Apparatus for detecting abnormality of oxygen sensor and controlling air/fuel ratio

This invention provides apparatus for detecting abnormality of an oxygen sensor accurately and also apparatus for appropriately controlling the air/fuel ratio of air and fuel mixture when an oxygen sensor is abnormal. The apparatus easily and properly detects a deteriorating oxygen sensor, with the use of which exhaust of nitrogen oxides or carbon monoxide increases, and when the oxygen sensor is determined to deteriorate, the feed back control of the air/fuel ratio of air and fuel mixture supplied to an internal combustion engine is preferably performed.

BACKGROUND OF THE INVENTION 
The present invention relates to an apparatus for detecting abnormality of 
an oxygen sensor which measures the oxygen concentration of exhaust gas 
discharged from an internal combustion engine and also for controlling the 
air/fuel ratio of air and fuel mixture supplied to the internal combustion 
engine according to data showing abnormality of the oxygen sensor. 
The air/fuel ratio of an air and fuel mixture supplied to an internal 
combustion engine is generally controlled based on a signal sent from an 
oxygen sensor provided in the exhaust system of the engine so as to lower 
the emission of exhaust discharge of the engine. As shown in FIG. 19, the 
air/fuel ratio is controlled in accordance with the output signal of the 
oxygen sensor in order to maintain the air/fuel ratio near the 
stoichiometric ratio at which purification of exhaust components reaches 
the optimum stage. 
When the oxygen sensor used for feed-back controlling the air/fuel ratio is 
abnormal, the emission of exhaust discharge may increase. Various 
techniques have hence been proposed for diagnosing abnormality of the 
oxygen sensor and furthermore for, when abnormality of the oxygen sensor 
is detected, compensating the feed-back control of the air/fuel ratio. 
Examples of apparatus for diagnosing abnormality of the oxygen sensor are 
illustrated in Japanese Published Unexamined Patent Applications No. 
Sho-62-151770 and No. Sho-53-95421; and apparatus for compensating the 
air/fuel ratio control are in Japanese Published Unexamined Patent 
Applications No. Sho-58-222939 and No. Sho-59-3137. 
When the oxygen sensor is contaminated by various substances, the sensor 
output shifts to lean or rich as shown in FIG. 20; that is, the 
performance of the oxygen sensor varies. The feed-back control of the 
air/fuel ratio according to an output signal of the oxygen sensor is 
thereby not performed satisfactorily, and thus the emission of exhaust 
discharge increases. 
For example, when the oxygen sensor contaminated by silicon is used for 
feed-back control of the air/fuel ratio, nitrogen oxides (NOx) in the 
exhaust discharge increase; and when the oxygen sensor contaminated by 
lead is used, carbon monoxide (CO) in the exhaust discharge increases. 
SUMMARY OF THE INVENTION 
One objective of the invention is to provide apparatus for accurately 
detecting abnormality of an oxygen sensor. 
Another objective of the invention is to provide apparatus for 
appropriately controlling the air/fuel ratio of air and fuel mixture when 
an oxygen sensor is abnormal. 
One embodiment of the present invention that realizes the first and other 
related objectives is an abnormality detecting device for oxygen sensors 
shown in FIG. 1, which detects abnormality of an oxygen sensor M2 sending 
a signal according to the oxygen concentration of exhaust gas discharged 
from an internal combustion engine M1. The abnormality detecting device 
includes air/fuel ratio setting means M3 for setting the air/fuel ratio of 
air and fuel mixture supplied to the internal combustion engine M1 lean or 
rich by open loop control; and abnormality detecting means M4 for 
determining that the oxygen sensor M2 is abnormal if an output signal of 
the oxygen sensor M2 is not less than a predetermined threshold when the 
air/fuel ratio is set to be lean by the air/fuel ratio setting means M3. 
Alternatively, the oxygen sensor is determined to be abnormal if an output 
signal of the oxygen sensor M2 is not greater than a predetermined 
threshold when the air/fuel ratio is set to be rich. 
In the abnormality detecting device for oxygen sensors shown in FIG. 1, the 
air/fuel ratio of air and fuel mixture supplied to the internal combustion 
engine M1 is set to be lean or rich by open loop control by the air/fuel 
ratio setting means M3. If an output signal of the oxygen sensor M2 is not 
less than a predetermined threshold when the air/fuel ratio is set lean, 
the abnormality detecting means M4 determines that the oxygen sensor M2 is 
abnormal. If, on the other hand, an output signal of the oxygen sensor M2 
is not greater than a predetermined threshold when the air/fuel ratio is 
set rich, the abnormality detecting means M4 also determines that the 
oxygen sensor M2 is abnormal. 
Another embodiment of the invention is an abnormality detecting device for 
oxygen sensors shown in FIG. 2, which detects an abnormality of an oxygen 
sensor M6 sending a signal according to the oxygen concentration of 
exhaust gas discharged from an internal combustion engine M5. The 
abnormality detecting device includes air/fuel ratio setting means M7 for 
periodically changing the air/fuel ratio of air and fuel mixture supplied 
to the internal combustion engine M1 between lean and rich by open loop 
control; limit value detecting means M8 for detecting the minimum and 
maximum values of an output signal sent from the oxygen sensor M6 when the 
air/fuel ratio is set to be rich or lean by the air/fuel ratio setting 
means M7; and abnormality detecting means M9 for determining that the 
oxygen sensor M6 is abnormal when at least one of the minimum and maximum 
values detected by the limit value detecting means M8 is within a 
predetermined output range. 
