GB2246715A

GB2246715A - Reducing light-off time in a catalytic converter

Info

Publication number: GB2246715A
Application number: GB9017635A
Authority: GB
Inventors: Thomas Tsoi-Hei
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1990-08-11
Filing date: 1990-08-11
Publication date: 1992-02-12
Also published as: GB9017635D0

Abstract

In order to reduce the light-off time, the exhaust gases from a cold engine are constrained by diverting screens to enter a catalyst matrix via a restricted area and to flow back and forth through a sequence of successive parallel channels therein. When the catalyst is warm all the channels are available for the gas to pass through only once. In Fig. 1 the diverting means comprises an extensible tube 18, which concentrates inlet gas on the centre of first matrix block 16a, and frustum 20 closed by valve 22 so that the gases pass through the core of matrix 16a, the inner of ring seals 28 and the core of 16b before being reflected by frustum 22 back through annular portions of 16a and b and the space between seals 28. Final passage is through the outsides of the matrices, around the frustum and out of inlet 14. A similar result is achieved by flap valves 32, 34 in Fig. 2. Microwaves may be supplied via connector 24 to heat the catalyst. <IMAGE>

Description

Title CATALYTIC CONVERTER Field of the invention The present invention relates to a catalytic converter for a motor vehicle.

Description of the known prior art Catalytic converters are used in motor vehicles to purify the exhaust gases by reducing content of carbon monoxide, unburnt hydrocarbons and noxious oxides of nitrogen. This is achieved by passing the gases over a matrix containing a catalyst, such as platinum, at a temperature at which they will react with each other and with surplus oxygen.

The temperature of the catalyst is important because below a certain so-called light off temperature, usually between 2500C and 3000C, the catalytic reaction does not occur to any appreciable extent.

Thus while the catalytic converter can significantly reduce pollution when it is hot, it has little effect at start-up from cold and it is desirable to take steps in this case to ensure that the light off temperature is reached as quickly as possible.

With this aim in mind, proposals have been made to heat the catalyst from an external energy source before and during start-up. This however requires additional equipment and in view of the large heat capacity of the converter, considerable electrical power is needed to raise its temperature in an acceptably short time.

It has also been suggested to use a separate start-up catalyst, which is smaller in size than the main converter and is located near to the exhaust ports. Because of its small size and its proximity to the engine, this auxiliary catalyst lights off earlier than the main catalyst but because it is subjected to a more severe environment, it tends to age more rapidly and to lose its effectiveness.

Object of the invention The invention seeks to enable a catalyst to reach its light off temperature more rapidly without adversely affecting the lifetime of the catalyst.

Summary of the invention According to the present invention, there is provided a catalytic converter for purifying engine exhaust gases, having a casing, inlet and outlet ducts leading to and from the casing respectively, and a matrix disposed within the casing between the inlet and outlet ducts to be traversed by engine exhaust gases, the matrix having a plurality of parallel channels and incorporating particles of a catalyst, characterised by flow diverting means arranged within the casing between the matrix and the inlet and outlet ducts, the flow diverting means having a first position in which exhaust gases are permitted to flow in parallel in the same direction through all channels of the matrix to traverse the length of the matrix only once and at least one second position in which the exhaust gases are constrained to follow a convoluted path, passing serially through different channels of the matrix and traversing the length of the matrix more than once.

In the invention, the flow diverting means within the catalytic converter allow the gases to flow in one of two ways through the matrix. After light off, the gases are made to pass through a matrix which is configured as a short wide matrix having all its channels connected in parallel with one another in the forward direction of gas flow. However, before light off, the gases are made to flow forwards and backwards along a convoluted path traversing the matrix several times but restricted to a small group of channels in each traverse. In other words, the converter appears as a long narrow matrix which near its entry point tends to heat up more rapidly since the gas concentration is increased.

Altering the flow geometry from a short wide matrix to a long thin matrix offers two additional advantages. First the gases travel a longer distance through the converter allowing the catalytic reaction, which is itself exothermic, to take place to a fuller extent. Second, the increased back pressure results in increased engine load which raises the exhaust temperature and exhaust flow rate, both of which assist in heating the central section even more rapidly.

Preferred features of the invention In one embodiment of the invention, the matrix is divided by the flow diverting means in the second position of the latter into a central section, surrounded by two concentric annular sections, the gases being directed first along the central section in the direction from the inlet to the outlet, then along the inner annular section in the direction from the outlet to the inlet and finally along the outer annular section towards the outlet.

Conveniently, in this case, the flow diverting means may comprise a telescopically collapsible section of inlet tubing capable of being extended from the inlet to a position adjacent the inlet end of the matrix, a truncated cone of wider base diameter than the inlet tubing mounted adjacent the outlet end of the matrix and a valve at the other end of the truncated cone.

The central section in this case will heat up most but the heat remaining in the exhaust gases after traversing the central section is not wasted but is used to heat the surround portions of the matrix which act as a jacket to reduce heat loss from the central section of the matrix.

