BACKGROUND TO THE INVENTION AND PRIOR ART

The invention relates to an arrangement and a method for reducing nitrogen oxides in exhaust gases from a combustion engine according to the preambles of claims 1 and 1 1. One way of reducing emissions of nitrogen oxides ΝΟχ from diesel engines is to use a technique called SCR (selective catalytic reduction). This technique involves a solution of urea being supplied in a specific dose to the exhaust gases in the exhaust line of a diesel engine. When urea solution is sprayed into the exhaust line, the finely divided urea solution becomes vaporised by contact with the hot exhaust gases, resulting in the formation of ammonia. The mixture of ammonia and exhaust gases is then led through an SCR catalyst. The nitrogen from the nitrogen oxides in the exhaust gases reacts there with the nitrogen in the ammonia to form nitrogen gas. The oxygen from the nitrogen oxides reacts with the hydrogen in the ammonia to form water. The nitrogen oxides in the exhaust gases are thus reduced in the catalyst to nitrogen gas and water vapour. With correct dosage of urea, the diesel engine's emissions of nitrogen oxides can be greatly reduced.

Nitrogen oxides ΝΟχ in exhaust gases comprise nitrogen monoxide NO and nitrogen dioxide N0 ₂ . The ability of SCR catalysts to reduce the amount of nitrogen oxides in exhaust gases is optimum when the exhaust gases contain equal amounts of nitrogen monoxide and nitrogen dioxide. The proportion of nitrogen dioxide has therefore to be 50%. Exhaust gases from diesel engines in particular usually contain a significantly smaller proportion of nitrogen dioxide than nitrogen monoxide. A known way of increasing the proportion of nitrogen dioxide is to provide an oxidation catalyst in the exhaust line at a location upstream of the SCR catalyst. An oxidation catalyst oxidises nitrogen monoxide to nitrogen dioxide. The proportion of nitrogen dioxide in the exhaust gases can thus be increased to improve the SCR catalyst's ability to reduce nitrogen oxides in the exhaust gases.

SUMMARY OF THE INVENTION The object of the present invention is to propose an arrangement and a method whereby the catalyst achieves substantially optimum reduction of nitrogen oxides in the exhaust gases in substantially all operating states of a combustion engine. This object is achieved with an arrangement of the kind mentioned in the introduction which is characterised by the features indicated in the characterising part of claim 1. For the catalyst to function optimally and achieve optimum reduction of the nitrogen oxides in the exhaust gases, the nitrogen oxides led to the catalyst have to contain an optimum proportion of nitrogen dioxide which is normally about 50%. According to the invention, at least one parameter related to the proportion of nitrogen dioxide out of the total amount of nitrogen oxides in the exhaust gases which reach the catalyst is detected. This parameter value detected is compared with a set-point value for the proportion of nitrogen dioxide. Such set-point values are with advantage stored in a way appropriate to the type of catalyst used in this case. If the measured parameter value does not correspond to the set-point value, the proportion of nitrogen dioxide is not the ideal proportion. In this case, the control unit, which may be a computer unit or the like with suitable software for the purpose, controls a component by which the parameter value can be adjusted. The component adjusts in this case the operation of the combustion engine in such a way that the parameter value assumes a value corresponding to the set-point value. As a rule, the combustion engine can be controlled in such a way without adversely affecting its operation. With such active control, the oxidation catalyst can substantially continuously supply the catalyst with nitrogen oxides which contain the ideal proportion of nitrogen dioxide. The catalyst may therefore achieve substantially optimum reduction of nitrogen oxides in the exhaust gases in substantially all operating states of the combustion engine.

According to a preferred embodiment of the present invention, said sensor is adapted to detecting a parameter which controls the oxidation catalyst's ability to oxidise nitrogen monoxide in nitrogen oxides to nitrogen dioxide. The oxidation catalyst's ability to oxidise nitrogen monoxide to nitrogen dioxide depends on several operating parameters. Said sensor may be adapted to detecting a parameter related to the exhaust flow which is led to the oxidation catalyst. The greater the exhaust flow through the oxidation catalyst, the smaller the proportion of nitrogen monoxide which the oxidation catalyst succeeds in oxidising to nitrogen dioxide. Said sensor may be a temperature sensor which measures the temperature of the exhaust gases close to the oxidation catalyst. At an exhaust temperature of about 300°C, the oxidation catalyst oxidises an optimum amount of nitrogen monoxide to nitrogen dioxide. Controlling the exhaust flow and the temperature of the exhaust gases makes it possible to control with good precision the oxidation catalyst's ability to oxidise nitrogen monoxide in the exhaust gases to nitrogen dioxide.

