Diagnostic apparatus for catalytic converter of an internal combustion engine

A catalytic diagnostic apparatus for determining whether a catalyst of an internal combustion engine has deteriorated as disclosed. Air-fuel ratio sensors are placed at the upstream and downstream sides of the catalytic converter, and the output signal from the upstream air-fuel sensor is used to control the air-fuel mixture provided to the engine via the fuel injectors, in a conventional closed loop operation. Diagnosis of the condition of the catalytic converter is performed only when selected engine operating parameters fall within a predetermined diagnostic range. When the engine is operated in the diagnostic range, the condition of the catalytic converter is analyzed by calculating a deterioration index based on a comparison of the variation of the output signals from the upstream and downstream air-fuel ratio sensors. In a preferred embodiment, a look-up table is provided to correct the deterioration index, based on the intake air flow rate to the engine, and on the cyclic frequency of the variation of the downstream air-fuel ratio sensor.

BACKGROUND OF THE INVENTION 
The present invention relates to a diagnostic apparatus for determining 
whether a catalyst of an internal combustion engine has deteriorated. 
The principal components of a system for cleaning exhaust gases of an 
internal combustion engine are a catalytic converter and an air-fuel ratio 
feedback control unit. The catalytic converter is coupled to an exhaust 
pipe for use in removing pollutants such as HC, NOx and CO contained in 
the exhaust gases. The air-fuel ratio feedback control unit includes an 
O.sub.2 sensor (oxygen sensor) which is arranged on the upstream side of 
the catalytic converter and used for detecting an air-fuel ratio. In other 
words, the quantity of fuel injected to such an internal combustion engine 
is controlled so that the air-fuel ratio may have a predetermined 
(stoichiometric) value, thus enabling the engine to operate with optimum 
efficiency and to output the minimum amount of pollutants. 
The efficiency of a conventional three-dimensional catalytic system in 
converting noxious components decreases as the performance of the 
catalytic converter itself deteriorates, even though the air-fuel ratio is 
precisely controlled by the air-fuel ratio feedback control unit. 
Therefore in order to prevent a degradation of the efficiency of the air 
cleaner system, it is necessary to monitor the condition of the catalytic 
converter so as to issue a warning when it has deteriorated. Japanese 
Patent Laid-Open No. 30915/1990, for example, discloses an apparatus for 
detecting the deterioration of a catalytic converter, entitled "Catalytic 
Deterioration Decision Apparatus for Internal Combustion Engine." This 
arrangement, includes two oxygen sensors (binary sensors), one situated on 
the upstream side of the catalytic converter and the other on the 
downstream side thereof, to measure the time lag between inversion of the 
output value of the upstream sensor and,that of the downstream sensor. The 
condition of the catalyst is determined by the scale of the time 
difference thus measured. More specifically, the smaller the time 
difference, the greater the deterioration of the catalyst. 
In such catalytic deterioration detection apparatus, however, the measured 
time difference fluctuates as the volume of exhaust gases fluctuates, 
without regard to deterioration of the catalytic converter itself, which 
reduces the accuracy of the deterioration assessment. As a result, a 
catalytic converter may be determined to be free from deterioration, even 
though it has in fact deteriorated. 
SUMMARY OF THE INVENTION 
An object of the present invention is to provide a catalytic diagnostic 
apparatus for an internal combustion engine that is capable of determining 
the condition of a catalytic converter accurately, without being affected 
by fluctuating elements such as variation in the output exhaust gas 
volume. 
In order to accomplish this object, the catalytic diagnostic apparatus for 
an internal combustion engine according to the present invention has a 
first air-fuel ratio sensor disposed upstream of the catalyst, which is 
used to detect the air-fuel ratio of exhaust gases of the internal 
combustion engine, and a second air-fuel ratio sensor disposed on the 
downstream side of the catalyst, which is used to detect the air-fuel 
ratio of exhaust gases on the downstream side thereof. The quantity of 
fuel injected into the internal combustion engine is controlled in 
response to the output of the upstream O.sub.2 sensor in a conventional 
closed loop, or feedback, control arrangement. For the purpose of 
diagnosing the condition of the catalytic converter according to the 
invention, a diagnostic area decision unit determines whether the engine 
is currently operating within a diagnostic decision area (that is, an area 
in which a valid diagnosis can be made) defined by predetermined engine 
operating parameters, and initiates diagnostic processing when the 
definitional criteria are satisfied. A downstream air-fuel ratio decision 
unit 20 then determines whether the output signal from the downstream 
sensor is on the rich side or the lean side of the air-fuel ratio relative 
to a stoichiometric mixture, and generates a signal which causes the 
feedback control apparatus to adjust the air-fuel mixture to make it 
leaner or richer accordingly. (When the air-fuel ratio is at the 
theoretical stoichiometric mixture, no such adjustment is necessary.) A 
deterioration decision unit decides whether the catalyst has deteriorated 
based on a deterioration index I calculated from the relationship between 
the respective output signals of the upstream and downstream air-fuel 
ratio sensors. 
