FR2844880A1 - Evaluation of the time response of a gas sensor for the detection of nitrogen oxides in internal combustion engine exhaust gas, involves using a change in concentration in the exhaust gas - Google Patents

Abstract

Process for evaluating the time response of a nitrogen oxide (NOx) gas sensor located in the path of the exhaust gas of an internal combustion engine involves using a change in concentration appearing in the exhaust gas to determine a time constant from a change in measurement signals and the time interval between the time instants at which the measurements were recorded. Evaluation of the time response of a gas sensor located in the path of a the exhaust gas of an internal combustion engine, and which delivers a measurement signal, utilizes a change in the concentration in the exhaust gas. The evaluation process is characterized by: (a) an average rate of average change in concentration greater than an expected maximum rate of change in the measurement signal; (b) during the change in the measurement signal caused by the change in concentration, a first value and a second value of the measurement signal are recorded respectively at a first and second instant; and (c) a time constant for the change in the measurement signal is determined from the first and second values as well as the interval of time separating the first and second instants. In the above process, a reduction in the concentration of exhaust gases is considered as the change, and the time interval is divided by the difference in the logarithms of the first and second values of the measurement signal. The reduction in the concentration is utilized by activating a shut-off in driving thrust. In the case of an internal combustion engine whose exhaust gas path comprises a catalytic converter for removal of nitrogen oxides (NOx) located upstream of the gas sensor, a reduction in the concentration of ammonia is utilized by making the internal combustion engine transfer from operation of regeneration with a sub-stoichiometric mixture to operation with a stoichiometric mixture. A change in the concentration occurring in any way during normal operation of the internal combustion engine can be utilized. Determination of the time constant and its mean is repeated several times for the same change in concentration.

Description

The invention relates to a method for evaluating the temporal response

  a gas sensor disposed in the exhaust path of an internal combustion engine and delivering a measurement signal, wherein a change of concentration in the exhaust gas of the internal combustion engine is used . The exhaust gases of internal combustion engines contain different polluting substances such as hydrocarbons, carbon monoxide and nitrogen oxides (NOx). The concentration of the polluting substances contained in the exhaust gases depends strongly on the operating parameters of the internal combustion engine, in particular the air / fuel ratio of the mixture with which the internal combustion engine operates. In order to improve the quality of the exhaust gas, reprocessing devices having one or more catalysts which convert the polluting substances are usually used. The operation of the internal combustion engine is then controlled by appropriate sensors which analyze the exhaust gas and is adjusted using these sensors. Typical sensors are, for example, probes

  lambda used to determine the air / fuel ratio.

  In order to further reduce the fuel consumption of Otto type internal combustion engines, lean combustion engines are increasingly being used. In the case of these internal combustion engines, it is necessary to take special measures to comply with the current limit values for NOx emissions. In the case of such internal combustion engines, more and more NOx sensors are used which detect the

  concentration of nitrogen oxides present in the exhaust gas.

  In order to ensure compliance with the gas emission limit values in force over the entire service life of an internal combustion engine, there is a growing demand for an autodiagostic (on-board diagnostic device). of all exhaust gases downstream of the treatment system. Such an on-board diagnostic device must control the operating capacity of the elements having an influence on the exhaust gas; the operating capacity of the

  Used gas sensors must also be checked. This control consists in particular in regularly evaluating the dynamic behavior of the measuring sensors used, so as to be able to determine a slowing of the response of the measuring sensor and, in case of insufficient dynamic behavior, to be able to diagnose a defective measuring sensor.

  Depending on the position of the sensor, this can sometimes be problematic. For example, do not the exhaust gases usually show large variations in

  concentration of substances downstream of a catalyst, so that it is difficult to detect a slow response of a measurement sensor located downstream of a catalyst arranged in the exhaust path of a combustion engine internal.

