Experimental investigation on the combustion and emissions in a pilot ignited direct injection natural gas engine using HCDI strategy

Abstract Pilot ignited direct injection natural gas engines have been increasingly attractive owing to their advantages in thermal efficiency and emissions. For conventional pilot ignited direct injection natural gas engines, soot emissions have been considered as an urgent issue, which could be possibly resolved by the adoption of homogenous charge direct injection strategy. To prove this viewpoint, this paper experimentally studied the effects of homogenous charge direct injection strategy on the combustion, emissions, thermal efficiency and operation stability of a pilot ignited direct natural gas engine. The results indicated that at low load, the application of homogenous charge direct injection strategies has negative effects on NOx, CO and HC emissions; though soot emissions could be improved, the negative effects seem to be more predominant. At high load, though NOx and HC emissions are impaired with the application of homogenous charge direct injection strategies, CO and soot emissions could be substantially improved with optimized injection parameters. Thermal efficiency could be slightly improved at low load condition and could be greatly raised at high load condition. Additionally, the adoption of homogenous charge direct injection strategies would increase the instabilities of engine operation, however, this behavior is more significant at low load condition.