A spark-ignition, air-compressing internal combustion engine has a combustion chamber in a piston movable toward a cylinder head of the engine. Fuel is injected onto the wall of the combustion chamber and air rotated in the chamber to remove the fuel from the combustion chamber wall gradually as a vapor. The cross-section of the combustion chamber wall is defined by two curved lines of specific radial relation generally indicated by the curved line extending from the opening into the combustion chamber having the smaller radius of curvature.
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