DE102009001843A1 - Method for operating a sensor element and sensor element - Google Patents

Abstract

A method for operating a lambda probe for determining the concentration of gas components in the exhaust gas of internal combustion engines is characterized by the following steps: determining the internal resistance; - Comparing the internal resistance with a predetermined resistance threshold; - Connecting a pump voltage when the resistance threshold is exceeded.

Description

The The invention relates to a method for operating a sensor element for determining the concentration of gas components in the exhaust gas of internal combustion engines and a sensor element for determining the concentration of gas components in Exhaust gas from internal combustion engines, which in such a method for Use can come.

object The present invention also relates to a computer program and a Computer program product used to carry out the method are suitable.

State of the art

Such sensors are also referred to as lambda probes and go, for example, from the book publication "Bosch Automotive Handbook" 25th edition, pages 133 ff out. A sensor for determining gas components and / or the concentration of gas constituents in gas mixtures, in particular in exhaust gases of internal combustion engines, with a reference electrode, which is acted upon by a reference gas channel with a reference gas, in particular air or an oxygen-containing gas, is further from the DE 100 43 089 C2 known.

sensor elements for lambda probes, which are usually planar are, have a reference gas channel, in which a reference electrode arranged is. These sensors are used for example as jump probes. The term "jump probe" is of the characteristic derived lambda sensors, which at an air ratio λ = 1 a "jump" from a first voltage value in the range of about 900 mV to a second voltage value in the range of a few mV. This jump is detected and for determining the correct air-fuel mixture at λ = 1, where optimal, stoichiometric combustion is present, evaluated.

About that In addition, these sensors are also pumped with a so-called Reference or by a loaded with a pumping voltage reference operated in the case of an imprinted anodic Stream the reference electrode out of the exhaust with oxygen is flowed through.

At the Operation of such lambda probes now the problem arises that unburned at the reference electrode or in an adjacent reference gas volume Hydrocarbons occur, for example, from polluted and / or overheated components or a leaking package originate from the probe. By these unburned hydrocarbons becomes a non-negligible part of the reference electrode supplied oxygen consumed, so that the oxygen concentration lowered at the reference electrode and thus the probe function is disturbed. This phenomenon is called CSD behavior ("Characteristic-Shift-Down") known. In this It is further disturbing that the unburned Hydrocarbons preferably in the hot, catalytic active areas, d. H. in particular at the reference electrode oxidized in the hot area of the probe ("hot spot area") become. In addition, the unburned hydrocarbons diffuse in the reference gas channel, although usually slower than oxygen, however typically uses a single hydrocarbon molecule more than a single oxygen molecule, so that the effective oxygen consumption rate by diffused unburned Hydrocarbons is greater than the Eindiffusion of oxygen. This results in a relative to the reference electrode Enrichment of unburned hydrocarbons or to a relative lack of oxygen. Finally, due to the Mechanisms explained the danger of CSD behavior in Reference gas channel significantly larger than in the internal volume in Probe housing that communicates with the reference gas channel stands.

the CSD behavior can now be counteracted by having a Electron current is applied through the sensor element, thereby forming a Oxygen flow drives. The oxygen flow goes from the reference electrode off and leads via the reference channel in the outdoor area of the sensor element. In this case, a sufficient oxygen partial pressure generated to oxidize or remove grease gas components, so that the CSD behavior is actively eliminated.

Of the Internal resistance of such lambda probes is beyond temperature dependent. If such probes with a pumping current operated, a pumping current leads to a voltage drop on the internal resistance and thus to a displacement of the measuring signal. With constant supply voltage and constant internal resistance (the caused by a constant temperature) is also the voltage drop constant and can thus be considered in advance in the control unit become. For unheated sensors, however, the internal resistance depends from the exhaust gas temperature. This can lead to a temperature-dependent Voltage drop on the internal resistance come, the signal delay equivalent. This is proportional to the pumping current.

Lambda sensors known from the prior art are usually operated without pumping current. On the one hand, this leads to a Verschwin due to the proportionality to the internal resistance that of the temperature-dependent signal delay. On the other hand, in this way no pumping action to eliminate the CSD behavior by flushing the reference channel can be achieved.

Of the The invention is based on the object, a method for operating an unheated sensor element, in particular a lambda probe, and to convey such a lambda probe, in which the CSD behavior eliminated.

Disclosure of the invention

Advantages of the invention

These The object is achieved by a method for operating a sensor element for determining the concentration of gas components in the exhaust gas of Internal combustion engine and such a sensor element with the features the independent claims 1 and 5 solved.

The basic idea The invention is the CSD behavior in unheated lambda probes, ie to minimize signal delay in these probes by: the control point to the respective internal resistance conditions is adjusted.

This happens by the fact that the internal resistance is determined, the internal resistance is compared with a predeterminable threshold and that a pumping voltage is switched on when the internal resistance falls below the threshold. In this way it is possible to use the lambda probe both operate with air reference as well as with pumped reference. in the Range of high resistances and lower temperatures the probe is conventionally operated with air reference. If the Internal resistance, however, has dropped with increasing temperature is switched on a pump voltage.

