GB2513447A

GB2513447A - An improved intercooler for an engine

Info

Publication number: GB2513447A
Application number: GB1402898.9A
Authority: GB
Inventors: Tom Robert George Thompson; David Davies
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2013-04-18
Filing date: 2014-02-19
Publication date: 2014-10-29
Anticipated expiration: 2034-02-19
Also published as: GB201307053D0; GB2513171B; DE102014105059A1; GB201402898D0; GB2513447B; GB2513446B; GB2513446A; RU152512U1; GB2513171A; GB201402897D0

Abstract

An air-to-air intercooler 5 comprising a charge air inlet 15 and outlet 16, a heat transfer unit 17, for example a matrix, having a front face via which ambient air enters and a rear face via which ambient air exits, and an ambient air flow control device 10, positioned towards the inlet, preferably on the rear face, and extending for less than the full length of the heat transfer unit, preferably less than half the full length, controlled based upon either the temperature of the charge air exiting the heat transfer unit or in the outlet. The flow control device preferably includes a number of flow control members, for example rotatable slats 11, opened and closed by a powered actuator 20, for example a rotary or solenoid actuator, in response to the temperature sensed by a sensor positioned in the outlet. The intercooler reduces the risk of condensation forming by avoiding overcooling of the charge air.

Description

An Improved Intercooler for an Engine This invention relates to an intercooler for an internal combustion engine and, in particular, to an intercooler for an engine of a motor vehicle.

Forced induction engines such as turbocharged and supercharged engines are configured to compress arrbient air entering the engine in order to increase power. Because compression of the air will cause an increase in temperature of the air it is known to use an intercooler or charge-air cooler to cool the air before it is indicted into the engine thereby increasing the density of the charge air.

is If the humidity of the ambient air is high, condensation in the form of water droplets may form on any internal surface of the intercooler that is cooler than the dew point of the compressed charge air. During certain engine operating conditions such as, for example, hard acceleration these water droplets may be blown out of the intercooler and into the cortustion chambers of the engine resulting in engine misfire, loss of torgue and potential damage to components of the engine.

The formation of such water droplets is often due to over cooling of the charge air when the engine is running at light load due to the need to design the intercooler to sufficiently cool the induction air when operating at maximum load and speed.

It is known from, for example, US Patent US6314950 to provide a cooler flow control device that uses a control system to control the position of flaps located on a front face of the cooler that extend across the entire width of the cooler and Patent publication W02008/002265 discloses a cooler flow control device that uses a pneumatic or hydraulic cylinder to move flaps in order to control the flow of ambient air through the cooler.

It is further known from, for example, Patent publications DE 102009 036 745 and US2002/0l95090 to provide a cooler flow control device having flaps on a front face of the cooler.

It is an object of the invention to provide an air to air intercooler for an engine in which the risk of the formation of condensation within the intercooler is reduced in a simple to control manner and with minimum impact on the underlying performance of the intercooler.

According to a first aspect of the invention there is provided an intercooler for an engine comprising an inlet via which charge air enters the intercooler, an outlet via which charge air exits the intercooler and a heat transfer unit through which the charge air flows from the inlet of the intercooler to the outlet of the intercooler, the heat transfer unit having an inlet end via which charge air enters the heat transfer unit, an outlet end via which charge air exits the heat transfer unit, a front face via which ambient air enters the heat transfer unit and a rear face via which ambient air exits the heat transfer unit, the intercooler further comprising an ambient air flow control device to vary the flow of ambient air through the part of the heat transfer unit that it overlies wherein the ambient air flow control device is positioned towards the inlet end of the heat transfer unit, extends for less than the full length of the heat transfer unit and is controlled based upon one of the temperature of the charge air exiting the heat transfer unit and the temperature of the charge air in the outlet of the intercooler.

The ambient air flow control device may extend for less than half of the length of the heat transfer unit.

The ambient air flow control device may be located on the rear face of the heat transfer nnit.

