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Abstract

Oxygen sensor monitoring a deterioration of a three-way

catalyst in natural gas fueled engines

Koji Moriya * , Takahiro Sako

Research and Development Department Osakagas Co., Ltd., 6-19-9, Torishima, Konohaha-Ku, Osaka-shi, Osaka 554-0051, Japan

Received 10 May 2000; received in revised form 20 October 2000; accepted 1 November 2000

Two oxygen sensors are used to provide high-precision air/fuel ratio control for a three-way catalyst system of natural gas fueled engines.

An optimized design for the sensor casing prevented sensor degradation, and an introduction of a subsidiary sensor in the exit side of the

catalyst with a window tracking effect stabilized the emission control performance. The window tracking effect was explained as follows:

the three-way catalyst deteriorates when used for long time in natural gas fueled engines. In most cases, the methane oxidation activity

gradually decreased by the sintering of platinum. The window tracking-type oxygen sensor, which has electrode with low methane

oxidation activity, detects the shifted window of the deteriorated catalyst because the window shift was brought about by the residual

methane in the exhaust gas. # 2001 Elsevier Science B.V. All rights reserved.

Keywords: Oxygen sensor; Three-way catalyst; Air/fuel ratio control; Window tracking; Long term stability

1. Introduction

Sensors and Actuators B 3646 (2000) 1–10

Utilization of fuels with methane as the primary component

such as natural gas and digestion gas is widely promoted

nowadays. Internal-combustion engines are typical

examples of equipments that use such fuels. The engines

(gas engines) are used as the driving units of many highefficiency

co-generation systems. The gas engines are

equipped with three-way catalyst systems [1,2] (consisting

of a three-way catalyst, oxygen sensors and a controller) to

control the emission of the exhaust gases.

Since the co-generation systems operate for many hours,

their three-way catalyst systems are required to provide a

long service life of 10 times as long as that of an automobile.

In addition, recent environmental regulations demand a

stable and high-performance emission control. For a

three-way catalyst, air/fuel ratio should be precisely controlled

to keep the stoichiometrically balanced composition

of the exhaust gas. Even a slight deviation in the air/fuel ratio

from the composition degrades the emission control performance

to a large extent.

In a basic three-way catalyst system, an oxygen sensor is

installed in the exhaust pipe between the engine and the

* Corresponding author. Tel.: þ81-6-6462-3410; fax: þ81-6-4804-0738.

E-mail address: moriya@osakagas.co.jp (K. Moriya).

0925-4005/00/$ – see front matter # 2001 Elsevier Science B.V. All rights reserved.

PII: S 0925-4005(00)00683-3

catalyst for feedback signal. SAAB automobile AB and

Robert Bosch GmbH industrialized this type of sensors

for a commercial application in a three-way catalyst system

for the first time [3]. Since then, the sensors have been

widely used as a part of the emission control system for

automobiles, in Europe [4], in Japan [5,6] and in US [7,8]

because of their excellent durability. To meet the strict

emission control regulations, the air/fuel ratio control systems

as well as the catalyst and the oxygen sensors has been

progressed [2]. One of them is the double oxygen sensor

feedback system which has an additional subsidiary sensor

on the exit side of the catalyst. The subsidiary sensor

provides a correction signal to adjust the control point from

the main sensor (the same in a basic system) installed

between the engine and catalyst for more accurate air/fuel

ratio control.

The first three-way catalyst system for gas engines was

commercialized through conversion of the basic system for

automobiles. A follow-up survey conducted to study commercial

equipments on the market revealed that the system

could not maintain stable performance for an extended

period of time. Three factors were pointed out as the main

causes of the performance degradation. One was the deterioration

of the catalyst itself to a level where it no longer

had a window. This severe deterioration resulted when

misfires continuously occurred in a gas engine and caused

an abnormal temperature increase in the catalyst [9]. Since

UNCORRECTED PROOF


2 K. Moriya, T. Sako / Sensors and Actuators B 3646 (2000) 1–10

this problem manifested itself as an irregular engine operating

condition, it could be solved simply by misfire-preventive

measures. The second was the detrimental degradation

of the oxygen sensors. The degraded sensors did not generate

an output, thus, disabling proper control. The third was

due to the discrepancy between the equivalence ratio that

was feedback-controlled by the main sensor signal and that

of the real catalyst window. This discrepancy reduced the

emission control efficiency. In this paper, the causes of

degradation in three-way catalyst performance resulting

from the oxygen sensor problems were determined, and

the characteristics that the subsidiary oxygen sensor must

provide in order to ensure long and stable performance of the

three-way catalyst systems were identified.

