GB2284770A - Engine exhaust gas catalytic converter - Google Patents

Abstract

An engine exhaust gas catalytic converter 22 has a catalyst matrix 28, the front face of which forms a boundary wall of a burner chamber 30. The latter front face is contoured in such a manner that the surface area of the boundary wall exposed to the flame in the burner chamber 30 is significantly greater than the cross-sectional area of the matrix as measured in a plane normal to the direction of flow of the exhaust gases through the matrix 28. <IMAGE>

Description

ENGINE EXHAUST GAS CATALYTIC CONVERTER Field of the invention The present invention relates to the design of an engine exhaust gas catalytic converter of the type that incorporates a burner chamber disposed upstream of a catalyst matrix.

Background of the invention Afterburners relying on the ignition of a mixture of exhaust gases and air have been proposed to reduce the light-off time of a catalytic converter. Fuel burners have been proposed in which a separate fuel supply injects fuel directly into the burner chamber. In any of these systems the intense heat generated by the flame in the burner chamber raises the temperature of the front face of the following catalyst matrix very rapidly but the temperature gradient within the matrix is very steep and the temperature rapidly drops off away from the front face. One consequence of this is that only a very small volume of the matrix reaches its light-off temperature and the overall conversion efficiency of the matrix remains low and the matrix is not capable of coping with large amounts of unreacted exhaust flow.Maintaining the flame within the burner chamber for a more prolonged period is no solution to this problem because the front face would be overheated and damaged.

Obiect of the invention The invention therefore seeks to provide a catalytic converter in which the temperature gradient is rendered less steep to permit deeper penetration into the catalytic matrix of the volume reaching light-off temperature when the flame is ignited in the preceding burner chamber.

Summarv of the invention According to the present invention, there is provided an engine exhaust gas catalytic converter having a catalyst matrix the front face of which forms a boundary wall of a burner chamber and wherein the said front face is contoured in such a manner that the surface area of the said boundary wall exposed to the flame in the burner chamber is significantly greater than the cross-sectional area of the matrix as measured in a plane normal to the direction of flow of the exhaust gases through the matrix.

A matrix is conventionally a honeycomb-like structure with axially extending capillary-like tubes that is formed by an extrusion process. Individual matrices are cut from such an extrusion such that their end faces are normal to the direction of gas flow through the fine tube. In such a configuration when the end face is heated, heat flow can only take place in a direction parallel to these tubes while the front face exposed to the flame in the burner chamber is of the minimum possible area. These two factors combined to cause the severe temperature gradient at the time the flame in the burner must be extinguished to prevent damage to the catalyst.

In the present invention the front face is contoured so as not to be normal to the fine tubes. As well as increasing the surface area exposed to the flame and thus reducing the heating density (as measured in joules per square millimetre) the inclination of the front face allows heat flow by conduction to take place within the matrix transversely as well as axially thereby increasing the overall volume of catalyst heated by the flame.

It would be possible to contour the surface by a sinuous cut but this leaves fragile edges. It is therefore preferred to form conical pits in the front face of the matrix, the diameter of the pits are significantly larger than the diameter of the fine tubes carrying the exhaust gases.

Such pits or craters may be spaced from one another or their edges may overlap so that they cover the entire front face of the matrix.

Forming such conical craters would have the result of shortening the fine tubes at the tips of the cones thereby reducing the flow resistance. If the depth of the craters is significant in comparison with the overall length of the matrix, then this may have the effect of altering the distribution of flow of exhaust gases to make full use of the entire volume of the matrix.

Such craters or conical pits may conveniently be formed by machining the end surface of the matrix.

If desired, both end surfaces of the matrix may be machined so as to avoid incorrect assembly. It is also possible by forming pits are both ends to ensure that the variation in flow resistance across the area of the matrix does not vary excessively.

Conveniently the burner chamber forms a part of the catalytic converter being housed between two matrices the front face of the second matrix being contoured as described above.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a block schematic drawing of an engine fitted with an exhaust gas ignition system, and Figure 2 is a section through the second brick of a catalytic converter of a second embodiment of the invention.

Detailed descrintion of the drawings Figure 1 shows an engine 10 having an intake manifold 12, a butterfly throttle 14, an air mass flow meter 16 and fuel injector 18. The exhaust system comprises a pipe 20 leading to a catalytic converter 22 that is made up of a housing 24 containing two matrices 26, 28 separated from one another by an afterburner chamber 30 containing an igniter 32. A pump 34 is connected to the exhaust pipe 20 upstream of the catalytic converter to introduce additional air into the exhaust gases.

