WO2009048408A1

WO2009048408A1 - Arrangement and method for recirculation of exhaust gases from a combustion engine

Info

Publication number: WO2009048408A1
Application number: PCT/SE2008/051104
Authority: WO
Inventors: Vesa Hokkanen; Astrid Simovits
Original assignee: Scania CV AB
Current assignee: Scania CV AB
Priority date: 2007-10-08
Filing date: 2008-10-01
Publication date: 2009-04-16
Anticipated expiration: 2010-04-08
Also published as: SE531599C2; SE0702248L

Abstract

The present invention relates to an arrangement and a method for recirculation of exhaust gases of a combustion engine (2). The arrangement comprises a return line (11), an EGR cooler device (14, 15) for cooling of recirculating exhaust gases which has an exhaust gas passage (14a, 15a) which comprises a first aperture (14b) and a second aperture (14c, 15b), and a cooling system which is adapted to cooling the recirculating exhaust gases in the EGR cooler device (14, 15) to the condensation temperature of the water vapour in the exhaust gases. The arrangement comprises a valve means (24) adapted to leading the recirculating exhaust gases alternately in opposite directions through the EGR cooler device (14, 15) so that condensate forms alternately within the exhaust gas passage (14a, 15a) of the EGR cooler device in the portion which comprises the first aperture (14b) and in the portion which comprises the second aperture (14c, 15b).

Description

Arrangement and method for recirculation of exhaust gases from a combustion engine

BACKGROUND TO THE INVENTION, AND STATE OF THE ART

The present invention relates to an arrangement and a method for recirculation of exhaust gases of a combustion engine according to the preambles of claims 1 and 11.

The technique called EGR (Exhaust Gas Recirculation) is a known way of leading part of the exhaust gases from a combustion process in a combustion engine back, via a return line, to a line for supply of air to the combustion engine. A mixture of air and exhaust gases is supplied via the air line to the engine's cylinders in which the combustion takes place. Adding exhaust gases to the air causes a lower combustion temperature resulting inter alia in the exhaust gases having a reduced content of nitrogen oxides NO_x. This technique is used both for Otto engines and for diesel engines.

The return line for the exhaust gases comprises inter alia an EGR valve which is settable so that a desired amount of exhaust gases is recirculated. An electrical control unit is adapted to controlling the EGR valve on the basis inter alia of information about the load of the combustion engine. The return line also comprises at least one EGR cooler adapted to cooling the exhaust gases in the return line before they are mixed with the air and led to the engine. In course of time, soot deposits from the exhaust gases inevitably build up on the inside surfaces of the EGR cooler, thereby impairing the heat transfer capacity of the EGR cooler and at the same time increasing the resistance to the flow of exhaust gases through the EGR cooler. The presence of the soot deposits reduces the performance of the combustion engine and increases the content of nitrogen oxides in the exhaust gases.

WO 2007/039702 refers to an arrangement for recirculation of exhaust gases of a supercharged combustion engine. The arrangement comprises a valve means which makes recirculation of exhaust gases possible in opposite directions through an EGR cooler. Soot deposits on the inside surfaces of the EGR cooler can be removed by changing the direction of flow of the exhaust gases through the EGR cooler. For it to be possible for the soot deposits to be removed fairly effectively, the exhaust gases need to be led through the EGR cooler at a very high velocity.

US 6,904,898 refers to an arrangement for recirculation of exhaust gases of a supercharged combustion engine in which the recirculating exhaust gases are cooled in a EGR cooler by means of a coolant. If the coolant is at a temperature below a threshold value, there is risk of the exhaust gases being cooled to a temperature such that condensate forms within the EGR cooler. During normal operation, to prevent the formation of condensate, no recirculation of exhaust gases through the EGR cooler is allowed when the coolant is at a temperature below said threshold value. In circumstances where the EGR cooler needs cleaning from soot deposits, however, exhaust gases are allowed to recirculate through the EGR cooler when the coolant is at a temperature below said threshold value. In such cases, condensate forms on the internal surfaces of the EGR cooler and effectively dissolves any soot deposits on them. However, the recirculating exhaust gases will be at above the condensation temperature during their main passage through the EGR cooler. Substantially the only result will be the formation of condensate within a final portion of the EGR cooler and the cleaning of the internal surfaces in this final portion of the EGR cooler.

SUMMARY OF THE INVENTION

The object of the present invention is to provide an arrangement and a method of the kind mentioned in the introduction whereby the internal surfaces of an EGR cooler device are kept clean of soot deposits from the exhaust gases in a simple and effective manner.

