GB2416001A

GB2416001A - Intake air and recirculated exhaust gas cooling system for a boosted i.c. engine

Info

Publication number: GB2416001A
Application number: GB0415206A
Authority: GB
Inventors: Stephen Joyce; Iain William Gouldson; Christopher David Bush; Noel Rodney Martin Henderson; Christopher David Whelan
Original assignee: Visteon Global Technologies Inc
Current assignee: Visteon Global Technologies Inc
Priority date: 2004-07-07
Filing date: 2004-07-07
Publication date: 2006-01-11
Anticipated expiration: 2024-07-07
Also published as: GB0415206D0; GB2416001B

Abstract

The intake air and recirculated exhaust gas cooling system of an eg automotive diesel engine comprises a combined cooler 201 for cooling a gas combination of compressed engine intake air and recirculated exhaust gas (EGR gas). Exhaust gas exits the combustion chamber 204 of an engine 200 and passes into an exhaust passage 203 which directs the exhaust gas to a mixing point 209 via a regulator 211 for varying the amount of exhaust gas that is recirculated and to a turbine 205 of a turbocharger 206. A rotary compressor 208 of the turbocharger 206 compresses the intake air and outputs it to the mixing point 209. The mixing point 209 mixes the compressed engine intake air and recirculated exhaust gas and outputs it to the cooler 201, which cools it and outputs it to an intake manifold 210 of the engine 200 and hence to the combustion chamber 204. Thus a single cooler is used instead of respective coolers for intake air and for EGR gas, saving space and reducing cost.

Description

Intake Air and Recirculated Exhaust Gas Cooling System The present

invention relates to an intake air and recirculated exhaust gas cooling system for a boosted internal combustion engine and to a method of cooling intake air and recirculated exhaust gas for a boosted internal combustion engine. A particular, but not exclusive, application of the invention is for an automotive diesel engine.

Many internal combustion engines and particularly automotive diesel engines are boosted by compressing engine intake air.

One way of doing this is to use a turbocharger, which typically has a rotary compressor for compressing the intake air driven by a turbine wheel powered by engine exhaust gas.

Another way is to use a supercharger, which might have a similar rotary compressor driven directly by the engine or by an electric motor or a positive displacement pump for compressing the intake air. In all cases, the result is that more air is sent into an engine's combustion chamber, increasing engine power.

When air is compressed, it is simultaneously heated. So, compressing engine intake air raises the temperature of air sent into the engine's combustion chamber. The raised air temperature increases the temperature of the combustion chamber and the surrounding engine components, which can increase thermal stress and reduce engine lifetime.

Compressed intake air is often therefore cooled to increase the amount by which it can be compressed without detrimentally increasing combustion chamber temperature. This cooling can also generally increase the amount of air - 2 provided in the combustion chamber at a given intake pressure, as cool air is denser than hot air. Intake air cooling can therefore help to increase engine power.

Emission regulations are now beginning to dictate that many automotive engines mix intake air with recirculated exhaust gas, as this can reduce NOX (e.g. Nitrogen Dioxide etc.) emissions. NOX is formed in far higher quantities above certain combustion temperatures. Mixing recirculated exhaust gas with engine intake air can lower the combustion temperature and therefore reduce NOX formation. However, exhaust gas is hot. Like compressed intake air, it therefore benefits from cooling before it enters an engine's combustion chamber. In particular, cooling recirculated exhaust gas can increase the amount of exhaust gas that can be provided in the combustion chamber at a given intake pressure (e.g. improve mass flow).

Coolers for cooling compressed intake air are usually referred to as charge air coolers or intercoolers.

Intercoolers can be cooled by engine coolant or other liquids, but are more commonly air cooled. An air cooled intercooler typically comprises an arrangement of tubes through which the compressed intake air can flow. Ambient air flows around the outside of the tubes and is therefore in a heat exchange relationship with the compressed intake air in the tubes and can cool it. The tubes are typically designed to maximise the area of the heat exchange surface between the compressed intake air and the ambient air in the smallest possible overall intercooler size. For most car and light truck engines, intercoolers can provide sufficient cooling capacity without being inconveniently large and have - 3 relatively straightforward, robust and maintenance free designs. They are typically made from aluminium or plastics, as they only have to deal with relatively low temperatures (less than around 200 C).

Coolers for cooling recirculated exhaust gas are usually referred to as Exhaust Gas Recirculation (EGR) coolers. An EGR cooler typically comprises a cylindrical shell containing one or more heat exchange tubes through which the exhaust gas can flow. Liquid engine coolant is passed through the shell around the tubes. The coolant is therefore in a heat exchange relationship with the exhaust gas and can cool it. Liquid cooling is used as it can typically provide greater cooling capacity than air cooling for a given heat exchange surface area. Thus, the heat exchange tubes can have a relatively large diameter and small surface area, which makes the EGR cooler tolerant to the build up of soot inside the tubes. EGR coolers are typically made from steel. One reason for this is that exhaust gas can be hot enough to damage other materials such as aluminium and plastics, but steel is more tolerant to high temperatures.

