Abstract: A fluid injection system for an internal combustion engine such as a spark-ignition engine in which an injection nozzle injects a finely divided spray of fluid, such as water or a water solution, into the engine in response to a flow of atomizing air. The nozzle is connected to a fluid supply reservoir and to an air compressor for supplying air to the nozzle at a rate in direct proportion to engine speed to induce the flow of fluid through the nozzle. A pressure responsive flow control device is provided that is responsive to engine load, as manifested by the pressure in the intake manifold for controlling the flow of air to the nozzle, and therefore the discharge of fluid from the nozzle. The pressure of air supplied by the compressor to the nozzle is varied in response to variations of the pressure in the exhaust manifold of the engine. As a result, the flow of atomizing air to the nozzle, and therefore the rate of fluid injection, is varied in response to variations in engine speed and in engine load.