WO2014016616A1

WO2014016616A1 - Method and apparatus for controlling an exhaust gas after-treatment system

Info

Publication number: WO2014016616A1
Application number: PCT/GB2013/052013
Authority: WO
Inventors: Paul Moore
Original assignee: Perkins Engines Company Limited
Priority date: 2012-07-27
Filing date: 2013-07-26
Publication date: 2014-01-30
Also published as: CN104718356A; EP2888458A1; US20150204226A1; GB201213443D0; WO2014016616A9

Abstract

A combustion engine is provided with an exhaust gas after treatment device, which comprises a first selective catalytic reduction (SCR) device (20), a second SCR device (40) downstream of the first SCR device and a diesel particulate filter (DPF) (30) located between the first SCR device and the second SCR device. The operating temperature range of the first SCR device is different to the operating temperature range of the second SCR device. A method of dosing the exhaust gas after treatment device comprises the steps of: allowing dosing of the first SCR device with reductant only when a temperature of the first SCR device is within the operating temperature range of the first SCR device; and allowing dosing of the second SCR device with reductant only when a temperature of the second SCR device is within the operating temperature range of the second SCR device; and reducing the level of dosing of the first SCR device when regeneration of the DPF is required. Furthermore, a controller configured to perform this method is provided.

Description

METHOD AND APPARATUS FOR CONTROLLING AN EXHAUST

GAS AFTER-TREATMENT SYSTEM

The present disclosure relates to an apparatus and method for exhaust gas after treatment.

Background

Exhaust gases generated by, for example, a boiler or an internal combustion engine, such as a diesel engine, may contain gases that are preferably reduced or removed from the gas stream before it is released into the atmosphere. For example, the exhaust gases may contain nitrous oxides (NOx) , which may be converted into less harmful emissions, such as nitrogen and water, by a Selective Catalytic

Reduction (SCR) system.

SCR systems may comprise a catalyst that facilitates a reaction between the NOx, which may be present in an exhaust gas stream passing through the SCR system, and a reductant to substantially remove the NOx from the exhaust gas.

The reductant may be added to the gas stream and absorbed onto the catalyst before it reacts with the NOx in the gas stream passing through the SCR system. Where the reductant is ammonia, it may be added to the gas stream as, for example, anhydrous ammonia, aqueous ammonia or urea, the last of which may thermally decompose into ammonia within the SCR system before being absorbed onto the catalyst. The storage state of the catalyst may depend upon the temperature of the catalyst. At low temperatures, the catalyst may not absorb any of the reductant, meaning that the SCR system may not remove NOx from the exhaust gas stream. Consequently, the SCR system may not be dosed with reductant until the catalyst has reached a temperature at which it may absorb the reductant, which may then react with NOx in the gas stream passing through the SCR system. If, however, the catalyst of an SCR device is dosed whilst it is very hot, the metals of the catalyst may be damaged.

Consequently, SCR devices may have an operating temperature range, outside of which they may not be dosed and cannot, therefore, remove NOx from the exhaust gas stream.

If an SCR device with a relatively high operating

temperature range is used, it may not be dosed and NOx may therefore remain untreated whilst exhaust gas temperatures are low, for example during low load or idling conditions, or after a cold start of the engine.

If an SCR device with a relatively low operating temperature range is used, it may not be dosed and NOx may therefore remain untreated whilst exhaust gas temperatures are high, for example during high load conditions or during roading.

Techniques may be used to adjust the engine operating parameters in order to increase or decrease exhaust gas temperatures for some operating conditions of the engine in order to maintain the SCR device within its operating temperature range for longer. However, these techniques result in a decrease in engine efficiency and may therefore be undesirable.

German Patent Application No. DE 10-2007-047-906 Al

describes an exhaust gas after treatment arrangement of a Diesel Particulate Filter (DPF) and an SCR device. The DPF is located upstream of the SCR device and may be present in order to reduce or remove diesel particulate matter or soot from the exhaust gas stream. The SCR device may then reduce or remove NOx from the exhaust gas stream before the exhaust gas is released into the atmosphere.

