FR2746142A1 - Monitoring system for catalytic converter fitted to vehicle exhaust - Google Patents

Abstract

The procedure provides monitoring of the operation of a catalytic converter mounted in the exhaust system from the internal combustion engine of a car. During certain operating modes of the engine, a chemical substance is stored within the catalytic converter, whilst in other operating modes this chemical substance is consumed. At each instant a determination is effected to establish the maximum amount of the chemical substance which can be stored within the converter, and also the amount which is currently stored. This information is derived by measuring the speed of rotation of the engine (N), the pressure at the inlet of the engine (P) and the speed of the exhaust gas passing through the converter. This information is supplied to a model of the engine operation in order to monitor the operating state of the catalytic converter.

Description

 The invention relates to internal combustion engines of motor vehicles equipped with a catalytic converter for treating exhaust gases.

 The invention relates more particularly to a motor vehicle, of the type in which the vehicle comprises an internal combustion engine equipped with a chemical catalytic converter for treating the exhaust gases downstream of the engine, of the type in which two probes are arranged in pipes upstream and downstream of the catalytic converter to each provide information to determine the amount of a chemical contained in the exhaust gas respectively upstream and downstream of the pot, the type in which for certain modes of Engine operation, the chemical is likely to be stored in the catalytic converter, while for other modes of operation, the previously stored chemical is consumed during chemical reactions of catalysis.

 Internal combustion engines make it possible to convert the energy resulting from the combustion of an air / fuel mixture into mechanical energy. If the initial mixture is determined according to the theoretical stoichiometric ratio, ie 1 gram of fuel per 14.7 grams of air, the products of combustion are carbon dioxide (CO2) and water ( H2O).

 However, in certain operating phases of the engine, the fuel mixture can be rich, that is to say have an excess of fuel, or poor, that is to say include an excess of air.

 In the case of a rich mixture, in the products of combustion, in addition to water and carbon dioxide, reductive molecules such as hydrocarbons (oc), carbon monoxide (CO) and hydrogen (H2).

In the case of a lean mixture, there is appearance in the exhaust gases of oxidizing molecules such as nitrogen oxides (NOx), and oxygen (2)
To limit the production of these polluting substances, it is therefore necessary that the engine operates as often as possible by burning a mixture dosed in stoichiometric proportions, but this is not always possible, nor even desirable.

 Indeed, when the engine is cold or when it is desired to obtain maximum power, for example during an acceleration of the vehicle, it is necessary to operate the engine with a rich mixture.

 On the contrary, during deceleration phases or when the need for power is low, it is advantageous to operate the engine with a lean mixture to reduce fuel consumption.

 For several years now, pollution control standards have required motor vehicles to use a catalytic converter designed to chemically treat exhaust gases to rid them of most of the polluting substances they contain.

 The catalytic converters generally used have the primary function of completing the combustion of the fuel mixture which is only incomplete within the engine. It is then a question of grouping on catalytic sites oxidizing molecules and reducing molecules present in the exhaust gases so that they combine in order to produce water and carbon dioxide. These sites are formed on a monolith which is a porous structure having a large area of contact with the exhaust gas passing through the pot, and which is coated with various chemical substances having catalytic properties.

 This function makes it possible to overcome the incomplete combustion inside the engine cylinders but only allows the polluting substances to be eliminated when the initial fuel mixture is a stoichiometric mixture.

 Also, the catalytic converters also have the ability to "store" oxygen atoms by oxidation of chemicals present in the catalyst, such as cerium.

 Thus, when the exhaust gases entering the catalytic converter are from the combustion of a lean mixture, they comprise excess oxygen atoms and these are stored in the catalytic converter. When the mixture burned in the engine is rich, the exhaust gases that arrive in the pot contain reducing molecules, but these can combine with the oxygen atoms, previously stored in the catalytic converter during a operating phase with a lean mixture, to produce water and carbon dioxide.

