Engine load parameter-calculating system and engine control system using the calculating system

An engine load parameter-calculating system for an internal combustion engine calculates an engine load parameter indicative of an amount of intake air drawn into the engine. The system determines a value of an opening area formed by a throttle valve and reflecting an amount of intake air, a reference value of the opening area in accordance with the rotational speed of the engine, and a value of the engine load parameter from the value of the opening area and the reference value of same. An engine control system calculates a basic control amount for controlling the engine by the use of the value of the engine load parameter.

BACKGROUND OF THE INVENTION 
This invention relates to an engine load parameter-calculating system, and 
an engine control system using the engine load parameter-calculating 
system. 
Conventionally, a method has been proposed by Japanese Provisional Patent 
Publication (Kokai) No. 63-143348, which comprises steps of detecting an 
amount of air drawn into an internal combustion engine by an intake air 
amount sensor or an intake pressure sensor, and controlling a fuel supply 
amount, ignition timing, etc. in accordance with the detected value of the 
amount of air drawn into the engine. When the engine is in a transient 
operating condition, a proper control amount cannot be obtained due to 
delay in detection of the amount of air by the intake air amount sensor or 
the intake pressure sensor. Therefore, according to the above method, when 
the engine is in a transient operating condition, an estimated value of 
intake pressure is obtained from detected values of throttle valve opening 
and the engine rotational speed, and a control amount is obtained based on 
the estimated value of intake pressure. 
However, according to this prior art, estimated values of intake pressure 
are stored in a storage device in the form of a map set in accordance with 
values of the throttle valve opening and the engine rotational speed. To 
obtain an accurate estimated value of intake pressure, the map is required 
to have many finely divided values (lattice points) of the throttle valve 
opening and the engine rotational speed. This requires the use of a 
storage device with a very large capacity. Further, it takes a longer time 
period to determine a control amount from such a very large amount of 
stored data, which results in degraded controllability of the engine. 
SUMMARY OF THE INVENTION 
It is a first object of the invention to provide an engine load 
parameter-calculating system which is capable of quickly calculating a 
parameter which is accurately indicative of load on the engine when it is 
in a transient operating condition, without requiring a very large amount 
of stored data. 
It is a second object of the invention to provided an engine control system 
using the engine load parameter-calculating system. 
To attain the first object of the invention, according to a first aspect of 
the invention, there is provided an engine load parameter-calculating 
system for an internal combustion engine having an intake passage, and a 
throttle valve arranged in the intake passage, the system calculating an 
engine load parameter indicative of an amount of intake air drawn into the 
engine. 
The engine load parameter-calculating system according to the first aspect 
of the invention is characterized by comprising: 
opening area value-determining means for determining a value of an opening 
area formed by the throttle valve; 
reference area value-determining means for determining a reference value of 
the opening area formed by the throttle valve in accordance with a 
rotational speed of the engine; and 
engine load parameter-determining means for determining a value of the 
engine load parameter from the value of the opening area formed by the 
throttle valve and the reference value of the opening area formed by the 
throttle valve. 
Preferably, the engine load parameter-determining means includes area 
ratio-calculating means for calculating a ratio between the value of the 
opening area formed by the throttle valve and the reference value, the 
value of the engine load parameter being determined based on the ratio. 
To attain the second object of the invention, according to a second aspect 
of the invention, there is provided an engine control system for an 
internal combustion engine including an intake passage, a throttle valve 
arranged in the intake passage, and an engine load parameter-calculating 
system for calculating an engine load parameter indicative of an amount of 
intake air drawn into the engine. 
The engine control system according to the second aspect of the invention 
is characterized by comprising basic control amount-calculating means for 
calculating a basic control amount for controlling the engine by the use 
of the value of the engine load parameter determined by the engine load 
parameter-determining means. 
Preferably, the engine control system includes an engine load sensor for 
detecting the engine load parameter, and transient operating 
condition-determining means for determining whether or not the engine is 
in a transient operating condition, and the basic control 
amount-calculating means calculates the basic control amount by the use of 
a value of output from the engine load sensor when the transient operating 
condition-determining means has determined that the engine is not in the 
transient operating condition. 
More preferably, the engine load sensor detects pressure within the intake 
passage at a location downstream of the throttle valve. 
Alternatively, the engine load sensor detects an amount of air drawn into 
the engine. 