However, as explained earlier, operation of the SCR device will be limited by its operating temperature range. This means that NOx may at times be released untreated into the atmosphere during normal engine operation, for example during idling or low-loading conditions for an SCR device with a high operating temperature range, or during high-load or roading conditions for an SCR device with a low operating temperature range.

Summary

The disclosure provides: a method of dosing an exhaust gas after treatment device comprising a first selective

catalytic reduction (SCR) device, a second SCR device downstream of the first SCR device and a diesel particulate filter (DPF) located between the first SCR device and the second SCR device, wherein an operating temperature range of the first SCR device is different to an operating

temperature range of the second SCR device, the method comprising the steps of: allowing dosing of the first SCR device with reductant when a temperature of the first SCR device is within the operating temperature range of the first SCR device, and preventing dosing of the first SCR device with reductant when the temperature of the first SCR device is outside the operating temperature range of the first SCR device; and allowing dosing of the second SCR device with reductant when a temperature of the second SCR device is within the operating temperature range of the second SCR device, and preventing dosing of the second SCR device with reductant when the temperature of the second SCR device is outside the operating temperature range of the second SCR device; and reducing the level of dosing of the first SCR device when regeneration of the DPF is required. The disclosure also provides: a controller to control the dosing of an exhaust gas after treatment device comprising a first selective catalytic reduction (SCR) device, a second SCR device downstream of the first SCR device and a diesel particulate filter (DPF) located between the first SCR device and the second SCR device, wherein an operating temperature range of the first SCR device is different to an operating temperature range of the second SCR device, the controller being configured to: allow dosing of the first SCR device with reductant when a temperature of the first SCR device is within the operating temperature range of the first SCR device, and prevent dosing of the first SCR device with reductant when the temperature of the first SCR device is outside the operating temperature range of the first SCR device; and allow dosing of the second SCR device with reductant when a temperature of the second SCR device is within the operating temperature range of the second SCR device, and prevent dosing of the second SCR device with reductant when the temperature of the second SCR device is outside the operating temperature range of the second SCR device; and reduce the level of dosing of the first SCR device when regeneration of the DPF is required. The disclosure also provides: an exhaust gas after treatment device comprising: a first selective catalytic reduction (SCR) device dosable by a first reductant injector upstream of the first SCR device; a second SCR device downstream of the first SCR device, the second SCR device being dosable by a second reductant injector; and a diesel particulate filter (DPF) located between the first and second SCR devices;

wherein, the second reductant injector is located between the DPF and the second SCR device; and an operating

temperature range of the first SCR device is different to an operating temperature range of the second SCR device.

Figures Figure 1 shows a schematic drawing of an engine unit

comprising two SCR devices;

Figure 2 shows a schematic drawing of an engine unit

comprising two SCR devices and a DPF;

Figure 3 shows a graphical representation of how the dosing of the SCR devices shown in Figures 1 and 2 may be

controlled;

Figure 4 shows a graphical representation of how the dosing of the SCR devices shown in Figure 2 may be controlled; and Figure 5 shows an example vehicle within which the engine units shown in Figures 1 and 2 may be used.

Detailed description

A Selective Catalytic Reduction (SCR) device may be used for a variety of applications where a reduction in NOx levels in an exhaust gas stream is desired. Such applications may include, but are not exclusive to, boilers, gas turbines and internal combustion engines, for example diesel engines.

Figure 1 shows an internal combustion engine 10 with a first SCR device 20 in the exhaust gas stream of the internal combustion engine 10 and a second SCR device 40 downstream of the first SCR device 20. According to this arrangement, exhaust gases expelled from the internal combustion engine 10 pass through the first SCR device 20 and then pass through the second SCR device 40 before being released into the atmosphere or before passing through other downstream components .

Each of the SCR devices may be independently dosed with reductant. The first SCR device 20 may be dosed with urea by a first injector 14, which may spray urea into the exhaust gas stream upstream of the first SCR device 20, and the second SCR device 40 may be dosed with urea by a second injector 34, which may spray urea into the exhaust gas stream upstream of the second SCR device 40.

