DE102007057707A1 - Method for operating a gas sensor for determining the concentration of gas components in the exhaust gas of internal combustion engines - Google Patents

Abstract

A method for operating a gas sensor for determining the concentration of gas components in the exhaust gas of internal combustion engines, arranged on a solid electrolyte (110) and separated from the gas mixture by at least one diffusion barrier, acted upon by a predetermined pumping current pumping electrodes (130, 150), characterized that the gas sensor is applied in the presence of a lean or stoichiometric gas mixture in the exhaust gas of the internal combustion engine with a pumping voltage (UP) which is at least so great that a decomposition of water contained in the exhaust gas and / or carbon dioxide takes place (decomposition voltage) and that from the thereby adjusting pumping current (IP) is closed to the pressure prevailing in the exhaust absolute pressure (p).

Description

The The invention relates to a method for operating a gas sensor for Determining the concentration of gas components in the exhaust gas of internal combustion engines according to the preamble of claim 1.

object The present invention also relates to a computer program a computer program product with a program code written on a machine-readable carrier is stored to carry of the procedure.

State of the art

electrochemical Gas sensors in the form of lambda probes are in large numbers used in exhaust systems of internal combustion engines in motor vehicles, around for the engine control signals over the Be able to provide exhaust composition. This way you can The engine can be operated so that the exhaust gases have an optimal composition for the Aftertreatment with catalysts usually present in the exhaust system today exhibit.

In 1 such a gas sensor is shown. The sensor element 100 has a gas entry hole 115 through which exhaust gas flows in and through a diffusion barrier 120 in a measuring room 130 arrives. A first electrode, also as outer electrode or outer pump electrode 150 is on the outside of the solid electrolyte 110 and under a porous protective layer 155 arranged exposed to the exhaust gas of an internal combustion engine (not shown). In the measuring space is a second electrode, also as an inner electrode or inner pumping electrode 140 designated arranged. Further, a reference electrode 145 is provided, which is supplied either by an open channel to the terminal contact area or by applying a pumping voltage with oxygen.

Between the inner pumping electrode 140 and the outer pumping electrode 150 a pump voltage U _{P is} applied so that a pump current I _P flows. A Nernst voltage between the inner pump electrode 140 and reference electrode 145 and the pumping current I _P are in a circuit unit, which is part of a control unit 190 is, generated and evaluated, wherein the pumping current I _P is adjusted so that there is a predetermined nominal Nernst voltage between the inner pumping electrode 140 and reference electrode 145 established. In the solid electrolyte 110 is also a in an insulating layer 162 embedded heating 160 arranged. Through this heater 160 the sensor element is heated to a temperature which is an optimal function of the sensor element 100 allowed.

A planar broadband lambda probe according to the limiting current principle can also be constructed as a single cell. Here are two electrodes on the Exhaust side arranged. At least the cathode is under a diffusion barrier arranged.

Further There are probes according to the limiting current principle, in which one electrode is arranged under a diffusion barrier in the exhaust gas while the others are arranged in a high-flow reference air channel is.

Such broadband lambda probes according to the limiting current principle are subjected to a fixed pumping voltage U _P. The fixed pumping voltage produces a positive pumping current for a lean exhaust gas, ie, an exhaust gas having an excess of air, which is uniquely related to the oxygen content of the exhaust gas and the transport or diffusion path of the oxygen to the cathode.

The Signal of such a gas sensor now depends next to the exhaust gas concentration also from the absolute pressure. This dependence is called "static pressure dependence" Knowledge of the absolute pressure is for a correction of the probe signal as well as the realization of others Functions in the vehicle of importance. To capture the pressure, can in principle be an additional one Pressure sensor to be arranged in the exhaust gas area. The arrangement of a However, such a pressure sensor is not only an increased installation costs and increased costs connected, such a pressure sensor also requires an increased circuit complexity and he has to - there it is an exhaust-relevant part - also continuously monitored for functionality be, for example, whether a defect of the pressure sensor is present or a line interruption of the supply or signal lines of the pressure sensor occured.

Of the The invention is therefore based on the object, a method for operating a gas sensor to mediate, which is not just a collection of Exhaust gas concentration, but also the absolute pressure on technical easy to implement manner allows.

