Fast Burning and Reduced Soot Formation via Ultra-High Pressure Diesel Fuel Injection 910225

The relation between the characteristics of a non-evaporating spray and those of a corresponding frame achieved in a rapid compression machine was investigated experimentally. The fuel injection pressure was changed in a range of 55 to 260 MPa and the other injection parameters such as orifice diameter and injection duration were changed systematically. The characteristics of the non-evaporating spray such as the Sauter mean diameter and the mean excess air ratio of the spray were measured by an image analysis technique. The time required for a pressure rise due to combustion was taken as an index to characterize the flame.

It was concluded that the mean excess air ratio of a spray is the major factor which controls the burning rate and that the high injection pressure is effective in shortening the combustion duration and reducing soot formation.

HIGH PRESSURE INJECTION seems to be advantageous in improving both engine efficiency and smoking tendency(1, 2, 3, 4 and 5), but the effect of high pressure injection on the mechanism of the spray and soot emission still remains unclear. Kato et al.(1) studied the effect of ultra-high injection pressures on combustion of a diesel engine with injection pressures of 100 to 220 MPa. They reported that ultra-high injection pressure such as 220 MPa results in a 80 % reduction of particulate emission at the same NOx level as compared to a conventional jerk pump system. Ohtani et al.(2) showed that when increasing the injection pressure from 50 to 150 MPa dry soot emission was decreased but SOF emission increased, and consequently total particulate emission did not decrease under the same NOx conditions. Ikegami et al.(3) reported that pollutants were reduced with increasing injection pressure up to 100 MPa when the orifice diameter was reduced according to the increase in injection pressure to keep the spray penetration constant. The trends in emissions resulting from the increased injection pressure reported in these studies partly contradict each other. This is because the effects of the combustion chamber geometry and the in-cylinder air motion on combustion differs depending on the increased injection pressure. For this reason it seems necessary to perform fundamental studies of the effects of injection pressure on combustion in a quiescent atmosphere.

In a previous study by Kamimoto et al.(4), the characteristics of the spray and flame in a quiescent atmosphere were investigated with injection pressures from 30 MPa to 110 MPa, where both injection amount and orifice diameter were held constant. It became clear that the increase in injection pressure improves the atomization of non-evaporating sprays and reduces soot concentration in the flame. However, the relation between the characteristics of a spray and those of a flame is still unclear.

This paper is the second report of the previous paper given above, and focuses on the effect of increased injection pressure on characteristics of the spray and flame using the same approaches as those in the previous paper. The spray and flame in a quiescent atmosphere were achieved in a rapid compression machine in a wider injection pressure range including a pressure as high as 250 MPa. In the previous study injection amount and orifice diameter both remained constant, and then the injection duration was naturally shortened when the injection pressure was raised. However, when one tunes the combustion in a practical engine under development, one would generally try to fix the injection duration for a given injection amount when specifications in the injection system are altered. This is a result of the compromise between NOx and particulate emissions. From this point of view the authors carried out experiments taking into account this condition. Namely the orifice diameter was changed when the injection pressure was changed so that the injection duration could remain constant.