Patent ID: 8534243

Claim:
A spark ignition four stroke internal combustion engine, comprising, for each cylinder: a combustion chamber, two intake conduits and two exhaust conduits ending up in said combustion chamber, two intake valves and two exhaust valves associated to said intake and exhaust conduits, arranged around a central spark plug, each of said valves comprising a stem mounted slidably along the axis thereof and a head in form of a circular disc cooperating with a respective valve seat, means for varying the lift of the intake valves, and a masking wall in said combustion chamber, which masks a curtain area of said intake valves on one side facing towards the exhaust valves, wherein: the means for varying the lift of the intake valves are actuating a lift value between a zero lift and a maximum lift H, as well as varying both the duration of the opening and the opening and closing instants of each intake valve, the axis of each intake valve is inclined with respect to the axis of a respective engine cylinder by an angle not exceeding 12°, said 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 included, a maximum value H of the lift of the intake valve; wherein said angle of inclination of each intake valve does not exceed 12°; wherein along each intake valve seat the masking wall has a tapered configuration towards a top edge, with an inner surface facing towards the axis of each intake valve, in form of cylindrical surface with axis coinciding with said axis of the intake valve, and an outer surface having a radiused profile, with a concave section at the base of the masking wall and a convex section at the top of the masking wall; wherein the inner surface of the masking wall facing towards the axis of each intake valve has a radial clearance with respect to the head of the intake valve comprised between 0.005 and 0.05 times the diameter of the head of the intake valve; wherein said 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 substantially equivalent to 0.25 times the maximum value H of the intake valve lift; wherein the inner surface of the masking wall facing towards the axis of each intake valve has a radial clearance with respect to the head of the intake valve substantially equivalent to 0.028 times the diameter of the head of the intake valve; and wherein along each intake valve seat the masking wall extends around the axis of the intake valve over an angle comprised between 90° and 200°, extremes excluded.