According to a preferred embodiment of the present invention, said sensor is adapted to detecting the amount of nitrogen oxides in the exhaust line at a location downstream of the oxidation catalyst and upstream of the catalyst. Such a sensor thus measures the amount of nitrogen dioxide led to the catalyst. In this case the control unit may be adapted to receiving information from a further sensor which detects the amount of nitrogen oxides in the exhaust line at a location upstream of the catalyst and to using this information to calculate the proportion of nitrogen dioxide out of the total amount of nitrogen oxides in the exhaust gases led to the catalyst. This proportion should correspond to a set-point value which in most cases is 0.50, i.e. the nitrogen oxides led to the catalyst contain in this case 50% nitrogen dioxide and 50% nitrogen monoxide.

According to another preferred embodiment, the catalyst is of the so-called SCR (selective catalytic reduction) type. In this case a solution of urea is sprayed into the exhaust line so that ammonia is formed upstream of the SCR catalyst. The mixture of ammonia and the exhaust gases is then led through the SCR catalyst. The nitrogen oxides in the exhaust gases are reduced in the SCR catalyst to nitrogen gas and water vapour. With correct dosage of urea, the diesel engine's emissions of nitrogen oxides can be greatly reduced. However, the invention is not limited to this type of catalyst but is applicable to any catalysts whose efficiency varies with the proportion of nitrogen dioxide out of the total amount of nitrogen oxides.

According to another preferred embodiment of the invention, said component is a bypass line which extends round the oxidation catalyst and a valve which is adapted to regulating the exhaust flow through the bypass line. Such a bypass line makes it possible for a portion of the exhaust gases to be led past the oxidation catalyst. This means that no oxidation of nitrogen monoxide to nitrogen dioxide takes place in the portion of the exhaust gases which is led past the oxidation catalyst. The bypass line is open in situations where the nitrogen oxides leaving the oxidation catalyst contain too large a proportion of nitrogen dioxide. This may occur in situations where the exhaust gases are at a high temperature while at the same time the exhaust flow through the oxidation catalyst is low. Alternatively, or in combination, said component may be an inlet throttle by which the air flow to the combustion engine can be regulated. An inlet throttle can be used to reduce the air flow to the combustion engine. This leads to the exhaust flow from the combustion engine being reduced while at the same time the temperature of the exhaust gases rises, thereby influencing the oxidation catalyst's ability to create nitrogen dioxide.

According to another preferred embodiment, said component is situated in the exhaust line, where it is capable of regulating the flow and/or the temperature of the exhaust gases led to the oxidation catalyst. An example of such a component is an exhaust brake. The temperature of the exhaust gases can be controlled by regulating a damper of the exhaust brake. A turbine with variable geometry is another component in the exhaust line which can be actively controlled to influence the exhaust flow and the temperature of the exhaust gases. Another example is a waste gate which comprises a bypass line with a waste gate valve to protect a turbine with fixed geometry from overspeeding. The waste gate valve can be used to control the exhaust flow and the temperature of the exhaust gases in the exhaust line. Alternatively, or in combination, said component may be inlet valves and/or exhaust valves of the combustion engine's cylinders. The exhaust flow and the temperature of the exhaust gases in the exhaust line can be controlled by varying the valve opening and closing times.

The object mentioned in the introduction is also achieved with the method according to claim 11.

BRIEF DESCRIPTION OF THE DRAWINGS

Preferred embodiments of the invention are described below by way of exampli reference to the attached drawings, in which:

Fig. 1 depicts an arrangement according to a first embodiment of the present

invention,

Fig. 2 is a flowchart illustrating the operation of the arrangement in Fig. 1 ,

Fig. 3 depicts an arrangement according to a second embodiment of the present invention and

Fig. 4 is a flowchart illustrating the operation of the arrangement in Fig. 3. DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS OF THE

