DE10300939A1 - Method and device for monitoring the NOx signal of a NOx sensor - Google Patents

Abstract

The method and the device for determining the NOx concentration in the exhaust gas of an internal combustion engine (1) by means of a NOx sensor (8, 8 ', 9, 10, 20) arranged in the exhaust system (2) of the internal combustion engine (1) that a NOx signal from the NOx sensor (8, 8 ', 9, 10, 20) is monitored and checked for plausibility depending on the operating parameters.

Description

The invention relates to a Method and device according to the preambles of the independent claims.

In motor vehicles with modern internal combustion engines, which have a lower fuel consumption in lean and stratified charge operation, an additional aftertreatment of the exhaust gases to reduce nitrogen oxide emissions is necessary in order to meet the legal exhaust gas regulations. To solve this problem, NOx storage catalysts are preferably used, and NOx sensors are used to monitor them. In order to achieve a high emission stability of the engine, the highest possible accuracy is required when measuring the NOx concentration in the exhaust gas, in particular for precise control of the lean operation and the storage cycle of the NOx catalytic converter. From the EP 0892265 A1 In this context, a gas sensor for measuring gas oxides is already known, in which exhaust gas is guided into a double measuring chamber system for measurement. The measuring chambers have Nernst cells for the measurement. While oxygen molecules are extracted from the gas mixture in the first measuring chamber, the gas oxide to be measured, for example nitrogen oxide, is broken down into nitrogen and oxygen in the second measuring chamber. A pump cell voltage applied to the first chamber is regulated to a constant value which corresponds to a constant oxygen concentration in this chamber.

Ideally, the output signal of a NOx sensor shows exactly the actual NOx concentration of the exhaust gas. In particular, a NOx sensor should deliver a NOx signal = 0 under conditions without NOx emission. This is important, for example, for regulating the storage cycle and for diagnosing a NOx storage catalytic converter with high storage capacity during lean operation, in which no NOx emissions occur downstream of the storage catalytic converter if the storage catalytic converter is not damaged. To this end, it is known to correct the output signals of a NOx sensor. So from the DE 19819462 A1 a method is known in which the output signals of a NOx sensor are corrected with the aid of correction values stored in characteristic maps. The maps are dependent in particular on the oxygen concentration and its rate of change in the exhaust gas and on the temperature of the sensor and the exhaust gas. The measured value of the NOx concentration is corrected here with a multiplicative correction value and an additive correction value.

Another possibility for the more precise determination of a NOx concentration is in the published patent application DE 199 46 628 described. Under defined operating conditions, the NOx signal of a sensor is recorded for a predeterminable time and compared with a predefinable target curve. A control variable is determined from the deviation in the course and compared with a threshold value, and an indication of the functionality of a NOx storage catalytic converter is derived. Furthermore, from the published application DE 44 41 432 It is known to investigate the behavior of the catalytic converter and the lambda probe by making a targeted variation of the pollutant concentration and evaluating the corresponding signal of an exhaust gas probe connected downstream of the catalytic converter.

In the published application DE 199 26 139 calibration of an output characteristic curve of a NOx sensor is proposed. A calibration point with an essentially known NOx concentration is determined, which corresponds to the minimum of the NOx value after regeneration of the storage catalytic converter. Alternatively, a value can be read from a map as a reference.

Also in the generic disclosure DE 199 11 664 calibration of the characteristic slope and the so-called offset value of an output characteristic of a NOx sensor is proposed. For this purpose, a calibration point is determined at which a known NOx concentration in the exhaust gas, preferably between 0 and 5 ppm, can be assumed, such as in a fuel cut-off phase of the internal combustion engine or in idle after regeneration of a NOx storage catalytic converter. However, on-board diagnosis in a vehicle with sufficient accuracy is difficult when determining the slope of the characteristic curve.

A disadvantage of the known stand The technology is that series spreads in the response behavior of the Sensors only with large Effort can be corrected. There are also dynamic deviations of the NOx signal from the exact one Measured value coming from non-stationary Fluctuations in the sensor control state result, but not with sufficient ones Accuracy can be represented. The analysis is complicated by cross-sensitivities of the sensors used, some of which cause significant measurement errors can.

Object of the present invention is therefore the creation of a method by which accuracy with which the NOx concentration in the exhaust gas of an internal combustion engine is determined, increased can be, as well as to provide an apparatus for performing the method.

This object is achieved with the Characteristics of the independent Claim solved.

According to the invention, a sensor signal of the NOx sensor is monitored and checked for plausibility depending on the operating parameters.

In a particularly cheap Design becomes an activity of the NOx sensor signal determined a characterizing the size of the activity Characteristic value formed and dependent the characteristic value of the NOx signal is evaluated by operating parameters. With implausible NOx signal a corrected signal is preferably used.

