Numerical Study of Fuel/Air Mixture Preparation in a GDI Engine

Numerical simulations are performed to investigate the fuel/air mixing preparation in a gasoline direct injection (GDI) engine. A two-valve OHV engine with wedge combustion chamber is investigated since automobiles equipped with this type of engine are readily available in the U.S. market. Modifying and retrofitting these engines for GDI operation could become a viable scenario for some engine manufactures. A pressure-swirl injector and wide spacing injection layout are adapted to enhance mixture preparation. The primary interest is on preparing the mixture with adequate equivalence ratio at the spark plug under a wide range of engine operating conditions. Two different engine operating conditions are investigated with respect to engine speed and load. A modified version of the KIVA-3V multi-dimensional CFD code is used. The modified code includes the Linearized Instability Sheet Atomization (LISA) model to simulate the development of the hollow cone spray. The model includes spray collision, coalescence, breakup and drop drag. Mixing for this engine is difficult due to the non-symmetric engine geometry and complicated flow structures in the cylinder. The results reveal that knowledge of the optimal timing interval between start of injection and ignition is important. Also, a flat piston and a radically designed piston with bowl cavity are compared in order to gain additional understanding of the in-cylinder air motion that affects fuel preparation.