GB2530509A

GB2530509A - A motor vehicle having a charge air cooler

Info

Publication number: GB2530509A
Application number: GB1416815.7A
Authority: GB
Inventors: Gerald Barr
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2014-09-24
Filing date: 2014-09-24
Publication date: 2016-03-30
Anticipated expiration: 2034-09-24
Also published as: GB2530509B; DE102015115211A1; GB201416815D0; RU2015138980A3; RU2690302C2; TR201511771A2; MX2015011082A; RU2015138980A

Abstract

A motor vehicle MV has a charge air cooler (CAC) 5 for cooling the charge air flowing to the engine 6. The CAC 5 is supplied with cold refrigerant from a refrigeration system, eg the vehicle's HVAC system 30, or a dedicated refrigeration system. The refrigerant may flow in a secondary cooling circuit containing the CAC 5 and the HVAC evaporator 10 and controlled by a valve 11. The cold refrigerant is used to cool the charge air as it passes through the CAC 5 by heat transfer from the charge air to the cold refrigerant. In the case where the engine is a forced induction engine 6 a conventional air-to-air intercooler 4 may be located upstream from the CAC 5 and downstream from a charge air compressor 3. An electronic controller 20 may control the HVAC 30 and the valve 11 based on a comparison of charge air cooling demand and cabin cooling demand. The conventional air-to-air intercooler may be omitted (fig.2). The engine may be normally aspirated (fig.3).

Description

A Motor Vehicle Having a Charge Air Cooler This invention relates to a motor vehicle having an engine and a refrigerant based heating and air conditioning system and, in particular, to the use of refrigerant from the heating and air conditioning system for oooling oharge air flowing into the engine.

It is known to increase the air or air/fuel mixture density of an engine by increasing the pressnre of the intake air stream prior to its entering a cylinder of an engine. The use of suoh foroed induction can be aohieved by either "turbocharging" or "supercharging" the engine and is primarily used to increase the power density (Watts per cubic metre) of an engine.

It will be appreciated that, because pressure is directly related to heat, as the compressor increases the pressure of the intake air stream the compressor also increases the intake air stream temperature. In addition, in the case of a turbocharger, the temperature of the air can also be increased due to heat transfer between the hot and cold sides of the turbocharger.

Furthermore, in modern engine designs it is a common practice to recirculate some of the exhaust gas exiting the engine to the intake side of the engine in a process that is often referred to as Exhaust Gas Recirculation (EGR) . Such recirculated exhaust gas is at a relatively high temperature and so will tend to increase the temperature of the charge air supplied to the engine.

There are several reasons why it is desirable to keep the temperature of the charge air as low as possible.

Firstly, cooler air is denser and so by cooling the intake charge air stream the mass of air inducted into the engine is increased resulting in potentially more power from the engine.

Secondly, the production of NOx emissions from an engine is highly dependent upon the temperature of combustion in the cylinders of the engine. It is therefcre desirable to cool the charge air (whether it is solely ambient air or a mix of ambient air and recirculated exhaust gas) in order to keep peak combustion temperatures as low as possible to prevent the production of excessive NOx from the engine.

It is known to provide charge air coding to reduce the temperature of the charge air by means of the use of an air to air intercooler. An intercooler is a heat exchanger placed into the air stream between the compressor and the engine intake manifold. The heat exchanger uses ambient air that is ducted into the intercooler to cool the charge air and can utilise fins to increase the rate at which heat is absorbed from the charge air and is re-radiated into the ambient environment.

There are a number of problems associated with known intercoolers which include the relatively large size reguired due to the relatively low cooling efficiency of such a device. The large size of a conventional air cooled intercooler often makes it difficult to package the intercooler in the engine compartment of a modern motor vehicle in an ideal position to receive a supply of ambient air. This problem is particularly so in the case of a small motor vehicle and small motor vehicles are increasingly using forced induction so as to facilitate the downsizing of the engine.

A further problem is that, even if the placement of the heat exchanger is optimized on the motor vehicle, the ambient air temperature entering the intercooler is an uncontrollable variable. It will be appreciated that the rate of heat transfer between the charge air and the ambient air is related to the difference in temperature between the ambient air entering the intercooler and the temperature of the charge air to be cooled. Therefore, for the same charge air temperature, if the ambient air temperature is high the rate of heat transfer will be considerably lower than if the ambient air temperature is low.

