BACKGROUND AND PRIOR ART The invention pertains to an arrangement and a method for the treatment of exhausts from a combustion engine according to the preambles of patent claims 1 and 11.

In order to reduce the emission of nitrogen oxides ΝΟχ from combustion engines, among others a technology called SCR (Selective Catalytic Reduction) is used. This technology entails that a fixed dose of a solution of urea is added to the exhausts in an exhaust pipe. The urea solution may be sprayed into the exhaust pipe following which the atomised urea solution is vaporized as it comes into contact with the hot exhausts so that ammonia is formed. The mixture of ammonia and exhausts is then led through an SCR catalyst. Here the nitrogen content of the nitrogen oxide in the exhausts reacts with the nitrogen in the ammonia, forming nitrogen gas. The oxygen in the nitrogen oxides reacts with the hydrogen in the ammonia, forming water. The nitrogen oxides in the exhausts are thus reduced to nitrogen gas and water steam in the catalyst. With the correct dosage of urea, the combustion engine's emission of nitrogen oxides may be reduced to a great extent.

Nitrogen oxides ΝΟχ in exhausts consist of nitrogen monoxide NO and nitrogen dioxide N0 ₂ . The ability of SCR-catalysts to reduce the amount of nitrogen oxides in exhausts is optimal when the exhausts contain equal amounts of nitrogen monoxide and nitrogen dioxide. The proportion of nitrogen dioxide should thus be 50%. Exhausts from diesel engines in particular usually contain a significantly smaller proportion of nitrogen dioxide than nitrogen monoxide. In order to increase the proportion of nitrogen dioxide, according to prior art a DOC oxidation catalyst (Diesel Oxidation Catalyst) is arranged in the exhaust pipe in a position upstream of the SCR-catalyst. An oxidation catalyst oxidises nitrogen monoxide NO into nitrogen dioxide N0 ₂ . Thus the proportion of nitrogen dioxide N0 ₂ in the exhausts may increase to the level at which the SCR-catalyst provides an optimal ability to reduce nitrogen oxides NO _x . An oxidation catalyst's ability to oxidise nitrogen monoxide NO into nitrogen dioxide N0 ₂ varies according to the temperature and flow of the exhausts. Since the temperature and flow of the exhausts vary during the operation of a combustion engine, an oxidation catalyst may not always deliver the desired ratio between the two types of nitrogen oxide. If an oxidation catalyst is dimensioned to oxidise nitrogen oxides in exhausts at a medium temperature of the exhausts, the proportion of nitrogen dioxide N0 ₂ obtained is too low at low exhaust temperatures and the proportion of nitrogen dioxide N0 ₂ is too high at high exhaust temperatures. Where the proportion of nitrogen dioxide N0 ₂ is too low, this results in poor efficiency of the SCR-catalyst. Where the proportion of nitrogen dioxide N0 ₂ is too high, this results in the formation of nitrous oxide on contact with the injected urea solution. Nitrous oxide is a potent greenhouse gas.

US 7,810,316 shows an exhaust pipe in a combustion engine with components for the after-treatment of exhausts. The exhaust pipe comprises an embodiment of two parallel pipes which are equipped with one oxidation catalyst each. One oxidation catalyst is used primarily to oxidise nitrogen monoxide into nitrogen dioxide at times when the combustion engine load is low, and the second oxidation catalyst is used primarily to oxidise nitrogen monoxide into nitrogen dioxide at times when the load of the combus- tion engine is high. A valve controls the exhaust flow to the respective parallel pipes before it is led to an SCR-catalyst and/or a particulate filter.

SUMMARY OF THE INVENTION The objective of the present invention is to provide an arrangement for after-treatment of exhausts from a combustion engine where the oxidation of nitrogen monoxide into nitrogen dioxide occurs in an amount, entailing the exhausts obtaining the desired proportions of nitrogen monoxide and nitrogen dioxide during different operating conditions of the combustion engine.