This object is achieved with the arrangement of the kind mentioned in the introduction which is characterised by the features indicated in the characterising part of claim 1. The exhaust gases are progressively cooled down from a relatively high temperature when they flow through an EGR cooler device. If the exhaust gases are cooled effectively, they reach at a position within the EGR cooler device a temperature at which the water vapour in the exhaust gases condenses. Condensate will therefore form from that position in the EGR cooler device to an aperture where the exhaust gases are led out from the EGR cooler device. According to the present invention, a valve means is used for leading the recirculating exhaust gases alternately in opposite directions through the EGR cooler device. Condensate will thus form alternately within the end portions of the EGR cooler device which comprise the first and second apertures of the exhaust gas passage. Condensate which forms on the internal wall surfaces of the EGR cooler device will dissolve any soot deposits. Soot deposits freed will be carried off by the exhaust gases and led to the combustion engine, in which they will burn. When the load of the combustion engine is low and the exhaust gases are at a lower temperature than during normal operation, the exhaust gases may cool relatively quickly to their condensation temperature within the EGR cooler device. In such cases, condensate will form in relatively large parts of the exhaust gas passage. With advantage, the capacity of the cooling system which cools the exhaust gases in the EGR cooler device will be such that condensate can form in half of the exhaust gas passage through the EGR cooler device. Thus one half of the exhaust gas passage can be cleaned of soot deposits when the exhaust gases are led in one direction through the EGR cooler device, and the remaining half of the exhaust gas passage when the exhaust gases are led in an opposite direction through the EGR cooler device. This means that the whole exhaust gas passage through the EGR cooler device can on separate operating occasions be subjected to effective cleaning of soot deposits.

According to a preferred embodiment of the present invention, the return line comprises a first line portion adapted to leading recirculating exhaust gases to the valve means, a second line portion extending between the valve means and the first aperture of the EGR cooler device, a third line portion extending between the valve means and the second aperture of the EGR cooler device, and a fourth line portion adapted to leading exhaust gases which have been cooled in the EGR cooler device towards the combustion engine. With such a configuration of the return line, the recirculating exhaust gases can be led in alternate directions through the EGR cooler device. The valve means may, in the first position, be adapted to connecting the first line portion to the second line portion and the third line portion to the fourth line portion, hi the first position, the warm exhaust gases are thus led into the EGR cooler device via the first aperture of the exhaust gas passage and out through the second aperture of the exhaust gas passage. The valve means may, in the second position, be adapted to connecting the first line portion to the third line portion and the second line portion to the fourth line portion. In the second position, the warm exhaust gases are thus led into the EGR cooler device via the second aperture of the exhaust gas passage and out through the first aperture of the exhaust gas passage. The valve means may comprise an internal space with a valve body arranged for rotation between the first position and the second position. With such a valve body the internal space can easily be divided so that the various line portions leading to the internal space of the valve means are connected with one another in a desired manner when the valve body is placed in the first position and the second position. Other types of valve means which have a corresponding function may also be used.

According to a preferred embodiment of the present invention, the EGR cooler device comprises at least one EGR cooler comprising said exhaust gas passage and a cooling-medium line adapted to leading the cooling agent through the EGR cooler. The exhaust gases in the EGR cooler device undergo effective cooling by the cooling agent which is led through the cooling-medium line. The EGR cooler device may comprise a first EGR cooler which has an exhaust gas passage which comprises the first aperture, and a second EGR cooler which has an exhaust gas passage which comprises the second aperture and a line portion which connects the two EGR coolers. The EGR coolers are thus arranged in series. The recirculating exhaust gases thus undergo two steps of cooling as they pass successively through the two EGR coolers. When the exhaust gases are led through the two EGR coolers, they are usually cooled to the condensation temperature of the water vapour at the pressure prevailing in the EGR cooler situated downstream. By changing the direction of the exhaust gases led through the two EGR coolers, condensate can be caused to form on the internal surfaces alternately in the two EGR coolers and clean the surfaces of soot deposits.

