GB2565580A

GB2565580A - A diesel engine exhaust gas aftertreatment system

Info

Publication number: GB2565580A
Application number: GB1713197.0A
Authority: GB
Inventors: Birri Anthony; Beresford-Knox Chris; Murray Iain; Stephen Brogan Mark
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2017-08-17
Filing date: 2017-08-17
Publication date: 2019-02-20
Also published as: GB201713197D0

Abstract

A diesel exhaust gas aftertreatment system 210 for a motor vehicle comprises a first NOx emission reduction device 12 close coupled to a diesel engine 5. A second NOx emission reduction device 14 is located downstream of the first device under a floor of the motor vehicle. At least one of the first and second devices has a bypass passage 213, 215; the flow through which is regulated by a bypass valve 240, 250 controlled by an electronic controller 20 in response to a number of predefined system operating conditions. A diesel particulate filter 18 may be positioned between the devices. A low pressure exhaust gas recirculation circuit 9 may have an inlet downstream of the filter and upstream of the second device, an outlet located on an air inlet side of the engine, and a valve 8 controlled by the controller. The devices may be Lean NOx Traps or Passive NOx absorbers. The predefined system operating conditions may include the temperature of the devices or the exhaust gas, NOx or sulphur loading of the devices, or the particulate loading of the filter.

Description

A Diesel Engine Exhaust Gas Aftertreatment System

This invention relates to the reduction of NOx emissions from a diesel engine and in particular to a system for reducing such emissions.

It is desirable to provide a diesel engine with more than one NOx emission reduction device so as to extend the engine temperature range over which NOx emissions can be effectively reduced. In order to achieve this aim it has been proposed to locate a first NOx reduction device very close to an outlet from the engine in what is often referred to as a 'close coupled arrangement' so as to enable it to effectively remove NOx immediately after the engine has been started from cold and locate a second NOx device a significant distance away from the engine in what is often termed an 'underfloor position'.

One problem inherent to a NOx emission reduction device such as a Lean NOx trap stems from the fact that the use of a fuel which contains sulfur will cause a corresponding accumulation of sulfur in the trap. This accumulation of sulfur decreases the conversion efficiency of the trap. Thus, the sulfur must be periodically burned off or desorbed by heating the trap to a temperature within a range of 600 to 690°C and sustaining that temperature for several minutes. One problem is that during this desulphation heating phase which may typically be required for periods of 3 to 10 minutes any aftertreatment devices such as the under floor trap located downstream from the close coupled trap will receive Sulphur species coming from the close coupled trap when it is being purged of Sulphur .

It is a problem with the use of an underfloor trap that trying to purge it of Sulphur can be difficult due to the significant temperature drop across the close coupled trap so that there are only a limited number of engine operating conditions where the temperature of the exhaust gas entering the underfloor trap is sufficiently high to effectively deSOx the trap.

It is an object of this invention to provide a diesel engine exhaust gas aftertreatment that overcomes at least some of the aforementioned problems.

According to a first aspect of the invention there is provided a diesel engine exhaust gas aftertreatment system for a motor vehicle comprising a diesel engine, an electronic controller, a first close coupled NOx emission reduction device arranged to receive a flow of exhaust gas directly from the diesel engine, a second NOx emission reduction device located downstream from the first NOx emission reduction device under a floor of the motor vehicle to which the system is fitted, at least one of the first and second NOx emission reduction devices having a valve controlled bypass passage that extends from a position upstream of the respective NOx emission reduction device to a position downstream of the respective NOx emission reduction device to selectively bypass the respective NOx emission reduction device when a predefined system operating condition is present, the flow of exhaust gas through the or each valve controlled bypass passage being regulated by a respective electronically controllable bypass valve the opening and closing of which is controlled by the electronic controller in response to one or more inputs indicative of the predefined system operating condition.

This has the advantage of improved system flexibility and system performance.

A diesel particulate filter may be positioned downstream from the first NOx emission reduction device and upstream from the second NOx emission reduction device .

The system may further comprise a low pressure exhaust gas recirculation circuit having an inlet located downstream from the diesel particulate filter and upstream from the second NOx emission reduction device and an outlet located on an air inlet side of the diesel engine and an electronically controlled exhaust gas recirculation valve to control the flow of gas through the exhaust gas recirculation circuit.

The electronic controller may be arranged to control opening and closing of the electronically controlled exhaust gas recirculation valve.

In accordance with a first embodiment, a first valve controlled bypass passage may be arranged to selectively bypass the first NOx emission reduction device .

In which case, the predefined system operating condition may be the sulphur loading of the second NOx emission reduction device and the first NOx emission reduction device is bypassed when it is required to purge the second NOx emission reduction device of sulphur .

Alternatively, the predefined system operating condition is one of the temperature of the exhaust gas entering the first NOx emission reduction device and the temperature of the first NOx reduction device and the first NOx emission reduction device is bypassed when it is required to prevent thermal damage from occurring to the first NOx emission reduction device.

As yet another alternative, the predefined system operating condition is the NOx loading of the first NOx emission reduction device and the first NOx emission reduction device is bypassed when it is required to prevent the amount of NOx stored in the first NOx emission reduction device exceeding a predefined limit.

In accordance with a second embodiment, a second valve controlled bypass passage may be arranged to selectively bypass the second NOx emission reduction device .

In which case, the predefined system operating condition may be the Sulphur loading of the first NOx emission reduction device and the second NOx emission reduction device is bypassed when it is required to purge the first NOx emission reduction device of sulphur .

Alternatively, the predefined system operating condition may be the particulate loading of the diesel particulate filter and the second NOx emission reduction device is bypassed when it is required to regenerate the diesel particulate filter.

