GB2278299A

GB2278299A - Combined catalytic converter and afterburner

Info

Publication number: GB2278299A
Application number: GB9311193A
Authority: GB
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1993-05-29
Filing date: 1993-05-29
Publication date: 1994-11-30
Anticipated expiration: 2013-05-29
Also published as: GB2278299B; GB9311193D0

Abstract

A combined catalytic converter and afterburner comprises an afterburner chamber 16 and a catalyst matrix 11 disposed in series with one another in a casing 10. The afterburner chamber 16 has an ignition source 18 and it serves, in use, to receive a combustible premixed charge, the charge being ignited by the ignition source to cause it to burn before entering the catalyst matrix 11. Interruptions such as rings 42, 44 are provided on the inner surface of the wall of the afterburner chamber 16 in the path of the gas flow. These disturb the boundary layer of the premixed charge adjacent the wall, thereby ensuring that gases in the boundary layer are diverted away from the wall into the path of the flame propagating in the afterburner chamber 16 from the ignition source 18 and are burnt before reaching the catalyst matrix 11. <IMAGE>

Description

COMBINED CATALYTIC CONVERTER AND AFTERBURNER Field of the invention The present invention relates to a combined catalytic converter and afterburner comprising an afterburner chamber and a catalyst matrix disposed in series with one another in a casing, wherein the afterburner chamber has an ignition source and is operative, in use, to receive a combustible premixed charge, the charge being ignited by the ignition source to cause it to burn before entering the catalyst matrix.

Description of the prior art A combined afterburner and catalytic converter as described above has already been proposed in W093/07365 for use in an exhaust gas ignition (EGI) system. In such a system, the engine is operated in a manner to produce an ignitable charge by enriching the mixture supplied to the combustion chamber to a point where sufficient hydrogen is present in the exhaust gases and adding air into the exhaust gases before they reach the afterburner. This creates the required combustible premixed charge which when ignited not only reduces the hydrocarbon and carbon monoxide emissions but also heats the catalytic converter to bring it rapidly to the light off temperature at which it can function efficiently.

It has been found when using the previously proposed combined catalytic converters and afterburners that unless a very long afterburner chamber was used, some of the premixed charge would manage to pass unreacted through the afterburner and catalyst matrix.

Oblect of the invention The present invention seeks therefore to mitigate the above disadvantages without resorting to an extended afterburner chamber, which is difficult to package beneath a vehicle.

Summarv of the invention According to the present invention, in such a combined catalytic converter and afterburner, means are provided on the inner surface of the wall of the afterburner chamber in the path of the gas flow for disturbing the boundary layer of the premixed charge adjacent the said wall, thereby ensuring that gases in the boundary layer are diverted away from the wall into the path of the flame propagating in the afterburner chamber from the ignition source and are burnt before reaching the catalyst matrix.

The invention is predicated on the realisation that the unburnt hydrocarbons are caused by the boundary layer that adheres to the wall of the afterburner chamber. The cold surface of the wall quenches the flame and these gases are prevented from burning fully. In the present invention this boundary layer is disturbed by obstructions placed in its path and is diverted inwards, away from the cold wall, where complete combustion can take place.

Preferably, the casing includes two catalyst matrices separated from one another by the afterburner chamber.

The means for disturbing the boundary layer may comprise at least one thin ring extending around the periphery of the afterburner chamber and projecting inwards into the afterburner chamber from the wall.

Conveniently, the ring is arranged to obstruct the flow of the boundary layer in close proximity to the catalyst matrix lying downstream of the ignition source, the ring being of sufficient height to cause the boundary layer to break away from the wall.

The ring may be separately formed of sheet metal and is secured to the wall of the afterburner chamber but as an alternative the wall of the afterburner may have integral surface interruptions that act as the means for disturbing the boundary layer.

The surface interruptions need not be continuous and may instead comprise a series of circumferentially discontinuous ridges that are disposed relative to one another in such a manner that most of the boundary layer flow is obstructed by at least one ridge during travel through the afterburner chamber.

Brief description of the drawing The invention will now be described further, by way of example, with reference to the accompanying drawing, which is a schematic block diagram of an internal combustion engine fitted with a combined catalytic convertor and afterburner of the present invention.

Description of the preferred embodiment In the drawing, an internal combustion engine 12 has an intake manifold incorporating an air mass flow meter 22.

The intake manifold also includes a butterfly throttle 24 and individual fuel injectors 20. The exhaust system of the engine includes a down pipe 14 leading to a combined catalytic converter and afterburner 10. The latter comprises two catalyst matrices or bricks 11 separated from one another by a chamber 16 in which is arranged an igniter represented in the drawing by a spark gap 18.

