Visualization of diesel spray and combustion from lateral side of two-dimensional piston cavity in rapid compression and expansion machine, second report: Effects of injection pressure and interval of split injection

Abstract

The effect of split injection on the fuel spray and combustion processes in a rapid compression and expansion machine was investigated using the visualization process. A two-dimensional piston cavity, designed with the cross section of a reentrant piston, was installed in the combustion chamber to observe the combustion process from the lateral side. Combustion experiments were conducted with injection pressures of 80 MPa, 120 MPa, and 180 MPa and an O2 concentration of 15%. The spray/wall interaction, mixture distribution, and ignition location were investigated using the shadow method. Along with natural flame luminescence, different spray impinging behaviors on combustion process were studied. Furthermore, the combustion characteristics of in-cylinder pressure, apparent heat release rate, and combustion phase were recorded and analyzed simultaneously. The results showed that both high injection pressure and split injection with a longer interval effectively improved the combustion performance. In addition, when the pilot injection was advanced further, the injection interval had a larger influence in reducing soot generation, while the effect of high injection pressure on heat release decreased. Flame separation was found to occur at high injection pressures. It was observed that the flame separation caused by the strong spray momentum was beneficial for reducing soot generation owing to the greater fuel-air interaction area. The spray and combustion processes were investigated in detail, and the significant effects of different injection pressures and injection intervals on combustion performance with the split injection method were highlighted.

Arrhenius type empirical ignition delay equations based on the phenomenology of in-cylinder conditions for wide operating ranges in modern diesel engines

“…This improvement also suggests the necessity for more detailed description of the factors related to the fuel injection pressure, which affects the microscopic mixture structure in the reaction zone and the ignition process. Moreover, the ignition occurs at the side parts of the fuel spray tip in the actual phenomena [70][71][72][73][74] while the prediction equation in this study assumes that the ignition occurs just at the spray tip to simplify the mechanisms and to limit the calculation load in practical use. The prediction accuracy should be improved with developing a more detailed description of the ignition region, something which is left for the future work.…”

Mechanism of the reduction in afterburning and thermal efficiency improvement with highly oxygenated fuels in diesel combustion

“…There are several explanations for the mechanism of increases in afterburning at high load operation in diesel engines, and visualizations of the spray flame with constant volume vessels, 20–26 the in-cylinder visualization engines, 27–30 and rapid compression and expansion machines (RCEM) 31,32 have been attempted to elucidate the mechanism. Kondo et al suggests that the increase in afterburning is mainly due to the accumulation of rich air-fuel mixture around the tip of the fuel spray, 33 and it is necessary to make the inside of the fuel spray leaner to reduce the afterburning.…”