System and method for improved fuel economy during vehicle deceleration conditions

1. A method of operating an engine of a vehicle having a powertrain, the method comprising:

during a driver tip-out condition where the vehicle is decelerating, reducing airflow to reduce engine torque output to a desired engine torque without reducing torque by retarding spark timing, following the driver tip-out, until airflow reaches a minimum value below which combustion may be degraded, wherein the desired engine torque controls engine deceleration while approaching a desired engine idle speed during vehicle deceleration; and

upon airflow reach said minimum value, retarding spark timing to further reduce engine output.

2. The method of claim 1 where an actual engine torque is estimated based on characteristics of torque converter coupled to said engine in the powertrain and based on engine operating conditions.

3. The method of claim 2 wherein said engine is coupled to a continuously variable transmission via said torque converter.

4. The method of claim 3 wherein said engine torque estimate takes into account transmission spin losses of said continuously variable transmission.

5. The method of claim 1 wherein during said deceleration, said engine output torque is adjusted to transition through a transmission lash zone at a reduced rate.

6. The method of claim 1 wherein during said deceleration, said engine output torque is adjusted based on whether an operator is applying brakes of the vehicle.

7. The method of claim 1 wherein during said deceleration, operating in at least a first mode where positive powertrain torque is provided to reduce operation through a transmission lash zone, and in response to actuation of a brake pedal by the driver, transitioning through said lash zone.

8. The method of claim 1 wherein during said deceleration, operating in at least a second mode where positive powertrain torque is provided to reduce operation through a transmission lash zone, and said positive torque is maintained while vehicle speed is below a threshold value.

9. The method of claim 1 wherein during said deceleration, operating in at least a third mode where positive powertrain torque is provided to reduce operation through a transmission lash zone, and said positive torque is maintained while turbine speed is below a predetermined value.

10. The method of claim 1 wherein during said deceleration, operating in at least a fourth mode where positive powertrain torque is provided to reduce operation through a transmission lash zone, and said positive torque is maintained while the transmission is in a non-braking gear.

11. The method of claim 1 wherein during said deceleration, operating in at least a first mode where positive powertrain torque is provided to reduce operation through a transmission lash zone, and in response to a change in a driver shift lever by the driver transitioning through said lash zone to generate braking torque.

12. A method of operating an engine of a vehicle having a powertrain having a transmission, the method comprising:

during a driver tip-out condition where the vehicle is decelerating, adjusting engine output based on a gear of the transmission, and reducing airflow to reduce engine torque output to a desired engine torque without reducing torque by retarding spark timing, following the driver tip-out, until airflow reaches a minimum value below which combustion may be degraded, wherein the desired engine torque controls engine deceleration while approaching a desired engine idle speed during vehicle deceleration; and

upon airflow reach said minimum value, retarding spark timing to further reduce engine output.

13. The method of claim 12 wherein during said deceleration, operating in at least a first mode where positive powertrain torque is provided to reduce operation through a transmission lash zone, and in response to actuation of a brake pedal by the driver, transitioning through said lash zone.

14. The method of claim 13 wherein during said deceleration, operating in at least a second mode where positive powertrain torque is provided to reduce operation through a transmission lash zone, and said positive torque is maintained while vehicle speed is below a threshold value.

15. The method of claim 14 wherein during said deceleration, operating in at least a third mode where positive powertrain torque is provided to reduce operation through a transmission lash zone, and said positive torque is maintained while turbine speed is below a predetermined value.

16. The method of claim 15 wherein during said deceleration, operating in at least a fourth mode where positive powertrain torque is provided to reduce operation through a transmission lash zone, and said positive torque is maintained while the transmission is in a non-braking gear.

17. A system for a passenger vehicle traveling on the road, the system comprising:

an engine;

a continuously variable transmission coupled to said engine via a torque converter; and

a controller configure to adjust engine output based on a gear ratio of the transmission during a driver tip-out condition where the vehicle is decelerating, and during said deceleration, reducing airflow to reduce engine torque output to a desired engine torque without reducing torque by retarding spark timing, following the driver tip-out, until airflow reaches a minimum value below which combustion may be degraded, wherein the desired engine torque controls engine deceleration while approaching a desired engine idle speed during vehicle deceleration, and upon airflow reach said minimum value, retarding spark timing to further reduce engine output.

18. The system of claim 17 wherein said controller is further configured to manage torque during transition through a lash zone.