In another embodiment of the invention, the downstream air-fuel ratio 
control unit is omitted, and a correction factor calculating unit detects 
the intake air flow rate of the engine as well as the frequency of the 
cyclic variation of the output signal from the upstream O.sub.2 sensor, 
and calculates a correction factor which is used to adjust the final 
deterioration index I. A third embodiment combines the features of the 
first and second. 
The downstream air-fuel ratio control unit preferably includes a lean-rich 
decision unit for determining whether the air-fuel ratio drifts to the 
lean or rich side, and an air-fuel ratio shift coefficient calculating 
unit for calculating an air-fuel ratio shift coefficient according to a 
decision signal from the lean-rich decision unit, so as to supply the 
coefficient thus calculated to the fuel injection quantity control unit. 
The deterioration decision unit preferably includes a correlation function 
calculating unit for calculating a cross correlation function between the 
output signal of the upstream air-fuel ratio sensor and that of the 
downstream air-fuel ratio sensor, and an autocorrelation function of the 
output signal of the upstream air-fuel ratio sensor. An instantaneous 
deterioration index calculating unit periodically calculates an 
instantaneous deterioration index as the ratio of the maximum value of the 
cross correlation function to that of the autocorrelation function, and a 
final deterioration index calculating unit calculates the mean value of a 
predetermined number of instantaneous deterioration indexes as the final 
deterioration index. A comparator then compares the final deterioration 
index with a predetermined reference deterioration index, and a decision 
unit determines catalytic deterioration according to an output signal from 
the comparator. 
In another preferred embodiment, the catalytic diagnostic apparatus for an 
internal combustion engine according to the invention has a catalytic 
diagnostic area decision unit which establishes predetermined limiting 
parameters of a catalytic diagnostic area, and the downstream air-fuel 
ratio control unit and the deterioration decision unit are operated only 
when the conditions of the catalytic diagnostic area have been satisfied. 
These conditions are determined to be satisfied when the engine speed 
(detected by an engine speed sensor) is greater than a predetermined speed 
level, and the catalytic temperature (estimated from the engine speed and 
the intake air quantity detected by an air flow rate detector), is also 
above a predetermined temperature level. 
Preferably, the upstream and downstream air-fuel ratio sensors in the 
catalytic diagnostic apparatus according to the invention are oxygen 
sensors; however, other types of sensors, such as for example, linear 
sensors, may be used. Also, the correction factor output unit includes an 
engine intake air quantity calculating unit, an output signal frequency 
calculating unit in the upstream air-fuel ratio sensor, a correction 
factor storage unit containing a look up table of predetermined correction 
factor values, and a reader unit for reading a correction factor from the 
storage unit, based on the engine intake air quantity and the frequency of 
the output signal of the upstream air-fuel ratio sensor. The correction 
factor thus determined is supplied to the final deterioration index 
calculating unit. 
When the output signal of the downstream air-fuel ratio sensor is outside 
the predetermined range due to fluctuations in exhaust gas volume, the 
fuel injection quantity control unit adjusts the fuel injection quantity. 
When the output signal of the downstream air-fuel ratio sensor is within 
the predetermined range, the diagnosis of the catalytic deterioration is 
carried out, based on the relationship between the respective output 
signals of the upstream and downstream air-fuel ratio sensors. Therefore, 
the catalytic deterioration diagnosis is carried out accurately without 
being affected by fluctuations in volume of exhaust gases. 
The correlation function calculating unit calculates the cross correlation 
function of the output signals of the upstream and downstream air-fuel 
ratio sensors, as well as the autocorrelation function of the output 
signal of the upstream air-fuel ratio sensor. Then the instantaneous 
deterioration index calculating unit periodically calculates the 
instantaneous deterioration index as the ratio of the maximum value of the 
cross correlation function to the value of the autocorrelation function. 
In the second and third embodiments of the invention, the correction 
factor output unit calculates and outputs a correction factor according to 
the intake air quantity of the engine and the frequency of the output 
signal of the upstream air-fuel ratio sensor, as described above. The 
final deterioration index calculating unit calculates the mean value of a 
predetermined number of periodic instantaneous deterioration indexes, and 
corrects the mean value thus calculated using the correction factor. The 
deterioration of the catalyst is determined using the corrected mean value 
as the final deterioration index. Therefore, the deterioration of the 
catalyst can be decided accurately, without being affected by fluctuations 
in not only the output frequency of the upstream air-fuel ratio sensor but 
also intake air quantity. 