The minimum and maximum values of an output signal may be the average of 
plural measurements. 
In the abnormality detecting device for oxygen sensors shown in FIG. 2, the 
air/fuel ratio of air and fuel mixture supplied to the internal combustion 
engine M5 is periodically changed between lean and rich by open loop 
control by the air/fuel ratio setting means M7. The minimum and maximum 
values of an output signal, sent from the oxygen sensor M6 when the 
air/fuel ratio is set rich or lean, are detected by the limit value 
detecting means M8. When at least one of the minimum and maximum values is 
within a predetermined output range, the abnormality detecting means M9 
determines that the oxygen sensor M6 is abnormal. 
A further embodiment of the invention is an abnormality detecting device 
for oxygen sensors shown in FIG. 3, which detects abnormality of an oxygen 
sensor M11 outputting a signal according to the oxygen concentration of 
exhaust gas discharged from an internal combustion engine M10. The 
abnormality detecting device includes air/fuel ratio controlling means M12 
for feed-back controlling the air/fuel ratio of air and fuel mixture 
supplied to the internal combustion engine M10 according to an output 
signal of the oxygen sensor M11; and abnormality detecting means M13 for 
determining that the oxygen sensor M11 is abnormal if an output signal of 
the oxygen sensor M11 is within a predetermined range when the feed-back 
control of the air/fuel ratio is executed by the air/fuel ratio 
controlling means M12. 
In the abnormality detecting device for oxygen sensors shown in FIG. 3, the 
feed-back control of the air/fuel ratio is performed based on an output 
signal sent from the oxygen sensor M11 by the air/fuel ratio controlling 
means M12. If the output signal of the oxygen sensor M11 is within a 
predetermined range when the feed-back control of the air/fuel ratio is 
executed, the abnormality detecting means M13 determines that the oxygen 
sensor M11 is abnormal. 
An embodiment of the present invention for realizing the first, second, and 
other related objectives is an air/fuel ratio controlling device shown in 
FIG. 4, which controls the air/fuel ratio of air and fuel mixture supplied 
to an internal combustion engine M14 according to an output signal sent 
from an oxygen sensor M15 provided in the exhaust system of the internal 
combustion engine M14. The air/fuel ratio controlling device includes 
abnormality detecting means M16 for determining that the oxygen sensor M15 
is abnormal according to the variation of an output signal of the oxygen 
sensor M15; air/fuel ratio setting means M17 for setting the air/fuel 
ratio of air and fuel mixture supplied to the internal combustion engine 
M14 lean and rich by open loop control; median computing mean M18 for 
determining the median of lean and rich signals outputted from the oxygen 
sensor M15 when the air/fuel ratio is set to be lean and rich by the 
air/fuel ratio setting means M17; and threshold setting means M19 for 
setting the median determined by the median computing means M18 as a 
threshold which discriminates between rich and lean states of the air/fuel 
ratio in feed-back control when abnormality of the oxygen sensor M15 is 
detected by the abnormality detecting means M16. 
In the air/fuel ratio controlling device of the invention shown in FIG. 4, 
the air/fuel ratio of air and fuel mixture supplied to the internal 
combustion engine M14 is controlled according to an output signal sent 
from the oxygen sensor M15 provided in the exhaust system of the internal 
combustion engine M14. When the abnormality detecting means M16 determines 
that the oxygen sensor M15 is abnormal, the air/fuel ratio of the mixture 
supplied to the internal combustion engine M14 is set lean or rich by open 
loop control by the air/fuel ratio setting means M17. Then the median of 
lean or rich signal sent from the oxygen sensor M15 is computed by the 
median computing mean M18. The threshold setting means M19 sets the median 
as a threshold which discriminates between rich and lean states of the 
air/fuel ratio in feed-back control. 
Here the abnormality detecting means M16 may be operated by variety of 
principles; for example, the means M16 may be substantially identical to 
any of the abnormality detecting means M4, M9 and M13. 
The open loop control is not feed-back control in which the air/fuel ratio 
of air and fuel mixture is controlled according to an output signal sent 
from an oxygen sensor, but is simple selection control in which the 
air/fuel ratio is simply set to a rich or lean state. 
The principles of the abnormality detecting devices for oxygen sensors are 
described now. 
(1) Abnormality detecting device for oxygen sensors shown in FIG. 1. 
As shown in FIG. 5, in a normal oxygen sensor, when the air/fuel ratio is 
shifted from lean (e.g., ratio of air excess .lambda.=1.03) to rich 
(.lambda.=0.97) by open loop control, the output signal of the oxygen 
sensor changes from lower than a first threshold V.sub.1 (e.g., 300 mV) 
and to higher than a second threshold V.sub.2 (e.g., 700 mV); namely an 
output signal of the oxygen sensor oscillates with a large variation in. 
