STARTER ALTERNATOR IN ZERO INERTIA CONFIGURATION IMPROVING THE ACCELERATION RESPONSE FROM OVERDRIVE

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1 International Journal of Automotive Technology, Vol. 4, No. 3, pp (2003) Copyright 2003 KSAE /2003/ STARTER ALTERNATOR IN ZERO INERTIA CONFIGURATION IMPROVING THE ACCELERATION RESPONSE FROM OVERDRIVE R. M. van DRUTEN 1)*, N. LIEBRAND 1), M. J. L. VERBAKEL 2) and M. RONDEL 2) 1) Technische Universiteit Eindhoven & Drivetrain Innovations DTI, Eindhoven, The Netherlands 2) TNO Automotive, Delft, The Netherlands (Received date ; revised date ) ABSTRACT The fuel economy of a vehicle equipped with an internal combustion engine can be improved by operating the engine at low rotational speeds where its optimum efficiency is found, also known as E-line tracking. However, if the engine is not oversized, this will significantly reduce the acceleration potential. An assist is then needed to guarantee an immediate response. A Starter Alternator (SA) - also known as an Integrated Starter Generator (ISG) - can replace the conventional alternator and starter motor. An additional benefit of the SA should be to provide electrical torque-assist. However, when mounted on the crankshaft in a CVT powertrain, the added inertia of the SA will take up more power during a fast downshift than the SA can deliver. This paper presents an alternative powertrain concept in which the SA is connected to the primary and secondary side of a continuously variable transmission (CVT) through a planetary gear set. In this so-called Zero Inertia configuration (ZI) [1], the inertia of the SA will act as a mechanical torque-assist by accelerating in the opposite direction of the engine during a fast downshift. Compared to the crankshaft mounted SA, this alternative powertrain will greatly improve the acceleration response without compromising the fuel economy of a CVT in eco-mode. The SA in ZI configuration is compared on the bases of performance and fuel consumption to (i) a standard CVT without assist, (ii) a crankshaft mounted SA providing electrical assist and (iii) the strictly mechanical ZI configuration. KEY WORDS : Continuously Variable Transmission, Starter Alternator, Power Assist, Transmission System 1. INTRODUCTION The main reason for not using the engine at its most efficient operating points, mostly at its lowest permissible engine speed, is the resulting lack of power response from the drivetrain. This problem is generally recognised with any transmission type, especially when a relatively small engine is used. Responsible for this dynamical behaviour is the inertia connected to the engine, which consumes a considerable amount of power when speeding up the engine to high power level. One way to minimise the problem is to reduce the primary inertias, however a certain minimum is needed to ensure a proper engine operation. Using the latest technology of integrated or belt driven Starter Alternators (SAs) will not improve the response because it will also increase the primary connected inertia. Especially at higher vehicle speeds (over 50 km/h) the electrical device will not be able to counteract this problem with an electrical assist due to its limited power. *Corresponding author. r.m.v.druten@tue.nl 2. ZERO INERTIA CONFIGURATION A solution for this dynamical drivetrain problem is proposed by van Druten (1999) and is known as the Zero Inertia (ZI) powertrain. In this powertrain concept, an additional inertia accelerates in the opposite direction of the engine. This changes the drivetrain dynamics significantly, resulting in a direct acceleration response when applying a fast downshift of the transmission and a direct deceleration response when shifting up. A proof of concept version of the ZI-powertrain is built and tested in a mid-size passenger car, see van Druten (2000). Mounting a SA in any conventional manner will require its inertia to accelerate together with the engine, thus decreasing its torque-assist potential. Mounting the SA in the ZI configuration will combine the advantages of an efficient electric machine with a powerful mechanical torque-assist. Figure 1 shows the position of the SA in relation to the rest of the drivetrain. At lower speeds, when the rotational speed of the SA is small, the electrical torque-assist dominates, at higher vehicle speeds the mechanical torque-assist will ensure a quick response. 1

