EP0233860A2 - Exhaust gas cleaning device - Google Patents

Abstract

A device for the cleaning of the exhaust gases from internal- combustion engines operated with significant excess oxygen, preferably diesel engines, especially of vehicles having at least one retaining device, especially a filter device, for fine-grain condensates, especially such as soot, which is arranged in an exhaust gas duct, can be brought into contact with at least part of the exhaust gases and, especially, has the exhaust gases flowing through it, and having at least one device for burning off condensates held by the retaining device, which can preferably be brought into use intermittently, the retaining device, especially the filter device (2), having at least one heat-resistant retaining body, especially a filter body (20), provided with a catalyst material, preferably on the basis of metals of the platinum group, if necessary also with Mo, W, Co and/or V, having at least one device (5) for initiating the burning off, preferably with an electrical resistance heating device (50), which is arranged in direct contact with the filter body and can be intermittently supplied with electricity, preferably from a power source (6) of a vehicle. <IMAGE>

Description

The invention relates to a device for the purification of the exhaust gases of internal combustion engines operated with a substantial excess of oxygen, preferably diesel engines, in particular vehicles, with at least one retention device arranged in an exhaust gas guide that can be brought into contact with at least a subset of the exhaust gases, in particular through which the exhaust gases flow Filter element, for finely divided condensates, such as soot in particular, and at least one device, which can preferably be put into operation in batches, for a burn-off of the Konoensate retained by the retaining element.

A number of methods have become known for reducing the amount of pollutants in the exhaust gases of vehicles operated with gasoline engines, according to which the conversion of CO, nitrogen oxides and unburned hydrocarbons into gases which are not very harmful to the environment can take place. In a number of industrialized countries, there are already legal regulations that mandate the installation of catalytic converters in motor vehicles.

Similar efforts to reduce the emissions caused by exhaust gases from internal combustion engines operated with excess oxygen, such as, in particular, diesel engines from vehicles, have also been underway for a number of years and have already made recommendations and in some states, for example the USA, legal regulations on pollutants. Emission limitation led. Fewer CO and nitrogen oxides form the special problem in the exhaust gases of diesel engines, but the extremely fine-particle condensates, which occur in large quantities, especially in the higher load range, pose a particular problem accumulate as soot particles, an unpleasant emission of the, where harmful to health, among other things, is attributed to their extreme fine particle size. Various methods have become known for reducing the soot emission, of which the most important are filtering on ceramic foams, in ceramic tube filters, with fine-meshed metal filament fleeces and also electrostatic deposition processes.

Greater experience in the practice of driving with motor vehicles has already been gained with filters based on cordierite. E.g. the composition and properties of such filters with honeycomb structure are described in DE-PS 26 31 875. These ceramic filters are elongated prismatic and have a large number of channels running side by side, the partition walls of which are gas-permeable, fine-porous and each sealed at the ends with a ceramic stopper. The exhaust gases flowing into the ducts that are open on the engine side are forced to make their way through the porous walls into the adjacent ducts that are closed on the engine side and open to the exhaust system. Soot and condensate particles are deposited on the walls of the channels, which can achieve soot separation of over 90%.

In order to prevent an excessive increase in the exhaust gas counterpressure when the filter is loaded more heavily, the filter must be regenerated at intervals, usually by burning off the soot, by ensuring that the temperature is sufficiently high to trigger the burnup. The burning, for which the existing exhaust gas in excess oxygen is used, is hampered by the relatively nied- ri ^g en temperatures of the exhaust gases of diesel engines. The necessary temperatures can be increased, for example Exhaust temperature can be reached by driving for longer periods in the full load range, they are around 600 - 900 ° C for uncoated ceramic bodies. A reduction in the temperature required for soot combustion can be achieved with catalytic coatings on the filters or catalytically active fuel additives.

