Method for controlling the air-fuel ratio in an internal combustion engine

A method and an apparatus for controlling the air-fuel ratio in an internal combustion engine in which the calculation of the presumed amount of fuel attached to the wall of the intake port of the engine is effected for correcting the amount of the fuel supplied to the engine in order to compensate for the variation of the air-fuel ratio of the air-fuel mixture used in the combustion of the engine.

FIELD OF THE INVENTION 
The present invention relates to a method for controlling the air-fuel 
ratio in an internal combustion engine for a motor car. 
DESCRIPTION OF THE PRIOR ART 
It is well known to correct the amount of fuel supplied to an internal 
combustion engine of a motor car detecting whether an idle switch is ON or 
OFF, or whether the rate of change of the air supplying rate or the rate 
of change of the pressure of the air in the intake manifold is over a 
predetermined value. Upon such a detection, the amount of the fuel 
supplied to the engine is increased in accordance with the temperature of 
the coolant of the engine. 
However, in such systems, the detection engine operating condition is not 
carried out to adequately correct the amount of the fuel supplied. For 
example, the temperature rise of the wall of the intake port passage 
constructed in the cylinder head (hereinafter referred to as the intake 
port) engine is not taken into consideration. As a result, the amount of 
fuel supplied to the engine is sometimes increased too much, while other 
times not enough. When the fuel increase is excessive, the exhaust gas 
becomes less desirable, while the fuel increase is not enough, the torque 
that is generated is insufficient so that the driving feeling is 
deteriorated and accordingly it is difficult to realize a desirable 
drivability of the engine. In addition, when the amount of the fuel 
supplied to the engine is corrected in relation to the length of time from 
the start of the engine, the car does not accelerate smoothly. 
SUMMARY OF THE INVENTION 
It is a main object of the present invention to provide an improved method 
for controlling the air-fuel ratio in an internal combustion engine in 
which the deterioration of the exhaust gas composition is prevented and 
desirable drivability of the engine is ensured even during the warm-up of 
the engine. 
According to the present invention, the amount of fuel supplied to the 
engine is controlled in accordance with the parameters of the engine. 
Thus, data relating to engine load and the engine warm-up condition is 
obtained. On the basis of the engine load condition and the engine warming 
condition, the presumed amount of fuel clinging to the intake port wall is 
calculated. Finally, the amount of the fuel supplied to the engine is 
corrected in accordance with the presumed amount of fuel attached to the 
wall. As a result, the variation of the air-fuel ratio of the air-fuel 
mixture used in the combustion of the engine is compensated.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Firstly, the analysis of the results of the experiments conducted by the 
inventors regarding the change of air-fuel ratio in the process of the 
operation of an internal combustion engine from the start of the engine 
will be described with reference to FIGS. 1A, 1B, 1C, 2A and 2B. FIGS. 1A, 
1B and 1C illustrate the changes of the air-fuel ratio at the inlet of the 
engine and at the outlet of the engine (FIG. 1A), the changes in 
temperature of the wall of the intake port of the engine and the coolant 
of the engine (FIG. 1B) and the change of the car speed (FIG. 1C) with 
respect to time (t). 
FIGS. 1A, 1B and 1C illustrate the changes in the case where the car has 
started running immediately after the engine has been re-started with the 
coolant temperature 40.degree. C., under which temperature the cleaning of 
the exhaust gas is usually considered to be difficult. In FIG. 1A, it is 
shown that the inlet air-fuel ratio (INLET A/F) is controlled to maintain 
the value 14.6 (stoichiometrically ratio, .lambda.=1) and the outlet 
air-fuel ratio (OUTLET A/F) varies to a great extent. Here, the inlet 
air-fuel ratio means the air-fuel ratio of air-fuel mixture controlled by 
the fuel injection system, and the outlet air-fuel ratio means the 
presumed air-fuel ratio of the combustion gas, which presumed air-fuel 
ratio is obtained by detecting a predetermined component in the exhaust 
gas. The outlet air-fuel ratio becomes large (LEAN) in the acceleration of 
the engine, and becomes small (RICH) in the deceleration of the engine. 
The values of the lean peak and the rich peak decrease with the lapse of 
time from the start of the engine. It can be seen that there exists a 
close correlation between the characteristic of FIG. 1A and the 
characteristic of FIG. 1B. In FIG. 1B, the changes of temperature 
(T.sub.w) of the wall of the intake port and the temperature (T.sub.c) of 
the coolant are shown. It can be seen that, by comparing FIG. 1C and FIG. 
