GB2412615A

GB2412615A - Regenerating a particulate filter

Info

Publication number: GB2412615A
Application number: GB0407634A
Authority: GB
Inventors: Miguel Angel Roca-Mompo; Peter Graham Eastwood
Original assignee: Ford Global Technologies LLC
Current assignee: Ford Global Technologies LLC
Priority date: 2004-04-03
Filing date: 2004-04-03
Publication date: 2005-10-05
Anticipated expiration: 2024-04-03
Also published as: GB2412615B; GB0407634D0

Abstract

A method for estimating the mass of soot burnt in a particulate filter processing exhaust gases from an internal combustion engine during a regeneration process, comprising estimating a mass of oxygen consumed in a particulate filter 36 during regeneration and using the estimated mass of consumed oxygen to calculate an estimated mass of soot burnt in the filter. The method can be employed using a system having sensors 38, 40 measuring the oxygen content of exhaust gases entering and leaving the particulate filter and supplying a signal indicating the oxygen content to an electronic controller 28. The controller may execute a soot estimation routine which estimates the amount of soot being burnt. Advantageously regeneration of a diesel vehicle particulate filter is performed when necessary and inappropriate filter cleaning times are avoided.

Description

A Method and System for Monitoring a Particulate Filter.

The present invention relates to a method and system for estimating the mass of soot burnt off during regeneration of a particulate filter attached to an exhaust flow from an internal combustion engine and in particular to the soot burnt off during regeneration of a diesel engine particulate filter (DPF).

0 As is known in the art, it is necessary to regularly regenerate a particulate filter attached to an engine in order to prevent the filter becoming blocked with soot.

Regeneration is an artificial procedure in which the temperature of the exhaust gasses are increased to a very high temperature in order to oxidize or burn off the soot collected in the filter. One example of a method for regenerating a particulate filter is described in European Patent Application EP-A-1304458.

SO Increasingly stringent US federal regulations limit the permissible levels for emissions. As such, vehicle manufacturers have developed various methods to reduce emissions while improving vehicle performance and fuel economy.

Forthcoming emissions legislation for Particulate Matter (PM) will almost certainly force the widespread introduction of the Diesel Particulate Filter (DPF). This is a device fitted to the exhaust system of a motor vehicle i> to trap most of the engine-out particulate prior to emission at the tailpipe. One of the challenges faced by every vehicle manufacturer is to know how much particulate matter has accumulated during the loading cycle and how much has been burnt during the regeneration process.

The obvious approach is simply to weigh the particulate filter but even with state of the art weighing scales, it is - 2 difficult to obtain measurements of sufficient precision when the mass of the particulate filter is approximately 10- 12kg and the mass of particulate matter trapped within it is often less than 50g. In addition it is clearly not practical for a particulate filter to be repeatedly removed from the motor vehicle to be weighed.

Furthermore, it is desirable to provide a means for estimating the particulate matter removed during lo regeneration with the particulate insitu during normal use of the motor vehicle. This is because regeneration of the particulate filter is a process which has some disadvantages and so which should ideally only be performed when necessary. For example in order to cause regeneration the temperature of the exhaust gasses have to be raised to a very high level and this is normally achieved by late injection of fuel into the cylinders of the engine. Such late injection is undesirable as it tends to cause dilution of the oil used to lubricate the engine by the late injected fuel. In addition, if particulate filter regeneration is performed too often this will have an adverse effect on fuel economy of the motor vehicle.

A-other problem associated with particulate filter regeneration is that if is sensitive to the amount of p:articuiate matter that Han been stored in the particulate tiller. If regeneration is performed when insufficient particulate matter}lab Accumulated then it is difficult to olit-,in stable self combuciticn and the particulate filter may nil- prcpr?rly regenerate wtqe-eas if too much particulate m-tt-r?r has been accumulated then the self combustion or e.:ot:l-errnic reaction will be too severe and can result in d.rn.-ge occurring to t:hf:' p--rticulate filter.

