Air-fuel ratio control system for engine

An air-fuel ratio control system for an engine in which a fuel injection amount is calculated according to the engine operating conditions and is corrected according to the output of an air-fuel ratio sensor, comprises a base fuel injection amount calculating section for calculating a base fuel injection amount corresponding to the stoichiometric air-fuel ratio on the basis of the intake air amount, a target air-fuel ratio calculating section for calculating a target air-fuel ratio according to the engine operating condition, a reference value calculating section for calculating a reference value which represents the target air-fuel ratio and is submitted to comparison with the output of the air-fuel ratio sensor, a feedback correction coefficient calculating section for calculating a feedback coefficient according to the deviation of the output of the air-fuel ratio sensor from the reference value, and a final fuel injection amount calculating section which corrects the base fuel-injection amount on the basis of the ratio of the stoichiometric air-fuel ratio to the target air-fuel ratio and the feedback coefficient to obtain a final fuel injection amount.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates to an air-fuel ratio control system for an engine in 
which the fuel injection amount is subjected to feedback correction 
according to the deviation of the actual air-fuel ratio detected by an 
air-fuel ratio sensor from a target air-fuel ratio determined according to 
the engine operating condition. 
2. Description of the Prior Art 
There has been known an air-fuel ratio control system in which the air-fuel 
ratio of the air-fuel mixture actually introduced into the engine is 
detected by an air-fuel ratio sensor, the detected air-fuel ratio is 
compared with a target air-fuel ratio and the amount of fuel to be 
injected is feedback-corrected according to the deviation of the detected 
air-fuel ratio from the target air-fuel ratio in order to improve control 
accuracy of the air-fuel ratio. Further, there also has been known an 
air-fuel ratio control system in which a lean sensor which outputs a 
signal substantially in proportion to the exhaust gas oxygen concentration 
is used as the air-fuel ratio sensor and the air-fuel ratio is 
feedback-controlled even when the actual air-fuel ratio is leaner than the 
stoichiometric air-fuel ratio, thereby improving fuel economy. (See 
Japanese Unexamined Patent Publication No. 59(1984)-208141, for example.) 
In such air-fuel ratio control systems, a base fuel injection amount for a 
given engine operating condition is first determined referring to a map in 
which the fuel injection amount or the width of the fuel injection pulse 
is related to the engine speed and the engine load, and the amount of fuel 
to be actually injected is determined by correcting the base fuel 
injection amount according to various conditions. In order to compare the 
output of the air-fuel ratio sensor with a target air-fuel ratio to obtain 
a feedback signal, a map in which reference values representing the target 
air-fuel ratios are related to engine operating conditions is used for 
determining the reference value according to a given engine operating 
condition and the output value of the air-fuel ratio sensor is compared 
with the reference value, and then the base fuel injection amount is 
corrected according to the deviation of the output value of the air-fuel 
ratio sensor from the reference value. Thus, the conventional air-fuel 
ratio control systems are disadvantageous in that various control maps of 
the type described above are needed and a large memory capacity is needed 
for storing the control maps. Further new control maps must be made when a 
new engine is developed, when the engine performance is changed, when the 
control characteristics are changed or when the injector is changed. 
Further, in the case of a map in which the width of the fuel injection 
pulse is related to the engine operating condition, widths of the fuel 
injection pulses optimal for engine operating conditions must be revised, 
thereby substantially adding to the manhours required for development. 
SUMMARY OF THE INVENTION 
In view of the foregoing observations and description, the primary object 
of the present invention is to provide an air-fuel ratio control system 
which can contribute to simplification of the control system and reduction 
of development time. 
