GB2428739A

GB2428739A - I.c. engine exhaust manifold with two gas flow paths

Info

Publication number: GB2428739A
Application number: GB0516107A
Authority: GB
Inventors: Duncan James Kay; Jon Edward Caine; Chris John Kaven; Roy Clissold; Marcus Timothy Davies
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2005-08-05
Filing date: 2005-08-05
Publication date: 2007-02-07
Anticipated expiration: 2025-08-05
Also published as: GB201000377D0; JP2007046599A; GB2466722A; GB0516107D0; GB2466722B; GB2428739B

Abstract

An exhaust manifold for an internal combustion engine 10 is disclosed having two exhaust gas flow paths 12, 13 defined therein. A first flow path 12 directs exhaust gasses with no appreciable cooling effect to a three way catalyst 16 located downstream and the second flow path 13 includes water cooling for the exhaust gasses passing therethrough using coolant drawn from a main engine cooling circuit. One or more valves 15 are used to control the proportion of flow through the first and second flow passages 12 and 13 so as to regulate the temperature of the exhaust gasses entering the three way catalyst 16.

Description

An Exhaust Manifold for an Internal Couibustion Engine This invention

relates to internal combustion engines and in particular to an exhaust manifold assembly for regulating the exhaust gasses flowing to a downstream emission control device.

Lean burn gasoline engines are one technology that can be used in order to reduce CO2 emissions. In order to comply with tailpipe NO emissions requirements, a NOx trap is required as part of the emission control system for these engines. Current NO trap technology is temperature sensitive and NO trapping performance will be degraded due to temperatures above say 650 C which cause ageing of the trap. In addition, the efficiency of a NO: trap is highly dependent on the trap temperature and, to obtain optimum NO trapping performance, the trap temperature must ideally be maintained within a predetermined range of a target temperature, which will depend on the NO trap formulation.

This predetermined range can be as small as plus or minus 25 C from the target temperature.

If there is a build up of sulphur deposits in the NO trap, these must be removed by a process known as de- suiphation' in which the temperature of the NO trap must be increased to more than 600 C and so the exhaust emission system must be able to facilitate this.

It is further known, in order to reduce the time it takes for the catalyst to light-off', to position a catalyst for an engine close to the exhaust outlet from the engine. Such catalysts are often known as close coupled catalysts because they are normally coupled directly to the exhaust manifold of the engine.

Such close-coupled catalysts are also subject to high temperature ageing effects, which can be mitigated by fl reducing incoming gas temperares. However any renuccion in exhaust gas temperatures must not be at the ex ns of achisving catalyst light-off' as quickly as possiule to ensure low tailpipe emissions. Catalyst light-off is achieved when the catalyst reaches an operating tencerature from a cold start at which it cegins to actively rcduce emissions.

These conflicting temperature requirements and cost constraints have led the inventor to seek a low cost method for controlling exhaust gas temperatures without compromising catalyst light off.

It is an object of the invention to provide an improved exhaust manifold assembly for an internal combustion engine and an emission control system utilising same.

According to a first aspect of the invention there is provided an exhaust manifold assembly for an internal combustion engine, the exhaust manifold assembly has an inlet into which exhaust gasses from the engine flow in use, an outlet from which exhaust gasses flow in use, a first exhaust gas flow path for transferring exhaust gasses from the inlet to the outlet, a second exhaust gas flow path for transferring exhaust gasses from the inlet to the outlet and a valve means to regulate the flow of exhaust gasses passing through the first and second flow paths wherein any exhaust gasses passing through the first flow path pass through the exhaust manifold assembly with minimal cooling and any exhaust gasses passing through the second flow path are subject to active cooling from an exhaust gas cooler so that the temperature of the exhaust gasses exiting the exhaust manifold assembly can be regulated.

The valve means may be operable to pass all or the majority of all the exhaust gasses from the engine through the first flow path when the engine is started from cold.

The valve means may be ope:able to pass all or majority of all the exhaust gasses from the engine thoough the first flow path at least when the temperature of the exhaust gasses is below a first predetermined temperstore.

The valve means is operable to pass at least sons of the exhaust gasses through the second flow path when one temperature of the exhaust gasses exceeds a second c predetermined temperature.

The valve means may be operable to vary the proportion of exhaust gasses flowing through the first and secoof flow paths in order to regulate the oernperature of the exheost gasses exiting the exhaust manifold assembly.

The exhaust gasses may be regulated in order to maintain the temperature of the exhaust gasses exiting the exhaust manifold assembly within a predetermined range of temperatures.

The valve means may be a single control valve positioned so as to regulate the flow of exhaust gasses through the first flow path.

The valve means may be positioned near to the outlet so as to selectively restrict the flow of exhaust gasses exiting the first flow path.

Alternatively, the valve means may be a single control valve positioned so as to regulate the flow of exhaust gasses through the second flow path.

The valve means may be positioned near to the outlet so as to selectively restrict the flow of exhaust gasses exiting the second flow path.

As yet another alternative the valve means may be a single control valve positioned so as to simultaneously regulate the flow of exhaust gasses through the first and second flow paths.

In which case, the valve means may be positioned near to the outlet so as to selectively vary the flow of exhaust gasses exiting the first and second flow paths.

The exhaust gas cooler may use coolant drawn from a main cooling system of the engine.

The exhaust manifold assembly may comprise of at least one inner exhaust conduit defining the first exhaust gas flow path, a first casing enclosing the inner exhaust conduit and being spaced therefrom so as to define the second exhaust gas flow path and a second casing enclosing the first casing and being spaced apart therefrom so as to define a gap through which the coolant is passed to form the exhaust gas cooler.

Each inner exhaust conduit may be connected to an outlet from the engine by a flange plate.

The or each flange plate may have a tubular sleeve extending therefrom with which an end of a respective inner exhaust conduit is engaged.

The exhaust manifold may have a single flange plate and a like number of tubular sleeves as there are cylinders in the engine to which the exhaust manifold assembly is connected.

Each tubular sleeve may have one or more apertures therein to provide an inlet to the second exhaust flow path.

Alternatively, each tubular sleeve may have one nr more apertures therein for co-operation with one or more corresponding apertures in an end portion of the inner exhaust conduit with which it engages to provide an inlet to the second exhaust flow path.

The manifold assembly may have a number of inner exhaust conduits forming the first flow path.

The inner exhaust conduits may flow independently from the engine to a junction where the separate inner exhaust conduits merge to form a single conduit extending to the outlet.

The first exhaust gas flow path may be formed by an exhaust conduit, the control means is a control valve positioned at an exit end of the exhaust conduit, the second exhaust gas flow path is formed by the exhaust conduit, a first cooling conduit connecting the control valve to an inlet to an exhaust gas cooler and a second cooling conduit connecting an outlet from the exhaust gas cooler to the control valve and the control valve is operable to regulate the flow of exhaust gasses through the first and second exhaust gas flow paths.

There may be several exhaust conduits that flow independently from the engine to a junction where they merge to form a single conduit extending to the control valve.

The control valve may be moveable from a first position in which all of the exhaust gasses flow from the inlet to the outlet through only the first exhaust flow path to a second position in which all of the exhaust gasses flow from the inlet to the outlet through the second exhaust flow path.

