SE0950429A1 - Arrangement and method for recirculating exhaust gases of an internal combustion engine - Google Patents

Abstract

The present invention relates to an arrangement and a method for recirculating exhaust gases of an internal combustion engine (2). Arrangement comprises a return line (10, 11) through which exhaust gases are recirculated from an exhaust line (4) to the internal combustion engine (2) and at least one EGR cooler (15) arranged in the return line (10, 11) in which the recirculating exhaust gases are adapted. to be cooled before being led to the internal combustion engine (2). The arrangement is an ammonia supply device (32-35) adapted to supply ammonia to the return line (10, 11), parameter estimating means (13a, 26, 27, 28, 36) for estimating the value of a parameter related to the presence of corrosive acids in the return line (10, 11) and a control unit (13) adapted to control the ammonia supply device (32-35) so that it supplies ammonia to the return line (10, 11) in an amount by means of adjacent parameter value so that corrosive acids in the return line (10, 11) are neutralized. (Fig1)

Description

a high sulfur content at the same time as the recirculating exhaust gases are cooled to such a low temperature that water in the liquid form precipitates inside the EGR cooler, sulfuric acid is formed. The EGR cooler and downstream lines for exhaust gas recirculation can thus receive corrosion damage.

SUMMARY OF THE INVENTION The object of the present invention is to prevent corrosion damage in EGR coolers and in downstream components for exhaust gas recirculation of an internal combustion engine.

This object is achieved with the arrangement of the kind mentioned in the introduction, which is characterized by the features stated in the characterizing part of claim 1. Thus, it is desirable to cool the recirculating exhaust gases to a low temperature in an EGR cooler before mixing with air and directing to the internal combustion engine. However, such cooling of the exhaust gases often results in the water vapor in the exhaust gases condensing in the EGR cooler. Liquid water together with the sulfur oxides in the exhaust gases form sulfuric acid. Liquid water together with the nitrogen oxides in the exhaust gases form nitric acid. Sulfuric acid and nitric acid are highly corrosive acids. According to the invention, ammonia is thus supplied to the return line. Arnmonia is an alkaline substance that has the ability to neutralize acids. The addition of ammonia to the exhaust gases results in the presence of acids which are rapidly neutralized. As a result, the return line and components will not be substantially exposed to acid corrosion. The return line and components are thus not required to be manufactured in expensive materials with extremely good corrosion properties. The control unit is advantageously an electrical control unit in the form of a computer unit which includes software for controlling the supply of ammonia to the return line during operation of the internal combustion engine.

According to an embodiment of the invention, the ammonia supply device is adapted to supply ammonia to the EGR cooler or to the return line in a position upstream of the EGR cooler with respect to the intended flow direction of the exhaust gases in the return line. In order for the added ammonia to be used to neutralize any acids present, it should preferably be added upstream of the places in the return line where condensate is normally formed so that the ammonia has time to mix with the exhaust gases before they reach the condensate. However, it is also possible to add the ammonia directly in areas where condensate normally forms. The recirculating exhaust gases initially have a very high temperature. Therefore, EG your EGR coolers are often used to cool the recirculating exhaust gases. In this case, the recirculating exhaust gases first in the most downstream EGR cooler of the return line obtain a temperature at which the water vapor in the exhaust gases can condense. In such an EGR cooler, the recirculating exhaust gases may be cooled by air at ambient temperature.

Alternatively, the exhaust gases in the EGR cooler may be cooled by coolant from a low temperature cooling system. Of course, the recirculating exhaust gases can also be cooled in only one EGR cooler and to a temperature at which the water vapor in the exhaust gases condenses.

According to another embodiment of the invention, the ammonia supply device is adapted to supply ammonia in the form of a liquid solution. Such an ammonia solution can be sprayed into the return line in a suitable place. The hot exhaust gases flowing in the return line quickly heat the ammonia solution to a temperature at which it evaporates to form ammonia gas. Ammonia solution can also be added in areas of the return line where condensate normally forms. In this case, the ammonia solution is mixed with water in liquid form. In order to provide a good mixture of said liquids, the return line in this area can be provided with flow means which give the exhaust gases a turbulent flow. Alternatively, the ammonia supply device may be adapted to supply ammonia in gaseous form. In this case, pure gaseous ammonia can be added which is rapidly mixed with the exhaust gases in the return line.

