Sand mode system and method for a vehicle

1. A method for operating a vehicle powertrain, comprising:

operating a first propulsion source providing propulsive effort to a front axle in a torque control mode;

operating a second propulsion source providing propulsive effort to a rear axle in a torque control mode, the second propulsion source different than the first propulsion source; and

increasing a proportion of a driver demand wheel torque provided via the second propulsion source from a base proportion and simultaneously decreasing the proportion of driver demand wheel torque provided via the first propulsion source in response to a steering angle being greater than a threshold angle.

2. The method of claim 1 , further comprising decreasing the proportion of the driver demand wheel torque provided via the second propulsion source and increasing the proportion of driver demand wheel torque provided via the first propulsion source in response to the steering angle being less than the threshold angle.

3. The method of claim 1 , further comprising adjusting the proportion of the driver demand wheel torque provided via the second propulsion source as a function of the steering angle.

4. The method of claim 3 , where adjusting the proportion of the driver demand wheel torque provided via the second propulsion source as the function of the steering angle includes increasing the proportion of the driver demand wheel torque provided to the second propulsion source as an exponential function of the steering angle.

5. The method of claim 3 , where adjusting the proportion of the driver demand wheel torque provided via the second propulsion source as the function of the steering angle includes increasing the proportion of the driver demand wheel torque provided to the second propulsion source as a linear function of the steering angle.

6. The method of claim 1 , where the first propulsion source and the second propulsion source are electric machines.

7. The method of claim 1 , where the first propulsion source or the second propulsion source is an electric machine, and where the other of the first propulsion source or the second propulsion source is an internal combustion engine.

8. A method for operating a vehicle powertrain, comprising:

operating a first propulsion source providing propulsive effort to a front axle in a torque control mode;

operating a second propulsion source providing propulsive effort to a rear axle in a torque control mode, the second propulsion source different than the first propulsion source;

increasing a proportion of a driver demand wheel torque provided via the second propulsion source from a base proportion in response to a request to enter a sand powertrain mode and simultaneously decreasing the proportion of the driver demand wheel torque provided via the second propulsion source in response to a vehicle slip angle exceeding a threshold angle.

9. The method of claim 8 , further comprising adjusting the threshold angle in response to driver input.

10. The method of claim 8 , further comprising adjusting the proportion of the driver demand wheel torque provided via the second propulsion source in response to a composition of a surface traveled across by a vehicle.

11. The method of claim 10 , where the surface traveled across by the vehicle is input to a human/machine interface.

12. The method of claim 8 , where the vehicle slip angle is estimated via one or more accelerometers.

13. The method of claim 8 , further comprising adjusting the proportion of the driver demand wheel torque provided via the second propulsion source in response to a steering angle being less than a threshold angle.

14. The method of claim 13 , further comprising adjusting the proportion of the driver demand wheel torque provided via the second propulsion source from the base proportion when the steering angle is less than the threshold angle or adjusting a proportion of the driver demand wheel torque provided via the first propulsion source when the steering angle is less than the threshold angle to maintain a speed of a wheel of the front axle to equal a speed of a wheel of the rear axle.

15. A vehicle system, comprising:

a first electric machine coupled to a wheel of a first axle;

a second electric machine coupled to a wheel of a second axle, wherein the second electric machine is different than the first electric machine, and wherein the second electric machine is decoupled from the first electric machine; and

a controller including executable instructions that cause the controller to:

simultaneously increase a proportion of a driver demand wheel torque provided via the second electric machine from a base proportion and decrease the proportion of driver demand wheel torque provided via the first electric machine in response to a steering angle being greater than a threshold angle.

16. The vehicle system of claim 15 , further comprising additional instructions to increase torque supplied to the wheel of the first axle in response to decreasing vehicle speed.

17. The vehicle system of claim 16 , further comprising additional instructions to increase torque supplied to the wheel of the second axle in response to increasing vehicle speed.

18. The vehicle system of claim 17 , where the first axle is a front axle, and where the second axle is a rear axle.

19. The vehicle system of claim 15 , further comprising increasing torque provided to the wheel of the second axle to induce slip of the wheel of the second axle while providing the driver demand wheel torque via the first electric machine and the second electric machine in response to a steering angle greater than a threshold angle.

20. The vehicle system of claim 15 , further comprising additional instructions to equalize a speed of the wheel of the second axle and a speed of the wheel of the first axle in response to a steering angle being less than a threshold angle.