Abstract:
A self-adapting control method for an exhaust system for internal combustion engines with controlled ignition, including an engine, a pre-catalyst, a trap for the collection of nitrogen oxides having a maximum initial capacity and a maximum available capacity not greater than this maximum initial capacity and a linear oxygen sensor disposed downstream of the trap for the collection of nitrogen oxides and generating at least a downstream composition signal substantially proportional to a downstream oxygen titer. The method comprises the stages of carrying out at least one regeneration process and carrying out at least one desulphurisation process of the trap for the collection of nitrogen oxides and also the stage of estimating, at least after each regeneration process, the maximum available capacity as a function of the maximum initial capacity and the downstream composition signal.

Description:
The present invention relates to a self-adapting control method for an exhaust system for internal combustion engines with controlled ignition. 
     BACKGROUND OF THE INVENTION 
     It is known that the composition of the exhaust gases produced in controlled ignition engines (for instance in petrol or gas engines in which the combustion of the air/fuel mixture is triggered, following command by the control system of the engine, by the ignition of a spark at a predetermined moment), depends, among other things, on the composition of the air/fuel mixture that is injected into the cylinders. These engines can in particular operate using a lean fuel mixture, i.e. having a ratio (A/F) greater than the stoichiometric ratio (A/F) ST , or, in an equivalent manner, having a titre λ, defined by the ratio (A/F)/(A/F) ST , greater than 1. In these circumstances, the exhaust gases form a highly oxidising atmosphere as a result of which a normal three-way catalyst (TWC) is not sufficient to remove the nitrogen oxide component NOx produced during combustion. As shown in FIG. 1, the efficiency of removal of nitrogen oxides η NOx  for a normal three-way catalyst is very high and close to 1 when the engine operates with a rich air/fuel mixture (having a ratio (A/F) lower than the stoichiometric ratio (A/F) ST  or, in an equivalent manner, a titre λ lower than 1), but deteriorates rapidly for values of the ratio (A/F) that are greater than the stoichiometric ratio (A/F) ST . Vice versa, the efficiency of removal of carbon monoxide η co , and, respectively, of non-combusted hydrocarbons η HC  is low in the presence of a rich air/fuel mixture and close to 1 for a lean air/fuel mixture. 
     A solution that is commonly used is to dispose, downstream of a three-way pre-catalyst, a main catalyst formed by a trap able to absorb and store the nitrogen oxides (a so-called NOx TRAP). When the trap is saturated, however, it is no longer able to perform this function and must therefore be emptied by means of a regeneration process which consists in creating, within the trap, an atmosphere such as to give rise to reduction reactions of the nitrogen oxides NOx. Molecular nitrogen N 2 , steam and other non-polluting products are released during these reactions. The reducing atmosphere is obtained by causing a mixture of exhaust gases composed chiefly of carbon monoxide CO and non-combusted hydrocarbons HC and substantially free from nitrogen oxides NOx to flow into the trap, as is the case when the engine operates with a rich air/fuel mixture. In this case, there is an overproduction of carbon monoxide CO and non-combusted hydrocarbons HC that the three-way catalyst is not able to remove as a result of the fact that it is not very efficient in the presence of a rich mixture, while the emissions of nitrogen oxides NOx are drastically reduced. The exhaust gas mixture thus produced reacts with the nitrogen oxides NOx present in the trap, thereby emptying it. During the regeneration process, moreover, the titre downstream of the trap is substantially stoichiometric. 
     The use of traps of the type described above raises a further problem connected with the fact that they also store sulphur oxides SOx. Even though the capture of sulphur oxides SOx is a slower process than the capture of nitrogen oxides NOx, provision must nevertheless also be made for desulphurisation cycles in order to maximise the available capacity and the efficiency of the trap. 
     Moreover, in order to ensure that the trap is highly efficient and to limit the consumption of fuel and polluting emissions, these regenerations and desulphurisations must be carried out according to well defined strategies. 
