Optimisation of an existing Automatic Transmission Calibration for maximizing Fuel Economy using AVL CRUISE Aditya Dhand and Martin O’Mahony AVL Powertrain UK Ltd., Basildon, SS156SR, United Kingdom
[email protected] KEYWORDS: Gear shift program, Fuel economy, Lock up clutch program, Cycle, Simulation ABSTRACT The stricter fuel consumption and emission regulations put the worldwide carmakers and suppliers under pressure to develop more efficient vehicle systems. Simulation-based design and virtual prototyping can insure greater product performance and quality of both the time and cost required by traditional build-and-test approach for the vehicle development process in general. Numerical simulation of the fuel consumption and emissions behaviour of an internal combustion engine fitted to a vehicle is today an indispensable aid to development. AVL’s vehicle systems and driveline analysis tool CRUISE can be used to perform these simulations and analysis in a user friendly and efficient manner. Additionally, a gear shift program (GSP) optimization tool has been developed in CRUISE for automatic transmissions to maximize fuel economy and/or minimize emissions over a given cycle. In the work reported here, the GSP and the lock up clutch program (LUCP) of an existing automatic transmission for an S-segment vehicle was optimized using AVL CRUISE to maximize fuel economy over two standard cycles. The sensitivity analysis was performed on the additional reduction gear ratio after the transmission and the cylinder deactivation strategy was also analysed. The GSP optimization tool provides us with a methodology to calibrate the transmission control unit (TCU) using simulation according to different criteria such as fuel economy, emissions and NVH. INTRODUCTION Increasingly stringent statutory emissions regulations are forcing automobile manufacturers to develop further the optimization potential of the vehicle system. High engine efficiency, increased comfort requirements, and stringent emission regulations are examples of the political and public conflicting requirements. Optimal powertrain integration and control design is essential to developing more fuel efficient vehicles. Vehicle systems are becoming increasingly complex as are expectations for both fuel economy and performance. Shorter product development times result in less time available to evaluate alternative powertrain configurations and control strategies [1]. Predicting the fuel economy and the emissions during a defined cycle is one of the most common simulation applications. AVL CRUISE is a comprehensive tool for prediction and improvement of fuel consumption, emissions and vehicle performance. It is used in drive train development to calculate fuel consumption, emissions, performance, transmission ratios,
etc. The modular structure of CRUISE permits modelling and simulation of all existing and future vehicle powertrain concepts and a parametric evaluation of the various components can be performed [2]. It can be easily linked with other simulation tools for the sub-system integration of vehicle thermal management systems, vehicle control systems and driving dynamics. One of the latest additions to CRUISE is a GSP generation and optimization toolbox. This toolbox enables rapid and efficient definition of GSPs for various powertrain configurations and driving styles. It can be used for optimization of GSP for a given driving cycle according to consumption/ emissions criteria as well as optimization of lock up management of torque converter according to consumption and/or emissions criteria. Export of GSP into the TCU in order to be used “on board” for vehicle calibration can also be undertaken [3]. Using CRUISE, the base vehicle was modelled and the fuel economy (FE) was calculated. The cylinder deactivation (CDA) as well as an additional reduction gear was implemented after the transmission in the base model. The GSP and the LUCP was optimized on different reduction gear ratios on the New European Driving Cycle (NEDC) and the FTP72 cycle. BASE VEHICLE MODELLING AND SIMULATION The vehicle modelled in CRUISE was an S-segment four wheel drive passenger car with an automatic transmission. The main components of the model are vehicle, engine, torque converter, gear box, final drive and tires. The vehicle is modelled as a single inertia. In addition to its mass it is characterized by a frontal area and drag coefficient, used to calculate aerodynamic resistance. The engine was modelled with torque-speed maps and fuelling maps. The torque converter sub model is characterized by the speed ratio, torque ratio and pump torque. The gear box is modelled as a kinematic ratio between the torque converter and the final drive. Inertia, the number of gears and gear ratios and mechanical efficiencies are also included. The final drive is characterized by gear ratio, inertia and efficiency. Finally the tires are specified by a dynamic radius, inertia and rolling resistance coefficient. The wheel slip was neglected during calculation. In between the central differential and the gearbox is an additional reduction gear and it is characterized by a fixed reduction ratio (RR). For the base version there is no reduction ratio applied. There are separate modules for GSP and LUCP. Figure 1 shows the model schematic.
Figure 1: Model Schematic The gasoline engine of the vehicle has the ability of cylinder deactivation below a certain engine torque and engine speed value. The losses in the auxiliaries are neglected. There was a base GSP and LUCP available which was input in the model. The engine throttle position correlation with engine torque and speed was input in the model. The load signal [%] for the GSP and the LUCP was assumed to be the engine throttle position. The vehicle modelling techniques used in the fuel economy studies can be divided into two categories according to the direction of the power flow calculation. One is backward, which calculates the required power from the wheel to the engine, and the other is forward, which calculates the power from the engine to the wheel. The main difference is that the forward simulation model requires a driver model and there is an error in the vehicle speed between the reference speed and the real simulation result. CRUISE features both forward and backward simulation solvers. For this study, the backward simulation was used as it provides faster and more accurate results. Also quasi static backward simulation is useful for initial component sizing, design of the controller. There are three reduction gear variants which are investigated in the simulations and compared with the base. 3% reduction 7% reduction 14% reduction The fuel economy has been calculated on the NEDC cycle and the FTP-72 cycle under hot and steady state conditions. The engine temperature is set to 80 deg C. The figures 3 and 4 show the NEDC and the FTP-72 cycle.
