Analysis of circulating power within hybrid electric vehicle transmissions

Since multiple paths of power flow exist in many transmissions, an undesirable situation of power circulation can arise in the system which leads to high mesh losses and less overall efficiency of the system. The paper presents a detailed analysis of circulating power within a planetary gear transmission used for hybrid vehicles. The fundamental relations connecting torques, speeds and power flows for a general planetary drive mechanism, are derived from the first principles for kinematic and power flow analysis. The power flow relations are investigated and clearly defined for the analysis of circulating power within complex chains of planetary drives and they also permit an immediate derivation of the power flow in all inversions of planetary drives. This study contributes to the development of a methodology for the circulating power-flow analysis of a planetary gear drive with two inputs and one output. This approach is generic and an illustration is presented to demonstrate its applicability to a hybrid planetary gear configuration. It is found that circulating power depends on planetary gear ratios and speed ratio of two power sources.