1A, the greater value of the lean peak and the rich peak appears when the 
acceleration or the deceleration is carried out more quickly. In FIG. 1C, 
the crooked portion (STAGGER) of the line indicates the staggering in the 
increase of the car speed immediately after the start of the car. 
FIG. 2A illustrates the relationship between the change (.DELTA.P.sub.i) of 
the pressure (P.sub.i) in the intake manifold and the peak values of the 
air-fuel ratio. FIG. 2B illustrates the relationship between the 
temperaure (T.sub.w) of the wall of the intake port and the air-fuel 
ratio. It can be seen that the lower the temperature (T.sub.w) of the wall 
of the intake port, the greater the values of the lean peak and the rich 
peak and that the greater the value of the acceleration or the value of 
the deceleration, the greater the value of the lean peak or the rich peak. 
The reason for the characteristic illustrated in FIGS. 2A and 2B is 
supposed to be that the transmission of the fuel into the combustion 
chamber of the cylinder is delayed because a portion of the fuel injected 
from the fuel injection value attaches itself to the wall of the intake 
port. During acceleration, the amount of the fuel supplied to the cylinder 
is deficient by the amount of the fuel attached to the wall and 
accordingly the effective air-fuel ratio becomes lean, while during 
deceleration, the amount of the fuel supplied to the cylinder is excessive 
due to the additional supply of the fuel as the result of evaporation of 
the fuel attached to the wall and accordingly the effective air-fuel ratio 
becomes rich. 
An apparatus for controlling the air-fuel ratio in accordance with an 
embodiment of the present invention is illustrated in FIGS. 3, 4 and 5. A 
cylinder of the internal combustion engine 1 of a four cycle spark 
ignition type for a motor car are supplied with air for combustion through 
an air cleaner 2, an intake pipe 3, a throttle valve 31. The fuel is 
supplied from the fuel reservoir through each of fuel injection valves 51, 
52, 53, 54, 55 and 56 to each of the cylinders of the engine. After 
combustion in the cylinders, the exhaust gas is discharged through an 
exhaust manifold 61 and an exhaust pipe 62. 
An air flow sensor 73 of a potentiometer type for detecting the air flow 
rate and producing the analog signal corresponding to the detected rate of 
air flow is provided in the intake pipe 3. A wall temperature sensor 74 
such as a thermistor for detecting the temperature of the wall of the 
intake port of the cylinder head is provided. A coolant temperature sensor 
75 such as a thermistor for detecting the temperature of the coolant of 
the engine may be provided. A rotational speed sensor 71 for detecting the 
rotational speed of the crank shaft of the engine and producing a pulse 
signal having a frequency corresponding to the detected rotational speed 
is provided. An ignition coil may be used for such rotational speed sensor 
in which the ignition pulse signal produced from the primary terminal of 
the ignition coil is used for the rotational speed signal. A control 
circuit 8 receives signals from the rotational speed sensor 71, the air 
flow sensor 73, the wall temperature sensor 74 and the coolant temperature 
sensor 75, calculates the amount of the fuel injection from the received 
signals and produces the control signal for electromagnetic fuel injection 
valves 51 through 56 to control the amount of the fuel injection. 
The details of the structure of the intake port 41, the intake pipe 3 with 
the fuel injection valve 51, an intake valve 412, the coolant 413 ad the 
wall temperature sensor 74 are illustrated in FIG. 4. The fuel injected 
from the fuel injection valve 51 is diffused at the injection angle 
.theta. toward the end 411 of the port 41. A portion of the diffused fuel 
is atomized, while considerable portion of the diffused fuel attaches 
itself to the surface of the intake valve 412 and the wall of the port 41. 
The wall temperature sensor 74 is located adjacent to the wall 411 of the 
port 41. 
The structure of the control circuit 8 is illustrated in FIG. 5. The 
control circuit 8 comprises a central processing unit (CPU) 800, a counter 
801 receiving a signal from the rotational speed sensor 71, an 
interruption controlling portion 802 receiving a signal which is 
synchronized with rotations of the engine from the counter 801 and sending 
the interruption signal to the CPU 800 through a common bus 812 upon 
receipt of the signal from the counter 801, and a digital input port 803 
receiving a signal from a starter switch 93. The starter switch 93 may be 
composed of starter contacts in a key switch 92. 
