WO1998025011A1

WO1998025011A1 - Engine exhaust system

Info

Publication number: WO1998025011A1
Application number: PCT/GB1997/002807
Authority: WO
Inventors: Thomas Tsoi-Hei Ma
Original assignee: Ford Global Technologies, Inc.
Priority date: 1996-11-30
Filing date: 1997-10-10
Publication date: 1998-06-11
Also published as: GB2319736A; GB9624971D0

Abstract

An exhaust system for an internal combustion includes an exhaust gas after-treatment device such as a catalytic converter (26) or a gas storage trap (28), and means for cooling the exhaust gases upstream of the after-treatment device. The cooling means include a turbine-blower (20, 30) comprising a turbine (20) driven by the exhaust gases from the engine and a blower (30) driven by the turbine (20) and connected by auxiliary pipes (32, 38) to draw gases from and return gases to a section (22) of the exhaust system downstream of the turbine (20). Recirculation of exhaust gases by the blower (30) through the auxiliary pipes (32, 38) and the section (22) of the exhaust system serves to cool the exhaust gases by heat loss through the walls of the auxiliary pipes (32, 38).

Description

ENGINE EXHAUST SYSTEM

The present invention relates to an exhaust system for an internal combustion engine including an exhaust gas after- treatment device such as a catalytic converter or hydrocarbon (HC) or oxides of nitrogen (NOx) storage trap.

Catalytic converters and NOx storage traps are temperature sensitive in their operation. Furthermore, limits must be placed on their temperatures during operation to prevent them from being permanently damaged. For these reasons, it is advantageous to be able to regulate the temperature of the exhaust gases reaching the after- treatment device, in particular to cool the gases under high load operating conditions before they reach the after- treatment device.

It has been proposed to provide two flow paths in parallel with one another upstream of the after-treatment device, cne flow path being longer than the other to allow increased heat loss through the walls of the exhaust pipe before the exhaust gases reach the after-treatment device. A shut-off valve is provided in the longer flow path to force all the gases to flow through the shorter flow path under low load conditions to conserve the exhaust gas temperature and maintain a catalytic converter or NOx trap at an efficient operating temperature. Under high load conditions, on the ether hand, the valve is opened to maximise the heat loss frc.t the exhaust gases upstream of the after-treatment device, to prevent the catalytic converter or NOx trap from being damaged from excessive heat.

Such a system is expensive to implement because of the requirement for a high temperature shut-off valve and such valves co not yet have proven reliability. The fact that a control system is required to operate the shut-off valve also ades to the system cost and complexity. Furthermore, a very long parallel flow path is required for efficient cooling and such a long pipe is difficult to package and install, in that there is not sufficient space available for it within the engine compartment.

With a view to mitigating the foregoing disadvantages, the present invention provides an exhaust system for an internal combustion including an exhaust gas after-treatment device such as a catalytic converter or a gas storage trap, and means for cooling the exhaust gases upstream of the after-treatment device, the cooling means including a blower connected by auxiliary pipes to draw gases from and return gases to a section of the exhaust system, recirculation of exhaust gases by the blower through the auxiliary pipes and the said section of the exhaust system serving to cool the exhaust gases by heat loss through the walls of the auxiliary pipes.

The mounting of a blower to force recirculation of gases around a loop lying upstream of the after-treatment device has several advantages over the prior art arrangement discussed above. In the first place, the gases do not just pass once through a length of pipe but keep recirculating around a loop. The loop may therefore be much shorter and easier to package. Because the exhaust gases will have been thoroughly mixed while flowing around the loop several times, the gases reaching the after-treatment device will be less prone to transient surges. To compensate for such surges, it is normally necessary to use a large catalytic converter with a high oxygen storage capacity and the reduction in these transient surges therefore permits a smaller and less costly catalytic converter to be used.

Preferably, the blower is driven by an engine exhaust gas turbine. A turbine-blower is available at a low cost because of high volume production for turbo-charger applications. Furthermore turbo-chargers, unlike shut-off valves, have proven reliability established over many years of use in high performance gasoline engines and diesel engines .

A turbine-blower offers advantages over a mechanically or electrically driven blower not only because of its proven reliability but also because it relies entirely on waste energy and does not take power from the engine.

A still further advantage of the use of a turbine- blower over a mechanically or electrically driven blower is that it is self-regulating and does not require a separate control system. Under low load operating conditions, there is little energy in the exhaust gases. The turbine will generate less power and the flow rate of the recirculating flow is correspondingly low. As the engine load increases, the output power of the turbine increases and the cooling effect of the recirculating flow is increased. In other words, when the exhaust gases are cool, then they are not further cooled and the cooling efficiency increases with rise in temperature of the exhaust gases. By suitable selection of the size of the auxiliary pipes and the turbine-blower, it is possible automatically to maintain the temperature of the exhaust gases at the after-treatment device within a narrower range of values over the entire operating range of the engine without the need for any separate control system. This reduces the cost of the system still further and minimises the risk of overheating of the after-treatment device.

