Methods and system for operating an engine

1. An engine operating method, comprising:

generating a vacuum request;

generating a catalyst heating request; and

combusting air and fuel within a cylinder of an engine at a retarded spark timing, wherein the retarded spark timing is selected based on the generated vacuum request and the generated catalyst heating request, wherein combusting air and fuel at the retarded spark timing provides engine torque insufficient to rotate the engine at an idle speed, while rotating the engine at the idle speed via torque produced by a motor in response to the generated catalyst heating request.

2. The method of claim 1 , the method further comprising selecting the retarded spark timing from a table based on a difference in desired vacuum and actual vacuum and further based on a difference between a catalyst temperature and a catalyst light off temperature, the method further comprising adjusting an amount of air entering the engine responsive to a desired rate of catalyst heating and further responsive to the generated vacuum request.

3. The method of claim 1 , further comprising increasing the torque produced by the motor as the engine torque decreases.

4. The method of claim 1 , where the engine torque is insufficient to rotate the engine at the idle speed, wherein the engine torque in concert with the torque produced by the motor is sufficient to rotate the engine at the idle speed.

5. The method of claim 1 , where the motor is a driveline integrated starter/generator.

6. The method of claim 1 , where the idle speed is based on engine temperature and further based on the vacuum request.

7. The method of claim 1 , further comprising running up speed of the engine from zero speed to the idle speed via the motor while combusting air and fuel within the cylinder of the engine and providing engine torque less than is needed to run-up engine speed to the idle speed.

8. An engine operating method, comprising:

generating a vacuum request;

generating a catalyst heating request;

supplying spark to cylinders of an engine at a timing advanced or retarded from minimum spark for best torque spark timing, wherein the timing advanced or retarded from minimum spark for best torque spark timing is selected based on both the generated vacuum request and the generated catalyst heating request, the timing advanced or retarded from minimum spark for best torque spark timing adjusted based on a difference in desired vacuum and actual vacuum; and

combusting air and fuel within a cylinder of the engine at the timing advanced or retarded from minimum spark for best torque spark timing producing insufficient engine torque to rotate the engine at a desired idle speed while compensating for the insufficient engine torque with torque produced by a motor to rotate the engine at the desired idle speed in response to the generated vacuum request and the generated catalyst heating request.

9. The method of claim 8 , further comprising weighting the timing based further on a difference in a catalyst light off temperature and an actual catalyst temperature.

10. The method of claim 9 , where the timing is retarded away from minimum spark for best torque when the difference in desired vacuum and actual vacuum is decreasing.

11. The method of claim 10 , where the timing is advanced toward minimum spark for best torque spark timing when the difference in the catalyst light off temperature and actual catalyst temperature decreases.

12. The method of claim 8 , where the generated catalyst heating request and the generated vacuum request are based on an engine start request.

13. The method of claim 8 , further comprising running up speed of the engine from zero speed to the desired idle speed via the motor while combusting air and fuel within the cylinder of the engine and providing engine torque less than is needed to run-up engine speed to the desired idle speed.

14. The method of claim 8 , further comprising reducing a load on an alternator in response to the difference in desired vacuum and actual vacuum.

15. A system, comprising:

an engine;

a transmission coupled to the engine;

a motor/generator coupled to the engine via a belt; and

a vehicle system controller including executable instructions stored in non-transitory memory to generate a vacuum request and a catalyst heating request, rotate the engine at an idle speed via the motor/generator while the engine is combusting air and fuel in response to the generated vacuum request and the catalyst heating request, the engine producing torque that is insufficient to rotate the engine at the idle speed, wherein a torque of the motor/generator increases based on the generated catalyst heating request, wherein the idle speed is based on both the generated vacuum request and the catalyst heating request.

16. The system of claim 15 , further comprising additional instructions to operate the engine at minimum spark timing for best torque while rotating the engine at the idle speed.

17. The system of claim 15 , further comprising additional instructions to retard spark timing from minimum spark timing for best torque while rotating the engine at the idle speed while the engine is producing torque that is insufficient to rotate the engine at the idle speed in response to the generated catalyst heating request.

18. The system of claim 15 , further comprising an ignition system and additional instructions to provide spark to the engine at minimum spark timing for best torque while the engine is producing torque that is insufficient to rotate the engine at the idle speed.

19. The system of claim 18 , further comprising additional instructions to close a throttle of the engine in response to the generated vacuum request.

20. The system of claim 15 , further comprising additional instructions to activate an aspirator in response to the generated vacuum request.