As yet a another alternative, the predefined system operating condition may be one of the temperature of the exhaust gas entering the second NOx emission reduction device and the temperature of the second NOx reduction device and the second NOx emission reduction device is bypassed when it is required to control the temperature of the second NOx emission reduction device.

As yet a further alternative, the predefined system operating condition may be the NOx loading of the second NOx emission reduction device and the second NOx emission reduction device is bypassed when it is required to prevent the amount of NOx stored in the second NOx emission reduction device exceeding a predefined maximum limit.

In accordance with a third embodiment, there may be two valve controlled bypass passages, a first valve controlled bypass passage arranged to selectively bypass the first NOx emission reduction device and a second valve controlled bypass passage arranged to selectively bypass the second NOx emission reduction device .

Alternatively, the predefined system operating condition may be one of the temperature of the exhaust gas entering the first NOx emission reduction device and the temperature of the first NOx reduction device and the first NOx emission reduction device is bypassed when it is required to prevent thermal damage from occurring to the first NOx emission reduction device.

As yet another alternative, the predefined system operating condition may be the NOx loading of the first NOx emission reduction device and the first NOx emission reduction device is bypassed when it is required to prevent the amount of NOx stored in the first NOx emission reduction device exceeding a predefined limit.

In another alternative, the predefined system operating condition may be the Sulphur loading of the first NOx emission reduction device and the second NOx emission reduction device is bypassed when it is required to purge the first NOx emission reduction device of sulphur.

In yet a further alternative, the predefined system operating condition may be the particulate loading of the diesel particulate filter and the second NOx emission reduction device is bypassed when it is required to regenerate the diesel particulate filter.

The predefined system operating condition may alternatively be one of the temperature of the exhaust gas entering the second NOx emission reduction device and the temperature of the second NOx reduction device and the second NOx emission reduction device is bypassed when it is reguired to control the temperature of the second NOx emission reduction device.

The predefined system operating condition may in yet a further alternative be the NOx loading of the second NOx emission reduction device and the second NOx emission reduction device is bypassed when it is required to prevent the amount of NOx stored in the second NOx emission reduction device exceeding a predefined maximum limit.

For all three embodiments, the first NOx emission reduction device may be one of a Lean NOx trap and a Passive NOx absorber.

For all three embodiments, the second NOx emission reduction device may be one of a Lean NOx trap and a Passive NOx absorber.

According to a second aspect of the invention there is provided a motor vehicle having a body including a body structure including a floor and a diesel engine exhaust gas aftertreatment system constructed in accordance with said first aspect of the invention wherein the first NOx emission reduction device is close coupled to the engine of the motor vehicle and the second NOx emission reduction device is mounted under the floor of the motor vehicle.

According to a third aspect of the invention there is provided a method of controlling a diesel engine exhaust gas aftertreatment system for a motor vehicle in accordance with the first embodiment of the first aspect of the invention wherein the method comprises bypassing the first NOx reduction device based upon one of a number predefined system operating conditions.

In which case, the predefined system operating conditions may comprise, the sulphur loading of the second NOx emission reduction device, the temperature of the exhaust gas entering the first NOx emission reduction device, the temperature of the first NOx reduction device and the NOx loading of the first NOx emission reduction device.

According to a fourth aspect of the invention there is provided a method of controlling a diesel engine exhaust gas aftertreatment system for a motor vehicle in accordance with the second embodiment of the first aspect of the invention wherein the method comprises bypassing the second NOx reduction device

- 8 based upon one of a number predefined system operating conditions .

In which case, the predefined system operating conditions may comprise, the sulphur loading of the first NOx emission reduction device, the particulate loading of the diesel particulate filter, the temperature of the exhaust gas entering the second NOx emission reduction device, the temperature of the second NOx reduction device and the NOx loading of the second NOx emission reduction device.

According to a fifth aspect of the invention there is provided a method of controlling a diesel engine exhaust gas aftertreatment system for a motor vehicle in accordance with the third embodiment of the first aspect of the invention wherein the method comprises bypassing the first NOx reduction device based upon one of a number first predefined system operating conditions and bypasses the second NOx reduction device based upon one of a number second predefined system operating conditions.

In which case, the first predefined system operating conditions may comprise, the sulphur loading of the second NOx emission reduction device, the temperature of the exhaust gas entering the first NOx emission reduction device, the temperature of the first NOx reduction device and the NOx loading of the first NOx emission reduction device and the second predefined system operating conditions comprise the sulphur loading of the first NOx emission reduction device, the particulate loading of the diesel particulate filter, the temperature of the exhaust gas entering the second NOx emission reduction device, the temperature of the second NOx reduction device and the NOx loading of the second NOx emission reduction device.

The invention will now be described by way of example with reference to the accompanying drawing of which: Fig.l is a schematic diagram of a first embodiment of a diesel engine exhaust gas aftertreatment system according to a first aspect of the invention fitted to a motor vehicle in accordance with a second aspect of the invention;

Fig.2 is a schematic diagram of a second embodiment of a diesel engine exhaust gas aftertreatment system according to the first aspect of the invention;

Fig.3 is a schematic diagram of a third embodiment of a diesel engine exhaust gas aftertreatment system according to the first aspect of the invention;

Fig.4 is a high level flow chart of a method in accordance with a third aspect of the invention for controlling a diesel engine exhaust gas aftertreatment system constructed in accordance with the first embodiment of the first aspect of the invention;

Fig.5 is a high level flow chart of a method in accordance with a fourth aspect of the invention for controlling a diesel engine exhaust gas aftertreatment system constructed in accordance with the second embodiment of the first aspect of the invention; and

Figs.6a & 6b form in combination a high level flow chart of a method in accordance with a fifth aspect of the invention for controlling a diesel engine exhaust gas aftertreatment system constructed in accordance with the third embodiment of the first aspect of the invention.

