It is recognized that stratified charge combustion in a spark ignited internal combustion engine can produce improved fuel economy. A historical summary of many significant patents and articles is given pertaining to the 3-valve form of this engine combustion principle. The present social need of reducing exhaust emissions, and the renewed goal of improving fuel economy, makes this engine form one practical approach to the “clean engine” being sought for vehicular use. It uses a rich prechamber charge to provide the proper stratification. The prechamber can vary considerably in size depending upon the goals sought and the engine application.A basic requirement is that the fuel-air equivalence ratios of the charges in the two combustion chambers are within certain limited ranges to achieve proper optimization. The ratios should be 15% rich for the prechamber and 15 to 30% lean for the main chamber at the moment of ignition. The fuel-air equivalence ratios of the two carburetors must be properly selected to achieve these ratios within the combustion chambers. With the use of a computer analysis of engine breathing and carburetor air-flow the proper fuel-air equivalence ratios of the two carburetors can be conveniently established.Typical valve gear and prechamber layouts relative to the auxiliary valve are illustrated for various types of engines. Controlled charge turbulence and good charge mixing are very important, in addition to developing the recommended fuel-air equivalence ratios, to achieve dependable ignition and combustion. Some air swirl within the main chamber is desirable for this reason. A thermal reactor for the exhaust gases is complementary to this engine structure as verified by laboratory tests.