Characterization of Mixture Formation in a Direct Injected Spark Ignition Engine 2001-01-1909

We have performed simulations and experiments to characterize the mixture formation in spray-guided direct injected spark ignition (DISI) gasoline engines and to help to understand features of the combustion process, which are characteristic for this engine concept. The 3-D computations are based on the KIVA 3 code, in which basic submodels of spray processes have been systematically modified at ETH during the last years. In this study, the break-up model for the hollow-cone spray typical for DISI engines has been validated through an extended comparison with both shadowgraphs and Mie-scattering results in a high-pressure-high-temperature, constant volume combustion cell at ambient conditions relevant for DISI operation, with and without significant droplet evaporation. Computational results in a single-cylinder research engine have been then obtained at a given engine speed for varying load (fuel mass per stroke), swirl and fuel injection pressure. In parallel, a new endoscopic technique has been developed and implemented in the engine to provide optical information on the liquid phase distribution in the combustion chamber at different times after start of injection. Quite a good, at least qualitative, agreement has thereby been observed between computed and measured results with regard to liquid fuel penetration in the engine for all operating conditions investigated. A detailed comparison between simulation and experiments has revealed several of the underlying causes for the characteristic dependence of engine performance on the varying operating conditions. Among them the spray-induced turbulence intensity, the fuel evaporation rate, the degree of charge stratification, both in the vicinity of the spark-plug and globally in the combustion chamber at different times after start of injection (SOI), as well as their interplaying with the air flow pattern have been identified as the most relevant parameters for this type of DISI engine combustion. The experimentally validated hollow cone spray break-up model can be now used to explore other system configurations for further optimization of the DISI mixture formation and combustion.