WO2008062254A1

WO2008062254A1 - Internal combustion engine comprising an exhaust gas recirculation system

Info

Publication number: WO2008062254A1
Application number: PCT/IB2006/004061
Authority: WO
Inventors: Nicolas Auffret; Nicolas Dronniou
Original assignee: Renault Trucks
Priority date: 2006-11-23
Filing date: 2006-11-23
Publication date: 2008-05-29
Also published as: US8479512B2; DE602007005511D1; US20130263831A1; EP2087223A1; WO2008062315A2; US8893494B2; US8302400B2; ATE462077T1; US20100064685A1; EP2087222A2; US20120255298A1; WO2008062315A3; EP2087222B1

Abstract

The internal combustion engine (1 ) comprises a plurality of cylinders (3), an air intake line (5) capable of carrying intake air towards an engine intake manifold (4), and an exhaust line (7) capable of collecting,, exhaust gas from an exhaust manifold (6). The internal combustion engine (1 ) also comprises an EGR line (12) capable of rerouting a part of the exhaust gas from the exhaust line (7), through an EGR cooler (14), towards the air intake line and at least a first turbocharger comprising a first turbine (15) driven by the exhaust gas flowing towards the atmosphere, mechanically linked to a first compressor (16) located on the air intake line (5). The internal combustion engine further comprises an EGR turbine (17) located on the EGR line (12) upstream from the EGR cooler (14), driven by the EGR gas flowing in the EGR line (12). Thus, EGR gas flowing from the exhaust manifold goes through the EGR turbine prior to entering the EGR cooler. Due to the pressure reduction occurring in the turbine, the EGR gas temperature is lowered at the intake manifold inlet, and less cooling power from the engine cooling system is required to cool down the EGR gas in the EGR cooler.

Description

INTERNAL COMBUSTION ENGINE COMPRISING AN EXHAUST GAS RECIRCULATION SYSTEM

Field of the invention

The present invention relates to an internal combustion engine comprising an exhaust gas recirculation system, especially an internal combustion engine dedicated to personal vehicles or industrial applications, such as industrial vehicles or machines.

Technological background

In many countries, environmental regulations impose an upper limit in engine NOx (nitrogen oxide and nitrogen dioxide) emissions, and. in future regulations, this limit will again be reduced.

One of the conventional ways of decreasing the level of NOx emissions in internal combustion engines is to recirculate a portion of the exhaust gas back to the engine cylinders. This results in lowering the combustion temperature and oxygen concentration and, as a consequence, limits NOx generation as NOx is generated by oxygen and high temperature. Cooling the exhaust gas recirculation (EGR) gas before reintroduction into the engine further reduces NOx emissions as this allows the introduction into the cylinders of a greater mass of exhaust gas and increases mixture heat capacity. To meet the current regulations, a typical internal combustion engine can comprise as depicted on Figure 1 :

- an air intake line 100 carrying intake air towards at least one engine intake manifold 101 connected to each cylinder 102, said air intake line 100 including an upstream low pressure compressor 103 and a downstream high pressure compressor 104 as well as an heat exchanger 105 (using the coolant of the engine cooling system) located between the high pressure compressor and the intake manifold 101 ;

- an exhaust line 106 having at least one exhaust manifold 107 collecting the exhaust gas formed in each cylinder 102; said exhaust line 106 can include two turbines 108, 109 driven by the exhaust gas flowing from the exhaust manifold 107, each turbine being mechanically connected to one of the compressors 103, 104;

- an exhaust gas recirculation (EGR) line 110 whose inlet is connected to the exhaust line 106 and whose outlet comes out in an EGR mixer 111 connected to the air intake line 100, before the intake manifold 101 and after the intake cooler 105, whereby part of the exhaust gas is mixed with intake air and then reintroduced into the engine cylinders 102.

A control valve 113 also referred to as EGR valve regulates the flow of exhaust gas rerouted from the exhaust manifold 107 into the intake manifold 101.

In such a known engine, the EGR gas is cooled before entering the cylinders 102 by means of an EGR cooler 112 located in the EGR line 110; this cooler 112 is usually an air / water heat exchanger using the coolant of the engine cooling system. Consequently, the heat transferred from the hot exhaust gas to the coolant can be significant, which can be detrimental to the cooling capacity of the vehicle cooling system.