According to another preferred embodiment of the present invention, said circulating cooling agent is intended to be circulated through the EGR cooler device by a reversible pump which, during operation of the combustion engine, is adapted to circulating the coolant alternately in opposite directions through the EGR cooler device. The EGR cooler device takes with advantage the form of a counterflow heat exchanger in which the exhaust gases can be cooled to an optimum low temperature in the EGR cooler device as a result of the exhaust gases led out from the EGR cooler device receiving their final cooling from the optimally cold cooling agent which has just been led into the EGR cooler device. The arrangement preferably comprises a separate cooling system comprising not only said cooling-medium line and said reversible pump but also a radiator element in which the circulating cooling agent is intended to be cooled. The location of the radiator element will with advantage be such that it has air at the temperature of the surroundings flowing through it. The radiator element may be fitted at a front portion of a vehicle. In such a radiator element the cooling agent of the separate cooling system can be cooled to a temperature close to the temperature of the surroundings. Where two EGR coolers are used, the arrangement may comprise a valve arrangement adapted to leading coolant from the cooling system of the combustion engine through the EGR cooler in which the recirculating exhaust gases undergo a first step of cooling and to leading cooling agent from the separate cooling system through the EGR cooler in which the recirculating exhaust gases undergo a second step of cooling. In this case, the separate cooling system is thus used for cooling one of the EGR coolers at a time, so the whole capacity of the separate cooling system can be used for subjecting the recirculating exhaust gases to a second step of cooling, thereby making it easier to cool the exhaust gases to a low temperature and facilitating the formation of water vapour in the respective EGR coolers.

The object indicated above is also achieved with the method of the kind mentioned in the introduction which is characterised by the features indicated in the characterising part of claim 11.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the present invention are described below by way of examples with reference to the attached drawings, in which: Fig. 1 depicts an arrangement for recirculation of exhaust gases of a supercharged combustion engine according to a first embodiment, Fig. 2 depicts an arrangement for recirculation of exhaust gases of a supercharged combustion engine according to a second embodiment and Fig. 3 depicts an arrangement for recirculation of exhaust gases of a supercharged combustion engine according to a third embodiment.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 depicts a vehicle 1 powered by a supercharged combustion engine 2. The vehicle 1 may be a heavy vehicle powered by a supercharged diesel engine. The exhaust gases from the cylinders of the combustion engine 2 are led via an exhaust manifold 3 to an exhaust line 4. The exhaust gases in the exhaust line 4, which will be at above atmospheric pressure, are led to a turbine 5 of a turbo unit. The turbine 5 is thus provided with driving power which is transmitted, via a connection, to a compressor 6. The compressor 6 compresses air which is led via an air filter 7 into an air line 8. A charge air cooler 9 is arranged in the air line 8. The charge air cooler 9 is arranged at a front portion of the vehicle 1. The purpose of the charge air cooler 9 is to cool the compressed air before it is led to the combustion engine 2. The compressed air is cooled in the charge air cooler 9 by surrounding air caused to flow through the charge air cooler 9 by a radiator fan 10. The radiator fan 10 is driven by the combustion engine 2 via a suitable connection.

The combustion engine 2 is provided with an EGR (Exhaust Gas Recirculation) system for recirculation of the exhaust gases. Adding exhaust gases to the compressed air led to the engine's cylinders lowers the combustion temperature and hence also the content of nitrogen oxides NO_x formed during the combustion processes. A return line 11 for recirculation of exhaust gases extends from the exhaust line 4 to the air line 8. The return line 11 comprises an EGR valve 12 by which the exhaust flow in the return line 11 can be shut off. The EGR valve 12 may also be used for steplessly controlling the amount of exhaust gases led from the exhaust line 4 to the air line 8 via the return line 11. The return line 11 comprises a first EGR cooler 14 and a second EGR cooler 15 for cooling the recirculating exhaust gases. In supercharged diesel engines 2, in certain operating situations, the pressure of the exhaust gases in the exhaust line 4 will be lower than the pressure of the compressed air in the inlet line 8. In such operating situations it is not possible to mix the exhaust gases in the return line 11 directly with the compressed air in the inlet line 8 without special auxiliary means. To this end it is possible to use, for example, a venturi 16 or a turbo unit with variable geometry. If instead the combustion engine 2 is a supercharged Otto engine, the exhaust gases in the return line 11 can be led directly into the inlet line 8, since the exhaust gases in the exhaust line 4 of an Otto engine in substantially all operating situations will be at a higher pressure than the compressed air in the inlet line 8. When the exhaust gases have mixed with the compressed air in the inlet line 8, the mixture is led via a manifold 17 to the respective cylinders of the combustion engine 2.

