Limit current sensor for determining the lambda value in gas mixtures

A limit current sensor for determining lambda values of a gas mixture includes a solid electrolyte layer comprised of a material which conducts oxygen ions; an anode provided on a surface of the solid electrolyte layer and having a surface opposite the solid electrolyte layer which is exposed to a gas which is one of the gas mixture being measured or a reference gas; a first pumping cell comprising the anode and a first cathode provided on a surface of the solid electrolyte layer opposite the anode; a second pumping cell comprising the anode and a second cathode provided on the surface of the solid electrolyte layer on which the first cathode is provided and spaced apart from the first cathode; a diffusion layer which is provided across the first cathode and the second cathode in contact therewith and along surfaces thereof opposite the solid electrolyte layer, and which is in communication with the gas mixture to be measured so that diffusion of the gas mixture to be measured through the diffusion layer occurs along a diffusion path which reaches the first cathode prior to reaching the second cathode; and means for activating one pumping cell at a time based on a predetermined threshold value of oxygen concentration so that the first cathode is activated at oxygen concentrations within a range near a stoichiometric gas mixture where .lambda.=1 and so that the second cathode is activated at higher oxygen concentrations.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The invention is based on a limit current sensor for determining the lambda 
value in gas mixtures of the generic type of the main claim. 
2. Description of the Related Art 
The German Offenlegungsschrift 39 08 393 discloses a limit current sensor 
in which, in order to reduce the response time, a second pumping cell is 
provided with which a constant concentration of oxygen in the diffusion 
channel can be obtained. The second pumping cell serves to achieve the 
steady state equilibrium condition of the diffusion current in the 
diffusion channel at an early point in time. Shortening the length of the 
diffusion channel would also bring about a rapid response time, but at the 
same time would increase the limit current too strongly. Because of the 
limited current loading capacity of the electrodes when the concentrations 
of oxygen are high in a lean gas mixture, a minimum length of the 
diffusion channel is necessary. 
For the use of the limit current sensor in the lean range (.lambda.&gt;1), the 
stoichiometric range (.lambda.=1) and up to the rich range (.lambda.&lt;1) of 
the fuel/air ratio, it is known, from EP-B1-190 750, to expose the anode 
of the pumping cell to a reference atmosphere. In the lean range these 
sensors operate like the known lean sensors. The oxygen molecules are 
reduced at the cathode so that the oxygen ions migrate from the cathode to 
the anode through the solid ZrO.sub.2 electrolyte. At the anode the ions 
are in turn converted into oxygen molecules and released into the 
atmosphere. Under stoichiometric conditions, there is a chemical 
equilibrium at the cathode so that there is no pumping current present. In 
the rich range, the oxygen ions are also fed from the cathode to the anode 
as a result of the applied pumping voltage. At the anode, they are in turn 
converted into oxygen molecules. The stream of oxygen ions flows in the 
opposite direction from that of the lean range. For this purpose, it is 
necessary to reverse the polarity of the pumping voltage. This is realized 
in that the level of the electromotive force occuring under stoichiometric 
conditions is used as switching signal. 
In limit current sensors, a limit current is generally measured with a 
constant voltage applied to the two electrodes of the limit current 
sensor. With an oxygen-containing measurement gas, the limit current is 
linearly dependent on the partial pressure of the oxygen for as long as 
the diffusion of the gas to the cathode determines the speed of the 
reaction which is underway. Such limit current sensors which are exposed 
in particular to the measurement gas are suitable for detecting the 
concentration of oxygen in lean measurement gases. Between the electrodes, 
the limit current goes into the lean range as soon as the oxygen molecules 
passing to the cathode through the diffusion layer are transported away 
rapidly in the form of ions. In the rich range, the limit current occurs 
when a diffusion barrier is placed in front of the anode and the diffusion 
of H.sub.2 and CO to the anode determines the speed of the entire 
reaction. 
When the pumping voltage grows slowly from the 0 volt value, there are 
ohmic conditions present between the electrodes so that, as the pumping 
voltage increases, the pumping current rises until the diffusion limit 
current brings about the limitation of the pumping current. If the cathode 
did not have a diffusion barrier or if it were exposed to the measurement 
gas with only a low diffusion resistance, in particular at high partial 
pressures a diffusion which limits the pumping current would not occur, as 
a result of which the current/voltage behavior of the sensor would 
continue to adhere to the ohmic conditions. As a result, the pumping 
voltage continues to rise so that finally, even at values greater than 1 
volt, it does not move into the limit current range and thus it does not 
become possible to measure the O.sub.2 content. Such high pumping voltages 
lead to the solid electrolyte and the electrode being destroyed. On the 
other hand, at low partial pressures, even a low diffusion resistance 
would be sufficient. However, in order to use the limit current sensor for 
detecting a wide range extending from lean to rich, a sufficient diffusion 
resistance must be ensured. A sufficient diffusion resistance which is 
determined by a corresponding diffusion path of the measurement gas has, 
in the vicinity of stoichiometric conditions, the disadvantage that there 
is hardly any difference in concentration any more and thus even small 
fluctuations of measurement gas falsify the sensor signal. Also, in this 
ease, even small voltages are sufficient to destroy the solid electrolyte. 