- C -

When the control valve s positioned between first and second positions, a proportion of the exhaust cascs may flow through the first flow path and a proportion cf The exhaust gasses may flow through the second flow paTh.

The position of the control valve between the first and second positions may be varied to regulate the temperature of the exhaust gasses leaving the exhaust gas manifold through the outlet.

When the control valve is in the first position it may substantially prevents the flow of exhaust gasses through the second cooling conduit.

A small leakage of exhaust gasses past the control valve may be permitted even when the control valve is in the first position so as to cool the control valve.

According to a second aspect of the invention there is provided an emission control system for an engine having an exhaust manifold assembly in accordance with said first aspect of the invention and a catalytic device coupled to the outlet from the exhaust manifold assembly.

The catalytic device may be a three way catalyst.

The emission control system may further comprise a lean NOx trap positioned downstream from the three way catalyst.

The three way catalyst and the lean NOx trap may be mounted in a common canister directly coupled to the outlet from the exhaust manifold assembly.

The emission control system may further comprise an electronic control unit and at least one exhaust gas temperature sensor operably connected to the electronic control unit to provide a signal indicative of the exhaust gasses downstream from the exhaust gas manifold.

The valve means may be a control valve and the position of the control valve may be controlled by the electron c control unit.

The position of the control valve may be controlled by the electronic control unit in response to one of a signal received from an exhaust gas temperature sensor and a temperature model.

The electronic control unit may be operable to move the control valve to a position in which exhaust gasses flow solely or primarily through the first flow path when the temperature of the exhaust gasses are sensed to be below a first predetermined temperature.

The first predetermined temperature may be a light off temperature of a three way catalyst coupled to the exhaust gas manifold.

A three way catalyst may be coupled to the outlet from the exhaust gas manifold and a lean NOx trap may be positioned downstream from the three way catalyst and the electronic control unit may be operable to control the position of the control valve so as to maintain the temperature of the exhaust gasses in the lean NOx trap within a predetermined range of temperatures.

The predetermined range may be bounded at its lower end by a lower temperature limit below which the lean NOx trap does not operate efficiently and may be bounded at its upper end by an upper temperature limit above which premature ageing of the lean NOx trap will occur.

The predetermined range may ce a range of temperan ares at which the lean NOx trap will. :oerate at peak efficiency.

The electronic control unit niay be operable to pernit S the temperature of the exhausts nesses to exceed the upoer temperature limit when it is necessary to remove sulphir from the lean NOx trap.

According to a third aspect of the invention there is provided method of controlling an emission control system having an exhaust manifold assemoly in accordance with said first aspect of the invention, a three way catalyst coupled to the outlet from the exhaust gas manifold assembly, and a lean NOx trap positioned downstream from the three way catalyst, wherein the method comprises causing the passage of all or a majority of the exhanst gasses through the first flow path after engine start-up order to produce rapid light-off of the three way catalyst until an exhaust gas temperature corresponding to the light-off temperature of 2C the three way catalyst has been reached.

The method may further comprise controlling the temperature of the exhaust gasses passing through the three way catalyst and lean NOx trap by varying the flow of exhaust gasses through the first and second flow paths so that the temperature of the exhaust gasses in the lean NOx trap stay within a range in which the lean NOx trap will operate at maximum efficiency.

The method may further comprise causing passage of all or a majority of the exhaust gasses through the second flow path when the temperature of the exhaust gasses approaches an upper temperature limit above which premature ageing of one of the three way catalyst and the lean NOx trap will occur.

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The method may further comprise allowing temperature excursions above the upper temperature limit when it is necessary to remove sulphur from the lean NOx trap.

The invention will now be described by way of example with reference to the accompanying draw:ng of which:- Fig.l is a schematic line drawing of an emassion control system according to one aspect of the invention; Fig.2 is a pictorial cutaway representation of a first embodiment of an exhaust manifold assembly according to a further aspect of the invention; is Fig.3 is a cutaway portion on an enlarged scale of the exhaust manifold assembly shown in Fig.2 showing a valve assembly used to regulate the flow of exhaust gasses through the exhaust manifold assembly; Fig.4 is a pictorial cutaway representation of a second embodiment of an exhaust manifold assembly according to the further aspect of the invention; Fig.5 is a pictorial cutaway representation of a third embodiment of an exhaust manifold assembly according to the further aspect of the invention; Fig.6 is a cross-section through a valve assembly forming part of the exhaust manifold assembly shown in Fig.5; and Fig.7 is a cross-section similar to that shown in Fig.6 but showing an alternative valve assembly.

With particular reference to Fig.l there is shown an emission control system for an internal combustion engine - 10 - and in the example given for a lean burn internal combuston engine 10.

The emission control system comprises of an exhaust manifold assembly defining first and second exhaust gas flow paths 12, 13, a three way catalyst 16 positioned downstream from the exhaust manifold assembly so as to receive exhaust gasses therefrom, a lean NOx trap 17 positioned downstream from the three way catalyst 16 so as to receive exhaust gasses from the three way catalyst 16, a valve means 15 to regulate the flow through the exhaust manifold assembly, an electronic control unit 5 to control the operation of the valve means 15 based upon the temperature of the exhaust gasses as measured in at least one position and shown diagrammatically by the temperature sensor 20.

Alternatively, control can be based on modelled exhaust gas temperatures that is to say by a temperature model.

A silencer 18 is positioned downstream from the lean NOx trap 17 to reduce the noise emitted from a tailpipe 19 of an exhaust system of which the emission control system forms a major part.

The exhaust manifold assembly has an inlet into which exhaust gasses from the engine flow and an outlet from which exhaust gasses flow into the three way catalyst 16. The valve means 15 is used to regulate the flow of exhaust gasses passing through the first and second flow paths 12, 13.

Although the valve means 15 is shown near to the end of the first exhaust gas flow path 12 it will be appreciated that it could be positioned towards the end of the second exhaust gas flow path 13, at the juncture of the two flow paths or there could be valves in both flow paths 12, 13.

Preferably, whatever valve means is used, it is positioned towards the outlet from the exhaust manifold assembly so as to reduce as much as possible the temperature at:h it operates.

The exhaust gasses flowing through the first exhaust gas flow path 12 do so with no appreciable cooling sc that the maximum possible exhaust gas temperature is transferred to the outlet from the exhaust manifold assembly. That is to say no attempt is made to cool the exhaust gasses flowing through the first flow path and so a minimal cooling of is these gasses will occur due to natural conduction and radiation losses.

Conversely, the exhaust gasses flowing or passing through the second flow path 13 are subject to active is cooling from an exhaust gas cooler 14 and so there is a significant difference in exhaust gas temperature becween the exhaust gasses entering the second exhaust gas flow path 13 and the exhaust gasses exiting the second exhaust gas flow path 13.

The exhaust gas cooler 14 can be of various forms but in all cases uses liquid coolant to cool the exhaust gases passing through the second exhaust gas flow path 13.