So, when it is desired to cool both engine intake air and recirculated exhaust gas, two separate coolers are conventionally provided; an intercooler and an EGR cooler.

According to a first aspect of the invention there is provided an intake air and recirculated exhaust gas cooling system for a boosted internal combustion engine, the system comprising: a mixing point for combining compressed engine intake air with recirculated exhaust gas to form a gas combination) - 4 an exhaust gas recirculation regulator for receiving exhaust gas from the engine and dividing flow of the exhaust gas exclusively between recirculation of the exhaust gas to the mixing point and output of the exhaust gas without recirculation; a controller for controlling the regulator such that more exhaust gas is recirculated at a lower engine load or speed than at a higher engine load or speed; and a combined cooler for receiving the gas combination, cooling it and outputting it to the engine.

In other words, the applicants have recognized that a single cooler can be used to cool both compressed intake air and recirculated exhaust gas. This has several advantages. For example, less space is taken up by a single combined cooler than by a separate intercooler and EGR cooler. Similarly, there are significant cost savings in using a single cooler rather than two separate coolers.

The system may have various specific arrangements for recirculating the exhaust gas. For example, exhaust gas is conventionally routed through a turbocharger when present.

Many conventional turbochargers incorporate variable exhaust gas flow systems, e.g. including waste gates and such like.

The applicants have therefore recognized that it may be convenient for the regulator to be housed in the turbocharger.

Similarly, the mixing point is typically positioned between a compressor of the boosting device (e.g. turbocharger or supercharger) and the combined cooler (although it might possibly be positioned in (or even upstream of) the - 5 compressor of the boosting device or in the combined cooler).

In a particularly preferred embodiment, the mixing point comprises a venturi through which the compressed intake air flows and at which the exhaust gas is added to the intake air. This provides very effective mixing.

In another specific arrangement, a bypass passage for allowing the combined gas to bypass the combined cooler may be provided. This allows cooling of the gas combination to be effectively switched off, e.g. under some specific operating conditions, such as during engine warm up.

Whilst a large amount of exhaust gas can be recirculated at low engine loads or speeds, only a small amount (or even none) is typically recirculated at high engine loads or speeds. For example, the controller can control the regulator to decrease the amount of exhaust gas that is recirculated from a lower engine load or speed to a higher engine load or speed. So, at low engine loads or speeds, reduction of NOx emissions can be maximised by recirculating exhaust gas.

However, at high engine loads or speeds, when more engine intake air is required to increase engine power, less exhaust gas is recirculated.

This is conventionally done in many automotive engines, particularly for cars and light trucks. However, the applicants have recognised that a result is that significant amounts of exhaust gas need only be cooled at low engine loads or speeds, when less engine intake air is used. At high engine loads or speeds, when more engine intake air is used, less or no exhaust gas needs to be cooled. So, the applicants have consequently recognised that the cooling capacity of the - 6 combined cooler of the invention can be less than the cumulative cooling capacity of a conventional EGR cooler and a conventional intercooler for the same engine. Indeed, for many engines the heat rejection capacity of the combined cooler need be no greater than the heat rejection capacity of a conventional intercooler for the same engine. In other words, the cooler can have a heat rejection capacity that is no greater than the heat rejection capacity required to cool only the maximum amount of compressed intake air to be output to the engine.

This is considered to be new in itself and, according to a second aspect of the present invention, there is provided an intake air and recirculated exhaust gas cooling system for a boosted internal combustion engine, the system having a combined cooler for receiving recirculated exhaust gas after combination with compressed engine intake air, cooling the gas combination and outputting it to the engine, wherein the cooler has a heat rejection capacity that is no greater than the heat rejection capacity required to cool only the maximum amount of compressed intake air to be output to the engine.

The combined cooler can therefore have more or less the same size as a conventional intercooler (although it might be slightly larger to accommodate any possible performance drop off caused by soot deposition). Furthermore, as the combined cooler can directly replace a conventional intercooler and at the same time eliminate the need for an EGR cooler, space and cost savings are possible.

Conventional intercoolers for car or light truck engines have heat capacities up to around 50 kW. So, likewise, the - 7 combined cooler of the invention can have a heat capacity of no more than around 50 kW. Of course, smaller engines or those that are boosted less and therefore use less intake air may use combined coolers of less heat rejection capacity. In particular, the combined cooler of the invention can have a heat capacity of no more than around 20 kW.