SUMMARY OF THE INVENTION 
The limit current sensor according to the invention has the advantage that 
the sensitivity of the limit current sensor is increased in the region 
around the stoichiometric ratio (.lambda.=1). 
With the measures disclosed in the subclaims, advantageous developments of 
the limit current sensor according to the invention are possible. It is 
particularly advantageous to realize the two pumping cells with different 
diffusion resistances. A simple way of realizing different diffusion 
resistances is achieved if the cathodes of the two pumping cells are 
arranged with different diffusion paths in the diffusion barrier. Good 
results can be achieved if the diffusion path of the pumping cell with the 
higher sensitivity corresponds to 0.1 to 0.7 times, preferably 0.3 times, 
the diffusion path of the pumping cell with the longer diffusion path. A 
cost effective design of the limit current sensor is possible by providing 
a common anode with a single connection line for the two pumping cells.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The limit current sensor according to FIG. 1 has a first solid electrolyte 
film 10, consisting for example of yttrium-stabilized zirconium oxide, 
with an anode 11 and a first cathode 12 and a second cathode 13. The first 
cathode 12 forms with the anode 11 a first pumping cell 14 and the second 
cathode 13 forms with the anode 11 a second pumping cell 15. Arranged 
parallel to the first solid electrolyte film 10 are a second solid 
electrolyte film 16 and a third solid electrolyte film 17. A heater 19 
which is embedded in an electrically insulating layer 18 is positioned 
between the two solid electrolyte films 16, 17. The insulating layer 18 
consists for example of Al.sub.2 O.sub.3. Instead of the solid electrolyte 
films 16 and 17, other ceramic films can also be used just as well, for 
example consisting of Al.sub.2 O.sub.3. Of course, when electrically 
insulating ceramic films are used, the insulating layer 18 for embedding 
the heater 19 can be dispensed with. 
In each case a gas-tight frame 22 which determines the distance and which 
also consists for example of zirconium oxide is provided between the first 
solid electrolyte film 10, the second solid electrolyte film 16 and the 
third solid electrolyte carrier 17. 
Between the first solid electrolyte film 10 and the second solid 
electrolyte film 16 a diffusion channel 20 which forms a diffusion barrier 
for the measurement gas is realized, which diffusion channel 20 is 
connected to the measurement gas via a diffusion hole 21. The anode 11 and 
the two cathodes 12, 13 extend around the diffusion hole 21 for example in 
an annular shape. The first cathode 12 is positioned here with a diffusion 
path 1.sub.1 nearer to the diffusion hole 21 than the second cathode 13 
with a diffusion path 1.sub.2. The diffusion path 1.sub.1 to the first 
cathode 12 is for example 0.3 times the diffusion path 1.sub.2 of the 
second cathode 13. In order to form an appropriate diffusion resistance, 
the diffusion channel 20 is filled with a porous material, consisting for 
example of Al.sub.2 O.sub.3. Here, the size of the pores determines, inter 
alia, the diffusion resistance. 
The anode 11 and the cathodes 12, 13 are connected to a pumping voltage 
source U, the connection to the cathodes 12, 13 being switchable as 
desired by means of a switch 24. In addition, an ammeter 23 is arranged in 
the circuit in order to measure the limit current I.sub.p. For practical 
application in a motor vehicle, instead of the ammeter 23, a control unit 
is provided for controlling the fuel/air mixture. 
FIG. 2 shows the characteristic curve of the pumping current I.sub.p of the 
two pumping cells 14, 15 plotted against the O.sub.2 concentration C. The 
limit current I.sub.p of the two pumping cells is measured by the ammeter 
23. When there is a high concentration of oxygen in the measurement gas 
(air=20.5%), the second cathode 13 is connected to the voltage source U 
via the switch 24. As the concentration C of oxygen decreases, the limit 
current I.sub.p2 of the second pumping cell 15 is reduced. As soon as a 
predetermined threshold value C.sub.s of the concentration C of oxygen is 
reached, the pumping voltage U is applied to the first cathode 12. For 
this purpose, the switch 24 is activated in accordance with the dotted 
line in FIG. 1. The activation of the switch 24 is carried out by a 
control circuit (not illustrated), the threshold value C.sub.s being 
defined by means of a current-proportional pumping voltage. The limit 
current I.sub.p1 which is now measured by the ammeter 23 is significantly 
higher with the same concentration of oxygen than the limit current 
I.sub.p2 of the second pumping cell 15. As the concentration of oxygen 
decreases, the limit current I.sub.p1 of the first pumping cell 14 becomes 
increasingly small until it becomes zero at a concentration of oxygen of 
10.sup.-10 bar, which corresponds to a stoichiometric ratio (.lambda.=1). 