According to another embodiment of the invention, said parameter estimating means comprises a sensor which senses the value of a parameter which is related to the presence of corrosive acids in the return line. With the aid of information from said sensor, the control unit can supply ammonia in an amount so that substantially all the corrosive acids present in the return line are neutralized. Said sensor may be positioned so that it is in contact with the exhaust gases in the return line. Such a sensor can detect the presence of a substance in the exhaust gases which is related to the presence of corrosive acids in the return line. Said sensor may be adapted to sense the amount of sulfur oxides and / or the amount of nitrogen oxides in the exhaust gases. The amount of sulfur oxides and the amount of nitrogen oxides in the exhaust gases are related to how much sulfuric acid and nitric acid must be neutralized with the help of ammonia to prevent acid corrosion in the return line. Such a sensor is advantageously arranged in the return line in a position upstream of the EGR cooler. Alternatively, said sensor is adapted to sense the amount of ammonia in the exhaust gases. Such a sensor is advantageously arranged downstream of the EGR cooler. This sensor thus detects the amount of ammonia that has not reacted with acid in the EGR cooler. To ensure that substantially no corrosive acids are present in the return line, ammonia should be added in an amount so that a certain excess of ammonia can be detected by said sensor.

According to another embodiment of the invention, said parameter estimating means is adapted to estimate a parameter value in the form of the sulfur content of the fuel which is led to the internal combustion engine. The more sulfur the fuel contains, the more sulfuric acid can be formed in the return line. When fuels with a high sulfur content are used, more ammonia must be added than when fuels with lower sulfur contents are used. Said parameter estimating means may include stored information regarding the sulfur content of the fuel. Such information can be provided with knowledge of the country in which the vehicle is to be used. Permitted sulfur content in the fuel varies between different countries. Alternatively, a driver himself can supply such stored information with knowledge of the fuel refueled in the vehicle. Said parameter estimating means may comprise a sensor which is positioned in contact with the fuel which is led to the internal combustion engine. In this case, for example, the sulfur content of the fuel can be sensed. With the help of this information, the control unit can supply an appropriate amount of ammonia to the return line.

According to another embodiment of the invention, said sensor is positioned in an area where condensate is normally formed in the return line. Condensate normally forms in the EGR cooler. Such a sensor can detect the presence of various substances in the condensate.

Said sensor may be adapted to sense the pH value in the condensate. The pH value is a measure of the hydrogen ion activity in the condensate. The addition of ammonia raises the pH value and reduces the hydrogen ion activity. A high pH indicates that the presence of acidic substances such as corrosive acids is low.

According to another embodiment of the invention, the arrangement comprises detection means for detecting whether condensate is present in the return line. A prerequisite for corrosive acids such as sulfuric acid and nitric acid to be able to form in, for example, the EGR cooler in the return line is that the exhaust gases are cooled to a temperature at which water vapor condenses. Said parameter estimating means may comprise an ent temperature sensor which senses the temperature of the exhaust gases in the return line in a position downstream of the EGR cooler. If the exhaust gases have a temperature that is equal to or lower than the condensing temperature of water vapor, the control unit can establish that water in the liquid form precipitates in the EGR cooler. In this case, the control unit supplies ammonia to the return line. Otherwise, the control unit does not need to supply ammonia to the return line regardless of the levels of sulfur oxides and nitrogen oxides in the exhaust gases.

The above-mentioned object is also achieved with the initially mentioned method which is characterized by what is stated in the characterizing part of claim 15.

BRIEF DESCRIPTION OF THE DRAWINGS In the following, exemplary embodiments of the invention are described, by way of example, with reference to the accompanying drawings, in which: Fig. 1 shows an arrangement according to a first embodiment of the invention, Fig. 2 shows an arrangement according to a second embodiment of invention and Fig. 3 shows a fate diagram describing an embodiment of a method according to the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION Fig. 1 shows a vehicle 1 driven by a supercharged internal combustion engine 2. The vehicle 1 may be a heavy vehicle driven by a supercharged diesel engine. The exhaust gases from the cylinders of the internal combustion engine 2 are led, via an exhaust gas collector 3, to an exhaust line 4.

The exhaust gases in the exhaust line 4, which have an overpressure, are led to a turbine 5 of a turbocharger. The turbine 5 then provides a driving force, which is transmitted, via a connection, to a compressor 6. The compressor 6 compresses air which is led into an inlet line 7. A charge air cooler 8 is arranged in the inlet line 7.

