This paper investigates the performance, exhaust emissions, and combustion characteristics of a dual fuel diesel engine fueled by CNG (compressed natural gas) as the main fuel. The experiments used standard ignition fuels prepared by n-hexadecane and heptamethylnonane which are used to define the ignitability of diesel combustion, and focused on the effects of fuels with better ignitability than ordinary gas oil such as fuels with higher cetane numbers, 70 and 100. Compared with gas oil ignition, a standard ignition fuel with C.N. 100 showed shorter ignition delays, and lower NOx exhaust concentrations, and engine noise. The results also showed that regardless of ignition fuel, misfiring occurred when the CNG supply was above 75%. While the CNG ratio where misfiring occurs lowered somewhat with increasing C.N., the combustion stability (defined as the standard deviation in the cycle to cycle variation of IMEP divided by the mean value of IMEP) was little influenced. In summary, the results show that the influence of the ignitability on the engine performance and emission characteristics of the dual fuel operation is relatively small when the ignition fuel has C.N., and similar to or higher than ordinary gas oil.

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