It is possible to connect a secondary air supply to the intake side of the blower so that air may be supplied to a catalytic converter without it passing through the engine. If such air is included with the primary intake air and measured by a mass air flow meter that is used to meter fuel to the engine to maintain an overall stoichiometric calibration, then the engine will automatically receive a mixture containing excess fuel that will pass unburnt into the exhaust system. The excess fuel and the secondary air - 4 -

will still form a stoichiometric mixture at the catalytic converter where they will react to give off heat to raise the temperature of the catalytic converter. Such a mode of operation is useful to heat the catalytic converter for example during prolonged idling. The same technique can also be used to desulphate an NOx trap, this process requiring the temperature of the NOx trap to be raised to approximately 650°C.

It is known to heat up a catalytic converter rapidly immediately after a cold start by exhaust gas ignition. This involves ensuring that the exhaust gases contain excess fuel and supplying secondary air into the exhaust gases to mix with the excess fuel to form an ignitable mixture. In the present invention, under fast idling conditions after a cold start, if the turbine-blower cannot produce sufficient secondary air to enable exhaust gas ignition, then more energy can be put into the exhaust gases by retarding the spark timing of the engine in order to increase the speed of the turbine-blower. Because the invention allows the temperature range of the exhaust gases reaching the after-treatment device to be compressed, it enables a catalytic converter and an NOx trap to be integrated onto a common substrate or in a common housing operating within the same temperature range. Hitherto, this could not be achieved because a catalytic converter would often operate at a temperature in excess of the safe maximum temperature for an NOx trap.

The invention will now be described further, by way of example, with reference to the accompanying drawing, which is a schematic diagram of an internal combustion engine fitted with an exhaust system of the present invention.

The single figure shows an engine 16 having an intake manifold 14 to which air is supplied by way of an air flow meter 10 and an intake throttle 12. The exhaust system comprises an exhaust manifold 18 and a downpipe 24 that contains a turbine 20 that drives a blower 30 through a shaft 23. The downpipe 24 extends to an after-treatment device that is shown to comprise two matrices 26, 28 separated by a chamber containing an igniter 44. The matrices 26, 28 may be a catalytic converter or an NOx storage trap.

The blower 30 is connected by auxiliary pipes 32 and 38 to a section 22 of the downpipe 24 upstream of the after- treatment device. The ends 34 and 36 of the auxiliary pipes 32 and 38 face each other across the section 22 of the downpipe 24 so as to define a continuous recirculation loop for exhaust gases when the blower 30 is in operation. It is not essential that the ends 34 and 36 of the auxiliary pipes 32 and 38 should face each other and it is alternatively possible for them to be spaced along the exhaust downpipe, the section of the downpipe between the auxiliary pipes 32 and 38 then forming part of the recirculation loop.

The intake side of the blower 30 is also connected to a secondary air supply pipe 42 that contains a regulating or shut-off valve 40 and opens in the intake manifold at a point between the air flow meter 10 and the intake throttle 12.

The valve 40 is completely shut under normal operating conditions of the engine. Air is supplied to the engine 16 through the intake manifold 14 and the quantity of fuel related to the measured air flow is supplied to the engine cylinders to be burnt in the engine . The exhaust gases are discharged through the exhaust manifold 18 and pass through the turbine 20 and the after-treatment device 26, 28 before being discharged to the ambient atmosphere.

At low engine loads, the turbine 20 does not rotate fast and the blower 30 has negligible effect. However as the engine load increases, the blower 30 is driven ever faster as the load increases and causes rapid circulation of an ever increasing proportion of the exhaust gases around the loop 30, 32, 34, 36, 38. This causes ever increasing heat loss from the walls of the auxiliary pipes 32, 38 thereby lowering the temperature of the exhaust gases before they reach the after-treatment device 26, 28. As a result the temperature range of the exhaust gases reaching the after- treatment device is compressed and it is possible to ensure that the gases never exceed a temperature at which an NOx trap will be damaged. Because of this, it is possible to incorporate an NOx trap within the same housing or even onto the same substrate as a catalytic converter.

The recirculation of exhaust gases around the loop 30 to 38 also ensures that the gases from several combustion cycles are thoroughly mixed with each other before they reach the after-treatment device 26, 28. Because of this there will be an averaging of the composition of exhaust gases discharged during consecutive cycles with the result that the amplitude of transient surges or spikes in the exhaust emissions will be severely attenuated. This is advantageous as it is the amplitude of such emission surges that dictates the required size and storage capacity of the after-treatment device. The reduction in the amplitude of transient surges achieved by the invention enables smaller and less expensive after-treatment devices to be employed.

