BACKGROUND OF THE INVENTION AND PRIOR ART

The present invention pertains to a valve arrangement to control the exhaust flow through an oxidation catalyst according to the preamble of claim 1. In order to reduce the emission of nitrogen oxides ΝΟχ from combustion engines, a technology called SCR (Selective Catalytic Reduction), among others, is used. This technology entails that a fixed dose of a solution of urea and water 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 vaporised 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 oxides in the exhausts reacts with the nitrogen in the ammonia, forming nitrogen gas. The oxygen in the nitrogen oxides reacts with the nitrogen in the ammonia, forming water. The nitrogen oxides in the exhausts are thus reduced to nitrogen gas and water vapour 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 conventional SCR catalysts to remove nitrogen oxides from exhausts is dependent on the relationship between nitrogen monoxide NO and nitrogen dioxide N0 ₂ . The ability of an SCR catalyst to reduce the amount of nitrogen oxides in exhausts is optimal when the exhausts contain equal amounts of nitrogen monoxide and nitrogen dioxide. 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 in the exhausts led to an SCR catalyst, a DOC oxidation catalyst (Diesel Oxidation Catalyst) is arranged in the exhaust pipe in a position upstream of the SCR catalyst, according to prior art. An oxidation catalyst oxidises nitrogen monoxide into nitrogen dioxide. The proportion of nitrogen dioxide in the exhausts may thereby be increased. An oxidation catalyst' s capacity to oxidise nitrogen monoxide into nitrogen dioxide varies according to the temperature and flow of the exhausts. The capacity of oxidation catalysts to oxidise nitrogen monoxide into nitrogen dioxide is greatest at exhaust temperatures around 300°C and low exhaust flows. Under such operating conditions, the oxidation catalyst oxidises nitrogen monoxide into nitrogen dioxide in such an amount that the SCR catalyst receives nitrogen oxides containing more nitrogen dioxide than nitrogen monoxide. The surplus of nitrogen dioxide means that the SCR catalyst's ability to eliminate nitrogen oxides drops drastically at the same time as nitrous oxide is formed, which is a very powerful greenhouse gas. The ammonia slip also increases. Accordingly, it is a problem where an oxidation catalyst delivers exhausts containing nitrogen oxides with a surplus of nitrogen dioxide to an SCR catalyst.

SUMMARY OF THE INVENTION

The objective of the present invention is to provide a simple and reliable valve arrangement, with which it is possible to reduce an oxidation catalyst's capacity to oxidise nitrogen monoxide into nitrogen dioxide at such operating times, when the oxidation catalyst is at risk of delivering nitrogen oxides with a surplus of nitrogen dioxide.

This objective is achieved with the valve arrangement of the type specified at the beginning, which is characterised by the features specified in the characterising portion of claim 1. The valve arrangement thus comprises a bimetal component which is impacted by the temperature of the exhausts. A bimetal component consists of two assembled, thin metal elements with different thermal expansion capability. When the bimetal component is heated, it is bent as a result of one metal expanding more than the other. Common metals that may be used in bimetals are, for example, copper and steel. Bimetals are simple components with a reliable function.

When exhausts with a high temperature are led to an oxidation catalyst, its capacity is enhanced, and accordingly there is a risk that it may oxidise nitrogen monoxide into nitrogen dioxide in too great an amount. Where the exhausts have a temperature above a predetermined value, the valve arrangement controls the exhaust flow through the oxidation catalyst, so that the oxidation catalyst' s capacity to oxidise nitrogen monoxide into nitrogen dioxide is reduced. The bimetal component impacts the valve arrangement, so that said control of the exhaust flow through the oxidation catalyst is obtained when the exhausts reach the predetermined value. The bimetal component may impact the valve arrangement, so that it abruptly flips when the temperature of the exhausts exceeds the predetermined value. The valve arrangement may, however, advantageously have a construction so that it gradually starts to reduce the capacity of the oxidation catalyst when the temperature of the exhausts exceeds the predetermined value. The valve arrangement may in this case reduce the oxidation catalyst's capacity, depending on by how much the exhausts' temperature exceeds the predetermined value. Such a reduction of the oxidation catalyst's capacity results in the SCR catalyst arranged downstream being able to receive, also at high exhaust temperatures, nitrogen oxides with a nitrogen monoxide/nitrogen dioxide ratio that provides a good reduction of the nitrogen oxides in the exhausts.