As shown in FIG. 1, the air-fuel ratio control system in accordance with 
the present invention comprises a base fuel injection amount calculating 
means 6 which determines a base fuel injection amount according to the 
amount of intake air detected by an intake air amount detecting means 9 so 
that the air-fuel ratio becomes equal to the stoichiometric value, a 
target air-fuel ratio calculating means 7 which determines a target 
air-fuel ratio according to the engine operating condition detected by an 
operating condition detecting means 10, a reference value calculating 
means 11 which determines a reference value corresponding to the target 
air-fuel ratio, a feedback coefficient calculating means 8 which compares 
an output signal Vs of an air-fuel ratio sensor 13 disposed in an exhaust 
passage 12 with an output signal Vr of the reference value calculating 
means 11 and determines a feedback coefficient according to the deviation 
of the output signal from the reference value, and a final fuel injection 
amount calculating means 5 which corrects the base fuel injection amount 
on the basis of the ratio of the stoichiometric air-fuel ratio to the 
target air-fuel ratio and the feedback coefficient to obtain a final fuel 
injection amount and controls an air-fuel ratio adjustment means 4. The 
air-fuel ratio adjustment means 4 receives the output signal of the final 
fuel injection amount calculating means 5 and at a predetermined timing 
delivers to fuel injectors 3 disposed in an intake passage 2 of an engine 
1 a fuel injection pulse having a width corresponding to the final fuel 
injection amount. 
In the air-fuel ratio control system in accordance with the present 
invention, the target air-fuel ratio determined by the target air-fuel 
ratio calculating means 7 according to the engine operating condition 
referring to a target air-fuel ratio map is used in both the final fuel 
injection amount calculating means 5 and the reference value calculating 
means 11, whereby the number of control maps can be reduced and the 
required memory capacity can be reduced. Further, since the engine 
operating condition is not directly related to the fuel injection amount 
or the width of the fuel injection pulse in any one of the control maps, 
revision of the control maps because of a specification change of the 
engine can be effected relatively easily. When the fuel injection amount 
or the width of the fuel injection pulse is directly related to the engine 
operating condition in a map, a large amount of information is packed in 
the map and accordingly, revision of the map requires significant time and 
labor. On the other hand, the relation between the engine operating 
condition and the target air-fuel ratio does not substantially depend upon 
the engine specification, the injector or the like, and accordingly the 
map used for calculating the target air-fuel ratio according to the engine 
operating condition can be relatively easily revised.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
In FIG. 2, a fuel injector 3 is disposed in an intake passage 2 
communicated with a combustion chamber 15 of an engine 1. Further, the 
intake passage 2 is provided with an air cleaner 16, an airflow sensor 17 
and a throttle valve 18. A catalytic convertor 19 is disposed in an 
exhaust passage 12 of the engine 1 and an air-fuel ratio sensor (lean 
sensor) 13 is disposed in the exhaust passage 12 upstream of the catalytic 
convertor 19. 
The air-fuel ratio control system of this embodiment controls the air-fuel 
ratio of air-fuel mixture to be introduced to the combustion chamber 15 by 
controlling the amount of fuel to be injected from the injector 3 which is 
controlled by a control signal output from a controller 20. In order to 
determine the engine operating condition, there are input into the 
controller 20 an intake air amount signal from the air flow sensor 17, a 
throttle opening signal representing the opening of the throttle valve 18 
from a throttle sensor 21, a crank angle signal generated by a distributor 
22 and an igniter 23, an intake air temperature signal from an intake air 
temperature sensor 24, a water temperature signal representing the 
temperature of the engine cooling water from a water temperature sensor 
25, and an air-fuel ratio signal from the air-fuel ratio sensor 13, and 
the controller 20 controls the amount of fuel to be injected from the 
injector 3 and the injection timing according to the engine operating 
condition. Reference numeral 26 denotes a battery. Further, the controller 
20 accomplishes the functions of the base fuel injection amount 
calculating means 6, the target air-fuel ratio calculating means 7, the 
reference value calculating means 11, the feedback coefficient calculating 
means 8, and the final fuel injection amount calculating means 5 shown in 
FIG. 1. That is, the controller 20 determines the base fuel injection 
amount (injection time) according to the amount of intake air so that the 
air-fuel ratio becomes equal to the stoichiometric value, determines the 
target air-fuel ratio according to the engine operating condition, 
determines the feedback coefficient according to the deviation of the 
output signal of the air-fuel ratio sensor 13 from the reference value 
corresponding to the stoichiometric value when the actual air fuel ratio 
is leaner than the stoichiometric value, and corrects the base fuel 
injection amount with various correction coefficients to obtain the final 
fuel injection amount. 