This objective is achieved with the arrangement of the type specified at the beginning, which is characterised by the features specified in the characterising portion of patent claim 1. In this case, a first oxidation catalyst is thus used, arranged in a bypass pipe to the exhaust pipe and a second oxidation catalyst is arranged inside the exhaust pipe downstream of the bypass pipe. The oxidation capacity of the first oxidation catalyst is dependent on the temperature and flow of the exhausts through the bypass pipe. With the help of said control means, the flow and temperature of the exhausts led through the first oxidation catalyst may be regulated. Based on the knowledge about the oxidation capacity of the second oxidation catalyst, the oxidation capacity of the first oxidation catalyst may be regulated, so that the first oxidation catalyst and the second oxida- tion catalyst jointly provide an oxidation of nitrogen monoxide into nitrogen dioxide in an amount entailing that the exhausts leaving the second oxidation catalyst have the desired proportions of nitrogen monoxide and nitrogen dioxide. According to the invention the bypass pipe extends around a turbine in the exhaust pipe. Thus exhausts may be led to the first oxidation catalyst from a position upstream of the turbine. Here the exhausts have a higher pressure and a higher temperature than the exhausts which are led to the second oxidation catalyst, which is thus located downstream of the turbine. Thus the first oxidation catalyst may provide a very good complementary oxidation capacity at those operating times when the second oxidation catalyst has a relatively small oxidation capacity.

According to a preferred embodiment of the present invention the second oxidation catalyst is dimensioned so that it has the capacity to oxidise nitrogen monoxide into nitrogen dioxide in an amount entailing the desired proportions of nitrogen monoxide and nitrogen dioxide being obtained in the exhausts leaving the second oxidation cata- lyst, at those operating times when optimal conditions for oxidising nitrogen monoxide into nitrogen dioxide prevail. Optimal conditions for oxidising nitrogen monoxide into nitrogen dioxide prevail when a small flow of exhausts with a temperature of around 300°C is led through the second oxidation catalyst. When optimal conditions prevail, no exhausts are led through the bypass pipe and the first oxidation catalyst, but instead the second oxidation catalyst provides the entire oxidation process. If the temperature of the exhausts falls and/or the exhaust flow increases, the second oxidation catalyst no longer has the capacity to oxidise nitrogen monoxide into nitrogen dioxide in an amount entailing the desired ratio being obtained. In this case, a suitable amount of exhausts is led through the first oxidation catalyst, providing a complementary oxida- tion, so that the desired ratio between nitrogen oxide and nitrogen dioxide is obtained downstream of the second oxidation catalyst.

According to one preferred embodiment of the present invention the first oxidation catalyst has a higher content of a precious metal coating than the second oxidation catalyst. Precious metal coatings of platinum, palladium and rhodium may be used as catalyst materials in oxidation catalysts. The oxidation capacity of the oxidation cata- lyst rises in proportion with the content of precious metals per area unit. By giving the first oxidation catalyst a high content of e.g. platinum, it may provide a much higher oxidation capacity when needed. The second oxidation catalyst may be given a lower oxidation capacity and thus provided with a lower content of platinum.

According to a preferred embodiment of the present invention, said control means comprises at least a valve element in the bypass pipe and a control device which is adapted to control the valve element, so that the first oxidation catalyst is perfused by a desired proportion of the exhaust flow in the exhaust pipe. Alternatively the valve ele- ment may sit in the exhaust pipe at the inlet or outlet of the bypass pipe. The valve element advantageously has a design allowing it to be placed in many different positions, so that the flow through the bypass pipe may be regulated in a stepwise or continuous manner. According to one preferred embodiment of the present invention said control means also comprise an exhaust brake, arranged inside the exhaust pipe in a position between the bypass pipe's inlet and outlet. The exhaust brake may be an existing exhaust brake in a vehicle. An exhaust brake is a valve which is arranged inside the exhaust pipe. In this case the control device may regulate the exhaust flow both through the bypass pipe and through the exhaust pipe. Thus several possibilities of regulating flow and temperature of the exhausts led through the bypass pipe arise, and the first oxidation catalyst's oxidation capacity may thus be regulated.

According to one preferred embodiment of the present invention said control means comprise at least one sensor which is adapted to detect a parameter, with which the control device estimates the flow and the temperature of the exhausts led through the first oxidation catalyst. Said sensor may be one or several suitably placed temperature sensors or flow sensors. Other types of sensors may obviously also be used. Based on the knowledge about the flow and the temperature of the exhausts being led through the exhaust pipe, the second oxidation catalyst's oxidation capacity may be determined. The control device may subsequently lead a suitable share of the exhaust flow in the exhaust pipe through the bypass pipe and the first oxidation catalyst, so that the desired ratio of nitrogen monoxide and nitrogen dioxide is obtained. According to one preferred embodiment of the present invention the turbine is a turbine in a turbocharger or a compound turbine. Many vehicles are driven by overloaded combustion engines. The turbine may in this case be a component in a turbocharger, which also comprises a compressor for compression of air led to the combustion engine. The bypass pipe with the first oxidation catalyst may advantageously be arranged inside an exhaust pipe around such a turbine. The bypass pipe may consist of an exist- ing wastegate in the turbine. In vehicles where the inlet air is compressed in two steps by a high pressure turbine and low pressure turbine, the bypass pipe with the first oxidation catalyst may be arranged around one of the said turbines. In vehicles with a compound turbine, the possibility of arranging the bypass pipe with the first oxidation catalyst so that it receives exhausts with a high temperature upstream of the compound turbine is also obtained.

