A Study of Combustion Structure and Implications on Post-Oxidation Under Homogeneous and Stratified Operation in a DISI Engine

An experimental investigation into the structure and flame propagation characteristics of stratified and homogeneous combustion has been performed in an optically-accessible, direct-injection spark ignition (DISI) engine using OH planar laser-induced fluorescence (PLIF) imaging. Homogeneous and stratified operation was achieved by employing either early or late injection timing strategies during the intake or compression stroke respectively.

Planar LIF OH images obtained revealed that for stratified operation, the 3D structure of the combustion zone is highly inhomogeneous and is predominantly due to high fuel concentration gradients which are formed as a result of local fuel mixture stratification. The images reveal a combustion structure which suggests that the flame propagation pathway is ultimately determined by the presence of these local fuel mixture inhomogeneities. In particular, the flame front and the burned gases appear to enclose the fresh/unburned gases which are locally either too lean or too rich to maintain flame propagation. The sensitivity to temporal and spatial local mixture stratification is also believed to be responsible for the significant cycle-to-cycle variations in terms of the early flame development just after ignition. Under homogeneous operation, the absence of fuel mixture inhomogeneities appears to reduce these cycle-to-cycle variations. Furthermore, LIF OH images reveal a completely different combustion structure under homogeneous operation. Following spark ignition, the flame propagates towards the combustion chamber walls, systematically consuming the fresh/unburned gases. The distinct gradient between the fresh/unburned gas and the reaction zone/burned gases clearly reveal the location of the flame front. Following the primary combustion process, significant post oxidation occurs in which it is believed that any remaining unburned fuel mixes with the hot, burned gases, forming distinct structures of high OH concentration which leads to secondary combustion during the expansion stroke. It is believed that the fuel which burns during this apparently slow oxidation phase originates from two possible sources. Under homogeneous operation, unburned fuel originates from the crevice volumes, drawn out at the start of the expansion stroke. Under stratified operation however, it is believed that any remaining unburned fuel arises from the locally fuel-lean or fuel-rich (i.e. quenched) zones which escape combustion and subsequently mix with the hot burned gases resulting in post oxidation later during the expansion stroke.