A particularly easy option for consideration fluctuations in oxygen concentration, which the NOx signal can influence exists in the use of lambda value signals from the NOx sensor itself to disposal be put.

The invention is explained below of an embodiment shown in the drawings, from which is also independent from the summary in the claims further features, details and advantages of the invention.

In a schematic representation:

1 an internal combustion engine with an associated exhaust system,

2 a NOx sensor,

3 the output signal of a NOx sensor depending on the NOx concentration with offset and gain.

1 shows an internal combustion engine in a schematic representation to illustrate different installation positions of NOx sensors 1 , for example a lean-running gasoline engine or a diesel internal combustion engine, with an exhaust system 2 and an engine control unit 3 , preferably for operating a motor vehicle. The internal combustion engine 1 has a number of cylinder banks 4 (corresponding components are only provided with a reference symbol), each of which has its own exhaust gas path 5 is connected downstream. In the exhaust system 2 is an exhaust gas cleaning device with a pre-catalyst for converting harmful or undesirable components of the exhaust gas into other components 6 and a main catalyst 7 arranged. The pre-catalyst 6 is as a 3-way catalyst and the main catalyst 7 designed as a NOx storage catalyst. Downstream of the cylinder banks 4 are in the exhaust gas paths 5 NOx sensors 8th arranged with which the NOx concentration of the exhaust system 2 led exhaust gas of the internal combustion engine 1 can be measured. Upstream of the pre-catalyst 6 is also another NOx sensor 8th' arranged. In an area of the exhaust system 2 between the pre-catalyst 6 and the main catalyst 7 , downstream of the pre-catalyst 6 and upstream of the main catalyst 7 , is another NOx sensor 9 arranged. Another NOx sensor 10 is downstream of the main catalytic converter 7 in the exhaust system 2 arranged. It goes without saying that in the case of an exhaust gas purification device having a plurality of parts, sensors can be arranged upstream or downstream of the respective parts.

In addition to the sensors mentioned are upstream and downstream of the pre-catalyst 6 and upstream of the main catalyst 7 Lambda probes 11 respectively. 12 and to determine the operating temperature of the catalytic converter temperature sensors 13 . 13 ' intended. It goes without saying that, alternatively or additionally, further temperature sensors can be provided for measuring the operating temperature of the exhaust gas cleaning device or parts thereof. The internal combustion engine has for exhaust gas recirculation 1 an exhaust gas recirculation device 14 with a controllable valve 15 on.

The engine control unit 3 detects operating parameters of the internal combustion engine in a manner known per se via further sensors (not shown) 1 , such as throttle valve position, exhaust gas recirculation rate, ignition timing, injection timing of pre / main / post-injections, injection pressure, tumble flap position, boost pressure, phaser of the camshaft, speed, accelerator pedal position, load, driving speed and the like, and this via actuators (not shown) influence if necessary, whereby for communication between the engine control unit 3 and the sensors or actuators a cable system 14 or the like is provided. The engine control unit also includes 3 a lambda control device for controlling the oxygen concentration in the exhaust gas or the lambda value. The engine control unit 3 receives NOx signals from the NOx sensors with which the NOx raw emissions of the internal combustion engine 1 or the NOx concentration downstream of the catalyst devices 6 and or 7 can be determined.

The NOx storage catalytic converter 7 is usually operated in a storage cycle which comprises at least one adsorption mode and one regeneration mode. The adsorptive storage takes place with a lambda value> 1, the storage at a later time with a lambda value <1 or = 1.

The in 2 NOx sensor shown 20 for measuring the NOx concentration in the exhaust gas consists of a ceramic 30 and is a double chamber sensor with a first and second measuring chamber 22 . 26 educated. Over a diffusion barrier 23 Part of the exhaust gas with NOx, O2 and other components can enter the first measuring chamber 22 to reach. The measuring chamber 22 has an oxygen measuring pump cell working according to the Nernst principle 21 with pump electrodes P1. By means of the latter, the oxygen content in the measuring chamber 22 to be changed. Another diffusion barrier 25 exhaust gas enters the second measuring chamber 26 , This one NOx measuring pump cell also working according to the Nernst principle 27 with pump electrodes P2. The nitrogen oxides contained in the exhaust gas are catalytically broken down into the components N2 and O2 by a special material of the inner P2 electrodes. An O2 reference cell is used to calibrate the system 24 used with electrodes P3.

To the oxygen measuring pump cell 21 a pump voltage UVS is applied. The current ICP is regulated in such a way that a constant, preferably stoichiometric value of the oxygen concentration in the first measuring chamber 22 results. A broadband lambda value UO2 is calculated from the pump current IP1. The voltage UVP corresponds to a lambda jump signal. Via the pump cell current IP2 of the measuring chamber 26 a voltage signal UNOX corresponding to the NOx concentration in the exhaust gas can be determined.

