FR3036438A1 - METHOD FOR VERIFYING THE QUANTITY OF NOX OUTSIDE AN ENGINE - Google Patents

Abstract

The invention relates to a method for verifying an estimate or a measurement of the amount of NOx at the output of an engine (1) in an exhaust line (2) incorporating a RCS system delivering in the line ( 2) predetermined amounts of reducing agent are sufficient to reduce NOx in a catalyst (5) with temperature rise in line (2) at a temperature from which the NOx are in the form of NO, injection of a quantity of reducing agent in the line (2) lower than the quantity necessary to reduce the previously measured or estimated amount of NO, new quantification of the entering NO by a new measurement of the amount of NO downstream added with the amount of NO reduced by the quantity of injected agent, verification whether this new quantification of incoming NO corresponds to the measurement or preliminary estimation of the amount of NOx.

Description

The present invention relates to a method for verifying an estimate or a measurement of the amount of nitrogen oxides, which can also be mentioned in the following: after the denomination of NOx, in a vehicle exhaust line at the output of a motor vehicle engine.  This verification process takes place after increasing the temperature in the exhaust line of the engine and advantageously during the regeneration phases of a particle filter integrated in the exhaust line.  This process can be followed by a step of resetting the estimation of the NOx measurement.  [0003] Compliance with current and future pollution control standards implies elimination of NOx, a grouping of nitric oxide, which may also be mentioned below under the name of NO, and nitrogen dioxide, which may also be mentioned. hereinafter referred to as NO2.  These NOx are produced by combustion in the engine.  These standards become more and more severe over time.  [0004] For the pollution control of NOx, a selective catalytic reduction system otherwise known under the abbreviation of RCS corresponding to the abbreviation of SCR for "selective Catalytic Reduction" is frequently used.  Thereafter the selective catalytic reduction system may also be cited by its abbreviation RCS.  In a system, it is used a liquid reducing agent intended to be introduced in a predetermined amount and by consecutive injections in an exhaust line of a motor vehicle.  The addition of this reducing agent depollution performs the treatment of oxides of nitrogen or NOx present in the exhaust line of the engine of a motor vehicle.  This reducing agent SCR is frequently ammonia or a precursor of ammonia, for example urea or a derivative of urea, especially in the aqueous phase, such as the mixture known under the trademark Adblue®.  A system typically has a reservoir containing a quantity of liquid reducing agent, a pump for supplying liquid agent to an exhaust line of a motor vehicle from an injector opening into the engine. exhaust line.  The liquid reducing agent is decomposed into ammonia gas, which may also be mentioned hereinafter by its chemical formula NH 3.  NH3 is stored in a selective SCR catalyst to reduce the NOx in the gases exhausted through the exhaust line.  In this RCS system, the amount of liquid reducing agent to be continuously injected is determined by a RCS control-command from many parameters including 10 including the amount of NOx entering the RCS system.  This quantity can be modeled or measured by the RCS control-command which can also be integrated into a control command of the motor.  [0009] In the long run, estimating the mass of NH3 actually stored in the selective catalytic reduction system is difficult and problematic.  Indeed, if the actual mass of NH3 is less than the estimated mass, no sufficient liquid reducing agent is injected with the risk of not treating the NOx sufficiently.  There is then a remaining surplus of untreated NOx discharged from the line.  [0010] Conversely, if the actual mass of NH3 is greater than the estimated mass, too much liquid reducing agent is injected with the risk of exceeding the maximum NH 3 storage capacity of the selective catalytic reduction system.  It follows that the surplus NH3 will leave the selective catalytic reduction system resulting in a loss of NH3 remaining unused.  This phenomenon is detrimental in two ways it reduces the autonomy of the vehicle by overconsumption of NH3 without gain on NOx reduction efficiency.  In addition, the unused NH3 surplus is released into the atmosphere with a risk of unpleasant odors for the driver and passengers of the vehicle.  In order to correct these two cases with respectively a residual surplus of untreated NOx and a loss of NH3 remaining unused, the control-command RCS permanently calculates the efficiency of the RCS system from: upstream NOx of the estimated or measured RCS system, - a measurement carried out by a nitrogen oxide sensor placed downstream of the selective catalytic reduction system.  Such a sensor, called a NOx sensor, measures both NOx and NH3 indiscriminately downstream of the RCS system.  To the NOx perceived by the NOx sensor downstream from the RCS system, the NH3 downstream of the selective catalytic reduction system must be added.  In known manner, if the measured efficiency is too low and if the conditions, for example temperature, flow, etc.  . . .  The RCS control-command is temporarily switched to closed-loop mode to determine whether there is a remaining surplus of untreated NOx or a loss of NH3 remaining unused.  This can not actually be done in the usual operating mode.  The objective of such a closed loop is either to modify the injection of liquid reducing agent by increasing or decreasing a gain on the injection setpoint, or to correct the upstream NOx measured or estimated by a multiplicative gain by modes of combustion.  