EP2411794A2

EP2411794A2 - Method for operating a sensor element, and corresponding sensor element

Info

Publication number: EP2411794A2
Application number: EP10709446A
Authority: EP
Inventors: Peer Kruse; Jens Schneider; Lothar Diehl
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2009-03-25
Filing date: 2010-02-18
Publication date: 2012-02-01
Also published as: CN102362174B; WO2010108731A3; DE102009001840A1; RU2541702C2; WO2010108731A2; RU2011142616A; CN102362174A

Abstract

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, especially a lambda probe. Said method is characterised by the following steps: the inner resistance of the sensor element is determined, and the pump voltage of the reference electrode is readjusted when the inner resistance falls as the temperature rises, in such a way that the regulating point of the sensor element does not change and the pump flow does not fall below a pre-determined minimal value.

Description

description

title

Method for operating a sensor element and sensor element

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 the exhaust gas of internal combustion engines.

The subject matter of the present invention is also a computer program and a computer program product which are suitable for carrying out the method.

State of the art

Such sensors, which are also referred to as lambda probes, are known, for example, from the book publication "Bosch Kraftfahrtechnisches Taschenbuch" 25th edition, pages 133 ff, 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 can be acted upon by a reference gas channel with a reference gas, in particular air or an oxygen-containing gas, is also known from DE 100 43 089 C2.

Sensor elements for lambda probes, which are usually constructed in a planar manner, have a reference gas channel in which a reference electrode is arranged. These sensors are used for example as jump probes. The term "jump probe" is derived from the characteristic curve of such lambda sensors which, with an air ratio λ = 1, make 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 used to determine the correct air-fuel mixture at λ = 1 at which optimal stoichiometric combustion is present.

In addition, these sensors are also operated with a so-called pumped reference or with a reference applied to a pumping voltage reference, in which the reference electrode is supplied from the exhaust gas with oxygen via an impressed anodic current.

When operating such lambda probes, the problem now arises that unburned hydrocarbons occur at the reference electrode or in an adjacent reference gas volume, resulting, for example, from contaminated and / or overheated components or a leaky packing of the probe. By means of these unburned hydrocarbons, a non-negligible part of the oxygen supplied to the reference electrode is consumed, so that the oxygen concentration at the reference electrode is reduced and thus the probe function is disturbed. This phenomenon is known as CSD behavior ("characteristic shift-down") In this context, it is further troublesome that the unburned hydrocarbons are preferably on the hot, catalytically active surfaces, ie in particular on the reference electrode in the hot region of the probe ("Hot spot area") are oxidized. Moreover, while the unburned hydrocarbons in the reference gas channel usually diffuse slower than oxygen, a single hydrocarbon molecule typically transcribes more than a single oxygen molecule so that the effective oxygen consumption rate of diffused unburned hydrocarbons is greater than the pure oxygen diffusion rate. This results in a relative enrichment of unburned hydrocarbons or a relative lack of oxygen at the reference electrode. Finally, due to the described mechanisms, the risk of CSD behavior in the reference gas channel is significantly greater than in the inner volume in the probe housing, which is in communication with the reference gas channel.

The CSD behavior can now be counteracted by the fact that the sensor element is acted upon by applying an electrical voltage with an electron current, thereby driving an oxygen ion current. The oxygen ion current passes into an oxygen flow at the reference electrode and leads via the reference channel into the outer region of the sensor element. tes. In this case, a sufficient oxygen partial pressure is generated in order to oxidize or transport off fat gas components, so that the CSD behavior is actively eliminated.

The internal resistance of such lambda probes is also temperature-dependent. If such probes are operated with a pumping current, a pumping current leads to a voltage drop at the internal resistance and thus to a displacement of the measuring signal. With constant supply voltage and constant internal resistance (which is due to a constant temperature), the voltage drop is constant and can thus be considered in advance in the control unit. For unheated sensors, however, the internal resistance depends on the exhaust gas temperature. This can lead to a temperature-dependent voltage drop across the internal resistance, which corresponds to a signal delay. This is proportional to the pumping current.

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

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

Disclosure of the invention

Advantages of the invention

This 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 engines and a sensor element having the features of the independent claims 1 and 4. The basic idea of the invention is to minimize the CSD behavior, ie a signal delay in the case of unheated lambda probes, by first determining the internal resistance of the sensor element and by adjusting the supply voltage of the sensor element with increasing temperature and thereby decreasing internal resistance in such a way that the control point of the sensor element does not change, that is, is located at the same point and while a predeterminable minimum pumping current is not exceeded.

The advantage of this measure is an increased pumping at high temperatures at which fat gas can increasingly evaporate from the pack.

Advantageous refinements and improvements of the method and the sensor element specified in the independent claims 1 and 4 are possible due to the measures listed in the dependent claims.

For example, in a first embodiment of the method, the internal resistance of the sensor element is measured.

In another embodiment of the method, the internal resistance over the temperature of the sensor element based on the operating parameters of the internal combustion engine is calculated or taken from a map. In this case, depending on, for example, the operating point of the internal combustion engine, the exhaust gas temperature, the exhaust gas amount ratio, the Abgasmassen- ström determined, and from these on the temperature and thus the internal resistance of the sensor element.

The sensor element according to the invention for determining the concentration of gas components in the exhaust gas of internal combustion engines with at least one electrolyte layer is characterized by the use of scandium-stabilized

Zirconia instead of yttria-stabilized zirconia in the area under the outer electrode with a thickness between 10 microns and 50 microns from. As a result, lower internal resistance values can be achieved, especially in the low-temperature range, because the resistance component of the ion-installation reaction is lowered. In order to produce an improved ionic conductivity, local areas with yttrium-stabilized zirconium oxide can be used. This applies in particular to the layers via which one or both electrodes are connected to the electrolyte.

