FIELD OF THE INVENTION

The invention relates to an exhaust post-treatment device for a vehicle according to the preamble of claim 1 and a method for post-treatment of exhaust gases from vehicles according to the preamble of claim 13. BACKGROUND TO THE INVENTION AND PRIOR ART

In diesel vehicles, particularly in heavy vehicles such as heavy trucks, the SCR method, i.e. a method for selective reduction of nitrogen oxides, is used to clean the vehicle's exhaust gases so that those released into the surroundings do not contain high contents of nitrogen oxides.

In the SCR method, a liquid urea solution injected into the exhaust pipe may be used as NO _x reducing agent.

Using liquid urea solution may entail problems at low exhaust temperatures in that the reducing agent injected does not become vaporised quickly enough, with consequent formation in the exhaust system of deposits which may lead to pressure drop, a problem which is solved by maintaining such a high exhaust temperature that the reducing agent injected becomes vaporised in the exhaust pipe. EP 2 078 834 A refers to a method and a system for cleaning of exhaust gases at such a high exhaust temperature that an additive becomes gasified such that the exhaust gases are warmed to shorten the time required before injection of liquid additive can commence, which involves using a great deal of energy. US 2005/0204733 Al refers to a device in which a thermoelectric element is used to generate electrical energy.

In the SCR method, gaseous ammonia injected into the exhaust pipe may also be used as reducing agent. Using a gaseous reducing agent reduces problems due to deposits of reducing agent in the exhaust pipe, since they only occur if the exhaust temperature before the exhaust gases reach the catalyst falls so low that the reducing agent is converted from gas to liquid or solid. DE 10 2007 058 768 Al refers to a device and a method for post-treatment of exhaust gases whereby gaseous reducing agent, e.g. ammonia, is injected into the exhaust pipe, and energy present in the exhaust gases is used to vaporise the reducing agent before it is injected in the exhaust pipe. SUMMARY OF THE INVENTION

The object of the present invention is to propose an exhaust post-treatment device for a vehicle and a method for post-treatment of exhaust gases from vehicles, which device can operate even if the temperature of the exhaust gases is low.

The above object is achieved according to the invention by an exhaust post-treatment device according to the characterising part of claim 1 and a method for post-treatment of exhaust gases from vehicles according to the characterising part of claim 13. The fact that an exhaust post-treatment device for a vehicle which is adapted to reducing nitrogen oxides present in the vehicle's exhaust gases by supplying liquid reducing agent to the exhaust gases in an exhaust pipe according to the characterising part of claim 1 presents the characteristic that the device comprises a Peltier element and is adapted to locally warming a surface within the exhaust pipe by means of a Peltier element by using thermal energy from the exhaust gases to vaporise liquid reducing agent which reaches the surface, and the fact that a method for post- treatment of exhaust gases from vehicles by supplying liquid reducing agent to the exhaust gases in an exhaust pipe in order to reduce nitrogen oxides present in them, according to the characterising part of claim 13, presents the characteristic of locally warming a surface within the exhaust pipe with an exhaust post-treatment device comprising a Peltier element by using thermal energy from the exhaust gases to vaporise liquid reducing agent which reaches the surface, achieve the advantage that formation of deposits of reducing agent within the exhaust pipe is thereby avoided even if the temperature of the exhaust gases is low. A further advantage is that the Peltier element has no movable parts and a consequently robust configuration. According to an embodiment of the invention, one side of the Peltier element is adapted to diverting thermal energy from the exhaust gases to the Peltier element by said energy being absorbed on one side of the Peltier element, and the other side of the Peltier element is adapted to locally warming a surface within the exhaust pipe by thermal energy being delivered from the Peltier element to the surface within the exhaust pipe, thereby achieving the advantage that the exhaust post-treatment device needs only moderate amounts of energy for its operation.

