FR2946750A1 - Method for monitoring reducing chemical solution i.e. aqueous urea solution, in internal combustion engine of motor vehicle, involves determining absorption spectrum produced by sample illuminated by incident spectrum of given wavelengths - Google Patents

Abstract

The method involves setting the temperature of a sample of chemical solution in a predetermined range of temperature, by measuring the temperature of the solution and heating the solution as long as the temperature of the solution is less than the predetermined range of temperature. An absorption spectrum produced by the sample illuminated by an incident spectrum of given wavelengths is determined. Presence of absorption peak of a predetermined chemical compound e.g. ammonia precursor or solvent such as water, is identified from the absorption spectrum. An independent claim is also included for a device for controlling a chemical solution, comprising a reservoir.

Description

A method of controlling a chemical solution comprising a reducing agent

TECHNICAL FIELD OF THE INVENTION The present invention relates to the control of a chemical solution comprising a reducing agent for the catalytic reduction of nitrogen oxides and a solvent.

BACKGROUND In motor vehicle engines, the combustion of fuel results in the creation of gases such as carbon monoxide (CO) or nitrogen oxides (NO).

These nitrogen oxides (NO) present a danger, on the one hand for the health of the human beings, and on the other hand for the environment since they contribute to the formation of fogs in the cities. Therefore, it is necessary to provide solutions to destroy these gases before they are emitted into the atmosphere.

Numerous solutions are known for chemically destroying these nitrogen oxides before their escape into the atmosphere, by selective catalytic reduction type chemical reactions, known as SCR (English Selective Catalytic Reduction). Such a reduction is carried out using ammonia contained in a reducing agent, the ammonia reacting with the nitrogen oxides in an SCR catalyst to form completely harmless dinitrogen (N2).

However, since ammonia is a toxic substance, it is preferred for automotive applications to extract urea, which is a non-toxic substance. Urea being highly soluble in water, the principle currently on the market is to inject a reducing solution that is to say a mixture of water as a solvent and urea as a reducing agent in the exhaust.

In addition, because of the toxicity of ammonia, it is important to finely control the amount of reducing agent injected into the exhaust line in order to limit during the operation of the SCR system the NH3 leakage through the catalytic system. . Indeed, if more reducing agent is added than it is converted during the reduction with the NOs, an undesired leakage of the NH3 may occur outside the exhaust line which will cause a nuisance to the fuel. 'environment.

On the other hand, today a growing number of combinations of reducing agents and different solvents are proposed. For example, document WO2008125767 discloses a reducing agent containing an alcohol and / or an ether as a solvent. Among the other molecules that can act as reducing agents, ammonium hydroxide, esters, amides, etc. are known, for example. These reducing agents are miscible with each other and can result in a reducing solution composition. complex.

In addition, in the case where the driver would have to fill himself the reservoir of reducing solution, the prior emptying of the tank is no longer possible and the mixture of reducing solution of quality and / or diverse composition would be inevitable. It therefore becomes necessary to provide a method for controlling the quality of the reducing solution contained in the SCR solution tank.

Several quality control processes have been proposed. Thus, for example, the document US6408619 proposes the use of a conductivity sensor for measuring the conductivity of the solution. The reducer concentration in the solution is then determined from the measured conductivity.

Another method is described in documents WO2004025286 and US20070163240, according to which a pulsed voltage is applied to a device for heating a reducing solution, the solution is heated locally and the concentration of reducing agent in the solution is determined from an output potential difference corresponding to a temperature difference between an initial temperature and a maximum temperature of a temperature sensor.

These methods are however not sufficiently accurate and reliable. In fact, the relatively low level of reductant in the solution makes the determination of the reducer concentration in the solution from the conductivity quite difficult. In addition, the determination of the concentration from the conductivity can be considerably influenced if there are impurities in the solution.

WO2007104779 discloses a method for determining the concentration of a component in a solution, wherein: - a sample of a solution is subjected to a temperature change in a temperature range comprising a characteristic temperature of the solution, the temperature corresponding characteristic, in case of sample cooling, to the freezing start temperature and, in the case of sample heating, to the end-of-melting temperature; a temperature change curve of the solution as a function of the elapsed time is drawn; the characteristic temperature is determined; the linearity of the temperature change curve above the characteristic temperature is analyzed; the concentration of the component in the solution is determined on the basis of the characteristic temperature and the linearity of the temperature change curve above the characteristic temperature.

