GB2547205A

GB2547205A - An exhaust system

Info

Publication number: GB2547205A
Application number: GB1602324.4A
Authority: GB
Inventors: Ford Kim; Stephen Brogan Mark; Graham Eastwood Peter; Birri Anthony; Ma Thomas
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2016-02-09
Filing date: 2016-02-09
Publication date: 2017-08-16
Anticipated expiration: 2036-02-09
Also published as: DE102017201597A1; GB2547205B; RU2017102418A3; RU2017102418A; GB201602324D0

Abstract

An exhaust system 8 for a vehicle 1 comprises an engine 2 with an exhaust duct 9 and a turbocharger 3 turbine 3b on the exhaust duct 9. A turbocharger bypass duct 12 comprises a first end 12a fluidly coupled to the exhaust duct 9 upstream of the turbine 3b and a second end 12b fluidly coupled to the exhaust duct 9 downstream of the turbine 3b. An exhaust after treatment device 10a, 10b is located downstream of the bypass duct 12. An injector 22 introduces a reactant into the bypass duct 12. The reactant injected may be hydrogen, a reformer gas, unburnt hydrocarbons or ammonia, and the treatment device 10a, 10b may be an SCR device or a lean NOx trap. A bypass valve 11 may be used to alter the flow or pressure of exhaust gases through the bypass duct 12. The reactant may be introduced when the engine 2 is operating under rich combustion conditions. A method of operating an exhaust system and a vehicle 1 comprising the exhaust system 8 are also claimed.

Description

An exhaust system

Technical Field

The present disclosure relates to an exhaust system for a motor vehicle, and is particularly, although not exclusively, concerned with an exhaust system configured to improve the efficacy of one or more exhaust treatment devices provided within the system.

Background

Vehicles, such as motor vehicles, are typically provided with exhaust after treatment devices configured to reduce the amount of polluting substances present in gases exhausted from the vehicle. The exhaust after treatment devices often comprise a catalyst, which is heated by the exhaust gases to a light off temperature before beginning to operate efficiently to remove pollutants from the exhaust.

As modern engines become more efficient, exhaust gases are being emitted from the engine at reduced temperatures. The amount of time taken for the exhaust after treatments devices to reach the desired operating temperature is therefore increasing.

Under Euro 6 emissions legislation, emissions on all real world drive cycles must conform with the legislated emissions limits and hence emissions must be controlled at both higher and lower temperatures.

An exhaust after treatment device, which can operate efficiently over an increased temperature range, is therefore desirable.

Statements of Invention

According to an aspect of the present invention, there is provided an exhaust system for a vehicle comprising: an exhaust duct configured to carry a bulk flow of exhaust gases from an engine of the vehicle; a exhaust bypass duct configured to carry a portion of the exhaust gases separately from the bulk flow and bypass a portion of the exhaust duct; an exhaust after treatment device provided downstream of the exhaust bypass duct; and an injector provided on the exhaust bypass duct and configured to introduce a reactant into the exhaust bypass duct.

According to another aspect of the present disclosure, there is provided an exhaust system for a vehicle comprising: an exhaust duct; a turbocharger turbine provided on the exhaust duct; a turbocharger bypass duct comprising first and second ends, wherein the first end is fluidly coupled to the exhaust duct upstream of the turbocharger turbine and the second end is fluidly coupled to the exhaust duct downstream of the turbocharger; an exhaust after treatment device provided downstream of the turbocharger bypass duct; and an injector provided on the turbocharger bypass duct and configured to introduce a reactant into the turbocharger bypass duct, e.g. directly into the turbocharger bypass duct.

The turbocharger bypass duct and/or the injector may be configured such that the reactant first mixes with the exhaust gases within the turbocharger bypass duct.

The exhaust after treatment device may comprise a catalyst configured to catalyse a reaction between the exhaust gases and the reactant. The catalyst may comprise a zeolite catalyst. Additionally or alternatively, the catalyst may comprise a platinum group metal catalyst, a selective catalytic reduction catalyst, and/or a diesel oxidation catalyst.