     The control systems available at present are based on units provided with a first oxygen sensor (LAMBDA sensor of linear type) disposed upstream of the catalyst TWC and a second oxygen sensor (LAMBDA sensor of on/off type) disposed downstream of the trap. The regeneration strategies currently used estimate the degree of filling of the trap solely from mapping of the engine and from physical and mathematical models, to whose parameters predetermined values are assigned at the calibration stage. The efficiency of control depends, among other things, on the accuracy of these values which cannot, however, subsequently be automatically updated during the operation of the system. 
     The systems described above are disadvantageous as they are not able to take account of any deviations with respect to nominal operating conditions. In particular, the performances of the various components are not constant over time, but show drifts due, for instance, to ageing or to the onset of malfunctions, as a result of which the values of the parameters of the physical and mathematical models set during calibration are not longer adapted correctly to describe the state of the system. In these circumstances, therefore, conventional regeneration strategies do not guarantee that measures to reset the efficiency of the trap are carried out when they are actually necessary. Consequently, it may be case that the trap remains saturated for longer than it should before it is emptied, with a substantial increase in polluting emissions from the vehicle. Moreover, the duration of the regenerations is also predetermined and cannot be modified if it proves to be inadequate. 
     SUMMARY OF THE INVENTION 
     The object of the present invention is to provide a self-adapting control method which is free from the drawbacks described above and which is, in particular, able to carry out a regeneration strategy on the basis of an estimation of the real conditions of the system. 
     The present invention therefore relates to a self-adapting control method for an exhaust system for internal combustion engines with controlled ignition, this exhaust system comprising an engine, a pre-catalyst, means for capturing nitrogen oxides having a maximum initial capacity and a maximum available capacity not greater than this maximum initial capacity, oxygen sensor means disposed downstream of the means for capturing nitrogen oxides and generating at least one downstream composition signal, this method comprising the stages of: 
     a) carrying out at least one process of regeneration of the means for capturing nitrogen oxides, 
     b) carrying out at least one process of desulphurisation of the means for capturing nitrogen oxides, 
     characterised in that it further comprises the stage of: 
     c) estimating, at least after each such process of regeneration of the means for capturing nitrogen oxides, the maximum available capacity as a function of the maximum initial capacity and the downstream composition signal, and in that the downstream composition signal is substantially proportional to a downstream oxygen titre. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is described in further detail below with reference to a preferred embodiment thereof, given purely by way of non-limiting example, made with reference to the accompanying drawings, in which: 
     FIG. 1 shows efficiency curves in a three-way catalyst; 
     FIG. 2 is a simplified block diagram of a control system of the present invention; 
     FIG. 3 is a more detailed block diagram relating to a part of the system of FIG. 2; 
     FIGS. 4 to  7  are flow diagrams of the control method of the present invention; 
     FIG. 8 shows possible curves of the downstream titre of the trap during a process of regeneration in the system of FIG. 2; 
     FIG. 9 is a detailed block diagram of a part of a system of the present invention according to a second embodiment; 
     FIG. 10 shows a flow diagram relating to the second embodiment of the control method of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In FIG. 1, a control system for the exhaust of an internal combustion engine  2  with controlled ignition is shown overall by  1 . The engine  2  is connected, via a first exhaust duct section  3   a,  to a pre-catalyst  4 , for instance a catalyst TWC. A second exhaust duct section  3   b  connects an output of the pre-catalyst  4  to an input of a trap  5  for the collection of nitrogen oxides NOx. The trap  5  is in particular composed of cells adapted to absorb and store molecules of nitrogen oxides NOx. 
     A first sensor of the concentration of oxygen in the exhaust gases, hereafter referred to as the upstream sensor  6 , and a second sensor of the concentration of oxygen in the exhaust gases, hereafter referred to as the downstream sensor  7 , are disposed upstream of the pre-catalyst  4  and, respectively along a third duct section  3   c  downstream of the trap  5 . Advantageously, both the oxygen concentration sensors are sensors of the linear LAMBDA or UEGO type. The sensors  6  and  7  generate an upstream composition signal V 1 , representative of an upstream titre λ M  at the output from the engine  2  and, respectively, a downstream composition signal V 2 , representative of a downstream titre λ V  at the output from the trap  5 . 