This mode of control operation has drawbacks.  The first disadvantage is that all the inaccuracies on the inputs of the RCS system, for example the upstream NOx, the NO2 / NO ratio, the temperature, the amount of reducing agent injected, etc. . .  is assumed to be correctable by adjusting the gain on injection or NOx upstream of the selective catalytic reduction system.  For example, if the real NOx are 30% higher than the modeled NOx, the successive closed loop operations must lead to 30% more injection than expected to compensate for this difference on the NOx.  This is achieved in practice only to a certain extent, which leads to an under-exploitation of the NOx treatment potential by the RCS system.  The second disadvantage is that it can happen that the closed loop can not reach a decision if there is a surplus of untreated NOx or a surplus of NH3 remaining unused.  This can be done for example for an instantaneous dynamics of the powertrain or an output of the conditions of realization of a closed loop, etc.  [0019] As a third disadvantage, it is possible that the correction is in the wrong direction.  Thus, one can be in actual situation surplus NH3 unused while the closed loop states that it is a surplus of untreated NOx, or vice versa.  Consequently, the problem underlying the invention is to verify in a simple and precise manner an estimate or a preliminary measurement of the amount of nitrogen oxides at the output of a motor vehicle engine. in an exhaust line incorporating a selective catalytic reduction system delivering via an injector in line predetermined amounts of a liquid reducing agent to be sufficient to reduce in a catalyst the nitrogen oxides present in the exhaust gas.  To achieve this objective, there is provided according to the invention a method for verifying an estimate or a preliminary measurement of the amount of nitrogen oxides at the output of a motor vehicle engine in a exhaust line incorporating a selective catalytic reduction system delivering injector in the line predetermined amounts of a liquid reducing agent to be sufficient to reduce in a catalyst the nitrogen oxides, characterized by the following steps: - rise the temperature in the exhaust line at a temperature from which the nitrogen oxides are only in the form of nitrogen monoxide, 15 - injection of a quantity of liquid reducing agent in the line lower than the amount estimated to be necessary to reduce the previously measured or estimated quantity of nitric oxide, - new quantification of nitric oxide entering the system by a no. a measure of the amount of nitric oxide downstream of the system plus the amount of nitric oxide to be reduced by the amount of reducing agent injected, - whether this new quantification of nitric oxide the system corresponds to the measurement or preliminary estimation of the quantity of nitrogen oxides.  The technical effect is to obtain accurate verification of measurements or estimates of quantities of nitrogen oxides or NOx at the engine outlet and upstream of the RCS system.  The rise in temperature converts all the nitrogen oxides contained in the gases of the exhaust line into nitric oxide, which is simpler for calculation, the various nitrogen oxides reacting differently with the reducing agent, this complicated the calculation with the obligation to have a precise ratio of nitrogen oxides / nitric oxide.  In addition, the fact of injecting a minimum amount of reducing agent makes it possible to no longer have a reducing agent converted into NH 3 which is surplus and not used for the reduction.  Such a surplus distorted the measurement, as the amount of surplus NH3 was counted among the nitrogen oxides in the sensor measurement downstream of the system.  Advantageously, when the new quantification of the nitric oxide does not correspond to the measurement or the preliminary estimate of the quantity of nitrogen oxides at the output of the engine, a system registration is carried out. selective catalytic reduction by determining a new amount of liquid reducing agent to be injected into the line, this new amount of reducing agent being a function of the new quantification of nitric oxide.  Advantageously, the determination of the new amount of liquid reducing agent to be injected takes into account a mean nitrogen dioxide / nitrogen monoxide ratio representative of the ratio corresponding to the exhaust gas passing through the line.  Indeed, nitrogen dioxides consume more reducing agent then converted to NH3 than nitrogen monoxides for their reduction.  This should be taken into account for an accurate determination of the new amount of liquid reducing agent to be determined.  Advantageously, when the amount of nitrogen oxides at the output of the engine was previously estimated by modeling, it is expected a modeling update 15 or when the amount of nitrogen oxides at the output of the engine was previously measured. by a sensor upstream of the system, the quantity of nitrogen oxides at the outlet of the system being also measured by a sensor downstream of the system, correction of the measurements of the sensors respectively upstream and downstream of the system is provided to to reduce the gap due to dispersions between the two sensors.  Advantageously, when the exhaust line comprises a particulate filter and the particle filter is periodically regenerated with an increase in the temperature in the line to reach a so-called regeneration temperature, the process of This is checked during the regenerations, the regeneration temperature being greater than or equal to the temperature for which the nitrogen oxides are only in the form of nitric oxide.  