According to a further embodiment of the invention, the Peltier element comprises at least two electrodes made of different semiconductive metals such that every second electrode is of N type and the alternate electrodes are of P type, which electrodes are arranged to extend from one side of the Peltier element to the other side of the Peltier element and are connected in series via conductors to a direct-current source, such that closing the electric circuit causes electrons in the N type electrode to move in the opposite direction to the current, and holes in the P type electrode to move in the direction of the current, both of them diverting heat from one side of the Peltier element to the other side of the Peltier element, thereby achieving the advantage that the configuration of the exhaust post-treatment device is adaptable, e.g.

depending on the desired effect of the Peltier element. According to further embodiments of the invention, the thermal energy absorption side of the Peltier element may be situated at least partly within the exhaust pipe or adjacent to the outside of the exhaust pipe, the thermal energy absorption side of the Peltier element may be situated downstream or upstream of the supply device for supply of liquid reducing agent to the exhaust gases in the exhaust pipe, as seen in their direction of flow, the thermal energy delivery side of the Peltier element may be situated adjacent to the outside of the exhaust pipe or at least partly within the exhaust pipe, the surface which is locally warmed within the exhaust pipe may be a surface of the thermal energy delivery side of the Peltier element which is situated within the exhaust pipe, or a surface on the inside of the exhaust pipe, a catalyst may be provided in the exhaust pipe downstream of the supply device for the liquid reducing agent, as seen in the direction of flow of the exhaust gases, and a control unit may be provided to control the Peltier element and the supply of liquid reducing agent, thereby achieving the advantage that the configuration of the exhaust post-treatment device is adaptable, e.g.

depending on whether high efficiency or simple configuration and easy assembly is prioritised. According to a further embodiment of the invention, the thermal energy absorption side of the Peltier element may be warmed by the exhaust gases,

direct current may be applied across the Peltier element, and

thermal energy may be delivered to the surface within the exhaust pipe from the thermal energy delivery side of the Peltier element,

thereby achieving the advantage that the exhaust post-treatment device needs only moderate amounts of energy for its operation. BRIEF LIST OF DRAWINGS

The invention is explained below in more detail with reference to the attached drawings, in which Fig. 1 depicts schematically an exhaust post-treatment device according to a first embodiment of the invention,

Fig. 2 depicts schematically an exhaust post-treatment device according to a second embodiment of the invention,

Fig. 3 depicts schematically an exhaust post-treatment device according to a third

embodiment of the invention,

Fig. 4 depicts schematically an exhaust post-treatment device according to a fourth embodiment of the invention,

Fig. 5 is a schematic flowchart of a method according to an embodiment of the invention, and

Fig. 6 is a schematic flowchart of a method according to a further embodiment of the invention. DESCRIPTION OF PREFERRED EMBODIMENTS OF THE INVENTION

Fig. 1 depicts schematically an exhaust post-treatment device 2 for a vehicle 4 according to a first embodiment of the invention, which device 2 comprises a Peltier element 6 also called a thermoelectric cooling element. The Peltier element 6 itself comprises at least one

"thermocouple" composed of two electrodes 8, 10 made of different semiconductive metals such that every second electrode 8 is of N type and the alternate electrodes 10 are of P type, which electrodes 8, 10 are arranged to extend from one side 28 of the Peltier element 6 to the other side 30 of the Peltier element 6 and are connected in series via conductors 12, preferably made of copper, to a direct-current source 14, such that closing the electric circuit causes electrons in the N type electrode to move in an opposite direction to the current, and holes in the P type electrode to move in the direction of the current, as illustrated in the diagram, both of them diverting heat from one side 28 of the Peltier element 6 to the other side 30 of the Peltier element 6. Applying a direct current to the electrodes 8, 10 in the direction of the arrows 22, 24, 26 causes thermoelectric effects in the Peltier element 6 which result in thermal energy being absorbed on one side 28 of the Peltier element 6 and being delivered on the other side 30 of the Peltier element 6, i.e. the Peltier element 6 has a heat absorption side 28 and a heat delivery side 30. The exhaust post-treatment device 2 comprises also a supply device 16 for supply to the exhaust gases 20 of liquid reducing agent 18 which is intended for reduction of nitrogen oxides present in the exhaust gases 20.

Connecting in series a number of "thermocouples" in a chain of alternate N type and P type electrodes results in a more powerful Peltier element.

The Peltier element 6 is thus adapted to diverting thermal energy from the exhaust gases 20 and to thereby locally warming a surface 31 which according to this embodiment is part of the inside wall of an exhaust pipe 32 by diverting thermal energy from the Peltier element 6 to the surface 31 within the exhaust pipe 32.