     A temperature sensor A is disposed along the second exhaust duct section  3   b  and generates a temperature signal V T . 
     The control system  1  further comprises a control unit  10  which receives as input the upstream and downstream composition signals V 1  and V 2  and the temperature signal V T  as well as a plurality of engine-related parameters which are not shown for the sake of simplicity, and supplies as output a plurality of operating quantities for respective engine control variables calculated in a known manner and not shown. 
     A block diagram relating to the control unit  10  is shown in greater detail in FIG.  2 . 
     An engine/pre-catalyst block  11  receives as input the downstream composition signal V 1  and a plurality of engine-related parameters and supplies as output an estimate of the composition of the exhaust gases at the output of the pre-catalyst  4 . In particular, three quantities relating to the exhaust gases being output from the pre-catalyst  4  are calculated: an upstream quantity of nitrogen oxides NOx M , an upstream quantity of carbon monoxide CO M  and an upstream quantity of non-combusted hydrocarbons HC M . These quantities take account of the efficiency of the pre-catalyst  4  in the respective removal of nitrogen oxides η NOx , carbon monoxide η CO  and non-combusted hydrocarbons η HC  as a function of the upstream titre η M  according to the curves shown in FIG.  1 . 
     The upstream quantities of nitrogen oxides NOx M , carbon monoxide CO M  and non-combusted hydrocarbons HC M  are supplied as input to a trap block  12  which also receives an estimate of the maximum capacity C MD , as will be explained below, the temperature signal V T  and a fuel flow value F. The trap block  12  which, as will be described in detail below, contains a model of the processes of capture of nitrogen oxides and sulphur by the trap  5 , calculates and supplies as output a capture efficiency NOx EFF , a quantity of nitrogen oxides stored NOx ST , a quantity of nitrogen oxides exchanged NOx CAP  and a quantity of sulphur oxides stored SOx ST . 
     The outputs from the trap block  12  are supplied as input to a regeneration control block  15 , which implements a regeneration control procedure and a desulphurisation control procedure, described in detail below, to check for the conditions that make it necessary to carry out a regeneration and/or a desulphurisation. The regeneration control block  15  also generates a plurality of signals that are supplied to a system supervisor, not shown for the sake of simplicity. In particular, the regeneration block  15  supplies a regeneration request signal RRQ, a desulphurisation request signal DRQ and a heating request signal HRQ. These signals are of a logic type and can therefore assume a logic value “TRUE” or a logic value “FALSE”. 
     The regeneration request signal RRQ is supplied as input to a parameter estimation block  16  which also receives the downstream composition signal V 2  and, as will be explained in detail below, implements an algorithm updating certain parameters of the models contained in the trap block  12 . In particular, the parameter estimation block  16 , when necessary, estimates the maximum available capacity C MD  and supplies it as input to the trap block  12  and to a diagnostic block  17 . Moreover, the parameter estimation block  16  generates a regeneration discontinuation signal REND, of logic type, that is supplied as input to the regeneration control block  15 . 
     With reference to FIG. 4, the diagnostic block  17  checks the state of ageing of the trap  5 , comparing the maximum available capacity C MD  with a threshold capacity C TH  (block  50 ). If the maximum available capacity C MD  is lower (output YES from the block  50 ), the diagnostic block  17  generates as output an error signal E (block  60 ), of logic type, setting it to the logic value “TRUE” in order to indicate a malfunction. 
     In detail, the calculation of the capture efficiency NOx EFF  and of the quantity of nitrogen oxides stored NOx ST  carried out in the trap block  12 , is based on an estimate of a residual capacity C R  of the trap  5  and on the upstream quantities of nitrogen oxides NOx M , carbon monoxide CO M  and non-combusted hydrocarbons HC M  calculated by the engine/pre-catalyst block  11 . The residual capacity C R  is deduced from the following equations: 
     