2 2 R. M. van DRUTEN, N. LIEBRAND, M. J. L. VERBAKE and M. RONDEL figure also shows less efficient operating lines, which result in an x % higher fuel consumption. Also depicted is the Wide-Open Throttle (WOT) line, which represents the maximum engine torque as a function of engine speed. In today's passenger cars, equipped with an automatic transmission (either a CVT or AT), an operating line roughly between the OL 15% and OL 20% lines is applied for stationary situations. The driveability corresponding to such a line is generally considered acceptable. Using such an operating line results in an acceptable driveability but clearly penalises the fuel economy. 4. DRIVEABILITY Figure 1. Starter Alternator in Zero Inertia configuration. This paper focuses on comparing the acceleration performance of four CVT based powertrains. A standard CVT powertrain will be used as a reference to three powertrains equipped with different torque-assist systems, i.e. the ZI configuration, the SA-ZI configuration and a configuration in which the SA is connected to the crankshaft (as seen in the Honda Insight and Civic). The simulations are performed for vehicle speeds within the range of 0 and 120 km/h. The simulations of the standard CVT powertrain and the ZI powertrain have been validatedusing a test vehicle, see van Druten (2001). 3. REFERENCE OPERATING LINE An engine can supply a certain amount of power at very different rotational speeds, but there is only one engine speed where the optimum efficiency is reached. Connecting all the optimum operating points provides the engine's Economy or E-line, see Figure 2. Besides the E-line, this Driveability is for the larger part determined by the instant availability of power when pressing the accelerator pedal. The so-called power reserve is the product of the actual engine speed and the torque reserve, where the torque reserve is defined as the difference between the actual and maximum (WOT) engine torque at the current engine speed (see Figure 2). In Figure 3, the power reserve is plotted against the actually delivered engine power in relation to the operating lines defined in Section 3. The driveability is improved tremendously when trading the E-line for an operating line resulting in a higher engine speed. For instance, choosing the OL 15% degrades fuel efficiency by 15% but increases the power reserve by a factor 4 to DRIVETRAIN SPECIFICATION AND MODELING The standard frontwheel drive CVT drivetrain, see Figure Figure 2. E-line and operating lines with x % higher fuel consumption. Figure 3. Power reserve as a function of stationary power for various operating lines.

3 STARTER ALTERNATOR IN ZERO INERTIA CONFIGURATION IMPROVING 3 Figure 5. Standard CVT drivetrain. Figure 7. ZI configuration. 5, consists of a gasoline engine, a torque converter, a drive neutral reverse set (DNR), an electro-hydraulically actuated belt-cvt, a final reduction gear and a differential gear. Figure 6 shows the SA-CVT drivetrain, consisting of a crankshaft mounted SA (10 kw peak power). It uses a NiMH battery for electrical energy storage and supply. Due to the electrical torque assist during vehicle launch, the torque converter can be left out, giving a km/h performance comparable to the reference vehicle. A dual mass flywheel serves to minimise driveline vibrations. Leaving out the torque converter lowers the primary inertia. For the same reason, the launch clutch is located at the secondary transmission side. This configuration is comparable to that of the Honda Insight CVT. The ZI configuration of Figure 7 is not different from Figure 8. SA-ZI configuration. Figure 6. SA-CVT configuration. the standard CVT powertrain, apart from the addition of a flywheel connected to the primary and secondary side of the variator through a planetary gear set. The SA-ZI configuration of Figure 8 can be described as a ZI configuration in which an SA replaces the flywheel. For a straightforward understanding of the additional effect of the SA on the ZI drivetrain, the inertia of the SA was chosen equal to the flywheel inertia. The kinematic parameters of the planetary gear set were also set equal to the optimised parameters for the ZI drivetrain. The parametric design of the ZI-powertrain is discussed in Serrarens (2001). All simulations have been performed with the use of ADVANCE, a modular vehicle simulation environment in Matlab/Simulink developed by TNO Automotive, see Eelkema (2002).

