Exhaust Gas Filter for Diesel Engines

Exhaust gas filters for Diesel engines have the purpose of filtering soot out of the exhaust gas of Diesel engines. The emission of soot from Diesel engines is critical because the soot particles contain adsorbed at their surface small amounts of substances some of which in high concentrations are considered to be harmful to human health.

The soot filters for the Diesel exhaust gases conventionally consist of a monolith of a porous ceramic material which is constructed of thin-walled ducts that are parallel to one another and extend over the whole length of the monolith in the flow direction of the exhaust gases. These ducts are alternately closed at their upstream or downstream end. The ducts are frequently open or closed in the manner of a chessboard pattern, so that each duct has an open and a closed end. Another way to make filters of this type consists of winding the filter in a spiral shape in the manner of manufacturing a corrugated board, to form a gas-permeable packet. In this case, the individual ducts are no longer developed only in the manner of a chessboard, but are also arranged in a spiral shape.

When the Diesel exhausts are led through a filter of this type, the exhaust gas entering through the ducts that are open on the inlet side is forced to pass through the porous duct walls and flow into the adjacent ducts that are open on the outlet side. In this case, the soot particles that can no longer pass through the duct walls collect in the ducts that are open upstream. When a certain final ignition temperature is reached and when a sufficient oxygen concentration exists in the exhaust gas, the collected soot burns off and the filter is regenerated. However, it was found that the porosity of the conventional soot burning filters decreases as times goes on, so that a rising backpressure builds up in front of the filter leading to a reduction of power in the engine and an increase of fuel consumption.

The objective of the present invention consists of finding a process for manufacturing an exhaust gas filter for Diesel engines made of a porous ceramic material whereby the decrease of porosity as well as the rising counterpressure in front of the filter that occur over the working life of the filter can be reduced considerably.

This objective is achieved by means of having a multi-zoned filter having different porosities in each zone, as well as by a process for manufacturing an exhaust gas filter whereby the filter is partially immersed in a solution or suspension of a porosity-reducing agent to create the above zones.

The filter block contemplated in the invention consists of several zones along the length of the filter, where the porosity of the duct walls in each zone decreases from zone to zone in the main flow direction of the gases from the gas inlet to the gas outlet. In each case, plugs are alternately arranged at the border of the zones. On the side of the gas inlet, every other duct is closed by a plug. The gas enters into the ducts that are open, which ducts are subsquently closed by plugs at the border to the next zone which has a decreased wall porosity. As a result, the gas is forced to pass through the porous wall of the first zone having walls with a relatively large wall porosity and thus arrives in the second duct, which duct is subsequently closed by a plug at the start of the next zone which has a further decreased wall porosity. As a result, the gas flow is forced to pass through the less porous walls of the second zone. It is now possible, after the passage through two zones with different porosity, to let the gases out of the filter. It is also possible to connect additional zones with correspondingly reduced porosity so that the gas flow is filtered several more times through walls having a decreasing porosity. It was found in this case that the best results are achieved when the gas flow must pass through two to four zones with different porosity. In this case, the expense for the manufacturing the filters is disproportionately high.

The staggering of porosities should be carried out in such a way that in the last filtering step through the wall having the least porosity, and sufficient gas passage is possible without excessive backpressure. A staggering of porosities has been especially successful where, in the flow direction of the gases, each successive zone is decreased to about 70 to 90% of the porosity of the preceding zone, in which case the porosity of the last zone normally should not be below 40%. While the porosity of the ceramic material in the case of the conventional filters is between 35 and 55% with an average pore diameter of about 10 to 35 micrometers, in the case of the present filter, the porosity of the first filter step is selected to be larger, approximately in the range of a porosity of 50 to 75%, with a correspondingly larger average pore diameter.

An especially simple process for the manufacturing of this filter consists of starting with a conventional filter block blank, the porosity of which corresponds to the porosity of the first filter step and thus represents the largest used porosity. Starting with the outlet end of the filter, this filter block will now be immersed up to the level of the first zone in a solution or suspension of an agent which reduces the porosity and then dried. The consistency or concentration of this agent must be selected to be such that, after the drying or possible required burning-in, the desired porosity of the part of the filter that was immersed is obtained. The porosity reducing characteristics of the corresponding agents must be able to determined easily beforehand by means of tests. If the filter is to have more than two zones, the filter may be immersed again, up to the level of the additional zone border, in the same or a different suspension or solution. The use of the same solution is suitable when the filter is dried before the second immersion, while the use of another solution is advantageous when the filter, without any intermediate treatment, i.e., still moist from the first immersion process, is subjected to the second immersion process. It may also be necessary to immerse the filter several times in a corresponding suspension or solution, if the desired reduction of porosity in a zone cannot be obtained by means of one immersion process.

