Interaction between Fuel Jets and Prevailing Combustion During Closely-Coupled Injections in an Optical LD Diesel Engine

Two imaging techniques are used to investigate the interaction between developed combustion from earlier injections and partially oxidized fuel (POF) of a subsequent injection. The latter is visualized by using planar laser induced fluorescence (PLIF) of formaldehyde and poly-cyclic aromatic hydrocarbons. High speed imaging captures the natural luminescence (NL) of the prevailing combustion. Three different fuel injection strategies are studied. One strategy consists of two pilot injections, with modest separations after each, followed by single main and post injections. Both of the other two strategies have three pilots followed by single main and post injections. The separations after the second and third pilots are several times shorter than in the reference case (making them closely-coupled). The closely-coupled cases have more linear heat release rates (HRR) which lead to much lower combustion noise levels. For all cases, POF is detected during the very weak HRR before the notable combustion of the first pilot injection’s fuel. When the subsequent fuel injections overlap with a local decrease in HRR, the prevailing combustion is to some degree extinguished during these phases. This is seen via the NL signal being replaced by the POF signal, indicating that hotter combustion products are being replaced by cooler ones. When the NL and POF regions spatially overlap, the POF signal decreases, and there is no injection, this marks the further oxidation of the POF into hotter combustion products. This extinguishing phenomenon can, to a lesser extent than closely-coupling, affect the combustion noise by affecting the magnitude of the local dips in the HRR, which via its influence on the pressure trace, affects combustion noise.