The technique is complicated since a fine control of the heating system is necessary (well insulated heating chamber in particular). This technique also consumes a lot of energy because the reducer solution is heated and cooled. In addition, the temperature inside and outside the heating chamber or the forced convection that changes the cooling gradient are not taken into account.

JP-A-2001020724 discloses means for detecting the urea concentration of a solution comprising a refractive index sensor for detecting the refractive index of the urea aqueous solution, a temperature sensor for detecting the temperature of the urea solution and calculating means for determining the urea concentration from the values detected by the two detectors.

This technique is relatively inexpensive and practical. This device is, however, limited to the measurement of the urea concentration and does not make it possible to determine the presence and the concentration of reducing agent other than the urea potentially present in the reducing solution contained in the reservoir.

The invention aims to overcome the drawback of the prior art by providing a method and a device capable of determining the presence and concentration of a plurality of reducing agents and / or solvents in a reducing chemical solution.

The invention therefore relates to a method of controlling a chemical solution comprising a reducing agent for the catalytic reduction of nitrogen oxides and a solvent, said method comprising a step of heating at least one sample of the solution in a predetermined temperature range, characterized in that, the sample being in said temperature range, it comprises a step of determining an absorption spectrum produced by the illuminated sample of an incident spectrum of wavelengths data. Indeed, an absorption spectrum advantageously contains on its own the information capable of allowing the identification of a plurality of chemical compounds. In addition, the invention may include one or more of the following features:

the warming step comprises a step of measuring the temperature of the solution and a step of heating the solution as long as the temperature of the solution is lower than the predetermined temperature range. Stabilization of the temperature makes it possible to measure the spectrum under experimental conditions where the chemical solution sample is assuredly liquid.

The method comprises a step of identifying the presence of an absorption peak of at least one predetermined chemical compound from the absorption spectrum. The presence in the spectrum of absorption peaks at determined wavelengths is indicative of the presence of chemical bonds characteristic of the desired chemical compound.

The predetermined chemical compound is a precursor reducing agent for ammonia or a solvent for the reducing agent.

the process comprises, according to the determined absorption peak, a step of determining the concentration of said chemical compound, in particular from the intensity of the absorption peak, the higher the concentration of the compound, the greater the absorption will be important.

the process comprises, according to the concentration determined, a step of diagnosis as to the presence or the concentration of said compound of the solution with respect to at least one reference state and / or emission of a suitable warning message characterizing the difference between the measured concentration or the detected presence of said compound and at least one reference state.

the incident spectrum is adapted to cover all the sensitive compounds of said solution and in particular the reducing agent and the solvent, which makes it possible to obtain in a single measurement all the information necessary for continuing the analysis of the chemical solution. 5

Furthermore, the subject of the invention is also a device for controlling a chemical solution comprising a reducing agent for the catalytic reduction of nitrogen oxides and a solvent, the device comprising a reservoir containing at least one sample of the solution, a heating element of the sample in a predetermined temperature range and, characterized in that it comprises a spectroscopic measuring device able to analyze the sample and more particularly to determine the concentration of at least one predetermined chemical compound of the sample in the tank and control means and calculations.

In a variant, the spectroscopic measuring device comprises a light source, a spectrometer, an optical sensor disposed in the reservoir connected by optical transmission means to the light source and the spectrometer, the control and calculation means being able to drive the operation of the light source, the spectrometer, the heating element and to derive from the information provided back by the spectrometer, said concentrations.

Finally, the invention relates to an internal combustion engine comprising an exhaust line provided with a selective catalytic reduction device for nitrogen oxides, characterized in that it comprises a device according to the invention.

BRIEF DESCRIPTION OF THE DRAWINGS Other features and advantages will appear on reading the following description of a particular embodiment, not limiting of the invention, with reference to the figures in which:

- Figure 1 is a schematic representation of an internal combustion engine having an exhaust line using a SCR type process. FIG. 2 represents an infrared absorption spectrum of urea. FIG. 3 represents a calibration curve of a reducing agent concentration as a function of a spectroscopic signal intensity. FIG. 4 is a flow chart for controlling a chemical solution comprising at least one reducing agent and at least one solvent.