The ambient air flow control device may include a number of flow control members that are mcveable between open and closed positions by a powered actuator that is controlled by an electronic controller to vary the flow of ambient air through the ambient air flow control device.

The position of the flow control members may be based upon the temperature of the charge air in the outlet of the intercooler.

The temperature of the charge air in the outlet of the intercooler may be sensed by a temperature sensor positioned in the outlet of the intercooler and opening and closing of the flow control members may be performed by the powered actuator in response to the temperature sensed by the temperature sensor.

The powered actuator may be one of an electrical rotary actuator and an electrical solenoid actuator.

The flow control members may be rotatable slats.

The heat transfer unit may be a heat transfer matrix.

According to a second aspect of the invention there is provided a motor vehicle having an intercooler constructed in accordance with said first aspect of the invention.

The invention will now be described by way of example with reference to the accompanying drawing of which:-Fig.1 is a diagrammatic rear view of a first embodiment of an Intercooler for an engine constructed in accordance with a first aspect of the invention showing an ambient air flow control device in a closed position; Fig.2 is a view similar to Fig.1 but showing the ambient air flow control device in an open position; Fig.3 is a diagrammatic plan view of the intercooler shown in Fig.1; Fig.4 is a diagrammatic plan view of the intercooler shown in Fig.2; Fig.5 is a diagrammatic rear view similar to that shown in Fig.l but showing a second embodiment of an intercooler for an engine constructed in accordance with the first aspect of the invention showing the ambient air flow control device in a closed position; Fig.6 is an enlarged view of the ambient air flow control device shown in Fig.5; and Fig.7 is a diagrammatic side view of a motor vehicle constructed in accordance with a second aspect of the invention having an intercooler constructed in accordance with the first aspect of the invention.

Referring firstly to Fig.7 there is shown a motor vehicle 1 having an internal combustion engine 2. An intercooler 5 is connected to an air induction system of the engine 2 by respective supply and return conduits indicated by a single line 3 on Fig.7. Hot charge air from the air induction system passes through passageways formed in the intercooler 5 and is cooled by ambient air through which the motor vehicle 1 is passing as indicated by the arrow D' In some cases an intercooler fan may be provided to maintain an air flow through the intercooler 5 even when the motor vehicle 1 is not moving. The charge air is returned after cooling to the air induction system of the engine 2.

To prevent the formation of condensation within the intercooler 5 an ambient air flow control device 10 is provided on a rear or ambient air downstream side of the intercooler 5. The ambient air flow control device 10 is operable to control or regulate the flow of ambient air through the intercooler 5 and can be of any suitable design.

The ambient air flow control device 10 is preferably located near to an inlet end of the intercooler 5 and extends for only part of the length of the intercooler 5.

When the ambient air flow control device 10 is in an open position, the flow of ambient air through the portion of the intercooler 5 over which it extends is substantially unaffected compared to a case where no ambient air flow control device is used. When the ambient air flow control device 10 is in a closed position, the flow of ambient air through the portion of the intercooler 5 over which it extends is significantly reduced compared to a case where no ambient air flow control device is used and substantially no ambient air can flow through the portion of the intercooler S over which it extends while the ambient air flow control device 10 is in the closed position.

Referring now to Figs.1 to 4 there is shown in more detail a first embodiment of the intercooler 5.

The intercooler 5 comprises inlet and outlet end tanks and 16 through which charge air respectively enters and leaves the intercooler 5 as indicated by the arrows VA and "B" and a central body portion 17 supporting an air to air heat transfer unit in the form of heat transfer matrix 6 in which heat transfer from the charge air to the ambient air takes place during use. The heat transfer matrix 6 has a length "L", height "H" and a width "W".

The inlet tank 15 is located at a charge air inlet end of the intercooler 5 and the outlet end tank 16 is located at a charge air ontlet end of the intercooler 5.

