The characteristics of the development process of an unsteady high-speed gaseous jet were investigated by means of an instantaneous schlieren method. Methane and helium were used as the test gas to examine the effect of molecular weight. The methane is the major component of natural gas, and the helium is the closest gas to hydrogen, where both natural gas and hydrogen would be the future fuels. To measure the jet penetration and the angle, instantaneous schlieren images of the gaseous jet were obtained by using a CCD camera with an image intensifire. It was shown that the jet angle decreases with time. The jet penetration could be predicted by a modified momentum theory considering the jet configuration, the change in the jet angle and the transient characteristics of the injector value. The behavior of the jet development of these gases in the early stage of the injection differed from that of a diesel spray which is a two-phase jet. The jet penetration was not affected by the injection pressure.