The applicants have also recognised that by recirculating more exhaust gas at a lower engine load than at a higher engine load the temperature of combined gas before cooling can be kept relatively low. At high engine loads, exhaust gas can reach around 800 C and compressed intake air can reach around 200 C. At lower engine loads, both exhaust gas and compressed engine intake air tend to be cooler, with exhaust gas falling in temperature to around 350 C and engine intake air falling in temperature to around that of the ambient air temperature, say around 25 C. Conventionally, EGR coolers have therefore had to deal with exhaust gas up to 800 C and certainly higher than 350 C. However, using the invention, the temperature of the gas combination can be kept below around 250 C. This is because the gas combination tends to contain a volume of intake air that is cooler than the exhaust gas. As the temperature of the exhaust gas and the intake air increases (with increased engine load), the proportion of exhaust gas decreases, keeping the temperature of the gas combination down. So, whilst conventionally EGR coolers have been made from steel to cope with the higher temperature exhaust gas, it is possible for the combined cooler of the invention to be fabricated from aluminium or an aluminium alloy, similar to a conventional intercooler. - 8 -

This is considered to be new in itself and, according to a third aspect of the present invention, there is provided an intake air and recirculated exhaust gas cooling system for a boosted internal combustion engine, the system having a combined cooler for receiving recirculated exhaust gas after combination with compressed engine intake air, cooling the gas combination and outputting it to the engine, wherein the cooler is fabricated from aluminium or an aluminium alloy.

Aluminium and aluminium alloys can be cheaper than steel, so provide cost savings. As they are lighter than steel, they are also particularly useful in automotive applications and such like to save weight.

Another advantage is that aluminium and aluminium alloys are suited to extrusion. So, the combined cooler may be extruded.

This improves ease of manufacture and adds to the cost savings. In particular, the combined cooler may have heat exchange tubes for passing the gas combination that comprise extruded aluminium or aluminium alloy. The heat exchange tubes may have internal fins.

In one example, the cooler may be air cooled. So, much like a conventional intercooler, it might have tubes through which the gas combination is passed and the tubes may be arranged to allow ambient air to pass over their outer surface to cool the gas combination. In another example, it is preferred that the cooler is liquid cooled. This might be achieved using engine coolant provided by an engine cooling system or by using a (separate) liquid coolant system dedicated to the combined cooler. In any case, liquid cooling can improve the heat rejection capacity of the cooler for a given overall - 9 - cooler size.

The applicants have recognised that one of the primary mechanisms of soot build up in the combined cooler of the invention is thermophoresis. The higher the temperature gradient between the gas combination in the cooler and the wall of the cooler, the greater the thermophoresis and the greater the soot build up. Keeping the maximum temperature of the combined gas as low as possible therefore mitigates soot build up. Indeed, there is no need for exhaust gas to be filtered or cleaned, as it is in some conventional systems.

In other words, the exhaust gas is preferably not filtered (e.g. between being expelled from the engine and entering the cooler). However, in a particularly preferred example, the cooler has a liquid coolant arranged to flow from an end of the cooler from which the gas combination is output by the cooler toward an end of the cooler into which the gas combination is received by the cooler.

Indeed, this is considered to be new in itself and, according to a fourth aspect of the present invention, there is provided an intake air and recirculated exhaust gas cooling system for a boosted internal combustion engine, the system having a combined cooler for receiving recirculated exhaust gas after combination with compressed engine intake air, cooling the gas combination and outputting it to the engine, wherein the cooler has a liquid coolant arranged to flow from an end of the cooler from which the gas combination is output by the cooler toward an end of the cooler into which the gas combination is received by the cooler.

So, where the gas combination is at its hottest as it enters the cooler, the coolant may also be hottest. Likewise, where the gas combination is at its coldest as it leaves the cooler, the coolant may also be coldest. This minimises the temperature difference between the gas combination and the cooler as the gas combination passes through the cooler. The temperature gradient between the gas combination and the cooler is therefore minimised and thermophoresis is reduced.

Less soot is therefore deposited in the cooler, both increasing the lifetime of the cooler and decreasing performance degradation over the lifetime of the cooler.

The cooler might be arranged so that the coolant flows substantially only in the opposite direction to the gas combination. This is known as a counter flow arrangement.

However, it is preferred that the coolant is also arranged to flow across the direction of flow of the gas combination (e.g. and from an end of the cooler from which the gas combination is output by the cooler toward an end of the cooler into which the gas combination is received by the cooler). This is known as a cross counter flow arrangement.

Preferred embodiments of the invention will now be described, by way of example only, with reference to the accompanying drawings, in which: Figure 1 is a block diagram of a boosted internal combustion engine utilising a conventional intercooler and conventional EGR cooler; Figure 2 is a schematic illustration of a conventional air cooled intercooler; Figure 3 is a schematic illustration of a conventional EGR cooler; Figure 4 is a block diagram of a boosted internal combustion engine utilising a combined intake air and exhaust gas recirculation system of the present invention; Figure 5 is an illustration of a combined cooler of the present invention; Figure 6 is a graph illustrating the required heat rejection capacity of the combined cooler of the present invention; Figure 7 is a graph illustrating the use of exhaust gas recirculation with variations in engine torque and speed; and Figure 8 is a graph illustrating the results of an experiment to demonstrate the effects of thermophoresis in the combined cooler of the invention.

