DE10244125B4 - Method for evaluating the time behavior of a NOx sensor - Google Patents

Abstract

Method for evaluating the time behavior of a gas sensor (8) arranged in the exhaust tract (2) of an internal combustion engine (1), which emits a measurement signal in which a concentration change in the exhaust gas of the internal combustion engine is utilized, wherein a mean rate of change of the concentration change is greater than an expected maximum rate of change of the measurement signal, characterized in that
- During the change caused by the change in concentration of the measurement signal at a first and a second time, a first or second value of the measurement signal detected and
- From the first and the second value and the time interval between the first and second time a time constant of the change of the measuring signal is determined, wherein
a) a change in concentration is used as a change and the time interval is divided by the difference of the logarithms of the first and second values of the measuring signal, or
b) at a third time a third value of the measuring signal is detected, wherein first and second and second ...

Description

The invention relates to a method according to the preamble of claim 1 for evaluating the time behavior of a arranged in the exhaust system of an internal combustion engine gas sensor which emits a measurement signal in which a concentration change in the exhaust gas of the internal combustion engine is utilized. Such a procedure is over DE 100 62 289 A1 known.

The Exhaust of internal combustion engines contains various pollutants, such as hydrocarbons, carbon monoxide and nitrogen oxides (NOx). The Pollutant concentration in the exhaust gas depends heavily on operating parameters of the internal combustion engine, especially the air / fuel ratio of the Mixture with which the internal combustion engine is operated. Usually are to improve the exhaust gas quality aftertreatment systems in the Use, comprising one or more catalysts, which convert the pollutants. The operation of the internal combustion engine is then monitored by suitable exhaust gas sensing sensors and under recourse controlled on the sensors. Typical sensors are e.g. Lambda probes for the Determination of the air / fuel ratio.

Around to further reduce the fuel consumption of gasoline engines, Combustion engines with lean combustion are becoming increasingly common Commitment. In these internal combustion engines need special measures be taken to specified limits in terms of emission of NOx. In such internal combustion engines are therefore reinforced NOx sensors are used, which control the concentration of nitrogen oxides in the Capture exhaust gas.

Around compliance with required exhaust emission limit values via the ensure the entire useful life of an internal combustion engine, is increasingly a self-diagnosis (on-board diagnosis = OBD) of the entire exhaust gas required by treatment system. Such OBD must be functional monitor exhaust-related components; also the functionality The gas sensors used must be checked. This review requires in particular the dynamic behavior of the sensors used regularly evaluate that a slowdown in response of the sensor detected and inadequate Dynamic behavior of a faulty sensor can be diagnosed.

Depending on the position of the sensor, this can sometimes be problematic. For example, the exhaust downstream of a catalyst typically does not have large concentrations of substance concentration, so that it is difficult to detect a slower response of a sensor seated downstream of a catalyst in the exhaust tract of an internal combustion engine. From the DE 198 28 929 A1 In this regard, it is known to utilize a regeneration phase of a NOx storage catalytic converter in which the internal combustion engine is operated with a substoichiometric mixture, in order to detect a rate of change of the measurement signal of a sensor. If the rate of change remains below a minimum value, a faulty sensor is diagnosed.

This Method has the disadvantage that the excitation of the NOx signal by a change be known or determined the NOx concentration at the sensor Conditions suffice got to. During operation of an internal combustion engine, it is however difficult to always be able to meet these basic conditions, so that either only a diagnosis of reduced quality can be carried out, or the frequency of performing strongly limited diagnosis is.

Of the The invention is therefore based on the object, a method for Evaluation of the timing behavior so that a more accurate Diagnosis possible is.

These Task is according to the invention with a A method according to claim 1 solved. It is envisaged that the mean rate of change of Concentration change is bigger, as an expected maximum rate of change of the measurement signal, and while the change caused by the concentration change of the measurement signal at a first and a second time first and second value of the measurement signal detected and from the first and the second value and the time interval between the first and second time determines a time constant of the change of the measurement signal becomes.

Thus, the method according to the invention no longer determines the rate of increase or mean change in the case of an exactly predetermined admission of the sensor with a change in the substance concentration, but instead evaluates the sensor signal in such a way that now the time constant is determined. The requirement for the change in the substance concentration is thereby relaxed so that now only a certain amount Minimum change is required. This slope must be greater than the expected maximum steepness of the measurement signal. For the sensor, the concentration change then represents a jump function.

The absolute values, i. the level at which the change are moving, are for the determination of the time constant irrelevant. The determination of Time constant only requires that in known temporal Spacing at least two values of the sensor signal are sampled.

