A PPCI-diffusion combustion strategy has shown the potential to achieve high efficiency, clean gasoline compression ignition (GCI) combustion across the full engine operating range. By conducting a 3-D CFD-led combustion system design campaign, this investigation was focused on developing a next generation (NextGen), step-lipped piston design concept in a 2.6L advanced light-duty GCI engine. Key geometric features of the NextGen piston bowl were parametrized and studied with customized spray targeting. A low lip positioning design with 128° spray targeting was found to provide the best performance.
Fuel injection strategy optimization was performed at a full-load operating point (OP), 2000 rpm/24 bar closed-cycle IMEP (IMEPcc). When combined with the optimized fuel injection strategy, the best NextGen design was predicted to produce a 1.3% ISFC improvement and 42.5% lower soot compared to the baseline piston bowl design due to faster diffusion combustion and enhanced late-stage air-utilization. Subsequently, at 2000 rpm/12 bar IMEPcc, the NextGen design was able to soften the first-stage PPCI combustion to reduce the negative work and lower the MPRR, leading to 2.1% better ISFC and 48.8% lower soot than the baseline design when combined with its benefit to improve the second-stage diffusion combustion. Finally, at 1500 rpm/6 bar IMEPcc, the NextGen design was found to appreciably reduce the in-cylinder heat transfer and enable a larger fuel injection quantity in the first fuel injection event while retaining its air utilization benefit compared to the baseline design. Therefore, it was predicted to produce 2.4% better ISFC and 49.3% lower soot.
