GB2555430B

GB2555430B - Vehicle brake cooling system and method

Info

Publication number: GB2555430B
Application number: GB1618157.0A
Authority: GB
Inventors: Sharp Keith; O Meachair Deaglan; Barber Daniel
Original assignee: Bentley Motors Ltd
Current assignee: Bentley Motors Ltd
Priority date: 2016-10-27
Filing date: 2016-10-27
Publication date: 2019-10-02
Anticipated expiration: 2036-10-27
Also published as: GB201618157D0; WO2018078373A1; GB2555430A

Description

VEHICLE BRAKE COOLING SYSTEM AND METHOD

Technical Field of the Invention

The present invention relates to brake cooling for a motor vehicle.

Background to the Invention

It is known to direct air flowing in and around a moving vehicle towards a vehicle brake in order to cool the brake. In one known arrangement for cooling a brake on a front wheel of a vehicle, a duct is provided in the lower surface of an under-tray below the engine compartment of the vehicle to direct a flow of air into a wheel arch where the wheel and associated brake are located. The duct has an inlet at its forward end adjacent the front bumper of the vehicle into which air flows when the vehicle is moving in a forward direction and an outlet which opens into the wheel arch. A deflector is mounted to the suspension system for the wheel which directs the air flowing from the duct towards the brake.

Whilst these ducting arrangements are effective, the velocity of the air flow and hence its mass flow rate tends to drop as it passes along the duct and is reduced further when the direction of air flow is changed by the deflector.

Other arrangements for directing cooling air flow on to a vehicle brake are also known. For example, US2002084150 discloses a splash shield for a vehicle disc brake assembly having vanes which pump cooling air across the surface of the rotor as the rotor rotates.

It is desirable to provide improved cooling arrangements for vehicle brakes as more efficient cooling can allow for the use of lower mass brake discs and drums, which in turn can lead to improved fuel economy and material cost savings. In particular, it is desirable to provide cooling arrangements for vehicle brakes which provide an increased mass flow rate of air passing over the brake than the known arrangements.

Summary of the Invention

According to a first aspect of the invention, there is provided a cooling system for a vehicle brake, the system comprising a duct having an inlet for receiving a primary air flow stream and an outlet, and a nozzle fluidly connected to an air supply for directing a secondary air flow stream into the duct, the nozzle being configured to direct the secondary air flow stream on to a surface of the duct so as to flow towards the outlet; wherein the air supply for the secondary air flow stream comprises a ram-air inlet fluidly connected with the nozzle, the ram-air inlet configured such that, in use, air flows through the inlet to the nozzle when the vehicle is moving in a forward direction.

The nozzle may be located between the inlet and the outlet of the duct.

The system may be configured such that, in use, the secondary air flow stream enters the duct at a higher velocity and pressure than the primary air flow stream upstream of the nozzle.

The system may be configured such that, in use, the combined air flow exiting the duct has greater velocity and flow rate than the primary air flow stream upstream of the nozzle.

The duct may have a Coanda profile.

In an embodiment, the duct extends along the underside of a panel forming part of the under surface of a body of the vehicle, the duct being concave and open towards the roadway in use. The duct may be provided in an under-tray of the vehicle, the duct having an inlet facing in a forward direction of the vehicle into which air is driven when the vehicle is travelling in a forward direction and an outlet which directs the combined air flow exiting the duct into a wheel arch of the vehicle in which a brake to be cooled is located. The duct may have a base and a pair of opposed side walls, and the nozzle may comprise a slot through the base, the slot extending in a lateral direction of the duct across the base and angled so as to direct the secondary air flow stream into the duct in a direction generally parallel to the base and towards the outlet of the duct. The nozzle may include a housing having an inlet connected with said air supply and which directs air from the supply through the slot.