Disclosure of the invention.

Advantages of the invention

These The object is achieved by a method for operating a gas sensor for determining the concentration of gas components of the initially described Sort of solved by the features of claim 1.

The basic idea The invention is to operate the gas sensor so that alone closed due to the sensor signals of the gas sensor to the absolute pressure can be. For this purpose, the invention provides, the gas sensor in presence of a stoichiometric or lean gas mixture in the exhaust gas of the internal combustion engine, ie a gas mixture with excess oxygen, with a pumping voltage to act on, which is at least so great that a decomposition takes place of water and / or carbon dioxide contained in the exhaust gas, and from the thereby adjusting pumping current to the prevailing in the exhaust gas To close absolute pressure. The pumping voltage, which is at least so great that a decomposition takes place of water and / or carbon dioxide contained in the exhaust gas, is hereinafter called "decomposition voltage" for short.

The The invention makes use of the fact that at λ ≥ 1, ie in the stoichiometric or lean operation, the entire fuel injected with the Oxygen to water and carbon dioxide reacts. Since the set Fuel consumption is known, it corresponds to the injected Fuel quantity, which is known, the production rate of the generated Water and carbon dioxide molecules determined, for example, be calculated. Assuming that the gas flow rate in the exhaust system at normal pressure is known or by, for example, a hot-film air mass flow meter can be measured on the water and / or carbon dioxide concentration be closed in the exhaust system at normal pressure. The exhaust gas concentration at λ = 1 can be independent be set exactly by the absolute pressure, since in this case no pumping current flows. additionally results in known fuel from the λ = 1 - setting the engine a fixed relationship of the fuel flow to the gas flow, which serves to secure the above provision.

If at this lambda value a constant positive pumping voltage in the Size of the decomposition voltage of the water is applied to the pumping cell, the pumping current is proportional Concentration of water molecules in the exhaust gas. This hangs beside the injection quantity also from the absolute pressure. A comparison of the this way measured water concentration with the calculated water concentration for normal pressure conditions based on the known per se static pressure dependence of the probe signal the information about the absolute pressure.

advantageously, can also be dispensed with any concentration calculation, by the pumping current, which at a defined decomposition voltage, defined injection quantity of fuel and defined oxygen addition in a defined operating state of the internal combustion engine, the is called adjusted at a defined exhaust gas flow, is compared with the in a desired pumping current stored under a data memory defined conditions and at normal pressure. Based on an empirical determined calibration curve is the deviation from the target pumping current associated with an absolute pressure.

advantageous Further developments of the method are the subject of claim 1 dependent claims. So Advantageously, the gas sensor is first with a pumping voltage which is so big that pumping out of oxygen takes place. The resulting pumping current is detected and as a background current of the on admission the gas sensor with the decomposition voltage resulting pumping current subtracted.

The Decomposition voltage is preferably between 1500 and 1800 mV.

The Pumping voltage for pumping out the oxygen is between 750 and 850, in particular 800 mV.

at Another very advantageous embodiment of the method is provided alternately between two different large decomposition voltages switch back and forth. The amount of lower voltage is chosen so that there, in addition to the possibly existing oxygen background, only water or carbon dioxide is decomposed. The amount is greater voltage chosen so that in addition the not yet decomposed product molecule, so either carbon dioxide or water, is decomposed. After subtracting an oxygen offset current, which can be determined leaves itself from the comparison of the thereby adjusting pumping currents independently of Absolute pressure the ratio from hydrogen to carbon atoms, that is, calculate and so on the fuel close, so the fuel identify.

The The method described above is preferably the subject of a computer program. which is stored in particular on a machine-readable carrier and on a computer, in particular a control unit of an internal combustion engine for Realization of the procedure expires. In that regard, it is also "retrofittable" with existing control devices, as an additional circuit complexity by additional Sensors completely eliminated in this process.

Brief description of the drawings

Further advantages and features of the invention are the subject of the following description and the drawings of embodiments of the invention.

In show the drawing:

1 a trained as a two-cell gas sensor in the form of a broadband lambda probe and;

2 a trained as Einzeller gas sensor in the form of a broadband lambda probe, in which the inventive method is used.