Tighter NOx emission regulations will therefore result in vehicle cooling systems needing more cooling power. Therefore, coolant pump design could be problematic, and fuel consumption could be significantly increased as cooling fans may have to be engaged more often to meet the extra cooling need.

Another technical issue that has to be taken into account is the engine pressure differential. In order words, for the EGR gas to be able to flow from the exhaust manifold to the intake manifold, the engine pressure differential (which is the difference between exhaust pressure and intake pressure, i.e. ΔP = P_eXhau_st - Pintake) must be positive and significant enough.

However, under specific engine operating conditions, exhaust backpressure can be lower than intake pressure (i.e. ΔP is negative) or not high enough. This generally occurs at low engine speeds or low loads. Consequently, under these engine operating conditions, no or too little EGR gas is rerouted into the intake manifold, and therefore NOx emissions cannot be reduced under the level imposed by regulations. This positive exhaust to intake pressure difference will also affect engine efficiency and increase fuel consumption. WO 01/14707 tackles the problem of EGR gas cooling and of engine cooling system overload. Under the teaching of this document, the EGR cooler has to be oversized, as EGR gas flowing from this EGR cooler goes through a compressor and then is reintroduced into the intake manifold without further cooling.

Moreover, since a single turbine is provided on the exhaust line to drive two compressors, namely an intake air compressor and an EGR gas compressor, the engine thermodynamic efficiency is not optimized.

Another engine provided with an EGR system is described in WO 98/35153. According to this document, EGR gas flows through a radiator, and then through a compressor before it is reintroduced into the intake manifold. Consequently, the EGR gas temperature increase taking place in the EGR compressor can be compensated by a prior temperature reduction in the radiator, in which the EGR gas cooling is achieved by air flow.

While this arrangement is profitable since it does not entail an overload of the engine cooling system, on the other hand it has several other drawbacks.

In particular, in order to achieve a sufficient decrease in EGR gas temperature, the radiator must be large enough, and located in a sufficiently open space to allow air to flow around it. However, a vehicle has a complex structure which includes a large number of components (engine, cooling system, suspension system, transmission system, hydraulic system etc.) which are very tightly arranged so as to minimize the overall size of the vehicle. The consequence is that the space dedicated to accommodate the radiator can be severely limited.

More generally, the engine arrangement described in WO 98/35153 involves many conduits, and consequently many associated components such as valves, etc. In addition to being complex, such a structure also lacks compactness.

It therefore appears that there is room for improvement in the exhaust gas recirculation system in internal combustion engines.

Summary of the invention

It is an object of the present invention to provide an improved internal combustion engine equipped with an exhaust gas recirculation system, which can overcome the drawbacks encountered in current engines. It is a further object of the present invention to provide an engine where EGR gas can be cooled enough without overloading the engine cooling system.

Another object of the invention is to provide an engine with a better thermodynamic efficiency.

A further object of the invention is to provide an engine whose pressure differential makes it possible for EGR gas to flow towards the intake manifold at any engine operating condition.

Thus, the present invention provides an internal combustion engine that comprises a plurality of cylinders, an air intake line capable of carrying intake air towards an engine intake manifold and an exhaust line capable of collecting exhaust gas from an exhaust manifold. The internal combustion engine also comprises an EGR line capable of rerouting a part of the exhaust gas from the exhaust line, through an EGR cooler, towards the air intake line and at least a first turbocharger comprising a first turbine driven by the exhaust gas flowing towards the atmosphere, mechanically linked to a first compressor located on the air intake line. The internal combustion engine further comprises an EGR turbine located on the EGR line upstream from the EGR cooler, driven by the EGR gas flowing in the EGR line. With this arrangement, EGR gas flowing from the exhaust manifold goes through the EGR turbine prior to entering the EGR cooler. Due to the pressure reduction occurring in the turbine, the EGR gas temperature is lowered, for example by as much as 100⁰C. Consequently, the EGR gas temperature at the EGR cooler inlet is lower than in the prior art engines. This makes it possible to reduce the load on the engine cooling system and to obtain a lower EGR gas temperature at the intake manifold inlet, which means an even more reduced NOx level in the exhaust gas. Moreover, to achieve this goal, the invention does not require a large and cumbersome radiator. It has to be noticed that, even if an air to water heat exchanger is still present in the engine to cool EGR gas, the heat rejection to the engine cooling system is however lowered thanks to the invention, since the expansion through the EGR turbine makes it possible to save a significant part of the vehicle cooling capacity.

