Method and apparatus for controlling the air-fuel ratio of an internal combustion engine

Air-fuel ratio learning is only carried out at the time of high exhaust temperatures, and is inhibited when the exhaust temperature is less than or equal to a predetermined temperature. In the latter case, air-fuel ratio feedback correction coefficient .alpha. is correctingly set, taking a correction level indicated by an air-fuel ratio learned correction coefficient K learned at the time of high exhaust temperatures as a true correction requirement level.

FIELD OF THE INVENTION 
The present invention relates to a method and apparatus for controlling the 
air-fuel ratio of an internal combustion engine, and more particularly to 
technology for maintaining air-fuel ratio control accuracy, by dealing 
with changes in oxygen concentration detection characteristics due to 
exhaust temperature. 
DESCRIPTION OF THE RELATED ART 
There is known a conventional type of air-fuel ratio control apparatus 
which judges the richness/leanness of the actual air-fuel ratio with 
respect to a target air-fuel ratio (stoichiometric air-fuel ratio), based 
on oxygen concentration in the exhaust detected by an oxygen sensor, and 
feedback controls a fuel supply amount to the engine based on the 
judgement result, so that the actual air-fuel ratio approaches the 
stoichiometric air-fuel ratio (target air-fuel ratio) (refer to Japanese 
Unexamined Patent Publication No. 60-240840). 
With this apparatus, the output characteristics of the detection signal 
produced by the oxygen sensor, change due to the sensor element 
temperature influenced by the exhaust temperature, so that even with the 
element active, if due to low exhaust temperatures the element temperature 
becomes relatively low, there is the possibility of for example an 
increase in lean output, with consequent variation of the control point 
for the air-fuel ratio feedback control to the lean side. 
Therefore, under low exhaust temperature conditions such as immediately 
after starting, or with low ambient temperatures and low load operation, 
there is the likelihood of a deterioration in engine operability and 
exhaust conditions, due to reduced accuracy in controlling to the target 
air-fuel ratio. 
SUMMARY OF THE INVENTION 
The present invention takes into consideration the above situation, with 
the object of controlling the air-fuel ratio stably and precisely without 
influence from the exhaust temperature. 
To achieve the above objective with the method and apparatus according to 
the present invention for controlling the air-fuel ratio of an internal 
combustion engine, an air-fuel ratio feedback correction value for 
correcting a fuel supply quantity of a fuel supply device, is set in a 
direction so that the air-fuel ratio of the engine intake mixture 
approaches a target air-fuel ratio, based on the oxygen concentration in 
the engine exhaust gas, while a correction requirement indicated by the 
air-fuel ratio feedback correction value is learned as an air-fuel ratio 
learned correction value for different operating conditions. Here, when 
the exhaust temperature is less than or equal to a predetermined 
temperature, learning of the air-fuel ratio learned correction value is 
inhibited, and the air-fuel ratio feedback correction value is 
correctingly set to be approximately equal to a correction level for a 
fuel supply quantity due only to an air-fuel ratio learned correction 
value for the relevant operating conditions. 
With such a construction, at the time of a low exhaust temperature with the 
likelihood of a change in oxygen concentration detection characteristics, 
air-fuel ratio learning is inhibited to avoid erroneous learning. On the 
other side, the air-fuel ratio feedback correction value is correctingly 
set using the learned result at the time of a high exhaust temperature as 
an appropriate correction level for the relevant operating conditions, so 
that erroneous control due to the beforementioned change in detection 
characteristics is prevented. 
With the method and apparatus according to the present invention for 
controlling the air-fuel ratio of an internal combustion engine, the 
air-fuel ratio learned correction value is learned for each of a plurality 
of operating conditions divided by engine rotational speed and engine 
load. 
With such a construction, the air-fuel ratio learned correction value can 
be learned in accordance with different correction requirements for engine 
rotational speed and engine load. 
Moreover, with the method and apparatus according to the present invention 
for controlling the air-fuel ratio of an internal combustion engine, the 
air-fuel ratio feedback correction value and the air-fuel ratio learned 
correction value are correction terms respectively multiplied by the basic 
fuel supply quantity, so that when the exhaust temperature is less than or 
equal to a predetermined temperature, the air-fuel ratio feedback 
correction value is correctingly set with the deviation of the air-fuel 
ratio learned correction value, and the multiplied result of the air-fuel 
ratio feedback correction value and air-fuel ratio learned correction 
value, as an additive correction value. 
