmission is carried out with RomaxDESIGNER and exported as a transient-capable, dynamic model to Adams. The GT-SUITE model of the engine and torque converter coupling is then used in a direct

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co-simulation environment with the Adams driveline/vehicle system model to perform the NVH evaluation. GT-SUITE is also used to predict the overall fuel economy effects.

The described methodology is applied to a transverse mounted, four cylinder engine with a six speed automatic transmission in an all-wheel drive vehicle configuration. The NVH prediction consists of evaluating three different torque converter damper designs: conventional damper, double damper, and pendulum absorber at different lockup clutch slip conditions across the engine speeds where lugging and rattle are prone to determine optimum trade-off balance between vehicle NVH and fuel economy.

2. GT-SUITE Model Development

GT-SUITE, from Gamma Technologies, is a multi-physics system simulation tool [1] used to model a four-cylinder, turbocharged engine with direct injection for this study. To make the engine model capable of performing transient maneuvers yet rapid simulation speed, a reduced model was created from an original, fully detailed GT-POWER engine model. A screenshot of the reduced engine model can be seen in Figure 1.

Figure 1: GT-SUITE Four Cylinder Dynamic Engine Model

As can be seen from the model screenshot, each cylinder is modeled separately, resulting in two engine torque pulsations per revolution. Since the engine model is physically-based, the engine torque is a function of the throttle angle and wastegate diameter, which control the air and fuel flow into the cylinders, as well as the engine speed.

Figure 2 shows the engine fuel efficiency map, as a function of the engine speed and brake torque. By examining the engine fuel efficiency map, it is clear that the engine is most efficient at higher loads and lower speeds. In recent years, to improve fuel economy, engine and transmission controllers have been designed to maximize engine fuel efficiency by operating in these areas. With advancements in engine controls and new engine technologies, the engine torque vibration amplitudes are becoming higher, and shifted toward lower engine speeds [2]. This trend has created new difficulties for the NVH engineer, whose goal is to isolate engine vibrations from the driveline. A

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typical four-cylinder GTDI engine mean torque is shown in Figure 3a with several crank-angle resolved engine torque curves from the engine model can be seen in Figure 3b.

Figure 2: GT-SUITE Four Cylinder Engine Fuel Efficiency Map

Figure 3a: A typical I4 GTDI engine mean torque

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Figure 3b: GT-SUITE Full-Load Engine Torque Curves vs. Crank Angle

For this particular engine, the maximum torque output is near 2500RPM with peak-to-peak torque pulse amplitudes increase with engine speeds in 1100-1900RPM, even at speeds near engine idle, torque vibration mitigation is necessary to prevent damage to the driveline components and isolate the driver's perception of undesirable NVH behavior.

There are several existing and newly emerging hardware and control software technologies that aim to reduce the transmitted torque vibrations from the engine to the transmission input shaft. For an automatic transmission with a torque converter, the torque converter itself can act as a fluid damper, since it is a hydrodynamic coupling. However, excessive torque converter slip is inefficient and therefore undesirable for fuel economy. Therefore, a bypass or lock-up clutch is used to mechanically lock the torque converter. Nevertheless, to improve NVH behavior, it is sometimes necessary under certain operating conditions to close, but not fully lock, the lock-up clutch. By maintaining a relatively small amount of controlled slip, it is possible to damp some of the engine vibrations without the fuel economy penalty of completely opening the clutch. In this study, several values of lock-up clutch slip are studied, to evaluate the fuel economy versus NVH trade-off of lock-up clutch slip.

In addition to lock-up clutch slip control, another solution for damping the engine torque vibrations is a mechanical damper within the torque converter unit. There are many configurations and the state of the art currently shows that the most effective ones involve a combination of torsion springs and a pendulum absorber.

Since the most effective of the new technologies can introduce high cost, simpler systems have been proposed in order to achieve a similar vibration isolation performance. For this study, three technologies have been chosen to evaluate: the conventional arc spring damper, the double damper, and the centrifugal pendulum vibration absorber. All of the configurations utilize a torsional arc spring set, which has the advantage of higher torsional travel compared to traditional straight torsional springs.

The conventional damper, seen schematically in Figure 4, uses one set of torsional arc springs that link the transmission hub to the lock-up clutch.

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Figure 4: Schematic Diagram of a Conventional Damper

The double damper, seen schematically in Figure 5, uses the one set of torsional arc springs linked to an intermediate plate which is connected to a hub through a set of linear torsional springs.