As is well known in the art, the heat transfer matrix 6 includes one or more charge air flow passages (not shown) through which the charge air flows in a generally longitudinal direction of the intercooler 5 from an inlet end of the heat transfer matrix 6 to an outlet end of the heat transfer matrix 6 and a plurality of fins (not shown) exposed to the flow of ambient air and conductively connected to the charge air flow passages so as to conduct heat away from the charge air flow passage or passages.

The air to air heat transfer unit can be of any known type and the invention is not limited to the use of a particular type of air to air heat transfer unit or to the use of a particular type of heat transfer matrix.

2\ri ambient air flow control device 10 is attached to a rear face of the intercooler 5 so as to be located on a downstream side of the heat transfer matrix 6.

In the example shown, the ambient air flow control device 10 is positioned close to the charge air inlet end of the intercooler 5 that is to say, towards the inlet end tank and overlies a portion of the heat transfer matrix 6 at an inlet end of the heat transfer matrix 6. The ambient air flow control device 10 is located on a rear face 7 of the heat transfer matrix 6.

The size of the ambient air flow control device 10 will depend upon a number of factors including, but not limited to, the total heat transfer capacity of the heat transfer matrix 6 and the amount of heat that needs to extracted from the charge air when the engine 2 is operating at low power to prevent condensation occurring.

It is not necessary for the ambient air flow control device 10 to extend along the heat transfer matrix 6 for the entire length "L" of the heat transfer matrix 6 in order to prevent condensation. In most cases the ambient air flow control device 10 need only extend for less than half the length "L" of the heat transfer matrix 6 and, in the example shown, the ambient air flow control device 10 extends for approximately one third of the length "L" of the heat transfer matrix 6.

The ambient air flow control device 10 comprises a frame 13 moveably supporting a number of rotatable louvres or slats U and an actuator 20 for moving the slats U In the example shown the slats 11 are arranged to extend in a vertical direction but it will be appreciated that they could alternatively be arranged to extend horizontally.

Each of the slats II can rotate from a fully closed position (as shown in Figs.l and 3) to a fully open position (as shown in Figs.2 and 4) . When in an open position, a number of ambient air flow passages 12 are produced between adjacent open slats 11 through which ambient air can flow (Only shown on Figs.2 and 4) All of the slats 11 are mechanically connected together and are moved by a single powered actuator in the form of a electrical rotary actuator 20 (only partially visible on Figs.3 and 4).

The electrical rotary actuator 20 is controlled by an electronic controller 50 in response to a measurement of the temperature of the charge air exiting the intercooler 5 measured by a temperature sensor 51 that senses the temperature of the charge air in the outlet tank 16.

therefore in this preferred embodiment the temperature sensed is the temperature of the charge air in the outiet tank 16 of the intercooler 5 and the temperature of the charge air exiting the heat transfer unit/ matrix 6 can be inferred from this measurement.

It is also be possible but more expensive/ complicated to locate one or more temperature sensors within the charge air flow passages of the heat transfer unit/ matrix 6 at or near to the outlet end of the heat transfer unit/ matrix 6 so as to directly measure the temperature of the charge air exiting the heat transfer unit/ matrix 6.

The electrical rotary actuator 20, which in this case is an electric motor, is mechanically coupled to all of the slats 11 via a drive mechanism (not shown) and is operable to rotate the slats 11 so as to open the slats 11 (or maintain the slats 11 in a fully open position if the slats 11 are already in the fully open position) when the temperature of the charge air exiting the intercooler 5 is such that condensation in unlikely to occur and rotate the slats 11 so as to close the slats 11 (or keep the slats 11 in the fully closed position) when the temperature of the charge air exiting the intercooler 5 is such that condensation could occur.

By controlling the slats 11 in this manner maximises engine performance by providing maximum cooling when required while preventing the formation of condensation within the charge air passages of the heat transfer matrix 6 by reducing the cooling effect when the charge temperature is lower.

Operation of the ambient air flow control device 10 is as follows. During operation of the motor vehicle 1 ambient air flows through the heat transfer matrix 6 in the direction of the arrows "5" shown on Figs. 3 and 4.

