ITBO20090702A1 - MIXER DEVICE FOR A LOW-PRESSURE ENGINE EGR SYSTEM WITH INTERNAL COMBUSTION - Google Patents

Abstract

A mixing device (23) for a low pressure EGR system (21) of an internal combustion engine (1); the mixing device (23) has: a first inlet conduit (25), which can be connected to an EGR conduit (22); a second inlet conduit (26), which can be connected to an intake of fresh air from outside; an outlet conduit (27), which can be connected to an intake conduit (6); a main butterfly valve (28), which is arranged within the first inlet conduit (25) to vary the section of the first inlet conduit (25) itself; and a secondary butterfly valve (29), which is arranged within the second inlet conduit (26) to vary the section of the second inlet conduit (26).

Description

DESCRIPTION

of the patent for Industrial Invention entitled:

? MIXER DEVICE FOR A LOW PRESSURE EGR SYSTEM OF AN INTERNAL COMBUSTION ENGINE?

TECHNIQUE SECTOR

The present invention? relating to a mixing device for a low pressure EGR system of an internal combustion engine.

ANTERIOR ART

In modern internal combustion engines? often there is an EGR (Exhaust Gas Recirculation) system that supplies part of the exhaust gases produced by combustion in the intake duct in order to mix the exhaust gases with fresh air in order to uniform the temperature in each combustion chamber and therefore reduce the pollutants present in the exhaust gases that are released into the atmosphere.

An EGR system comprises an EGR duct, which connects the exhaust duct to the intake duct, and a mixing device which? arranged at the intersection between the EGR duct and the intake duct and has the function of regulating the mixing of the exhaust gases coming from the exhaust duct with the fresh air present in the intake duct. Typically, the mixing device comprises a butterfly valve, which? arranged in correspondence with the EGR duct,? driven by an electric motor and has the function of varying the passage opening of the EGR duct to vary the flow rate of the exhaust gases that are introduced into the intake duct.

The above-described structure of the mixing device has the advantage of being economical and at the same time allows to regulate in a fairly precise way the flow rate of the exhaust gases which are introduced into the intake duct; however, the above-described structure of the mixing device does not allow to precisely adjust the flow rate of the exhaust gases that are introduced into the intake duct in all operating conditions and in particular when a flow rate of exhaust gases is to be introduced into the intake duct. particularly high discharge. Furthermore, known mixing devices have problems of reliability. over time, as the electric motor can? be subject to excessive overheating caused by the heat coming from the EGR duct which is in turn heated by the exhaust gases flowing through the EGR duct itself and which indicatively have a temperature of at least 350-400 ° C.

DESCRIPTION OF THE INVENTION

Purpose of the present invention? providing a mixing device for a low pressure EGR system of an internal combustion engine, which mixing device is free of the drawbacks described above and, in particular, is easy and economical to implement.

According to the present invention, a mixing device is provided for a low pressure EGR system of an internal combustion engine as claimed in the attached claims.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will come now described with reference to the attached drawings, which illustrate a non-limiting example of embodiment, in which:

? figure 1? a schematic view of a supercharged internal combustion engine provided with an EGR system which uses a mixing device made in accordance with the present invention;

? figure 2? a sectional view of the mixing device of Figure 1;

? figure 3? a view with parts removed for clarity of an actuator device of the mixing device of Figure 1;

? figure 4? a graph illustrating the positions of two butterfly valves of the mixing device of Figure 1;

? figure 5? a perspective view of the mixing device of Figure 1;

? Figures 6 and 7 are two different perspective and exploded views of the mixing device of Figure 1; And ? figure 8? a perspective view of an insert made of refractory material disposed along a shaft of a main throttle valve of the mixing device of Figure 1.

PREFERRED EMBODIMENTS OF THE INVENTION

In figure 1, with the number 1? indicated as a whole an internal combustion engine supercharged by means of a turbocharger supercharging system 2.

The internal combustion engine 1 comprises four cylinders 3, each of which? connected to an intake manifold 4 via at least one respective intake valve (not shown) and to an exhaust manifold 5 via at least one respective exhaust valve (not shown). The intake manifold 4 receives fresh air (that is, air coming from the external environment) through an intake duct 6, which is equipped with a 7 air filter and? regulated by an 8 throttle valve. Along the intake duct 6? an intercooler 9 has the function of cooling the intake air. To the exhaust manifold 5? connected an exhaust duct 10 which supplies the exhaust gases produced by combustion to an exhaust system, which emits the gases produced by combustion into the atmosphere and normally comprises at least one catalyst 11 and at least one silencer (not shown) arranged downstream of the catalyst 11.

