EP1952136A1

EP1952136A1 - Gas sensor

Info

Publication number: EP1952136A1
Application number: EP06807290A
Authority: EP
Inventors: Lothar Diehl; Thomas Seiler
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 2005-11-14
Filing date: 2006-10-16
Publication date: 2008-08-06
Also published as: JP4709905B2; WO2007054421A1; US20090152112A1; JP2009516158A; DE102005054144A1; US8828205B2

Abstract

Disclosed is a gas sensor, particularly a lambda probe, for determining the oxygen concentration in the exhaust gas of an internal combustion engine that is operated using a fuel-air mixture. Said gas sensor comprises a pump cell with an outer electrode (150) that is exposed to the exhaust gas, an inner electrode (140) located in a measuring chamber (13) which is separated from the exhaust gas by means of a first diffusion barrier (120), and an electronic circuit (190) for generating a voltage (Upump) applied between the outer electrode (150) and the inner electrode (140) as well as for measuring and evaluating a pump current (Ipump) that is generated in said process in order to draw a conclusion therefrom about the composition of the fuel-air mixture. The inventive gas sensor is characterized in that the outer electrode (150) is arranged in an independent measuring volume (230) which is separated by means of an additional, second diffusion barrier (220) whose coefficient of diffusion is different from that of the first diffusion barrier (120) while the circuit (190) is configured so as to preferably repeatedly reverse the polarity of the voltage (Upump).

Description

gas sensor

description

The invention relates to a gas sensor according to the preamble of claim 1.

State of the art

Oxygen sensor gas sensors in the form of lambda probes are used in large numbers in exhaust systems of internal combustion engines in motor vehicles in order to be able to provide signals for the engine control via the exhaust gas composition. In this way, the engine can be operated so that the exhaust gases have an optimum composition for the aftertreatment with today in the exhaust system usually present catalysts.

In Fig. 1 a known from the prior art gas sensor is shown. The sensor element 100 has a gas inlet hole 115 through which exhaust gas flows and passes through a diffusion barrier 120 into a measuring space 130. In the measuring space, an inner pumping electrode 140 is arranged. An outer pumping electrode 150 is exposed on the outside of the solid electrolyte 110 and under a porous protective layer 155 to the exhaust gas of an internal combustion engine (not shown).

Between the inner pumping electrode 140 and the outer pumping electrode 150, a pumping voltage U _pUm p is applied, so that a _pumping current l _pUm p flows. In the solid electrolyte 110, a heater 160 embedded in an insulating layer 162 is further arranged. By this heater 160, the sensor element on heats a temperature that allows optimal operation of the sensor element 100.

This planar broadband lambda probe according to the limiting _{current principle is} subjected to a fixed pumping voltage U _pUm p. With a lean exhaust gas, ie with an excess of exhaust gas, the fixed pumping voltage generates a positive _pumping current I _pUm p, which is clearly related to the oxygen content of the exhaust gas. However, in a rich exhaust gas, ie an exhaust gas with excess fuel, a positive pumping current also occurs due to the decomposition of the water contained in the exhaust gas. Although the applied pumping voltage Up _to p is well below the decomposition voltage of the water, but since hydrogen exists in the exhaust gas, the water decomposition is energetically possible, because at the outer pumping electrode 150, water is generated from the reaction of the hydrogen with the oxygen ions. The _pumping current l _pUm p is therefore limited by the hydrogen content in the case of rich exhaust gas. Since this pumping current Ipu _m p in the rich exhaust gas has the same direction as the _pumping current I _pUm p with lean exhaust gas, it is no longer _{possible to deduce} the exhaust gas composition from the _pumping current I _pum p.

Object of the present invention is to develop a generic gas sensor such that both in the presence of a lean exhaust gas and in the presence of a rich exhaust gas can be clearly concluded on the exhaust gas composition.

Advantages of the invention

This object is achieved by a gas sensor having the features of claim 1. The basic idea of the invention is to enable a defined gas diffusion towards the outer exhaust gas electrode by arranging the outer pumping electrode in its own measuring volume and providing a second diffusion barrier with a diffusion coefficient which differs from that of the first diffusion barrier, thus unambiguously defining the pumping current depending on the exhaust gas composition and thus in turn Ström to close the exhaust gas composition. In this case, the electronic circuit that generates the pumping voltage and the pumping current must be designed so that a reversal of the pumping voltage is possible. By reversing the polarity of the pump voltage, the diffusion direction of the oxygen or of the hydrogen in the exhaust gas can be reversed, from which conclusions about the exhaust gas composition are possible in the manner described in more detail below.