('mpositional]y, rhc pr-rticulate matter collected by ti-rticulatc fi:lt, er roan Off? considered to consist of four t!-I'-')-: i fungi, rarboriaceous, a'-;}l, organics and sulphates. The - 3 - purpose of regeneration is to burn out the carbonaceous and organic fractions, the incombustible ash remains behind and the sulphate fraction escapes by evaporation and/or decomposition. The term 'soot' as meant herein refers to the carbonaceous and organic fractions of particulate matter trapped in a particulate filter and it is these fractions that are burnt off during regeneration.

It is therefore desirable to obtain an estimate of how much soot has been burnt during regeneration in order to better estimate when the particulate filter should next be regenerated.

It is an object of this invention to provide a method and system for estimating the mass of soot burnt off during regeneration of a particulate filter for an internal combustion engine.

According to a first aspect of the invention there is provided a method for estimating the mass of soot burnt in a particulate filter processing exhaust gasses from an internal combustion engine during regeneration of the particulate filter, the method comprising estimating the mass of Oxygen consumed in the particulate filter during regeneration of the particulate filter and using the e,tirnated mass of)ygr- onsumed to calculate an estimated n-ss calf soot burnt ire Lh- particulate filter.

The method may further comprise determining when r;eneration has start?-] and determining when regeneration Alas finished and estinatirlg the soot burnt during the intervening period.

egeneratiorl may he determined to have started when the lmp-rture of the e.h-:ust gases entering the particulate Filter exc?ed a pr-let-rmined temperature. - 4

Regeneration may be determined to have finished when the temperature of the exhaust gases entering the particulate filter falls below the predetermined temperature.

Alternatively, regeneration may be determined to have finished when a pressure drop across the particulate filter falls below a predetermined level or as yet another alternative, regeneration may be determined to have finished 0 when a predetermined period of time has elapsed since the regeneration was determined to have started.

The method may further comprise converting a sensed air fuel ratio of the exhaust gases entering the particulate filter into a first Oxygen content and converting a sensed air/fuel ratio of the exhaust gases exiting the particulate filter into a second Oxygen content. Alternatively, the method may further comprise measuring a first Oxygen content of the exhaust gases entering the particulate filter and JO measuring a second Oxygen content of the exhaust gases exiting the particulate filter.

In either case, the method may further comprise measuring the mass air flow entering the engine and using If, the measured mass air flow to convert the first Oxygen content into a first Oxygen mass flow rate and the second Oxygen content into a second Oxygen mass flow rate and subtracting the second Oxygen mass flow rate from the first Oxygen mass flow rate to produce an estimated mass of Oxygen :' being consumed in the particulate filter per second.

The method may further comprise integrating during the period when regeneration is determined to be occurring the mass of Oxygen being consumed in the particulate filter per it, second to provide an estimated mass of Oxygen consumed during regeneration. 5

The method may further comprise converting the estimated mass of Oxygen consumed during regeneration into an estimated mass of soot burnt during regeneration.

The method may further comprise using the estimated mass of soot burnt during regeneration to provide an improved estimate of when regeneration of the particulate filter should next occur.

0 Converting the mass of Oxygen consumed during regeneration into an estimated mass of soot burnt during regeneration may further comprise converting the mass of Oxygen into an equivalent number of moles of Oxygen and using the number of moles of Oxygen to estimate an equivalent number of moles of soot burnt and estimating a mass of soot burnt from the estimated number of moles of soot burnt.

According to a second aspect of the invention there is provided a system for estimating the mass of soot burnt in a particulate filter connected to an exhaust stream from an internal combustion Engine during regeneration of the particulate filter, the system comprising a mass air flow sensor to measure the mass flow rate of air entering the engine and supply a signal indicative of the mass air flow rate to an electronic. ontroller, a first sensor to measure i h(:? (oxygen content of tt-e ex} laust gasses entering the -;arti.c-:ulate filter and Supply Ha signal indicative of such :orterlt to the elect:ori.c controller, a second sensor to 30!nu-asure the Oxygen <::ont-i.-rlt of the exhaust gases exiting the ,arti.:-ulate filter ant Supply a signal indicative of such >ntcnt to the electronic controller, a temperature sensor i<.' senc] a signal t<-' the electronic controller indicative of -it- temperature of tint--. -<ha.st gasses entering the 35..-rt::ic:ulate filter, the electronic controller being operable :omparQ tics Tenneco r.emperat.:re of the exhaust gasses ur--ring the particui:tc filter with a predetermined - 6 - temperature and start a soot estimation routine to estimate the mass of soot being burnt in the particulate filter when the temperature of the exhaust gasses entering the particulate filter exceeds the predetermined temperature indicating that regeneration is occurring and terminate the soot estimation routine when regeneration of the particulate filter is determined to have ceased and is further operable to provide an output indicative of the total mass of soot burnt during the period when regeneration was determined to be occurring wherein the estimation of the mass of soot burnt during regeneration is based upon the difference in Oxygen content of the exhaust gasses entering and exiting the particulate filter and the mass air flow to the engine.