Preferably, this coolant is extracted from a main cooling circuit (not shown) of the engine 10 and then returned to the main cooling circuit. This is because the exhaust gas cooler 14 is then able to recover some of the heat from the exhaust gasses and return it to the main cooling circuit. This is advantageous after starting the engine from cold as it speeds up engine warm up, thereby reducing engine warm up emissions, increasing warm up fuel economy and improving cabin heater performance. However, it will be appreciated that a separate coolant supply circuit having a radiator to dissipate the heat absorbed from the exhaust gasses and a pump to circulate the coolant could be used if required.

- 12 - The three way catalyst 16 can be located a short distance downstream from the exhaust manifold assembly but it is preferable for it to be cc'pled directly to the outlet from the exhaust manifold assembly. That is to say the three way catalyst 16 is preferacly a close coupled catalyst.

Similarly, although the lean NOx trap can be a separate : unit positioned downstream from The three way catalyst 16 it is advantageous, and possible by utilising an exhaust manifold assembly according to this invention, for the three way catalyst 16 and the lean NOx trap 17 to be mounted in a common canister coupled directly to the outlet from the :5 exhaust manifold assembly. This is because such an arrangement is less expensive to produce and is easier to package on a motor vehicle than cwo separate canisters. In such an arrangement the common canister would contain a three way catalyst brick at an inlet end of the canister and 2C a lean NOx trap brick located downstream from the three way catalyst brick near an outlet from the common canister.

Although a single temperature sensor is shown at the entrance to the lean NOx trap 17 it will be appreciated that several temperature sensors could be used to provide signals indicative of the temperature of the exhaust gasses at various positions such as exiting the exhaust manifold assembly, exiting the three way catalyst 16, entering the lean NOx trap 17 and leaving the lean NOx trap 17.

Alternatively temperatures of the exhaust gasses could be modelled. However, provided at least one signal is supplied to the electronic control unit 5 indicating the approximate temperature of the exhaust gasses leaving the three way catalyst 16 and entering the lean NOx trap 17 effective control of the valve means 15 can be obtained.

Operation of the emission control system is as follows.

- 13 - Upon engine start-up the primary need is to light-off the three way catalyst 16 as soon as possible and to light- off the lean NOx trap 17 although this is not as critical as it will be some time before the engine 10 is able to run in a lean mode of operation. Typically a lean burn engine will need to run for approximately three minutes before it is sufficiently warm to be operated lean.

Therefore during this phase of operation the electronic control unit 5 is operable to control the valve means 15 such that a large percentage of the exhaust gasses entering the exhaust manifold assembly pass through the first exhaust gas flow path 12 by opening fully the valve means 15. There will be a small flow through the second exhaust gas flow path but this will not significantly affect the temperature of the exhaust gasses exiting the exhaust manifold assembly.

It will be appreciated that the valve means 15 does not restrict the flow through the second exhaust gas flow path but that path has a higher restriction to flow than the first exhaust gas flow path when the valve means 15 is fully open. This process will continue until the signal from the temperature sensor 20 indicates that the temperature within the three way catalyst 16 has reached the light-off temperature of the three way catalyst 16. This temperature is used as a first predetermined temperature below which exhaust gasses are directed primarily through the first exhaust gas flow path. With the arrangement shown this temperature may be derived by a combination of direct measurement using the temperature sensor 20 and modelling.

For example if a light-off temperature of 275 C is required for the three way catalyst 16 the electronic control unit 5 may be operable to control the valve means to direct flow through the first exhaust gas flow path until the sensed temperature reaches 325 C by using a look up table relating measured temperature to actual brick temperature.

Alternatively, the predetermined temperature set in the - 14 - electronic control unit 5 could he based upon recev:ng a signal indicative of 325 C it being known that a 5OC temperature difference exists between the position of measurement and the core brick temperature.

s soon as the three way catalyst is lit-off then there is less need for rapid heating and so the electronic control unit 5 enters a second phase of operation in which it endeavours to maintain the temperature of the exhaust gasses leaving the exhaust manifold assembly within a predetermined range which in its broadest sense is bounded at its lower end by a lower temperature limit below which one of the three way catalyst and the lean NOx trap does not operate efficiently and bounded at its upper end by an upper temperature limit above which premature ageing of one of the three way catalyst and the lean NOx trap will occur.

In practice during this phase of operation the electronic control unit 5 operates so as to maintain the temperature of the exhaust gasses in the lean NOx trap within a range of temperatures where it will operate at maximum efficiency. If the exhaust gas temperature approaches the upper end of this range, then the electronic control unit 5 will operate the valve means to cause more of the exhaust gasses to flow through the second flow path 13 by closing the valve means 15 either fully or partially and, if the temperature of the exhaust gasses approach the lower end of this range, then the electronic control unit 5 will increase the percentage of exhaust gasses flowing through the first flow path by opening the valve means 15. In this way, at least while the engine 10 is operating in a lean burn mode of operation, the electronic control unit 5 is able to keep the temperature of the exhaust gasses entering the lean NOx trap 17 within the required temperature range.

If the temperature of the exhaust gasses entering the lean NOx trap 17 exceed the upper limit of the preferred temperature range then the vave means 15 will he corated to cause the maximum volume of exhaust gasses to flow through the second exhaust cas flow path 13 by clos:ng completely the valve means 5. This will keep the temperature of the exhaust casses exiting the exhaust manifold assembly as close to the upper temperature limit of the range as possible so as:o reduce the effects of temperature ageing on the lean NOx trap 17. It will be appreciated that during engine running at high load and high speed the exhaust gas temperature exiting the engine 10 will rise considerably and the cooling requirement is likely to temporarily exceed the cooling capacity of the exhaust gas cooler 14. During these conditions the aim is therefore to minimise the temperature of the gasses entering the lean NOx trap 17.

During such conditions it is also possible that the three way catalyst 16 would also be adversely affected and so the electronic control unit 5 may also be operable to operate the valve means 15 to cause the maximum flow through the second exhaust gas flow path 13 and the minimum through the first exhaust gas flow path 12 when the temperature of the exhaust gasses in the three way catalyst 16 are above a predetermined upper limit. This is particularly relevant if there is a considerable distance between the three way catalyst 16 and the lean NOx trap 17 because then the temperature of the exhaust gasses in the three way catalyst 16 could be considerably higher than in the lean NOx trap 17.

The electronic control unit 5 is however also programmed to permit the temperature of the exhaust gasses entering the lean NOx trap 17 to exceed the upper limit of the preferred temperature range when it is determined that sulphur needs to be removed from the lean NOx trap 17. This is because to remove sulphur from the lean NOx trap 17, the - 16 - lean NOx trap 17 has to be heated to a high temperature such as 650 C, which is well above its normal operating range.

When such a de-sulphation operation is required the electronic control unit 5 controls the valve means 15 to permit the temperature entering the lean NOx trap 17 to rise to the required temperature which will normally require permitting a significant proportion of the exhaust gasses to flow through the first exhaust gas flow path 12 and limiting the flow through the second exhaust gas flow path 13. If the temperature of the exhaust gasses exiting the engine are approximately 675 C then no cooling is required and the flow will be primarily through the first flow path 12 but if the temperature of the exhaust gasses is approximately 800 C then some cooling effect is desirable in order to protect the three way catalyst 16 and particularly the lean NOx trap 17 from this undesirable temperature. In this case the valve means 15 would permit flow to occur through both of the exhaust gas flow paths 12, 13 so as to produce the desired de-suiphation temperature.