With particular reference to Fig.l there is shown a motor vehicle 1 having a first embodiment of a diesel engine exhaust gas aftertreatment system 10 comprising a diesel engine 5 having an inlet arranged to receive an inlet flow of air as indicated by the arrow 2, a first NOx emission reduction device 12 that is close coupled to the engine, a diesel particulate filter 18 arranged to receive a supply of exhaust gas from the first NOx emission reduction device and a second NOx emission reduction device 14 arranged to receive a supply of exhaust gas from the diesel particulate filter 12. After passing through the second NOx emission reduction device 14 the exhaust gasses flow to atmosphere as indicated by the arrow 3. It will be appreciated that one or more sound deadening devices and/ or further exhaust gas treatment devices could be located between an outlet from the second NOx emission reduction device 14 and atmosphere.

The diesel particulate filter 18 is positioned downstream from the first NOx emission reduction device 12 and upstream from the second NOx emission reduction device 14 that is to say it is interposed between the two NOx emission reduction devices 12, 14.

A low pressure exhaust gas recirculation circuit 9 has an inlet located downstream from the diesel particulate filter 18 and upstream from the second NOx emission reduction device 14 and an outlet located on an air inlet side of the diesel engine 5 and an electronically controlled exhaust gas recirculation valve 8 to control the flow of gas through the exhaust gas recirculation circuit 9.

A first valve controlled bypass passage 13 is arranged to selectively bypass the first NOx emission reduction device 12. The first valve controlled bypass passage 13 extends from a position upstream of the first NOx emission reduction device 12 to a position downstream of the first NOx emission reduction device 12. A first electronically controllable bypass valve 40 is used to regulate the flow of exhaust gas through the first valve controlled bypass passage 13.

The first NOx emission reduction device 12 is one of a Lean NOx trap and a Passive NOx absorber and the second NOx emission reduction device 14 is one of a Lean NOx trap and a Passive NOx absorber.

The system further comprises an electronic controller 20 that is used to control opening and closing of the first electronically controllable bypass valve 40 and the exhaust gas recirculation valve 8.

The electronic controller 20 receives a number of inputs (not shown) that provide information regarding various predefined system operating conditions such as the operating state of the engine 5, temperatures and the operating states of the first and second NOx emission reduction devices and uses these inputs to control the opening and closing of the first electronically controllable bypass valve 40 and the exhaust gas recirculation valve 8.

The control methodology used for controlling the exhaust gas recirculation valve 8 is conventional in nature and so will not described in further detail.

Regarding control of the first electronically controllable bypass valve 40 in accordance with one control strategy the predefined system operating condition is the Sulphur loading of the second NOx emission reduction device and the first electronically controllable bypass valve 40 is opened by the electronic controller 20 so as to bypass the first NOx emission reduction device 12 when it is required to purge the second NOx emission reduction device 14 of sulphur .

That is to say, based upon one or more inputs received by the electronic controller 20 when it is determined that desulphation of the second NOx emission reduction device 14 is required, the first NOx emission reduction device 12 is bypassed by opening the first electronically controllable bypass valve 40 which permits exhaust gas to flow through first valve controlled bypass passage 13 while preventing the flow of exhaust gas through the first NOx emission reduction device 12.

This has the effect of reducing the temperature drop between the engine 5 and the second NOx emission reduction device 14 thereby increasing the temperature within the second NOx reduction device 14.

In accordance with a further control strategy the predefined system operating condition is one of the temperature of the exhaust gas entering the first NOx emission reduction device 12 and the temperature of the first NOx reduction device 12 and the first electronically controllable bypass valve 40 is opened by the electronic controller 20 so as to bypass the first NOx emission reduction device 12 when it is required to control the temperature of the first NOx emission reduction device 12 so as to avoid thermal damage .

It will be appreciated that because the first NOx emission reduction device 12 is close coupled to the engine 5 that is to say, there is no appreciable temperature drop from the time exhaust gas exits the engine 5 to the time it enters the first NOx emission reduction device 12, there are engine operating conditions where the temperature of the exhaust gas exiting the engine 5 is above a safe temperature limit for the first NOx emission reduction device 12 and there is a risk that thermal damage will occur to the first NOx emission reduction device 12. Examples of this are when the engine 5 is being operated at high load and speed for a significant period of time and when the engine 5 is being operated so as to regenerate the diesel particulate filter 18.

In accordance with a further control strategy the predefined system operating condition is the NOx loading of the first NOx emission reduction device 12 and the first electronically controllable bypass valve 40 is opened by the electronic controller 20 so as to bypass the first NOx emission reduction device 12 so as to reduce the quantity of NOx being stored in the first NOx emission reduction device 12 thereby preventing the amount of NOx exceeding a predefined NOx loading limit.

It will be appreciated that it is desirable to maintain a relatively low level of NOx fill in the first NOx emission reduction device 12. This is because following a start-up from cold the first NOx emission reduction device 12 will reach an efficient working temperature far sooner than the second NOx emission reduction device 14 due to the close proximity of the first NOx emission reduction device 12 to the engine 5.

It is therefore advantageous to maintain sufficient headroom in the first NOx emission reduction device 12 to capture NOx following a start-up when the second NOx emission reduction device 14 will likely be either not working or working at a low efficiency.

Therefore during normal running of the engine 5 the first electronically controllable bypass valve 40 is opened by the electronic controller 20 so as to bypass the first NOx emission reduction device 12 when the level of NOx in the first NOx emission reduction device 12 reaches a predefined level provided that the second NOx emission reduction device 14 is operating efficiently.

With particular reference to Fig.2 there is shown a motor vehicle 1 having a second embodiment of a diesel engine exhaust gas aftertreatment system 110 that is in most respects identical to that previously described and so will not be described again in detail except with reference to the differences between the two embodiments.

