Effect of Load on Close Coupled Post Injection Efficacy for Soot Reduction in an Optical Heavy Duty Diesel Research Engine

Abstract

The use of closecoupled post injections is an incylinder sootreduction technique that has much promise for high efficiency heavyduty diesel engines. Closecoupled post injections, short injections of fuel that occur soon after the end of the main fuel injection, have been known to reduce engineout soot at a wide range of engine operating conditions, including variations in injection timing, exhaust gas recirculation (EGR) level, load, boost, and speed. While many studies have investigated the performance of post injections, the details of the mechanism by which soot is reduced remains unclear. In this study, we have measured the efficacy of post injections over a range of load conditions, at constant speed, boost, and rail pressure, in a heavyduty opticallyaccessible research diesel engine. Here, the base load is varied by changing the maininjection duration. Measurements of engineout soot indicate that not only does the efficacy of a post injection decrease at higher engine loads, but that the range of postinjection durations over which soot reduction is achievable is limited at higher loads. Optical measurements, including the natural luminescence of soot and planar laserinduced incandescence of soot, provide information about the spatiotemporal development of incylinder soot through the cycle in cases with and without postinjections. The optical results indicate that the post injection behaves similarly at different loads, but that its relative efficacy decreases due to the increase in soot resulting from longer maininjection durations.