In addition, the lowest temperature obtainable from an air to air intercooler for the charge air must always be greater than the temperature of the ambient air. Therefore, if the ambient air temperature is high, there is a physical restriction on the temperature of charge air that can be provided by a conventional intercooler. In an effort to overcome this limitation, it is known from German patent publication 102011056616 to provide an intercooler for an engine that uses an evaporator of a refrigeration circuit to cool the air entering the intercooler. Such an arrangement enables the temperature of the air used to cool the charge air to be reduced below ambient temperature.

However, there remains a need for a charge air cooler that is compact, inexpensive to manufacture and can function efficiently irrespective of where it is mounted in the engine compartment.

It is an object of the invention to provide a charge air cooler for a motor vehicle that is compact, is not reliant on ambient air flow to provide cooling for the charge air and can be provided in an economical manner.

According to a first aspect of the invention there is provided a motor vehicle having an engine and a charge air cooler upstream from the engine to cool the charge air flowing to the engine wherein the charge air cooler is supplied with cold refrigerant from a refrigeration system located on the motor vehicle and the cold refrigerant is used to cool the charge air by heat transfer therebetween.

Mi intercooler may be located upstream from the charge air cooler.

The engine may be a forced induction engine and a charge air compressor is located upstream from the charge air cooler.

The charge air compressor may be located upstream from the intercooler.

The refrigeration system may be a refrigerant based heating and air conditioning system of the motor vehicle.

The refrigerant based heating and air conditioning system may include an primary evaporator to provide a flow of cool air to a cabin of the motor vehicle and the charge air cooler is a secondary evaporator connected in parallel with the primary evaporator.

The heating and air conditioning system further comprises an electronically controlled valve and an electronic controller wherein the electronic controller is operable to control the flow of refrigerant through the primary and secondary evaporators.

The controller may be operable to control the flow of coolant through the primary and secondary evaporators based upon a comparison of the respective need for air charge cooling with a need for cabin cooling.

The charge air cooler may cool the charge air to a temperature that is lower than the temperature of the ambient air in which the motor vehicle is operating.

According to a second aspect of the invention there is provided a method for the cooling the charge air supplied to an engine of a motor vehicle comprising locating a charge air cooler in a charge air flow path to the engine and supplying cold refrigerant from a refrigeration system to the charge air cooler to cool the charge air before it is inducted into the engine.

The method may further comprise mounting an intercooler upstream from the charge air cooler so as to partially cool the charge air before it flows through the charge air cooler.

The method further may comprise using a refrigerant based heating and air conditioning system for a cabin of the motor vehicle to provide the cold refrigerant to the charge air cooler.

The method may further comprise comparing a need for charge air cooling with a need for cabin cooling and controlling the flow of refrigerant based upon the comparison.

The method may further comprise controlling the flow of refrigerant to give preference to charge air cooling if the NOx emissions from the engine exceed a predefined NOx limit.

The method may further comprise controlling the flow of refrigerant to give preference to cabin cooling if the temperature of the cabin exceeds a predefined temperature limit.

The method may further comprise controlling the flow of refrigerant to give preference to cabin cooling if a user cabin temperature demand is not being met.

The invention will now be desoribed by way of example with reference to the accompanying drawing of which:-Fig.1 is a schematic block diagram of a motor vehicle according to a first embodiment of a first aspect of the invention; Fig.2 is a schematic block diagram of a motor vehicle according to a second embodiment of the first aspect of the invention; and Fig.3 is a schematio block diagram of a motor vehicle according to a third embodiment of the first aspect of the invention.

With reference to Fig.1 there is shown a first or preferred embodiment of a motor vehicle MV constructed in accordance with the first aspect of the invention.

The motor vehicle MV has an engine 6 and a refrigerant based heating and air conditioning system (HVAC) 30.

A supply of ambient air indicated by the arrow A' is inducted via an air filter into an induction system of the engine 6 and passes via a duct to an inlet of a compressor 3. A supply of recirculated exhaust gas (EGR) is mixed with the ambient air upstream of the compressor 3. The flow of EGR is controlled by an electronically controllable EGR valve 8 as is well known in the art.