 The catalytic converter therefore has the function not only of promoting chemical reactions between substances contained in the exhaust gas, but it also has the function of being a buffer stock of oxygen molecules which makes it possible to regulate the composition of the gas emitted into the atmosphere at the outlet of the pot.

 However, the size of this buffer stock is not unlimited. If the engine runs for a long time burning a lean mixture, the catalytic converter becomes saturated with oxygen, that is to say, for example that all the cerium atoms have been oxidized. Excess oxygen atoms from the combustion will then be emitted into the atmosphere, combined with nitrogen atoms in the form of nitrogen oxides (NOx) due to the high temperature of these gases.

 On the other hand, if the engine runs for too long while burning a rich mixture, the oxygen atom storage of the catalytic converter is exhausted and there is then emission, in the air, of hydrocarbon molecules (HC) and carbon monoxide (CO).

 Thus, to limit the emission of pollutants and to make the best use of the capacities of the catalytic converter, it is necessary to control as precisely as possible the air / fuel ratio of the fuel mixture which is burned in the engine.

 It is known for this purpose to use a probe arranged in the exhaust gas circuit between the engine and the catalytic converter. Such a probe, generally of the "EGO" or "lambda" type, or of the "UEGO" type makes it possible to determine the oxygen content of the exhaust gases.

 Such a probe makes it possible to determine whether the fuel mixture introduced into the cylinder is rich or poor and this information is transmitted to a computer which controls the injectors of the engine to modify the amount of fuel injected accordingly.

 However, this corrective process ignores the chemical reactions that take place in the catalyst.

 However, the catalyst, like any chemical system, has characteristics that vary with time and temperature.

 In fact, the catalytic converters used only make it possible to obtain an effective action only from a threshold temperature. The catalytic converter is also subject to aging which corresponds to a decrease in the oxygen storage capacity and a decrease in the number of catalytic sites on which the harmful chemical substances can react with each other.

 Thus, the air / fuel ratio control devices having only one probe arranged upstream of the catalytic converter can not take into account these phenomena and it has been proposed to have a second probe downstream of the catalyst in the exhaust system .

 Such a probe also measures the richness of the exhaust gas at the outlet of the catalytic converter, that is to say in fact the comparison between the actual ratio between oxidizing molecules and reducing molecules and the stoichiometric ratio. This information is generally used to correct the control loop of the first probe.

 The second probe thus makes it possible to detect the presence of excess oxygen atoms in the exhaust gases which betray a saturated state of the buffer stock formed by the pot.

 Various methods have been proposed to cause at that moment a "purge" at least partial pot.

 However. All the devices using an upstream probe and a downstream probe only propose a corrective treatment that is triggered after the emission of a certain quantity of polluting substances.

 The invention therefore aims to provide a method of monitoring the operation of the catalytic converter that optimizes the control of the air / fuel ratio of the fuel mixture.

 For this purpose, the invention proposes a process of the type previously described, characterized in that, from the information provided by the probes, the maximum quantity of chemical substance that is likely to be stored in the pot is determined at each instant. catalytic, or stock size, and the amount of chemical that is actually stored in the pot, or stock status.

