2006 年 72 巻 716 号 p. 1037-1043
Experiment and numerical simulation indicated that the large NOx formation in diesel engine is due to the weak mixing intensity in the spray tip region, where the flow and turbulence structure is quite different from the continuous jet flames. The fact indicates that there is a possibility of reducing NOx, from diesel engines by enhancing mixing intensity at the spray tip region to the level of continuous jet flame. As one of the attempts to make the velocity profile of diesel spray similar to the steady jet, an inert gas was injected prior to the fuel injection. With the pre-injection of inert gases, the flame apparently became less luminous, and the NOx emission index decreased to two-thirds of the non pre-injection case. Numerical simulation showed that NO decreases with the increase of the pre-injection period, although the maximum temperatures are almost constant in all cases. This suggests that the NOx reduction is not due to the temperature difference but due to the enhanced mixing and decreased time at high temperature by the pre-injection. The report presents the above results together with photographic analysis of enhanced mixing of spray tip region by pre-injection of water.