With such a construction, even if air-fuel ratio feedback correction is 
carried out while using the air-fuel ratio learned correction value 
learned at the time of high exhaust temperature, the air-fuel ratio is 
corrected at an appropriate level approximately equivalent to this 
air-fuel ratio learned correction value. 
Furthermore, with the method and apparatus according to the present 
invention for controlling the air-fuel ratio of an internal combustion 
engine, the exhaust temperature is indirectly detected on the basis of at 
least one of; cooling water temperature, ambient temperature, engine load, 
and elapsed time from start. 
With such a construction, the exhaust temperature can be indirectly 
detected using a previously installed sensor, thus obviating the need to 
newly install a sensor for directly detecting the exhaust temperature. 
Moreover, with the method and apparatus according to the present invention 
for controlling the air-fuel ratio of an internal combustion engine, the 
beforementioned predetermined temperature may be approximately 400 degrees 
C. 
With such a construction, when the exhaust temperature less than or equal 
to approximately 400 degrees C., with the likelihood of error in the 
control point for the air-fuel ratio feedback control due to the change in 
the oxygen concentration detection characteristics, learning can be 
inhibited, and the air-fuel ratio feedback correction value can be 
correctingly set based on the high temperature learned results. 
Other objects and aspects of the present invention will become apparent 
from the following description of embodiment given in conjunction with the 
appended drawings.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
As follows is a description of embodiment of the present invention. 
With the embodiment shown in FIG. 2, an internal combustion engine 1 draws 
in air from an air cleaner 2 by way of an intake duct 3, a throttle valve 
4, and an intake manifold 5. Fuel injection valves 6 are provided as fuel 
supply devices (see FIG. 1) for each cylinder, in respective branch 
portions of the intake manifold 5. 
The fuel injection valves 6 are electromagnetic type fuel injection valves 
which open with power to a solenoid and close with power shut-off. The 
injection valves 6 are driven open in response to an injection pulse 
signal provided by a control unit 12 (to be described later) so that fuel 
pressurized by a fuel pump (not shown), and controlled to a predetermined 
pressure by means of a pressure regulator, is injected intermittently to 
the engine 1. 
Ignition plugs 7 are provided for each combustion chamber of the engine 1, 
for spark ignition of a mixture therein. Exhaust from the engine 1 is 
discharged by way of an exhaust manifold 8, an exhaust duct 9, a three-way 
catalytic converter 10 and a muffler 11. 
The control unit 12 incorporates a microcomputer having for example a CPU, 
ROM, RAM, A/D converter and input/output interface. Input signals from the 
various sensors are received by the control unit 12, and computational 
processing carried out (as described later) to thereby control the 
operation of the fuel injection valves 6. 
For the various sensors there is provided in the intake duct 3, an airflow 
meter 13, which outputs a signal corresponding to an intake air quantity Q 
of the engine 1. 
Also provided is a crank angle sensor 14 which outputs a reference crank 
angle signal REF for each reference piston position, and a unit crank 
angle signal POS for each 1.degree. or 2.degree. crank angle. The period 
of the reference crank angle signals REF or the number of unit crank angle 
signals POS within a predetermined period, is measured to compute the 
engine rotational speed Ne. 
Moreover, a water temperature sensor 15 is provided for detecting the 
cooling water temperature Tw in the water jacket of the engine 1. 
There is also an oxygen sensor oxygen sensor 16, provided as an oxygen 
concentration detection device (see FIG. 1), at a junction portion of the 
exhaust manifold 8. 
The oxygen sensor 16 is a known zirconium oxide tube type oxygen 
concentration cell which generates an electromotive force corresponding to 
a ratio of the oxygen concentration in the exhaust to that in the 
atmosphere (reference oxygen concentration). The oxygen sensor 16 is one 
which detects only the stoichiometric air-fuel ratio (rich or lean with 
respect to a target air-fuel ratio) utilizing the fact that the 
concentration of oxygen in the exhaust gas drastically changes around the 
stoichiometric air-fuel ratio (the target air-fuel ratio in the present 
embodiment). With the present embodiment, the oxygen sensor 16 is provided 
with a heater to keep it in an active condition, even under low exhaust 
temperature conditions such as immediately after starting. 