Provided it is so arranged that the final deterioration index is corrected 
based the frequency of the output signal of the upstream air-fuel ratio 
sensor and the intake air quantity of the engine while the output signal 
of the downstream air-fuel ratio sensor falls within the predetermined 
diagnostic range at the time the catalyst is diagnosed, the deterioration 
of the catalyst can be decided accurately without being affected by 
fluctuations in volume of exhaust gases, the output frequency of the 
downstream air-fuel ratio sensor or the intake air quantity. 
If intake air quantity, water temperature and fuel injection quantity are 
taken into account, diagnostic information may be made even more reliable. 
Other objects, advantages and novel features of the present invention will 
become apparent from the following detailed description of the invention 
when considered in conjunction with the accompanying drawings.

DETAILED DESCRIPTION OF THE DRAWINGS 
FIG. 1 is a schematic block diagram of a first embodiment of the present 
invention, in which a fuel injection control unit 7 having an air-fuel 
ratio feedback computer 8, a fuel injection quantity computer 9 and an 
output unit 10, controls the quantity of fuel supplied to an engine 1. An 
upstream O.sub.2 sensor, (that is, an air-fuel ratio sensor) 3 is placed 
on the engine side of a catalytic converter 2. This sensor is a lambda 
(.lambda.) sensor whose detection element is made of, for example, 
zirconia or titania. 
The fuel injection quantity computer 9 calculates the fundamental injection 
quantity FO from the following equation (1) based on the intake air 
quantity Qa detected by an intake air quantity sensor 5 and the rotational 
speed Ne of the engine detected by a speed sensor 6: 
##EQU1## 
where kO=a predetermined proportionality constant. 
On the other hand, the air-fuel ratio feedback computer 8 samples output 
signals of the upstream O.sub.2 sensor 3 at predetermined intervals, 
calculates a correction coefficient .alpha. so as to set the air-fuel 
ratio to a predetermined value according to what has been detected, end 
supplies the coefficient .alpha. to the fuel injection quantity computer 
9. 
The fuel injection quantity computer 9 then calculates an injection 
quantity F from the following equation (2) by adding the correction 
coefficient .alpha. to the fundamental injection quantity FO. 
##EQU2## 
The fuel injection quantity computer 9 supplies a signal indicating the 
injection quantity F thus calculated to the output unit 10, which applies 
a voltage duty signal corresponding to the injection quantity F, to a fuel 
injection valve 26. With this manner of feedback air-fuel ratio control, 
the input air-fuel ratio varies cyclically about an approximately 
stoichiometric mixture on the upstream side of the catalytic converter 2. 
According to the present invention, the fluctuation of the air-fuel ratio 
under the air-fuel ratio feedback control is utilized for a test signal to 
diagnose the deterioration of the catalytic converter. In other words, 
owing to oxidation-deoxidization of the catalyst, the air-fuel ratio 
changes less on the downstream side of the catalytic converter 2 unless 
the catalytic converter 2 is deteriorated. If, however, the catalytic 
converter 2 deteriorates, the cyclic variation of the air-fuel ratio on 
the downstream will approximate that on the upstream side. Such 
deterioration is thus diagnosed by giving special attention to the 
similarity of the cyclic variation of the air-fuel ratio on both sides of 
the catalytic converter. 
For this purpose, a deterioration diagnostic unit 11 is provided with a 
catalytic diagnostic area decision unit 22, a downstream air-fuel ratio 
control unit 28 and a deterioration decision unit 29. The downstream 
air-fuel ratio control unit 28 includes an output decision unit 19, a 
lean-rich decision unit-20 and an air-fuel ratio shift coefficient 
calculating unit 21. Further, the deterioration decision unit 29 includes 
a correlation function calculating unit 18, an instantaneous deterioration 
index .PHI.i calculating unit 16A, a final deterioration index I 
calculating unit 16B, a comparator 16E and a catalytic deterioration 
decision output unit 17. The catalytic diagnostic area decision unit 22 is 
supplied with output signals respectively from the intake air quantity 
sensor 5 and the speed detection sensor 6. 
A determination of whether or not the catalytic converter 2 has 
deteriorated is made only when the engine operating parameters fall within 
certain predetermined ranges, referred to as a diagnostic area Da. In 
particular, for example, the engine 1 must be driven at not less than a 
predetermined speed, and the temperature of the catalytic converter 2 must 
be at least 500.degree. C. as shown in FIG. 5. The catalytic diagnostic 
area decision unit 22 determines whether or not these conditions have been 
met. When they are, a diagnostic execution signal is supplied from the 
catalytic diagnostic area decision unit 22 to the output decision unit 19 
and the correlation function calculating unit 18, which initiate the 
diagnosis in response thereto. 
FIG. 2 illustrates the operation of the calculating units 18, 16A, 16B, the 
comparator 16E and the catalytic deterioration decision output unit 17, 
which collectively make up the deterioration decision unit 29. 
