FR2899277A1 - METHOD FOR PILOT CONTROL OF A LAMBDA COEFFICIENT. - Google Patents

Abstract

Pilot control method of a lambda coefficient during the heating phase of the exhaust gas cleaning installation of an internal combustion engine comprising at least one catalyst and at least one lambda probe. of the lambda pilot control (10) during the catalyst heating phase, at least from time to time a higher frequency modulation so as to predefine in this phase an average value of the lambda coefficient as a function time (12) such that lambda> 1 and that one reaches at least briefly a coefficient lambda such as lambda <1.

Description

STATE OF THE ART The present invention relates to a control method

  pilot of a lambda coefficient during the heating phase of the exhaust gas cleaning system of an internal combustion engine comprising at least one catalyst and at least one lambda sensor. To comply with the strict standards relating to In the exhaust of internal combustion engines, not only must the gross emissions be reduced, but the catalyst must be warmed up as quickly as possible to the correct operating temperature before starting. For this, various means are known such as for example the increase in the temperature of the exhaust gas by the ignition delay, the enrichment of the mixture combined with the secondary air injection, the use of a glow plug in the exhaust gas line upstream of the catalyst. In the case of turbocharged turbo-charge engines and charge pressure control valve, the charge pressure control valve can be opened to rapidly heat the catalyst. All these measurements depend on the exhaust gas concept (rich or lean) (for example the load-displacement flap, the secondary air injection, the camshaft adjustment, etc.). .) and influence the quality of the exhaust gas. The regulations require that the effectiveness of these measurements in motor vehicles can be checked by onboard diagnostics. If during cold start any of these measures could not be adjusted, the result would be that the limit values of the exhaust gases would not be respected. On the other hand, from the point of view of the conversion of the components of the exhaust gas, the oxygen storage capacity by the catalyst OSC (Oxygen Storage Capacity) is particularly interesting. The accumulation capacity of an exhaust gas cleaning plant for oxygen is used to take oxygen in the lean phases and to restore oxygen in the rich phases. This makes it possible to convert the harmful oxy- gen components of the exhaust gases. But the oxygen storage capacity decreases with the aging of the exhaust cleaning system. Under these conditions, during the rich phases, it may no longer have enough oxygen to clean the exhaust gas and remove the harmful components; the lambda probe downstream of the exhaust cleaning system detects these components to be oxidized. In addition, this lambda sensor detects oxygen in the lean, prolonged phases, oxygen that is no longer stored in the exhaust cleaning system. Many countries prescribe by regulation the control of the exhaust cleaning system during operation by the engine control (on-board diagnosis). An active catalyst diagnosis thus serves to detect an unacceptable drop in the conversion resulting in an unacceptable increase in the exhaust coefficients and to display this situation by a control indicator. The diagnosis of the catalyst with oxygen measurement is carried out using a wide-band probe proceeding as follows: Oxygen is first removed from the catalyst by a rich mixture (X <1). After this conditioning phase, a lean exhaust gas (X> 1) is introduced and the quantity of oxygen supplied is integrated. If during this lean phase, the probe downstream of the catalyst displays a lean mixture (that is to say containing oxygen), the integrated amount corresponds to the current oxygen storage capacity by the catalyst; this ability is a measure of quality. This process is applied several times successively. DE 41 12 478 C2 describes a method for evaluating the aging state of a catalyst according to which the lambda coefficient is measured upstream and downstream of the catalyst. It is examined whether in the case of a regulation oscillation upstream of the catalyst passing from a rich mixture to a lean mixture or inversely, the lambda coefficient downstream of the catalyst has a corresponding variation and if this is the case it is determined the mass flow rate of the gases passing through the catalyst, the integral as a function of time of the product of the mass flow rate of the gases and the lambda coefficient upstream of the catalyst are calculated, the integral as a function of time of the product of the mass flow rate of the gases is calculated and the lambda coefficient downstream of the catalyst and, as a measure of the state of aging of the catalyst, either the difference between the two integrals or the quotient of the two integrals or the quotient of the difference by one of the two integrals is used. . The disadvantage of the method thus described is that it is necessary to measure the lambda coefficient before the exhaust cleaning installation using an expensive broadband lambda probe to determine the quantity of exhaust gas. oxygen supplied or withdrawn using the integration of the product of the current value of the lambda coefficient by the mass flow rate of the gases. EP 0546 318 B1 discloses another method for determining the oxygen storage capacity in an exhaust cleaning plant. An evolution of the lambda coefficient is applied to the system, the lack of oxygen at the beginning then being greater than the oxygen storage capacity of the catalyst. The oxygen supply is chosen so that in the lean phases, the catalyst is each time filled up to its capacity limit. In order to determine the oxygen capacity of the catalyst, the average value of the lambda coefficient is displaced upstream of the catalyst during the oscillations of the lambda coefficient of the system, in a controlled manner in the direction of a lean mixture, thereby reducing the oxygen uptake. from one phase to another. By determining the number of rich mixture / lean mixture transitions displayed by the lambda probe installed downstream of the catalyst, the oxygen storage capacity can be determined; an increase in the number of phases corresponds to a decrease in the accumulation capacity. The disadvantage of this process is the emission of uncleaned exhaust gases during lean phases.

