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A sequential port injection, lean-burn, fully electronically-controlled compressed natural gas (CNG)/Diesel dual-fuel engine has been developed based on a turbo-charged and inter-cooled direct injection (D.I.) diesel engine. During the optimization of engine overall performance, the effects of pilot diesel and pre-mixed CNG/air mixture equivalence ratio on emissions (CO, HC, NOx, soot), knocking, misfire and fuel economy are studied. The rich and lean boundaries of the pre-mixed CNG/air mixture versus engine load are also provided, considering the acceptable values of NOx and THC emissions, respectively. It is interesting to find that there is a critical amount of pilot diesel for each load and speed point, which proved to be the optimum amount of pilot fuel. Any decrease in the amount of pilot diesel from this optimum amount results in an increase of NOx emissions, because the pre-mixed CNG/air mixture must be made richer, otherwise THC emissions would increase.

A numerical simulation was performed to investigate the pilot ignited natural gas combustion process in a direct injection natural gas engine. Various mixture distribution characteristics were compared in terms of the evolution of mixture equivalent ratio distributions and mixture concentration stratifications around top dead center (TDC). Based on above, the pilot injections were specially designed to investigate ignition core formation and its effects on natural gas combustion process. The result shows that pilot ignition sites have great impacts on pilot fuel ignition process and natural gas combustion process. The pilot ignition site on the region with rich NG/Air mixture is disadvantageous to the pilot fuel ignition due to a lack of oxygen, which is not beneficial to ignition core formation.