2. Experimental

Table 1

Typical gas composition of exhaust gas from natural gas fueled engine

Equivalence Gas concentration(vol.%/N 2 base)

ratio

CO 2 CO H 2 O 2 NO CH 4

Before catalyst

A 1.020 10.82 0.780 0.483 0.440 0.286 0.160

B 1.010 11.02 0.510 0.316 0.450 0.312 0.158

C 1.005 11.09 0.390 0.241 0.470 0.324 0.160

D 1.003 11.13 0.340 0.210 0.471 0.332 0.159

E 1.000 11.17 0.290 0.180 0.472 0.338 0.157

F 0.995 11.22 0.210 0.130 0.495 0.352 0.154

G 0.980 11.22 0.080 0.049 0.680 0.392 0.142

After catalyst

H 1.017 11.76 0.180 0.254 0.010 0.000 0.087

I 1.008 11.63 0.018 0.059 0.010 0.000 0.074

J 1.004 11.66 0.065 0.028 0.010 0.000 0.044

K 1.002 11.73 0.004 0.017 0.008 0.000 0.023

L 1.000 11.82 0.001 0.002 0.006 0.002 0.002

M 0.992 11.63 0.000 0.000 0.040 0.296 0.005

N 0.985 11.53 0.000 0.000 0.190 0.360 0.013

Table 2

Oil ash composition

Engine Oil Concentration (wt.%)

Ca P Zn Others

SA6N170 Spark30R 18 16 22 44

3408CTA GX30 10 2 4 84

3412CTA Pegasus40 12 29 13 58

PA6 GHP10W-30 13 32 5 50

SA6N170 L970 16 28 8 64

To analyze the changes in sensing performance, characteristics

of the oxygen sensors collected from commercial

equipments that had operated for certain lengths of time

were measured (field test). The sensors tested were OZA-

25F, OZA-21F and OZA-31F manufactured by NGK Spark

Plug Co., Ltd. The gas engines equipped with those sensors

were SA6N170 manufactured by KOMATSU Ltd., PA6 by

Nissan Diesel Motor Co., Ltd., 3408CTA and 3412CTA by

Caterpillar Inc., MSG12V12E by MAN B&W Desentrail

Energie Systeme GmbH and L5108GSI by Waukesha

Engine Division: Dresser Ind. Inc. The sensors were

installed in exhaust pipes connected to the engine exhaust

manifolds (before and after catalytic converters), and were

exposed to exhaust gases. Since those engines were designed

to recover the exhaust gas heat, the exhaust pipes were

covered with heat insulator. The temperature of the exhaust

gases at the sensor installation locations ranged from 820 to

920 K. The model compositions of exhaust gases (nitrogen

base gas) before and after the catalyst are listed in Table 1.

There were no noticeable differences in gas composition by

the engine type.

The compositions of the ash that was blown onto the

sensor in the field test were determined by EPMA (see

Table 2). Lubrication oils used were Spark 30R produced

by Japan Energy Corporation, GX-30 and L970 by Nisseki

Mitsubishi Co., Ltd., Pegasus 40 by Mobil Sekiyu Private

Limited and GHP10W30 by Idemitsu Petrochemical Co.,

Ltd.

Static characteristics, internal resistance and response

characteristics were measured. For static characteristics

measurements, the compositions of the exhaust gases of

the gas engines were analyzed and a representative exhaust

gas composition was determined. Fig. 1 shows the schematic

diagram of the static characteristics measuring system.

The gas mixtures were produced by mixing several

standard gases with mass-flow controllers. To produce a

humidified gas composition, the oxygen and nitrogen mixture

was passed through a bubbling water bath. A sensor was

installed in a quartz holder (see Fig. 2) and heated. A

temperature-measuring sensor was fabricated by imbedding

thermocouples (0.5 mm in diameter) in the sensor electrode.