This preferred embodiment of the invention is very advantageous, since it uses the necessary regenerations for the particulate filter to perform the implementation of the verification process, which allows to use the temperature rise during regenerations for the implementation of two different processes.  The rise in temperature necessary for regeneration is then taken as a temperature rise according to the verification method and the small amount of reducing agent usually injected during regeneration to protect the injector also serves as a quantity. injected to be less than the estimated amount necessary to reduce the previously measured or estimated amount of nitric oxide.  The treatments or verifications of two different means of depollution are therefore implemented simultaneously which is economical, namely mainly a single temperature rise and a single injection of reducing agent for the regeneration of the particulate filter and the verification method according to the invention.  In addition, there is less disruption of the operation of the engine than if these treatments or verifications had been implemented separately.  [0030] Advantageously, the method is implemented with a period of time lagging on the start of the regeneration and a period of time in prolongation after the end of the regeneration, the periods of time late and in prolongation being determined. according to the thermal inertia of the line and the selective catalytic reduction system.  [0031] Advantageously, the liquid reducing agent injected during the process is at a lower temperature than in current operation of the selective catalytic reduction system 15 for the protection of the injector against the rise in temperature, the lower temperature being similar the ambient temperature prevailing in an environment outside the exhaust line.  [0032] The invention also relates to an exhaust line comprising a catalytic reduction system with a reservoir, an injector and a catalyst located in the line downstream of the injector, as well as a control unit comprising means for calculating quantities of agent to be injected and integrating or connected to control means and for measuring or estimating the temperature in the line, comprising means for storing and comparing temperatures, characterized in that unit uses such a method comprising means for calculating a new quantification of the nitrogen oxides entering the system after the temperature control means have fixed in the line a temperature from which the oxides nitrogen are only in the form of nitric oxide.  The control unit is therefore able to implement such a method as soon as the comparison means determine that the temperature in the line is greater than or equal to the estimated temperature sufficient for the nitrogen oxides contained only in the form of nitric oxide Advantageously, the means for quantifying the nitrogen oxides in the line comprise, on the one hand, a nitrogen oxide sensor downstream of the system and, d on the other hand, an upstream nitrogen oxide sensor or a nitrogen oxide determination model upstream of the system, this model being integrated into the system control unit.  Advantageously, the means for calculating a new quantification of the nitrogen oxides entering the system, then in the form of nitric oxide, is carried out by a new measurement of the amount of nitric oxide downstream. of the system with the amount of nitric oxide to be reduced by the amount of reducing agent injected.  Advantageously, the unit comprises means of comparison between the new quantification of nitric oxide and the measurement or the preliminary estimate of the amount of nitrogen oxides and means for calculating a new quantity of nitrogen. nitrogen oxides entering the system from the new quantification of nitric oxide, taking into account a nitric oxide / nitrogen dioxide ratio, as well as means for calculating a new amount of nitrogen. reducing agent to be injected from the new amount of nitrogen oxides calculated.  Advantageously, the unit also comprises means for resetting the nitrogen oxide sensors or the model from the new quantization at the input of the system.  Advantageously, the line comprises a particulate filter for the depollution of particulate exhaust gases, in particular soot particles, the filter having to undergo regenerations triggered by a particulate pollution control control unit. a particulate pollution control control unit driving temperature rise means inside the line at a regeneration temperature greater than the estimated temperature sufficient for the nitrogen oxides contained in the exhaust gases to be solely in the form of nitric oxide.  Other features, objects and advantages of the present invention will appear on reading the detailed description which follows and with reference to the appended drawings given by way of non-limiting examples and in which: FIG. a schematic representation of a longitudinal section of a heat engine and of its exhaust line comprising various means of depollution, in particular a RCS system whose operation can be controlled according to a verification method according to the present invention, 3036438 8 - FIG. 2 shows various curves relating to a respective parameter of the exhaust line at the output of the heat engine as a function of time, these parameters being able to be taken into account in the verification method according to the present invention.  It should be borne in mind that the figures are given by way of examples and are not limiting of the invention.  They constitute schematic representations of principle intended to facilitate the understanding of the invention and are not necessarily at the scale of practical applications.  