Abstract: A spark ignition four stroke internal combustion engine includes two intake valves and two exhaust valves for each cylinder, arranged around a central spark plug and provided with means for varying the lift of the intake valves between a zero value and a maximum value H. A masking wall which masks the curtain area of the intake valves on one side facing towards the exhaust valves is provided in the combustion chamber associated to each cylinder. The axis of each intake valve is inclined with respect to the axis of the respective engine cylinder by an angle not exceeding 12° and the abovementioned masking wall has along the seat of each intake valve a height h in the direction parallel to the axis of the intake valve which is comprised between 0.1 and 0.5 times, extremes excluded, the maximum value H of the lift of the intake valves.

Abstract: A spark ignition four stroke internal combustion engine includes two intake valves and two exhaust valves for each cylinder, arranged around a central spark plug and provided with means for varying the lift of the intake valves between a zero value and a maximum value H. A masking wall which masks the curtain area of the intake valves on one side facing towards the exhaust valves is provided in the combustion chamber associated to each cylinder. The axis of each intake valve is inclined with respect to the axis of the respective engine cylinder by an angle not exceeding 12° and the abovementioned masking wall has along the seat of each intake valve a height h in the direction parallel to the axis of the intake valve which is comprised between 0.1 and 0.5 times, extremes excluded, the maximum value H of the lift of the intake valves.