The primary difference between the first embodiment and this second embodiment is that instead of the first NOx emission reduction device 12 being selectively bypassed by means of the first valve controlled bypass passage 13 and the first electronically controllable bypass valve 40 in the case of this second embodiment it is the second NOx emission reduction device 14 that can be selectively bypassed by means of a second valve controlled bypass passage 15 and a second electronically controllable bypass valve 50 .

The second valve controlled bypass passage 15 extends from a position upstream of the second NOx emission reduction device 14 to a position downstream of the first NOx emission reduction device 14. The second electronically controllable bypass valve 50 is used to control the flow of exhaust gas through the second valve controlled bypass passage 15.

As before, the first NOx emission reduction device 12 is one of a Lean NOx trap and a Passive NOx absorber and the second NOx emission reduction device 14 is one of a Lean NOx trap and a Passive NOx absorber.

As before the electronic controller 20 receives a number of inputs (not shown) that provide information regarding various predefined system operating conditions such as the operating state of the engine 5, various temperatures, the states of the first and second NOx emission reduction devices and the state of the diesel particulate trap 18 and uses these inputs to control opening and closing of the second electronically controllable bypass valve 50 and the exhaust gas recirculation valve 8.

The control methodology used for controlling the exhaust gas recirculation valve 8 is conventional in nature and so will not described further.

Regarding control of the second electronically controllable bypass valve 50 in accordance with one control strategy the predefined system operating condition is the Sulphur loading of the first NOx emission reduction device 12 and the second electronically controlled valve 50 is opened by the electronic controller 20 so as to bypass the second NOx emission reduction device 14 when it is required to purge the first NOx emission reduction device 12 of sulphur .

That is to say, based upon one or more inputs received by the electronic controller 20 when it is determined that desulphation of the first NOx emission reduction device 12 is required the second NOx emission reduction device 14 is bypassed by opening the second electronically controllable bypass valve 50 which permits exhaust gas to flow through second valve controlled bypass passage 15 while preventing the flow of exhaust gas through the second NOx emission reduction device 14.

This has the advantageous effect of preventing Sulphur contamination of the second NOx emission reduction device 14 when the first NOx emission reduction device 12 is being purged of sulphur contaminants. The sulphur contaminated exhaust gas can bypass the second NOx emission reduction device 14 thereby preventing sulphur contamination of the second NOx emission reduction device 14 during sulphur purging of the first NOx emission reduction device 12.

In accordance with a further control strategy the predefined system operating condition is the particulate loading of the diesel particulate filter and the second electronically controllable bypass valve 50 is opened by the electronic controller 20 so as to bypass the second NOx emission reduction device 14 when the diesel particulate trap 18 is being regenerated to remove soot from the diesel particulate trap 18.

This prevents particulate burn-off and high temperature lean exhaust gas from causing damage to the second NOx emission reduction device 14.

In accordance with yet a further control strategy the predefined system operating condition is one of the temperature of the exhaust gas entering the second NOx emission reduction device and the temperature of the second NOx reduction device and the second electronically controllable bypass valve 50 is opened by the electronic controller 20 so as to bypass the second NOx emission reduction device 14 when it is required to control the temperature of the second NOx emission reduction device 14. It will be appreciated that a NOx emission reduction device such as a Lean NOx trap operates efficiently within a limited temperature range and so if the exhaust gas temperature is too low or too high the exhaust gas can be permitted to bypass the second NOx emission reduction device 14 by opening the second electronically controllable bypass valve 50.

In accordance with yet one more control strategy the predefined system operating condition is the NOx loading of the second NOx emission reduction device 14 and the second electronically controllable bypass valve 50 is opened by the electronic controller 20 so as to bypass the second NOx emission reduction device 14 in order to control the NOx loading of the second NOx emission reduction device 14.

That is to say, if the amount of NOx stored in the second NOx emission reduction device 14 reaches a predefined upper or maximum limit and the conditions are currently not suitable for purging the second NOx emission reduction device 14 of NOx but there is capacity in the first NOx emission reduction device 12 and the first NOx emission reduction device 12 is operating efficiently then the second NOx emission reduction device 14 can be bypassed by opening the second electronically controllable bypass valve 50 until the conditions for purging NOx from the second NOx emission reduction device 14 are present.

This has the advantage that NOx slip from the second NOx emission reduction device 14 can be avoided.

With particular reference to Fig.3 there is shown a motor vehicle 1 having a third embodiment of a diesel engine exhaust gas aftertreatment system 210 that is a combination of the two embodiments previously described and so will not be described in detail except with reference to the various control strategies employed by the electronic controller 20.

It will be appreciated that in the case of this third embodiment there are two valve controlled bypass passages, a first valve controlled bypass passage 213 arranged to selectively bypass the first NOx emission reduction device 212 and a second valve controlled bypass passage 215 arranged to selectively bypass the second NOx emission reduction device 214.

A first electronically controllable bypass valve 240 is used to control the flow of exhaust gas through the first valve controlled bypass passage 213 and a second electronically controllable bypass valve 250 is used to control the flow of exhaust gas through the second valve controlled bypass passage 215.

The first and second electronically controllable bypass valves 240, 250 are both controlled by the electronic controller 20 in response to a number of inputs received by the electronic controller 20 that provide information regarding various predefined system operating conditions such as, the operating state of the engine 5, the temperature of the aftertreatment devices 12, 14, 18, the temperature of the exhaust gas flowing through the aftertreatment devices 12, 14, 18 and the operating status of the aftertreatment devices 12, 14, 18.

As before low pressure exhaust gas recirculation is also controlled by the electronic controller 20 by controlling the operation of the electronically controlled exhaust gas recirculation valve 8.