In particular, the dimensions of the various elements illustrated are not representative of reality.  For example, the exhaust line shown in Figure 1 is bent, which is not limiting.  On the other hand, in this figure, the selective catalytic reduction catalyst is integrated in the particulate filter, which may not be the case.  [0041] FIG. 1 shows depollution means placed in the exhaust line 2 coming out of a heat engine 1.  In this embodiment, starting from the exhaust of the engine, the exhaust gases can pass through an oxidation catalyst 3 or DOC, which is optional.  The exhaust line 2 then passes through a catalyst 5 of a SCR system, this catalyst 5 being, in this embodiment, integrated in a particle filter 6 or FAP.  The particulate filter 6 advantageously has a porous substrate for filtering exhaust gases, the pore size of the substrate corresponding to the minimum size of the particles to be retained which are present in the exhaust gases, frequently soot particles. .  Advantageously, the porous substrate is impregnated with the active phase of the SCR catalyst 5 on which the transformation of the nitrogen oxides is carried out under the action of the addition of reducing agent in the catalyst 5.  In another embodiment, the SCR catalyst 5 can be placed before the particle filter 6, these two elements being or not grouped together in the same set.  The reducing agent SCR contributes to the chemical reduction of nitrogen oxides, this by initial conversion, if necessary, of the starting reducing agent into ammonia or NH 3.  Most frequently, the reducing agent is an aqueous solution containing urea known as AdBlue®.  In the SCR system, this aqueous solution is contained in a urea tank 7 and is first injected by an injector 8 into a mixing box 9 or 3036438 9 mixer advantageously located in the exhaust line 2 upstream of the SCR catalyst 5.  This mixing box 9 is intended to increase the residence time of the reducing agent with the exhaust gas and to atomize the droplets of the aqueous solution injected via one or more impactors.  Due to the high temperature, the urea decomposes into ammonia NH3 and carbon dioxide CO2 according to the chemical reaction: CO (NH2) 2 + H2O → 2 NH3 + CO2 The ammonia reacts in turn with nitrogen oxides to form, by a reduction reaction, dinitrogen and water.  For example, with nitric oxide, the reaction is written: 4 NO + 4 NH3 + O2 -> 4 N2 + 6 H2O Another reaction with nitric oxide and nitrogen dioxide is written: The disadvantage of a SCR system is that the reducing agent is stored in a tank 7 near the exhaust line 2 and then injected into the tank 7. line 2 exhaust.  There may thus be two possible causes of a malfunction of a RCS system.  The first cause is the aging and / or the damage of the system, for example the aging of the reducing agent, the aging and / or the damage of the injector, the booster pump of the injector, etc.  The second cause is the fouling, for example of the mixing box, in particular its impactors, cause to which it is remedied during a regeneration of the particulate filter 6.  The RCS system includes or is connected to a control unit which determines a target amount of RCS reducer injected into the exhaust line 2.  This target quantity is determined according to the operation of the engine using a previously established injection mapping forming part of a model integrated in the control unit.  [0049] However, a RCS system, for various reasons, in particular because of its aging, exhibits performance that changes over time, generally in the sense of a decrease in its effectiveness.  This has the consequence that the amount of target SCR reducer previously calculated may no longer correspond to the amount of SCR reducer to be effectively injected to obtain proper operation of the SCR system.  The target amount may indeed be lower or greater than that which should actually be injected.  When the target quantity is lower, the pollution control with nitrogen oxides is no longer optimal because of a lack of reducing agent SCR.  When the target amount is greater, there is too much SCR reducing agent supplied to the SCR catalyst 5, which results in unnecessary SCR reducer expense as well as environmental pollution by this reducer.  This results in poor control of the amount of injected reducer, more generally a lack of efficiency of the depollution function.  With reference to FIGS. 1 and 2, the present invention proposes a method for verifying an estimate or a preliminary measurement of the quantity of nitrogen oxides at the output of a vehicle thermal engine 1. In an exhaust line 2 incorporating a selective catalytic reduction system delivering via injector 8 in line 2, amounts of a predetermined liquid reducing agent are sufficient to reduce the nitrogen oxides in a catalyst.  At the output of the engine 1 is to be taken as being in the vicinity of the output of the engine 1, in any case upstream of the RCS system 15 and its catalyst 5.  The method comprises a step of raising the temperature in the exhaust line 2 at a temperature from which the nitrogen oxides are only in the form of nitrogen monoxide or NO.  This step is followed by a step of injecting an amount of liquid reducing agent into line 2 less than the estimated amount necessary to reduce the previously measured or estimated amount of nitric oxide.  There will therefore be no surplus of reducing agent stored in the SCR catalyst or leaving line 2 without having been used for the reduction of nitric oxide.  Since the amount of liquid reducing agent injected is less than the amount necessary to treat the NOx, once the agent has been converted into NH 3 by entering the selective catalytic reduction system, the NH 3 is only used to treat the NOx, then only in the form of NO due to the rise in temperature.  