A first temperature sensor 12 may be located upstream of the first SCR device 20 and a second temperature sensor 32 may be located upstream of the second SCR device. Readings from these sensors may be used by a controller 50 to determine the temperature of the first 20 and second 40 SCR devices and control the first 14 and second 34 injectors

accordingly . When the temperature of the catalyst within the first SCR device 20 or the second SCR device 40 is within its

respective operating range, the injected urea may be stored on the catalyst as ammonia. When ammonia is stored on the catalyst, it may react with NOx in the exhaust gas passing through the SCR device and remove the NOx in the exhaust gas. However, if the temperature of the catalyst within the SCR device is outside of its operating temperature range, the SCR device may not be dosed and may not operate to remove NOx from the exhaust gas stream.

The first SCR device 20 may be optimised to operate at low temperatures; for example, it may have an operating

temperature range of 180°C-350°C. This may be achieved, for example, by using a catalyst that comprises a copper zeolite (CuZe) . The second SCR device 40 may be optimised to operate at high temperatures; for example, it may have an operating temperature range of 300°C-600°C. This may be achieved, for example, by using a catalyst comprising an iron zeolite (FeZe) .

One of the primary factors that determines the temperature of the catalyst in each of the first 20 and second 40 SCR devices may be the exhaust gas temperature. By optimising the first 20 and second 40 SCR devices as explained above, the first SCR device 20 may operate at low exhaust gas temperatures, for example, soon after the internal

combustion engine 10 has been started from cold, or during idling of the internal combustion engine 10 or low-load conditions. This may result in good low temperature NOx conversion. The second SCR device 40 may operate at higher exhaust gas temperatures, for example during high-load conditions or roading. This may result in good high

temperature NOx conversion. The operating temperature ranges of the first SCR device 20 and the second SCR device 40 may overlap such that at times both are being dosed. For example, the upper bound of the operating temperature range of the first SCR device 20 may be greater than the lower bound of the operating temperature range of the second SCR device 40 and lower than the upper bound of the operating temperature range of the second SCR device 40. The upstream exhaust gas temperatures may be higher than the downstream exhaust gas temperatures, as each component that the exhaust gas stream passes through may reduce the

temperature of the exhaust gas. By making the first SCR device 20 low temperature optimised, it may enter its operating temperature range more readily after a cold start or during engine idling. If the operating temperature ranges of the two SCR devices are set to overlap, as the exhaust gas temperatures rise, the temperature of the first SCR device 20 may approach the upper bound of its operating temperature range at about the time that the temperature of the second SCR device 40 exceeds the lower bound of its operating temperature range.

Consequently, by optimising the operating temperature ranges for the first 20 and second 40 SCR devices, it may be possible to operate at least one SCR device across a greater range of temperatures, for example, between 180°C-600°C. Various modifications to the arrangement shown in Figure 1 may be apparent to the skilled person.

For example, the first SCR device 20 may be optimised to have a high temperature operation range and the second SCR device 40 may be optimised to have a low temperature

operation range.

The first 20 and second 40 SCR devices could be arranged to operate in parallel to each other, with a valve that directs the exhaust gas flow to either or both of the first 20 or second 40 SCR devices according to the exhaust gas

temperature range. Instead of dosing the first 20 and second 40 SCR devices with urea, they may be dosed with any other suitable dosing agent, for example anhydrous or aqueous ammonia.

Furthermore, the dosing agent may be added to each of the first 20 and second 40 SCR devices using any suitable technique well known to the skilled person. The dosing agent and application technique may be different for each of the first 20 and second 40 SCR devices.

Figure 2 shows an engine unit that is similar to that shown in Figure 1, but further includes a Diesel Particulate

Filter (DPF) 30 that is located between the first SCR device 20 and the second SCR device 40.

The DPF 30 may reduce or remove diesel particles or soot from the exhaust gas stream passing through it. The DPF 30 may be arranged to be downstream of the first SCR device 20 and upstream of the second SCR device 40 (as shown in Figure 2), or may be upstream of both the first 20 and second 40 SCR devices, or may be downstream of both the first 20 and second 40 SCR devices. The DPF 30 may function most effectively when it is at a relatively high temperature, so it may be preferable to locate it as far upstream as possible so that the

temperature of the exhaust gases passing through the DPF has not been significantly reduced by components upstream of the DPF .

