Vehicle and vehicle engine start-up control method

1. A vehicle comprising:

an engine and an electric machine each configured to propel the vehicle;

a starter-generator coupled to the engine and configured to adjust engine speed during an engine start-up event; and

a controller programmed to,

in response to engine speed increasing towards a target speed during an engine start-up event, generate a target drag torque with the starter-generator to reduce overshoot of the target engine speed, and

in response to generating the target drag torque with the starter-generator during the engine start-up event, decrease an amount of spark retard of the engine.

2. The vehicle of claim 1 , wherein the target drag torque is based on a difference between a current engine torque and an engine inertia torque required to increase engine speed.

3. The vehicle of claim 2 , wherein the engine inertia torque is based on a difference between the target engine speed and a current engine speed.

4. The vehicle of claim 1 , wherein the target drag torque is based on a difference between a target engine speed and a current engine speed.

5. The vehicle of claim 4 , wherein the target drag torque is based on a proportional gain term and an integral gain term.

6. The vehicle of claim 1 , wherein the starter-generator is configured to convert kinetic energy into electrical energy to charge a battery when generating the target drag torque to reduce overshoot of the target engine speed.

7. The vehicle of claim 1 , wherein the starter-generator is selectively coupled to a crankshaft of the engine via a belt.

8. The vehicle of claim 1 , further comprising a disconnect clutch configured to couple the engine the electric machine, and wherein the controller is programmed to, in response to the engine obtaining the target speed, close the disconnect clutch.

9. The vehicle of claim 8 , wherein the target speed of the engine is a current operating speed of the electric machine.

10. A controller comprising:

an input channel configured to receive signals indicative of engine speed during an engine start-up event;

an output channel configured to provide commands to operate a starter-generator that is coupled to an engine; and

control logic programmed to,

in response to increasing engine speed during a start-up event, generate the command to operate the starter-generator to apply a target drag torque to the engine to reduce overshoot of a target engine speed, and

in response to the starter-generator operating to apply the target drag torque, decrease an amount of spark retard of the engine.

11. The controller of claim 10 , wherein the target drag torque is based on a difference between a current engine torque and an engine inertia torque required to increase engine speed.

12. The controller of claim 11 , wherein the engine inertia torque is based on a difference between the target engine speed and a current engine speed.

13. The controller of claim 10 , wherein the target drag torque is based on a difference between the target engine speed and a current engine speed.

14. The controller of claim 13 , wherein the target drag torque is based on a proportional gain term and an integral gain term.

15. A method of controlling a vehicle powertrain comprising:

in response to a speed of an engine increasing towards a target speed during a start-up event, increasing a load on the engine by generating a target drag torque with a starter-generator to reduce overshoot of the target speed; and

in response to increasing the load on the engine by generating the target drag torque, decreasing an amount of spark retard in the engine.

16. The method of claim 15 , further comprising in response to the engine obtaining the target speed, closing an engine disconnect clutch to couple the engine to an electric machine.

17. The method of claim 16 , wherein the target speed of the engine is a current operating speed of the electric machine.