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The main purpose of this study is to reveal the mechanism of stratified- mixture formation in gasoline direct-injection engines. So far the authors have developed a computer code "GTT" for numerically simulating the fuel spray behavior in fuel injection engines, and have proposed the physical models for droplet breakup, spray impingement and liquid film formation on a wall, and evaporation of a droplet and liquid film, which have been applied mainly to the sprays injected from hole nozzles for diesel engines. In this study, in order to numerically simulate the hollow-cone sprays injected from a swirl injector for gasoline direct-injection engines, a physical model for hollow-cone sprays has been proposed. The injection boundary condition and the model coefficients for the droplet breakup model (improved wave breakup model) have been determined appropriately.

The mixture formation process in a premixed compression ignition engine was numerically analyzed. This study aimed to find out effective injection conditions for lean mixture formation with high homogeneity, since the NOx and soot emissions in the engine are closely related to the mixture homogeneity. To calculate fuel spray behavior, a practical computer code GTT (Generalized Tank and Tube) was employed. In a model for the premixed compression ignition engine, the effects of injection parameters, such as injection timing, initial droplet size, spray angle, injection velocity, nozzle type (pintle and hole) and injection position / direction, on the mixture homogeneity near ignition timing (or TDC) were investigated. To evaluate the homogeneity of the mixture, an index was defined based on the spatial distribution of fuel mass fraction. The fuel vapor mass fractions as well as the homogeneity indices, obtained as a function of time, were compared under various boundary conditions.

To find more effective lean mixture preparation methods for smokeless and low NOx combustion, a numerical study of the effects of in-cylinder flow field before injection on mixture formation in a premixed compression ignition engine was conducted. Premixed compression ignition combustion is a very attractive method to reduce both NOx and soot emissions, but it still has some problems, such as high HC and CO emissions. In case of early direct injection, it is important to avoid wall wetting by spray impingement, which can cause higher HC and CO emissions. Since it is not easy to examine the effects of initial flow and injection parameters on mixture formation over the wide range by practical engine tests, a computer program named “GTT (Generalized Tank and Tube)” code was used to simulate the in-cylinder phenomena before autoignition.