According to the invention, the valve arrangement is adapted to control the exhaust flow through the oxidation catalyst, so that the oxidation catalyst' s capacity to oxidise nitrogen monoxide into nitrogen dioxide is reduced when the exhausts have a flow falling below a predetermined value. An oxidation catalyst's capacity to oxidise nitrogen monoxide into nitrogen dioxide is thus also dependent on the exhaust flow through the oxidation catalyst. A small, and therefore slow, flow of exhausts through the oxidation catalyst entails that the exhausts come into contact with the active catalyst material for a longer period, which means that a greater proportion of the nitrogen monoxides are oxidised into nitrogen dioxides before leaving the oxidation catalyst. In this case, the valve arrangement thus takes both the temperature and the flow of the exhausts into consideration. The predetermined values of the exhausts in relation to temperature and flow are related to each other, and define different operating modes at which the oxidation catalyst's capacity should be reduced to avoid that the nitrogen oxides in the exhausts, which the oxidation catalyst delivers to the SCR catalyst, contain too large a proportion of nitrogen dioxide. According to a preferred embodiment of the invention, the valve arrangement is adapted to control the flow through the oxidation catalyst, so that the oxidation catalyst' s capacity is reduced to a level where the SCR catalyst arranged downstream receives nitrogen oxides containing a maximum of 50% of nitrogen dioxide. The ability of an SCR catalyst to reduce the amount of nitrogen oxides is optimal when it contains 50% of nitrogen monoxide and 50% of nitrogen dioxide. If the proportion of nitrogen monoxide is greater than the proportion of nitrogen dioxide, the SCR catalyst functions relatively well, even if it does not have optimal capacity. If the proportion of nitrogen dioxide is greater than the proportion of nitrogen monoxide, the SCR catalyst' s capacity to eliminate nitrogen oxides is markedly reduced, while it emits nitrous oxide and ammonia. It is suitable for an oxidation catalyst to be dimensioned so that it is able to deliver nitrogen oxides with a proportion of nitrogen dioxide within a range of 40-50% in most operating modes, and to reduce the capacity of the oxidation catalyst with the valve arrangement at operating modes where the exhausts have a high temperature and a low flow, so that the proportion of nitrogen dioxide in the nitrogen oxides never exceeds 50%.

According to the invention, the valve arrangement comprises a channel for receiving exhausts in connection with the oxidation catalyst and a valve which controls the exhaust flow through the channel, and thus the exhaust flow through the connecting oxidation catalyst. With the help of such an alternative flow channel for the exhausts, the exhaust flow may be changed through the oxidation catalyst in such a manner that it obtains a reduced capacity to oxidise nitrogen monoxide into nitrogen dioxide.

According to one embodiment of the invention, said channel is a bypass conduit with which exhausts may be led past the oxidation catalyst. In this case, a portion of the exhausts is led past the oxidation catalyst instead of through it. Since only a reduced portion of the exhausts is led through the oxidation catalyst, the proportion of nitrogen dioxide in the exhaust conduit in a position downstream of the oxidation catalyst becomes lower. Advantageously, said channel extends through the oxidation catalyst. Said channel may be formed by way of a bore through e.g. a central section of the oxidation catalyst. Thus, the channel is not bulky. Said channel may also consists of a pipe which extends around the oxidation catalyst.

According to another embodiment of the invention, said channel is connected with the oxidation catalyst, so that the exhaust flow through the channel results in a

corresponding exhaust flow in an area of the oxidation catalyst. Since the valve blocks the exhaust flow through the channel, no exhaust flow is obtained in said area of the oxidation catalyst. The exhausts are in this case forced to be led through a limited area of the oxidation catalyst, resulting in a reduction of the oxidation catalyst's capacity to oxidise nitrogen monoxide into nitrogen dioxide. Said channel may be formed by a wall element which is fastened at the outlet side of the oxidation catalyst. Preferably, several such channels are arranged on the oxidation catalyst' s outlet side, so that they may stop or reduce the flow through a relatively large area of the oxidation catalyst.