In principle, the one-time reversal of the pump voltage suffices to draw conclusions about the exhaust gas composition. A particularly advantageous embodiment provides an electronic circuit in which a rectangular alternating pumping voltage is generated. Preferably, the electronic circuit generates a rectangular alternating pumping voltage, preferably in the frequency range between 2 and 500 Hz, in particular between 20 and 50 Hz. By rectangular polarity reversal of the pumping voltage, a corresponding inversion of the pumping direction, which provides qualitative information, whether fat or lean gas is present , The amount of current in one of the two pumping directions allows a quantitative concentration determination.

In principle, it suffices if the diffusion coefficient of the second diffusion barrier differs from that of the first diffusion barrier. An embodiment provides that the diffusion coefficient of the second diffusion barrier is smaller than the diffusion coefficient of the first diffusion barrier. In this case, a smaller current is produced with a lean exhaust gas than with a richer exhaust gas. The current is in a sense proportional to whether there is a lean or rich exhaust.

For this purpose, the second diffusion barrier can be made more open-pored than the first diffusion barrier.

Another embodiment provides that the second diffusion barrier in the flow direction of the exhaust gas has a shorter length than the first diffusion barrier. drawing

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

In the drawing show:

1 shows a known from the prior art gas sensor.

Fig. 2 shows an embodiment of a gas sensor according to the invention and

Fig. 3 shows another embodiment of a gas sensor according to the invention.

Description of the embodiments

A gas sensor known from the prior art has a sensor element 100, which is formed by a solid electrolyte 110. In the solid electrolyte 110, a measuring chamber 130 is formed, in which an inner pumping electrode 140 is arranged. The exhaust gas of an internal combustion engine (not shown) flows through a gas inlet hole 115 via a diffusion barrier 120 in the measuring volume 130. On the outside of the sensor element 100 exposed to the exhaust gas, an outer pumping electrode 150 is arranged, which is covered by an open-pored protective layer 155. By means of a schematically illustrated electronic circuit 190, a constant pumping voltage U _pUm p is generated between the outer pumping electrode 150 and the inner pumping electrode 140 arranged in the measuring volume 130. In the case of a lean exhaust gas, this results in a positive _pumping current I p _pm , which results in oxygen ions O ^{2 "being} pumped from the measuring volume 130 into the exterior of the sensor element, ie, into the exhaust gas. However, due to the decomposition of the water contained in the exhaust gas, the excess of exhaust gas also results in a positive pumping current of the water. However, since hydrogen exists in the exhaust gas, the decomposition of water is energetically possible, because at the outer pumping electrode 150 is generated from H ₂ and O ₂ water. The current is thus limited in a rich exhaust gas composition by the hydrogen content at the outer electrode. Since the _pumping current I _pUm p in the case of rich exhaust gas composition has the same direction as the _pumping current I _pUm p with lean exhaust gas composition, it can not be concluded from the _{pumping flow} that the exhaust gas composition is readily.

In order to be able to conclude the exhaust gas composition even with a rich exhaust gas composition, the invention provides that the outer pumping electrode 150 is arranged in a further own measuring volume 230 into which exhaust gas flows through the gas inlet hole 115 via a second diffusion barrier 220. In this case, the second diffusion barrier 220 has a different diffusion coefficient than the first diffusion barrier 120. It is, for example, thinner and more open-pored than the first diffusion barrier 120.