The steepness of the characteristic curve of the limit current I.sub.p1 of 
the first pumping cell 14 alone indicates that even small fluctuations in 
the O.sub.2 concentration of the measurement gas in the vicinity of 
.lambda.=1 bring about a significant change in the limit current I.sub.p1. 
Finally, this signifies a higher sensitivity of the limit current sensor 
in the region near to .lambda.=1. The size of the threshold value C.sub.s 
at which the switchover from the second pumping cell 15 to the first 
pumping cell 14 takes place is dependent on the positioning of the first 
cathode 12 in the diffusion channel 20. 
A second exemplary embodiment of a limit current probe which can be used as 
a broadband sensor from the lean range to the rich range of a gas mixture 
is shown in FIG. 3. In this limit current sensor, the anode 11 is arranged 
in a reference channel 25. The reference channel 25 is connected for 
example to the atmosphere. The measurement gas is fed, as in the case of 
the sensor according to FIG. 1, via the diffusion hole 21 and the 
diffusion barrier 20 to the two cathodes 12 and 13. The arrangement of the 
cathodes 12 and 13 and their diffusion path 1.sub.1 and 1.sub.2 
corresponds to the embodiment according to FIG. 1. However, in the present 
exemplary embodiment, the cathodes 12, 13 are arranged on the second solid 
electrolyte film 16. The first solid electrolyte film 10 contains the 
diffusion hole 21, as in the first exemplary embodiment. The anode 11 is 
adjoined by a further ceramic film 26 in which the reference channel 25 is 
provided. In the present exemplary embodiment, the heater 19 with the 
insulating layer 18 is connected directly to the reference channel 25 for 
better heat conduction. However, it is equally conceivable to provide an 
additional ceramic film between the insulation layer 18 and the reference 
channel 25. 
The characteristic curve illustrated in FIG. 4 shows the profile of the 
pumping current I.sub.p of a concentration of oxygen in the lean exhaust 
gas (.lambda.&gt;1) plotted against the concentration of oxygen ranging 
between .lambda.=1 and a concentration of oxygen in the rich exhaust gas 
(.lambda.&lt;1). The concentration of oxygen in the rich exhaust gas 
indicates the incorrect amount of oxygen which is necessary to set the gas 
mixture to .lambda.=1. In this context, the concentrations of oxygen are 
to be understood as those which have proven to have negative values in the 
coordinate system. The profile of the characteristic curve in the lean 
exhaust gas corresponds to the profile according to FIG. 2. Given further 
approximation to .lambda.=1, the pumping voltage U.sub.p according to FIG. 
5 is held at a constant value of for example 300 millivolts. 
When .lambda.=1, an electromotive force (Nernst voltage) which is opposed 
to the outer pumping voltage is obtained, as already described, as a 
result of which the limit current I.sub.p1 which is measured by the 
ammeter 23 becomes zero. In this case, the partial pressure of the oxygen 
in the diffusion channel 20 becomes approximately 10.sup.-10 bar. At the 
transition into rich exhaust gas (.lambda.&lt;1), the electromotive force 
with approximately 900 millivolts predominates. However, this voltage is 
not effective since, on the one hand, it operates against the pumping 
voltage U.sub.p which is applied from outside and, on the other hand, it 
is reduced, principally at relatively large pumping currents, by the 
internal resistance of the voltage source of the electromotive force. If 
the pumping voltage U.sub.p applied from outside is not selected to be too 
large and the internal resistance of the voltage source of the 
electromotive force is small, an anodic limit current I.sub.p1, develops 
under the influence of the electromotive force for .lambda.&lt;1, the second 
cathode 13 being in turn switched to by means of the switch 24 when a 
specific, adjustable threshold value C.sub.S, is exceeded. Here, the limit 
current I.sub.p1, measured by the meter 23, according to the dot-dash line 
drops suddenly to a lower value until the anodic limit current I.sub.p2, 
is obtained at the second cathode 13. As the concentration of oxygen 
drops, the anodic limit current I.sub.p2, continues to rise, but with a 
lower gradient than the anodic limit current I.sub.p1, of the first 
pumping cell 14. 
In order to switch over from the first cathode 12 to the second cathode 13, 
and vice versa, a threshold value for the limit current can also be set. 
When the corresponding pumping cell is operating, the system again 
operates with current-proportional pumping voltage. 
The limit current sensor according to the invention is manufactured in a 
known way using lamination and screen printing technology and by means of 
subsequent co-sintering.