According to other features of the invention
- the size and state of the chemical stock are determined using a mathematical modeling of the chemical reactions occurring in the catalytic converter, and the modeling takes into account in particular the kinetics of the reactions in question, which depend on the concentrations of the chemical species that 'they bring into play, and gas flow through the catalytic converter
the method comprises a step of estimating the concentration of the chemical substance in the exhaust gas downstream of the catalytic converter and the state of the stock as a function, in particular, of a first value of the size of the stock, the estimated value; the concentration of the chemical in the exhaust gas downstream of the pot is compared with the value actually measured by the downstream probe, and the value of the first estimate of the stock size is corrected to cancel the difference between the values , estimated and measured, of the concentration of the chemical in the exhaust gas downstream of the catalytic converter
the chemical substance stored in the catalytic converter is oxygen and the process comprises the steps of
measure the rotational speed of the engine and the intake pressure of the fuel mixture in the engine
calculate the flow velocity of the exhaust gases in the catalytic converter as a function, in particular, of a modeling of the flow of gases in the exhaust system, of the engine rotation speed and of the intake pressure
measuring, with the probes, the concentration of oxygen in the exhaust gases upstream and downstream of the catalytic converter, and deduce standardized values so as to be negative or positive depending on the gases analyzed that reveal the fuel mixture which was burned had an excess or deficit of air compared to the stoichiometric ratio of combustion
deduce the concentration, in the exhaust gases downstream of the pot, of the oxidants and the reducing agents of the chemical reactions of storage and storage of oxygen according to the formulas
if v> O: Ox = v / K and Red = 0
if v <O: Ox = 0 and Red = - v / K '
where K and K 'are characteristic parameters of the downstream probe
to express the chemical kinetics and oxygen storage and de-storage reactions in the catalytic converter as a function of the measured concentrations of oxidants and reducing agents downstream of the pot and as a function of the size and state of the oxygen stock according to the following formulas
c, = k1 (Ox) a '(0.5 y - x) bi = C1 (x, y, v) a2 b2
and c2 = k2 (Red) a2 (0.5 y + x) b2 = C2 (y, v)
where k ,, a1, b1 and k2, a2, b2 are characteristic parameters of both reactions
write the law of variation over time of the estimated concentration of oxygen in the exhaust gases downstream of the pot
dz / dt = - V / L [z (t) - u (t)] + (K / 2e) [C1 (x, y, v) - C2 (x, yv)]
where L is the length of the site reactions monolith
and e is the void fraction of this monolith
write the law of variation of the state of the stock
dx / dt = C1 (x, y, v) - C2 (x, y, v)
define the law of variation over time of the adjustment of the size of the stock as a function of the error between the concentrations, calculated and measured, of oxygen downstream of the pot
dy / dt = - G f (t) [v (t) - z (t) j
where f (t) is a function of the sign of (v) and G is a parameter which makes it possible to determine the convergence speed of the method; and
solve the system of differential equations thus obtained to obtain the calculated values of x, y and z, applying a recursive method to cancel the difference between the calculated and measured values of the oxygen concentration downstream of the pot
the chemical substance stored in the catalytic converter is a nitrogen oxide.

 The invention also relates to a method for detecting the aging of a catalytic converter of a motor vehicle, of the type in which the catalytic converter is capable of storing a chemical substance present in the exhaust gas of the engine, comprising the step of comparing at a set point the maximum quantity of chemical substance that can be stored by the catalytic converter which is determined according to a monitoring method according to the teachings of the invention.

 The invention also relates to a method for controlling the air-fuel ratio of a fuel mixture for an internal combustion engine of a motor vehicle equipped with a catalytic converter for treating exhaust gases downstream of the engine, of the type in which two probes are arranged upstream and downstream of the pot to each provide information to determine the amount of a chemical contained in the exhaust gas respectively upstream and downstream of the pot, of the type in which, for some values of the air / fuel ratio, the chemical substance is likely to be stored in the catalytic converter, while for other values, the stored chemical substance is consumed during chemical reactions of catalysis, and of the type in which the ratio air / fuel is modified according to the information provided by the probes in order to reduce the polluting emissions of the engine, the air / fuel ratio of the fuel mixture is controlled in particular according to the amount of chemical substance actually stored in the catalytic converter which is determined according to a monitoring method according to the teachings of the invention.

 When the monitored substance is oxygen, the engine is controlled to regulate the amount of oxygen molecules actually stored around a value close to half the size of the stock.

 If the monitored chemical is nitrogen oxide, when the amount of nitrogen oxide molecules actually stored reaches a value close to the size of the stock, the engine is controlled to operate with excess fuel up to the amount of nitrogen oxide molecules actually stored is almost zero.

