3-D Modeling of Conventional and HCCI Combustion Diesel Engines

Operation of a Diesel engine in HCCI combustion mode is typically restricted to moderate engine loads. For practical applications, an engine switching from HCCI to conventional Diesel combustion mode at higher loads appears to be the most plausible solution. In this light, modeling applicable to both combustion modes is favourable. The performance of Diesel engines is affected by chemical kinetics of fuel oxidation through correct prediction of auto-ignition, especially at complicated injection scenarios such as those for HCCI. Thus, the modeling exploits the advantage of the detailed chemistry approach which incorporates a generalized partially stirred reactor, (PaSR), model coupled with a surrogate diesel fuel oxidation mechanism as a substitute of real hydrocarbon fuel. The surrogate Diesel fuel is assumed to be a 70/30 % mixture of n-heptane and toluene, consisting of 68 species participating in 279 reactions. PaSR assumes that chemical processes proceed in two successive steps: the reaction act follows after the micro-mixing simulated on a sub-grid scale. Consistently evaluated time scales for micro-mixing and limiting reaction rates gives a combustion model in a "well-closed" form incorporated into the KIVA-3V code. The results of the model were verified by experimental results obtained from a direct injection Diesel engine operated in conventional and HCCI combustion mode. The engine is a single cylinder version of a passenger car type common rail engine with a displacement of 480 cc. Split injection was used for conventional Diesel operation. For HCCI operation, fuel was injected in multiple steps during the compression stroke creating a homogeneous mixture avoiding spray interaction with the cylinder liner and piston. As a measure to properly phase the combustion process in the engine cycle and to prevent knock, a combination of a reduced geometric compression ratio and high rates of cooled EGR was investigated.