As shown in FIG. 2, the correlation function calculating unit 18 includes 
data sampling units 12A, 12B, memories 13A, 13B, an autocorrelation 
calculating unit 14A, a cross correlation function calculating unit 14B 
and a cross correlation function calculating unit 14D. The output signal 
S3 of the upstream O.sub.2 sensor 3 is supplied to the sampling unit 12A, 
where it is sampled at fixed time intervals (or angles). The sampled 
values are temporarily stored in the memory 13A. The output signal S4 of a 
downstream O.sub.2 sensor (air-fuel ratio sensor) is supplied via the 
output decision unit 19 (not shown in FIG. 2) to the sampling unit 12B and 
sampled at fixed time intervals (or angles) with the sampled values being 
temporarily stored in the memory 13B. 
Output signals x, y are supplied respectively from the memories 13A, 13B to 
the cross correlation function calculating unit 14B, which carries out 
calculations according to the following equations (3-1).about.(3-n): 
##EQU3## 
In other words, the products of the signal y.sub.0 of the sensor 4 at time 
t2 in FIG. 3(B) and those of the sensor 3 in FIG. 3(A), ranging from 
x.sub.odn at a point in time tO up to x.sub.0 thereof, are calculated, 
respectively. Then the products of the signal Y.sub.01 at time t3 and 
those values of x ranging from the signal x.sub.odn1 up to the signal 
x.sub.01 thereof are calculated, respectively. The products of the signal 
Y.sub.On and those ranging from x.sub.odnn up to x.sub.On are thus 
calculated until the desired result is obtained. In FIG. 3, time .tau. 
from t0 to t1 is equivalent to the delay time (phase difference) of the 
output signal S4 of the sensor 4 relative to the output signal S3 of the 
sensor 3. The product of the signals x, y is maximized when x is shifted 
by a delay of .tau.. 
The results obtained from Eqs. (3-1).about.(3-n) are supplied to the cross 
correlation function calculating unit 14D which calculates a cross 
correlation function Y according to the following equation (4). 
##EQU4## 
The output signal x from the memory 13A is also supplied to the 
autocorrelation calculating unit 14A, which carries out a calculation 
according to the following equation (5). 
##EQU5## 
The autocorrelation function X and the cross correlation function Y that 
have thus been calculated are supplied to the calculating unit 16A, which 
periodically calculates the instantaneous deterioration index .PHI.i from 
the following equation (6) 
EQU .PHI..sub.i =Y.sub.M /X (6) 
where YM=maximum value in a term constituting the cross correlation 
function Y. 
Since the similarity of the variation of the air-fuel ratio on both sides 
of the catalytic converter 2 increases when the catalytic converter 2 
deteriorates, the periodic instantaneous deterioration index .PHI.i also 
becomes higher (close to 1) as shown in FIG. 4. In other words, a 
deterioration index .PHI.ia when the catalytic converter 2 is 
substantially deteriorated has a greater maximum value than a 
deterioration index .PHI.ib when the deterioration of the catalytic 
converter is slight. 
The instantaneous deterioration indices .PHI.i are thus calculated 
periodically and supplied to the final deterioration index I calculating 
unit 16B. 
As shown by the following equation (7), the arithmetic mean value of n 
instantaneous deterioration indices .PHI.i is calculated in the 
calculating unit 16B, and is set as the final deterioration index I for 
the catalytic converter 2. 
##EQU6## 
When the final deterioration index I is calculated, correction factors 
under various operating conditions may be added thereto; for example, a 
correction coefficient k1 deriving from the engine load and a correction 
coefficient k2 deriving from the catalyst temperature may be added so as 
to make I=(.SIGMA.k1k2.PHI.i)/n. In this case, the correction coefficients 
k1, k2 are stored in a look up table. 
The final deterioration index I thus calculated is supplied to the 
comparator 16E, and compared with a predetermined deterioration reference 
level ID. When the final deterioration index I is equal to or greater than 
the deterioration reference level ID, a deteriorated condition of the 
catalytic converter is determined to exist, and a signal to that effect is 
supplied to the decision output unit 17. Then the decision output unit 17 
drives a display (not shown), indicating that the catalytic converter 2 
has been deteriorated. 
The reason for use of the mean value as the final deterioration index I 
instead of using the periodically calculated instantaneous deterioration 
index .PHI.i directly is that if either the engine speed or load 
fluctuates during traveling, the instantaneous deterioration index .PHI.i 
may also fluctuate in response. Therefore, the instantaneous deterioration 
indexes .PHI.i obtained during a fixed period of time, at a fixed speed or 
at a fixed load zone are accumulated and by making their mean value the 
final deterioration index I, erroneous readings due to transient 
conditions (such as rapid vehicle acceleration) are suppressed, and it is 
possible to evaluate deterioration of the catalyst over the whole driving 
range. However, the instantaneous deterioration index .PHI.i may be 
employed directly for making such a decision as long as the driving 
condition is properly controlled. 