According to the invention, the control method comprises the steps of
filter a stock status setpoint using a first filter
filtering the variable from the first filter using a second filter that allows an approximate modeling of the behavior of the engine and the catalytic converter in terms of oxygen storage and de-stocking, the second filter essentially comprising an integrator, an amplifier gain, and a delay modeling the delay between fuel injection and the passage of the corresponding molecules in the catalytic converter
compare by subtraction the variable resulting from the second filter with the estimated value of the state of the stock
filter the result of this subtraction with a third filter; and
compare by subtraction the variables from the first and the third filter to obtain a value of the correction to be made to the control of the injection device.

Other characteristics and advantages of the invention will appear on reading the detailed description which follows for the understanding of which reference will be made to the appended drawings in which
FIG. 1 is a schematic representation of a device implementing the method according to the invention;
FIG. 2 is a representation illustrating the sequence of the main steps of the method according to the invention;
- Figure 3 is a representation illustrating more precisely a method according to the teachings of the invention.

 FIG. 4 represents a flowchart of the control method of an injection device using the results of the pot monitoring method according to the invention.

 FIG. 1 schematically shows an internal combustion engine 10 for a motor vehicle.

 The engine 10 is powered at 12 with a fuel mixture whose air / fuel ratio is metered by an injection device 14.

 The exhaust gases from combustion in the engine exit the engine at 16 and are conveyed by an exhaust pipe 18 towards the inlet of a catalytic exhaust pipe 20 which comprises a monolith (not shown) coated with substances capable of storing oxygen, for example in the form of metal oxides.

 At the outlet 24 of the catalytic converter, the exhaust gases are directed to silencers (not shown) before being released into the atmosphere.

 In accordance with the teachings of the invention. two probes are provided, upstream 26 and downstream 28, which make it possible to determine, directly or indirectly, the concentration or oxygen content of the exhaust gases respectively upstream and downstream of the catalytic converter 20.

 The probes 26, 28 are, for example, probes "EGO", also called "lambda" or non-proportional probes or "UEGO" probes said proportional or linear, and provide information that, after treatment, allows to determine the concentration oxygen in the exhaust gas. For the treatment, the information is transformed into a variable u which is negative when the fuel mixture, which results in the gas analyzed, is poor, and the value of the variable u is positive when the exhaust gas is derived from the combustion a rich mixture.

 It is also possible to replace the information provided by the upstream probe 26 with an estimated value of the oxygen content of the exhaust gas upstream of the catalytic converter, this value then being estimated by means of a map taking into account certain engine operating parameters such as intake pressure P, engine rotation speed N, coolant temperature, etc.

 The information provided by the downstream probe 28 is also transformed into a variable v which is positive or negative depending on the fuel mixture, from which combustion these gases result. was rich or poor.

 The variables u and v are then introduced into a computer 30, as well as the pressure of the feed gas P and the rotation speed N, in order to determine by the method according to the invention. x and y variables which describe the instantaneous state of the catalytic converter 20.

The variable y is a quantity indicative of the maximum amount of oxygen that can be stored inside the catalytic converter 20, for example in the form of cerium oxide. This quantity therefore expresses the size of the oxygen stock and is expressed in mol / m3
The variable y is therefore directly indicative of the state of aging of the pot 20 since, as we have seen above, the maximum oxygen storage capacity of the pot tends to decrease with time.

 The variations of y are, however, relatively slow.

 The variable x is indicative of the amount of oxygen that is actually stored in the catalytic converter at a given instant t. It is expressed as a function of the maximum storage capacity y at the same instant t in such a way that x = -1 / 2y if there is no oxygen stored in the oxide form in the catalytic converter, c that is, if the stock is empty, and x = 1 / 2y if the catalytic converter is saturated with oxygen, that is, if the stock is full.

 The method for determining the values x and y is a recursive method which is shown diagrammatically in FIG. 2 and which first consists in calculating an estimated value z, indicative of the oxygen content of the exhaust gases downstream of the catalysts. based on an arbitrary initial value y0 of the stock size.

 This value z is in particular calculated as a function of the oxygen content u downstream of the catalytic converter 20, supplied by the downstream probe 26, the speed of rotation of the engine N, the intake pressure P, and the state of the stock x.