The charge cooler 8 is arranged in an area A at a front part of the vehicle 1.

The purpose of the charge cooler 8 is to cool the compressed air before it is led to the internal combustion engine 2. The compressed air is cooled in the charge cooler 8 with ambient air flowing through the charge air cooler 8 by means of a cooler fl älct 9.

The radiator fan 9 is driven by the internal combustion engine 2 by means of a suitable connection.

The combustion engine 2 is equipped with an EGR (Exhaust Gas Recirculation) system for recirculation of the exhaust gases. By mixing exhaust gases with the compressed air that is led to the engine cylinders, the combustion temperature and thus also the content of nitrogen oxides (NOX) that are formed during the combustion processes is lowered. An exhaust line 10 for recirculating exhaust gases extends from the exhaust line 4 to the inlet line 7. The return line 10 comprises an EGR valve 12, with which the exhaust gas flow in the return line 10 can be switched off if necessary. The EGR valve 12 can also be used to steplessly control the amount of exhaust gases led from the exhaust line 4, via the return line 10, to the inlet line 7. A control unit 13 is adapted to control the EGR valve 12 with information on the operating condition of the internal combustion engine 2. The return line includes a first EGR cooler 14 for cooling the exhaust gases in a first stage and a second EGR cooler 15 for cooling the exhaust gases in a second stage.

In supercharged diesel engines 2, during certain operating conditions, the exhaust gas pressure in the exhaust line 4 is lower than the compressed air pressure in the inlet line 7. Under such operating conditions it is not possible to directly mix the exhaust gases in the return line 10 with the compressed air in the inlet line 7 without special aids. In this case, for example, a venturi 16 or a turbine with a variable geometry can be used. If the combustion engine 2 is instead an overcharged otto engine, the exhaust gases in the return line 10 can be led directly into the inlet line 7 as the exhaust gases in the exhaust line 4 of an otto engine have a substantially higher pressure than the compressed air in the inlet line 7 after mixing the exhaust gases the air in the inlet line 7, the mixture is led, via a line 11 and a branch 17, to the respective cylinders of the internal combustion engine 2.

The internal combustion engine 2 is cooled in a conventional manner with a cooling system comprising a circulating coolant. The coolant is circulated in the cooling system by means of a coolant purge 18. The cooling system also comprises a thermostat 19 which is adapted to direct the coolant to a cooler 20 when the coolant has reached a temperature at which it needs to be cooled. The cooler 20 is mounted at a front portion of the vehicle 1 in a position downstream of the charge cooler 8 and the second EGR cooler 15 with respect to the intended flow direction of the air in the area A. The coolant in the cooling system is used to cool the recirculating exhaust gases in the first stage of the first EGR cooler 14.

The cooling system comprises a line 21 which initially leads coolant to the first EGR cooler 14 for cooling the recirculating exhaust gases in the first stage. The first EGR cooler 14 can be mounted on or in connection with the internal combustion engine 2. The recirculating exhaust gases can be cooled here fi from a temperature of approximately S00 - 600 ° C to a temperature close to the coolant temperature which is usually in the range 70 - 90 ° C. After the coolant has passed through the first EGR cooler 14, it is led, via a line 22, to a line 23 where it is mixed with hot coolant from the internal combustion engine 2. The coolant is led, via the line 23, to the cooler 20 where it is cooled before used again for cooling the internal combustion engine 2 or the recirculating exhaust gases in the first EGR cooler 14. The compressed air in the charge cooler 8 and the recirculating exhaust gases in the second EGR cooler 15, which are mounted on a front surface of the vehicle 1, are flowed through by lult with ambient temperature. It is thus possible to cool the compressed air and the recirculating exhaust gases to a temperature close to the ambient temperature. The air and the recirculating exhaust gases have a lower volume when they are cooled, ie. they occupy a smaller volume per unit weight. By cooling the compressed air and the exhaust gases to a temperature which substantially corresponds to the ambient temperature, a substantially optimal amount of lu fi and recirculating exhaust gases can be led to the cylinders of the internal combustion engine 2.

Exhaust gases contain relatively abundant water vapor. When the recirculating exhaust gases are cooled in an EGR cooler 15 which is cooled by air at ambient temperature, they in many cases obtain a temperature which is lower than. water vapor condensation temperature.