In order to ensure a minimum temperature of, for example, 740 degrees, required for a NOx measurement, the NOx sensor 20 heating elements 28 to which a heating voltage UH is supplied by a heating device, not shown. is. An associated temperature measuring device, not shown, emits a temperature signal from which the NOx sensor temperature can be determined.

The regulation of the oxygen measuring pump cell 21 typically takes place over shorter times (<= 1 sec) than that of the NOx measuring pump cell 27 (3-4 sec). The NOx signal depends on many influencing factors such as temperature fluctuations on the sensor, stationary gas pressure on the sensor, oxygen concentration, NH3 emission or the like. The effect of these influencing variables can be compensated for in a manner known per se as soon as stationary disturbances are involved. However, the deviations of the NOx signal from the exact measured value caused by non-stationary disturbances or fluctuations can briefly be larger than those caused by stationary disturbances, since the oxygen measuring pump cell occurs during these dynamic processes 21 and the NOx measuring pump cell 27 are not balanced. Non-stationary fluctuations in the sensor control state are mainly caused by fluctuations in the exhaust gas composition. Other variables influencing the sensor control state are above all the exhaust gas velocity near the sensor, the sensor temperature and the exhaust gas temperature. Because the oxygen inflow ICP of the oxygen measuring pump cell 21 is set as an integral of the potential gradient across the entire cell, in the event of dynamic changes in the exhaust gas composition, in particular in the case of changes in the oxygen concentration in the cell, oxygen passages which are higher or lower than the control target value can enter the NOx measuring pump cell 27 come.

The NOx signal of known NOx sensors has an offset component and a further component, which results from the neglect of other cross influences depending on a NOx concentration, as in 3 is shown. The offset component OS corresponds to the intercept of the output signal S _out , which corresponds to the voltage signal UNOX. The slope of the output signal above the NOx concentration at point K, hereinafter referred to as gain, which is essentially linear in the region of interest, reflects the activity of the NOx sensor, ie the change in the NOx sensor signal when the NOx concentration changes in the exhaust, again. The activity of the gain of the NOx sensor can be determined in a linear range.

With aging of the NOx sensor or in the event of a defect, changes in the characteristics shown can result, which lead to a NOx signal that differs significantly from reality. Furthermore, the characteristic is influenced by cross-sensitivities of the NOx sensor. For example, in the case of on-board diagnosis, influences which relate to the composition of the exhaust gas are unavoidable, in particular when NH _{3 occurs} , which occurs in the catalytic converters when the mixture is rich 6 . 7 can arise and in the sensor 20 then converted to NOx and detected. There are also dynamic changes in the oxygen concentration, resulting in a temporarily unsteady pumping capacity of the oxygen measuring pump cell 21 cause. These lead to a change in the setpoint of oxygen entering the NOx measuring chamber 26 arrives and is incorrectly detected there as NOx.

According to the invention, the NOx signal is generated by at least one of the NOx sensors 8th . 8th' . 9 . 10 . 20 monitored and checked for plausibility. For this purpose, the engine control unit 3 a control unit 3a which evaluates the NOx signals depending on operating parameters. An activity of the NOx sensor signal is preferably determined and a characteristic value characterizing the size of the activity is formed. Depending on the operating parameters, the characteristic value of the NOx signal is evaluated, and an error signal is generated if the characteristic value is implausible. It is cheap, at least the NOx sensor 10 downstream of the main catalytic converter 7 to be used for the plausibility check.

The plausibility is preferably examined as a function of variables that describe influences on the NOx signal, such as operating points of the internal combustion engine 1 , Another influencing variable that influences the measurement signal is a quantity of NOx at the installation location of the NOx sensor 8th . 8th' . 9 . 10 . 20 , for example depending on a catalyst state value determined elsewhere.

The activity of the NOx signal can be evaluated, for example, by statistical mathematical methods known per se, in which a characteristic value for the activity of the NOx signal is determined. Preference is given to statistical mathematical methods such as those used for filtering the NOx signal is subjected to obtain a signal with reduced fluctuations. Low-pass filtering, which eliminates higher-frequency fluctuations in the NOx signal, is preferably used. In particular, a moving average value can be formed, in which the values of the signal with reduced fluctuations are determined iteratively according to the following calculation rule: current output value = previous output value + (current NOx signal value - previous output value) * F, F = filter factor. With a filter factor F close to 1, the respective current NOx signal value is hardly filtered. In the case of stronger fluctuations, a smaller filter factor is selected, which effects a stronger smoothing of the NOx signal. The NOx signal values used for forming the mean value can be NOx signal values of a predetermined time interval or the NOx signal values to which the same or similar operating parameter values are assigned. The characteristic value for the activity of the NOx signal then results from the mean value. If the NOx signal hardly reacts, ie if the gain is low, this corresponds to a low activity value. If the gain is greatly increased, this corresponds to a high activity value. However, if the influencing factors indicate that the activity of the NOx signal should be low, an increased gain is recognized as an error. The activity value can therefore be used to check the plausibility of the NOx signal and thus to diagnose it.

