Configuration Selection for Hybrid CVT Patrick Debal, Saphir Faid and Steven Bervoets, Punch Powertrain
Abstract— This paper reports the intensive study performed at Punch Powertrain that compared the PRE-configuration (electric motor1 in front of the variator) and POSTconfiguration (electric motor behind the variator) for a CVT based hybrid powertrain. This study led to the decision to further develop the POST-configuration into the current hybrid powertrain. The main deciding factor in the decision is the possible fuel saving that can be realized with either of both configurations. The other factors also taken into account in the decision are also treated. For both configurations a hybrid strategy was elaborated in depth. After optimizing the strategy for both configurations simulations were run on different type approval as well as real world drive cycles. The POST configuration yields a better fuel consumption reduction over the NEDC cycle as well as real traffic cycles. Additionally, the optimized strategy for the PREconfiguration is a lot more complex than for the POSTconfiguration. This may require more powerful controller hardware both in processor power and memory capacity. Additionally, the EV-range for the POST configuration is substantially longer than for the PRE-configuration. This longer EV-range of the POST-configuration is attributed to the fact that the electric power to the wheels while driving and from the wheels while braking does not incur the variator losses. Moreover, the braking torque that can be transferred by the CVT belt is not completely clear. Prior experience has shown that the allowable torque transfer is a lot lower than the permissible driving torque. Next to the fuel consumption the distinct advantages of one of the configurations over the other were studied.
I. INTRODUCTION When Punch Powertrain started the development of its hybrid powertrain two configuration options were available. The decision of which configuration to develop needed to match in the ambitious development targets set forward. The first option is to link the electric motor/generator with the powertrain in front of the CVT variator. This configuration is applied in mild hybrids like the Honda Insight/Civic and Hyundai Accent (aka Kia Rio). Hyundai applied the VT2 CVT from Punch Powertrain in its hybrid. Patrick Debal is project leader of the hybrid powertrain development at Punch Powertrain in Sint-Truiden, Belgium (phone: +32 11 679 266; fax: +32 11 679 230; e-mail:
[email protected]). Saphir Faid is project leader of the switched reluctance motor development at Punch Powertrain in Sint-Truiden, Belgium (phone: +32 11 679 193; fax: +32 11 679 230; e-mail:
[email protected]). Steven Bervoets is project engineer for the development of the switched reluctance motor development at Punch Powertrain in Sint-Truiden, Belgium (phone: +32 11 679 215; fax: +32 11 679 230; e-mail:
[email protected]). 1 When electric motor is used in this paper, it is a subsystem that can operate as motor as well as generator. In figures it may be abbreviated to EMG which stands for electric motor/generator.
Lotus in the UK developed the EVE, a full hybrid demonstrator based on a Proton passenger car also using the VT2 CVT from Punch Powertrain. These examples use a thin, sandwiched flywheel motor. Another possibility is to link an electric motor with a more standard form factor with gears or a chain to the input shaft of the transmission. At Punch Powertrain the configuration with the motor before the CVT is called the “PRE”-configuration.
PRE-Configuration
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Figure 1: PRE-Configuration Schematic The other configuration, hence called the “POST”configuration, links the electric motor/generator behind the variator. This configuration was not yet seen on the market, neither with CVT or stepped transmissions. It offers a direct connection of the electric motor to the wheels. POST-Configuration
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Figure 2: POST-Configuration Schematic Punch Powertrain investigated next to the fuel consumption reduction different other aspects of both configurations. This resulted in the clear decision to go for the best one. II. POTENTIAL EFFICIENCY IMPROVEMENT The major reason for vehicle constructors to develop hybrid vehicles is fuel saving. The hybrid passenger cars on the market have a low fuel consumption that is partially attributed to the hybrid powertrain and partially to additional measures like highly efficient engines, streamlining, mass
reduction and other measures. Punch Powertrain, as a powertrain developer and supplier, only has impact on the hybridisation of the powertrain especially when existing engines have to be used. For both configurations Punch Powertrain elaborated a hybrid strategy in depth for a full hybrid offering all hybrid powertrain modes. The powertrain modes are EV, conventional, assist and generate mode. In EV-mode the vehicle is propelled as an electric vehicle, only by using the electric motor. This is during acceleration, cruising and braking. In conventional mode only the engine is used. In assist and generate mode the electric motor operates as motor to help the engine using energy stored in the battery respectively as generator to convert excess power into battery charge. The power flows for each configuration in each mode except conventional mode are depicted in Figure 3. PRE-Configuration
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Figure 3: Power Flows in Different Hybrid Modes until Power Split Point The efficiency of both configurations can be compared on a qualitative basis for each mode. A. Generate Mode In generate mode the engine produces more torque than required for propelling the vehicle. This mode is used to operate the engine at higher efficiency. This occurs when a relative low torque is required from the engine and its efficiency can be raised by increasing the torque. The energy generated in excess is stored in the battery. In the PRE-configuration the mechanical energy is directly transferred from the ICE to the electric motor that at that time acts as a generator. This transfer incurs no variator losses. In case of the POST-configuration mechanical energy is