Thus, liquid water falls out inside the second EGR cooler 15 and in the downstream lines 10, 11 which lead the recirculating exhaust gases to the internal combustion engine 2. The control unit 13 is, in this case, adapted to estimate whether water vapor condenses in the second EGR. the cooler 15. The control unit 13 then receives information from a temperature sensor 25 which is adapted to sense the temperature of the recirculating exhaust gases in the return line 10 in a position downstream of the second EGR cooler 15. If the recirculating exhaust gases have been cooled to a temperature equal to or lower than the condensing temperature of the water vapor, the control unit 13 can ascertain that precipitation of liquid water takes place in the second EGR cooler 1 5.

Fuels such as diesel oil and petrol are available in different environmental classes with varying sulfur contents. In some countries and parts of the world, relatively large amounts of sulfur are accepted in the fuel. When a fuel is burned in the internal combustion engine, sulfur oxides are formed in the exhaust gases in relation to the amount of sulfur in the fuel. Sulfur oxides form in contact with water in liquid form sulfuric acid which is a highly corrosive substance. Exhaust gases also contain nitrogen oxides which, in contact with liquid water, form nitric acid.

Nitric acid is also a very corrosive substance. Thus, when the exhaust gases are cooled in the second EGR cooler, both sulfuric acid and nitric acid can be formed in the second EGR cooler and downstream parts of the return line 10. As a result, the material in the second EGR cooler 15 and the downstream parts of the return line can 10 and the line 11 receive serious corrosion damage.

To prevent corrosion damage, the vehicle 1 is equipped with a tank 32 containing ammonia. Ammonia is an alkaline substance that can neutralize acids such as sulfuric acid and nitric acid. The ammonia in this case is in gaseous form and stored with an overpressure in the tank 32. A line 33 extends between the tank 32 and the return line. Control unit 13 is adapted to control a valve 34 in the line 33. The line 33 comprises an injection means 35 which is adapted to inject ammonia into the return line 10. In this case the ammonia is injected into the return line 10 in a position substantially immediately downstream of the EGR valve 12. The ammonia is thus injected into a position upstream of the two EGR coolers 14, 15. The ammonia gas is mixed relatively quickly with the exhaust gases in the return line 10. The mixture of the exhaust gases and ammonia is led to the first EGR cooler 14 where they are cooled in a first step. The exhaust gases and the ammonia are then led to the second EGR cooler where they are cooled in a second stage. In cases where the exhaust gases are cooled to a temperature below the condensation temperature of the water, water is formed in the liquid form in the second EGR cooler 15. When the sulfur oxides and nitrogen oxides in the exhaust gases come into contact with the condensed water, sulfuric acid and nitric acid are formed. the ammonia in the exhaust gases. This prevents the second EGR cooler 15 and the downstream parts of the return line 10 and the line 11 from being exposed to corrosion.

In order for the ammonia to be able to neutralize substantially all of the sulfuric acid and nitric acid formed in the return line 10, it is required that ammonia must be added in a certain amount to the exhaust gases in the return line 10. The amount of ammonia that needs to be added to neutralize sulfuric acid and nitric acid is related to the amount of acid formed in the return line 10. During operation of the vehicle, the control unit 13 therefore receives information fi from at least one sensor which detects the value of a parameter related to the amount of sulfuric acid and / or nitric acid present in the return line 10. The control unit 13 may in this case receive information from a sensor 26 which senses the sulfur content of the fuel combusted in the internal combustion engine 2.

The sulfur content is a parameter that is related to the amount of sulfur oxides in the exhaust gases and the amount of sulfuric acid that can be formed in the return line 10. Fig. 1 shows such a sensor 26 which is arranged in a fuel tank 29 in the vehicle 1. Alternatively, such a sulfur sensing sensor 26 may have a substantially arbitrary location in contact with the fuel driving the internal combustion engine 2. The sensor 26 may alternatively be located in a fuel line 30 which conducts fuel from the fuel tank 29 to the dry combustion engine 2. The sensor 26 may in this case be arranged in connection with a fuel pump 31 which transports fuel from the fuel tank 29 to the internal combustion engine 2.