DETAILED DESCRIPTION FIG. 1 is a schematic representation of an internal combustion engine 1, for example for a motor vehicle, comprising an exhaust line 2.

The exhaust gases produced by the engine 1 are treated in the exhaust line 2 by successively passing through an oxidation catalyst 3, by a catalyst SCR 4, then by a particulate filter 5. The catalyst SCR 4 allows the reduction of the NOX nitrogen oxides present in the exhaust gas, by selective catalytic reaction between the NOX and a reducing chemical solution injected into the exhaust line, upstream of the catalyst SCR 4 by an injector 6. The reducing chemical solution , which may comprise at least one solvent mixed with at least one reducing agent, is stored in a tank 7.

The reducing solution may for example be an aqueous solution of urea, the solvent being water, the reducing agent urea. An NOX detector 8 can be placed between the engine and the oxidation catalyst 3.

The injector 6 connected to the reservoir 7 is able to inject a predetermined quantity of the reducing solution into the exhaust gas upstream of the catalyst SCR 3. A temperature sensor 9 can be placed between the injector 6 and the catalyst SCR 4 The temperature sensor 9 may be a thermocouple.

The SCR catalyst 4 stores the NH3 formed by the decomposition of the reducing agent. NH3 reacts with NOX to convert to N2. The SCR process is characterized by a high reduction rate, often greater than 90%, for a dosage close to stoichiometry. Another NOX detector 10 at the outlet of the SCR catalyst 4 can, in conjunction with the NOX detector 8, evaluate the efficiency of the SCR process.

At the outlet of the particulate filter 4, a pressure sensor 11 can measure the back pressure created through the particulate filter 4.

A spectrometric measuring device 12 is provided for the analysis of the reducing solution. The spectrometric device 12 comprises a light source 13 and a spectrometer 14. The light source 13 may be a broad-spectrum light source, that is to say, emitting over a plurality of wavelengths, in the field of ultra-light. violets, visible, but preferably in the infrared wavelength domain. The infrared range extends from approximately 600 nm to 4000 nm. Indeed, an infrared light source is an economical light source and adapted to the spectroscopic identification of the molecules. Also, the light source 13 may be several monochromatic emitters which collectively target several molecular characteristics of the molecules that can be used as reducing molecules or solvents.

The spectrometric device 12 further comprises an optical sensor 15 disposed in the reducing agent tank 7. The optical sensor 15 is connected to the light source 13 on the one hand and to the spectrometer 14 on the other hand by optical transmission means 16 such as optical fibers. The optical sensor 15 comprises a transmitting part 17 and a receiving part 18 spaced apart from a space E intended to be occupied by the reducing solution present in the tank 7. Thus, the light of the light source 13 diffused via the transmitter The optical fiber 17 passes through the reducing solution on this space E, a fraction of the emitted light is absorbed by the reducing solution and the unabsorbed fraction is collected by the optical receiver 18 of the optical sensor 15 to be transmitted to the spectrometer 14. In some cases, it is possible that the light passes through a gas (empty tank) or a solid (very low outside temperature), but the principle remains the same.

The spectrometer 14 breaks down the light collected by the optical receiver 18 in order to determine an absorption light spectrum, known per se, as shown in FIG. 2, that is to say a distribution of the light intensity collected as a function of the wavelengths. Such an absorption spectrum exhibits, at determined wavelengths, light absorption peaks 22 characteristic of the presence of specific functional groups present in the reducing chemical solution.

By functional group is meant here a chemical bond such as for example N-H for urea. Each type of chemical bond is capable of absorbing radiation over at least one known wavelength. Thus the detection of one or more specific functional groups makes it possible to establish the presence of a specific chemical compound.

The determination of the absorption light spectrum is preferably carried out at a determined temperature range. This temperature range may be chosen to be in conditions where the reducing agent can not crystallize, even partially. The heating of the solution to the chosen temperature range is provided by a heating element 19 included in the reservoir 7. In addition, the reservoir 7 comprises a temperature sensor 20 of the reducing chemical solution. The temperature sensor 20 may be a thermocouple.

