Simulation Study on High Expansion Ratio Dedicated Hybrid Engine for Hybrid Commercial Vehicle Application

The fuel economy and emission of the hybrid vehicle depend largely on the selected engine. And the dedicated hybrid engine (DHE) can be controlled to operate in the optimal operating range because DHE can be decoupled from the vehicle transmission system. The main purpose of this paper is to improve the thermal efficiency of the diesel engine under common operating conditions combined with high compression ratio (CR) and early or late intake valve closing (IVC) angle. According to the vehicle road spectrum data, the optimal operating range of the engine is determined to be 1200-1400 rpm and 70%-90% load. Then CR and IVC angle are optimized by using the calibrated one-dimensional thermodynamic model of the engine under limited peak combustion pressure (P_max). The results show that the adjustment of IVC angle and CR can control the thermal state at the end of compression stroke. The combination of CR and IVC angle can achieve the optimal fuel consumption improvement. The minimum brake special fuel consumption (BSFC) is reduced from 189.2 g/kWh to 184 g/kWh. Based on the thermal process analysis of internal combustion engines, the effects of CR, IVC, and boost pressure on engine performance are analyzed. The improvement of thermal efficiency caused by higher CR would be compensated by the decrease of combustion constant degree. What is more, early or late IVC angle can reduce combustion phase loss and gas exchange loss. Further, a more efficient turbocharger can be matched to achieve higher thermal efficiency.