An emission control system according to the invention therefore has several advantages including but not limited to improved engine warm up due to the recovery of heat from the exhaust gasses resulting in increased fuel economy and reduced emissions during start-up, less risk of premature ageing of any catalytic devices due to the ability to reduce the maximum temperature of the exhaust gasses entering these devices, the opportunity to combine a three way catalyst and lean NOx trap in a single unit that is close coupled to the exhaust manifold of the engine and improved lean NOx trap performance due to the ability to maintain the temperature of the exhaust gasses within a preferred range where optimum efficiency is obtained.

Although the invention has been described with particular reference to an emission control system for a - 17 - lean burn engine because its use is particularly advantageous for such an engine, it will he appreciated that it could be applied to other tyes of engine. In this case the lean NOx trap would be replaced by a secondary catalyst or particulate trap depending uon the type of engine.

With particular reference no Figs.2 and 3 there is shown in greater detail a first embodiment of an exhaust manifold assembly 30 suitable for use in forming part of the so emission control system previously described.

The exhaust manifold assemoly 30 comprises of four inner exhaust conduits in the form of exhaust pipes 33 enclosed by a first casing in the form of an inner clamshell 32, which is spaced away from the exhaust pipes 33 to define a closed volume. The inner clan.shell 32 is itself enclosed within a second casing in the form of an outer clamshell 31, which is spaced away from the inner clamshell 32 to define a closed volume. The inner and outer clamshells 32 and 31 are in this case pressed metal parts each of which has two or more components welded together to define the respective closed volume. It will however be appreciated that alternative forms of construction could be used.

The exhausts pipes 33 define a first exhaust gas flow path through theexhaust manifold assembly 30, this path being designed to minimise cooling of the exhaust gasses.

This may be achieved through miriimising the thermal mass of the pipes since the outer clamshells 32 and 31 will provide the required structural rigidity for the manifold. The volume formed between the exhaust pipes 33 and the inner clamshell 32 forms a second exhaust gas flow path 13 and the volume formed between the inner and outer clamshells 32 and 31 forms an exhaust gas cooler 14.

Coolant from a main cooling circuit of an engine to which the exhaust manifold assembly 30 is connected in use - 18 - is circulated through the volume formed between the rrmer and outer clamshells 32 and 31 from an inlet 25 to an outlet 26 so as to transfer heat away from the inner clam shell 32.

That is to say, heat is removed from any exhaust gasses flowing through the second exhaust gas flow path 13 by conduction through the inner clamshell 32.

A single exhaust flange 39 is used in this case to connect the exhaust manifold assembly 30 to an engine (not shown) . However, if each of the exhaust pipes were to be separately cooled then it is possible that several exhaust flanges would be used.

The inner and outer clamshells 32 and 31 are welded to the exhaust flange 39 so as to secure them thereto. The exhaust flange 39 has a number of tubular sleeves 34 extending outwardly therefrom for co-operation with the exhaust pipes 33.

Each of the exhaust pipes 33 is fitted into a respective one of the tubular sleeves 34 to form a slip joint therebetween. It will be appreciated that, alternatively, the exhaust pipes could be fitted to the outside of the tubular sleeves.

The slip joints allow movement to take place during heating and cooling of the exhaust pipes 33 but provide a location to align the exhaust flange 39 and the exhaust pipes 33. Each of the tubular sleeves 34 is positioned so that it aligns with an aperture 39a (of which only one is visible on Fig.2) formed in the exhaust flange 39. The apertures 39a form an inlet to the exhaust manifold assembly from the engine and are aligned with exhaust ports (not shown) of the engine.

Each of the tubular sleeves 34 has a number of apertures 35 formed in it. These apertures are not - 19 - obstructed by the exhaust pipes 33 since only a few millimetres of overlap between the exha'st pipes 33 and the tubular sleeves 34 is required. The apertures 35 in the tubular sleeves 34 form an inlet to the second exhaust flow path defined between the exhaust pipes 33 and the inner clamshell 32.

Alternatively, the apertures 35 formed in the tubular sleeves 34 cooperate with complementary apertures (not shown) in the ends of the exhaust pipes 33. The apertures in the tubular sleeves 34 and the complementary apertures in the exhaust pipes 33 form an inlet to the second exhaust flow path defined between the exhaust pipes 33 and the inner clamshell 32.

The exhaust pipes 33 extend independently from the engine to a junction 36 where the separate exhaust pipes 33 merge to form a single conduit extending to an outlet from the exhaust manifold assembly 30.

An exit aperture 37 is formed at an outlet end of the second exhaust gas flow path 13 to allow exhaust gasses to flow out of the closed volume to the outlet from the exhaust manifold assembly 30.

A valve means in the form of a valve assembly 40 is positioned slightly upstream from the outlet of the exhaust manifold assembly 30 at a position just prior to a position at which the flows from the first and second exhaust gas flow paths merge to form a single mixed outlet flow. The valve assembly 40 is in this case shown controlling the flow through only the second exhaust gas flow path 13 but it will be appreciated that it could equally be positioned (as shown in Fig.1) to control the flow through the first exhaust gas flow path or be located at the position where the first and second exhaust gas flow paths merge so that it controls the flow through both flow paths.

- 20 - The valve assembly 40 comprises of a butterfly uilve 41 which is moveable by a vacuum actuator 42 via a lin:ae 43.

The vacuum actuator 42 is electronically controlled in use by an electronic control unit such as that shown in Fig.l.

It will he appreciated that instead of using a vacuun actuator and electric actuator or other suitable actuation means could be used.

The valve assembly 40 is mounted in an outlet casing 38 of the exhaust manifold assembly 30 which in this case is also used as one end of a canister for a close coupled catalytic device such as a three way catalytic converter (not shown) . As before a lean NOx trap could also be mounted in the same canister used for the three way catalyst. The outlet casing has interior passages (not shown) which direct coolant from the inlet 25 to regions surrounding the bearings (not shown) of the butterfly valve 41. This permits the use of less expensive bearing materials and in addition because the butterfly valve 41 only regulates the flow of gasses through the second flow path it is exposed to lower temperatures than it would be if located at an outlet from the first flow path.

Operation of the exhaust manifold assembly 30 is as follows. The butterfly valve 41 is controlled by the electronic control unit to adopt a position dependent upon a desired temperature of exhaust gasses downstream from the exhaust manifold assembly 30.

If the butterfly valve 41 is moved to the position shown in F.ig.3 then flow through the second exhaust gas flow path is unrestricted and so exhaust gasses will flow through the first exhaust gas flow path through the exhaust pipes 33 directly through the exhaust manifold assembly 30 with no appreciable loss of temperature and also through the second exhaust gas flow path where it is cooled. The two exhaust - 2: - gas flows will then mix before leaving the exhaust manifold assembly 30. The temperature of the resulting gas flow will depend upon the respective tecceratures and volume flow rates of the two flows but will be considerably less than the temperature of the exhaust gasses entering the exhaust manifold assembly 30. By varyong the ratio of these two flows using the valve assembly 40 the exhaust manifold assembly 30 can be used to vary the temperature of the exhaust gasses leaving the exhaust manifold assembly 30.