Moreover, an exhaust temperature sensor 17 is provided in the exhaust 
system, as an exhaust temperature detection device (see FIG. 1) for 
detecting the temperature of the engine exhaust. 
The CPU of the microcomputer in the control unit 12 computes the fuel 
injection quantity (fuel injection pulse width) Ti for the fuel injection 
valves as; 
EQU Ti.rarw.Tp.times.CO.times..alpha.K+Ts 
Here Tp is the basic fuel injection quantity (basic fuel injection pulse 
width) computed based on the intake air quantity Q and the engine 
rotational speed Ne, while CO is the respective correction coefficients 
for correcting the basic fuel injection quantity Tp, corresponding to 
engine operating conditions such as cooling water temperature, and 
transient operation. Moreover .alpha.(originally equal to 1.0) is the 
air-fuel ratio feedback correction coefficient (air-fuel ratio feedback 
correction value) for correcting the basic fuel injection quantity Tp in a 
direction so that the air-fuel ratio detected by the oxygen sensor 16 
approaches the stoichiometric air-fuel ratio. This may be set for example, 
by proportional-plus-integral control. 
Furthermore, K is an air-fuel ratio learned correction coefficient 
(air-fuel ratio learned correction value), which is stored, in rewritable 
form, for each of a plurality of operating conditions divided by basic 
fuel injection quantity Tp and engine rotational speed Ne. A correction 
level indicated by the air-fuel ratio feedback correction coefficient 
.alpha. is learned for each of the operating conditions and the stored 
data rewritten. More specifically, correction requirements indicated by 
the air-fuel ratio feedback correction coefficient oc, are learned and 
stored as air-fuel ratio learned correction coefficients K for each of the 
operating regions, so that the air-fuel ratio obtained by correction using 
the air-fuel ratio learned correction coefficient K, is stabilized in the 
vicinity of the stoichiometric air-fuel ratio, without correction by the 
air-fuel ratio feedback correction coefficient .alpha.. 
Moreover, Ts is a voltage correction amount for correcting a change in the 
ineffective injection period of the fuel injection valve 6 due to a change 
in battery voltage. 
Incidentally, even with the oxygen sensor 16 heated by a heater, there will 
still be a change in the output characteristics of the oxygen sensor 16 
with a drop in element temperature (see FIG. 5), under low exhaust 
temperature conditions such as immediately after starting, or with low 
ambient temperatures, or with low load operation. Moreover, the resultant 
change in output characteristics will influence the air-fuel ratio 
feedback control which uses the oxygen sensor 16, causing a variation of 
the control point from the target air-fuel ratio (see FIG. 4). 
With the present embodiment, the control unit 12 avoids deterioration in 
air-fuel ratio control accuracy occurring with low exhaust temperature 
conditions, by control as illustrated by the flow chart in FIG. 3. 
In this respect, the functions of the air-fuel ratio feedback correction 
value setting device, the air-fuel ratio learning device, the low exhaust 
temperature correction device, and the low exhaust temperature learning 
inhibit device (see FIG. 1) are realized by software illustrated by the 
flow chart of FIG. 3 and stored in the control unit 12. 
In the flow chart of FIG. 3, initially in step 1 (with "step" denoted by S 
in the figures), it is judged if the heater provided for the oxygen sensor 
16 is faulty. More specifically, a diagnosis is made of the heater power 
circuit for disconnections or short circuit, and if the heater is 
operating normally control proceeds to step 2. 
In step 2, the oxygen sensor 16 is checked for faults by judging its 
output. When the output is normal, control proceeds to step 3. 
In step 1 or step 2, if a heater fault or oxygen sensor 16 fault is 
determined, control proceeds to step 4 where the air-fuel ratio feedback 
control using the oxygen sensor 16 is inhibited, giving an open control 
condition. 
In step 3, it is judged if the exhaust temperature detected by the exhaust 
temperature sensor 17 is less than or equal to a predetermined temperature 
(for example 400.degree. C.). 
