GB2538961A

GB2538961A - A method of adaptively controlling purging of a lean NOx trap

Info

Publication number: GB2538961A
Application number: GB1509378.4A
Authority: GB
Inventors: Wright James; Ford Kim
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2015-06-01
Filing date: 2015-06-01
Publication date: 2016-12-07
Anticipated expiration: 2035-06-01
Also published as: GB2538961A8; RU2710658C2; RU2016119297A3; GB2538961B; DE102016109981A1; GB201509378D0; MX2016007049A; RU2016119297A

Abstract

A means of adaptively controlling the purging strategy of a lean NOx trap (LNT) 112 when the operating conditions do not support a standard (optimal) strategy. An ECU 130 may monitor upstream or downstream sensors 115, 116 or may estimate NOx levels present. When a parameter indicative of purge effectiveness falls below a target level, the purge strategy is adapted by changing at least one of the frequency of purge events, the length of each purge event and the richness of the feedgas entering the lean NOx trap during a purge event so as to restore the purge effectiveness to the target level. The parameter indicative of purge effectiveness may be a value of NOx conversion efficiency at the end of a purge event, the average purge time or amount of reductant used during preceding purge events.

Description

A Method of Adaptively Controlling Purging of a Lean NOx Trap This invention relates to a motor vehicle having an 5 emission control system including a lean NOx trap and, in particular, to a method for adapting a strategy used to periodically purge the lean NOx trap of stored NOx.

It is well known to use a lean NOx trap (LNT) to remove 10 oxides of nitrogen from the exhaust gases flow from a combustion engine of a motor vehicle.

Current LNT purging strategies or LNT regeneration strategies operate the combustion rich at a predefined air/ fuel ratio for a predefined period of time in order to purge stored NOx from the LNT.

Euro 6.2 emission legislation requires that the emissions produced when operating in any real world drive cycle or a reference drive cycle representing such a real world drive cycle must conform with the respective emission limits legislated for that drive cycle.

The ability to purge an LNT in the real world is limited by a number of factors including the temperature of the LNT and the ratio of flow rate of feedgas to LNT catalyst bed volume (space velocity) and both of these are dependent on vehicle operating conditions such as vehicle speed.

In order to produce a rich feedgas flow to the LNT the engine torque needs to be above a certain threshold and, if a particular drive cycle is very transient and/or the particular driver uses the accelerator pedal aggressively (i.e. on/off) then the engine torque may not meet the necessary threshold for a sufficient length of time to convert a significant amount of stored NOx in the LNT.

Such ineffective purging of the LNT will ultimately result in a build up of NOx in the LNT and loss of NOx conversion efficiency.

It is an object of this invention to provide a method of adaptively controlling purging of a LNT so that purging of the LNT remains effective.

According to a first aspect of the invention there is provided a method of adaptively controlling purging of a lean NOx trap arranged to receive a flow of feedgas from a combustion engine comprising establishing whether operating conditions required for purging of the lean NOx trap are present and, provided the operating conditions required for purging of the lean NOx trap are present, comparing a parameter indicative of purge effectiveness with a target value for purge effectiveness and, if the parameter indicative of purge effectiveness is less than the target value, adapting a standard purge strategy used for controlling purging of the lean NOx trap by changing at least one of the frequency of purge events, the length of each purge event and the richness of the feedgas entering the lean NOx trap during a purge event and using the adapted purge strategy to purge the lean NOx trap.

The purge strategy may be adapted by increasing the frequency of purge events compared to the standard purge strategy.

The purge strategy may be adapted by increasing the richness of the feedgas entering the lean NOx trap compared to the standard purge strategy.

The purge strategy may be adapted by increasing the frequency of purge events and increasing the richness of the feedgas entering the lean NOx trap for each purge event compared to the standard purge strategy.

The length of each purge event may be reduced compared to the standard purge strategy.

If the parameter indicative of purge effectiveness is more than the target value, the standard purge strategy is used.

The parameter indicative of purge effectiveness may be NOx conversion efficiency at the end of a purge event and the target value may be a target value of NOx conversion efficiency.

The parameter indicative of purge effectiveness may be an average purge time for a predefined number of preceding purge events and the target value may be a target value of average purge time.

The parameter indicative of purge effectiveness may be an estimate of the reductant supplied during a predefined number of preceding purge events and the target value is a target value of reductant supplied for the same number of purge events.

