Computational Study of Soot Entrainment via Thermophoretic Deposition and Crevice Flow in a Diesel Engine

Abstract

Soot deposition on cylinder liner and the entrainment into engine oil have been correlated to oil starvation and damage of the engine piston. In this computational work, different injection strategies are implemented to investigate their respective effects on spatial evolution of combustion soot and the soot entrainment process in a light-duty diesel engine, taking into account the thermophoretic soot deposition on the cylinder liner as well as entrance of soot into the crevice region. Numerical computation of diesel combustion is undertaken by means of linking a plug-in chemistry solver namely, CHEMKIN-CFD into ANSYS FLUENT 12, a commercial Computational Fluid Dynamics (CFD) software. A chemical reaction mechanism of n-heptane combined with soot precursor formation mechanism is employed as the diesel surrogate fuel model. Here, turbulence-chemistry interaction is represented by the Eddy-Dissipation Concept (EDC) model. The inclusion of top land volume in the computation mesh and implementation of crevice model in the CFD solver allow inspections of soot entrainment. Soot mass in the top land volume obtained from the simulation is used to represent soot mass transport into crevice via blowby. By assuming thermophoresis as the primary mechanism of soot deposition on the liner, the mass of soot deposited is obtained through calculation of thermophoretic deposition velocity. During the closed cycle combustion process, the mass of soot deposited on liner via thermophoresis is more dominant than those entrained into crevice region through blowby. Variation of start of injection (SOI) does not have considerable effect on the amount of soot entrainment. On the other hand, the amount of soot entrainment into engine oil is not favourable when the split-main injection is employed with late SOI, although the total soot formed during the combustion is reduced. While thermophoresis mechanism relies on the temperature gradient at the wall, the soot concentration near the wall and its temporal evolution are deduced to have a more significant effect on the deposited soot mass. Variation of the injection strategy is found to affect the soot entrainment process by influencing the in-cylinder gas motion, the location of combustion and the evolution of soot cloud. This simulation study provides an insight to the effects of injection parameters on key in-cylinder, which can assist in engine design for minimising soot deposition in the future.