Thus the exhaust post-treatment device 2 is adapted to reducing nitrogen oxides present in the exhaust gases 20 of the vehicle 4 by supplying liquid reducing agent 18 to the exhaust gases 20 in an exhaust pipe 32, and to locally warming a surface 31 within the exhaust pipe 32 by means of a Peltier element 6 by using thermal energy from the exhaust gases 20 to vaporise liquid reducing agent 18 which reaches the surface 31, and thereby prevent formation of deposits of reducing agent within the exhaust pipe. Using a heat pump as above makes it possible to increase the temperature locally on the surface 31 within the exhaust pipe 32 where liquid reducing agent 18 is supplied, e.g. by being injected, so that vaporisation of the reducing agent 18 can take place.

The liquid reducing agent 18 is an NO _x reducing agent, e.g. a liquid NO _x reducing agent such as a liquid urea solution, e.g. the additive marketed under the trade name "Ad Blue" which contains 32.5 wt% of urea in distilled/deionised water.

Where a liquid urea solution is used, the surface 31 in the exhaust pipe 32 needs to be kept at a temperature of at least about 200°C for vaporisation to be assured.

Supplying a liquid urea solution in the exhaust pipe with exhaust gases at temperatures over 180°C causes the water to boil away, followed by the urea melting and becoming vaporised by so-called thermolysis. This stage involves formation inter alia of ammonia by the reaction (NH2)2CO => NH3 + HNCO.

After the thermolysis of the urea, hydrolysis takes place in a downstream catalyst or at a temperature over 400°C by the reaction

HNCO + H20 => NH3 + C02. NH3 reduces ΝΟχ by the reaction

NH3 + ΝΟχ => N2 + H20 (unbalanced formula in that x may have various different values).

Recondensation of ammonia takes place at about -30°C, so this is not a major problem in a vehicle exhaust pipe during operation.

At least one control unit 34 is preferably provided to control the Peltier element 6 and the supply of liquid reducing agent 18, depending on which reducing agent is used and on the temperature of the exhaust gases in the exhaust pipe 32, so that these activities only take place when necessary. If the Peltier element 6 and the supply of liquid reducing agent 18 are controlled by separate control units, the latter have to be synchronised such that the control unit for controlling the Peltier element 6 preferably serves as a "slave unit" to the control unit for controlling the supply of liquid reducing agent 18. If the exhaust gases 20 are very cold, e.g. just after a cold start at very low outside temperatures, it may be impossible for the reducing agent to become gasified despite local warming of a surface 31 in the exhaust pipe 32 by the Peltier element 6, in which case the control unit 34 will block the supply of liquid reducing agent 18 in the exhaust pipe 32. Similarly, if the exhaust gases 20 are very warm, i.e. warmer than 400°C, the liquid reducing agent 18 will become gasified without need for a surface 31 in the exhaust pipe 32 to be warmed locally as above, in which case the control unit 34 will block operation of the Peltier element 6. The control unit 34 will ensure that reducing agent 18 is supplied to the exhaust gases 20 and that the Peltier element 6 is in operation if the exhaust temperature is so high that the reducing agent 18 which does not reach the surface 31 within the exhaust pipe 32 can become gasified, but not so high that the reducing agent 18 which reaches, and thereby cools, a surface 31 within the exhaust pipe 32 also becomes gasified. This temperature range may be set by the control unit 34, depending on which reducing agent is used. The exhaust temperature has to be high enough for the reducing agent 18 which does not reach a surface 31 within the exhaust pipe 32 to become gasified, which temperature is different for different reducing agents 18. The exhaust temperature required for liquid NO _x reducing agent which reaches and thereby cools a surface 31 within the exhaust pipe 32 to nevertheless become gasified depends on various factors such as the temperature of the liquid reducing agent at the time of supply, the dimensions of the exhaust pipe, the velocity of the exhaust flow, the size of the area reached by liquid reducing agent, etc. The exhaust gases may normally be at a temperature of between about 200°C and about 700°C during operation.

It is also possible to use the exhaust post-treatment device without a control unit, but in that case deposits will form on the inside surface of the exhaust pipe and subsequently burn away when the temperature rises, and the Peltier element will be run even when not required for the gasification of urea, so a device with no control unit is not advisable.