The combustion engine 2 is cooled in a conventional manner by a cooling system which contains a coolant. A coolant pump 18 circulates the coolant in the cooling system. When the coolant has cooled the combustion engine 2, it is led in a line 26 to a thermostat 19 of the cooling system. When the coolant has reached a normal operating temperature, the thermostat 19 is adapted to leading the coolant to a radiator 20 in order to be cooled. The radiator 20 is fitted at a forward portion of the vehicle 1. The radiator 20 is here fitted downstream of the charge air cooler 9 and a radiator element 21 with respect to the intended direction of air flow at the forward portion of the vehicle 1. The radiator element 21 is part of a separate cooling system whose purpose is to cool the exhaust gases in the EGR coolers 14, 15. With such positioning of the radiator element 21 and the charge air cooler 9, the compressed air and the recirculating exhaust gases can be cooled to a temperature close to the temperature of the surroundings. The air and the exhaust gases are cooled so that they occupy a smaller specific volume, thereby making it possible to supply a larger amount of air and recirculating exhaust gases to the cylinders of the combustion engine.

The separate cooling system is dimensioned to be able to cool the exhaust gases from a temperature of about 500-600⁰C to a temperature below the condensation temperature of the water vapour in the exhaust gases at the prevailing exhaust pressure. Each of the EGR coolers 14, 15 has running through it an exhaust gas passage 14a, 15a. The separate cooling system comprises a line circuit 22 adapted to leading the coolant through the two EGR coolers 14, 15. The coolant is intended to flow through the respective EGR coolers 14, 15 in a substantially opposite direction relative to the direction of flow of the exhaust gases through the exhaust gas passage 14a, 15a. The EGR coolers 14, 15 are thus adapted to serving as a counterflow heat exchanger. The separate cooling system comprises a reversible coolant pump 23 which is settable in a first position and a second position. When the coolant pump 23 is placed in the first position, it circulates the coolant from the radiator element 21 in a direction in the line circuit 22 such that it reaches first the second EGR cooler 15 and thereafter the first EGR cooler 14. When the coolant pump 23 is placed in the second position, it circulates the coolant from the radiator element 21 in a direction in the line circuit 22 such that it reaches first the first EGR cooler 14 and thereafter the second EGR cooler 15. The cold coolant from the radiator element 21 can thus be led to the respective EGR coolers 14, 15 in different successions depending on the direction of flow of the coolant through the line circuit 22. The return line 11 comprises a valve means 24 which has a rotatable valve body 24a settable in at least a first position and a second position. The return line 11 comprises four line portions connected to the valve means 24. A first such line portion 1 Ia is adapted to leading warm exhaust gases to the valve means 24. A second such line portion 1 Ib is adapted to leading exhaust gases between the valve means 24 and an aperture 14b of the exhaust gas passage 14a which extends through the first EGR cooler 14. A third such line portion 1 Ic is adapted to leading exhaust gases between the valve means 24 and an aperture 15b of the exhaust gas passage 15a which extends through the second EGR cooler 15. A fourth such line portion 1 Id is adapted to leading the cooled exhaust gases from the valve means 24 towards the combustion engine 2. A fifth line portion l ie extends between the first EGR cooler 14 and the second EGR cooler 15.

When the rotary body of the valve means 24 is placed in a first position represented by a continuous line in Fig. 1, the line portions 11a, 1 Ib are connected to one another while at the same time the line portions l ie, 1 Id are connected to one another. When the valve means 24 is placed in a second position represented by a broken line in Fig. 1, the line portions 11a, l ie are connected to one another while at the same time the line portions l ib, 1 Id are connected to one another. An electrical control unit 25 in the form of a computer unit provided with suitable software is adapted to controlling the valve means 24 and to placing the valve body 24a in the two positions alternately during operation of the combustion engine 2. The control unit 25 is also adapted to controlling the reversible coolant pump 23 and the EGR valve 12.