One of the advantages of all three embodiments is that the exhaust gas to be recirculated is that the point of extraction is downstream from the diesel particulate trap 18 and therefore soot or other particulate matter will have been extracted before the exhaust gas is recirculated to an inlet side of the engine 5.

A further advantage of all three embodiments is that by positioning the diesel particulate trap 18 between the first NOx emission reduction device 12 and the second NOx emission reduction device 14 there is always a significant temperature difference between the exhaust gas flowing through the first NOx emission reduction device 12 and the second NOx emission reduction device 14. This increases the opportunities for operating at least one of the two NOx emission reduction devices 12, 14 within a maximum operating efficiency temperature window.

Various operating strategies can be used with an arrangement in accordance with this third embodiment.

For example :a/ the first NOx emission reduction device 12 can be bypassed by opening the first electronically controllable bypass valve 240 to allow exhaust gas to flow through the first bypass passage 213 when it is required to purge the second NOx emission reduction device 14 of sulphur;

b/ the first NOx emission reduction device 12 can be bypassed by opening the first electronically controllable bypass valve 240 to allow exhaust gas to flow through the first bypass passage 213 when it is required to control the temperature of the first NOx emission reduction device 12;

c/ the first NOx emission reduction device 12 can be bypassed by opening the first electronically controllable bypass valve 240 to allow exhaust gas to flow through the first bypass passage 213 when it is required to control the amount of NOx being stored in the first NOx emission reduction device 12;

d/ the second NOx emission reduction device 14 can be bypassed by opening the second electronically controllable bypass valve 250 to allow exhaust gas to flow through the second bypass passage 215 when it is required to purge the first NOx emission reduction device 12 of sulphur.

e/ the second NOx emission reduction device 14 can be bypassed by opening the second electronically controllable bypass valve 250 to allow exhaust gas to flow through the second bypass passage 215 when it is required to regenerate the diesel particulate filter 18 . ;

f/ the second NOx emission reduction device 14 can be bypassed by opening the second electronically controllable bypass valve 250 to allow exhaust gas to flow through the second bypass passage 215 when it is required to control the temperature of the second NOx emission reduction device; and g/ the second NOx emission reduction device 14 can be bypassed by opening the second electronically controllable bypass valve 250 to allow exhaust gas to flow through the second bypass passage 215 when it is required to prevent the amount of NOx stored in the second NOx emission reduction device 14 exceeding a predefined maximum limit.

With reference to Fig.4 there is shown a method for controlling the diesel engine exhaust gas aftertreatment system 10 shown in Fig.l.

The method starts in box 1100 with a key-on event and progresses to box 1200 to check whether the second NOx emission reduction device 14 requires desulphation (a deSOx purge). If the second NOx emission reduction device 14 requires to be purged of Sulphur then the method advances to box 1250 where the first electronically controllable bypass valve 40 is opened so as to bypass the first NOx emission reduction device 12 and then advances to box 1280 where Sulphur is removed from the second NOx emission reduction device

14. The method then returns to box 1200.

It will be appreciated that, although not shown on Fig.4, the first electronically controllable bypass valve 40 will be closed when Sulphur purging of the second NOx emission reduction device 14 is complete.

If when checked in box 1200 there is no need to purge Sulphur from the second NOx emission reduction device 14 then the method will advance from box 1200 to box 1300 where it is checked whether the temperature of the exhaust gas entering the first NOx emission reduction device 12 or a temperature within the first NOx emission reduction device 12 is above a predefined temperature limit above which thermal damage could occur to the first NOx emission reduction device 12.

If the temperature when checked is above the predefined temperature limit then the method advances to box 1350 the first electronically controllable bypass valve 40 is opened so as to bypass the first NOx emission reduction device 12.

From box 1350 the method returns to box 1200 and will cycle around boxes 1200, 1300, 1350 until when checked in box 1300 the temperature has fallen below the predefined limit at which time the first electronically controllable bypass valve 40 is closed.

If when checked in box 1300 the temperature when checked is not above the predefined limit then the method advances to box 1400.

In box 1400 it is checked whether the amount of NOx (the NOx loading) in the first NOx emission reduction device 12 is above a predefined fill limit.

If the amount of NOx in the first NOx emission reduction device 12 is above the predefined fill limit then the method will advance to box 1450 where the first electronically controllable bypass valve 40 is opened so as to bypass the first NOx emission reduction device 12 and then returns to box 1200. Although not shown this will only happen if the second NOx emission reduction device 14 is operating efficiently so that it is able to remove NOx from the exhaust gas flow. It will be appreciated that the return to box 1200 from box 1450 may also include the step of removing or purging NOx from the first NOx emission reduction device 12.

By opening the first electronically controllable bypass valve 40 in such circumstances overfilling of the first NOx emission reduction device 12 is prevented which is important because upon start-up it is the first NOx emission reduction device 12 that is required to capture NOx because the first NOx emission reduction device 12 reaches a temperature range where it can efficiently capture NOx well before the second NOx emission reduction device 14 due to its location close to the engine 5 where there is no appreciable reduction in exhaust gas temperature.

If when checked in box 1400 the amount of NOx in the first NOx emission reduction device 12 is not above the predefined fill limit then the method will advance to box 1800 with the first electronically controllable bypass valve 40 closed.

In box 1800 it is checked whether a key-off event has occurred and, if it has, the method ends in box 1900 otherwise it returns to box 1200.

It will be appreciated that the checks of boxes 1200, 1300 and 1400 could be performed simultaneously on a continuous basis and need not be executed sequentially as shown in Fig.4.

With reference to Fig.5 there is shown a method for controlling the diesel engine exhaust gas aftertreatment system 110 shown in Fig.2.