  Document DE 10240977 A1 describes a method for managing an internal combustion engine, particularly a motor vehicle, with a direct fuel injection system comprising an exhaust gas treatment system with at least one catalyst. For heating at least one of the catalysts it is intended to operate the internal combustion engine cyclically in lean mode with X> 1 to fill the oxygen accumulator of at least one of the catalysts and in the rich with <1 so that the heat of reaction of the unburned components of the fuel with the oxygen of at least one of the catalysts is used for heating the catalyst. Firstly, by a sudden variation of the lambda coefficient, the oxygen storage capacity of each catalyst separately or of a group of catalysts is determined and, with the aid of the oxygen storage capacity thus obtained for the catalyst or catalysts is determined a lower heating coefficient relative to oxygen, and taking into account a lambda coefficient of the exhaust gas and mass flow of the exhaust gas is determined intake in energy and thus the temperature in at least one catalyst. Thus, a regulation and control of the heating of the catalyst (s) is carried out by adjusting the rich and / or poor phases.

  According to the state of the art, to promote the rapid heating of the catalyst, after start a slightly lean mixture is controlled with X> 1. As the dew point, especially for the cold start of the lambda probe is not If it is still passed downwards, the operating state of the lambda probe is reached only after the start. For this purpose, the lambda coefficient is controlled directly after start-up on a value> 1 and which is generally around 1.05. This ensures that in spite of the deviations related to the dispersions of the series of manufacture, in the dosage of fuel one always respects a slightly lambda coefficient lambda. If the coefficient = 1 is exceeded, this would lead to a sharp increase in CO and HC emissions and, where applicable, to the non-compliance of the regulatory emission limits. However, thanks to a lambda lean coefficient, even after exceeding the so-called ignition temperature, temperature from which the catalyst is at an optimum operating temperature and is in principle able to function as a three-way catalyst, it would not It is not possible to convert the nitrogen oxides contained in the exhaust gas.