Figure 5: Schematic Diagram of a Double Damper

The centrifugal pendulum vibration absorber, seen schematically in Figure 6, adds a centrifugal pendulum to the series turbine damper. Whereas tuned mass dampers, such as the conventional and the series turbine, are tuned for a given operating frequency, the pendulum absorber can be designed to be effective for a given engine order.

Figure 6: Schematic Diagram of a Centrifugal Pendulum Absorber

To properly capture the arc spring dynamics in a simulation environment, it has been shown that the arc spring needs to be discretized into several masses [3]. For the purposes of this study, the arc spring is discretized into five mass elements to capture the non-linear behavior accurately.

In Figure 7, it is clearly visible that the arc spring behavior is non-linear and hysteretic as a function of rotation speed and angular displacement. This is due to the friction between the arc spring and the outer housing, which varies as a function of the transmitted torque and rotational speed.

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Figure 7: GT-SUITE Arc Torque vs. Displacement and Angular Speed

Each of the aforementioned dampers is modeled using the standard mechanical library components in the GT-SUITE software. For the conventional and double dampers, 1-D rotational springs and inertias were used. For the pendulum absorber, a mix of 1-D rotational and also 2-D planar mechanical models is used. Figure 8 shows the final model of the pendulum absorber.

Figure 8: GT-SUITE Pendulum Absorber Model

To compare the performance of each damper, an isolation test is performed, where the engine model excites the damper and driveline with the lock-up clutch in the fully closed position. By comparing the engine acceleration amplitude with the transmission input shaft acceleration amplitude, one can calculate the isolation performance of the damper. The steady-state comparison of the three damper technologies can be seen in Figure 9.

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Figure 9: GT-SUITE Absorber Steady-State Isolation Performance Curves

From inspection, it can be seen that the pendulum absorber exhibits the best isolation of Figure 9 across the speed range. To predictively analyze how these dampers affect the rest of the vehicle, a RomaxDESIGNER transmission model and Adams driveline/vehicle model are coupled to GT-SUITE in a co-simulation environment.

3. RomaxDESIGNER Model Development

RomaxDESIGNER is a software tool for the simulation and analysis of geared transmission systems [4]. RomaxDESIGNER models are parametrically defined, fully flexible, full-system models based on materials, geometry and detailed component data for gears and bearings etc.

Dynamic FUSION [5] is a software tool that is used to create multibody dynamic (MBD) models from the parametric RomaxDESIGNER models. This automated process creates a generic MBD model which is optimized to contain enough fidelity to accurately represent dynamic performance in a specified frequency range while optimising model size and simulation performance by removing unnecessary degrees of freedom. This MBD model is then translated to the MSC Adams format to be used as part of the system drivetrain Adams model.

For this study, the transmission is a 6-speed planetary automatic transaxle unit, seen in Figure 10. It is comprised of 3 planetary stages and 5 clutches for speed selection, coupled by a chain drive to a differential with planetary speed reduction which drives the front wheels. A hypoid gear power transfer unit (PTU) sends power to the driveshaft and provides all-wheel drive capability when required by engaging a clutch in the rear axle unit.

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Figure 10: Six-Speed Transaxle Unit with PTU Modeled in RomaxDESIGNER

In addition to the shafts and planet carriers, components such as bushings and rolling element bearings are modeled in all degrees of freedom as non-linear torque and misalignment-dependent stiffnesses. Gears are modeled using their macro-geometry details. For this study it is sufficient to model the clutches and brakes in the transmission as simple friction units and the chain drive as an elastic belt.

Once the model is complete in RomaxDESIGNER it can be converted to a multibody form using Dynamic FUSION. To improve simulation performance the bearing and bushing stiffnesses are linearized at operating conditions. The resulting multibody model is shown in Figure 11 and it can be seen that the model has been reduced to a relatively small number of lumped bodies (represented by brown discs which include all the required mass, inertia and gear ratio information.

Figure 11: Generic Multibody Model of Transaxle in Romax DynamicFUSION

This generic transaxle multibody model is then merged into a complete full vehicle and driveline Adams model. The resulting model of the transmission in Adams is shown in Figure 12.

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Figure 12: Transaxle Model Translated to Adams Format in Adams/View

4. Adams Model Development

Adams software is used to analyse how dynamic loads and forces are distributed throughout a mechanical systems [6]. The chassis and drivetrain of the vehicle are modeled using Adams/Chassis and Adams/Driveline respectively. Subsystems included in the chassis model are: the vehicle body (hidden), the steering, the front, rear suspension, brakes and wheels as shown in Figure 13.

