GB2532977A

GB2532977A - A method of scheduling the regeneration of a lean NOx trap

Info

Publication number: GB2532977A
Application number: GB1421560.2A
Authority: GB
Inventors: Arevalo Andres; Ford Kim; Wright James; Demory Romain
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2014-12-04
Filing date: 2014-12-04
Publication date: 2016-06-08
Anticipated expiration: 2034-12-04
Also published as: GB201421560D0; MX2015016533A; DE102015121060A1; GB2532977B; BR102015029832A2; RU2015151492A; RU2015151492A3; RU2708173C2

Abstract

A method of scheduling the regeneration of a lean NOx trap is provided in which purging of the lean NOx trap is based on a need to minimise the emission of NOx from a tailpipe of a vehicle. This is accomplished by using the relationship between NOx emissions from the tailpipe, and the engine running time or distance travelled. This way it is possible to predict the amount of time the engine can run, or the distance the car can travel between purge events that will minimise the amount of tailpipe NOx being produced by the engine. Therefore, regeneration, or purging, of the NOx trap can be scheduled for either when the engine has been running for a certain amount of time, or the car has travelled a certain distance..

Description

-I -

A Method of Scheduling the Regeneration of a Lean NOx Trap This invention relates to the regeneration of a lean NOx trap for an engine of a road vehicle and, in particular, to a method and apparatus for scheduling the purging of NOx from the lean NOx trap (LNT) during operation of the road vehicle.

A LNT is an exhaust after-treatment device for lean burn engines. The LNT has to be purged periodically to release and convert the oxides of nitrogen (NOx) stored in the LNT during lean operation of the engine. To accomplish the purge, the engine is operated at an air-to-fuel ratio that is rich of stoichiometric. As a result of the rich operation, substantial amounts of carbon monoxide (CO) and hydrocarbons (HC) are generated to convert the stored NOx. Typically, the purge mode is activated on the basis of estimated LNT loading. That is, when the estimated mass of NOx stored in the LNT exceeds a predetermined threshold, a transition to the purge mode is initiated. The rich operation will be continued for several seconds until the LNT is emptied of the stored NOx, whereupon the purge mode is terminated and the normal lean operation of the engine is resumed.

Since the engine has to be operated rich of stoichiometric during the purge operation, purging will have a significant negative effect on fuel economy compared to the fuel economy advantage of lean operation and so it is conventional practice to optimise the timing of the purge in order to reduce the loss in fuel economy.

It is further known that, because the regeneration of a LNT aftertreatment device requires the engine to be operated rich, this will result in some fuel mixing with the oil used to lubricate the engine thereby resulting in dilution of the -2 -oil. It is therefore also common practice to delay regeneration of a LNT until the level of NOx stored in the LNT reaches a pre-determined high level in order to maximise the amount of NOx stored in the LNT and reduce the number of regeneration events that are required thereby reducing fuel dilution.

The inventors have realised that such prior art approaches are disadvantageous in that the NOx slippage from a LNT increases as the LNT is filled with NOx. When a LNT is substantially empty of stored NOx even large variations in engine operation such as sudden demands for increased torque will not result in any significant NOx slippage from the LNT. However, when the LNT fills with NOx the resistance of the LNT to NOx slippage reduces and so as the filling approaches a level close to that at which regeneration is conventionally initiated even relatively small variations in engine operation can result in significant NOx slippage from a LNT.

Any NOx slippage from an LNT will result in a spike in NOx emissions from the vehicle and so is undesirable. In addition, the instantaneous increase in NOx emissions due to the NOx spike will also have a detrimental effect on the cumulative NOx emissions from the vehicle.

With increasingly more stringent emission targets, particularly for NOx, any increase in cumulative NOx emissions may result in a vehicle being found to be non-conforming due to the NOX slippage whereas during normal running of the vehicle the tailpipe level of NOx is well within permitted legislative levels.

One example of more stringent emission regulations in 35 Europe is the introduction of Real Driving Emissions (RDE)requirement to report emissions using a Portable Emission System (PFMS) under real driving conditions on public roads.

It is an object of the invention to provide a method of 5 scheduling the regeneration of a Lean Max trap that minimises tailpipe NOx emissions.