In an alternative embodiment, the duct has a generally annular body defining a central through bore having an inlet end and an outlet end, the nozzle comprising a ring nozzle for directing the secondary air flow stream on to the surface of the bore. The bore may have a Coanda profile and the nozzle may be arranged such that the secondary air flow stream follows the profile of the bore towards the outlet end. The bore may have a converging inlet section, a cylindrical central section downstream of the inlet section, and a diverging outlet section downstream of the central section. The inlet section may be shorter that the central section and the outlet section. The inlet section may be toroidal and the outlet section may be conical. The ring nozzle may be located proximal the inlet end of the bore and may enter the bore in the inlet section. In an embodiment, an annular chamber is defined in the body of the duct, the annular chamber being connected to said air supply, the ring nozzle providing a throttled outlet from the annular chamber into the bore. The duct may be mounted to a dirt shield for a disc brake rotor, the duct directing air onto an inner surface of the disc brake rotor. The disc brake rotor may be a ventilated rotor and the duct may be arranged to direct a flow of air directly into air flow passages in the ventilated rotor. A funnel-like cowling may be attached to the annular body or formed integrally therewith surrounding the inlet to the bore.

The ram-air inlet for the secondary air flow stream supply may comprise a housing having an inlet thorough which air enters the housing and an outlet fluidly connected with the nozzle, the cross sectional area of the inlet being larger than the cross sectional area of the outlet.

In accordance with a second aspect of the invention, there is provided a vehicle having a brake cooling system according to the first aspect of the invention and/or a dirt shield for a disc brake rotor according to the second aspect of the vehicle.

In accordance with a fourth aspect of the invention, there is provided a method of cooling a brake on a vehicle, the method comprising: a. providing a duct having an inlet for receiving a primary air flow stream and an outlet for directing an air flow towards a brake; b. introducing a secondary flow of air into the duct through a nozzle, the secondary flow of air being directed on to a surface of the duct and having a higher velocity and pressure than the primary air flow stream; wherein the secondary air flow is provided by a ram-air inlet fluidly connected with the nozzle and mounted in the vehicle such that air flows through the inlet to the nozzle when the vehicle is moving in a forward direction.

The air flow exiting the duct may comprise a combination of the primary and secondary air flow streams, the combined air flow stream having a higher velocity and flow rate than the primary air flow stream upstream of the nozzle.

The nozzle may be configured as a Coanda nozzle such that the secondary air flow stream follows the surface of the duct.

Detailed Description of the Invention

In order that the invention may be more clearly understood several embodiments thereof will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 is a perspective view from above and to one side of a part of a motor vehicle illustrating a first embodiment of a brake cooling system in accordance with the invention;

Figure 2 is a plan view from above of the part of a motor vehicle of Figure 1;

Figure 3 is a view form the front of the part of the motor vehicle of Figure 1;

Figure 4 is a plan view from below of the part of a motor vehicle of Figure 1;

Figure 5 is a cross-sectional view through an under-tray duct and a nozzle forming part of the brake cooling system illustrated in Figures 1 to 4;

Figure 6 is a view similar to that of Figure 2 but omitting some details of the vehicle and having arrows to indicate the direction of air flow in the brake cooling system;

Figure 7 is a further cross-sectional view through the under-tray duct and a nozzle forming part of the brake cooling system illustrated in Figures 1 to 6 in which arrows have been added to indicate the direction of air flow through the duct and nozzle;

Figure 8 is a perspective view from above and to one side of a part of a motor vehicle illustrating a second embodiment of a brake cooling system in accordance with the invention;

Figure 9 is a plan view from below of the part of the motor vehicle of Figure 8;

Figure 10 is a cross sectional view through an air amplifier duct forming part of the brake cooling system in accordance with the second embodiment.

Figures 1 to 7 illustrate a first embodiment of a brake cooling system 10 for a motor vehicle 12 in accordance with an aspect of the invention. The brake cooling system 10 is configured to supply a flow of cooling air to a disc brake at the front of the vehicle when the vehicle is moving in a forward direction. A brake cooling system 10 in accordance with the invention will usually be provided on both sides of the vehicle in order to cool both disc brakes at the front of the vehicle. For convenience, only a brake cooling system 10 for cooling a disc brake on the left-hand side of the vehicle is shown and described. It should be appreciated that a corresponding brake cooling system 10 is provided for the disc brake on the right-hand side of the vehicle. Furthermore, whilst the brake cooling system 10 is described in relation to cooling of a front disc brake, the principles of the system could be applied to a system for cooling brakes anywhere on a vehicle and/or to cooling for other types of brakes, such as drum brakes.