The supercharging system 2 of the internal combustion engine 1 comprises a turbocharger 12 provided with a turbine 13, which? arranged along the exhaust duct 10 to rotate at high speed? under the action of the exhaust gases expelled from the cylinders 3, and a compressor 14, which? arranged along the intake duct 6 and d? mechanically connected to the turbine 13 to be driven into rotation by the turbine 13 itself so? to increase the pressure of the air fed into the supply duct 6.

Along the exhaust duct 10? a bypass duct 15 is provided, which? connected in parallel to the turbine 13 in order to present its ends? connected upstream and downstream of the turbine 13 itself; along the bypass duct 15? arranged a 16 wastegate valve, which? adapted to regulate the flow rate of the exhaust gases flowing through the bypass duct 15 and d? controlled by a pneumatic actuator 17. Along the intake duct 6? provided a bypass duct 18, which? connected in parallel to the compressor 14 in order to present its ends? connected upstream and downstream of the compressor 14 itself; along the bypass duct 18? arranged a valve 19 Poff, which? adapted to regulate the flow rate of the exhaust gases flowing through the bypass duct 18 and d? piloted by an electric actuator 20.

The internal combustion engine 1 comprises a low pressure EGR system 21, which recirculates a part of the exhaust gases present in the exhaust duct 10, reintroducing these exhaust gases into the intake duct 6. The EGR system 21 comprises an EGR duct 22, which connects the exhaust duct 10 to the intake duct 6, and a three-way mixing device 23 which? arranged at the intersection between the exhaust duct 10 and the intake duct 6 and has the function of regulating the mixing of the exhaust gases coming from the exhaust duct 10 with the fresh air present in the intake duct 6. According to a preferred embodiment, along the EGR duct 22? a heat exchanger 24 is arranged to cool the exhaust gases coming from the exhaust duct 10.

The EGR 21 system? of low pressure, that is? the EGR duct 22 originates from the exhaust duct 10 downstream of the catalyst 11 so as to take the exhaust gases which are already been treated by the catalyst 11 itself and which have a pressure only slightly higher than the atmospheric pressure; in this way, the exhaust gases recirculated by the EGR 22 are more? ? clean ?, that is? have a smaller quantity? of pollutants. Such a configuration? also called? Long-Route? EGR, as the EGR 22 duct must be more? longer than normal to get downstream of the catalyst 11.

The three-way mixing device 23 connects the intake duct 6 with the EGR duct 22. According to what is better illustrated in Figure 2, the mixing device 23 comprises an inlet duct 25 which is connected in series to the EGR duct 22, a duct 26 which is connected in series to a duct coming from the air filter 7, and an outlet duct 27 which is connected in series to the intake duct 6.

As illustrated in FIG. 3, the inlet conduit 25 forms a right angle with the outlet conduit 27 and forms an obtuse angle with the inlet conduit 26, and the inlet conduit 26 forms an acute angle with the outlet conduit 27. Along the inlet duct 25? a main throttle valve 28 is arranged which varies the passage opening of the inlet duct 25 itself, and along the inlet duct 26? a secondary throttle valve 29 is arranged which varies the passage opening of the inlet duct 26 itself. The main throttle valve 28 comprises a throttle plate 30 and a shaft 31 which supports the throttle plate 30 and? mounted swivel. Similarly, the secondary throttle valve 29 comprises a throttle plate 32 and a shaft 33 which supports the throttle plate 32 and? mounted swivel.