INVENTION

Fig. 1 depicts a combustion engine in the form of a diesel engine 1. The diesel engine 1 may be intended to power a heavy vehicle. The exhaust gases from the cylinders of the diesel engine 1 are led via an exhaust manifold 2 to an exhaust line 3. The exhaust line 3 is provided with a turbo unit which comprises a turbine 4 and a compressor 5. The turbine 4 is intended to convert the energy of the exhaust gases in the exhaust line 3 to mechanical work for driving the compressor 5. The compressor 5 is intended to compress air which is led into an inlet line 6 to the diesel engine 1. A charge air cooler 7 is provided in the inlet line 6 to cool the compressed air before it is led to the respective cylinders 9 of the diesel engine 1 via a manifold 8. Each cylinder 9 is provided with a schematically depicted inlet valve 9a and an exhaust valve 9b. The exhaust line 3 is here provided with catalytic exhaust cleaning by the method known as SCR (selective catalytic reduction). This method involves supplying a urea solution to the exhaust gases in the diesel engine's exhaust line 3. The urea solution is stored in a tank 10 and is led to the exhaust line 3 via a line 11. A control unit 12, which may be a computer unit with suitable software, controls the supply of the urea solution by activation of a pump 13. The pump 13 conveys urea solution to an injection means 14 which injects urea solution in the exhaust line 3. The control unit 12 may use information concerning specific engine parameters to calculate the amount of urea solution which needs to be added to achieve optimum reduction of the proportion of nitrogen dioxide in the exhaust gases. The urea solution supplied is heated by the exhaust gases in the exhaust line 3 and is thereby vaporised and converted to ammonia. The mixture of ammonia and the exhaust gases is then led to an SCR catalyst 15. In the SCR catalyst 15, the nitrogen from the nitrogen oxides in the exhaust gases reacts with the nitrogen in ammonia to form nitrogen gas. The oxygen from the nitrogen oxides reacts with the hydrogen in the ammonia to form water. The nitrogen oxides in the exhaust gases are thus reduced in the SCR catalyst 15 to nitrogen gas and water vapour.

The nitrogen oxides ΝΟχ in the exhaust gases comprise nitrogen monoxide NO and nitrogen dioxide N0 ₂ . An SCR catalyst 15 reduces the amount of nitrogen oxides optimally when the exhaust gases led through the SCR catalyst 15 are at a temperature of about 300°C and contain equal amounts of nitrogen monoxide and nitrogen dioxide. An SCR catalyst thus reduces nitrogen oxides optimally when the proportion of nitrogen dioxide x is 50 % of the total amount of nitrogen oxides. However, exhaust gases from diesel engines 1 contain a significantly larger proportion of nitrogen monoxide than nitrogen dioxide. For this reason, an oxidation catalyst 16 is provided in the exhaust line 3 at a location upstream of the SCR catalyst 15. An oxidation catalyst 16 has the ability to oxidise nitrogen monoxide to nitrogen dioxide. The ability of the oxidation catalyst 16 to oxidise nitrogen monoxide to nitrogen dioxide depends on several operating parameters. The most important operating parameters are the exhaust flow q through the oxidation catalyst 16 and the temperature T of the exhaust gases in the oxidation catalyst 16.

A temperature sensor 18 is provided in the exhaust line 3 at a location immediately upstream of the oxidation catalyst 16. The temperature sensor 18 detects the temperature of the exhaust gases when they are led into the oxidation catalyst 16. This temperature corresponds substantially to the temperature T of the exhaust gases in the oxidation catalyst 16. In this case the diesel engine 1 is equipped with an inlet throttle 19. An inlet throttle 19 is a valve by which the supply of air to the diesel engine 1 can be regulated. The control unit 12 is adapted to controlling the inlet throttle 19. The control unit 12 is also adapted to controlling the inlet valve 9a and the outlet valve 9b of the respective cylinders 9 of the diesel engine 1. The turbine 4 is in this case a turbine with variable geometry. The control unit 12 is also adapted to controlling the flow through the turbine 4. The inlet line 6 comprises a flow sensor 20 which measures the air flow to the combustion engine 1. The air flow to the combustion engine 1 is related to the exhaust flow q in the exhaust line 3.