The standard purge strategy may comprise an optimised frequency of purge events, an optimised length for each purge event and an optimised richness of feedgas entering the lean NOx trap for each purge event.

The method may further comprise comparing a long term average purge time value with a long term average purge time threshold and, if the value for the long term average purge time value is less than the long term average purge time threshold using this as an indication that an adapted purge -4 -strategy should be immediately adopted as soon as purging is permitted.

According to a second aspect of the invention there is 5 provided a motor vehicle having a combustion engine arranged to supply feedgas to a lean NOx trap and an electronic control system to control the operation of the engine wherein the electronic control system is arranged to establish whether operating conditions required for purging 10 of the lean NOx trap are present and, provided the operating conditions required for purging of the lean NOx trap are present, is arranged to compare a parameter indicative of purge effectiveness with a target value for purge effectiveness and, if the parameter indicative of purge effectiveness is less than the target value, is arranged to adapt a standard purge strategy used for controlling purging the lean NOx trap by increasing at least one of the frequency of purge events and the richness of the feedgas entering the lean NOx trap and is further arranged to use the adapted purge strategy to purge the lean NOx trap.

The invention will now be described by way of example with reference to the accompanying drawing of which:-Fig.1 is a high level flow chart of a method of adaptively controlling purging of a lean NOx trap according to a first aspect of the invention; Fig.2 is a schematic diagram of a motor vehicle constructed in accordance with a second aspect of the invention; Fig.3 is a chart showing the relationship between NOx efficiency and length of purge time for a standard optimal purge strategy, a drive cycle/ style affected purge strategy and an adapted purge strategy in accordance with this invention; _ 5 _ Fig.4 is a graph showing the relationship between percentage NOx conversion and NOx stored between purges for the three strategies shown in Fig.3; Fig.5 is a graph showing the relationship between NOx storage rate and time for the standard purge strategy shown in Fig.3; Fig.6 is a graph showing the relationship between NOx storage rate and time for the drive cycle/ style affected purge strategy shown in Fig.3; and Fig.7 is a graph showing the relationship between NOx storage rate and time for the adapted purge strategy shown in Fig.3.

With reference to Fig.1 there is shown a method for adaptively controlling the NOx purging of a LNT 112 forming part of a motor vehicle 100. The motor vehicle 100 has a combustion engine 110 that supplies feedgas to the LNT 112 via an exhaust system 111. The feedgas after passing through the LNT 112 flows through one or more downstream exhaust components 114 which may include, for example, a silencer and a particulate filter before exiting to atmosphere via a tailpipe 111t of the exhaust system 111. An electronic control system 130 is provided to control the operation of the engine 110 and, in particular, to control the operation of the engine 110 during purging of the LNT 112. The electronic control system 130 receives in the case of the example shown inputs from upstream and downstream NOx sensors 115 and 116 but it will be appreciated that the NOx levels upstream and downstream from the LNT 112 could be derived by modelling based upon knowledge of the historical and current operation of the engine 110. -6 -

In all cases the electronic control system 130 also receives inputs from a number of other sensors shown collectively as 140 that are required to control the normal operation of the engine 110.

The electronic control system 130 includes a number of electronic processors (not shown) used to control the operation of the engine 110 in accordance with stored instructions and logic in order to perform the adaptive purging method shown in Fig.1.

Referring now to Fig.1 an adaptive purging method for a LNT is shown. The method commences at box 10 with a Key-on event. That is to say, the engine 110 of the motor vehicle 100 is started and, as shown in box 15, is running normally.

Then in box 20 it is checked whether purging of the LNT 112 is required. This step could be based on the time that has elapsed since last purge or could be based on an analysis of the operation of the LNT 112 since the last purge such as, for example, an estimation of the current NOx loading of the LNT 112.

Irrespective of the means used to establish whether purging of the LNT 112 is required if purging is not currently required the method returns to box 15 with the engine 110 running. However, if when checked in box 20, purging of the LNT 112 is required the method advances from box 20 to box 30 where it is checked whether suitable conditions currently exist for the purging of NOx from the LNT 112.

