Air-fuel ratio control device for internal combustion engine

An air-fuel ratio control device for an internal combustion engine controls an air-fuel ratio to the lean side rather than to a stoichiometric air-fuel ratio by the use of a factor determined in accordance with inlet pipe pressure and engine speed. A throttle opening degree sensor is provided to detect a degree of throttle opening, and on the basis of the degree of throttle opening, the factor determined in accordance with the inlet pipe pressure and the engine speed is corrected in a high load range of the engine. Since the output of the throttle opening degree sensor in the high load range of the engine is more accurate than the output of a pressure sensor for detecting the inlet pipe pressure, the air-fuel ratio can be controlled accurately in the high load range by correcting the factor in accordance with the degree of throttle opening.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to an air-fuel ratio control device for an 
internal combustion engine of the lean-burn control type wherein the 
air-fuel ratio is controlled to become a target air-fuel ratio on the lean 
side rather than a stoichiometric air-fuel ratio; in other words, a type 
wherein a lean mixture is used. 
2. Description of the Related Art 
Generally, a basic fuel injection time is determined on the basis of engine 
speed and inlet pipe pressure or intake air quantity. The basic fuel 
injection time thus determined is corrected in accordance with engine 
cooling water temperature, intake air temperature, and so on to determine 
execution fuel injection time. On the basis of this execution fuel 
injection time, fuel injection is performed. In addition, a lean-burn 
control system is known in which the air-fuel ratio is controlled on the 
lean side rather than on a stoichiometric air-fuel ratio. Since the peak 
of NOx is normally set somewhat on the lean side, deviating from the 
stoichiometric air-fuel ratio, the air-fuel ratio in the lean-burn control 
system is controlled beyond a level corresponding to the peak of NOx and 
to the lean side for the purpose of reducing NOx so as to improve fuel 
consumption. 
Japanese Patent Application Laid-Open No. 62-199943 discloses a system in 
which lean-burn control is performed by determining a lean correction 
factor on the basis of inlet pipe pressure and engine speed and 
multiplying the basic fuel injection time by the lean correction factor. 
A pressure sensor for detecting inlet pipe pressure is accurate in low and 
medium load ranges where a degree of opening of a throttle valve is small; 
however, in a high load range, the change of output of the sensor is small 
as compared to the change of opening of the throttle valve. That is, the 
resolving power of the sensor becomes degraded. Particularly, while a 
vehicle is running at high altitudes (high-altitude atmospheric pressure 
PA is lower than low altitude atmospheric pressure PAo), the output of the 
pressure sensor in the high load range (where inlet pipe pressure PM is 
substantially equal to the atmospheric pressure PA) changes little and not 
in proportion to the change of opening of the throttle valve. That is, an 
air quantity being sucked into a combustion chamber of the engine cannot 
be detected accurately in the high load range by the pressure sensor. 
Therefore, an adequate lean correction factor cannot be obtained in the 
high load range, with the result that lean-burn control cannot be 
performed accurately. Such a problem arises also where the lean correction 
factor is determined using an airflow meter for detecting intake air 
quantity rather than inlet pipe pressure. 
SUMMARY OF THE INVENTION 
It is an object of the present invention to provide an air-fuel ratio 
control device for an internal combustion engine which can accurately 
perform lean-burn control in the high load range as well as in the low and 
medium load ranges. 
To achieve the foregoing object, the present invention provides an air-fuel 
ratio control device for an internal combustion engine which, as shown in 
FIG. 1A, includes a detection means (first sensor) A for detecting either 
inlet pipe pressure or intake air quantity, a detection means (second 
sensor) B for detecting engine speed, means (throttle opening degree 
detection sensor) D for detecting the degree of a throttle opening, a 
basic fuel injection time calculating means for calculating a basic fuel 
injection time on the basis of engine speed and either inlet pipe pressure 
or intake air quantity, a correction factor calculating means for 
calculating a correction factor on the basis of engine speed and either 
inlet pipe pressure or intake air quantity that is used in controlling the 
air-fuel ratio to the lean side rather than to a stoichiometric air-fuel 
ratio, a air-fuel ratio controlling means for controlling the air-fuel 
ratio on the basis of the basic fuel injection time and the correction 
factor, and a correction means E for correcting the correction factor on 
the basis of at least the degree of the throttle opening in a high load 
range of the engine. 
The basic fuel injection time calculating means, correction factor 
calculating means, and air-fuel ratio controlling means are included in a 
control means C. 
According to the present invention, when the detection value of the 
throttle opening degree detection sensor exceeds a given level indicating 
the high load range, the correction factor determined on the basis of 
engine speed and either inlet pipe pressure or intake air quantity is 
corrected in accordance with a correction value determined in accordance 
with at least the degree of the throttle opening. Since the degree of the 
throttle opening is detected accurately in the high load range, an 
inadequate correction factor based on inlet pipe pressure can be corrected 
and changed to an adequate correction factor in the high load range, 
whereby lean-burn control can be performed accurately. 
