A CFD Study of Post Injection Influences on Soot Formation and Oxidation under Diesel-Like Operating Conditions

One in-cylinder strategy for reducing soot emissions from diesel engines while maintaining fuel efficiency is the use of close-coupled post injections, which are small fuel injections that follow the main fuel injection after a short delay. While the in-cylinder mechanisms of diesel combustion with single injections have been studied extensively and are relatively well understood, the in-cylinder mechanisms affecting the performance and efficacy of post injections have not been clearly established. Here, experiments from a single-cylinder heavy-duty optical research engine incorporating close-coupled post injections are modeled with three dimensional (3D) computational fluid dynamics (CFD) simulations. The overall goal is to complement experimental findings with CFD results to gain more insight into the relationship between post-injections and soot. This paper documents the first stage of CFD results for simulating and analyzing the experimental conditions. In this stage, an engineering CFD model with a two-stage soot sub-model facilitates development of new and appropriate analysis methods. The methods include new ways to visualize and quantify soot formed, soot oxidized and net soot. Parameters used to evaluate formation and oxidation, like fuel and oxygen concentrations, are also visualized and quantified to provide a deeper understanding of the in-cylinder evolution of soot. Experiments found and CFD replicated a trend where engine-out soot first decreases, then increases with increasing postinjection duration when both the main injection duration and dwell between injections are held constant. To help understand this trend, a number of factors that influence soot formation and oxidation are analyzed, including changes in temperature, pressure, oxygen, fuel vapor and soot distributions. Fuel vapor distribution and burn rate variation appear to be dominant factors in determining whether soot increases or decreases with post injections. The prime conclusion regarding the in-cylinder mechanism of soot reduction by post injections is that the simulations predict that short post injections increase the rate of fuel burning, thereby reducing the soot precursor species (vapor fuel) concentration, leading to lower soot formation. The model does not predict any appreciable increase in soot oxidation with a short post injection. Indeed, late in the cycle, soot oxidation with a short post injection is slower than with only the main injection because less oxygen is available for soot oxidation after combustion of the larger injected fuel mass and because there is less soot to oxidize.