According to the invention, the valve comprises a spring element, adapted to control the exhaust flow through the channel depending on the size of the incoming exhaust flow. The spring element, which is impacted by the incoming exhaust flow, and the bimetal component, which is impacted by the temperature of the exhausts, may be arranged so that they provide, at a high exhaust temperature and a low exhaust flow, an exhaust flow through the channel, resulting in the oxidation catalyst obtaining a reduced capacity to oxidise nitrogen monoxide into nitrogen dioxide.

According to one embodiment, the spring element consists of said bimetal component. In this, both the ability of the bimetal component to change shape at temperature changes and its resilient properties when impacted by the incoming exhausts are used. The valve's constituent parts may thus be reduced.

According to one embodiment of the invention, the valve comprises a valve body, which is adapted to control the flow through the channel by shifting between an open state and a substantially closed state. The substantially closed state entails that a minimal exhaust flow is led through the channel, so that the bimetal component is able to substantially continuously detect the temperature of the incoming exhausts. The bimetal component detecting the temperature of the exhausts may provide a power which acts on the valve body in one direction, and the spring element being impacted by the exhaust flow may provide a power that acts on the valve body in an opposite direction. Alternatively, the valve body may be comprised in said bimetal component. In this case, the bimetal component is not only able to impact the valve, but may also constitute the valve body that blocks the channel in a substantially closed state and reveals it in an open state. The invention also pertains to an exhaust system comprising a valve arrangement according to one of the embodiments defined above.

BRIEF DESCRIPTION OF THE DRAWINGS Below is a description of, as examples, preferred embodiments of the invention with reference to the enclosed drawings, in which: Fig. 1 shows a part of an exhaust conduit which comprises a valve arrangement according to a first embodiment of the present invention, Fig. 2 shows a valve in the valve arrangement in Fig. 1 in more detail,

Fig. 3 shows an alternative valve in the valve arrangement in Fig. 1,

Fig. 4 shows a valve arrangement according to a second embodiment of the present invention,

Fig. 5 shows a valve in the valve arrangement in Fig. 4 in more detail and

Fig. 6 shows an alternative valve for the valve arrangement in Fig. 4.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 shows a combustion engine in the form of a diesel engine 1. The diesel engine 1 may be designed as a driver motor for a heavy goods vehicle. The diesel engine 1 is equipped with an exhaust conduit 2 comprising a container 3 for exhaust-treating components. The exhaust-treating components may obviously be arranged in several separate containers. The container 3 may be a silencer. The container 3 in this case comprises a first exhaust-treating component in the form of an oxidation catalyst DOC 4 (Diesel Oxidation Catalyst). An oxidation catalyst 4 comprises elongated channels with an internal layer of a catalyst material in the form of a precious metal. Thus, the oxidation catalyst 4 has the ability to e.g. oxidise nitrogen monoxide NO into nitrogen dioxide N0 ₂ . Accordingly, the proportion of nitrogen dioxide N0 ₂ in the exhausts may be increased. Exhausts from diesel engines in particular contain a significantly smaller proportion of nitrogen dioxide than nitrogen monoxide. The ability of an oxidation catalyst 4 to oxidise nitrogen monoxide NO into nitrogen dioxide N0 ₂ varies according to the temperature and flow of the exhausts.

Downstream of the oxidation catalyst 4 the container 3 comprises a second exhaust- purifying component in the form of a particulate filter 5, which may be called DPF

(Diesel Particulate Filter). A particulate filter 5 comprises elongated parallel channels with stop surfaces arranged in suitable places. The stop surfaces force the exhausts to be led into adjacent elongated channels in the particulate filter 5. The walls of the channels are made of a porous material with fine pores, allowing the passage of exhausts but not of soot particles. The soot particles thus get stuck inside the particulate filter 5. The particulate filter 5 is regenerated continuously without any active measures, since the soot particles are oxidised with N0 ₂ and/or actively oxidised through heat-increasing measures, which accelerate the oxidation with either N0 ₂ or oxygen. Downstream of the particulate filter 5 the container 3 comprises a third exhaust- purifying component in the form of an SCR catalyst 6 for catalytic exhaust purification according to the method called SCR (Selective Catalytic Reduction). This method entails that a reductant in the form of a urea solution is injected into the exhausts. In this case, urea solution is stored in a tank 7 and is led, via a conduit 8, to an injection element 9 that injects urea solution into a space 3a in the container. A control device 10 controls the supply of the urea solution with information regarding specific engine parameters 11. A pump 12 transports the urea solution to the injection element 9.

