This report is part of a research program carried out at Ecole Polytechnique de Montreal and aimed at the use of natural gas in automobiles.A computer simulation, modeling the behavior of the conventional equipment (conversion kit) used to convert a gasoline engine to natural gas, was carried out under varying temperature and storage pressure conditions. The results of this study clearly indicated the need for an approach which would compensate for the variations of densities of both air and fuel. The design of an electronically controlled injection system with enough flexibility to allow its use on most engines with only minor modifications was thus undertaken. The ultimate goal of this research is to operate the engine with a closed loop fully auto-adaptative system but the first step in the design was limited to an open loop semi-empirical system that has shown good preliminary test results. The results obtained by simulation of the conventional conversion kit and with the open loop injection system are reported in this paper.