When the correlation function is used to determine catalytic deterioration, 
the correlation function can be calculated accurately only if the air-fuel 
ratio at the downstream O.sub.2 sensor 4 (FIG. 6(B)) fluctuates at (on 
both sides of) the stoichiometric line ST. The output S3 of the upstream 
sensor 3 is maintained close to the stoichiometric point (FIG. 6(A)) by 
virtue of the feedback control described previously. However, the output 
signal S4 of the downstream O.sub.2 sensor 4 tends to drift to the rich or 
lean side as shown in FIG. 6(C) because of variation in exhaust quantity, 
and consequently the correlation function may not be calculated 
accurately. Thus, the output decision unit 19 compares the output time of 
the rich side of the signal S.sub.4 with that of the lean side, and from 
this comparison determines whether S.sub.4 is proximate to the 
stoichiometric line. (Approximately equal output times would indicate that 
it is.) If S.sub.4 is not near the stoichiometric line the signal S.sub.4 
is supplied to decision unit 20, which determines whether it is on the 
lean side or rich side, and supplies the result to the air-fuel ratio 
shift coefficient calculating unit 21. The air-fuel ration shift 
coefficient calculator generates a fuel ratio adjustment signal which is 
supplied to the air-fuel ratio feedback computer 8 to modify the air-fuel 
mixture to make it leaner or richer. 
As shown in FIG. 7(A), the air-fuel ratio shift coefficient calculator unit 
21 supplies a signal to the air-fuel ratio feedback computer 8 which 
causes the output signal S4 of the downstream O.sub.2 sensor 4 to move 
into proximity with the stoichiometric line by shifting it to the lean 
side when the air-fuel ratio is on the rich side, and shifting it to the 
rich side when it is on the lean side in the diagnostic area Da (from t1 
up to t2). The correlation function of the output signal S3 of the 
upstream O.sub.2 sensor 3 shown in FIG. 7(B) with the downstream O.sub.2 
sensor 4 shown in FIG. 7(C) can then be calculated accurately. 
Therefore, in a preferred embodiment of the invention, instead of simply 
taking the mean value of N instantaneous degradation indices as described 
above, the final deterioration index calculation is performed by the final 
deterioration index calculator 16B by taking the mean of the m largest 
values of .PHI.i, according to the following formula: 
##EQU7## 
where max.sub.M (.PHI.i) represents the M largest values of .PHI.i within 
the designated time interval. For example, if M=3, then 
##EQU8## 
This calculation achieves greater diagnostic accuracy than a straight mean 
value calculation because the maximum deterioration index occurs at a time 
when the air-fuel ratio, as detected and indicated by the output signal 
S.sub.4 of the downstream O.sub.2 sensor 4, varies about the 
stoichiometric point, as shown in FIG. 7(D). 
FIG. 8 is a flowchart which shows the operation of the deterioration 
diagnostic unit 11 as described above. At Step 100 of FIG. 8, a decision 
is made on whether the catalytic diagnostic area decision unit 22 is in 
the diagnostic area Da and if so, at step 102 a determination is made by 
the lean-rich decision unit 20 whether S.sub.4 is on the lean side or on 
the rich side. If it is rich, at Step 103 the air-fuel ratio shift 
coefficient calculating unit 21 calculates a coefficient for shifting the 
air-fuel ratio AF from the rich to lean side, and supplies the calculated 
result to the air-fuel ratio feedback computer 8. IF S.sub.4 is on the 
lean side, an opposite adjustment is made at step 103 B. (The air-fuel 
ratio feedback computer 8 corrects the correction coefficient .alpha. 
according to the coefficient supplied from the air-fuel ratio shift 
coefficient calculating unit 21 and supplies the corrected value to the 
fuel injection quantity computer 9, so that the air-fuel ratio is 
controlled as stated above.) Next, at Step 104 the autocorrelation 
function X and the cross correlation function Y are calculated by the 
correlation function calculating unit 18, so that the periodic 
instantaneous deterioration index .PHI.i can be calculated. 
At Step 105 the calculating unit 16B calculates the final deterioration 
index I and the comparator 16E compares it with a reference value QID 
(step 106) to determine whether or not the catalytic converter 2 has 
deteriorated. If the catalytic converter 2 is judged deteriorated an alarm 
is activated (step 108). On the other hand, if it is found not to be 
deteriorated, a determination is made in step 109 whether the air-fuel 
ratio has been inverted (changed sign). If it has, air-fuel adjustment is 
suspended at step 210; and if not, steps 102-106 are repeated. 
