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

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