However, as explained earlier, it may be preferable for the first SCR device 20 to be optimised for low temperature operation and be located as far upstream as possible.

Therefore, the DPF 30 may be located downstream of the low temperature optimised first SCR device 20 and upstream of the high temperature optimised second SCR device 40 (as shown in Figure 2), in order to strike a balance between the first SCR device 20 needing to be located where the exhaust gas temperatures are at their highest, and the DPF 30 needing to run at relatively high temperatures. In the arrangement shown in Figure 2, if the DPF 30 is a passive regeneration DPF (i.e., it is regenerated by NOx in the exhaust gas stream that may react with, and remove, soot from the DPF), it can be regenerated by controlling the dosing of the upstream SCR device (s) . When regeneration of the DPF 30 is required, dosing of the first SCR device 20 is reduced or stopped in order to allow some NOx particles in the exhaust gas stream to pass through the first SCR device 20 and into the DPF 30 and regenerate the DPF 30. Any remaining NOx particles which exit the DPF 30 may be removed by the second SCR device 40, provided the second SCR device 40 is within its operating temperature range. The operating temperature range of the first SCR device 20 may be arranged to be below the temperature range at which the DPF 30 may passively regenerate. This means that when the temperature of the catalyst of the first SCR device 20 exceeds its operating temperature range, dosing of the first SCR device 20 stops and the first SCR device 20 stops operating. NOx may then pass through the first SCR device 20 un-reacted on to the DPF 30, where it may passively regenerate the DPF 30 as the DPF 30 enters its passive regeneration temperature range. If the operating

temperature range of the second SCR device 40 has been set such that as the temperature of the first SCR device 20 reaches and exceeds the upper bound of its operating

temperature range, the second SCR device 40 is already within its operating temperature range, any remaining NOx particles output from the DPF 30 may be removed by the second SCR device 40.

If the DPF 30 is an active regeneration DPF (i.e., it is regenerated by an elevated DPF temperature that causes oxidation and removal of soot from the DPF), dosing of the first 20 and second 40 SCR devices may continue as normal, taking account of the temperatures of the first 20 and second 40 SCR devices.

Although not shown in the Figures, an ammonia slip catalyst may also be arranged downstream of one or both of the first 20 and second 40 SCR devices in order to prevent ammonia slip. A single ammonia slip catalyst may be located

downstream of the second SCR device 40. Alternatively, a single ammonia slip catalyst may be located downstream of the first SCR device 20, or a first ammonia slip catalyst may be located downstream of the first SCR device 20 and a second ammonia slip catalyst may be located downstream of the second SCR device 40. Figure 3 shows the method steps that might be undertaken by the controller 50 in order to determine when the first 20 or second 40 SCR devices may be dosed with reductant.

In Step S310, the temperatures of the catalysts in the first 20 and second 40 SCR devices may be determined from the exhaust gas temperature measured upstream of the first SCR device 20. The catalyst temperatures may also be determined from the exhaust gas temperature measured downstream of the first SCR device 20 and upstream of the second SCR device 40, or the exhaust gas temperature measured downstream of the second SCR device 40, or from some combination of measurements taken at at least two of those locations.

The temperature of the catalysts may alternatively be obtained from a temperature sensor within at least one of the catalysts; for example, a temperature sensor may be located within the catalyst of the first SCR device 20, the measurement of which might also be used to estimate the temperature of the catalyst in the second SCR device 40. Alternatively, the temperature of the catalysts of the first 20 and second 40 SCR devices may be estimated from measured internal combustion engine parameters, such as at least one of engine speed, fuel injection quantity, altitude and ambient temperature. The temperatures of the catalyst may alternatively be obtained from any other direct or indirect temperature measurement or estimation technique that would be known to the skilled person. In Step S320, it may be determined whether or not the first SCR device 20 is within its operating temperature range. For example, the first SCR device 20 may be optimised to operate at catalyst temperatures of between 180°C-350°C. If the temperature of the first SCR device 20, as determined in Step S310, is below that range, it may not be able to absorb ammonia onto its catalyst and may not, therefore, be dosed. Thus, the control method progresses to Step S330, where dosing of the first SCR device 20 is prevented. If the temperature of the first SCR device 20, as determined in Step S310, is above the operating temperature range, it may not be dosed because operation of the device may result in damage to the catalyst. Thus, the control method progresses to Step S330, where dosing of the first SCR device 20 is prevented .