The predetermined temperature is the minimum temperature at which the 
expected output characteristics of the oxygen sensor 16 can be obtained. 
Therefore, when the exhaust temperature rises above this predetermined 
temperature, the actual air-fuel ratio can be controlled to the target 
air-fuel ratio (the stoichiometric air-fuel ratio) by setting the air-fuel 
ratio feedback correction coefficient .alpha. based on the output of the 
oxygen sensor 16. Accordingly, when judged in step 3 that the exhaust 
temperature exceeds the predetermined temperature, control proceeds to 
step 5 where, in the predetermined feedback control regions, the air-fuel 
ratio feedback correction coefficient .alpha. is set based on the output 
of the oxygen sensor 16, and normal air-fuel ratio control is carried out 
with the correction level indicated by the air-fuel ratio feedback 
correction coefficient .alpha. being learned as the air-fuel ratio learned 
correction coefficient K. 
On the other hand, when judged in step 3 that the exhaust temperature is 
less than or equal to the predetermined temperature, this is the condition 
wherein the oxygen sensor 16 will not realize its expected output 
characteristics due to low exhaust temperature. Hence, if air-fuel ratio 
feedback control is carried out as usual, there is the possibility of 
deterioration in operability and exhaust performance, due to control point 
variation from the target air-fuel ratio (see FIG. 4). 
Therefore in step 3, when judged that the exhaust temperature is less than 
or equal to the predetermined temperature, control proceeds instead to 
step 6 and the subsequent steps, and not to step 5, and control is carried 
to deal with changes in the output characteristics of the oxygen sensor 
16. 
In step 6 it is judged if the predetermined operating region for carrying 
out air-fuel ratio feedback control exists. If not, control proceeds to 
step 4 to give an open control condition wherein setting of the air-fuel 
ratio feedback correction coefficient .alpha. is not carried out (ie. the 
correction coefficient .alpha. is clamped). 
When judged in step 6 that the air-fuel ratio feedback control region 
exists, control proceeds to step 7, where the air-fuel ratio feedback 
correction coefficient .alpha. is set based on the output of the oxygen 
sensor 16. 
Here, if conditions were normal, learning and updating of the air-fuel 
ratio learned correction coefficient K would be carried out based on the 
air-fuel ratio feedback correction coefficient .alpha. set in step 7. 
However, since it was predicted in step 3 that due to the low exhaust 
temperature, there will be a change in the output characteristics of the 
oxygen sensor 16, then in the next step 8, the learning and updating of 
the air-fuel ratio learned correction coefficient K is inhibited, and 
air-fuel ratio learning correction is carried out using the air-fuel ratio 
learned correction coefficient K learned for the high temperature 
conditions without updating. 
That is to say, when the output characteristics of the oxygen sensor 16 are 
changed due to the low exhaust temperature (see FIG. 5), if the air-fuel 
ratio learned correction coefficient K is learned and updated based on the 
air-fuel ratio feedback correction coefficient .alpha. at that time, then 
due to variation of the air-fuel ratio feedback control point from the 
target air-fuel ratio (see FIG. 4), learning will be made with this 
control point variation from the target air-fuel ratio. As a result, when 
the exhaust temperature rises, the air-fuel ratio learned correction will 
be carried out based on the erroneously learned result, with deterioration 
in the air-fuel ratio control accuracy. Therefore, when a low exhaust 
temperature condition wherein the change in output characteristics of the 
oxygen sensor 16 is predicted, the learning and updating of the air-fuel 
ratio learned correction coefficient K is inhibited to prevent erroneous 
learning. 
In the next step 9, the average value of the air-fuel ratio feedback 
correction coefficient .alpha.(the average of the maximum and minimum 
values ) is computed, and in step 10, the air-fuel ratio learned 
correction coefficient K corresponding to the current basic fuel injection 
quantity Tp and engine rotational speed Ne is read from a map. Then, since 
as mentioned before, air-fuel ratio learning is inhibited at the time of 
low exhaust temperatures, the read air-fuel ratio learned correction 
coefficient K, becomes the learned value for the high exhaust temperature 
conditions. 