When the feed-back control of the air/fuel ratio is executed based on an 
output signal of an oxygen sensor contaminated by silicon, exhaust of 
nitrogen oxides (NOx) increases. In the oxygen sensor contaminated by 
silicon, the output signal (voltage) is higher than those of the normal 
oxygen sensor when the air/fuel ratio is in lean state. On the other hand, 
when the feed-back control of the air/fuel ratio is executed based on an 
output signal of an oxygen sensor contaminated by lead, exhaust of carbon 
monoxide (CO) increases. In the oxygen sensor contaminated by lead, the 
output signal (voltage) is lower than those of the normal oxygen sensor 
when the air/fuel ratio is in rich state. 
When the output signal of the oxygen sensor becomes not less than the first 
threshold V.sub.1 in the lean air/fuel ratio, the oxygen sensor is 
determined to deteriorate so as to cause the internal combustion engine to 
discharge a large amount of NOx. On the other hand, when the an output 
signal of the oxygen sensor become not greater than the second threshold 
V.sub.2 in the rich air/fuel ratio, the oxygen sensor is determined to 
deteriorate so as to cause the internal combustion engine to discharge a 
large amount of CO. 
(2) Abnormality detecting device for oxygen sensors shown in FIG. 2. 
As shown in FIG. 6, in a normal oxygen sensor, when the air/fuel ratio is 
periodically changed between lean and rich states by open loop control, 
the output signal oscillates with a large variation in; the minimum of the 
output signal becomes lower than a first threshold V.sub.1 and the maximum 
becomes higher than a second threshold V.sub.2. 
In an oxygen sensor contaminated such that exhaust of NOx increases, the 
output signal has a high voltage and oscillates around the second 
threshold V.sub.2 with a small amplitude. In an oxygen sensor contaminated 
such that exhaust of CO increases, the output signal has a low voltage and 
oscillate around the first threshold V.sub.1 with a small amplitude. 
When either the minimum or the maximum of the output signal sent from the 
oxygen sensor is within a predetermined range between the first threshold 
V.sub.1 and the second threshold V.sub.2, the oxygen sensor is determined 
to be abnormal. 
(3) Abnormality detecting device for oxygen sensors shown in FIG. 3. 
As shown in FIG. 7, in a normal oxygen sensor, when the feed-back control 
of the air/fuel ratio is executed, the output signal sent from the oxygen 
sensor oscillates with a large variation in. 
In an oxygen sensor deteriorated such that exhaust of either NOx or CO 
increases, when the feed-back control of the air/fuel ratio is executed, 
the output signal oscillates with a small amplitude near a slice level 
V.sub.0 located between threshold V.sub.L and threshold V.sub.O. 
When the output signal of the oxygen sensor is within a predetermined range 
around the slice level V.sub.0, the oxygen sensor is determined to be 
abnormal.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
Preferred embodiments of the invention are now described referring to the 
drawings. Since there may be many modifications without departing from the 
scope of the invention, the embodiments below are not intended to limit 
the invention to the embodiments, but are intended to illustrate the 
invention more clearly. 
FIG. 8 is a schematic view illustrating the invention; i.e., an apparatus 
for detecting abnormality of an oxygen sensor and for feed-back 
controlling the air/fuel ratio. 
The apparatus 1 includes an electronic control unit (hereinafter referred 
to as ECU) 3 for detecting the conditions of an engine 2 and executing 
various operations, e.g., controlling the air/fuel ratio and diagnosing 
abnormality of the oxygen sensor. 
The engine 2 has a combustion chamber 7 including a cylinder 4, a piston 5, 
and cylinder head 6. The combustion chamber further includes an ignition 
plug 8. 
The inlet system of the engine 2 includes an intake valve 9, an inlet port 
10, an inlet pipe 11, a surge tank 12 for absorbing surges of intake air, 
a throttle valve 14 for controlling the amount of intake air, and an air 
cleaner 15. 
The exhaust system of the engine 2 includes an exhaust valve 16, an exhaust 
port 17, an exhaust manifold 18, a catalytic converter 19 filled with a 
three-way catalyst, and an exhaust pipe 20. 
The ignition system of the engine 2 includes an igniter 21 for generating a 
high voltage sufficient for ignition and a distributor 22 connected to a 
crank shaft (not shown) for selectively distributing the high voltage 
generated by the igniter 21 to the ignition plug 8. 
The fuel system of the engine 2 includes an electromagnetic fuel injection 
valve 25 for injecting fuel sent from a fuel tank (not shown) into the 
inlet port 10. 
The engine 2 further has sensors for detecting the driving conditions; 
i.e., a manifold air pressure sensor 31 for detecting the pressure of 
intake air, an intake air temperature sensor 32 for detecting the 
temperature of intake air, a throttle position sensor 33 for detecting the 
opening of the throttle valve 14, a water temperature sensor 35 for 
detecting the temperature of cooling water, and an upstream oxygen sensor 
36 (hereinafter referred to as an oxygen sensor) for detecting the oxygen 
concentration of exhaust gas before it flows into the catalytic converter 
19. A downstream oxygen sensor 37 (hereinafter referred to as a sub-oxygen 
sensor) may be provided if necessary for detecting the oxygen 
concentration of exhaust gas after it flows out of the catalytic converter 
19. A cylinder discrimination sensor 38 for outputting a standard signal 
at every rotation of a cam shaft of the distributor 22 and an engine speed 
sensor 39 for outputting a signal of rotation angle at every 1/24 rotation 
of the cam shaft of the distributor 22 are provided. 