  With the aid of DE 198 28 929 AI, it is known in this connection to take advantage of a regeneration phase of a NOx storage catalyst, during which the internal combustion engine operates with a sub-stoichiometric mixture, in order to detect a rate of change of the measurement signal of a measurement sensor. If the rate of change remains below a minimum value, a faulty sensor is diagnosed. The disadvantage of this method lies in the fact that the excitation of the NOx signal by means of a change of NOx concentration must be known at the level of the measuring sensor or must satisfy given conditions. When an internal combustion engine is in operation, however, it is not always possible to meet these basic conditions so that either the diagnostic performed is of lower quality or the frequency with which a diagnosis is made. is

greatly reduced.

  The problem which is therefore at the basis of the invention is to improve a method making it possible to evaluate the temporal response so as to allow a more precise diagnosis. This problem is solved according to the invention by a method making it possible to evaluate the temporal response of a gas sensor arranged in the exhaust gas path of an internal combustion engine and which delivers a measurement signal, a method for of which a change of concentration is used in the exhaust gas of the internal combustion engine, the method being characterized in that an average rate of change of the concentration change is greater than a maximum expected change rate of the measurement signal. during the change of the measurement signal caused by the change of concentration, a first value and a second value of the measurement signal are respectively entered at a first and a second time, and a time constant for the change of the measurement signal. measurement signal is determined from the first and the second value as well as from the interval of

  time separating the first and the second moment.

  The method according to the invention may further comprise one or more of the following characteristics: a decrease in the concentration is used as a change and the time interval is divided by the difference of the logarithms of the first and second value of the measurement signal; the method is used in the case of an internal combustion engine whose exhaust gas path comprises an NOx storage catalyst arranged upstream of the gas sensor, and a decrease in a concentration of NH 3 is used in passing the internal combustion engine from regeneration operation with a substoichiometric mixture to operation with a stoichiometric mixture; a decrease in the concentration is used by activating a power supply cut-off; a third value of the measurement signal is entered at a third instant, the first and the second instant as well as the second and third instant being respectively separated by an identical time interval, an increase in the concentration is used as a change, and the time constant is calculated according to the following formula: T = (y3-y2) 2y2-y3-yl T being the time constant, the time interval, y1 the first value, y2 the second value and y3 the third value, and the time interval d being less than an expected value for the time constant T; a third value of the measurement signal is entered at a third instant, the first and the second instant and the second and third instant being respectively separated by an identical time interval, an increase in the concentration is used as a change, and the time constant is calculated according to the following formula: ## EQU1 ## where T is the time constant, the time interval and Z being given by the following equation, wherein y1 denotes the first value, y2 the second value and y3 the third value: y3-yl j1 (3- y2 y2-y3 Z = -0,5y-y + y-) y yl-y2-a4 yly2 yl-y2 'only being used for Z values that are greater than 0 and less than 1; an increase in a concentration of NH 3 is used as a change by running the internal combustion engine into operation with a substoichiometric mixture; - A full charge enrichment is triggered to pass the internal combustion engine in operation with a sub-stoichiometric mixture; an increase in NOx concentration is used as a change by passing the internal combustion engine from stoichiometric mixing to operation with an overbased mixture; for the evaluation, a change in the concentration occurring anyway during the normal operation of the internal combustion engine is used; the determination of the time constant and its mean is repeated several times; - the repeated determination of the time constant and its mean is

  achieved during the same concentration change.

  The method according to the invention therefore no longer determines the rate of rise or the mean change when the sensor is subjected to an accurately predetermined change of substance concentration but evaluates the sensor signal so that henceforth only the time constant is determined. The requirement for changing the concentration of substances is relaxed in that it now suffices simply that the change has a given minimum slope or rate of change. This rate of change must be greater than the maximum expected variation speed of the measurement signal. The sensor then perceives this

  change of concentration as a step function.

  Absolute values, that is, the level at which the change is made, are not relevant for determining the time constant. The determination of the time constant only presupposes that at least two values of the

  sensor are sampled in a known time interval.