:o This is useful for controlling the temperature of exhaust gasses entering a downstream catalytic device or lean NOx trap so as to operate these deices at or near to peak operating efficiency.

:5 If the butterfly valve 41 is moved to a position at right angles to that shown, then flow through the second exhaust gas flow path will be blocked and so exhaust gasses will flow solely through the exhaust pipes 33 directly through the exhaust manifold assembly 30 with no appreciable loss of temperature. This is useful if there is a need to provide maximum temperature to a downstream device in order to say light-off a catalyst or a lean NOx trap.

One of the drawbacks of positioning the valve 41 in this position at the end of the second exhaust gas flow path is that at all times exhaust gasses flow through the first exhaust gas flow path and there is no way of limiting this flow only of increasing the flow through the second exhaust gas flow path. This limits the maximum temperature reduction that can be achieved because there is always un- cooled exhaust gasses flowing. However, as previously mentioned the butterfly valve 41 is exposed to lower temperatures by being so positioned and so potentially less expensive materials can be used in its manufacture.

With reference to Fig.4 there is shown a second embodiment of an exhaust manifold assembly according to the - 22 - invention which in many respects is the same as the: previously described.

The exhaust manifold assembly comprises of four inner exhaust conduits in the form of exhaust pipes 133 enclosed by a first casing in the form of an inner clamshell 132 which is spaced away from the exhaust pipes 133 to define a closed volume. The inner clamshell 132 is itself enclosed within a second casing in the form of an outer clamshell 131 which is spaced away from the inner clamshell 132 to define a closed volume. The inner and outer clamshells 132 and 131 are, as before, pressed metal parts welded to define the respective closed volumes.

The exhaust pipes 133 define a first exhaust gas flow path through the exhaust manifold assembly 30. The volume formed between the exhaust pipes 133 and the inner clamshell 132 forms a second exhaust gas flow path 13 and the volume formed between the inner and outer clamshells 132 and 131 forms an exhaust gas cooler 14.

Coolant from a main cooling circuit of the engine (not shown) to which the exhaust manifold assembly is connected in use is circulated through the volume formed between the inner and outer clamshells 132 and 131 from an inlet 125 to an outlet 126 so as to transfer heat away from the inner clam shell 132.

A single exhaust flange 139 is used in this case to connect the exhaust manifold assembly to the engine.

The inner and outer clamshells 132 and 131 are welded to the exhaust flange 139 so as to secure them thereto. The exhaust flange has a number of tubular sleeves (not visible) extending outwardly therefrom for cooperation with end portions of the exhaust pipes 133.

- 23 - Each of the exhaust pipes 133 is fitted to the:tside of the tubular sleeves to form a slip joint which all tws movement to take place during hearing and cooling oi the exhaust pipes 133.

Each of the tubular sleeves is positioned so that it aligns with an aperture (not shown) formed in the exhaust flange 139. The apertures form an inlet to the exhaust manifold assembly from the engine and are aligned with exhaust ports (not shown) of the engine.

Each of the tubular sleeves has a number of apertures formed in it for cooperation with complementary apertures in the ends of the exhaust pipes 133. The apertures in the tubular sleeves and the complementary apertures 135 in the exhaust pipes 133 form an inlet to the second exhaust flow path defined between the exhaust pipes 133 and the inner clamshell 132.

The exhaust pipes 133 extend independently from the engine for a short distance to a manifold where the separate exhaust pipes 133 merge to form a single conduit extending to an outlet from the exhaust manifold assembly.

An exit aperture 137 is formed at an outlet end of the second exhaust gas flow path 13 to allow exhaust gasses to flow out of the closed volume to the outlet from the exhaust manifold assembly.

A valve assembly 140 is positioned slightly upstream from the outlet of the exhaust manifold assembly at a position just prior to a position at which the flows from the first and second exhaust gas flow paths merge to form a single mixed outlet flow. The valve assembly 140 controls the flow through only the first exhaust gas flow path.

-

The valve assembly 140 comprises of a butterfv alve 141 which is moveable by a vacuum actuator 142 via a linkage 143. The vacuum actuator 142 iS electronically ccn:rolled in use by an electronic control unit such as that stown in Fig.1.

The valve assembly 140 is mounted in an outlet casing 138 of the exhaust manifold assembly which in this case is also used as one end of a canister for a close couplet catalytic device such as a three way catalytic converter (not shown) . As before a lean NOx trap could also be mounted in the same canister used for the three way catalyst. The outlet casing 138 is a casting and has interior passages (not shown) which direct coolant from the inlet 125 to regions surrounding the bearings (not shown) of the butterfly valve 141.

Operation of the exhaust manifold assembly is as follows. The butterfly valve 141 is controlled by the electronic control unit to adopt a position dependent upon a desired temperature of exhaust gasses downstream from the exhaust manifold assembly.

If the butterfly valve 141 is moved to the position shown in Fig.4 then flow through the first exhaust gas flow path is unrestricted and so exhaust gasses will flow through the exhaust pipes 133 directly through the exhaust manifold assembly with no appreciable loss of temperature and also through the second exhaust gas flow path where it is cooled.

The two exhaust gas flows will then mix before leaving the exhaust manifold assembly. The temperature of the resulting gas flow will depend upon the respective temperatures and volume flow rates of the two flows and because of differences in resistance to flow the majority of the flow will be through the first exhaust gas flow path. By varying the ratio of these two flows the exhaust manifold assembly is able to be used to vary the temperature of the exhaust - 25 - gasses leaving the exhaust manifold assembly which is useful in maintaining a downstream catalytic device or lean NOx trap at or near to peak operating efficiency.

If the butterfly valve 141 is moved to a position at right angles to that shown in Fig.4 then flow through the first exhaust gas flow path will be blocked and so exhaust gasses will be forced to flow primarily through the second exhaust gas flow path so as to benefit from maximum cooling.

This arrangement is useful for reducing the temperature of the exhaust gasses below a temperature where premature ageing of any devices mounted downstream from the exhaust manifold assembly will occur or at least minimising any temperature excursions above such a temperature.

It will be appreciated that with the butterfly valve 141 so positioned, the proportion of exhaust gasses that flow through the second exhaust gas flow path must be set such that when the valve 141 is in the position shown in Fig.4 sufficient exhaust gas temperature can be provided to effect rapid light-off of any catalyst located downstream.

This can be done by varying the size and number of apertures which will change the resistance to flow through the second exhaust gas flow path or by changing the gap between the exhaust pipes 133 and the inner clamshell 132 or by a combination of these.

With reference to Figs. S and 6 there is shown a third embodiment of an exhaust manifold assembly according to the invention.

The exhaust manifold assembly comprises of four exhaust conduits in the form of exhaust pipes 233 which define a first exhaust gas flow path through the exhaust manifold assembly, a valve assembly 240 having a moveable valve member 241 positioned towards the end of the first gas flow path, a first cooling conduit 213a providing a flow path - 26 - from the valve member 241 to ar. exhaust gas cooler 21- and a second cooling conduit 213b prciding a flow path fror an outlet from the exhaust gas cocier 214 hack to the vaive member 241.