The compressor 3 compresses the mixed ambient air and EGR and supplies the resultant high pressure charge air to a conventional air to air intercooler 4 where it is cooled by interaction with a supply of ambient air from outside the motor vehicle MV as is well known in the art. The cooled charge air passes from the intercooler 4 to a heat exchanger in the form of a charge air cooler 5 located downstream from the intercooler 4 where further cooling of the charge air takes place before the charge air enters the engine 6 via an inlet manifold Exhaust gas from engine 6 exits the engine 6 via an exhaust manifold and passes through an exhaust system 7 that may include one or more aftertreatment devices before exiting to atmosphere as indicated by the arrow E' It will be appreciated that if the compressor 3 is part of a turbocharger then a turbine of the turbocharger will be positioned at the exhaust side of the engine 6 to drive the compressor 3.

The heating and air conditioning system (HVAC) 30 is a two phase refrigerant circuit comprising in this case of an evaporator 10, an electronically controlled valve 11, a refrigerant pump 12, a condenser 13, an expansion valve 14 and an electronic controller 20.

The evaporator 10 is a heat exchanger used to extract heat from air passing therethrough and transfer it to refrigerant gas passing through the evaporator 10. The air passing through the evaporator 10 can be selectively used to cool a passenger compartment or cabin of the motor vehicle MV and a blower or fan (not shown) is used to adjust the flow of air through the evaporator 10.

The condenser 13 is a heat exchanger used to extract heat from liguid refrigerant passing through the condenser 13 and transfer it to air passing through the condenser 13.

The air passing through the condenser 13 can be selectively used to heat the cabin of the motor vehicle MV. It will be appreciated that a blower or fan is normally used to adjust the flow of air through the condenser 13 so as to vary the flow rate of heated air entering the passenger cabin.

It will also be appreciated that in some heating and air conditioning systems the cooled air from the evaporator and the heated air from the condenser 13 are mixed along with inducted ambient air in a blending unit before being supplied to the oabin.

The refrigerant pump 12 is controlled by the electronic controller 20 to regulate the flow of refrigerant around the refrigerant circuit of the HVAC 30 as is well known in the art. The refrigerant pump 12 can be driven via a dis-connectable drive by the engine 6 or be driven by an electric motor. The refrigerant leaves the refrigerant pump 12 as a hot, high pressure gas and flows into the condenser 13. Due to the heat exchange with the air flowing through the condenser 13, the temperature of the refrigerant when it leaves the condenser 13 is much lower and it has changed phase from a gas to a liquid.

The liquid refrigerant then flows to the expansion valve 14 where it expands as it passes through the expansion valve 14. This is because downstream of the expansion valve 14 there is a low pressure area produced by the refrigerant pump 12 drawing refrigerant through the expansion valve 14 to the downstream side of the expansion valve 14 and the fact that the expansion valve considerably restricts the flow of refrigerant therethrough. This expansion of the refrigerant causes it to boil and undergo a phase change in which it vaporises into a cold gas. The cold gaseous refrigerant then passes from the downstream side of the expansion valve 14 through the evaporator 10 and the control valve 11 back to the refrigerant pump 12 and the process is repeated.

Upstream from the evaporator 10 there is an inlet to a secondary cooling circuit that is connected to a conduit joining the expansion valve 14 to the evaporator 10. The secondary cooling circuit includes the charge air cooler 5 and has an outlet conduit connected to the electronically controlled valve II.

One of the main features of the invention is that the charge air passing through the charge air cooler 5 is cooled by cold refrigerant extracted from the HVAC 30 and not by ambient air.

The charge air cooler 5 is a heat exchanger and can be of many differing constructions but in all cases operates as a secondary evaporator connected to the HVAC 30 and includes one or more passages through which refrigerant extracted from the H\AC 30 flows. As the charge air flows through the charge air cooler 5 into the engine 6 it is cooled by heat transfer to the refrigerant flowing through the passages.

It will be appreciated that only the main components of the HVAC 30 are shown and described and that the invention is not limited to the use of a HVAC exactly as shown and described.

