Control of engine start for a hybrid system

1. A control-module implemented method to control a flying engine start in a powertrain comprising an electric motor providing a propulsion torque to a transmission input shaft, an internal combustion engine, and an engine disconnect clutch selectively providing torque transfer between the engine and the motor, the method comprising:

monitoring an output torque request;

determining the propulsion torque to the transmission input shaft based upon the output torque request;

monitoring a desired input speed;

determining a synchronization speed based upon the desired input speed;

determining, by a controller, a desired engine activation speed profile based upon the synchronization speed, comprising:

determining a first stage wherein a compensation torque is provided from the motor to the engine through the engine disconnect clutch to accelerate the engine from a stopped state to an engine firing speed;

once the engine firing speed is achieved, determining a second stage wherein the engine provides torque to accelerate the engine while simultaneously reducing the compensation torque provided from the motor to the engine;

determining a third stage subsequent to the second stage wherein the compensation torque is provided to the engine by the motor to control the engine to the synchronization speed; and

determining a fourth stage wherein the engine disconnect clutch is locked; and

controlling the motor to provide a motor torque based upon a sum of the propulsion torque and the compensation torque.

2. The method of claim 1 , wherein monitoring the desired input speed comprises monitoring a desired input speed profile; and

wherein determining the synchronization speed comprises determining the synchronization speed based upon the desired input speed profile and a capability of the engine to accelerate.

3. The method of claim 1 , wherein the engine firing speed comprises a minimum engine firing speed.

4. The method of claim 3 , wherein the compensation torque is determined using an open-loop clutch control term and a closed-loop clutch control term; and

wherein determining the engine activation speed profile further comprises determining the second stage wherein an engine torque command is limited to be less than the open-loop clutch control term.

5. The method of claim 1 , wherein the compensation torque is determined using an open-loop clutch control term and a closed-loop clutch control term; and

wherein determining the engine activation speed profile further comprises determining the second stage wherein an engine torque command is limited to be less than the open-loop clutch control term.

6. The method of claim 1 , wherein determining the engine activation speed profile further comprises determining the second stage wherein a torque capacity of the engine disconnect clutch is reduced to a nonzero minimum value.

7. The method of claim 1 , wherein the engine firing speed comprises a minimum engine firing speed.

8. The method of claim 1 , wherein controlling the motor is based upon a sum of the propulsion torque, the compensation torque, and a driveline active damping control torque.

9. The method of claim 1 , wherein determining the compensation torque comprises:

determining a torque required to turn the engine; and

determining the compensation torque based upon the torque required to turn the engine.

10. The method of claim 1 , wherein determining the compensation torque comprises determining an open-loop clutch control term and a closed-loop clutch control term.

11. The method of claim 10 , wherein determining the open-loop clutch control term comprises utilizing an engine disconnect clutch pressure-to-torque feed-forward compensation value.

12. The method of claim 1 , wherein determining the compensation torque comprises:

determining the compensation torque based upon an engine starting position.

13. The method of claim 1 , further comprising reducing a torque capacity of a transmission clutch to dampen an effect of the flying engine start upon an output torque of the transmission, wherein the transmission clutch includes a first element coupled to the motor and a second element coupled to the transmission input shaft for selectively providing torque transfer between the motor and the transmission when the transmission clutch is engaged.

14. The method of claim 1 , further comprising reducing a torque capacity of the engine disconnect clutch to dampen an effect of the flying engine start upon an output torque of the transmission.

15. The method of claim 1 , further comprising controlling a second motor to provide a second motor torque based upon the sum of the propulsion torque and the compensation torque.

16. System to control a flying engine start in a powertrain comprising an electric motor providing a propulsion torque to a transmission input shaft of a transmission, an internal combustion engine, and an engine disconnect clutch selectively providing torque transfer between the engine and the motor, the system comprising:

the motor; and

a control module configured for:

monitoring an output torque request;

determining the propulsion torque to the transmission input shaft based upon the output torque request; monitoring a desired input speed;

determining a synchronization speed based upon the desired input speed;

determining a desired engine activation speed profile based upon the synchronization speed, comprising:

determining a first stage wherein a compensation torque is provided to the engine through the engine disconnect clutch to accelerate the engine from a stopped state to an engine firing speed, once the engine firing speed is achieved,

determining a second stage wherein the engine provides torque to accelerate the engine while simultaneously reducing the compensation torque provided to the engine,

determining a third stage subsequent to the second stage wherein the compensation torque is provided to the engine by the motor to control the engine to the synchronization speed, and

determining a fourth stage wherein the engine disconnect clutch is locked; and

controlling the motor to provide a motor torque based upon a sum of the propulsion torque and the compensation torque.