According to the above first embodiment of the present invention, the 
output signal S4 of the downstream O.sub.2 sensor 4 is examined when the 
deterioration of the catalytic converter 2 is to be diagnosed, and if it 
is not in the neighborhood of the stoichiometric line, the air-fuel ratio 
is adjusted so that it is set close thereto. Whether or not the catalytic 
converter 2 has deteriorated is decided based on a correlation function of 
the signals S3, S4. It is therefore possible to provide a catalytic 
diagnostic apparatus capable of detecting a deteriorated condition of the 
catalytic converter 2 accurately for an internal combustion engine, 
without being affected by fluctuating elements such as variations in 
volume of the output exhaust gases. 
The cycling frequency of the output signal S3 of the upstream O.sub.2 
sensor 3 decreases as it deteriorates. (That is, the waveform shown by a 
solid line in FIG. 9(A) changes to that shown by a broken line.) When the 
frequency of the output signal S3 of the upstream O.sub.2 sensor 3 is 
high, the air-fuel ratio AF fluctuates within a predetermined range 
(between the upper limit UL and the lower limit LL) as shown by the solid 
line of FIG. 9(B). When the frequency of the output signal S3 decreases, 
however, the fluctuation of the air-fuel ratio AF exceeds the above range, 
as shown by a broken line of FIG. 9(B), and an error tends to occur in the 
deterioration index I. 
More specifically, as shown in FIG. 10, the detected deterioration index Ib 
tends to increase as the air-fuel ratio control frequency becomes lower, 
and decreases as it rises, relative to a true deterioration index IO 
(approximately 0.3). Further, the deterioration index varies with the 
quantity of exhaust gas G passing through the catalyst. That is, the 
deterioration index becomes relatively higher than the deterioration index 
Ib when the quantity of exhaust gas G is large (curve Ia), and the 
deterioration index becomes lower than the deterioration index Ib when the 
quantity of exhaust gas G is small (curve Ic). 
FIG. 11 is a schematic block diagram of a second embodiment of the 
invention which compensates for fluctuations of the frequency and the 
intake air quantity by adding a correction factor derived from a look up 
table based on detected values of these parameters. (Like reference 
characters designate like or corresponding component parts of the first 
embodiment.) In FIG. 11, a deterioration diagnostic unit tlA is provided 
with a catalytic diagnostic area decision unit 22, a deterioration 
decision unit 29 and a correction factor output unit 30. The correction 
factor output unit 30 includes an intake air quantity Qa calculating unit 
27, a frequency f calculating unit 23, a correction factor reader unit 24 
and a look up table 25. 
The intake air quantity Qa calculating unit 27 calculates the intake air 
quantity Qa based on to the output signal from the intake air quantity 
sensor 5. (Qa in this case represents intake air quantity per unit time or 
per unit rotation of engine.) Further, the frequency f calculating unit 23 
calculates the frequency f of the output signal S3 of the upstream O.sub.2 
sensor 3. The reader unit 24 reads a correction factor from the look up 
table 25 based on the an intake air quantity signal and the frequency 
signal from the calculating units 27, 23 respectively, which compensates 
for the deviation of the calculated deterioration index from the true 
value, as shown in FIG. 10. The correction factor is input to the final 
deterioration index I calculating unit 16B where it is added to the 
deterioration index I to compensate for fluctuations of the cycling 
frequency of the upstream O.sub.2 sensor 3 and of the intake air quantity 
as noted. 
The correction factors stored in the look up table 25 are determined 
experimentally beforehand. As shown in FIG. 12, the correction factors 
take the form of a family of characteristic curves for differing values of 
air flow Q.sub.a. The representative values shown in FIG. 12 indicate, for 
example, that at a frequency of 1.0 Hz, the value of the correction factor 
for an air flow of 150 Kg/hr is 1.2, and for an air flow of 50 Kg/hr is 
0.7. 
FIG. 13 is a flowchart which shows the operation of the deterioration 
diagnostic means 11A. At Step 200, a decision is made on whether the 
catalytic diagnostic area decision unit 22 is in the diagnostic area Da. 
If so, the reader unit 24 reads a correction factor from the look up table 
25 (Step 201) according to the output signals from the calculating units 
27 and 23 respectively, and supplies the correction factor to the 
calculating unit 16B. In Step 202 the calculating unit 16B calculates the 
final deterioration index I by adding the correction factor to the 
instantaneous deterioration index .PHI.i, calculated by calculating units 
18, 16A. Next, in step 203, the comparator 16E compares the final 
deterioration index I with the decision value QID and the reference output 
unit 17 determines whether the catalytic converter 2 has deteriorated. If 
the catalytic converter 2 is found not to be deteriorated (step 204), Step 
200 is repeated. If however, it is found deteriorated, a display is 
activated in step 205 indicating the deteriorated condition. 
