Combustion performance and stability of a dual-fuel diesel-natural-gas engine

Experimental investigations on the combustion performance and the stability of a dual-fuel engine fuelled with diesel and natural gas were conducted. The effects of the pilot diesel quantity and the pilot diesel injection timing were analysed. The results show that the maximum (peak) in-cylinder pressure increases and appears earlier with increasing pilot diesel quantity. The heat release rate has two peaks during the process of combustion. Earlier pilot diesel injection resulted in an earlier pressure evolution and a higher peak in-cylinder pressure. Correspondingly, the ignition of the pilot diesel was advanced and the heat release rate of diesel ignition increases. Earlier diesel injection timing increases the diesel combustion heat release, and the combustion of the whole charge in cylinder is more drastic. The pilot diesel quantities have different effects on the cycle-by-cycle variations (i.e. the coefficient of variation). At a low load, both very high and very low pilot diesel quantities lead to a high coefficient of variation in the indicated mean effective pressure, and the effect becomes weaker with increasing load. The coefficient of variation in the indicated mean effective pressure decreases and the coefficient of variation in the the peak in-cylinder pressure increases with increasing load. Additionally, the coefficient of variation in the indicated mean effective pressure decreases as the pilot diesel injection was advanced. The largest value of the coefficient of variation in the peak in-cylinder pressure occurs at 20 degrees crank angle before top dead centre. The maximum rate of pressure rise increases dramatically with earlier pilot diesel injection timing. Nitrogen oxide emissions increase and hydrocarbon emissions decrease with increasing pilot diesel quantity. The hydrocarbon emissions decrease at earlier diesel injection timing; however, the nitrogen oxide emissions increase.