This paper describes a method and apparatus intended to minimize hydrocarbons (HC), carbon monoxide (CO), and nitric oxide (NO) in spark ignition engine exhaust by utilizing the unused displacement of the engine at part loads as an internal thermal reactor. The method used is to induct exhaust gas plus air into one portion of the cylinder and unthrottled fuel-air charge into the balance. The fuel-air charge is rich to minimize NO formation, but, as a result, the products of combustion contain HC and CO. Air is added to these products before re-induction to provide additional oxygen to complete the oxidation which is promoted by the high pressures and temperatures of compression and combustion. Load control is achieved by varying the relative amounts of fuel-air charge and recirculated exhaust.Experimentally, it was shown that the necessary stratification existed until the spark occurred but not thereafter. The very low NO concentrations expected were realized, but HC and CO values were higher than desired.