FR2808839A1 - PARTICLE FILTER AND METHOD FOR REGENERATING A PARTICLE FILTER - Google Patents

Abstract

The invention relates to a particulate filter (10) for an exhaust gas cleaning system of internal combustion machines, with a filter element (16) made of silicon carbide, as well as a method for the regeneration of gas. Such a particulate filter (10). It is provided that an electric heating element (18) of silicon carbide is associated with the filter element (16). According to the method, provision is made for regeneration of the particulate filter (10) to be initiated by the application of a voltage to a silicon carbide electric heater (18) which is associated with the filter element. (16).

Description

The invention relates to a particulate filter for a purification system of

  exhaust gas of internal combustion machines, with an electrically conductive ceramic filtering element, in particular silicon carbide, and a method for the regeneration of a

such a particulate filter.

  To purify an exhaust gas of internal combustion engines, especially spontaneous ignition engines, it is known to install so-called purification systems

  exhaust gas in a section of exhaust gas.

  Such exhaust gas cleaning systems may include both catalyst systems and particulate filters. These are particularly important in diesel engines because relatively large smoke emissions are

this case.

  Particle filters are normally integrated, with a housing that includes them, into the exhaust section of the internal combustion engine. They consist of a monolithic or segmented filter element which is traversed by the exhaust gas to be filtered. The porosity of the particulate filter is then chosen according to the properties of the components to be separated and the conditions imposed by the fluid technique. The particulate filter is blocked in the exhaust section of the internal combustion machine by

  example using elements called source mats.

  With appropriate porosity, smoke particles above a certain size are retained in the particulate filter during normal operation. Smoke accumulated in the particulate filter should be removed at regular intervals to prevent clogging of the filter. Such regeneration is normally carried out by oxidation with oxygen in the air but

  requires an ignition temperature of at least 550 OC.

  However, the ignition temperature can be decreased by the addition of additives but such a measurement requires a particular add device as well as control methods for it. If the regeneration is to be carried out without addition of additives, it is at least necessary to build the particulate filter in a refractory material and stable at high temperature. Silicon carbide (carborundum) can be used in a known manner, which has a high chemical stability and a high stability to

  oxidation for temperatures above 1000 C.

  Often, even when the particulate filter is mounted near the engine, the necessary ignition temperatures

  can not be reached during normal operation.

  In order to still perform the regeneration of the particulate filter, there are known methods and devices with which the exhaust gas temperature is increased. On the one hand, it imposes an increase in temperature by intervening directly in the management of the engine, which is obtained most of the time at the cost of an unwanted modification of the torque and an increase in fuel consumption . On the other hand, the exhaust gas can also be heated directly by electric convection heating, which however can be done efficiently only with considerable electric heating power due to the high flow velocity of the gas exhaust and its poor thermal conductivity. The use of so-called burner elements leads only to indirect heating of the particulate filter and fuel consumption.

  So here, too, necessarily increases significantly.

  The object of the present invention is to provide a particulate filter and a method for the regeneration thereof with which the necessary ignition temperatures can be reached the most.

effectively possible.

  This problem is solved according to the invention by the particle filter for the purification system of the exhaust gas of internal combustion machines, with an electrically conductive ceramic filtering element, in particular made of silicon carbide, characterized in that an electric heating element, constructed in particular in the same electrically conductive ceramic, is associated with the filter element; as well as by the process for the regeneration of such a particulate filter characterized in that a regeneration of the particulate filter is triggered by the application of a voltage on an electric heating element which is constructed in particular in the same ceramic electrical conductor and which is associated with the filter element. Since an electric heating element of silicon carbide is associated with the filter element, it can be heated directly. Due to the very small coefficient of thermal expansion of the silicon carbide, the heating element can be directly associated with the filter element because only small material stresses occur even for high temperature gradients. Silicon carbide is also very suitable as a resistance material for

  heating due to its electrical properties.

  According to the method, the regeneration of the particulate filter is initiated by applying a voltage to the electric heating element. In doing so, the voltage is preferably between 8 and 12 V. Preferably, it is also possible to use the 12 V voltage provided by an onboard system, which is notably

  in a range of 8 to 14V, especially 12 to 14V.

  Preferably, the voltage requirement is controlled according to particulate filter fouling, filter temperature, exhaust gas temperature, desired ignition temperature and / or temperature. a mass flow of exhaust gas. In this way, it is possible to adapt the voltage individually to the actual data in the filter area

with particles.