The control circuit 8 also comprises an analog input port 804 which 
consists of an analog multiplexer and an analog to digital converter, 
converts analog signals from the air flow sensor 73, the wall temperature 
sensor 74 and the coolant temperature sensor 75 to the digital signals and 
causes the CPU 800 to read-in the converted data. The output signals of 
the counter 801, the portion 802, the port 803 and the port 804 are 
transmitted to the CPU 800 through the common bus 812. A power source 
circuit 805 supplies power to a random access memory (RAM) 807. The power 
source circuit 805 is connected directly to a battery 91, so that the RAM 
807 is supplied always with power from the battery 91, regardless of the 
key switch 92. A power source circuit 806 which is connected to the 
battery via the key switch 92 supplies power to portions of the control 
circuit 8 except for the RAM 807. The RAM 807 is a non-volatile memory to 
which power is always supplied from the battery 91 through the power 
source circuit 805, and the content of the RAM 807 does not disappear when 
the engine is stopped due to the switching off of the key switch 92. The 
memory 808 is a read only memory (ROM) in which information regarding the 
program, various constants, the maps shown in FIGS. 10 and 11 which will 
be explained later, and the like are stored. A counter 809, for 
controlling the time for the fuel injection and including registers, 
consists of the counter of the count-down type. The counter 809 converts a 
digital signal representing the valve open time of the electromagnetic 
fuel injection valves 51 through 56, i.e. the amount of the fuel 
injection, into a pulse signal determining the actual valve open time of 
the electromagnetic fuel injection valves 51 through 56. The power 
amplifier 810 produces the signal for driving the electromagnetic fuel 
injection valves 51 through 56. A timer circuit 811 measures the elapsed 
time, and the measured elapsed time is transmitted to the CPU 800. 
The counter 801 for counting the number of rotations of the engine using 
the rotational speed sensor 71 supplies an interruption instruction signal 
to the interruption control circuit 802 when the counting of the counter 
801 is terminated. Receiving the interruption instruction signal, the 
interruption control circuit 802 produces an interruption signal which 
causes the interruption process routine to start, in which process of the 
calculation of the amount of fuel injection is carried out. 
An example of the operation of the CPU 800 in the control circuit 8 of FIG. 
5 is illustrated in the flow chart of FIG. 6. Due to the interruption 
signal from the interruption control circuit 802, the number or speed Ne 
of rotation of the engine is read-in from the counter 801 in the step 
S101. The air flow rate Q.sub.a is read-in from the analog input port 804 
in the step S102. The base amount of the fuel injection, i.e. the base 
pulse width W.sub.o for the electromagnetic fuel injection, is calculated 
from the engine speed Ne and the air flow rate Q.sub.a in the step S103 
using equation (1) below. 
EQU W.sub.o =f.multidot.Q.sub.a /N.sub.e (1) 
where "f" is a constant. 
The temperature T.sub.w of the wall 411 of the intake port is read-in from 
the analog input port 804 in the step S104. The detection of the load 
condition of the engine is carried out in steps S105 and S106 using 
equation (2) of the damped function and equation (3), below. 
EQU W.sub.n =1/32(W.sub.n-1 .times.31+W.sub.o) (2) 
EQU .DELTA.W=W.sub.o -W.sub.n (3) 
Equation (2) represents the process of damping the change of the width of 
the pulse for the fuel injection. W.sub.n is the value of the damped 
function for the present rotation period of the engine, while W.sub.n-1 is 
the value of the damped function for the preceding rotational period of 
the engine. 
The determination whether .DELTA.W is negative, zero or positive is 
executed in step S107. 
When .DELTA.W&lt;0, the signal for increasing the amount of the fuel injection 
is cleared and the signal for decreasing the amount of fuel injection is 
produced, and the value D for correcting the decrease of the fuel 
injection is calculated in accordance with the value .DELTA.W and the 
temperature T.sub.w using equation (4) below, in step S108. 
EQU D=.beta..times.d (4) 
where .beta. is the factor of the decrease of the fuel injection, and d is 
the base amount of the decrease of the fuel injection. 
When .DELTA.W=0, both the signals for increasing and decreasing the amount 
of fuel injection are cleared and the values D and E for correcting the 
decrease and the increase of the fuel injection are rendered zero, in step 
S109. 
When .DELTA.W&gt;0, the signal for decreasing the amount of the fuel injection 
is cleared and the signal for increasing the amount of fuel injection is 
produced, and the value E for correcting the increase of the fuel 
injection is calculated in accordance with the value .DELTA.W and the 
temperature T.sub.w using equation (5) below, in step S110. 
