A mathematical model has been developed which describes the evaporation and trajectories of fuel droplets during their flight in the inlet manifold of port-injected gasoline engines. Based on the model, a computer simulation program was developed, and sample results of this program are given in this paper. Extensive results obtained using the simulation program are described in a companion paper. The simulation program can be used as a tool to improve understanding of mixture preparation in port injected engines. Such effects as: engine load and speed, air and fuel temperature, fuel type, fuel injection velocity, injection timing, and spray droplet size can be investigated.