4 4 R. M. van DRUTEN, N. LIEBRAND, M. J. L. VERBAKE and M. RONDEL The control for all drive trains aims at tracking the E- line for all stationary situations, whereas in kick-down transients the engine torque tracks the WOT line. Because of the limited transmission ratio range, exactly tracking the E-line will not always be possible for stationary conditions. A detailed efficiency map is used for the conversion of energy by the SA in combination with a validated battery model. 6. COMPARISON OF ACCELERATION PERFORMANCE This paragraph presents the simulated performance of the configurations described in Section 5. Four different transients will be considered: the acceleration from standstill to 100 km/h, from 30 to 50 km/h, km/h and km/h. Each transient is presented by three separate plots: vehicle acceleration level, engine speed and the lead which each torque-assist equipped configuration will develop over the standard CVT configuration. Except for the acceleration from standstill, all transients are simulated using a realistic but prescribed engine speed trajectory to keep the engine power equal and therefore the comparison more clear. Because of the different launch-devices that were used (torque converter and friction clutch) during the acceleration from standstill, the engine speed could not be prescribed. In this case, the engine speed is determined by the drivetrain dynamics. During all transients (except from standstill), the torque converter lock-up clutch remains engaged. The two SA torque-assist systems can maintain a 6- second boost within its thermal limitations. An assist with less power is possible for a longer period but the object in this comparison is to realise an optimum response. However, because the ZI configuration can not deliver maximum power for more than ca. 2 seconds, the results of the SA systems will also be plotted with a 2-s boost. These results are visualised with grey lines Acceleration from standstill to 100 km/h Most noticeable from Figure 9 is the difference between the SA-CVT configuration with friction clutch and the other configurations, which use a torque converter as launch device. The high acceleration level of the SA- CVT configuration around t = 0.5 s is caused by the decelerating inertia of the engine, thus releasing its kinetic energy into the drivetrain. The other engine speed trajectories are all limited around 2000 rpm during the first two seconds when the torque converter stalls. In this case, the mechanical torque-assist of the ZI configurations is not available because the inertia of the flywheel and of the SA will accelerate together with the vehicle. A swift downshift is needed to enable this mechanical torqueassist. The ratios of the planetary gear set are chosen such Figure 9. Response during acceleration from standstill. Table 1. Acceleration times: from standstill to 100 km/h. configuration time [s] relative [%] CVT ZI SA-CVT 2 s assist SA-CVT 6 s assist SA-ZI 2 s assist SA-ZI 6 s assist that the accelerating inertia will hardly compromise the launch of the vehicle. The small difference between the ZI configuration and the standard CVT is mainly caused by the increased vehicle weight (30 kg) and to a smaller extent by the increased drivetrain inertia (0.4 kg m 2 ). Acceleration times are listed in Table 1, showing a maximum deviation of 4.5% compared to the standard CVT vehicle. Although the difference in time is small, the SA-CVT configuration with a 6-s boost manages to get 10 meter ahead of the CVT configuration. After a 2-s torque-assist, it can be seen that the SA configurations can not increase their lead any further with regard to the standard CVT because of the increased vehicle weight (60 kg). The ZI vehicle will eventually lag 5 meter behind after 12.6 s Acceleration from 30 to 50 km/h Immediately after the kick-down at 30 km/h, the CVT will start to shift down. The resulting acceleration of the primary inertias will consume a significant amount of engine power. This is clearly demonstrated by the delayed response of the standard CVT vehicle, see Figure 10, also known as jet-start behaviour. During the first 0.5

5 STARTER ALTERNATOR IN ZERO INERTIA CONFIGURATION IMPROVING 5 Figure 10. Acceleration from 30 to 50 km/h. Figure 11. Acceleration from 50 to 80 km/h. Table 2. Acceleration times: from 30 to 50 km/h. configuration time [s] relative [%] CVT ZI SA-CVT 2 s assist SA-CVT 6 s assist SA-ZI 2 s assist SA-ZI 6 s assist s the SA-ZI configuration can benefit from both the mechanical power-assist as well as from the electric torque provided by the SA. Despite the fact that around t = 0.65 the electric assist is shut down, the SA-ZI acceleration increases suddenly. This paper can not provide an elaborate explanation of the different ways in which power from the engine and SA can be transmitted through the belt and through the planetary gear set to the wheels. The reason for temporarily shutting down the electric boost is to minimise the total losses within the drivetrain and thus optimise the power at the wheels. For the simulations all ratios of the planetary gear set have been chosen equal to the ZI configuration to