In FIG. 3, an intake air amount signal Tp from the airflow sensor 17 is 
first compensated for the temperature of intake air by an intake air 
temperature correction coefficient C.sub.air determined on the basis of 
the output of the intake air temperature sensor 24, and then submitted to 
calculation of a base fuel injection pulse (Tp.times.Ck). At the same 
time, a first air-fuel ratio AF1 is read from a first map M.sub.1 
according to the engine speed Ne derived from a crank angle signal and the 
compensated intake air amount signal. Further, a second air-fuel ratio AF2 
is read from a second map M.sub.2 according to a throttle opening Ta 
output from the throttle sensor 21 and the engine speed Ne. Then a target 
air-fuel ratio AF is derived from the first and second air-fuel ratios AF1 
and AF2. 
The target air-fuel ratio AF is first compensated for the temperature of 
the engine cooling water by a cooling water temperature correction 
coefficient C.sub.w determined on the basis of the output of the water 
temperature sensor 25, and then submitted to calculation of a reference 
value Vr and correction of the base fuel injection pulse. 
The output signal of the air-fuel ratio sensor 13 is amplified and compared 
with the reference value Vr by a comparator. The output signal of the 
comparator is submitted to calculation of a feedback correction 
coefficient Cfb through P.I. control. Peak values upon signal inversions 
in the P.I. control are averaged to obtain a study correction coefficient 
Sstdy. Acceleration or deceleration of the vehicle is detected by way of 
the rate of change of the intake air amount signal Tp or the rate of 
change of the throttle opening Ta, and an acceleration increase correction 
coefficient Cacc or a deceleration increase correction coefficient Cdec is 
calculated. Further, cranking of the engine is detected through the crank 
angle signal and an after-cranking increase correction coefficient Cs is 
calculated taking into account the cooling water temperature. Further, a 
recirculation reduction correction coefficient Crec is calculated. The 
base fuel injection pulse is corrected on the basis of the correction 
coefficients thus obtained, and at the same time, an ineffective injection 
time Tv depending on the battery voltage is calculated and the base fuel 
injection pulse is further corrected on the basis of the ineffective 
injection time Tv to obtain a final fuel injection pulse. The final fuel 
injection pulse thus obtained is delivered to the injector 3. The fuel 
injection timing is controlled by a separate control system. 
The operation of the controller 20 will be described in more detail with 
reference to the flow chart shown in FIG. 4. This flow chart shows only a 
main part of the routine for calculating the final fuel injection pulse. 
The controller 20 first initializes the system in step S1, and reads, in 
step S2, the outputs of the sensors described above in order to detect the 
operating condition of the engine 1. In step S3, the base fuel injection 
time To (=Tp.times.Ck) is calculated on the basis of the intake air amount 
signal Tp compensated for the intake air temperature. The base fuel 
injection time To corresponds to the fuel injection amount proportionate 
to the intake air amount for controlling the air-fuel ratio to the 
stoichiometric value (A/F=14.7), and the coefficient Ck is a matching 
coefficient for the airflow sensor 17 and the injector 3. 
Then in step S4, a base target air-fuel ratio AF is calculated. The base 
target air-fuel ratio AF is basically related to the engine speed Ne and 
the engine load (the intake pressure Pb) to be rich in a heavy load range 
and lean in intermediate and light load ranges as shown in FIG. 5. In 
accordance with the base target air-fuel ratio characteristics shown in 
FIG. 5, a slight change in the engine load across the boundary a between 
the rich range and the lean range leads to an abrupt change of the 
air-fuel ratio. In order to precisely control the air-fuel ratio without 
generating shock when the engine operating condition moves across the 
boundary a, the first and second maps M.sub.1 and M.sub.2 are used for 
calculating the base target air-fuel ratio AF. 