Thus, in the reservoir 7, if the temperature sensor 20 detects a temperature below the lower threshold of the predetermined temperature range, the heating element 19 is started. The range of the temperature range may be a few degrees Celsius in the range of -20 ° C to 20 ° C. This temperature range is chosen to prevent the solidification of the reducing chemical solution, which may cause a malfunction of the injector 6. For example, if the chemical solution comprises as a urea reducing agent, the temperature range of the solution is never less than 5 ° C, to avoid crystallization of urea. When the temperature sensor 20 detects a temperature above the upper threshold of the temperature range, the heating element 19 is turned off.

It is expected that the various sensors and detectors (8, 9, 10, 20), the injector 6, the light source 13, the spectrometer 14 and the heating element 19 are connected to a control member 21. The control member 21 may be a Central Electronic Unit (or ECU). The value of the temperature measured by the temperature sensor 20 as well as the light spectrum are sent to the control member 21. The control member 21 further comprises calculation means able to deduce from the absorption light spectrum the concentration of the reducing agent contained in the reducing solution. Thus, if it is desired to know if the reducing chemical solution contained in the tank 7 contains urea and to determine its concentration, the control member 21 identifies the presence of absorption peaks characteristic of the urea from the absorption spectrum and deduces therefrom a quantity characterizing an absorption level from, for example, the determination of a peak height H as shown in FIG. 2 or an integral I of one or several peaks, as further shown in the hatched portion in FIG. 2, for one or more wavelengths absorbed by the NH functional group. The control member 21 determines, in a second step, from a calibration curve establishing the relationship between the urea concentration and the measured absorption level, the concentration of urea present in the reservoir 7.20 The calibration curve establishing, a relation between the concentration of urea and the level of light absorption measured by the spectrometer 14, on the characteristic wavelength of the functional group, can be determined beforehand on engine test bench or by numerical simulation and loaded in memory in the control member 21.

The description illustrates the process of the invention in the simplified case of a reducing chemical solution consisting of water and urea, but the invention is not limited to this reductive chemical solution composition. In order to detect the possible presence of other reducing agents in the tank, and / or other solvents, it is expected that the control member 21 comprise a calibration curve specific to each reducing agent and / or potentially sensitive solvents for an SCR process, these reducing agents being chemical precursor ammonia compounds, that is to say capable of generating ammonia by chemical reaction, such as, for example, and without limitation, guanadinium salts; The solvents of these reducing agents can be, for example, belong to the family of alcohols.

Thus, to each of these chemical compounds precursors of ammonia or solvents is determined beforehand on engine bench or by numerical simulation a calibration curve establishing a relationship between the concentration of said reducing agent or said solvent and the light intensity measured on one or several wavelengths absorbed by a functional group specific to the desired chemical compound.

The control member 21 can then determine a certain number of actions to be performed.

Thus, the control member 21 finely determines the amount of reducing solution to be injected as a function of the concentration measurements made in order to guarantee a good yield of the SCR process. The control member 21 can determine whether the heating element 19 is to be started or not.

FIG. 3 shows an example of a calibration curve established with respect to the concentration of solvent in the tank 7. The solvent is here, for example, water. The calibration curve relates the solvent concentration, on the ordinate of FIG. 3, to an absorption level of a reducing agent over a chosen wavelength, on the abscissa of FIG. 3. The higher the concentration of reducing agent is high. the higher the level of absorption, and the lower the concentration of the solvent.

Depending on the level of absorption measured, different conclusions can be drawn. Thus, three zones 23, 24, 25 are delimited by vertical dotted lines in FIG.

In zone 23, the indicated point 26 corresponds to an absorption level measured in the presence of air. This means that the tank does not contain enough reducing solution and the user is therefore warned of the need to fill the tank 7.

In zone 23, the indicated point 27 corresponds to an absorption level measured in the presence of water, without reducing agent.

In zone 23, there is not enough reducer in the tank 7 and too much solvent. This can happen when the user has added only water in the tank 7 without reducer. This can have the consequence of rendering the SCR system ineffective. The control member 21 then signals the user to put more reducing agent in the tank.

In zone 24, the concentration of reducing agent ensures a good operation of the SCR process and therefore a good reduction of NO. In addition, the injector 6 is not degraded.

