The study and the evolution of a simple air-assisted injection system for two-cycle, S.I. engines is described.In the first stage of the research the validity of the system was investigated. During this time the system underwent several improvements and modifications.Later, the research was carried on with the following aims: a) to distinguish the spray characteristics by means of a high speed photographic technique; b) to investigate the dependence of such characteristics on some parameters of the injection system, and their influence on engine performance, with special regard to fuel economy and exhaust emissions.Finally, the case of fuel vaporisation at the outlet of the electroinjector was investigated. In fact, vaporisation and ideal atomisation are similar under some aspects and the knowledge of b.s.f.c. and HC emission in the limit case of perfect atomisation would give a precise idea of the improvements obtainable through betterments in spray characteristics.