transferred from the ICE to the electric motor over the variator. This transfer incurs variator losses. In generate mode the torques at the CVT input shaft are relatively small. The incurred losses are at relatively high CVT efficiencies. Therefore the CVT losses in generate mode for the POSTconfiguration are relatively small. B. Assist Mode In assist mode the engine produces not enough torque to propel the vehicle according to the driver demand. The electric motor fills the gap. Energy stored in the battery is used. In the PRE-configuration mechanical energy from the electric motor/generator is transferred with the mechanical energy of the ICE over the variator to the wheels. This occurs when high torque is required. Therefore the additional electric torque incurs increased losses in the CVT. At these high torque levels, for a given ratio, the CVT operates at a lower efficiency due to a more than linear increase of the losses. Therefore the marginal losses are higher. This is in contrast with the POST-configuration where the electric motor/generator creates the torque after the variator. Consequently no additional variator losses are incurred on this energy. C. Electric Vehicle Mode The electric torque passes over the variator in the PREconfiguration. Consequently variator losses are incurred. The POST-configuration electric torque is created after the variator. No variator losses are incurred except for the electric oil pump that provides sufficiently oil for the belt tensioning and some drag losses. This can be avoided when the variator can be disconnected by a clutch on the secondary shaft. When braking in electric mode the PRE-configuration regenerative braking torque has to pass over the variator. Also now variator losses are incurred. A higher brake torque limit for electric braking may be applied to compensate the extra losses. But the variator belt may limit the allowable regenerative braking torque to lower limits. So it is not certain whether the PRE-configuration offers the same regenerative braking potential as the POST-configuration. In the POST-configuration during braking the regenerative braking torque does not pass over the variator. No losses are incurred in the variator except for the energy provided for belt tensioning. Also the electric brake torque is limited only by the electric motor and the battery and not by the push belt. D. Conventional Mode In conventional mode both powertrains have the same power flows. So performance and efficiency will be similar. The main differences can be found in the drag losses of the electric drive. E. Strategy Development and Simulation Results A first step in this strategy development is to create a
holistic view on the optimisation principles. To reach the ambitious fuel reduction targets a system optimisation rather than a component optimisation is required. New mathematical models were developed to derive the operating areas of the different modes by applying the optimisation principles for the different hybrid modes for both configurations. After optimising the strategy for both configurations simulations were run on different type approval as well as real world drive cycles. The POST configuration yields 8 to 10% more fuel consumption reduction over the NEDC cycle as well as real traffic cycles. Additionally, when operating the vehicle in EV-mode, both the difference in efficiency while driving and while braking has an even larger effect on EV-range for the same battery capacity. Without limiting the regenerative braking torque for a potential lower allowable push belt torque during braking, the PRE-configuration has an EV-range 35% lower than the POST-configuration2. Additionally, the optimised strategy for the PREconfiguration is a lot more complex than for the POSTconfiguration. This may require more powerful controller hardware both in processor power and memory capacity. III. PRACTICAL IMPLICATIONS Next to a difference in performance and efficiency, each configuration has one or more practical advantages over the other. This section gives an overview of those implications as they were identified by Punch Powertrtain. A. Electric Motor with Fixed or Variable Ratio In the PRE-configuration the electric motor has a variable ratio to the wheels. This implies that more flexibility to provide torque and speed from the electric motor to the wheels. A smaller electric motor can be used to obtain the same high torque at low speeds but the smaller motor and the transmission ratio result in a much lower torque at high speeds. This implies that hybrid operation at high speeds is limited. If this matches the use of the vehicle this is not an issue. This implies that e.g. during highway driving the hybrid powertrain will primarily operate as a conventional powertrain. In most cases highway driving does not benefit from the hybrid nature of a powertrain. On the other hand the characteristics of the engine and the electric motor/generator need to match to obtain a good optimization. The electric motor in the POST-configuration has a fixed ratio with the wheels. The electric motor speed is directly linked with the wheel speed, like with the large electric motor in the Prius. A larger electric motor is required to provide a similar launch torque but then the powertrain can address full hybrid operation at all speeds. As a result the use of the powertrain is for more kinds of traffic (city, rural, 2
Based on a 3 kWh battery pack and a usable SoC window of 65%.