The EGR turbine being located on the EGR line is solely driven by EGR gas rerouted into the intake manifold, the EGR turbine is not driven by exhaust gas flowing towards the atmosphere. In other words, the EGR turbine is an EGR dedicated turbine, arranged in parallel with the first turbine of the first turbocharger; all the flow passing through the EGR turbine is fed to the air intake line.

According to a preferred implementation of the invention, the internal combustion engine further comprises an energy recovering means linked to the EGR turbine and capable of recovering the energy provided by the EGR turbine.

This important arrangement of the engine makes it possible to recover the energy produced by the EGR turbine in an appropriate energy recovering means, which can directly use this energy or store it for future use. Consequently, thanks to this implementation of the invention, on top of a better EGR temperature decrease, a better engine thermodynamic efficiency can be achieved.

Preferably, the EGR line outlet is connected to the air intake line upstream from at least one compressor.

In that way, the invention makes it possible to manage engine air pressure differential ΔP, since the compressor forces EGR gas to flow towards the intake manifold even at engine operating conditions when ΔP would be opposite or favourable but too low. Therefore, the engine back pressure is significantly limited: EGR gas will naturally flow from a high pressure source to a low pressure source and fuel consumption can be improved. Because there always exists an EGR gas recirculation, engine NOx emissions can be effectively reduced under the imposed level, whatever the engine operating conditions. According to a first embodiment of the invention, the energy recovering means is a second compressor mechanically connected to the EGR turbine and capable of compressing gas flowing from the first compressor outlet towards the intake manifold.

In this embodiment, the engine comprises two turbochargers whose turbine, driven by exhaust gas or EGR gas, provides energy for compressing intake air or a mix of intake air and EGR gas. The gas flowing in the air intake line towards the intake manifold can then pass through a two- stage turbocharger. This arrangement may be implemented in order to provide an intake pressure which is high enough to create a favourable engine pressure differential. The second compressor can be located on the air intake line, downstream from the first compressor. When needed, the air intake line may further comprise an additional compressor located downstream from the first compressor. This additional compressor is preferably situated between the first and the second compressors, and may be part of an additional turbocharger, the turbine of which being located on the exhaust line upstream from the first turbine.

Alternatively, the second compressor is arranged in parallel with an additional compressor located on the air intake line downstream from the first compressor.

According to a second embodiment of the invention, the energy recovering means may be an energy storage component (such as a battery), a crankshaft mechanically or electrically connected to the EGR turbine, or an electrical device connected to the EGR turbine (such as an electric motor or an alternator).

This second embodiment can be implemented when no compressor linked to the EGR turbine is required on the air intake line to obtain a satisfactory engine pressure differential. Consequently, the energy provided by the EGR turbine can either be directly used by another energy recovering means, or stored in an energy recovering means for a future use or for a use by another device located farther.

According to a third embodiment of the invention, the EGR turbine is linked to a shaft of the at least first turbocharger said shaft connecting the first turbine and the first compressor. In other words, this embodiment of the invention incorporates a single compressor which is driven by two turbines namely an EGR turbine driven by EGR gas and a turbocharger turbine driven by engine exhaust gas. In this embodiment of the invention, the energy that is recovered on the EGR turbine is added to the energy that is recovered by the turbine of the engine turbocharger. In order to increase the intake pressure, the air intake line may further comprise an additional compressor located downstream from the first compressor. This additional compressor can be for example part of an additional turbocharger the turbine of which is driven by exhaust gas.

The EGR line outlet can be connected to the air intake line upstream from the only compressor or from the compressor located most upstream. This would result in a better mixing of EGR gas and intake air, and thus a better cooling of EGR gas since they may flow through more coolers and compressors. Additionally, if several compressors are provided, EGR gas pressure would also be higher, which favours NOx emission reduction.

Alternatively, the EGR line outlet can be connected to the air intake line downstream from the compressor located most upstream and upstream from at least one other compressor. With this disposition, the EGR pipes are shorter, the engine being less expensive and more compact.

