Method for controlling gaseous fuel pressure

1. A method for controlling gaseous fuel pressure in an accumulator of a fuel system for a combustion engine of a vehicle with the steps of:

determining a nominal amount of gaseous fuel to be introduced into the accumulator; the nominal amount depending on the torque demand,

introducing less gaseous fuel into the accumulator than the determined nominal amount as a response to decreasing fuel demand by reducing or closing an inlet valve, which inlet valve is adapted to regulate input of gaseous fuel to the accumulator;

while the inlet valve is reduced or closed, performing at least one injection of gaseous fuel coming from the accumulator into at least one combustion chamber of the combustion engine by at least one injector of the fuel system, which at least one injection contributes to combustion in the combustion engine, thereby reducing pressure in the accumulator, and

mitigating any excessive torque caused by the reducing of pressure by braking the vehicle.

2. A method according to claim 1 , wherein the at least one injection is performed without venting gaseous fuel from the accumulator to a tank of the fuel system.

3. A method according to claim 1 , wherein the fuel system further comprises a release valve adapted to regulate venting of gaseous fuel from the accumulator to a tank of the fuel system, and wherein the at least one injection is performed while the release valve is closed.

4. A method according to claim 3 , wherein the inlet valve and the release valve are individually and electrically operated.

5. A method according to claim 1 , wherein the reducing of pressure is performed in response to a current accumulator pressure need.

6. A method according to claim 1 , wherein the at least one injection has a near minimum duration.

7. A method according to claim 1 , further comprising:

predicting a future accumulator pressure need,

wherein the reducing of pressure is performed in advance in response to the predicted future accumulator pressure need.

8. A method according to claim 7 , further comprising:

mitigating the in advance reducing of pressure by prolonging the duration of the at least one injection.

9. A method according to claim 1 , wherein the inlet valve is reduced or closed and the at least one injection is performed such that the inlet valve has a reduced gaseous fuel mass flow rate compared to the gaseous fuel mass flow rate of said at least one injection.

10. A method according to claim 1 , wherein the accumulator is a common rail in fluid communication with the at least one injector.

11. A method according to claim 1 , wherein the gaseous fuel is natural gas or biogas.

12. A fuel system for a combustion engine of a vehicle, wherein the fuel system comprises:

an accumulator;

an inlet valve adapted to regulate input of gaseous fuel to the accumulator;

at least one injector;

means configured to determine a nominal amount of gaseous fuel to be introduced into the accumulator; the nominal amount depending on the torque demand, and

a control unit configured to reduce or close the inlet valve so as to introduce, as a response to decreasing fuel demand, less gaseous fuel into the accumulator than the nominal amount determined by said means, and to trigger at least one injection of gaseous fuel coming from the accumulator into at least one combustion chamber of the combustion engine by the at least one injector, while the inlet valve is reduced or closed, which at least one injection contributes to combustion in the combustion engine, thereby reducing pressure in the accumulator, and configured to mitigate any excessive torque.

13. A computer program comprising program code means for performing the steps of claim 1 when said program is run on a computer.

14. A computer readable medium carrying a computer program comprising program code means for performing the steps of claim 1 when said program product is run on a computer.

15. A control unit for controlling gaseous fuel pressure in an accumulator of a fuel system for a combustion engine of a vehicle, the control unit being configured to perform or trigger the steps of the method according to claim 1 .

16. A vehicle comprising a fuel system according to claim 12 .

17. The fuel system of claim 12 wherein the control unit is further configured to mitigate any excessive torque caused by the reducing of pressure by braking the vehicle.

18. The fuel system of claim 12 wherein the control unit is further configured to mitigate any excessive torque caused by the reducing of pressure by recuperating the energy of the excessive torque in an electric machine and/or energy storage of the vehicle.