If, however, the temperature of the first SCR device 20, as determined in Step S310, is within the operating temperature range, it may be dosed so that it may operate to reduce or remove NOx from the exhaust gas stream. Thus, the control method progresses to Step S340, where dosing of the first SCR device 20 is allowed. The level of dosing that may be applied may be dependent upon a number of factors, which may include, but are not exclusive to, at least one of: catalyst temperature, exhaust gas flow rate, NOx concentration, type of catalyst and estimated ammonia storage state on the catalyst . In Step S350, it may be determined whether or not the second SCR device 40 is within its operating temperature range, for example, 300°C-600°C. This step is analogous to Step S320. For the same reasons as explained in respect of the first SCR device 20 above, if the determined temperature of the second SCR device 40 is outside of its operating temperature range, the control method proceeds to Step S360 where dosing of the second SCR device 40 is prevented. However, if the determined temperature of the second SCR device 40 is within the operating temperature range, the control method proceeds to Step S370 where dosing of the second SCR device 40 is allowed. The level of dosing applied to the second SCR device 40 may be set by considering a number of different factors, as explained earlier in respect of the first SCR device .

Figure 4 shows further method steps undertaken by the controller 50 in order to determine when the first 20 or second 40 SCR devices may be dosed in the arrangement shown in Figure 2.

Steps S310-S370 are the same as those shown in Figure 3. After Step S370, the control method proceeds to Step S410, where it is determined whether or not DPF regeneration should take place. Whether or not DPF regeneration should take place may be determined by any suitable means, for example the controller 50 may consider the status of a flag that is set when the operator demands DPF regeneration, or when measurement of soot content in the DPF 30 has

determined that regeneration should take place, or after a set period of time since the previous regeneration has elapsed .

If it is determined that regeneration should not take place, the control method proceeds back to Step S310. If it is determined that regeneration should take place, the control proceed proceeds to Step S420 where the level of dosing of the first SCR device 20 is reduced, for example by a

particular percentage reduction, or a reduction by a

particular quantity of reductant, or by stopping the dosing entirely. This should increase the amount of NOx passing through the first SCR device 20 and reaching the DPF 30, which should improve regeneration of the DPF 30. The DPF 30 may have a minimum temperature for regeneration and the DPF 30 and first SCR device 20 may be configured such that the minimum temperature for regeneration is greater than the upper bound of the first SCR device 20 operation temperature range. In this way, when it is determined that the DPF 30 should be regenerated, when the DPF 30 has reached its minimum temperature for regeneration the first SCR device 20 will already have ceased to be dosed with reductant, in which case reduction of the dosing level in Step S420 will maintain reductant dosing at a zero level.

Alternatively, the minimum temperature for regeneration may be less than the upper bound of the first SCR device 20. In Step S420, it may be arranged that the level of dosing of the first SCR device 20 is only reduced when regeneration of the DPF 30 is desired and the DPF temperature is above its minimum regeneration temperature. If the DPF temperature is below its minimum regeneration temperature, the reductant dosing level may not be reduced for DPF regeneration until the DPF temperature has risen above its minimum regeneration temperature. The temperature of the DPF 30 may be determined by any suitable means, for example using a temperature sensor (s) located upstream, within and/or downstream of the DPF 30, and/or using at least one of the temperature sensors 12 and 32, and/or may be estimated from measured internal

combustion engine parameters, such as at least one of engine speed, fuel injection quantity, altitude and ambient

temperature. The temperature of the DPF 30 may

alternatively be obtained from any other direct or indirect temperature measurement or estimation technique that would be known to the skilled person.