The vehicle has a front bumper 14 which curves around the front of the vehicle and partially along either side as far as the front wheel arches 16, only a left-hand one of which is shown. Located within the wheel arch 16 is part of a suspension system 18 supporting a wheel hub 20 to which a vehicle wheel 2lean be mounted in a known manner. The wheel 21 is omitted in Figures 1,2 and 4 so that details of the brake cooling system can be seen. A disc brake rotor 22 is mounted on the hub 20 and forms part of a disc brake together with a disc brake caliper (not shown) in the usual way. The rotor 22 is a ventilated rotor having a pair of friction plates 22a, 22b arranged coaxially in a parallel, spaced-apart relationship and a plurality of vanes 22c extending between of friction plates. The vanes 22c define flow passages or channels 22d between each adjacent pair of vanes and are arranged so that as the brake rotor rotates about its axis, air flow is induced from the inner circumference of the rotor, through its interior, and out to the external circumference. This air flow through the disc draws heat from the brake rotor and expels it radially outward to transfer heat away from the brake rotor through convection. Air is able to enter the flow passages 22d from the side of the disc which faces inwardly of the vehicle. A lower edge region of the front bumper 14 curves inwardly underneath the vehicle body for a short distance. An under-tray 24 is mounted to the lower edge of the front bumper 14. The under-tray 24 forms part of the underbody of the vehicle enclosing an engine compartment 26 of the vehicle from below. In practice, various components of the vehicle, such as a power unit, transmission and steering rack for example, are mounted in the engine compartment. However, these and other components have been omitted from the Figures in order that details of the brake cooling system 10 can be seen.

The under-tray 24 helps to protect the components located above it from the ingress of water, dirt and debris which may be thrown up as the vehicle moves but is also used to manage the flow of air underneath the vehicle. To this end, the lower/outer surface 28 of the under-tray which faces the roadway may be contoured to direct air flowing underneath the vehicle to improve the aerodynamic performance of the vehicle.

As part of the brake cooling system 10, the under-tray 24 is contoured to define a first duct 32. The first duct 32 is defined in the lower surface 28 and is concave, opening towards the roadway in use. The first duct 32 extends from a forward end region of the under-tray 24 toward a rearward end, generally in a longitudinal direction of the vehicle. The first duct 32 has an inlet 34 at its forward end and an outlet 36 at its reward end which opens into the wheel arch 16. When the vehicle is moving forwardly, air flowing underneath the front bumper, or alternatively through an opening in the front bumper, enters the first duct through the inlet 34 as a primary air flow stream 38 which travels along the duct towards the outlet 36.

The first duct 32 is generally channel shaped having a largely planar base 40 and a pair of opposed side walls 42 and is wider than it is deep. The duct 32 increases in depth from the inlet end 34 towards the outlet end 36, having a wedge-like shape. A nozzle 44 for introducing a secondary air flow 46 into the duct is mounted to the base 40 on the upper surface of the under-tray. The nozzle 44 has an outlet 48 in the form of a slot extending through the base 40 in a lateral direction of the duct 32 and substantially over its full width. As can be seen best in Figure 5, the nozzle outlet 48 is tapered, narrowing towards the outer surface 28 of the under-tray 24, and is angled so as to direct the secondary air flow 46 into the duct 32 substantially parallel to the base 40 and in the same general direction as the primary air flow 38, that is to say towards the outlet 36 of the duct 32. The nozzle 44 is located towards a rearward end of the duct 32, close to the outlet 36. The primary and secondary air flows 38, 46 form a combined air flow 50 which exits the duct 32 through the duct outlet 36 into the wheel arch.