EQU E=.alpha..times.e 
Where .alpha. is the factor for the increase of the fuel injection, and e 
is the base amount of the increase of the fuel injection. 
The correction of the value W.sub.o of the width of the base fuel injection 
pulse is effected and the working width of the fuel injection pulse is 
obtained in step S111. The obtained working width of the fuel injection 
pulse is fixed in the counter 809 in step S112. Thus the procedure of the 
routine of FIG. 6 is completed. 
The basic characteristics of the operation of the apparatus of FIGS. 3, 4 
and 5 will now be described with reference to FIGS. 8, 9, 10 and 11. 
FIG. 8 illustrates the relationship between the accumulated number 
.SIGMA.N.sub.e of rotations of the engine and the car speed and the width 
(W.sub.o, W.sub.n) of the fuel injection pulse. W.sub.n represents the 
damped function which is obtained by damping the change of the value 
W.sub.o of the width of the pulse for the fuel injection by means of the 
filtering process. In each of the regions of .SIGMA.N.sub.e, the change of 
the value W.sub.n converges to the corresponding value W.sub.o. The 
hatched portion H.sub.1 represents the value to be corrected for the 
increase of the fuel injection during a period of acceleration where the 
supply of fuel is deficient. The hatched portion H.sub.2 represents the 
value to be corrected for the decrease of the fuel injection during a 
period of constant speed where the supply of fuel is excessive. The 
hatched portion H.sub.3 represents the value to be corrected for the 
decrease of the fuel injection during a period of deceleration where the 
supply of fuel is excessive. 
FIG. 9 illustrates the relationship between the accumulated number 
.SIGMA.N.sub.e of rotations of the engine and the car speed (I), the 
pulses (II) for the fuel injection having the width W.sub.o of the base 
fuel injection, the value W.sub.n (III) of the damped function obtained by 
damping the value W.sub.o through the digital filtering process and the 
value .DELTA.W (IV) which corresponds to the presumed amount of fuel 
attached to the wall of the intake port. 
The value W.sub.o is represented by the above mentioned equation (1). The 
value W.sub.n is represented by the above mentioned equation (2). The 
value .DELTA.W is represented by the above mentioned equation (3). 
FIG. 10 is a map defining the relationship between .DELTA.W and the load 
controlling factor (.alpha., .beta.). ".alpha." is the load controlling 
factor for an increase of fuel, while ".beta." is the load controlling 
factor for a decrease of fuel. 
FIG. 11 is a map defining the relationship between the temperature T.sub.w 
(.degree.C.) of the wall of the intake port and the base correction factor 
(d, e) of the amount of the fuel injection. "d" is the base correction 
factor (%) for a decrease of fuel, while "e" is the base correction factor 
(%) for an increase in the fuel injection. 
The maps of FIGS. 10 and 11 are stored in the ROM 808 of the control 
circuit 8 of FIG. 5. As described with reference to the steps S108 and 
S110 in the flow chart of FIG. 6, the value D for correcting the decrease 
in the fuel injection and the value E for correcting the increase in the 
fuel injection are calculated in accordance with equations (4) and (5), 
respectively. 
Another example of the operation of the CPU 800 in the control circuit 8 of 
FIG. 5 is illustrated in the flow chart of FIG. 7. The process from the 
step S201 through the step S203 is the same as that from step S101 to step 
S103 in FIG. 6. The temperature T.sub.c of the coolant is read-in from the 
analog input port 804 in the step S204. 
The value .DELTA.T.sub.n, which is the presumed value of the difference 
between the temperature T.sub.c of the coolant and the temperature T.sub.w 
of the wall of the intake port, is calculated by using the values K.sub.1 
and K.sub.2 in step S205. The value K.sub.1 is a constant determined by 
the temperature of the coolant at the start of the engine. The value 
K.sub.1 is read out from the map of FIG. 13 which is stored in the ROM 
808. The value K.sub.2 is a constant inherent in the present engine. The 
calculation is expressed in equation (6) below. 
EQU .DELTA.T.sub.n =1/K.sub.1 {(K.sub.1 -1).DELTA.T.sub.n-1 +K.sub.2 }(6) 
Equation (6) represents the process of damping the change of the difference 
between the temperatures T.sub.w and T.sub.c. .DELTA.T.sub.n is the value 
of the damped function for the present rotational period of the engine, 
while .DELTA.T.sub.n-1 is the value of the damped function for the 
preceding rotational period of the engine. .DELTA.T.sub.o is equal to 
zero. 