All solutions and suspensions of porosity-reducing agents that can withstand the later operational temperature of the filter block and are correspondingly abrasion-proof with respect to the gas flow may be used. For this purpose, only essentially inorganic substances can be used. Because they are easy to handle, water glass solutions have proven to be especially useful. In the case of higher demands on the temperature stability, slurries or suspensions of materials resistant to high temperatures were also found to be useful, especially materials that are also used for the manufacturing of the filter, such as cordierite or china clay/feldspar/quartz mixtures. If necessary, these suspensions may also contain an inorganic binding agent, such as a small quantity of water glass. It is advantageous to adjust the concentration of the suspension or solution in such a way that with a single immersion, the desired reduction of porosity by 10 to 30% can be achieved after the drying and possible burning-in of the blank.

As soon as the filter block blank is provided with the desired number of zones having duct walls with decreasing porosity along the length of the filter in the flow direction, the corresponding alternately arranged plugs, at the filter inlet, at the zone borders and at the filter outlet, may be arranged in the ducts. At the filter inlet and at the filter outlet, this takes place by conventional methods. Inside the filter at the zone borders, this placement may take place by pressing out a pasty sealing mass consisting, for example, of a mass of talc and water glass, and inserting the mass into the filter duct at the corresponding zone border by means of a hollow needle, thereby closing the duct at this point.

Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of the invention when considered in conjunction with the accompanying drawings.

The drawings, in diagram form, show two embodiments of an exhaust gas filter according to the invention.

FIG. 1 is a longitudinal sectional view through an exhaust gas filter formed by two zones;

FIG. 2 is a sectional view through an exhaust gas filter consisting of three zones.

In the case of the filter shown in FIG. 1, the gas flows through the filter from the left to the right. On the side of the inlet, every other duct is closed by a first row 1 of plugs. The exhaust gas flows into the open ducts 2, 2', 2", etc., and by means of the row 3 of the plugs by which the ducts 2, 2', 2", etc., are closed, is forced to flow through the porous walls 4, 4', etc., into the ducts 5, 5', etc. In the ducts 5, 5', etc., the exhaust gas now flows a distance until it encounters the plugs 6, 6', etc., inserted at the outlet side, whereupon the gas is forced to enter the ducts 8, 8', etc., through the porous walls 7, 7', etc., the exhaust being able to leave the filter through said ducts 8, 8', etc. The path of the gas is shown in diagram form in FIG. 1 by means of arrows drawn into the ducts. The porosity of the walls 4, 4', i.e. therefore of the first filter zone, is 70%, while the porosity of the walls 7, 7', i.e., of the second filter zone, is 50%.

FIG. 2 shows an exhaust gas filter with three different zones. At the inlet of the filter, every other duct again is closed by a row 21 of plugs. The exhaust gas enters the ducts 22, 22', etc., is forced by the row 23 of plugs located at the first zone border to enter the ducts 25, 25', etc., through the porous walls 24, 24', etc., of the first zone. By means of the third row 26 of plugs, the exhaust gas is forced to enter the ducts 28, 28', etc., through the porous walls 27, 27', etc., of the second zone. The ducts 28, 28', etc., at the filter end, are closed by the row 30 of plugs so that the gas is again forced to enter the ducts 31, 31', etc., through the porous walls 29, 29', etc., of the third zone, the gas being able to leave the filter from these ducts 31, 31'. The filter is constructed in this case in such a way that the porosity of the first zone is at about 70%, the porosity of the second zone is at 60%, and the porosity of the third zone is at 50%. As also shown in FIG. 2, the individual zones may have different sizes in order to be able to adapt them to different conditions.

Tests have shown that the porosity of a conventional soot burning filter, i.e., a filter having only one filter zone, having a filter length of 6 inches and an initial porosity of 50%, over a period of 20,000 miles of exhaust gas flow, is reduced as follows: The porosity decreases in the range of two inches from the outlet plug to 45%; in the range of four inches from the outlet plug to 40%, and in the range six inches from the outlet plug to 30%. That is, the porosity decreases the most in the area of the gas inlet. The overall porosity of the filter is therefore reduced from 50% to about 38.3%, resulting in a considerable rise in backpressure.

For a filter according to the invention having the same dimensions that is divided into two equally large zones of 70% and 50porosity, the following results are obtained after an identical time performance test and with the same filter efficiency: The porosity of the first zone decreases from 70% to about 50%, and the porosity of the second zone decreases from 50% to about 45%. Thus an overall porosity of 47.5% is obtained which is a considerable improvement in comparison to the overall porosity of only 38.3% that can be obtained by means of a conventional filter.

From the preceding description of the preferred embodiments, it is evident that the objects of the invention are attained, and although the invention has been described and illustrated in detail, it is to be clearly understood that the same is by way of illustration and example only and is not to be taken by way of limitation. The spirit and soope of the invention are to be limited only by the terms of the appended claims.