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

Abstract 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° BTDC, Optimal Injection (OI) at 180° BTDC, and Late Injection (LI) at 130° 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.