An output signal from the sensors is sent to the ECU 3. According to the 
input signal, the engine speed control, the air/fuel ratio control, and 
other controls are executed. The ECU 3 forms a logical operation circuit 
including a central processing unit (CPU) 3a, a read only memory (ROM) 3b, 
a random access memory (RAM) 3c, a backup RAM 3d, and a timer 3e; the 
components in the CPU are connected to an input/output port 3g through a 
common bus 3f and further connected to peripheral devices. The CPU 3a 
receives detection signals sent through an A/D converter 3h and the 
input/output port 3g from the manifold air pressure sensor 31, the intake 
air temperature sensor 32, the throttle position sensor 33, the water 
temperature sensor 35, the oxygen sensor 36, and the sub-oxygen sensor 37. 
The CPU also receives signals sent from the cylinder discrimination sensor 
38 and the engine speed sensor 39 through a waveform shaping circuit 3i 
and the input/output port 3g. The CPU 3a drives and controls the igniter 
21, the fuel ejection valve 25, and a check lamp 40 for informing an 
operator of an abnormality of the oxygen sensor 36. 
Electricity is supplied to the backup RAM 3d of the ECU 3 without running 
through an ignition switch (not shown); thus various data, such as 
thresholds for feed-back control, are thus maintained irrespective of the 
conditions of the ignition switch. 
Processes of first through fourth embodiments for detecting abnormality of 
the oxygen sensor 36 executed by the ECU 3 are now explained based on the 
corresponding flow charts. Devices of the first through fourth embodiments 
have a substantially similar construction to that shown in the schematic 
view of FIG. 8. 
The first embodiment will now be discussed with reference to FIG. 1. 
Processing for determining if the oxygen sensor 36 is contaminated by 
silicon and thus deteriorated such that the use of the sensor 36 increases 
nitrogen oxides (NOx) of exhaust discharge in feed-back control is 
explained based on the flow chart of FIG. 9. This processing starts after 
warm-up of the engine 2. 
At step 100, the feed-back control of the air/fuel ratio stops and open 
loop control starts. At step 110, the air/fuel ratio is set to lean in the 
open loop control by driving and regulating the fuel ejection valve 25. 
The opening time period of the fuel ejection valve 25 is shortened, and 
the air/fuel ratio is set to lean, for example, at air excess rate 
.lambda.=1.03, and is maintained for a certain time period. The output 
signal sent from the oxygen sensor 36 is detected at step 120. When the 
output signal of the oxygen sensor 36 is not less than a predetermined 
threshold V.sub.3 (e.g., 300 mV), at step 130 the oxygen sensor is 
determined to be contaminated by silicon. The exhaust of nitrogen oxides 
will therefore be excessive. The check lamp 40 is then lit at step 140 and 
program exits from the processing. 
This process enables deteriorating oxygen sensors that are contaminated 
such that exhaust of NOx is excessive to be easily discriminated. 
The second embodiment will also be discussed with reference to FIG. 1. 
Processing for determining if the oxygen sensor 36 is contaminated by lead 
and thus deteriorated such that the use of the sensor 36 increases carbon 
monoxide (CO) of exhaust discharge in feed-back control is explained based 
on the flow chart of FIG. 10. 
At step 200, the feed-back control of the air/fuel ratio stops and open 
loop control starts. At step 210, the air/fuel ratio is set to rich in the 
open loop control by driving and regulating the fuel ejection valve 25. 
The opening time period of the fuel ejection valve 25 is increased, and 
the air/fuel ratio is set rich, for example to .lambda.=0.97, and is 
maintained for a certain time period. The output signal sent from the 
oxygen sensor 36 is detected at step 220. When the output signal of the 
oxygen sensor 36 is not greater than a predetermined threshold V.sub.4 
(e.g., 700 mV), at step 230 the oxygen sensor is determined to be 
contaminated by lead. The exhaust of carbon monoxide will therefore be 
excessive. The check lamp 40 is then lit at step 240 and program exits 
from the processing. 
This process enables deteriorating oxygen sensors that are contaminated 
such that exhaust of CO is excessive to be easily discriminated. 
The third embodiment will be described with reference to FIG. 2. Processing 
for determining if the oxygen sensor 36 is contaminated by silicon or lead 
and thereby deteriorated is explained based on the flow chart of FIG. 11. 
At step 300, the feed-back control of the air/fuel ratio stops and open 
loop control starts. At step 310, the air/fuel ratio is periodically 
changed between lean and rich in the open loop control by driving and 
regulating the fuel ejection valve 25. The opening time period of the fuel 
ejection valve 25 is adjusted, and the air/fuel ratio is periodically 
changed between rich, e.g., .lambda.=0.97 and lean, e.g., .lambda.=1.03 at 
the cycle of 2 Hz. The output signal sent from the oxygen sensor 36 is 
detected at step 320. The program proceeds to step 330 at which the 
minimum and maximum of the output signal are determined. Then, at step 340 
and step 350, it is determined if the minimum and the maximum of the 
output signal of the oxygen sensor 36 are within a predetermined output 
range. When either the minimum or the maximum of the output signal is 
determined to be within the predetermined range, that is, when the minimum 
is not less than a first threshold V.sub.1 (step 340) or when the maximum 
is not greater than a second threshold V.sub.2 (step 350) as shown in 
FIG. 6, the oxygen sensor 36 is determined to be contaminated and thus its 
operation is degraded. The check lamp 40 is then lit at step 360 and the 
program exits from the processing. 