  OBJECT OF THE INVENTION It is therefore the object of the present invention to develop a process which, as soon as the catalyst heating phase is complete, ensures at least partly a conversion of the NOx nitrogen oxides without, in this phase, exceeds the regulatory limits of the components to be oxidized. DESCRIPTION AND ADVANTAGES OF THE INVENTION For this purpose, the present invention relates to a process of the type defined above, characterized in that the lambda coefficient of the pilot control lambda is applied to the chronogram, during the heating phase of the catalyst, at least from time to time a higher frequency modulation so as to predefine in this phase an average value of the coefficient lambda as a function of time such that> 1, and that we arrive at least briefly at a lambda coefficient such that X <1. This control strategy of the lambda coefficient per-puts at this stage to ensure a partial conversion of nitrogen oxides because at least from time to time we arrive at a coefficient lambda such as <1. At the same time Meanwhile, the lambda coefficient, which remains on average lean, does not negatively influence the conversion of components to be oxidized such as HC hydrocarbons or carbon monoxide CO. It has even been observed that this pilot control strategy aided to a lesser extent the conversion of the components to be oxidized. For this, by a pilot control on a coefficient> 1, for example = 1.05, it is possible to cover with certainty the dispersion related to mass production so that there is no risk of exceeding the limited by the components to be oxidized as indicated above. The addition of a high frequency modulation is according to a preferred variant of the method for the timing of the Lambda coefficient Lambda pilot control is controlled with an asymmetrical working ratio between the phases corresponding to 2> 1 and those corresponding to 2 <1, by relatively relatively short order, a Lambda coefficient significantly lower than 35 1 and in part by leaving the coefficient 2> 1 relatively long, to ensure the conversion of the components to be oxidized such as hydrocarbons HC and carbon monoxide CO. In contrast, a Lambda coefficient significantly lower than 1 accelerates the conversion of nitrogen oxides.

  In addition, it is intended to modify the amplitude and / or frequency high frequency modulation according to the operating point of the internal combustion engine, which allows a stable pilot control of the Lambda coefficient. Since, for example, the oxygen storage capacity (OSC) of the catalyst (s) undergoes some aging, the invention provides that the amplitude of the high frequency modulation is adapted to the present value of the storage capacity of the catalyst. oxygen (OSC) of the catalyst (s). This pilot control of the Lambda coefficient ensures, on the one hand, a conversion rate as constant as possible of the nitrogen oxides and, on the other hand, HC hydrocarbons and carbon monoxide CO, which guarantees the respect of the limit values for harmful components even after some aging. In general, it is advantageous and advantageous to increase the frequency of the high frequency modulation as a function of the decrease in the oxygen storage capacity (OSC) of the catalyst (s). This reduces the risk of overshooting by spikes of rich mixing. The method provides for determining the present value of the oxygen storage capacity (OSC) of the catalyst (s) by a method for diagnosing oxygen storage capacity (OSC) and recording this value. This value can then be used directly to calculate the optimal modulation from the point of view of the frequency and the amplitude and, where appropriate, the working ratio between the phases corresponding to a rich mixture 2> 1 and the phases corresponding to lean mixture. 2 <1. If in addition to the oxygen storage capacity (OSC) of the catalyst (s), there is a volumetric weighting of the catalyst temperature and if this is taken into account for amplitude modulation and / or or Lambda coefficient timing of the Lambda pilot control during the heating phase of the catalyst (s), it is possible to determine from there a capacity and a rate of oxygen accumulation which is a function of the temperature in the the catalyst (s). Since the catalyst or catalysts are heated back and forth during the heating phase (depending on the direction of passage of the gases), this volumetric weighting can determine a catalyst characteristic temperature for the whole of the catalyst or catalysts. features for other cal-culs. A particularly preferred variant of the method provides for measuring the value of the oxygen storage capacity (OSC) of the catalyst (s) and the value of the volumetric weighting of the catalyst temperature and of recording this value in the characteristic fields. This makes it possible to represent or exploit advantageously even complex functional relationships.

  According to the method described above, the amplitude of the modulation of the Lambda coefficient timing diagram of the Lambda pilot device can be limited. This is particularly interesting for the regularity of operation if one leads especially a lean Lambda coefficient.