Preferably, the engine control system includes difference-calculating means 
for calculating a difference between a value of output from the engine 
load sensor and the value of the engine load parameter determined by the 
engine load parameter-determining means, when the transient operating 
condition-determining means has determined that the engine is not in the 
transient operating condition, and the basic control amount-calculating 
means corrects the value of the engine load parameter determined by the 
engine load parameter-determining means, by the difference from 
calculating the basic control amount, when the transient operating 
condition-determining means has determined that the engine is in the 
transient operating condition. 
To attain the first object of the invention, according to a third aspect of 
the invention, there is provided an engine load parameter-calculating 
system for an internal combustion engine having an intake passage and a 
throttle valve arranged in the intake passage, the system calculating an 
engine load parameter indicative of an amount of intake air drawn into the 
engine, 
the system comprising: 
throttle valve opening-detecting means for detecting a value of angle 
assumed by the throttle valve; 
reference value-determining means for determining a reference value of the 
angle of the throttle valve in accordance with a rotational speed of the 
engine; and 
engine load parameter-determining means for determining a value of the 
engine load parameter indicative of the amount of intake air, from the 
detected value of the angle assumed by the throttle valve and the 
reference value of the angle assumed by the throttle valve. 
Preferably, the engine load parameter-determining means includes means for 
calculating a ratio between the value of the angle assumed by the throttle 
valve and the reference value of the angle assumed by the throttle valve, 
the engine load parameter being determined based on the ratio. 
To attain the second object of the invention, according to a fourth aspect 
of the invention, there is provided an engine control system for an 
internal combustion engine including an intake passage, a throttle valve 
arranged in the intake passage, and the engine load parameter-calculating 
system according to the third aspect of the invention, 
the engine control system comprising basic control amount-calculating means 
for calculating a basic control amount for controlling the engine by the 
use of the value of the engine load parameter obtained by the engine load 
parameter-determining means. 
The above and other objects, features, and advantages of the invention will 
become more apparent from the ensuing detailed description taken in 
conjunction with the accompanying drawings.

DETAILED DESCRIPTION 
The invention will now be described in detail with reference to the 
drawings showing an embodiment thereof. 
Referring first to FIG. 1, there is shown the whole arrangement of an 
engine control system according to the embodiment of the invention. In the 
figure, reference numeral 1 designates an internal combustion engine for 
automotive vehicles. Connected to the cylinder block of the engine 1 is an 
intake pipe 2 across which is arranged a throttle body 3 accommodating a 
throttle valve 3' therein. A throttle valve opening (.theta..sub.TH) 
sensor 4 is connected to the throttle valve 3' for generating an electric 
signal indicative of the sensed throttle valve opening (the angle assumed 
in the throttle valve 3') and supplying same to an electronic control unit 
(hereinafter called "the ECU") 5. 
Fuel injection valves 6, only one of which is shown, are inserted into the 
interior of the intake pipe at locations intermediate between the cylinder 
block of the engine 1 and the throttle valve 3' and slightly upstream of 
respective intake valves, not shown. The fuel injection valves 6 are 
connected to a fuel pump, not shown, and electrically connected to the ECU 
5 to have their valve opening periods controlled by signals therefrom. 
Spark plugs 12, which are provided for respective cylinders of the engine 
1, are electrically connected to the ECU 5 to have ignition timing 
.theta..sub.IG thereof controlled by a signal therefrom. 
On the other hand, an intake pipe absolute pressure (P.sub.BA) sensor 8 is 
provided in communication with the interior of the intake pipe 2 through a 
conduit 7 at a location immediately downstream of the throttle valve 3' 
for supplying an electric signal indicative of the sensed absolute 
pressure within the intake pipe 2 to the ECU 5. 
An engine rotational speed (Ne) sensor 9 and a cylinder-discriminating 
(CYL) sensor 10 are arranged in facing relation to a camshaft or a 
crankshaft of the engine 1, neither of which is shown. The engine 
rotational speed sensor 9 generates a pulse as a TDC signal pulse at each 
of predetermined crank angles whenever the crankshaft rotates through 180 
degrees, while the cylinder-discriminating sensor 10 generates a pulse at 
a predetermined crank angle of a particular cylinder of the engine, both 
of the pulses being supplied to the ECU 5. 
An atmospheric pressure sensor 11 for detecting atmospheric pressure is 
electrically connected to the ECU 5, and supplies a signal indicative of 
the detected atmospheric pressure thereto. 
