GB2250216A

GB2250216A - A catalytic converter

Info

Publication number: GB2250216A
Application number: GB9025992A
Authority: GB
Inventors: Geoffrey Keene Wilson
Original assignee: MG Rover Group Ltd
Current assignee: MG Rover Group Ltd
Priority date: 1990-11-29
Filing date: 1990-11-29
Publication date: 1992-06-03
Also published as: GB9025992D0

Abstract

A catalytic converter includes a matrix (15) comprising a ceramic substrate (18) defining longitudinal passages (19). The wall of each passage is lined by a resistor comprising a coating (22) of positive temperature coefficient of resistivity material which in turn has an active coating (23) impregnated with a catalyst. Current in the resistor heats the matrix and allows catalytic reaction of exhaust gases from an internal combustion engine to begin soon after engine start-up. The rise in temperature in use raises the resistance of the heater so that little current passes when the converter has reached its running temperature. <IMAGE>

Description

A CATALYTIC CONVERTER The invention relates to catalytic converters for the exhaust gas of internal combustion engines.

Catalytic converters are required to ensure the complete combustion of carbon monoxide and hydrocarbons and to chemically reduce oxides of nitrogen. Known catalytic converters perform this function adequately once the catalyst reaches a temperature of about 300 degress Celcius. Unfortunately, it takes a little time for the exhaust gas to reach this temperature from a cold start, during which time the engine is operating relatively inefficiently.

According to the invention there is provided a catalytic converter for the exhaust gas of an internal combustion engine, the converter comprising a matrix including a substrate defining a plurality of passages for the flow of exhaust gas, an active coating comprising a catalyst on the walls of the passages, and electric heating means for the matrix.

The electric heating means provides for preheating of the catalyst idependently of the exhaust gas so that catalytic action can begin much sooner after engine start up.

The electric heating means may comprise a resistor, preferably comprising a positive temperature coefficient of resistivity (PTC) material. This provides a convenient heating means without a need for sophisticated sensors and controls since the electric current becomes self-limiting once the catalytic action starts and itself raises the temperature of the catalyst.

The resistor may be incorporated in the active coating or, where the substrate is ceramic, in the substrate itself.

Alternatively, the resistor may comprise an additional coating between the substrate and the active coating. Where the active coating has a surface layer impregnated with a metallic catalyst, the resistor may comprise a further layer below the catalyst.

A pair of electrodes for the flow of current through the resistor may be provided one at each end of the passages. Preferably the electrodes are perforated to allow the flow of exhaust gas.

Alternatively the electrodes are formed by coating the end surfaces of the matrix with a conductive layer or by impregnating the ends of the substrate and/or the active coating to form a conductive layer.

Where the substrate is conductive it may itself form an electrode for current flow through the resistor.

The invention will now be described by way of example and with reference to the accompanying drawings, of which Fig 1 is a longitudinal cross-section through a catalytic converter according to the invention.

Fig 2 is an enlarged cross-sectional view of part of the converter shown in Fig 1.

A catalytic converter 11 is of generally known construction. A metallic casing 12 has inlet and outlet ducts 13 and 14 respectively and houses two matrices 15 and 16. The second matrix 16 which is downstream of the first matrix 15 is conventional and comprises a ceramic substrate defining a plurality of longitudinal passages 17 the walls of which have an active coating or wash coat impregnated with a catalyst, eg platinum.

The first matrix 15 is generally similar in construction to the second matrix 16 and again comprises a ceramic substrate 18 defining a plurality of longitudinal passages 19. The matrices 15 and 16 are generally cylindrical and are supported in the casing 12 by insulating mats 21 of fibrous material.

As shown in Fig 2, the wall of each passage 19 in the ceramic substrate 18 is lined by a resistor comprising a coating 22 of an electrically resistive PTC material, eg doped barium titanate, which in turn has an active coating 23 or wash coat of alumina impregnated with a platinum group metal.

Electric current is made to flow through the PTC material by means of a pair of electrodes 24 one at each end of the passages 19.

These electrodes are perforated to allow the flow of exhaust gas.

The inlet duct 13 is in use connected to the exhaust of an internal combustion engine and the outlet duct 14 to a silencer and tail pipe. The electrodes 24 can be connected to an electrical source, eg a vehicle battery. The connection can be made using the vehicle ignition switch so that current flows in the PTC material under all engine running conditions. By heating the catalyst the catalytic action starts soon after the engine is started and is made to continue during extensive periods of engine idling or tickover.

Heat from the exhaust gas and the reaction promoted by the catalyst causes the PTC material to increase its electrical resistance and reduce the current so that additional controls may be unnecessary. However, alternative resistor materials may be used with the current controlled by sensing the temperature of the gas in the outlet duct 14.

In a first alternative arrangement (not shown) the ceramic substrate is made conductive and itself forms a heating resistor, either with or without PTC characteristics.

In a second alternative arrangement (not shown) the active coating is made electrically conductive and forms a heating resistor, either with or without PTC characteristics. The electric conductivity may be imparted to the active coating throughout, eg by additional materials in the wash coat, or may be impregnated into a surface layer prior to or during impregnation of the catalyst.

If an electrically conductive coating is applied to a conductive substrate the substrate may itself form one electrode and a further coating, eg the surface impregnated with the metallic catalyst, may form the other electrode.

The conductive substrate may be a known metallic type or may be formed by impregnating a known ceramic type.

Claims

1. A catalytic converter for the exhaust gas of an internal combustion engine, the converter comprising a matrix including a substrate defining a plurality of passages for the flow of exhaust gas, an active coating comprising a catalyst on the walls of the passages, and electric heating means for the matrix.

2. A catalytic converter according to Claim 1, wherein the electric heating means comprises a resistor.

3. A catalytic converter according to Claim 2, wherein the resistor comprises a positive temperature coefficient of resistivity (PTC) material.

4. A catalytic converter according to Claim 2 or Claim 3, wherein the resistor is incorporated in the active coating.

5. A catalytic converter according to Claim 2 or Claim 3, wherein the substrate is ceramic and the resistor is incorporated in the substrate.

6. A catalytic converter according to Claim 2 or Claim 3, wherein the resistor comprises an additional coating between the substrate and the active coating.

7. A catalytic converter according to Claim 2 or Claim 3 wherein the active coating has a layer impregnated with a metallic catalyst and the resistor comprises a further layer below the catalyst.

8. A catalytic converter according to any of Claims 2 to 7, wherein a pair of electrodes for the flow of current through the resistor is provided one at each end of the passages.

9. A catalytic converter according to Claim 8, wherein the electrodes are perforated to allow the flow of exhaust gas.

10. A catalytic converter according to Claim 8, wherein the electrodes are formed by coating the end surfaces of the matrix with a conductive layer.

11. A catalytic converter according to Claim 8, wherein the electrodes are formed by impregnating the ends of the substrate and/or the active coating to form a conductive layer.

12. A catalytic converter according to Claim 4 or Claim 6 wherein the substrate is conductive and itself forms an electrode for current flow through the resistor.

13. A catalytic converter according to any preceding claim and including a further matrix downstream of the matrix provided with electric heating means, the further matrix including a substrate defining a plurality of passages for the flow of exhaust gas, and an active coating comprising a catalyst on the walls of the passages wherein the further matrix is not provided with electric heating means.

14. A catalytic converter for the exhaust gas of an internal combustion engine substantially as described herein with reference to the accompanying drawings.