By those listed in the dependent claims Measures are advantageous developments and improvements of specified in the independent claims 1 and 5 Method and the sensor element possible.

So the internal resistance of the sensor element can be measured directly.

at In another embodiment of the method, the internal resistance is over the temperature of the sensor element based on the operating parameters of Internal combustion engine calculated or taken from a map. In doing so, depending on, for example, the operating point the internal combustion engine, and the exhaust gas temperature and / or the exhaust gas amount ratio and / or the exhaust gas mass flow determined, and from these to the temperature and thus closed the internal resistance of the sensor element.

When Threshold for the resistance, for example, a Resistance value between 500 and 3500 Ω, especially 1.5 kΩ.

The Sensor element according to the invention for determination the concentration of gas components in the exhaust gas of internal combustion engines, in particular a lambda probe with a printed electrolyte layer, is characterized in that the printed electrolyte layer made of scandium-stabilized zirconia. For one thing, one printed electrolyte against a produced by means of a green sheet Electrolytes are made much thinner, thereby the internal resistance decreases considerably. About that In addition, by the use of scandium-stabilized Zirconia instead of yttria-stabilized zirconia especially in the low temperature range with the same layer thickness a lower one Internal resistance reached. Such a sensor element can for Implementation of the method according to the invention particularly preferably used.

Around can produce improved ionic conductivity while local areas with yttria-stabilized zirconia used become.

Farther is intended to reduce the DC internal resistance, the To maximize electrode areas and the reference electrode Position it near the surface, in between arranged electrolyte as well as possible to the hot To couple exhaust gas.

The Sensor element is advantageously via a voltage divider operated, the variable divider resistors having. This can be technically easy to implement Way switched a pump voltage and thus the sensor element operated with pumped reference.

A Such lambda probe can also with a very low pumping current to be operated as possible low voltage delay leads and still a CSD and Shunt resistance guaranteed. The pumping currents are advantageously in the range between 0 μA and 10 μA, preferably between 2 μA and 5 μA.

Short description of the drawing

embodiments The invention are illustrated in the drawing and in the following description explained in more detail.

1 shows a sensor element according to the invention schematically in section;

2 shows a circuit arrangement for operating a sensor element.

Embodiments of the invention

In 1 schematically a sensor element is shown, which by an electrolyte 100 formed from scandium-stabilized zirconia, which is supported on a support 105 , For example, by screen printing, may be applied. The electrolyte has a thickness of about 500 microns. The printing technology for the production of the electrolyte in the field 101 under the outer electrode 110 is used to achieve a small layer thickness of scandium-stabilized zirconia and thereby minimize the internal resistance due to the ion incorporation reaction.

The lambda probe has a reference electrode exposed to the exhaust gas (not shown) 110 on, which with a control unit SG over one in the 1 only schematically illustrated electrical line 111 is connected and one in a reference gas volume 130 arranged reference electrode 120 who also have a line 140 connected to the control unit SG. To reduce the DC internal resistance, the electrode surface of the electrode exposed to the exhaust gas 110 chosen as large as possible, ideally chosen in the ideal case, taking into account the structural conditions. The reference electrode 120 is positioned as close as possible to the outer surface of the probe exposed to the exhaust gas in order to couple the electrolyte arranged therebetween as well as possible to the hot exhaust gas. The probe can be operated with a pump current that is chosen to be as small as possible in order to cause a small voltage delay and still ensure the CSD and shunt capability. The pump currents are in the range between 0 μA and 10 μA, in particular and preferably in the range between 2 μA and 5 μA.

An outlet 132 of the pumping gas is dimensioned small in order to advance from fat gas to the reference electrode 120 as possible to prevent. However, it must be chosen so large that a pressure equalization with the environment is guaranteed. In this case, porous layers with high flow resistance must be avoided. To prefer is an open channel with a correspondingly small cross-section. The reference channel can be realized by a simple printing layer with a sacrificial layer of thickness 20 to 30 microns and a channel width between 0.5 and 1 mm (not shown). In principle, it is also possible to use a not quite tight printed electrode lead as a reference channel (not shown). In addition, it can be provided with a porous printing layer 133 In the input region of the reference channel further Eindrin gene of fat gas components in the reference gas channel 130 to suppress and at the same time adjust the flow resistance and thus the pressure build-up in the reference range.

In the following, a method for operating such a lambda sensor to compensate for the signal distortion, which results from a CSD behavior, in conjunction with 2 described.

In 2 schematically a voltage divider consisting of the resistors R ₁ and R _{2 is} shown, which generates a probe voltage U _S on the probe via a series resistor R _V. For this purpose, the voltage U _{0 is divided} by the voltage divider, wherein a divider voltage U _T via the series resistor R _{V is applied} as a probe voltage U _S to the probe, which in 2 is schematically represented by an internal resistance R _I and a voltage source in the form of Nernst voltage U _N.