The temperature T.sub.w of the wall of the intake port is calculated in 
step S206 in accordance with equation (7) below. 
EQU T.sub.w =T.sub.c +.DELTA.T.sub.n (7) 
The obtained value T.sub.w is used in the following step as in the case of 
the flow chart of FIG. 6 where the temperature T.sub.w is obtained through 
measurement by the sensor 74. Accordingly the procedure from step S207 
through step S214 are the same as that from step S105 through S112 in the 
flow chart of FIG. 6. 
The basic characteristics of the operation expressed in the flow chart of 
FIG. 7 are illustrated in FIGS. 12 and 13. The changes of the temperature 
T.sub.c of the coolant and the temperature T.sub.w of the wall of the 
intake port and the changes of the difference between T.sub.c and T.sub.w 
with respect to the accumulated number .SIGMA.N.sub.e of the rotation of 
the engine are illustrated in FIG. 12. Since it is found that there exists 
a close correlation between the temperature difference .DELTA.T.sub.n 
(=T.sub.w -T.sub.c) and the accumulated number .SIGMA.N.sub.e of the 
rotation of the engine, the calculation of the presumed value of the 
temperature T.sub.w of the wall can be carried out in accordance with 
equation (6), above. Although the accumulated number .SIGMA.N.sub.e of 
rotation of the engine is assigned to the abscissa of FIG. 12, it is also 
possible to use the accumulated width .SIGMA.W of the fuel injection pulse 
or the elapsed time "t" from the start of the engine for the value 
assigned to the abscissa. The relationship between the temperature T.sub.c 
of the coolant and the constant K.sub.1 is illustrated in FIG. 13. 
Although the specific embodiments of the present invention are described 
hereinbefore, it is also possible to provide variously modified 
embodiments of the present invention. 
For example, although in the specific embodiment the degree of digital 
filtering in the formation of the damped function W.sub.n corresponding to 
the width W.sub.o of the fuel injection pulse is maintained to be 
constant, it is also possible in modified embodiments to vary the degree 
of digital filtering. The degree of digital filtering is expressed by the 
value L in equation (8) below. 
EQU W.sub.n =1/L{(L-1).multidot.W.sub.n-1 +W.sub.o } (8) 
In the modified embodiments, the value of L may be selected, for example, 
from 8, 16, 32 and 64. In the modified embodiments, the value of L may be 
varied in accordance with the operation condition of the engine, for 
example, temperature of the coolant, the rotational speed of the engine, 
degree of vacuum in the intake manifold, air flow rate, presence/absence 
of the air-fuel ratio feedback control, and the like. The change of the 
damped function W.sub.n in accordance with the selection of the value L is 
illustrated in FIG. 14. W.sub.n and .DELTA.W in the case where L=32 is 
illustrated in line M.sub.32, while W.sub.n and .DELTA.W in the case where 
L=16 is illustrated in line M.sub.16. 
Also, the degree of digital filtering may be varied by adjusting the 
frequency of the calculation, for example, by changing from the 
calculation per each rotation of the engine to the calculation per every 
two rotations of the engine. 
Although in the above described embodiments digital filtering is used in 
the formation of the damped function W.sub.n, it is also possible to use 
analog filtering. 
Although in the above described embodiments the corrections for both the 
acceleration and the deceleration are effected, it is also possible to 
effect the correction only for the acceleration or only for the 
deceleration. 
In another modified embodiment of the present invention, it is possible to 
effect additionally the increase/decrease control of the amount of the 
fuel injection in accordance with the temperature of the coolant of the 
engine in the steady running state of the engine, not only in the 
acceleration or deceleration state of the engine. 
In another modified embodiment of the present invention, it is possible to 
determine the state of the engine load from the changes in the pressure 
P.sub.i in the intake manifold, the rotational speed Ne of the engine, the 
rate Q.sub.a of the amount of the air intake, and the like, instead of the 
determination from the change in the width W.sub.o of the base fuel 
injection pulse corresponding to the rotational speed N.sub.e of the 
engine and the rate Q.sub.a of the amount of the air intake. 
In another modified embodiment of the present invention, it is possible to 
determine the change in the engine load by detecting the difference 
between the preceding value and the present value, instead of the digital 
filtering method. 
Also, in the analog method used in another embodiment of the present 
invention, it is possible to use a damping circuit using a capacitor for 
obtaining the difference between the damped value and the ordinary value 
and determining the value of the correction.