 The state of the stock x is itself determined in particular from the arbitrary initial value y0 of the stock size and from the oxygen content v of the exhaust gas downstream of the catalytic converter 20, supplied by the probe. downstream 28.

 The estimated value z thus calculated is compared by subtraction at 32 to the value v measured by the downstream probe 28, the oxygen content downstream of the catalytic converter 20. Depending on the sign of this difference, the value y0 is modified to achieve a new calculation of the variables x and z.

 When the measured z and measured values v of the oxygen content of the exhaust gases downstream of the pot 20 are equal, the values, at this given instant t, of the size of the stock y and the state of the stock x are obtained. .

 FIG. 3 shows more precisely a calculation algorithm that makes it possible to determine, in accordance with the teachings of the invention, the x, y values of the state and the size of the oxygen stock formed by the pot. catalytic 20.

 This method is based on the modeling of the chemical behavior of the system formed by the catalytic converter 20.

 The method, according to the invention, firstly requires to know the mass flow rate of the exhaust gas, which can be measured, directly, by a flowmeter or, indirectly, by measuring the rotation speed N of the engine and the pressure P of the gases in the intake manifold of the engine 10. These parameters are generally necessary for the operation of the fuel injection device 14 and are measured in all vehicles equipped with an electronic injection device 14.

 The upstream 26 and downstream 28 probes used to determine, as has been seen previously, the values u and v normalized oxygen content of the gases, respectively at the inlet 22 and the outlet 24 of the catalytic converter 20.

Depending on the sign of the information v provided by the downstream probe 28. the concentration of oxidants and reducing agents, depending on whether the fuel mixture is poor or rich, contained in the exhaust gas downstream of the catalytic converter can be determined. These concentrations are given by the following formulas
if v> 0: Ox = v / K and Red = 0
if v <0: Ox = 0 and Red = -v / K '
where K and K 'are characteristic parameters of the downstream probe 28.

 It is then necessary to express the chemical kinetics c 1 and c 2 of oxygen storage and storage reactions in the catalytic converter as a function of the concentrations measured in oxidants and reducing agents downstream of the pot 20 and depending on the size. and the state x of the oxygen store in the pot.

These kinetics can be written in the following form
c, = k1 (Ox) a '(0.5 y - x) bi = Cl (x, y, v)
have bi
and c2 = k2 (Red) al (0.5 y + x) b1 = C2 (x, y, v)
where k1, a1, b, and k2, a2, b2 are characteristic parameters of the two oxygen storage and de-storage reactions.

We can then write the law of variation over time of the theoretical oxygen concentration downstream of the pot
dz / dt = - V / L [z (t) - u (t)] + (K / 2e) [C1 (x, y, v) - C2 (x, yv)]
where L is the length of the site reactions monolith
and where e is the void fraction of this monolith.

We then write the law of variation over time of the state of the stock x
dx / dt = C1 (x, y, v) - C2 (x, y, v).

Then, we define the law of variation of the adjustment of the size of the stock y as a function of the error between the oxygen contents, calculated z and measured v, after the pot
dy / dt = - G f (t) [v (t) - z (t) j
where f (t) is a function of the sign of v and G a parameter which makes it possible to determine the convergence speed of the method.

The function f (t) can be for example
f (t) = v (t) or
f (t) = sgn v (t) or again
f (t) = v2 (t) [sgn v (t)].

 We thus obtain a system of three differential equations with three unknown functions x, y, z that are solved numerically for example by the recursive method described above so as to obtain the instantaneous values x of the state of the stock and y of the size of the stock.

 Various applications of this process are contemplated.

 A first application, the most immediate, is to monitor the aging of the catalytic converter 20.

 Indeed, knowing the maximum size of the oxygen stock that can be stored in the pot 20, is to know, at each instant t and directly, the ability of the catalytic converter to perform its function of cleanup of gases oxidation exhaust of unburned hydrocarbons.