Whilst the first duct 32 extends generally in a longitudinal direction of the vehicle, it is angled slightly so that the combined air flow 50 exiting the duct 32 is directed laterally outwardly in the general direction of the brake disc rotor 22 to some extent, though the combined air stream 50 still flows largely in a rearward direction of the vehicle. In order that the combined air stream 50 is more accurately targeted at the brake disc rotor 22, it is directed onto a deflector 52 mounted to the suspension 18 which turns the combined air flow 50 so that it flows laterally outwardly of the vehicle towards the inner diameter of brake disc rotor 22 where it is able to enter the flow passages 22d of the ventilated disc.

The nozzle 44 is fluidly connected with a source of pressurised air so that the secondary air flow stream entering the first duct 32 is at a higher pressure and higher velocity than the primary air flow stream 38 approaching the nozzle 44. In accordance with the invention, in an arrangement as illustrated in the accompanying drawings, relatively high pressure air is ducted from the front of the vehicle. To this end the nozzle 44 is fluidly connected, say by means of a length flexible pipe 54, with a funnellike secondary air flow inlet housing 56 mounted to the inner surface of the front bumper 14. The inlet housing 56 has forwardly facing inlet opening 57 at a front end into which high-velocity air 58 flows through an opening in the front bumper 14 when the vehicle is travelling in a forward direction, particularly when the vehicle is traveling at speed. The inlet housing 56 tapers to an outlet 60 at the rear of the housing to which the pipe 54 is connected. The nozzle 44 comprises a housing having an nozzle inlet 62 to which the pipe 54 is fluidly connected and a main body or cowl 64 which covers nozzle outlet slot 48 so that the air flowing through the pipe is directed into the outlet slot 48 and passes through the slot 48 to enter the duct 32 as the secondary air flow stream 46.

The secondary air flow inlet housing 56 acts like a ram inlet to provide a supply of pressurised air to the nozzle when the vehicle is moving in a forward direction. The cross-sectional areas of the outlet 60 of the secondary air flow inlet housing 56 and the pipe 54 are considerably smaller than the cross-sectional area of the inlet to the secondary air flow inlet housing 56 so that the pressure of the air flow increases as it passes through the inlet housing 56 to enter the pipe 54. The increase in pressure helps to maintain the velocity of the air flow so that it is at a relatively high velocity and pressure as it enters the duct 32. In contrast the primary air flow 38 has a lower velocity where it enters the duct 32 than the air 58 entering the secondary air flow inlet housing 56 and the velocity and pressure of the primary air flow 38 may fall further as it passes along the duct 32, especially if the cross-sectional area of the duct increases between its inlet 36 and the position of the nozzle outlet 48, as may often be the case. As a consequence, the secondary air flow 46 entering the duct 32 through the nozzle outlet slot 48 has a higher velocity and higher pressure than the primary air flow 38 upstream of the nozzle. The introduction of a higher velocity, higher pressure secondary air stream 46 into the duct 32 increases the overall velocity of the combined air stream 50 flowing through the duct 32 downstream of the nozzle outlet 48 compared to the velocity of the primary air flow stream 38 upstream of the nozzle. Due to its increased velocity, the combined air flow 50 exiting the duct 32 is able to travel further with an increased mass-flow rate than would be the case for the primary flow stream only without the secondary air flow 48. This improves cooling of the brake as increased mass-flow rate improves heat transfer from the brake disc rotor 22 into the air stream.

The nozzle 44 is designed as a Coanda nozzle so that the high-velocity secondary air flow stream 46 enters parallel to the base 40 of the duct and follows the surface of the duct. The secondary air flow 46 induces an increased flow rate in the primary air flow 38, thus increasing the overall mass flow rate of the combined air flow stream 50 through the duct 32 when compared to the flow through an equivalent duct without the secondary air flow 46. The secondary air flow 46 also enhances the directional integrity of the combined air flow 50. This arrangement enables a relatively small volume of high velocity, high pressure secondary air flow 46 to be used to increase the mass flow rate of the much larger volume primary flow stream 38. A second embodiment of a brake cooling system for a vehicle, indicated generally at 100, will now be described with reference to Figures 8 to 10. Components of the second embodiment which are the same as those in the first embodiment, or which perform the same function, will be given the same reference number but increased by 100 in each case.