Fig. 2 is a flowchart illustrating how nitrogen oxides are removed from the exhaust gases by the arrangement depicted in Fig. 1. The process starts at 21. At 22, the control unit 12 receives information from the flow sensor 20 concerning the air flow in the inlet line 6 to the diesel engine 1. The control unit 12 uses this information to determine the exhaust flow q in the exhaust line 3. The control unit 12 receives information from the temperature sensor 18 concerning the temperature T of the exhaust gases before they are led into the oxidation catalyst 16. At 23, the control unit 12 has access to stored information concerning paired set-point values qB on the exhaust flow and set-point values T _B on the exhaust temperature, at which values the oxidation catalyst 16 oxidises nitrogen monoxide to nitrogen dioxide in such an amount as to result in the ideal distribution of nitrogen monoxide and nitrogen dioxide. At 24, the control unit 12 does a comparison to see whether the value received concerning the exhaust flow q and the value received concerning the exhaust temperature T correspond to any of the paired stored set-point values qe on the exhaust flow and set-point values TB on the exhaust temperature. If such is the case, the control unit 12 may find that the oxidation catalyst 16 oxidises nitrogen monoxide to nitrogen dioxide in such an amount that the ideal proportions of nitrogen monoxide and nitrogen dioxide are led to the SCR catalyst 15, in which case the latter effects optimum reduction of the nitrogen oxides in the exhaust gases. Thereafter the process starts again at 21.

If such is not the case, the control unit 12 finds that the oxidation catalyst 16 does not oxidise nitrogen monoxide to nitrogen dioxide N0 ₂ in such an amount that the ideal combination of nitrogen monoxide and nitrogen dioxide is led to the SCR catalyst 15. In this case, at 25, the control unit 12 controls for example the inlet throttle 19 to limit the air flow to the diesel engine 1 in such a way that the exhaust flow q is adjusted to the set-point value qe and the exhaust temperature T is adjusted to the set-point value T _B . As there is a relationship between the exhaust flow q and the exhaust temperature T, these two parameters assume new values when the air flow is altered. By suitable adjustment of the inlet throttle 19 so that said set-point values are achieved it is possible for an ideal combination of nitrogen monoxide and nitrogen dioxide to be led from the oxidation catalyst 16 to the SCR catalyst 15. The SCR catalyst 15 thus effects optimum reduction of the amount of nitrogen oxides in the exhaust gases at the prevailing exhaust temperature T. Thereafter the process starts again at 21.

Alternatively, or in combination, the control unit 12 may influence the opening and closing times of the inlet valves 9a and/or the exhaust valves 9b in order to adjust the exhaust flow to the set-point value qe and the exhaust temperature T to the set-point value TB. Alternatively, or in combination, the control unit 12 may also control the variable-geometry turbine 4. By such control the exhaust flow q may also be adjusted to the set-point value qs and the exhaust temperature T to the set-point value T _B . The control unit 12 may thus use one or more of said components 4, 9a, 9b, 19 to adjust the exhaust flow q and the exhaust temperature T to said set-point values q _B , Τβ.

Fig. 3 depicts an alternative arrangement for reducing the nitrogen oxides in the exhaust gases from a diesel engine 1. The same reference notations are used in Fig. 3 for similar components already depicted in Fig. 1. A nitrogen oxide sensor 26 is provided in the exhaust line 3 upstream of the oxidation catalyst 16. The nitrogen oxide sensor 26 is adapted to detecting the content of nitrogen oxides led to the oxidation catalyst 16. A nitrogen dioxide sensor 27 is provided in the exhaust line 3 downstream of the oxidation catalyst 16 and upstream of the catalyst 15. The nitrogen dioxide sensor 27 is adapted to detecting the content of nitrogen oxides in the exhaust gases after they have passed through the oxidation catalyst 16. In this embodiment, a flow sensor 20 is likewise used to measure the air flow to the combustion engine 1, and a temperature sensor 18 to detect the temperature T of the exhaust gases before they are led into the oxidation catalyst 16. This embodiment also comprises a turbine 4 with fixed geometry. Such a turbine 4 is provided in a conventional way with a waste gate 29. A waste gate 29 is a bypass line provided with a waste gate valve. A waste gate 29 is used to protect fixed-geometry turbines from overspeeding. An exhaust brake 30 in the form of a damper is provided in the exhaust line 3. A bypass line 31 with a valve 32 is situated close to the oxidation catalyst 16. Fig. 4 is a flowchart illustrating how nitrogen oxides can be removed from the exhaust gases in an optimum way by means of the arrangement in Fig. 3. The process starts at 33. At 34, the control unit 12 receives information from the nitrogen oxide sensor 26 concerning the content of nitrogen oxides in the exhaust gases led to the oxidation catalyst 16. At 35, the control unit 12 receives information from the nitrogen dioxide sensor 27 concerning the content of nitrogen dioxides in the exhaust gases led to the SCR catalyst 15. At 36, the control unit 12 calculates the proportion of nitrogen dioxide x out of the total amount of nitrogen oxides. At 37, the control unit 12 does a comparison to see whether the proportion x of nitrogen dioxide corresponds to a stored set-point value Χβ. For the SCR catalyst 15 to be able to effect optimum reduction of the nitrogen oxides, the proportion of nitrogen dioxide x should be about 50% of the total amount of nitrogen oxides. The set-point value x _B is therefore in this case 0.50. If the proportion of nitrogen dioxide x in the nitrogen oxides corresponds to the set- point value XB, the control unit 12 finds that the nitrogen oxides led to the SCR catalyst 15 contains the ideal proportion of nitrogen dioxide and that the nitrogen oxides in the exhaust gases will undergo optimum reduction in the SCR catalyst 15. Thereafter the process starts again at 33.