Regeneration may be determined by the electronic controller to have ceased when the sensed temperature of the exhaust gasses falls below the predetermined temperature.

Alternatively, regeneration may be determined by the electronic controller to have ceased when a predetermined length of time has elapsed after starting the soot estimation routine.

The system may farther -comprise a differential pressure sensor to determine the pressure drop across the particulate filter and supply a, igr.1 indicative of the pressure drop to the electronic controller, in which case, regeneration may be determined by r..e electronic controller to have ceaseci when the pressure drop across the particulate filter o '-ills below a preder.errnlneci limit.

The first and second -sensors may be both Oxygen sensors ncl the electronic ccntr<:>llc-,r may be operable to convert an from the first sc;ns,or into an equivalent Oxygen ->nt.cnt. and convert th<-- c'tpt from the second sensor into ,cllvalerit Oxygen c:ort.ent. In which case, the output r:rjm.h? first -; enscr may be a voltage and the output from - 7 the second sensor may be a voltage and the conversion may be performed by means of a look up table stored in a memory of the electronic controller relating voltage to Oxygen content. Alternatively, the conversion may be performed using a polynomial equation stored in a memory of the electronic controller representing a relationship between voltage and Oxygen content.

As yet another alternative, the first and second sensors may be both UEGO sensors and the electronic controller may be operable to convert an air/fuel ratio derived from the first sensor into an equivalent Oxygen content and convert the air/fuel ratio determined from the second sensor into an equivalent Oxygen content. In which is case, the conversion may be performed by means of a look up table stored in a memory of the electronic controller relating air/fuel ratio to Oxygen content or may be performed using a polynomial equation stored in a memory of the electronic controller representing a relationship JO between air/fuel ratio and Oxygen content.

The electronic controller may be operable to use the mass air flow to the engine to convert an Oxygen content determined by the first sensor into a first Oxygen mass flow >t, rate and to convert an Oxygen content determined by the second sensor into a second Oxygen mass flow rate and may be further operable to subtract the second Oxygen mass flow rate from the first Oxygen mass flow rate to provide a signal indicative of the estimated mass of Oxygen being !' consumed in the particulate filter per second.

The electronic controller may be further operable to integrate during the period when the soot estimation routine is operating the signal indicative of the mass of Oxygen i, being consumed in the particulate filter per second to provide a signal indicative of the estimated mass of Oxygen consumed during regeneration. - 8 -

The electronic controller may be further operable to convert the estimated mass of Oxygen consumed during regeneration into a signal indicative of the estimated mass of soot burnt during regeneration.

The electronic controller may be further operable to use the signal indicative of the estimated mass of soot burnt during regeneration to estimate when regeneration of 0 the particulate filter should next occur.

The invention will now be described by way of example with reference to the accompanying drawing of which: FIG.1 is a block diagram of a motor vehicle having a system for estimating the mass of soot burnt off during regeneration of a diesel engine particulate filter according to a second aspect of the invention; FIG.2 is a high level flow chart illustrating part of a method for estimating the mass of soot burnt off during regeneration of a particulate filter according to a second aspect of the invention; FIG.3 is a flow chart showing how the measurement Hi: oxygen content of.-.xhaust gasses entering and exiting the -'aLiculate filter are converted into an estimation of mass f;.:o,t burnt; LIG.4 is a graE:tl::howing a typical relationship between Air /L'ucl Ratio and Oxygen content for use in determining (oxygen content in exhaust gasses; and EI(;.5 i-; a graph st:'wirg the relationship between moles :35 I,{ oxygrrl consumed zinc] the mass of soot in grams for various <,m--;itions. - 9

Referring now to FIG.1 a block diagram illustrating a system for estimating the mass of soot burnt during regeneration is shown.

