Interaction between Fuel Jets and Prevailing Combustion during Closely-Coupled Injections in an Optical LD Diesel Engine 2019-01-0551
Two imaging techniques identify the interface between developed combustion from earlier injections and unburnt fuel of a subsequent injection. Planar laser induced fluorescence (PLIF) excites a fuel tracer, 3-pentanone, which displays the spatial distribution of the unburnt fuel. High speed imaging captures the natural luminescence 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. The falling edge of the second pilot’s heat release rate (HRR) does not coincide with a fuel injection and is used as the reference case. Here, the decline in the HRR is due to an exhaustion of the fuel source. 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 difference between them is that one has slightly shorter separations than in the other. In both triple-pilot cases, the decline in HRR is not due to exhaustion of the fuel source, but instead coincides with the subsequent fuel injection. The magnitude of the drop in HRR is too large to be accounted for by the latent heat of vaporization. For both closely-coupled strategies, there are two mechanisms responsible for the drop in HRR: Physical displacement of the larger combustion regions away from the center and extinguishing of small scale combustion regions. The larger scale displacement moves the prevailing combustion into areas where more of the oxygen has been used up, slowing combustion. The smaller scale extinguishing is likely due to very high strain rates along the path of the high velocity fuel jet.