The brake cooling system 100 in accordance with the second embodiment may be combined with the brake cooling system 10 of the first embodiment or it may be used independently.

As with the first embodiment, the brake cooling system 100 will be described with reference to cooling of a front brake disc on the left-hand side of a motor vehicle. It should be appreciated that a corresponding brake cooling system is provided for the front disc brake on the right-hand side of the vehicle so that both disc brakes at the front of the vehicle are cooled. It should also be appreciated that the principles of the brake cooling system 100 can be adapted for cooling of a brake anywhere on a vehicle and for cooling other types of a brakes, such as drum brakes.

Figures 8 and 9 illustrate a left-hand front quarter section of a motor vehicle 112 similar to that illustrated in Figures 1 to 4, in which a front bumper 114 extends about the front of the vehicle and down the side as far as a wheel arch 116. Located in the wheel arch 116 is part of a suspension system 118 supporting a wheel hub 120 to which a vehicle wheel (not shown) can be mounted in a known manner. A ventilated disc brake rotor 122 is mounted on the hub 120. The disc brake rotor 122 forms part of a disc brake together with a disc brake caliper (not shown) in the usual way. A lower edge region of the front bumper 114 curves inwardly underneath the vehicle body for a short distance and an under-tray 124 is mounted to the lower edge of the front bumper 114 to form part of the underbody of the vehicle enclosing an engine compartment of the vehicle from below.

In common with the first embodiment, the brake cooling system 100 has a first duct 132 defined in the lower surface 128 of the under-tray 124 which introduces a flow of air 168 into the wheel arch 116 when the vehicle is traveling in a forward direction. The air flow 168 exiting the first duct 132 is directed on to a deflector 152 mounted to the suspension which turns the air fl owl 68 so that it travels laterally outwardly towards the brake disc rotor 122. The first duct 132 and the deflector are constructed and operate in substantially the same way as the first duct 32 and deflector 52 described above in relation to the first embodiment, except that in the second embodiment there is no nozzle for introducing a secondary air flow into the first duct. The depth of the first duct 132 increased from its inlet end towards the outlet so that the duct is wedge shaped, as can be seen most clearly in Figure 8 which shows how the base 140 and side walls of the duct are inset into the under-tray, especially towards the deeper outlet end. As discussed previously, much of the velocity of the air flow 168 from first duct is lost as its direction is changed by the deflector 152 so that the velocity of air flow approaching the disc brake rotor 122 from the first duct 132 is relatively low. A dirt shield 170 is mounted to the suspension 118 on the inside of the brake disc rotor 122 in a known manner to protect the rotor from dirt and water. An air amplifier duct 174 is mounted to the inner face of the dirt shield 170 and directs air through an aperture in the dirt shield and directly into the flow passages 122d in the ventilated rotor 122. The air amplifier duct 174 has a generally cylindrical body 175 defining a central through bore 176. The bore 176 has a converging, toroidal inlet section 178, a cylindrical central section 180 downstream of the inlet section, and a diverging conical outlet section 182 downstream of the central section 180. Air surrounding the air amplifier duct 174 enters the bore 176 through the inlet section 178 to form a primary air flow 138 through the bore 176. In the present embodiment, the primary air flow 138 includes at least some of the air flow 168 from the first duct 132 and the deflector 152. The inlet section 178 is relatively short in comparison with either the central cylindrical section 180 or the outlet section 182. As illustrated in Figure 8, a funnel-like cowling can be mounted to the inlet end of the body 175 to help direct air from the deflector 152 and elsewhere in the vicinity into the bore 176.