In order to avoid the full load driving described, a number of auxiliary devices have been proposed by means of which the soot burn-off on the FiltEr can be triggered and promoted. E.g. has become known to arrange externally supplied gas burners themselves in the exhaust gas stream, which heat it up, so that the exhaust gas heats the filter to the temperature required for soot combustion. This type of regeneration is complex and considerable burner outputs are required to heat the exhaust gas, e.g. with a two-liter engine, around 20 kW are then required at 2500 rpm. When using condensate filters provided with catalysts, the temperature required for burning off the soot can be reduced, as mentioned, but the duration of a regeneration process is usually very long, usually more than ten minutes. The further development is therefore aimed at a further reduction in the soot burn-off temperatures, with the aim being to burn away evenly through the filters. ;

The aim of the invention is to provide a device of the type mentioned at the outset which requires low specific energy outputs when initiating and maintaining regeneration of the condensate filter, the latter being carried out in the shortest possible time.

The invention relates to an exhaust gas cleaning device. do ^g oer type mentioned at the outset, which essentially consists in the retention, in particular filter element, of at least one heat-resistant retention provided with catalyst material, preferably based on the metals of the platinum group, optionally additionally with Mo, W, Co and / or V. , in particular a filter body with at least one combustion initiation member, preferably with an electric resistance heating member, arranged directly adjacent to it, preferably from a power source of a vehicle, which can be supplied with batches of electricity. The new device has the advantage that in every load range and with little effort with a problem-free switching process, preferably powered by the vehicle battery - if desired - regeneration of the filter by soot burn-up can be initiated at relatively short intervals, which increases the occurrence Back pressures in the exhaust system due to higher loading of the filter with soot can be avoided. The device can be accommodated, for example, in the front silencer housing or in another, advantageously close extension of the exhaust pipe to the engine. Together with the catalysts, for example based on platinum or also in their place, oxidation catalysts, such as based on V ₂ O ₅ or those with a spinel structure, can be arranged on the filter body. Particularly important for the effectiveness of the new filter device is the intimate contact of the burn-off initiation member or its surface with the material of the catalytically coated filter body, which is achieved through the direct, preferably flat contact, a high number of contact points being achieved, thereby ensuring targeted, avoiding energy losses and immediate start of the soot burn-up front simultaneously at many centers, where the filter body and the initiator are in close contact, as a result reached uninhibited heat transfer and the erosion front .. is initiated on a broad basis and then traverses the filter body.

According to a particularly preferred embodiment within the scope of the invention, in which the transition of the erosion into the filter body can take place particularly intensively when the initiation device is switched on, it is provided that the erosion initiation member has catalyst material, in particular on the basis, on its surface adjacent to the filter body the metals of the platinum group, is preferably provided with a heat-resistant sheath, which is arranged directly adjacent to or in the retaining, in particular filter body, and is provided with a catalyst on the outside and in which at least one resistance heating conductor is arranged. In this case, there is the advantage that the condensate and soot deposit during engine operation on the burn-initiator tionsor ^g of itself on its catalytically coated outside in the same manner as on the catalytically coated surface of the filter body, so that when heating the Resistance heating element at a relatively low temperature, the burn-off of the soot deposited there is initiated at the initiation element itself and jumps out of the common contact points or surfaces with the filter body without any inhibition into the filter body.

It can also be provided that the shell of the erosion initiation member is formed by a part of the retaining body itself, that is to say it is formed integrally therewith. For example, the heating conductor can be arranged directly in one or more of the longitudinal honeycombs of a ceramic honeycomb filter. If the erosion initiation member, which advantageously has an elongated shape, is arranged to extend into the interior of the retaining body, in particular the filter body, the greatest possible contact area between the initiation member and the filter body is provided, as a result of which the exhaust gas front has a particularly large starting area.