The injector may be configured to introduce the reactant during a regeneration event of the exhaust after treatment device. Additionally or alternatively, the injector may be configured to introduce the reactant during a DeNOx and/or DeSOx event of the after treatment device.

The exhaust system may further comprise a hydrogen source configured to provide hydrogen to the injector. The hydrogen source may comprises a hydrogen generator configured to produce hydrogen when required by the exhaust system. The hydrogen generator may be required to generate the quantity of hydrogen required to be injected by the injector. Additionally or alternatively, the hydrogen generator may be configured to generate hydrogen to be stored within a reservoir of the hydrogen source. The hydrogen generator may only be used to generate hydrogen when insufficient hydrogen is available in the reservoir.

The exhaust system may further comprise a bypass valve. The bypass valve may be configured to control the flow of exhaust gases through the turbocharger bypass duct. Additionally or alternatively, the bypass valve may be configured to control the pressure of exhaust gases within the turbo charger bypass duct.

The exhaust after treatment device may comprise a selective catalytic reduction device. Additionally or alternatively, the exhaust after treatment device may comprise a lean NOx trap.

According to another aspect of the present disclosure, there is provided a method of operating a vehicle, the vehicle comprising: an engine; an exhaust duct; a turbocharger turbine provided on the exhaust duct; a turbocharger bypass duct comprising first and second ends, wherein the first end is fluidly coupled to the exhaust duct upstream of the turbocharger turbine and the second end is fluidly coupled to the exhaust duct downstream of the turbocharger; an exhaust gas after treatment device provided downstream of the turbocharger bypass duct, the exhaust gas treatment device comprising a catalyst; and an injector provided on the turbocharger bypass duct; the method comprising: introducing a reactant into the turbocharger bypass duct using the injector.

The method may further comprise operating the engine under rich combustion conditions. The reactant may be introduced whilst the engine is operating under rich combustion conditions.

Additionally or alternatively, the method may further comprise operating the engine to increase the temperature of the exhaust gases. The reactant may be introduced whilst the engine is operating to increase the temperature of the exhaust gases.

In other words, the reactant may be introduced whilst the engine is running under hot and/or rich combustion conditions.

The vehicle may further comprises a bypass valve, configured to control the flow of exhaust gases through the turbocharger bypass duct. The method may further comprise operating the bypass valve to alter a flow rate of exhaust gases through the bypass duct whilst the reactant is being introduced. For example, the flow rate of bypass exhaust gases may be increased or decreased whilst the reactant is being injected.

Additionally or alternatively, the method may comprise operating the bypass valve to alter a pressure of exhaust gases within the bypass duct whilst the reactant is being introduced. For example the pressure of the bypass exhaust gas may be increased or decreased whilst the reactant is being injected.

The bypass valve may be controlled in order to minimise reactions between the reactant and the exhaust gases upstream of the catalyst.

The reactant may comprise any substance configured to react with the exhaust gases in the presence of the catalyst. For example, the reactant may comprise hydrogen gas, unburnt hydrocarbons, a reformer gas and/or ammonia gas.

According to another aspect of the present disclosure, there is provided a vehicle comprising the exhaust system according to a previously mentioned aspect of the disclosure.

To avoid unnecessary duplication of effort and repetition of text in the specification, certain features are described in relation to only one or several aspects or embodiments of the invention. However, it is to be understood that, where it is technically possible, features described in relation to any aspect or embodiment of the invention may also be used with any other aspect or embodiment of the invention.

Brief Description of the Drawings

For a better understanding of the present invention, and to show more clearly how it may be carried into effect, reference will now be made, by way of example, to the accompanying drawings, in which:

Figure 1 is a schematic view of an engine and exhaust system according to arrangements of the present disclosure; and

Figure 2 shows a method of operating a vehicle according to arrangements of the present disclosure.

Detailed Description

With reference to Figure 1, a vehicle 1 comprises an engine 2, a turbocharger 3, an air inlet 4, an exhaust gas recirculation (EGR) system 6, and an exhaust system 8. As depicted in Figure 1, the engine 2 comprises a turbocharged diesel engine, however it is equally envisaged that the present invention could also apply to any other type of vehicle engine, such as a petrol engine. Additionally or alternatively, the engine may be naturally aspirated or comprise a supercharger and/or be provided with some other form of enhanced induction. The vehicle may comprise an additional motor, such as an electric motor, and the engine 2 may be part of a hybrid drive system.

In Figure 1, inlet air enters via the inlet 4 and is compressed by a turbocharger compressor 3a before being supplied to an inlet manifold 2a of the engine. A throttle 5 may be provided upstream of the inlet manifold 2a to control the flow of air to the inlet manifold. Air is drawn into the engine 2, via the inlet manifold 2a. The inlet air is mixed with fuel within the engine 2 and the mixture is combusted to provide power to drive the vehicle, as well as to power any ancillary systems, such as electrical systems, provided on the vehicle. The combustion of the fuel and inlet air produces exhaust gases including water vapour, carbon dioxide (CO2), carbon monoxide (CO), nitrous oxides (NOx), sulphur oxides (SOx), Particulate Matter (PM), and other substances, which are exhausted via an exhaust manifold 2b.

The EGR system 6 comprises recirculation duct 6a and a recirculation valve 6b configured to control a flow of exhaust gases within the recirculation duct 6a. As depicted in Figure 1, the EGR system 6 comprises a high pressure EGR system configured to recirculate exhaust gases prior to the gases being expanded through a turbocharger turbine 3b. However, it is also envisaged that the EGR system 6 may comprise a low pressure EGR system, in which exhaust gases are recirculated after being expanded through the turbocharger turbine 3b. The vehicle may comprise a combination of low pressure and high pressure EGR systems. Alternatively, the vehicle may not comprise an EGR system. The EGR system 6 allows a portion of the exhaust gases to be recirculated back to the inlet of the engine 2. Replacing a portion of the oxygen rich air with burnt exhaust gases reduces the proportion of the contents of each cylinder that is available for combustion. This results in a lower heat release and lower peak cylinder temperature and thereby reduces the formation of ΝΟχ.

The exhaust system 8 comprises an exhaust duct 9, a turbocharger turbine 3b, a lean NOx trap (LNT) 10a and/or a selective catalytic reduction (SCR) device 10b upstream of an exhaust outlet 14.

Exhaust gases, which are not recirculated via the EGR system 6, enter a high pressure portion 9a of the exhaust duct 9. The turbocharger turbine 3b is provided on the exhaust duct 9, such that exhaust gases flowing through the exhaust duct, e.g. the bulk flow of exhaust gases, are expanded through the turbocharger turbine 3b to reach a low pressure portion 9b of the exhaust duct 9. The turbocharger turbine 3b is provided on the same shaft as the turbocharger compressor 3a and is configured to drive the compressor to compress the inlet air and provide enhanced induction for the engine 2.

The exhaust system 8 further comprises a turbocharger bypass duct 12. The turbocharger bypass duct is fluidly coupled, at a first end 12a, to the high pressure portion 9a of the exhaust duct, e.g. at a position on the exhaust duct 9 upstream of the turbocharger turbine 3b. The turbocharger bypass duct 12 is fluidly coupled, at a second end 12b, to the low pressure portion 9b of the exhaust duct, e.g. at a position on the exhaust duct 9 downstream of the turbocharger turbine 3b. The turbocharger bypass duct allows a portion of the exhaust gases to bypass the turbocharger turbine 3b. A turbocharger bypass valve 11 may be provided to control the proportion of the exhaust gases which flow though the turbocharger turbine and/or the turbocharger bypass duct. Control of the exhaust gases in this way allows the level of boost provided by the turbocharger to be controlled. As depicted in Figure 1, the turbocharger bypass valve 11 may be provided on the exhaust duct 9 and may be configured to control the flow of exhaust gases through the turbocharger turbine 3b, e.g. such that more or less exhaust gases pass through the turbocharger bypass duct. Additionally or alternatively, the turbocharger bypass valve 11 may be provided on the turbocharger bypass duct 12 and may be configured to control the flow of exhaust gases through the turbocharger bypass duct 12 directly. The turbocharger bypass valve 11 may be provided at or towards the first end 12a of the bypass duct. Additionally or alternatively, the turbocharger bypass valve 11 may be provided at or towards the second end 12b of the bypass duct. Again additionally or alternatively, the bypass valve 11 may be provided at any location between the first and second ends 12a, 12b. For example, the turbocharger bypass valve 11 may be provided upstream and/or downstream of the injector 22, described below.

When the bypassed exhaust gases leave the bypass duct 12, e.g. downstream of the second end 12b, the bypassed exhaust gases may mix with the bulk flow of exhaust gases within the low pressure portion 9b. Alternatively, due to the relative flow properties, such as the pressures, velocities and/or flow regimes, of the bulk exhaust flow and the bypassed exhaust flow, the exhaust gases may not mix downstream of the second end 12b before reaching the LNT 10a and/or the SCR device 10b. In either case, both the bulk flow of exhaust gases and the bypassed exhaust gases may pass through the LNT 10a and the SCR device 10b.

The LNT 10a typically contains a zeolite catalyst, which enables NOx compounds (particularly NO and NO2) to be adsorbed from the exhaust gases. Zeolite is able to adsorb a finite amount of NOx and as the trap fills the rate of NOx adsorption is reduced. Once the trap is full, no more NOx can be adsorbed and the zeolite must be purged in order to allow more NOx to be adsorbed.

In order to purge the LNT 10a, the engine may be operated under rich combustion conditions, which may lead to the generation of an increased concentration of reducing substances within the exhaust gases, such as unburnt hydrocarbons (HC). The engine may also be controlled to increase the temperature of the exhaust gases. The increased concentration of reducing substances and high temperature may lead to the captured NOx being converted into nitrogen and water, which can be exhausted from the vehicle.

In use, the LNT 10a may also capture SOx from the exhaust gases. The captured SOx may also be stored in the zeolite catalyst. Storage of the SOx may reduce the availability of the catalyst to store NOx. Hence, as the amount of SOx stored within the zeolite increases, it may be necessary to increase the frequency with which the LNT 10a is purged. In order to reduce the amount of SOx stored in the catalyst it may be desirable to perform a desulfurisation (DeSOx) procedure. The DeSOx procedure may also comprise operating the engine under rich combustion conditions with an increased exhaust gas temperature; however, in order to remove the stored SOx, it may be necessary for the LNT 10a to be heated to higher temperatures than during a purge event.

The LNT 10a may further comprise an oxidation catalyst. For example, the LNT may comprise a platinum group metal catalyst, such as a platinum, palladium, osmium, iridium, ruthenium or rhodium catalyst. The LNT may thereby be configured to capture nitrogen oxides and catalyse the oxidation of other substances within the exhaust gases. During operation of the vehicle 1, the oxidation catalyst may itself become oxidised. When the LNT is purged, as described above, the increased concentration of reducing substances in the exhaust gases may lead to the oxidation catalyst being reduced. This may allow the catalyst to continue catalysing the oxidation reaction of substances within the exhaust gases.

The SCR device 10b, in the exhaust system depicted in Figure 1, comprises a catalyst configured to catalyse a reduction reaction to lower the concentration of polluting substances, such as NOx, within the exhaust gas. A reductant is typically injected upstream of the SCR device, for example by an SCR dosing system (not shown), which reacts with the exhaust gases in the presence of the SCR catalyst. For example, NOx may be reduced by the reductant into nitrogen gas and water vapour. In a typical SCR device anhydrous ammonia is used as the reductant. The dosage of reductant can be controlled to determine the efficiency at which NOx is removed from the exhaust gases.

In another arrangements (not shown) the exhaust system 8 may comprise a Passive NOx Adsorber (PNA), which may be known as an LNT lite. The PNA may be configured to capture NOx, from the exhaust gases, at or below a first temperature and release NOx, into the exhaust gases, at or above a second temperature. The PNA may be further configured to oxidise hydrocarbons, carbon monoxide and/or other substances in the exhaust gases by virtue of a catalyst.

In another arrangement (not shown), the exhaust system 8 may comprise a diesel particulate filter (DPF) comprising a filter structure configured trap particulate matter and soot from the exhaust gases. The filter structure may be a single use filter. Alternatively, the DPF may be configured to regenerate the filter by breaking down the trapped particulates and releasing them. The particulate filter may comprise a catalyst configured to enhance the removal of trapped particulates from the filter during regeneration.

Additionally or alternatively, the exhaust system 8 may comprise a diesel oxidation catalyst (DOC) (not shown). The DOC typically comprises a substrate coated with a catalyst, such as a platinum group metal catalyst, configured to catalyse an oxidation reaction of substances within the exhaust gases such as CO, HC and PM.

Again additionally or alternatively, the exhaust system may comprise a combined after treatment device such as a combined selective catalytic reduction and diesel particular filter (SCR/DPF) device, which combines the functions of the devices described above. Any other exhaust after treatment device may be included as desired. The exhaust after treatment devices may be provided in any order within the exhaust system.

Once the exhaust gases have passed though the LNT 10a and the SCR 10b, they may be exhausted from the vehicle via the outlet 14. An outlet emissions sensor 16, such as a NOx sensor, may be provided upstream of the outlet 14 to determine the emissions being produced by the vehicle. The emissions values recorded by the emissions sensor 16 may be used to together with a measurement from an engine emissions sensor 18 to determine the efficiency with which the LNT 10a and the SCR 10b are operating to remove pollutants from the exhaust gases. The determined efficiency may be used to determine the amount of reductant to be added to the SCR device 10b and/or when the LNT 10a should be purged.

The vehicle 1 further comprises a hydrogen source 20. The hydrogen source 20 may comprise a reservoir configured to store hydrogen, which can be refilled as required, e.g. when depleted. Additionally or alternatively, the hydrogen source 20 may comprise a hydrogen generator configured to generate hydrogen through electrolysis, reformation or any other suitable process. Hydrogen may be generated during operation of the vehicle when it is required for use by systems of the vehicle. The hydrogen generator may be configured to produce the quantity of hydrogen required for the operation of the vehicle. Alternatively, excess hydrogen, e.g. hydrogen which is not immediately required, may be produced and may be stored within the reservoir. The hydrogen generator may not operate to produce hydrogen when sufficient hydrogen is available within the reservoir. Additionally or alternatively, hydrogen may be generated, even when not immediately required, and may be stored within the reservoir to be used when needed.

As depicted in Figure 1, the exhaust system 8 further comprises an injector 22. The injector 22 is provided on the turbocharger bypass duct 12 and is configured to introduce hydrogen into the bypass duct, e.g. directly into the bypass duct 12. The injector may be fluidly coupled to the hydrogen source 20, e.g. via a hydrogen duct 24, such that hydrogen may be selectively introduced into the turbocharger bypass duct 12 from the hydrogen source 20. The injector may be controllable to selectively vary the pressure at which hydrogen is introduced into the bypass duct.

Providing the injector 22 on the turbocharger bypass duct 12 may be beneficial for engine packaging and routing of the hydrogen duct 24 from the hydrogen source 20. Additionally or alternatively, providing the injector 22 on the turbocharger bypass duct 12 may be beneficial to the performance of the exhaust system 8, as described below.

Hydrogen gas, which is introduced into the turbocharger bypass duct 12 may flow within the exhaust gases to reach the LNT 10a and/or the SCR device 10b. Hydrogen reaching the SCR device 10b may act as a reductant, and may react with substances in the exhaust, such as NOx, in the presence of the SCR catalyst or another catalyst provided in the SCR device 10b. The reaction between the hydrogen and the exhaust gases may proceed at a lower temperature than the reaction between ammonia and the exhaust gases and hence by utilising hydrogen as a reductant in the SCR device 10b, the temperature range over which the SCR device is able to operate may be increased. Additionally or alternatively, hydrogen may react with oxygen within the SCR device 10b to increase the temperature of the exhaust gases and/or the SCR device 10b. The SCR device may therefore reach a temperature at which ammonia may be used effectively as a reductant at an earlier point, e.g. time, in the drive cycle.

Hydrogen may be introduced into the turbocharger bypass duct 12 in preference to ammonia being introduced into the SCR, e.g. at all temperatures. Alternatively, hydrogen and ammonia may be introduced at different points in a drive cycle and/or may be introduced together, e.g. in combination. The relative amounts of hydrogen and ammonia, which are introduced may vary according to the vehicle operating conditions.

In some conditions, such as when the exhaust gases and/or the SCR device 10b are at a low temperature, it may be desirable to only introduce hydrogen into the turbocharger bypass duct 12. This may allow the SCR device to begin operating, using the hydrogen as a reductant, to reduce the concentrations of polluting substances, such as NOx, at the low temperature. The hydrogen may also react with oxygen in the exhaust gases to increase the temperature of the SCR device.

When the temperature of the SCR device is high, it may not be beneficial to introduce hydrogen. For example, the rate of the reaction between the hydrogen and oxygen present within the exhaust gases will be increased and the hydrogen may less effectively reduce NOx or other pollutants present in the exhaust gases. Hence, at high temperatures it may be desirable for ammonia to be used as the reductant.

At intermediate temperatures, it may be desirable to introduce both hydrogen and ammonia. By introducing both hydrogen and ammonia, the efficiency of the SCR device may be increased (compared to if either hydrogen or ammonia was used exclusively) and the temperature of the SCR may continue to increase due to the reaction between the hydrogen and oxygen.

As mentioned above, under normal operation, the LNT 10a adsorbs NOx from the exhaust gases and may also catalyse an oxidation reaction of other substances in the exhaust gases, such as CO or HC. Hence, during normal operation of the LNT 10a, it may not be beneficial to introduce hydrogen into the device. However, when the LNT 10a is purged, as described above, introduction of hydrogen may be desirable. For example, hydrogen introduced during purging may react with the captured NOx, which may reduce the time required for the purge event, e.g. the time over which the engine is required to operate under rich combustion conditions. Additionally, the hydrogen may reduce the temperature at which the adsorbed NOx can be converted into nitrogen and water.

As mentioned above, it may also be necessary to perform the DeSOx procedure in order to remove SOx, which has been stored within the zeolite catalyst. Introducing hydrogen during DeSOx may improve the efficiency of the DeSOx procedure. Hydrogen may diffuse more deeply and/or more rapidly into the zeolite catalyst, which may lead to an increased rate of SOx removal. Additionally or alternatively, when introducing hydrogen it may be possible to perform the DeSOx procedure at lower temperatures and with the engine being operated under less rich conditions.

If the LNT 10a comprises an oxidation catalyst, as described above, introducing hydrogen during purging of the LNT 10a or a DeSOx event may increase the rate at which the oxidation catalyst is reduced, e.g. regenerated in order to be able to catalyse future oxidation reactions.

When providing hydrogen for the purposes described above, the hydrogen may be injected into the exhaust gases upstream of the exhaust after treatment device 10a, 10b, e.g. into the exhaust duct 9. The hydrogen may therefore by permitted to mix with the bulk flow of exhaust gases before entering the after treatment device, e.g. the LNT 10a and/or the SCR device 10b. Mixing of the hydrogen with the exhaust gases may be beneficial, as it may increase the likelihood that hydrogen will be available to react with the exhaust gases adjacent to the catalyst within the exhaust after treatment device (10a, 10b), e.g. in order to reduce NOx or other pollutants within the exhaust gases. However, mixing of the hydrogen and the exhaust gas upstream of the exhaust after treatment device may lead to oxidation of the hydrogen before the exhaust gases reach the catalyst. Hence it may be desirable to control the mixing of the injected hydrogen with the bulk exhaust gases.

When hydrogen is injected into the turbocharger bypass duct 12, as shown in Figure 1, the hydrogen may initially mix with a controlled portion of the exhaust gases within the turbocharger bypass duct 12 before entering the bulk flow of exhaust gases within the exhaust duct 9. Mixing of the hydrogen with a limited portion of the exhaust gases is beneficial, as it prevents excessive dilution of the hydrogen and/or oxidation of the hydrogen by the exhaust gases.

The turbocharger bypass valve 11 may be adjusted to control the flow rate of exhaust gases, which mix with the hydrogen within the turbocharger bypass duct 12. Adjusting the turbocharger bypass valve 11 will also effect the pressure of exhaust gases within the turbocharger bypass duct 12 and/or the low pressure portion 9b of the exhaust duct.

Controlling the relative pressures of the hydrogen, the bypassed exhaust gases and/or the bulk exhaust flow allows the dispersion of hydrogen within the exhaust gases at the LNT 10a and/or SCR 10b to be controlled. Furthermore, exhaust gases passing through the turbocharger bypass duct 12 may be less turbulent than the bulk exhaust flow. The less turbulent nature of the bypass flow, may limit diffusion of the hydrogen within the bulk gases. The rate of diffusion and/or mixing of the hydrogen may also be affected by the relative flow properties, e.g. pressure, velocity and/or flow regime, of the two flows of exhaust gases, e.g. the bypassed exhaust gases and hydrogen, and the bulk exhaust flow. Limiting the rate of the diffusion of hydrogen may reduce the rate of oxidation of the hydrogen and may increase its efficacy in improving the performance of the LNT 10a and/or the SCR device 10b, as described above.

With reference to Figure 2, a method 200 of operating a vehicle will now be described. As described above, the vehicle may comprise the engine 2, the exhaust duct 9, the turbocharge turbine 3b, the turbocharger bypass duct 12 the LNT 10a and/or the SCR device 10b, and the injector 22 provided on the turbocharger bypass duct 12 as shown in Figure 1. The method 200 comprises a first step 202 in which a reactant, e.g. hydrogen, is introduced into the turbocharger bypass duct 12 using the injector 22.

The method 200 may include a second step 204 in which the engine is operated under hot and/or rich combustion conditions. As described above, such engine operating conditions may be required in order to regenerate the LNT 10a and/or the SCR 10b and hence it may be beneficial to introduce hydrogen whilst the engine is operating under hot and/or rich combustion conditions.

The method 200 may comprise a fourth step 204, in which the turbocharger bypass valve 11 is operated in order to control the flow of bypassed exhaust gases within the bypass duct 12. As described above, control of the bypass flow in this way may affect the flow rate and/or pressure of exhaust gases within the bypass duct, which may affect the mixing and/or diffusion of the injected hydrogen with the exhaust gases. The turbocharger bypass valve 11 may thereby be controlled in order to minimise reactions between the hydrogen and the exhaust gases upstream of the catalyst, which may maximise the efficacy of the hydrogen in improving the performance of the LNT 10a and/or the SCR device 10b.

When the turbocharger bypass valve 11 is operated, e.g. in the fourth step 204, to control the flow rate and/or pressure of exhaust gases within the turbocharger bypass duct 12, the flow of exhaust gases through the turbocharger turbine 3b may also be affected. Additionally or alternatively, the process of injecting hydrogen into the bypass duct, may itself affect the flow of exhaust gases within the bypass duct 12, and hence the flow through the turbocharger turbine 3b. A change in the flow of exhaust gases through the turbocharge turbine 3b affects the power provided to the turbocharger compressor 3a by the turbocharger 3b, which affects the level of enhanced induction, e.g. the boost level, provided to the engine 2. The turbocharger bypass, the vanes and/or nozzle of the turbocharger turbine 3b may be adjusted to compensate for the change in flow rate. Additionally or alternatively, the throttle 5 may be adjusted in order to compensate for the change in boost level, e.g. to allow the engine to continue operating to provide the same power as before. Again, additionally or alternatively, the recirculation valve 6b may be adjusted in order to adjust the amount of exhaust gas recirculation and compensate for the change in boost level.

Although the above description has been given with reference to the introduction of hydrogen into the exhaust gases, it is equally envisaged that any other reactant may be introduced using the injector 22, for example, HC, reformer gas, ammonia or a combination of different gases may be introduced into the turbocharger bypass duct 12.

It will be appreciated by those skilled in the art that although the invention has been described by way of example, with reference to one or more examples, it is not limited to the disclosed examples and alternative examples may be constructed without departing from the scope of the invention as defined by the appended claims.