With a construction wherein the air-fuel ratio feedback correction 
coefficient .alpha. is set using proportional control during air-fuel 
ratio inversion, and integral control between inversions, the average 
value in step 9 is obtained by averaging the maximum and minimum values of 
the correction coefficient .alpha. obtained for each proportional control 
for each air-fuel ratio inversion. 
Moreover, in step 11, the deviation of the air-fuel ratio learned 
correction coefficient K, and the current air-fuel ratio correction value 
(equal to the average value of the air-fuel ratio feedback correction 
coefficient .alpha. multiplied by the air-fuel ratio learned correction 
coefficient K), is set as a correction value A. Then in step 12, the 
correction value A is added to the air-fuel ratio feedback correction 
coefficient .alpha. to correctingly set the correction coefficient .alpha. 
. Now when the average value is obtained for each air-fuel ratio 
inversion, then the beforementioned correction of the correction 
coefficient .alpha. is carried out for each air-fuel ratio inversion. 
The air-fuel ratio learned correction coefficient K read in step 10, is the 
value which is learned for the required correction level to obtain the 
target air-fuel ratio occurring under current engine operating conditions 
(conditions with the same basic fuel injection quantity Tp and engine 
rotational speed Ne) although the exhaust temperature condition is 
different from that at the learning control. Here if learning continues, 
then under high temperature conditions, the target air-fuel ratio is 
obtained by changing the correction coefficient .alpha. about the original 
value of 1.0, and correction requirements are indicated by the air-fuel 
ratio learned correction coefficient K only. 
On the other hand, since the air-fuel ratio feedback correction coefficient 
.alpha. set in step 7, has its value set using the oxygen sensor 16 which 
outputs oxygen concentration detection signals with characteristics 
different from the expected output characteristics due to the low exhaust 
temperature conditions, then some variation from the control point can be 
predicted. 
Since the overall correction level indicated by the air-fuel ratio feedback 
correction coefficient .alpha. and the air-fuel ratio learned correction 
coefficient K, should be nearly constant and not dependent on the exhaust 
temperature, then the deviation of the air-fuel ratio learned correction 
coefficient K, and the average value of the air-fuel ratio feedback 
correction coefficient .alpha. multiplied by the air-fuel ratio learned 
correction coefficient K, indicates the error in the control point 
produced by the change in the output characteristics of the oxygen sensor 
16, due to the low exhaust temperature conditions. 
Accordingly the air-fuel ratio learned correction coefficient K is taken as 
showing the true correction requirement level, and the abovementioned 
deviation is added to the air-fuel ratio feedback correction coefficient 
.alpha. so that correction of an approximately equivalent level to that 
for the high exhaust temperature condition is carried out. As a result the 
variation in the air-fuel ratio feedback control point due to the change 
in the output characteristics of the oxygen sensor 16 under low exhaust 
temperature conditions is corrected. 
Consequently, even under conditions such as immediately after starting, or 
with low ambient temperatures, or low load operation and under conditions 
wherein exhaust temperatures are low and the expected output 
characteristics of the oxygen sensor 16 are not obtained, feedback control 
approaching the target air-fuel ratio is possible, so that engine 
operability and exhaust performance can be improved. 
In the above embodiment, the air-fuel ratio feedback correction coefficient 
.alpha. is proportional-plus-integral controlled. However, the invention 
is not limited to this control method, and other methods such as for 
example proportional-plus-integral-plus-differential control are also 
possible. 
Moreover, a construction is also possible wherein, as well as stopping the 
learning and updating of the air-fuel ratio learned correction coefficient 
K under low exhaust temperature conditions, the learning correction using 
the air-fuel ratio learned correction coefficient K is also stopped. In 
this case, correction setting of the correction coefficient .alpha. may be 
carried out with the deviation of the learned correction coefficient K 
learned at the time of high exhaust temperature, and the air-fuel ratio 
feedback correction coefficient .alpha. for the time of low exhaust 
temperature, as the correction value A. 
Moreover, with the present embodiment a sensor which directly detects the 
exhaust temperature is provided. However a construction is also possible 
wherein exhaust temperature is indirectly detected from information such 
as cooling water temperature, ambient temperature, engine load, and 
elapsed time from starting. Furthermore, the exhaust temperature 
conditions may be estimated from the output level of the oxygen sensor 16.