During operation of the combustion engine 2, the compressed air in the charge air cooler 9 and the coolant of the separate cooling system in the radiator element 21 are cooled by a cooling air flow which is at the temperature of the surroundings. It is thus possible to cool the compressed air in the charge air cooler 9 and the coolant in the radiator element 21 to a temperature close to the temperature of the surroundings. The coolant leaving the radiator element 21 is therefore substantially always at a low enough temperature to enable it to cool the recirculating exhaust gases to a temperature at which the water vapour in the exhaust gases will condense within one of the EGR coolers 14, 15. The control unit 25 is adapted to placing the valve means 24 in the first position and second position alternately at suitable intervals during operation of the combustion engine 2. When the control unit 25 places the valve means 24 in the first position, it simultaneously places the reversible coolant pump 23 in the first position so that it circulates the cold coolant in the line circuit 22 in a direction such that it reaches first the second EGR cooler 15 and thereafter the first EGR cooler 14. When the valve means 24 is in the first position, it leads the warm exhaust gases, which may be at a temperature of 500-600⁰C, from the first line portion 1 Ia to the second line portion 1 Ib. The exhaust gases are led into the first EGR cooler 14 via the aperture 14b. The exhaust gases undergo a first step of cooling in the first EGR cooler 14 by the coolant of the separate cooling system. When the exhaust gases have been cooled in the first EGR cooler 14, they are led via the line portion 1 Ie to the second EGR cooler 15, in which they undergo a second step of cooling by optimally cold radiator coolant which comes directly from the radiator element 21. The cold coolant is led into the second EGR cooler 15 at a position close to the aperture 15b. Within the second EGR cooler 15, the exhaust gases usually reach a temperature at which the water vapour in the exhaust gases begins to condense on the internal surfaces of the second EGR cooler 15. From this position to the aperture 15b of the exhaust gas passage, condensate forms on the heat transfer surfaces of the second EGR cooler 15. The precipitated condensate dissolves any soot deposits on the internal surfaces of the second EGR cooler 15. The soot deposits come away from the walls and are carried off out of the second EGR cooler 15 by the exhaust flow. When the exhaust gases have undergone a second step of cooling in the second EGR cooler 15, they are led back to the valve means 24 via the line portion l ie. The valve means 24 leads the cooled exhaust gases to the line portion 1 Id for further transfer towards the combustion engine 2. When the valve means 24 and the reversible coolant pump 23 are in their respective first positions, the result is the formation of condensate within the second EGR cooler 15 in a portion of the exhaust gas passage 15a which is situated close to the aperture 15b. The formation of condensate results in good cleaning of the internal surfaces in this portion of the second EGR cooler 15.

When the control unit 25 places the valve means 24 in the second position, it simultaneously places the reversible coolant pump 23 in the second position so that the cold coolant is circulated in a direction in the line circuit 22 such that it is led first to the first EGR cooler 14 and thereafter to the second EGR cooler 15. When the valve means 24 is in the second position, it leads the warm exhaust gases from the first line portion 1 Ia to the second EGR cooler 15 via the third line portion l ie. The exhaust gases are cooled in the second EGR cooler 15 by the coolant of the separate cooling system. When the exhaust gases have undergone a first step of cooling in the second EGR cooler 15, they are led via the line portion 1 Ie to the first EGR cooler 14, in which they undergo a second step of cooling by coolant which comes directly from the radiator element 21. The cold coolant is led into the first EGR cooler at a position close to the aperture 14b of the first EGR cooler. At a position within the first EGR cooler 14 the exhaust gases usually reach a temperature at which the water vapour in the exhaust gases begins to condense on the internal surfaces of the first EGR cooler 14. From this position to the aperture 14b of the exhaust gas passage, condensate will thus form on the heat transfer surfaces. The precipitated condensate dissolves soot deposits on the internal surfaces of the first EGR cooler 14. The soot deposits can thus come away from the walls and be carried off out of the first EGR cooler 14 by the exhaust flow. When the exhaust gases have undergone a second step of cooling in the first EGR cooler 14, they are led via the line portion 1 Ib to the valve means 24. The valve means 24 leads the cooled exhaust gases to the line portion 11 d for further transfer towards the combustion engine 2. When the valve means 24 and the reversible coolant pump 23 are in their respective second positions, the result is the formation of condensate in a portion of the exhaust gas passage which is situated close to the aperture 14b. The formation of condensate results in good cleaning of the internal surfaces in this portion of the first EGR cooler 14.

The temperature of the exhaust gases varies during operation, depending inter alia on the load of the combustion engine. The separate cooling system is with advantage so dimensioned that in operating situations where the exhaust gases are at a low temperature it subjects them to cooling such that they reach their condensation temperature substantially immediately when they are led into the EGR cooler 14, 15 situated downstream. Such dimensioning of the separate cooling system results in the formation of condensate throughout the second EGR cooler 15 when the valve means is in the first position in situations where the load of the combustion engine is low. This means that the whole of the second EGR cooler 15 can be cleaned effectively by condensate. In a corresponding manner, the formation of condensate occurs throughout the first EGR cooler 14 when the valve means is in the second position and at the same time the load of the combustion engine is low. The result is that the whole of the first EGR cooler 14 can likewise be cleaned effectively by condensate.

