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

This study explores methods to extend the lean combustion limit, improve efficiency, and reduce engine-out emissions in a hybrid-dedicated homogeneous lean combustion engine. Under lean combustion conditions, slow laminar flame speed restricts flame kernel growth, leading to instability and limiting the lean combustion threshold. To address this challenge, a combustion system was developed that leverages the engine’s internal flow to enlarge the spark channel at the spark gap and promote the formation of a larger-scale flame kernel. The combustion chamber was optimized by redesigning the intake port geometry and piston bowl to generate a high tumble flow. This new design significantly increased airflow around the spark plug gap, accelerating the initial combustion process and effectively expanding the lean combustion limit. Additionally, multiple ignition was implemented to prevent spark channel shortening caused by increased electrical resistance, further enhancing combustion stability. A novel spray geometry was developed to improve mixture homogeneity and robustness under lean conditions, leading to better fuel efficiency and lower emissions. Furthermore, an electric supercharger was integrated to precisely control air charging, providing sufficient boost to power a D-segment SUV in combination with hybrid motors. The synergy of these technologies enabled stable combustion at a maximum lambda value of 2.0, achieving a 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. By addressing the inherent challenges of lean combustion, this research contributes to the development of more efficient and environmentally sustainable internal combustion engines.