Using the gas temperature and the calibration curves from

the temperature-measuring sensor, the surface temperature

of the electrode was determined. The sensor output was

measured with a data logger having input impedance of

more than 10 10 ohms.

Internal resistance was determined by the following process.

The sensor was exposed to a mixed gas of rich

composition (composition A in Table 1) for 5 min. After

open circuit voltage (OCV) was measured, a resistance (Rl)

was connected, and closed circuit voltage (CCV) was measured

after a 5 min stabilizing period. The internal resistance

of the test sensor was calculated from the OCV, CCV and Rl

values.

Fig. 3 shows the schematic diagram of the response curve

measurement system. By rapidly changing the ambience of a

sensor, the sensor response speed was measured. The standard

mixed gas compositions used in the experiment are

shown in Table 3. One cylinder contained a model mixed gas

UNCORRECTED PROOF


of exhaust gas at fuel rich operation (rich model gas), while

the other contained a lean model gas. The flow rate of the

mixed gas from each cylinder was maintained constant by

the mass-flow controllers. A four-way valve was installed to

K. Moriya, T. Sako / Sensors and Actuators B 3646 (2000) 1–10 3

Fig. 1. Schematic diagram of the static characteristics measurement apparatus.

Fig. 2. Structures of the quarts holder.

send a part of the mixed gas to the sensor holder (see Fig. 4)

and the other part to the exhaust side. The cross-sections of

the sensor electrode and the solid electrolyte were observed

with SEM and EPMA.

UNCORRECTED PROOF


4 K. Moriya, T. Sako / Sensors and Actuators B 3646 (2000) 1–10

Table 3

Gas composition for response measurement

Stage

Gas concentration(vol.%/N 2 base)

CO 2 CO H 2 O 2 CH 4

Lean 11.300 0.079 0.048 0.624 0.138

Rich 11.300 0.758 0.493 0.451 0.163

3. Results and discussion

3.1. Deterioration of oxygen sensors

As shown in Fig. 5, in this sensor, a YSZ is sintered into a

tube shape and electrodes were formed on both sides by

plasma spraying process. The YSZ tube is installed in a

metal case and lead wires from the electrodes are settled by

Fig. 3. Schematic diagram of the sensor response measurement apparatus.

Fig. 4. Structures of the stainless holder for response measurement.

waterproof sealing rubber on the metal case. Table 2 lists the

compositions of the ashes blown onto the sensor case. As

indicated in the table, the compositions and their amounts

varied with the lubrication oil type but they did not affect on

the sensor characteristics. Table 4 shows the inner resistance

and the rapid rising points (the values of equivalence ratio

when the sensor outputs were 0.45–0.7 V). Fig. 6 indicates

typical response curves. Sensors a1, b1 and c1 in Table 4

were new. Sensors a2 through a5, b2, c2 and c3 were used for

many or several hours in actual applications, but their inner

resistance and the rapid rising points were close to those of

the new sensors. Other sensors (degraded sensors) had

higher internal resistance than the new sensors, and the

rapid rising points were different. In all cases where the

sensor characteristics showed deviations, the system performances

were low. But even when the characteristics of

sensor was same as the new sensor’s, the performance of

UNCORRECTED PROOF


the system was low in some cases. In the following, the

former case is explained. Performance degradation in the

latter case is explained in the Section 3.2.

Fig. 6(a) indicates the response curves of sensors a1 and

a3 measured at 823 K in mixed gases A through F listed in

Table 1. The sensors were exposed to a mixed gas of each

composition for 7 min and the dry air for 3 min. Sensor a3

showed the same characteristics as a new sensor (a1),

although it operated more than 16,000 h in actual usage.

A number of new sensors also showed the identical characteristics

within the range of the measuring device precision.

Fig. 6(b) shows the response curves of sensors b1 and

b5. While new sensors a1 and b1 showed the same response

curves, sensor b5, which was used for 13,000 h prior to the

measurement, revealed a different characteristics. When the

exposure gas was changed from air to a rich model gas

(equivalence ratio > 1), the new sensors showed an output

remained at a constant level, which appeared as a plateau.