The invention will now be described by way of example with reference to the accompanying drawing of which:-Fig.1 is a schematic plan view of a motor vehicle according to a third aspect of the invention having a system for scheduling the regeneration of a LMT in accordance with a second aspect of the invention; Fig.2 is a block diagram of the system shown in Fig.1 showing in more detail various functional components of the system; Fig.3 is a schematic diagram of a first embodiment of an exhaust system for an engine; Fig.4 is a schematic diagram of a second embodiment of an exhaust system for an engine; and Fig.5 is a chart showing the relationships between 002, fuel in oil, tailpipe NOx including purge and a ratio of NOx reduction to CO2 ratio verses distance to purge.

With particular reference to Figs.1 and 2 there is shown a motor vehicle 1 having a lean burn engine in the form of a diesel engine 2. The motor vehicle 1 has in this case four road wheels 5, 6 of which the front two road wheels 5 are driven by the engine 2 via a driveline 3 which includes a multi-ratio transmission, a differential and a -4 -pair of halfshafts. A forward direction of travel of the motor vehicle 1 is indicated by the arrow F on Fig.l.

The engine 2 is connected to an exhaust system 7 via an exhaust manifold 8 which, although not shown, could include a turbocharger. Exhaust gas flows from the engine 2 via the manifold 8 into the exhaust system 7 and passes through a lean NOx trap (LNT) 10, a passive selective catalytic reduction catalyst (pSCR) 11 and a silencer 12 before exiting to atmosphere via a tailpipe 9. It will be appreciated that the exhaust system 8 could include additional exhaust aftertreatment devices or more than one silencer and so is not limited to the exact configuration shown in Fig.l.

An electronic system 20 is provided to control the operation of the engine 2 and to schedule regeneration or purging of the LNT 10.

The electronic system 20 comprises a central processor 21 arranged to receive an input 22 indicative of the nature of the exhaust gas passing through the exhaust system 7 from one or more exhaust gas sensors and an input 23 indicative of the operation of the engine 2 and/or the vehicle 1.

The central processor 21 comprises in this case of a central processor unit capable of executing a computer program and a number of memory devices in which one or more programs and operating instructions for the central processor unit are stored.

The central processor 21 and in particular the central processor unit is arranged to process the signals received from the inputs 22, 23 and provide an output to an engine control unit 25 used to control the operation of the engine 2. -5 -

So far as this invention is concerned the electronic system 20 functions so as to schedule regeneration of the LNT 10 in accordance with a method of scheduling the regeneration of a lean Max trap as provided by this invention. It will however be appreciated that the electronic system 20 and, in particular, the engine control unit 25 may also be operable to control the operation of the engine 2 during normal running of the engine 2.

With reference to Figs.3 and 4 there are shown two embodiments of the invention and in particular the various sensors forming the input 22 of Fig.2 along with one of the sensors forming the input 23 of Fig.2.

With reference to Fig.3 there is shown the exhaust system 7 in more detail. The central processor 21 connected to a number of exhaust gas sensors forming the input 22. The exhaust gas sensors in this case comprise an air to fuel ratio sensor or lambda sensor 22a, a first exhaust gas temperature sensor 22c, a second exhaust gas temperature sensor 22d and a third exhaust gas temperature sensor 22e.

A single mass airflow (MAF) sensor 23a is shown on Fig.3 constituting the input 23 however it will be appreciated that various other sensors could be included as part of the input 23 such as, for example and without limitation, an engine speed sensor, an accelerator position sensor, vehicle speed sensor, a vehicle distance travelled sensor and a sensor or other means of providing an indication of fuel usage.

Operation of the electronic system 20 is as follows. A measurement of the intake mass airflow is provided by the 35 MAF sensor 23a, a value of the air to fuel ratio of the exhaust exiting the engine 2 is provided by the lambda -6 -sensor 22a and first and second exhaust gas temperature sensors 22c and 22d provide values of exhaust gas temperature upstream and downstream from the LNT 10. Using the inputs from the sensors 22a, 22c, 22d and 23a allows the central processor 21 to calculate engine out NOx and also storage of this NOx within the LNT.

Modelled feedgas NOx inputs produced using the inputs 22 and 23 are provided to a LNT storage model. The LNT storage model provides an estimate of the NOx stored in LNT as a function of modelled LNT temperature and modelled gas flow rate. The modelled LNT storage is then used to predict when NOx slip will occur due to a low storage rate of the LNT 10.

