Analyzing the impact of injection timing variations on the performance of multi-cylinder CNG direct injection spark-ignition engine

Objective. This study investigates the impact of Compressed Natural Gas (CNG) injection timing on the performance of a retrofitted CNG Direct Injection (CNG-DI) engine. The aim is to understand how varying injection timing affects key performance metrics such as torque, power, thermal efficiency, and fuel consumption in CNG-DI engines, providing insights for system optimization. Methods. A 1298 c.c., four-cylinder, four-stroke spark ignition engine with a compression ratio of 9 was retrofitted for CNG-DI operation. Modifications included a redesigned cylinder head for high-pressure injectors, controlled by a programmable gas ECU. The engine was tested with three injection timings: Early Injection (EI) at 230 degrees BTDC, Optimal Injection (OI) at 180 degrees BTDC, and Late Injection (LI) at 130 degrees BTDC. Performance, Emission and Combustion parameters were evaluated across engine speeds from 1000 to 3100 rpm. Results. Late Injection (LI) timing improved torque, power, BMEP, thermal efficiency, and volumetric efficiency by 4-16% within the 1000-2200 rpm range, attributed to better combustion dynamics. Optimal Injection (OI) timing achieved 4-12% higher performance in the 2500-2800 rpm range due to improved mixture formation. Early Injection (EI) timing resulted in 3-8% improvements at speeds above 2800 rpm, driven by faster combustion and increased peak power. Conclusion. This study highlights the crucial role of injection timing in optimizing CNG-DI engine performance: Late Injection improves fuel efficiency at lower speeds, Optimal Injection enhances thermal efficiency at mid-range RPMs, and Early Injection maximizes power at higher RPMs. However, the study is limited by its focus on a single engine configuration and does not consider emissions or Life Cycle Assessment (LCA). Future research should investigate the environmental impact of injection timing through LCA and assess the long-term durability of engine performance.