OPTIMAL OPERATION OF POWER-SPLIT HYBRID ELECTRIC POWERTRAIN: COMPARISON BETWEEN TWO PERFORMANCE INDICES

Among all the current environmentally-friendly ground vehicles proposed to reduce fuel consumption and emissions, power-split hybrid electric vehicles represent one of the most promising solutions. Their operation is based on the cooperation of the thermal engine and the electric unit enabled by mechanical transmissions consisting of planetary and ordinary gear sets. However, implementing proper energy management strategies to realise the best powertrain operations is crucial to maximise the environmental gain without compromising performance and drivability. For this purpose, an initial comprehensive analysis of the powertrain response is required. In this respect, this paper relies on a unified parametric approach, available in the literature, to investigate the acceleration response of an output-split hybrid electric powertrain by also considering the transmission mechanical power losses. The efficiency maps of the engine and electric machines are introduced to assess two performance indices. These are the powertrain real global efficiency and a fictitious equivalent efficiency where the efficiency of each actuator is normalised to the respective maximum value. The aim is to compare the optimal powertrain operation resulting from the maximisation of each performance index for a given vehicle speed and acceleration.