A motor vehicle 10 includes a diesel internal combustion engine 12 having an intake or inlet manifold 14 and an exhaust manifold 16. An exhaust pipe 34 couples the exhaust manifold 16 to a particulate filter in the form of a diesel particulate filter 36 (DPF). It will be appreciated lo by those skilled in the art that the exhaust pipe 34 may also be connected to one or more catalytic converters in order to further improve exhaust emission performance.

A conventional fuel supply 26 provides fuel which is injected directly into the engine where it mixes with the air from the intake manifold 14 to provide a combustion mixture. The injection of fuel to the engine 12 is controlled by an electronic controller in the form of an engine control module (ECM) 28. An accelerator pedal 24 is used to provide an inculcation of driver demand.

The motor vehicle lO is further provided with a system for estimating the mass of soot burnt during regeneration of ne particulate filter 36 and with means to cause regeneration of the particulate filter 36.

The system inclucies an electronic controller which in i L.; Case is include;-! Is an integral part of the engine corlt.ol,module 283 but Could be formed by one or more -;-p-ar-ate electronic or,trollers. The electronic controller 23 is arranged to receive signals from a number of sensors fcrmin part of the system as described hereinafter and is r!r-rmmed to perforrr certain functions in order to provide Oreo cstimate of thc mar; of soot burnt during regeneration.

The term signal Is meant herein is to be interpreted in It, wriest -,ens.and;:-n include but is not limited to a - 10 digital signal, data, an analogue signal, a measurement of voltage or current, a measurement of frequency or variations in frequency.

The first sensor is an air sensor 22 which measures the temperature of the air entering the engine 12 through the intake manifold 14. The air sensor 22 is operable to send a signal is sent to the electronic controller 28 indicative of the sensed temperature. The airflow through the intake To manifold 14 is measured by means of a mass air flow (MAP) sensor 18 and a signal indicative of the mass air flow is sent to the electronic controller 28.

An engine coolant temperature sensor 30 and an engine ib speed (RPM) sensor 32 communicates engine temperature 'T' and engine speed information 'RPM' respectively to the electronic controller 28 which can be used to estimate the duty cycle of the engine 12.

The particulate filter 36 is monitored via an upstream or first exhaust gas sensor 38 and a downstream or second exhaust gas sensor 40. The upstream and downstream sensors 38 and 40 are both exhaust gas oxygen sensors which are commonly referred to as UEGO sensors. These provide an A, indication of the presence or absence of oxygen in the exhaust stream to the electronic controller 28.

The upstream sensor 38 is located between the engine 12 and the particulate filter 36 and the downstream sensor 40 In is located between the particulate filter 36 and an exit from the;exhaust pipe 34 to atmosphere.

Although the invention is described with reference to a method and system utilising two UEGO sensors it will be If-, appreciated that any suitable Oxygen sensor may be utilised to determine the Oxygen content of the exhaust gasses. So that for example, the first and second sensors could both be Oxygen sensors which provide a voltage related to Oxygen content and the electronic controller could be operable to convert the output from the first sensor into an equivalent Oxygen content and convert the output from the second sensor into an equivalent Oxygen content. In which case, the conversion could be performed by means of a look up table stored in a memory of the electronic controller relating voltage to Oxygen content or the conversion could be performed using a polynomial equation stored in a memory of the electronic controller representing a relationship between voltage and Oxygen content.

An exhaust gas temperature sensor 37 is attached to the exhaust pipe 34 to provide a signal to the electronic controller 28 indicative of the temperature of the exhaust gasses flowing to the particulate filter 36.

A differential pressure sensor 39 is used to provide a signal to the ECM 28 indicative of the pressure drop across the particulate filter 36.

The electronic controller 28 includes at least one

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microprocessor and various computer readable storage media, which may include but is not limited to a read only memory (ROM), a random access memory (RAM), and a keep-alive memory (KAM). The computer readable storage media may be implemented by any of a number of known volatile and nonvolat.ile storage devices including but not limited to PROM, EL'i-<OM, EEPROM, flash memory, and the like, all of which are wii known in the art. RAM is typically used for temporary .i..i-a storage of various operating variables which are lost when the engine ignition is turned off, such as counters, liners, status flags, arid the like. KAM is generally used ! at, store learned c.: r adaptive values which may change over t-i.me. The contents of KAM are maintained as long as some -.,w-r is provided to the electronic controller 28.

rely, one Or more ROMS within the electronic controller 28 contain control logic implemented by program instructions executed by the microprocessor along with various system parameter values and calibrations.

