GB2334496A

GB2334496A - Hybrid vehicle with controlled downhill electrodynamic braking

Info

Publication number: GB2334496A
Application number: GB9803494A
Authority: GB
Inventors: Carl Charles Bourne
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1998-02-20
Filing date: 1998-02-20
Publication date: 1999-08-25
Anticipated expiration: 2018-02-20
Also published as: GB9803494D0; GB2334496B

Abstract

In a vehicle which comprises an internal combustion engine 20 driving a generator 18, the power from which is used to drive motors 16a for each of the driven wheels 12a of the vehicle, the motors 16a can also be used to brake the wheels 12a, in which case the electric power generated is used to power a heater 34 which dissipates the power into the cooling system 32, 30 of the engine 20. The control of the braking when the vehicle is on a downhill slope can be such that the vehicle is prevented from accelerating and the speed limited so that the rate of electrical energy dissipation by the heater does not exceed a maximum rate.

Description

A Vehicle Power train The present invention relates to electric vehicles and has particular application in hybrid vehicles.

Our co-pending UK patent application No. 9711407.8 describes a power train for a hybrid electric vehicle comprising an internal combustion engine and an electric motor each arranged to provide power to drive the wheels of the vehicle, a cooling system for the engine including a circuit for fluid engine coolant and a heat dissipating means, a heater arranged to heat the coolant, and control means which can control the motor so as to apply a braking torque to the wheels thereby generating electrical power, which is used to power the heater thereby to dissipate the power in the cooling system.

The present invention relates to the control of an electric vehicle when it is descending a hill. It is known from WO96/11826 to provide a vehicle braking system in which the brakes are controlled so as to control the speed of the vehicle when descending a hill.

The present invention provides a vehicle power train comprising one or more electric motors capable of providing a driving torque to drive the vehicle and a braking torque to brake the vehicle, and energy dissipating means which can dissipate electrical energy generated by the motor or motors during braking up to a maximum dissipation rate, and control means arranged to control the amount of braking produced by the motor or motors when the vehicle is descending a slope to control the speed of the vehicle such that the motor or motors can produce enough braking to prevent the vehicle accelerating without the electrical power produced by the braking exceeding said maximum dissipation rate.

Preferably the power train further comprises an engine such as an internal combustion engine and the heat dissipating means may then conveniently use the engine cooling system.

Preferred embodiments of the present invention will now be described by way of example only with reference to the accompanying drawings in which: Figure 1 is a schematic representation of a vehicle power train according to the invention; and Figure 2 is a schematic representation of the cooling system of the embodiment of Figure 1.

Referring to Figures 1 and 2, a series hybrid vehicle 10 has a front pair of wheels 12a, 12b and a rear pair of wheels 14a, 14b. Each of the wheels 12, 14 is driven by an independent electric motor 16a, 16b, 16c, 16d. The power for the motors 16 is derived via a generator 18 from an internal combustion engine 20 and distributed in the desired proportions to the motors 16 by a control unit 22. Each of the motors can also be used to apply a braking torque to its respective wheel 12. This is achieved by using the motor 16 as a generator so that rotation of the wheel 12 is used to produce an electric current, the power from which needs to be dissipated as is explained below.

The direction and magnitude of the torque supplied to each wheel is therefore controllable independently. A battery 24 is also provided to act as a buffer for the storage of excess energy produced and to smooth over irregularities in the power of the engine 20. A brake pedal 26 has a potentiometer 28 connected to it which produces a brake torque demand signal dependent on the pedal position and inputs it to the control unit 22.

The brake pedal also controls a conventional disc brake system for the vehicle which operates together with the motors 16a, 16b, 16c, 16d to provide the required braking torque for the vehicle.

Referring to Figure 2, the engine 20 is cooled by a water cooling system comprising a radiator 30, a coolant circuit 32 for circulating water through the engine 20 and the radiator and an electric pump 33 for pumping the water round the cooling circuit. The water cools the engine in the conventional manner by transferring heat from the engine to the radiator where it is dissipated into the air that passes over the radiator.

An electric heater 34 is also provided in the cooling system such that water circulating in the system can be heated by it. The power supply to the heater is controlled by the control unit 22. The control unit can direct power generated in the motor 16a by braking of the wheel 12a either to the battery 24 or to the heater 34.

Referring back to Figure 1, in order to be able to control the speed of the vehicle automatically when it is descending a hill, the control unit 22 needs to be able to monitor the speed of the vehicle. It does this by monitoring the fluctuations in current flowing to and from the motors 16. The control unit is also connected to an inclinometer 32 which enables it to determine the steepness of the slope which the vehicle is descending. The control unit is also connected to a switch 34 which the driver of the vehicle can operate to put the control unit into and out of a hill descent control mode which will be described below. Finally the control unit 22 is connected to a potentiometer 35 which is connected to the vehicle accelerator pedal 36 and provides a signal indicative of its position.

In hill descent mode, the control unit controls the motors 16 so as to produce sufficient braking to keep the vehicle speed substantially constant whilst it descends a hill without any input from the driver through the accelerator or brake pedals. It does this by monitoring the vehicle speed as determined from the individual wheel speeds and increasing or decreasing the braking torque from the motors 16 as required to keep the vehicle speed equal to a predetermined speed. This speed as set at a nominal value, of about 5 kph, but can be varied by the driver by using the accelerator and brake pedals 26, 36 to increase or decrease the speed of the vehicle under the control of the braking torque from the motors 16.

Because in the normal hill descent mode all of the braking is provided by the motors 16, all the power generated by them during braking will be converted into electrical power. In order that hill descent mode can be maintained for long periods of time, the power produced during braking needs to be dissipated by the heater 34. The maximum dissipating power of the heater therefore dictates the maximum braking power that can be used.

The braking torque required to prevent the vehicle from accelerating increases with the angle 0 of the slope which the vehicle is descending.

However the power produced in the motors 16 is related to the product of the braking torque and the rotational speed of the motors, i.e. the speed of the vehicle. Therefore, on any given slope the braking power required to keep the vehicle at constant speed increases with the speed. Therefore on any given slope there is a maximum speed Vmax above which the braking power required to maintain a constant speed cannot be dissipated in the engine cooling system. This defines the maximum speed that the vehicle is allowed to reach under the hill descent control system.

Figure 3 shows how Vmax varies with the angle of slope 0. In this embodiment the control unit limits the vehicle speed to a lower speed Viim thereby allowing some deceleration of the vehicle to be provided by the motors 16 without the power produced exceeding that which can be dissipated.

Claims

CLAIMS 1. A vehicle power train comprising one or more electric motors capable of providing a driving torque to drive the vehicle and a braking torque to brake the vehicle, and energy dissipating means which can dissipate electrical energy generated by the motor or motors during braking up to a maximum dissipation rate, and control means arranged to control the amount of braking produced by the motor or motors when the vehicle is descending a slope to control the speed of the vehicle such that the motor or motors can produce enough braking to prevent the vehicle accelerating without the electrical power produced by the braking exceeding said maximum dissipation rate.
2. A power train according to claim 1 wherein the control unit is arranged to limit the speed of the vehicle to a limit speed which is lower than that at which the power produced to keep the vehicle speed constant would equal the maximum dissipation rate.
3. A power train according to claim 1 or claim 2 wherein the control means is arranged to receive signals from a slope measuring means and to control the speed in dependence on the measured slope.
4. A power train according to any foregoing claim further comprising an engine having a cooling system wherein the dissipation means is arranged to dissipate heat in the engine cooling system.
5. A vehicle power train substantially as hereinbefore described with reference to the accompanying drawings.