The operation of the system is already known per se and needs only be described briefly for an understanding of the present invention. The catalytic converter does not function until it matrices reach a light-off temperature of around 3500C. This means that during cold start the catalytic converter is ineffective. To reduce the light-time of the catalytic converter, the engine is operated for a few second after starting with an excessively rich mixture to cause carbon monoxide and hydrogen to be present in significant concentrations in the exhaust gases.

When additional air is introduced, the resulting mixture can be ignited even when the engine is cold and this is done by the igniter 32 in the afterburner chamber 30.

A catalytic converter matrix is normally formed of an extruded matrix of ceramic material with honeycomb-like passages for the gases to pass through. The matrices are cut from such an extrusion and their front face is therefore conventionally flat. The result of this is that the area facing the afterburner chamber is very small and within a very short time of the ignition, the front face of the matrix reaches a temperature high enough to endanger the catalytic material. This means that the flame in the afterburner must be extinguished but because the total amount of heat stored in the matrix is relatively small and confined to the front face of the matrix.The conversion efficiency of the catalytic converter is therefore low at the time the afterburner flame is extinguished because though the temperature at the front face is high, the degree of penetration and the total volume of the matrix that is active is still very small.

To mitigate this problem, the present invention provides for the contouring of the front face of the second matrix bounding the afterburner chamber. One way of achieving this, as shown in Figure 2, is to form conical craters or pits 34 in the front face of the matrix. By such contouring the total area of the matrix that is exposed to the flame is increased and therefore the rate of temperature rise at any point is reduced. Because more time is allow for conduction the temperature gradient is also reduced. Lastly heat flow between the craters can now take place radially as well as axially to an increased penetration of the light-off region.

As a result of all these effects, at the time the afterburner flame is extinguished, not only is there a larger area of matrix that is active but the penetration of the light-off zone is increased, so the overall conversion efficiency of the catalytic matrix at the end of the exhaust gas ignition phase is increased.

The improvement in conversion efficiency can be used to allow the exhaust gas ignition phase to be reduced in duration thereby mitigating any undesired side-effects of over fuelling such as plug fouling in the engine.

On the other hand, if the exhaust gas ignition phase is maintained for as long as the catalyst is not overheated, then the total amount of heat stored in the catalyst matrix is increased and this reduces the risk of the matrix being cooled down below the light-off temperature by the still cold exhaust gases, immediately after the extinguishing of the flame in the afterburner chamber 30.

The craters or conical pits 34 may conveniently be formed by machining the end surface of the matrix and the edges of the craters may either be spaced apart or overlapping.

Forming such conical craters would have the result of shortening the fine tubes at the tips of the cones thereby reducing the flow resistance. If the depth of the craters is significant in comparison with the overall length of the matrix, then this may have the effect of altering the distribution of flow of exhaust gases to make full use of the entire volume of the matrix.

In the embodiment illustrated in the Figure 2, both end surfaces of the matrix are formed with craters 34 in order to avoid incorrect assembly. Furthermore, by intentionally mis-aligning the craters 34 at both ends it is possible to ensure that the variation in flow resistance across the area of the matrix does not vary excessively.

Claims (7)

1. An engine exhaust gas catalytic converter having a catalyst matrix the front face of which forms a boundary wall of a burner chamber and wherein the said front face is contoured in such a manner that the surface area of the said boundary wall exposed to the flame in the burner chamber is significantly greater than the cross-sectional area of the matrix as measured in a plane normal to the direction of flow of the exhaust gases through the matrix.

2. A catalytic converter as claimed in claim 1, wherein conical pits are formed in the front face of the matrix, the diameter of the pits being significantly larger than the diameter of the fine tubes carrying the exhaust gases.

3. A catalytic converter as claimed in claim 2, wherein the pits are spaced apart from one another.

4. A catalytic converter as claimed in claim 3, wherein the pits are formed by machining the end surface of the matrix.

5. A catalytic converter as claimed in any preceding claim, wherein both ends of the matrix are contoured.

6. A catalytic converter as claimed in any preceding claim, wherein the burner chamber forms a part of the catalytic converter and is defined between two matrices, of the catalytic converter, the contoured surface being the front face of the second matrix.

7. An engine exhaust gas catalytic converter, constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.