According to this embodiment, the heat absorption side 28 of the Peltier element 6 is situated within an exhaust pipe 32, so warm exhaust gases 20 pass a surface 36 of the heat absorption side 28 of the Peltier element 6, leading to warming of the heat absorption side 28 of the Peltier element 6. The exhaust post-treatment device 2 according to this embodiment works according to the following principle during operation of the Peltier element 6, based on the example of using a liquid urea solution as reducing agent.

The heat delivery side 30 of the Peltier element 6 according to this embodiment is situated adjacent to the wall of the exhaust pipe 32 where there is risk of deposits, i.e. preferably adjacent to the region where the liquid reducing agent 18 from the supply device 16 reaches the inside surface of the exhaust pipe, e.g. centrally to the supply device 16 for liquid reducing agent, where the heat delivery side 30 of the Peltier element 6 delivers thermal energy to a surface 31 within the exhaust pipe 32, in this case to the wall of the exhaust pipe, and thereby locally warms the inside surface of the exhaust pipe as a result of the heat delivery side 30 of the Peltier element 6 being in thermal contact with the exhaust pipe 32. The aforesaid method makes it possible for the thermal energy present in the exhaust gases to be utilised, despite the exhaust temperature being only moderate, e.g. about 100-300°C, to substantially completely gasify the reducing agent 18. Without the local warming described above of a surface 31 within the exhaust pipe 32, moderate exhaust temperatures might cause parts of the reducing agent reaching this surface 31 to solidify, potentially causing deposits and consequent pressure drop in the exhaust system. Increasing pressure drop may preferably be used to detect build-up of deposits. Such deposits certainly burn away if the exhaust temperature thereafter becomes very high, but on long runs at low load and steady speed, e.g. when driving an unladen vehicle combination in flat terrain, the exhaust temperature will remain at low levels for long periods, potentially leading to deposits. Warming instead the whole exhaust flow, e.g. by electricity or diesel fuel or heat pump, would consume a great deal more energy than the aforesaid utilisation of the thermal energy present in the exhaust gases.

Preferably, a catalyst 38 is provided in the exhaust pipe 32 downstream of the supply device 16 for the liquid reducing agent 18, to ensure that not only thermolysis of urea but also hydrolysis as above takes place. The diagram shows the catalyst 38 upstream of the heat absorption side 28 of the Peltier element 6 but it may also be situated downstream of the heat absorption side 28 of the Peltier element 6, with respect in either case to the exhaust flow direction A in the exhaust pipe 32. Alternatively, the exhaust post-treatment device may be used with no catalyst, but this is not advisable because it might lead to periodic build-up of deposits.

Fig. 2 depicts schematically an exhaust post-treatment device 2 for a vehicle 4 according to a second embodiment of the invention, which device 2 comprises a Peltier element 6. The device 2 comprises also a supply device 16 for supply to the exhaust gases 20 of liquid reducing agent 18 intended for reduction of nitrogen oxides present in them.

The embodiment depicted in Fig. 2 differs from that described with reference to Fig. 1 in that in this case the heat absorption side 28 of the Peltier element 6 is adjacent to the outside of an exhaust pipe 32, and that a possible catalyst 38 is provided downstream of the heat absorption side 28 of the Peltier element 6. The catalyst 38 may however also be situated upstream of the heat absorption side 28 of the Peltier element 6, as in Fig. 1, or alternatively the exhaust post-treatment device may be used with no catalyst. In other respects, the description with respect to Fig. 1 applies also, e.g. as regards a possible control unit, in Fig. 2.

Fig. 3 depicts schematically an exhaust post-treatment device 2 for a vehicle 4 according to a third embodiment of the invention, which device 2 comprises a Peltier element 6. The device 2 likewise comprises a supply device 16 for supply to the exhaust gases 20 of liquid reducing agent 18 intended for reduction of nitrogen oxides present in them.

The embodiment depicted in Fig. 3 differs from that described with respect to Fig. 1 in that in this case the heat absorption side 28 of the Peltier element 6 is upstream of the supply device 16 for supply of liquid reducing agent 18 to the exhaust gases 20. In other respects, the description with respect to Fig. 1 applies also, e.g. as regards a possible control unit, to the embodiment depicted in Fig. 2 but any catalyst 38 has to be situated downstream of the supply device 16 for supply of liquid reducing agent 18 to the exhaust gases 20.