Exhaust gasses which flow from the engine through the exhaust manifold assembly via the exhaust pipes 233 ard then through the first and second cooling conduits 213a, 213b and the exhaust gas cooler are said to follow a second exhaust :o gas flow path.

Coolant from a main cooling circuit of the engine (not shown) to which the exhaust manifold assembly is connected in use is circulated through the exhaust gas cooler 214 in :5 order to cool any exhaust gasses passing therethrough.

single exhaust flange 239 is used to connect the exhaust manifold assembly to the engine and each of the exhaust pipes 233 is welded directly to the exhaust flange 239.

The exhaust pipes 233 extend independently from the engine for a short distance to a manifold where the separate exhaust pipes 233 merge to form a single conduit extending to the valve assembly 240.

The valve assembly 240 is positioned slightly upstream from the outlet of the exhaust manifold assembly at a position at which the flows from the first and second exhaust gas flow paths merge to form a single mixed outlet flow. The valve assembly 240 controls the flow through both of the exhaust gas flow paths or to be more precise determines how much of the exhaust gasses are diverted through the exhaust gas cooler 214.

The valve assembly 240 comprises of a butterfly valve member 241 which is moveable by a vacuum actuator 242 via a - 27 - linkage 243. The vacuum actuator 242 is electronically controlled in use by an electronic control unit such as that shown in Fig.1.

The valve assembly 240 is mounted in an outlet casing 238 of the exhaust manifold assembly which in this case is also used as one end of a canister for a close coupled catalytic device such as a three way catalytic converter (not shown) . As before a lean NOx trap could also be mounted in the same canister used for the three way catalyst. The outlet casing 238 is in this case a casting and is cast as a single part with the two cooling conduits 213a, 213b. However, it will be appreciated that other forms of construction and design could be used.

Operation of the exhaust manifold assembly is as follows. The butterfly valve 241 is controlled by the electronic control unit to adopt a position dependent upon a desired temperature of exhaust gasses downstream from the exhaust manifold assembly.

If the butterfly valve 241 is moved to the position shown in Figs.5 and 6 then flow through the first and second exhaust gas flow paths is unrestricted but the majority of the exhaust gasses will flow through the exhaust pipes 133 and directly through the exhaust manifold assembly with no appreciable loss of temperature. This is because of the higher resistance to flow that exists if the exhaust gasses flow through the exhaust gas cooler 214 and because the inertia of the exhaust gasses will carry them past the entrance to the first coolant conduit 231a.

Therefore when the valve member 241 is in this position there is very little difference in the exhaust gas temperature entering and leaving the exhaust manifold assembly because the exhaust gasses pass primarily through the first exhaust gas flow path. A small flow of exhaust - 28 gas will however flow through the exhaust gas cooler 214 which can be used to help warm up the coolant in the main engine cooling circuit.

The two exhaust gas flows will then mix before leaving the exhaust manifold assembly. The actual temperature of the resulting gas flow will depend upon the respective temperatures and volume flow rates of the two flows.

If the butterfly valve is moved to a position at 45 degrees to that shown in Fig.5 as indicated by the reference number 214a on Fig.6, then flow through the first exhaust gas flow path will be blocked and all of the exhaust gasses have to flow through the second exhaust gas flow path. It will be appreciated that the second exhaust gas flow path includes not only the exhaust pipes 233 but also the first and second cooling conduits 213a, 213b and the exhaust gas cooler 214. This valve position is useful to prevent the temperature of the exhaust gasses exiting the exhaust manifold assembly from causing premature ageing of any devices mounted downstream from the exhaust manifold assembly by minimising the temperature of the exhaust gasses exiting the exhaust manifold assembly. That is not to say that the exhaust gasses will not reach a high temperature because that is possible if the cooling capacity of the exhaust gas cooler 214 is exceeded by running the engine near to peak power but they are minimised. For example, if the exhaust gas cooler is able to produce a temperature drop of 150 C then, if the exhaust gas temperature rises to 800 C, the temperature of the exhaust gasses exiting the exhaust manifold assembly will still be 650 C but this will have a less serious ageing effect than would a temperature of 800 C.

Therefore by varying the position of the butterfly valve 241, 214a between these two positions the ratio of exhaust gasses that are cooled and not cooled can be - 29 - changed. This means that the exhaust manifold asseticly is able to vary the outlet temperature of the exhaust. gesses which travel downstream which can be used to maintain a catalytic device or lean NOx trap at or near to peak operating efficiency.

With reference to Fig.7 there is shown an alternative valve arrangement which is intended to be a direct replacement for that shown in Fig.6 In this case a penny valve 341 is used instead of a butterfly valve.

The primary difference between this valve arrangement is and that previously described is that when the valve member 341 is in a first position, as indicated by the reference numeral 341, no flow is permitted through the second flow path because the outlet from the second cooling conduit 2l3b is covered. When the valve member is in a second position, as indicated by the reference numeral 341a on Fig.7, all of the exhaust gasses flow through the second exhaust gas flow path and none can flow only through the first exhaust gas flow path. Therefore this arrangement allows the flow through both of the exhaust gas flow paths to be selectively stopped. This enables a higher exhaust gas temperature to be provided during engine warm up than is the case with the arrangement shown in fig.6.

s before the valve member 341 is moveable to permit the temperature of the exhaust gasses exiting the exhaust manifold assembly to be regulated to meet various operational requirements as previously described.

One advantage of the valve arrangement shown in Fig.7 is that the bearings for the valve member 341, 341a are located at the exit from the second cooling conduit 2l3b and so are exposed for some of the time to the cooler exhaust - 30 - gasses exiting from the second cc:ling conduit 213b thereby allowing less expensive bearing raterials to be used. Also it is possible to allow for a small leakage past the valve member 341 so that even when the ast majority of exhaust gasses are flowing un-cooled froro the exhaust manifold assembly a flow of cooled exhausc gasses is able to cool a rear face of the valve member 34T It will be appreciated by those skilled in the art that io although the invention has been cescribed by way of example with reference to one or more embodiments it is not limited to the disclosed embodiments and chat modifications to the disclosed embodiments or alternative embodiments could be constructed without departing froro the scope of the invention.

Claims

- 31 - Claims 1. An exhaust manifold assembly for an internal combustion

engine, the exhaust manifold assembly has an inlet into which exhaust gasses from the engine flow in use, an outlet from which exhaust gasses flow in use, a first exhaust gas flow path for transferring exhaust gasses from the inlet to the outlet, a second exhaust gas flow path for transferring exhaust gasses from the inlet to the outlet and a valve means to regulate the flow of exhaust gasses passing through the first and second flow paths wherein any exhaust gasses passing through the first flow path pass through the exhaust manifold assembly with minimal cooling and any exhaust gasses passing through the second flow path are subject to active cooling from an exhaust gas cooler so that the temperature of the exhaust gasses exiting the exhaust manifold assembly can be regulated.
2. An exhaust manifold assembly as claimed in claim 1 wherein the valve means is operable to pass all or the majority of all the exhaust gasses from the engine through the first flow path when the engine is started from cold.
3. An exhaust manifold assembly as claimed in claim 1 or in claim 2 wherein the valve means is operable to pass all or the majority of all the exhaust gasses from the engine through the first flow path at least when the temperature of the exhaust gasses is below a first predetermined temperature.
4. An exhaust manifold assembly as claimed in claim 3 wherein the valve means is operable to pass at least some of the exhaust gasses through the second flow path when the temperature of the exhaust gasses exceeds a second predetermined temperature.

