DE3803100A1 - Method and device for the regeneration of soot filters - Google Patents

Abstract

During the regeneration of a soot filter (4) for diesel engines the exhaust gas mass flow flowing through the soot filter (4) is greatly reduced by means of an exhaust flap (6) and the excess portion of the exhaust gas is diverted into a bypass (7). Due to the great reduction of the exhaust gas mass flow, the electrical heating (5) and the ceramic body (4) and the soot layers are only slightly cooled so that very little additional heat energy has to be fed in in order to reach the ignition temperature, which allows the heating coil (5) to be fed from the vehicle battery. Due to reduction of the oxygen mass flow, the layers of soot in the ceramic body (4) are burnt off at reduced speed and at relatively low temperatures, so that a long material service life is achieved owing to the low thermal stresses. <IMAGE>

Description

The invention relates to a method for regenerating Soot filters for diesel engines according to the preamble of Claim 1 and devices for performing the Method according to the preamble of patent claim 6.

Experience has shown that diesel engines emit three to eight times as many particles as comparable petrol engines. The Particles consist predominantly of carbon on which organic substances of higher molecular weight are deposited. These particles are undesirable. Your mutagenic effect is proven to be carcinogenic. The Because of their small size, particles are respirable and become therefore viewed as a trigger of lung diseases. In addition, the emitted particles increase the total amount Suspended particles in the atmosphere and contribute to smog formation at.

Adequate reduction in particle emissions is particularly in the case of diesel engines with a larger displacement internal engine measures based on the current state of the art Technology not possible in the short term. That is why required to post-treat the diesel exhaust.

Of the possible methods and devices for cleaning The diesel tax has been approved for use in vehicles so far only filters can enforce. One of the filter systems used in practice use one honeycomb body extruded from a porous ceramic as Filter element. The pores in the honeycomb walls are like this fine that the particles are retained. With increasing operating time clog the pores, the Exhaust back pressure increases, the engine loses power and Fuel consumption increases.  

Because of this, these ceramic filters must be timely be regenerated. In practice, this is after one Operating time of about 5 hours. This is done by external supply of heat energy the filtered Carbon is heated to the extent that it is in the exhaust gas contained oxygen reacts and burns. The Ignition temperature is above 500 degrees C. The temperature of the Diesel emissions themselves usually remain below that Ignition temperature or reaches it too rarely and then only too briefly.

There are various methods known for the for the Thermal energy required to reach the ignition temperature produce. One method uses the ceramic body upstream fuel or gas powered burners.

Another method is to get hot air through the Blown filter element.

A third method is described in EP-A-02 20 588. Here the thermal energy is from an electrically heated one Creates resistance. In addition, in the area of Resistance heating creates a carbon-air atmosphere adding dusty carbon from either an external Storage added or with the help of compressed air from the Filter itself is whirled up. This Carbon-atmospheric oxygen mixture is used by the electrical Heating coil ignited and then ignites the Soot layers on the walls of the ceramic filter. The so The flame front formed then migrates with the exhaust gas flow through the filter.

It is also known to have a chemical on the soot filter Spray on oxidizing agent, which the The combustion temperature of the soot is reduced catalytically.  

The known methods allow a very fast one Regenerate the soot filter, usually in about 2 minutes. However, the temperature rises locally to values over 1000 degrees C. This overheating zone moves quickly through the filter body. Experience has also shown that they form so-called ignition nests that lead to another local Cause the filter body to overheat, at worst until to the melting temperature of the material. Therefore one observes repeatedly caused by thermal stresses The filter body breaks if it is made of ceramic. The bypass formed at the breaking point reduces the Filter effect.

In addition, it is cumbersome to keep one fuel - fed burner, a storage container for Carbon dust or oxidizing agent and optionally carry a compressed air tank or a compressor have to.

Also the provision of sufficient electrical energy for heating the exhaust gas-coal dust mixture to the Ignition temperature as mentioned above is in any case Vehicle quite problematic because of the heating coil and all additional filter parts due to the relatively cold exhaust gas flow be cooled down strongly. The one in the above Proposed EP-A-02 20 588, the heating coil, so to speak to be placed in the slipstream of a suitable cover plate, may reduce the direct cooling of the heating coil; on the cooling of the filter body and the soot layers this doesn't change anything.

The present invention is based on the object Specify the procedure of the type mentioned, which only little thermal energy is required to reach the ignition temperature and which above all with controlled soot burn-off allows only slight excess temperatures.