In an advantageous way, the exhaust manifold is arranged in two parts, each connected to a corresponding EGR pipe, the two EGR pipes meeting upstream from the first turbine. This prevents the EGR turbine from getting energy from only one part of the exhaust manifold, receiving only a few exhaust pulses, which would lead to an irregular driving of said turbine and to a poor efficiency.

Besides, the air intake line can further comprise at least one cooler located downstream from the EGR line outlet. This makes it possible to lower the EGR gas temperature in at least one cooler before it is reintroduced into the intake manifold.

For example, the air intake line can include:

- an intake cooler located downstream from the compressor situated most downstream, and upstream from the intake manifold;

- and/or at least one intake cooler located between two compressors when at least two compressors are present.

These and other advantages will become apparent upon reading the following description in view of the drawing attached hereto representing, as non-limiting examples, embodiments of an engine according to the invention.

Brief description of the drawing

The following detailed description of several embodiments of the invention is better understood when read in conjunction with the appended drawing being understood, however, that the invention is not limited to the specific embodiments disclosed. In the drawing,

Figure 1 is a schematic drawing of an internal combustion engine of the prior art; Figures 2 - 5 are schematic drawings of an internal combustion engine according to several variants of a first embodiment of the invention;

Figures 6 and 7 are schematic drawings of an internal combustion engine according to several variants of a second embodiment of the invention, Figure 8 is a schematic drawing of an internal combustion engine according to a third embodiment of the invention.

Detailed description of the invention

An internal combustion engine 1 typically comprises an engine block 2 defining a plurality of cylinders 3, namely six cylinders in the embodiments illustrated in the figures. The number and arrangement of cylinders as illustrated in the drawings is of course purely indicative.

Intake air is carried towards an intake manifold 4 feeding the cylinders 3, through an air intake line 5. The gas formed in each cylinder 3 can be collected by an exhaust manifold 6 arranged in two halves. An exhaust line 7 connected to the exhaust manifold 6 carries one part of the gas (exhaust gas) towards the atmosphere. The other part of the gas (EGR gas) is carried by two circuits 8, 9, each connected to one half of the exhaust manifold 6. These circuits 8, 9, each comprising a valve 10, 11 , meet and form a single EGR line 12 whose outlet comes out in an EGR mixer 13 connected to the air intake line 5. The EGR line is also provided with an EGR cooler 14 using the engine coolant, situated downstream from the meeting point of conduits 8, 9.

The engine 1 can further include at least a first turbocharger. The first turbocharger can be a low pressure turbocharger having a turbine 15 located on the exhaust line 7, and a compressor 16 located on the air intake line 5. The engine 1 can further comprise a second turbine which is a dedicated EGR turbine 17 located on the EGR line 12, downstream from the meeting point of conduits 8, 9 and upstream from the EGR cooler 14. Several embodiments of the invention will now be described; the same elements in each embodiment bear the same reference numerals.

In a first embodiment of the invention, illustrated in Figures 2 - 5, EGR turbine 17 is part of a high pressure turbocharger, the compressor 18 of which is located on the air intake line 5 downstream from the low pressure compressor 16. The air intake line 5 can be provided with a first intake cooler 19 located between low pressure compressor 16 and high pressure compressor 18, and with a second intake cooler 20 located downstream from the high pressure compressor 18 and upstream from the intake manifold 4. Both coolers 19, 20 can suitably use an engine coolant from the engine cooling system.

As depicted in Figures 2 and 3, the engine 1 is equipped with a two-stage turbocharger and, thus, a high boost pressure can be achieved.

In the embodiment described in connection with Figure 2, the EGR mixer 13 is located upstream from low pressure compressor 16; intake air mixed with EGR gas flow through two coolers 19 and 20 and two compressors 16 and 18; this has the advantage of a better mixing between intake air and EGR gas, a better cooling of EGR gas and a higher gas pressure at the intake manifold 4. Another significant advantage is that it is possible to get high EGR rates even if boost pressure, i.e. intake pressure, is higher that exhaust pressure.

In the embodiment described in connection with Figure 3, the EGR mixer 13 is located downstream from low pressure compressor 16 and upstream from first intake cooler 19. With this arrangement, the EGR circuit is shorter, and then more compact. And since EGR gas still flows through one compressor and two coolers, efficient mixing and cooling can be obtained, as well as a high enough intake pressure; it should also be noted that this embodiment has the further advantage of a good balance of the low pressure turbocharger as almost the same mass flow goes through the low pressure compressor 15 and through the low pressure turbine 16.

