Vehicle and method of control thereof

1. A speed control system for a hybrid electric vehicle, the vehicle having a powertrain comprising at least one engine and at least one electric machine, wherein the system is configured to:

allow a driver to set a target vehicle speed;

operate the powertrain in first and second speed control modes so as to change a vehicle speed of the hybrid electric vehicle to maintain a prescribed distance behind a followed vehicle without exceeding the target vehicle speed,

wherein in the first speed control mode the system limits operation of the powertrain to an electric vehicle (EV) mode in which the electric machine produces positive torque while the engine produces no positive torque; and

determine that at least one of a prescribed one or more conditions is met with reference to data obtained from a sensor of the hybrid electric vehicle or from an external source, the sensor or external source monitoring traffic or locations external to the vehicle,

wherein in the second speed control mode the system allows operation of the powertrain in hybrid electric vehicle (HEV) mode, and EV inhibit mode, in which the engine produces positive torque, and

wherein when the at least one of the prescribed one or more conditions is met, the system switches from the first speed control mode to the second speed control mode to allow operation of the powertrain in the HEV mode and EV inhibit mode so as to change the vehicle speed to maintain the prescribed distance behind the followed vehicle without exceeding the target vehicle speed.

2. The speed control system of claim 1 , further operable to determine a required rate of acceleration of the vehicle in order to maintain the prescribed distance behind the followed vehicle, the system being operable in the first speed control mode to limit operation of the powertrain to the EV mode by limiting a maximum rate of acceleration that is commanded by the system to a rate that is achievable in the EV mode.

3. The speed control system of claim 2 , further operable in the first speed control mode to limit the maximum rate of acceleration to a rate corresponding substantially to the maximum rate achievable in the EV mode.

4. The speed control system of claim 1 , further operable in the first speed control mode to limit operation of the powertrain to the EV mode by limiting a maximum amount of drive torque that is commanded by the system to an amount that is achievable in the EV mode.

5. The speed control system of claim 4 , further operable in the first speed control mode to limit the maximum amount of drive torque that is commanded by the system to a rate corresponding substantially to the maximum amount achievable in the EV mode.

6. The speed control system of claim 1 , wherein the prescribed one or more conditions includes a condition that a difference between the target vehicle speed and a current vehicle speed exceeds a prescribed delta speed value.

7. The speed control system of claim 6 , wherein the prescribed delta speed value is determined in dependence on gradient of a surface over which the vehicle is driving.

8. The speed control system of claim 6 , wherein the prescribed one or more conditions includes the condition that a difference between the target vehicle speed and a current vehicle speed exceeds the prescribed delta speed value and a followed vehicle is accelerating at a rate exceeding a prescribed followed vehicle acceleration rate.

9. The speed control system of claim 8 , wherein the prescribed followed vehicle acceleration rate is determined in dependence on gradient of a surface over which the vehicle is driving.

10. The speed control system of claim 8 , wherein the prescribed followed vehicle acceleration rate corresponds substantially to the maximum rate of acceleration that the system is permitted to command when the system is operating in the first speed control mode and none of the prescribed one or more conditions is met.

11. The speed control system of claim 1 , wherein the prescribed one or more conditions includes a condition that the driver demands a rate of acceleration of the vehicle which exceeds a prescribed threshold value.

12. The speed control system of claim 1 , wherein the prescribed one or more conditions includes a condition that the driver demands an amount of drive torque which exceeds a prescribed threshold value.

13. The speed control system of claim 1 , further operable automatically to assume the second speed control mode if, whilst in the first speed control mode, the at least one engine is started more than a prescribed number of times within a prescribed period of time.

14. The speed control system of claim 1 , further operable automatically to assume the second speed control mode if, whilst in the first speed control mode, it is determined that a density of traffic exceeds a prescribed value.

15. The speed control system of claim 14 , further operable to determine the density of traffic responsive to traffic density data received by the system from a traffic information service.

16. The speed control system of claim 14 , further operable to determine the density of traffic responsive to measurement of a number of vehicles passing the vehicle within a prescribed period of time.

17. The speed control system of claim 14 , further operable to determine the density of traffic responsive to measurement of a number of vehicles passed by the vehicle within a prescribed period of time.

18. The speed control system of claim 1 , further operable automatically to assume the second speed control mode if, whilst in the first speed control mode, the driver increases a rate of acceleration of the vehicle above a maximum value available in the EV mode; one or more times in a given time period, a prescribed number of times over a given distance, or a prescribed number of times over a given journey.

19. The speed control system of claim 1 , wherein the prescribed one or more conditions includes a condition that a maximum rate of acceleration achievable by the vehicle in the EV mode is below a prescribed threshold value.

20. The speed control system of claim 1 , wherein the prescribed one or more conditions includes a condition that a maximum amount of torque achievable by the vehicle in the EV mode is below a prescribed threshold value.

21. The speed control system of claim 1 , further comprising a driver-operable control to select one of the first and second speed control modes.

22. The speed control system of claim 1 , further operable to store data in respect of geographical locations at which an engine start has been commanded by the system, when the vehicle subsequently approaches a same location in substantially a same direction of travel the system being configured to start the at least one engine in advance of arrival at the location such that power generated as a consequence of starting the at least one engine is delivered to a driveline of the vehicle substantially at said location.

23. The speed control system of claim 22 , further configured to start the at least one engine in advance of arrival at the same location such that drive torque from the engine is delivered to the driveline of the vehicle substantially at said same location.

24. The speed control system of claim 22 , further configured to store said data in respect of geographical location only when the at least one engine was started in the absence of driver intervention causing an engine start.

25. The speed control system of claim 22 , further configured to store said data in respect of geographical location only when the at least one engine was started when traffic density was below a threshold value.

26. The speed control system of claim 22 , further configured to store said data in respect of geographical location only when a followed vehicle was not accelerating at a rate exceeding a prescribed rate thereby triggering an engine start.

27. The speed control system of claim 22 , wherein the at least one engine was started when traffic density was below a threshold value.

28. The speed control system of claim 1 , wherein the data is indicative of electromagnetic or ultrasonic radiation reflected from traffic and detected by the sensor.

29. A method of controlling speed of a hybrid electric vehicle via a speed control system, wherein the vehicle has a powertrain comprising an engine and an electric machine, the method comprising:

allowing a driver to set a target vehicle speed; and

operating the powertrain in first and second speed control modes to change a vehicle speed of the hybrid electric vehicle to maintain a prescribed distance behind a followed vehicle without exceeding the target vehicle speed;

wherein in a first speed control mode of the control system, operation of the powertrain is limited to an electric vehicle (EV) mode in which the electric machine produces positive torque while the engine produces no positive torque; and

determining that at least one of a prescribed one or more conditions is met with reference to data obtained from a sensor of the hybrid electric vehicle or from an external source, the sensor or external source monitoring one or more of traffic or locations external to the vehicle,

wherein in a second speed control mode of the control system, operation of the powertrain is allowed in hybrid electric vehicle (HEV) mode and EV inhibit mode, in which the engine produces positive torque, and

wherein when the at least one of the prescribed one or more conditions is met, the system switches from the first speed control mode to the second control speed control mode to allow operation of the powertrain in the HEV mode and EV inhibit mode so as to change the vehicle speed to maintain the prescribed distance behind the followed vehicle without exceeding the target vehicle speed.