There are occasions when it is desired intentionally to heat the after-treatment device. For example if a catalytic converter temperature is about to drop below the light-off temperature during prolonged idling, then steps should be taken to prevent it from doing so. Likewise, immediately after a cold start, it is desirable to heat a catalytic converter to bring it to its light-off temperature as quickly as possible. In the case of an NOx trap once again it is necessary to operate above a minimum temperature if it is to be effective and also it may be necessary to raise its temperature very significantly for short periods at regular intervals to desulphate it. In all these cases, one can generate heat in the after-treatment device by reacting unburnt fuel and secondary air in the exhaust system. If the catalytic converter is already hot, then one can rely on the exothermic reaction taking place within it to generate the necessary additional heat but if the catalytic converter is cold, then the igniter 44 can be used to initiate ignition of fuel and air present in the exhaust gases.

Conventionally secondary air had been provided directly into the exhaust system for the purpose outlined above by the use of an electrical blower. The displacement of such a blower must be significant and must typically match the idle flow rate to the engine cylinders. Such blowers have proved expensive and difficult to package within the restricted space of a vehicle engine compartment. They also take power from the engine and increase the fuel consumption of the vehicle . The turbine driven blower present in the exhaust system of the present invention can be used in place of such a source of secondary air by opening the valve 40 connected in the pipe 42. The blower 30 will now draw secondary air directly from the intake system and mix it with the recirculating exhaust gases without the air passing through the engine 16. Because such secondary air will have been metered by the air flow meter 10 together with the primary intake air, then if the fuelling system is calibrated for stoichiometry, the mixture supplied to the engine 16 will contain excess fuel. The excess fuel will pass unburnt through the engine and then react stoichiometrically with the secondary air in the after-treatment device as described above. This will raise the temperature of the after- treatment device without discharging unburnt hydrocarbons to the ambient atmosphere.

It is also possible to use a turbine drive blower to cool the exhaust gases upstream of the turbine. In this case, the auxiliary pipes draw gases from and return gases to the section of the exhaust pipe upstream of the turbine. It is also possible in such an embodiment for the returned gases to be injected by the blower directly into the individual branches of the exhaust manifold and for the gases to be directed at the exhaust valves associated with the respective branches. If additional air is introduced into the recirculating gases, then the air can serve the function of combining with hydrocarbons that collect in the exhaust ports to reduce the hydrocarbon emissions especially during cold starts.

Claims

1. An exhaust system for an internal combustion including an exhaust gas after-treatment device such as a catalytic converter (26) or a gas storage trap (28), and means for cooling the exhaust gases upstream of the after- treatment device, the cooling means including a blower (30) connected by auxiliary pipes (32,38) to draw gases from and return gases to a section (22) of the exhaust system, recirculation of exhaust gases by the blower (30) through the auxiliary pipes (32,38) and the said section (22) of the exhaust system serving to cool the exhaust gases by heat loss through the walls of the auxiliary pipes (32,38) .

2. An exhaust system as claimed n claim 1, wherein the blower (30) is driven by an engine exhaust gas turbine (20) .

3. An exhaust system as claimed in claim 1 or 2, wherein a secondary air supply (42) is connected to the auxiliary pipe (38) leading to the suction side of the blower (30) .

4. An exhaust system as claimed m claim 3, wherein a flow regulating valve (40) is provided in the secondary air supply (42) to shut off or to regulate the flow of secondary air drawn into the auxiliary pipe (38) by the blower (30) .

5. An exhaust system as claimed in any preceding claim, wherein the connections (34,36) between the auxiliary pipes (32,38) and the said section (22) of the exhaust system lie facing one another on opposite sides of a pipe of the exhaust system.

6. An exhaust system as claimed in any preceding claim, wherein the after-treatment device comprises a single housing containing both a catalytic converter (26) and an NOx trap (28) .

7. An exhaust system as claimed in claim 6, wherein the catalytic converter and the NOx trap are formed integrally on a common substrate.

8. An exhaust system as claimed in any one of claims 1 to 5, wherein the exhaust gas after-treatment device comprises a catalytic converter connected in series with an NOx trap and wherein the auxiliary pipes are connected to the section of the exhaust system extending between the catalytic converter and the NOx trap.

9. An exhaust system as claimed in any one of claims 1 to 4, wherein the auxiliary pipes are connected to a section of the exhaust system upstream of the turbine.

10. An exhaust system as claimed in claim 9, wherein the blower is connected to return gases separately to individual branches of the exhaust manifold, the auxiliary return pipe having branches connected to the exhaust manifold, each branch being operative to direct a stream of gases towards the exhaust valve of the associated exhaust port .

11. An internal combustion engine comprising an exhaust system as claimed in claim 3 or any claim appended thereto, wherein the secondary air is drawn from a point in the intake manifold lying upstream of the main throttle and downstream of an air flow meter that forms part of a fuel metering system that is operative to supply to the engine a quantity of fuel stoichiometrically related to the measured air flow supplied both to the engine cylinders and directly to the engine exhaust system.

12. An internal combustion engine as claimed in claim 11, wherein an igniter (44) is provided upstream of an after-treatment device of the exhaust system.

13. An internal combustion engine as claimed in claim 11 or 12, wherein means are provided for selectively retarding the spark timing of the engine to increase the exhaust gas temperature and thereby increase the speed of the turbine-blower.