Preferably used for monitoring of activity of the NOx signal uses a model that shows expected behavior of internal combustion engine and exhaust system and thus a corresponding behavior of the NOx signal.

The respective values of the operating parameters of the internal combustion engine 1 and / or the vehicle, which is from the internal combustion engine 1 is driven, a certain change in the exhaust gas composition and amount of exhaust gas can be expected, which should result in a corresponding NOx signal. The plausibility assessment is preferably carried out in the case of operating conditions with increasing dynamic fluctuations in oxygen concentration and / or in the case of operating conditions with an increased ammonia concentration in the exhaust gas and / or as a function of a NOx catalyst status value. A lambda value signal, preferably a two-point and / or a broadband lambda signal of the NOx sensor, can be used to record the fluctuations in oxygen concentration 8th . 8th' . 9 . 10 . 20 be used.

Dynamic values of the oxygen concentration fluctuations, operating states with an increased ammonia concentration in the exhaust gas and / or a deteriorated NOx catalyst state are expected to increase the values of the NOx signal. Are these from control unit 3a registered, the NOx signal is assessed as plausible. If no increased value of the NOx signal is registered under the conditions mentioned, the NOx signal is assessed as implausible. After the registration of such implausible values of the NOx signal, the NOx signal in question can be disregarded for the engine control for a predetermined time interval and / or replaced by a corrected signal.

A faulty NOx sensor 8th . 8th' . 9 . 10 . 20 can be easily recognized if, with several existing NOx sensors 8th . 8th' . 9 . 10 . 20 , the behavior of the individual sensors 8th . 8th' . 9 . 10 . 20 is compared with each other. If everyone behaves the same implausibly, this can be used to assess a possible fault condition of one of the catalysts 6 . 7 be used.

If a gain that is too high or too low is measured and this value is implausible, a value that is reduced or increased by a desired factor can be specified instead of the implausible NOx signal measured. It is particularly expedient to correct the NOx signal as a function of the catalyst state value. An error signal can also be generated and in the engine control unit 3 are processed. Expediently, if an implausible NOx signal occurs in the engine control unit several times in succession 3 Information is deposited that the NOx sensor 8th . 8th' . 9 . 10 . 20 and / or the catalysts 6 . 7 should be checked for their function.

Claims (12)

experienced to monitor a NOx signal in the exhaust system ( 2 ) an internal combustion engine ( 1 ) arranged NOx sensor ( 8th . 8th' . 9 . 10 . 20 ), characterized in that the NOx signal of the NOx sensor ( 8th . 8th' . 9 . 10 . 20 ) depending on the operating parameters of the internal combustion engine ( 1 ) is checked for plausibility. A method according to claim 1, characterized in that to investigate the plausibility - an activity of the NOx signal is determined; - on the size of the activity is a characteristic parameter is formed and - in dependence the characteristic value of the NOx signal is evaluated by operating parameters. A method according to claim 1 or 2, characterized in that that the plausibility in operating conditions with dynamic fluctuations in oxygen concentration occurring is examined. Method according to at least one of the preceding Expectations, characterized in that the plausibility in operating states with an ammonia concentration in the exhaust gas is examined. Method according to at least one of the preceding claims, characterized in that the plausibility is dependent on a Ka Analyzer state value is examined. Method according to at least one of the preceding Expectations, characterized in that the characteristic value of the activity with statistical mathematical method is obtained. Method according to at least one of the preceding Expectations, characterized in that a corrected NOx signal is formed in the case of an implausible NOx signal. Method according to at least one of the preceding Expectations, characterized in that an implausible NOx signal generates an error signal becomes. Method according to at least one of the preceding claims, characterized in that a lambda value signal, preferably a two-point and / or a broadband lambda signal of the NOx sensor (for detecting fluctuations in oxygen concentration, 8th . 8th' . 9 . 10 . 20 ) is used. Device for carrying out the method according to at least one of the preceding claims, characterized in that means for examining the plausibility of a NOx signal of the NOx sensor ( 8th . 8th' . 9 . 10 . 20 ) are provided. Apparatus according to claim 10, characterized in that a control unit ( 3a ) is provided for evaluating the NOx signals as a function of operating parameters. Device according to claim 10 or 11, characterized in that at least one NOx sensor ( 10 ) downstream of a main catalytic converter ( 7 ) is intended to investigate the plausibility.