Four-stroke engine

1. A four-stroke engine ( 1 ) comprising:

a cylinder ( 2 );

a piston ( 3 ) movably mounted in the cylinder in order to rotatingly drive a crankshaft ( 4 ) via a connecting rod ( 5 );

the cylinder and the piston conjointly delimiting a combustion chamber ( 6 ); the cylinder having an intake opening ( 7 ) and an intake valve ( 8 ) for opening and closing the intake opening;

the cylinder having an exhaust opening ( 9 ) and an exhaust valve ( 10 ) for opening and closing the exhaust opening, followed by an exhaust duct ( 28 );

a mixture-preparation device ( 15 ) for supplying an air/fuel-lubricant mixture or for supplying in two steps an air/lubricant mixture and an air/fuel mixture;

a crankcase ( 11 ) connected to the cylinder and delimiting together with the underside of the piston ( 3 ) a crankcase volume ( 12 ), to which at least air ( 13 ) and lubricant ( 14 ) is supplied by the mixture preparation device;

an intake channel ( 16 ) connecting to the intake opening ( 7 ) with intake valve ( 8 ) to supply to the intake at least air;

a valve drive assembly ( 18 ) driven by said crankshaft ( 4 ) for actuating the intake valve ( 8 ) and the exhaust valve ( 10 ), characterized in that it further comprises:

a valve drive assembly housing ( 19 , 20 , 21 , 21 ′, 22 ) comprising one or more sections ( 19 , 20 , 21 , 21 ′, 22 ), and the intake channel ( 16 ) is supplied with a connecting duct ( 17 ) communicating with the crankcase volume ( 12 ) either by being connected to the crankcase or cylinder, or by being connected to the valve drive assembly housing ( 19 , 20 , 21 , 21 ′, 22 ), which communicates with the crankcase volume ( 12 ), and the communication from the intake channel ( 16 ) to the crankcase volume is arranged through at least one valve ( 49 ) that opens and shuts only once per engine cycle.

2. A four-stroke engine ( 1 ) according to claim 1 , wherein the intake channel ( 16 ) is supplied with the mixture preparation device ( 15 ) to supply at least air ( 13 ) and lubricant ( 14 ) to the intake and to the connecting duct ( 17 ).

3. A four-stroke engine ( 1 ) according to claim 2 , wherein the mixture preparation device ( 15 ) is in the form of a carburettor or a low-pressure injection system providing an air-fuel-lubricant mixture to the intake and the connecting duct ( 17 ).

4. A four-stroke engine ( 1 ) according to claim 1 , wherein the at least one valve ( 49 ) is constituted by parts driven by the cam shaft or driven at the same speed as the cam shaft, i.e. half the speed of the crankshaft.

5. A four-stroke engine according to claim 4 , wherein the valve is a rotary valve ( 49 ).

6. A four-stroke engine according to claim 1 , wherein atop section ( 22 ) of the valve drive assembly housing is in flow connection only with a lower section ( 21 , 20 ) and therefore there is no flow from the crankcase via a lower section ( 19 , 20 ) of the valve drive assembly housing to the top section ( 22 ) and further via the intake channel ( 16 ) to the combustion chamber.

7. A four-stroke engine ( 1 ) according to claim 1 , wherein the connecting duct ( 17 ) is connected to the crankcase ( 11 ) or cylinder ( 2 ), and the valve drive assembly housing ( 19 , 20 , 21 , 21 ′, 22 ) is in communication exclusively with the crankcase volume ( 12 ).

8. A four-stroke engine ( 1 ) according to claim 1 , wherein the section/s ( 19 , 20 , 21 , 21 ′, 22 ) is/are separate from the crankcase volume but in communication with the crankcase volume through at least one passage ( 23 , 24 , 25 , 26 , 27 ).

9. A four-stroke engine ( 1 ) according to claim 8 , wherein the at least one passage ( 23 ) is free from valves.

10. A four-stroke engine ( 1 ) according to claim 9 , wherein there is only a single passage ( 23 ) free from valves.

11. A four-stroke engine ( 1 ) according to claim 8 , wherein the at least one passage ( 24 ) or duct connecting the crankcase volume and the valve drive assembly housing ( 19 , 20 , 21 , 21 ′, 22 ) is provided with a check valve.

12. A four-stroke engine ( 1 ) according to claim 11 , wherein the check valve is arranged to allow flow only into the valve drive assembly housing.

13. A four-stroke engine according to claim 11 , wherein the check valve is arranged to allow flow only from the valve drive assembly housing.

14. A four-stroke engine ( 1 ) according to claim 8 , wherein the at least one passage ( 25 ) is opened and shut by the piston ( 3 ).

15. A four-stroke engine according to claim 8 , wherein the at least one passage ( 26 , 27 ) is controlled by a rotary valve.

16. A four-stroke engine ( 1 ) according to claim 8 , wherein the at least one passage ( 23 , 24 , 25 , 26 , 27 ) is of small size and has a combined cross-section area smaller than the cross-section area of an aperture with a diameter of 40% of the cylinder diameter, and preferably of 30% of the cylinder diameter.

17. A four-stroke engine ( 1 ) according to claim 8 , wherein the at least one passage ( 23 , 24 , 25 , 26 , 27 ) is of small size and has a combined cross-section area smaller than the cross-section area of an aperture with a diameter of 20% of the cylinder diameter, and preferably of 10% of the cylinder diameter.

18. A four-stroke engine ( 1 ) according to claim 1 , wherein an intake duct ( 47 ) is added connecting to the crankcase or cylinder and supplied with a mixture preparation device ( 15 ) to supply at least air ( 13 ) and lubricant ( 14 ) to the crankcase volume ( 12 ).

19. A four-stroke engine ( 1 ) according to claim 18 , wherein the mixture preparation device ( 15 ) is in the form of a carburettor or a low-pressure injection system providing an air-fuel-lubricant mixture to the crankcase volume ( 12 ).

20. A four-stroke engine ( 1 ) according to claim 15 , wherein the rotary valve is constituted by the crank mechanism ( 31 , 31 ′).