GB2316017A

GB2316017A - Exhaust gas purification device

Info

Publication number: GB2316017A
Application number: GB9616482A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1996-08-06
Filing date: 1996-08-06
Publication date: 1998-02-18
Also published as: GB9616482D0

Abstract

An exhaust gas purification device is described that comprises a housing 10, a chemically active matrix 20 arranged in the housing 10 to define within the housing a supply 14 and a discharge 16 plenum chamber on opposite sides of the flow end faces of the matrix, and supply 11 and discharge 18 pipes connected to the respective plenum chambers. In the invention, the supply pipe 11 is positioned near to the edge at a side wall of the matrix 20 to direct the flow of exhaust gases into the supply plenum chamber 14 generally parallel to the front face of the matrix 20 and the discharge pipe 18 is connected to the discharge plenum chamber 16 near to the opposite edge at the same side wall of the matrix as the supply pipe 11.

Description

EXHAUST GAS PURIFICATION DEVICE The present invention relates to an exhaust gas purification device comprising a housing, a chemically active matrix arranged in the housing to define within the housing a supply and a discharge plenum chamber on opposite sides of the flow end faces of the matrix, and supply and discharge pipes connected to the respective plenum chambers.

The exhaust gas purification device may be a catalytic converter, a storage trap, or any other device used to clean the exhaust gases of an internal combustion engine by passing the gases through a matrix.

In known catalytic converters using a matrix with a honeycomb of capillary tubes to provide a large reaction surface for contacting the exhaust gases, the design of catalytic converter is intended to ensure that the gas flow is uniform over the entire flow end cross section of the matrix so that no hot spots are created on account of the gas flow favouring one part of the matrix over another. The gas flow in the supply plenum and in the matrix are in line with one another and the plenum is designed to distribute the flow out of the supply pipe evenly over the front face of the matrix. Similar considerations are also adopted in the design of NOx and HC storage traps.

A disadvantage associated with such a construction is that the front face of the matrix is prone to accumulation of contamination, thereby affecting the durability of the purification device.

In catalytic converters, the even distribution of the gases increases the light off time because it is necessary to supply enough heat to heat the entire front face of the catalytic matrix.

With a view to mitigating the foregoing disadvantages, in the present invention the supply pipe is positioned near to the edge between the front face and a side wall of the matrix to direct the flow of exhaust gases into the supply plenum chamber generally parallel to the front face of the matrix and the discharge pipe is connected to the discharge plenum chamber near to the edge between the same side wall and the rear face of the matrix.

Because the exhaust gases is directed across the front face of the matrix and returns in a reverse direction towards the discharge pipe, the gases must perform a U-turn in the purification device and pass through the matrix during their change in direction. The supply gases will fill the supply plenum from one side, seep laterally through the capillary tubes of the matrix and then leave the purification device from the same side as they entered.

Because of this geometry, the usage of the flow cross section of the matrix will vary with the flow rate of the exhaust gases. At low flow rates, most of the flow volume will pass through the section of the matrix near the side of the matrix close to the supply and discharge pipes. With progressively increasing flow rates, more and more capillary tubes will come into play and the usage area of the matrix will spread from the side of the matrix close to the supply and discharge pipes towards the side remote from the supply and discharge pipes.

It is preferred that the flow cross section of the matrix should be long and narrow, the supply and discharge pipes being positioned by one of the narrow side walls of the matrix.

The effect of spreading the usage of the matrix is enhanced by directing the supply pipe, and preferably also the discharge pipe, parallel to the longer side of the matrix. At high flow rate, the momentum of the supply gases will cause the gases to penetrate deeper into the supply plenum chamber before they turn through a right angle to pass through the matrix.

Because different parts of the matrix will come into play at different flow rates, the matrix properties may be modified along its length as may also be the design of the housing.

For example, NOx is produced predominantly under high engine loads and one may therefore provide an NOx storage material only on the section of the matrix remote from the supply and discharge pipes. If desired, the matrix may be constituted by several bricks, having different chemical compositions, arranged side by side with one another.

Furthermore, in a close coupled catalytic converter, the need for rapid light off during cold starts and low load operation conflicts with the need to avoid over-heating of the catalyst under high load operation. In the present invention, it is possible to provide cooling fins in the housing to extract heat from the exhaust gases only at the part of the housing remote from the supply and discharge pipes. Thus the exhaust gases will not be cooled during low load operation and the small area of the catalyst that is effective during such operation will light off rapidly. At higher loads, however, the risk of over-heating is reduced by extracting heat from the exhaust gases having sufficient momentum to reach the cooling fins.

An external fan may preferably be provided to force cool the fins at or near the maximum load conditions. This allows the exit gas temperature from catalytic converter to be regulated to match more closely the temperature requirement of another purification device downstream, for example an NOx trap, which has a narrower operating temperature range than a catalytic converter.