Referring to Figure 1, a turbocharged engine 100 incorporating conventional intake air cooling and recirculated exhaust gas cooling has an intercooler 101 and an Exhaust Gas Recirculation (EGR) cooler 102.

An exhaust passage 103 on the output side of a combustion chamber 104 of the engine 100 is arranged to receive exhaust gas as it exits the combustion chamber 104. The exhaust passage 103 is connected to direct the exhaust gas to a - 12 turbine wheel 105 of a turbocharger 106 and to the EGR cooler 102. The turbine wheel 105 of the turbocharger 106 is coupled by a shaft 107 to a rotary compressor 108 for compressing engine intake air. The turbine wheel 105 can therefore drive the rotary compressor 108 under power of the exhaust gas exiting the combustion chamber 104. The rotary compressor 108 is arranged to output the compressed engine intake air to the intercooler 101.

Both the intercooler 101 and the EGR cooler 102 are connected to a mixing point 109 upstream of an intake manifold 110 on the intake side of the combustion chamber 104. The intercooler 101 is arranged to receive hot compressed engine intake air from the rotary compressor 108, cool it, and output cooled compressed engine intake air to the mixing point 109. Similarly, the EGR cooler 102 is arranged to receive hot exhaust gas output by the combustion chamber 104 via the exhaust passage 103, cool it, and output cooled recirculated exhaust gas to the mixing point 109. A regulator 111 is positioned between the EGR cooler 102 and the mixing point 109 to control the amount of exhaust gas that is recirculated to the mixing point 109 and hence output to the intake manifold 110 and combustion chamber 104.

Referring to Figure 2, the intercooler 101 has an input manifold 120 for receiving compressed engine intake air from the compressor 108 and an output manifold 121 for outputting cooled compressed engine intake air to the mixing point 109.

Extending between the input manifold 120 and the output manifold 121 are heat exchange tubes 122. Each tube 122 has fins 123 mounted on its external surface extending into the space outside the tubes 122, through which space ambient air - 13 can flow. The tubes 122 therefore provide a heat exchange surface between the compressed engine intake air and the ambient air. The fins 123 increase the surface area of the tubes 122 in heat exchange relationship with the ambient air, improving the heat rejection capacity of the intercooler 101.

The intercooler 101 is generally manufactured from aluminium or plastics and requires a maximum heat rejection capacity of up to around 20kW for an engine 100 having power of around kW.

Referring to Figure 3, the EGR cooler 102 comprises a cylindrical shell 130 containing heat exchange tubes 131 through which exhaust gas can flow. The heat exchange tubes 131 extend between an exhaust gas inlet 132 at one end of the shell 130 and an exhaust gas outlet 133 at the other end of the shell 130 and run along the length of the shell 130. A coolant inlet 134 toward the exhaust gas inlet end of the shell 130 is arranged to introduce engine coolant into the shell 130 and a coolant outlet 135 toward the exhaust gas outlet end of the shell 130 is arranged to allow engine coolant to exit the shell 130. The tubes 131 therefore provide a heat exchange surface between the exhaust gas and the engine coolant. The EGR cooler 102 is made from steel and requires a maximum heat rejection capacity of up to around 5kW for an engine 100 having power of around 110 kW.

In use, the combustion chamber 104 of the engine 100 outputs exhaust gas to the exhaust passage 103. The exhaust gas flows along the exhaust passage 103 to the turbine 105 of the turbocharger 106 and to the EGR cooler 102. The pressure of the exhaust gas drives the turbine 105 and consequently the rotary compressor 108 of the turbocharger 106 via the shaft - 14 107. The compressor 108 compresses engine intake air and outputs it to the intercooler 101.

The intercooler 101 cools the hot compressed engine intake air received from the compressor 108 and outputs it to the mixing point 109. More specifically, hot compressed engine intake air flows from the compressor 108 into the intercooler 101 through the input manifold 120 of the intercooler 101 in the direction of arrow A in Figure 2. The hot compressed engine intake air passes from the input manifold 120, along heat exchange tubes 122 to output manifold 121. As the air passes along the tubes 122 it is in heat exchange relationship with ambient air flowing around the outside of the tubes 122 and over the fins 123 in the direction of arrows U in figure 2. So, as the intake air passes along the tubes 122, it is cooled by the ambient air. The cooled compressed engine intake air then exits the output manifold 121, in the direction of arrow B in figure 2, and passes to the mixing point 109.