  To compute the time constant from these two values taking into account the time interval separating said values, model assumptions relating to the temporal response of the sensor are usefully employed. In most cases, it will be necessary to take, for this purpose, a characteristic called PT1, that is to say corresponding to a first-order timer element. In this case, the following equation can be used to describe the time course of the S signal during the excitation caused by the change of an amplitude A of the magnitude to be measured: S (t) = A (1e) S0 e '(1) In this equation, T is the sensor's time constant and SO is the initial value of the sensor signal at the time of the change. Such a temporal response of a PT1 element makes it easy to calculate, in simple reports, the time constant T in the event of an upward or downward flank, that is to say in the case of a decrease or increase in the concentration. to be measured, which satisfies the

condition according to the invention.

  In the case of a decrease in the concentration, the time constant T is obtained for a signal rate according to equation (1) by dividing the time interval of the measurement values by the difference of the logarithms of the values. measurement. It is thus possible, by means of a simple calculation, to calculate the time constant T from the measured values and the known interval. Of course, it is also possible to use, in place of the difference in logarithms, the logarithm of the quotient obtained from the two measurement values, the logarithm of a quotient of two values being equivalent, in a known manner, unlike logarithms

values.

  It is therefore possible, while at the same time sparing the computing resources, to calculate the time constant by resorting to a characteristic diagram.

  in which is recorded the time constant determined for a corresponding pair of values, from a predefined time interval used to obtain the two values of the measurement signal. Usefully, such a characteristic diagram will make the difference between changes with increasing or decreasing concentration of substances.

  Unexpectedly, however, it turned out that it was possible, even with relatively simple processes, to calculate the time constant of

mathematical way.

  That this calculation can be done in an approach of this type was a priori surprising, the amplitude A, the initial value SQ and the time constant T constituting three unknowns in the equation in question so that one could not not actually expect, using two measures, to successfully solve the equation to get

  the time constant T sought.

  The invention, however, takes advantage of the fact that equation (1) is simplified in certain special cases. When "triggering" a concentration of substances, i.e. of an upward flank, the initial value SO may be zero. The same goes for the opposite operation; the amplitude A is then equal to zero. Equation (1)

  is thus simplified accordingly.

  The signal of the gas sensor is known at each given instant t. On the other hand, neither the amplitude A, nor the initial value SQ, nor the time constant T are known. In addition, the time interval separating the two instants where the measurement has been performed is known. The time constant T thus computable now provides direct information on the dynamic properties of the signal and therefore of the gas sensor. Thanks to a cleverly made connection, it is not necessary, to perform the calculation, to know the amplitude A of the excitation, in other words the change of concentration of substances, which makes it possible to obtain greater independence during the determination of the time constant T. The method according to the invention, which makes it possible to evaluate the dynamics of a gas sensor, does not therefore depend on the amplitude A of the concentration change, the method thus being able to be used universally. This property is particularly valuable for establishing the diagnosis of gas sensors located in an exhaust system downstream of a catalyst, the substance concentration changes generally not being known at this point in time.

  quantitatively or fixedly adjustable.

  The method is equally suitable for linear lambda probes as for diagnosing NOx sensors. For the process according to the invention, it suffices to know the concentration change qualitatively. At this

  In this regard, several changes of concentration are taken into account.

  In the case of an internal combustion engine whose exhaust path comprises a NOx storage catalyst arranged upstream of the NOx sensor, these NOx accumulating catalysts generally operate in a discontinuous manner. In a storage phase, they adsorb the NOx compounds present in the exhaust gas, which appear when the internal combustion engine operates with an over-stoichiometric air / fuel mixture. The NOx accumulator catalyst whose absorption capacity is only limited is emptied during so-called regeneration phases, during which the internal combustion engine operates momentarily with a sub-stoichiometric air / fuel mixture. . The carbon oxides and non-burned hydrocarbons thus appearing in the exhaust gas serve as reducing agent for the NOx storage catalyst. During such regeneration phases, NH 3 inside the catalyst also appears, a simple intermediate product which does not leave said catalyst. This only appears when the regeneration phase is over and there is no more NOx in the accumulator catalyst. Then, the NH 3 as well as the exhaust gas components generated with the sub-stoichiometric air / fuel mixture reach the exit of the catalyst. As a result, the internal combustion engine switches back to over-stoichiometric operation, so that NOx are again present in the exhaust gases and the conditions necessary for the emission of NH 3 are no longer present.

  made at the outlet of the catalyst.