The electronically controlled valve 11 is controlled by the electronic controller 11 to vary the flow of refrigerant through the charge air cooler 5 and the evaporator 10 based upon predefined rules stored as an executable program in the electronic controller 20.

The electronic controller 20 receives inputs indicated generally by the reference numeral 21. The inputs 21 are normally outputs from sensors and are used by the electronic control unit 20 to control the operation of the PP/AC 30 and, in particular, running of the refrigerant pump 12 and operation of the electronically controlled valve 11.

The inputs 21 include, for example and without limitation, an output from ambient air temperature sensor, an output from a cabin temperature sensor, outputs from one -10 -or more charge air sensors, an output from an exhaust gas temperature sensor, an output from a refrigerant temperature sensor, an output from an exhaust gas NOx sensor and an input indicative of a vehicle user climate control demand.

The electronic controller 20 uses the inputs 21 to produce an optimum balance between cabin cooling and charge air cooling.

For example, if the temperature of the cabin of the motor vehicle MV is above a required temperature the electronically controlled valve 11 will normally bias the flow of refrigerant so as to increase cooling of the cabin of the motor vehicle MV. This is particularly the case if the level of NOx emissions from the engine 6 is within acceptable limits or the temperature of the charge air entering the engine 6 is within acceptable limits.

Whereas, if the cabin of the motor vehicle MV is within an acceptable temperature range but either the level of NOx emissions from the engine 6 is above a predefined limit or the temperature of the charge air entering the engine 6 is above a preferred temperature, the electronic controller 20 will control the electronically controlled valve 11 to bias the flow of refrigerant so as to increase cooling of the charge air entering the engine 6 in preference to cooling the cabin of the motor vehicle MV.

Therefore the electronic controller 20 is operable to control the HVAC 30 and, in particular, the electronically controlled valve 11 based upon a comparison of charge air cooling demand and cabin cooling demand as indicated by one or more of the inputs 21.

The electronically controlled valve 11 is preferably a regulator valve in which the proportion of refrigerant flowing through the charge air cooler 5 and the evaporator -11 -can be varied. However, in some embodiments the electronically controlled valve 11 is a shut-off valve that either permits refrigerant to flow through the charge air cooler 5 or does not.

In use, ambient air enters the air inlet system of the engine via the air filter 1, is mixed with recirculated exhaust gas and then compressed in the compressor 3. The temperature of the charge air exiting the compressor 3 is normally in the region of 200°C and this hot charge air enters the intercooler 4 where it is cooled by the transfer of heat to the ambient air flowing throllgh the intercooler 4. The cooler charge air then flows through the charge air cooler 5 where further cooling takes place due to heat transfer with the refrigerant flowing through the charge air cooler 5. This cooled charge air the flows into the engine 6 via the inlet manifold 9 and fuel is added to the charge air and is combusted in the cylinder or cylinders of the engine 6. It will be appreciated that in the case of most modern engines the fuel is normally added by the direct injection of fuel into each cylinder of the engine 6 but the invention is not limited to the method or apparatus used to add the fuel.

A very small reduction in the temperature of the charge air entering the engine 6 such as, for example 5°C has been found to produce a significant reduction in NOx emissions from a test engine operating as a diesel engine.

Using only a conventional intercooler the temperature of the charge air can be reduced by the intercooler from, for example, 180°C to 40°C. However, by the addition of a refrigerant charge air cooler placed downstream from the intercooler as proposed by this invention, a further reduction of charge air temperature is possible.

-12 -For example, the charge air temperature can be reduced from 4000 referred to above to 30°C by using the combination of an intercooler 4 and a downstream charge air cooler 5.

Such a reduction in charge air temperature will have a significant effect in reducing NOx emissions from the engine 6.

One of the advantages of positioning the charge air cooler 5 downstream from the intercooler 4 is that it is possible for the temperature of the charge air to be reduced to a temperature lower than the temperature of the ambient air in which the motor vehicle MV is operating. Whereas, if the charge air cooler 5 were to be located upstream from the intercooler 4 it would not be possible to produce a charge air supply having a temperature below the temperature of the ambient air due to the subseguent interaction of the charge air with ambient air as it passes through the intercooler.

With reference to Fig.2 there is shown a second embodiment of a motor vehicle MV constrllcted in accordance with the first aspect of the invention.

