A range extended electric vehicle (REEV) has the benefit of zero pipeline emission for most of the daily commute driving using the full electric mode while maintaining the capability for a long-range trip without the requirement of stop-and-charge. This capability is provided by the on-board auxiliary power unit (APU) which is used to maintain the battery state of charge at a minimum limit. Due to the limited APU package size, a small capacity engine with low-cylindercount is normally used which inherently exposes more severe torque pulsation, that arises from a low firing frequency.
By using vector control, it is feasible to vary the generator in-cycle torque to counteract the engine torque oscillation dynamically. This allows for a smoother operation of the APU with the possibility of reducing the size of the engine flywheel. In this paper, a series of motor/generator control torque patterns were applied with the aim of cancelling the engine in-cycle torque pulses. The correlation between the electric machine torque profile and the engine in-cycle speed variation was investigated. As more aggressive use of the electric machine was made to achieve better system operation characteristic, the electric losses become more significant compared to constant torque strategy, as does the thermal impact. The results showed that within the target APU specification, the optimum speed fluctuation reduction of 21.9% could be achieved, while the M/G’s electric loss percentage rose from 3.5 to 4.7% at 4500rpm. An upgrade for cooling circuit specification was not considered necessary since the electric loss only increased by 2.5%. The Pareto frontier was also plotted for total electric loss against peak-to-peak engine speed variation.