As illustrated in Figure 3, the mixing device 23 comprises a common actuator device 34 which controls both the main throttle valve 28 and the secondary throttle valve 29. In particular, the actuator device 34 comprises a single common electric motor 35 which gives motion to both throttle valves 28 and 29, and a connection element 36, which establishes a mechanical connection between the secondary throttle valve 29 and the main throttle valve 28 in such a way that the positions of the two throttle valves 28 and 29 are mechanically constrained to each other. According to a preferred embodiment, the common electric motor 35 transmits motion to the main throttle valve 28 and therefore directly controls the position of the main throttle valve 28 itself; in turn, the main throttle valve 28 transmits motion to the secondary throttle valve 29 through the connecting element 36 which constrains the position of the main throttle valve 28 to the position of the secondary throttle valve 29. In other words, the main throttle 28 acts as the? Master? while the secondary throttle valve 29 acts as a? slave? copying the position of the main throttle valve 28 thanks to the action of the connecting element 36. In particular, the actuator device 34 comprises a gear transmission 37 which transmits motion from a shaft of the electric motor 35 to the shaft 31 of the main throttle valve 28.

The gear transmission 37 comprises an initial toothed wheel 38 integral with the shaft of the electric motor 35, a final toothed wheel 39 integral with the shaft 31 of the main throttle valve 28, and an intermediate and idle toothed wheel 40 interposed between the wheel 38 initial toothed tooth integral with the shaft of the electric motor 35 and the final toothed wheel 39 integral with the shaft 31 of the main throttle valve 28. The intermediate toothed wheel 40 has a first ring of small diameter teeth which? coupled to the initial toothed wheel 38 and a second ring of large diameter teeth which? coupled to the final toothed wheel 39. Overall, the gear transmission 37 has a gear ratio that reduces the speed? rotation of the electric motor 35 (ie the electric motor 35 rotates faster than the shaft 31 of the main throttle valve 28).

According to a preferred embodiment illustrated in the attached figures, the connecting element 36 comprises a rigid arm 41 having one end. 42 what? connected in an eccentric way to the shaft 31 of the main throttle valve 28 and one end? 43 what? opposite to the extremity? 42 and? eccentrically connected to shaft 33 of the secondary throttle valve 29. The end 43 of the arm 41 has an elongated seat 44 which? engaged in a rotatable and sliding manner by an eccentric pin 45 which? integral with shaft 33 of the secondary throttle valve 29 and? supported by a plate 46 integral with the shaft 33 itself; moreover, the extremity? 42 of arm 41? hinged in an eccentric way to the shaft 31 of the main throttle valve 28 and in particular? hinged to the final toothed wheel 39 which? integral with the shaft 31 itself. The rigid arm 41 has a first section which includes the end? 42, and a second trait that includes the extremity? 43 and the seat 44 elongated,? pi? short of the first section and forms an obtuse angle with the first section. According to a different embodiment not shown, the connecting element 36 comprises a gear cascade which constrains the angular position of the shaft 33 of the secondary throttle valve 29 to the angular position of the shaft 31 of the main throttle valve 28.

In the illustrated embodiment, the actuator device 34 imparts to the throttle valves 28 and 29 a law of motion in which when the main throttle valve 28? the secondary throttle valve 29 completely closed? completely open and vice versa; furthermore, the actuator device 34 keeps the secondary butterfly valve 29 completely open until the main butterfly valve 28 reaches a predetermined position of partial opening. When described above? shown in Figure 4, where as a function of the position? of the shaft of the electric motor 35 (shown on the abscissa) the trend of the position (i.e. of the degree of opening / closing) of the secondary throttle valve 29 is shown with a dotted line and with a continuous line the trend of the position (i.e. opening / closing degree) of the main throttle valve 28. In Figure 4? evident that when the main throttle 28? the secondary throttle valve 29 completely closed? completely open and vice versa; moreover, the secondary butterfly valve 29 remains completely open until a predetermined position of partial opening of the main butterfly valve 28 is reached and only subsequently begins to close.

AND? It is important to note that the relationship between the position of the main throttle 28 and the position of the secondary throttle 29? easily modifiable by varying the conformation (shape and / or dimensions) of the arm 41 of the connecting element 36; then the relationship between the position of the main throttle 28 and the position of the secondary throttle 29 shown in FIG. 4? just an example of one of the many relations obtainable by means of the actuator device 34. For example, the relationship between the position of the main throttle valve 28 and the position of the secondary throttle 29 can? be easily modified simply by varying the angle between the two portions of the arm 41 and / or by varying the length of the two portions of the arm 41. In this way, with a simple modification to the arm 41 of the connecting element 36? It is also possible to substantially modify the characteristics of the mixing of the exhaust gases present in the inlet duct 25 with the fresh air present in the inlet duct 26.