  If the above-described measures are applied during catalyst heating and before the Lambda probe is ready for operation, the activity of the catalyst (s) can be further increased for this measure to be advantageous. even before reaching the triggering phase, and it is known to increase the activity of the catalyst (s) for the conversion of the components to be oxidized (HC, CO) by a modulation of the Lambda coefficient from the regular mode 2 ,, = 1. If the measurements are applied to internal combustion engines with direct injection of gasoline or of injection in the intake pipe, it is of interest, in particular for engines with direct fuel injection, to combine the control strategy known operating mode called split homogeneous mode because here improves the ability to operate in lean mode compared to the pure homogeneous mode. In principle, it is also possible to combine an alternating mode of operation. For example, if the combustion process allows it, the stratified mode can be selected in the lean part of the amplitude whereas in the rich part of the amplitude the homogeneous mode is used. If for an injection in the intake pipe, the ability to operate in lean mode is ensured in the required ranges, it allows to apply this strategy also here. Drawings The present invention will be described hereinafter in more detail with the aid of an exemplary embodiment shown in the accompanying drawings, in which: FIG. 1a is a schematic view of a symmetrical pilot control of the Lambda coefficient, FIG. 1b is a diagrammatic representation of an asymmetric pilot control of the Lambda coefficient, and FIG. 2 is a schematic view of the timing diagram of the Lambda coefficient, the oxygen concentration and different concentrations of pollutant. DESCRIPTION OF EMBODIMENTS The technical environment in which the process of the invention is carried out is, for example, an internal combustion engine formed of an engine block and an air supply channel supplying the engine block. with combustion air. The amount of air in the air supply channel is determined using an air measurement system. The exhaust gases of the internal combustion engine pass through an exhaust cleaning plant whose main component is an exhaust gas channel; according to the direction of passage of the exhaust gas, the channel comprises a first Lambda probe upstream of a catalyst and a second Lambda probe downstream of the catalyst. The exhaust gas cleaning plant may also have a second catalyst downstream of the second Lambda probe. The Lambda probes are connected to a control system that calculates the mixture from the data provided by the lambda probes and the data from the feed air metering system to control a fuel metering system. measure the fuel with corresponding injectors in the air supply channel. The Lambda sensor installed in the exhaust gas duct downstream of the engine block enables a Lambda coefficient to be set using the control system, which allows the exhaust gas cleaning system to achieve an optimum cleaning effect. The Lambda probe can be a simple snap-in probe or a complicated, broad-band probe that can determine the Lambda coefficient in a wide band. If the Lambda probe is a probe with sudden variation, it is more economical but allows only regulation on a set value 2 ,, = 1. Thus, during the determination of the capacity of oxygen accumulation, one does not can activate the regulation of the Lambda coefficient. In this operating phase, there is only a pilot control of the fuel mixture with a Lambda coefficient predefined by the control system. The second Lambda probe installed in the exhaust gas channel downstream of the first catalyst can also be used by the control installation and used to determine in a process according to the state of the art, the ability to accumulate oxygen through the exhaust cleaning system. The control system can also be connected to a memory unit and display which allows for example to display a malfunction of the exhaust cleaning system. In addition, this also indicates whether, for example in the case of an on-board diagnosis, a catalyst oxygen storage capacity has already been determined which would already be classified as a limit for complying with the exhaust gas regulations. In order to determine the oxygen storage capacity, generally during the on-board diagnosis, the internal combustion engine is first run for a sufficient period of time with excess fuel (rich mixture) to reduce all the oxygen contained in it. the catalyst. During the operating mode with lean, consecutive mixture, oxygen accumulates in the catalyst; With the Lambda probe, it is determined whether downstream of the catalyst oxygen-rich exhaust gases are present, since the oxygen storage capacity OSC is exceeded. According to the state of the art, a sudden variation between the value 2 <1 and the value 2> 1 is predefined. For this pilot control it is possible to have defects generated for example by the manufacturing dispersion of the various injectors or defects in filling input. These defects distort the value of the Lambda coefficient and thus also result in errors in the determination of the oxygen storage capacity.