The gas purification device of the invention has the advantage that because the matrix has a larger cross section and a shorter length between its flow end faces for a given volume, it offers less resistance to flow than conventional devices. The engine can therefore breath more freely and operate more efficiently.

It is also easier to package a purification device of the invention in a vehicle because the housing will be generally long and narrow in place of the short squat construction of conventional catalytic converters and storage traps. Such a device may either be fitted horizontally within the tunnel that is normally provided in the floor pan of the vehicle, or it may be mounted vertically within the engine compartment.

As earlier mentioned, the supply plenum chamber should be long and narrow and ideally its cross sectional area in the direction of the supply gas flow should be comparable with that of the supply pipe. Consequently, there is no need to provide entry and exit cones to connect respectively the supply and discharge pipes to the housing and this not only saves on manufacturing cost but also reduces pressure losses. Such pressure loss is known to occur on account of turbulence caused by the separation of the boundary layer which occurs within the conical sections.

The invention will now be described further, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a section of a gas purification device of the invention taken along the direction of gas flow in the supply plenum chamber, Figure 2 is a section of the device in Figure 1 taken along the line II-II in Figure 1, Figure 3 is a section of the device in Figure 1 taken along the line III-III in Figure 1, Figures 4, 5 and 6 are perspective views of various embodiments of the invention.

In Figures 1 to 3, a catalytic converter is shown that comprises a housing 10 which is of generally elliptical cross section and contains a catalytic matrix 20. The matrix 20 is an elongate block surrounded by a sealing mat 22. The matrix 20 is mounted with its flow end faces parallel to the minor axis of the elliptical housing and defines at opposite ends of the major axis a supply plenum chamber 14 and a discharge plenum chamber 16 each extending along the length of the housing 10. On the left hand side of the housing as view in Figures 1 and 3, a supply pipe 11 and a discharge pipe 18 communicate with the respective plenum chambers 14 and 16, the supply pipe 11 being fitted at its end with a nozzle 12 to direct a stream of exhaust gases along the length of the supply plenum chamber 14 parallel to the front face of the matrix 20. Heat exchange fins 17 project into the supply plenum chamber 14 but only at the end of the chamber remote from the pipe 11 and similarly cooling fins 15 project outwards from the housing 10 at the same end.

Under low flow rates, the bulk of the exhaust gases entering the supply plenum chamber 14 will find their way to the discharge pipe 18 while flowing only in the section of the matrix 20 lying near the supply and discharge pipes 11 and 18. In other words, the flow distribution through the capillary tubes of the matrix 20 will be decidedly nonuniform and concentrated at the left hand side of the matrix 20 as shown in Figures 1 and 3. With ever increasing flow rates, the momentum of the exhaust gases will carry them further and further into the supply plenum chamber 14.

Furthermore the capillary tubes at the left hand side of the matrix 20 will be unable to carry the full flow and more and more capillary tubes are called into play starting from the left hand side of the matrix 20 as the flow rate increases.

Only at or near the maximum flow rate will the entire cross section of the matrix 20 be used equally.

This variable flow distribution through the matrix 20 that depends upon the gas flow rate has several advantages in the case of a catalytic converter as well as in the case of a storage trap. Dealing first with the catalytic converter implementations of the present invention, an advantage is that during cold start and low flow rate operation only a small section of the matrix 20 is used so that its light off time is reduced. At higher flow rates the heat generation is more evenly distributed over a larger section of the catalytic matrix 20 so that no section tends to over-heat. This redistribution of flow and heat not only safeguards the catalyst under high load conditions, but it also reduces the contamination of the front face of the section of the matrix 20 that is active under low flow conditions.

The heat exchange fins 17 and the cooling fins 15 are effective in removing heat from the exhaust gases only under high flow rate conditions because they are located in a region of the supply plenum chamber 14 remote from the supply pipe 11. This has particular advantages for a close coupled catalytic converter in that it permits the catalyst to survive under high load without impairing its ability to light off rapidly under low load. Also it allows the exit gas temperature from the catalytic converter to be regulated to match more closely the temperature requirement of another purification device downstream, for example an NOX trap which has a narrower operating temperature range than the catalytic converter.

Because different sections of the matrix 20 are effective at different flow rates, one can take advantage of this to modify the chemical composition of the matrix to suit the exhaust gas composition under the prevailing engine operating conditions. Thus the section of the matrix 20 remote from the supply pipe 11 will only be effective under high load conditions and as it is under these conditions that NOX is predominantly produced, that section of the matrix 20 can be designed to contain an NOX storage medium.

By the same token, the nearer section of the matrix 20 to the supply pipe 11 may be designed to contain a redox three way catalyst medium to cope with emissions during idle and low load conditions.

