A historical review of the patents and literature pertaining to 3-valve stratified charge engines is presented in this paper. This very old invention appears to be a practical approach for the “clean engine” being sought for vehicular use since it has the intrinsic capability of simultaneously giving good fuel economy and producing minimal objectionable exhaust emissions. The prime requisites of this engine are a rich prechamber charge and a very lean main chamber charge regardless of prechamber volume, nozzle diameter, valving and spark plug location.Fuel-air equivalence ratios of the charges in the two combustion chambers are significantly important in order to achieve the proper optimization. These ratios should be about 15% rich for the prechamber and 15 to 30% lean for the main chamber at the moment of ignition. The proper values for the two carburetor fuel-air equivalence ratios and the two intake manifold pressures, as well as several less influential engine design parameters, are methodically established by computer analysis of engine breathing and charge mixing.An air-swirl inducing main intake port and a lean thermal reactor are complementary to this engine structure as verified by improved results in laboratory tests.