Effect of the diesel injection timing and the pilot quantity on the combustion characteristics and the fine-particle emissions in a micro-diesel pilot-ignited natural-gas engine

Natural gas is a potential alternative to conventional liquid fuels for utilization in automotive internal combustion engines. Its utilization can significantly reduce the levels of nitrogen oxide and particulate matter emissions from dual-fuel diesel/natural-gas engines. However, this reduction in the particulate matter emissions may induce an increase in the quantity of particles of smaller size, which are harmful to human health. The engine parameters, namely the diesel injection timing and the pilot quantity directly, affect the homogeneity of the fuel–air mixture, the spatial distribution of initial ignition centres and the quantity of initial ignition centres. In this paper, the effects of these two engine parameters on the combustion and the particle emission characteristics of micro-diesel pilot-ignited natural-gas fuel are investigated. The combustion parameters were calculated from the cylinder pressure data. An electrical low-pressure impactor was employed to measure the particle number concentration and particle size distribution. Experimental results indicate that the ignition delay period is prolonged and the particle number concentration progressively increases with advance of the injection timing, while the particle mass concentration undergoes a steady decrease and then a rapid increase. On the other hand, increasing the pilot diesel quantity can shorten the ignition delay period and enhance the burning rate. Meanwhile, the particle mass concentration increases evidently but the particle number concentration decreases unexpectedly. It should be noted that the total number of fine particles is sensitive to advancing injection and decreasing pilot mass, especially above the 20° crank angle before top dead centre injection timing and below the basic pilot mass.