 By using a monitoring criterion directly related to the physicochemical phenomenon exploited by the system, the number of parameters to be measured or estimated is reduced and thus the risks of dispersions due to measurement uncertainties are reduced.

 Of course, it is also advantageous to use the knowledge of the x and y variables of the state and the size of the stock to improve the control logic of the fuel injection device 14.

 Indeed, the knowledge at any time t of the state of the oxygen stock makes it possible to best use the buffering effect of this stock to obtain more latitude in the choice of the air / fuel ratio.

 Indeed, if at a time t the oxygen store is almost empty, and the operating conditions of the engine 10 and the power demand are such that it is possible to use a lean fuel mixture, the device injection 14 can be controlled to reduce the amount of fuel per mass of air to limit the consumption of the engine 10. Excess oxygen atoms are then stored in the pot 20.

 On the contrary, for example after a certain period of operation with a lean mixture, if the driver requests a significant acceleration of the vehicle, it will be possible to operate the engine 10, during acceleration, with a rich mixture, the hydrocarbons in excess in the exhaust gas then being recombined with the oxygen previously stored in the catalytic converter 20 to prevent the emission of pollutants.

In parallel, the method according to the invention also makes it possible to know, after a cold start of the engine, the instant from which the pot is primed, that is to say
The moment from which the exhaust gases are actually treated by the pot 20.

 Indeed, as long as the catalytic converter 20 has not reached a threshold temperature, the method according to the invention detects a storage capacity y of the pot 20 which is zero. When the storage capacity begins to grow, it is because the chemical reactions in the pot have begun.

 The process according to the invention has been described for monitoring the state and the size of the oxygen stock formed by the catalytic converter 20.

 However, some catalytic converters have the ability to store and destock other chemicals in a similar manner. In particular, it is possible to store NOx nitrogen oxides produced during lean-burn operation phases.

 In a similar way to what has been seen above. it is possible to model the chemical storage and storage reactions of nitrogen oxides NOx in such a pot, called "NOx-trap". Such a device however requires the use of a downstream probe sensitive to nitrogen oxides.

On the contrary, the upstream probe 26 may be a sensor of the EGO or UEGO type, sensitive to the oxygen content,
The information u delivered by the upstream probe is then used to know, using a map, the content of nitrogen oxides upstream of the pot.

The method according to the invention makes it possible to "predict"
The emission of pollutant and therefore react by acting on the control of the injection device 14 before such emission, while the method according to the prior art could only ascertain the emission of pollutant downstream of the pot.

 Thus, it is for example less than 90% pos, the stock gradually filling up with nitrogen oxides, then to control the transition to operation with a rich mixture for a short time. to bring the stock status back to almost zero.

 The method according to the invention makes it possible to trigger a "purge" sequence of the stock of nitrogen oxides NOx, by imposing the operation of the engine with a rich mixture, before the nitrogen oxides are released into the atmosphere .

 Such a control strategy makes it possible to operate during most of the lean-time because the rate of production of nitrogen oxide molecules is then much lower than the rate of production of reducing substances in rich-mix operation.

 FIG. 4 shows schematically an exemplary embodiment of a device 34 particularly suitable for implementing the air / fuel ratio control method using the knowledge of the stock status x in the case where the substance monitored chemical is oxygen.

 Such a device is the application to the problem of the device described by K. J. ASTROM, C. C. HANG and B. C. LIM in the journal IEEE Transactions on Automatic Control, Vol 39, No. 2, February 1994.

 This device 34 has three filters F1, F2, and F3.

 The first filter F1, for example a filter of the first order, is intended to filter a set value Cs of the stock status.

 The filtered value of the set point Cs is introduced into the second filter F2 which essentially comprises an integrator, a gain amplifier L and a phase delay T.

This second filter forms a simplified model of the behavior of the engine 10 and the catalyst 20 in terms of modification of the richness of the gases and storage / destocking of the molecules in the pot 20. In particular, the delay T corresponds to the estimated delay between the injection of the fuel mixture in the engine and the passage of the corresponding molecules in the pot 20.

