Automatic variation of internal combustion engine valve timing in response to load and speed is an effective technique for reducing emission of oxides of nitrogen and unburned hydrocarbons. The variation employed is cam advance, which causes part of the exhaust gases normally expelled near the end of the exhaust stroke to be retained and mixed with the fresh air-fuel mixture during the following inlet stroke. Part of the unburned hydrocarbons normally emitted are contained in these recirculated end-of-stroke gases, and are oxidized further by re-exposure to combustion. The peak flame temperature is reduced by the diluent, and formation of oxides of nitrogen is suppressed.A prototype vehicle equipped with an automatic cam advance system shows that emissions measured by the Federal Test Procedure are reduced by the following amounts: nitric oxide 64%, unburned hydrocarbons 32%, and carbon monoxide 16.5%.Only minimal changes in vehicle operating characteristics are caused, and potential durability is good since the existing engine valves and pistons are used to handle the recirculated exhaust. The cylinder-to-cylinder distribution of the diluent is inherently uniform, and the amount of recirculation is repeatable.