A system is described for reducing and controlling certain undesired byproducts from a combustion process in an engine by forcing supplemental air, preferably preheated, into the intake manifold of the engine at a point downstream from a conventional carburetor device. In addition, liquid vapor may be introduced into the intake manifold at approximately the same point. Preferably, a blower means for delivering supplemental air and a pumping means for pumping liquid are both operated by a single variable speed electric motor so that the functions of both devices can be related to changes in engine demands during operation.