A gasoline homogeneous charged compression ignition (HCCI) called the controlled auto ignition (CAI) engine is an alternative to conventional gasoline engines with higher efficiency and lower emission levels. However, noise and vibration are currently major problems in the CAI engine. The problems result from fast burning speeds during combustion, because in the CAI engine combustion is controlled by auto-ignition rather than the flame.
Thus, the ignition delay of the local mixture has to vary according to the location in the combustion chamber to avoid noise and vibration. For making different ignition delays, stratification of temperature or mixing ratio was tested in this study. In charge stratification, which determines the difference between the start of combustion among charges with different properties, two kinds of mixtures with different properties flow into two intake ports.
In this study, calculations start from the intake valve opening (IVO) to consider flow motion, turbulent, and heat transfer effects. In gasoline engines, inducted gases are mixed due to piston motion, turbulent flow, strong tumble, and weak swirl motion. In order to consider these effects, a full engine mesh, which has valve and piston motions, was used in this study. A Multi-zone method was adopted to calculate the chemical reaction in the full engine mesh to save computational time. A reduced chemical kinetic mechanism, which has 47 species and 67 reactions, was used with simplified assumptions. A gasoline surrogate, composed of iso-octane, n-heptane, and toluene, was used to describe the gasoline fuel.
By non-symmetric fuel injection, fuel stratification was accomplished. Stratified cases show lower maximum pressure and smaller pressure rising rates than those of the homogeneous case. And fuel stratification was helpful to extend the combustion duration.