Effects of pre-injection on ignition, combustion and emissions of spray under engine-like conditions

Abstract

Large-eddy simulation (LES) of multiple-injection spray combustion under engine-like conditions is performed using a coupled model of an Eulerian stochastic field transported probability density function (ESF T-PDF) and a flamelet generated manifold (FGM). This coupled model allows the use of the T-PDF methods in modeling the interaction of turbulence and chemistry at affordable computational costs for engine applications. Simulation results are compared with the available experimental data for spray flames with multiple-injection and at a high level of exhaust gas recirculation (EGR) under conditions relevant to internal combustion engines. It is shown that the T-PDF/FGM LES model is capable of reproducing not only global combustion characteristics, such as the pressure rise and ignition delay time but also replicating the evolution of liftoff length and the spray flame structure.The effects of pre-injection strategies are then investigated, by systematically varying the pre-injection duration and the dwell time between two injections, while keeping the total injected fuel mass constant. The LES results reveal different mechanisms by which a pre-injection can change the ignition delay time, the combustion mode and the emissions in spray flames, depending on the injection timing. It is shown that even an extremely short non-igniting pre-injection can substantially change the ignition and emissions characteristics of the main-injection. It is shown that the combustion mode of a single-injection can be altered by splitting the injection to pre- and main-injections. The current study also demonstrates that decreasing the dwell time, within the range that is examined here, at a given pre-injection, will potentially increase the soot oxidation rates while it does not significantly change the rate of soot formation.