A pump 30 is also provided to introduce additional air into the exhaust system by way of a regulating valve 32 and a HEGO sensor 38 located near the exhaust manifold is used in the control of the fuelling of the engine to maintain stoichiometry.

In normal use the HEGO sensor ensures that a stoichiometric mixture is supplied to the combustion chambers of the engine and at this setting the three-way catalyst functions with optimum performance when it is hot to minimise the noxious emissions, namely carbon monoxide, unburnt hydrocarbons and oxides of nitrogen.

The purpose of the afterburner chamber 16 is to assist in heating the second catalyst brick 11 to its light off temperature immediately after cold start, during a so called EGI (exhaust gas ignition) phase. In this phase the mixture supplied to the engine is made extremely rich (typically 1.7 to 2 times the fuel for stoichiometry). Such over-fuelling results in the production of a significant proportion of hydrogen. This allows the gaseous mixture to be re-ignited and to burn spontaneously when air is added. The pump 30 supplies sufficient air to permit complete combustion of the hydrogen, carbon monoxide and hydrocarbons present in the exhaust gases and on reaching the afterburner chamber 16 the mixture of exhaust gases and additional air is ignited by the igniter 18 and burns as a hot flame to raise the temperature of at least the front face of the second catalyst brick 11.

The system as so far described is known and in the past it was found that despite the presence of a flame in the afterburner chamber 16 during the EGI phase, unburnt hydrocarbons and carbon monoxide persisted in finding their way in small quantities into the gases discharged to the ambient.

The present invention is concerned with reducing the emission of these small quantities of noxious gases during the EGI phase and provides in the described embodiment projections in the form of rings 42 and 44 on the inner surface of the afterburner chamber downstream of the igniter 18.

The invention is based on the realisation that the boundary layer of the gases in the afterburner tends not to be burnt because the flame in the afterburner is quenched by the cold surfaces of the casing. By providing interruptions to the gas flow, such as by means of the rings 42 and 44, the invention forces the boundary layer to break away from the interior wall of the afterburner chamber so that no part of the pre-mixed charge can find its way through the afterburner chamber without being exposed to the flame.

The persons skilled in the art will appreciate that raised rings are only one way of forcing the boundary layer to break away and that other circumferentially continuous or discontinuous interruptions in the surface of the afterburner chamber can be used to prevent the boundary layer from adhering to the wall of the chamber over its entire length between the two catalyst matrices 11.

The surface interruptions may for example comprise a series of circumferentially discontinuous ridges that are disposed relative to one another in such a manner that most of the boundary layer flow is obstructed by at least one ridge during travel through the afterburner chamber.

The method of construction of the housing and its assembly will also be clear to the person skilled in the art, it being most convenient to form the casing in the form of two mating half shells with the surface interruptions pressed or permanently secured to the two halves of the casing.

Claims

1. A combined catalytic converter and afterburner comprising an afterburner chamber and a catalyst matrix disposed in series with one another in a casing, wherein the afterburner chamber has an ignition source and is operative, in use, to receive a combustible premixed charge, the charge being ignited by the ignition source to cause it to burn before entering the catalyst matrix, characterised in that means are provided on the inner surface of the wall of the afterburner chamber in the path of the gas flow for disturbing the boundary layer of the premixed charge adjacent the said wall, thereby ensuring that gases in the boundary layer are diverted away from the wall into the path of the flame propagating in the afterburner chamber from the ignition source and are burnt before reaching the catalyst matrix.

2. A combined catalytic converter and afterburner as claimed in claim 1, wherein the casing includes two catalyst matrices separated from one another by the afterburner chamber.

3. A combined catalytic converter and afterburner as claimed in claim 1 or 2, wherein the means for disturbing the boundary layer comprises at least one thin ring extending around the periphery of the afterburner chamber and projecting inwards into the afterburner chamber from the wall.

4. A combined catalytic converter and afterburner as claimed in claim 3, wherein the ring is arranged to obstruct the flow of the boundary layer in close proximity to the catalyst matrix lying downstream of the ignition source, the ring being of sufficient height to cause the boundary layer to break away from the wall.

5. A combined catalytic converter and afterburner as claimed in claim 3 or 4, wherein the ring is separately formed of sheet metal and is secured to the wall of the afterburner chamber.

6. A combined catalytic converter and afterburner as claimed in claim 3 or 4, wherein the wall of the afterburner has integral surface interruptions that act as the means for disturbing the boundary layer.

7. A combined catalytic converter and afterburner as claimed in claim 6, wherein the surface interruptions comprise a series of circumferentially discontinuous ridges that are disposed relative to one another in such a manner that most of the boundary layer flow is obstructed by at least one ridge during travel through the afterburner chamber.

8. A combined catalytic converter and afterburner, constructed arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.