GB2254014A

GB2254014A - Fast warm-up catalytic converter

Info

Publication number: GB2254014A
Application number: GB9106551A
Authority: GB
Inventors: Ian Berry
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1991-03-27
Filing date: 1991-03-27
Publication date: 1992-09-30
Also published as: GB9106551D0

Abstract

An engine exhaust system comprising a catalytic converter 12 and a by-pass duct 20 connected in parallel with the catalytic converter 12 to form a closed recirculation loop around the converter 12. A fan 22 causes forced circulation of gases around the loop and an electrically operated heater 24 is provided along the loop for heating the recirculating gases in order to reduce light off time. <IMAGE>

Description

Title Fast Warm-up Catalytic Converter Filed of the invention The present invention relates to a motor vehicle exhaust system incorporating a catalytic converter.

Description of the prior art It is known to use a catalytic converter in the exhaust system of a motor vehicle in order to reduce undesired emissions such as carbon monoxide, oxides of nitrogen and unburnt hydrocarbons. Such converters are known only to operate effectively after they have reached a certain operating temperature, termed the light-off temperature, which is typically around 3250C.

Governmental regulations in different countries now set the standard to which motor vehicles must adhere and as the emissions depend on the mode in which the engine is operating, these regulations stipulate a drive cycle over which the emissions must be measured. The drive cycle normally requires the vehicle to start from cold and to be driven in a set pattern including idling, acceleration, cruising and deceleration modes.

The catalytic converter can cope well once it has reached its light off temperature but a consequence of this is that a major proportion of the emissions measured over the drive cycle are attributable to the fairly short time that the engine and the exhaust system are still cold.

Obiect of the invention The present invention therefore seeks to provide a motor vehicle exhaust system in which the time taken for the catalytic converter to reach its light off temperature is reduced.

Summary of the invention According to the present invention, there is provided an engine exhaust system comprising a catalytic converter, a by-pass duct connected in parallel with the catalytic converter to form a closed recirculation loop around the converter, means for causing forced circulation of gases around the loop and an electrically operated heater along the loop for heating the recirculating gases.

There have been proposed earlier systems for heating the catalytic converter to enable it to reach its light off temperature more rapidly. To this end, electrical heating elements have been built into the converter and it has also been suggested to use microwave energy to heat the catalyst. The disadvantage of building a heating element into the converter is that the exhaust gases flow over it at all times, even after the light off temperature has been reached, and lifetime of the heating element is therefore limited. Microwave energy on the other hand requires a costly magnetron and presents some packaging difficulties.

In the present invention, the heater is arranged in a by-pass through which gases do not flow once the light off temperature has been reached. Furthermore the recirculation permits a heater of modest power to be used because even before engine start up, the air in the closed loop around the catalytic converter can be heated and because the same air is passing over the heater in each cycle around the loop and little heat is lost to ambient during this time effective pre-heating of the converter can be achieved.

Such a mode of operation requires a delay between the driver initiating starting and the engine firing and such a delay may not always be acceptable. Of course, a more powerful heater can be used to enable the light off temperature to be reached after the engine has fired but it may also be possible to activate the heating of the catalyst as soon as the driver enters the vehicle, his presence being detected by a door switch or seat switch, so as to make use of the time taken by the driver to settle himself into the vehicle.

The recirculation of the exhaust gases around the closed loop after the engine has fired has the further advantages of passing some exhaust gases more than once through the catalyst, thereby cleaning them more thoroughly before they are discharged and as the catalytic conversion process is itself exothermic this in turn reduces further the time taken to reach the light off temperature.

The shape of the junctions at which the by-pass duct joins the exhaust system can be such that when circulation is not forced around the closed loop, the exhaust gases will naturally tend to flow through the catalytic converter towards the ambient atmosphere. However, it is possible to incorporate valves or deflectors at these junctions to ensure that gases do not by-pass the converter during normal operation.

The means for causing forced circulation of the gases around the closed loop is preferably a fan having an impeller disposed in the by-pass duct and driven by an electric motor disposed outside the by-pass duct.

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 block diagram of an exhaust system of the invention.

Detailed descriDtion of the Preferred embodiment The exhaust system in the drawing comprises a catalytic converter 12 of conventional design arranged between two sections 10 and 14 of the engine downpipe. A by-pass duct 20 is arranged in parallel with the converter 12 being connected to each of the sections 10 and 14 upstream and downstream of the converter 12. At the junctions between the by-pass duct 20 and the downpipe sections 10, 14, there are arranged thermostatically operated valves 16, 18 which are closed when the engine is hot and opened when the engine is cold.

Within the by-pass duct 20, there are arranged a fan 22 and a heater 24 which are operated only when the valves 16 and 18 are open.

With the valves 16 and 18 closed, the exhaust system operates in the same manner as any conventional system fitted with a catalytic converter, the exhaust gases entering along the arrow 26, following the route of the arrow 36 and exiting along the arrow 28 to the silencer.

When, on the other hand, the valves 16 and 18 are opened and the fan 22 is operated some of the air downstream of the converter 12 is drawn into the by-pass duct 20 through the valve 18 by the fan 22 and after being heated by the electrical heater 24 is returned through the valve 16 to the section of the downpipe 10 upstream of the catalytic converter 12. In this case, a recirculation loop is created as represented by the arrows 36, 30, 32 and 34.

The flow along the recirculation loop can take place either before the engine has started and/or during the warm-up phase after the engine has started. In the former case, clean air passes around the closed loop transferring heat from the heater 24 to the converter 12 so that when the engine is started the converter will already have been pre-heated. This pre-heat mode can either be initiated intentionally by the driver or it may come into operation automatically when the presence of the driver is detected, even before the ignition has been turned on.

If recirculation through the catalytic converter 12 takes place while the engine is firing, part of the exhaust gases will be recirculated. This will have the effect of improving the cleaning of the gases and of reducing the light off both on account of the heat transfer from the heater 24 and the improved exothermic reaction.

Recirculation during engine firing can be arranged to come into effect only for a fixed length of time after ignition but because it is possible for the temperature of the exhaust system to drop below the light off temperature during normal operation, for example at idling or when an engine is operated in a lean burn mode, it is preferred to sense the temperature of the converter and to bring the recirculation into effect whenever the catalyst temperature drops.

Claims

1. An engine exhaust system comprising a catalytic converter, a by-pass duct connected in parallel with the catalytic converter to form a closed recirculation loop around the converter, means for causing forced circulation of gases around the loop and an electrically operated heater along the loop for heating the recirculating gases.

2. An exhaust system as claimed in claim 1, wherein valves are arranged between the by-pass duct and the catalytic converter to isolate the by-pass duct from the catalytic converter when the light off temperature of the catalytic converter is exceeded.

3. An exhaust system as claimed in claim 2, wherein the valves are thermostatically operated.

4. An exhaust system as claimed in any preceding claim, wherein the means for causing forced circulation of gases around the loop comprises a fan disposed in the by-pass duct.

5. An exhaust system as claimed in any preceding claim, comprising means for automatically initiating recirculation of gases through the by-pass duct in response to operation of a door switch or seat switch.

6. An exhaust system as claimed in any preceding claim, wherein means are provided for automatically initiating recirculation of gases through the by-pass duct in response to the temperature of the catalytic converter dropping below a predetermined value.

7. An engine exhaust system constructed, arranged and adapted to operate substantially as herein described with reference to and as illustrated in the accompanying drawing.