Abstract
In diesel combustion it is commonly known that NO_X emissions increase when the fuel injection velocity increases. On the other hand, increased fuel velocity reduces NO_X in steady jet flames, due to a decreased residence time in the flame region. To answer this contradiction, the authors have made variety of experiment and numerical simulation. The results indicated that the large NO_X 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 NO_X 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 is injected prior to the fuel injection. Figure 1 shows an experimental result comparing the flame appearance and NO_X emission index between the cases with and without a nitrogen injection prior to the propane injection. The flame apparently becomes less luminous by the pre-injection of nitrogen, and the NO_X emission index is two-thirds of the non pre-injection case. Figure 2 is the result of numerical simulation, showing the effect of pre-injection period and types of gases for the reduction of NO_X. It is seen 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 NO_X reduction is not due to the temperature difference but due to the enhanced mixing and decreased residence time at high temperature by the pre-injection. The paper will present the above results together with photographic analysis of enhanced mixing of spray tip region by pre-injection of water.