The ECU 5 comprises an input circuit 5a having the functions of shaping the 
waveforms of input signals from various sensors, shifting the voltage 
levels of sensor output signals to a predetermined level, converting 
analog signals from analog-output sensors to digital signals, and so 
forth, a central processing unit (hereinafter called "the CPU") 5b, memory 
means 5c storing various operational programs which are executed in the 
CPU 5b, and for storing results of calculations therefrom, etc., and an 
output circuit 5d which outputs driving signals to the fuel injection 
valves 6. 
The CPU 5b operates in response to the engine parameter signals from the 
sensors described above, and not shown, to determine operating conditions 
in which the engine 1 is operating, and calculates, based upon the 
determined operating conditions, the valve opening period or fuel 
injection period T.sub.OUT over which the fuel injection valves 6 are to 
be opened, by the use of the following equation (1) in synchronism with 
inputting of TDC signal pulses to the ECU 5: 
EQU T.sub.OUT =T.sub.i .times.K.sub.1 +K.sub.2 (1) 
where T.sub.i represents a basic value (hereinafter referred to as "Ti 
value") of the fuel injection period T.sub.OUT of the fuel injection 
valves 6, which is read from a Ti map in which Ti values are set in 
accordance with the engine rotational speed Ne and the intake pipe 
absolute pressure P.sub.BA. In retrieving the Ti value, there are used a 
value of the engine rotational speed Ne actually detected by the engine 
rotational speed sensor 9, and a value of the intake pipe absolute 
pressure P.sub.BA (hereinafter referred to as "detected P.sub.BA value") 
actually detected by the intake pipe absolute pressure sensor 8, or 
alternatively a calculated value of P.sub.BA (hereinafter referred to as 
"calculated PBA value") which is calculated in a program shown in FIG. 2, 
referred to hereinafter. 
K.sub.1 and K.sub.2 are other correction coefficients and correction 
variables, respectively, which are calculated based on various engine 
parameter signals to such values as to optimize characteristics of the 
engine such as fuel consumption and driveability depending on operating 
conditions of the engine. 
The CPU 5b further retrieves a basis value .theta.i (hereinafter referred 
to as ".theta.i value") of ignition timing .theta..sub.IG from an ignition 
timing map in which .theta.i values are set in accordance with the engine 
rotational speed Ne and the intake pipe absolute pressure P.sub.BA. In 
retrieving a .theta.i value, there are used a value of Ne actually 
detected by the engine rotational speed sensor 9, and a detected P.sub.BA 
value, or alternatively, a calculated PBA value. The ignition timing 
.theta..sub.IG is calculated by correcting the .theta.i value in 
accordance with operating conditions of the engine. 
The CPU 5b supplies through the output circuit 5d, the fuel injection 
valves 6 and the spark plug 12 with driving signals corresponding to the 
calculated fuel injection period T.sub.OUT and ignition timing .theta.i 
determined as above, respectively. 
FIGS. 2a-2c show a program for calculating the Ti value and the .theta.i 
value. 
At a step S1, it is determined whether or not the engine is cranking. If 
the answer to this question is affirmative (Yes), a flag F.sub.PBTH is set 
to a value of 0 at a step S28, and the .theta.i value and the Ti value are 
calculated by the use of the P.sub.BA value detected by the intake pipe 
absolute pressure sensor 8 and the engine rotational speed Ne at a step 
S29. The flag F.sub.PBTH is set to a value of 1 when the .theta.i and Ti 
values are calculated by the use of the calculated P.sub.BA value PBATH, 
as described hereinafter. 
If the answer to the question of the step S1 is negative (No), i.e. if the 
engine is not cranking, it is determined at a step S2 whether or not the 
engine rotational speed Ne is equal to or higher than a predetermined 
value Ne.sub.PB (e.g. 4000 rpm). If the answer is affirmative (Yes), i.e. 
if Ne.gtoreq.Ne.sub.PB, the program proceeds to the step S28. When the 
engine rotational speed is high, detection of the P.sub.BA value in a 
transient operating condition of the engine is not delayed beyond a 
satisfactory level, which makes unnecessary the calculation of a value of 
P.sub.BA based on throttle valve opening .theta..sub.TH. 
If both the answers to the questions of the steps S1 and S2 are negative 
(No), i.e. if the engine is not cracking and at the same time 
Ne&lt;Ne.sub.PB, full load throttle valve opening THWTPB is calculated based 
on the engine rotational speed Ne at a step S3. The full load throttle 
valve opening THWTPB is the minimum value of throttle valve opening at 
which the intake pipe absolute pressure P.sub.BA assumes a value which is 
substantially the same as assumed when the throttle valve is fully opened. 