This process enables an oxygen sensor whose operation is degraded by 
contamination to be easily discriminated. 
The fourth embodiment is in accordance with the feature of FIG. 3. 
Processing for determining if the oxygen sensor 36 is contaminated by 
silicon or lead and thereby deteriorated is explained based on the flow 
chart of FIG. 12. This process for detecting abnormality of the oxygen 
sensor 36 is executed while the feed-back control of the air/fuel ratio is 
being executed. 
At step 400, an output signal sent from the oxygen sensor 36 are detected 
while the feed-back control of the air/fuel ratio is being executed. The 
program proceeds to step 410 at which the minimum and maximum of the 
output signal are determined. Then at step 420 and step 430, it is 
determined if the minimum and the maximum of the output signal are within 
a predetermined range around a slice level V.sub.0 between threshold 
V.sub.1 and threshold V.sub.0. When the minimum is not less than a 
threshold V.sub.L lower than the slice level V.sub.0 at step 420 and when 
the maximum is not greater than a threshold V.sub.H higher than the slice 
level V.sub.0 at step 430 as shown in FIG. 7, the oxygen sensor 36 is 
determined to be contaminated and its operation thus degraded. The check 
lamp 40 is then lit at step 440 and program exits from the processing. 
The above processes for detecting abnormality of the oxygen sensor 36 may 
be executed when a car with the oxygen sensor 36 stops at a traffic light 
or is checked and examined in a garage. In the above first through fourth 
embodiments, deterioration of the oxygen sensor 36 is detected, but the 
same processes are applicable to detecting deterioration of the sub-oxygen 
sensor 37. 
As described above, in the apparatus for detecting abnormality of an oxygen 
sensor shown in FIG. 1, the oxygen sensor is determined to be abnormal and 
its operation degraded if an output signal of the oxygen sensor is not 
less than a predetermined threshold when the air/fuel ratio is set to 
lean, or if an output signal of the oxygen sensor is not greater than a 
predetermined threshold when the air/fuel ratio is set to rich. 
Deteriorating oxygen sensors which are contaminated by silicon or lead and 
therefore resulting in an increased exhaust of NOx or CO in the feed-back 
control of the air/fuel ratio are easily and accurately detected. 
In the apparatus for detecting abnormality of an oxygen sensor shown in 
FIG. 2, the minimum and maximum of a signal, output from the oxygen sensor 
when the air/fuel ratio is set to lean or rich by open loop control are 
determined. The oxygen sensor is determined to be abnormal and its 
operation degraded when at least one of the minimum and maximum values is 
within a predetermined output range. Deteriorating oxygen sensors are also 
easily and accurately detected. 
In the apparatus for detecting abnormality of an oxygen sensor shown in 
FIG. 3, the feed-back control of the air/fuel ratio is performed based on 
an output signal sent from the oxygen sensor. When the output signal of 
the oxygen sensor is within a predetermined output range, the oxygen 
sensor is determined to be abnormal and thus its operation degraded. 
Deteriorating oxygen sensors are as easily and accurately detected by the 
above apparatus. 
Now examples in which abnormality of the oxygen sensor 36 is detected by 
the above processes are explained. 
In the examples below, the normal oxygen sensor or deteriorating oxygen 
sensor 36 is mounted on the exhaust system of a vehicle. An output signal 
of the oxygen sensor 36 are detected under various conditions, e.g., the 
variation of the engine speed or the air/fuel ratio. 
(EXAMPLE 1) 
Voltages of the signals output from plural oxygen sensors in the lean 
air/fuel ratio are measured at variety of engine speeds. The exhaust 
amount of nitrogen oxides varies depending on the oxygen sensor. Table 1 
shows the measurement conditions and the results. In Table 1, A and B 
denote automobile models on which the oxygen sensors are mounted, and C 
and D denote measurement conditions. The conditions of C are as follows: a 
large flow rate of exhaust discharge; engine speed 1,500 rpm; and the air 
excess rate .lambda.=1.04. The conditions of D are as follows: a small 
flow rate of exhaust discharge; engine speed 800 rpm; and the air excess 
rate .lambda.=1.03. Samples No. 1 and No. 2 are normal oxygen sensors and 
No. 3 through No. 5 are deteriorating sensors which increase the exhaust 
of nitrogen oxides. Each resulting value in Table 1 is the average of 
three measurements. 
TABLE 1 
______________________________________ 
Emission of NOx in exhaust gas 
Sensor output voltage (mV) 
(g/mile) C D 
Automobile models 
1,500 rpm 800 rpm 
No. A B .lambda. = 1.04 
.lambda. = 1.03 
______________________________________ 
1 0.20 0.40 80 75 
2 0.52 1.20 280 200 
3 0.70 1.60 450 380 
4 1.20 3.50 550 450 
5 1.71 5.10 700 650 
______________________________________ 
As clearly seen in Table 1, in the normal oxygen sensors, No. 1 and No. 2, 
the sensor outputs in the lean air/fuel ratio range are maintained small 
irrespective of the engine speed. In the deterioration oxygen sensors, No. 