The method starts in box 2100 with a key-on event and progresses to box 2200 to check whether the first NOx emission reduction device 12 requires desulphation (a deSOx purge). If the first NOx emission reduction device 12 requires to be purged of Sulphur then the method advances to box 2250 where the second electronically controllable bypass valve 50 is opened so as to bypass the second NOx emission reduction device 14 and then advances to box 2280 where Sulphur is removed from the first NOx emission reduction device 12. The method then returns to box 2200.

It will be appreciated that, although not shown on Fig.5, the second electronically controllable bypass valve 50 will be closed when Sulphur purging of the first NOx emission reduction device 12 is complete.

If when checked in box 2200 there is no need to purge Sulphur from the first NOx emission reduction device 12 then the method will advance from box 2200 to box 2300 where it is checked whether the temperature of the exhaust gas entering the second NOx emission reduction device 14 or a temperature within the second NOx emission reduction device 12 is within a range where efficient operation of the second NOx emission reduction device 14 will occur.

If the temperature of the second NOx emission reduction device 14 when checked in box 2300 is outside of the efficient operating range then the method advances to box 2350 and the second electronically controllable bypass valve 50 is opened so as to bypass the second NOx emission reduction device 14.

Although not shown on Fig.5 this opening of the second electronically controllable bypass valve 50 will depend upon the first NOx emission reduction device 12 being able to operate efficiently in storing NOx.

From box 2350 the method returns to box 2200 and will cycle around boxes 2200, 2300, 2350 until the temperature when checked in box 2300 has fallen within the required temperature range at which time the second electronically controllable bypass valve 50 is closed provided no other requirements for it to be open are present.

If when checked in box 2300 the temperature is within the required temperature range then the method advances to box 2400.

In box 2400 it is checked whether the NOx level in the second NOx emission reduction device 14 is above a predefined maximum fill limit. If the amount of NOx in the second NOx emission reduction device 14 is above the predefined maximum fill limit then the method will advance to box 2450 where the second electronically controllable bypass valve 50 is opened so as to bypass the second NOx emission reduction device 14 and then returns to box 2200. Although not shown on Fig.5 this will only happen if the first NOx emission reduction device 12 is operating efficiently so that it is able to remove NOx from the exhaust gas flow.

It will be appreciated that the return to box 2200 from box 2450 may also include the step of removing or purging NOx from the second NOx emission reduction device 14.

By opening the second electronically controllable bypass valve 50 in such circumstances overfilling of the second NOx emission reduction device 14 is prevented which is important because overfilling may result in NOx slip from the second NOx emission reduction device 14 directly to atmosphere.

If when checked in box 2400, the amount of NOx in the second NOx emission reduction device 14 is not above the predefined maximum fill limit then the method will advance to box 2500 with the second electronically controllable bypass valve 50 closed.

In box 2500 it is checked whether the diesel particulate filter 18 needs to be purged of accumulated particulate matter such as soot. If the diesel particulate filter 18 needs to be purged of particulate matter then the method advances to box 2550 where the second electronically controllable bypass valve 50 is opened so as to bypass the second NOx emission reduction device 14 and then advances to box 2580 where particulate matter is purged from the diesel particulate filter 18. The method then returns from box 2450 to box 2200.

It will be appreciated that, although not shown on Fig.5, the second electronically controllable bypass valve 50 will be closed when purging of the diesel particulate filter 18 is complete unless there is another requirement to keep it open.

If when checked in box 2500 there is no need to purge the diesel particulate filter 18 then the method will advance from box 2500 to box 2800.

In box 2800 it is checked whether a key-off event has occurred and, if it has, the method ends in box 2900 otherwise it returns to box 2200.

It will be appreciated that the checks of boxes 2200, 2300, 2400 and 2500 could be performed simultaneously on a continuous basis and need not be executed sequentially as shown in Fig.5.

With reference to Figs.6a and 6b there is shown a method of controlling the diesel engine exhaust gas aftertreatment system 210 shown in Fig.3.

The method starts in box 3100 with a key-on event and progresses to box 3200 to check whether the second NOx emission reduction device 14 requires desulphation (a deSOx purge). If the second NOx emission reduction device 14 requires to be purged of Sulphur then the method advances to box 3250 where the first electronically controllable bypass valve 240 is opened so as to bypass the first NOx emission reduction device 12 and then advances to box 3280 where Sulphur is removed from the second NOx emission reduction device

14. The method then returns to box 3200. It will be appreciated that although not shown the first electronically controllable bypass valve 240 will be closed when Sulphur purging of the second NOx emission reduction device 14 is complete unless there is another requirement to keep it open.

If when checked in box 3200 there is no need to purge Sulphur from the second NOx emission reduction device 14 then the method will advance from box 3200 to box 3300 where it is checked whether the temperature of the exhaust gas entering the first NOx emission reduction device 12 or a temperature within the first NOx emission reduction device 12 is above a predefined temperature limit above which thermal damage could occur to the first NOx emission reduction device 12.

If, when checked in box 3300, the temperature is above the predefined temperature limit the method advances to box 3350 where the first electronically controllable bypass valve 240 is opened so as to bypass the first NOx emission reduction device 12.

From box 3350 the method returns to box 3200 and will cycle around boxes 3200, 3300, 3350 until the temperature when checked in box 3300 falls below the predefined temperature limit at which time the first electronically controllable bypass valve 240 is closed unless there is another requirement to keep it open.

If when checked in box 3300 the temperature is not above the predefined limit then the method advances to box 3400.

In box 3400 it is checked whether the NOx level in the first NOx emission reduction device 12 is above a predefined fill limit. If the amount of NOx in the first NOx emission reduction device 12 is above the predefined fill limit then the method will advance to box 3450 where the first electronically controllable bypass valve 240 is opened so as to bypass the first NOx emission reduction device 12 and then returns to box 3200. Although not shown on Fig.6a this will only happen if the second NOx emission reduction device 14 is operating efficiently so that it is able to remove NOx from the exhaust gas flow.