The full load throttle valve opening THWTPB is calculated by the use of a 
Ne-THWTPB table shown in FIG. 3. In FIG. 3, predetermined values THWTPB 1 
to THWTPB 5 of the full load throttle valve opening are provided, which 
correspond to predetermined values Ne.sub.1 to Ne.sub.5, respectively. 
Values of the full load throttle valve opening THWTPB corresponding to 
values of the engine rotational speed falling between adjacent ones of the 
predetermined valves Ne.sub.1 to N.sub.5 are calculated by interpolation. 
As can be learned from the figure, the lower the engine rotational speed, 
the smaller the value of throttle valve opening at which there is obtained 
substantially the same value of P.sub.BA as assumed when the throttle 
valve is fully opened. 
At a step S4, it is determined whether or not the present value 
.theta..sub.TH n of throttle valve opening detected in the present loop is 
smaller than a value of the full load throttle valve opening THWTPB 
calculated at the step S3. If the answer to this question is negative 
(No), i.e. if .theta..sub.TH n.gtoreq.THWTPB, the calculated P.sub.BA 
value, referred to hereinafter, is set to atmospheric pressure P.sub.A at 
a step S16, since the intake pipe absolute pressure is then equal to a 
value assumed when the throttle valve is fully opened, and a correction 
coefficient KPBTH for use in calculation at a step S 25, referred to 
hereinafter, is set to a value of 0 at a step S17, followed by the program 
proceeding to a step S21. Alternatively, when the atmospheric pressure 
sensor is not provided, the calculated P.sub.BA value may be set to normal 
atmospheric pressure (760 mmHg) at the step S16. 
If the answer to the question of the step S4 is affirmative (Yes), i.e. if 
.theta..sub.TH n&lt;THWTPB, the correction coefficient KPBTH is set to a 
predetermined value (which is close to 0, e.g. 0.2) at a step S5. Then, at 
a step S6, an opening area SWOTTH (hereinafter referred to as "reference 
area value") formed by the throttle valve corresponding to the full load 
throttle valve opening HWTPB is calculated, and at a step S7, an opening 
area STH (hereinafter referred to as "intake air-reflecting area value) 
formed by the throttle valve corresponding to the present value 
.theta..sub.TH n of throttle valve opening is calculated by a 
.theta..sub.TH -STH table shown in FIG. 4. In the figure, predetermined 
values STH.sub.1 to STH.sub.7 of throttle valve opening area are provided 
which correspond to predetermined values .theta..sub.TH1 to 
.theta..sub.TH7 of throttle valve opening, respectively. Values of 
throttle valve opening area corresponding to values of throttle valve 
opening falling between adjacent ones of the predetermined values 
.theta..sub.TH1 to .theta..sub.TH7 are calculated by interpolation. Then, 
at a step S8, a ratio KS of the intake air-reflecting area value STH to 
the reference area value SWOTTH is calculated, and at a step S9, it is 
determined whether or not the ratio KS is smaller than 1.0. If the answer 
to this question is negative (No), the ratio KS is corrected to 1.0, 
whereas if the answer is affirmative (Yes), the program immediately 
proceeds to a step S11. Although the ratio KS is naturally expected to 
assume a value smaller than 1.0, this fact is confirmed by the step S9. 
At the step S11, an estimated P.sub.BA value PBTH corresponding to the 
ratio KS is calculated by a KS-PBTH table shown in FIG. 5. The KS-PBTH 
table is set based on actually measured data shown in FIG. 6. 
Specifically, the relationship between the ratio KS and the intake pipe 
absolute pressure P.sub.BA was actually measured at engine rotational 
speeds Ne of 1000, 2000, 3000, 4000, 5000, 6000, and 7000 rpm, under 
normal atmospheric pressure (760 mmHg). As a result, it was found that 
data obtained at any engine rotational speed Ne are included within a 
hatched area shown in FIG. 6, which means that the ratio KS and P.sub.BA 
have an approximately constant relationship irrespective of the engine 
rotational speed Ne. The KS-PBTH table in FIG. 5 is based on this 
relationship. Predetermined values PBTH0 to PBTH6 of the estimated 
P.sub.BA values are provided, which correspond to predetermined values KS0 
to KS6 of the ratio KS, respectively. Values of the estimated P.sub.BA 
value corresponding to values of the ratio KS falling between adjacent 
ones of the predetermined values KS0 to KS6 are calculated by 
interpolation. 
