Non-conventional operating conditions and fuels in diesel engines can produce longer ignition delays compared to conventional diesel combustion. If those extended delays are longer than the injection duration, the ignition and combustion progress can be significantly influenced by the transient following the end of injection (EOI), and especially by the modification of the mixture field. The objective of this paper is to assess how those long ignition delays, obtained by injecting at low in-cylinder temperatures (e.g., 760-800K), are affected by EOI. Two multi-hole diesel fuel injectors with either six 0.20mm orifices or seven 0.14mm orifices have been used in a 2.34L single-cylinder optical diesel engine. We consider a range of ambient top dead center (TDC) temperatures at the start of injection from 760-1000K as well as a range of injection durations from 0.5ms to 3.1ms. Ignition delays are computed through the analysis of both cylinder pressure and chemiluminescence imaging. A simplified one-dimensional (1-d) model of the diesel jet, able to match the behavior of a transient injection and entrainment processes, is used to estimate the ensemble-averaged mixture fraction fields during the injection event and at the ignition kernel locations.
At TDC temperatures of 850K or higher, the injection duration is longer than ignition delay, and thus EOI has no effect on ignition delay. At TDC temperatures of 800K or lower, for short injection durations (<1.3ms), ignition occurs after EOI and ignition delay decreases with decreasing injection duration is observed. In addition, the 1-d spray model predicts a decrease of the mixture fraction at ignition kernels with decreasing ignition delay. This is in contrast to the expected trend of increasing kinetic time with decreasing mixture fraction for well-mixed reactors. This suggests that mixture fraction alone is not the first-order parameter influencing the timing and position of ignition sites. The history of the ignition kernel(s) and/or of the scalar dissipation may also need to be considered.