Fig. 4 depicts schematically an exhaust post-treatment device 2 for a vehicle 4 according to a fourth embodiment of the invention, which device 2 comprises a Peltier element 6. The device 2 likewise comprises a supply device 16 for supply to the exhaust gases 20 of liquid reducing agent 18 intended for reduction of nitrogen oxides present in them. The embodiment depicted in Fig. 4 differs from that described with respect to Fig. 1 in that in this case the heat absorption side 28 of the Peltier element 6 is upstream of its heat delivery side 30 and is adjacent to the outside of an exhaust pipe 32. This embodiment further differs from that described with respect to Fig. 1 in that the heat delivery side 30 of the Peltier element 6 is at least partly situated within an exhaust pipe 32 so that it has within the exhaust pipe 32 a surface 31 which is warmed locally. In other respects, the description with respect to Fig. 1 applies also, e.g. as regards a possible control unit, to the embodiment depicted in Fig. 2 but any catalyst 38 has to be situated downstream of the supply device 16 for supply of liquid reducing agent 18 to the exhaust gases 20.

The liquid reducing agent 18 is preferably kept warm enough not to freeze, which in the case of "Ad Blue" would take place at about -11°C.

Figures 5 and 6 illustrate examples of the method for post-treatment of exhaust gases.

Fig. 5 is a schematic flowchart of a method according to an embodiment of the invention, and the reference notations 2, 4, 18, 20, 31, 32 used here are those of Figures 1-4. The method for post-treatment of exhaust gases from vehicles by 100 supplying liquid reducing agent 18 to the exhaust gases 20 in an exhaust pipe 32 in order thereby to reduce nitrogen oxides present in the exhaust gases 20 of the vehicle 4 comprises also the step of 200 locally warming a surface 31 within the exhaust pipe 32 with an exhaust post-treatment device 2 comprising a Peltier element 6 by using thermal energy from the exhaust gases 20 to vaporise liquid reducing agent 18 which reaches the surface 31, and thereby prevent formation of deposits of reducing agent within the exhaust pipe.

Fig. 6 is a schematic flowchart of a method according to a further embodiment of the invention, and the reference notations 6, 20, 28, 30, 31, 32 used here are those of Figures 1-4. The embodiment in Fig. 6 comprises not only the steps of Fig. 5 but also those of using the exhaust gases 20 to warm 110 the heat absorption side 28 of the Peltier element 6,

120 applying a direct current across the Peltier element 6, and

200 delivering thermal energy to the surface 31 within the exhaust pipe 32 from the heat delivery side 30 of the Peltier element 6. The invention is in no way restricted to the embodiments described but may be varied freely within the scopes of the claims. Parts from the various embodiments may be combined. The heat absorption side 28 of the Peltier element 6 may thus be situated adjacent to the outside of the exhaust pipe 32 or at least partly within the exhaust pipe 32. The absorption side 28 of the Peltier element 6 may thus be situated either downstream or upstream of the supply device 16 for supply of liquid reducing agent 18 to the exhaust gases 20 in the exhaust pipe 32, as seen in their direction of flow, but a downstream location is advisable purely functionally in that having the heat absorption side 28 of the Peltier element 6 downstream of the supply device 16 for supply of liquid reducing agent 18 means that energy is only absorbed from the exhaust gases 20 after the supply of liquid reducing agent 18 to the exhaust gases 20, thereby avoiding a corresponding lowering of the exhaust temperature before supply of liquid reducing agent 18, which would be disadvantageous for its vaporisation. The heat delivery side 30 of the Peltier element 6 might thus be situated either adjacent to the outside of the exhaust pipe 32 or at least partly within the exhaust pipe 32. The surface 31 which is warmed might thus be part of the inside of the exhaust pipe 32 or be a surface of the heat delivery side 30 of the Peltier element 6 which is situated within the exhaust pipe 32 or be some other surface within the exhaust pipe which is in thermally conductive contact with the heat delivery side 30 of the Peltier element 6.