Thus, according to the present embodiment of the invention, the estimated 
P.sub.BA value is calculated based on the ratio of the intake 
air-reflecting area value STH to the reference area value SWOTTH, and is 
not dependent on the engine rotational speed Ne. Therefore, it is possible 
to estimate a value of intake pipe absolute pressure as an engine load 
parameter indicative of an amount of intake air drawn into the engine by 
the use of a one-dimensional table (KS-PBTH table) in which the number of 
data used is far smaller than the number of data used in conventional 
methods. It goes without saying that it is also possible to obtain the 
estimated P.sub.BA value from a map from which the estimated P.sub.BA 
value can be retrieved according to the intake air-reflecting area value 
STH and the reference area value SWOTTH, instead of calculating the ratio 
of the former to the latter, In this case as well, accurate estimation of 
a value of P.sub.BA can be effected by the map in which the number of data 
used is smaller than the number of data used in conventional methods. 
At a step S12, the estimated P.sub.BA value PBTH thus obtained is corrected 
according to atmospheric pressure by the use of the following equation 
(2): 
EQU PBTH=PBTH-K(PAPBTH-P.sub.A) (2) 
where PAPBTH represents normal atmospheric pressure (760 mmHg), and K a 
coefficient which is set, e.g. to 1.0. 
Then, at a step S13, a difference between the present value .theta..sub.TH 
n of throttle valve opening and an immediately preceding value 
.theta..sub.TH n-1 of same obtained in the immediately preceding loop is 
calculated as an amount of change DTH. It is determined at a step S14 
whether or not an absolute value .vertline.DTH.vertline. of the amount of 
change DTH is larger than a predetermined value DTHG. If the answer to 
this question is negative (No), i.e., if 
.vertline.DTH.vertline..ltoreq.DTHG, which means that the engine is not in 
a transient operating condition, it is determined at a step S18 whether or 
not an absolute value of an amount of change DP.sub.BA in P.sub.BA is 
larger than a predetermined value P.sub.BA G. The amount of change 
DP.sub.BA is calculated similarly to the amount of change DTH as a 
difference between the present value of P.sub.BA and an immediately 
preceding value of P.sub.BA obtained in the immediately preceding loop. 
If the answer to the question of the step S18 is negative (No), i.e. if 
.vertline.DP.sub.BA .vertline..ltoreq.DP.sub.BA G, it is judged that the 
engine is in a steady operating condition, and then a difference 
DPB.sub.cal between the detected P.sub.BA value and the estimated P.sub.BA 
value PBTH at a step S19 is calculated, followed by the program proceeding 
to a step S28. The difference DPB.sub.cal corresponds to deviation in 
table values in each of the tables in FIGS. 3 to 5 due to aging of the 
engine, or when the engine has an additional intake passage bypassing the 
throttle valve, it corresponds to a deviation due to the opening area of 
the additional intake passage. The difference DPB.sub.cal is used in 
correction of the estimated P.sub.BA value at a step S15, referred to 
hereinafter. 
If the answer to the question of the step S18 is affirmative (Yes), i.e. if 
.vertline.DP.sub.BA .vertline.&gt;DP.sub.BA G, it is determined at a step S20 
whether or not the flag F.sub.PBTH is equal to 1. If the answer to this 
question is negative (No), i.e. if F.sub.PBTH =0, which means that in the 
immediately preceding loop, the detected P.sub.BA value was used in 
calculating the .theta.i value and the Ti value, the program proceeds to 
the step S28, where the .theta.i value and the Ti value are calculated by 
the use of the detected P.sub.BA valve. On the other hand, if the answer 
to the question of the step S20 is affirmative (Yes), i.e. if F.sub.PBTH 
=1, which means that in the immediately preceding loop, the calculated 
P.sub.BA value PBATH was used in calculating the .theta.i value and the Ti 
value, the program proceeds to a step S21. When the calculated P.sub.BA 
value P.sub.BATH was used in the immediately preceding loop, the 
calculated P.sub.BA value P.sub.BATH is continuously used if the amount of 
change .vertline.DP.sub.BA .vertline. of the P.sub.BA value is large 
(.vertline.DP.sub.BA .vertline.&gt;DP.sub.BA G), even if the amount of change 
.vertline.DTH.vertline. of throttle valve opening is small 
(.vertline.DTH.vertline..ltoreq.DTHG). 