- 32 -
5. An exhaust manifold assembly as claimed in any of claims 1 to 4 wherein the valve means is operable to v&ry the proportion of exhaust gasses flowing through the first and second flow paths in order to regulate the temperature of the exhaust gasses exiting the exhaust manifold assembly.
6. An exhaust manifold assembly as claimed in claim 5 wherein the exhaust gasses are regulated in order to maintain the temperature of the exhaust gasses exiting the exhaust manifold assembly within a predetermined range of temperatures.
7. An exhaust manifold assembly as claimed in any of claims 1 to 6 wherein the valve means is a single control valve positioned so as to regulate the flow of exhaust gasses through the first flow path.
8. An exhaust manifold assembly as claimed in claim 7 wherein the valve means is positioned near to the outlet so as to selectively restrict the flow of exhaust gasses exiting the first flow path.
9. An exhaust manifold assembly as claimed in any of claims 1 to 6 wherein the valve means is a single control valve positioned so as to regulate the flow of exhaust gasses through the second flow path.
10. An exhaust manifold assembly as claimed in claim 9 wherein the valve means is positioned near to the outlet so as to selectively restrict the flow of exhaust gasses exiting the second flow path.
11. An exhaust manifold assembly as claimed in any of claims 1 to 6 wherein the valve means is a single control valve positioned so as to simultaneously regulate the flow of exhaust gasses through the first and second flow paths.

- 33 -
12. An exhaust manifold assembly as claimed cc. claim 11 wherein the valve means is positioned near to the cutlet so as to selectively vary the flow of exhaust gasses exiting the first and second flow paths.
13. An exhaust manifold assembly as claimed in any of claims 1 to 12 wherein the exhaust gas cooler uses coolant drawn from a main cooling system of the engine.

is
14. An exhaust manifold assembly as claimed in any of claims 1 to 13 wherein the exhaust manifold assembly comprises of at least one inner exhaust conduit defining the first exhaust gas flow path, a first casing enclosing the inner exhaust conduit and being spaced therefrom so as to define the second exhaust gas flow path and a second casing enclosing the first casing and being spaced apart therefrom so as to define a gap through which the coolant is passed to form the exhaust gas cooler.
15. A method substantially as described herein. * *S * S * *555 * S S... a. * S S.. I 5 S. S

S S S * .*

15. An exhaust manifold assembly as claimed in claim 14 wherein each inner exhaust conduit is connected to an outlet from the engine by a flange plate.

16. An exhaust manifold assembly as claimed in claim 15 wherein the or each flange plate has a tubular sleeve extending therefrom with which an end of a respective inner exhaust conduit is engaged.

17. An exhaust manifold assembly as claimed in claim 14 wherein the exhaust manifold has a single flange plate and a like number of tubular sleeves as there are cylinders in the engine to which the exhaust manifold assembly is connected.

18. An exhaust manifold assembly as claimed in claim 16 or in claim 17 wherein each tubular sleeve has one or - 34 - more apertures therein to provide an inlet to the second exhaust flow path.

19. An exhaust manifold assembly as claimed in claim s 16 or in claim 17 wherein each tubular sleeve has one or more apertures therein for cooperation with one or more corresponding apertures in an end portion of the inner exhaust conduit with which it engages to provide an inlet to the second exhaust flow path.

20. An exhaust manifold assembly as claimed in any of claims 14 to 19 wherein the manifold assembly has a number of inner exhaust conduits forming the first flow path.

21. An exhaust manifold assembly as claimed in claim wherein the inner exhaust conduits flow independently from the engine to a junction where the separate inner exhaust conduits merge to form a single conduit extending to the outlet.

22. An exhaust manifold assembly as claimed in any of claims 1 to 13 wherein the first exhaust gas flow path is formed by an exhaust conduit, the control means is a control valve positioned at an exit end of the exhaust conduit, the second exhaust gas flow path is formed by the exhaust conduit, a first cooling conduit connecting the control valve to an inlet to an exhaust gas cooler and a second cooling conduit connecting an outlet from the exhaust gas cooler to the control valve and the control valve is operable to regulate the flow of exhaust gasses through the first and second exhaust gas flow paths.

23. An exhaust manifold assembly as claimed in claim 22 wherein there are several exhaust conduits that flow independently from the engine to a junction where they merge to form a single conduit extending to the control valve.

- 35 - 24. An exhaust manifold assembly as claimed in claim 22 or in claim 23 wherein the cooorol valve is moveable from a first position in which all of the exhaust gasses flow from the inlet to the outlet through only the first exhaust flow path to a second position in which all of the exhaust gasses flow from the inlet to the outlet through the second exhaust flow path.

25. An exhaust manifold assembly as claimed in claim 24 wherein, when the control valve is positioned between the first and second positions, a proportion of the exhaust gasses flow through the first flow path and a proportion of the exhaust gasses flow through the second flow path.

26. An exhaust manifold assembly as claimed in claim wherein the position of the control valve between the first and second positions is varied to regulate the temperature of the exhaust gasses leaving the exhaust gas manifold through the outlet.

27. An exhaust manifold assembly as claimed in any of claims 24 to 26 wherein when the control valve is in the first position it substantially prevents the flow of exhaust gasses through the second cooling conduit.

28. An exhaust manifold assembly as claimed in claim 27 wherein a small leakage of exhaust gasses past the control valve is permitted even when the control valve is in the first position so as to cool the control valve.

29. An emission control system for an engine having an exhaust manifold assembly as claimed in any of claims 1 to 28 and a catalytic device coupled to the outlet from the exhaust manifold assembly.

30. An emission control system as claimed in claim 28 wherein the catalytic device is a three way catalyst.

- 36 - 31. An emission control system as claimed in claim 30 wherein the emission control system further comprises a lean NOx trap positioned downstream from the three way catalyst.

32. An emission control system as claimed in claim 31 wherein the three way catalyst and the lean NOx trap are mounted in a common canister directly coupled to the outlet from the exhaust manifold assembly.

33. An emission control system as claimed in any of claims 29 to 32 wherein the emission control system further comprises an electronic control unit and at least one exhaust gas temperature sensor operably connected to the electronic control unit to provide a signal indicative of the exhaust gasses downstream from the exhaust gas manifold.

34. An emission control system as claimed in claim 33 wherein the valve means is a control valve and the position of the control valve is controlled by the electronic control unit.

35. An emission control system as claimed in claim 34 wherein the position of the control valve is controlled by the electronic control unit in response to one of a signal received from an exhaust gas temperature sensor and a temperature model.

36. An emission control system as claimed in claim 35 wherein the electronic control unit is operable to move the control valve to a position in which exhaust gasses flow solely or primarily through the first flow path when the temperature of the exhaust gasses are sensed to be below a first predetermined temperature.