According to a possible embodiment not shown, the actuator device 34 also comprises a return spring which tends to push the butterfly valves 28 and 29 towards a neutral position in which the secondary butterfly valve 29? fully open and the main throttle 28? completely closed therefore in the complete absence of exhaust gases recirculated in the intake duct 6. In the event of malfunction of the electric motor 35, the return spring ensures that the throttle valves 28 and 29 are moved and maintained in the neutral position free from exhaust gases recirculated in the intake duct 6.

As illustrated in Figure 6, the shaft 31 of the main butterfly valve 28 has an external portion 47 which protrudes from the inlet duct 25 (through a through hole provided with a gasket) to couple with the actuator device 34; similarly, the shaft 33 of the secondary butterfly valve 29 also has an external portion 48 which protrudes from the inlet duct 26 (through a through hole provided with a gasket) to couple with the actuator device 34. The actuator device 34 comprises a box 49, inside which the outer portion 47 of the shaft 31 of the main throttle valve 28 and the outer portion 48 of the shaft 33 of the secondary throttle valve 29 penetrate. The connection element 36, the electric motor 35, and the gear transmission 37 are also housed inside the box 49. Box 49? equipped with a removable lid 50 which? equipped with an annular gasket to ensure adequate sealing. Furthermore, the box 49 has a cylindrical housing 51 which extends perpendicularly to a bottom wall 52 of the box 49 and houses the electric motor 35 inside it. The bottom wall 52 of the box 49 has two through holes (not shown) to allow the passage of the shafts 31 and 23 of the butterfly valves 28 and 29.

According to a preferred embodiment, the box 49? supported solely by the inlet duct 26 (that is, it does not in any way touch the inlet duct 25 and on the contrary is arranged at a certain distance from the inlet duct 25 itself). Also, box 49? independent and separable from the inlet duct 26 and? fixed to the inlet duct 26 by screws.

Thanks to the fact that the box 49? separated from the inlet duct 25, the heat of the inlet duct 25 can? be transmitted to box 49 only by radiation or convection and not by conduction; so ? It is possible to limit the overheating of the box 49. To further reduce the transmission of heat from the inlet conduit 25 to the box 49 and to the gear transmission 37, at least a part of the outer portion 47 of the shaft 31 of the main throttle valve 28? made of thermally insulating material; in particular, the outer portion 47 of the shaft 31 of the main butterfly valve 28 comprises an insert 53 of thermally insulating material and in particular of refractory material (typically ceramic). AND? It is important to note that the insert 53 has both the function of thermal insulation (that is, of hindering the transmission of heat by conduction along the shaft 31 and towards the box 49 and the transmission 37 with gears), and the function of transmitting the motion (that is to transmit the rotation movement along the shaft 31 between the butterfly plate 30 and the gear transmission 37).

As illustrated in Figure 8, the insert 53 made of thermally insulating material constitutes a part of the outer portion 47 of the shaft 31 of the main throttle valve 28 and? interposed between two sections of the outer portion 47 so as to thermally separate the two sections of the outer portion 47 (i.e. preventing heat transmission by conduction) while maintaining continuity. mechanics of the shaft 31 to transmit the rotational movement along the shaft 31 between the butterfly plate 30 and the gear transmission 37. The insert 53 made of thermally insulating material is mechanically interlocked on both sides on the corresponding sections of the external portion 47; the joints between the insert 53 and the two sections of the external portion 47 also have an angular constraint so as to prevent relative rotation between the insert 53 and the two sections of the external portion 47 to transmit the rotation movement along the shaft 31 between the throttle plate 30 and the gear transmission 37. According to the embodiment illustrated in Figure 8, each joint between the insert 53 and a section of the outer portion 47 comprises a cross-shaped protuberance 54 which rises axially from the insert 53 and fits inside a seat 55 which ? obtained in the section of the external portion 47 and reproduces in negative the shape of the protuberance 54.

As illustrated in figures 5, 6 and 7, to increase the cooling of the electric motor 35 (which heats up both due to the effect of the heat coming from the inlet duct 25, and to the effect of the heat generated in use inside the electric motor 35 itself), the cylindrical housing 51? provided with a plurality? of cooling fins 56, which thermally connect a wall of the cylindrical housing 51 to a wall of the inlet duct 26. Specifically, each cooling fin 54? arranged radially with respect to the cylindrical housing 51 and completely embraces the cylindrical housing 51 itself.

