Method for determining pilot injection mass

1. A method for determining a pilot injection mass during variable displacement operation of an internal combustion engine, the method comprising:

skipping selected cylinders;

determining a pilot injection mass of a fuel from a source based upon a temperature difference between a desired temperature of contents of a skipped cylinder and an estimated temperature of the contents of the skipped cylinder that is to be fired;

injecting the pilot injection mass of the fuel into the skipped cylinder that is to be fired;

injecting a main injection of the fuel from the source in the skipped cylinder that is to be fired, wherein the pilot injection mass is injected into the skipped cylinder before the main injection is injected; and

firing the skipped cylinder.

2. The method of claim 1 , wherein the pilot injection mass is computed based on a number of cycles the skipped cylinder has been skipped.

3. The method of claim 1 , further comprising:

injecting the main injection into the skipped cylinder a predetermined amount of time after the pilot injection mass is injected into the skipped cylinder.

4. The method of claim 1 , further comprising:

if the pilot injection mass exceeds a threshold, splitting the pilot injection mass into multiple pilot injections; and

injecting the multiple pilot injections into the skipped cylinder.

5. The method of claim 1 , further comprising reducing the main injection to account for torque produced by the pilot injection mass to maintain total output torque.

6. The method of claim 1 , further comprising determining a timing for injecting the pilot injection mass such that an end of injecting the pilot injection mass is a specified number of crank angle degrees before injecting the main injection.

7. The method of claim 3 , wherein the predetermined amount of time is chosen to achieve oxidation of the pilot injection mass of the fuel before injecting the main injection.

8. An engine controller in an internal combustion engine operated in a skip fire manner, wherein the engine controller configured to:

selectively skip certain cylinders;

determine a pilot injection mass of a fuel from a source based upon a temperature difference between a desired temperature of contents of a skipped cylinder and an estimated temperature of the contents of the skipped cylinder that is to be fired;

inject the pilot injection mass of the fuel into the skipped cylinder that is to be re-fired;

inject a main injection of the fuel from the source in the skipped cylinder that is to be re-fired, wherein the pilot injection mass is injected into the skipped cylinder before the main injection is injected; and

fire the skipped cylinder.

9. The engine controller of claim 8 , wherein the pilot injection mass is computed based on a number of cycles the skipped cylinder has been skipped.

10. The engine controller of claim 8 , wherein the engine controller is further configured to inject the main injection into the skipped cylinder a predetermined amount of time after the pilot injection mass is injected into the skipped cylinder.

11. The engine controller of claim 8 , wherein the engine controller is further configured to:

split the pilot injection mass into multiple pilot injections if the pilot injection mass exceeds a threshold; and

inject the multiple pilot injections into the skipped cylinder.

12. The engine controller of claim 8 , wherein the engine controller is further configured to reduce the main injection to account for torque produced by the pilot injection mass to maintain total output torque.

13. The engine controller of claim 8 , wherein the engine controller is further configured to determine a timing for injecting the pilot injection mass such that an end of injecting the pilot injection mass is a specified number of crank angle degrees before starting the main injection.

14. The engine controller of claim 10 , wherein the predetermined amount of time is chosen to achieve oxidation of the pilot injection mass of the fuel before starting the main injection.

15. A non-transitory, computer-readable medium having instructions recorded thereon which, when executed by a processor, cause the processor to:

selectively skip certain cylinders;

determine a pilot injection mass of a fuel from a source based upon a temperature difference between a desired temperature of contents of a skipped cylinder and an estimate temperature of the contents of the skipped cylinder that is to be fired;

inject the pilot injection mass of the fuel into the skipped cylinder that is to be re-fired;

inject a main injection of the fuel from the source in the skipped cylinder that is to be re-fired, wherein the pilot injection mass is injected into the skipped cylinder before the main injection is injected; and

fire the skipped cylinder.

16. The non-transitory, computer-readable medium of claim 15 , wherein the pilot injection mass is computed based on a number of cycles the skipped cylinder has been skipped.

17. The non-transitory, computer-readable medium of claim 15 , wherein the instructions further cause the processor to inject the main injection into the skipped cylinder a predetermined amount of time after the pilot injection mass is injected into the skipped cylinder.

18. The non-transitory, computer-readable medium of claim 15 , wherein the instructions further cause the processor to:

split the pilot injection mass into multiple pilot injections if the pilot injection mass exceeds a threshold; and

inject the multiple pilot injections into the skipped cylinder.

19. The non-transitory, computer-readable medium of claim 15 , wherein the instructions further cause the processor to reduce the main injection to account for torque produced by the pilot injection mass to maintain total output torque.

20. The non-transitory, computer-readable medium of claim 15 wherein the instructions further cause the processor to determine a timing for injecting the pilot injection mass such that an end of injecting the pilot injection mass is a specified number of crank angle degrees before starting the main injection.

21. The non-transitory, computer-readable medium of claim 17 , wherein the predetermined amount of time is chosen to achieve oxidation of the pilot injection mass of the fuel before starting the main injection.