In the variant described in connection with Figure 4, the engine 1 is equipped with a three-stage turbocharger, since an additional turbocharger is provided by the EGR turbine 17. This additional turbocharger comprises a turbine 21 located on the exhaust line 7 upstream from the turbine 15 of the low pressure turbocharger and a compressor 22 located on the air intake line 5 downstream from first intake cooler 19 and upstream from high pressure compressor 18. An additional cooler 23 can also be provided on the air intake line 5 between intermediate compressor 22 and high pressure compressor 18. In other words, the implementation of Figure 4 corresponds to a conventional two-stage turbocharger architecture (with two turbines on the exhaust line and two compressors on the air intake line) with a third compressor linked to the EGR turbine, in order to increase the final boost pressure. The three compressors are connected in series.

In the variant of Figure 5, there is also provided an additional turbocharger, comprising a turbine 24 located on the exhaust line 7 upstream from the turbine 15 of the low pressure turbocharger and a compressor 25 located on the air intake line 5, downstream of the low pressure compressor 16 and of the first intake cooler 19 and upstream from second intake cooler 20. However, compressor 18 is located on a parallel line 26 whose inlet is connected to the air intake line 5 upstream from high pressure compressor 25 and whose outlet is connected to air intake line 5, through a valve 27, downstream from intermediate compressor 25. A single direction flow valve 27 has to be used to prevent gas from the high pressure compressor 25 from flowing towards the compressor 18 outlet, which could generate surge.

It has to be noticed that Figure 5 shows no pipe 8, 9 meeting in EGR line 12, but an EGR line 12 whose inlet is connected to the exhaust manifold 6 and which is provided with a single valve 28. However an arrangement with two conduits 8, 9 can be envisaged (not shown). A further arrangement could be to introduce the EGR mixer upstream of the low pressure compressor (not shown). A further embodiment of the invention is shown in Figures 6 and 7.

In this embodiment, EGR turbine 17 is not linked to a compressor located on the air intake line, but to another type of energy recovering means 30 which is capable of recovering the energy provided by the turbine. Depending on the vehicle needs, this energy recovering means 30 may be: - an energy storage component (such as a battery);

- a crankshaft mechanically or electrically connected to turbine 17;

- an electrical device connected to turbine 17 (such as an electric motor or an alternator);

- or any other component capable of directly using the energy provided by turbine 17 or storing this energy for a future user for a use by another device located farther. This embodiment can be very attractive as vehicle electric energy needs tend to grow to power more vehicle electrical equipments such as air conditioning, GPS system etc. For example, the component could provide electricity to an engine accessory such as the oil pump or the water pump, which would help to distribute coolant to EGR cooler, etc. The mechanical connection may be achieved by means of a viscous coupling or other appropriate coupling.

In the arrangement of Figure 6, a single turbocharger is provided suitably comprising turbine 15 and compressor 16. The air intake line 5 is equipped with only one cooler (first intake cooler 19), and the EGR mixer 13 is situated upstream from the compressor 16. However, other implementations can be envisaged.

Figure 7 shows an alternative arrangement where the engine 1 is provided with a two-stage turbocharger: - a low pressure turbocharger comprising turbine 15 and compressor 16;

- a high pressure turbocharger comprising an additional turbine 31 located on exhaust line 7 upstream from turbine 15 and a high pressure compressor 32 located downstream from low pressure compressor 16, a first intake cooler 19 being situated between the two compressors 16, 32.

In this arrangement, the EGR mixer 13 may be situated upstream from low pressure compressor 16. However, other implementations can be envisaged (more coolers, different location for EGR mixer, etc.).

It is possible to envisage that the energy recovering means 30 could control the EGR rate, if an electric motor is used, by accelerating or decelerating the EGR dedicated turbine. Another possible implementation could be to introduce a variable geometry turbine which would help controlling the EGR gas mass rerouted into the intake manifold.

Figure 8 illustrates a further embodiment of the invention whereby the EGR turbine 17 can be directly linked to the shaft of the standard turbocharger. In other words the compressor 15 can be driven by the turbine 16 and by the EGR turbine 17. This embodiment of the invention incorporates a single compressor 15 and two separate turbines 16 and 17; the engine includes two turbine inlet ports: one turbine inlet port is dedicated to the standard exhaust system, and the other turbine inlet port is dedicated to the EGR circuit.