The EGR cooler 102 cools the hot exhaust gas received from the exhaust passage 103 and outputs it to the mixing point 109. More specifically, hot exhaust gas flows from the exhaust passage 103 into the EGR cooler 102 through the exhaust gas inlet 132 of the EGR cooler 102 in the direction of arrow C in Figure 3. The hot exhaust gas passes from the inlet 132, along heat exchange tubes 131 to outlet 133.

Engine coolant is received in the shell 130 of the EGR cooler 102 through coolant inlet 134 in the direction of arrow V in Figure 3, passes along the shell 130 and out of the shell 130 through coolant outlet 135 in the direction of arrow W in Figure 3. As the exhaust gas passes along the tubes 131 it is in heat exchange relationship with coolant passing through the shell 130. So, as the exhaust gas passes along the tubes 131, it is cooled by the coolant. The cooled exhaust gas then exits the outlet 133, in the direction of arrow D in Figure 3, and passes to the mixing point 109.

The amount of exhaust gas that is recirculated to the mixing point 109 is controlled by varying the amount of exhaust gas that flows through the regulator 111 positioned between the EGR cooler 102 and the mixing point 109. The amounts of both intake air and recirculated exhaust gas that pass through the intercooler 101 and the EGR cooler 102 are varied with engine load. So, the amount of heat that needs to be removed from intake air by the intercooler 101 and from exhaust gas by the EGR cooler 102 also varies with engine load.

Referring to Figure 4, a turbocharged engine 200 incorporating intake air cooling and recirculated exhaust gas cooling according to an embodiment of the invention has a combined cooler 201 for cooling both intake air and recirculated exhaust gas together as a gas combination.

An exhaust passage 203 on an output side of a combustion chamber 204 of the engine 200 is arranged to receive exhaust gas as it exits the combustion chamber 204. The exhaust gas passage 203 is connected to direct the exhaust gas to a turbine wheel 205 of a turbocharger 206 and to a regulator 211 for varying the amount of exhaust gas that is recirculated. The regulator 211 is connected to pass the exhaust gas to a mixing point 209 for mixing the compressed engine intake air with the recirculated exhaust gas to form a gas combination. The turbine wheel 205 of the turbocharger - 16 206 is coupled by a shaft 207 to a rotary compressor 208 for compressing engine intake air. The turbine wheel 206 can therefore drive the rotary compressor 208 under power of exhaust gas exiting the combustion chamber 204. The rotary compressor 208 is arranged to output the compressed engine intake air to the mixing point 209.

The mixing point 209 comprises a venturi (not shown). More specifically, the mixing point has a constricted throat for passing the engine intake air. An inlet is provided in the side wall of the constricted throat for admitting the exhaust gas and the regulator 211 is connected to pass the exhaust gas to the inlet. The mixing point is connected to the input side of the combined cooler 201 to pass the gas combination to the cooler 201 and the combined cooler 201 is connected to an intake manifold 210 on the intake side of the combustion chamber 204.

In this embodiment, the regulator 211 is simply positioned between the exhaust passage 203 and the mixing point 209. It comprises a poppet valve and actuator and is controlled by an electronic engine control module of the engine 200. In another embodiment (not shown), the regulator 211 is housed in the turbocharger 206. More specifically, the regulator 211 is cast in the casing of the turbocharger 206 around the turbine 205.

In this embodiment, all the gas combination (e.g. all the intake air and recirculated exhaust gas) always passes through the cooler. However, in another embodiment (not shown), a bypass passage is connected from upstream of the cooler 201 to downstream of the cooler 201. More specifically, the bypass passage can be connected from between the mixing point 209 and the cooler 201 to the intake manifold 210 of the engine 200. The bypass passage allows combined gas cooling to be bypassed during engine warm up and such like.

Referring to Figure 5, the combined cooler 201 has an input manifold 220 at one end for receiving the gas combination from the mixing point 209. It also has an output manifold 221 at another end for outputting cooled gas combination to the intake manifold 210 of the combustion chamber 204. Extending between the input manifold 220 and the output manifold 221 are heat exchange tubes 222. In this embodiment, there are eight heat exchange tubes 222 that run straight along the length of the combined cooler 201 between the input manifold 220 and the output manifold 221.

A coolant inlet 223 toward the outlet manifold end of the cooler 201 is arranged to introduce coolant into a coolant passage 224 of the combined cooler 201. A coolant outlet 225 toward the inlet manifold end of the cooler 201 is arranged to allow coolant to exit the coolant passage 224 of the cooler 201. The coolant passage 224 and tubes 222 are arranged to provide a heat exchange surface between the coolant in the passage 224 and the gas combination in the heat exchange tubes 222. In this embodiment, the passage 224 crosses back and forth across the length of the heat exchange tubes 222, with the result that the coolant is in a cross counter flow arrangement with respect to the gas combination.