keep the comparison as straightforward as possible. An acceleration level higher than the standard CVT over the entire range is possible with the SA-ZI configuration by using different ratios. The total acceleration time for the SA-ZI configuration increases almost 6% with respect to the standard CVT. Nevertheless, the driver will presume that the SA-ZI is the faster one, because of the instant response at the start and because the differences in vehicle lead are very small Acceleration from 50 to 80 km/h As the vehicle speed increases, the differences between the mechanical (power) assist and the electrical (torque) assist become more apparent, see Figure 11. The standard CVT vehicle is showing the typical dynamical behaviour associated with such a transmission operating in a fuelefficient mode (eco-mode). Despite the fact that the engine speeds up as normal, a delay of about 0.7 s can not be prevented. The electrical torque-assist of the SA-CVT configuration can improve the response only slightly during the first 0.5 s. The combination of electrical and mechanical assist - the SA-ZI configuration - shows an immediate and persistent response. After 2.5 s from the start the electrical torque assist is turned on again after being shut down just before t = 1 s resulting in a sudden increase in the vehicle acceleration level. This was done to minimise the total losses within the drivetrain and thus optimise the power at the wheels. Notice the differences between the acceleration levels between t = 1 and t = 2 s. These differences are caused by the increase in vehicle weight of 30 kg for the ZI Table 3. Acceleration times: from 50 to 80 km/h. configuration time [s] relative [%] CVT ZI SA-CVT 2 s assist SA-CVT 6 s assist SA-ZI 2 s assist SA-ZI 6 s assist

6 6 R. M. van DRUTEN, N. LIEBRAND, M. J. L. VERBAKE and M. RONDEL Table 4. Acceleration times: from 80 to 120 km/h. configuration time [s] relative [%] CVT, eco-mode CVT, sport-mode ZI SA-CVT 2 s assist SA-CVT 6 s assist SA-ZI 2 s assist SA-ZI 6 s assist Figure 12. Acceleration from 80 to 120 km/h. configuration and 60 kg for the two SA configurations. During this period, only the SA-CVT can benefit from an assist (the assist is temporarily turned of for the SA-ZI configuration) and can therefore maintain a slightly higher acceleration than the standard CVT. These differences however, are barely noticeable to the driver who will mainly focus on an instant response. Despite the fact that the electric torque-assist is turned off for almost 1.5 s, the SA-ZI configuration improves the acceleration time with more than 11% with respect to the standard CVT, and will take a lead of about 2.5 m after 4.1 s Acceleration from 80 to 120 km/h A vehicle equipped with a CVT usually provides the driver the choice between different gear control strategies. The simulations described in the previous sections were all based on a fuel-efficient strategy or economy mode (eco-mode). Figure 12 shows the effect of a sport-mode in which the controller maintains a higher engine speed during stationary situations, in this case 45%. The results clearly show a significant improvement of the sport-mode relative to the eco-mode. The effect on the fuel consumption will be discussed in section 7. This transient very clearly shows the difference between the two configurations equipped with an SA torqueassist. The SA-ZI configuration combines the high-power ZI assist with the longer lasting electric assist, which significantly improves the functionality of the SA. The ZI configuration is intended to improve only the immediate response; the flywheel does not provide enough energy to maintain a higher vehicle acceleration level. The acceleration time is therefore slightly increased as a result of the higher vehicle mass. Because of the quick response, the ZI configuration will keep a lead relative to the standard CVT. The lead of the SA-ZI configuration grows to an impressive 10 m. 7. COMPARISON OF FUEL CONSUMPTION The purpose of the torque-assist systems presented in this paper is to enable the engine to operate as fuel-efficiently as possible without compromising the driveability of the vehicle. Figure 13 shows the fuel consumption of the various configurations over the NEDC cycle relative to the standard CVT vehicle in sport-mode. The sport-mode results in a 45% higher engine speed at stationary vehicle speeds as long as the maximum rotational speed of the engine has not been reached. The two SA configurations are able to recover some of their braking energy through regenerative braking. Brake energy recovery can only be applied to the front wheels, and a brake distribution of 70/30 (front/back) is assumed. The amount that the SA-ZI configuration is able to recuperate equals an average electric accessory load of 275 W that can be used to supply for example the ignition, lights, radio etc. with electric power. The non-sa configurations have been simulated to supply this same amount of electrical power with the use of a conventional alternator, which is assumed to convert mechanical to Figure 13. Fuel consumption relative to the standard CVT in sport mode over the NEDC cycle.

