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High pressure injection of natural gas is being investigated as a mean of fueling diesel engines and meeting increasingly stringent EPA regulations on emissions of nitrogen oxides and particulates. In the work described in this paper, the penetration into air of a sonic jet of methane emerging from a suddenly opened poppet valve has been modelled analytically and measured using flow visualization. The injection pressure ratios were in the range 1.5 to 5 and the conical jet sheet Reynolds numbers were in the range 7000 to 56000. Schlieren photographs revealed that the conical sheet gas jet exhibits an unstable behaviour between the upper and lower plates which simulate the fire deck and the piston. The integral model developed indicates the principal parameters on which the gaseous jet penetration depends and establishes the requirements for scaling. The conical sheet jet penetration is found to be approximately 30% less than that of round holes, given the same flow area.