       
           C   MD   =K   AG    C   M   (1) 
       
     
     
       
           C   L   =C   MD   −SOx   ST   (2) 
       
     
     
       
           C   R   =C   D   −NOx   ST   (3) 
       
     
     in which C M  is the maximum capacity of the trap  5 , C MD  is the maximum available capacity and C L  is the free capacity. In particular, the maximum capacity C M  and the maximum available capacity C MD  represent the maximum quantities of nitrogen oxides NOx that the trap  5  can store at the beginning of its life and, respectively, at the current moment, while the free capacity C L  is that part of the maximum available capacity C MD  not occupied by sulphur oxides SOx. The maximum available capacity C MD  is not greater than the maximum capacity C M  as, at a given moment, a proportion of the cells making up the trap  5  is not able to capture molecules of nitrogen oxides NOx, for two main reasons. Firstly, some cells are irreversibly damaged, as a result of ageing, for instance because they are obstructed by solid deposits. The coefficient of ageing K AG  which appears in equation (1) and is updated by an adaptation algorithm described in detail below, takes account of the reduction of the maximum capacity C M  due to the wear of the trap  5 . Secondly, the trap  5  can also store sulphur oxides SOx, as discussed above. Consequently, a proportion of the cells of the trap  5 , corresponding to the quantity of sulphur oxides stored SOx ST , is temporarily unavailable to interact with the nitrogen oxides NOx until a desulphurisation process is carried out. The residual capacity C R , lastly, represents the cells of the trap  5  that have not captured any molecules and are therefore actually available to interact with molecules of nitrogen oxides NOx. 
     The quantity of nitrogen oxides stored NOx ST  is calculated on the basis of the following equations: 
     
       
           NOx   CAP   =NOx   M   K   TN   K   CRN    K   NOx   (4) 
       
     
     
       
           NOx   CO   =CO   M   K   T1   K   CO   (5) 
       
     
     
       
           NOx   HC   =HC   M   K   T1   K   HC   (6) 
       
     
     
       
           NOx   ST   =NOx   OLD   +NOx   CAP   −NOx   CO   −NOx   HC   (7) 
       
     
     with the constraint: 
     
       
           NOx   ST   ≦C   D   (8) 
       
     
     In equations (4), (5), (6) and (7), NOx CAP  is the fraction of the upstream quantity of nitrogen oxides NOx M  that is captured by the trap  5  at the current moment, NOx OLD  is the quantity of nitrogen oxides stored up to the current moment, and NOx CO  and NOx HC  represent the fractions of nitrogen oxides present in the trap  5  which, at the current moment, are reacting in a known manner with carbon monoxides and, respectively, non-combusted hydrocarbons, thereby freeing the corresponding cells. Moreover K TN  and K T1  are coefficients that take account of the temperature dependence of the reaction to capture nitrogen oxides NOx and, respectively, of the reduction reactions of the nitrogen oxides NOx which take place in the trap  5  and are calculated in a known manner on the basis of the temperature signal V T ; K CRN  is a coefficient of residual capacity that modifies the probability of capture of individual molecules of nitrogen oxides NOx as a function of the residual capacity C R ; K NOx  is a coefficient of absorption of nitrogen oxides NOx by the trap  5 , and K CO  and K HC  are empirical correction coefficients that are determined experimentally. 
     The capture efficiency NOx EFF  is given by the following equation: 
     
       
           NOx   EFF   =NOx   CAP   /NOx   M   (9) 
       
     
     The quantity of sulphur oxides stored SOx ST  is calculated by means of a model similar to that illustrated by equations (3) to (6). The following equations in particular apply: 
     
       
           SOx   CAP   =SOx   M   K   TS   K   CRS   K   SOx   (10) 
       
     
     
       
           SOx   CO   =CO   M   K   T2   K   CO ′  (11) 
       
     
     
       
           SOx   HC   =HC   M    K   T2    K   HC ′  (12) 
       
     
     
       
           SOx   ST   =SOx   OLD   +SOx   CAP   −SOx   CO   −SOx   HC   (13) 
       