  During the heating phase, directly after the start of the internal combustion engine, in addition to the assistance for the rapid heating of the catalyst, it is planned to control a slightly lean mixture after starting with a coefficient of 2> 1. especially when cold start of the Lambda probe, it is still below the dew point, the Lambda probe will be ready to operate significantly after startup. As shown schematically in FIGS. 1a and 1b, during the heating phase of the exhaust cleaning system of the internal combustion engine, it is intended to apply a timing diagram of the Lambda coefficient 11 of the pilot control 10 of the coefficient Lambda during the heating phase of the catalyst, at least from time to time with a higher frequency modulation so that in this phase, a mean value of the Lambda 12 coefficient in time such that 2> 1 is predefined; in the example presented, we have 2 ,, = 1.05, and at least briefly, we arrive at a Lambda value such that 2 <1. Figure la shows a variant of the method with a Lambda pilot control. 10, symmetrical. FIG. 1b shows a variant of the method for a timing diagram of the Lambda coefficient 10 of the pilot control Lambda 10 with an asymmetrical working ratio between the phases 2> 1 and 2 <1. The conversion ranges of the NOx nitrogen oxides Are within the range of the Lambda timing chart 11 for which there is 2 <1 insofar as the catalyst (s) are in the triggered phase, i.e. have reached the operating temperature. NOx oxides reduced in rich phase can no longer be reconstituted in the catalysts nor downstream in the exhaust pipe for reasons of kinematic reaction. In this context, it should nevertheless be noted that a conversion of NOx nitrogen oxides with yields greater than 99% can not be obtained as is possible for a regulated mode of operation on 2 ,, = 1 of a 3-way catalyst but this is not the objective of the strategy described. According to the invention, the high-frequency modulation is modified in amplitude 13 and / or in frequency according to the operating point of the internal combustion engine. The amplitude 13 of the higher frequency modulation can be adapted to the present value of the oxygen storage capacity (OSC) of the catalyst (s). It is also possible to increase the frequency of high frequency modulation as a function of the decrease in the value of the oxygen storage capacity (OSC) of the catalyst or catalysts. The present value of the oxygen storage capacity (OSC) of the catalyst (s) can be determined by a method for diagnosing oxygen storage capacity (OSC) as described above. and this value is recorded. In addition to the oxygen storage capacity (OSC) of the catalyst (s), it is also possible to volumetrically weight a catalyst temperature, and to account for it during amplitude modulation and / or or frequency of the timing of the Lambda-11 coefficient of the Lambda pilot control during the heating phase of the catalyst or catalysts. To calculate the amplitude 13 and frequency of the modulation in the preferred process variant, the value of the oxygen storage capacity (OSC) of the catalyst (s) and the value of the volumetric weighting of the catalyst are preferably measured. the temperature of the catalyst, and deposited in fields of characteristics. The above-described strategy can be applied hereinafter during various phases of catalyst heating as shown by the following combination presented in tabular form and applying the measurements during catalyst heating or as the probe Lambda is able to work.

  Operating point- Phase Status suitable for operation Pilot control temperature for the operation of Lambda probe coefficient Lambda Lambda probe according to reach? the invention Start A No, the end of the No Inactive dew point is not reached Heating of B No, the end of No Active or inactive catalyst dew point is not reached Heating of C No, the end of Yes Active, catalyst dew point is not reached Heating D Yes Yes Inactive; regulated catalyst mode À = 1 Operation E Yes Yes Inactive, regular mode (no heating operation of the catalyst) regulated = 1 It may be advantageous to apply in phase B that is to say before reaching the temperature the operation of the catalytic converter (triggering) insofar as, in addition to improving the conversion of nitrogen oxides NOx, the activity of the catalyst (s) to convert the components to be oxidized (HC; ). In addition, depending on the design of the system or application, phase D can be omitted so that it goes directly to regular operation without further catalyst heating. This can then be done in the phase with a regulated or unregulated Lambda coefficient depending on whether the Lambda probe is ready for operation or not. It is also possible to switch to phase D with a Lambda sensor capable of operating, to transmit the control strategy to the control strategy. The predefined value of the Lambda coefficient can not be entered directly into the control of the Lambda coefficient as a set value. FIG. 2 schematically shows the Lambda timing diagram 11, the oxygen concentration chronogram 40 as well as different pollutant concentrations 30, 60. According to the setting of the frequency and the amplitude 13, in the example presented, by comparison with the mode controlled with a constant 2 ,, = 1.04, it is possible to carry out a NOx conversion of nitrogen oxides of greater than 50%. It should be noted that with the increase of the amplitude 13, the modulation of the pilot control Lambda 10 successively decreases the concentration of nitrogen oxide NOx 30. It appears in FIG. 2 that for a too small amplitude 13 for the Lambda coefficient is no longer controlled below unity, a decrease in the concentration of nitrogen oxides NOx 30 is observed. It is only with the increase in amplitudes 13 that the concentration of Nitrogen oxides NOx 30 decreases. The oxygen concentration 40 remains constant over wide ranges, for example about 0.6%. The Oxygen Accumulator (OSC), which is always equipped with a 3-way catalytic converter, makes it possible to receive the rich mixing peaks of the Lambda coefficient driven on the rich mode so that with a good determination of the frequency and the amplitude 13 CO and HC are not expected to be significant, as shown by the chronograms of concentration of carbon oxides CO, 50 and that of unburned hydrocarbons HC, 60. The process allows that even if the regulation of the Lambda coefficient does not is not yet ready for operation, after the start of the internal combustion engine, a pilot control of the Lambda coefficient is carried out which, as soon as the catalyst heating phase is complete, ensures a partial conversion of the nitrogen oxides without causing a significant deterioration of the components to be oxidized. Another advantage of this procedure is that we are not bound