  Moreover, the regeneration is carried out periodically and / or depending on the clogging of the filter. According to a preferred embodiment of the invention, the heating element extends over a longitudinal axis of the particulate filter or coaxially with it. The heating element may be in the form of a tube or bar and may have a cross-sectional area of 1 to 3 cm 2. In addition, the heating element is preferably mounted in an area of the filter element in which there is a space velocity of the gas

  exhaust very large, especially the largest.

  When the filter element consists of individual segments that are glued together, it has proven advantageous to arrange the heating element in the angle of adjacent segments. In this way, a heat transfer between the heating cross section traversed by the current and the segments

adjacent can be optimized.

  The heating element is preferably designed so that heating element temperatures T> 500 C can already be reached for heating power.

  electric P <1 kW. In particular, the area of

  The descent of the heating element may not extend over the entire filter length. Often with such small heating elements, surface temperatures on the heating element of greater than 1000 ° C. are obtained with the adjacent filter material being heated to temperatures well above 700 ° C. The ignition temperature of the smoke accumulated on the particulate filter is thus exceeded locally. If there is a minimum amount of smoke on the particulate filter that is between 10 and 20 g depending on the volume of the filter, the heat of reaction released during the combustion of local smoke leads to a spread of smoke combustion. over the entire volume of the filter and therefore to a complete regeneration. According to two other preferred arrangements: the heating element is mounted in an area of the filtering element in which a large exhaust gas space velocity, especially the largest, for a regular distribution of the exhaust ducts 'flow; and the heating element is made at least partially in one piece with the heating element.

filtering.

  Other advantageous forms of embodiment of

  the invention can be deduced from the following description

of the invention.

  The invention is described below with reference to an exemplary embodiment, with reference to the appended drawings, in which: FIG. 1 is a schematic perspective view of a particulate filter in an exhaust gas section. an internal combustion engine; Figure 2 is a section along a longitudinal axis of the particulate filter; and FIG. 3 is a section transversely to the axis

  longitudinal particle filter.

  Figure 1 shows a perspective view of a particulate filter 10 as used, for example, for purifying an exhaust gas from a diesel engine. The particulate filter 10 is mounted in the exhaust gas section 12 of an internal combustion machine and it must filter the smoke particles formed during the combustion of an air and fuel mixture. The porosity, the shape and the storage volume of such particulate filters can be adapted to the requirements of the

respective fluids.

  The particulate filter 10 is mounted with a source mat 14 in the exhaust section 12 so as not to slip. In doing so, the filter 10 may consist of several filter segments glued together or, as shown here, a monolithic filter element 16. The element 16 itself is made of silicon carbide because this material is particularly refractory and has a very stable form due to its small coefficient of thermal expansion. In addition, this material is characterized by its very high thermal stability with respect to the gaseous, liquid and solid constituents present in the exhaust gas. A bar-shaped heating element 18 extends along a longitudinal axis of the filter element 16. The heating element 18 is connected to ground via its front end 20 by means of a contact element 22 and a voltage source via its rear end 24 by means of another contact element 26. Figure 2 shows a section along the longitudinal axis through the filter In place of the solid silicon carbide material used herein, a tube-shaped heating element 18 may also be used. It has been found advantageous to limit the size of the cross-sectional area of the heating element 18 to an area of 1 to 3 cm 2. The heating element 18 is also made of silicon carbide, as already mentioned. If appropriate, doping with other suitable elements can be performed to improve the electrical properties. To ensure good electrical contact of the ends of the heating bar, the bar ends are provided with a small electrical resistance. Due to the properties of the substantially identical materials between the heating element 18 and the filter element 16, one can also expect only small mechanical stresses in the event of high temperature gradients. A design of the heating element can then be such that heating element temperatures T> 500 OC can already be reached for electric heating power.

P <1 kW.

  Figure 3 shows a cross-section of the particulate filter 10. The heating element 18 is mounted in the center of the filter element 16 and thus lies on the longitudinal axis thereof. But it is also conceivable to arrange the heating element 18 in other areas of the filter element 16. Due to considerations relating to the fluid technique, the heating element 18 should extend to less in most cases coaxially with respect to the longitudinal axis. Naturally, it is also possible to envisage multiple arrangements of heating elements 18. In addition, a selection of the mounting point can also be carried out so that the heating element 18 is mounted in an area of the heating element. filtering 16 in which one must expect a space velocity of the exhaust gas

  particularly large or even the largest.