The downstream NOx sensor 10 of the selective catalytic reduction system therefore measures only NOx which are in the unique form of NO.  A new quantification of the NO entering the system is then performed by a new measurement of the amount of NO downstream of the system plus the amount of NO to be reduced by the amount of reducing agent injected.  From the measurement of NOx, again then in the form of NO, at the output of the selective catalytic reduction system and the amount of NH 3 entering, it is possible, by input and output budget, to calculate the amount of NOx then in the form of of NO actually entering the selective catalytic reduction system.  Then, it is verified whether this new quantification of NO entering the system corresponds to the measurement or the preliminary estimate of the amount of nitrogen oxides.  Thus, the method according to the invention is based on two physico-chemical principles encountered at high temperatures as is the case during a regeneration of the particulate filter.  The first principle referred to by the first step of the process according to the invention is that the ratio NO2 / NO or nitrogen dioxide / nitric oxide is zero in the whole of line 2.  Indeed, the NOx are only in the form of nitrogen monoxide or NO because the formation of nitrogen dioxide or NO2 is impossible thermodynamically at high temperature.  It follows that the NOx decontamination reaction in the selective catalytic reduction system only concerns NO reacting with NH3 and this reaction is at 1/1 stoichiometry (1 molecule of NO reacts with 1 molecule of NH3).  This is not the case with reactions involving NO2, as it has been possible to verify the equations given previously.  Thus, the calculation of the amount of NOx input corresponds to the sum of NH3 20 from the injected agent and then converted which has reduced some of the NOx and NOx measured by the NOx sensor downstream 10, so no reduced in the RCS system.  Since the measurement of the downstream NOX sensor 10 is sensitive to the amount of NO2 in the NOx to be measured, having a zero ratio in line 2 makes it possible to overcome this problem.  The measurement of NO alone is therefore more accurate than a random mixture of NO and NO2.  Since there is no NO2, there is no NOx removal reaction by HC in the oxidation catalyst, which could be detrimental with an excess of HC in the line 2 as occurs during a regeneration of the particulate filter 6.  Thus, the amount of NOx at the inlet of the selective catalytic reduction system corresponds exactly to the amount of NOx at the outlet of the engine.  This amount may have been modeled by a NOx model of a control unit or the amount of NOx may have been measured by a NOx sensor upstream of the selective catalytic reduction system.  In the case of a turbocharger, for installation constraints in the environment of the exhaust line 2 of the vehicle the NOx sensor may be placed at the outlet of the turbine.  The second principle is that the storage capacity of the selective catalytic reduction system is zero.  Indeed, since less liquid reducing agent is injected than is necessary to treat all the NOx, the injected liquid reducing agent only serves to treat NOx and the downstream NOX sensor 10 only measures NOx which are then only as NO or nitric oxide.  In the case where the new quantification of the nitric oxide does not correspond to the measurement or the preliminary estimate of the quantity of nitrogen oxides at the output of the engine, a system registration is carried out. selective catalytic reduction by determining a new amount of liquid reducing agent to be injected in line 2, this new amount of reducing agent being a function of the new quantification of nitric oxide.  In this case, the determination of the new quantity of liquid reducing agent to be injected 8 takes account of a mean ratio of nitrogen dioxide / nitric oxide representative of the ratio corresponding to the exhaust gas passing through the line. 2.  When the amount of nitrogen oxides at the output of the engine 1 was previously estimated by modeling, it can be expected to update the modeling.  Since the prior checking is done in a closed loop, it is possible to verify that the correction previously applied by the closed loop RCS is coherent or even modify it if necessary.  When the quantity of nitrogen oxides at the outlet of the engine 1 was previously measured by a sensor upstream of the system, the quantity of nitrogen oxides at the outlet of the system being also measured by a downstream sensor 10 of the system, it is then possible to verify that the values brought up by the two sensors coincide or not and correction is provided for the measurements of the sensors respectively upstream and downstream of the system in order to reduce the difference due to the dispersions between both sensors.  [0067] A particularly advantageous embodiment will now be described.  When the exhaust line 2 comprises a particle filter 6 and the regeneration of the particle filter 6 with a rise in temperature in line 2 is periodically carried out to reach a so-called regeneration temperature, the verification method is carried out during the regenerations, the regeneration temperature being equal to or higher than the temperature for which NOx nitrogen oxides are only in the form of nitric oxide NO.  Indeed, as the particulate filter 6 retains the particles and fills quickly, it is necessary to regenerate frequently, generally every 300 or 5 500 kms traveled.  The regeneration phase consists of bringing the particle filter to a very high temperature, the filter substrate being raised to more than 550 ° C.  