Thermal Efficiency and Emissions of a Commercial Vehicle Hybrid-Dedicated Diesel Engine With Miller/Atkinson Cycle and High Compression Ratio: An Experimental Study

Abstract

Abstract. To address energy security and global environmental challenges, this study conducted bench tests to enhance the thermal efficiency of commercial vehicle hybrid-dedicated diesel engines, developing powertrain systems with high fuel economy and competitive total cost of ownership. Focusing on key conditions for thermal efficiency optimization, this study employed the Miller/Atkinson cycle and high compression ratio configurations. Through experimental and thermodynamic analyses, the study investigated how effective compression ratio (ECR) influences combustion processes and emission characteristics, proposing a technical route to improve thermal efficiency under limited peak cylinder pressure. Experimental results demonstrated that ECR modifications effectively regulate the in-cylinder thermal environment, with reduced temperature and pressure providing margin for advancing the start of injection, thereby minimizing combustion losses. Without a significant increase in nitrogen oxide emissions relative to the baseline, the late intake valve closing strategy, combined with a high geometric compression ratio, significantly reduced soot emissions at low load. Compared to the baseline, large soot particles are reduced by over 85%. The increased expansion ratio improved thermal-work conversion efficiency while reducing pumping losses, resulting in an approximately 1.2% increase in the maximum theoretical thermal efficiency. The maximum brake thermal efficiency of the optimized diesel engine increased by 2.5%, and the minimum brake specific fuel consumption (BSFC) dropped from 192.6 g/(kW · h) to 187.5 g/(kW · h). Furthermore, the BSFC under low-load conditions was significantly reduced by over 10 g/(kW · h).