It is not essential that the individual traverses of the convoluted path should be concentric and in alternative embodiments of the invention, the path may be folded into one or more folds all of which lie in the same plane. In such a case, the flow diverting means may conveniently be formed by pivotable flaps arranged at the inlet and outlet ends of the converter. The flaps preferably lie in a streamlined position parallel to the gas flow when the catalyst is hot and cooperate with profiled surfaces on the matrix to divert the gas flow along the desired path prior to light off.

The matrix of a catalytic converter is often formed of two blocks, called bricks, which are separated from one another by a gap. This method of construction is desirable in that it simplifies manufacture but it also has the advantage of breaking up the laminar flow of the gases along the narrow channels of the matrix to increase the reaction rate.

If the matrix is constructed of two or more bricks in the present invention, then it is important to provide partitions within the gaps between bricks to prevent the flow of the gases in the convoluted path from mixing with one another at the junction between bricks.

The flow diverting means are preferably automatically controlled in dependence upon exhaust gas outlet temperature. One may provide a temperature sensor for controlling the flow diverting means through a suitable servo control system but more simply bimetal devices may be used to effect automatic mechanical change over of the position of the flow diverting means, as a function of exhaust gas outlet temperature.

If an external heat supply is used to pre-heat the catalytic converter or supplement the heat from the exhaust gases during start up, then in the present invention it suffices to heat only the entry section of the convoluted flow path through the catalyst. This reduces the power requirement of the external source.

Furthermore, if microwave energy is used to effect such auxiliary heating, then the flow diverting means may additionally function as a waveguide to direct the microwave energy to the entry section of the matrix.

Brief description of the drawings The invention will now be described further, by way of example, with reference to the accompanying drawings, in which Fig 1 is an axial section through a first catalytic converter embodying the invention, and Figs 2 and 3 are similar sections through two alternative embodiments of the invention.

Detailed description of the preferred embodiments The converter in Figure 1 comprises a housing 10 having an inlet pipe 12 and an outlet pipe 14. Within the housing 10, there is mounted a honeycomb matrix 16 formed of two bricks 16a and 16b separated from one another by a small air gap 16c. The matrix 16 is supported in the housing 10 by means of a mat 26 which acts as padding to hold the matrix in place within the housing 10.

The inlet pipe 12 has slidably and telescopically mounted within it a tube 18 which can be moved from a retracted position shown in dotted lines to the position shown in solid lines in which it abuts the matrix 16.

At the outlet end of the matrix 16, a cone 20 is mounted within the housing 10 with its base abutting the matrix 16. The narrow end of the cone can be closed by means of a butterfly valve 22.

When the catalyst is warm, the tube 18 is retracted and the valve 22 is opened, as shown in dotted lines in the drawing. The exhaust gases enter through the inlet pipe 12 and pass in parallel through all the channels of the matrix 16 and leave through the outlet pipe 14. This is the same as the path normally followed through a conventional catalytic converter.

When the catalyst is cold, the tube 18 is moved forwards to abut the matrix 16 and the valve 22 is closed as shown in solid lines in the drawings. The air gap 16c between the bricks 16a and 16b in this embodiment contain partition walls 28 in the form of two concentric rings to prevent radial flow of exhaust gases and divide the matrix into a central cylindrical section surrounded by two concentric annular sections. With the tube 18 abutting the matrix 16, the exhaust gases can only enter into the inner section of the matrix. The gases flow through the channels in the central section of the matrix brick 16a and enter the gap 16c. Here they are prevented from radial flow by the inner partition and they continue their axial flow through the second brick 16b until they exit into a closed chamber defined by the interior of the cone 20 which is closed off by the butterfly valve 22.

The base of the cone 20 is of the same diameter of as the second partition ring in the gap 16c which is larger than the central section. The gases can therefore perform a Uturn and pass a second time through the matrix, this time passing though the inner of the two concentric annular sections until they again reach the inlet end of the matrix 16 where they will enter another closed chamber surrounding the tube 18. The gases will now turn around once more and travel a third time through the matrix, flowing during this traverse through the outer one of the concentric annular sections. On leaving the outer annular sections, the gases can reach the outlet pipe 14 through the space surrounding the cone 20 at the outlet end of the matrix 16.

Thus, with the flow diverting means constituted by the tube 18 and the valve 22, the gases may selectively be directed to pass through a short wide matrix under normal conditions or a long thin matrix when the catalyst is still cold.

A connector 24 is provided on the inlet pipe to enable a microwave source to be connected to the converter. When the tube 18 is in the position shown in solid lines, microwave energy can be supplied through the connector 24 and be guided along the inlet pipe 12 and the tube 18 to heat only the central section of the catalyst. This concentration of the energy permits a magnetron of lower power to be used to heat the catalyst.

The embodiment illustrated in Figures 2 and 3 operate on the same principle of enabling the gases to follow either a straight path or a convoluted path through the matrix and they differ from the embodiment of Figure 1 in the construction of the flow diverting means and the geometry of the convoluted flow path.