Such suitable purge conditions could include whether the motor vehicle 100 is moving or is moving above a predefined speed, whether a currently selected gear falls within a predefined range of suitable gears for purging such as, for example and without limitation, third or higher, -7 -whether the current temperature of the feedgas exiting the engine 110 or entering the LNT 112 is within a predefined temperature range such as, for example, 250 to 450°C whether the temperature of the engine 110 is above a lower limit such as, for example, whether an engine coolant temperature is above 60°C, whether a predefined engine speed or torque level is being met or whether the vehicle is accelerating.

If at least one and normally a combination of several of the above conditions or other suitable conditions for purging of the LNT 112 are met then the method advances from box 30 to box 40 and otherwise the method returns to box 15 because purging is not currently possible.

In box 40 it is determined whether a standard purge strategy is acceptable. A purge strategy as meant herein is a combination of the frequency at which purge events will occur, the length of time for each purge event and the air/fuel ratio for each purge event.

A standard purge strategy is one that has been optimised to produce a desired improvement in LNT performance while minimising fuel usage and CO2 emissions during the purge.

Referring now back to Fig.1 box 40 numerous methods for determining whether the standard purge strategy is acceptable can be used but in each case a parameter indicative of purge effectiveness is compared to a target value.

In this case the parameter of purge effectiveness is the NOx conversion efficiency at the end of a purge event. In this case, sensors 115, 116 are used to directly measure the NOx conversion efficiency. The NOx level in the feedgas is measured upstream of the LNT 112 by the NOx sensor 115 -8 -and this is compared with the NOx level in the feedgas downstream of the LNT 112 as measured by the sensor 116.

Alternatively, modelled values of upstream and downstream NOx can be used or one of the upstream and downstream NOx levels can be measured and the other be modelled. Whatever method is used to deduce the end of purge NOx conversion efficiency this value is compared with a predefined target value such as, for example and without limitation, 70%. If the end of purge conversion efficiency is greater than 70% the standard purge strategy is used as indicated by the advancement of the method from box 40 to box 70. In box 70 the standard purge strategy is used to purge the LNT 112 and then the method advances to box 80 to check whether a key-off event has occurred and, if it has not, returns to box 20 with the engine running. However, if a key-off event has occurred when checked in box 80 the method ends as indicated by the box 99.

However, if when checked in box 40 the end of purge efficiency is less than 70% the standard purge strategy will not produce an acceptable LNT performance and so an adaption of the standard purge strategy is required and the method advances to box 50.

Various other methods could be used in box 40 to establish whether the standard purge strategy can be used. For example and without limitation, the parameter indicative of purge effectiveness could be an average purge time for a predefined number of preceding purge events and the target value would be in such a case a target value of average purge time such as, for example, the standard 8 second period. In such a case the length of purge time is measured for each purge and, if after a number of purge events, such as three, the average purge time is less than 8 seconds then the standard purge strategy would considered to be not acceptable. It will be appreciated that for these purge _ 9 _ events the air/fuel ratio and the frequency of purge events would be as per the standard purge strategy.

The average purge time could also be calculated for one or more entire key-on cycles resulting in a long term average purge time value PT" that can be stored in a memory of the electronic control system 130. This long term average purge time value PT" can then be compared to a long term average purge time threshold PTThresii and, if the value for the long term average purge time value PTa.qis less than the long term average purge time threshold PTTInesh, this could be used as an indication that an adapted purge strategy should be immediately adopted as soon as purging can be permitted. In such a case the method would, at least when conducting the first execution of box 30, jump from box 30 directly to box 60 and use a predefined adapted purge strategy based upon the long term average value PT",. The box 30 could contain a check to see whether a flag indicating the need to use the adapted purge strategy has been set. The flag would be set if the long term average purge time value PT,,, is less than the threshold PTTI,,,A. The use of such a long term purge time average has the advantage that there is no need to see whether an adapted purge strategy needs to be applied it can be set from the beginning of a drive cycle rather than waiting to check the current purge effectiveness which may result in poor NOx control at the very beginning of a drive cycle.

As yet another alternative, the parameter indicative of purge effectiveness could be a desired amount of reductant for a successful purge event and so the parameter indicative of purge effectiveness could be whether a target mass of reductant has been supplied in one or more previous purge events.