The electronic controller 28 receives signals from the upstream and downstream exhaust gas sensors 38 and 40, the exhaust gas temperature sensor 37, the mass air flow sensor 18 and the differential pressure sensor 39 and uses them to estimate the mass of soot burnt during regeneration of the 0 particulate filter 36 as will be described hereinafter in greater detail.

Operation of the system is best understood with reference to the method which is used by the system to provide the estimate of soot burnt.

The method is shown in Figs.2 and 3. The method starts at 100 which corresponds to starting of the engine 12. The first step 110 is the measurement of the exhaust gasses flowing through the exhaust pipe 34 to the particulate filter 36 This is done using the exhaust gas sensor 37 and the signal provided Lo the electronic controller 28 is used by the electronic controller 28 in step 120 to determine whether the temperature of the exhaust gas has exceeded a predetermined temperature. In this case the predetermined temperature is 4500(2,ut it will be appreciated that it c.c-'uk] be some other temperature depending upon the characteristics of ttlO particulate filter used.

If the rnc-asurecl r?mperature is less than 450 C then the cycle is. repeated arici it is assumed that regeneration is not ;ccurrirlg. As soon as t he test of step 120 confirms that the temperature of the exhaust gasses exceeds 450 C then the cnertloclrnoves on to step 130. Step 130 starts a soot stimation routine shown in detail in Fig.4 to estimate the mar;,s cat soot that is burnt during the time period when rererat,ion of the:artic,ulate filter 36 is occurring.

This routine is continued until a first one of a number of conditions indicates that regeneration has finished.

The first of these conditions is that the temperature of the exhaust gasses, as measured by the exhaust gas sensor 37, has fallen below the predetermined temperature of 450 C indicating that regeneration has been halted.

i0 The second of these conditions is an elapsed time since regeneration started. At step 130 a timer is started in the electronic controller and the output from the timer is summed until a predetermined period of time, which in this case is 600 seconds, has elapsed. A this point regeneration Is is assumed to be complete because, after such an extended period of high temperature, it is likely that all of the soot has been burnt.

The third condition is based upon a comparison of the JO differential pressure across the particulate filter, as measured by the sensor 39, with a predetermined value of pressure difference. If the measured pressure differential

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falls below the predetermined value of pressure difference then regeneration is assumed to be complete. This is ;s because, with such a low pressure differential, it is unlikely that any substantial soot deposits remain in the particulate filter 36.

When the first of the three conditions is met the soot estimation routine is terminated at step 150. At this point an estimate of the soot burnt is known and as indicated in step 160 this figure is supplied to the electronic controller for use in producing a more accurate estimate of when next to regenerate the particulate filter 36. The 3, control of the actual regeneration of the particulate filter 36 and the determination of when to regenerate the particulate filter 36 does not form part of this invention but in general terms this is based upon various parameters including but not limited to the duty cycle of the engine the differential pressure across the particulate filter and the mileage covered by the motor vehicle after completion of the previous regeneration.

The routine used to produce the estimate of the mass of soot burnt will now be described in greater detail with reference to Figs. 3 to 5.

At step 130 the routine is started and the first step is the estimation of the Oxygen content or concentration of the exhaust gasses entering and leaving the particulate filter 36. This is obtained by using the output signals from the two UEGO sensors 38, 40. Each of these sensors 38, produces, with the use of appropriate transfer functions taking into account the backpressure on the sensor and the temperature of the sensor, a value indicative of the air/ fuel ratio of the exhaust gasses.

Each of these values of air/fuel ratio is then converted into an equivalent Oxygen content in one of two ways, either directly by use of a look up table of the type shown in Fig.4 stored in a memory of the electronic controller 28 or by means of direct calculation using a polynomial correlation bet-weep air/fuel ratio and Oxygen % content For the example given an appropriate polynomial expression 1S: Y = -0.0325X + 2.184X - 24.605 where: Y is the Oxygen:'; content; and X is the-? measured air/fuel ratio.