The response curve of sensor b5, on the other hand, showed

an output (b in Fig. 6) which is close to that of the new sensor

when the sensor was exposed to a rich model gas, but this

output decreased gradually with time (c), and there was no

Table 4

Characteristics of the sensor used in the commercially used engine

K. Moriya, T. Sako / Sensors and Actuators B 3646 (2000) 1–10 5

Fig. 5. Structures of the oxygen sensor.

plateau on the graph. When the air was introduced after the

exposure to the rich model gas, the sensor output showed a

negative value (d). The negative output decreased gradually

with time (e). The decreased output value(V b V c ) was close

to the absolute value of V d . We suspected the deterioration of

the reference electrode. Assuming a condition in which the

supply of oxygen ions from the reference electrode is

interrupted, therefore, the oxygen move only through the

exhaust gas-side electrode. When the air is introduced to the

exhaust gas-side electrode section (a in Fig. 6, initial stage),

the oxygen ion concentration in the zirconia is the same level

when normal sensor is exposed to air. This is because the

oxygen ion is provided through exhaust gas-side electrode

and diffused in the zirconia. In the early stage of introducing

the reducing gas to the exhaust gas-side electrode (b), the

oxygen ion concentration in the zirconia near the reference

electrode is close to the concentration level in the initial

stage just like in a new sensor. In contrast, the oxygen ion

concentration near the exhaust gas-side electrode decreases

owing to the oxygen consumption by the oxidation reaction

of the reducing gas. Because of this, the sensor generates an

output similar to that of a normal sensor for a short time.

Sensor resistance (ohm) Type Engine Exposure (h) Equivalence

ratio

Output ¼ 0.7 V Output ¼ 0.4 V

a1 OZA-25F New a 0 1.011 1.002 6.0 10 2

a2 OZA-25F MAN-MSG12V12E 10650 1.014 1.002 6.0 10 2

a3 OZA-25F MAN-MSG12V12E 16590 1.007 1.001 1.0 10 3

a4 OZA-25F CAT-3408CTA 2784 1.003 0.998 4.0 10 2

a5 OZA-25F WA-L5108GSI 2228 1.013 1.004 3.5 10 2

b1 OZA-21F New a 0 1.005 1.000 4.5 10 2

b2 OZA-21F KOM-SA6N170 3483 1.011 1.005 1.5 10 3

b3 OZA-21F CAT-3408CTA 3081 1.024 1.008 6.5 10 4

b4 OZA-21F CAT-3408CTA 3000 1.032 1.007 1.0 10 6

b5 OZA-21F KOM-SA6N170 12899


6 K. Moriya, T. Sako / Sensors and Actuators B 3646 (2000) 1–10

However, when the sensor is kept exposed to the reducing

gas for a while, oxygen continues to be consumed through

the exhaust gas-side, thus, forcing the oxygen ions near the

reference electrode to diffuse to the exhaust gas-side electrode.

As a result, the oxygen ion concentration near the

reference electrode lowers (c). In that situation the electrons

flow from the exhaust gas-side electrode to the reference

electrode through measurement apparatus. The output of a

concentration cell type oxygen sensor is expressed by the

following Eq. (1).

V ¼ RT

4F ln P ref

(1)

P sens

where R is the gas constant, T the temperature, F the Faraday

constant, P sens is the partial pressure of oxygen at the

detection electrode side.

When the oxygen concentration at the detection electrode

side is constant, the output decreases in accordance with the

drop of the oxygen concentration at the reference electrode

side. If the exhaust gas-side electrode is exposed to the air

after the previously described condition, the oxygen concentration

at the detection electrode side suddenly increases,

but the oxygen concentration at the reference electrode side

remains low until the oxygen ions are replenished by ions

diffused from the exhaust gas-side electrode. This produces

a condition in which the oxygen concentration at the exhaust

gas-side electrode is low and that at the reference electrode

side is high. In this condition, the oxygen sensor show a

negative output (d) that a normal sensor would not generate.

Fig. 7 shows an SEM image of the sensor electrode. Fig. 8

shows the EPMA line analysis chart of the area bordered by

the solid line in Fig. 7. As indicated in Fig. 7, a thick film like

deposit on the electrode of sensor b5 was observed. The

EPMA result revealed that this deposit was the silicon

oxides. No OZA-25 sensors marked ‘‘a’’ in Table 4

Fig. 6. Response curves of the sensors exposed to exhaust gases.

degraded, while some OZA-21 and OZA-31 sensors marked

‘‘b’’ and ‘‘c’’ in Table 4 exhibited degradation.