     
     The symbols have the same meaning as the corresponding symbols of equations (4) to (7). 
     In detail, SOx M  is an upstream quantity of sulphur oxides entering the trap  5  and calculated by multiplying the fuel flow F by an average concentration value of sulphur in petrols, while SOx OLD  is the quantity of sulphur oxides stored up to the current moment. In addition, SOx CO  and SOx HC  represent the fractions of sulphur oxides present in the trap  5  which, at the current moment, are reacting in a known manner with carbon monoxide and, respectively, non-combusted hydrocarbons, thereby freeing the corresponding cells. The coefficients K TS  and K T2  take account of the temperature dependence of the reaction to capture sulphur oxides SOx and, respectively, of the reduction reactions of the sulphur oxides SOx which take place in the trap  5  and are calculated in a known manner on the basis of the temperature signal V T ; K CRS  is a coefficient of residual capacity that modifies the probability of capture of a molecule of sulphur oxides SOx as a function of the residual capacity C R ; K SOx  is a coefficient of absorption of sulphur oxides SOx by the trap  5 , and K CO ′ and K HC ′ are empirical correction coefficients that are determined experimentally. 
     With reference to FIGS. 5 and 6, the regeneration and, respectively, desulphurisation control procedures implemented by the regeneration control block  15  will now be described. 
     As shown in FIG. 5, at the beginning of the regeneration control procedure, the quantity of nitrogen oxides stored NOx ST  and the capture efficiency NOx EFF  are calculated according to equations (7) and (9) respectively (block  100 ). 
     A test is then carried out to check whether the capture efficiency NOx EFF  is greater than a predetermined threshold capture efficiency value NOx EFF * (block  105 ). If so, the regeneration control procedure is discontinued (block  170 ), otherwise a regeneration request is made, in particular by setting the regeneration request signal RRQ to the logic value “TRUE” (block  110 ). Subsequently, a sequence of four tests is conducted cyclically until at least one of the conditions examined is satisfied. In detail, it is checked whether the quantity of nitrogen oxides stored NOx ST  is lower than a threshold quantity of nitrogen oxides stored NOx ST * (block  120 ); it is checked whether the value of the downstream titre λ V  has fallen significantly below 1, in particular by checking whether a deviation Δ, given by the time integral, for a regeneration time τ N  that has elapsed from the beginning of regeneration, of a quantity obtained on the basis of a known function of the difference (1−λ V ), is greater than a threshold value Δ TH  (block  130 ); it is therefore checked whether the regeneration time τ N  is greater than a safety regeneration time τ DN  (block  140 ) and, lastly, whether a discontinuation of regeneration has been externally requested, for instance by checking whether the regeneration discontinuation signal REND has been set to the logic value “TRUE” (block  150 ). In all four cases, if the condition examined is verified the regeneration is discontinued (block  160 ) and the regeneration control procedure is terminated (block  170 ). If, however, the outcome of the check is negative, after each of the tests relative to the blocks  120 ,  130  and  140 , the subsequent test is carried out, while after the test corresponding to the block  150  the quantity of nitrogen oxides stored NOx ST  is calculated again, according to the equation (7) (block  155 ) and there is therefore a return to the block  120 . 
     With reference to FIG. 6 a,  the desulphurisation control procedure starts with the calculation of the quantity of sulphur oxides stored SOx ST , according to equation (13) (block  200 ). 
     A test is then carried out to check whether the conditions for desulphurisation have been met (block  210 ), as illustrated in detail below. If so, a desulphurisation request is made, setting the desulphurisation request signal DRQ to the logic value “TRUE”, (block  250 ), otherwise the desulphurisation control procedure is concluded (block  290 ). 
     Following the desulphurisation request (block  250 ), a test of the emptying of the trap  5  is carried out to check whether, during desulphurisation, the quantity of sulphur oxides stored SOx ST  has fallen below a lower threshold SOx INF  (block  260 ). If so, the desulphurisation control procedure is terminated (block  290 ), otherwise it is checked whether a desulphurisation time τ S  that has elapsed since the beginning of desulphurisation is greater than a safety desulphurisation time τ DS  (block  270 ). If this is the case, the desulphurisation control procedure is concluded (block  290 ), otherwise the quantity of sulphur oxides stored SOx ST  is calculated again in accordance with equation (13) (block  280 ) and a return is made to carry out the test of the emptying of the trap  5  (block  260 ). 
     As shown in FIG. 6 b,  checking of the conditions for the conduct of a desulphurisation starts with a test to check whether the quantity of sulphur oxides stored SOx ST  is greater than a first upper threshold SOx SUP1  (block  215 ). 
     If not, the desulphurisation control procedure is concluded (block  290 , FIG. 6 a ), otherwise a second test is conducted to check whether the temperature of the exhaust gases T at the input of the trap  5  exceeds a threshold temperature T S  (block  220 ). 
