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

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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
to the stoichiometric point. With the slightly deteriorated catalyst, methane was present at the stoichiometrically balanced composition owing to the low methane oxidation activity of the catalyst. As a result, some residual oxidant K. Moriya, T. Sako / Sensors and Actuators B 3646 (2000) 1–10 9 Fig. 11. Block diagram of the NO x reduction system using dual oxygen sensors. Fig. 12. Conceptual drawings of catalyst window and oxygen sensor characteristics: (a) fresh catalyst; (b) deteriorated catalyst; solid line, traditional oxygen sensor; dotted line, window tracking type oxygen sensor. existed in an amount corresponding to the equivalent volume of residual methane. In other words, nitrogen oxides was not reduced at the stoichiometric point but reduced at the composition of fuel rich side to the stoichiometric point Fig. 13. Characteristics of the oxygen sensors to the model exhaust gases: (a) model gas of fresh catalyst; (b) model gas of used catalyst after 9000 h operation. UNCORRECTED PROOF
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Oxygen sensor monitoring a deterioration of a three-way ... - UMEL

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