In this embodiment, the combined cooler 201 is cooled using engine coolant. The coolant inlet 223 and coolant outlet 235 - 18 are therefore connected to an engine cooling system. In another embodiment, the combined cooler 201 has its own liquid cooling system. This incorporates a liquid coolant circuit connected to the inlet 223 and outlet 225 of the cooler 201, a pump and a heat exchanger for cooling the coolant. One advantage of this is that coolant can be supplied to the cooler 201 at a lower mean temperature than typical engine coolant, say 50 C instead of 80 C.

In this embodiment, the combined cooler 201 is made from aluminium, although in other embodiments various aluminium alloys are used. It is simply required that the cooler can resist combined gas temperatures of up to around 250 C, as described in more detail below. More specifically, the heat exchange tubes 222 are extruded from aluminium, but the othercomponents may be fabricated from other materials. The heat exchange tubes 222 can therefore be straightforwardly provided with fins (not shown) that extend from the inside surface of the tubes 222. These fins typically extend along the main axis of the tubes 222. They increase the area heat exchange surface of the tubes 222 in contact with the gas combination (in comparison to the inside surface of the tubes 222 alone), but do not constrict the tubes 222 (as this might increase soot build up).

In use, the combustion chamber 204 of the engine 200 outputs exhaust gas to the exhaust passage 203. The exhaust gas flows along the exhaust passage 203 to the turbine 205 of the turbocharger 206 and to the mixing point 209 via the regulator 211. The pressure of the exhaust gas drives the turbine 205 and consequently the rotary compressor 208 of the turbocharger 206 via the shaft 207. The compressor 208 compresses engine intake air and outputs it to the mixing point 209.

The compressed engine intake air and recirculated exhaust gas are mixed at mixing point 209. More specifically, the constricted throat of the venturi increases the pressure of the intake air at its narrowest part and allows the pressure to decrease downstream of the throat (or the narrowest part of the throat). Exhaust gas admitted at the inlet is therefore drawn into the flow of intake air and the disturbed flow caused by the pressure change encourages mixing.

The combined cooler 201 cools the gas combination output by the mixing point 209. After cooling, the cooler 201 outputs the gas combination to the intake manifold 210 of the engine and hence to the combustion chamber 204. More specifically, hot gas combination flows from the mixing point 209 into the cooler 201 through the input manifold 220 of the cooler 201 in the direction of arrow E in Figure 5. The hot gas combination passes from the input manifold 220, along heat exchange tubes 222 to output manifold 221. At the same time, engine coolant flows into the coolant passage 224 through coolant inlet 223, in the direction of arrow X in Figure 5, along the passage 224, in the direction of arrows Y in Figure 5, and out through coolant outlet 225, in the direction of arrow Z in Figure 5. As the gas combination passes along the tubes 222, it is in heat exchange relationship with engine coolant flowing through the coolant passage 224. So, as the gas passes along the tubes 222, it is cooled by the engine coolant. The cooled gas combination then exits the output manifold 221, in the direction of arrow F in - 20 figure 5, and passes to the intake manifold 210 of the engine 200.

The amount of exhaust gas that is recirculated is controlled by varying the amount of exhaust gas that flows through the regulator 211 positioned between the exhaust passage 203 and the mixing point 209. The regulator 211 varies the amount of exhaust gas in a more or less conventional manner, as described with reference to Figures 1 to 3. However, the exhaust gas is combined with the compressed engine intake air before cooling. So, the heat rejection required from the combined cooler 201 varies roughly according to the cumulative heat rejection required from the conventional intercooler 101 and the EGR cooler 102 together.

In more detail, Figure 6 illustrates the heat rejection required from the conventional intercooler 101 and EGR cooler 102 described above and from the combined cooler 201 embodying the invention. Similarly, Table 1 below lists for a typical engine 100, 200 (having a maximum power of around 110 kW) for different engine loads at a speed of 2000 rpm: the percentage of exhaust gas in the intake air and recirculated exhaust gas output to the engine 100, 200; the temperature of the exhaust gas before cooling) the temperature of the intake air before cooling; the temperature of the gas combination at the mixing point 209 embodying the invention; the heat rejection required by the conventional EGR cooler 102; and the heat rejection required by the of conventional intercooler 101.

Load (%) 10 SO 30 40 50 60 30 100 EGR Rate _ (%) 40 30 20 10 0 0 0 0 EGR Temp (degC) 350 450 500 550 600 650 _ 700 750 Alr temp _ (degC?_ 25 60 75 80 85 110 150 175 Mlxed Ternp In (degC) 155 165 160 130 85 110 150 175 EGR Heat ReJ. (kW) 2.7 3.2 3.0 19 0 0 0 0 0 0 0.0 Alr Heat ReJ (kW) 0 0 0.9 1.6 2.2 2.8 4.4 8.1 10.5

TABLE 1

The line in Figure 6 plotted with squares shows the heat rejection required from the conventional intercooler 101 (Charge Air Cooler (CAC)). Substantially no heat rejection is required from the intercooler 101 at engine loads of around 10% or less, as the engine intake air is not compressed significantly by the compressor 108 and is typically only the same temperature as the ambient air, say around 25 C. As the engine load increases, the amount of intake air progressively increases and hence its compression rate and temperature increases, although the intake air temperature does not rise above around 175 C in this example. The heat rejection required from the intercooler 101 therefore rises progressively, for example reaching around 10.5kW at 100% engine load.