In the above second embodiment of the present invention, the deterioration 
index I is compensated using a correction factor determined by the 
frequency f of the output signal S3 of the upstream O.sub.2 sensor 3 and 
the intake air quantity Qa at the time the deterioration of the catalytic 
converter 2 is diagnosed. The compensated deterioration index I is then 
used to decide whether or not the catalytic converter 2 has deteriorated. 
It is therefore possible to provide a catalytic diagnostic apparatus 
capable of detecting deterioration of the catalytic converter 2 accurately 
for an internal combustion engine, without distortion by the fluctuations 
of the frequency of output signal S3 of the upstream O.sub.2 sensor 3 and 
the intake air quantity Qa. 
FIG. 14 is a schematic block diagram of a third embodiment of the present 
invention, which combines the features of the first and second 
embodiments. In this embodiment, the .deterioration diagnostic unit 11B 
has a catalytic diagnostic area decision unit 22, a correction factor 
output unit 30, a downstream air-fuel ratio control unit 28 and the 
deterioration decision unit 29. 
FIG. 15 is an operational flowchart for the deterioration diagnostic unit 
11B. At Step 300 of FIG. 15, a decision is made by the catalytic 
diagnostic area decision unit 22 whether the vehicle is operating in the 
diagnostic area Da; and if it is, a decision is made by the output 
decision unit 19 on whether the output signal S4 of the downstream O.sub.2 
sensor 4 is proximate to the stoichiometric line. (This decision is made 
by comparing the output time of the rich signal of the signal S4 with that 
of the lean signal thereof.) If the signal S4 is not near the 
stoichiometric line, the signal S4 is supplied to the lean-rich decision 
unit 20 which decides whether the signal S4 is on the lean side or rich 
side (step 302). If it is on the lean side, an adjustment is made toward 
the rich side in step 303B, and if it is rich an adjustment is made to the 
lean side in step 303A, as described previously. 
If it is on the rich side, in Step 303A the air-fuel ratio shift 
coefficient calculating unit 21 calculates a coefficient to shift the 
air-fuel ratio AF to lean side (or vice versa in step 303B), and the 
calculated result is supplied to the air-fuel ratio feedback computer 8, 
which corrects the correction coefficient .alpha. according to the 
coefficient supplied from the air-fuel ratio shift coefficient calculating 
unit 21 and supplies the correction signal .alpha. thus corrected to the 
fuel injection quantity computer 9. The air-fuel ratio is thus controlled 
as described above. 
At Step 304A, the correlation function calculating unit 18 calculates the 
autocorrelation function X and the cross correlation function Y, to 
determine the periodic instantaneous deterioration index .PHI.i, and at 
step 304B, the reader unit 24 reads a correction factor from the look up 
table 25 according to the output signals from the calculating units 27 and 
23 respectively, and supplies the correction factor to the calculating 
unit 16B. The calculating unit 16B calculates the final deterioration 
index I as described previously (step 305) and adds the correction factor 
from the correction factor reader unit 24. The comparator 16E compares the 
resulting deterioration index I with the reference value QID (step 306) to 
determine whether or not the catalytic converter 2 has deteriorated. If 
the catalytic converter 2 is found not to be deteriorated, a determination 
is made in step 309 whether the air-fuel ratio has been inverted. If it 
has, air-fuel adjustment is suspended at step 310; and if it has not, 
steps 302-306 are repeated. If on the other hand, the catalytic converter 
2 is determined to be deteriorated in step 306, the deteriorated condition 
is displayed by a display unit (not shown). 
According to the third embodiment of the present invention, a decision is 
made on whether the output signal S4 of the downstream O.sub.2 sensor 4 is 
in proximity to the stoichiometric line at the time the deterioration of 
the catalytic converter 2 is diagnosed; if it is not, the air-fuel ratio 
is adjusted to shift it close thereto. Then a correlation function of the 
signal S4 close to the stoichiometric line and the signal S3 is 
calculated. The deterioration index I is compensated by a correction 
factor determined by the frequency f of the output signal S3 of the 
upstream O.sub.2 sensor 3 and the intake air quantity Qa; the compensated 
value is then used to determine whether the catalytic converter 2 has 
deteriorated. It is therefore possible to provide a catalytic diagnostic 
apparatus capable of detecting deterioration of the catalytic converter 2 
accurately for an internal combustion engine, without distortion due to 
fluctuations in volume of exhaust gases, in the output signal S3 of the 
upstream O.sub.2 sensor 3, in the intake air quantity Qa and the like. 
Although the method and apparatus according to the invention are intended 
primarily to determine only whether the catalytic converter 2 has 
deteriorated, in the embodiments shown the upstream O.sub.2 sensor 3 may 
be arranged so that it is also possible to determine when it is 
deteriorated. That is, the response speed of the upstream O.sub.2 sensor 3 
declines as its deteriorates and the autocorrelation function X decreases. 