Downstream of the SCR catalyst 6 the container 3 comprises a fourth exhaust- purifying component in the form of an ammonia slip catalyst 13 ASC (Ammonia Slip Catalyst). The task of the ammonia slip catalyst 13 is to eliminate any surplus ammonia which is not reduced in the SCR catalyst. An ammonia slip catalyst generally comprises a precious metal coating such as a platinum coating, which oxidises ammonia into nitrogen, nitrogen oxides and nitrous oxide.

During the operation of the combustion engine 1, the control device 10 calculates, with information regarding engine parameters 11 such as load and speed, the amount of urea solution which needs to be added in order for the nitrogen oxides in the exhausts to be reduced optimally. The control device 10 activates the pump 12, which transports the calculated amount of urea solution to the injection element 9, which injects the urea solution into the exhausts. The added urea solution is heated by the exhausts inside the container 3, so that it is vaporised and transformed into ammonia. The mixture of ammonia and exhausts is then led to the SCR catalyst 6. Inside the SCR catalyst 6 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 nitrogen in the ammonia, forming water. The nitrogen oxides in the exhausts are thus reduced to nitrogen gas and water vapour in the catalyst 6.

The ability of the SCR catalyst 6 to reduce nitrogen oxides is related to the temperature of the exhausts. An optimal temperature may be within the range 300-450°C. At higher and lower exhaust temperatures the capacity of the SCR catalyst to reduce nitrogen oxides is reduced. The exhaust flow through the SCR catalyst is also a factor that impacts the capacity of the SCR catalyst. The faster the exhausts pass through the SCR catalyst, the smaller the proportion of the exhaust's nitrogen oxide content that is able to be reduced. The nitrogen oxides ΝΟχ in exhausts consists of nitrogen monoxide NO and nitrogen dioxide N0 ₂ . The ability of an SCR catalyst 6 to remove nitrogen oxides from exhausts also depends on the ratio of nitrogen monoxide NO and nitrogen dioxide N0 ₂ . The ability of an SCR catalyst to reduce the amount of nitrogen oxides in exhausts is optimal when the exhausts contain equal amounts of nitrogen monoxide NO and nitrogen dioxide N0 ₂ . The task of the oxidation catalyst 4 is to oxidise nitrogen monoxide NO into nitrogen dioxide N0 ₂ in such an amount, that the SCR catalyst 6 receives nitrogen oxides NOx which ideally contains equal amounts of nitrogen monoxide and nitrogen dioxide N0 ₂ .

The oxidation catalyst 6 has been equipped with a through channel 14, centrally arranged. A valve 15 has been arranged inside the channel 14. The valve 15 is adapted to control the exhaust flow through the channel 14. Of the exhausts that reach the oxidation catalyst 4, some will be led through the through passages in the oxidation catalyst 4, in contact with the active layers of catalyst materials in the oxidation catalyst 4, and obtain an oxidation of nitrogen monoxide NO into nitrogen dioxide N0 ₂ . A remaining part of the exhausts is led through the channel 14 in unchanged form, with the help of the valve 15. When the valve 15 is in a closed state,

substantially the entire exhaust flow is led through the oxidation catalyst 4. With the help of the valve 15, a variable portion of the exhausts may thus be led past the oxidation catalyst 4. The greater the portion of the exhaust flow that is led through the channel 14 in an unchanged form, the smaller the amount of nitrogen monoxide which is oxidised into nitrogen dioxide in the oxidation catalyst 4.