The line plotted in Figure 6 with diamonds shows the heat rejection required from the EGR cooler 102. Heat rejection of around 2.7 kW is required from the EGR cooler at an engine load of around 10%. Whilst under these conditions around 40% of the air and gas output to the engine might be recirculated exhaust gas, the temperature of the exhaust gas is only around 350 C. As the engine load increases, the amount of exhaust gas that is recirculated is progressively decreased. - 22

For example, when the load on the engine reaches around 20%, only around 30% of the air and gas output to the engine might be recirculated exhaust gas. However, the exhaust gas may have risen to a temperature of around 450 C and the overall amount of air and gas required by the engine 100 has begun to increase. The heat rejection required from the EGR cooler 102 therefore rises to around 3.2 kW. When engine load increases further, the required heat rejection tends to fall. Indeed, above an engine load of around 50%, no exhaust gas is recirculated and no heat rejection is required from the EGR cooler 102. So, whilst the exhaust gas temperature may reach 750 C at higher loads, the heat rejection required from the EGR cooler 102 decreases above around 20% engine load and is zero above 50% engine load.

At other engine speeds the total amount of heat rejection required from the intercooler 101 and the EGR cooler 102 varies. In particular, as the engine speed increases, the heat rejection requirement from the intercooler 101 increases across the whole range of engine loads. The maximum heat rejection required from the intercooler 101 is as much as 20 kW. For example, heat rejection of 17kW is required to cool (at 80% efficiency) 550 kg of intake air per hour from a temperature of 160 C and output from the compressor 108 at 240 kPa (2.4 bar). Similarly, the maximum heat rejection required from the EGR cooler is as much as 5kW (equivalent to cooling at 60% efficiency 75 kg per hour of exhaust gas from 500 C).

The line plotted with triangles in Figure 6 shows the heat rejection required from the combined cooler 201 embodying the - 23 invention. This is roughly the sum of the line plotted for the intercooler 101 the line plotted for the EGR cooler 102.

Importantly, it can be seen from the graph that the heat rejection required from the combined cooler 201 tends towards the same heat rejection required from the conventional intercooler 101 as engine load is increased and, in this example, is virtually the same above 50% engine load when exhaust gas recirculation is switched off. In addition, the heat rejection capacity of the combined cooler 201 is less at lower engine loads than at higher engine loads. The maximum heat rejection capacity of the combined cooler 201 is only therefore roughly the same as that of the conventional intercooler 101.

It is also apparent from Table 1 that, in this example, the temperature of the gas combination before cooling does not exceed 175 C. Indeed, the maximum temperature of the gas combination occurs when no exhaust gas is recirculated at 100% engine load. This means that the maximum temperature experienced by the combined cooler 201 is the same as the maximum temperature experienced by the conventional intercooler 101. So, the combined cooler 201 only needs to be able to tolerate temperatures of up to around 250 C.

Again, at other engine speeds the total amount of heat rejection required from the combined cooler 201 varies.

Typically, the amount of engine intake air that is required increases with increasing engine speed. However, at engine speeds above around 3000 rpm, exhaust gas recirculation is typically turned off. In more detail, referring to figure 7, in a typical example, exhaust gas is only recirculated at - 24 engine torques and loads below the line plotted with diamonds. So, the maximum required heat rejection is still dictated by the heat rejection required at higher loads, when the only engine intake air is cooled, and the maximum heat capacity of the combined cooler 201 is substantially the same as the conventional intercooler 101, e.g. up to around 20kW (although it may be greater, e.g. for a more powerful engine 200).

Referring to Figure 8, the amount of particulate matter (e.g. soot) that is deposited in the combined cooler 201 is plotted against temperature difference dT between the gas combination and the heat exchange tubes 222. It can be seen that the weight of deposited particulate matter increases with the temperature difference dT between the gas combination and the heat exchange tubes 222. However, as the temperature of the gas combination is minimised in the combined cooler 201 and the coolant of the combined cooler 201 is in a cross counter flow configuration with respect to the gas combination, the temperature gradient and hence particulate deposition is minimised in the combined cooler 201.

The engine 200 of the described embodiments of the invention is a diesel engine. However, the invention can equally be applied to a petrol (or "gasoline") engine, Liquid Petroleum Gas (LPG) gas engine or such like. Similarly, the described engine 200 is intended for automotive applications, usually for cars and light trucks. However, it may of course be used in a broad range of other applications, such as for an electrical generator.