Whilst Steps S410 and S420 are shown in Figure 4 as taking place after Steps S310-S370, they may in fact take place at any time during the control method shown in Figure 4. For example, Steps S410 and S420 may take place before Step S310 and determine whether or not to apply a reduction to the current first SCR device 20 dosing level or the first SCR device 20 dosing level that will be set in Step S330 or S340. As a further example, Steps S410 and S420 may take place after Step S330/S340 and before Step S350.

Figures 1 and 2 show a controller 50 in accordance with an embodiment of the present disclosure.

The controller 50 may be configured to carry out the method steps described in the present disclosure.

The controller 50 may have a number of inputs and outputs that may be used to control the dosing of the first 20 and second 40 SCR devices. For example, the inputs might include, but are not exclusive to: a measurement from the temperature sensor 12 upstream of the first SCR device 20 and a measurement from the temperature sensor 32 upstream of the second SCR device 40. The controller 50 may also have a number of outputs, including, but not exclusive to, a dosing level control signal for the first injector 14 and a dosing level control signal for the second injector 34.

The controller 50 may be implemented in an engine control unit, for example the Caterpillar® A4:E4 or A5:E2, or as a standalone control unit.

Figures 1 and 2 also show an engine unit comprising the first 20 and second 40 SCR devices. Figure 4 shows a vehicle within which the engine unit shown in Figures 1 and 2 could be used.

Various alternatives and modifications of the above

described aspects may be appreciated. For example, there may be provided an exhaust gas after treatment device comprising: a first selective catalytic reduction (SCR) device; and a second SCR device; wherein, an operating temperature range of the first SCR device is different to an operating temperature range of the second SCR device.

Furthermore, there may be provided a method of dosing an exhaust gas after treatment device comprising a first selective catalytic reduction (SCR) device and a second SCR device, wherein an operating temperature range of the first SCR device is different to an operating temperature range of the second SCR device, the method comprising the steps of: allowing dosing of the first SCR device with reductant when a temperature of the first SCR device is within the

operating temperature range of the first SCR device, and preventing dosing of the first SCR device with reductant when the temperature of the first SCR device is outside the operating temperature range of the first SCR device; and allowing dosing of the second SCR device with reductant when a temperature of the second SCR device is within the

operating temperature range of the second SCR device, and preventing dosing of the second SCR device with reductant when the temperature of the second SCR device is outside the operating temperature range of the second SCR device.

Furthermore, there may be provided a controller to control the dosing of an exhaust gas after treatment device

comprising a first selective catalytic reduction (SCR) device and a second SCR device, wherein an operating

temperature range of the first SCR device is different to an operating temperature range of the second SCR device, the controller being configured to: allow dosing of the first SCR device with reductant when a temperature of the first

SCR device is within the operating temperature range of the first SCR device, and prevent dosing of the first SCR device with reductant when the temperature of the first SCR device is outside the operating temperature range of the first SCR device; and allow dosing of the second SCR device with reductant when a temperature of the second SCR device is within the operating temperature range of the second SCR device, and prevent dosing of the second SCR device with reductant when the temperature of the second SCR device is outside the operating temperature range of the second SCR device . Industrial Applicability

The present disclosure finds application in converting NOx within an exhaust gas stream into less harmful products across a greater range of operating temperatures.

A first, upstream SCR device may have a lower operating temperature range than a second, downstream SCR device. In this way, the upstream SCR device may reach its operating temperature more quickly after engine start-up by virtue of its low operating temperature range and close proximity to the engine exhaust output and begin NOx conversion quickly. NOx emissions may also be reduced or eliminated over a greater range of engine operating conditions by having a first SCR device with a low operating temperature range and a second SCR device with a high operating temperature range, for example from engine idling through to high-speed/high- load conditions, without having to adjust the engine

operating parameters in order artificially to adjust exhaust gas temperatures, which might reduce engine efficiency.