The calculation of the time constant from these two values taking into account the time interval of the values expediently uses model assumptions with regard to the time behavior of the sensor. In most cases, a so-called PT1 characteristic will be assumed. In this case, in the excitation effected by changing the quantity to be measured by an amplitude A, the timing of the signal S will be described by the following equation

In In this equation, T denotes the time constant of the sensor and S0 is the initial value of the sensor signal at the change. Such a time behavior allows a PT1 element in a simple way with a rising or falling edge, i. with a reduction or increase in the concentration to be measured, the condition of the invention Fulfills, after simple connections to calculate the time constant T.

in the The case of a reduction in the concentration is obtained during a signal course according to equation (1) the time constant T by the time interval of the measured values by the difference of the logarithms of the measured values is divided. Consequently can calculate the time constant T from the measured values by simple calculation and the known distance. Of course, instead of the Difference of the logarithms the logarithm of the quotient from the Both measurements are used, since the known Logarithm of a quotient of two values is equivalent to the difference of the logarithmic values.

The Calculation of the time constant can be particularly with regard to Performance friendly and easy to use take place on a suitable map in which starting from a predetermined time interval with which the two values of the measurement signal be won for corresponding pairs of values the time constant is stored. Conveniently, Such a map will be between changes with increasing and decreasing substance concentration.

It However, surprisingly pointed out that even with relatively simple operations a mathematical calculation of the time constant is possible.

That in such an approach a calculation is possible a priori could not can be expected since with the amplitude A and the initial value S0 as well given the time constant T three unknowns in the mentioned equation, so that actually two measurements are not a successful one resolution can expect the equation according to the searched time constant T.

The However, invention makes use of the knowledge that equation (1) simplified in special cases. For "switching on" a substance concentration, i.e. a rising edge, the starting value S0 can be set to zero become. something similar applies to the "turn off", then the amplitude A equals zero. Equation (1) then simplifies accordingly.

The Signal of the gas sensor is known at any time t. Unknown On the other hand, the amplitude A, the starting value S0 and the time constant are T. Next, one has knowledge of the time interval between the two Times of measurement. The thus calculable time constant T now provides a direct statement about the dynamic properties the signal and thus the gas sensor. By skillful linking becomes the amplitude A of the excitation, i. the concentration of substance concentration, for the Calculation not needed, which a big independence in the determination of the time constant T arises.

The inventive method for evaluating the dynamics of a gas sensor is thus independent of the amplitude A of the concentration change, whereby a universal applicability of the method is achieved. This property is particularly valuable for the diagnosis of gas sensors that are located in an exhaust system downstream of a catalytic converter, as there are usually the substance concentration changes quantitatively unknown or fixable.

The Procedure is for Linear lambda probes are just as suitable as for the diagnosis of NOx sensors. For the inventive method enough it, the concentration change to know qualitatively. In this regard, come different concentration changes in question.

at an internal combustion engine, upstream of the NOx sensor, a NOx storage catalytic converter in the exhaust tract is, such NOx storage catalysts are usually discontinuous operated. In a storage phase, they adsorb NOx compounds the exhaust gas, which in operation of the internal combustion engine with more than stoichiometric Air / fuel mixture are operated. The emptying of the only limited receptive NOx storage catalyst takes place in so-called regeneration phases, in which the internal combustion engine temporarily with substoichiometric Air / fuel mixture is operated. The thereby contained in the exhaust gas Carbon monoxide and unburned hydrocarbons serve the NOx storage catalytic converter as a reducing agent. While Such regeneration phases also arise within the catalyst NH3, which is only an intermediate and the catalyst does not leave. This only occurs when the regeneration phase is over, and NOx is no longer present in the storage catalytic converter.

Then beats NH3 as well as the exhaust components of the substoichiometric air / fuel mixture through to the catalyst outlet. Therefore, the internal combustion engine now again in superstoichiometric Switched operation, which is again in the exhaust gas NOx and the requirements for the emission of NH3 at the catalyst outlet are no longer present.

There NOx sensors are usually cross-sensitive to NH3 They reduce the occurring NH3 concentration, so that the desired signal change results, which is evaluated to determine the time constant.

For the inventive method is it therefore possible to utilize a reduction in NH3 concentration that results when the internal combustion engine of a regeneration operation with substoichiometric Mixture in a stoichiometric operation Mixture is switched.