Figure 13: Adams/Chassis Model

Parts are modeled using rigid bodies and connected through joints or compliance elements, such as bushings, springs and dampers. Flexibility is introduced in the front and rear anti-roll bars using discrete beams. The chassis model provides low frequency vehicle response due to engine torque excitation. Vibrations can be measured at various locations such as the steering wheel and seat track.

The drivetrain model contains the following subsystems: the engine and gearbox, the front, rear half shafts, the driveshaft and the rear differential unit as shown in Figure 14. The engine and transmission internal components are not included since the details will be included in the GT-SUITE and RomaxDESIGNER models, respectively.

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Figure 14: Adams/Driveline Model

Compliances are introduced in the front and rear half shafts and the driveshaft. The driveline system is mounted on the vehicle body and front, rear sub frames through bushings.

The two models are then merged into a single model in the Adams/Driveline environment as shown in Figure 15. This merged model is then further connected with the GT-SUITE and RomaxDESIGNER models, detailed in the next section.

Figure 15: Merged Chassis and Driveline Model

5. Model Co-Simulation and Results

To evaluate the effect of torque converter slip and damper design on fuel economy and driveline torsional vibrations a dynamic scenario is simulated. To model this scenario, the engine is initialized to 800RPM, and the engine accelerates the vehicle under full load until the engine speed reaches 3000RPM. The transmission is set to operate in fifth gear, and a lock-out clutch in the rear drive unit (RDU) is locked.

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For this study, several levels of torque converter lock-up clutch slip in the conventional damper are simulated, from 0RPM to 60RPM in increments of 10RPM. The dynamic results for the transmission output shaft speed amplitude, a common NVH metric, with the simulated levels of torque converter clutch slip are shown in Figure 16.

Figure 16: Transmission Speed Amplitude as a Function of Torque Converter Slip

As can be observed from the above figure, increasing the torque converter slip reduces the amplitude of speed fluctuations, improving NVH. The figure also shows two common driveline NVH responses: lugging and rattle. The vehicle lugging mode, seen between 20-25 miles per hour, is mainly contributed by the torque converter damper vibration. The rattle seen between 40-50 miles per hour, occurs when backlash in the driveline gears is excited by driveline compliances.

Figure 17 shows an analogous plot for another common NVH metric, driver seat track acceleration, around the lugging response. This determines the effect of torque converter slip on vehicle seat track acceleration response.

Figure 17: Seat Track Acceleration as a Function of Torque Converter Slip

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Therefore, it is observed that the torque converter slip also reduces the vehicle seat track vibration. However, it is well known that increasing torque converter slip will negatively impact fuel economy [7].

To examine the trade-off between fuel economy and NVH for torque converter slip, it is also necessary to quantify the fuel economy penalty for slipping the torque converter clutch. This computation pertains only to the engine/drivetrain/vehicle configuration used for this study. Figure 18 shows the fuel economy penalty for slipping the torque converter lock-up clutch, compared to 0RPM slip.

Figure 18: Fuel Economy Penalty for Torque Converter Slip

The result in Figure 18 accounts not only for the inefficiency due to the lockup clutch slip, but also non-linear engine fuelling behavior as a function of engine speed and torque, which are also dynamically changing in the simulation. As the vehicle speed increases, the fuel economy penalty for slipping the torque converter decreases, because the ratio of torque converter slip to input shaft speed decreases, but also because the engine behavior shown in Figure 2 becomes more linear.

It should be noted that the fuel economy penalty for this study is only established in the low-speed, high-load region of the engine operation, and does not represent a complete driving cycle fuel economy assessment. This topic will be discussed further in the following section. Nevertheless, the result shows that slipping the torque converter lock-up clutch at higher speeds does result in a corresponding fuel economy penalty.

Figure 19 demonstrates that the fuel economy penalty of the double damper and pendulum absorber is negligible compared to a locked torque converter clutch. However, the fuel economy benefit is clear compared to a conventional damper with torque converter slip of 40RPM.

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Figure 19: Fuel Economy Penalty of Different Dampers

Because the double damper and pendulum absorber exhibit minimal fuel consumption penalty compared to a conventional damper with locked torque converter clutch, they are attractive alternatives if the NVH metrics are met.

Figure 20 shows the transmission output shaft speed root-mean-square (RMS) amplitudes for different levels of torque converter slip, compared to the double damper, pendulum absorber, and the target maximum NVH level.