It will be appreciated that the return to box 3200 from box 3450 may also include the step of removing or purging NOx from the first NOx emission reduction device 12.

By opening the first electronically controllable bypass valve 240 in such circumstances overfilling of the first NOx emission reduction device 12 is prevented which is important because upon start-up it is the first NOx emission reduction device 12 that is required to capture NOx because the first NOx emission reduction device 12 reaches a temperature range where it can efficiently capture NOx well before the second NOx emission reduction device 14 due to its closeness to the engine 5.

If when checked in box 3400 the amount of NOx in the first NOx emission reduction device 12 is not above the predefined fill limit then the method will advance to box 3800 with the first electronically controllable bypass valve 240 closed and will then advance to box 4100 and from there to box 4200.

In box 4200 it is checked whether the first NOx emission reduction device 12 requires desulphation (a deSOx purge). If the first NOx emission reduction device 12 requires to be purged of Sulphur then the method advances to box 4250 where the second electronically controllable bypass valve 250 is opened so as to bypass the second NOx emission reduction device 14 and then advances to box 4280 where Sulphur is removed from the first NOx emission reduction device 12. The method then advances to box 4850 and from there will return to box 3200.

It will be appreciated that, although not shown on Fig.6b, the second electronically controllable bypass valve 250 will be closed when Sulphur purging of the first NOx emission reduction device 12 is complete unless there is another requirement to keep it open.

If when checked in box 4200 there is no need to purge Sulphur from the first NOx emission reduction device 12 then the method will advance from box 4200 to box 4300 where it is checked whether the temperature of the exhaust gas entering the second NOx emission reduction device 14 or a temperature within the second NOx emission reduction device 14 is within a range where efficient operation of the second NOx emission reduction device 14 will occur.

If when checked in box 4300 the temperature is outside of the efficient operating range then the method advances to box 4350 and the second electronically controllable bypass valve 250 is opened so as to bypass the second NOx emission reduction device 14. Although not shown on Fig.6b this will depend upon the first NOx emission reduction device 12 being able to operate efficiently in storing NOx.

From box 4350 the method returns to box 3200 via box 4850 and will cycle through boxes 4300, 4350 and 4850 until the temperature falls within the required temperature range at which time the second electronically controllable bypass valve 250 is closed provided that there no other requirements for it to be open.

If when checked in box 4300 the temperature is within the required temperature range then the method advances to box 4400.

In box 4400 it is checked whether the NOx level in the second NOx emission reduction device 14 is above a predefined maximum fill limit. If the amount of NOx in the second NOx emission reduction device 14 is above the predefined maximum fill limit then the method will advance to box 4450 where the second electronically controllable bypass valve 250 is opened so as to bypass the second NOx emission reduction device 14 and then returns to box 3200 via box 4850. Although not shown on Fig.6b this will only happen if the first NOx emission reduction device 12 is operating efficiently so that it is able to remove NOx from the exhaust gas flow.

It will be appreciated that the return to box 4850 from box 4450 may also include the step of removing or purging NOx from the second NOx emission reduction device 14.

By opening the second electronically controllable bypass valve 250 in such circumstances overfilling of the second NOx emission reduction device 14 is prevented which is important because overfilling may result in NOx slip from the second NOx emission reduction device 14 directly to atmosphere.

In box 4400, if when checked, the amount of NOx in the second NOx emission reduction device 14 is not above the predefined maximum fill limit then the method will advance to box 4500 with the second electronically controllable bypass valve 250 closed.

In box 4500 it is checked whether the diesel particulate filter 18 needs to be purged of accumulated particulate matter such as soot. If the diesel particulate filter 18 needs to be purged of particulate matter then the method advances to box 4550 where the second electronically controllable bypass valve 250 is opened so as to bypass the second NOx emission reduction device 14 and then advances to box 4580 where particulate matter is purged from the diesel particulate filter 18. The method then returns to box 3200 via box 4850. It will be appreciated that, although not shown on Fig.6b, the second electronically controllable bypass valve 250 will be closed when purging of the diesel particulate filter 18 is complete unless there is another requirement to keep it open.

If when checked in box 4500 there is no need to purge the diesel particulate filter 18 then the method will advance from box 4500 to box 4800.

In box 4800 it is checked whether a key-off event has occurred and, if it has, the method ends in box 4900 otherwise it returns to box 3200 via box 4850.

It will be appreciated that the checks of boxes 3200, 3300, 3400, 4200, 4300, 4400 and 4500 could be performed simultaneously on a continuous basis and need not be executed sequentially as shown in Figs.6a and 6b.

Therefore, in summary, the invention provides two NOx emission reduction devices arranged in series each of which can be selectively bypass when required to improve system performance.

Preferably a diesel particulate trap is located between the two NOx emission reduction devices in order to ensure that there is a significant temperature difference therebetween.

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 embodiments it is not limited to the disclosed embodiments and that alternative embodiments could be constructed without departing from the scope of the invention as defined by the appended claims.

Claims

1. A diesel engine exhaust gas aftertreatment system for a motor vehicle comprising a diesel engine, an electronic controller, a first close coupled NOx emission reduction device arranged to receive a flow of exhaust gas directly from the diesel engine, a second NOx emission reduction device located downstream from the first NOx emission reduction device under a floor of the motor vehicle to which the system is fitted, at least one of the first and second NOx emission reduction devices having a valve controlled bypass passage that extends from a position upstream of the respective NOx emission reduction device to a position downstream of the respective NOx emission reduction device to selectively bypass the respective NOx emission reduction device when a predefined system operating condition is present, the flow of exhaust gas through the or each valve controlled bypass passage being regulated by a respective electronically controllable bypass valve the opening and closing of which is controlled by the electronic controller in response to one or more inputs indicative of the predefined system operating condition.

