A spark ignition engine concept employing sonic throttling intake valves has demonstrated that the rapid, turbulent combustion process so produced permits extremely lean part-load operation. Light load mixture ratios in excess of 20:1 are possible without incurring the consequences of cyclic variations in cylinder pressure development and torque. A resulting improvement in fuel consumption is achieved.The impetus for throttling the engine by means of the intake valves originated in the hypothesis that increasing the intensity of the small scale turbulence within the combustion chamber would effect an acceleration of combustion propagation rates during the initial period immediately after ignition. Throttling the engine by means of the intake valves results in sonic inflow velocity throughout the idle, road load, and acceleration operating ranges up to approximately two-thirds load. The mechanical means of operating engines on this principle have been demonstrated on both single and multicylinder engines. An extensive test program has established the operating characteristics of this method with respect to thermal efficiency and controllability. Preliminary vehicle tests have demonstrated stable operation at these lean mixture ratios.