Referring back now to box 50 on Fig.l, the degree and type of adaption required is either based upon data stored -10 -in one or more look up tables, can be estimated using a modelled LNT system, be calculated using one or more algorithms or be deduced based upon a combination of two or more of these. The method for establishing the required adaption can use historical data of how the vehicle has been operated in the recent past to assist with producing a suitable adapted strategy. For example, how the driver has operated the accelerator pedal, how long a required level of torque has been produced or any other factor affecting the availability of conditions suitable for purging the LNT 112.

Irrespective of the method used to quantify the magnitude and type of adaption required, the adaption is made by changing at least one of the frequency of purge events, the length of each purge event and the richness of the feedgas entering the lean NOx trap during a purge event.

So for example, the purge strategy can be adapted by increasing the frequency of purge events compared to the standard purge strategy.

Alternatively, the purge strategy can be adapted by increasing the richness of the feedgas entering the lean NOx trap compared to the standard purge strategy. That is to say, the air/ fuel ratio (A) is reduced.

As yet another alternative, the purge strategy can be adapted by increasing the frequency of purge events and increasing the richness of the feedgas entering the lean NOx trap for each purge event compared to the standard purge strategy.

Because it is often the case that the ineffectiveness of the standard purge strategy is due to the unavailability of sufficient purge time for each purge event, the length of each purge event is often reduced compared to the standard purge strategy.

Returning to Fig.1, from box 50 the method advances from box 50 to box 60 where the adapted purge strategy is used to purge the LNT 112. The method then advances from box 60 to box 80 to check whether a key-off event has occurred and, if it has not, returns to box 20 with the engine running. However, if when checked in box 80, a key-off event has occurred the method ends as indicated by the box 99.

If the method returns to box 20 from box 80 then the boxes 20 and 30 are executed as previously described and, if purging is still required and suitable conditions exist, the boxes 40 through 80 are also executed again and this process will continue until there is a key-off event detected which in this case will occur at box 80. It will be appreciated that in practice the method will end whenever a key-off event occurs and that the use of a single key-off check shown in Fig.1 is provided simplify understanding of the invention.

In Figs.3, 4 and 5 the values for a typical standard purge strategy are shown indicated by the reference numeral 1. In the case of this example the standard purge strategy comprises a frequency of one purge every 180 seconds, a purge length of 8 seconds and an air/fuel ratio of 0.96.

In Figs.3, 4 and 5 the values for drive cycle or driver behaviour affected purging are shown indicated by the reference numeral 2. In the case of this example the frequency of purging remains one every 180 seconds, the air/fuel ratio remains at 0.96 but the effective purge length has reduced to 2 seconds. This is because the vehicle 100 is currently being driven erratically with sharp but short periods of acceleration and so the period required for purging using the standard purge strategy is not available. The effect of this is to reduce the NOx -12 -conversion efficiency to the level 2 shown on Figs.3 and 4 resulting in a much lower NOx storage rate as shown in Fig.6 compared to the ideal NOx storage rate provided when it is possible to use the standard purge cycle as shown in Fig.5.

It is this type of compromised purging that is addressed by this invention.

In Figs.3, 4 and 5 the values for an adapted purge strategy in accordance with this invention are shown indicated by the reference numeral 3. In the case of this example the adapted purge strategy comprises a frequency of one purge every 120 seconds, a purge length of 3 seconds and an air/fuel ration of 0.92.

In this case the adaption applied has resulted in the frequency being increased, the air/fuel ratio being reduced (feedgas made richer) and the purge time being reduced compared to the standard purge strategy. With such an adapted purge strategy only 3 seconds are required for each purge event rather than 8 seconds if the standard purge strategy is used. Therefore there is an increased probability of more successful purge events being completed compared to the standard purge strategy. Therefore, as shown in Fig.3 by the reference numeral 3, the NOx efficiency of the LNT 112 will be restored using the adapted purge strategy to the required or target level whereas if the vehicle 100 were to be operated using the standard purge strategy the NOx efficiency would drop to the level indicated by the reference numeral 2 on Fig.3.

Therefore in summary, the purging of an LNT is not based upon a predefined and fixed purge strategy but is adapted based upon the dynamic operation of the motor vehicle. When possible a standard or optimised purge strategy is used because this will produce the best compromise between fuel usage and CO2, however, when a parameter indicative of purge effectiveness falls below a -13 -target level, the purge strategy is adapted by changing at least one of the frequency of purge events, the length of each purge event and the richness of the feedgas entering the lean NOx trap during a purge event so as to restore the purge effectiveness to the target level.