The idea of this arrangement is the following. In the lean, the current l _pUm p is _limited by the diffusion of oxygen to the inner pumping electrode 140 (cathode). In the case of a rich exhaust gas composition, the _pumping current I _pUm p is _limited by the diffusion of the hydrogen to the outer pumping electrode 150 (anode). Now, if the gas supply to the outer pumping electrode 150 via a thinner or more open-pored diffusion barrier 220 than the gas supply to the inner pumping electrode 150, which is a thicker or less open-pore diffusion barrier 120, then a reversal of the pumping direction results in that in a lean mixture composition Current l _pum p is smaller, since now the oxygen diffusion limitation passes from the inner pumping electrode 140 to the outer pumping electrode 150, which is difficult to achieve for inflowing gas. Thus, at the electrode at which the rate-determining reaction step, namely the O ₂ reduction, takes place, a lower O ₂ concentration is available than before the polarity reversal and as a result the current decreases. With a rich exhaust gas composition, the _pumping current l _pUm p _becomes larger, since now the hydrogen diffusion limitation passes from the outer pumping electrode 150 (anode) to the inner pumping electrode 140 (cathode), which is easier to reach for inflowing gas. As a result, a higher H ₂ concentration is available at the electrode at which the rate-determining reaction step, namely the H ₂ oxidation to H ₂ O takes place, than before the polarity reversal and as a result the current increases.

By a polarity reversal can be concluded qualitatively on the exhaust gas composition from the observation of whether the _pumping current l _pUm p rises or falls.

The electronic circuit 190 performs such a current _comparison in the case of a rectangular, high-frequency reversal of the polarity of the pump current U pUm inversion of the pumping direction. By this comparison, qualitative information can be obtained as to whether fat or lean gas is present. Due to the magnitude of the current I _pUm p in one of the two pumping directions, a quantitative concentration determination can be made.

One embodiment makes use of the fact that it is basically sufficient to set a positive pump voltage constantly and only if l _pum p = 0 has been reached to generate a short inversion pulse, that is briefly a negative pump voltage U _pum p between inner pumping electrode 140 and outer Adjust pump electrode 150. This inversion pulse then provides information as to whether one is in the rich or lean load of the characteristic until the next zero crossing of the current.

In a further advantageous embodiment, shown in Fig. 3, the same elements are designated by the same reference numerals as in the embodiment shown in Fig. 2, so that for the description thereof to the above fully incorporated by reference.

In contrast to the exemplary embodiment illustrated in FIG. 2, in the exemplary embodiment illustrated in FIG. 3, the inner pumping electrode 140, ie the cathode, is connected to the ground connection of the heater 160. In this case For example, the potential of the anode, ie the outer pumping electrode 150, is alternately higher or lower than the heater mass by the amount of the pumping voltage U _pUm p. The frequency at which the pumping direction is reversed must be higher than the heater timing in order to avoid interference. The advantage of this embodiment is that a connecting cable is eliminated.

Claims

R. 311974Patent claims

1. A gas sensor, in particular lambda probe, for determining the oxygen concentration in the exhaust gas of an internal combustion engine operated with a fuel-air mixture comprising a pumping cell with an outer electrode exposed to the exhaust gas (150), with one in the exhaust gas through a first diffusion barrier ( 120) separate measuring space (130) arranged inside electrode (140) and with an electronic circuit (190) for generating a between outer electrode (150) and inner electrode (140) voltage applied (U _pUm p) and for measuring and evaluating a thereby adjusting pumping current (l _pUm p). to conclude from this on the composition of the fuel-air mixture, characterized in that the outer electrode (150) is arranged in a separate measuring volume (230), which is separated by a further, second diffusion barrier (220), the diffusion coefficient of differs from the first diffusion barrier (120), and that the circuit (190) for preferably repeated reversal of the voltage (U _pUm p) is formed.

2. Gas sensor according to claim 1, characterized in that the electronic circuit (190) generates a rectangular alternating pumping voltage.

3. Gas sensor according to claim 2, characterized in that the frequency of the alternating pump voltage between 2 and 500 Hz, in particular between 20 and 50 Hz.

4. Gas sensor according to one of the preceding claims, characterized in that the electronic circuit (190) generates an inversion pulse in the pumping voltage (Upump) after each zero crossing of the _pumping current (l _pUm p).

5. Gas sensor according to one of claims 1 to 4, characterized in that the diffusion coefficient of the further, second diffusion barrier (220) is smaller than the diffusion coefficient of the first diffusion barrier (120).

6. Gas sensor according to one of claims 1 to 5, characterized in that the second diffusion barrier (220) is open-pored than the first diffusion barrier (120).

7. Gas sensor according to one of claims 1 to 6, characterized in that the second diffusion barrier (220) in the flow direction of the exhaust gas has a shorter length than the first diffusion barrier (120).

8. Gas sensor according to one of the preceding claims, characterized in that the inner electrode (140) is electrically conductively connected to the ground terminal of a heater (160).