In the forgoing description, numerous specific details are - 25 set forth in order to provide a thorough understanding of the invention. It will be evident, however, to one skilled in the art that the present invention may be practiced without using these specific details. In other instances, well-known methods and structures have not been described in detail so as not to unnecessarily obscure the present invention.

Likewise, the described embodiments of the invention are only examples of how the invention may be implemented.

Modifications, variations and changes to the described embodiments will occur to those having appropriate skills and knowledge. These modifications, variations and changes may be made without departure from the scope of the invention defined in the claims. - 26

Claims

Claims: 1. An intake air and recirculated exhaust gas cooling system for a

boosted internal combustion engine, the system comprising: a gas mixing point for combining compressed engine intake air with recirculated exhaust gas to form a gas combination; an exhaust gas recirculation regulator for receiving exhaust gas from the engine and selectively dividing flow of the exhaust gas exclusively between recirculation of the exhaust gas to the mixing point and output of the exhaust gas without recirculation) a controller for controlling the regulator such that more exhaust gas is recirculated at a lower engine load than at a higher engine load; and a combined cooler for receiving the gas combination from the mixing point, cooling it and outputting it to the engine.
2. An intake air and recirculated exhaust gas cooling system as claimed in Claim 1, further comprising a turbocharger housing the regulator.
3. An intake air and recirculated exhaust gas cooling system as claimed in Claim 1 or Claim 2, wherein the mixing point comprises a venturi through which the compressed intake air flows and at which the exhaust gas is added to the intake air.
4. An intake air and recirculated exhaust gas cooling system as claimed in any preceding claim, further comprising a bypass passage for allowing the gas combination to bypass - 27 the combined cooler.
5. An intake air and recirculated exhaust gas cooling system as claimed in any preceding claim, wherein the heat rejection capacity of the combined cooler is no greater than the heat rejection capacity required to cool only the maximum amount of compressed intake air to be output to the engine.

6. An intake air and recirculated exhaust gas cooling system for a boosted internal combustion engine, the system having a combined cooler for receiving recirculated exhaust gas after combination with compressed engine intake air, cooling the gas combination and outputting it to the engine, wherein the cooler has a heat rejection capacity that is no greater than the heat rejection capacity required to cool only the maximum amount of compressed intake air to be output to the engine.
6. An intake air and recirculated exhaust gas cooling system as claimed in any preceding claim, wherein the combined cooler has a heat rejection capacity of no more than kW.
7. An intake air and recirculated exhaust gas cooling system as claimed in any preceding claim, wherein the combined cooler has a heat rejection capacity of no more than kW.
8. An intake air and recirculated exhaust gas cooling system as claimed in any preceding claim, in which the controller controls the regulator to reduce the recirculation of exhaust gas as engine load increases so that the - 28 temperature of the gas combination is kept below around 250 C.
9. An intake air and recirculated exhaust gas cooling system as claimed in any preceding claim, wherein the combined cooler is fabricated from aluminium or an aluminium alloy.
10. An intake air and recirculated exhaust gas cooling system for a boosted internal combustion engine, the system having a combined cooler for receiving recirculated exhaust gas after combination with compressed engine intake air, cooling the gas combination and outputting it to the engine, wherein the cooler is fabricated from aluminium or an aluminium alloy.
11. An intake air and recirculated exhaust gas cooling system as claimed in any preceding claim, wherein the combined cooler comprises extruded heat exchange tubes for passing the gas combination.
12. An intake air and recirculated exhaust gas cooling system as claimed in any preceding claim, wherein the cooler is liquid cooled.
13. An intake air and recirculated exhaust gas cooling system as claimed in any preceding claim, wherein the cooler has a liquid coolant arranged to flow from an end of the cooler from which the gas combination is output by the cooler toward an end of the cooler into which the gas combination is received by the cooler. - 29
14. An intake air and recirculated exhaust gas cooling system for a boosted internal combustion engine, the system having a combined cooler for receiving recirculated exhaust gas after combination with compressed engine intake air, cooling the gas combination and outputting it to the engine, wherein the cooler has a liquid coolant arranged to flow from an end of the cooler from which the gas combination is output by the cooler toward an end of the cooler into which the gas combination is received by the cooler.
15. An intake air and recirculated exhaust gas cooling system as claimed in Claim 13 or Claim 14, wherein the liquid coolant is arranged to flow across the direction of flow of the gas combination.
16. An intake air and recirculated exhaust gas cooling system as claimed in any preceding claim, wherein the exhaust gas is not filtered.
17. A method of cooling intake air and recirculated exhaust gas using the system of any preceding claim.
18. An intake air and recirculated exhaust gas cooling system substantially as herein described, with reference to or as shown in the accompanying drawings.
19. A method of cooling intake air and recirculated exhaust gas substantially as herein described, with reference to or as shown in the accompanying drawings.