 The value resulting from the second filter F2 is subtracted from the calculated value x of the state of the stock according to the method seen above. The result of this comparison is filtered by a third filter F3 to provide an estimate of an instantaneous perturbation 6 (t) to be corrected.

The correction a (t) to be supplied to the injection device 14 is then equal to the value filtered by the filter F1 of the instruction
Cs, from which the estimate of the disturbance is subtracted

Claims (10)

 1. A method for monitoring the operation of a catalytic converter of a motor vehicle, of the type in which the vehicle comprises an internal combustion engine (10) equipped with a catalytic converter (20) for treating the exhaust gas downstream of the engine (10), of the type in which two probes (26, 28) are arranged in pipes upstream (22) and downstream (24) of the catalytic converter (20) to each provide information for determining the concentration (u, (v) a chemical substance contained in the exhaust gas, respectively upstream and downstream of the pot (20), of the type in which, for certain operating modes of the engine, the chemical substance is likely to be stored in the catalytic converter (20), whereas for other modes of operation the previously stored chemical substance is consumed in catalytic chemical reactions, characterized in that from the information (u, v) provided by the probes (26, 28), the maximum amount of chemical substance that is likely to be stored in the catalytic converter, or the size of the stock (y), and the quantity of chemical substance that is actually stored in the pot, or state, are determined at each instant (t) the stock (x).  2. The monitoring method as claimed in claim 1, characterized in that the size (y) and the state (x) of the chemical substance stock are determined by using a mathematical modeling of the chemical reactions occurring in the catalytic converter (20). and in that the modeling takes into account in particular the kinetics (cl, c2) of the reactions in question, which depend on the concentrations (Ox, Red) chemical species they bring into play, and the flow of gas through the catalytic converter.  3. Monitoring method according to claim 2, characterized in that it comprises a step of estimating the concentration (z) of the chemical in the exhaust gas downstream of the catalytic converter (20) and the state (x) the inventory in particular according to a first value of the size (y) of the stock, in that the estimated value (z) of the concentration of the chemical in the exhaust gas downstream of the pot (20) ) is compared with the value (v) effectively measured by the downstream probe (28), and in that the value of the first estimate of the size (y) of the stock is corrected to cancel the difference between the values, estimated (z) ) and measured (v), the concentration of the chemical in the exhaust gas downstream of the catalytic converter (20).  4. The monitoring method as claimed in claim 3, characterized in that the chemical substance stored in the catalytic converter (20) is oxygen and in that the process comprises the steps of  measuring the rotational speed (N) of the engine (10) and the pressure (P) for admitting the fuel mixture into the engine (1 0);  calculate the flow velocity (V) of the exhaust gases in the catalytic converter (20) as a function, in particular, of a modeling of the flow of gases in the exhaust circuit, of the engine rotation speed ( 10) and the pressure (P) of admission  measuring with the probes (26, 28), the concentration of oxygen in the exhaust gases upstream (22) and downstream of the catalytic converter (20), and deduce therefrom values (u, v) normalized in a manner to be negative or positive depending on whether the gases analyzed reveal that the fuel mixture which has been burned has an excess or deficit of air relative to the stoichiometric ratio of the combustion  - deduce the concentration (Ox, Red), in the exhaust gas downstream of the pot (20), the oxidants and reductants of the chemical reactions of storage and storage of oxygen according to the formulas  if v> 0 Ox = v / K and Red = 0  if v <0: Ox = 0 and Red = - v / K '  where K and K 'are characteristic parameters of the downstream probe (28);  expressing the chemical kinetics (c1) and (c2) of oxygen storage and storage reactions in the catalytic converter (20) as a function of the concentrations (Ox Red) measured of oxidants and downstream reductants (24) of the pot (20) and depending on the size (y) and the state (x) of the oxygen store according to the following formulas  c, = k1 (Ox) a '(0.5 y - x) bl = C1 (x, y, v)  a2  and c2 = k2 (Red) a2 (0.5 y + x) b2 = C2 (y, v)  where