In the first map M.sub.1, the first air-fuel ratio AF.sub.1 is related to 
the engine speed Ne and the intake air amount signal Tp as shown in FIG. 
6, the figure in each area in FIG. 6 representing the value of the first 
air-fuel ratio AF.sub.1. In the second map M.sub.2, the second air-fuel 
ratio AF.sub.2 is related to the engine speed Ne and the throttle opening 
Ta as shown in FIG. 7, the figure in each area in FIG. 7 representing the 
value of the second air-fuel ratio AF.sub.2 (correction air-fuel ratio). 
The base target air-fuel ratio AF is obtained by subtracting the second 
air-fuel ratio AF.sub.2 from the first air-fuel ratio AF.sub.1, that is, 
AF=AF.sub.1 -AF.sub.2. 
For example, when the engine operating condition related to the engine 
speed Ne and the intake air amount is as represented by point b in FIG. 6 
and the throttle opening Ta is 60%, the first air-fuel ratio AF.sub.1 is 
determined to be 22 from the first map M.sub.1 and the second air-fuel 
ratio AF.sub.2 is determined to be 8 from the second map M.sub.2, thereby 
obtaining a base target air-fuel ratio of 14 (22-8=14). When the throttle 
opening Ta is in the range of 40 to 20%, the second air-fuel ratio AF is 
finely set to be 8 to 2 so that the base target air-fuel ratio AF is 
gradually increased into the lean range. 
In step S5, the cooling water temperature correction coefficient C.sub.w is 
calculated on the basis of the detection signal of the water temperature 
sensor 25. In step S6, the base target air-fuel ratio AF calculated in the 
step S4 is corrected on the basis of the cooling water temperature 
correction coefficient C.sub.w to obtain a corrected target air-fuel ratio 
AFD. The cooling water temperature correction coefficient C.sub.w is of a 
value not larger than 1 and decreases with lower cooling water temperature 
as shown in FIG. 8 so that the corrected target air-fuel ratio AFD is 
enriched with lower cooling water temperature. When the temperature of the 
cooling water rises substantially above 45.degree. C., the cooling water 
temperature correction coefficient C.sub.W is approximated to 1 and the 
corrected target air-fuel ratio AFD becomes substantially equal to the 
base target air-fuel ratio AF. 
In step S7, it is determined whether the engine operating condition is such 
as requires feedback control of the air-fuel ratio. In the step S7, when 
the corrected target air-fuel ratio is not smaller than 14.7 (lean), it is 
determined that the feedback control is to be carried out, and otherwise, 
it is determined that an open loop control is to be carried out. 
When it is determined in the step S7 that the feedback control is to be 
carried out, a reference value Vr for comparing the corrected target 
air-fuel ratio AFD with the output Vs of the air-fuel ratio sensor 14 
(i.e., a slice level) is calculated in step S8. As shown in FIG. 9, the 
reference value Vr is a voltage which is related to the corrected target 
air-fuel ratio AFD to be increased with increase of the corrected target 
air-fuel ratio AFD. In step S9, the reference value Vr corresponding to 
the corrected target air-fuel ratio AFD is compared with the output Vs of 
the air-fuel ratio sensor 13 and a feedback correction coefficient Cfb is 
calculated. In the next step S10, a study correction coefficient Cstdy is 
calculated. 
The feedback correction coefficient Cfb is calculated on the basis of the 
following formula in order to effect a P.I. control. 
EQU Cfb=P+.intg..DELTA.Id.theta. 
In this control, the feedback correction coefficient Cfb is determined so 
as to enrich the air-fuel mixture to be introduced into the engine when 
the sensor output Vs is larger than the reference value Vr (that is, when 
the detected air-fuel ratio is leaner than the corrected target air-fuel 
ratio AFD), and to make the air-fuel mixture leaner when the sensor output 
Vs is smaller than the reference value Vr. The value P in the above 
formula is a value to be uniformly added or subtracted when the order of 
values of the sensor output Vs and the reference value Vr is inverted, and 
the value .DELTA.I is a value to be subtracted or added every 
predetermined crank angle. The values P and .DELTA.I are set as follows so 
that the feedback correction coefficient Cfb or the air-fuel ratio of the 
air-fuel mixture to be fed to the engine gradually changes during idling. 