This problem is solved by the characteristic features of claim 1.

The present invention leaves the usual way by Supply of a lot of thermal energy and a lot of atmospheric oxygen, be it the atmospheric oxygen in the exhaust gas, especially in the Idle, or by adding heated air to it Start regeneration and keep going and throttles during the regeneration that flows through the filter body Exhaust gas volume to a very low value in the order of magnitude from 10-30% of the idle exhaust gas mass flow. Through the Reducing the amount of exhaust gas will cool down the Heat source that is preferred Embodiment of the invention around an electric heating coil acts, but also the filter body and the soot layers, significantly reduced. This allowed for an experimental model the energy required to reach the ignition temperature between 60 and 70% can be reduced.

Finally, by reducing the amount of exhaust gas Soot oxidation reaction rate reduced. Thereby the regeneration process is prolonged, which, as already mentioned, about every 5 hours is required from approx 2 minutes to about 8 minutes; which thereby achieved Reduction of temperatures and temperature gradients leads however, a drastic reduction in thermal Tensions in the ceramic filter body and thus allows longer service life and simplification of the gastight storage of the same.

The reduction of the electrical energy required for sufficient heating of the heating coil on the Ignition temperature allows the heater to be switched off Operate on-board battery. Have performed trials shown that for the regeneration of a soot filter for one  Diesel engine with 3 l displacement only one electric Connected load of approx. 1.7 kW and only 7 Ah in total are needed.

It is in principle with the method according to the invention possible the regeneration process in all load ranges perform. The soot filter during regeneration The amount of exhaust gas flowing through only has to be set in this way be that to maintain a controlled Burning soot required, relatively low Amount of oxygen is available. Because the oxygen content of diesel exhaust strongly depends on the load condition of the engine a fixed value can practically not be specified. Because of the defined relationships regarding Oxygen content, temperature and exhaust gas mass flow recommended it is possible to carry out the regeneration at idle. Then the regeneration can also be carried out instead of with a regulation be controlled by a simple controller.

In the simplest case, it is sufficient during the regeneration a corresponding proportion of the exhaust gas on the soot filter to pass by. Will this split the exhaust gas flows too Fixed start of the regeneration process, then the amount of exhaust gas that flows through the filter in the increase to the same extent as the soot oxidizes and the regenerated part of the filter body in a desired manner cool.

According to a preferred embodiment of the invention, the Electrically generated heat in the soot layers in the direct Contact supplied, results in with the least effort optimal ignition effect. The reaction starts quickly and the electrical energy consumption remains low.

Another object of the present invention is to Suitable implementation of the method according to the invention  Specify devices.

This task is built into a soot filter Housing with a and the exhaust gas to be cleaned a pipe carrying the cleaned exhaust gas and with a heat source in front of the face of the Ceramic body solved by the characteristic features of Claim 6.

This solution is very simple. To realize them uses the conventional and proven exhaust flaps will.

According to a preferred embodiment of the invention Bypass used a second soot filter, the Exhaust valve outside regeneration the exhaust gas on both Soot filter distributed. This solution prevents that during the very short regeneration phase practically unpurified diesel exhaust gas can get into the atmosphere. There is also the advantage that instead of one single large filter body uses two smaller ones can be. Small filter bodies are relatively cheaper and relatively less susceptible to thermal stress than large filter body.

It goes without saying that the principle according to the invention also applies can be expanded to three or more filter bodies.

The invention for the generation of thermal energy preferred electrical resistance heating is characterized special simplicity and operational reliability. Because of the reduction in the amount of exhaust gas during regeneration the required connection power is easy to obtain.

If the heating resistor is connected to the front of the Filter body is in contact, is the heat transfer  optimal, the required electrical power is minimal.

The resistance heater preferably has a plurality of parallel resistance wires. In this way, the required electrical power also from the vehicle electrical system Motor vehicle with 12 volts or 24 volt accumulator to be provided.

According to a preferred embodiment of the invention Filter body is a ceramic honeycomb body and the wires of the Resistance heaters have hairpin bends Loops, these loops in the channels of the Honeycomb structure of the ceramic body are inserted. Thereby have the glowing resistance wires over a longer one Route direct contact with the soot layers so that the Heat transfer from the wire to the soot layer takes place optimally.

Another filter variant has the shape of a Piece of pipe that is mounted between two end plates, that the exhaust gas flows through the pipe wall. Such filter bodies are made of foam ceramic or wound and pressed ceramic fibers or metal wires. At this The heating coil is preferably in the form of a filter in front of the inner wall appropriate.

