Development of Combustion System for a Hybrid-Dedicated Homogeneous Lean Burn Engine

This study explores strategies to extend the lean combustion limit, improve thermal efficiency, and reduce engine-out emissions in a hybrid-dedicated homogeneous lean-burn engine. Under lean combustion conditions, slow laminar flame speed hinders flame kernel growth, leading to combustion instability and limiting lean limit of air excess ratio. To address this challenge, the combustion system is developed to generate high-intensity in-cylinder flow promoting plasma channel expansion at the spark plug gap and enabling the formation of larger initial flame kernel. A newly designed intake port and piston bowl geometry were introduced to enhance tumble flow, significantly raising convective flow speed at the spark plug gap. This accelerated the initial combustion process and effectively expanded the lean combustion limit. A high-energy multiple ignition was also implemented to prevent spark channel blow-off or short circuit caused by increased electrical resistance, further improving combustion stability under lean operation. Additionally, new spray pattern was developed to enhance mixture homogeneity and robustness, thereby reducing HC and NOx emissions. An electric supercharger was integrated to precisely manage air dilution and to provide a sufficient boost for powering a D-segment SUV when combined with hybrid electric motors. The synergy of these technologies enabled stable combustion at air excess ratio exceeding λ=2.0, achieving the maximum peak thermal efficiency of 45% and NOx emissions below 0.3 g/kWh at 2000 rpm and 8.5 bar. This study highlights the impact of each technology on combustion characteristics and underscores the potential of lean-burn engines as a viable solution for meeting stringent future emission regulations with significant carbon reduction.