The invention concerns a circuit arrangement for operating a guide probe that is arranged downstream of a catalytic converter with the features that are stated in the generic term of claim 1.
An exhaust gas probe is known from DE 41 00 106 C1, at which the electrode that is exposed to the exhaust gas is covered by a porous ceramic protective layer, in which catalytically active materials are allocated discreetly and homogeneously in such a way, that the discreetly allocated catalytically active materials, preferably platinum, are active at higher temperatures, whereby the homogeneously allocated active components, preferably rhodium, are active at lower temperatures. Due to the lower amount of these substances especially an improvement of the sensor regulation position is achieved, especially at lower temperatures. The sensor has furthermore a simple manufacturing technology.
In such exhaust gas sensors with oxygen ion conducting solid electrolytes the transmission from a rich to a lean mixture is measured by measuring the potential between the exhaust gas probe and the reference electrode, which is exposed to a gas with defined oxygen content, as for example the ambient air. This transmission expresses itself in a strong jump of the probe voltage at the transmission from a rich to a lean mixture, which is often also called lambda jump. The exhaust gas probe is divided by a porous protective layer, which covers the exhaust gas probe.
The protective layer serves not only as mechanical protection of the exhaust gas electrode, but it also increases the so-called contamination resistance.
For controlling exhaust gas compositions downstream of a catalytic converter such jump probes are used as guide probes. Theses guide probes serve the controlling of the catalytic converter and are additionally used for balancing the probe that regulates the mixture formation upstream before the catalytic converter, the so-called pre-catalyst-probe. The regulation and the controlling of such a guide probe downstream of the catalytic converter is based on a control point, which is slightly moved away from the stoichiometric point (lambda=1) into the rich area. Thereby control voltages in the range of 600 mV to 700 mV are deployed.
A disadvantage at the adjustment of such a high control point is that even at a constant lambda the probe voltage depends on the proportion of the rich gas components carbon monoxide (CO) and hydrogen (H2). Furthermore the gas composition at the control point also depends strongly on the probe temperature. That strong gas- and temperature dependency causes an increased effort for an optimal coordination of the control system. The catalytic converter can adjust the gas balances variably well after a rich/lean change over a longer period of time. Under certain circumstances there are working areas, in which no controlling onto a constant lambda value in the system is possible due to the different gas composition.
The invention is therefore based on the task to provide a circuit arrangement, which enables the increase of the accuracy of the rich gas measurement in a very small range with low rich gas concentrations. Furthermore the temperature dependency of the measuring signal shall be reduced.
The circuit arrangement for operating a guide probe that is arranged behind the catalytic converter according to the invention has the advantage that with the aid of a familiar jump probe rich gas components can be proven in the exhaust gas. Due to the resistor, which is arranged between the reference electrode and the exhaust gas electrode, and which purposefully influences an oxygen ion transport between the reference electrode and the exhaust gas electrode, a linear characteristic curve behavior at rich gas concentration is achieved in a very advantageous manner. It is also an extraordinary advantage that jump probes can be used as guide probes, which do not require additional circuit effort. The output signal is based on the familiar measurement of the probe voltage of such a jump probe.
The resistor is selected in such a way that the probe voltage that drops above it is lower than the Nernst voltage of the guide probe. Advantageous values of the resistor vary between 5000 and 20000 ohm.
Preferably the resistor and the porous coating are coordinated in such a way that the rich gas molecules that are accumulating in the porous coating are completely oxidized by oxygen ion transport that is caused by the resistor.
The porosity and the thickness of the porous coating are advantageously adjusted in such a way that an oxidation current flows in the range of 20 to 60 μA at an hydrogen content of 100 ppm. The values for the resistors and the oxidation current apply to the used electrode size. When changing the geometric area of the exhaust gas electrode the values have to be adjusted correspondingly.
By selecting applicable electrodes that are catalytically less active the sensitivity towards CO can be reduced. The output signal of the guide probe is then proportional to the hydrogen partial pressure.
Further advantages and features of the invention are subject matter of the following description as well as the drawing of embodiments. It is schematically shown in:
a and 2b show circuit arrangements for operating a guide probe that are using the invention;
a and 3b show a probe voltage as a function of the lambda value at typical post-catalyst gas compositions, wherein
An exhaust gas probe, shown in
The current of the oxygen ion (O2−-ions) from the reference electrode 110 to the exhaust gas electrode 120, as well as the current of carbon monoxide CO through the porous coating 130 to the exhaust gas electrode 120 are schematically shown in
CO+O2−→CO2+2e−.
Furthermore another reaction of the rich gas hydrogen H2 takes place in the exhaust gas electrode 120:
H2+O2−→H2O+2e−.
A circuit arrangement for operating a probe that is shown in
At a corresponding adjustment of the diffusion resistance of the protective layer 130 and at an optimized value of the parallel applied resistor Rx operating conditions can be set, at which basically each rich gas molecule that arrives in the protective layer 130 is oxidized. The current that flows through the arrangement is then proportional to the concentration to the component in the exhaust gas. Thereby the probe voltage Us is also proportional to the concentration in the exhaust gas.
The same probe with a resistor Rx of 15 kΩ and the wiring from
It shall be pointed out that the pump capability of the reference air duct (not shown) has to be considered hereby. If sufficiently enough oxygen cannot be delivered in addition over the reference air, the previously stated reaction behavior is limited insofar.
In order to achieve a diffusion control, thus a targeted diffusion current, the resistor Rx is generally selected in such a way that the resulting probe voltage Us is significantly lower than the corresponding Nernst voltage of the probe in dead state. This condition limits the upper voltage onto 0.45 V to 0.5 V. At probe voltages lower than 0.2 V oxygen is released as a further electrode reaction:
O2−→ 1/2 O2+2e31 .
By the parallel reaction the current or the probe voltage Us is increased.
When using the guide probe downstream after the catalytic converter hydrogen and carbon monoxide occur almost exclusively as rich gas components. Due to the faster diffusion of hydrogen it is proven with a significantly higher sensitivity. Familiar electrodes are partially catalytically more inactive regarding the electrode reaction with carbon monoxide. It is thereby possible with applicable selected catalytically inactive electrode materials to produce an increase of the selectiveness regarding H2.
In order to avoid that the after-transport of oxygen over the reference air duct limits the pre-described measurements, as described above, protective layers 130 are used at this jump probe that is applied as a guide probe, which are thicker than protective layers at familiar jump probes. Alternatively or additionally a bigger reference air duct can also be provided. Hereby the pre-described linear area can be increased and optimized.
Number | Date | Country | Kind |
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10 2006 041 184.6 | Sep 2006 | DE | national |
Filing Document | Filing Date | Country | Kind | 371c Date |
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PCT/EP07/57948 | 8/1/2007 | WO | 00 | 12/17/2008 |