If, as further provided in an advantageous manner, the combustion initiation member is arranged to extend essentially from the exhaust gas upstream end of the retaining body, in particular the filter body, essentially in the main direction of the flow of exhaust gas, the radial component of the propagation of the soot combustion front is reinforced, thereby ensuring can be that the filter body is regenerated more uniformly over its cross section. To spread the erosion front, a plurality of erosion initiation members can also be arranged in the main exhaust gas flow direction over the filter body cross section. The advantage of axial alignment, that the lying from a cartridge heater asstromabwärtig ^g portion of the filter body which can exert at least virtually no filter function honeycomb filters, is kept low. In order to use this area, based on the cross-section of the filter body in the same position as the heating cartridge described, at least one further heating cartridge with the same orientation can be arranged behind it, according to a further variant, an elongated combustion initiation element can almost or completely penetrate the filter body in the main direction of the flow of exhaust gas.

If the combustion initiation element, preferably in the interior of the upstream exhaust gas area of the retention, in particular filter body, is arranged to extend essentially transversely to the main exhaust gas flow direction net is easier accessibility, especially given cartridge-like training.

To be surprisingly effective, the new device has been found, when the case retention, in particular filter body, essentially the entire cross section of the exhaust guide _'engaging with to heat and preferably to carbon and / or oxygen resistant three-dimensional, with metallic filaments, wire, ribbon or the like. -Elements, in particular made of steel, with interstitial support, the filaments, ribbons, wires at least partially an intermediate coating with at least one layer applied from the gas phase of at least one carbide and / or nitride and / or oxide , and optionally borides, preferably an oxycarbonitride, have at least one of the elements from the group Ti, Zr, V, Ta, Nb, Mo, W, Al, Si, over which coating the catalyst material is arranged. The applied by vapor deposition to the metallic carrier body frame intermediate layer of said hard materials, whereby their thickness _/ um is usually less than 10, can be, as it turned out, caking of the metallic filter body skeleton due to local overheating during the soot-burning avoided. In addition, as has been shown, the intermediate coating brings about a synergistic increase in the action of the catalyst on the filter body, as a result of which the burn-off temperature is reduced and the soot burn-off is accelerated. This effect is probably due to the fact that the intermediate layer brings about an advantageous change in the microstructure of the catalyst layer arranged on it.

It has been shown that the already mentioned effect of the uninhibited Übersprin ^g ens of Abbrandfront heightened who can, if in an advantageous manner the sheath of the erosion initiation organ, which extends into the interior of the retaining body, in particular the filter body, is formed with heat-resistant metal, in particular steel, the outer surface of which, at least in part, comprises an intermediate coating with at least one the gas phase applied intermediate coating of at least one carbide, nitride and / or oxide, and optionally borides, preferably an oxycarbonitride, of at least one element from the group comprising Ti, Zr, Hf, V, Nb, Ta, Mn, W, Al and Si which coating the catalyst material is arranged. In addition to the good thermal conductivity of the metal sheath, jumping the burn-off front into the filter body is further facilitated by adapting the properties of the microstructure of the catalytic coating to those of the filter body.

In the sense of an additional improvement in the starting and waning of the erosion front by the filter body, it is particularly preferred if at least one layer of the intermediate coating of the support of the retaining body, in particular filter body, formed by wire or band elements, in particular steel, and / or erosion Initiation organ, in particular from its shell, preferably grown epitaxially on the substrate, is essentially crystalline. This form of training improves the starting of the soot combustion, as has been shown.

In order to achieve economical use of the regeneration energy during driving operation, it can be provided according to a particularly simple construction that, for example, is taken care of by means of a control device is that the burn-off initiator is automatically switched on at adjustable regular intervals, so that the soot burn-off is carried out regardless of the particular loading of the filter.

In order to keep the use of energy for the erosion initiation organ optimally low, it is advantageous in the context of a preferred embodiment of the device according to the invention to limit the duration or energy supply to the erosion initiation organ. Accordingly, it is preferred if the retaining body, in particular the filter body, has at least one temperature sensor in its exhaust gas downstream region, which is operatively connected to a switching element of the power supply of the erosion initiation element via a control element that can be set by temperature thresholds. With such a device can in any case, if the Re ^g enerationsfront substantially the end portion of the filter has been reached, which then leads to a sharp increase in temperature, the power supply to be switched off for initiation organ, so that supply of power after the completion of combustion does not occur.

