Method to determine the fuel temperature in a common rail injection system

1. A method to determine temperature of a fuel in a common rail injection system of an engine, comprising

determining fuel temperature in a common rail based on a fuel temperature measured by a temperature sensor located upstream from the fuel rail with at least one fuel pump arranged in line between the temperature sensor and the common rail;

wherein the determination of the fuel temperature in the common rail includes:

determining whether the engine was cold started or warm started based on a comparison of a determined temperature of cooling water in the engine and a determined air temperature at an air inlet of the engine, and

in response to determining a warm start of the engine, accounting for leftovers of hot fuel in the common rail in the determination of the fuel temperature in the common rail.

2. The method according to claim 1 , wherein a heat exchange between the fuel and its environment is accounted for the determination of the temperature of the fuel.

3. The method according to claim 2 , wherein the heat exchange between the fuel and the engine is determined from a fuel temperature at a place of the common rail injection system and a temperature of a cooling water of the engine.

4. The method according to claim 2 , wherein the heat exchange between the fuel and the air in the environment of the engine is determined from a fuel temperature at a place of the common rail injection system, a driving speed of the vehicle and an air temperature in the environment of the vehicle.

5. The method according to claim 2 , wherein the dependency of the heat exchange between the fuel and the environment on the flow speed of the fuel is determined from a fuel temperature at a place of the common rail injection system, a temperature of the cooling water of the engine and the fuel flow speed in the common rail.

6. The method according to claim 2 , wherein the heat generated by compressing and expanding the fuel is determined from a fuel temperature at a place of the common rail injection system and a pressure of the fuel.

7. The method according to claim 1 , wherein for determination of said fuel temperature at least one of the following is selected from the group consisting of: a heat exchange between the fuel and the engine, a heat exchange between the fuel and the air in the environment of the engine, a dependency of the heat exchange between the fuel and the environment on a flow speed of the fuel, a heat generated by compressing, and expanding the fuel.

8. The method according to claim 7 , wherein for the determination of the temperature of the fuel dynamical effects are accounted by low pass filters.

9. The method according to claim 8 , wherein different low pass filters are used for the cases of the engine warming up and the engine cooling down.

10. The method according to claim 1 , wherein a cold start and a warm start of the engine are distinguished by comparing the difference of the temperature of the cooling water of the engine and the temperature of the air in the environment of the vehicle with a predefined threshold value after the start of the engine.

11. The method according to claim 10 , wherein the current values of the temperature of the cooling water of the engine, the temperature of the air in the environment of the vehicle, and the determined values of the temperature of the fuel are saved to a non-volatile memory before shutting down the engine.

12. The method according to claim 10 , wherein accounting for leftovers of hot fuel in the common rail in the determination of the fuel temperature in the common rail in response to determining a warm start of the engine comprises calculating a time at which the engine was previously switched off based on a stored value of a temperature of the cooling water of the engine before the previous switch-off of the engine, a value of a temperature of the cooling water at the start of the engine, and a value of the temperature of the air in the environment of the vehicle.

13. The method according to claim 12 , wherein the temperature of the fuel after the start of the engine is determined from a value of the fuel temperature stored in the non-volatile memory before switching off the engine, a measured value of the temperature of the air in the environment of the vehicle and the time the engine was switched off.

14. The method according to claim 12 , wherein after a warm start of the engine the determination of the temperature of the fuel accounts the determined value of the temperature of the fuel at the start of the engine for at least some time.

15. The method according to claim 1 , wherein the temperature of the fuel in the leakage line is determined.

16. The method according to claim 15 , wherein the temperature of the fuel in the leakage line is calculated from the temperature of the fuel in the common rail.

17. A control unit for determining temperature of a fuel in a common rail injection system of an engine, the control unit being operable to:

determine fuel temperature in a common rail based on a fuel temperature measured by a temperature sensor located upstream from the fuel rail with at least one fuel pump arranged in line between the temperature sensor and the common rail;

wherein the determination of the fuel temperature in the common rail includes:

determining whether the engine was cold started or warm started based on a comparison of a determined temperature of cooling water in the engine and a determined air temperature at an air inlet of the engine, and

in response to determining a warm start of the engine, accounting for leftovers of hot fuel in the common rail in the determination of the fuel temperature in the common rail.

18. The control unit according to claim 17 , wherein a heat exchange between the fuel and its environment is accounted for the determination of the temperature of the fuel.

19. The control unit according to claim 17 , further operable to calculate a time at which the engine was previously switched off based on (a) a stored value of a temperature of the cooling water of the engine before the previous switch-off of the engine, (b) a value of a temperature of the cooling water at the start of the engine, and (c) a value of the temperature of the air in the environment of the vehicle.