Coolant system

1. Engine cooling system with a cooling circuit comprising a coolant pump for supplying an engine with a coolant and for circulating the coolant in the cooling circuit, and at least one heat exchanger for cooling the coolant downstream of the engine, wherein an expansion tank is connected to the cooling circuit upstream of the coolant pump, wherein the cooling system is pressurized by pressure regulating means arranged to pressurize coolant supplied to the cooling circuit from the expansion tank during at least one predetermined operating mode of the engine and the expansion tank is closed to the ambient atmosphere during all normal engine operation modes.

2. Engine cooling system according to claim 1 , wherein the pressure regulating means is located in the expansion tank.

3. Engine cooling system according to claim 2 , wherein the pressure regulating means is arranged to displace the coolant in the expansion tank.

4. Engine cooling system according to claim 3 , wherein the pressure regulating means is supplied with a pressurized fluid from an external source of pressure.

5. Engine cooling system according to claim 4 , wherein the pressurized fluid is contained in a volume separated from the coolant.

6. Engine cooling system according to claim 5 , wherein the pressure regulating means comprises a diaphragm.

7. Engine cooling system according to claim 4 , wherein the cooling system further comprises a deaeration chamber located upstream of the coolant pump.

8. Engine cooling system according to claim 7 , wherein the expansion tank is connected to the deaeration chamber.

9. Engine cooling system according to claim 2 , wherein the pressure regulating means is connected to a source of fluid pressure via a controllable valve.

10. Engine cooling system according to claim 9 , wherein the coolant and the fluid from the source of fluid pressure are maintained in a non-contacting relationship.

11. Engine cooling system according to claim 9 , wherein the valve is a pressure controlled valve.

12. Engine cooling system according to claim 11 , wherein the valve is controlled by the pressure in the expansion tank.

13. Engine cooling system according to claim 9 , wherein the valve is controllable to pressurize the expansion tank to a predetermined pressure during start-up of the engine.

14. Engine cooling system according to claim 9 , wherein the valve is controllable to maintain a predetermined pressure in the expansion tank during normal operation of the engine.

15. Engine cooling system according to claim 1 , wherein one operating mode is a start-up of the engine.

16. Engine cooling system according to claim 1 , wherein the pressure regulating means is located in a supply conduit connecting the expansion tank to the cooling circuit.

17. Engine cooling system according to claim 16 , wherein the cooling system further comprises a deaeration chamber located upstream of the coolant pump.

18. Engine cooling system according to claim 17 , wherein the expansion tank is connected to the deaeration chamber via a controllable valve.

19. Engine cooling system according to claim 18 , wherein the valve is controlled by the pressure in the deaeration chamber.

20. Engine cooling system according to claim 18 , wherein the pressure regulating means is a pump.

21. Engine cooling system according to claim 18 , wherein the pressure regulating means is an injector.

22. Engine cooling system according to claim 1 , wherein the volume of the expansion tank is at least 10% of the total volume of the cooling system.

23. Engine cooling system according to claim 1 , wherein the volume of the expansion tank is up to 30% of the total volume of the cooling system

24. A vehicle comprising an engine cooling system according to claim 1 .