The matrix 20 need not be a catalyst and may be a storage trap or a plasma reactor. In the case of a storage trap which has a large matrix, the main advantages of the invention reside in reduced pressure losses and simplified packaging in the vehicle on account of the lateral flow across the housing 10 and the long and narrow shape of the housing 10. These advantages are common to all the embodiments of the invention.

Plasma reactors with a matrix comprising a packed bed of pellets of dielectric material have been proposed as a means of purifying exhaust gases and these involve the application of a high voltage to create an electrical field within the matrix to promote chemical reaction between the constituents in the exhaust gases. The disadvantage of such reactors is their high power consumption and in the present invention this problem may be mitigated by dividing the matrix into sections and only applying a voltage across the sections that are active at any one time having regard to the flow rate of gases through the purification device.

Various geometries can be adopted for the external layout and positioning of the purification device of the invention. Figure 4 shows a configuration for a close coupled catalytic converter. The supply pipe lia is in this case directly connected to the exhaust manifold of the engine and enters the housing 10a from the top. The discharge pipe 18a exits laterally from the top of the housing 10a and is bent through an elbow to continue as a downpipe towards the underside of the vehicle. In this case the exhaust gas flow at low speed is generally L-shaped descending into the supply plenum chamber and immediately turning right as viewed to pass through the catalyst matrix and exit via the pipe 18a. At high speeds, the flow is more U-shaped penetrating lower into the supply plenum chamber in the housing 10a and utilising a larger section of the catalyst matrix. This embodiment lends itself particularly well to additional cooling using the fins 15a which are effective only during high flow rates. An external fan (not shown) may also be provided to force cool the fins 15a at or near the maximum flow conditions.

The housing 10b in the case of the embodiment of Figure 5 may be an NOX trap and is intended to be mounted vertically in the engine compartment of the vehicle. The downpipe of the exhaust system is connected to the horizontal supply pipe 11b and the discharge pipe 18b leads horizontally to the exhaust silencer. Because the pipes llb and 18b are crossed, the gas flow is turned as it enters the supply plenum chamber in the housing 10b so that its momentum directs it away from the front face of the matrix.

The embodiment of Figure 6 is intended to be mounted in line with the exhaust system within the tunnel normally provided in the floor pan of the vehicle. To obtain an inline configuration while supplying and discharging gases from the same end of the housing, the discharge pipe 18c is designed in this case to penetrate into the housing 10c as represented by the dotted lines to withdraw gases from the end of the housing near the supply pipe llc. The charge plenum chamber in the housing 10c is in this case flared around the discharge pipe 18c so as not to be obstructed by the discharge pipe 18c.

Claims

1. An exhaust gas purification device comprising a housing, a chemically active matrix arranged in the housing to define within the housing a supply and a discharge plenum chamber on opposite sides of the flow end faces of the matrix, and supply and discharge pipes connected to the respective plenum chambers, characterised in that the supply pipe is positioned near to the edge between the front face and a side wall of the matrix to direct the flow of exhaust gases into the supply plenum chamber generally parallel to the front face of the matrix and the discharge pipe is connected to the discharge plenum chamber near to the edge between the same side wall and the rear face of the matrix.

2. A device as claimed in claim 1, wherein the flow cross section of the matrix is substantially longer than it is wide, and wherein the supply and discharge pipes are positioned by one of the narrower sides of the matrix.

3. A device as claimed in claim 1 or 2, wherein the supply pipe is directed parallel to the longer side of the matrix.

4. A device as claimed in claim 1, 2 or 3, wherein the properties of the matrix vary with distance from the side near to the supply and discharge pipes.

5. A device as claimed in claim 4, wherein a redox three way catalyst material is included in a section of the matrix near to the supply and discharge pipes.

6. A device as claimed in claim 4 or 5, wherein an NOX storage material is included in a section of the matrix remote from the supply and discharge pipes.

7. A device as claimed in claim 4, 5 or 6, wherein the matrix is constituted by two or more bricks arranged side by side one another.

8. A device as claimed in any preceding claim, wherein fins are provided to extend into the supply plenum chamber and/or to extend outwards from the housing in order to extract heat from the exhaust gases only at the part of the housing remote from the supply and discharge pipes.

9. A device as claimed in claim 8, wherein a fan is provided to blow air over the external fins.

10. A device as claimed in claim 9, wherein means are provided for controlling the fan to regulate the temperature of the gases leaving the device.

11. A device as claimed in claim 1, wherein the matrix form part of a plasma reactor and means are provided for applying a high voltage across the matrix.

12. A device as claimed in claim 11, wherein the matrix is divided into sections and means are provided for applying a high voltage to only selected sections of the matrix, the sections varying with the gas flow rate through the purification device.

13. A gas purification device constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawings.