GB2584845A

GB2584845A - An after treatment system and method of operating an after-treatment system for an engine

Info

Publication number: GB2584845A
Application number: GB1908620.6A
Authority: GB
Inventors: Murphy Patrick
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2019-06-17
Filing date: 2019-06-17
Publication date: 2020-12-23
Anticipated expiration: 2039-06-17
Also published as: GB2584845B; GB201908620D0

Abstract

An aftertreatment system for an engine comprises a first aftertreatment device 56 upstream of a turbocharger turbine (14b, fig 3), a second aftertreatment device (20, fig 3) downstream of the turbocharger turbine, a bypass passage 19b that bypasses the first aftertreatment device, and at least one valve 60, 62 that controls the relative exhaust flow through the first aftertreatment device and the bypass passage. A method of operating the aftertreatment system comprises permitting the exhaust flow through the first aftertreatment device, determining a performance parameter of the second aftertreatment device, and controlling the valve to restrict exhaust flow through the first aftertreatment device when the performance parameter of the second aftertreatment device has reached a threshold value. The method is carried out by a controller of the aftertreatment system. Software is executed by a computing device and causes the computing device to carry out the method.

Description

AN AFTER TREATMENT SYSTEM AND METHOD OF OPERATING AN AFTER-

TREATMENT SYSTEM FOR AN ENGINE

The present disclosure relates to an after treatment system and a method of operating an after-treatment system for an engine, and particularly, although not exclusively, relates to a system and a method in which an upstream after-treatment device is bypassed.

Background

Many vehicle engines are fitted with one or more exhaust after-treatment devices to remove polluting substances from exhaust gases before they are released. These exhaust after-treatment devices are often heated by the exhaust gases themselves to a temperature at which removal of the polluting substances can proceed effectively. If the engine also includes a turbocharger, after-treatment usually occurs downstream of the turbocharger.

It is known that a time delay exists before after-treatment devices reach their light-off temperature due to thermal inertia. This time delay is exacerbated in modern engines having lower typical exhaust gas temperatures, and in engines having a turbocharger, since enthalpy loss occurs as the exhaust gases pass through the turbocharger turbine. During this time delay, pollutants may be released into the atmosphere.

To combat this, it has previously been proposed to pass exhaust gases through a lower thermal inertia after-treatment device located upstream of the turbocharger. However, such upstream after-treatment devices suffer from a lack of durability and have not made it to mass production.

Accordingly, improvements are required in the art of exhaust gas after-treatment.

Statements of Invention

According to a first aspect of the present invention there is provided a method of operating an after-treatment system for an engine, the after-treatment system comprising a first after treatment device upstream of a turbocharger turbine; a second after-treatment device downstream of the turbocharger turbine; a bypass passage that bypasses the first after-treatment device; and at least one valve that controls the relative exhaust flow through the first after-treatment device and the bypass passage. The method comprises permitting the exhaust flow through the first after-treatment device; determining a performance parameter of the second after-treatment device; and controlling the valve so as to restrict exhaust flow through the first after-treatment device when it is determined that the performance parameter of the second-after-treatment device has reached a threshold value.

The method may additionally comprise controlling the valve so as to restrict exhaust flow through the bypass passage when it is determined that the performance parameter of the second after-treatment device has dropped below the threshold value.

The method may additionally comprise controlling the valve so as to restrict exhaust flow through the first after-treatment device when it is determined that the performance parameter of the second after-treatment device has again reached the threshold value.

The after-treatment system may additionally comprise at least one other after-treatment device upstream of the turbocharger turbine. The after-treatment system may additionally comprise at least one other after-treatment device downstream of the turbocharger turbine.

The first after-treatment device may be a pre-turbo catalyst. The valve may be arranged such that the valve selectively blocks either the bypass passage or flow through the first after-treatment device.

The after-treatment system may additionally comprise first and second valves that control the relative exhaust flow through the first after-treatment device and the bypass passage.

The first valve may be upstream of the first after-treatment device and the bypass passage and the second valve may be downstream of the first after-treatment device and the bypass passage. The first valve may selectively permit flow through the first after-treatment device and the second valve may selectively permit flow through the bypass passage.

The first after-treatment device and bypass passage may be in parallel. The first after-treatment device may have a lower thermal mass than the second after-treatment device.

The after-treatment system may further comprise a sensor configured to detect a property of the second after-treatment device. The method may further comprise determining the performance parameter using the sensor. The sensor may comprise a temperature sensor configured to determine a temperature of the second after-treatment device.

The performance parameter may be, or may be a function of, the property of the second after-treatment device. The method may comprise determining a temperature of the first after-treatment device and bypassing the first after-treatment device if the temperature of the first after-treatment device is determined to be greater than a threshold value.

According to another aspect of the present invention there is provided software which, when executed by a computing device, causes the computing device to perform the method according to any preceding claim.

According to another aspect of the present invention there is provided an after-treatment system for an engine of a motor vehicle, the after-treatment system comprising a first after treatment device upstream of a turbocharger turbine; a second after-treatment device downstream of the turbocharger turbine; a bypass passage that bypasses the first after-treatment device; at least one valve that controls the relative exhaust flow through the first after-treatment device and the bypass passage; and a controller configured to carry out the method according to any preceding claim.

The bypass passage and first after-treatment device may be housed within a canister cast into a cylinder head or a turbine housing of the engine.

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 the air and exhaust paths in an engine having a turbocharger and two downstream after-treatment devices; Figure 2 is a schematic view of the air and exhaust paths in an engine having a turbocharger, two downstream after-treatment devices and one upstream after-treatment device; Figure 3 is a schematic view of the air and exhaust paths in an engine having a turbocharger, two downstream after-treatment devices and two upstream after-treatment devices; Figures 4a-d are schematic views of the exhaust paths through the upstream after-treatment device and bypass passage according to a range of valve positions; Figures 5a and 5b are schematic views of exhaust paths through an alternative arrangement of an upstream after-treatment device and bypass passage; and Figure 6 is a flowchart showing a method of the present invention.

Detailed description

With reference to Figures 1, 2 and 3, an engine assembly 2 for an internal combustion engine 10 of a motor vehicle according to arrangements of the present disclosure is described. Air may enter an air inlet duct 46 through an inlet 12 and then pass through an air filter 13. The air may then pass through a compressor 14a of a turbocharger 14. The turbocharger 14 may improve the engine power output and reduce emissions. Typically, the turbocharger 14 is arranged with an exhaust gas-driven turbine 14b driving the compressor 14a mounted on the same shaft. A charge air cooler 16 may be provided downstream of the turbocharger compressor 14a. The charge air cooler 16 may further increase the density of the air entering the internal combustion engine 10, thereby improving its performance. The air may then enter the internal combustion engine 10 via a throttle 18 configured to vary the mass flow of air into the internal combustion engine.

In a particular example of the present disclosure, the internal combustion engine 10 comprises a diesel engine, however, it is equally envisaged that the engine 10 may be a spark ignition engine. As is depicted in Figures 1, 2 and 3, the internal combustion engine 10 may comprise a number of cylinders 10a-d and the air may flow into each of these cylinders at an appropriate time in the engine's cycle as determined by one or more valves (not shown).

The exhaust gases leaving the internal combustion engine 10 may enter an exhaust duct 19 configured to receive exhaust gases from the engine and exhaust them via an exhaust outlet 28. Exhaust gases within the exhaust duct 19 may pass through the turbine 14b of the turbocharger.

A first exhaust gas recirculation (EGR) loop 22 configured to selectively recirculate exhaust gases from the internal combustion engine 10 back into the internal combustion engine may also be provided. The first EGR loop 22 may be provided about the turbocharger 14 such that exhaust gases leaving the turbine 14b may be recirculated into the inlet of compressor 14a. The first EGR loop 22 may further comprise a first recirculation valve 24, which may control the amount of recirculation via the first EGR loop 22 through recirculation duct 42 and back into the air inlet duct 46.

A second EGR loop 32 configured to selectively recirculate exhaust gases from the internal combustion engine 10 back into the internal combustion engine may also be provided. The second EGR loop 32 may be provided about the engine 10 with exhaust gases leaving the engine 10 being recirculated to the air inlet of the engine 10. The second EGR loop 32 may comprise a second EGR duct 33, which may branch from the main exhaust flow path, e.g. gases may be diverted from the main exhaust flow path to flow through the second EGR duct 33. The second EGR duct 33 may branch from the main exhaust flow path at a point between the engine 10 and the turbine 14b of the turbocharger. Accordingly, the exhaust gases in the second EGR loop 32 may be at a higher pressure than the exhaust gases in the first EGR loop 22. The second EGR loop 32 may comprise a second recirculation valve 34 which may control the amount of recirculation in the second EGR loop 32.

The first EGR loop 22 may comprise a recirculation treatment device (not shown). Similarly, the second EGR loop 32 may comprise a recirculation treatment device (not shown). Both the first EGR loop 22 and the second EGR loop 32 are optional and one or both may be omitted.

A downstream after-treatment device 20 may be provided downstream of the turbine 14b, e.g. to reduce emissions from the engine exhaust. One or more further downstream after-treatment devices 21 may be provided, e.g. downstream of the after-treatment device 20. As shown in Figure 2, the exhaust gases may pass through an upstream after-treatment device 56 located in the exhaust duct 19 upstream of the turbocharger 14. Alternatively, for example as shown in Figure 3, there may be one or more additional after-treatment devices 54 located upstream of the upstream after-treatment device 56. The upstream after-treatment device 56 may be a pre-turbo catalyst.

With reference to Figure 3, after-treatment device 54 is located in the second EGR loop 32. After-treatment device 54 may alternatively be located proximally to after-treatment device 56 along the exhaust duct 19, rather than along optional recirculation loop 32. The exhaust after-treatment devices 20, 21, 54, 56 may be of any suitable type; the after-treatment devices 20, 21, 54, 56 may comprise one or more of an oxidation catalyst, e.g a diesel oxidation catalyst, and a particulate filter, e.g a diesel particulate filter.

The upstream after-treatment devices 54, 56 may have a lower thermal mass, and thus lower thermal inertia, than the downstream after-treatment devices 20, 21. In this way, compared with the downstream after-treatment devices 20, 21, the upstream after-treatment devices 54, 56 may require a lower total mass flow of exhaust gas in order to heat up to their light-off temperatures such that the removal of polluting substances can proceed effectively. As such, the upstream after treatment devices 54, 56 reach their light-off temperatures after lower total mass flow of exhaust gas than the downstream after-treatment devices 20, 21. Total mass flow of exhaust gas often correlates with time, meaning that the upstream after-treatment devices 54, 56 reach light-off sooner than downstream after-treatment devices 20, 21.

The upstream after-treatment devices therefore aid the downstream after-treatment devices in removing pollutants during any period of time in which the downstream after-treatment devices are unable to remove pollutants at a desired efficiency. This is particularly relevant, for example, upon engine start-up, when pollutants may pass through the downstream after-treatment devices 20, 21 whilst the downstream after-treatment devices 20, 21 are heating up.

With reference to Figures 4a-d, collectively Figure 4, the exhaust duct 19 at a location upstream of the turbocharger 14 bifurcates to form a first passage 19a and a bypass passage 19b. Within the first passage 19a is housed an upstream after-treatment device 56. The bypass passage 19b does not contain an after-treatment device. Alternatively, the exhaust duct 19 may branch into greater than two passages (not shown), each passage bar one containing an upstream after-treatment device of a different thermal mass. In additional alternative embodiments not shown, one or more additional upstream after-treatment devices may be housed in series with after-treatment device 56 in first passage 19a, in other passages in parallel with passages 19a, 19b, or in series with after-treatment device 56 in a non-branched portion of the exhaust 19 either upstream or downstream of after-treatment device 56.

This branching may occur at any location along the exhaust gas path between the beginning of the exhaust gas duct 19 and turbocharger turbine 14b. In the present example, the branching occurs upstream of after-treatment device 56.

Immediately preceding the branching of the exhaust gas path is a first valve device 60, which is capable of restricting the flow through each passage 19a, 19b. The first valve device 60 may be capable of controlling the relative flow through each passage 19a, 19b continuously between 0% and 100% of the total gas flow through the exhaust gas duct 19. The skilled person will understand that restricting the flow through one passage 19a, 19b will divert the remaining exhaust gas flow through the other passage 19a, 19b. For example, fully closing one passage 19a, 19b will divert 100% of the exhaust gases through the remaining passage 19a, 19b. Additionally, the first passage 19a and the bypass passage 19b may each be at least partially open at the same time, such that the exhaust gases can flow through passages 19a, 19b simultaneously.

The first passage 19a and the bypass passage 19b merge downstream of after-treatment device 56 and upstream of the turbocharger 14 to reform the exhaust duct 19.

As shown in Figure 4 there may additionally (or alternatively) be a second valve device 62 located at the downstream end of the two passages 19a, 19b to prevent backwards flow. This second valve device may match the state of the first valve device in controlling the flow through each passage 19a, 19b. This second valve device is optional and is not essential to the functioning or operation of the present invention. Merging of the exhaust gas flow occurs immediately downstream of second valve device 62, if present.

Figures Sa and SID, collectively Figure 5, show an alternative arrangement of an upstream after treatment device 56 and bypass passage 19b. The arrangement of Figure 5 is configured to operate in a similar manner to the arrangement shown in Figure 4 and as described elsewhere in the present disclosure. The same reference numerals are used in Figure 5 as in Figure 4 and elsewhere in the description to indicate similar components and operation. Figure 5 differs from Figure 4 in that the valve devices 60, 62 are of a type that 'flips' between the first passage 19a and the bypass passage 19b, with limited control therebetween of the exhaust gas flow path and relative flow rates through each passage 19a, 19b. In other words, a single valve may close passage 19a or 19b at either or both ends.

The first valve device 60 and the optional second valve device 62 are in communication with a controller 70, as indicated by the dashed lines in Figures 4 and 5. The controller 70 may be configured to perform the method of the present disclosure shown in Figure 6.

In manufacture, the passages 19a, 19b may form part of a canister which is cast into the cylinder head or turbine housing of the engine assembly 2.

The engine assembly 2 may comprise a number of sensors (not shown). These sensors may be coupled to, located or embedded within any number of the upstream 54, 56 and/or downstream 20, 21 after-treatment devices, for example in thermal communication with the after-treatment devices 20, 21, 54, 56. Additionally, these sensors may be located within the exhaust duct 19, for example at the exhaust outlet 28 and/or immediately upstream and downstream of each after-treatment device. The sensors are able to determine the properties of each after-treatment device at any time during engine operation, either directly or indirectly. The sensors may be of any suitable type for this purpose, for example, temperature sensors to detect whether each after-treatment device has reached a known light-off temperature, gas sensors to detect pollutant concentrations (e.g. nitrogen oxides, carbon monoxide or particulates) or exhaust gas flow rate and pressure, and the state of operation of the turbocharger turbine.

According to the properties that these sensors detect, performance parameters for each after-treatment device may be determined. Each performance parameter may depend on any property of the after-treatment device and/or the properties of the exhaust gases immediately upstream and downstream of each after-treatment device. For example, this performance parameter may be dependent upon the temperature of each after-treatment device in relation to its light-off temperature. Additionally or alternatively, this performance parameter may be dependent upon other measurements, such as the concentration of pollutants entering and leaving each after-treatment device, or the concentration of pollutants at the exhaust outlet 28.

In use, the valve devices 60, 62 are controlled by the controller 70 according to the determination of the performance parameters of the after-treatment devices 20, 21, 54, 56, using the properties detected by the sensors embedded within the after-treatment devices 20, 21, 54, 56 as well as within the exhaust duct 19 and at the exhaust outlet 28.

To better demonstrate the present invention, the method and system will now be described in the context of typical operation of a motor vehicle engine in combination with the schematic representations of exhaust gas flow paths shown in Figures 4a-d.

Figure 4a demonstrates an engine-off condition, in which there is no exhaust gas flowing through the exhaust duct 19 and thus no exhaust gas flowing through either the first passage 19a or the bypass passage 19b. The first and second valve devices 60, 62 may be in either the opened or closed positions or anywhere therebetween.

Upon cold-start of an internal combustion engine 10, the after-treatment devices 20, 21, 54, 56 will initially be at ambient temperature. A performance parameter is determined for each downstream after-treatment device 20, 21 using the measurements of the sensors. In cold-start conditions this performance parameter will initially be below a threshold value. As such, valve devices 60, 62 permit at least part of the exhaust gas flow through upstream after-treatment device 56.

An example of such an operating condition is shown in Figure 4b. The first valve device 60 has closed off the bypass passage 19b and opened the first passage 19a such that all exhaust gases pass through the first passage 19a including the upstream after-treatment device 56. The optional second valve device 62 has also closed off the bypass passage 19b and opened the first passage 19a. In this state, all exhaust gases will pass through the upstream after-treatment device 56 before reaching downstream after-treatment devices 20, 21. As such, the concentration of pollutants being emitted is reduced.

The performance parameter may be related to the temperature of the downstream after-treatment device 20, 21. Initially at ambient temperature, as exhaust gases flow through the downstream after-treatment device 20, 21, it gradually accumulates heat. Whilst this gradual heating occurs, the after-treatment device 20, 21 is below its light-off temperature, and so it is not removing pollutants from the exhaust gas flow at the desired efficiency. As such, the performance parameter will remain below its threshold value until pollutants are being removed at the desired efficiency, such as when the light off temperature is reached.

The properties of the downstream after-treatment devices 20, 21, in addition to the exhaust gases, are constantly or periodically being detected by the sensors. As such, performance parameters of the downstream after-treatment devices 20, 21 are being determined. As the exhaust gases continue to pass through the downstream after-treatment devices 20, 21, their temperatures gradually increase, and their performance parameters approach threshold values.

When the performance parameter of the downstream after-treatment devices 20, 21 is determined to have reached a threshold value, for example when the light-off temperature is reached, the first valve device 60 (including the optional second valve 62) restricts the flow to first passage 19a and fully opens bypass passage 19b. Exhaust gases therefore bypass upstream after-treatment device 56 limiting its exposure to the exhaust gases to only instances when the downstream after-treatment device 20, 21 is below its threshold value.

Figure 4c demonstrates this operating condition. The first valve device has closed off the first passage 19a and opened the bypass passage 19b such that all exhaust gases pass through the bypass passage 19b. The after-treatment device 56 is thus removed from the exhaust gas flow. The optional second valve device has also closed off the first passage 19a and opened the bypass passage 19b, such that the upstream after-treatment device 56 is not subjected to the exhaust gases by means of reverse flow from downstream. These valve states may occur, for example, once the downstream after-treatment devices 20, 21 have reached their light-off temperatures, thus reaching the required operating efficiency.

It may be the case that as the downstream device approaches the threshold performance parameter, the first and second valve devices 60, 62 may gradually permit flow through the bypass passage 19b. Figure 4d demonstrates an engine-on condition in which the first and second valve devices have at least partially opened both the first passage 19a and the bypass passage 19b. In this way, exhaust gases are able to flow through both passages 19a, 19b.

As such, the present method and system ensure that the upstream after-treatment device is bypassed, either partially or fully, when the downstream after-treatment devices 20, 21 are operating sufficiently such that the combined performance of the upstream and downstream after-treatment devices 54, 56, 20, 21 does not permit greater than acceptable levels of pollutants to be emitted from the vehicle.

The above method of operating an after-treatment system is shown in Figure 6. At step 602, the first valve device 60 permits exhaust flow through the first after-treatment device 56. If present, the optional second valve device 62 may match the state of the first valve device 60 to allow the exhaust gases to exit the passage 19a. At step 604, a performance parameter of the second after-treatment device 20 is determined. At step 606, the first valve device 60 is controlled so as to restrict the exhaust flow through the first after-treatment device. The optional second valve device 62 if present may match the state of the first valve device 60.

Whilst the method of Figure 6 is shown in a particular order, it will be appreciated by the skilled person that the steps may be reorganised or occur concurrently without departing from the scope of the present disclosure. For example, the determination of the performance parameter of the second after-treatment device 20 may occur concurrently with steps 602 and 606, and indeed may precede step 602 and/or succeed step 606.

The continuous or periodic monitoring of the downstream after-treatment device 20, 21 may be carried out in combination with continuous or periodic monitoring of the exhaust gas characteristics downstream of the downstream after-treatment device 20, 21, for example at the exhaust outlet 28.

Having reached the threshold values, and having fully restricted flow through first passage 19a including upstream after-treatment device 56, the performance parameters of the downstream after-treatment devices 20, 21 may still be monitored.

If the performance parameters fall below the threshold value, for example, if it is determined by sensors downstream of the downstream after-treatment devices 20, 21 that insufficient quantities of pollutants are being removed from the exhaust gases, then at least part of the exhaust gas flow may be permitted to flow through the upstream after-treatment devices. This scenario may occur, for example, during instances of high engine power output or high levels of exhaust gas recirculation via the first and second EGR loops 22, 32, when downstream after-treatment devices 20, 21 may be unable to remove sufficient quantities of pollutants from the exhaust gases, resulting in the emission of unacceptable levels of pollutants into the environment. As a result, the valve devices 60, 62 may at least partially open first passage 19a to allow the flow of at least some exhaust gas through the upstream after-treatment device 56. In this way, the additional after-treatment capacity provided by the upstream after-treatment device 56 is utilised when it is determined that the capabilities of the downstream after-treatment devices 20, 21 are being overrun by exhaust gas pollutant concentration.

Once the performance parameters have again reached the threshold value, for example, when engine power output drops or exhaust gas recirculation is reduced, the valves 60, 62 can be controlled such that all exhaust gases flow through the bypass passage 19b.

In addition to cold-start conditions, the present invention may be equally applicable to stop-in-gear or stop-in-neutral engine restart conditions, when the engine is only temporarily on standby. In these circumstances, after-treatment devices 20, 21, 54, 56 may cool down to below their light-off temperatures, but above ambient temperatures. As such, there may still be a time period in which the performance parameters of downstream after-treatment devices 20, 21 have not yet reached their threshold values.

The method and system of the present invention ensure that the quantity of pollutants emitted whilst the downstream after-treatment devices are heating up, for example upon cold start, is greatly reduced, and the amount of time the upstream after-treatment devices spend in the exhaust gas flow is minimised to only what is necessary. The lifespan of the upstream after-treatment devices may therefore be maximised.

It is noted that the present invention may determine when to divert the gas flow away from the first passage 19a and through the bypass passage 19b according to the exact moment when the downstream catalysts reach light-off. Other methods which the present invention aims to improve upon may determine this point according to when the gas temperatures reach the relevant temperature. However, due to the high thermal masses of the downstream after-treatment devices, the moment at which the exhaust gases reach the light-off temperature and moment at which the downstream after-treatment devices reach light-off are unlikely to be coincident. The present invention removes the possibility of leaving the upstream after-treatment devices in the exhaust flow for too long or diverting away from the upstream after-treatment devices too early, thus enhancing upstream after-treatment device lifetime and reducing emission levels.

Claims

Claims 1. A method of operating an after-treatment system for an engine, the after-treatment system comprising: a first after-treatment device upstream of a turbocharger turbine; a second after-treatment device downstream of the turbocharger turbine; a bypass passage that bypasses the first after-treatment device; and at least one valve that controls the relative exhaust flow through the first after-treatment device and the bypass passage, the method comprising: permitting the exhaust flow through the first after-treatment device; determining a performance parameter of the second after-treatment device; and controlling the valve so as to restrict exhaust flow through the first after-treatment device when it is determined that the performance parameter of the second-after-treatment device has reached a threshold value.
2. The method of claim 1, wherein the method additionally comprises: controlling the valve so as to restrict exhaust flow through the bypass passage when it is determined that the performance parameter of the second after-treatment device has dropped below the threshold value.
3. The method of claim 2, wherein the method additionally comprises: controlling the valve so as to restrict exhaust flow through the first after-treatment device when it is determined that the performance parameter of the second after-treatment device has again reached the threshold value.
4. The method of any preceding claim, wherein the after-treatment system additionally comprises at least one other after-treatment device upstream of the turbocharger turbine.
5. The method of any preceding claim, wherein the after-treatment system additionally comprises at least one other after-treatment device downstream of the turbocharger turbine.
6. The method of any preceding claim, wherein the first after-treatment device is a pre-turbo catalyst.
7. The method of any preceding claim, wherein the valve is arranged such that the valve selectively blocks either the bypass passage or flow through the first after-treatment device.
8. The method of any preceding claim, wherein the after-treatment system additionally comprises first and second valves that control the relative exhaust flow through the first after-treatment device and the bypass passage.
9. The method of claim 8, wherein the first valve is upstream of the first after-treatment device and the bypass passage and the second valve is downstream of the first after-treatment device and the bypass passage.
10. The method of claim 8, wherein the first valve selectively permits flow through the first after-treatment device and the second valve selectively permits flow through the bypass passage.
11. The method of any preceding claim wherein the first after-treatment device and bypass passage are in parallel.
12. The method of any preceding claim, wherein the first after-treatment device has a lower thermal mass than the second after-treatment device.
13. The method of any preceding claim, wherein the after-treatment system further comprises a sensor configured to detect a property of the second after-treatment device, the method further comprising determining the performance parameter using the sensor.
14. The method of claim 13, wherein the sensor comprises a temperature sensor configured to determine a temperature of the second after-treatment device.
15. The method of claim 13 or 14, wherein the performance parameter is, or is a function of, the property of the second after-treatment device.
16. The method of any preceding claim, wherein the method comprises: determining a temperature of the first after-treatment device; and bypassing the first after-treatment device if the temperature of the first after-treatment device is determined to be greater than a threshold value.
17. Software which, when executed by a computing device, causes the computing device to perform the method according to any preceding claim.
18. An after-treatment system for an engine of a motor vehicle, the after-treatment system comprising: a first after treatment device upstream of a turbocharger turbine; a second after-treatment device downstream of the turbocharger turbine; a bypass passage that bypasses the first after-treatment device; at least one valve that controls the relative exhaust flow through the first after-treatment device and the bypass passage; and a controller configured to carry out the method according to any preceding claim.
19. The after-treatment system of claim 18, wherein the bypass passage and first after-treatment device are housed within a canister cast into: a cylinder head; or a turbine housing, of the engine.