The value of X in a case where the upstream O.sub.2 sensor 3 is 
deteriorated is therefore determined and stored as a deterioration 
reference value XO. It is thus possible to detect deterioration of the 
upstream O.sub.2 sensor 3 by comparing the value of X calculated by an 
autocorrelation function calculating unit 13 with the reference value XO. 
FIG. 16 is a block diagram of an embodiment of the invention in which an 
upstream O.sub.2 sensor deterioration decision unit 15 has been added to 
determine whether the upstream O.sub.2 sensor 3 has deteriorated. In this 
embodiment, the value of X calculated by the autocorrelation function 
calculating unit 13 is supplied to the upstream O.sub.2 sensor 
deterioration decision unit 15 as well as to the periodic instantaneous 
deterioration index .PHI.i calculating unit 16A. The decision unit 15 
decides whether the calculated value of X is greater than or equal to 
reference value XO. If it is less than the reference value XO, the 
upstream O.sub.2 sensor is determined to be deteriorated and a signal to 
that effect is supplied to the decision output unit 17 and displayed by a 
display means (not shown). The decision output unit 17 then outputs a 
signal indicating whether the catalytic converter 2 or the upstream 
O.sub.2 sensor 3 has deteriorated. 
The example shown in FIG. 16 is applicable to any one of the first, second 
and third embodiments, and is capable of determining the deterioration of 
not only the catalytic converter 2 accurately, but also the upstream 
O.sub.2 sensor 3, without the addition of further apparatus. 
Although a description has been given of detecting deterioration of the 
upstream O.sub.2 sensor 3 (which is prone to deterioration), a decision 
may also be made on deterioration of the downstream O.sub.2 sensor 4 as 
well. Although the catalytic diagnostic area decision unit 22 has been 
configured to estimate the catalytic temperature from the engine speed Ne 
and the intake air quantity Qa in the embodiments above, it may also be 
acceptable to detect the temperature of the catalytic converter 2 by 
fitting a temperature sensor to it. Moreover, the temperature of the 
catalytic converter 2 may also be estimated from the integrated value of 
fuel injection quantity and engine starting time. 
Although the catalytic temperature and engine speed are used to judge 
whether the vehicle is operating in the catalytic diagnostic area in the 
embodiments above, vehicle speed, engine speed, cooling water 
temperatures, engine loads and the like may also be used to for this 
purpose. 
In the first, second and third embodiments, deterioration of catalyst is 
determined according to the correlation function of the respective output 
signals S3, S4 of the upstream and downstream O.sub.2 sensors by 
controlling the output signal S4 of the downstream O.sub.2 sensor 4 to 
move it into proximity with the stoichiometric line. Deterioration of the 
catalytic converter 2 may also be decided according to the inversion time 
difference between the output signal of the upstream and downstream 
sensors or the difference between the output signal frequency of the 
upstream O.sub.2 sensor and that of downstream O.sub.2 sensor, by 
controlling the output signal of the downstream O.sub.2 sensor to move it 
into proximity with the stoichiometric line. Furthermore, deterioration of 
the catalytic converter 2 may also be determined based on the difference 
between the output signal amplitude of the upstream and downstream O.sub.2 
sensors or by the ratio of the former to the latter. 
Although the output signal S4 of the downstream O.sub.2 sensor 4 is 
supplied via the output decision unit 19 to the correlation function 
calculating unit 18 in the first and third embodiments above, the output 
signal S4 of the downstream O.sub.2 sensor may be supplied directly to the 
correlation function calculating unit 18. In this case, a signal 
permitting the correlation function calculating unit 18 to start 
calculation may be supplied from the output decision unit 19 to the 
correlation function calculating unit 18. 
In the second and third embodiments above, the correction factor has been 
determined by the frequency f of the output signal S3 of the upstream 
O.sub.2 sensor 3 and the intake air quantity Qa. Such a correction factor 
may also be determined, however, based on the calculated inclination of 
the waveform of the output signal S3 instead of the frequency f and the 
intake air quantity Qa, as shown in FIG. 17. In that embodiment, signal S3 
is passed through a differential filter 23A and its slope is calculated, 
in slope calculator 23B. The detected slope value is then used with the 
intake air quantity signal Q.sub.a from detector 27, to read a correction 
factor from the look up table 25. 
Finally, while oxygen sensors have been used as air-fuel ratio sensors in 
the embodiments above, any other kinds of sensors may be employed as long 
as they are used for detecting air-fuel ratios. 
Although the invention has been described and illustrated in detail, it is 
to be clearly understood that the same is by way of illustration and 
example, and is not to be taken by way of limitation. The spirit and scope 
of the present invention are to be limited only by the terms of the 
appended claims.