A particularly economical use of the energy required to initiate and maintain the erosion is particularly guaranteed if the initiation element is actually switched on depending on whether the filter body is covered with soot and condensate beyond a certain threshold value, which is reflected in the pressure in the Exhaust system manifested in front of the filter body. According to a further advantageous variant, provision is made for a pressure sensor to be arranged in the exhaust gas duct upstream of the retaining element, in particular the filter element, and the power supply via a control element which can be set using pressure thresholds and a switching element tion of the burn-off initiation organ is operatively connected. The pressure sensor can work, for example, on a piezo basis.

It is particularly favorable if, in the last-described embodiment, an upper and a lower pressure threshold value can be input to the control element, so that when the upper predetermined exhaust gas counterpressure is exceeded, the combustion is started by switching on the power supply and, if the counterpressure lower limit is undershot after the filter has been exposed, the heating element is switched off.

An exhaust gas purification device (1) according to the invention is explained in more detail using the drawing.

In a housing 10 provided on both sides with flanges 111 for connection to an exhaust system, having two conical projections 12 and a cylindrical middle part, through which the exhaust gas flows in the flow direction r, and which is double-walled in the cylindrical part, the intermediate space being filled with mineral wool as thermal insulation 101 , is held by a mineral thermal insulation material 102, the filter body 20 of the particle retention or filter element 2, which is formed, for example, by a wound steel fleece material with catalyst-coated metal filaments 23 with interstitials 24 or a catalytic coated honeycomb filter. From the end upstream of the exhaust gas protrudes - shown in full line - an approximately cartridge-like combustion initiation element 5 with a shell 53, which has a layer zone 54 consisting of an approximately 3 μm thick catalyst carrier layer with the constituents Ti, Al, C, N and oxygen and a Pt and Pd-containing catalyst coating, and within which there is a resistance heating conductor 50, in the exhaust gas upstream area 21 of the filter body 20. The power lines 51, 52 leading to the heating conductor 50 are connected to a power source 6, for example a vehicle battery. The switch 55 located in line 52 is connected to a control element 78 which can be set to the desired temperature (t) or pressure (p) threshold values, the pressure threshold part p of the control element 78 being connected via an active line 71 to a pressure sensor 7, For example, on a piezo-crystal base, which is arranged upstream of the exhaust body of the filter body 20. The temperature threshold part is a t Wirkleitun ^g 81 having an end portion located in the abgasstromabwärtigen 22 of the filter body 20 temperature sensor 8 located connected. If the exhaust gas pressure rises above a respectively set threshold value during operation of the diesel engine as a result of loading the catalytically coated filter body 20 with condensate particles, the pressure sensor 7 via line 71 by means of control or ^g ans78 via the switch 55 switches the current flow through the heating conductor 50 the heating cartridge 5 is switched on, the casing 53 is heated and the catalytic coating, on which the soot has also deposited, causes the burning front which occurs there to jump into the filter body 20. Walking through the burn-up front takes place within a few minutes. The heating cartridge 5, which is to be switched off immediately after regeneration has taken place, can be switched off automatically by the control element being adjustable to a lower threshold value of the exhaust gas counterflow. As the regeneration progresses, the pressure drops and when the value falls below the lower threshold, the control element 78 switches off the power supply to the cartridge 5. The switch-off can take place in another way, by the temperature sensor at the point in time when the exothermic combustion front has reached the end region 22 of the filter body 20 8 a temperature threshold value is exceeded and the control element 78 interrupts the power supply to the initiation element 5 by actuating the switch 55. The drawing schematically indicates how a second heating cartridge 5 "protrudes from the exhaust gas downstream end in the same position, based on the cross section of the filter body 2, in the filter body 2, with which, for example, the regeneration dies before reaching the downstream end of the filter body A broken line also indicates how a heating cartridge 5 'can also protrude radially into the upstream exhaust gas region 21 of the filter body 20 and can be arranged in contact with the filter body 20. This type of arrangement shown has advantages in the case of repairs or replacements shown device can achieve an extremely economical condensate filter regeneration.