As will be explained, the air-fuel ratio control device for an internal 
combustion engine according to the present invention can perform optical 
lean-burn control in the high load range as well as in low and medium load 
ranges.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
An internal combustion engine equipped with a control device according to 
the present invention will now be described in detail with reference to 
the drawings. 
FIG. 1B schematically shows an internal combustion engine. An intake air 
temperature sensor 14 for detecting an intake air temperature is provided 
in the vicinity of an air cleaner 10. Downstream, a throttle valve 12 is 
provided whose opening is controlled by an accelerator pedal. Attached to 
the throttle valve 12 is a throttle opening degree sensor 16 for 
delivering a signal proportional to the degree of opening of the throttle 
valve 12. 
One end of a pipe 15 is connected downstream from the throttle opening 
degree sensor 16 to an inlet pipe so as to communicate with the inlet 
pipe. Attached to other end of the pipe 15 is a semiconductor pressure 
sensor 13 which detects the absolute pressure of the inlet pipe or in 
other words, inlet pipe pressure. 
Downstream from the throttle valve 12 is a surge tank 18 which communicates 
with a combustion chamber(s) formed in an engine body through an intake 
manifold 20. A fuel injection valve 22 for each cylinder projects into the 
intake manifold 20. 
The combustion chamber formed in the engine body communicates with a 
catalyst unit 25 filled with catalytic converter rhodium through an 
exhaust manifold 24. Attached to the exhaust manifold 24 is an O.sub.2 
sensor 26 which detects the density of residual oxygen in exhaust gas and 
delivers a signal whose polarity is inverted at the point of a 
stoichiometric air-fuel ratio. Attached to an engine block of the engine 
body is a water temperature sensor 28 for detecting an engine cooling 
water temperature and which projects through the engine block into a water 
jacket. 
Each cylinder of the engine body is provided with a spark plug 46, which 
projects through a cylinder head into the combustion chamber and is 
connected via a distributor 48 and an ignitor 50 to a control circuit 52. 
Provided inside the distributor 48 is a rotational angle sensor 54 which 
comprises a signal rotor secured to a distributor shaft and a pickup 
secured to a distributor housing. The rotational angle sensor 54 outputs 
an engine speed signal to the control circuit 52 in the form of a pulse 
train with one pulse being generated for example, every 30 degrees, of CA 
(crank angle). 
The control circuit 52 includes a microcomputer. Specifically, as shown in 
FIG. 2, the control circuit 52 comprises a RAM 56, a ROM 58, an MPU 60, 
and input/output port 62, an input port 64, output ports 68 and 70, and a 
bus 72 including a data bus, a control bus, etc. The input/output port 62 
is connected to an analog-to-digital converter (A-D converter) 74 and a 
multiplexer 76. The multiplexer 76 is respectively connected through a 
buffer 75 to the inlet pipe pressure sensor 13, through a buffer 78 with 
the water temperature sensor 28, through a buffer 80 with the throttle 
opening degree sensor 16, and through a buffer 821 with the intake air 
temperature sensor 14. 
The MPU 60 controls the A-D converter 74 and the multiplexer 76 via the 
input/output port 62, and successively converts the outputs of the 
pressure sensor 13, water temperature sensor 28, intake air temperature 
sensor 14, and throttle opening degree sensor 16 from analog to digital, 
and stores the outputs in digital form in the RAM 56. The O.sub.2 sensor 
26 is connected through a comparator 84 and a buffer 86 to the input port 
64. The rotation angle sensor 54 is connected through a waveform shaping 
circuit 88 to the input port 64. 
The output port 68 is connected through a drive circuit 92 to the ignitor 
50. The output port 70 is connected through a drive circuit 94 provided 
with a down counter to the fuel injection valve 22. In the drawings, 96 is 
a clock, and 98 is a timer. Previously stored in the ROM 58 are a control 
routine program, a basic ignition timing table, a basic fuel injection 
time table, and the like. 
Basic fuel injection time TP is calculated using the basic fuel injection 
time table and on the basis of the inlet pipe pressure defined by the 
output of the inlet pipe pressure sensor 13 and the engine speed defined 
by the output of the rotational angle sensor 54 as will be described 
later. This basic fuel injection time TP is corrected on the basis of the 
outputs of the intake air temperature sensor 14, the O.sub.2 sensor 26, 
and the water temperature sensor 28, whereby an execution fuel injection 
time TAU is obtained. 
Similarly to the calculation of the basic fuel injection time TP, a basic 
ignition timing A.sub.BASE is calculated using the basic ignition timing 
table and on the basis of the outputs of the inlet pipe pressure sensor 13 
and the rotational angle sensor 54, and corrected on the basis of the 
outputs of the intake air temperature sensor 14, the water temperature 
sensor 28, and the like, whereby an execution ignition timing SA is 
obtained. 
A control routine of the embodiment will now be described with reference to 
the flow chart (FIG. 3). Calculation and execution routines for the 
execution ignition timing SA are identical with those used in controlling 
a conventional electronically-controlled internal combustion engine and 
thus will not be described. 