  The filter element 16 may be monolithically designed or consist of different filter segments 28 which are glued together. The heating element 18 should be mounted, if possible, in the angle of adjacent filter segments 28 of a total of four segments in this case. In this way, the greatest possible contact area between the heating element 18 and the filter segments 28 can be provided so that the heating

  can be achieved particularly effectively.

  Regeneration of the particulate filter 10 is effected by applying a voltage. In doing so, surface temperatures of the heating element 18 of about 1000 OC should be reached locally if possible. In the immediate vicinity of the heating element 18, the temperatures then rise significantly above 700 ° C. so that the ignition temperature for the smoke accumulated on the particulate filter 10 is exceeded at least locally. Depending on the volume of the filter, a certain minimum amount of smoke (normally between 10 and 20 g) is required in order for the heat of reaction released during the local combustion of smoke to lead to a spread of smoke combustion all over the place. filter volume. The prescribing of the voltage can be carried out using control devices not represented here according to particulate filter fouling, filter temperature, exhaust gas temperature, the desired ignition temperature and / or a mass flow of exhaust gas. All these parameters allow an appropriate adaptation of the voltage to real conditions. For example, the prescribing of an unnecessarily high voltage can be avoided when the ignition temperature is already reached with a reduced expense. Regeneration is usually done periodically. Devices and methods for controlling regeneration are known by the state of the

  therefore do not have to be detailed here.

  Although the invention has been particularly presented and described with reference to a preferred embodiment thereof, it will be readily understood by those skilled in the art that changes in form and detail can be made without difficulty. out of the mind or the framework of the invention.

LIST OF REFERENCES

  Particle filter 12 Exhaust gas section 14 Source mat 16 Filter element (monolithic or segmented) 18 Heating element Front end 22 Contact element 24 Rear end 26 Contact element 28 Filter segments

Claims (13)

  Particle filter (10) for an exhaust gas cleaning system of internal combustion machines, with a filter element (16) made of electrically conductive ceramic, in particular of silicon carbide, characterized in that an electric heating element (18), in particular in the same electrically conductive ceramic, is associated with the element of filtering (16).   Particle filter according to Claim 1, characterized in that the heating element (18) extends along a longitudinal axis of the particulate filter (10) or coaxially to this one.   Particle filter according to Claim 1 or 2, characterized in that the heating element (18) is tube or bar shape.   Particle filter according to Claim 3, characterized in that the heating element (18) has a   cross-sectional area of 1 to 3 cm2.   5. Particle filter according to any one of   preceding claims, characterized in that the element   heater (18) is mounted in an area of the filter element (16) in which a space velocity of a large exhaust gas, especially the largest,   for a regular distribution of the flow channels.   6. Particle filter according to any one of   preceding claims, characterized in that the element   filtering (16) is monolithic or consists of   different segments (28) which are glued together.   7. Particle filter according to any one of   preceding claims, characterized in that the element   heater (18) is mounted in the corner of segments of   adjacent filter (28) when the filter (10) is segmented.   8. Particle filter according to any one of   preceding claims, characterized in that the element   the heating element (18) is designed so that heating element temperatures T> 500 oC can already be   achieved for electric heating power P <1 kW.   9. Particle filter according to any one of   preceding claims, characterized in that the element   heater (18) is made at least partially of a   holding alone with the filter element (16).   10. Process for the regeneration of a filter   particles (10) for an exhaust gas purification system.   of internal combustion machines, with a filter element (16) electrically conductive ceramic, in particular silicon carbide, characterized in that a regeneration of the particulate filter (10) is triggered by the application of a voltage on an electric heating element (18) which is constructed in particular in the electrically conductive ceramic and which is associated with the filter element (16).   11. The method of claim 10, characterized in that the voltage corresponds to the edge voltage, in particular in a range of 8 to 14 V, especially 12 to 14 V.   12. Process according to any one of the claims   and 11, characterized in that the voltage requirement is based on particle filter fouling, filter temperature, exhaust gas temperature, ignition temperature desired and / or a mass flow of exhaust gas.   13. Process according to any one of the claims   12, characterized in that the regeneration is effected   killed periodically and / or depending on the clogging of the filter.