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Figure 3: FTP-72 Cycle The FE benefit of using CDA vs. a non CDA engine on the NEDC is 15.5%, which is mainly because the engine operates mostly in the CDA region during the NEDC. For all further results the base version is considered to have CDA engine. The following table 1 gives the result of the fuel economy results from the simulations of the as percentage change from the base version. From the table it is seen that 14% reduction gives the best benefit in terms of FE. FE Benefit [%] RR 3% RR 7% RR 14% Table 1: FE Results
NEDC 1.1 2.4 4.9
FTP72 0.9 2.0 3.7
OPTIMIZATION OF THE GSP ON THE NEDC CYCLE The GSP optimization toolbox in CRUISE calculates optimal gear shifts to be used over a cycle run. It performs optimization with regards to consumption and emission limits and also takes into consideration the driveability ranges. For Auto Transmission, it also calculates the optimal status of lock up of torque converter during the cycle. It optimizes the parts of the cycle where the vehicle is accelerating/cruising and ignores the deceleration part as it assumes the deceleration fuel shut off is active. Also it takes into consideration the lower NVH limit which can be defined in terms of engine speed for each gear. As soon as the engine falls below this lower limit a downshifting is suggested by the optimizer. In this work, the GSP optimization was carried out with the aim of maximizing fuel economy over the NEDC cycle. The three variants of the RR were used in the optimizations. The NVH limitation was set to 1000 rpm. The GSP toolbox outputs the optimal gear at each point in the cycle and the corresponding state of the lock up clutch. The figure 4 shows the output optimal gears on the NEDC cycle for the RR 14% from the GSP toolbox.
Figure 4: GSP optimized cycle Using the output of the GSP toolbox, the base GSP and LUCP is changed accordingly in order to produce the optimal gears and clutch state during the cycle. The figure 5 shows the base and the optimized gear up-shifting program for the RR 14% respectively. The solid lines show the original up-shifting map for 7 gear shifts and the dotted lines show the new upshifting map changed according to optimizer. From the figure 5 it can be seen is that the region of low load signal is mostly changed as during NEDC the engine operates mostly in low torque region. The figure 6 shows the urban driving cycle (UDC) part of the NEDC with the gear positions described by the base GSP and the optimized GSP. The early up-shifting and higher maximum gear can be seen for the optimal GSP. Also the optimized GSP ensures that the
engine speed never falls below the NVH limit during acceleration and cruising phases of the cycle. The table 2 gives the FE benefit after the GSP has been optimized individually for each of the RR variants as a percentage of the original FE with no reduction (RR 1) and the original GSP and LUCP. 100
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Figure 5: Base and Changed Up-shifting Map Urban Driving Cycle [UDC] for RR 14% 8
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RR 3% RR 7% RR 14% Table 2: FE benefit with optimized GSPs
From the table 2 above it can be seen that post optimization the FE is the best for the RR 7%. This is as a result of the fact that the NVH limitation has a less negative impact on the FE in this case than in the case of RR 14%. The figure 7 shows the effect of RR on the optimized GSP in the UDC. In the second speed hill, with a RR of 14% the highest gear is limited to 4th gear where as in the other case it is the 5th gear. This effect illustrates the importance of the NVH limitation on the GSP optimization. Urban Driving Cycle [UDC] 8
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Figure 7: UDC with different RR OPTIMIZATION OF THE GSP ON FTP-72 CYCLE The GSPs optimized on the NEDC cycle for the three variants of RR were used to simulate the FTP- 72 cycle. The table 3 gives the FE benefit obtained as compared with the base version with no reduction and original GSP and LUCP.
RR 3% RR 7% RR 14% Table 3: FE Benefit on FTP-72 Cycle
FTP-72 FE Benefit [%] 5.2 7.2 6.3
As it can be seen from the table 3 the RR 7% gives the best FE benefit. This case was then taken and the GSP and LUCP were optimized using the toolbox. The procedure
followed was identical to the one described in the previous section. The FE benefit after optimization was 6.5%. The fuel economy benefit is actually reduced after the optimization as compared to the one with NEDC optimized GSP. This is due to the fact that with the NEDC optimized GSP the NVH limitation is not met at a number of points in the cycle as shown in figure 8 where as in the GSP optimized on the FTP cycle the NVH limitation is respected.
Figure 8: Cycle with NEDC Optimized GSP From the figure 8, it can be seen that in case of the NEDC optimized GSP around time 868.02 s the engine speed falls below 1000 rpm where as the transmission is still in 7th gear. This is not case in the FTP optimized GSP as is seen in the figure 9. This also stresses the importance of the NVH limitation in GSP optimization. Also the changes in the gear position can be seen in the figures 8 and 9.
Figure 9: FTP-72 Optimized GSP Cycle CONCLUSIONS GSP optimization toolbox developed in AVL CRUISE has the ability to virtually determine an optimal combination of gear and torque converter states over specified drive cycles which offers significant potential in the design of more efficient vehicle systems. This paper describes the GSP optimization work for an existing automatic transmission on NEDC and FTP-72 cycle. The results show that significant FE benefit can be achieved by simultaneous optimization of the GSP and the LUCP of the transmission. The process also takes into account the NVH limitation for the transmission. Further work is required to completely automate the process which currently includes manual adjustment of the GSP based on the results of the optimizer. ACKNOWLEDGMENTS The technical support and advice of Aljosa Perger, Adrien Balihe, Nicholas Stack and Arno Van der Heijden is highly appreciated. REFERENCES [1]. Melody Baglione and Mark J. Duty, “Development of a Powertrain Matching Analysis Tool”, SAE paper 2010-01-0490 [2]. Pavan Potluri, Vishal Desai, David Maillard, Nirav Shah, Baekhyun Cho, “Which Hybrid Powertrain would be Suitable for your Vehicle to Reduce CO2 emissions?”, EVS24, Stavanger, Norway, May 13-16, 2009 [3]. AVL CRUISE User’s Guide, Edition 07/2009
