Device for clearing a supply pipe of a reducing agent that has previously gone from a first phase to a second phase then back again

Abstract

In a device for keeping a valve controlled flow passage for supplying a reduction agent to a catalytic converter in an exhaust system of a diesel engine, wherein the reduction agent is gaseous above a certain temperature but solid below this temperature, the valve being a control valve for dosing the flow of the gaseous reduction agent and including a valve seat and a movable valve control surface, the valve surface is formed by a controllable element which can be energized selectively to be cooled or heated so that, with the valve closed and the valve surface cooled to form a condenser element gaseous reduction agent in the flow passage is solidified at the valve surface but, with the valve surface heated is readily again decomposed so as to become gaseous. As the cooling of the valve surface forms adjacent the cooled valve surface a cold trap the resolidifed reduction agent precursor formed at the valve surface where it can easily be again decomposed by the heating of the valve surface.

Description

This is a continuation-in-part application of international application PCT/IB2004/050469 filed Apr. 19, 2004 and claiming the priority of German application 203 08 348.2 filed May 26, 2003.

BACKGROUND OF THE INVENTION

The invention resides in a device for cleaning a supply pipe of a reducing agent which is used in the exhaust system of a diesel engine including an SCR catalytic converter.

In addition to carbon monoxide (CO), particles and hydrocarbons (HC) in particular nitrogen oxides (NO_x) are the primary pollutants directly emitted in the operation of internal combustion engines, particularly diesel engines. Three-way catalytic converters, as they are used in gasoline and gas engines, can not be used with diesel engines because of the excess of oxygen in the diesel engine exhaust gas. For this reason, Selective Catalytic Reduction, SCR catalytic converters, which operate selectively, have been developed for the reduction of the nitrogen oxide emissions of diesel engines have been developed wherein the nitrogen oxide emissions often are reduced to N₂ and H₂O with the aid of added reduction agents, namely ammonia (NH₃).

A device for supplying gaseous ammonia to the exhaust duct of an internal combustion engine of a motor vehicle is known from DE 197 20 209 C1. This device comprises a gas tight and pressure resistant converter which includes a thermolytically NH₃ releasing compound or a thermolytically NH₃ releasing material mixture, a so-called NH₃ precursor. A NH₃ precursor, for example ammonium carbonate, may be provided which is a solid material. The converter is connected to the exhaust duct of a diesel engine by way of a supply line which joins the exhaust duct in the flow direction of the exhaust gas ahead of the SCR catalytic converter. As a dosing device, a clocked valve, is provided which is operated by a control unit in such a way that the NH₃ is introduced into the exhaust gas flow in the required amount. The converter consists essentially of a pressure resistant reaction container which is surrounded by a heating device in the form of heater pipes. The heating device is connected to the cooling water circuit of the diesel engine via a supply and a return line.

Upon heating of the ammonium carbonate, which may for example be used as the NH₃ precursor the ammonium carbonate breaks down into NH₃ and CO₂, whereby the reduction agent is converted from a first solid phase to a second gaseous phase. The gas mixture is collected in the pressure resistant reactor container until a corresponding internal pressure has built up. When a certain internal pressure has been reached in the reaction container, an equilibrium state is reached so that no additional ammonium carbonate is decomposed. Under the engine operating conditions wherein the engine cooling water flowing through the heating device normally has a temperature of between 800 and 100° C., the pressure in the reaction container in the equilibrium state is about 3-4 bar. In order to provide a sufficient NH₃ amount for introduction into the exhaust gas flow during dynamic operation of the diesel engine, the converter also serves as a reaction gas storage device providing for a certain reserve volume for the reaction gas or, respectively the NH3 reduction agent contained therein.

The reaction container is generally arranged at a certain distance from the exhaust duct. It has been found that at lower temperatures the reaction gas mixture may recombine in the supply pipe to the exhaust duct and return to the solid state depending on the type of precursor used. In the most disadvantageous case this may result in a blockage of the supply pipe. The same applies for the control valves arranged in the supply pipe, such as the clocked valve used as a dosing device. However, the clogging of the supply line and particularly of the clocked valve disposed therein driving shut down of the internal combustion engine is not particularly problematic in connection with same reduction agents such as, for example, with ammonium carbonate. Since the NH₃ precursor deposited in the supply line, and particularly in the valve or valves, is again decomposed when the engine is again operated and temperatures rise again so that the supply line and the valve or valves are again freed. In order to render the supply line more rapidly operational, it has already been proposed to heat the supply line and the valves disposed therein. However, depending on the distance of the reaction container from the exhaust duct, the realization of such a heating concept concerning the whole supply line including the clocked valves disposed therein is quite expensive.

