Pathways to Exceed 50% Thermal Efficiency in Direct-Injection Gasoline Engines

Researches on high efficiency and low emission control strategies are crucial for addressing energy security and pollution challenges for combustion engines of vehicles. This paper investigates the effects of increasing compression ratio and air-fuel ratio (λ) under naturally aspirated conditions, as well as enhancing intake flow and λ under boosted intake combined with the Miller cycle, on combustion efficiency and pollutant emissions. Experiments were conducted on a high-compression-ratio (up to 16.6) single-cylinder thermodynamic gasoline engine. Under natural aspiration, the effective compression ratio was raised via valve timings adjustments, while λ was increased using integrated passive and active pre-chamber systems. Under boosted conditions, intake flow was controlled via a flow meter, and λ was controlled via an active pre-chamber to analyze the λ distribution and thermal efficiency at high-efficiency operating points. Results indicate that under natural aspiration, increasing the effective compression ratio to 15.8 and λ to 1.4 improved the indicated thermal efficiency (ITE) to 40.3%. Further deployment of an active pre-chamber enabling ultra-lean combustion (λ = 2.0) achieved an ITE of 43.3% while reducing NOx emissions to 53×10⁻⁶. Under boosted intake with Miller cycle, elevating intake pressure to 282kPa and achieving ultra-lean combustion (λ = 2.0–2.2) resulted in ITE over 50%, with NOx emissions consistently below 50×10⁻⁶.