Method of controlling operation of an engine having both an exhaust fluid recirculation apparatus and an exhaust fluid treatment apparatus

1. A method of controlling operation of an engine, the engine comprising:

a combustion unit comprising at least one combustion cylinder, the combustion unit having an inlet and an outlet;

an exhaust fluid treatment apparatus configured to receive fluid from the outlet;

an exhaust fluid recirculation apparatus comprising a conduit for fluid communication between the combustion unit outlet and the combustion unit inlet and having a valve movable between open and closed positions, wherein the closed position is configured to prevent fluid flowing in the conduit between the combustion unit outlet and the combustion unit inlet; and

an exhaust fluid recirculation controller configured to provide a primary signal for controlling the position of the valve,

the method comprising:

confirming that the valve is in the closed position;

providing a secondary signal that the exhaust fluid recirculation valve should remain in the fully closed position, wherein the secondary signal overrides the primary signal;

injecting fuel upstream of the valve for combustion in the exhaust fluid treatment apparatus;

detecting when the primary signal indicates a desired position of the valve which is other than the fully closed position;

stopping the step of injecting fuel;

waiting a first predetermined period; and

allowing the primary signal to override the secondary signal.

2. The method of claim 1 wherein the step of injecting fuel upstream of the valve for combustion in the exhaust fluid treatment apparatus comprises injecting fuel downstream of the combustion unit.

3. The method of claim 1 wherein the first predetermined period is a fixed time.

4. The method of claim 1 wherein the first predetermined period is dependent on measured or predicted mass flow of fluid through the combustion unit.

5. The method of claim 1 wherein the first predetermined period is dependent on a volume of injected fuel injected in a preceding number of injections.

6. The method of claim 1 wherein the step of injecting fuel upstream of the valve for combustion in the exhaust fluid treatment apparatus comprises injecting fuel into at least one of the at least one combustion cylinder.

7. The method of claim 6 wherein the step of injecting fuel upstream of the valve for combustion in the exhaust fluid treatment apparatus occurs when an exhaust valve of the at least one combustion cylinder is open.

8. A method of controlling operation of an engine, the engine comprising:

a combustion unit comprising at least one combustion cylinder, the combustion unit having an inlet and an outlet;

an exhaust fluid treatment apparatus configured to receive fluid from the outlet;

an exhaust fluid recirculation apparatus comprising a conduit for fluid communication between the combustion unit outlet and the combustion unit inlet and having a valve movable between open and closed positions, wherein the closed position is configured to prevent fluid flowing in the conduit between the combustion unit outlet and the combustion unit inlet; and

an exhaust fluid recirculation controller configured to provide a primary signal for controlling the position of the valve,

the method comprising:

confirming that the valve is in the closed position;

providing a secondary signal that the exhaust fluid recirculation valve should remain in the fully closed position, wherein the secondary signal overrides the primary signal;

injecting fuel upstream of the valve for combustion in the exhaust fluid treatment apparatus;

detecting when the primary signal indicates a desired position of the valve which is other than the fully closed position;

stopping the step of injecting fuel;

awaiting a first predetermined period;

checking whether it is true or false that the primary signal continues to indicate a desired position of the valve which is other than the fully closed position; and,

when it is true: enabling the primary signal to override the secondary signal;

when it is false: enabling the secondary signal to override the primary signal.

9. The method of claim 8 wherein the first predetermined period is a fixed time.

10. The method of claim 8 wherein the first predetermined period is dependent on measured or predicted mass flow of fluid through the combustion unit.

11. The method of claim 8 wherein the first predetermined period is dependent on a volume of injected fuel injected in a preceding number of injections.

12. The method of claim 8 wherein the step of injecting fuel upstream of the valve for combustion in the exhaust fluid treatment apparatus comprises injecting fuel into at least one of the at least one combustion cylinder.

13. A method of controlling operation of an engine, the engine comprising:

a combustion unit comprising at least one combustion cylinder, the combustion unit having an inlet and an outlet;

an exhaust fluid treatment apparatus configured to receive fluid from the outlet;

an exhaust fluid recirculation apparatus comprising a conduit for fluid communication between the combustion unit outlet and the combustion unit inlet and having a valve movable between open and closed positions, wherein the closed position is configured to prevent fluid flowing in the conduit between the combustion unit outlet and the combustion unit inlet; and

an exhaust fluid recirculation controller configured to provide a primary signal for controlling the position of the valve,

the method comprising:

confirming that the valve is in the closed position;

providing a secondary signal that the exhaust fluid recirculation valve should remain in the fully closed position, wherein the secondary signal overrides the primary signal;

injecting fuel upstream of the valve for combustion in the exhaust fluid treatment apparatus;

receiving a tertiary signal indicative of a requirement for exhaust fluid recirculation;

stopping injecting fuel upstream of the valve for combustion in the exhaust fluid treatment apparatus;

awaiting a second predetermined period; and

enabling the primary signal to override the secondary signal.

14. The method of claim 13 wherein the tertiary signal is caused by a turbocharger associated with the engine increasing in speed above a threshold.

15. The method of claim 13 wherein the second predetermined period is dependent on a prediction of a period taken for the injected fuel to have passed into the exhaust fluid treatment apparatus.