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Homogeneously charged natural gas was burned in a diesel engine assisted by a small amount of gas oil injection. The effects of the equivalence ratio of natural gas, the intake temperature and the EGR rate on ignition of gas oil, the burning rate of natural gas and the knock limit were clarified experimentally. As results, a marked improvement in the trade-off between smoke and NOx was obtained maintaining a high thermal efficiency by a combination between the natural gas charge rate, the EGR rate, the intake preheating and the nozzle orifice size suitable at each engine load.

In order to improve the trade-off between smoke and NOx in a DI diesel engine, the effect of premixed natural gas on combustion and exhaust emission has been investigated experimentally. The effects of the CNG charging rate and the EGR rate on ignition and combustion rate are mainly examined, in addition, the effects of CNG and EGR on NOx reduction was analyzed from the viewpoint of water content in the EGR gas. As a result, it is found that (1) the burning rate of natural gas pre-mixture is larger under the higher temperature condition, thus, the larger burning rate results in shortening the combustion duration and leads to lower fuel consumption, (2) ignition and the burning rate of natural gas are suppressed by EGR, (3) significant NOx reduction is obtained by charging natural gas under the low load condition, and (4) the NOx reduction rate due to EGR is about twice of the one due to the port water injection.