37. An emission control system as claimed in claim 36 wherein the first predetermined temperature is a light off - 37 - temperature of a three way catalyst coupled to the exhaust gas manifold.

38. An emission control system as claimed in claim 35 wherein a three way catalyst is coupled to the outlet from the exhaust gas manifold and a lean NOx trap is positioned downstream from the three way catalyst and the electronic control unit is operable to control the position of the control valve so as to maintain the temperature of the exhaust gasses in the lean NOx trap within a predetermined range of temperatures.

39. An emission control system as claimed in claim 38 wherein the predetermined range is bounded at its lower end by a lower temperature limit below which the lean NOx trap does not operate efficiently and bounded at its upper end by an upper temperature limit above which premature ageing of the lean NOx trap will occur.

40. An emission control system as claimed in claim 38 wherein the predetermined range is a range of temperatures at which the lean NOx trap will operate at peak efficiency.

41. An emission control system as claimed in claim 38 wherein the electronic control unit is operable to permit the temperature of the exhausts gasses to exceed the upper temperature limit when it is necessary to remove sulphur from the lean NOx trap.

42. A method of controlling an emission control system having an exhaust manifold assembly as claimed in any of claims 1 to 28, a three way catalyst coupled to the outlet from the exhaust gas manifold assembly, and a lean NOx trap positioned downstream from the three way catalyst, wherein the method comprises causing the passage of all or a majority of the exhaust gasses through the first flow path after engine start-up in order to produce rapid light-off of - 38 - the three way catalyst until an exhaust gas temperature corresponding to the light-off temperature of the three way catalyst has been reached.

43. A method as claimed in claim 42, further comprising controlling the temperature of the exhaust gasses passing through the three way catalyst and lean NOx trap by varying the flow of exhaust gasses through the first and second flow paths so that the temperature of the exhaust gasses in the lean NOx trap stays within a range in which the lean NOx trap will operate at maximum efficiency.

44. A method as claimed in claim 42 or 43, further comprising causing passage of all or a majority of the exhaust gasses through the second flow path when the temperature of the exhaust gasses approaches an upper temperature limit above which premature ageing of one of the three way catalyst and the lean NOx trap will occur.

45. A method as claimed in claim 44, further comprising allowing temperature excursions above the upper temperature limit when it is necessary to remove sulphur from the lean NOx trap.

46. An exhaust manifold assembly substantially as described herein with reference to the accompanying drawing.

47. An emission control system substantially as described herein with reference to the accompanying drawing.

48. A method substantially as described herein.

Amendments to the claims have been filed as follows Claims 1. An exhaust manifold assembly for an internal combustion engine, the exhaust manifold assembly has an inlet into which exhaust gasses from the engine flow in use, an outlet from which exhaust gasses flow in use, a first exhaust gas flow path for transferring exhaust gasses from the inlet to the outlet, a second exhaust gas flow path for transferring exhaust gasses from the inlet to the outlet and io a single valve means to regulate the flow of exhaust gasses passing through the first and second flow paths wherein any exhaust gasses passing through the first flow path pass through the exhaust manifold assembly with minimal cooling and any exhaust gasses passing through the second flow path are subject to active cooling from an exhaust gas cooler so that the temperature of the exhaust gasses exiting the exhaust manifold assembly can be regulated.

2. An exhaust manifold assembly as claimed in claim 1 wherein the exhaust manifold assembly comprises of at least one inner exhaust conduit defining the first exhaust gas flow path, a first casing enclosing the inner exhaust conduit and being spaced therefrom so as to define the second exhaust gas flow path and a second casing enclosing the first casing and being spaced apart therefrom so as to define a gap through which the coolant is passed to form the exhaust gas cooler. * ** * * * I.. .

3. An exhaust manifold assembly as claimed in claim 2 * * wherein each inner exhaust conduit is connected to an outlet from the engine by a flange plate, each flange plate having * a tubular sleeve extending therefrom with which an end of a * respective inner exhaust conduit is engaged. * *SS

*. : 4. An exhaust manifold assembly as claimed in claim 3 * wherein each tubular sleeve has one or more apertures therein for co- operation with one or more corresponding apertures in an end portion of the inner exhaust conduit with which it engages to provide an inlet to the second exhaust flow path.

5. An exhaust manifold assembly as claimed in any of claims 2 to 4 wherein the manifold assembly has a number of inner exhaust conduits forming the first flow path and the inner exhaust conduits flow independently from the engine to a junction where the separate inner exhaust conduits merge to form a single conduit extending to the outlet.

6. An exhaust manifold assembly as claimed in claim 1 wherein the first exhaust gas flow path is formed by an exhaust conduit, the single valve means is positioned at an exit end of the exhaust conduit, the second exhaust gas flow path is formed by the exhaust conduit, a first cooling conduit connecting the control valve to an inlet to an exhaust gas cooler and a second cooling conduit connecting an outlet from the exhaust gas cooler to the control valve and the control valve is operable to regulate the flow of exhaust gasses through the first and second exhaust gas flow paths.

7. An exhaust manifold assembly as claimed in claim 6 wherein there are several exhaust conduits that flow independently from the engine to a junction where they merge to form a single conduit extending to the control valve. * SI * S S S...

8. An exhaust manifold assembly as claimed in claim 6 * S or in claim 7 wherein the control valve is moveable from a first position in which all of the exhaust gasses flow from * the inlet to the outlet through only the first exhaust flow ** * path to a second position in which all of the exhaust gasses s flow from the inlet to the outlet through the second exhaust * laS flow path.

S I I * S.

9. An emission control system for an engine having an exhaust manifold assembly as claimed in any of claims 1 to 8 and a catalytic device coupled to the outlet from the exhaust manifold assembly.

10. An emission control system as claimed in claim 9 wherein the emission control system further comprises an electronic control unit and the position of the single valve means is controlled by the electronic control unit in response to one of a signal received from an exhaust gas temperature sensor and a temperature model.

11. A method of controlling an emission control system having an exhaust manifold assembly as claimed in any of claims 1 to 8, a three way catalyst coupled to the outlet from the exhaust gas manifold assembly, a lean NOx trap positioned downstream from the three way catalyst, an electronic control unit to control the operation of the single valve means and at least one exhaust gas temperature sensor operably connected to the electronic control unit to provide a signal indicative of the exhaust gasses downstream from the exhaust gas manifold wherein the method comprises controlling the position of the valve means to cause the passage of all or a majority of all of the exhaust gasses through the first flow path after engine start-up in order to produce rapid light-off of the three way catalyst until an exhaust gas temperature corresponding to the light-off : ** temperature of the three way catalyst has been reached. *.S. e.

S

12. A method as claimed in claim 11 wherein the method further comprises controlling the temperature of the exhaust : gasses passing through the three way catalyst and lean NOx S..

* trap by using the valve means to vary the flow of exhaust gasses through the first and second flow paths so that the * ..

: 35 temperature of the exhaust gasses in the lean NOx trap stay * within a range in which the lean NOx trap will operate at maximum efficiency.

13. An exhaust manifold assembly substantially as described herein with reference to the accompanying drawing.

14. An emission control system substantially as described herein with reference to the accompanying drawing.