This information regarding storage rate is then used to schedule a LNT purge to return the LNT 10 to a state of low NOx slip and high storage rate. The prediction of slippage of NOx from the LNT 10 is used to trigger an early purge of the LNT 10 and therefore reduce the spillage of NOx. The optimal conversion point of the LNT 10 is also modelled to bias the scheduling of the purges to a point of maximum NOx efficiency or maximum NOx conversion. The efficiency of conversion of stored NOx is modelled as a function of modelled LNT temperature, modelled gas flow rate, modelled NOx storage and lambda during purge.

In the case where a downstream passive SCR 11 is present in the exhaust system 7, the scheduling of the purges can also be modified to maximise NH3 production at the expense of CO2 in order to improve the conversion of NOx in the passive SCR 11 which uses NH3 as a reductant.

The modelled conversion efficiency can also be used to 35 bias the placement of the LNT purges towards maximum conversion of stored NOx for a minimal fuel/CO2 penalty. -7 -

Mapped fuel economy per LNT purge can also be used to trigger LNT purges in order to minimise fuel/CO2 penalty.

In some embodiments the electronic system 20 is operable to use more than one purge schedules depending upon the state of the LNT 10 or other requirements such as emission regulations.

In such a case a first purge schedule is used bias the purging of the LNT 10 in order to minimise tailpipe NOx and a second purge schedule is used to minimise a ration of Max conversion compared to the CO2 produced.

It will be appreciated that there are numerous emission regulations that have to be complied with and that in some cases in order to meet such regulations it is necessary to bias the purging of the LNT 10 to minimise tailpipe NOx. For example, there may be a requirement to limit the mass of NOx per Km travelled to a predefined level. Euro 6 emission standards for example requires there to be less than 0.08g/Km of NOx emissions for diesel cars and less than 0.06 g/Km of NOx emissions for petrol cars. Therefore if the NOx emissions are greater than an emission factor Ef equal to the actual NOx emission output from the vehicle divided by the NOx emissions permitted by the regulation, that is to say Ef >1.0 then the emissions regulations will not be being complied with. Therefore in order to prevent this a predefined level for the emission factor such as, for example 0.85 can be set and if E, > 0.85 the first purge schedule is used and if EL < 0.85 the second purge schedule is used.

Similarly, in relation to the European Real Driving Emissions reporting requirement a conformance factor between the real on road emissions and the limits defined for the -8 -regulated New European Driving Cycle (NEDC)must be calculated and this must be less than a to be declared limit, if the usage of the vehicle indicates that the predicted value for the emission factor using the second schedule would be above the factor or some limit below the factor then the first schedule could be used.

However, in circumstances where the NOx emissions from a vehicle is well within any required regulations it is preferred to use the second schedule because that optimises the fuel/CO2 penalty because not only will this schedule reduce the running costs of a vehicle it will also reduce fuel in oil dilution.

It will also be appreciated that as an LNT ages its storage efficiency will drop compared to a new LNT. Therefore when the LNT is new it may be possible to use a purge schedule such as the second purge schedule because there is no risk of any risk of tail pipe emissions exceeding prescribed limits but as the LNT ages the LNT is unable to meet such regulations unless the first purge schedule is used. Therefore by setting a parameter such as equivalent age indicative of a predicted thermal age of the LNT and setting an age limit it is possible to switch from the second purge schedule to the first purge schedule based upon whether the current value of the parameter is above the limit or below the limit. For example if the equivalent age is greater than the age limit then the first purge schedule is used and otherwise the second purge schedule is used.

With reference to Fig.4 there is shown an alternative embodiment to that shown in Fig.3 that in most respects is the same as the first embodiment.

The central processor 21 is, as before, connected to a number of exhaust gas sensors forming the input 22 but in _ 9 _ this case in addition to the air to fuel ratio sensor or lambda sensor 22a, the first exhaust gas temperature sensor 22c, the second exhaust gas temperature sensor 22d and the third exhaust gas temperature sensor 22e there is also provided a first Max sensor 22b located upstream of the LNT 10 and a second NOx sensor 22f located downstream of the passive SCR 11 so as to directly sense the NOx emissions exiting the tailpipe 9.

io As before a single mass airflow (MAF) sensor 23a is shown on Fig.4 constituting the input 23 however it will be appreciated that various other sensors could be included as part of the input 23 such as, for example and without limitation, an engine speed sensor, an accelerator position sensor, vehicle speed sensor, a vehicle distance travelled sensor and a sensor or other means for providing an output indicative of fuel used.