example

On a test stand with a two-liter passenger car diesel engine with conventional exhaust gas routing, instead of the front silencer in a thermally insulated housing, a spirally rolled filter body made of steel wire fleece with a catalyst coating with platinum was arranged with a specific surface area of 0.40 m ² / g, which about 20 cm in diameter and 35 cm in length. An electrical heating cartridge of 3 cm in diameter and 30 cm in length was built into the filter body from the exhaust gas inflow side, projecting centrally and axially and in contact with it, which was fed by an accumulator supplied by the motor. The maximum power consumption of the heating cartridge at 12 V was 1 kW. In the area of the exhaust gas downstream end of the filter body

a thermocouple was placed 3 cm from the side of the filter, which was connected to an automatic recorder. The diesel engine was operated at 25, 50 and 75% of the full load and the exhaust gas back pressure was measured with a pressure transmitter arranged in front of the filter; If this exceeded 0.3 bar, the heating cartridge was switched on and the time between the start-up of the heating cartridge and the temperature increase indicated by the thermocouple was determined. The heating cartridge was switched on at 2 minutes. For the three specified engine loads, average times of 8.6, 6.4 and 5.2 minutes of passage of the burn-up front through the filter body from the switching on of the heating cartridge were determined from three test series.

When using a heating cartridge, the steel casing of which, according to the particularly preferred embodiment according to the invention, was coated on the outside with the same platinum-based catalyst as the steel wool filter body, the otherwise specified test conditions allowed the time stated between switching on the heating cartridge and ending the migration of the regeneration front through the filter body to 7.2, 5.8 and 4.5 minutes.

Example 2:

Analogously to Example 1, a filter made of ceramic foam and coated with Pt and Pd was used. An electrical immersion heater with a diameter of 30 mm, a length of 120 mm and a power consumption of 1 kW (at 12 V) was inserted axially into the filter body and penetrating it. The heating cartridge was provided with a 2 µm thick crystalline coating made of TiN with low levels of carbon and oxygen. On this coating was in the finest form by ther Mix decomposition deposited a well-adhering catalytically effective coating of Pt and Pd.

When the filter device was operating, an exhaust gas back pressure of 320 mbar was recorded at full load at a speed of 1500 rpm. The particle emission measured according to the density number SZ (Bosch) was 0.22. After an operating time of 35 minutes, the back pressure rose to 355 mbar and after another 10 minutes to 370 mbar.

After the heating cartridge was switched on, the back pressure dropped to 350 mbar over a period of 4 minutes and to 330 mbar after a further 2 minutes. The power supply was then cut off. An energy expenditure of 0.1 kWh was therefore necessary for the regeneration of the exhaust gas cleaning device.

Example 3:

Analogously to Example 1, an exhaust gas purification device according to the invention was tested on a test stand, which was installed in the exhaust pipe of a two-liter passenger car diesel engine instead of the front silencer. A ceramic honeycomb filter body coated with Pt and Pd, which had a diameter of about 20 cm and a length of 35 cm, was used with two heating cartridges with a diameter of 15 cm and length 15 cm, also catalytically coated with Pt and Pd, which were arranged axially one behind the other in the middle. The maximum power consumption of the heating cartridge was 1.1 kW. The diesel engine was operated at 75% of the full load, and the exhaust gas back pressure was measured with a pressure transmitter located in front of the filter. At the start of the test, this was 115 mbar. If the back pressure exceeded 300 mbar, the heating cartridges were switched on and the time between starting up the heating cartridge and falling below the back pressure of 200 mbar indicated by the pressure transmitter certainly. The average time required for regeneration was 5.2 min.

