A numerical study is carried out of engine in-cylinder air-fuel mixing phenomena using multidimensional computer models. Submodels to describe turbulence and convective wall heat transfer of engine flows and to treat the disintegration processes and vaporization of hollow-cone sprays are developed and validated using available experiments. These models are implemented in the KIVA codes and are applied to diesel and direct-injection spark-ignition (DISI) engines.
A modified RNG k-$\varepsilon$ turbulence model and a temperature wall model are proposed to include the effects of flow density variation. In addition, a sheet spray model is developed for computing sheet atomization and the subsequent drop breakup processes of hollow sprays. The predicted spray characteristics of a gasoline-engine pressure-swirl atomizer such as spray structures, spray tip penetrations, drop Sauter mean diameters and drop size distributions agree well with experimental data.
The present models are first applied to diesel engine combustion simulations. The demonstrated improvements in combustion and emissions predictions of both single and split fuel injection schemes obtained using the present models lead to a study of the mechanisms of soot and NOx reduction using multiple injections. It is shown that the reduced soot formation using multiple injections is due to the fact that the soot-producing fuel-rich regions at the spray tip are not replenished when the injection is terminated and then restarted. With multiple injections, the subsequently injected fuel burns rapidly and does not contribute significantly to soot production.
As the final application of the models, a direct-injection spark-ignition engine is simulated to study the air-fuel mixing processes. The induction and compression processes with early fuel injection are computed in which realistic engine port/chamber geometry, intake-valve motion and pressure-swirl atomized sprays are considered. The in-cylinder gas flow structures, gas/spray interactions and evolution of the liquid and vapor phases of the injected fuel are characterized. The effects of fuel injection timing on gas/spray interactions, turbulence generation, spray/wall impingement, vaporization rate, air-fuel distribution and thermodynamic characteristics of the mixture are studied.