Embodiments of the invention

An in 1 illustrated gas sensor, which operates on the double cell principle and in which the erfindungemäße method is used, has a sensor element 100 on, by a trained in layered solid electrolyte 110 is formed. On the outside of the sensor element 100 is exposed to the exhaust gas a first electrode, also as an outer pumping electrode 150 designated, arranged by an open-pored protective layer 155 is covered. In the solid electrolyte 110 is a measurement volume 130 formed in which a second electrode, also as an inner pumping electrode 140 designated, is arranged. In addition, in the solid electrolyte 110 a reference electrode 145 arranged, which is supplied either via a terminal contact side reference channel (not shown) or by internal pumping with oxygen.

In the solid electrolyte 110 is also a in an insulating layer 162 embedded heating 160 arranged. Through this heater 160 the sensor element is heated to a temperature which is an optimal function of the sensor element 100 allowed. The exhaust gas of an internal combustion engine (not shown) flows through a gas inlet hole 115 via a diffusion barrier 120 into the measuring volume 130 ,

By a schematically illustrated electronic circuit, for example, part of a control unit 190 can be between the outer pumping electrode 150 and in the measurement volume 130 arranged inner pumping electrode 140 generates a constant pumping voltage U _P , which is controlled so that due to the pumping current, a λ = 1 - gas mixture in the measuring volume 130 established. This is achieved in that the pumping current to a Nernst voltage U _N = 450 mV between the inner pumping electrode and the "oxygen-purged" reference electrode 145 regulated. becomes.

The situation is different for lambda probes according to the limiting current principle, which are constructed as so-called single cells and in which the two electrodes 140 . 150 on the exhaust side under a diffusion barrier 159 are arranged, wherein a fixed pumping voltage U _{P applied} to these electrodes. In this case, as in 2 represented the two pumping electrodes 140 . 150 under a common diffusion barrier 159 be arranged. But it is also possible to arrange only the cathode under a diffusion barrier or to arrange an electrode in the exhaust gas under a diffusion barrier and the other electrode in a reference air channel with a high limiting current. In these probes, the pump pump voltage at lean exhaust produces a positive pumping current which is clearly related to the oxygen content of the exhaust gas. Such a probe also has a layer 162 with heating elements 160 on, for example, meandering heating elements, the Heizerzuleitungen 161 can be supplied with current and voltages. In 2 are the same elements with the same reference numerals as in 1 ,

The Signal such broadband lambda probes now depends next to the exhaust gas concentration also from the absolute pressure. This dependence is called "static Pressure dependence " the absolute pressure is for a correction of the probe signal as well as for other functions for control the internal combustion engine of importance. In principle, the Absolute pressure in the exhaust system measured by installing a pressure sensor become. Such a sensor would however, an increased Assembly costs, which in turn is associated with additional costs, entail. About that would be out Such a pressure sensor susceptible to failure and should continuously monitored for faults as part of an on-board diagnosis become.

The basic idea of the present invention is to operate gas sensors described above in such a way that, with their help, the absolute pressure in the exhaust gas can also be measured. For this purpose, two modes are provided. The one operating mode takes place at a stoichiometric exhaust gas, ie at a value λ = 1. In this case, a constant positive pumping voltage U _{P is} applied, which has such a high value that decomposition of the water and / or carbon dioxide takes place. This decomposition voltage is, for example, between 1500 and 1800 mV. With the help of this voltage, a complete decomposition of the water contained in the exhaust gas and / or the carbon dioxide takes place. An exhaust gas concentration at λ = 1 can be set exactly, regardless of the absolute pressure, since no pumping current flows in this case. If a constant positive pump voltage at the level of the decomposition voltage of the water and / or the carbon dioxide is applied to the pump cell at this lambda value, the pumping current I _{P is} proportional to the concentration of the water and / or carbon dioxide moles in the exhaust. This depends not only on the injection quantity, which is known, but also on the absolute pressure. A comparison of the water and / or carbon dioxide concentration measured in this way with a previously determined water and / or carbon dioxide concentration for normal pressure conditions provides the information about the absolute pressure p in the exhaust gas based on the known per se static pressure dependence of the probe signal. The water and / or carbon dioxide concentration under normal pressure conditions can also be measured in the lean exhaust gas. In this case, all of the injected fuel reacts with the oxygen to form water and carbon dioxide. Since the injection quantity is known, the production rate of the generated water and / or carbon dioxide molecules can be calculated. Taking into account the likewise known gas flow rate in the exhaust gas line at normal pressure, the water and / or carbon dioxide concentration in the exhaust gas line at atmospheric pressure can be calculated in this way.