Fig. 2 shows the valve 15 which is arranged in the channel 14 in more detail. The valve 15 comprises a valve house 15a. The valve house 15a comprises one or several inlet openings 15b for receipt of exhausts. A bimetal component 15c is arranged internally of the inlet openings 15b. The bimetal component 15c consists of metal tin sheet comprising two , thin metal plates made of different materials with different heat expanding features, arranged side by side. The bimetal component 15c is fastened in the house 15a in a curved state. In a cross- sectional direction, the bimetal component 15c is thinner than the internal width of the house 15a, so that exhausts may pass the bimetal components 15 inside the house 15a. Alternatively, the bimetal component 15 may comprise through openings for passage of exhausts. The bimetal component 15 is fastened between a wall surface in connection with the inlet opening 15b of the valve house and a first side of a valve body 15d. The valve body 15d is moveably arranged between a closed state and a more or less open state in relation to a valve seat 15e. The valve body 15d is arranged on a valve rod 15f, which is moveably attached in a hole extending through a wall in connection with the inlet opening 15b of the valve house, and a wall in connection with the outlet opening 15g of the valve house. A spring element 15h is attached between a second side of the valve body 15d and an internal wall surface of the valve house 15a in connection with the outlet opening 15g. The bimetal component 15c acts with a resilient force on the valve body 15d, seeking to move it towards the valve seat 15e and thus to close the valve arrangement 15. The two metal plates comprised in the bimetal component are arranged in relation to each other, so that the resilient force with which the bimetal component 15c acts on the valve body 15d decreases with a rising exhaust temperature. The spring element 15h acts with a resilient force on the valve body 15d, seeking to move it from the valve seat 15e and thus to open the valve arrangement 15. This resilient force is substantially constant during all operating modes. The exhaust flow acts with a force on the valve body 15d, seeking to move the valve body 15d towards the valve seat 15e and thus to close the valve arrangement 15. The force with which the exhaust flow acts on the valve body is related to the size of the exhaust flow. The position of the valve body

15c in relation to the valve seat 15e is thus determined by the temperature and flow of the exhausts, i.e. the same parameters that impact the oxidation catalyst's capacity to oxidise nitrogen monoxide NO into nitrogen dioxide N0 ₂ . The task of the oxidation catalyst 4 is to oxidise nitrogen monoxide NO into nitrogen dioxide N0 ₂ . The capacity of the oxidation catalyst 4 to perform this task is highest when the exhausts have a high temperature and a low flow. In such operating modes, a conventional oxidation catalyst oxidises nitrogen dioxide N0 ₂ in such an amount that there is a risk, that the nitrogen dioxide content may exceed 50% of the total amount of nitrogen oxides ΝΟχ leaving the oxidation catalyst 4 and being led to the SCR catalyst 6. The ability of the SCR catalyst to reduce nitrogen oxides ΝΟχ in the exhausts drops radically, however, if the nitrogen dioxide N0 ₂ content in the exhausts exceeds 50% of the total amount of nitrogen oxides ΝΟχ. The oxidation catalyst 4 should, for this reason, deliver nitrogen oxides ΝΟχ to the SCR catalyst 6 containing no more than 50% nitrogen dioxide N0 ₂ . Suitably, it delivers nitrogen oxides with a proportion of nitrogen dioxide N0 ₂ within the range 45-50%. The bimetal component 15c and the spring element 15h are dimensioned so that the bimetal component 15c, jointly with the incoming gas flow, maintains the valve arrangement 14 in a closed state at operating modes when there are low exhaust temperatures and low exhaust flows, high exhaust temperatures and high exhaust flows and low exhaust temperatures and high exhaust flows. In such operating modes, the capacity of the oxidation catalyst to oxidise nitrogen monoxide into nitrogen dioxide is not entirely optimal, and it delivers a nitrogen dioxide N0 ₂ content that falls below 50% of the total amount of nitrogen oxides ΝΟχ. In operating modes where the exhausts have high temperatures and low flows, the oxidation catalyst 4 has an optimal capacity. The high exhaust temperature impacts the bimetal component 15c, so that it exerts a reduced resilient force on the valve body 15d, while the exhaust flow exerts a relatively small force on the valve body 15d towards the closed state. The spring element 15h now has the capacity to shift the valve element 15d towards an open state, so that a part of the exhausts is led through the open valve 15 and the channel 14, without oxidising in the oxidation catalyst 4. Since part of the exhausts pass by the oxidation catalyst 4, the nitrogen dioxide N0 ₂ content in the nitrogen oxides ΝΟχ reaching the SCR catalyst 6, arranged downstream, is reduced. The valve body 15c is moved by the spring element 15h to an open state, which may be more or less open depending on the temperature and flow of the exhausts. The flow through the valve arrangement 15 may thus vary steplessly within a certain range. The components of the valve arrangement 15 are dimensioned so that the valve body 15d opens, and leads past a part of the exhausts in the channel 14, so that the nitrogen dioxide N0 ₂ content in the exhausts downstream of the oxidation catalyst 4 is at most 50% of the total amount of nitrogen oxides ΝΟχ.

