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

8 K. Moriya, T. Sako / Sensors and Actuators B 3646 (2000) 1–10
Table 6
Relationship between the performance of the system and the sensor degradation
Sensor type Engine Time (h) (Abnormal: (); normal: (*))
Sensor condition
System performance
a2 OZA-25F MAN-MSG12V12E 10650 *
a3 OZA-25F MAN-MSG12V12E 16590 * *
a4 OZA-25F CAT-3408CTA 2784 * *
a5 OZA-25F WA-L5108GSI 2228 * *
b2 OZA-21F KOM-SA6N170 3483 * *
b3 OZA-21F CAT-3408CTA 3081
b4 OZA-21F CAT-3408CTA 3000
b5 OZA-21F KOM-SA6N170 12899
c2 OZA-31F WA-L5108GSI 5 *
c3 OZA-31F KOM-SA6N170 2237 *
c4 OZA-31F CAT-3408CTA 545
c5 OZA-31F MAN-MSG12V12E 860
the fuel rich side. But the detection point still remains at the
stoichiometric point. Those make a deviation between the
window and the detection point of the equivalence ratio that
results in a degradation in the emission control performance
of the system. This finding supported the field test results
that degradation of the emission control performance
occurred even when the sensor characteristics were normal,
as in the case of sensors a2, c2 and c3 described previously.
Fig. 11 shows an example of a system in which two oxygen
sensors are used: one (main sensor) is set in front of the
catalyst to detect the air/fuel ratio quickly as installed in the
ordinal three-way catalyst system, and the other (subsidiary
sensor) is installed after the catalyst to detect characteristics
changes of the catalyst for correcting the detection point.
This system, which was first commercialized by General
Motor Corp., incorporates a function to detect the heavy
deterioration of the catalysts used with gasoline fueled
engines. In a gas fueled engine that uses the methane rich
fuel, the catalyst window gradually changes, as described
previously.
Therefore, in order to keep the system performance
normal for long time to the extent of the slight catalyst
deterioration level, the sensor installed in the exit side of the
catalyst should correct the detection point by monitoring the
change of the catalyst window. Although a combination of a
CO sensor and a NO x sensor can be used to monitor the
change of the window, there is no low-cost NO x sensor that
operates for a prolonged period without maintenance. Fig. 12
indicates the principle of the window tracking type sensor
that could detect the slight catalyst deterioration. Chart (a)
shows a composition model of the exhaust gas after the fresh
catalyst, while chart (b) shows a composition model of the
exhaust gas after the slightly deteriorated catalyst. With the
fresh catalyst, methane, representing hydrocarbons, and
carbon monoxide are oxidized and nitrogen oxides is
reduced simultaneously when the air/fuel ratio is held close
Fig. 10. Relationship between the output of oxygen sensors and the windows of the three-way catalyst in gas engine exhaust. Sensor OZA-21-F, catalyst: (a)
fresh catalyst (Pt ¼ 1:2 g/l Rh ¼ 0:3 g/l); (b) after 9000 h operation in the exhaust. Gas engine: Yanmar Diesel Engine Co., Ltd. 4GP100-C,
temperature ¼ 600 8C, GHSV ¼ 40;000 h 1 .
UNCORRECTED PROOF