The second embodiment is in most respects identical to the motor vehicle MV of the first embodiment described above, functions in a like manner and for which like reference numerals refer to the parts having the same construction and operation.

The only significant difference between the first embodiment shown in Fig.l and this second embodiment is that, in the case of this second embodiment, a conventional intercooler is not located between the charge air cooler 5 and the compressor 3. Therefore the only cooling for the charge air flowing to the engine 6 is provided by the charge air cooler 5. Such an arrangement is useful in the case of a motor vehicle where there is no room on the motor vehicle to package a conventional intercooler but there is a need to -13 -reduce the level of NOx emissions from the engine of the motor vehicle. Some examples of such a motor vehicle are, a motor vehicle having very small dimensions and/or a motor vehicle having an engine located under the floor of the motor vehicle.

As before the charge air cooler 5 directly cools the charge air by heat transfer with the cold refrigerant passing through the charge air cooler. However, in the case of this embodiment the charge air is not cooled to or below ambient temperature it is merely cooled to a temperature lower than the temperature at which it leaves the compressor 3. This is because the large reduction in temperature required to cool the hot charge air from a compressor to ambient would need a very large charge air cooler and the required supply of refrigerant available from a standard heating and air conditioning system would potentially reduce the cooling available for the cabin of the motor vehicle to an unacceptable level. However, as stated above, even a relatively small reduction in the temperature of the charge air entering an engine can have a significant effect on reducing NOx emissions from the engine.

With reference to Fig.3 there is shown a third embodiment of a motor vehicle MV constructed in accordance with the first aspect of the invention.

The third embodiment is in most respects identical to the motor vehicle MV of the third embodiment described above, functions in a like manner and for which like reference numerals refer to the parts having the same construction and operation.

The only significant difference between the second embodiment shown in Fig.2 and this third embodiment is that, in the case of this second embodiment, the engine is not a forced induction engine it is a normally aspirated engine -14 -and so there is no compressor located upstream from the charge air cooler 5.

Although the use of charge air cooling is particularly relevant to forced induction engines having a compressor to compress the charge air the inventor has realised that the use of charge air cooling is also relevant to a normally aspirated engine operating in high ambient temperature environment. Proposed future exhaust emission reguirements mean that it will be increasingly difficult to meet proposed NOx targets particularly in countries where the ambient temperature is high.

By using a charge air cooler of the type used in this invention to cool the charge air it is possible to reduce the temperature of the charge air to a temperature below ambient temperature and this will result in reduced NOx emissions from the engine.

Therefore the use of a refrigerant based charge air cooler on a normally aspirated engine it is potentially easier to meet such proposed stringent NOx targets in high ambient temperature environments. For example, if the ambient temperature is 40°C, a considerable reduction in NOx emissions from the engine can be obtained by reducing the charge air temperature by 10°C to 30°C.

Although in the three embodiments described above there is an exhaust gas recirculation flow to a position upstream from the charge air cooler it will be appreciated that the invention is equally advantageous for engines having no recirculation of exhaust gas.

One of the advantages of the invention is that the majority of the hardware reguired to facilitate use of the invention is already present on any vehicle having a refrigerant based heating and air conditioning system.

-15 -Therefore, the additional costs due to the addition of the charge air cooler compared to the potential NOx emission gains are relatively low.

A further advantage of the invention is that the charge air cooler is either fitted within the inlet duct to the engine or is formed as part of the inlet duct. Very little additional space is therefore reguired for the charge air cooler and so it can be more easily packaged in the engine compartment.

A further advantage of the invention is that cooling of the charge air is not reliant on ambient air temperature and so, even in a hot ambient air environment, NOx emissions can be significantly reduced.

Although it is extremely advantageous from cost and packaging viewpoints to extract the refrigerant from an existing heating and air conditioning system it will be appreciated that & separate refrigeration system could be provided solely for the purpose of providing cold refrigerant for the charge air cooler.

In accordance with a second aspect of the invention a method for improving the cooling of charge air for an engine of a motor vehicle is provided.

The method comprises locating a charge air cooler in a charge air flow path to an engine and supplying cold refrigerant to the charge air cooler to directly cool the charge air before it is inducted into the engine.

