Abstract: A gasoline engine fed with lean mixture only is improved for practical use. The cylinder head of the engine has an area providing intense turbulance in cooperation with the top surface of the piston. It is provided, further, with a torch chamber which emits torch flames sharing the combustion of lean mixture so that the flame propagation distance is decreased. Among the torch flame, one passes through a portion of the combustion chamber just below the center of the head of the suction valve, and another passes through a portion of the combustion chamber just below the center of the head of the exhaust valve.