Fig. 3 shows an alternative embodiment of a valve 16, which may be arranged in a channel 14 extending through an oxidation catalyst 4. In this case, the valve

arrangement 16 only comprises a bimetal component 16 in the form of metal tin sheet. The metal tin sheet has an internal edge surface 16a, which is attached in the channel 14, and a free edge surface 16b, which is located in a position upstream of the fixed edge surface 16a with respect to the direction of the flow of exhausts in the channel 14. The bimetal component 16 here has the feature that it curves, i.e. bends downward towards a successively more open position at rising exhaust temperatures. The exhaust flow in the channel 14, however, exerts a force that strives to bend the bimetal component 16 upwards, i.e. towards a closed state. The bimetal component 16 is dimensioned so that it holds the channel 14 in a closed state in operating modes where there are low exhaust temperatures and low exhaust flows, high exhaust temperatures and high exhaust flows and low exhaust temperatures and high exhaust flows. In such operating modes, the capacity of the oxidation catalyst to oxidise nitrogen monoxide into nitrogen dioxide is not entirely optimal, and it delivers a nitrogen dioxide N0 ₂ content that falls below 50% of the total amount of nitrogen oxides ΝΟχ. In operating modes where the exhausts have high temperatures and low flows, the oxidation catalyst 4 has an optimal capacity. The high exhaust temperature impacts the bimetal component 16 so that it bends, and the free end surface is bent downward. The low exhaust flow does not have the capacity to exert a force to lift the free end surface 16b upward towards a closed state, in which the free end surface abuts to an upper surface in the channel 14. Thus, an opening for the exhausts is created between the free end surface 16b and the upper surface of the channel 14. This opening obtains a size that varies according to the temperature and flow of the exhausts. The size of the opening defines how large a part of the exhausts is led past the oxidation catalyst 4 via the channel 14. How great a portion of the exhausts that is led through the channel 14 is determined by the reduction of the oxidation catalyst's capacity.

Fig. 4 shows another principle for preventing an oxidation catalyst 4 from oxidising nitrogen oxides in such an amount that nitrogen oxides ΝΟχ, containing no more than 50% of nitrogen dioxide N0 ₂ , is delivered to an SCR catalyst arranged downstream. In this case, a part of the outlet side 4a of the oxidation catalyst 4 has been equipped with a wall element 17, defining short channels 18 for receipt of exhausts that leave certain areas of the oxidation catalyst 4. Each one of the channels 18 comprises a valve 19, with which the through-passing flow is controlled. When the valves 19 are in a closed state, the exhaust flow through the connecting areas of the oxidation catalyst 4, and thus the oxidation of nitrogen oxides NO into nitrogen dioxide N0 ₂ , is substantially blocked in these areas by the oxidation catalyst 4. The valves 19 are adapted to be closed in operating modes where there is a risk that the proportion of nitrogen dioxide may exceed 50% of the total amount of nitrogen oxides ΝΟχ reaching the SCR catalyst 6. Such operating modes arise where the exhausts have a high temperature and a low flow. In this case, the valve 19 is adapted to close and blocks connecting areas of the oxidation catalyst 4. Accordingly, the oxidation catalyst 4 delivers nitrogen oxides ΝΟχ with a proportion of nitrogen dioxide which is at most 50% to the SCR catalyst 6 located downstream

Fig. 5 shows two such valves 19 in further detail. In this case the valves 19 comprise only a bimetal component 19 in the form of a metal tin sheet. The bimetal component 19 has an internal edge surface 19a, which is attached in the channel 18, and a free edge surface 19b located in a position downstream of the fixed edge surface 19a, with regard to the direction of the flow of exhausts in the channel 18. The bimetal component 19 here has the feature that it curves, i.e. it bends upward towards a closed state at rising temperatures. The exhaust flow in the channel 14 exerts a force that strives to bend the bimetal component 18 downward, i.e. towards an open state. The bimetal component 19 is dimensioned so that it maintains the channel 18 in an open state in operating modes where there are low exhaust temperatures and low exhaust flows, high exhaust temperatures and high exhaust flows and low exhaust temperatures and high exhaust flows. In such operating modes, the capacity of the oxidation catalyst to oxidise nitrogen monoxide into nitrogen dioxide is not entirely optimal, and it delivers a nitrogen dioxide N0 ₂ content that falls below 50% of the total amount of nitrogen oxides ΝΟχ. In operating modes where the exhausts have high temperatures and low flows, the oxidation catalyst 4 has an optimal capacity. The high exhaust temperature impacts the bimetal component 19 so that it bends and the free end surface is bent upwards. The low exhaust flow does not have the capacity to exert a force that may press down the free end surface 19b from the substantially closed state.