The method described below for operating in 1 represented sensor element, ie a lambda probe, could also be referred to as "hybrid model" - the operation of the probe with air reference or with pumped reference, depending on the temperature-related internal resistance of the probe. In the area of high internal resistance and low temperatures, the probe is conventionally operated with air reference. If the resistance falls below a predefinable threshold value, which, as mentioned above, can be 1 or 1.5 kΩ, then the temperature has risen far enough, a pump voltage is switched on. The internal resistance can be measured directly or be determined by a resistance model, in which case the exhaust gas temperature, the exhaust gas mass flow and other operating variables of the internal combustion engine can be used as input variables. These variables can be done for example by means of other sensors or based on the knowledge of a stored in the control unit SG map in terms of speed and load. The connection of the pump voltage via the voltage divider, for example, by opening the switch S and thus switching off a resistor R ₂ ^P of 1 kΩ, which is parallel to the resistor R ₂ . Due to this measure, due to the low internal resistance present at elevated temperatures, only a small and still tolerable distortion of the probe voltage occurs in comparison to the voltage swing due to the Nernst voltage. This method of operating the sensor element is advantageous in particular with regard to evaporation of fatty-gas components from the sealing pack (so-called "hot CSD").

The following is a dimensioning example for the in 2 illustrated circuit described. For a tolerable delay of the probes Voltage of 10 mV and 5 μA pump current, the internal resistance may still change by R = U / I = 2 kΩ when the pump current is switched on. Known lambda sensors have at 850 ° C an internal resistance of R _I = 16 kΩ. The internal resistance increases by 2 kΩ on cooling to 500 ° C and further up to about 70 kΩ at 350 ° C, before the probe material goes into a non-conductive state. The above-described hybrid model can be used without compensation of the control point by switching a pump voltage from 500 ° C of the sensor element temperature. If the internal resistance of the sensor element is further reduced on the sensor element by further measures, the temperature range in which largely distortion-free pumping is possible is correspondingly increased.

The resistors of the voltage divider are to be selected for temperatures less than 500 ° C in the ratio 1:10, that is, for example, R ₁ = 10 kΩ and R ₂ = 1 kΩ. At temperatures above 500 ° C., a resistance of, for example, 2.17 kΩ is connected to R ₂ so that a divider voltage U _T of 1.78 V results for a supply voltage U ₀ of 5 V. The resistances can be arbitrarily reduced as long as the ratio is maintained and the power source is not overloaded. However, the series resistor R _V is not increased because of the shunt resistance.

By the connection of the pumping current above the resistance threshold value corresponding temperature can be reduced already existing CSD and prevent further emergence.

The The method described above can be used, for example, as a computer program be implemented in the control unit of the internal combustion engine and run out there. The program code may be on a machine-readable Carrier be stored, the control unit SG can read.

QUOTES INCLUDE IN THE DESCRIPTION

This list The documents listed by the applicant have been automated generated and is solely for better information recorded by the reader. The list is not part of the German Patent or utility model application. The DPMA takes over no liability for any errors or omissions.

Cited patent literature

• - DE 10043089 C2 [0003]

Cited non-patent literature

• - "Bosch Automotive Handbook" 25th edition, pages 133 ff [0003]

Claims (11)

Method for operating a lambda probe for Determination of the concentration of gas components in the exhaust gas of internal combustion engines, characterized by the following steps: - Determine the internal resistance; - Compare the internal resistance with a predefinable resistance threshold; - connect a pumping voltage when falling below the resistance threshold. Method according to claim 1, characterized in that that the internal resistance is measured or by means of a model is determined. Method according to claim 1 or 2, characterized that as a threshold for resistance a resistance value between 500 and 3500 Ω, in particular 1.5 kΩ provided becomes. Method according to one of claims 1 to 3, characterized in that the switching on of the pumping voltage by switching off resistors (R ₂ ^P ) of a voltage divider, via which the pumping voltage is set takes place. Sensor element for determining the concentration of gas components in the exhaust gas of internal combustion engines, in particular lambda probes with at least one printed electrolyte layer ( 100 . 101 ), characterized in that the printed electrolyte layer ( 101 ) consists of scandium-stabilized zirconia. Sensor element according to claim 5, characterized in that to minimize the DC internal resistance of the sensor element, the electric denflächen ( 110 . 120 ) so on the electrolyte ( 101 ) are formed so that they have a maximum possible surface. Sensor element according to claim 5 or 6, characterized in that the reference electrode ( 120 ) is positioned near the outer, exhaust-exposed surface of the sensor element. Sensor element according to one of claims 5 to 7, characterized in that it can be operated via a voltage divider with variable divider resistors (R ₁ , R ₂ , R ₂ ^P ). Sensor element according to one of claims 5 to 8, characterized in that it is acted upon by a pumping current which is between 0 μA and 10 μA, preferably between 2 μA and 5 μA. Computer program that shows all the steps of a procedure according to one of claims 1 to 4 executes when it on a computing device, in particular the control unit an internal combustion engine runs. Computer program product with program code based on a machine-readable carrier is stored for execution of the method according to one of claims 1 to 4, when the Program on a computer or a control unit of a vehicle is performed.