An annular chamber 184 is defined in the body 175 of the air amplifier duct 174 in a region surrounding the central section 180 of the bore. The annular chamber 184 has an inlet 186 which is connected to a source of pressurised air, that is to say air which is at a higher pressure than the primary air flow 138 entering the bore 176. An outlet from the annular chamber 184 is provided by a small ring nozzle 188 defined in the body of the air amplifier duct 174 between the annular chamber 184 and the inlet section 178 of the through bore 176. The ring nozzle 188 forms a throttled outlet through which pressurised air from the annular chamber 184 enters the bore 176 at high velocity to form a secondary air flow 148. The air amplifier duct 174 is constructed as a Coanda nozzle with the bore 176 defining a Coanda profile so that the high-velocity secondary air flow 148 follows the contour of the bore 176, which directs the secondary air flow 148 towards the outlet 182. A low pressure region 190 is created at the centre of the bore surrounded by the high pressure, high velocity secondary air flow 148. The presence of the low pressure region 190 induces the primary air flow 138 to pass through the bore 176 to the outlet 182 with an increased velocity. The combined flow 150 of the secondary air flow 148 and the primary air flow 138 exhausts from the air amplifier duct 174 in a high volume, high velocity flow 150 which is directed into the flow passages 122d of the vented rotor.

The air amplifier duct 174 uses the secondary air flow 146 to create a combined flow of air 150 through the rotor 122 which is at a higher volume and higher flow rate than would be possible if relying only on the first duct 132 and deflector 152. The increase in velocity and mass flow rate of the air passing through the rotor from the air amplifier duct 174 efficiently cools the disc brake rotor. Furthermore, increasing the velocity of air passing through the disc 122 also increases the velocity of the air exiting the disc. Air surrounding the disc will be entrained into the air flow exiting the disc which will draw more air over the outer surfaces of the rotor. This will also add to the increased cooling effect on the rotor 122.

To provide a source of pressurised air for the secondary air flow, relatively high pressure air is ducted from the front of the vehicle in a manner similar to the first embodiment and as illustrated in Figures 8 and 9. In this arrangement, the inlet 186 to the annular chamber is fluidly connected, say by means of a flexible pipe 154, with a funnel-like secondary air flow inlet housing 156 mounted to the inner surface of the front bumper 114. The housing 156 is similar to the housing 56 in the first embodiment described. Accordingly, a detailed description of the housing 156 will not be provided and the user should refer to the description of the secondary air flow inlet housing 56 of the first embodiment for details.

Whilst the combination of a first duct 132 in the under-tray to deliver a flow of cool air into the wheel arch 116 and an air amplifier duct 174 is particularly effective at cooling the brake rotor 122, it will be appreciated that the air amplifier duct 174 could be used on a vehicle without the first duct 132. In this case, the secondary air flow 146 will draw in air surrounding the inlet 176 to form the primary air flow 138 or alternative means could be used to direct a flow of air towards the inlet of the air amplifier duct. It should also be recognised that the air amplifier duct could be used in combination with a first duct 32 in accordance with the first embodiment, in which the first duct 32 is also provided with nozzle 44 for introducing a higher pressure, higher velocity secondary air flow into the first duct in order to accelerate the flow of air from the first duct into the wheel arch. In this case, the same source of pressurised air could be used to supply both the nozzle 44 for the first duct 32 and the annular chamber 184 of the air amplifier duct 174 or they could be supplied from different sources.

The above embodiments are described by way of example only. Many variations are possible without departing from the scope of the invention as defined in the appended claims.

Claims

1. A cooling system for a vehicle brake, the system comprising a duct having an inlet for receiving a primary air flow stream and an outlet, and a nozzle fluidly connected to an air supply for directing a secondary air flow stream into the duct, the nozzle being configured to direct the secondary air flow stream on to a surface of the duct so as to flow towards the outlet; wherein the air supply for the secondary air flow stream comprises a ram-air inlet fluidly connected with the nozzle, the ram-air inlet configured such that, in use, air flows through the ram-air inlet to the nozzle when the vehicle is moving in a forward direction.

2. A cooling system for a vehicle brake as claimed in claim 1, wherein the nozzle is located between the inlet and the outlet of the duct.