The passage of the ambient air through the heat transfer matrix 6 will cool the charge air as it passes through the heat transfer matrix 6 from the front face 8 of the heat transfer matrix 6 to the downstream or rear face 7 of the heat transfer matrix 6 and the temperature of the ambient air will increase. The temperature of the charge air exiting the intercooler 5 is used in this embodiment as an indication of the probable charge air temperature in the heat transfer matrix 6 and, in particular, whether the charge air temperature is likely tc be low enough at any position within the heat transfer matrix 6 to produce a risk of condensation occurring within the charge air passages of the heat transfer matrix 6.

The charge air enters the intercooler 5 via the inlet end tank 15 and exits the intercooler 5 via the outlet end tank 16 as indicated by the arrows "A" and "B".

If the temperature of the charge air exiting the intercooler 5 is high then it can be inferred that there is no risk of condensation forming and so the ambient air flow control device 10 assumes a fully open state as shown in Figs.2 and 4 in which virtually no restriction to ambient airflow takes place.

However, if the temperature of the charge air exiting the intercooler 5 falls below a certain temperature there is a risk of condensation occurring within the charge air flow passages at or near to the outlet end of the heat transfer matrix 6 or within the outlet end tank 16. Cooling of the charge air must therefore be reduced in order to reduce the risk of condensation occurring and so the electrical rotary actuator 20 is operable to rotate the slats 11 from their respective fully open positions towards their respective -10 -closed positions in order to reduce the cooling effect of the heat transfer matrix 6 In the region over which the ambient air flow control device 10 is positioned. If the temperature of the charge air continues to fall or is still too low, the electrical rotary actuator 20 will continue to move the slats 11 towards their respective closed positions until, eventually, the slats 11 are in their respective closed positions (as shown in Figs.1 and 3). That is to say, the slats 11 are adaptively moved based upon the actual sensed temperature of the charge air exiting the intercooler so as to reduce the risk of condensation forming in the heat transfer unit matrix 6. If the temperature of the charge air exiting the intercooler 5 rises above a temperature where there is no risk of condensation forming then the electrical rotary actuator 20 will rotate the slats 11 towards their open position.

In an alternative embodiment, the slats 11 are either fully open or fully closed and the electrical rotary actuator 20 maintains the slats in a fully open position when the temperature of the charge air exiting the intercooler 5 exceeds a first upper temperature threshold and fully closes the slats 11 when the temperature of the charge air exiting the intercooler S falls below a second lower temperature threshold and holds the slats 11 in their respective fully closed positions until the temperature of the charge air exiting the intercooler S rises above the second lower temperature threshold and so restricts cooling of the charge air only when there is a real risk of condensation occurring.

When the slats 11 are in their closed positions substantially no ambient air can flow through the part of the heat transfer matrix 6 covered by the ambient air flow control device 10. Therefore, when the ambient air flow control device 10 is in a closed state, the effective cooling area of the heat transfer matrix 6 is significantly

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reduced compared to the situation when ambient air flow control device 10 is in an open state. When the ambient air flow control device 10 is in the open state, there is only a small reduction in the cooling area of the heat transfer matrix 6 due primarily to the presence of the frame 13 of the ambient air flow control device 10.

It will be appreciated that the cooling effect of the heat transfer matrix 6 per unit area is considerably higher at the charge air inlet end of the intercooler 5 than it is at the charge air outlet end of the intercooler 5 due to the greater temperature difference at the charge air inlet end of the intercooler 5 compared to the oharge air outlet end of the intercooler 5.

In addition, due to the very high cooling effect of the heat transfer matrix 6 towards the charge air inlet end of the intercooler 5, if the ambient air flow control device 10 were to be positioned close to the charge air outlet end of the intercooler 5 there is a possibility that the charge air could be overcooled that is to say, cooled below the dew point before the charge air reaches the ambient air flow control device 10, thereby risking condensation within the charge air flow passages.