If the answer to the question of the step S14 is affirmative (Yes), i.e. if 
.vertline.DTH.vertline.&gt;DTHG, which means that the engine is in a 
transient operating condition, the difference DPB.sub.cal calculated at 
the step S19 is added to the estimated P.sub.BA value PBTH to thereby 
calculate the calculated P.sub.BA value PBATH at a step S15, followed by 
the program proceeding to a step S21. 
At the step S21, a different DPB between the calculated P.sub.BA value 
PBATH and the detected P.sub.BA value is calculated. The detected P.sub.BA 
value used in this calculation is a value of output from the intake pipe 
absolute pressure sensor 8. Alternatively, a P.sub.BA value corrected in 
compensation for a time lag caused by filtration of the sensor 8 or by 
mechanically removing pulsation of the intake air (as disclosed in 
Japanese Provisional Patent Publication (Kokai) No. 62-93471) may be used. 
When the step S21 is reached via the step S20, an immediately preceding 
value of the calculated P.sub.BA value PBATH obtained in the immediately 
preceding loop is used. 
Then, at a step S22, it is determined whether or not the different DPB 
between the calculated P.sub.BA value PBATH and the detected P.sub.BA 
value obtained at the step S21 is larger than a predetermined positive 
value GPBTHP. If the answer to this question is negative (No), it is 
determined at a step S23 whether or not the difference DPB is smaller than 
a predetermined negative value GPBTHM. If both the answers to the 
questions of the steps S22 and S23 are negative (No), i.e. if 
GPBTHM.ltoreq.DPB.ltoreq.GPBTHP, it is judged that the detected P.sub.BA 
value substantially represents an actual value of the intake pipe absolute 
pressure, and then the program proceeds to the step S28. 
On the other hand, if either the answer to the question of the step S22 or 
the answer to the question of the step S23 is affirmative (Yes), i.e. if 
DPB&gt;GPBTHP or DPB&lt;GPBTHM, which means that the difference between the 
calculated value and the detected value is very large, the flag F.sub.PBTH 
is set to a value of 1 at a step S24, and the calculated P.sub.BA value 
PBATH is corrected at a step S25 according to the difference DPB by the 
following equation (3): 
EQU PBATH=PBATH-DPB.times.KPBATH (3) 
When the amount of change .vertline.DTH.vertline. of throttle valve opening 
.theta..sub.TH is large, the calculated P.sub.BA value becomes slightly 
larger than the actual value of the intake pipe absolute pressure during 
acceleration of the engine (slightly smaller than the actual value during 
deceleration of the engine). Therefore, the correction by the equation (3) 
is carried out for correcting this deviation. 
At the following step S26, limit checking is carried out by the use of a 
value of atmospheric pressure, since the calculated P.sub.BA value PBATH 
cannot be larger than the value of atmospheric pressure. Then, at a step 
S27, the .theta.i value and the Ti value are calculated by the use of the 
calculated P.sub.BA value PBATH. 
FIG. 7 shows changes in the calculated P.sub.BA value ((b) of the figure) 
and the basic air-fuel ratio A/F ((c) of same), when the throttle valve is 
opened ((a) of same). The one-dot-chain lines in (b) and (c) of the figure 
represent theoretically expected changes in the intake pipe absolute 
pressure and the desired value of the basic air-fuel ratio. Here, the 
basic air-fuel ratio is an air-fuel ratio obtained when K.sub.1 of the 
equation (1) is set to 1 and K.sub.2 of same is set to 0, i.e. when 
T.sub.OUT =Ti. 
The calculated P.sub.BA value according to the present embodiment of the 
invention, which is indicated by the solid line in (b) of the figure, is 
substantially equal to the theoretically expected value of the intake pipe 
absolute pressure. In contrast, the detected P.sub.BA value, which is 
indicated by the broken line, changes with a delay relative to the 
theoretically expected value of the intake pipe absolute pressure. 