According to one preferred embodiment of the present invention, the arrangement comprises an SCR-catalyst, which is arranged inside the exhaust pipe in a position downstream of the second oxidation catalyst with respect to the exhausts' intended flow direction in the exhaust pipe. In an SCR-catalyst the nitrogen oxides in the exhausts are reduced to nitrogen gas and water steam. The reduction is most efficient when the nitrogen oxide contains equal amounts of nitrogen monoxide and nitrogen dioxide. In exhausts that leave a combustion engine, the proportion of nitrogen dioxide is clearly lower than the proportion of nitrogen monoxide. With the help of the first oxidation catalyst and the second oxidation catalyst, the proportion of nitrogen dioxide may be increased at the expense of the proportion of nitrogen monoxide, so that an optimal relationship between nitrogen monoxide and nitrogen dioxide may essentially always be achieved during varying operating conditions. The arrangement may in this case comprise components for the supply of urea solution in a position upstream of the SCR-catalyst with respect to the exhausts' intended flow direction in the exhaust pipe. A urea solution is injected into the exhaust pipe. When the urea solution is evaporated by the hot exhausts, ammonia is formed, which is led jointly with the exhausts through the SCR-catalyst. With the correct dosage of urea, the combustion engine's emission of nitrogen oxides may be reduced to a great extent.

According to one preferred embodiment of the present invention, the arrangement comprises a particulate filter, arranged in a position downstream of the second oxidation catalyst with respect to the exhausts' intended flow direction in the exhaust pipe. In a particulate filter soot particles in the exhausts get caught and are burned. The op- erating temperature of the exhausts is not always sufficiently high to continuously maintain a temperature in the particulate filter at which the soot particles are burned. The combustion temperature of the soot particles may, however, be reduced significantly when the exhausts contain a high proportion of nitrogen dioxide. In order to reduce the ignition temperature of the soot particles, an oxidation catalyst may advantageously be used. With a suitable proportion of nitrogen dioxide in the exhausts, an ignition temperature may be obtained at which the soot particles are burned essentially continuously during the varying operating conditions of the combustion engine. If both a particulate filter and an SCR-catalyst are used in the exhaust pipe, the particulate filter is placed between the oxidation catalyst and the SCR-catalyst. Another way of increasing the temperature of the exhausts is to inject unburned fuel in a position upstream of the first or second oxidation catalyst. Thus the temperature of the exhausts may be increased markedly in the oxidation catalysts, which guarantees a combustion of the soot particles in the particulate filter arranged downstream.

The objective mentioned in the introduction is also achieved with the method specified in patent claim 11.

BRIEF DESCRIPTION OF THE DRAWING

Below is a description, as an example, of preferred embodiments of the invention with reference to the enclosed drawings, in which:

Fig. 1 shows an arrangement for the treatment of exhausts from a combustion engine according to a first embodiment of the invention,

Fig. 2 shows an arrangement for the treatment of exhausts from a combustion en- gine according to a second embodiment of the invention and

Fig. 3 shows an arrangement for the treatment of exhausts from a combustion engine according to a third embodiment of the invention.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVEN- TION

Fig. 1 schematically shows a vehicle 1 which is driven by an overloaded combustion engine 2 which may be a diesel engine. The vehicle may be a heavy goods vehicle. The exhausts from the combustion engine 2 are led out via an exhaust pipe 3. The ex- hausts leaving the combustion engine 2 have an overpressure. The overpressure of the exhausts is used to drive a turbine 4 in a turbocharger. The turbine 4 drives a compres- sor 5 in the turbocharger. The compressor 5 compresses air which is led into an inlet pipe 6 for air to the combustion engine 2. The inlet pipe 6 comprises an intercooler 7 where the compressed air is cooled before it is led to the combustion engine 2. A return pipe 8 for recirculation of exhausts extends from the exhaust pipe 3 and up to the inlet pipe 6. The return pipe 8 comprises an EGR-cooler 9 for cooling of the recirculated exhausts before they are mixed with the compressed air and led to the combustion engine 2.