Referring now to Figs.5 and 6 there is shown a second embodiment of an intercooler 105 that is intended to be a direct replacement for the intercooler 5 shown in Figs.1 to 4 and Fig.7.

The intercooler 105 comprises inlet and outlet end tanks 115 and 116 through which charge air respectively enters and leaves the intercooler 105 as indicated by the arrows "A" and "B" and a central body portion 117 supporting an air to air heat transfer unit in the form of a heat transfer matrix 106 in which heat transfer from the charge air to the ambient air takes place during use. The inlet -12 -tank 115 is located at a charge air inlet end of the intercooler 105 and the outlet end tank 116 is located at a charge air outlet end of the intercooler 105.

As before, the heat transfer unit can be of any known type and is not limited to the use of an air to air heat transfer matrix.

Mi ambient air flow control device 110 is attached to a rear face of the intercooler 105 so as to be located on an ambient air downstream side of the heat transfer matrix 106.

The ambient air flow control device 110 is positioned close to the charge air inlet end of the intercooler 105 that is to say, towards the inlet end tank 115 and overlies a portion of the heat transfer matrix 106 located at an inlet end of the heat transfer matrix 106.

As before, the size of the ambient air flow control device 110 will depend upon a number of factors and but will normally not need to extend for more than half of the length of the heat transfer matrix 106 as measured from the inlet end of the heat transfer matrix 106 in order to prevent condensation from occurring. In the example shown the ambient flow control device 110 extends for approximately one sixth of the length of the heat transfer matrix 106.

The ambient air flow control device 110 comprises of a frame 113 moveably supporting a number of rotatable louvres or slats and in this case three rotatable slats 111. The slats 111 are arranged to extend in a horizontal direction but it will be appreciated that the slats could alternatively be arranged to extend vertically as for the first embodiment.

Each of the slats 111 can rotate from a closed position (as shown in Fig.5 and 6) to an open position (not shown) In the open position a number of ambient air flow passages -13 -are produced between the open slats 111 through which ambient air can fiow.

All of the siats 111 are mechanically connected together and are moved by a single powered actuator which in this case is in the form of an electrical sclenoid actuator 120. The term solenoid actuator' as meant herein includes all electrical actuators that produce a substantially linear actuating output.

The solenoid actuator 120 has an output rod 121 which is connected to a rotatable arm 123. The arm 123 is connected via a linkage (nct shown) to all cf the slats 111 so that axial movement of the output rod 121 produces opening and closing of the slats 111.

The solenoid actuator 120 is operatively connected to an electronic controller or control unit 150 which may be an electronic control unit 150 that is provided only for controlling operation of the ambient air flow control device or couid be an electronic controller or control unit having other control functions such as, for example, an engine control unit (ECU) A temperature sensor 151 provides a signal indicative of the temperature of the charge air exiting the heat exchange matrix 106 and senses the temperature of the charge air in the outlet tank 116. The temperature sensor 151 could be unigue to a control system including the ambient air flow control device 110 or could be shared with one or more other systems. Advantageously, the temperature sensed is the temperature of the charge air in the outlet tank 116 of the intercooler 105 from which the temperature of the charge air exiting the heat transfer unit/ matrix 106 can be inferred. It would also be possible to locate one or more temperature sensors within the charge air flow passages of the heat transfer unit/ matrix 106 at or near to the outlet -14 -end of the heat transfer unit/ matrix 106 so as to directly measure the temperature of the charge air exiting the heat transfer unit/ matrix 106.

Therefore, in this second embodiment a control system for controlling the temperature of the oharge air is formed by the ambient air control device 110, the solenoid actuator and its associated linkages, the electronic control unit and the temperature sensor 151.

Control of the slats 111 is based upon the temperature of the charge air exiting the heat exchange matrix or the interoooler 105 and the solenoid actuator 120 is operable, in response to a control signal from the electronic control unit 150, to open the slats 111 (or maintain the slats 111 fully open if already in the fully open position) when the charge air out temperature is high and close the slats 111 (or keeps them in the fully closed position) when the charge air oft temperature falls such that condensation is likely to occur.