3 through No. 5, on the other hand, the sensor outputs are relatively 
large. With a predetermined threshold (e.g., 300 mV), oxygen sensors are 
thus easily determined to be normal ones or deteriorating ones, in other 
words, those increase exhaust of NOx. 
Table 2 shows the preferable measurement conditions. 
TABLE 2 
______________________________________ 
Condition 1 
Condition 2 Condition 3 
______________________________________ 
Engine speed 
500 to 1,000 
1,000 to 1,500 
1,500 to 2,000 
rpm 
Air excess rate 
1.0 to 1.03 
1.01 to 1.04 1.02 to 1.05 
(.lambda.) 
______________________________________ 
(EXAMPLE 2) 
Voltages of the signals output from plural oxygen sensors in the rich 
air/fuel ratio are measured at variety of engine speeds. The exhaust 
amount of carbon monoxide varies depending on the oxygen sensor. Table 3 
shows the measurement conditions and the results. In Table 3, A and B are 
the same as Example 1, and C and D are also the same except the air excess 
rate .lambda.=0.97. Samples No. 1 and No. 2 are normal oxygen sensors and 
No. 3 and No. 4 are deteriorating sensors which increase carbon monoxide. 
Each resulting value in Table 1 is the average of three measurements. 
TABLE 3 
______________________________________ 
Emission of CO in exhaust gas 
Sensor output voltage (mV) 
(g/mile) C D 
Automobile models 
1,500 rpm 800 rpm 
No. A B .lambda. = 0.97 
.lambda. = 0.97 
______________________________________ 
1 5.0 2.5 890 900 
2 7.2 4.1 800 820 
3 9.8 6.2 580 600 
4 11.8 8.9 360 390 
______________________________________ 
As clearly seen in Table 3, in the normal oxygen sensors, No. 1 and No. 2, 
the sensor outputs in the rich air/fuel ratio are maintained large 
irrespective of the engine speed. In the deterioration oxygen sensors, No. 
3 and No. 4, on the other hand, the sensor outputs are relatively small. 
With a predetermined threshold (e.g., 700 mV), oxygen sensors are thus 
easily determined to be normal ones or deteriorating ones that allows an 
increase in exhaust of CO. 
Table 4 shows the preferable measurement conditions. 
TABLE 4 
______________________________________ 
Condition 1 
Condition 2 Condition 3 
______________________________________ 
Engine speed 
500 to 1,000 
1,000 to 1,500 
1,500 to 2,000 
(rpm) 
Air excess rate 
0.99 to 0.97 
0.99 to 0.96 0.99 to 0.96 
(.lambda.) 
______________________________________ 
(EXAMPLE 3) 
In Example 3, the air/fuel ratio is periodically changed between lean and 
rich. The minimum and the maximum of the voltages of the signals output 
from various oxygen sensors are measured at variety of engine speeds. 
Table 5 shows the measurement conditions and the results for NOx, and 
Table 6 shows those for CO. In Tables 5 and 6, A and B are the same as 
Example 1, and the engine speed for C and D are also the same as Example 
1. The air excess rate .lambda. and the changeover cycle (Hz) are the same 
in both Table 5 and Table 6. Samples No. 1 and No. 2 are normal oxygen 
sensors and Nos. 3 through No. 5 are deteriorating sensors. 
TABLE 5 
______________________________________ 
Sensor output voltage (mV) 
Emission of NOx in exhaust gas 
C D 
(g/mile) .lambda. = 1.03 
.lambda. = 1.03 
Automobile models 
-0.96 -0.97 
No. A B 2 (Hz) 1.2 (Hz) 
______________________________________ 
1 0.20 0.40 910-130 900-130 
2 0.52 1.20 830-250 810-250 
3 0.70 1.60 870-360 880-350 
4 1.20 3.50 900-740 840-630 
5 1.71 5.10 870-840 810-780 
______________________________________ 
TABLE 6 
______________________________________ 
Sensor output voltage (mV) 
Emission of CO in exhaust gas 
C D 
(g/mile) .lambda. = 1.03 
.lambda. = 1.03 
Automobile models 
-0.96 -0.97 
No. A B 2 (Hz) 1.2 (Hz) 
______________________________________ 
1 5.0 2.5 910-130 900-130 
2 7.2 4.1 780-160 810-130 
3 9.8 6.2 520-190 580-170 
4 11.8 8.9 400-210 440-180 
______________________________________ 
As clearly seen in Table 5 and Table 6, in the normal oxygen sensors, No. 1 
and No. 2, the difference of the sensor outputs between in the lean 
air/fuel ratio and in the rich air/fuel ratio is large irrespective of the 
engine speed. In the deterioration oxygen sensors, No. 3 through No. 5, on 
the other hand, the difference of the sensor outputs is relatively small. 
With two predetermined thresholds (e.g., 300 mV and 700 mV), oxygen 
sensors are thus easily determined to be normal ones or deteriorating ones 
that increase the exhaust of NOx or CO. 
Table 7 shows the preferable measurement conditions. 