The turbine 4 is equipped with a so-called wastegate comprising an exhaust passage 10 with a wastegate- valve 11 , with which it is possible to lead some of the exhausts past the turbine 4. A wastegate valve 11 is normally opened when the turbocharger's charge air pressure becomes too high. The exhaust pipe 3 in this case comprises a second turbine in the form of a compound turbine 12, located downstream of the turbocharger's turbine 4. A compound turbine 12 is used to generate energy from the exhausts for the operation of the vehicle. An exhaust brake 13 is arranged inside the exhaust pipe 3 downstream of the compound turbine 12. The exhaust brake 13 is a valve in the form of a damper in the exhaust pipe 3, with which it is possible to regulate the exhaust flow in the exhaust pipe 3 in an essentially continuous manner. The exhaust pipe 3 is equipped with a bypass pipe 14. The bypass pipe 14 comprises an inlet 14a, where exhausts from the exhaust pipe 3 are received in a position upstream of the compound turbine 12 and the exhaust brake 13. The bypass pipe 14 comprises an outlet 14b where exhausts are led back to the exhaust pipe 3 in a position downstream of the compound turbine 12 and the exhaust brake 13. The bypass pipe 14 comprises a first oxidation catalyst 15. An oxidation catalyst oxidises nitrogen monoxide NO into nitrogen dioxide N0 ₂ . An oxidation catalyst's ability to oxidise nitrogen monoxide into nitrogen dioxide is dependent of several parameters. The temperature of the exhausts is such a parameter. When the exhausts have a high temperature, a more efficient oxidation is achieved than when the exhausts have a lower temperature. An optimal oxidation capacity is obtained at around 300°C. Another parameter is the flow of exhausts through the oxidation catalyst. When the exhaust flow is great, a smaller part of the nitrogen monoxide in the exhausts may be oxidised into nitrogen dioxide than what happens with a smaller exhaust flow. A third parameter is the content of precious metal in the oxidation catalyst. The oxidation cata- lyst may comprise precious metal coatings of platinum, palladium or rhodium. A coating with a high content of a precious metal results in a more efficient oxidation process than a coating with a lower content of the precious metal. The bypass pipe 14 is also equipped with a valve element 16. With the help of the valve element 16 the exhaust flow through the bypass pipe 14 may be controlled. Only such exhausts in the exhaust pipe 3 which are led into the bypass pipe 14 thus pass through the first oxidation cata- lyst 15.

A control device 17 is adapted to control the wastegate valve 1 1, the exhaust brake 13 and the valve element 16. The control device 17 receives information from a number of sensors 18, detecting the parameters with which the temperature and flow of the exhausts may be determined. The sensors 18 may detect the temperature, flow and pressure of the exhausts in a suitable number of locations inside the exhaust pipe 3 and the bypass pipe 14. The control device 17 may also receive information from sensors 18, detecting the content of nitrogen oxide, nitrogen dioxide or nitrous oxide in suitable locations inside the exhaust pipe 3. The control device 17 may be a computer de- vice with a software suitable for this purpose. The exhaust pipe 3 comprises a second oxidation catalyst 19, arranged inside the exhaust pipe 3 in a position downstream of the bypass pipe's outlet 14b. All exhausts inside the exhaust pipe 3 are thus led through the second oxidation catalyst 19.