If the proportion of nitrogen dioxide x in the nitrogen oxides does not correspond to the set-point value x _B , the SCR catalyst 15 therefore does not receive nitrogen oxides which contain a desired proportion of nitrogen dioxide. In this case, at 38, the control unit 12 receives information from the flow sensor 20 concerning the exhaust flow q in the exhaust line 3 and information from the temperature sensor 18 concerning the temperature T of the exhaust gases when they are led into the oxidation catalyst 16. Here again the control unit 12 contains stored information concerning paired set-point values qB, TB on the exhaust flow and the exhaust temperature, at which values the oxidation catalyst 16 oxidises nitrogen monoxide to nitrogen dioxide in such an amount that the nitrogen oxides led to the SCR catalyst 15 contain the ideal proportion of nitrogen dioxide. At 39, the control unit 12 determines an appropriate such set- point value qe for the exhaust flow and an appropriate such set-point value TB for the temperature of the exhaust gases.

In this case, at 40, the control unit 12 controls for example the valve of said waste gate 29 in such a way that the exhaust flow q is adjusted to the set-point value qe and the exhaust temperature T is adjusted to the set-point value TB. The result is an exhaust flow and an exhaust temperature at which the oxidation catalyst 16 oxidises nitrogen monoxide to nitrogen dioxide in such an amount as to result in the ideal combination of nitrogen monoxide and nitrogen dioxide. Alternatively, or in combination, the control unit 12 may control the exhaust brake 30. The control unit 12 may use the exhaust brake to adjust the exhaust temperature T to a relatively great extent and the exhaust flow q to a smaller extent. In situations where the oxidation catalyst 16 oxidises too large an amount of nitrogen monoxide to nitrogen dioxide, the control unit 12 may open the valve 32 so that a certain amount of the exhaust gases is led through the bypass line 31 and therefore past the oxidation catalyst 16. The exhaust flow q through the oxidation catalyst 16 is thus reduced. The control unit 12 may therefore here again use one or more of the components 29, 30, 31, 32 to adjust the exhaust flow q and the exhaust temperature T to desired set-point values qe, TB at which the oxidation catalyst 16 delivers nitrogen oxides with the ideal proportion of nitrogen dioxide. The SCR catalyst 15 thereafter effects optimum reduction of the nitrogen oxides in the exhaust gases. The invention is not limited to the embodiment described above but may be varied freely within the scopes of the claims. One or more of the components 4, 9a, 9b, 19 used in the embodiment in Fig. 1 to adjust the exhaust flow q and the exhaust temperature T to desired set-point values qs, TB may of course be used in the embodiment in Fig. 3 for the same purposes. Similarly, one or more of the

components 20, 29, 30, 31, 32 used in the embodiment in Fig. 3 to adjust the exhaust flow q and the exhaust temperature T to desired set-point values qe, TB may of course be used in the embodiment in Fig. 1. In most circumstances it is ideal that the nitrogen oxides led to catalysts contain 50% nitrogen monoxide and 50% nitrogen dioxide. In certain operating conditions, deviations from this distribution may be advantageous.