The upper bound of the first SCR device operating

temperature range may be greater than the lower bound of the second SCR device temperature range, such that the two operating temperature ranges overlap. In this way, as the first SCR device stops working as its temperature reaches the upper bound of its operating temperature range, the second SCR device may already be within its operating temperature range so that unbroken NOx conversion may take place over a wide range of temperatures. Where a diesel particulate filter (DPF) is arranged between the first upstream SCR device and the second downstream SCR device, dosing of the first SCR device may be reduced or stopped altogether when regeneration of the DPF is desired. This allows an increased amount of NOx to pass through the first SCR device to the DPF to aid regeneration of the DPF, thus improving DPF regeneration. It may further be arranged that the minimum regeneration temperature of the DPF is greater than the upper bound of the first SCR device operating temperature range, such that when the DPF is at regeneration temperature, dosing of the first SCR device will already have stopped and a reduction of dosing for regeneration of the DPF may simply be maintenance of dosing at a zero level.

Claims

1. A method of dosing an exhaust gas after treatment device comprising a first selective catalytic reduction (SCR) device, a second SCR device downstream of the first SCR device and a diesel particulate filter (DPF) located between the first SCR device and the second SCR device, wherein an operating temperature range of the first SCR device is different to an operating temperature range of the second SCR device, the method comprising the steps of:

allowing dosing of the first SCR device with reductant when a temperature of the first SCR device is within the operating temperature range of the first SCR device, and preventing dosing of the first SCR device with reductant when the temperature of the first SCR device is outside the operating temperature range of the first SCR device; and

allowing dosing of the second SCR device with reductant when a temperature of the second SCR device is within the operating temperature range of the second SCR device, and preventing dosing of the second SCR device with reductant when the temperature of the second SCR device is outside the operating temperature range of the second SCR device; and reducing the level of dosing of the first SCR device when regeneration of the DPF is required.

2. The method of claim 1, wherein a lower bound of the operating temperature range of the first SCR device is below a lower bound of the operating temperature range of the second SCR device.

3. The method of either claim 1 or claim 2, wherein an upper bound of the operating temperature range of the first SCR device is greater than a lower bound of the operating temperature range of the second SCR device and less than an upper bound of the operating temperature range of the second SCR device.

4. The method of any preceding claim, wherein an upper bound of the operating temperature range of the first SCR device is below a regeneration temperature of the DPF .

5. A controller to control the dosing of an exhaust gas after treatment device comprising a first selective

temperature range of the second SCR device, the controller being configured to:

allow dosing of the first SCR device with reductant when a temperature of the first SCR device is within the operating temperature range of the first SCR device, and prevent dosing of the first SCR device with reductant when the temperature of the first SCR device is outside the operating temperature range of the first SCR device; and

allow dosing of the second SCR device with reductant when a temperature of the second SCR device is within the operating temperature range of the second SCR device, and prevent dosing of the second SCR device with reductant when the temperature of the second SCR device is outside the operating temperature range of the second SCR device; and reduce the level of dosing of the first SCR device when regeneration of the DPF is required.

6. An engine unit comprising the controller of claim 5.

7. A vehicle comprising the engine unit of claim 6.

8. An exhaust gas after treatment device comprising:

a first selective catalytic reduction (SCR) device dosable by a first reductant injector upstream of the first SCR device;

a second SCR device downstream of the first SCR device, the second SCR device being dosable by a second reductant injector; and

a diesel particulate filter (DPF) located between the first and second SCR devices;

wherein, the second reductant injector is located between the DPF and the second SCR device; and

an operating temperature range of the first SCR device is different to an operating temperature range of the second SCR device.

9. The exhaust gas after treatment device of claim 8, wherein a lower bound of the operating temperature range of the first SCR device is below a lower bound of the operating temperature range of the second SCR device.

10. The exhaust gas after treatment device of either claim 8 or claim 9, wherein an upper bound of the operating temperature range of the first SCR device is greater than a lower bound of the operating temperature range of the second SCR device and less than an upper bound of the operating temperature range of the second SCR device.

11. The exhaust gas after treatment device of any one of claims 8 to 10, wherein a regeneration temperature of the

DPF is above an upper bound of the operating temperature range of the first SCR device.

12. An internal combustion engine comprising the exhaust gas after treatment device of any one of claim 8 to 11.

13. A vehicle comprising the internal combustion engine of claim 12.