Preferably, the refrigerant is extracted from a refrigerant based heating and air conditioning system used to provide at least cooling to a cabin of a motor vehicle.

-16 -The method advantageously further comprises mounting an intercooler upstream from the charge air cooler so as to partially cool or pre-cool the charge air before it flows through the charge air cooler.

The method further oomprises balanoing the needs for charge air cooling with the need for cabin cooling and controlling the flow of refrigerant through the charge air cooler to meet such needs.

Balancing the needs for charge air cooling with the need for oabin cooling may comprise comparing a need for charge air cooling with a need for oabin cooling and controlling the flow of refrigerant based upon the comparison.

Balancing the needs for charge air cooling with the need for cabin cooling and controlling the flow of refrigerant through the oharge air oooler may oomprise giving preference to charge air cooling if the Max emissions from the engine exceed a predefined NOx limit.

Balancing the needs for charge air cooling with the need for cabin cooling and controlling the flow of refrigerant through the charge air cooler may also comprise giving preference to cabin cooling if the temperature of the cabin exceeds a predefined temperature limit or a user cabin temperature demand is not being met.

It will be appreciated by those skilled in the art that althollgh the invention has been desoribed 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

-17 -Claims 1. A motor vehicle having an engine and a charge air cooler upstream from the engine to cool the charge air flowing to the engine wherein the charge air cooler is supplied with cold refrigerant from a refrigeration system located on the motor vehicle and the cold refrigerant is used to cool the charge air by heat transfer therebetween.
2. A motor vehicle as claimed in claim 1 wherein an intercooler is located upstream from the charge air cooler.
3. A motor vehicle as claimed in claim 1 or in claim 2 wherein the engine is a forced induction engine and a charge air compressor is located upstream from the charge air cooler.
4. A motor vehicle as claimed in claim 3 when dependent upon claim 2 wherein the charge air compressor is located upstream from the intercooler.
5. A motor vehicle as claimed in any of claims 1 to 4 wherein the refrigeration system is a refrigerant based heating and air conditioning system of the motor vehicle.
6. A motor vehicle as claimed in claim 5 wherein the refrigerant based heating and air conditioning system includes an primary evaporator to provide a flow of cool air to a cabin of the motor vehicle and the charge air cooler is a secondary evaporator connected in parallel with the primary evaporator.
7. A motor vehicle as claimed in claim 6 in which the heating and air conditioning system further comprises an electronically controlled valve and an electronic controller wherein the electronic controller is operable to control the -18 -flow of refrigerant through the primary and secondary evaporators.
8. A motor vehicle as claimed in claim 7 wherein the controller is operable to control the flow of coolant through the primary and secondary evaporators based upon a comparison of the respective need for air charge cooling with a need for cabin cooling.
9. A motor vehicle as claimed in any of claims 1 to 8 wherein the charge air cooler cools the charge air to a temperature that is lower than the temperature of the ambient air in which the motor vehicle is operating.
10. A method for the cooling the charge air supplied to an engine of a motor vehicle comprising locating a charge air cooler in a charge air flow path to the engine and supplying cold refrigerant from a refrigeration system to the charge air cooler to cool the charge air before it is inducted into the engine.
11. A method as claimed in claim 10 wherein the method further comprises mounting an intercooler upstream from the charge air cooler so as to partially cool the charge air before it flows through the charge air cooler.
12. A method as claimed in claim 10 or in claim 11 wherein the method further comprises using a refrigerant based heating and air conditioning system for a cabin of the motor vehicle to provide the cold refrigerant to the charge air cooler.
13. A method as claimed in claim 12 wherein the method further comprises comparing a need for charge air cooling with a need for cabin cooling and controlling the flow of refrigerant based upon the comparison.