Operation of the electronic system 20 is as follows. A measurement of the intake mass airflow is provided by the MAF sensor 23a, a value of the air to fuel ratio of the exhaust exiting the engine 2 is provided by the lambda sensor 22a and first and second exhaust gas temperature sensors 22c and 22d provide values of exhaust gas temperature upstream and downstream from the LNT 10. Using the inputs from these sensors 22a, 22c, 22d and 23a allows the central processor 21 to calculate engine out NOx and also storage of this NOx within the LNT.

The upstream NOx sensor 22b provides a direct measurement of the NOx to the LNT 10 and so a NOx feedgas model is not required.

The tailpipe NOx sensor 22f is used in this case to 35 provide in combination with the upstream NOx sensor 22b a direct measurement of NOx stored in the LNT 10 without the -10 -need to model this. It will be appreciated that the NOx stored in the LNT 10 is the difference between the NOx measured by the NOx sensor 22b entering the LNT 10 and the NOx measured by the NOx sensor 22f leaving the tailpipe 9 if there is were no passive SCR 11 downstream from the LNT 10.

That is to say:-LNT NOx storage = Feedgas NOx -Tailpipe NOx.

In a case where there is a passive SCR 11, as shown in Fig.4, between the upstream and tailpipe NOx sensors 22b and 22f then the NOx stored in the LNT 10 is the difference between the NOx measured by the NOx sensor 22b entering the LNT 10 and the NOx measured by the NOx sensor 22e leaving the tailpipe 9 minus the NOx removed by the passive SCR.

Therefore for the case where there is a passive SCR downstream from the LNT:-LNT NON sLorage = [(Feedgas NOx -NOx removed in passive SCR) -Lailoipe NOx].

The passive SCR NOx reduction could be modelled.

In addition, the tailpipe NOx sensor 22f can also be used to provide a direct measurement of NOx slip and this can be used to trigger a purge of the LNT 10.

By using the second embodiment shown in Fig.4 the robustness of the scheduling can be enhanced because of the use of upstream and tailpipe Max sensors 22b, 22f that provide actual measurements of the NOx entering the LNT 10 and the NOx slippage from the LNT 10 rather than use predictions based upon system models.

As before, the optimal conversion point of the LNT 10 can be modelled to bias the scheduling of the purges to a point of maximum NOx efficiency or maximum NOx conversion. The efficiency of conversion of stored NOx can be modelled as a function of measured LMT temperature, gas flow rate from the MAF sensor 23a, calculated NOx storage from the two NOx sensors 22b and 22f and the air to fuel ratio measured by the lambda sensor 22a during a purge.

As before, when a downstream passive SCR 11 is present in the exhaust system 7, the scheduling of a purge can also be modified to maximise NH3 production at the expense of CO2 in order to improve the conversion of Max in the passive SCR 11 which uses NH3 as a reductant.

As before, the conversion efficiency can also be used to bias the placement of the LNT purges towards maximum conversion of stored NOx for a minimal fuel/CO2 penalty.

As before, mapped fuel usage per LNT purge can also be used to trigger LNT purges in order to minimise fuel/002 penalty.

The electronic system 20 could as previously referred 25 to be operable to use more than one different purge schedule.

For example there could be a first purge schedule to bias the purging of the LNT 10 to minimise tailpipe NOx; a second purge schedule to bias the purging of the LNT 10 to minimise a ration of NOx conversion to CO2 penalty and a third purge schedule to modify the purge schedule to maximise NH3 production at the expense of 002.

-12 -With reference to Fig.5 there are shown examples of key relationships with respect to the distance travelled by a vehicle between LNT regeneration/ LNT purge (DeN0x) events.

It will be appreciated that instead of distance travelled between purge events the key relationships could be referenced against engine running time between purges.