A another possibility the desired Concentration change it is an overrun fuel cut to be used as a reduction in concentration to activate at the internal combustion engine. When fuel cut is the combustion chambers the internal combustion engine no longer supplied fuel, but only air. The air gets into the engine through the engine Exhaust system pumped. Since air is negligible or negligible Concentrations of NOx or NH3 contains, creates a drastic Decrease of NOx or NH3 in the exhaust gas. This reduction in substance concentration then serves to diagnose the sensor dynamics.

wobei nur Werte für Z verwendet werden, die größer 0 und kleiner 1 sind.Of course, a concentration change can also be evaluated with an increase in the concentration for diagnosis. If, in this case, one wishes to carry out a calculation of the time constant on an analytical basis, then it is also expedient here to assume a PT1 time behavior of the NOx sensor. In this case, it is preferred that a third value of the measurement signal is detected at a third time, wherein first and second as well as second and third time are each at the same time interval to each other, as change uses an increase in concentration and the time constant according to the following formula T = -d / ln (Z) where T is the time constant and d is the time interval and Z is given by the equation where y1 is the first, y2 is the second and y3 is the third value,

using only values for Z that are greater than 0 and less than 1.

wobei T die Zeitkonstante, d der zeitliche Abstand, y1 der erste, y2 der zweite und y3 der dritte Wert ist und wobei der zeitliche Abstand d kleiner als ein Erwartungswert für die Zeitkonstante T ist.In a calculation-saving approach, by approximating the e-function in equation (1) by difference equations, the time constant can also be determined with an increase in concentration by detecting a third value of the measurement signal at a third time, wherein first and second as well as second and second the third time each lie in the same time interval to each other, as change uses an increase in concentration and the time constant is calculated according to the following formula:

where T is the time constant, d the time interval, y1 the first, y2 the second and y3 the third value and wherein the time interval d is smaller than an expected value for the time constant T.

There NOx sensors, as usual, usually a strong cross-sensitivity on NH3 may also advantageously NH3 concentration change be recycled. An increasing NH3 concentration may e.g. generated be by the internal combustion engine in the short term with stoichiometric air / fuel mixture is operated. Such an operation is usually always then before, when a full load enrichment is turned on, so it preferred is such a full load enrichment phase for diagnosis to use. something similar applies to the transition an internal combustion engine into a catalyst protecting Operation or for the rinse a catalyst after fuel cut.

Of course you can also an increasing change the NOx concentration be recovered by the internal combustion engine of stoichiometric in superstoichiometric Operation is switched. However, such a measure usually only results then to an increasing NOx concentration at a NOx sensor, if this no NOx storage catalyst is connected upstream, since such a catalyst otherwise absorb NOx and thus no concentration change would let through the NOx sensor.

Basically, the required concentration changes by targeted intervention in the operation of an internal combustion engine after initiating a diagnostic function. Alternatively it is it possible to wait until a corresponding during normal operation of the internal combustion engine Event with Concentration Change occurs, e.g. at sudden Power request, which is a full load request with switching in stoichiometric Operation results in longer Downhill with fuel cut, etc.

One Another advantage of the method lies in the Possibility, the calculated result, namely the time constant T, or over several calculation processes to mediate. This can be a review or Recalculation and averaging immediately after the existence of the first Results take place, i. within the same concentration change.

For example the second value of a first determination of the time constant T immediately as the first value for a second determination can be used. The repetition of the provision the time constant and the averaging of the calculated time constant values elevated the accuracy of the result and effectively protects against misdiagnosis. The inventive method is then particularly robust.

The The invention will be described below with reference to the drawings for example, even closer explained. In the drawings shows:

1 a schematic representation of an internal combustion engine 1 and

2 Time curves of a substance concentration in the exhaust gas and a signal of a NOx sensor.

In the 1 illustrated internal combustion engine 1 has an exhaust tract 2 in which an exhaust aftertreatment system having OBD capabilities is located. The OBD function is performed by a control unit 3 This also applies to the normal control of the operation of the internal combustion engine 1 responsible is. The control unit 3 Among other things, it controls an injection system 6 on, that of the internal combustion engine 1 Fuel allocates, reads the values of the probe 7 and the sensor 8th and performs the OBD.

In the exhaust tract 2 the internal combustion engine is a three-way pre-catalyst 4 and a NOx storage catalyst 5 arranged. It is also possible a single catalyst, showing both properties. Upstream of these two catalysts is a pre-catalyst lambda probe 7 and downstream of it, a NOx sensor 8th ,

The control unit checks the dynamic behavior of the NOx sensor 8th as follows:
During the check, it is assumed that when the NOx sensor is excited by a change in the measured exhaust gas concentration, the sensor signal approximately corresponds to a PT1 characteristic, as shown in the above equation (1).

2 shows in a curve 9 the course of the excitation, wherein here, for example, the change in an NH 3 concentration is plotted. Curve 10 shows the PT1 characteristic sufficient measurement signal S of the NOx sensor 8th ,

The increasing NH3 concentration is generated by brief switching of the internal combustion engine in substoichiometric operation in a regeneration phase. NH3 is produced during such a regeneration phase in the NOx storage catalytic converter 5 as an intermediate, which is not at the exit of the NOx storage catalyst for so long 5 exits, as stored in this NOx. Is the NOx storage catalyst 5 emptied of NOx, NH3 passes through to the output and reaches the NOx sensor with a strongly increasing concentration 8th , This is the rising edge at time t0 of the concentration of NH3 as it curves 9 shows.