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

-14 -Claims 1. A method of adaptively controlling purging of a lean NOx trap arranged to receive a flow of feedgas from a combustion engine comprising establishing whether operating conditions required for purging of the lean NOx trap are present and, provided the operating conditions required for purging of the lean NOx trap are present, comparing a parameter indicative of purge effectiveness with a target value for purge effectiveness and, if the parameter indicative of purge effectiveness is less than the target value, adapting a standard purge strategy used for controlling purging of the lean NOx trap by changing at least one of the frequency of purge events, the length of each purge event and the richness of the feedgas entering the lean NOx trap during a purge event and using the adapted purge strategy to purge the lean NOx trap.
2. A method as claimed in claim 1 wherein the purge strategy is adapted by increasing the frequency of purge events compared to the standard purge strategy.
3. A method as claimed in claim 1 wherein the purge strategy is adapted by increasing the richness of the feedgas entering the lean NOx trap compared to the standard purge strategy.
4. A method as claimed in claim 1 wherein the purge strategy is adapted by increasing the frequency of purge events and increasing the richness of the feedgas entering the lean NOx trap for each purge event compared to the standard purge strategy.
5. A method as claimed in any of claims 2 to 4 35 wherein the length of each purge event is reduced compared to the standard purge strategy.

-15 -
6. A method as claimed in any of claims 1 to 5 wherein, if the parameter indicative of purge effectiveness is more than the target value, the standard purge strategy is used.
7. A method as claimed in any of claims 1 to 6 wherein the parameter indicative of purge effectiveness is NOx conversion efficiency at the end of a purge event and the target value is a target value of NOx conversion efficiency.
8. A method as claimed in any of claims 1 to 6 wherein the parameter indicative of purge effectiveness is an average purge time for a predefined number of preceding purge events and the target value is a target value of average purge time.
9. A method as claimed in any of claims 1 to 6 wherein the parameter indicative of purge effectiveness is an estimate of the reductant supplied during a predefined number of preceding purge events and the target value is a target value of reductant supplied for the same number of purge events.
10. A method as claimed in any of claims 1 to 9 wherein the standard purge strategy comprises an optimised frequency of purge events, an optimised length for each purge event and an optimised richness of feedgas entering the lean NOx trap for each purge event.
11. A motor vehicle having a combustion engine arranged to supply feedgas to a lean NOx trap and an electronic control system to control the operation of the engine wherein the electronic control system is arranged to establish whether operating conditions required for purging of the lean NOx trap are present and, provided the operating conditions required for purging of the lean NOx trap are -16 -present, is arranged to compare a parameter indicative of purge effectiveness with a target value for purge effectiveness and, if the parameter indicative of purge effectiveness is less than the target value, is arranged to adapt a standard purge strategy used for controlling purging the lean NOx trap by increasing at least one of the frequency of purge events and the richness of the feedgas entering the lean NOx trap and is further arranged to use the adapted purge strategy to purge the lean NOx trap.
12. A method of adaptively controlling purging of a lean NOx trap arranged to receive a flow of feedgas from a combustion engine substantially as described herein with reference to the accompanying drawing.
13. A motor vehicle substantially as described herein with reference to the accompanying drawing.AMENDMENTS TO CLAIMS HAVE BEEN FILED AS FOLLOWSClaims 1. A method of adaptively controlling purging of stored NOx from a lean NOx trap arranged to receive a flow of feedgas from a combustion engine comprising establishing whether operating conditions required for purging of the lean NOx trap are present and, provided the operating conditions required for purging of the lean NOx trap are present, determining whether a standard purge strategy optimised to produce a desired improvement in lean NOx trap performance while minimising fuel usage and CO2 emissions during the purge and comprised of a combination of an optimised frequency of purge events, an optimised length for each purge event and an optimised richness of feedgas entering the lean NOx trap for each purge event will produce (r) effective purging of the NOx trap by comparing a parameter indicative of purge effectiveness with a target value for purge effectiveness and selecting one of the standard purge strategy and an adapted purge strategy based upon the C3) 20 comparison, the standard purge strategy being used to purge C\J the lean NOx trap if the parameter indicative of purge effectiveness is more than the target value and the adapted purge strategy being used if the parameter indicative of purge effectiveness is less than the target value, wherein the standard purge strategy is adapted to produce the adapted purge strategy based upon the dynamic