Consequently, when the Ti value is calculated by the use of the detected 
P.sub.BA value, the basic air-fuel ratio A/F, as indicated by the broken 
line in (c) of the figure, is largely deviated toward the lean side. In 
contrast, when the Ti value is calculated by the use of the calculated 
P.sub.BA value, the basic air-fuel ratio A/F, as indicated by the solid 
line in (c) of same, is substantially equal to the desired value of the 
basic air-fuel ratio. Therefore, when the fuel supply is increased upon 
acceleration of the engine, for example, an amount of fuel to be increased 
can be properly determined, whereby deviation of the air-fuel ratio from a 
desired value can be prevented when the engine is in such a transient 
operating condition. 
Further, according to the present embodiment, the basic value .theta.i of 
ignition timing is also calculated by the calculated P.sub.BA value when 
the engine is in a transient operating condition. Therefore, the ignition 
timing can be properly determined. 
In addition, when the engine is in a steady operating condition, the 
detected P.sub.BA value accurately represents an actual value of the 
intake pipe absolute pressure, so that by the use of the detected P.sub.BA 
value, accurate control of ignition timing and fuel supply can be 
effected. 
Further, the difference DPB.sub.cal between the estimated P.sub.BA value 
and the detected P.sub.BA value is obtained when the engine is in a steady 
operating condition, and the calculated P.sub.BA value is calculated using 
the difference DPB.sub.cal when the engine is in a transient operating 
condition. Therefore, it is possible to eliminate adverse effects of a 
deviation of the estimated P.sub.BA value due to aging of the related 
component parts or those of an intake passage bypassing the throttle 
valve. 
Although, in the above described embodiment, the engine load parameter is 
calculated by the use of the intake pipe absolute pressure sensed by the 
intake pipe absolute pressure sensor 8, this is not limitative, but it may 
be calculated by the use of an amount of intake air Qa which is sensed by 
means of an airflow meter. In such a case, the KS-PBTH table in FIG. 5 
should be replaced by a KS-QatH (an estimated value of the amount of 
intake air) table, and a detected value of the amount of intake air should 
be used instead of the P.sub.BA value. 
Further, although in the above described embodiment, first, the intake 
air-reflecting area value STH and the reference area value SWOTTTH are 
calculated based on the throttle valve opening .theta..sub.TH and the full 
load throttle valve opening HWTPB, respectively, and then the area ratio 
KS is calculated as STH/SWOTTH, followed by calculating the estimated 
P.sub.BA value according to the ratio KS, this is not limitative, but if 
the shape of the throttle valve is changed such that the relationship 
between the throttle valve opening and the intake air-reflecting area 
value is linear (e.g. a variable venturi type), the steps (steps S6 and 
S7) for calculating the intake air-reflecting area value can be omitted, 
and the estimated PBA value can be obtained using a ratio in angle between 
the throttle valve opening .theta..sub.TH and the full load throttle valve 
opening THWTPB. 
In the present embodiment, the difference DPB.sub.cal between the estimated 
P.sub.BA value PBTH and the detected P.sub.BA value is calculated at the 
step S19, and the difference DPB.sub.cal is used for correcting the 
estimated P.sub.BA value PBTH to the calculated P.sub.BA value PBATH, 
whereby the following two deviations from the actual P.sub.BA value can be 
compensated for: 
A first deviation is caused, in an arrangement where an intake passage 
bypassing the throttle valve is provided, when the opening of a control 
valve provided in the intake passage bypassing the throttle valve is 
increased. A second deviation is caused due to carbon attached to the 
throttle valve and associated component parts thereof in the course of 
long term service, which substantially decreases the intake air-reflecting 
area value. The first deviation can also be compensated for by storing in 
advance changes in the intake pipe absolute pressure resulting from 
degrees of opening of the control valve provided in the intake passage 
bypassing the throttle valve, in a table of correction values, and 
correcting the estimated PBA value by the use of the correction values in 
accordance with detected values of opening of the control valve (or an 
instruction signal for opening the control valve). Therefore, the 
correction at step S15 may be effected by the use of such a table of 
correction values instead of using the difference DPB.sub.cal. However, in 
this case as well, the compensation for the second deviation must be 
carried out, as in the present embodiment, by the use of the difference 
between the detected P.sub.BA value and the estimated P.sub.BA value, 
which is calculated when the engine is in a steady operating condition. 
Furthermore, although, in the present embodiment, the correction 
coefficient KPBTH for use in the step S25 is set to a predetermined value 
except when the throttle valve is fully opened (the answer to the question 
of the step S4 is negative (No)), this is not limitative, but it may be 
set to different values depending on whether the the engine is 
accelerating or decelerating, or may be varied depending on the engine 
coolant temperature.