In zone 25, the solvent level is too low. The reducing agent, if it is for example urea, can crystallize and there is a serious risk of clogging of the injector 6. The control member 21 indicates to the user that the level of solvent is too low and it is necessary to add solvent in the reducing solution. The indicated point 28 corresponding to an absorption level measured in the presence of solid reducing agent, if it is urea.

Figure 4 shows a control flowchart of the reducing chemical solution. The decision sequences are based on the information of the temperature sensor 20 and the spectrometer 14. The information is turned into action: identification of the reducing agent, of the solvent, amount of reducing solution to be injected, heating of the reducing solution , information to the driver, etc.

The decision sequence 29 relates to warming up at least one sample of the solution in a predetermined temperature range in the reservoir 7. The decision sequence 29 comprises a comparison test and a step 31. The comparison test corresponds to the comparison of the measurement of the temperature of the reducing chemical solution in the tank 7 by the temperature sensor 20 at a temperature range. When the temperature of the reducing solution in the tank 7 is lower than the lower threshold of the set temperature range, step 31 is carried out: the heating element 19 is actuated. The temperature of the reducing chemical solution is thus maintained above the lower threshold. The lower threshold can be fixed at any point between -20 ° C and 50 ° C. This threshold is chosen to ensure that there are no reducing agent crystals. Thus, in the case where the urea used as a reducing agent, a threshold of 20 ° C. is preferably chosen.

The sequence 32 comprises two steps 33 and 34 and several comparison tests 35, 36, 37 with respect to reference states. The decision sequence 32 makes it possible to determine a step to be performed as a function of the determination of the concentration of reducing agent and / or solvent. This is particularly the case of the diagnostic sequence 38 as to the presence or concentration of said compound of the solution with respect to at least one reference state and / or emission of a suitable warning message characterizing the the difference between the measured concentration or the detected presence of said compound and at least one reference state.

When the temperature of the reducing solution contained in the reservoir 7 is maintained at the desired temperature, step 33 is carried out then step 34. Step 33 consists in determining an absorption light spectrum using the spectrometer 14 The absorption spectrum is produced by the illuminated sample of an incident spectrum of given wavelengths of the light source 13. The step 34 consists of identifying the presence of a peak of absorption of minus a predetermined chemical compound from the absorption spectrum, to determine the absorption levels for the wavelengths of the selected functional groups and, according to the determined absorption peak 22, to determine the concentration of said chemical compound according to of the determined absorption level, using the control member 21.

The concentration can then be compared in the comparison test 35. If the concentration is less than or equal to a first reference state E1, step 39 is performed. In step 39, the control member 21 finds that the level of reducing solution is too low in the reservoir 7 and that filling is necessary. Optionally, step 39 is accompanied by a warning message to the user indicating that the level of the reducing solution is too low and a filling of the tank is necessary.

In the case where the concentration is greater than the first reference state E1, a new comparison test 36 is performed.

If the concentration is less than or equal to a second reference state E2, step 40 is performed. In step 40, the control member 21 finds that the concentration of reducing agent in the reducing solution is too low and filling the reservoir 7 with reducing agent is necessary. Optionally, step 40 is accompanied by a warning message to the user indicating that the concentration or the level of reducing agent is low and that a filling of the tank 7 with urea is necessary.

In the case where the concentration is greater than the second reference state E2, a new comparison test 37 is performed.

If the concentration is greater than a third reference state E3, step 41 is performed. In step 41, the control member 21 finds that the concentration of solvent in the reducing solution is too low and that a filling of the reservoir 7 with solvent is necessary. Optionally, step 41 may be accompanied by a warning message to the user indicating that the concentration or the rate of solvent of the reducing solution is too low and that a filling of the tank 7 with water is necessary.

Whatever the result of the various comparison tests, the results are sent to the control member 21 which controls the injector 6.

This control member 21 receives further information 42 and 43. The information 42 is a table of amount of reducing agent to be injected for a quantity of NOX determined to reduce.