The first relationship is that of CO2 with respect to the distance between purge events and the second is a relationship between the amount of fuel transferred to engine oil with respect to the distance between purge events. Because both of these relationships have a similar characteristic they are shown as a single curve although the actual values would of course be different.

It can be seen that, as the distance between purge events increases, the amount of CO2 and fuel in oil reduces. This is not surprising because both CO2 and fuel in oil are related to the additional fuel used to purge the LNT by operating the engine rich (Lambda < 1). Therefore, if the only requirement is to reduce fuel in oil or 002, it would be desirable to increase the distance between purge events to the maximum distance possible.

The third relationship is between tailpipe NOx including the effect of purging and the distance travelled between purge events. The curve is substantially U-shaped having a minimum value indicated by an arrow 'a'. The point 'a' corresponds to the distance used for the first purge schedule.

For distances between purge events less than the optimum point 'a', the amount of tail pipe NOx increases as the distance reduces due to the effect of the increased frequency of purge events and the negative effect these have -13 -on tailpipe NOx because of the rich running (Lambda < 1) of the engine during a purge.

For distances greater than the optimum point 'a', the amount of tailpipe Max increases as the distance increases due to the effect of increased NOx slippage in combination with the NOx produced by the purge event both of which adversely effect tailpipe NOx.

io The fourth relationship is a ratio E between NOx reduction and CO2 production with respect to the distance travelled between purge events.

In this case the relationship falls as the distance travelled between purge events increases until at a distance indicated by an arrow 'b' it is at a minimum. From the minimum point 'b' it begins to increase again as the distance between purge events is further increased due to the fact that NOx slippage is having an adverse effect on the reduction of NOx and because the effect of this NOx slippage is so significant that it overpowers the decreasing level of CO2 as the distance between purge events is increased. The point 'b' represents a minimal fuel penalty/ CO2 approach to the scheduling of LNT purging whereby the purging is initiated when the ratio E is at a minimum.

Tailpipe NOx = Function (FG NOx + NOxsll, + NOxpur, + Where:-FG Max = Feedgas NOx = Max due to slip; NOxp,,, = NOx due to purge; and NOxcorw% = NOx due conversion efficiency of LNT.

If the method includes more than one purge schedule, the point 'b' corresponds to the distance used for the -14 -second purge schedule and the method switches between purging at point 'a' and purging at point 'b' as previously described.

However, from Fig.5 it can be seen that if the purging is delayed until this distance has been travelled since the last purge event then the level of tailpipe NOx taking into account any purge events and NOx slippage is considerably higher than the optimum point 'a' as indicated on Fig.5 by point P. The inventors have therefore realised that with increasingly more onerous NOx targets a method that optimises tailpipe NOx as indicated by point 'a' will allow a vehicle that would otherwise fail to meet a legislative limit can do so. For example, if the level of tailpipe NOx that must be maintained to comply with a legislative limit is set at a level L, a vehicle utilising the method proposed by this invention would pass whereas the same vehicle optimised for the ratio e would fail because point P is above the limit 'L'.

Therefore in summary, conventional diesel LNT control strategy is based upon on feedgas NOx, LNT storage, NOx reduction in the LNT and minimisation of the fuel purge penalty. This requires using a large percentage of the storage capacity of the LNT in order to reduce the number of purges and the cost of these purges. The consequence of this strategy is that the LMT tends to be filled to a high percentage of its capacity in the order of 90 to 100% and this will increase the rate and occurrence of NOx slip, release or spillage due to exhaust gas temperature and flow changes. In other words, the tailpipe NOx emission are not minimised due to the slip or spillage of NOx from the LNT.

By using a method as proposed by this invention purges are not scheduled as with prior art systems when the LNT is -15 -virtually full but instead scheduled to produce minimal tailpipe NOx. This is achieved by increasing the purge frequency (reducing the distance between purge events) in order to trigger purges when the LNT is only partially full.

Such an approach maximises the storage rate of the LNT and virtually eliminates any risk of NOx slippage from the LNT.

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 and that it is not limited to these disclosed embodiments being only limited by the scope of the invention as defined by the appended claims.