The particles contained in the substrate are then burned, transformed into gas and can pass through the substrate towards the exit of the exhaust line 2.  This temperature of 550 ° C is sufficient to convert all NOx into nitric oxide or NO.  In the known manner, during the regeneration phases of the particulate filter, no attempt was made to depollute the exhaust gases with NOx but only to protect the injector 8 of liquid reducing agent from the high temperatures encountered in the process. line 2 exhaust.  For this, it was customary to inject a small amount of liquid and cold reducing agent to limit the temperature of the head of the injector 8 and maintain its integrity.  The present invention uses these features in a preferred implementation of the verification method as previously mentioned.  Preferably, the process can be carried out with a time period late on the start of the regeneration and a period of time in prolongation after the end of the regeneration, the periods of time lagging and in prolongation being determined according to the thermal inertia of line 2 and the selective catalytic reduction system.  This is illustrated in FIG. 2 by comparing the regeneration duration shown with the high curve and the computing validity taking duration shown at the penultimate curve at the bottom.  In addition, preferably, in the verification method according to the present invention, as during a regeneration of the particulate filter of the state of the art, the rise in temperature is high, for example more than 500. ° C, the liquid reducing agent injected during the process may be at lower temperature lower than in current operation of the selective catalytic reduction system.  This serves to protect the injector 8 against the rise in temperature, the low temperature being close to the ambient temperature prevailing in an environment outside the exhaust line 2.  Indeed, since it can be seen at the two upper curves in FIG. 2, the temperature in the RCS system shown at the second top curve rises with delay compared to the beginning of the regeneration shown at the first time. curve of the top and drop with delay compared to the end of the regeneration, the regeneration being symbolized by the passage in 1 of the curve of the top.  [0073] The invention also relates to an exhaust line 2 incorporating the above-mentioned characteristics for a catalytic reduction system.  The RCS system comprises a control unit RCS controlling the amount of reducing agent to be injected from, on the one hand, a quantification of the nitrogen oxides.  This quantification can be done according to measurements of a nitrogen oxide sensor downstream of the system and, on the other hand, measurements of a nitrogen oxide sensor being upstream or downstream. a model for determining the nitrogen oxides upstream of the system, this model being integrated in the control unit of the RCS system.  According to the invention, the control command unit RCS integrates temperature control means in line 2.  The unit is also connected to means for measuring or estimating the temperature in the line 2 by including means for storing an estimated temperature sufficient for the nitrogen oxides contained in the exhaust gases to be only in the form of nitric oxide and means for comparing the temperature in line 2 with the estimated sufficient temperature.  Such a unit implements a verification method such as previously described as soon as the comparison means determine that the temperature in line 2 is greater than or equal to the estimated temperature sufficient, the control unit comprising means for calculating a quantity of liquid reducing agent to be injected in the line 2 less than the estimated quantity necessary to reduce the previously measured or estimated quantity of nitric oxide contained in the exhaust gas.  The unit may comprise means for calculating a new quantification of nitrogen oxides entering the system after the temperature control means have fixed in line 2 a temperature from which the oxides nitrogen are only in the form of nitric oxide.  This new quantification can be done by a new measurement of the amount of nitric oxide downstream of the system plus the amount of nitric oxide to be reduced by the amount of reducing agent injected, the means comparing the new quantification of nitric oxide entering the system with the measurement or preliminary estimation of the amount of nitrogen oxides then in the unique form of nitric oxide.  The unit may comprise means for resetting the nitrogen oxide sensors or the model from the new quantification at the input of the system and means for calculating a new quantity of nitrogen oxides. entering the system from the new quantification of nitric oxide taking into account a nitrogen dioxide / nitrogen monoxide ratio, as well as means for calculating a new amount of reducing agent to be injected at from the new amount of calculated nitrogen oxides.  In the preferred embodiment of the present invention, when the exhaust line 10 comprises a particulate filter 6 to undergo regenerations triggered by a control unit particle pollution control, this unit pilot means for raising the temperature within line 2 at a regeneration temperature higher than the estimated temperature sufficient so that the nitrogen oxides contained in the exhaust gas are only in the form of nitric oxide.  The particulate pollution control control unit may be connected to the control control unit RCS so that only one of these two units controls the rise in temperature.  Advantageously, the particle pollution control control unit performs this function.  An example of operation of the verification method according to a preferred embodiment of the invention, for which the verification is done during a regeneration of the particulate filter, will be given below with reference to FIGS. 1 and 2 and more. particularly in Figure 2.  