TABLE 7 
______________________________________ 
Condition 1 
Condition 2 
Condition 3 
______________________________________ 
Engine speed 500 to 1,000 
1,000 to 1,500 
1,500 to 2,000 
(rpm) 
Frequency 0.8 to 1.4 
1.2 to 1.8 
1.6 to 2.2 
(Hz) 
.lambda. rich .gtoreq.0.97 
.gtoreq.0.97 
.gtoreq.0.96 
lean .ltoreq.1.03 
.ltoreq.1.03 
.ltoreq.1.04 
______________________________________ 
(EXAMPLE 4) 
In Example 4, the output signal is measured open loop control but in the 
feed-back control of the air/fuel ratio. The minimum (in the lean air/fuel 
ratio) and the maximum (in the rich air/fuel ratio) of the voltages of 
signals output from various oxygen sensors is measured during the 
feed-back control of the air/fuel ratio. Table 8 shows the measurement 
conditions and the results for NOx, and Table 9 shows those for CO. In 
Tables 8 and 9, C and D denote measurement conditions; that is, automobile 
model A is driven at a constant speed. Samples No. 1 and No. 2 are normal 
oxygen sensors and No. 3 and No. 4 are deteriorating sensors. 
TABLE 8 
______________________________________ 
Sensor output voltage (mV) 
C D 
Emission of NOx in exhaust gas 
Driving conditions 
(g/mile) 80 km/hr 40 km/hr 
Automobile models 
8 ps 2 ps 
No. A B rich-lean 
rich-lean 
______________________________________ 
1 0.20 0.40 900-120 910-110 
2 0.52 1.20 850-200 860-190 
3 1.20 3.50 840-300 840-280 
4 1.71 5.10 820-360 840-360 
______________________________________ 
TABLE 9 
______________________________________ 
Sensor output voltage (mV) 
C D 
Emission of CO in exhaust gas 
Driving conditions 
(g/mile) 80 km/hr 40 km/hr 
Automobile models 
8 ps 2 ps 
No. A B rich-lean 
rich-lean 
______________________________________ 
1 5.0 2.5 900-130 910-110 
2 7.2 4.1 850-200 880-160 
3 9.8 6.2 760-350 750-280 
4 11.8 8.9 600-400 580-350 
______________________________________ 
As clearly seen in Table 8 and Table 9, in the normal oxygen sensors, No. 1 
and No. 2, the difference of the sensor outputs between the lean air/fuel 
ratio and the rich air/fuel ratio (i.e., the difference between the 
maximum and the minimum) is large. In the deteriorating oxygen sensors, 
No. 3 and No. 4, on the other hand, the difference of the sensor outputs 
is relatively small. With two predetermined thresholds V.sub.L and V.sub.H 
(e.g., 250 mV and 850 mV), oxygen sensors are thus easily determined to be 
normal ones or deteriorating ones, in other words, those increase exhaust 
of NOx or CO. 
Processes of fifth through seventh embodiments for controlling the air/fuel 
ratio executed by the ECU 3 are now explained based on the corresponding 
flow charts. Devices of the fifth through seventh embodiments have a 
substantially identical construction as shown in the schematic view of 
FIG. 8. 
The fifth embodiment will be discussed with reference to FIG. 4. Processing 
for maintaining the air/fuel ratio lean and then rich, measuring the 
output signal of the oxygen sensor 36 in lean and rich states, and 
determining the median of the output signal is explained based on the flow 
chart of FIG. 13. This processing starts after warm-up of the engine 2. 
At step 500, the feed-back control of the air/fuel ratio stops and open 
loop control starts. At step 510, the air/fuel ratio is set to lean (e.g., 
the air excess rate .lambda.=1.02) in the open loop control by driving and 
regulating the fuel ejection valve 25 and is maintained for a certain time 
period. An output signal D.sub.L of the oxygen sensor 36 for the lean 
state is detected at step 520. 
Then at step 530, the air/fuel ratio is set to rich (e.g., .lambda.=0.98) 
in the open loop control by driving and regulating the fuel ejection valve 
25 and is maintained for a certain time period. An output signal D.sub.R 
of the oxygen sensor 36 for the rich state is detected at step 540. 
When the output signal D.sub.L of the oxygen sensor 36 in the lean state is 
not less than a predetermined threshold V.sub.L (e.g., 400 mV), the oxygen 
sensor is determined to be abnormal at step 550 and the check lamp 40 is 
then lit at step 560. On the other hand, when the output signal D.sub.R of 
the oxygen sensor 36 in the rich state is not greater than a predetermined 
threshold V.sub.R (e.g., 700 mV), the oxygen sensor is determined to be 
abnormal at step 570 and the check lamp 40 is then lit at step 560. 
When the oxygen sensor 36 is determined to be abnormal at either step 550 
or step 570, the median V.sub.TH of the output signal D.sub.L in lean 
state and D.sub.R in rich state is determined at step 580. The program 
proceeds to step 590 at which the median V.sub.TH is set as a threshold 
(slice level) for discriminating lean and rich in the feed-back control of 
the air/fuel ratio and then exits from the processing. 
As shown in FIG. 14A, when the voltage of the output signal D.sub.L in 
.lambda.=1.02 is 500 mV and that of the output signal D.sub.R in 
.lambda.=0.98 is 900 mV, the median V.sub.TH is equal to 700 mV. The 
median V.sub.TH is used as the threshold in the feed-back control of the 
air/fuel ratio. Even if the output signal of the oxygen sensor 36 
oscillates at a higher voltage or a lower voltage, virtually the center of 
the oscillation becomes equal to the threshold. Thus lean and rich states 
of the air/fuel ratio are appropriately discriminated from each other and 
are converted into binary signals of 0 V and 5 V as shown in FIG. 14B. 