7 STARTER ALTERNATOR IN ZERO INERTIA CONFIGURATION IMPROVING 7 electrical energy with a constant efficiency of 50%. Because the power delivered by the engine is only about 4.5 kw on average during this cycle, the accessory load has a considerable effect on the fuel consumption (compare excl. vs. incl. accessories bars). The highest fuel economy is achieved with the SA- CVT configuration, despite its 60 kg additional vehicle weight. Its fuel consumption could be reduced even further because this configuration can support an electrical load of about 310 W through regenerative braking instead of 275 W. The reason for the less effective recovery of braking energy of the SA-ZI configuration is that the SA usually regenerates at a lower rotational speed with a lower efficiency. This can be improved by choosing other ratios for the planetary gear set. The SA-CVT configuration has clear advantages over the standard CVT in eco-mode in terms of performance as well as fuel economy. However, the reduced performance of both is not acceptable to many drivers, especially at higher vehicle speeds. The two ZI configurations are able to combine a high performance (immediate response) with a significant reduction in fuel consumption compared to the standard CVT in sport-mode. Moreover, as vehicle speed increases, the performance of the ZI configurations will keep improving relative to the standard CVT in sport-mode. Figure 13 shows how much the relative fuel consumption is increased because of the added vehicle weight (1-2%) and because of the added flywheel or SA inertia (1-2%). These figures can be derived by comparing the ZI, SA-CVT and SA-ZI configuration to the CVT in ecomode. The added fuel consumption of the ZI systems are caused by speeding up the flywheel or SA after each vehicle stop. Friction losses of the flywheel or SA are not taken into account. A stop and go strategy (engine shutdown at vehicle standstill) can decrease the fuel consumption of the vehicles with torque-assist by 4%. It was assumed that the NEDC cycle would be started with a cold engine. Therefore, the stop-go strategy was initiated 5 minutes after the start. This reduces the number of engine shutdowns by 5 out of 14 opportunities. The two ZI drivetrains will be able to employ an impulse-start by decoupling the inertia connected to the planetary gear set before the vehicle comes to a halt and therefore will not consume additional energy. Direct cranking will be possible for both SA configurations. If the 10 kw SA can crank up the engine in 0.2 s, and 8 starts are required, then the average electric load over the total duration of the cycle (1180 s) equals almost 14 W. The energy needed for a direct start of the SA-CVT configuration is more than compensated by the higher electric load it can support through regenerative braking. 8. CONCLUSION AND OUTLOOK The addition of an SA to a drivetrain can significantly reduce the fuel consumption of a vehicle through regenerative braking (5%) and a stop-go strategy (4%). Lowering the engine speed or E-line tracking can further decrease fuel consumption by 8%. This strategy will only be accepted by the driver if the driveability is not compromised. The SA-CVT configuration, see Figure 6, can not meet this requirement because the SA does not have enough power to enable an immediate response at higher vehicle speeds. The functionality of the SA is greatly improved by mounting the SA in a Zero Inertia configuration, see Figure 8. The inertia of the SA will act not only as an electric torque-assist but also as a mechanical powerassist by accelerating in the opposite direction of the engine during a fast downshift of the transmission. This configuration is therefore able to maintain a low engine speed and still have an immediate acceleration response available at higher vehicle speeds (>30 km/h). Compared to the standard CVT in sport-mode, an improvement in fuel economy of up to 15% is possible. Currently under investigation for further enhancement of the SA-ZI transmission are: Reducing the torque transmitted through the CVT belt during launching and acceleration by applying an electric torque of the SA. This increases the torque capacity of the CVT. Optimising the ratios of the planetary gear set to increase the effectiveness of the power-assist. Replacing the CVT with an Automated Manual Transmission (AMT), while eliminating the torque interruption using the inertia, i.e. applying the energy stored in the rotating inertia of the SA. Downsizing of the engine to further reduce fuel consumption, weight and packaging space. ACKNOWLEDGEMENT This study is part of ImpulseDrive which is a research project at the Technische Universiteit Eindhoven in The Netherlands within the section Control Systems Technology of the Department of Mechanical Engineering. The project is subsidized by the NWO Technology Foundation within the Innovational Research Incentives Scheme 2000/2001. REFERENCES van Druten, R. M., Vroemen, B. G., Rosielle, P. C. J. N. and Schouten, M. J. W. (1999). Design of a powertrain for optimal performance and fuel economy using a CVT and a flywheel, In Proc. of the Internat. Congress on Continuously Variable Power Transmission CVT'99,

8 8 R. M. van DRUTEN, N. LIEBRAND, M. J. L. VERBAKE and M. RONDEL pp van Druten, van Tilborg, P., Rosielle, P. C. J. N. and Schouten, M. J. W. (2000). Design and construction aspects of a Zero Inertia CVT for passenger cars (F2- A058). In Proc. of FISITA'00, Seoul, Korea. van Druten, R. M., Serrarens, A. F. A., Vroemen B. G., van den Tillaart, E. and de Haas, J. (2001). Mild hybrids with CVT: Comparison of electrical and mechanical torque assist. VDI Berichte 1610, pp Eelkema, J., et al. (2002). ADVANCE, a modular vehicle simulation environment in Matlab/Simulink, MathWorks' International Automotive Conference, Stuttgart, Germany. Serrarens, A. F. A. (2001). Driveability control of the ZI powertrain, Chapter 3, In Integrated Powertrains and their Control; Editors: N. D. Vaughan, pp , Professional Engineering Publishing Ltd.