Alternatively, the Control Unit may receive information from a sensor 27 arranged in contact with the exhaust gases in the return line 10 in a position upstream of the second EGR cooler 15. The sensor 27 may here detect a parameter which is directly related to the amount of sulfur oxides and / or the amount of nitrogen oxides in the exhaust gases. Such a sensor 2? may alternatively be arranged in the exhaust line 4. According to a further alternative, the Control Unit 13 may obtain information from a sensor 28 which is arranged in the return line in a position downstream of the second EGR cooler 15. Sensor 28 may be adapted to sense the amount of ammonia in the exhaust gases. As long as there is ammonia left in the exhaust gases downstream of the second EGR cooler 15, which has not reacted with acid, the control unit 15 can state that ammonia has been added in a sufficient amount to neutralize any acids present in the second EGR cooler 15. If the sensor 28 indicates an excess amount of ammonia above a certain value, the control unit 13 can state that an unnecessary amount of ammonia has been supplied to the exhaust gases in the return line 10.

The control unit 13 can in this case control the valve 34 so that a small amount of ammonia is injected into the return line 10. The addition of ammonia in the return line 10 is thus prevented from corroding the material in the second EGR cooler 15 and downstream parts of the return line 10 and line 11. These components thus do not need to be manufactured in expensive materials with extremely good corrosion properties.

Fig. 2 shows an alternative embodiment of the invention. In this case, the vehicle is equipped with a tank 32 containing ammonia solution in liquid form which, for example, may consist of 25% ammonia and 75% water. A line 33 extends between the tank 32 and the return line 10. The control unit 13 is adapted to control a valve 34 in the line 33.

The line 33 comprises an injection means 35 which is adapted to inject ammonia into the second EGR cooler 15 in an area where condensate is normally formed. In order for the ammonia solution to mix with the condensate, it is convenient to provide this area of the second EGR cooler 15 with air rectifiers which create turbulent flow of the exhaust gases 1. The turbulent exhaust gas flow creates movements in the condensate so that the ammonia solution obtains a good mixture in the condensate. Thus, when sulfur oxides and nitrogen oxides in the exhaust gases come into contact with the condensate in the second EGR cooler, sulfuric acid and nitric acid are formed. However, the ammonia in the condensate is rapidly neutralized by these acids. In order to estimate the amount of ammonia that needs to be added, the control unit 13 in this case receives information from a pH sensor 36 which senses the pH value in the condensate. The lower the PH value that the PH sensor 36 senses, the more acidic the condensate and the greater the amount of ammonia solution that needs to be added. Alternatively, the control unit includes pre-programmed information or information entered by the driver regarding the sulfur content of the fuel. This information can be stored in a part 13a of the control unit 13. Using this information and a number of engine parameters, the control unit 13 estimates the amount of ammonia needed to neutralize any acids. in the return line 10.

Fig. 3 shows a flow chart describing the operation of the arrangements shown in Fig. 2. The flow process starts at step 38. The control unit 13 here receives information from the temperature sensor 25 regarding the exhaust gas temperature as they leave the second EGR cooler 15. The control unit 13 evaluates, at step 39, if the recirculating exhaust gases are cooled to a temperature in the second EGR cooler 15 equal to or lower than the condensing temperature of the water vapor If the exhaust gases are cooled to a temperature higher than the condensing temperature, no condensation of water vapor occurs in the second EGR cooler 15 Since the formation of sulfuric acid and nitric acid requires access to liquid water, in this case no such acids are formed in the second EGR cooler 15. Then the process starts again at step 38. If the control unit 13 receives information from the temperature sensor 25 indicating that the exhaust gases are cooled to a temperature equal to or lower than the water vapor condensation temperature occurs cond ensation of water vapor in the second EGR cooler 15. The control unit 13 receives, at step 40 ,. information from the sensor 35 regarding the pH value of the condensate. At step 41, the control unit 13 calculates the amount of ammonia solution that needs to be added in order for the p acids in the condensate to be neutralized. It is convenient to add ammonia with a certain excess so that any acids in the condensate are guaranteed to be eliminated. It is no problem to lead an excess of ammonia to the internal combustion engine 2. The ammonia is converted to water and nitrogen gas during combustion in the internal combustion engine 2 and thus does not result in any particulate emissions in the exhaust gases. Thereafter, the process restarts at step 38. The mixture of ammonia in the exhaust gases also results in exhaust gases reaching the internal combustion engine oil sump being buffered and neutralized. The engine oil therefore does not need to contain the same amount of buffering bases, in the form of, for example, 11 lime, which today account for a large part of the engine's particulate emissions. Ammonia solutions have a low freezing point so it is no problem to store and supply ammonia solutions in cold climates.