ALt-er Completing t--is conversion two values of Oxygen ,,t;-t- are c-'btained, Lir;t one corresponding to the position upstream of the particulate filter 36 and a second one corresponding to Oxygen content downstream from the particulate filter 36.

The next step is to convert these Oxygen contents into a mass flow of Oxygen consumed in the particulate filter 36 during regeneration. This is achieved at step 205 in two stages.

The first stage is to convert the first and second Oxygen contents into mass flow rates. To achieve this, the signal from the mass airflow sensor 18 is used to provide a value indicative of the mass of air being consumed by the engine 12. This is then used by multiplying the mass flow rate of air with the Oxygen content expressed as a decimal fraction to calculate the mass of Oxygen entering and leaving the particulate filter 36.

The second stage is to subtract the second mass flow rate of Oxygen from the first flow rate of Oxygen to provide a signal indicative of the actual mass per second of Oxygen being consumed in the particulate filter 36.

The next step 210 is to perform signal conditioning on the signal indicative of the mass of oxygen consumed per s-coní. The signal conditioning may comprise of several -,ta-es but in the example shown comprises of filtering of the signal to remove- noise and the correction of any steady t-ate errors.

At step 215, the conditioned signal indicative of the mass of Oxygen being consumed per second is integrated over time to produce a signal indicative of the total mass of Oxygen consumed during r-egeneration.

En more detail, at step 130 an integrator embedded as -.trt. -t t:'l? electronic controller 28 is started which sums the signal indicative of the mass of Oxygen being consumed per second in the particulate filter 36 during regeneration.

This integration continues until regeneration is determined at step 150 to have ceased. The output from the integration step 215 is a value equating to the estimated mass of Oxygen consumed during regeneration of the particulate filter.

The final step 220 is to convert the total mass of Oxygen consumed into a total mass of soot burnt. This is done in three stages.

The first stage 221 is to convert the mass of Oxygen into an equivalent number of moles of Oxygen. This is done by dividing the mass figure by the molecular mass of Oxygen which, as is well known in the art, is 32.

The second stage 222 is to estimate an equivalent number of moles of soot for the moles of Oxygen consumed.

Because it is known that both CO and CO2 are given off during regeneration, a generation combustion equation can be written as: C + O2 > 2( -0.5)CO2 +(1- )CO from which one would expect the molar ratio of O2 to C to fall somewhere between unity and 0.5. The molar ratio can be calculated by taking the molecular mass of O2 as 32 (J and the atomic mass of C as 12.

Because it is known that soot contains some hydrogen and a common empirical formula is C1oH or C,H for the carbonaceous fraction. It seems reasonable to adopt a if, limiting condition of CH2 for the organic fraction, as this corresponds to the fuel. As these two fractions cannot be resolved, it is more appropriate to write CyH for the total - 17 particulate, with y falling somewhere between (say) 9 and 0.5. High and low values correspond, respectively, to particulate that are predominantly carbonaceous and those which are predominantly organic. The point of this additional consideration is that hydrogen will be oxidised to H2O and thus account for some oxygen as well and so this needs to be taken into account as in equation (1).

0 CvH+(XtY+4)02)2(a-2)yCO2 +2(1-)yCO±- 2O (1) where: cx=1 if only CO2 is formed; and cx=0.5 if only CO is formed. from which it can be seen that by dividing the number of moles of Oxygen

by:- 1: aby+4) an equivalent number of moles of soot having the formula CyH can be estimated.

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The number of moles of soot are then converted into an equivalent mass of soot by multiplying the number of moles with a figure indicative If the estimated molecular weight of the soot. The estimation for this molecular weight will depend upon the way in which the engine has been operated but will range from C,,,H if the soot is expected to be wet JO t: C,- if dry soot is predicted.

The estimated mass of soot burnt is then stored in memory at step 150 and at step 160 is output from the soot e Carnation.,ystem to another system or process for further up. - 18

As an alternative to steps 221 to 223 the number of moles of Oxygen consumed can be converted directly into a mass of soot burnt by using a look up table stored in the memory of the electronic controller 28 and an estimated soot composition based upon the duty cycle of the engine. A typical chart is shown in Fig.5 from which it can be seen that once a number of moles of Oxygen is known and a composition is estimated it is relatively straight forward to produce a figure indicative of burnt soot.