Furthermore, to reduce the DC internal resistance, it is intended to maximize the electrode areas and to position the reference electrode close to the outer surface of the probe in order to couple the electrolyte therebetween as well as possible to the hot exhaust gas.

Such a lambda probe is also operated with a very low pumping current, which leads to the lowest possible voltage distortion and still ensures a CSD and shunt resistance. The pump currents are in the range between 0 μA and 10 μA, preferably between 2 μA and δ μA.

Short description of the drawing

Embodiments of the invention are illustrated in the drawings and explained in more detail in the following description.

In Fig. 1, a sensor element according to the invention is shown schematically in section.

Embodiments of the invention

1 shows schematically a sensor element which is formed by an electrolyte 100 which can be applied to a carrier 105, for example by screen printing. The electrolyte has a thickness of about 500 microns. The printing technique of the invention for the preparation of the electrolyte in

Area 101 under the outer electrode 110 is used to achieve a small layer thickness of about 10 μm to 50 μm of scandium-stabilized zirconium oxide, thereby minimizing the internal resistance due to the ion incorporation reaction. The lambda probe has an outer electrode 110 which is exposed to the exhaust gas (not shown) and which is connected to a control unit SG via an electrical line 11 1 shown only schematically in FIG. 1 and a reference arranged in a reference gas volume 130 (reference gas channel) - Electrode 120, which is also connected via a line 140 to the control unit SG. In order to reduce the DC internal resistance, the electrode surface, in particular of the electrode 1 10 exposed to the exhaust gas, is chosen to be as large as possible, ideally it is chosen to be maximum, taking into account the structural conditions and the resulting temperature distributions. The reference electrode 120, whose surface adjoins the electrode exposed to the exhaust gas

1 10 is positioned as close as possible to the outer surface of the probe 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 ensure a low voltage delay while still ensuring 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.

Purely in principle, it is also possible to use a pump current only at a higher temperature, for example> 500 ⁰ C, to switch on, which serves to bring about a "Abreak- tion" of evaporating from the packing fat gas. An outlet 132 of the pumping gas is designed to be small, in order to prevent the penetration of grease gas to the reference electrode 120 as far as possible, however, it must be so large that a pressure equalization with the environment is ensured, whereby porous layers with a high flow resistance must be avoided, an open channel with a correspondingly small diameter being preferred The reference channel can be realized by a simple pressure layer with a sacrificial layer of thickness 20 to 30 μm and a channel width between 0.5 and 1 mm (not shown) .in principle, it is also possible to use a not quite tightly printed electrode feed line as reference channel to use

(not shown). In addition, it can be provided to suppress further penetration of fatty-gas components into the reference gas channel 130 with a porous pressure layer 133 in the input region of the reference channel and at the same time to set the flow resistance and thus the pressure build-up in the reference region. The following describes a method for operating such a lambda sensor to compensate for the signal distortion resulting from the pumping current for suppressing CSD behavior. The compensation of the signal delay presupposes the knowledge of the internal resistance of the sensor element, which is first determined. This can be determined, for example, by measurements or by calculation or, for example, by means of a characteristic map, depending on the operating parameters of the internal combustion engine, such as the exhaust gas mass flow, the exhaust gas quantity ratio, the exhaust gas temperature and the like. There is now an adaptation of the supply voltage of the probe to the sinking with increasing temperature internal resistance such that the supply voltage is readjusted with increasing temperature, so that the control point is always in the same place, that is not changed and a minimum pumping current is not exceeded , In this case, an increased pumping at high temperatures is achieved in an advantageous manner, in which fat gas can increasingly evaporate from the pack.

By changing the control point, which takes place within a control software in the control unit SG, CSD-related signal distortions can be compensated.

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

Claims

claims

1. A method for operating a sensor element for determining the concentration of gas components in the exhaust gas of internal combustion engines, in particular a lambda probe, characterized by the following steps: - It determines the internal resistance of the sensor element;

- It is the pumping voltage of the reference electrode at (120) with increasing temperature decreasing internal resistance readjusted such that the control point of the sensor element does not change and a predetermined minimum pumping current is not exceeded.

2. The method according to claim 1, characterized in that the internal resistance of the sensor element is determined by a Ri-pulse internal resistance measurement.

3. The method according to claim 1, characterized in that the internal resistance of the sensor element is determined by calculation based on the operating parameters of the internal combustion engine or a map is taken.

4. Sensor element for determining the concentration of gas components in the exhaust gas of internal combustion engines having at least one electrolyte layer, characterized by an electrolyte layer (101) below the exhaust gas E- electrode between 10 and 50 microns thick, consisting of

a. Scandium stabilized zirconia and / or b. a mixture of scandium- and yttrium-stabilized zirconium oxide and / or c. using a mixture of scandium to yttrium stabilized zirconia.

5. Sensor element according to claim 4, characterized in that for minimizing the DC internal resistance of the sensor element, the electric denflächen (1 10, 120) are formed on the electrolyte so that they have a geometrically maximum possible surface.

6. Sensor element according to claim 4 or 5, characterized in that the reference electrode (120) is arranged near the outer surface exposed to the exhaust gas of the sensor element.

7. Sensor element according to one of claims 4 to 6, characterized in that it is acted upon by a pump current which is between 0 uA and 10 uA, preferably between 2 uA and 5 uA.

8. Computer program that executes all the steps of a method according to one of claims 1 to 3, when it runs on a computing device, in particular the control unit of an internal combustion engine.

A computer program product with program code stored on a machine-readable medium for carrying out the method according to one of claims 1 to 3, when the program is executed on a computer or a control unit of a vehicle.