Performance comparison of series–parallel hybrid transmissions with multiple gears and modes based on efficiency model

Although multi-gear and multi-mode series–parallel hybrid transmissions (MGMM-SPHTs) can significantly improve the economy and power performance of hybrid electric vehicles (HEVs), few studies have elucidated their inherent mechanism. The present study quantitatively explores how different numbers of gears, the speed ratio range, and the working mode improve the economy and power performance of SPHTs. Moreover, the cost factor is considered in the comprehensive evaluation index. First, the speed ratio range of SPHT with different gears is defined, and the possible working modes of SPHT with different gear combinations are examined. Second, the efficiency model of the SPHT components is constructed on the basis of the energy flow, which provides a foundation for the subsequent construction of the power loss model. Moreover, by taking the minimum power loss in the power transmission process as the objective function, the dynamic programming method is used to evaluate the energy utilization performance of SPHT with different numbers of gears. Finally, on the basis of the aforementioned research, the comprehensive performance of MGMM-SPHTs is quantitatively compared, and the various influencing factors are analyzed in detail. The comparative analysis shows that increasing the number of engine direct drive gears has a significant effect on improving the economy. In particular, when the number of gears is increased from one to two, the fuel consumption is reduced by 4.007% on average; by contrast, when the number of gears is increased from two to three, the fuel consumption is reduced by only 1.173%. In addition, increasing the number of engine and motor direct drive gears has a significant effect on improving the power performance. Furthermore, this study elucidates the energy-saving mechanism of MGMM-SPHT, which can effectively guide the product design and development of SPHT.