3. A cooling system for a vehicle brake as claimed in claim 1, wherein the system is configured such that, in use, the secondary air flow stream enters the duct at a higher velocity and pressure than the primary air flow stream upstream of the nozzle.

4. A cooling system for a vehicle brake as claimed in any one of claims 1 to 3, wherein the system is configured such that, in use, the combined air flow exiting the duct has greater velocity and flow rate than the primary air flow stream upstream of the nozzle.

5. A cooling system for a vehicle brake as claimed in any one of claims 1 to 4, wherein the duct has a generally annular body defining a central through bore having an inlet end and an outlet end, the nozzle comprising a ring nozzle for directing the secondary air flow stream on to the surface of the bore.

6. A cooling system for a vehicle brake as claimed in claim 5, wherein the nozzle is arranged such that the secondary air flow stream follows the profile of the bore towards the outlet end.

7. A cooling system for a vehicle brake as claimed in claim 5 or claim 6, wherein the bore has a converging inlet section, a cylindrical central section downstream of the inlet section, and a diverging outlet section downstream of the central section.

8. A cooling system for a vehicle brake as claimed in any one of claims 5 to 7, wherein the ring nozzle is located proximal the inlet end of the bore.

9. A cooling system for a vehicle brake as claimed in any one of claims 5 to 8, wherein an annular chamber is defined in the body of the duct, the annular chamber being connected to said air supply, the ring nozzle providing a throttled outlet from the annular chamber into the bore.

10. A cooling system for a vehicle brake as claimed in any one of claims 5 to 9, wherein the duct is mounted to a dirt shield for a disc brake rotor, the duct directing air onto an inner surface of the disc brake rotor.

11. A cooling system for a vehicle brake as claimed in claim 10, wherein the disc brake rotor is a ventilated rotor and the duct directs a flow of air directly into air flow passages in the ventilated rotor.

12. A cooling system for a vehicle brake as claimed in any one of claims 1 to 4, wherein the duct extends along the underside of a panel forming part of the under surface of a body of the vehicle.

13. A cooling system for a vehicle brake as claimed in claim 12, wherein the duct is provided in an under-tray of the vehicle, the duct having an inlet into which air can flow when the vehicle is travelling in a forward direction and an outlet which directs the combined air flow exiting the duct into a wheel arch of the vehicle in which a brake to be cooled is located.

14. A cooling system for a vehicle brake as claimed in claim 12 or claim 13, wherein the duct has a base and a pair of opposed side walls, the nozzle comprising a slot through the base, the slot extending in a lateral direction of the duct across the base and angled so as to direct the secondary air flow stream into the duct in a direction generally parallel to the base and towards the outlet of the duct.

15. A cooling system for a vehicle brake as claimed in claim 14, wherein the nozzle comprises a housing having an inlet connected with said air supply and which directs air from the supply through the slot.

16. A cooling system for a vehicle brake as claimed in any one of the preceding claims, wherein the air supply ram-air inlet comprises a housing having an inlet thorough which air enters the housing and an outlet fluidly connected with the nozzle, the cross sectional area of the inlet being larger than the cross sectional area of the outlet.

17. A vehicle having a cooling system for a vehicle brake as claimed in any one of claims 1 to 16.

18. A method of cooling a brake on a vehicle, the method comprising: a. providing a duct having an inlet for receiving a primary air flow stream and an outlet for directing an air flow towards a brake; b. introducing a secondary flow of air into the duct through a nozzle, the secondary flow of air being directed on to a surface of the duct and having a higher velocity and pressure than the primary air flow stream; wherein the secondary air flow is provided by a ram-air inlet fluidly connected with the nozzle and mounted in the vehicle such that air flows through the inlet to the nozzle when the vehicle is moving in a forward direction.

19. A method as claimed in claim 18, wherein the air flow exiting the duct comprises a combination of the primary and secondary air flow streams, the combined air flow stream having a higher velocity and flow rate than the primary air flow stream upstream of the nozzle.

20. A method as claimed in claim 18 or claim 19, wherein the nozzle is configured such that the secondary air flow stream follows the surface of the duct.