Accordingly, the exhaust flow through the areas of the oxidation catalyst 4 having outlet openings in connection with said channels 18 is substantially blocked.

Fig. 6 shows an alternative valve 20 that may be used to control the exhaust flow through the channels 18. The valve 20 comprises the same components as the valve in Fig. 2, but they have been arranged so that the valve closes instead of opening in operating modes where the exhausts have a high temperature and a low flow. The valve arrangement 20 comprises a valve house 20a. The valve house 20a comprises one or several inlet openings 20b for receipt of the exhausts. A bimetal component 20c is arranged internally of the inlet openings 20b. In a cross- sectional direction the bimetal component 20c is narrower than the internal width of the house 20a, so that exhausts may pass by the bimetal component 20c inside the house 20a. The bimetal component 20c is attached between a wall surface in connection with the inlet opening 20b of the valve house, and a first side of a valve body 20d. The valve body 20d is moveably arranged in relation to a valve seat 20e between a substantially closed state and a more or less open state. The valve body 20d is arranged on a valve rod 20f, which is moveably attached in a hole extending through a wall in connection with the inlet opening 20b of the valve house and a wall in connection with the outlet opening 20g of the valve house. A spring element 20h is attached between a second side of the valve body 20d and an internal wall surface of the valve house 20a in connection with the outlet opening 20g.

The bimetal component 20c acts with a resilient force on the valve body 20d, striving to move it from the valve seat 20e and thus towards an open state. The resilient force with which the bimetal component 20 acts on the valve body 20d decreases as the exhaust temperature increases. The spring element 20h acts with a resilient force on the valve body 20d, striving to move it towards the valve seat 20e and thus to close the valve arrangement 20. This resilient force is substantially constant during all operating modes. The exhaust flow acts with a force on the valve body 20d, striving to move the valve body 20d towards an open state. The force with which the exhaust flow acts on the valve body 20d is related to the size of the exhaust flow. The position of the valve body 20d in relation to the valve seat 20e is thus determined by the temperature and flow of the exhausts.

Thus, the oxidation catalyst 4 has the highest capacity when the exhausts have a high temperature and a low flow. The bimetal component 20c and the spring element 20h are dimensioned so that they keep the valve arrangement 20 in an open state in operating modes where there are low exhaust temperatures and low exhaust flows, high exhaust temperatures and high exhaust flows and low exhaust temperatures and high exhaust flows. In such operating modes, the capacity of the oxidation catalyst to oxidise nitrogen monoxide into nitrogen dioxide is not entirely optimal, and it delivers a nitrogen dioxide N0 ₂ content that falls below 50% of the total amount of nitrogen oxides ΝΟχ. In operating modes where the exhausts have high temperatures and low flows, the oxidation catalyst 4 has an optimal capacity. The high exhaust temperature impacts the bimetal component 20c so that it exerts a reduced spring force on the valve body 20d, while the exhaust flow exerts a relatively small force on the valve body 20d towards the open state. The spring element 20h now has the capacity to shift the valve element 20d towards the closed state, so that the flow of exhausts through the valve 20 and the channel 18 are substantially blocked. Accordingly, the full capacity of the oxidation catalyst 4 may not be used and it thus delivers a reduced nitrogen dioxide N0 ₂ content in the nitrogen oxides ΝΟχ to the SCR catalyst 6 arranged downstream. The flow through the valve arrangement 20 may be varied steplessly from an entirely open state to a successively substantially closed state, where a minimal flow of exhausts is led through the valve arrangement 20. The components of the valve arrangement 20 are dimensioned so that the valve body 20d closes and blocks the flow in parts of the oxidation catalyst 4, so that the nitrogen dioxide N0 ₂ content in the exhausts downstream of the oxidation catalyst 4 is at most 50% of the total amount of nitrogen oxides ΝΟχ. The invention is not limited to the embodiment described above, but may be varied freely within the scope of the patent claims.