Fig. 2 depicts an alternative embodiment in which only one EGR cooler 14 is used for cooling the recirculating exhaust gases in the return line 11. The EGR cooler 14 has running through it an exhaust gas passage 14a with two apertures 14b, c. During operation of the combustion engine 2, a control unit 25 places alternately in a first position and a second position a valve means 24 and a reversible coolant pump 23. When the control unit 25 places the valve means 24 in the first position, it also places the reversible coolant pump 23 in the first position. The cold coolant from the radiator element 21 is thus circulated in a direction in the line circuit 22 such that it is led into the EGR cooler 14 at a position close to the aperture 14c and out from the EGR cooler 14 close to the aperture 14b. When the valve means 24 is in its first position, the warm exhaust gases are led from the first line portion 11 a to the second line portion l ib. The exhaust gases are led thereafter into the EGR cooler's passage 14a via the aperture 14b and out via the aperture 14c. At a position within the EGR cooler 14 the exhaust gases are usually cooled to a temperature at which the water vapour in the exhaust gases begins to condense on the internal surfaces of the EGR cooler 14. From this position to the exhaust gases being led out via the aperture 14c of the exhaust gas passage, condensate forms on the internal surfaces of the exhaust gas passage 14a. Soot deposits can thereby be freed from the walls and carried off out of the EGR cooler 14 by the exhaust flow. The cooled exhaust gases are thereafter led through the line portion 1 Ic to the valve means 24. The valve means 24, which is thus in the first position, leads the exhaust gases to the line portion 1 Id for further transfer towards the combustion engine 2. When the valve means 24 and the reversible coolant pump 23 are in their respective first positions, the result is a formation of condensate in a portion of the exhaust gas passage 14a which is situated close to the aperture 14c. The formation of condensate results in good cleaning of the internal surfaces in this portion of the EGR cooler 14.

When the control unit 25 places the valve means 24 in the second position, it also places the reversible coolant pump 23 in the second position. The cold coolant from the radiator element 21 is thus circulated in a direction in the line circuit 22 such that it is led into the EGR cooler 14 at a position close to the aperture 14b and out from the EGR cooler 14 close to the aperture 14c.

When the valve means 24 is in its second position, the warm exhaust gases are led from the first line portion 1 Ia to the third line portion l ie. The exhaust gases are led thereafter into the exhaust gas passage 14a through the EGR cooler via the aperture 14c and out via the aperture 14b. At a position within the EGR cooler 14 the exhaust gases will usually have been cooled to a temperature at which the water vapour in the exhaust gases begins to condense on the internal surfaces of the exhaust gas passage 14a. From this position to the exhaust gases being led out via the aperture 14b, condensate will form on the heat transfer surfaces of the EGR cooler. Soot deposits can therefore be freed from the walls and be carried off out of the EGR cooler 14 by the exhaust flow. The cooled exhaust gases are led thereafter through the second line portion 1 Ib to the valve means 24. The valve means 24, which is thus in the second position, leads the exhaust gases to the fourth line portion 1 Id for further transfer towards the combustion engine 2. When the valve means 24 and the reversible coolant pump 23 are in their respective second positions, the result is therefore the formation of condensate in a portion of the EGR cooler 14 which is situated close to the aperture 14b. The formation of condensate results in good cleaning of the internal surfaces in this portion of the EGR cooler 14.

The separate cooling system is here so dimensioned that, in operating situations where the exhaust gases are at a low temperature, it provides cooling of the exhaust gases such that they reach their condensation temperature when they have passed through about half of the EGR cooler 14. Such dimensioning of the separate cooling system results in the formation of condensate in the half of the EGR cooler 14 which comprises the aperture 14c when the valve means is in the first position in situations where the load of the combustion engine is low. This means that half of the EGR cooler 14 can be cleaned effectively by condensate. In a corresponding manner, there is formation of condensate in the other half of the EGR cooler 14 which comprises the aperture 14b when the valve means is in the second position and at the same time the load of the combustion engine is low. This means that the second half of the EGR cooler 14 can likewise be cleaned effectively by condensate. By switching the valve means 24 and the reversible coolant pump 23 between the first and second positions at suitable intervals, the whole of the EGR cooler 14 can be cleaned with condensate during operation of the combustion engine 2.