It is therefore the object of the present invention to provide an arrangement with which the problem of the NH₃ precursor deposition in the supply line can be counteracted in a relatively simple way.

SUMMARY OF THE INVENTION

In a device for keeping a valve controlled flow passage for supplying a reduction agent to a catalytic converter in an exhaust system of a diesel engine, wherein the reduction agent is gaseous above a certain temperature but solid below this temperature, the valve being a control valve for dosing the flow of the gaseous reduction agent and including a valve seat and a movable valve control surface. The valve surface is formed by a controllable element which can be energized selectively to be cooled or heated so that, with the valve closed and the valve surface cooled to form a condenser element gaseous reduction agent in the flow passage is solidified at the valve surface but, with the valve surface heated is readily again decomposed so as to become gaseous. As the cooling of the valve surface forms adjacent the cooled valve surface a cold trap, the re-solidified reduction agent precursor formed at the valve surface valve can easily be again decomposed by the heating of the valve surface.

This device in the form of a dosing valve comprises a condenser element forming a cold trap. The condenser element is connected to a control unit and is operated as such with a corresponding control when the internal combustion engine is shut down and/or when the valve is closed. The switching on of the condenser element may also depend on other operating parameters, for example, the momentary temperature of the valve and/or of the supply line. Upon operation of the condenser element, its temperature is reduced to form the cold trap with the result that reduction compounds which are in the second phase (gaseous) are returned around the condenser element to the first phase (solid), for example with the use of ammonium carbonate or to an intermediate phase. In this process the reduction agents which are present in the second phase also in the area of the cold trap are pulled back into the cold trap and also converted into the first phase or an intermediate phase. By this effect reduction agents still present in the second phase are removed from the supply line and any valves included therein or in clutches or similar compounds by the cold trap formed by the condenser element as reduction agents which are contained at predetermined locations in the supply duct and are still in the second phase are collected and regenerated. With a renewed use of the supply line for supplying reduction agents to the SCR catalytic converter arranged in the exhaust duct of the internal combustion engine then first or intermediate phase reduction agents deposited only in the area of the condenser element need to be removed by an appropriate heating. For this reason, the valve also includes a heating device so that also the reduction agent deposited in the valve by the activation of the conductor element can be reconverted to the second phase and supplied to the SCR catalytic converter disposed in the exhaust duct of the internal combustion engine.

The condenser element is arranged expediently either at a valve seat or at the movable valve which cooperates with the respective valve seat. The heating device is either assigned to the same valve element or the respective complementary valve element. The advantage of this device is that it can at the same time act as a dosing valve. Preferred is an embodiment wherein the movable valve area of the valve is in the form of a heatable condenser element so that this part of the valve can form the cold trap as well as the reconversion means of the reduction agents. Expediently, one or several Peltier elements are used for this purpose which, dependent on how they are controlled, operate either to cool or to heat. It is advantageous in this connection if the movable valve component itself represents the heat exchange surface of the Peltier element or elements.

An embodiment wherein the stationary valve seat includes two adjacent connecting openings which can be jointly closed by the movable valve structure. The valve structure which is effective as a condenser element and which closes both connecting openings when the valve is closed results in the formation of a cold trap for each connecting line whereby reduction agents present in the respective connecting lines in the second phases are particularly effectively withdrawn. One of these connecting lines is connected to a storage container including the reduction agent or a reduction agent precursor whereas the other connecting line of the valve is connected to the exhaust duct.

The arrangement described above is particularly suitable for installation in a supply line through which the gaseous reduction agent flows which has been generated from a solid NH₃ precursor by thermolytic decomposition. Ammonium carbonate is particularly suitable as such a precursor. The arrangement is also usable in connection with a system wherein urea is used as NH₃ precursor.

