Abstract: Control apparatus is described for metering a liquid additive to be vaporized and introduced to the fuel mixture for an internal combustion engine. Improved enhancement of the combustion process is obtained by supplying the additive in accurate and reliable dependence upon the condition of operation of the engine, as represented by the pressure in the intake manifold. Such control is made possible by a pressure responsive valve having a valve member movable with low friction along the length of a narrow valve orifice of uniform width. The resulting flow is then linearly related to the movement of the valve member, and provides sensitive and accurate control even at extremely low flow rates. A complete system for combustion enhancement employing such a valve and associated control features is described. Independently adjustable springs at opposite ends of the valve member provide separate adjustment of the flow for engine operation at high and low power conditions.