The material that differed in the ‘‘a’’- and ‘‘b’’-series

sensors was the seal rubber that was the supporting parts of

the lead wires connected to the electrodes. Fig. 9 shows a

thermal gravimetory pattern of the seal rubber of the ‘‘b’’

series sensor at 5 K/min in the airflow. The weight loss

appeared near 373 K, and 1% loss was at 620 K. Table 5

shows the concentrations of decomposed materials from the

Fig. 7. SEM images of cross-sections of sensors.

UNCORRECTED PROOF


Fig. 8. EPMA line analysis patterns (Si) of the sensor cross-sections in

Fig. 7.

seal rubber that was heated for 1 h at 423 K. As shown in

Table 5, a polysiloxane, which is an organic silicon compound,

was detected. These experimental results showed

that the degradation of oxygen sensor was caused by the

deposit of silicon oxides on the reference electrode that was

from the seal rubber. As shown in Table 4, the internal

resistance of sensor b5 was larger than that of the new

sensors. This could be also attributed to the deterioration of

the reference electrode.

3.2. Window tracking-type oxygen sensors

Table 6 shows the relationship between the performance

of the three-way catalyst system and the sensor degradation.

As indicated in the table, when the sensor was critically

damaged, the system cannot be controlled. In the cases of

sensors a2, c2 and c3, for example, the emission was not

properly controlled even though the sensors were not

degraded. It was identified that the main cause of the catalyst

deterioration was reduced methane oxidation activity owing

to the sintering of platinum when the catalyst was used for

natural gas fueled engines [9]. Here, a window was defined

K. Moriya, T. Sako / Sensors and Actuators B 3646 (2000) 1–10 7

Table 5

Gas composition from the seal rubber

Retention

time (min)

Fig. 9. Thermal gravimetory pattern of the seal rubber material.

Concentration (ppm, w/w)

OZA-31F

OZA-21F

Composition

12.9 5 – Include

dimethylcyclopolysiloxane

13–32.9 257 5 C 14 H 30 C 18 H 38 ,C 16 H 18 ,

BHT, DEP, DBP

33–48 1

Total 310 6

as a region where the NO x conversion rate was 98% or higher

and the CO conversion rate was 95% or higher. As shown in

Fig. 10, with a new catalyst (LL catalyst manufactured by

Cataler Corporation), the window had a range of 1.000–

1.015 in terms of equivalence ratio. After 9000 h of use in

the field, the window narrowed to a range of 1.002–1.004

with the same type of catalyst. In this condition, the conversion

rates of NO x and CO remained nearly 100%, but the

methane conversion rate was lower (hereafter referred to a

slight deterioration). When miss fire occurs continuously,

the catalyst surface was melt down because of the high

temperature (ca. 1100 K) caused by the oxidization of the

fuel on the catalyst. The catalyst containing platinum and

rhodium was occasionally poisoned by the lead compounds.

In those cases the catalyst was deteriorated to the level of no

window.

The oxygen sensor or lambda sensor, on the other hand,

shows a rapid rise where equivalence ratio is 1.000, which is

a typical l characteristics. The output of the sensor rise at the

stoichiometric point (rapid rising point) from the level under

0.2 V to that of over 0.7 V, therefore, an output of 0.45 V is

normally used as the detection point in the air/fuel ratio

control system. With a new catalyst, the detection point is

located within the window and the emission control performance

remains normal. When the slight deterioration of the

catalyst occurs, the lean edge of the window shifts towards

UNCORRECTED PROOF


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


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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Springer, Berlin, 1984, p. 119.

[3] B. Freundenberger, Mortor Service, March 1993, pp. 50–56.

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paper 770295, Detroit, MI, USA, 1977.

[5] S. Nakagawa, T. Yamaguchi, Toyota emission control systems

meeting Japanese 1978 emission standards, J. Soc., Automotive

Eng. Jpn. 31 (1977) 1175–1181.

[6] K. Kondo, Nissan anti-pollution systems for Japanese 1978 emission

standards with three-way catalyst and lambda sensor, J. Soc.,

Automotive Eng. Jpn. 31 (1977) 1182–1188.

[7] R.P. Canale, C.R. Carlson, S.R. Winegarden, D.L. Miles, General

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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

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