Investigation of two-stage fuel injection combustion insensity and soot formation under diesel engine conditions

The use of pilot injection has shown significant benefits in the reduction of combustion emissions. However, there has been challenge in understanding the soot and NOx formation mechanism due to the complication of secondary flow-induced air-fuel mixture formation and subsequent combustion mechanism under multi-stage injection. Several injection parameters such as injection timing, injection prssure, interval between injections, and injection quality can affect directly emission formation processe. The goal of this paper is to advance an understanding of the effect of pilot and post injection on the combustion intensity and subsequently soot formation of n-heptane, which has been widely known as diesel surrogate. Experiment was performed by changing the interval time between the pilot and main injections under conditions of 23 kg/ m3 density with 0% O2 (non-combusting) and 15% O2 (combusting). Hydrid shadowgraph and Mie scattering imaging technique in a nearly silultaneous fashion mode along the same line of a sight was used to visualize the spray. Pilot/main spray flame properties such as ignition delay, and ignition location were characterized based on the injection interval. It was shown that the pilot+main injection reduced ignition timing while remaining the combustion duration comparing to single injection.