The electronic control unit 150 may be operable to maintain, so far as possible, the temperature of the charge air exiting the intercooler 105 within a predefined range by continuously varying the flow of ambient air through the ambient air flow control device 110 to either increase the temperature of the charge air or reduce it depending upon the sensed temperature.

Such control of the slats 111 will maximise engine performance by providing high cooling when the charge air is hot while preventing the formation of condensation within the charge air passages of the heat transfer matrix 106 when the charge air is cool.

Operation of the ambient air flow control device 110 is as follows. During operation of the motor vehicle 1, -15 -ambient air will flow through the heat transfer matrix 106 from a front face of the heat transfer matrix 106 and exit via a rear face 107 upon which the ambient air flow control device 110 is located.

The passage of the ambient air through the heat transfer matrix 106 will cool the charge air as it passes through the heat transfer matrix 106. The charge air enters the intercooler 105 via the inlet end tank 115 and exits the intercooler 105 via the outlet end tank 116, as indicated by the arrows "A" and "B".

If the charge air out temperature sensed by the temperature sensor 151 is high, there is no risk of condensation forming and the ambient air flow control device is controlled by the electronic control unit 150 to adopt a fully open state in which virtually no restriction to ambient airflow takes place.

However, as the ambient air out temperature falls there is an increasing risk of condensation occurring within the charge air flow passages at or near to the outlet end of the heat transfer matrix 106 or within the outlet end tank 116 and cooling of the charge air must therefore be reduced in order to reduce the risk of condensation occurring.

The solenoid actuator 120 is therefore controlled by the electronic control unit 150 to move the slats 111 from their respective fully open positions towards their respective closed positions in order to reduce the cooling effect of the heat transfer matrix 106 in the region over which the ambient air flow control device 10 is positioned.

If the temperature of the charge air exiting the intercooler 105 is still too low or continues to fall, the solenoid actuator 120 will continue to move the slats 111 towards their respective closed positions until eventually -16 -all of the slats 111 are in their respective closed positions (as shown in Figs.5 and 6) When the slats 111 are all in the closed position substantially no ambient air can flow through the part of the heat transfer matrix 106 covered by the ambient air flow control device 110. Therefore, when the ambient air flow control device 110 is in a closed state, the effective cooling area of the heat transfer matrix 106 is significantly reduced compared to the situation when the ambient air flow control device 110 is in a fully open state. When the ambient air flow control device 110 is in the fully open state, only a small reduction in heat transfer matrix cooling area occurs due primarily to the presence of the frame 113 of the ambient air flow control device 110.

It will be appreciated that the cooling effect of the heat transfer matrix 106 per unit area is considerably higher at the charge air inlet end of the intercooler 105 than it is at the charge air outlet end of the intercooler due to the greater temperature difference at the charge air inlet end of the intercooler 105 compared to the charge air outlet end of the intercooler 105.

In addition, due to the very high cooling effect of the heat transfer matrix 106 towards the charge air inlet end of the intercooler 105, if the ambient air flow control device were to be positioned close to the charge air outlet end of the intercooler 5 there is a possibility that the charge air could be overcooled before it reaches the ambient air flow control device 110 thereby risking condensation within the charge air flow passages.

It will be appreciated that the temperature sensor used to measure the temperature of the charge air needs to be positioned close to the outlet end of the heat transfer -17 -matrix because it is normally towards the outlet end of the heat transfer matrix or the outlet end of the intercooler that condensation is most likely to occur.

It will also be appreciated that, if a temperature sensor is used to measure the temperature of the charge air exiting the intercooler and a powered actuator is used to control operation of an ambient air flow control device based upon this sensed temperature, then the ambient air flow control device could alternatively be positioned on a front face of the intercooler. However the ambient air flow control device should still be positioned at or very close to an inlet end of the iritercooler 5 to prevent over cooling of the charge air before it reached the ambient air flow control device.

