The performance of gasoline direct injection (GDI) engine is simulated using the AVL-FIRE code. The effects of four piston shapes on mixture formation and combustion are compared and analyzed. For GDI engines with forward tumble and multihole high-pressure injector, a piston with flat top surface and pit is beneficial for the maintenance of in-cylinder turbulence intensity. Such structure aids in the formation of a combustible mixture around the spark plug at the time of ignition. A piston with a smoother top surface strengthens the turbulence kinetic energy at the time of ignition, thereby accelerating combustion and increasing in-cylinder temperature and pressure. The higher in-cylinder temperature and pressure promote NO emission, whereas soot emission decrease because of subsequent oxidation. (c) 2015 Elsevier Ltd. All rights reserved.