Fig. 3 depicts a further alternative embodiment of the present invention. For the sake of clarity, it shows only the portions of the vehicle which are relevant for the function of this embodiment. The undepicted portions have with advantage the same configuration as in Figs. 1 and 2. In situations where the control unit 25 places the valve means 24 in the first position, it simultaneously places the reversible coolant pump 23 in the first position. The cold coolant from the radiator element 21 is therefore led by the coolant pump 23 in the line circuit 22 to a first valve 22a. The first valve 22a is placed in a position such that the coolant is led straight on in Fig. 3. The coolant is thus led on in the line circuit 22 to the second EGR cooler 15. The coolant is led into the second EGR cooler 15 at a position close to the aperture 15b. As the valve means 24 is in the first position, which is represented by a continuous line, the recirculating exhaust gases will have already been cooled in the first EGR cooler 14 before they reach the second EGR cooler 15. The cold coolant from the radiator element 21 subjects the exhaust gases in the second EGR cooler 15 to a second step of cooling to a temperature at which the water vapour in the exhaust gases begins to condense on the internal surfaces of the second EGR cooler 15. From this position to the aperture 15b where the exhaust gases are led out from the second EGR cooler 15, condensate will form on the heat transfer surfaces of the second EGR cooler 15. The coolant leaving the second EGR cooler 15 is led to a second valve 22b which is placed in a position such that the coolant is led back to the radiator element 21 via a section of the line circuit 22 which comprises a third valve 22c. At the same time as the valves 22a-c are placed in the abovementioned positions so that coolant from the separate cooling system can cool the exhaust gases in the second EGR cooler 15, a valve 26b in the line 26 of the combustion engine's cooling system is placed in a position such that at least part of the warm coolant in the line 26 is led via a line 26bi to the first EGR cooler 14. The coolant from the combustion engine's cooling system here subjects the recirculating exhaust gases to a first step of cooling. The coolant led out from the first EGR cooler 14 reaches a fourth valve 22d which is placed in a position such that it leads the coolant back to the line 26 via a line 26b₂. In situations where the control unit 25 places the valve means 24 in the second position, it simultaneously places the reversible coolant pump 23 in the second position. The cold coolant from the radiator element 21 is thus led in an opposite direction in the line circuit 22 by the coolant pump 23. The cold coolant from the radiator element 21 will now be led first to the third valve 22c which is placed in a position such that it leads the coolant in the line circuit 22 in a direction upwards in Fig. 3 and into the first EGR cooler 14. The coolant is led into the first EGR cooler 14 at a position close to the aperture 14b. As the valve means 24 is in the second position, which is represented by a broken line, the recirculating exhaust gases will have already been cooled in the second EGR cooler 15 before they reach the first EGR cooler 14. The cold coolant in the first EGR cooler 14 subjects the exhaust gases to a second step of cooling to a temperature at which the water vapour in the exhaust gases begins to condense on the internal surfaces of the first EGR cooler 14. From this position to the aperture 14b of the exhaust gas passage of the first EGR cooler 14, condensate will form on the heat transfer surfaces of the first EGR cooler 14. The coolant leaves the first EGR cooler 14 and is led back to the radiator element 21 in the line circuit 22 via the fourth valve 22d and the first valve 22a. At the same time as the valves 22a, c, d are placed in the abovementioned positions so that the cold coolant from the separate cooling system 21 can cool the exhaust gases in the first EGR cooler 14, a valve 26a in the line 26 of the combustion engine's cooling system is placed in a position such that at least part of the coolant in the line 26 is led via a line 26ai to the second EGR cooler 15. The coolant from the combustion engine's cooling system here subjects the recirculating exhaust gases to a first step of cooling in the second EGR cooler 15. The coolant is led out from the second EGR cooler 15 and back to the line 26 via the second valve 22b and a line 26a₂.

In this embodiment, a valve arrangement 22a-d is thus used to lead the cold coolant from the separate cooling system through whichever of the EGR coolers 14, 15 is at the time subjecting the recirculating exhaust gases to the second step of cooling. A valve arrangement 26a-b and a number of extra lines 26ai, 26a₂, 26bi, 26b₂ are used for leading the relatively warm coolant from the combustion engine's cooling system through whichever of the EGR coolers 14, 15 is at the time subjecting the recirculating exhaust gases to the first step of cooling. In this case it is therefore possible for the whole of the separate cooling system to be used for subjecting the recirculating exhaust gases to the second stage of cooling, thereby making it easier to cool the exhaust gases to a low temperature and facilitating the formation of water vapour within the respective EGR coolers 14, 15.

The present invention is in no way limited to the embodiment illustrated in the drawings but may be varied freely within the scopes of the claims.