The optimum threshold is set according to the output signal of the oxygen 
sensor 36 as explained above. Even when the oxygen sensor 36 is 
contaminated and its output is degraded, the lean and rich states are 
properly detected and the air/fuel ratio is preferably controlled. 
In the fifth embodiment, abnormality of the oxygen sensor 36 is detected in 
a similar manner as the first or the second embodiment. Other methods, 
however, may be applied for detecting abnormality of the oxygen sensor. 
For example, those of the third and fourth embodiments are applicable. 
The sixth embodiment will also be described with reference to FIG. 4. 
Processing for controlling the air/fuel ratio by using the minimum and 
maximum of the output signal of the oxygen sensor 36 are explained based 
on the flow chart of FIG. 15. 
At step 600, the feed-back control of the air/fuel ratio stops and open 
loop control starts. At step 610, the air/fuel ratio is periodically 
changed between rich and lean in the open loop control by driving and 
regulating the fuel injection valve 25. The output signal of the oxygen 
sensor 36 in rich and lean states is detected at step 620. The minimum 
V.sub.MIN and maximum V.sub.MAX of the output signal are then determined 
at step 630. When even one of the minimum or maximum of the output signal 
is within a predetermined output range, the oxygen sensor 36 is determined 
to be abnormal at step 640 and the check lamp 40 is then lit at step 650. 
When the oxygen sensor 36 is determined to be abnormal at step 640, the 
median V.sub.TH between the minimum V.sub.MIN and the maximum V.sub.MAX 
are determined at step 660. The program proceeds to step 670 at which the 
median V.sub.TH is set as a threshold for discriminating lean and rich in 
the feed-back control of the air/fuel ratio and then exits from the 
processing. 
As shown in FIG. 16A, when output signal of the oxygen sensor 36 oscillates 
at a voltage higher than a predetermined threshold V.sub.0, the oxygen 
sensor 36 is determined to be abnormal, and the median V.sub.TH between 
the minimum V.sub.MIN and the maximum V.sub.MAX is determined to be a 
threshold. Even if the output signal of the oxygen sensor 36 is abnormal, 
lean and rich states of the air/fuel ratio in the feed-back control of the 
air/fuel ratio are appropriately discriminated from each other and are 
converted into binary signals of 0 V and 5 V as shown in FIG. 16B. 
The optimum threshold is set according to the output signal of the oxygen 
sensor 36 as explained above. Thus, even when the oxygen sensor 36 is 
contaminated and its output shifts to a higher or lower voltage, the 
air/fuel ratio is preferably controlled. 
The seventh embodiment will also be explained with reference to FIG. 4. An 
alternative processing for control using the median V.sub.TH of the output 
signal of the oxygen sensor 36 based on the flow chart of FIG. 17. 
When abnormality of the oxygen sensor 36 is detected at step 700 in the 
same manner as the fifth or the sixth embodiments explained above, the 
median V.sub.TH is determined at step 710. The program proceeds to step 
720 at which the voltages of the signals output from the oxygen sensor 36 
in the feed-back control of the air/fuel ratio are proportionally 
converted based on the value of the median V.sub.TH, thus allowing the 
output signal to be converted into a normal signal with a large variation 
in amplitude, and the program then exits from the processing. 
The voltage generated as an output signal of the oxygen sensor is converted 
as shown in FIG. 18 and Table 10. 
TABLE 10 
______________________________________ 
Voltage measured (mV) 
Voltage converted (mV) 
______________________________________ 
500 0 
900 1,000 
700 500 
600 250 
800 750 
______________________________________ 
For example, when the voltage of the output signal is higher than a 
predetermined threshold V.sub.0, a signal of 500 mV in the lean air/fuel 
ratio (.lambda.=1.02) is converted into that of 0 V, and a signal of 900 
mV in the rich air/fuel ratio (.lambda.=0.98) into that of 1 V. The center 
of the amplitude of the abnormal signal output from the oxygen sensor is 
corrected to the predetermined threshold V.sub.0 or 500 mV; namely, the 
voltage of an abnormal signal is proportionally converted into that of a 
normal signal with a large variation in. In this embodiment, when X 
denotes voltage measured and Y denotes voltage converted, the conversion 
is performed based on the following equation for conversion. 
EQU Y=2.5X -1250 
Since an output signal is compensated in the above manner, even when the 
signal is shifted to a higher voltage or a lower voltage or have only a 
small amplitude, the air/fuel ratio is adequately detected using the 
predetermined threshold V.sub.0 and thus is preferably controlled. 
As described above, in the apparatus for controlling the air/fuel ratio of 
the invention, the air/fuel ratio is set lean or rich by open loop 
control, and the median of an output signal of the oxygen sensor in the 
lean or rich state is determined. When the oxygen sensor is determined to 
be abnormal, the median is set as a threshold for discriminating between 
rich and lean of the air/fuel ratio in the feed-back control. Thus, even 
when the oxygen sensor deteriorates by contamination and outputs an 
abnormal signal, the feed-back control of the air/fuel ratio is preferably 
performed.