Claims (11)

1. A device for cleaning the exhaust gases of operated with excess oxygen combustion engines, preferably diesel engines, in particular of vehicles, with at least one arranged in an exhaust system, at least with a partial quantity of the exhaust gases into contact brin ^g- cash, through which flow, in particular from the same retention. filter element, in particular, for finely divided condensates, such as particularly soot, and at least one device, which can preferably be put into operation in batches, for an edge of the condensates retained by the retention element, characterized in that the retention element, in particular filter element (2), has at least one with catalyst material, preferably on the basis of the metals of the platinum group, optionally additionally provided with Mo, W, Co and / or V, heat-resistant retention bodies, in particular filter bodies (20) with at least one body arranged directly adjacent to it, preferably from a power source (6) of a vehicle having a power-supply burn-off initiation member (5), preferably an electrical resistance heating member (50). 2. Device according to claim 1, characterized in that the erosion initiation member (5). its side on the filter body (20) is provided with catalyst material (54), in particular based on the metals of the platinum group, preferably a heat-resistant and with catalyst (54) arranged on or in the retention, in particular filter body (20) ) provided sleeve (53) in which at least one resistance heating conductor (50) is arranged. 3. Device according to claim 1 or 2, characterized is characterized in that the shell (53) of the erosion initiation member (5) with the retaining, in particular filter body (20) is integrally formed. 4. Apparatus according to claim 1, 2 or 3, characterized in that at least one burn-off initiation member (5) is arranged extending into the interior of the retaining, in particular filter body (20). 5. Device according to one of claims 1 to 4, characterized in that at least one burn-off initiation member (5), preferably from the exhaust gas upstream end (21) of the retaining, in particular filter body (20) substantially in the main exhaust gas flow direction (r) inside is arranged extending. 6. Device according to one of claims 1 to 5, characterized in that at least one burn-off initiation member (5), preferably in the upstream region (21) of the retaining, in particular filter body (20), substantially transversely to the main exhaust gas flow direction (r) in the interior of which is arranged to extend. 7. Device according to one of claims 1 to 6, characterized in that the retaining, in particular filter body (20), essentially taking the entire cross section of the exhaust gas duct with three-dimensional, resistant to heat and preferably carbon and / or oxygen, with metallic filaments , Wire, bandod. Elements (23), in particular made of steel, are formed with a support constructed of interstitials (24), the filaments, ribbons, wires (23) being at least partially an intermediate coating with at least one layer of at least one carbide applied from the gas phase and / ooer nitrides and / or oxides, and optionally borides, preferably an oxycarbonitride, at least one of the elements from the group Ti, Zr, V, Ta, Nb, Mo, W, Al, Si, over which coating the catalyst material is arranged. 8. Device according to one of claims 1 to 7, characterized in that in the interior of the retaining, in particular filter body (2), extending, on this abutting casing (53) of the burn-off initiation member (5) with heat-resistant metal, in particular steel, is formed, the outside of which is at least partially an intermediate coating with at least one layer of at least one carbide, nitride and / or oxide applied from the gas phase, and optionally borides, preferably an oxycarbonitride, at least one element from the group Ti, Zr , Hf, V, Nb, Ta, Mo, W, Al and Si, over which coating the catalyst material (54) is arranged. 9. Device according to one of claims 1 to 8, characterized in that at least one layer of the intermediate coating of the wire or band elements (23), in particular made of steel, formed the support, in particular filter body (20) and / or burnup Ini _t ia tion member (5) is preferably grown in particular of its sleeve (53) epitaxially grown on the substrate formed of the substantially crystalline. 10. The device according to one of claims 1 to 9, characterized in that a pressure sensor (7) is arranged in the exhaust gas duct upstream of the retaining element, in particular filter element (2), which has a control element (78) with a switching element (55 ) the power supply of the burn-off initiation member (5) is operatively connected. 11. The device according to one of claims 1 to 10, characterized in that the retaining, in particular filter body (20) in its exhaust gas downstream region (22) has at least one temperature sensor (8) which has a control element (78) with a switching element via an adjustable temperature threshold value (55) or the power supply of the burn-off initiation member (5) is operatively connected.

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