The goal of tills study was to investigate the effects of injector characteristics and injection timing on fuel mass delivery to the valve in a fired spark ignition engine. The relationship between injection timing and fuel arrival at the intake valve must be known for designing an injection system for increased liquid entrainment into the gas phase and minimized fuel wetting of the port walls. Fuel spray behavior from a conventional injector design and prototype vacuum-assist and air-assist injector designs is investigated by measuring fuel drop size and velocity at the intake valve using phase Doppler interferometry. The results of this study indicate the amount of time required for the fuel spray to reach the intake valve is constant and independent of injection timing. However, the width of the temporal fuel spray arrival distribution is a strong function of injection timing. injection timing and the corresponding gas-phase dynamics have the strongest impact on the smallest drops, resulting in a redistribution of tile drops with respect to size. In addition, the fuel spray behavior was investigated at the start of tile intake valve event when hot cylinder gases flow into the intake port. The arithmetic mean diameter of the fuel film atomized off the back of the intake valve during this period was found to range from 55 to 65 mu m.