Simulations of In-Cylinder Processes in a Diesel Engine Operated with Post-Injections Using an Extended CMC Model 2014-01-2571

In this study, numerical simulations of in-cylinder processes associated to fuel post-injection in a diesel engine operated at Low Temperature Combustion (LTC) have been performed. An extended Conditional Moment Closure (CMC) model capable of accounting for an arbitrary number of subsequent injections has been employed: instead of a three-feed system, the problem has been described as a sequential two-feed system, using the total mixture fraction as the conditioning scalar. A reduced n-heptane chemical mechanism coupled with a two-equation soot model is employed.

Numerical results have been validated with measurements from the optically accessible heavy-duty diesel engine installed at Sandia National Laboratories by comparing apparent heat release rate (AHRR) and in-cylinder soot mass evolutions for three different start of main injection, and a wide range of post injection dwell times. Good agreement with the experimental results is reported for the AHRR, while qualitative reproduction of in-cylinder soot mass evolutions have been obtained, the computed soot mass is considerably underestimated. Subsequently, numerical investigations concerning the effects of different post injection timings on soot formation and oxidation processes are presented, with particular emphasis on the role of the increased mixing by post injections. The simulation results revealed two main competing phenomena which govern the evolution of the in-cylinder soot mass during and after post-injections: I) the accelerated oxidation of the soot from the main injection, and II) the dependency of soot formed during post-injection on the in-cylinder temperatures. Overall, the findings suggest that the extended CMC framework is a promising candidate for the simulation of multiple injections in diesel engines, allowing for deeper understandings of the associated in-cylinder processes.