Below is an embodiment of the invention which will be described with reference to the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows schematically an arrangement for supplying ammonia as a reduction agent to an SCR catalytic converter disposed in the exhaust duct of an internal combustion engine including a valve disposed in the supply line and shown in an open position; and,

FIG. 2 shows the valve in a closed position.

DESCRIPTION OF A PREFERRED EMBODIMENT

A device for supplying ammonia (NH₃) to a reduction catalytic converter disposed in the exhaust duct of a diesel engine of a motor vehicle is indicated in FIG. 1 overall by the reference numeral 1. The device 1 comprises a container 2 for generating ammonia (NH₃) by thermolytic decomposition of an NH₃ precursor, wherein a pressed body 3 of ammonium carbonate is used in the shown embodiment as NH₃ precursor. The ammonium carbonate body 3 is cylindrical and is formed to the shown shape by compressing powdered ammonium carbonate. The pressed ammonium carbonate body 3 has a round cross-sectional area. The container 2 includes a heating device which is designated by the reference numeral 4. The heating device 4 is in the form of a radiation heater.

The radiation heater is formed in the shown exemplary embodiment by three heating elements 5, 5′ 5″ which are each in the form of an Archimedes spiral and banked relative to one another. The individual heating elements 5, 5′, 5″ of the heating device 4 are controllable independently of one another so that the heating energy generated and/or also the course of a heating phase can be controlled. The heating device 4 is arranged in a lower part of the container 2. Below the heating elements 5, 5′ 5″ a heat barrier 6 is arranged. Above the radiation heater and at a small distance therefrom there is a heating plate 7 which forms another part of the heating device 4. The heating plate 7 consists in the shown embodiment of a transparent glass-ceramic material through which the heat radiation generated by the heating device 4 can pass. The side of the heating plate 7 facing the pressed ammonium carbonate body 3 is surface structured with knobs. At its lower surface facing the container bottom, the heating plate 7 is provided with a tube 8, which forms a connecting channel 9. The connecting channel 9 is connected to a supply line which is generally designated by the reference numeral 10.

FIG. 1 shows the device 1 and, in particular, the container 2 with the pressed ammonium carbonate body 3 disposed therein before the initial operation thereof but with the supply line 10 already open. At this point, the lower surface of the pressed ammonium carbonate body 3 is disposed on top of the knob structure of the heating plate 7. With the first operation of the device 1, upon energizing the heating device 4, a certain amount of the ammonium carbonate body 3 is thermolytically decomposed. As a result, the knobs extend onto the ammonium carbonate body 3 and fix the body 3 in place.

The heating device is so designed that at the side of the heating plate 7 facing the ammonium carbonate body 3 the maximum temperatures generated are lower than the decomposition temperature of the reaction gas or gas mixture formed during the thermolysis of the ammonium carbonate. This is achieved to a large extent by the glass-ceramic heating plate 7. It allows the heating device 4 to combine the advantages of a radiation heater with regard to a rapid reaction capability and the incidental spontaneous decomposition of ammonium carbonate with the advantages of heating devices with heat transfer by direct contact.

The container 11 is provided with a rolling sleeve piston 11 which divides the interior of the container into a first container section and a second container section. By the rolling sleeve piston 11, the two container sections are separated in a gas-tight manner. The first container section contains the heating device 4 and the pressed ammonium carbonate body 3 as well as the outlet formed by the connecting channel 9. The second container section is connected to a compressed air system D. By way of the compressed air system which also includes a compressor providing the needed compressed air to the system a pressure is established in the second section as it is needed for establishing the desired engagement pressure between the pressed ammonium carbonate body 3 and the upper side of the heating plate 7.

In the container 2, the solid pressed ammonium carbonate body 3 is stored as NH₃ precursor. Upon energization of the heating device 4, the ammonium carbonate disposed on the heating plate 7 is decomposed so that the NH₃ precursor is converted to the second phase—the gaseous phase. The gaseous reduction agent is supplied via the connecting channel 9 and the supply line 10 to the exhaust duct as, respectively, the SCR catalytic converter.