Furthermore, in order to try to cool the inlet duct 25 in the vicinity? of the box 49 in order to reduce the transmission of heat from the inlet duct 25 to the box 49 (therefore to the gear transmission 37 and above all to the electric motor 35), the inlet duct 25? provided with a plurality? of cooling fins 57 preferably arranged axially (ie parallel to the central axis of the inlet duct 25).

The above-described mixing device 23 has numerous advantages.

First, the aforementioned mixing device 23? of simple and economical construction, since with a single electric motor 35 it allows to adequately control both the main throttle valve 28 and the secondary throttle valve 29.

Furthermore, thanks to the presence of the main throttle valve 28 along the inlet duct 25 and of the secondary throttle valve 29 along the inlet duct 26,? It is always possible to optimally adjust the mixing of the fresh air present in the inlet duct 26 with the exhaust gases present in the inlet duct 25. Specifically, by closing the secondary throttle valve 29 completely or almost completely? It is possible to introduce a particularly high flow rate of the exhaust gases into the intake duct 6 which is always adjustable with extreme precision.

In the mixing device 23 described above, it is easy to modify the characteristics of the mixing of the exhaust gases present in the EGR duct 22 with the fresh air present in the intake duct 6 by modifying the relationship between the position of the main throttle valve 28 and the position of the valve. 29 to secondary throttle by simply replacing the arm 41 of the connecting element 36 (arm 41 which has a very low replacement cost).

The above described mixing device 23 has a high reliability. over time, as the 35 electric motor? adequately protected from excessive overheating essentially due to the heat transmitted by the inlet duct 25 which is heated by the exhaust gases which indicatively have a temperature of at least 350-400 ° C. The thermal protection of the 35 electric motor? obtained by creating a separation between the box 49 and the inlet duct 25, inserting the insert 53 of thermally insulating material in the external portion 47 of the shaft 31 of the main throttle valve 28, making the cooling fins 56 between the housing 51 and the inlet duct 26, and making the cooling fins 57 of the inlet duct 25.

Finally, the mixing device 23 has a modular structure, since the actuator device 34? completely contained in the box 49 that? separable from the inlet ducts 25 and 26 and from the outlet duct 27; in this way, the same box 49 (that is, the same actuator device 34) can? be mounted on ducts 25, 26 and 27 of variable size so as to create a wide range of mixing devices 23 which differ from each other in the useful flow rate.

Claims (22)

