Abstract: A hot fuel gas generator for an internal combustion engine simultaneously vaporizes gasoline and water in a novel heat exchanger mounted in an exhaust manifold of an internal combustion engine. A control valve meters the amount of a water and gasoline mixture supplied to the heat exchanger which delivers the resulting superheated steam and gaseous fuel to the intake manifold by way of an adapter downstream of the location of the usual carburetor. The control valve is actuated by devices responsive in operation to inlet manifold pressure of the internal combustion engine.