k1, a1, b1 and k2, a2, b2 are characteristic parameters of the two reactions  - write the law of variation over time of the estimated concentration (z) of the oxygen in the exhaust gases downstream of the pot  dz / dt = - V / L [z (t) - u (t)] + (K / 2e) [C1 (x, y, v) - C2 (x, y v)]  where L is the length of the site reactions monolith  and e is the void fraction of this monolith  - write the law of variation of the state of the stock  dx / dt = C1 (x, y, v) - C2 (x, y, v)  define the law of variation over time of the adjustment of the size (y) of the stock as a function of the error between the concentrations, calculated (z) and measured (v), of oxygen downstream (24) of the pot (20)  dy / dt = - G f (t) [v (t) - z (t)]  where f (t) is a function of the sign of (v) and G is a parameter which makes it possible to determine the convergence speed of the method; and  solving the system of the differential equations thus obtained to obtain the calculated values of x, y and z, by applying a recursive method to cancel the difference between the computed (z) and measured (v) values of the oxygen concentration downstream of the pot (20).  5. Monitoring method according to any one of claims 1 to 3, characterized in that the chemical substance stored in the catalytic converter is a nitrogen oxide.  6. A method for detecting the aging of a catalytic converter of a motor vehicle, of the type in which the catalytic converter (20) is capable of storing a chemical substance present in the exhaust gas of the engine (10), characterized in that it comprises the step of comparing to a set value the maximum quantity (y) of chemical substance that can be stored by the catalytic converter (20) which is determined according to a monitoring method according to any one of the claims 1 to 5.  7. A method for controlling the air-fuel ratio of a fuel mixture for an internal combustion engine (10) of a motor vehicle equipped with a catalytic converter (20) for treating exhaust gases downstream of the engine (10) , of the type in which two probes (26, 28) are arranged upstream and downstream of the pot (20) to each provide information to determine the amount of a chemical contained in the exhaust gas, respectively upstream and downstream of the pot (20), of the type in which, for certain values of the air / fuel ratio, the chemical substance is likely to be stored in the catalytic converter (20), whereas for other values, the substance stored chemical is consumed in the course of catalytic chemical reactions, and of the type in which the air / fuel ratio is changed according to the information provided by the probes (26, 28) for the purpose of reducing the pollutant emissions of the engine (10) , characterized in that that the air / fuel ratio of the fuel mixture is controlled in particular according to the amount of chemical substance actually stored in the catalytic converter which is determined according to a monitoring method according to any one of the preceding claims.  8. A method of controlling the air / fuel ratio according to claim 7 taken in combination with claim 4, characterized in that the motor (10) is controlled to regulate the amount of oxygen molecules actually stored (x) around a value close to half the size of the stock (y).  9. A method of controlling the air / fuel ratio according to claim 7 taken in combination with claim 5, characterized in that when the amount of nitrogen oxide molecules (NOx) actually stored (x) reaches a value close to the size of the stock (y), the engine (10) is controlled to operate with excess fuel until the amount of nitrogen oxide molecules actually stored (x) is almost zero.  Air / fuel ratio control method according to one of claims 7 or 8, characterized in that the control method comprises the steps of  - filter a set value (Cs) of the stock status (x) using a first filter (F1)  filtering the variable resulting from the first filter (Fi) using a second filter (F2) which allows an approximate modeling of the behavior of the engine and the catalytic converter in terms of storage and storage of oxygen, the second filter (F2) essentially comprising an integrator, a gain amplifier (L), and a delay (I) modeling the delay between the fuel injection and the passage of the corresponding molecules in the catalytic converter (20)  - compare by subtraction the variable resulting from the second filter (F2) with the estimated value (x) of the state of the stock  - Filter the result of this subtraction with a third filter (F3);  - Comparing by subtraction the variables from the first (F1) and the third (F3) filter to obtain a value of the correction (a (t)) to bring to the control of the injection device (14).