  As the NOx sensors generally have a transverse sensitivity to NH3, they record the decrease in the NH3 concentration that occurs so that the desired change in the signal takes place, which is evaluated for

determine the time constant.

  The process according to the invention makes it possible to take advantage of a reduction in a concentration of NH 3, which occurs when the internal combustion engine 15 passes from a regeneration operation with a sub-stoichiometric mixture to

  operation with a stoichiometric mixture.

  Another possibility of using the desired concentration change as a reduction of the concentration consists in activating a power failure of the internal combustion engine. During a power failure, the combustion chambers of the internal combustion engine are no longer

  fueled but only in the air. The air is forced back into the exhaust system by the internal combustion engine. The air does not contain NOx or NH3 - or only in negligible concentrations - there is a sharp decrease of NOx or NH3 in the exhaust. This decrease in the concentration of substances is then used to diagnose the dynamics of the sensor.

  Of course, the diagnosis can also be established by evaluating a concentration change in the direction of an increase in concentration. If, in this case, it is desired to perform a calculation of the time constant on an analytical basis, it is here also judicious to start from a time response PT1 of the NOx sensor. In this case, it is preferable to enter a third value of the measurement signal at a third instant, the first and the second instant as well as the second and third instant being respectively separated by an identical time interval, of using as a change an increase in the concentration and calculate the time constant according to the following formula: T = 1n (Z) T being the time constant, the time interval and Z being given by means of the following equation, wherein y1 is the first value, y2 is the second value and y3 is the third value: y3-yl + y (y3yl y2-y3 yl-y2 4y-yl) yl-y2 '

  only Z is used for values greater than 0 and less than 1.

  In an approach based on simple calculations, it is possible to determine the time constant by means of the approximate simulation of the exponential function e in Equation (1) by equations with differences also realized during an increase in the concentration. in that a third value of the measurement signal is entered at a third instant, the first and the second instant as well as the second and third instant respectively being separated by an identical time interval, in that an increase of the concentration is used as a change, and in that the time constant is calculated according to the following formula: T = (y3-y2) 2 y2-y3 yl 'T being the time constant, of the interval time, yl the first value, y2 the second value and y3 the third value, the time interval d being less than an expected value for the time constant T. The NOx sensors having as a rule Because of the high transverse sensitivity to NH.sub.3, it is also possible to advantageously use a change in concentration of NH.sub.3. An increasing concentration of NH3 may for example be generated in that the internal combustion engine operates momentarily with a sub-stoichiometric air / fuel mixture. In general, such operation is present whenever full charge enrichment is initialized, so that it is preferable to use such a full load enrichment phase to establish the diagnosis. The same is true when an internal combustion engine goes into operation protecting the catalyst or when a catalyst is cleaned afterwards.

  a power failure in thrust.

  Of course, it is also possible to use a change to increase the NOx concentration by passing the internal combustion engine from stoichiometric operation to over-stoichiometric operation. In general, however, such a measurement results in an increasing concentration of NOx at a NOx sensor only if no NOx storage catalyst is mounted upstream of it, since a catalyst of NOx this type would otherwise absorb NOx and thus

  concentration change at the NOx sensor.

  In principle, the necessary changes in concentration can be achieved by intervening accurately in the operation of an internal combustion engine after initialization of a diagnostic function. An alternative would be to wait, during normal operation of the internal combustion engine, to see an event involving a change in concentration, for example during a sudden load of power resulting in a full load load with a transition to sub-operation. stoichiometric, as is the case during a longer mountain descent with power failure, etc. Another advantage of the method according to the invention resides in the possibility of controlling the calculated result, namely the time constant T - and of calculating the average over several calculation processes. Such control or recalculation with averaging can be done just after obtaining the first

  result, i.e. during the same change in concentration.