In a further development of the invention, the Filter body a temperature sensor can be arranged. This Temperature sensor measures the during the regeneration phase Temperature of the gases leaving the filter. So that's it possible to detect whether the ignition temperature is exceeded the regeneration process is in progress. So that's it but also possible to regulate the position of the exhaust flap, to the regeneration process with controlled To let speed run.

According to a further preferred embodiment, can  and / or arranged behind the filter body pressure sensor be. This allows the filter to be loaded with soot determined and the completeness of the regeneration process be monitored.

Based on the drawing, the invention in the form of Embodiments are explained in more detail. Show it:

Fig. 1 shows an exhaust system thereof with a soot filter and a device for regeneration,

Fig. 2, an exhaust system with two parallel soot filter and means for regeneration thereof,

Fig. 3 shows an energy-saving design of an electric heater and

Fig. 4 shows an exhaust system with a second type of soot filter and a device for regeneration thereof.

In Fig. 1 it can be seen a coming from the diesel engine, leading to clean exhaust pipe 1, a built-in a partly cut-shown housing 2 soot filter 4 of a ceramic body having honeycomb structure and microporous walls, and the purified exhaust gas dissipating tube 3. An electrical heating coil 5 is shown in front of the face of the ceramic body 4 against which the soot layers in the filter body 4 can be heated to the ignition temperature.

In the pipe 1 supplying the exhaust gas, an exhaust gas flap 6 is installed. During the normal operating state, the flap is in the position 6.0 , so that the entire exhaust gas flows through the filter body 4 . At the beginning of the regeneration process, the flap is brought into position 6.1 , so that the vast majority of the exhaust gas is passed into a bypass line 7 . A small part of the exhaust gas in the order of 10-30%, based on the amount of exhaust gas when the engine is idling, continues to flow through the filter body 4 . This small amount of exhaust gas is hardly able to cool the heating coil 5 or the filter body 4 and the soot layers deposited thereon, so that the required ignition temperature is achieved with relatively small electrical connections. The oxidation of the soot layers then takes place relatively slowly and in a controlled manner thanks to the small amount of oxygen, so that only relatively low temperatures and, above all, reduced temperature gradients occur. As a result, the ceramic material, but also the housing 2, are subjected to less thermal stress, so that their service life increases and the gas-tight mounting of the ceramic body 4 in the housing 2 can be achieved with simpler and cheaper means.

In Fig. 2 can be seen a two arranged in its housing 2.1, 2.2 soot filters 4.1, 4.2. existing filter system. In the line 1 coming from the engine and carrying the exhaust gas to be cleaned, the changeover valve 6 is again inserted, the flap of which now has three positions. During normal operation, this flap is in the middle position 6.0, for example. As a result, the exhaust gas flow is divided between the two filter bodies 4.1 , 4.2 . For regeneration, the flap is either pivoted into position 6.1 or 6.2 , so that the filter body 4.1 , 4.2 to be regenerated only receives the amount of exhaust gas required for regeneration. In this way, no uncleaned exhaust gas is released into the atmosphere, even during the regeneration phase.

Temperature sensors 8.1 , 8.2 are arranged behind the filter bodies 4.1 , 4.2 . These measure the temperature of the exhaust gas flowing through the filter body. In this way it can be recognized whether the ignition temperature typical for the regeneration process has been exceeded and how long this has been the case. The flap position can also be adjusted with the help of the measured exhaust gas temperature, so that the regeneration process takes place at the desired controlled speed.

Furthermore, pressure sensors 9.1 , 9.2 , 10.1 , 10.2 can be arranged in front of and behind each filter body 4.1 , 4.2 . In this way, the pressure difference can be determined via the filter bodies 4.1 , 4.2 and thus the state of loading of the filter surfaces with soot. As soon as the pressure difference exceeds a certain value, either a signal is displayed or the regeneration process is initiated fully automatically. An electronic circuit 11 symbolized as a block is used for this purpose, which for example emits an electronic signal 12 to the outside.

