Oxygen sensor monitoring a deterioration of a three-way ... - UMEL

10 K. Moriya, T. Sako / Sensors and Actuators B 3646 (2000) 1–10
Fig. 14. Methane oxidation activity of the oxygen sensor electrode. Sensor NTK OZA25-F; heating rate ¼ 2:5 8C/min, GHSV ¼ 30;000 h 1 .
where the oxidants completely reacted. Therefore, if the
control point is shifted to the rich side in accordance with the
equivalent volume of residual methane, the controlled air/
fuel ratio can remain in the window of the catalyst within
slight catalyst deterioration. The electrodes of ordinal oxygen
sensors for the three-way catalyst systems are mainly
made of platinum because of its high oxidation activity.
Because the space velocity for the electrodes is supposed to
be much smaller than that for the catalyst, the sensors should
detect the stoichiometric point even if they are set in the
exhaust gases of engines using methane as the fuel (see solid
line in Fig. 12). Figs. 13 and 14 indicates the characteristics
curves of a commercial oxygen sensor exposed to the model
exhaust gas after the catalyst, which used to be set in the
three-way catalyst system for gas fueled engine. Because the
detection point of the sensor was at the stoichiometric point
in spite of the exhaust gas composition, it is suspected that
the oxygen sensor do not detect the window of the slight
deteriorated catalyst. As discussed earlier, since the window
of a slightly deteriorated catalyst shifts toward the rich side
according to the equivalent volume of methane, the oxygen
sensor having the ability of detecting shifted window (the
window tracking-type oxygen sensor) should be equipped
with the electrode with low methane oxidation activity (see
dotted line in Fig. 12).
4. Conclusion
The three-way catalyst system is an effective emission
control system of internal combustion engines. It was found
that the oxygen sensor was one of the essential elements
having long service life when the sensor casing is preferably
designed. It was also found that the window tracking-type
oxygen sensor with methane-inactivated electrode could
detect the shifted window of the slightly deteriorated catalyst.
When a three-way catalyst system was assembled with a
feedback system using the window tracking subsidiary sensor,
the system performance should be stable for long time.
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Koji Moriya has been a research fellow at Osaka Gas Research and
Development department since 1985. He received a BE degree in
Industrial Chemistry in 1983 and a ME degree in Molecular Engineering
from Kyoto University. His current research work is focused on the
development of chemical sensors.
Takahiro Sako has been a research fellow at Osaka Gas Research and
Development department since 1987.
UNCORRECTED PROOF