The invention is in no way limited to the described embodiments but can be varied freely within the scope of the claims. It is possible, for example, to control the supply of ammonia only with information from one or more of said sensors 26, 27, 36.

It is also possible to supply liquid ammonia solution in the return line 10 in a position upstream of the second EGR cooler 15. The ammonia solution which is advantageously sprayed into the return line 10 is rapidly heated by the hot exhaust gases and evaporated before it reaches the second EGR cooler 15.

Claims (15)

10 15 20 25 30 35 12 Patent claims An arrangement for recirculating exhaust gases of an internal combustion engine (2), the arrangement comprising an exhaust line (4) adapted to discharge exhaust gases from the internal combustion engine (2), a return line (10, 11) through which exhaust gases are recirculated from the exhaust line (4). ) to the internal combustion engine (2) and at least one EGR cooler (15) arranged in the return line (10, 11) in which the recirculating exhaust gases are adapted to be cooled before being led to the internal combustion engine (2), characterized in that the arrangement comprises an ammonia supply device (32-3 5) adapted to supply ammonia to the return line (10, 11), pair estimating means (13a, 26, 27, 28, 36) adapted to estimate the value of a parameter related to the presence of corrosive the acid return line (10, 11) and a control unit (13) adapted to receive information from said parameter estimating means regarding the estimated parameter value and to control it to ammonia. leading the device (32-3 5) so that it supplies ammonia to the return line (10, 11) in an amount by means of said estimated parameter value so that any corrosive acids in the return line (10, 11) are neutralized. Arrangement according to claim 1, characterized in that the ammonia supply device is adapted to supply ammonia to the EGR cooler (15) or to the return line (10) in a position upstream of the EGR cooler (15) with respect to the intended flow direction of the exhaust gases in the return line ( 10). Arrangement according to one of the preceding claims, characterized in that the ammonia supply device (32-3 5) is adapted to supply ammonia in the form of a liquid solution. Arrangement according to one of Claims 1 or 2, characterized in that the ammonia supply device (32-3 5) is adapted to supply ammonia in gaseous form. Arrangement according to any one of the preceding claims, characterized in that said parameter estimating means comprises a sensor (26, 28, 32, 33) which is adapted to sense the value of a parameter which is related to the presence of corrosive acids in the return line (10, 11). , 15). 10 15 20 25 30 35 13 Arrangement according to claim 5, characterized in that said sensor (27, 28) is positioned so that it is in contact with the exhaust gases in the return line (10, 11, 15). Arrangement according to claim 6, characterized in that said sensor (27) is adapted to sense the amount of sulfur oxides and / or the amount of nitrogen oxides in the exhaust gases. Arrangement according to claim 6, characterized in that said sensor is adapted to sense the amount of ammonia in the exhaust gases. Arrangement according to any one of the preceding claims 1-4, characterized in that said parameter estimating means (13a, 26) is adapted to estimate a parameter net value in the form of the sulfur content of the fuel which is led to the internal combustion engine. Arrangement according to claim 5, characterized in that said parameter estimating means comprises stored information (13a) regarding the sulfur content of the fuel. Arrangement according to claim 5, characterized in that said parameter estimating means comprises a sensor (26) which is positioned in contact with the fuel which is led to the internal combustion engine (2). Arrangement according to claim 5, characterized in that said sensor (36) is positioned in an area of the return line (10, 11, 15) where condensate is normally formed. Arrangement according to claim 12, characterized in that said sensor (36) is adapted to sense the pH value in the condensate. Arrangement according to one of the preceding claims, characterized in that it comprises detecting means (25) for detecting whether condensate is formed in the return line (10, 11). A method for recirculating exhaust gases of an internal combustion engine (2), the combustion engine comprising an exhaust line (4) adapted to discharge exhaust gases from the internal combustion engine (2), a return line (10, 11) through which exhaust gases are recirculated from the exhaust line ( 4) to the dry combustion engine (2) and at least one EGR cooler (15) arranged in the exhaust passage (10, 11) in which the recirculating exhaust gases are adapted to be cooled before they reach the combustion engine (2), characterized by the steps of estimating the value of a parameter related to the presence of corrosive acids in the return line (10, 11, 15) and supplying ammonia to the recirculating exhaust gases in the return line (10, 11, 15) in an amount using said parameter value so that any corrosive acids in the return line (10, 11, 15) is neutralized.