R I V E N D I C A Z I O N I 1) Mixing device (23) for a low pressure EGR system (21) of an internal combustion engine (1); the mixer device (23) comprises: a first inlet conduit (25) which? connectable to an EGR duct (22); to a fresh air intake from outside; an outlet duct (27) connectable to a suction duct (6); and a main throttle (28) which? arranged inside the first inlet duct (25) to vary the passage opening of the first inlet duct (25) itself; the mixer device (23)? characterized in that it comprises a secondary throttle valve (29) which? arranged inside the second inlet duct (26) to vary the passage opening of the second inlet duct (26). 2) Mixing device (23) according to claim 1 comprising an actuator device (34) which controls the main butterfly valve (28) and the secondary butterfly valve (29) and? provided with a connecting element (36), which establishes a mechanical connection between the main throttle valve (28) and the secondary throttle valve (29) so that the positions of the two throttle valves (28, 29) are mechanically linked to each other. 3) Mixing device (23) according to claim 2, wherein the actuator device (34) comprises a single common motor (35), which varies the position of both butterfly valves (28, 29). 4) Mixing device (23) according to claim 3, wherein the actuator device (34) comprises a gear transmission (37) which transmits motion from the motor (35) to a first shaft (31) of the valve (28) to main butterfly. 5) Mixing device (23) according to claim 2, 3 or 4, wherein the connecting element (36) comprises a rigid arm (41) having a first end? (43) connected in an eccentric way to a second shaft (33) of the secondary throttle valve (29), and a second end? (42) what? opposite to the first extremity? (43) and? eccentrically connected to a first shaft (31) of the main throttle valve (28). 6) Mixing device (23) according to claim 5, wherein: the first end? (43) of the arm (41) has an elongated seat (44) which? engaged in a rotatable and sliding manner by an eccentric pin (45) integral with the second shaft (31) of the main butterfly valve (28); and the first extremity? (42) of the arm (41)? eccentrically hinged to the first shaft (31) of the main throttle valve (28). 7) Mixing device (23) according to claim 6, in which the rigid arm (41) has a first section which comprises the first end? (43) and the seat (44) elongated, and a second section which includes the second extremity? (42) and,? pi? long of the first section, and forms an obtuse angle with the first section. 8) Mixing device (23) according to claim 5, 6 or 7, in which modifying the conformation of the arm (41) of the connecting element (36)? It is possible to modify the relationship between the position of the main throttle valve (28) and the position of the secondary throttle valve (29) to modify the characteristics of the mixing of the exhaust gases present in the first inlet duct (25) with the fresh air present in the second inlet duct (26). 9) Mixing device (23) according to one of claims 2 to 8, wherein due to the action of the actuator device (34) when the main throttle valve (28)? the secondary throttle valve (29) completely closed? completely open and vice versa. 10) Mixing device (23) according to claim 9, wherein the actuator device (34) keeps the secondary throttle valve (29) fully open until the main throttle valve (28) reaches a predetermined position of partial opening . 11) Mixing device (23) according to one of claims 2 to 8, wherein the main butterfly valve (28) comprises a first butterfly plate (30) and a first shaft (31) which supports the first plate (30) butterfly and has a first external portion (47) which protrudes from the first inlet duct (25) to couple with the actuator device (34); at least a part of the first outer portion (47) of the first shaft (31) of the main throttle valve (28)? made of thermally insulating material. 12) Mixing device (23) according to claim 11, wherein the first outer portion (47) of the first shaft (31) of the main butterfly valve (28) comprises an insert (53) made of thermally insulating material and in particular of material refractory. 13) Mixing device (23) according to claim 12, wherein the insert (53) made of thermally insulating material constitutes a part of the first outer portion (47) of the first shaft (31) of the main butterfly valve (28) and? interposed between two sections of the first external portion (47) so as to thermally separate the two sections of the first external portion (47) while maintaining continuity. mechanics of the first shaft (33). 14) Mixing device (23) according to claim 13, in which the insert (53) made of thermally insulating material engages mechanically from both sides on the corresponding sections of the first external portion (47) by means of joints which also have an angular constraint in so as to prevent relative rotation between the insert (53) and the two sections of the first outer portion (47) to transmit the rotational movement along the first shaft (31). 15) Mixing device (23) according to one of claims 2 to 14, wherein: the main butterfly valve (28) comprises a first butterfly plate (30) and a first shaft (31) which supports the first butterfly plate (30) and has a first external portion (47) which protrudes from the first duct (25 ) input to couple with the actuator device (34); the secondary butterfly valve (29) comprises a second butterfly plate (32) and a second shaft (33) which supports the second butterfly plate (32) and has a second external portion (48) which protrudes from the second conduit (26 ) input to couple with the actuator device (34); and the actuator device (34) comprises a box (49), inside which the first outer portion (47) of the first shaft (31) of the main throttle valve (28) and the second outer portion (48) of the second penetrate shaft (33) of the secondary throttle valve (29). 16) Mixing device (23) according to claim 15, wherein the box (49)? supported only by the second inlet duct (26). 17) Mixing device (23) according to claim 16, wherein the box (49)? independent and separable from the second inlet duct (26) and? fixed to the second inlet duct (26) by means of screws. 18) Mixing device (23) according to claim 15, 16 or 17, wherein: the actuator device (34) comprises an electric motor (35) which? inserted in a cylindrical housing (51) of the box (49); And the housing (51) cylindrical? provided with a plurality? of first cooling fins (56), which thermally connect a wall of the cylindrical housing (51) to a wall of the second inlet duct (26). 19) Mixing device (23) according to claim 18, wherein each first cooling fin (56)? arranged radially with respect to the cylindrical housing (51) and completely embraces the cylindrical housing (51) itself. 20) Mixing device (23) according to one of claims 1 to 19, wherein the first inlet duct (25)? provided with a plurality? of second cooling fins (57) preferably axially arranged. 21) EGR system (21) of an internal combustion engine (1) comprising a mixer device (23) according to one of claims 1 to 20. 22) EGR system (21) according to claim 21 and comprising an EGR duct (22) which originates from an exhaust duct (10) downstream of a device for reducing pollutants.