The supply line 10 includes a valve 12 which is shown in FIG. 1 in an open position. The valve 12 comprises as movable element a Peltier element 13 which can be moved by an activating member 14 to the open position. The activating number 14 may be for example an electromagnet. The valve can be moved into the closed position as shown in FIG. 2 by the Peltier element 13 using the energy stored in the compression spring 15. The surface 16 of the Peltier element 13 forms the movable valve area and consequently the control element of the valve 12. A membrane 17 circumferentially seals the valving chamber 18. In the open position of the valve 12, the reaction gas generated in the container 2 by the thermolysis flows through the valving chamber 18 into the supply line 10 via the valve 12 which is in communication with the exhaust duct in order to supply the gaseous reduction agent to the SCR catalytic converter. When the valve 12 is closed, as shown in FIG. 2, the Peltier element is at the same time energized for cooling the heat exchange surface 16 thereof. By the abutment of the surface 16 of the Peltier element 13 at the valve opening for the connecting channel 9 and the channel extending from the valve 12 to the exhaust duct with both valve openings being sealed by seals 19 with respect to the surface 16, the channels adjacent the valve 12 form cold traps so that reduction agent gas still present in the two channels is drawn to the cold surface 16 closing the channels. As a result the channels 10 are kept open as the chances of a reformation of ammonium carbonate from the gaseous reaction agent in the supply line 10 is avoided to a large degree. Upon opening the valve 12 or shortly before opening, the Peltier element is so energized that it becomes a heating element whereby the ammonium carbonate deposited on the surface 16 during cooling thereof is rapidly decomposed and the valve becomes operative for controlling the gaseous reaction agent flow.

In the embodiment described in the Figures, the two flow opening of the valve 12 with the seats 19 are arranged side-by-side. Both openings are therefore closed at the same time by the surface 16 of the Peltier element 13 which serves as a valve control surface. The Peltier element 13 is energized for cooling expediently only shortly after the valve 12 has been closed.

Ammonium carbonate is the preferred NH₃ precursor material for operating the device as described. It is advantageous in connection with the use of ammonium carbonate that its thermolytic decomposition begins to a large extent already at temperatures above 70° C. This relatively low temperature has the advantage that the ammonium carbonate deposited on the valve surface 16 during cooling thereof can again be decomposed with relatively little energy input for freeing the valve from the ammonium carbonate deposited and reformed on the surface 16 during cooling as the ammonium carbonate is again rapidly decomposed.

Claims

1. A device for keeping a valve controlled flow passage supplying a reduction agent to a catalytic converter in an exhaust system of a diesel internal combustion engine, said reduction agent being gaseous above a certain temperature, but solid below said certain temperature, said valve (12) being a control valve for dosing the flow of the gaseous reduction agent to the catalytic converter, said control valve including a valve seat (19) and a movable valve surface (16) which abuts the valve seat when the valve is closed, said valve surface (16) being formed by a controllable element which, when energized is selectively cooled or heated so that, upon energization of the controllable element with the valve closed and said valve surface being cooled, a cold trap structure is formed in the flow passage adjacent the valve surface (16) for solidifying gaseous reduction agent present in the flow passage on the cold valve surface and, upon subsequent energization of the controllable element, the solidified reduction agent is again converted to a gaseous phase, and a control unit for selectively controlling the energization of the controllable element for cooling or heating the controllable element.

2. A device according to claim 1, wherein the movable valve surface (16) is in the form of a heatable condensation element (13).

3. A device according to claim 1, wherein the valve surface (16) is formed by a heat exchange surface of at least one Peltier element (13).

4. A device according to claim 1, wherein the valve (12) includes a stationary valve seat provided with two adjacent connecting openings, of which are in communication of a container (2) in which the reduction agent or a reduction agent precursor is stored and the other connecting opening is in communication with exhaust duct of the diesel engine.

5. A device according to claim 4, wherein the closed portion of the valve (12) both connecting openings of the valve seat are covered by the valve surface (16).

6. A device according to claim 5, wherein the container (2) which is in communication with one of the connecting openings of the valve (12) includes a precursor (3) compound which, when heated decomposes to form the reduction agent or reduction agent mixture including NH3 and which, with a reduction of the temperature thereof is reformed to a solid state, said container (2) including a heating structure (4) for causing the thermolytic decomposition of the precursor compound wherein NH3 is formed for supply to the catalytic converter in the exhaust system.

7. A device according to claim 6, wherein the NH3 precursor stored in the container (2) is pressed ammonium carbonate body (3).