Abstract: An external auto-ignited four stroke cycle gas engine with a combustion chamber which is arranged in the piston rotation symmetrically with regard to the cylinder axis and which in the upper dead center point of the piston absorbs nearly the total gas-air mixture. The engine, furthermore, includes means for imparting turbulence upon the gas-air mixture about the longitudinal cylinder axis, and also has a trough which is arranged in the cylinder head, and into which a spark plug extends. The combustion chamber in the form of a truncated cone open and widening toward the cylinder head has a substantially plane bottom while the greatest diameter of this truncated cone-shaped combustion chamber equals from 55 to 70% of the piston diameter and the depth of this combustion chamber is in a range from 15 to 20% of the piston diameter. The mantle surface of the combustion chamber is at an angle of from 10.degree. to 30.degree. inclined relative to the longitudinal axis of the cylinder.