The control unit 3 recognizes the end of the NOx regeneration phase in a manner known to those skilled in the art and switches the operation of the internal combustion engine 1 back to stoichiometric air / fuel mixture. This means that no more NH3 is produced in the NOx storage catalytic converter 5 , whereby at time t1 the NOx sensor 8th to detect a strongly decreasing concentration of NH3.

The signal S of the NOx sensor 8th that the control unit 3 is added, this excitation returns, following the curve 10 , At time t0, the concentration increases from a start value S (t0) to a maximum value S (t1) reached at time t1. Due to the then falling concentration of NH3, which returns from its maximum value NH3 (t1) back to the initial value NH3 (t0), the curve falls 10 the measurement signal of the NOx sensor 8th also off again.

The control unit 3 Now, at two points in time after the introduction of the concentration change, the signal S is detected. This takes place at times t2 and t3, so that values S (t2) and S (t3) are present. The times t2 and t3 are spaced in time by a distance d. From these values, the time constant is then calculated using the equation: T = d / ln (S (t2) / S (t3)). (2)

In order to the time constant T can be calculated by a simple calculation.

equation (2) is based on a suitable transformation and resolution of equation (1). Of course you can instead, on an analytic solution of equation (2) for the times t2 and t3 based calculation of the time constant T also a suitable Numerical method used to evaluate the equation (1) become.

In an alternative approach, three values S (t4), S (t5), S (t6) are acquired at equidistant times t4, t5, t6 with an increasing change in concentration, and the time constant is then given by the following equation:

Of the time interval d = t6 - t5 = t5 - t4 is chosen small against the expected time constant, so that applies d <T. In particular, a value for d be used, the 10d <T enough.

ist. Hierbei werden natürlich Werte mit Z ≤ 0 oder Z ≥ 1 verworfen.If this condition can not be fulfilled for the time interval d, the following equation allows the calculation of the time constant: T = -d / ln (Z), where

is. Of course, values with Z ≦ 0 or Z ≥ 1 are discarded here.

Claims (9)

Method for evaluating the time behavior of an exhaust tract ( 2 ) an internal combustion engine ( 1 ) arranged gas sensor ( 8th ), which emits a measurement signal in which a concentration change in the exhaust gas of the internal combustion engine is utilized, wherein a mean rate of change of the concentration change is greater than an expected maximum rate of change of the measurement signal, characterized in that - during the by the concentration change caused change in the measurement signal at a first and a second time a first or second value of the measurement signal detected and A time constant of the change of the measuring signal is determined from the first and the second value and the time interval between the first and the second time point, a) using as change a reduction of the concentration and the time interval by the difference of the logarithms of the first and the second Or b) at a third time a third value of the measurement signal is detected, wherein the first and second and second and third time are each at the same time interval to each other, as change uses an increase in concentration and the time constant according to the following formula is calculated:

where T is the time constant, d the time interval, y1 the first, y2 the second and y3 the third value and wherein the time interval d is less than an expected value for the time constant T, or c) at a third time a third value of Measuring signal is detected, with the first and second and second and third time in each case at the same time interval to each other, as change an increase in the concentration is used and the time constant is calculated according to the following formula: T = -d ln (Z) . where T is the time constant and d is the time interval and Z is given by the following equation, where y1 denotes the first, y2 the second and y3 the third value

using only values for Z that are greater than 0 and less than 1. Method according to Claim 1 in an internal combustion engine ( 1 ), in whose exhaust tract ( 2 ) the gas sensor ( 8th ) a NOx storage catalyst ( 5 ), characterized in that a reduction of an NH3 concentration is used by the internal combustion engine ( 1 ) is switched from a regeneration operation with a stoichiometric mixture in a stoichiometric mixture operation. Method according to claim 1, characterized in that that a reduction in concentration is used by a Fuel cutoff is activated. A method according to claim 1, characterized in that an increase of an NH 3 concentration is used as a change, in which the internal combustion engine ( 1 ) is switched in operation with stoichiometric mixture. A method according to claim 4, characterized in that a full-load enrichment is turned on to the internal combustion engine ( 1 ) in operation with substoichiometric mixture to switch. A method according to claim 1, characterized in that an increase of a NOx concentration is used as a change by the internal combustion engine ( 1 ) is switched from stoichiometric to superstoichiometric operation. Method according to one of the above claims, characterized in that for the evaluation an already in normal operation of the internal combustion engine ( 1 ) change in concentration is used. Method according to one of the above claims, characterized in that the time constant (T) is determined repeatedly and is averaged. Method according to claim 7, characterized in that that repeated determination within the same concentration change carried out becomes.