  The second value of a first determination of the time constant T can for example be used, at the same time, of first value for a second determination. The repetition of the determination of the time constant and the calculation of the average of the calculated values of the time constants improve the accuracy of the result and make it possible to cope effectively with the diagnostic errors. The process

  according to the invention is then particularly robust.

  The invention will be discussed below in more detail, in the form of examples, using the drawings. In the drawings, FIG. 1 is a diagrammatic representation of an internal combustion engine 1 and FIG. 2 represents the time course of the concentration of substances present in the exhaust gases as well as a signal emitted by a

NOx sensor.

  The internal combustion engine shown in FIG. 1 has an exhaust gas path 2, in which is disposed an exhaust gas reprocessing system which also serves as an on-board diagnostic device. The on-board diagnostic function is supported by a control unit 3, which is also responsible for the normal control of the operation of the internal combustion engine 1. Thus, the control unit 3 controls, inter alia, an injection system 6 which supplies fuel to the internal combustion engine 1, reads the values coming from the probe 7 and the sensor 8, and carries out the

"embedded" diagnosis.

  In the exhaust gas path 2 of the internal combustion engine are arranged a three-way precatalyst 4 and a NOx accumulator catalyst 5. It is also possible to have a single catalyst combining the two properties. Upstream of these two catalysts is a precatalytic lambda probe 7 and, in

  upstream of these, a NOx sensor 8.

  The control unit verifies the dynamic response of the NOx sensor 8 as follows: During this verification, it is assumed that when the NOx sensor is excited by a change in the measured exhaust gas concentration, the sensor signal approximately satisfies a characteristic PT1, such that

  given by equation (1) above.

  Figure 2 shows a curve 9 showing the pace of the excitation, the example reported here showing the change in a concentration of NH3. Curve 10 represents the measurement signal S of the NOx sensor 8 having the characteristic PT1. The increasing concentration of NH3 is obtained in a regeneration phase, momentarily passing the internal combustion engine into sub-stoichiometric operation. NH3 appears during such a regeneration phase in the NOx storage catalyst 5 as an intermediate, which does not leave the NOx storage catalyst as long as it contains NOx. Once the NOx accumulator catalyst no longer contains NOx, the NH3 goes to the outlet and arrives at the NOx sensor 8 with a greatly increasing concentration. This constitutes the upward flank to

  the time t0 of the concentration of NH3, as shown by curve 9.

  The control unit 3 recognizes the end of the NOx regeneration phase in the manner known to the professional and operates the internal combustion engine 1 again with a sub-stoichiometric air / fuel mixture. Thus no more NH3 is produced in the NOx accumulator catalyst 5, so that the NOx sensor 8 detects, at time t1, a strongly

decreasing NH3.

  The signal S of the NOx sensor 8 transmitted to the control unit 3 reproduces this excitation by following the curve 10. At the instant t0, the concentration 35 increases by passing from an initial value S (t0) to a maximum value S (tl), which is reached at time tl. Then the concentration of decreasing NH3, which returns from its maximum value NH3 (tl) to the initial value NH3 (t0), the curve 10 of the signal of

  measurement emitted by the NOx sensor 8 also decreases.

  The control device 3 then inputs the signal S in two instants located after the initialization of the change of the concentration. This occurs at times t2 and t3 so that values S (t2) and S (t3) are obtained. The instants t2 and t3 are separated by a time interval d. The time constant is then calculated from these values using the following equation: T = d / ln (S (t2) / S (t3)). (2) It is thus possible to calculate the time constant T by means of a simple calculation. Equation (2) is based on an appropriate transformation and resolution of equation (1). Of course, it is also possible, instead of calculating the time constant T based on an analytical resolution of equation (2) for times t2 and t3, to use an appropriate numerical method for the evaluation. of

equation (1).