One potential Cross-influence due to fluctuations of the nitrogen / oxygen ratio in air can be measured by measuring the oxygen limit in air Pushing operation determined and computationally compensated.

The Determination of the absolute pressure p for a lean exhaust gas mixture, d. H. at λ ≥ 1, that first the pumping electrodes are subjected to a pumping current, the for pumping of the oxygen from the measuring cavity in the case of a two-cell trained broadband lambda probe or from the cathode layer is sufficient in the case of a single cell. A required pumping voltage is about 800 mV. Then, at the same lambda value, a constant positive Pump voltage in height the decomposition voltage of the water and / or carbon dioxide applied, in addition the water contained in the exhaust gas and / or the carbon dioxide too to reduce. The occurring pumping current is measured and from this will be the background current, i. H. the pumping current that occurs during Pumping off the oxygen stops, subtracts. The difference is a measure of the absolute pressure p in the exhaust gas.

A Another beneficial application aims to alternate between two different sizes Switching decomposition voltages back and forth. The amount lower Tension is chosen that there, in addition to the possibly existing oxygen background, only water or only carbon dioxide is decomposed. The magnitude larger voltage is chosen that in addition the not yet decomposed product molecule, so either carbon dioxide or water, is decomposed. After deduction of an oxygen offset current lets out the comparison of the resulting pumping currents, regardless of Absolute pressure, the ratio from hydrogen to carbon atoms in the fuel and calculate so identify the fuel type.

The method described above can be used, for example, as a computer program on a computing device, in particular a control device 190 be implemented an internal combustion engine and run there. The program code may be stored on a machine-readable medium that the controller 190 can read. It is understood that the invention is not limited thereto, but purely in principle the method can also be implemented as a circuit, which in this case preferably part of the control unit 190 is. The circuit can be constructed as an integrated circuit or as a discrete circuit.

Claims (7)

Method for operating a gas sensor for determining the concentration of gas components in the exhaust gas of internal combustion engines, with on a solid electrolyte ( 110 ) and separated from the gas mixture by at least one diffusion barrier, acted upon by a predeterminable pumping current pumping electrodes ( 130 . 150 ), characterized in that the gas sensor in the presence of a lean or stoichiometric gas mixture in the exhaust gas of the internal combustion engine with a pumping voltage (U _P ) is applied, which is at least so great that a decomposition of water contained in the exhaust gas and / or carbon dioxide takes place (decomposition voltage ) and that from the thereby adjusting pumping current (I _P ) is closed to the pressure prevailing in the exhaust absolute pressure (P). A method according to claim 1, characterized in that the gas sensor is first subjected to a pumping voltage (U _P ) which is so great that pumping out of oxygen takes place, that the thereby adjusting pumping current (I _P ) is detected and as a background current of the is subtracted when exposed to the gas sensor with the decomposition voltage resulting pumping current (I _P ). Method according to claim 1, characterized in that that the decomposition voltage is between 1500 and 1800 mV. Method according to one of the preceding claims, characterized in that the pumping voltage (U _P ) for pumping out the oxygen is between 750 and 850, in particular 800 mV. Method according to one of the preceding claims, wherein the pumping current in a first, the Determination of water or carbon dioxide serving Zer settlement voltage and at a second, the additional determination of carbon dioxide or water serving decomposition voltage is measured and is concluded from the comparison of the thereby adjusting two pumping currents to the hydrogen-carbon ratio of the fuel. Computer program, which carries out all the steps of a method according to one of claims 1 to 5, when it is stored on a computing device, in particular the control device ( 190 ) runs an internal combustion engine. Computer program product with program code, which is stored on a machine-readable carrier, for carrying out the method according to one of claims 1 to 5, when the program is stored on a computer or a control unit ( 190 ) of an internal combustion engine is executed.