One feature of the invention is that the temperature sensed for controlling operation of the ambient air flow control device can be the temperature of the charge air exiting the intercooler. This is advantageous because it is cost effective and simple to execute and it is possible to infer or estimate whether condensation is likely to occur within the intercooler based upon this temperature.

By using a an ambient air flow control device that is less than half of the length of the heat transfer unit it is easier to control the charge gas outlet temperature compared to a full length ambient air flow control device due to the different relationships between charge gas outlet temperature and opening position for full length and partial length air flow control devices.

It will be appreciated that the single powered actuator described in both embodiments could also be controlled to close the slats as part of any additional strategy in the control system such as, for example, to enable faster engine warm up, to support hotter gas temperatures to achieve -18 -particulate and or catalyst light off, to minimise aerodynamic drag or to minimise the risk of icing in cold start and/ or wet ambient conditions.

One advantage of locating the ambient air flow control device on the rear face of the intercooler is that, if the control linkage fails during use, the pressure of the ambient air flowing through the intercooler will tend to open the rotatable slats. This is particularly so if the slats are mounted asymmetrically relative to the axis of rotation.

Although the flow control members are described above with respect to rotatable slats it will be appreciated that other types of flow control members could be used such as for example one or more slideable slats or one or more rotary disc valves or any other suitable type of flow control member.

It will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to one or more embodiments it is not limited to the disclosed embodiments and that alternative embodiments could be constructed without departing from the scope of the invention as defined by the appended claims.

Claims

-19 -Claims 1. An intercooler for an engine comprising an inlet via which charge air enters the intercooler, an outlet via which charge air exits the intercooler and a heat transfer unit through which the charge air flows from the inlet of the intercooler to the outlet of the intercooler, the heat transfer unit having an inlet end via which charge air enters the heat transfer unit, an outlet end via which charge air exits the heat transfer unit, a front face via which ambient air enters the heat transfer unit and a rear face via which ambient air exits the heat transfer unit, the intercooler further comprising an ambient air flow control device to vary the flow of ambient air through the part of the heat transfer unit that it overlies wherein the ambient air flow control device is positioned towards the inlet end of the heat transfer unit, extends for less than the full length of the heat transfer unit and is controlled based upon one of the temperature of the charge air exiting the heat transfer unit and the temperature of the charge air in the outlet of the intercooler.
2. An intercooler as claimed in claim 1 wherein the ambient air flow control device extends for less than half of the length of the heat transfer unit.
3. An intercooler as claimed in claim 1 or in claim 2 wherein the ambient air flow control device is located on the rear face of the heat transfer unit.
4. An intercooler as claimed in any of claims 1 to 3 wherein the ambient air flow control device includes a number of flow control members that are moveable between open and closed positions by a powered actuator that is controlled by an electronic controller to vary the flow of ambient air through the ambient air flow control device.

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5. An intercooler as claimed in claim 4 wherein the position of the flow oontrol members is based upon the temperature of the charge air in the outlet of the intercooler.
6. An intercocler as claimed in claim 5 wherein the temperature of the charge air in the outlet of the intercooler is sensed by a temperature sensor positioned in the outlet of the intercooler and opening and closing of the flow control members is performed by the powered actuator in response to the temperature sensed by the temperature sensor.
7. An intercooler as claimed in any of claims 4 to 6 wherein the powered actuator is one of an electrical rotary actuator and an electrical solenoid actuator.
8. An intercooler as claimed in any of claims 4 to 7 wherein the flow control members are rotatable slats.
9. An intercooler as claimed in any of claims 1 to 8 wherein the heat transfer unit is a heat transfer matrix.10. A motor vehicle having an intercooler as claimed in any of claims 1 to 9.
10. An intercooler for an engine substantially as described herein with reference to the accompanying drawing.
11. A motor vehicle substantially as described herein with reference to the accompanying drawing.