Figure 20: Transmission Output Shaft RMS Speed

It is observed that for this powertrain configuration, the target maximum NVH level is only violated during lugging. Therefore, Figure 21 is shown which shows more detail in this area.

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Figure 21: Transmission Output Shaft RMS Speed during Lugging

From the above figure, it can be observed that the target NVH metric can be met in the conventional damper with 40RPM or higher torque converter clutch slip. The double damper exhibits superior isolation compared to the conventional locked converter damper in higher speed regions, but during lugging it is shown to require additional torque converter slip to meet the target metric. The pendulum damper NVH behavior compares favourably to the conventional locked converter damper but below 22 miles per hour would require some slip to meet the NVH metric, but above this speed its performance quickly improves and is superior even to the maximum slip case using the conventional converter damper.

The behavior described above can also be verified by comparing the seat track vibration in the vehicle lugging mode in Figure 22.

Figure 22: Seat Track Acceleration during Lugging

The above figure demonstrates that in the region shown, the pendulum absorber exhibits lower seat track vibration amplitudes compared to the conventional locked

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torque converter clutch, but are still higher compared to the conventional torque converter with 40RPM slip in this low speed region.

Figure 23 compares PTU gear contact torques during the rattle for 0 and 40 RPM slips with double damper and pendulum absorber designs. It was shown that the PTU rattle can be mitigated by the convertor slip or damper design while the pendulum absorber provides the most effective rattle mitigation.

Figure 23: PTU Rattle Torque Comparisons

6. Conclusions and Next Steps

In this study, a co-simulation methodology was presented to predictively model the trade-offs between fuel economy and NVH for three torque converter damper types with controlled lock-up clutch slip. The benefit of this approach is that specialized, dedicated tools for each portion of the model (i.e., GT-SUITE for the engine and torsional damper, Romax Dynamic FUSION for the transmission, and MSC Adams for the drivetrain/vehicle model) can be all used together through co-simulation to evaluate the full system response.

The results of the study indicate that while slipping the torque converter clutch is a beneficial method to reduce seat track and transmission output shaft vibrations during lugging, the fuel economy is negatively impacted. To improve fuel economy, two additional damper types were introduced, the double damper and pendulum absorber, which exhibit a negligible fuel economy penalty compared to the conventional damper, and require less torque converter clutch slip to achieve the NVH target. However, the added complexity of the double damper or pendulum absorber in the torque converter unit comes with a cost impact.

This new methodology provides a system evaluation approach allowing the engineer to better balance the multiple attribute challenges during vehicle development process and determine the most effect solution with respect to NVH, fuel economy and cost.

While the results of this study focus on short transient vehicle acceleration, complete evaluation of vehicle fuel economy is achieved only by examining a set of government-mandated driving schedules. The engine speed and load requirements for these driving schedules vary between city and highway cycles, and depending upon the given vehicle

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can differ from the presented transient simulation. To fully appreciate the fuel economy benefits of reducing torque converter slip, future work will include examining the torque converter slip map for different operating conditions over complete driving cycles. An additional benefit of improved NVH behavior at low speeds which will be evaluated in the future is the ability to modify the transmission shift strategy, to allow earlier shifting.

7. References

1. GT-SUITE User Manual, Version 7.5, 2014. 2. Lindemann, P., Swank, M. "Dynamic Absorbers for Modern Powertrains". SAE

Technical Paper 2011-01-1554, 2011. 3. Blessing, U., Sawodny, O., and Schaper, U. "Modeling and Torque Estimation of an

Automotive Dual Mass Flywheel". 2009 American Control Conference, pp. 1207-1212. 4. Cheng, Y., Abe, T., and Wilson, B., "Automatic Transmission Gear Whine Simulation

and Test Correlation," SAE Technical Paper 2005-01-2290, 2005. 5. Platten, M., Lin, J., Jain, S., Eccles, M., "Automated and Optimized Multi-Body Models

for Gearbox and Driveline Dynamic Simulation," JSAE Technical Paper 20145179, 2014.

6. Adams User Manual, Version 2013.2, 2014. 7. Robinette, D., Grimmer, M., Horgan, J., Kennell, J., Vykydal, R., “Torque Converter

Clutch Optimization: Improving Fuel Economy and Reducing Noise and Vibration,” SAE Technical Paper 2011-01-0146, 2011

8. Acknowledgments

The authors would like to thanks W. Chen, E. Pesheck, B. Juang, J. Iqbal, Y. Chen and H. Huang of Ford Motor Company for their technical contributions to the project.