Claims

-16 -Claims 1. A method of scheduling the regeneration of a lean Max trap arranged to receive exhaust gas from a lean burn engine of a vehicle wherein the method comprises using a first purge schedule based upon a relationship between tailpipe NOx emissions and one of engine running time between purge events and distanced travelled between purge events to predict one of an engine running time and a distance travelled between purge events that will result in a minimum amount of tailpipe Max being produced by the engine and purging the lean NOx trap at one of the predicted engine running time and the predicted distance travelled in order to minimise tailpipe NOx emissions produced by the engine.
2. A method as claimed in claim 1 wherein the method further comprises generating a lean NOx trap storage model and using the lean Max trap storage model to predict when NOx slip will occur.
3. A method as claimed in claim 2 wherein the lean NOx trap storage model provides an estimate of Max stored in the lean NOx trap as a function of modelled lean NOx trap temperature and modelled exhaust gas flow rate.
4. A method as claimed in claim 2 wherein the lean Max trap storage model provides an estimate of Max stored in the lean NOx trap based upon a measurement of NOx entering 30 the lean NOx trap and modelled exhaust gas flow rate.
5. A method as claimed in claim 1 wherein the method further comprises using a Max sensor located upstream from the lean NOx trap and a NOx sensor located downstream of the -17 -lean NOx trap to directly measure NOx and using the measurements from the upstream and downstream NOx sensors to predict when NOx slip will occur.
6. A method as claimed in claim 1 wherein the method further comprises using a NOx sensor located downstream of the lean NOx trap to directly measure NOx and using the measurement from the downstream NOx sensor to provide a direct NOx slip measurement for use in scheduling a purge.
7. A method as claimed in any of claim 1 to 6 wherein the method further comprises using the first purge schedule for purging the lean NOx trap when a parameter indicative of one of lean NOx trap age and an emission factor is above a predefined level and using a second purge schedule for purging the lean NOx trap when the parameter indicative of one of lean NOx trap age and the emission factor is below the predefined level.
8. A method as claimed in claim 7 wherein the second purge schedule purges the lean NOx trap so as to minimise a ratio of NOx conversion compared to CO2 produced.
9. A method as claimed in claim 1 wherein the method further comprises selectively modifying the purge schedule to maximise NH3 production for use by a downstream passive selective reductant catalyst.
10. A an electronic system for scheduling the purging of a lean NOx trap arranged to receive exhaust gas from a lean burn engine wherein the system is operable to generate a first schedule based upon a relationship between tailpipe NOx emissions and one of engine running time between purge -18 -events and distanced travelled between purge events to predict one of an engine running time and a distance travelled between purge events that will result in a minimum amount of tailpipe NOx being produced by the engine and the system is further operable to purge the lean NOx trap at one of the predicted engine running time and the predicted distance travelled in order to minimise the amount of tailpipe NOx emissions produced by the engine.
11. A system as claimed in claim 10 wherein the system includes a central processor and the central processor is operable to produce the first purge schedule.
12. A system as claimed in claim 10 or in claim 11 wherein the system includes a central processor and the central processor is operable to control the engine to purge the lean NOx trap in accordance with the first purge schedule.
13. A system as claimed in any of claims 10 to 12 wherein the electronic system is further operable to use the first purge schedule for purging of the lean NOx trap when a parameter indicative of one of lean NOx trap age and an emission factor is above a predefined level and to use a second purge schedule for purging the lean NOx trap when the parameter indicative of one of lean NOx trap age and the emission factor is below the predefined level.
14. A system as claimed in claim 13 wherein the system includes a central processor and the central processor is operable to produce the second purge schedule.