  precisely at a fixed value of the Lambda coefficient as is necessary for example for 2 ,, = 1, for which a Lambda probe ready for regulation would be required.

Claims (3)

  1) Pilot control method of a lambda coefficient during the heating phase of the exhaust gas cleaning installation of an internal combustion engine comprising at least one catalyst and at least one lambda probe, characterized in what is applied to the timing diagram of the lambda coefficient (11) of the lambda pilot control (10), during the heating phase of the catalyst, at least from time to time a higher frequency modulation so as to predefine in this phase a mean value of the lambda coefficient as a function of time (12) such that> 1 and that one reaches at least briefly a lambda coefficient such that X <1. 2) Method according to claim 1, characterized in that one drives the timing of the lambda coefficient (11) of the pilot control lambda (10) with a non-symmetrical working ratio between the phases corresponding to> 1 and those corresponding to < 1. 3) Process according to claim 1, characterized in that according to the operating point of the internal combustion engine, the high frequency modulation amplitude (13) and / or frequency 4 is modified. The method according to claim 3, characterized in that the amplitude (13) of the high frequency modulation is adapted to the present value of the oxygen storage capacity (OSC) of the catalyst (s). 5) Process according to claim 3, characterized in that increases the frequency of the high frequency modulation as a function of the decrease of the oxygen storage capacity (OSC) of the catalyst or catalysts. 6) Process according to claim 4, characterized in that the current value of the oxygen storage capacity (OSC) of the catalyst (s) is determined and recorded by a method of diagnosis of the accumulation capacity of oxygen (OSC). 7) Process according to claim 3, characterized in that in addition to the oxygen storage capacity (OSC) of the catalyst or catalysts, a volumetric weighting of the catalyst temperature is carried out, and is taken into account for the amplitude and / or frequency modulation of the timing diagram of the lambda coefficient (11) of the lambda pilot control (10) during the heating phase of the catalyst (s). 8) Process according to Claim 7, characterized in that the value of the oxygen storage capacity (OSC) of the catalyst (s) and the value of the volumetric weighting of the catalyst temperature are measured and recorded. in a field of characters. 9) Process according to claim 1, characterized in that one limits the amplitude (13) of the modulation of the chronogram of the coefficient lambda (11) of the lambda pilot control (10). 10) Application of the method according to one of claims 1 to 9, characterized in that the measurements are applied during the heating of the catalyst and before reaching the operating temperature of the lambda probe. 511) Application of the method according to one of claims 1 to 10, characterized in that the measurements are applied to internal combustion engines with direct injection of gasoline or injection into the intake duct. 10