Influence of number of gear step on engine and motor operation characteristics for parallel HEV

In this study, influence of number of gear step on the engine and motor operation is investigated for a parallel hybrid electric vehicle(HEV). 6-speed automatic transmission (AT), 8-speed AT and continuously variable transmission(CVT) are selected for the target parallel HEV and a comparative analysis is performed. In order to obtain the maximum fuel economy potential, backward simulators are developed based on dynamic programming by considering the transmission efficiency, power electronics system and hydraulic system loss. From the simulation results, it is found that as the number of gear step increases, the engine operation is performed more closely on the optimal operating line for the minimum fuel consumption. Accordingly, the amount of motor load leveling power decreases and the power electronics system loss is reduced. On the other hand, the hydraulic system loss becomes larger with the increased number of gear step due to the additional elements, which cause lubrication and sealing losses. For CVT HEV, even if the engine thermal efficiency is improved and PE loss can be reduced by CVT function, the fuel economy is rather reduced because of the high hydraulic system loss required to maintain the 3∼4 times higher line pressure than AT.