In this embodiment of the invention, EGR gas that have been cooled by flowing through the EGR turbine 17, can be further cooled by an EGR cooler 14 before joining the air intake line 5 upstream from the compressor 16. By connecting on the same shaft, the turbine 15 driven by the engine exhaust gas and the EGR turbine 17 driven by EGR gas, an extra driving power is provided to the compressor 16. Overall this embodiment of the invention is less demanding on the vehicle cooling system while making an optimized use of the energy contained in the exhaust gas and in the EGR gas. It can also be envisaged that this latter embodiment of the invention which is provided with a twin-turbine turbocharger could integrate a two stage turbocharged arrangement where said twin-turbine turbocharger would be used in cooperation with a low pressure turbocharger in an arrangement similar the one depicted on Figure 4.

Of course, the invention is not restricted to the embodiments described above by way of non-limiting examples, but on the contrary it encompasses all embodiments thereof.

Claims

1. An internal combustion engine (1) having a plurality of cylinders (3), comprising: - an air intake line (5) capable of carrying intake air towards an engine intake manifold (4); an exhaust line (7) capable of collecting exhaust gas from an exhaust manifold (6); an EGR line (12) capable of rerouting a part of the exhaust gas from the exhaust line (7), through an EGR cooler (14), towards the air intake line; at least a first turbocharger comprising a first turbine (15) driven by the exhaust gas flowing towards the atmosphere, mechanically linked to a first compressor (16) located on the air intake line (5); characterized in that the engine (1) further comprises an EGR turbine (17) located on the EGR line (12) upstream from the EGR cooler (14), driven by the EGR gas flowing in the EGR line (12).

2. The internal combustion engine according to claim 1 , characterized in that it further comprises an energy recovering means linked to the EGR turbine (17) and capable of recovering the energy provided by the EGR turbine (17).

3. The internal combustion engine according to claim 1 or claim 2, characterized in that the EGR line outlet is connected to the air intake line upstream from at least one compressor.

4. The internal combustion engine according to claim 2 or 3, characterized in that the energy recovering means is a second compressor (18) mechanically connected to the EGR turbine (17) and capable of compressing gas flowing from the first compressor outlet towards the intake manifold (4).

5. The internal combustion engine according to claim 4, characterized in that the second compressor (18) is located on the air intake line (5), downstream from the first compressor (16).

6. The internal combustion engine according to claim 4, characterized in that the air intake line (5) further comprises an additional compressor (22) located downstream from the first compressor (16).

7. The internal combustion engine according to claim 4, characterized in that the second compressor (18) is arranged in parallel with an additional compressor (25) located on the air intake line (5) downstream from the first compressor (16).

8. The internal combustion engine according to claim 2 or 3, characterized in that the energy recovering means (30) is an energy storage component such as a battery.

9. The internal combustion engine according to claim 2 or 3, characterized in that the energy recovering means (30) is a crankshaft mechanically or electrically connected to the EGR turbine (17).

10. The internal combustion engine according to claim 2 or 3, characterized in that the energy recovering means (30) is an electrical device connected to the EGR turbine (17).

11. The internal combustion engine according to claim 2 characterized in that the EGR turbine (17) is linked to a shaft of the at least first turbocharger said shaft connecting the first turbine (15) and the first compressor (16).

12. The internal combustion engine according to any one of claims 8 to 11 , characterized in that the air intake line (5) further comprises an additional compressor (32) located downstream from the first compressor (16).

13. The internal combustion engine according to any one of claims 1 to 12, characterized in that the EGR line outlet is connected to the air intake line (5) upstream from the only compressor or from the compressor located most upstream.

14. The internal combustion engine according to any one of claims 4 to 7 or 12, characterized in that the EGR line outlet is connected to the air intake line (5) downstream from the compressor located most upstream and upstream from at least one other compressor.

15. The internal combustion engine according to any one of claims 1 to 14, characterized in that the exhaust manifold (6) is arranged in two parts, each connected to a corresponding EGR conduit (8,9), the two EGR conduits (8,9) meeting upstream from the first turbine (17).

16. The internal combustion engine according to any one of claims 1 to 15, characterized in that the air intake line (5) further comprises at least one cooler (19,20) located downstream from the EGR line outlet.