-19 -
14. A method as claimed in claim 12 or in claim 13 wherein the method further comprises controlling the flow of refrigerant to give preference to charge air cooling if the NOx emissions from the engine exceed a predefined NOx limit.
15. A method as claimed in any of claims 12 to 14 wherein the method further comprises controlling the flow of refrigerant to give preference to cabin cooling if the temperature of the cabin exceeds a predefined temperature limit.
16. A method as claimed in any of claims 12 to 14 wherein the method further comprises controlling the flow of refrigerant to give preference to cabin cooling if a user cabin temperature demand is not being met.
17. A motor vehicle substantially as described herein with reference to the accompanying drawing.
18. A method for the cooling the charge air supplied to an engine of a motor vehicle.Amendment to the claims have been filed as follows Claims 1. A motor vehicle having an engine and a charge air cooler upstream from the engine to cool the charge air flowing to the engine, the charge air cooler being supplied with cold refrigerant from a refrigeration system located on the motor vehicle and the cold refrigerant is used to cool the charge air by heat transfer therebetween, the refrigeration system being a refrigerant based heating and air conditioning system of the motor vehicle comprising a primary evaporator to provide a flow of cool air to a cabin of the motor vehicle, a secondary evaporator forming the charge air cooler connected in parallel with the primary evaporator, an electronic controller and an electronically controlled valve used to control the flow of refrigerant through the primary and secondary evaporators wherein the electronic controller is operable to control the flow of refrigerant through the primary and secondary evaporators using the electronically controlled valve based upon a comparison of the respective need for charge air cooling with a need for cabin cooling.2. A motor vehicle as claimed in claim 1 wherein an intercooler is located upstream from the charge air cooler.3. A motor vehicle as claimed in claim 1 or in claim 2 wherein the engine is a forced induction engine and a charge air compressor is located upstream from the charge air cooler.4. A motor vehicle as claimed in claim 3 when dependent upon claim 2 wherein the charge air compressor is located upstream from the intercooler.5. A motor vehicle as claimed in any of claims 1 to 4 wherein, if the temperature of the cabin of the motor vehicle is within acceptable limits and the level of NOx emissions from the engine of the motor vehicle is above a predefined limit, the electronically controlled valve is operable to bias the flow of refrigerant so as to increase cooling of the charge air entering the engine in preference to cabin cooling.6. A motor vehicle as claimed in any of claims 1 to 4 wherein, if the temperature of the cabin of the motor vehicle is within acceptable limits and the temperature of the charge air entering the engine is above a preferred temperature, the electronically controlled valve is operable to bias the flow of refrigerant so as to increase cooling of the charge air entering the engine in preference to cabin cooling.7. A motor vehicle as claimed in any of claims 1 to 6 wherein, if the temperature of the cabin of the motor vehicle is above a required temperature, the electronically controlled valve is operable to bias the flow of refrigerant so as to increase cabin cooling.8. A motor vehicle as claimed in any of claims 1 to 7 wherein the charge air cooler cools the charge air to a temperature that is lower than the temperature of the ambient air in which the motor vehicle is operating.9. A method for the cooling the charge air supplied to an engine of a motor vehicle comprising locating a charge air cooler in a charge air flow path to the engine and supplying cold refrigerant from a refrigerant based heating and air conditioning system for a cabin of the motor vehicle to provide the cold refrigerant to the charge air cooler to cool the charge air before it is inducted into the engine wherein the method further comprises comparing a need for charge air cooling with a need for cabin cooling and controlling the flow of refrigerant based upon the comparison.10. A method as claimed in claim 9 wherein the method fnrther comprises mounting an intercooler npstream from the charge air cooler so as to partially cool the charge air before it flows through the charge air cooler.11. A method as claimed in claim 9 or in claim 10 wherein the method further comprises controlling the flow of refrigerant to give preference to charge air cooling if the NOx emissions from the engine exceed a predefined NOx limit and the temperature of the cabin is within an acceptable temperature range.12. A method as claimed in claim 9 or in claim 10 wherein the method further comprises controlling the flow of refrigerant to give preference to charge air cooling if the temperature of the charge air entering the engine is above a preferred temperature and the temperature of the cabin is within an acceptable temperature range.13. A method as claimed in any of claims 9 to 12 wherein the method further comprises controlling the flow of refrigerant to give preference to cabin cooling if the temperature of the cabin exceeds a predefined temperature limit.14. A method as claimed in any of claims 9 to 12 wherein the method further comprises controlling the flow of refrigerant to give preference to cabin cooling if a user cabin temperature demand is not being met.15. A motor vehicle substantially as described herein with reference to the accompanying drawing.16. A method for the cooling the charge air supplied to an engine of a motor vehicle substantially as described herein with reference to the accompanying drawing.