Claims

1. An arrangement for recirculation of exhaust gases of a combustion engine (2), which arrangement comprises a return line (11) adapted to returning exhaust gases to the combustion engine (2), an EGR cooler device (14, 15) for receiving and cooling of recirculating exhaust gases in the return line (11), which has an exhaust gas passage (14a, 15a) comprising a first aperture (14b) and a second aperture (14c, 15b), and a cooling system with a circulating cooling agent adapted to making it possible to cool the recirculating exhaust gases in the EGR cooler device (14, 15) to the condensation temperature of the water vapour in the exhaust gases, characterised in that the return line (11) comprises a valve means (24) settable in a first position whereby it leads the recirculating exhaust gases in a first direction through the exhaust gas passage (14a, 15 a) of the EGR cooler device, and in a second position whereby it leads the recirculating exhaust gases in an opposite direction through the exhaust gas passage (14a, 15 a) of the EGR cooler device, and that the valve means (24) is adapted to being placed in the first position and second position alternately during operation of the combustion engine (2) so that condensate forms alternately within the EGR cooler device in a portion of the exhaust gas passage (14a, 15a) which comprises the first aperture (14b) and in a portion of the exhaust gas passage (14a, 15a) which comprises the second aperture (14c, 15b).

2. An arrangement according to claim 1 , characterised in that the return line (11) comprises a first line portion (1 Ia) adapted to leading recirculating exhaust gases to the valve means (24), a second line portion (1 Ib) extending between the valve means (24) and the first aperture (14b) of the EGR cooler device, a third line portion (1 Ic) extending between the valve means (24) and the second aperture (14c, 15b) of the EGR cooler device, and a fourth line portion (l id) adapted to leading exhaust gases which have been cooled in the EGR cooler device (14, 15) towards the combustion engine (2).

3. An arrangement according to claim 2, characterised in that the valve means (24), in the first position, is adapted to connecting the first line portion (1 Ia) to the second line portion (1 Ib) and the third line portion (1 Ic) to the fourth line portion (l id).

4. An arrangement according to claim 3, characterised in that the valve means (24), in the second position, is adapted to connecting the first line portion (1 Ia) to the third line portion (1 Ic) and the second line portion (1 Ib) to the fourth line portion (l id).

5. An arrangement according to claim 4, characterised in that the valve means (24) comprises an internal space to which said line portions (1 la-d) lead and a valve body (24a) arranged for rotation between the first position and the second position.

6. An arrangement according to any one of the foregoing claims, characterised in that the EGR cooler device comprises at least one EGR cooler (14, 15) which comprises said passage (14a, 15a) for the exhaust gases and a cooling-medium line (22) adapted to leading said cooling agent through the EGR cooler (14, 15).

7. An arrangement according to claim 6, characterised in that the EGR cooler device comprises a first EGR cooler (14) which has an exhaust gas passage (14a) which comprises the first aperture (14b) and a second EGR cooler (15) which has an exhaust gas passage (15a) which comprises the second aperture (15b) and a line portion (1 Ie) which connects the two EGR coolers (14, 15).

8. An arrangement according to claim 6 or 7, characterised in that said cooling system comprises a reversible pump (23) which, during operation of the combustion engine (2), is adapted to circulating said cooling agent alternately in two opposite directions through the EGR cooler device (14, 15).

9. An arrangement according to claim 8, characterised in that the combustion engine is cooled by a cooling system with a circulating coolant and that said cooling system, which is adapted to cooling the valve means (24) to the condensation temperature of the water vapour in the exhaust gases, is a cooling system separate from the combustion engine's cooling system.

10. An arrangement according to claims 7 and 9, characterised in that the arrangement comprises a valve arrangement (26a-b) adapted to leading coolant from the combustion engine's cooling system through the EGR cooler (14, 15) in which the recirculating exhaust gases undergo a first step of cooling, and a valve arrangement (22a-d) adapted to leading cooling agent from the separate cooling system through the EGR cooler (14, 15) in which the recirculating exhaust gases undergo a second step of cooling.

11. A method for recirculation of exhaust gases of a combustion engine (2), which method comprises the steps of returning exhaust gases to the combustion engine (2), of cooling the recirculating exhaust gases by means of an EGR cooler device (14, 15) which has an exhaust gas passage (14a, 15a) which comprises a first aperture (14b) and a second aperture (14c, 15b), and of cooling the recirculating exhaust gases in the EGR cooler device (14, 15) to the condensation temperature of the water vapour in the exhaust gases, characterised by the step of leading recirculating exhaust gases alternately in opposite directions through the exhaust gas passage (14a, 15a) of the EGR cooler device so that condensate forms alternately within the EGR cooler device in a portion of the exhaust gas passage (14a, 15a) which comprises the first aperture (14b) and in a portion of the exhaust gas passage (14a, 15a) which comprises the second aperture (14c, 15b).