SCR dosing system

1. A method for purging a selective catalytic reduction dosing system having a urea reservoir, a urea delivery module comprising at least one pump, a dosing injector arranged to spray urea into an engine exhaust pipe and a feedline extending from the dosing injector to the urea delivery module, the method comprising the steps of:

(1) activating a reverse pump or a pump in a reverse direction to reduce urea pressure in preparation to draw fluid/gas into the urea delivery module;

(2) opening the dosing injector to allow fluid/gas to flow into the feedline from the engine exhaust pipe;

(3) stopping the reverse pump or the reverse direction of the pump and closing the dosing injector;

(4) activating a forward pump or a forward direction of the pump;

(5) stopping the forward pump or the forward direction of the pump and opening the dosing injector; and

(6) activating the reverse pump or the reverse direction of the pump.

2. The method according to claim 1 , wherein steps (1) and (2) take place simultaneously.

3. The method according to claim 1 , wherein in step (3), the reverse pump or the reverse direction of the pump is stopped before the dosing injector is closed.

4. The method according to claim 1 , wherein the reverse pump or the reverse direction of the pump is stopped at the same time that the dosing injector is closed.

5. The method according to claim 1 , wherein step (3) includes stopping the reverse pump, step (4) includes activating the forward pump, and steps (3) and (4) take place simultaneously.

6. The method according to claim 1 , wherein in step (5), the activation of the forward pump or the forward direction of the pump is stopped after the dosing injector is opened.

7. The method according to claim 1 , wherein activation of the forward pump or the forward direction of the pump is stopped at the same time that the dosing injector is opened.

8. The method according to claim 1 , wherein the method comprises an additional step (7) of closing the dosing injector.

9. The method according to claim 1 , wherein the method comprises operation of a single pump in the forward direction and the reverse direction.

10. The method according to claim 9 , wherein the method comprises operation of a purge valve to control direction of flow in the forward direction or the reverse direction.

11. The method according to claim 10 , wherein the method further comprises a precursor step of activating a crossover mode of the purge valve before steps (1) and (6).

12. The method according to claim 11 , wherein the method further comprises an intervening step of activating a normal mode of the purge valve before step (4) and/or the precursor step.

13. The method according to claim 1 , wherein the method comprises operation of the forward pump and the reverse pump.

14. A selective catalytic reduction dosing system comprising:

a urea reservoir;

a urea delivery module comprising at least one pump;

a dosing injector arranged to spray urea into an engine exhaust pipe;

a feedline extending from the dosing injector to the urea delivery module; and

an electronic programmable control unit configured to:

(1) activate a reverse pump or a pump in a reverse direction to reduce the urea pressure in preparation to draw fluid/gas into the urea delivery module;

(2) open the dosing injector to allow fluid/gas to flow into the feedline from the exhaust pipe;

(3) stop the reverse pump or the reverse direction of the pump and close the dosing injector;

(4) activate a forward pump or a forward direction of the pump;

(5) stop the forward pump or the forward direction of the pump and open the dosing injector; and

(6) activate the reverse pump or the reverse direction of the pump.

15. The selective catalytic reduction dosing system according to claim 14 , comprising a pressure sensor operable to detect at least a first predetermined pressure.