In the case of a use of urea as a reducing agent, the nitrogen oxides are completely eliminated when the ratio between the mole fraction of urea and the mole fraction of the nitrogen oxides is substantially 0.5 because on the one hand, ammonia is formed by thermohydrolysis of the urea according to the following reaction:

(NH 2) CO (NH 2) + H 2 O - 2 NH 3 + CO 2 (1) Thus one mole of urea gives two moles of ammonia, and on the other hand the reaction of elimination of the oxides of nitrogen with the ammonia is equimolar. The emission of nitrogen oxides at the outlet of the engine 1 thus defines the amount of reducing agent to be injected into the exhaust line 2. The same applies if there is a plurality of reducing agents identified by to the absorption spectrum. From the determination of their respective concentration and knowing their reaction equation, it is easy to determine the amount of reducing solution to be injected for a quantity of NO, determined to reduce.

The information 43 is a NO / NO 2 ratio determined at the outlet of the oxidation catalyst 3. The information 43 can be determined from the information 44, 45, 46. The information 46 is the quantity of NOX emitted in the emissions The information 46 can be determined from the information 45 of a NOX emission table in the emissions of the engine 1 or from data measured by a NOX detector 8 or 10. information 44 is a table of NO / NO2 ratios.

With the knowledge of all this information, the control member 21 is able to manage the reducing solution and in particular its injection into the exhaust line.

The control method of the reducing solution thus makes it possible to accurately and reliably determine the concentration of a plurality of reducing agents and / or solvents in the reducing chemical solution, from a single measuring device.

In a variant not shown, the optical sensor 15, the heating element 19 and the temperature sensor 20 are arranged in an auxiliary tank connected to the main tank 7. The auxiliary reservoir is of smaller volume than the main reservoir and comprises a sample of the reducing chemical solution. It is thus possible to heat the sample to a temperature different from the rest of the solution contained in the main tank 7. Since the volume of the solution sample is small, the energy expended for heating is advantageously less.

Claims (10)

Revendications1. A method of controlling a chemical solution comprising a reducing agent for the catalytic reduction of nitrogen oxides and a solvent, said method comprising a step (29) of heating at least one sample of the solution within a predetermined range characterized in that, the sample being in said temperature range, it comprises a step (33) for determining an absorption spectrum produced by the illuminated sample of an incident spectrum of wavelengths data. 2. Method according to claim 1, characterized in that the heating step comprises a step (30) of measuring the temperature of the solution and a step (31) of heating the solution as long as the temperature of the solution is less than the predetermined temperature range. 3. Method according to claim 1 or claim 2, characterized in that it comprises a step (34) of identifying the presence of an absorption peak (22) of at least one predetermined chemical compound from absorption spectrum. 4. Method according to claim 3, characterized in that the predetermined chemical compound is an ammonia precursor reducing agent or a solvent of the reducing agent. 5. Method according to claim 3 or claim 4, characterized in that it comprises, according to the absorption peak (22) determined, a step (34) for determining the concentration of said chemical compound. 6. Method according to claim 5, characterized in that it comprises, according to the determined concentration, a diagnostic step (39, 40, 41) as to the presence or the concentration of said compound of the solution with respect to at least one reference state (E1, E2, E3) and / or emission of a suitable warning message characterizing the difference between the measured concentration or the detected presence of said compound and at least one reference state. 7. Method according to any one of the preceding claims, characterized in that the incident spectrum is adapted to cover all the sensitive compounds of said solution and in particular the reducing agent and the solvent. 8. Device for controlling a chemical solution comprising a reducing agent for the catalytic reduction of nitrogen oxides and a solvent, the device comprising a reservoir (7) containing at least one sample of the solution, a heating element (19) ) of the sample in a predetermined temperature range and, characterized in that it comprises a spectroscopic measuring device (12) able to analyze the sample and more particularly to determine the concentration of at least one predetermined chemical compound of the sample present in the reservoir (7) and control and calculation means (21). 9. Device according to claim 8, characterized in that the spectroscopic measuring device comprises a light source (13), a spectrometer (14), an optical sensor (15) disposed in the reservoir (7) connected by transmission means optical (16) to the light source (13) and the spectrometer (14), the control and calculation means (21) being able to control the operation of the light source (13), the spectrometer (14), the heating element (19) and deriving from the information supplied back by the spectrometer (14), said concentrations. 10. Internal combustion engine comprising an exhaust line (2) provided with a selective catalytic reduction device of the nitrogen oxides, characterized in that it comprises a device according to claim 8 or claim 9.