Abstract: A method and apparatus for reducing the harmful products of combustion comprises an enclosed vessel adapted to contain a quantity of volatile liquid fuel and a means for extracting gaseous vapors from the fuel and for mixing the vapors with the combustion supporting gas for combustion in an internal combustion engine. A thermal reactor is included into which the combustion exhaust gases from the internal combustion engine are discharged. A flow limiting valve is interposed in the path of the exhaust gases from the thermal reactor for discharging a first portion of the exhaust gases to the atmosphere and returning a second portion of the exhaust gases to the thermal reactor vessel. Also provided is a valve for discharging the second portion of the exhaust gases from the internal combustion engine at a temperature below the boiling range of the liquid fuel into the vessel below the surface of the liquid fuel therein and an ultrasonic generator for imparting ultrasonic energy to the exhaust gases.