motorway) and its use is more universal. There is more flexibility to match the engine operation with the electric motor operation. As indicated above this results in very efficient operation of the complete powertrain with a straightforward strategy. B. Engine Starting with the Motor The extra torque to start the engine needs to come directly from the electric motor. This implies that this torque is not available for driving the vehicle. This is true for both configurations. The PRE-configuration allows starting the engine at standstill if the motor can be disconnected from the wheels. This implies an additional clutch but avoids the cost of a starter appropriate for start/stop systems. While driving the engine can also be started by the motor. If the motor has the spare torque available, the engine starting procedure will not affect driveability. Otherwise the effect will be present, but small and short. In the POST-configuration the variator can adapt its ratio to reduce the level of additional torque at the electric motor shaft required to start the engine. This can result in better driveability. This method can only be applied when the vehicle speed is high enough (> 8 km/h). At lower speeds the engine cannot be started with the traction motor. This issue is identified as the major drawback of the POST-configuration. Punch Powertrain is elaborating a solution by adding an auxiliary motor that can be used as starter/alternator and airco drive. By combining these different functions in one unit it can still be cost effective. ICE EMG AuxDr
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High voltage solution being developed
Figure 4: High Voltage Starter/Alternator/Airco Drive C. Mechanical Construction Maintaining the total powertrain length without engine downsizing is one of the targets at Punch Powertrain. Therefore integrating a sandwich electric motor between the engine and the transmission is no option for the PREconfiguration. There is no motor technology available that offers a motor powerful enough for a full hybrid that fits in the existing bell housing. An electric motor placed on top of the transmission is then the way to go for both configurations. To allow a maximum carry over of existing components, already produced in large volumes, the new design needs to adhere close to the VT2 CVT. A chain drive between the motor and the transmission is then the most feasible option. In both configurations the chain drive offers its challenges to be a cost-effective and reliable design.
IV. CONFIGURATION SELECTION Due to a better fuel consumption reduction and some important benefits when considering practical implications Punch Powertrain selected the POST-configuration for the development of the hybrid powertrain. Meanwhile the hybrid powertrain is developed as prototype for powertrain test bench assessment and calibration for drivability in a demonstrator vehicle. Different aspects of the development have already been reported [1], [2]. The demonstrator performs nicely.
Figure 5: Chain Drive in the POST-Configuration D. Applicable Engine Range In the PRE-configuration the combined torque of the engine and the electric motor is transferred over the push belt. At the high torque side of applications it makes no sense to allow engines up to the torque limit of the transmission. Engines have their most efficient operation at approximately 75% of the maximum torque. If the engine is close to the transmission torque limit, this does not leave enough room for optimization with the motor. So in general, smaller engines will be used. In the POST-configuration, only the engine torque is restricted by the transmission torque limit. Therefore, the applicable engine range is the same as with the conventional powertrains. The combined power of the engine and the motor allows the POST-configuration hybrid powertrain being used in a larger vehicle segment than the PREconfiguration. E. Oil Pump The POST-configuration needs an electric oil pump to provide hydraulic and lubrication oil for the CVT during EV-mode. The required hydraulic pressure is low because minimal clamping is required. The PRE-configuration also needs an electric oil pump in EV-mode at least until the primary speed is high enough for the conventional oil pump to provide oil at the required pressure and flow. Due to the clamping requirements the electric oil pump needs to provide oil at a higher pressure. Without too much studying of the details of a system combining a conventional with an electric oil pump, Punch Powertrain decided to go for a single electric oil pump operating at high voltage. This oil pump offers the benefit that the delivered oil flow can be made to match the requirements. The related efficiency gains are high enough for the incurred costs.
Figure 6: The POST-configuration Hybrid Powertrain in Front of the Demonstrator Vehicle The figure above shows the hybrid powertrain in front of the demonstrator vehicle. This is a B-segment vehicle offering little space in the engine compartment. The whole hybrid powertrain fits in this envelope without engine downsizing. The strategy validation as well as fuel consumption measurement still needs to be performed. V. CONCLUSION The POST-configuration offers the best efficiency gain while it presents the least practical issues to its developers. Punch Powertrain has managed to develop a compact and performing hybrid with this configuration. ACKNOWLEDGMENT The development of the hybrid powertrain at Punch Powertrain is supported by the Flemish Government as an IWT industrial research and development projects. The IWT is the Institute for the promotion of Innovation by Science and Technology in Flanders. REFERENCES [1]
[2]
P. Debal, S. Faid, S. Bervoets, L. Tricoche and B. Pauwels, “Development of a Post-Transmission Hybrid Powertrain”, in Proc. Electric Vehicle Symposium (EVS24), Stavanger, 2009. P. Debal, S. Faid, L. Tricoche and S. Bervoets, “CVT-Based Full Hybrid Powertrain Offering High Efficiency at Lower Cost”, SAEPaper 2010-01-1313, Detroit, 2010.