A particulate filter 20 is arranged downstream of the second oxidation catalyst 19. The task of the particulate filter 20 is to catch soot particles in the exhausts. The soot particles are then burned in the particulate filter 20. One way of guaranteeing a good combustion of the soot particles in the particulate filter is to lead exhausts with a high content of nitrogen oxide through the particulate filter. A high content of nitrogen oxide in the exhausts reduces the combustion temperature of the soot particles. With the help of the first oxidation catalyst 15 and the second oxidation catalyst 19, the exhausts may be supplied with nitrogen dioxide in an amount reducing the combustion temperature to a temperature level which is maintained in the particulate filter during the normal operation of the combustion engine. Thus the soot particles may be burned in essentially a continuous manner in the particulate filter. In this case a device 21 for the sup- ply of unburned fuel HC in the bypass pipe 14 is also arranged in a position upstream of the first oxidation catalyst 15. By injecting unburned fuel at suitable times, the temperature of the exhausts may be significantly increased in the first oxidation catalyst 15, and thus in the particulate filter 20 arranged downstream, thus ensuring the combustion of the soot particles. The exhaust pipe is equipped with an SCR-catalyst 23 for the catalytic exhaust purification according to the method called SCR (Selective Catalytic Reduction). This method entails a urea solution being injected into the exhausts. A urea solution may be stored inside a tank and led, via a pipe, to an injection element 22, which injects the urea solution into the exhaust pipe. The control device 17 or another separate control device may control the supply of the urea solution. Such a control device may, with information regarding specific engine parameters, calculate the amount of urea solution which needs to be added in order for the nitrogen oxide in the exhausts to be reduced optimally. The added urea solution is heated by the exhausts inside the exhaust pipe, so that it is vaporised and transformed into ammonia. The mixture of ammonia and exhausts is then led to the SCR catalyst 23. Inside the SCR-catalyst 23 the nitrogen in the nitrogen oxides in the exhausts reacts with the nitrogen in the ammonia, forming nitrogen gas. The oxygen in the nitrogen oxides reacts with the hydrogen in the ammonia, forming water. The nitrogen oxides in the exhausts are thus reduced to nitrogen gas and water steam in the SCR-catalyst 23. The nitrogen oxides ΝΟχ in the exhausts consist of nitrogen monoxide NO and nitrogen dioxide N0 ₂ . An SCR-catalyst 23 reduces the amount of nitrogen oxides in the exhausts optimally when the exhausts led through the SCR-catalyst 23 have equal amounts of nitrogen monoxide and nitrogen dioxide. An SCR-catalyst thus reduces nitrogen oxides optimally when the proportion of nitrogen dioxide is 50% of the total amount of nitrogen oxides. Exhausts from combustion engines generally contain a significantly larger proportion of nitrogen monoxide than of nitrogen dioxide.

It is thus important to increase the proportion of nitrogen dioxide in the exhausts, both in order to provide an essentially continuous combustion of the soot particles in the particulate filter 20 and to reduce the nitrogen oxides in the exhausts in the SCR- catalyst 23. The exhaust pipe also comprises an ammonia-catalyst 24 where any potential surplus of ammonia and nitrogen dioxide is converted into nitrogen gas and nitrous oxide. Nitrous oxide is a potent greenhouse gas, the emission of which into the environment should be prevented as far as possible. It is thus important that the oxidation catalysts 15, 19 may essentially always oxidise nitrogen monoxide into nitrogen dioxide in an amount, making the nitrogen oxide which reaches the SCR-catalyst 23 contain as much nitrogen dioxide N0 ₂ as nitrogen monoxide NO.

During the operation of the combustion engine 2 the exhausts are led out through the exhaust pipe 3. The control device 17 essentially continuously receives information from the sensors 18 regarding the temperature, pressure, flow, etc. of the exhausts. Based on this information and information regarding the combustion engine's 2 speed and load, the control device 17 determines, with the help of maps or other types of stored information, how large a part of the nitrogen monoxide in the exhausts needs to be oxidised into nitrogen dioxide, in order for the exhausts led to the SCR-catalyst 23 to contain equal amounts of nitrogen monoxide NO and nitrogen dioxide N0 ₂ .

The second oxidation catalyst 19 is dimensioned so that it may oxidise nitrogen monoxide NO into nitrogen dioxide N0 ₂ in an amount entailing equal amounts of nitrogen monoxide NO and nitrogen dioxide N0 ₂ being led to the SCR-catalyst 23 when the exhausts have a temperature of around 300°C and the exhaust flow is low. Under such optimal operating times, it is sufficient for the exhausts to be led only through the second oxidation catalyst 19. When the control device 17 receives information from e.g. said sensors 18 indicating that there is such an optimal operating time, it closes the valve element 16. Thus no exhausts are led through the bypass pipe 14 and the first oxidation catalyst 15. Here the second oxidation catalyst by itself oxidises nitrogen monoxide NO into nitrogen dioxide N0 ₂ .