A particularly energy-saving design of the electric heater can be seen in FIG. 3 in a schematic representation. One can see on average the face of a filter body 4 , which is flown by the exhaust gas, with the filter channels 4.2 delimited by porous walls 4.1 . The channel walls 4.1 are covered with soot 4.3 . The electrical heater 5 can be seen in front of the end face of the ceramic body 4 which is flown by the exhaust gas to be cleaned. It consists of two connection electrodes 5.1 , between which a plurality of wires 5.2 is tensioned, of which only one is shown in the drawing. The resistance wire 5.2 is provided with a series of hairpin-shaped loops 5.3 . These loops 5.3 are inserted into the channels 4.2 which are open at the end, so that the glowing wire 5.2 comes into direct contact with the soot layers 4.3 . As a result, the transfer of heat to the soot layers is optimal for reaching the ignition temperature. In addition, the wires 5.2 are mechanically supported in the area of the loops 5.3 .

In FIG. 4 one recognizes an exhaust system for diesel engines comprising a tubular filter body 4.4 of foam ceramic or wound and pressed ceramic fibers or metal wires in a schematic representation. The exhaust gas flows through the filter body 4.4 from the inside to the outside, which is achieved by gas-tight end plates 2.4 , 2.5 . The electrical heating coil 5.4 is located in the central bore of the filter body 4.4 .

It goes without saying that the exhaust flap 6 shown in FIGS . 1, 2 and 4 can be used not only in the line 1 supplying the exhaust gas but also in the line 3 discharging the cleaned exhaust gas.

Claims (14)

1. A method for regenerating soot filters ( 4 ) for diesel engines during their operation, wherein thermal energy is supplied to reach the ignition temperature of the soot, characterized in that during the regeneration the amounts of exhaust gas which flow through the soot filter ( 4 ) strongly, ie reduced to approx. 10-30% of the quantities that leave the engine at idle. 2. The method according to claim 1, characterized in that a corresponding proportion of the exhaust gas on the soot filter ( 4 ) is bypassed. 3. The method according to claim 1 or 2, characterized in that between two soot filters ( 4.1 , 4.2 ) is switched. 4. The method according to claim 1, 2 or 3, characterized in that the thermal energy is introduced by an electrical resistance heater ( 5 ) into the filter ( 4 ) or into the exhaust gas stream shortly before it enters the filter ( 4 ). 5. The method according to claim 4, characterized in that the thermal energy is supplied to the soot layers ( 4.3 ) in direct contact. 6. Device for performing the method according to at least one of claims 1 to 5 with a soot filter ( 4 ), installed in a housing ( 2 ) with a supplying the exhaust gas to be cleaned and a cleaned exhaust gas pipe ( 1 , 3 ) and with a heat source ( 5 ) in front of the face of the filter body ( 4 ), characterized in that an exhaust gas flap ( 6 ) is inserted into one of the exhaust gas lines ( 1 , 3 ) and bypasses the greater part of the exhaust gas quantity during regeneration ( 7 , 1.2 ) conducts, and that an electrical resistance heater ( 5 ) is provided to generate the thermal energy. 7. The device according to claim 6, characterized in that a second soot filter ( 4.2 ) is used in the bypass and that the exhaust flap ( 6 ) distributes the exhaust gas to both soot filters ( 4 ) outside the regeneration. 8. Apparatus according to claim 6 or 7, characterized in that the heating resistor ( 5 ) with the end face of the filter body ( 4 ) is in contact. 9. Apparatus according to claim 6, 7 or 8, characterized in that the heating resistor ( 5 ) has a plurality of parallel resistance wires ( 5.2 ). 10. The device according to claim 8 or 9, characterized in that the filter body ( 4 ) is a ceramic honeycomb body, that the resistance wires ( 5.2 ) of the resistance heater ( 5 ) have hairpin-shaped loops ( 5.3 ) and that these loops ( 5.3 ) in the Channels ( 4.2 ) of the honeycomb structure of the ceramic body ( 4 ) are inserted. 11. The device according to at least one of claims 6 to 9, characterized in that the filter body ( 4.4 ) has the shape of a tube piece which is mounted between end walls ( 2.4 , 2.5 ) such that the exhaust gas stream flows through the tube wall from the inside to the outside, and that the heating coil ( 5.4 ) is mounted in front of the inner wall of the filter body ( 4.4 ). 12. The apparatus according to claim 11, characterized in that the filter body ( 4.4 ) consists of foam ceramic or of wound and pressed ceramic fibers or metal wires. 13. The device according to at least one of claims 6 to 12, characterized in that a temperature sensor ( 8 ) is arranged behind the filter body ( 4 ). 14. The device according to at least one of claims 6 to 13, characterized in that pressure sensors ( 9 , 10 ) are arranged in front of and / or behind the ceramic body ( 4 ).