In usual operation of the RCS system, it is injected with the liquid reducing agent to clean the NOX exhaust gas.  In this case, after conversion of the reducing agent into NH 3, the mass of NH 3 stored in the catalyst of the selective catalytic reduction system is non-zero.  During the regeneration of the particle filter or RG FAP operation, the regeneration being shown by the top curve in Figure 2 for the durations 2, 3 and 4, the reduction agent injection is reduced because goes into injector protection.  NOx is continued to be processed by the NH3 stored in the selective catalytic reduction system and the injected NH3 as injector protection.  During regeneration, as shown in the second curve from the top, the temperature or temperature intra-RCS rises in the selective catalytic reduction system and continues after the end of the regeneration for durations 3, 4 and the beginning of the duration 5 by thermal inertia.  The storage capacity of the NH3 catalyst SC tends to 0, the descent being during durations 2 and 3.  The NH3 stored, or NH3 mass stored at the third curve from the top leaves the selective catalytic reduction system quickly.  The fourth curve shows a peak of unused NH3 surplus which could disturb the measurement of the downstream NOx sensor during the verification process during the duration 3.  That is why the validity of the calculation according to the method, illustrated in the sixth curve from the top does not concern this duration 3 but the durations 4 and 5.  After this peak, the NOx downstream of the selective catalytic reduction system, then in the form of NO due to the rise in temperature, also increase since there is no longer available NH3 available and no does not inject enough liquid reducing agent to treat everything.  Once the NH3 surplus peak has been passed, as shown in the fourth curve for duration 3, the downstream NOx sensor 10 of the selective catalytic reduction system only measures NO coming out of the selective catalytic reduction system.  It is therefore possible to evaluate, with the amount of liquid reducing agent injected to protect the injector delivered according to the fifth curve during the durations 2, 3, 4 and the beginning of the duration 5 before the temperature does not return to its usual level after regeneration, the amount of NOx actually entering the selective catalytic reduction system.  The partial oxidation of the NH 3 entering the NO selective catalytic reduction system, possible at high temperature by reaction between NH 3 and O 2, has no impact on the NO + NH 3 balance.  At the fifth curve, insufficient quantity injection is performed to reduce all the NO during times 2 to 4 and then start time 5 when the temperature is still high.  During the rest of the duration 5, the injection of reducing agent is canceled and then returned to a normal level from the duration 6.  This suspension of the injection for depolluting during a determined period of time is used to reset the NOx estimate on different modes of combustion.  Indeed, in regeneration of the particulate filter, one is on a specific mode of combustion.  After the regeneration, it is possible to reset one or more modes of combustion by activating them successively.  At the regeneration output of the particulate filter, the temperature in the RCS system remains high at a certain moment, it is possible to continue to compare the upstream NOx recalculated with the estimated NOx as long as the flow rate of the liquid reducing agent is limited. , which is the case of the duration 2 at the end of the duration 5, as shown in the sixth curve.  When the temperature of the line has dropped sufficiently, the protection of the injector is deactivated and no liquid reducing agent is injected.  By recalibrating the NOx output of the engine after regeneration, it overcomes the accuracy of the injection but we lose the benefits of the regeneration of the particulate filter because the NO2 / NOx ratio becomes non-zero with consumption of a small part of the NOx in the catalyst and an influence of the ratio on the stoichiometry of the NOx decontamination reactions as well as an influence of the ratio on the accuracy of the downstream NOx sensor 10.  [0091] As a part of the duration 6, the mode of resetting of the NOx at the output of the engine is left: the injection flow rate is no longer limited and the temperature in the line and in particular in the RCS 5 catalyst has sufficiently decreased for the catalyst to recover an NH3 storage capacity.  We can no longer compare the upstream NOx recalculated with NOx estimated by the control.  If necessary, we correct the NOx estimation model on the different combustion modes tested to make it closer to reality.  One of the main advantages of the present invention is the improvement in the accuracy of measuring the amount of NOx upstream of the selective catalytic reduction system estimated or measured by the RCS control unit.  A better estimate of the mass of NH3 in the SCR catalyst over time is also obtained as well as a reduction in the occurrence of excess NOx in the untreated as well as a better NOx treatment efficiency.  At the same time, a decrease in the occurrences of unused NH3 surplus is obtained, resulting in a reduction in the losses of liquid reducing agent and therefore a potential increase in the vehicle's autonomy as well as a reduction in the risks of d smell for the driver or the passengers of the vehicle, when the surplus of unused NH3 is important.  The invention is in no way limited to the described and illustrated embodiments which have been given by way of example only.