If the temperature of the exhausts falls to a lower value and/or the exhaust flow through the exhaust pipe increases, it may be noted that the second oxidation catalyst 19 does not have the capacity to oxidise nitrogen monoxide into nitrogen dioxide to an extent entailing equal amounts of nitrogen monoxide NO and nitrogen dioxide N0 ₂ being led to the SCR-catalyst 23. When the control device 17 receives information indicating that this is the case, it estimates how large a part of the exhausts inside the exhaust pipe 3 that need to be led through the bypass pipe 14 and the first oxidation catalyst 15, in order for equal amounts of nitrogen monoxide NO and nitrogen dioxide N0 ₂ being led to the SCR-catalyst 23. Since the bypass pipe 14 has an inlet 14a in a position upstream of the compound turbine 12, the first oxidation catalyst 15 may receive exhausts with a higher pressure and a higher temperature than the second oxida- tion catalyst 19. The oxidation of nitrogen monoxide NO into nitrogen dioxide N0 ₂ thus becomes more efficient in the first oxidation catalyst 15 than in the second oxidation catalyst 19. In order for the first oxidation catalyst 15 to obtain an additionally increased oxidation capacity, it may contain a higher content of a precious metal than the second oxidation catalyst 19. The first oxidation catalyst 15 thus provides a high oxidation capacity so that it has the ability - even in unfavourable operating conditions, when the exhausts have a low temperature and the exhaust flow is high - to essentially always deliver the desired composition of nitrogen oxides to the SCR-catalyst 23 jointly with the second oxida- tion catalyst 19. The first oxidation catalyst 15 may thus provide a variable oxidation capacity within a relatively large area. During certain operating times, the control device 17 may lead the entire exhaust stream through the bypass pipe 14 and the first oxidation catalyst 3. During most operating times, however, a part of the exhausts is led through the bypass pipe 14 while a remaining part is led, via the ordinary exhaust pipe 3, to the compound turbine 12. The control device 17 advantageously has the capacity to steer the valve element 16 to more or less open positions, so that the exhaust flow through the bypass pipe 14 may be regulated in a continuous manner or in a relatively large number of fixed steps. The control device 17 may regulate the temperature and flow of the exhausts to the first oxidation catalyst 15 in several different manners. On occasions when the exhaust flow through the first oxidation catalyst 15 must be optimised, the control device 17 closes the exhaust brake 13 and opens the valve element 16 to a maximum, leading the entire exhaust flow through the first oxidation catalyst 15. In this case the turbine 4 arranged upstream also obtains an increased capacity, likewise generating a higher exhaust flow. On occasions when the exhaust temperature must be increased, the control device 17 opens the wastegate valve 11, leading warm exhausts which have not expanded in the turbine 4, into the bypass pipe 14 and the first oxidation catalyst 15. The bypass pipe 14 extends around the compound turbine 12 and the exhaust brake 13. The control device 17 may thus use the exhaust brake 13 in the exhaust pipe 3 and the valve element 16 in the bypass pipe 14 to increase the back pressure of the exhausts, and thus to impact the temperature of the exhausts led through the first oxidation catalyst 15. Fig. 2 shows a combustion engine 2 with an exhaust pipe 3, which to a large extent is equipped with the same components as in Fig. 1. Therefore the joint components are not reviewed again. One difference is that the exhaust pipe 3 does not comprise a compound turbine 12. The bypass pipe 14 here instead extends around a turbine 4 in the turbocharger. The bypass pipe 14 here has an inlet 14a which is located upstream of the turbine 4 and an outlet which is located downstream of the turbine 4. The bypass pipe 14 comprises a first oxidation catalyst 15 and a valve element 16, with which the exhaust flow through the bypass pipe may be adjusted. The bypass pipe 14 may here consist of a separate device or constitute a part of an existing wastegate and the valve element 16 of a wastegate valve 11. Fig. 3 shows a combustion engine with an exhaust pipe 3 which to a large extent is equipped with the same components as in Fig. 1 and 2. Therefore the joint components are not reviewed again. The combustion engine 2 in this case is an Exxon engine. The air led to the combustion engine 2 is compressed in two steps by a low pressure compressor 26 and a high pressure compressor 5. A high pressure turbine 4 drives the high pressure compressor 5 and a low pressure turbine 27 drives the low pressure compressor 26. The bypass pipe 3 in this case also does not comprise any compound turbine 12. The bypass pipe 14 here extends around the low pressure turbine 27 in the turbo- charger. The bypass pipe 14 here has an inlet 14a, which is located upstream of the low pressure turbine 27, and an outlet, which is located downstream of the low pressure turbine 27. The bypass pipe 14 comprises a first oxidation catalyst 15 and a valve element 16.

The invention is not limited to the embodiment described above, but may be varied freely within the framework of the patent claims.