-19 -
15. A system as claimed in claim 13 or in claim 14 wherein the system includes a central processor and the central processor is operable to control the engine to purge the lean NOx trap in accordance with the second purge schedule.
16. A system as claimed in any of claims 13 to 15 wherein use of the second purge schedule minimises a ratio of NOx produced by the engine compared to 002 produced.
17. A method of scheduling the regeneration of a lean NOx trap arranged to receive exhaust gas from a lean burn engine of a vehicle substantially as described herein with reference to the accompanying drawing.
18. A system for scheduling the purging of a lean NOx trap substantially as described herein with reference to the accompanying drawing.
19. A vehicle having a lean burn engine substantially as described herein with reference to the accompanying drawing.Amendment to the claims havebeen filed as follows Claims 1. A method of scheduling the regeneration of a lean Max trap arranged to receive exhaust gas from a lean burn engine of a vehicle, the method comprising using at least one purge schedule for scheduling the regeneration of the lean NOx trap wherein, when a parameter indicative of one of lean NOx trap age and an emission factor is above a predefined level, a first purge schedule is used for purging the lean NOx trap, the first purge schedule being based upon a relationship between tailpipe Max emissions and one of engine running time between purge events and distanced travelled between purge events to predict one of an engine running time and a distance travelled between purge events LC) 15 that will result in a minimum amount of tailpipe NOx being produced by the engine and purging the lean NOx trap at one of the predicted engine running time and the predicted distance travelled in order to minimise tailpipe Max CO emissions produced by the engine and, when the parameter C\J 20 indicative of one of lean NOx trap age and the emission factor is below the predefined level, a second purge schedule is used for purging the lean NOx trap, the second purge schedule minimising a ratio of NOx conversion compared to 002 produced by the engine.2. A method as claimed in claim 1 wherein the method further comprises generating a lean NOx trap storage model and using the lean NOx trap storage model to predict when NOx slip will occur for use in minimising tailpipe NOx emissions.3. A method as claimed in claim 2 wherein the lean Max trap storage model provides an estimate of Max stored in the lean NOx trap as a function of modelled lean NOx trap temperature and modelled exhaust gas flow rate.4. A method as claimed in claim 2 wherein the lean NOx trap storage model provides an estimate of Max stored in the lean NOx trap based upon a measurement of NOx entering 5 the lean NOx trap and modelled exhaust gas flow rate.5. A method as claimed in claim 1 wherein the method further comprises using a NOx sensor located upstream from the lean NOx trap and a NOx sensor located downstream of the lean NOx trap to directly measure NOx and using the measurements from the upstream and downstream NOx sensors to predict when NOx slip will occur for use in minimising LC) tailpipe NOx emissions.6. A method as claimed in claim 1 wherein the method further comprises using a NOx sensor located downstream of CO the lean NOx trap to directly measure NOx and using the C\J measurement from the downstream NOx sensor to provide a direct NOx slip measurement for use in scheduling a purge to 20 minimise tailpipe NOx emissions.7. A method as claimed in claim 1 wherein a passive selective catalyst is located downstream from the lean NOx trap and the first purge schedule is selectively modified to maximise NH3 production for use by a downstream passive selective reductant catalyst.8. An electronic system for scheduling the purging of a lean NOx trap arranged to receive exhaust gas from a lean burn engine, the electronic system being operable to generate at least one purge schedule for scheduling purging the lean NOx trap wherein, when a parameter indicative of one of lean NOx trap age and an emission factor is above a LC) 15COC\J 20 predefined level, the electronic system is operable to generate and use a first purge schedule based upon a relationship between tailpipe NOx emissions and one of engine running time between purge events and distanced travelled between purge events to predict one of an engine running time and a distance travelled between purge events that will result in a minimum amount of tailpipe NOx being produced by the engine and the electronic system is further operable to use the first purge schedule to purge the lean NOx trap at one of the predicted engine running time and the predicted distance travelled in order to minimise the amount of tailpipe NOx emissions produced by the engine and, when the parameter indicative of one of lean NOx trap age and the emission factor is below the predefined level, the electronic system is operable to generate and use a second purge schedule to minimise a ratio of NOx produced by the engine compared to 002 produced by the engine.9. A system as claimed in claim 8 wherein the electronic system includes a central processor and the central processor is operable to generate the first purge schedule.10. A system as claimed in claim 8 wherein the electronic system includes a central processor and the central processor is operable to control the engine to purge the lean NOx trap in accordance with the first purge schedule.11. A system as claimed in claim 8 wherein the electronic system includes a central processor and the central processor is operable to generate the second purge schedule.12. A system as claimed in claim 8 wherein the electronic system includes a central processor and the central processor is operable to control the engine to purge the lean NOx trap in accordance with the second purge schedule.13. A method of scheduling the regeneration of a lean NOx trap arranged to receive exhaust gas from a lean burn engine of a vehicle substantially as described herein with reference to the accompanying drawing.14. A system for scheduling the purging of a lean NOx trap substantially as described herein with reference to the LC) accompanying drawing. is15. A vehicle having a lean burn engine substantially as described herein with reference to the accompanying CO drawing. C\J