In step 100, engine speed NE, inlet pipe pressure PM, and throttle opening 
TA are read. 
In step 102, a correction factor KAFB is read from an NE-PM characteristic 
map as shown in FIG. 5 on the basis of the inlet pipe pressure. In step 
104, a correction factor KTAAF is read form an NE-TA characteristic map as 
shown in FIG. 6 on the basis of the degree of throttle opening. 
In step 106, the KAFB read in step 102 is multiplied by the KTAAF read in 
step 104, whereby a lean control factor KAF is obtained as below: 
EQU KAF=KAFB.multidot.KTAAF (1) 
As shown in FIG. 6, the correction factor KTAAF based on the degree of 
throttle opening is one (1) when the degree of throttle opening TA is 
smaller than a given valve. Therefore, when the degree of throttle opening 
is smaller than a given valve, the lean correction factor KAF of the 
expression one (1) is influenced by only the correction factor KAFB based 
on the inlet pipe pressure. When the degree of throttle opening exceeds a 
given valve, the correction factor KTAAF based on the degree of throttle 
opening becomes smaller than one (1); therefore, the lean control factor 
KAF is influenced by both the correction factor KAFB based on the inlet 
pipe pressure and the correction factor KTAAF based on the degree of 
throttle opening. Accordingly, in a range where the degree of throttle 
opening is larger than a given valve, the lean control factor decreases as 
the degree of the throttle opening increases even if the inlet pipe 
pressure PM and the engine speed NE show no change. As shown in FIG. 6, 
the degree of throttle opening corresponding to the correction factor 
KTAAF being smaller than one (1) increases as the engine speed NE 
increases. Further, at "wide open throttle (WOT)" or a degree of throttle 
opening TA2 near "full load", the correction factor KTAAF is zero (0). 
When the correction factor KTAAF becomes zero (0), the lean control factor 
KAF becomes zero (0); therefore, as will be understood from expressions 
(2) and (3) as described later, the air-fuel ratio is controlled to the 
stoichiometric air-fuel ratio. 
In step 108, an execution air-fuel ratio correction factor KAFS is 
calculated in accordance with the following expression: 
EQU KAFS=(1-KAF) (2) 
In step 110, the basic fuel injection time TP is calculated on the basis of 
inlet pipe pressure PM and engine speed NE. The basic fuel injection time 
TP is corrected on the basis of the engine cooling water temperature (the 
output of the water temperature sensor 28), the intake air temperature 
(the output of the intake air temperature sensor 14), and the like, 
whereby the execution fuel injection time TAU is obtained. In this 
embodiment, lean-burn control is performed using the air-fuel ratio 
correction factor KAFS. That is, the execution fuel injection time TAU is 
calculated in accordance with the following expression: 
EQU TAU=(A.multidot.TP).multidot.KAFS+B (3) 
where A and B are correction factors determined in accordance with the 
engine cooling water temperature, the intake air temperature, and the 
like. 
After the execution fuel injection time TAU is calculated, the fuel 
injection execution routine controls the fuel injection valve 22 on the 
basis of the execution fuel injection time TAU, whereby fuel injection is 
performed. 
The characteristic of the lean control factor KAF calculated in accordance 
with the expression (1) which is dependent on a load change will now be 
described with reference to FIG. 4. This includes two types corresponding 
to high attitude running and low attitude running. 
When the degree of throttle opening TA becomes equal to a given opening 
TA1, the inlet pipe pressure becomes such that the pressure during low 
attitude running (for example, the atmospheric pressure PAo) is higher 
than the pressure during high attitude running (for example, the 
atmospheric pressure PA). During high attitude running, the KAF reaches 
peak value when TA=TA1. When the degree of throttle opening TA exceeds a 
given valve TA1, the correction factor KTAAF based on the degree of 
throttle opening TA is influenced, so that the KAF decreases gradually 
from its value before being influenced by the high attitude running mode 
during high attitude running, or from its value before being influenced of 
the low attitude running mode during low attitude running, and becomes 
zero (0) when TA=TA2. 
In this way, in the high load range, the setting of the lean control factor 
by the correction factor based on the inlet pipe pressure is not switched 
to the setting of the lean control factor by the correction factor based 
on the degree of throttle opening. Instead, in the high load range, the 
correction factor based on the inlet pipe pressure is influenced by the 
correction factor based on the degree of throttle opening. Therefore, the 
target air-fuel ratio can be varied smoothly irrespective of whether the 
attitude is high or low. 
As described above, according to the present embodiment, the lean-burn 
control process in the high load rang (wherein it could not be performed 
accurately by the use of the correction factor based on the inlet pipe 
pressure) is influenced by the correction factor based on the degree of 
throttle opening. Therefore, accurate lean-burn control can be performed 
in all load ranges, thereby resulting in improved driveability, driving 
force output, fuel consumption, etc. 
It should be noted that the intake air quantity may be used in place of 
inlet pipe pressure, and the correction factor KTAAF may be determined in 
accordance with only the degree of throttle opening.