     If this is the case, a desulphurisation request is generated (block  250 , FIG. 6 a ) and, in the opposite case, the quantity of sulphur oxides stored SOx ST  is compared with a second upper threshold SOx SUP2  (block  225 ), greater than the first upper threshold SOx SUP1 . 
     If the quantity of sulphur oxides stored SOx ST  is greater than the second upper threshold SOx SUP2  (output YES from the block  225 ), heating of the trap  5  is requested, by setting the heating request signal HRQ to the logic value “TRUE” (block  230 ), otherwise (output NO from the block  225 ) the test to check the temperature of the exhaust gases T is again carried out (block  220 ). 
     Following the heating request (block  230 ), a new test is carried out to check whether the temperature of the exhaust gases T has exceeded the threshold temperature T S  (block  235 ). 
     If this is the case (output YES from the block  235 ), the heating of the trap  5  is discontinued, by setting the heating request signal HRQ to the logic value “FALSE” (block  240 ) and the desulphurisation request is generated (block  250 , FIG. 6 a ) If, in contrast, the temperature of the exhaust gases T is lower than the threshold temperature T S  (output NO from the block  235 ), a further test checks whether a heating time τ H , that has elapsed from the commencement of heating of the trap  5  is greater than a safety heating time τ DH  (block  245 ). 
     If so, the desulphurisation procedure is discontinued (block  290 , FIG. 6 a ), otherwise the heating request for the trap  5  is confirmed (block  230 ). 
     With reference to FIG. 7, the updating algorithm implemented by the parameter estimation block  16  will be described below; during the regeneration stages, this block  16  checks the accuracy of the estimate of the maximum available capacity C MD  and, if necessary, updates its value by calculating an updated coefficient of ageing K AGN , which is used in equation (1) in place of the coefficient of ageing K AG . 
     In particular, the flow of carbon monoxide downstream CO V  should be zero during the regeneration, since all the carbon monoxide entering the trap  5  reacts with the stored nitrogen oxides NOx, until they are completely eliminated. As a result of the deterioration to which the trap  5  is subject with use, it may nevertheless be the case that the estimate of the maximum available capacity C MD  used in the model for the calculation of the quantity of nitrogen oxides stored NOx ST  is greater than the actual capacity of the trap  5 . In these circumstances, the nitrogen oxides NOx stored in the trap  5  are completely eliminated before the regeneration control block  16  concludes the regeneration process underway. Consequently, the carbon monoxide produced by the engine  2  passes through the trap  5  and gives rise to a flow of carbon monoxide downstream CO V  which is not zero, causing, at the output from the trap  5 , the downstream titre λ V  to deviate from the stoichiometric value. At a time τ O  which precedes a regeneration completion instant τ R  and is indicative of the fact that all the nitrogen oxides NOx stored have been eliminated, the downstream sensor  7  detects a reduction of the downstream titre λ V  (reference is made to FIG. 8 in which the downstream titre λ V  is shown by a dashed line, and the upstream oxygen titre λ M  is shown by a continuous line). On the basis of the downstream composition signal V 2  provided by the downstream sensor  7  and a measurement or estimate of the flow of exhaust gases G V , that can be obtained in a known manner, it is possible to ascertain the flow of carbon monoxide downstream CO V  and, by integrating the latter over time, a downstream carbon monoxide mass CO VTOT  which represents an index of the error committed in the estimate of the maximum available capacity C MD . By comparing the downstream carbon monoxide mass CO VTOT  with a threshold mass CO TH  it is possible to decide whether it is necessary to adapt the current value of the maximum available capacity C MD . 
     In detail, the updating algorithm starts with a test to check whether a regeneration process is underway, for instance by monitoring whether the regeneration request signal RRQ is set to the logic value “TRUE” and, at the same time, whether the regeneration discontinuation signal REND is set to the logic value “FALSE” (block  300 ). 
     If this is not the case, the updating algorithm is terminated (block  360 ) in the opposite case, the flow of carbon monoxide downstream CO V  is calculated (block  310 ), according to a known function of the flow of exhaust gases G V  and the downstream titre λ V . 
     The downstream carbon monoxide mass CO VTOT  is then calculated by integrating over time the flow of carbon monoxide downstream CO V  (block  320 ) and compared with the threshold mass CO TH  (block  330 ). If the downstream carbon monoxide mass CO VTOT  is lower than the threshold mass CO TH  (output NO from the block  330 ) the test is again carried out to check whether a regeneration process is underway (block  300 ). If not (output YES from the block  330 ), the value of the maximum available capacity C MD  is corrected by means of an adaptation of the coefficient of ageing K AG  (block  340 ). In particular, the updated coefficient of ageing K AGN  is calculated by decreasing the coefficient of ageing K AG  by a predetermined value K DEC  and then used to calculate an updated value of the maximum available capacity C MD  according to the equation: 
     
