A new engine concept has been proposed wherein a very lean premixed fuel-air mixture is inducted and compressed to a very high compression ratio with fuel of such an octane quality that the autoignition occurs throughout the charge simultaneously at or slightly after TDC (the homogenous charge simultaneous autoignition engine or HCSA engine). The HCSA engine promises very high efficiency due to high compression ratio and high air-fuel ratio and less pollutants due to high air-fuel ratio. A detailed and tested two-dimensional mathematical model with a tested sub-model for the autoignition process was used to prove that the HCSA engine will operate efficiently in a wide range of engine speed and load. The indicated thermal efficiency had its average value around 40%. A preliminary investigation proved the feasibility of the concept by operating a two-stroke engine in a speed range of 700-2000 rev/min producing a brake mean effective pressure in the range of 0-2.6 bar with a compression ratio in the range, of 19-27 with lean fuel- air mixtures. A mixture of gasoline and commercial hexane was used to vary the octane quality of the fuel.