______________________________________ 
throttle full closed 
otherwise 
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P 0.025 0.047 
.DELTA.I 0.0021 0.0041 
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The study correction coefficient Cstdy is obtained by adding up the values 
of the feedback correction coefficient Cfb at the time increase and 
decrease of the value of the feedback correction coefficient Cfb is 
inverted and taking an average of the values when a predetermined number 
of the values have been added up. However, if the newest study correction 
coefficient Cstdy' is used, as it is, for correcting the base fuel 
injection time To, a wrong study will lead to a significant change of the 
air-fuel ratio, and accordingly, a value obtained by adding a quarter of 
the newest study correction coefficient Cstdy' to the preceding study 
correction coefficient Cstdy is actually adopted as the study correction 
coefficient Cstdy. 
In step S11, other correction coefficients, such as the acceleration 
correction coefficient Cacc, the deceleration correction coefficient Cdec, 
the after-cranking correction coefficient Cs, and the recirculation 
correction coefficient Crec, as well as the ineffective injection time Tv 
are calculated. Then, in step S12, a final fuel injection pulse width Ti 
is calculated, and fuel is injected for a time corresponding to the final 
fuel injection pulse width Ti at a predetermined timing (step S13). 
The final fuel injection pulse width Ti is obtained by multiplying the base 
fuel injection time To calculated in the step S3 by the ratio of the 
stoichiometric air-fuel ratio (14.7) to the corrected target air-fuel 
ratio AFD calculated in the step S6, thereby obtaining the fuel injection 
time corresponding to the corrected target air-fuel ratio AFD, obtaining a 
corrected fuel injection time by multiplying the fuel injection time 
corresponding to the corrected target air-fuel ratio by a value obtained 
by adding to or subtracting from 1 the various relevant correction 
coefficients, and adding the ineffective injection time Tv to the 
corrected fuel injection time. 
In the particular embodiment described above, the base target air-fuel 
ratio calculated according to the engine operating condition is 
compensated for the engine cooling water temperature (the engine 
temperature) to obtain the corrected target air-fuel ratio and then the 
reference value representing the corrected target air-fuel ratio and to be 
submitted to comparison with the output of the air-fuel ratio sensor is 
calculated. This is advantageous over the conventional system in which the 
reference value (representing the base target air-fuel ratio) to be 
submitted to comparison with the output of the air-fuel ratio sensor is 
first calculated and then compensated for the engine cooling water 
temperature. That is, though the relation of the output of the air-fuel 
ratio sensor to the exhaust gas oxygen concentration is linear, the 
relation of the output of the air-fuel ratio sensor to the air-fuel ratio 
is not linear, and accordingly, if the base target air-fuel ratio is first 
calculated and thereafter compensated for the engine temperature, the 
value of the air-fuel ratio actually changed for a given value of the 
correction coefficient can vary depending on the value of the base target 
air-fuel ratio before the correction. This adversely affects the control 
accuracy. On the other hand, in the system of the embodiment in which the 
base target air-fuel ratio calculated according to the engine operating 
condition is first compensated for the engine temperature to obtain the 
corrected target air-fuel ratio and then the reference value representing 
the corrected target air-fuel ratio and to be submitted to comparison with 
the output of the air-fuel ratio sensor is calculated, the control 
accuracy cannot be affected by properties of the air-fuel ratio sensor. It 
is also preferred that compensation for various engine conditions such as 
change in the atmospheric pressure, change with age of the engine, or 
which mode is selected, power mode or economy mode, be effected before 
calculation of the reference value. 
Further, the corrections other than the feedback correction made in the 
embodiment described above may be omitted if desired.