Claims

1. An exhaust system for a vehicle comprising: an exhaust duct; a turbocharger turbine provided on the exhaust duct; a turbocharger bypass duct comprising first and second ends, wherein the first end is fluidly coupled to the exhaust duct upstream of the turbocharger turbine and the second end is fluidly coupled to the exhaust duct downstream of the turbocharger; an exhaust after treatment device provided downstream of the turbocharger bypass duct; and an injector provided on the turbocharger bypass duct and configured to introduce a reactant into the turbocharger bypass duct.

2. The exhaust system of claim 1, wherein the exhaust after treatment device comprises a catalyst configured to catalyse a reaction between the exhaust gases and the reactant.

3. The exhaust system of claim 1 or 2, wherein the injector is configured to introduce the reactant during a regeneration event of the exhaust after treatment device.

4. The exhaust system of any of the preceding claims, wherein the injector is configured to introduce the reactant during a DeNOx and/or DeSOx event of the after treatment device.

5. The exhaust system of any of the preceding claims, wherein the exhaust system further comprises a hydrogen source configured to provide hydrogen to the injector.

6. The exhaust system of claim 5, wherein the hydrogen source comprises a hydrogen generator configured to produce hydrogen when required by the exhaust system.

7. The exhaust system of any of the preceding claims further comprising a bypass valve configured to control the flow of exhaust gases through the turbocharger bypass duct.

8. The exhaust system of any of the preceding claims further comprising a bypass valve configured to control the pressure of exhaust gases within the turbo charger bypass duct.

9. The exhaust system of any of the preceding claims, wherein the catalyst comprises a zeolite catalyst.

10. The exhaust system of any of the preceding claims, wherein the catalyst comprises a platinum group metal catalyst.

11. The exhaust system of any of the preceding claims, wherein the exhaust after treatment device comprises a selective catalytic reduction device.

12. The exhaust system of any of the preceding claims, wherein the exhaust after treatment device comprises a lean NOx trap.

13. A method of operating a vehicle, the vehicle comprising: an engine; an exhaust duct; a turbocharger turbine provided on the exhaust duct; a turbocharger bypass duct comprising first and second ends, wherein the first end is fluidly coupled to the exhaust duct upstream of the turbocharger turbine and the second end is fluidly coupled to the exhaust duct downstream of the turbocharger; an exhaust gas after treatment device provided downstream of the turbocharger bypass duct, the exhaust gas treatment device comprising a catalyst; and an injector provided on the turbocharger bypass duct; the method comprising: introducing a reactant into the turbocharger bypass duct using the injector.

14. The method of claim 13, wherein the method further comprises: operating the engine under rich combustion conditions; wherein the reactant is introduced whilst the engine is operating under rich combustion conditions.

15. The method of claim 13 or 14, wherein the method further comprises: operating the engine to increase the temperature of the exhaust gases; wherein the reactant is introduced whilst the engine is operating to increase the temperature of the exhaust gases.

16. The method according to any of claims 13 to 15, wherein the vehicle further comprises a bypass valve, configured to control the flow of exhaust gases through the turbocharger bypass duct, wherein the method comprises operating the bypass valve to alter a flow rate of exhaust gases through the bypass duct whilst the reactant is being introduced.

17. The method according to any of claims 13 to 16, wherein the vehicle further comprises a bypass valve, configured to control the flow of exhaust gases through the turbocharger bypass duct, wherein the method comprises operating the bypass valve to alter a pressure of exhaust gases within the bypass duct whilst the reactant is being introduced.

18. The method according to claim 16 or 17, wherein the bypass valve is controlled in order to minimise reactions between the reactant and the exhaust gases upstream of the catalyst.

19. The method according to any of claims 13 to 18, wherein the reactant comprises hydrogen.

20. The method according to any of claims 13 to 19, wherein the reactant comprises a reformer gas.

21. The method according to any of claims 13 to 20, wherein the reactant comprises unburnt hydrocarbons.

22. The method according to any of claims 13 to 21, wherein the reactant comprises ammonia.

23. A vehicle comprising the exhaust system according to any of claims 1 to 12.

24. An exhaust system or vehicle substantially as described herein, with reference to and as shown in the drawings.

25. A method of operating a vehicle substantially as described herein and with reference to the drawings.