In Figure 2, two flap valves 32, 34 are provided at the inlet and outlet ends of the matrix 16. At normal temperature, the flap valves lie in the positions shown in dotted lines where they present no obstruction to the gas flow. When the catalyst is cold, however, they occupy the positions shown in solid lines in which they define a Z-shaped convoluted path. The partitions 28 in this case are two straight spaced walls dividing the matrix into an upper, a central and a lower section. The exhaust gases in this case pass in the forward direction (i.e. from inlet to outlet) through the upper section, in the reverse direction through the central section and in the forward direction in the lower section.

In this embodiment, as in the case of the embodiment of Figure 3, it is necessary to contour the end faces of the matrix 16 appropriately to provide an effective seal with the flap valves.

The embodiment of Figure 1 is preferred over that of Figure 2 in that the entry section of the convoluted path, that is to say the central section, is thermally insulated by the surrounding annular sections. In Figure 2, the entry section, being the upper section, is adjacent the housing 10 and is not therefore as well thermally insulated.

The embodiment of Figure 3 is an effective compromise between the other two embodiments in this respect. As will be evident without the need for detailed description from comparison with Figure 2, when the flap valves 42, 44, 46 and 48 are in the dotted line position, they do not interfere with the gas flow but in the solid line positions, the gases are direction along first a central section in the forward direction, then two parallel adjacent section in the reverse direction, then finally in the forward direction through two parallel outer sections lying adjacent the housing. The central section is still not insulated on two sides but it is partially surrounded by the convolutions which in this case are in the form of two Z-shapes disposed one above the other.

It will be appreciated that an external heat source can also be used in the case of the last two embodiments to improve the light off time still further.

Claims

CLAIM8

1. A catalytic converter for purifying engine exhaust gases, having a casing, inlet and outlet ducts leading to and from the casing respectively, and a matrix disposed within the casing between the inlet and outlet ducts to be traversed by engine exhaust gases, the matrix having a plurality of parallel channels and incorporating particles of a catalyst, characterised by flow diverting means arranged within the casing between the matrix and the inlet and outlet ducts, the flow diverting means having a first position in which exhaust gases are permitted to flow in parallel in the same direction through all channels of the matrix to traverse the length of the matrix only once and at least one second position in which the exhaust gases are constrained to follow a convoluted path, passing serially through different channels of the matrix and traversing the length of the matrix more than once.

2. A catalytic converter as claimed in claim 1, wherein the matrix is divided by the flow diverting means in the second position of the latter into a central section, surrounded by two concentric annular sections, the gases being directed first along the central section in the direction from the inlet to the outlet, then along the inner annular section in the direction from the outlet to the inlet and finally along the outer annular section towards the outlet.

3. A catalytic converter as claimed in claim 2, wherein the flow diverting means comprise a telescopically collapsible section of inlet tubing capable of being extended from the inlet to a position adjacent the inlet end of the matrix, a truncated cone of wider base diameter than the inlet tubing mounted adjacent the outlet end of the matrix and a valve at the other end of the truncated cone.

4. A catalytic converter as claimed in claim 1, wherein the gas flow path is folded by the flow diverting means into one or more Z-folds.

5. A catalytic converter as claimed in claim 4, wherein the flow diverting means are formed by pivotable flaps arranged at the inlet and outlet ends of the converter.

6. A catalytic converter as claimed in claim 5, wherein the flaps lie in a streamlined position parallel to the gas flow when the catalyst is hot and cooperate with profiled surfaces on the matrix to divert the gas flow along the desired path prior to light off.

7. A catalytic converter as claimed in any preceding claim, wherein the matrix comprises two bricks separated from one another by a gap and wherein partitions are provided within the gap between bricks to prevent the flow of the gases in the convoluted path from mixing with one another at the junction between the bricks.

8. A catalytic converter as claimed in any preceding claim, wherein all movable elements of the flow diverting means at the inlet and outlet of the converter are ganged for simultaneous operation.

9. A catalytic converter as claimed in claim 8, wherein the flow diverting means are automatically controlled in dependence upon exhaust gas outlet temperature.

10. A catalytic converter as claimed in claim 9, wherein a temperature sensor is provided for sensing the exhaust gas temperature at the outlet of the converter and for controlling the change over of the flow diverting means through a servo control system.

11. A catalytic converter as claimed in claim 9, wherein bimetal devices are used to effect automatic mechanical change over of the position of the flow diverting means as a function of exhaust gas outlet temperature.

12. A catalytic converter as claimed in any preceding claim, wherein an external supply of heat is provided to pre-heat the catalytic converter or supplement the heat from the exhaust gases during start up, the heat supply being arranged to heat only the entry section of the convoluted flow path through the catalyst.

13. A catalytic converter as claimed in claim 12, wherein the external heat source is a source of microwave energy and wherein the flow diverting means additionally function as a waveguide to direct the microwave energy to the entry section of the matrix.

14. A catalytic converter constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in Figures 1 to 3 of the accompanying drawings.