A Study on Dynamic Torque Cancellation in a Range Extender Unit

As electric hybrid powertrains become more widely adopted in passenger vehicles, a variety of electric machine and internal combustion engine architectures has been developed to meet diverse requirements. Research suggests that less than 20 percent of daily driving mileages are over 70 km but that longer trips are relatively common. This fact makes the range extended electric vehicle (REEV) potentially more favourable to typical family usage. To fulfil an everyday full electric commute a smaller battery pack can be used, which reduces cost. An on-board auxiliary power unit (APU) is designed to recharge the batteries for occasional long range trips. Usually, a low-cylinder-count small capacity engine coupled with an electric generator is used in the APU. However, this type of engine inherently exposes the system to more severe torque pulsations due to the lower firing frequency. This is not desirable for intermittent APU operation. The noise and vibration could be noticeable to the passengers once the APU starts running. The torque output of a synchronous electric machine can be accurately controlled by the invertor through vector control theory. Thus, it is feasible to dynamically vary the generator in-cycle torque pattern to counteract the engine torque oscillation. Consequently, a smoother operation is achievable. In this paper, a series of motor/generator control torque patterns were applied to cancel the engine in-cycle torque pulsations. The correlation between the electric machine torque profile and the engine in-cycle speed variation was investigated. As more aggressively use of the electric machine was made to achieve better system operation characteristic, the electric losses become more significant compared to constant torque demand strategies as does the thermal impact. Therefore, the electric losses and thermal effect were also studied for a given motor/generator design when a dynamic torque control strategy is deployed.