2. A system as claimed in claim 1 wherein a diesel particulate filter is positioned downstream from the first NOx emission reduction device and upstream from the second NOx emission reduction device.

3. A system as claimed in claim 2 wherein the system further comprises a low pressure exhaust gas recirculation circuit having an inlet located downstream from the diesel particulate filter and upstream from the second NOx emission reduction device and an outlet located on an air inlet side of the diesel engine and an electronically controlled exhaust gas recirculation valve to control the flow of gas through the exhaust gas recirculation circuit.

4. A system as claimed in claim 3 wherein the electronic controller is arranged to control opening and closing of the electronically controlled exhaust gas recirculation valve.

5. A system as claimed in any of claims 1 to 4 wherein a first valve controlled bypass passage is arranged to selectively bypass the first NOx emission reduction device.

6. A system as claimed in any of claims 1 to 4 wherein a second valve controlled bypass passage is arranged to selectively bypass the second NOx emission reduction device.

7. A system as claimed in any of claims 1 to 4 wherein there are two valve controlled bypass passages, a first valve controlled bypass passage arranged to selectively bypass the first NOx emission reduction device and a second valve controlled bypass passage arranged to selectively bypass the second NOx emission reduction device.

8. A system as claimed in claim 5 or in claim 7 wherein the predefined system operating condition is the sulphur loading of the second NOx emission reduction device and the first NOx emission reduction device is bypassed when it is required to purge the second NOx emission reduction device of sulphur.

9. A system as claimed in claim 5 or in claim 7 wherein the predefined system operating condition is one of the temperature of the exhaust gas entering the first NOx emission reduction device and the temperature of the first NOx reduction device and the first NOx emission reduction device is bypassed when it is required to prevent thermal damage from occurring to the first NOx emission reduction device.

10. A system as claimed in in claim 5 or in claim 7 wherein the predefined system operating condition is the NOx loading of the first NOx emission reduction device and the first NOx emission reduction device is bypassed when it is reguired to prevent the amount of NOx stored in the first NOx emission reduction device exceeding a predefined limit.

11. A system as claimed in claim 6 or in claim 7 wherein the predefined system operating condition is the sulphur loading of the first NOx emission reduction device and the second NOx emission reduction device is bypassed when it is reguired to purge the first NOx emission reduction device of sulphur.

12. A system as claimed in claim 6 or in claim 7 wherein the predefined system operating condition is the particulate loading of the diesel particulate filter and the second NOx emission reduction device is bypassed when it is reguired to regenerate the diesel particulate filter.

13. A system as claimed in claim 6 or in claim 7 wherein the predefined system operating condition is one of the temperature of the exhaust gas entering the second NOx emission reduction device and the temperature of the second NOx reduction device and the second NOx emission reduction device is bypassed when it is required to control the temperature of the second

NOx emission reduction device.

14. A system as claimed in in claim 6 or in claim 7 wherein the predefined system operating condition is the NOx loading of the second NOx emission reduction device and the second NOx emission reduction device is bypassed when it is reguired to prevent the amount of NOx stored in the second NOx emission reduction device exceeding a predefined maximum limit.

15. A system as claimed in any of claims 1 to 14 wherein the first NOx emission reduction device is one of a Lean NOx trap and a Passive NOx absorber.

16. A system as claimed in any of claims 1 to 14 wherein the second NOx emission reduction device is one of a Lean NOx trap and a Passive NOx absorber.

17. A motor vehicle having a body including a body structure including a exhaust gas aftertreatment claims 1 to 16 wherein the device is close coupled to vehicle and the second NOx mounted under the floor of floor and a diesel engine system as claimed in any of first NOx emission reduction the engine of the motor emission reduction device is the motor vehicle.

18. A method of controlling a diesel engine exhaust gas aftertreatment system for a motor vehicle as claimed in claim 5 wherein the method comprises bypassing the first NOx reduction device based upon one of a number predefined system operating conditions.

19. A method as claimed in claim 18 wherein the predefined system operating conditions comprise the sulphur loading of the second NOx emission reduction device, the temperature of the exhaust gas entering the first NOx emission reduction device, the temperature of the first NOx reduction device and the NOx loading of the first NOx emission reduction device.

20. A method of controlling a diesel engine exhaust gas aftertreatment system for a motor vehicle as claimed in claim 6 wherein the method comprises bypassing the second NOx reduction device based upon one of a number predefined system operating conditions.

21. A method as claimed in claim 20 wherein the predefined system operating conditions comprise the sulphur loading of the first NOx emission reduction device, the particulate loading of the diesel particulate filter, the temperature of the exhaust gas entering the second NOx emission reduction device, the temperature of the second NOx reduction device and the NOx loading of the second NOx emission reduction device .

22. A method of controlling a diesel engine exhaust gas aftertreatment system for a motor vehicle as claimed in claim 7 wherein the method comprises bypassing the first NOx reduction device based upon one of a number first predefined system operating conditions and bypasses the second NOx reduction device based upon one of a number second predefined system operating conditions.

23. A method as claimed in claim 22 wherein the first predefined system operating conditions comprise the sulphur loading of the second NOx emission

5 reduction device, the temperature of the exhaust gas entering the first NOx emission reduction device, the temperature of the first NOx reduction device and the NOx loading of the first NOx emission reduction device and the second predefined system operating conditions

10 comprise the sulphur loading of the first NOx emission reduction device, the particulate loading of the diesel particulate filter, the temperature of the exhaust gas entering the second NOx emission reduction device, the temperature of the second NOx reduction device and the

15 NOx loading of the second NOx emission reduction device .