State of the art high-pressure fuel injectors offer the ability to inject multiple times per cycle, and can reach very low fuel amounts per injection event. This behaviour allows the application of pilot injections in diesel engine applications or dual fuel engines. In both diesel and dual fuel engines, the amount of pilot fuel affects the engine efficiency. The understanding of the underlying ignition mechanism of the pilot fuel is required to optimize injection parameters and the engines’ fuel consumption.
The present work focuses on the differences of ignition mechanisms between long and short injections. The investigation has been performed numerically, using CFD with a well-proven combustion model. The setup used employs a well characterized single orifice injector, injecting into a high temperature, pressurized environment with a composition of 15% oxygen. The duration of injection (DOI) has been gradually decreased form 1.5 ms to 0.3 ms, while a square mass flow rate profile has been employed.
The analysis distinguishes between long, medium and short DOIs: the reference ignition delay is defined by the long DOIs, for which constant ignition delays are found. Shortening the injection duration leads to a decrease in ignition delay for medium DOIs (accelerated case), while for short DOIs an ignition delay trend reversal is observed (prolonged case). The ignition location is close to the spray tip for the long and the short cases, while it moves towards the spray tail for the medium DOIs. The ignition delays of the medium DOIs are shorter due to enhanced mixing after end of injection. The relatively larger areas of leaner mixture increase the temperature and in particular the contribution of the low temperature reactions, which promotes ignition. In the short DOIs, the ignition delay is longer due to overmixing, which reduces the mixture reactivity and prolongs the ignition delay. The ignition moves towards the most fuel rich location in the spray, which lies close to the spray tip at a certain time after the end of injection.