Therefore in summary the inventors have produced a

system and method that can be used in-situ on a motor vehicle to provide an estimate of the soot burnt in a particulate filter during regeneration. This figure of soot burnt can then be used to provide a more accurate estimation of when next to regenerate the particulate filter thereby reducing the risk of premature or late regeneration.

It will be appreciated by those skilled in the art that although the invention has been described by way of example with reference to a number of specific embodiments it is not limited to these embodiments and that various alternative embod meets or modifications to the disclosed embodiments could be made without departing from the scope of the invention. - 19

Claims

1. A method for estimating the mass of soot burnt in a particulate filter processing exhaust gasses from an internal combustion engine during regeneration of the particulate filter, the method comprising estimating the mass of Oxygen consumed in the particulate filter during regeneration of the particulate filter and using the estimated mass of Oxygen consumed to calculate an estimated To mass of soot burnt in the particulate filter.

2. A method as claimed in claim 1 wherein the method further comprises determining when regeneration has started and determining when regeneration has finished and is estimating the soot burnt during the intervening period.

3. A method as claimed in claim 2 wherein regeneration is determined to have started when the temperature of the exhaust gases entering the particulate filter exceed a predetermined temperature.

4. A method as claimed in claim 3 wherein regeneration is determined to have finished when the temperature of the exhaust gases entering the particulate 2r, filter fall below the predetermined temperature.

5. A method as claimed in any of claims 2 to 4 wherein regeneration is determined to have finished when a pressure drop across the particulate filter falls below a Bra predetermined level.

6. A method as claimed in any of claims 2 to 5 wherein regeneration is determined to have finished when a predetermined period of time has elapsed since the it, regeneration was determined to have started. -

7. A method as claimed in any of claims 1 to 6 wherein the method further comprises converting a sensed air fuel ratio of the exhaust gases entering the particulate filter into a first Oxygen content and converting a sensed air/fuel ratio of the exhaust gases exiting the particulate filter into a second Oxygen content.

8. A method claimed in any of claims 1 to 6 wherein the method further comprises measuring a first Oxygen content of the exhaust gases entering the particulate filter and measuring a second Oxygen content of the exhaust gases exiting the particulate filter.

9. A method as claimed in claim 7 or in claim 8 wherein the method further comprises measuring the mass air flow entering the engine and using the measured mass air flow to convert the first Oxygen content into a first Oxygen mass flow rate and the second Oxygen content into a second Oxygen mass flow rate and subtracting the second Oxygen mass flow rate from the first Oxygen mass flow rate to produce an estimated mass of Oxygen being consumed in the particulate filLer per second.

10. A method as claimed in claim 9 wherein the method further comprises integrating during the period when regeneration is determined to be occurring the mass of O,ygen being consumed in the particulate filter per second to provide an estimatectrnass of Oxygen consumed during regeneration.

il.. A method as claimed in claim 10 wherein the method further comprises converting the estimated mass of Oxygen consumed during regeneration into an estimated mass of soot turret Turing regeneration.

12. A method a'; claimed in claim 11 wherein the method lrt.her,-ompri. ses using the estimated mass of soot burnt - 21 during regeneration to provide an improved estimate of when regeneration of the particulate filter should next occur.

13. A method as claimed in claim 11 or in claim 12 wherein converting the mass of Oxygen consumed during regeneration into an estimated mass of soot burnt during regeneration further comprises converting the mass of Oxygen into an equivalent number of moles of Oxygen and using the number of moles of Oxygen to estimate an equivalent number 0 of moles of soot burnt and estimating a mass of soot burnt from the estimated number of moles of soot burnt.