  In an alternative approach, during a concentration change towards an increase, three values S (t4), S (t5), S (t6) are recorded at times t4, t5, t6 equidistant from each other, and the time constant is then obtained by means of the following equation: T = d (S (t6) -S (t5)) 2. S (t5) -S (t6) -S (t4) To do this, The time interval d = t6-t5 = t5-t4 is chosen to be small relative to the expected time constant, so that d <T. He is in

  particular possible to use for d a value satisfying the condition 1 Od <T.

  If the time interval d can not satisfy this condition, it is then possible to calculate the time constant using the following equation: -d T = l (, in which Z = -o'5 S ( t6) - S (t4) - 1 S (t6) - S (t4) - S (t5) - S (t6) S (t4) - S (t5) 14 S (t4) - S (t5)) S (t4) t4) - S (t5) In this regard, it goes without saying that any value o Z <O or Z> I is rejected. 35

Claims (12)

  A method for evaluating the time response of a gas sensor (8) disposed in the exhaust path (2) of an internal combustion engine (1) and delivering a measurement signal, process in which a change of concentration in the exhaust gas of the internal combustion engine is used, characterized in that - an average rate of change of the concentration change is greater than an expected maximum change rate of the measurement signal, 1 0 - during the change of the measurement signal caused by the change of concentration, a first value and a second value of the measurement signal are respectively entered at a first and a second instant, and - a time constant for the change of the signal measurement is determined from the first and the second value as well as from the interval of   1 time separating the first and the second moment.   Method according to claim 1, characterized in that a decrease in the concentration is used as a change and in that the time interval is divided by the difference of the logarithms of the first and second values. the measurement signal.   3. Method according to one of the preceding claims used in the case of a   internal combustion engine (1) whose exhaust gas path (2) comprises an NOx accumulator catalyst (5) arranged upstream of the gas sensor (8), characterized in that a decrease of a concentration of NH3 is used by passing the internal combustion engine (1) from regeneration operation with a substoichiometric mixture to operation with a stoichiometric mixture.   4. Method according to one of claims I or 2, characterized in that   Decrease in concentration is used by activating a push power cut. 5. Method according to claim 1, characterized in that a third value of the measurement signal is entered at a third instant, the first and the second instant as well as the second and third instant respectively being separated by an identical time interval. in that an increase in the concentration is used as a change, and in that the time constant is calculated according to the following formula: 2 * y3-y2) T being the time constant, d the time interval , yl the first value, y2 the second value and y3 the third value, and the time interval d being less than an expected value for the time constant T. 6. Method according to claim 1, characterized in that a third value of the measurement signal is entered at a third instant, the first and the second instant as well as the second and third instant respectively being separated by an identical time interval. in that an increase in the concentration is used as a change, and in that the time constant is calculated according to the following formula: -d ln (Z) T being the time constant, d the time interval, time and Z being given by the following equation, where y1 is the first value, y2 is the second value and y3 is the third value: Z = -0.5. '"+ (3 q 2 yl-y2 4 ly2) yl-y2   only Z values are used which are greater than 0 and lower than 11.   7. Process according to claim 1, 5 or 6, characterized in that   Increasing a concentration of NH3 is used as a change by passing the internal combustion engine (1) into operation with a sub-stoichiometric mixture.   8. Method according to claim 7, characterized in that a full charge enrichment is triggered to pass the internal combustion engine (1) in   operation with a substoichiometric mixture.   9. A method according to claim 1, 5 or 6, characterized in that an increase of a NOx concentration is used as a change by passing the internal combustion engine (1) through operation with a mixture   stoichiometric to operation with an over-stoichiometric mixture.   10. Method according to one of the preceding claims, characterized in that   for evaluation, a change in the apparent concentration of   anyway during normal operation of the internal combustion engine (1).   11. Method according to one of the preceding claims, characterized in that   that the determination of the time constant (T) and its mean is repeated several times.   12. Method according to claim 11, characterized in that the repeated determination is carried out during the same concentration change.