       
           C   MD   =K   AGN    C   M   (1′) 
       
     
     The regeneration process is then discontinued, by setting the regeneration discontinuation signal REND to the logic value “TRUE” (block  350 ) and the parameter updating algorithm is terminated (block  360 ). 
     In a second embodiment, which will now be described with reference to FIG. 9, the method is based on a system in which the downstream sensor  5  is formed by a sensor of nitrogen oxides NOx rather than by a sensor of UEGO type. Since the sensor of nitrogen oxides NOx also contains a linear oxygen sensor, it is able to provide as output a signal representative of the concentration of nitrogen oxides NOx and also of the downstream titre λ V . 
     The simplified block diagram of FIG. 9 shows a control unit  10 ′ similar to the control unit  10 , except that a parameter estimation block  16 ′ also supplies as output an updated coefficient of absorption K NOxN  which is supplied as input to the trap block  12 . 
     With reference to FIG. 10, the parameter estimation block  16 ′ calculates a downstream concentration of nitrogen oxides NOx V  (block  400 ), as a function of the quantity of nitrogen oxides upstream NOx M  and the quantity of nitrogen oxides exchanged NOx CAP  and uses it, together with a measured concentration of nitrogen oxides NOx MIS , to calculate an estimation error NOx ERR  (block  410 ) given by the equation: 
     
       
           NOx   ERR   =NOx   V   −NOx   MIS   (14) 
       
     
     The estimation error NOx ERR  is then used to calculate a correction term ΔK NOx  (block  420 ) which is added to the coefficient of absorption K NOx  to obtain the updated coefficient of absorption K NOxN  (block  430 ). 
     The proposed method has the following advantages. 
     Firstly, the possibility of updating the value of the maximum available capacity C MD  by using the curve of the downstream composition signal V 2  during regeneration makes it possible more accurately to estimate the degree of filling of the trap. Consequently, it is possible precisely to determine the instants of onset of conditions that make it necessary to carry out a regeneration process, irrespective of the state of ageing of the trap  5 . This avoids the possibility that, during operation, the trap  5  remains saturated for unacceptable periods and therefore reduces the risk of substantial emissions of nitrogen oxides NOx. Moreover, the duration of the regeneration process may be calculated such that this process is not protracted beyond the moment in which the trap  5  is actually emptied, so as to avoid emissions of non-combusted hydrocarbons HC and carbon monoxide CO, as discussed above, as well as higher consumption. 
     It is also advantageous, particularly during the performance of the parameter updating algorithm, to use a sensor of UEGO type downstream of the trap  5 . This sensor provides an accurate measurement of the exhaust titre, on the basis of which it is possible to determine the quantity of carbon monoxide CO in the exhaust gases and therefore to find out in good time when emptying of the trap  5  has taken place. The information obtained by the UEGO sensor thus makes it possible to provide an efficient criterion for the updating of the maximum available capacity C MD . 
     According to the variant described, a further advantage lies in the use of a sensor of nitrogen oxides NOx. In this case, it is possible to check whether the model used for the calculation of the quantity of nitrogen oxides stored NOx ST  and the capture efficiency NOx EFF  is correct and, if necessary, to modify it by calculating the updated coefficient of absorption K NOxN . Consequently, the estimate of the degree of filling of the trap  5  is more reliable and the probability of polluting emissions is reduced. 
     It will lastly be appreciated that modifications and variations that do not depart from the scope of protection of the present invention may be made to the method as described.