Claims (10)

REVENDICATIONS1. Method for verifying an estimate or a preliminary measurement of the quantity of nitrogen oxides at the outlet of a motor vehicle thermal engine (1) in an exhaust line (2) incorporating a catalytic reduction system selective delivery by an injector (8) in the line (2) amounts of a liquid reducing agent predetermined to be sufficient to reduce in a catalyst (5) oxides of nitrogen, characterized by the following steps: - rise of the temperature in the exhaust line (2) at a temperature from which the nitrogen oxides are only in the form of nitric oxide, - injecting a quantity of liquid reducing agent into the lower line (2) the estimated quantity required to reduce the previously measured or estimated amount of nitric oxide, - new quantification of nitric oxide entering the system by a new measurement of the amount of nitric oxide downstream of the system with the amount of nitric oxide to be reduced by the amount of reducing agent injected, - whether this new quantification of nitric oxide entering the system corresponds to the measurement or estimate prior to the amount of nitrogen oxides. 2. Method according to claim 1, wherein, when the new quantification of nitric oxide does not correspond to the measurement or the preliminary estimate of the amount of nitrogen oxides at the output of the engine, it is proceeded to a registration of the selective catalytic reduction system by determining a new quantity of liquid reducing agent to be injected in line (2), this new quantity of reducing agent being a function of the new quantification of nitric oxide. 3. The method of claim 2, wherein the determination of the new amount of liquid reducing agent to be injected (8) takes into account a mean ratio of nitrogen dioxide / nitric oxide representative of the ratio corresponding to the gases of exhaust crossing the line (2). 3036438 19 4. The method of claim 2 or 3, wherein, when the amount of nitrogen oxides at the output of the engine (1) was previously estimated by modeling, it is expected a reactualization modeling or when the amount of oxides The nitrogen at the outlet of the engine (1) was previously measured by a sensor upstream of the system, the quantity of nitrogen oxides at the outlet of the system being also measured by a sensor downstream (10) of the system. correction of the sensor measurements respectively upstream and downstream (10) of the system is provided in order to reduce the difference due to the dispersions between the two sensors. 5. A method according to any one of the preceding claims, wherein when the exhaust line (2) comprises a particulate filter (6) and periodic regenerations of the particulate filter (6) are carried out. with increasing temperature in the line (2) to reach a so-called regeneration temperature, the verification process is carried out during the regenerations, the regeneration temperature being equal to or greater than the temperature from which the oxidation oxides nitrogen are only in the form of nitric oxide. 6. The method of claim 5, which is implemented with a delay time period on the start of the regeneration and a period of time in prolongation after the end of the regeneration, the periods of time late and in prolongation being determined according to the thermal inertia of the line (2) and the selective catalytic reduction system. 7. A method according to any one of claims 5 or 6, wherein the liquid reducing agent injected during the process is at a lower temperature than in current operation of the selective catalytic reduction system for the protection of the injector (8). ) against the rise in temperature, the lower temperature being close to the ambient temperature prevailing in an environment outside the line (2) exhaust. 8. Process according to any one of the preceding claims, in which an injection delay time is expected to proceed with the injection of a quantity of liquid reducing agent in line (2) lower than the estimated quantity. necessary, this injection delay time being calculated to empty the catalyst (5) of the amount of liquid reducing agent previously introduced by estimating the amounts of liquid reducing agent still to be in the catalyst (5) as a function of amounts of liquid reducing agent previously introduced in the line (2). 3036438 20 9. Exhaust line (2) comprising a catalytic reduction system with a reservoir (7), an injector (8) and a catalyst (5) located in the line (2) downstream of the injector (8) and a control unit comprising means for calculating the amount of agent to be injected with means for quantifying the nitrogen oxides entering the system, the unit integrating or being connected to control means and for measuring or estimating the temperature in the line (2) by comprising means for storing and comparing temperatures, characterized in that it implements a method according to any one of the preceding claims comprising means for calculating a new quantification of the nitrogen oxides entering the system after the temperature control means have fixed in line (2) a temperature from which the nitrogen oxides are only in the form of d nitric oxide. 10. Line (2) according to claim 9, which comprises a particulate filter (6) for the depollution of particulate exhaust gases, especially soot particles, the filter (6) to undergo regenerations triggered by a particulate pollution control control unit, the particulate pollution control control unit controlling temperature rise means within the line (2) at a regeneration temperature higher than the estimated temperature sufficient for the The nitrogen oxides contained in the exhaust gases are only in the form of nitric oxide.