operation of the combustion engine by using at least one of an increased frequency of purge events compared to the standard purge strategy, a reduced length for each purge event compared to the standard purge strategy and an increased richness for the feedgas entering the lean NOx trap during each purge event compared to the standard purge strategy and using the adapted purge strategy to purge the lean NOx trap.2. A method as claimed in claim 1 wherein the purge strategy is adapted by increasing the frequency of purge events compared to the standard purge strategy while reducing the length of each purge event compared to the standard purge strategy.3. A method as claimed in claim 1 wherein the purge strategy is adapted by increasing the richness of the feedgas entering the lean NOx trap compared to the standard purge strategy while reducing the length of each purge event compared to the standard purge strategy.4. A method as claimed in claim 1 wherein the purge strategy is adapted by increasing the frequency of purge events and increasing the richness of the feedgas entering the lean NOx trap for each purge event compared to the standard purge strategy.(r) 5. A method as claimed in claim 1 wherein the purge strategy is adapted by increasing the frequency of purge events compared to the standard purge strategy, while increasing the richness of the feedgas entering the lean NOx trap for each purge event compared to the standard purge C\J strategy and reducing the length of each purge event compared to the standard purge strategy.6. A method as claimed in any of claims 1 to 5 wherein the parameter indicative of purge effectiveness is NOx conversion efficiency at the end of a purge event and the target value is a target value of NOx conversion efficiency.7. A method as claimed in any of claims 1 to 5 wherein the parameter indicative of purge effectiveness is an average purge time for a predefined number of preceding purge events and the target value is a target value of average purge time.8. A method as claimed in any of claims 1 to 5 wherein the parameter indicative of purge effectiveness is an estimate of the reductant supplied during a predefined number of preceding purge events and the target value is a target value of reductant supplied for the same number of purge events.9. A motor vehicle having a combustion engine arranged to supply feedgas to a lean NOx trap and an electronic control system to control the operation of the engine, the electronic control system being arranged to establish whether operating conditions required for purging stored NOx from the lean NOx trap are present and, provided the operating conditions required for purging of the lean NOx trap are present, to determine whether a standard purge strategy optimised to produce a desired improvement in lean NOx trap performance while minimising fuel usage and 002 (r) emissions during the purge and comprised of a combination of an optimised frequency of purge events, an optimised length for each purge event and an optimised richness of feedgas entering the lean NOx trap for each purge event will produce C3) 20 effective purging of the NOx trap by comparing a parameter C\J indicative of purge effectiveness with a target value for purge effectiveness and selecting one of the standard purge strategy and an adapted purge strategy based upon the comparison is arranged to use the standard purge strategy if the parameter indicative of purge effectiveness is more than the target value and, use the adapted purge strategy if the parameter indicative of purge effectiveness is less than the target value, wherein the standard purge strategy is adapted by the electronic controller to produce the adapted purge strategy based upon the dynamic operation of the combustion engine by using at least one of an increased frequency of purge events compared to the standard purge strategy, a reduced length for each purge event compared to the standard purge strategy and an increased richness for the feedgas entering the lean NOx trap during each purge event compared to the standard purge strategy and using the adapted purge strategy to purge the lean NOx trap.10. A motor vehicle as claimed in claim 9 wherein the standard purge strategy is adapted by the electronic controller using one of:-a/ an increased frequency of purge events compared to the standard purge strategy along with a reduced length for each purge event compared to the standard purge strategy; b/ an increased richness of the feedgas entering the lean NOx trap compared to the standard purge strategy along with a reduced length for each purge event compared to the standard purge strategy; c/ an increased frequency of purge events along with an increased richness for the feedgas entering the lean NOx (r) trap for each purge event compared to the standard purge 1-- strategy; and CD d/ an increased frequency of purge events compared to the standard purge strategy along with an increased richness C)J of the feedgas entering the lean NOx trap for each purge event compared to the standard purge strategy and a reduced length for each purge event compared to the standard purge strategy.11. A method of adaptively controlling purging of a lean NOx trap arranged to receive a flow of feedgas from a combustion engine substantially as described herein with reference to the accompanying drawing.12. A motor vehicle substantially as described herein with reference to the accompanying drawing.