14. A system for estimating the mass of soot burnt in a particulate filter connected to an exhaust stream from an internal combustion engine during regeneration of the particulate filter, the system comprising a mass air flow sensor to measure the mass flow rate of air entering the engine and supply a signal indicative of the mass air flow rate to an electronic controller, a first sensor to measure the Oxygen content of the exhaust gasses entering the particulate filter and supply a signal indicative of such content to the electronic controller, a second sensor to measure the Oxygen content of the exhaust gases exiting the particulate filter and supply a signal indicative of such content to the electronic controller, a temperature sensor to send a signal to the electronic controller indicative of the temperature of the exhaust gasses entering the particulate filter, the electronic controller being operable to compare the sensed temperature of the exhaust gasses entering the particulate filter with a predetermined temperature and start a soot estimation routine to estimate the mass of soot being burnt in the particulate filter when tt-re temperature of the extlaust gasses entering the particulate filter exceeds the predetermined temperature indicating that regeneratioris occurring and terminate the sc<-, estimation routine when regeneration of the particulate filter i; determined le.' havr- ceased and is further operable - 22 to provide an output indicative of the total mass of soot burnt during the period when regeneration was determined to be occurring wherein the estimation of the mass of soot burnt during regeneration is based upon the difference in Oxygen content of the exhaust gasses entering and exiting the particulate filter and the mass air flow to the engine.

15. A system as claimed in claim 14 wherein regeneration is determined by the electronic controller to 0 have ceased when the sensed temperature of the exhaust gasses falls below the predetermined temperature.

16. A system as claimed in claim 14 or in claim 15 wherein regeneration is determined by the electronic controller to have ceased when a predetermined length of time has elapsed after starting the soot estimation routine.

17. A system as claimed in any of claims 14 to 16 wherein the system further comprises a differential pressure JO sensor to determine the pressure drop across the particulate filter and supply a signal indicative of the pressure drop to the electronic controller and regeneration is determined

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by the electronic controller to have ceased when the pressure drop across the particulate filter falls below a 2 A' predetermined limit.

18. A system as claimed in any of claims 14 to 17 wherein the first and second sensors are both Oxygen sensors and the electronic controller is operable to convert an >,o output from the first sensor into an equivalent Oxygen content and convert the output from the second sensor into an equivalent Oxygen content.

19. A system as claimed in claim 18 wherein the output If, from the first sensor is a voltage and the output from the second sensor is a voltage and the conversion is performed - 23 by means of a look up table stored in a memory of the electronic controller relating voltage to Oxygen content.

20. A system as claimed in claim 18 wherein the output from the first sensor is a voltage and the output from the second sensor is a voltage and the conversion is performed using a polynomial equation stored in a memory of the electronic controller representing a relationship between voltage and Oxygen content.

21. A system as claimed in any of claims 14 to 17 wherein the first and second sensors are both UEGO sensors and the electronic controller is operable to convert an air/fuel ratio derived from the first sensor into an equivalent Oxygen content and convert the air/fuel ratio determined from the second sensor into an equivalent Oxygen content.

22. A system as claimed in claim 21 wherein the conversion is performed by means of a look up table stored in a memory of the electronic controller relating air/fuel ratio to Oxygen content.

23. A system as claimed in claim 21 wherein the conversion is performed using a polynomial equation stored in a memory of the electronic controller representing a relationship between air/fuel ratio and Oxygen content.

24. A system as claimed in any of claims 14 to 23 wherein the electronic controller is operable to use the mass airflow to the engine to convert an Oxygen content determined by the first sensor into a first Oxygen mass flow rate and to convert an Oxygen content determined by the s->c:-onc] sensor into a second Oxygen mass flow rate and is further operable to subtract the second Oxygen mass flow rate from the first Oxygen mass flow rate to provide a - 24 signal indicative of the estimated mass of Oxygen being consumed in the particulate filter per second.

25. A system as claimed in claim 24 wherein the electronic controller is further operable to integrate during the period when the soot estimation routine is operating the signal indicative of the mass of Oxygen being consumed in the particulate filter per second to provide a signal indicative of the estimated mass of Oxygen consumed during regeneration.

26. A system as claimed in claim 25 wherein the electronic controller is further operable to convert the estimated mass of Oxygen consumed during regeneration into a signal indicative of the estimated mass of soot burnt during regeneration.

27. A system as claimed in claim 26 wherein the electronic controller is further operable to use the signal indicative of the estimated mass of soot burnt during regeneration to estimate when regeneration of the particulate filter should next occur.

28. A system substantially as described herein with reference to the accompanying drawing.

29. A method substantially as described herein with reference to the accompanying drawing.