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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an air-fuel ratio control method for internal
combustion engines, and more particularly to a method of controlling the
air-fuel ratio of an air-fuel mixture supplied to an internal combustion
engine when the engine is in a high load operating condition.
2. Prior Art
It is conventionally known to control the air-fuel ratio of an air-fuel
mixture supplied to an internal combustion engine (hereinafter referred to
as "the supply air-fuel ratio") to a stoichiometric air-fuel ratio or its
vicinity when load on the engine is relatively low, and enrich the supply
air-fuel ratio to prevent the temperature of the engine from rising to an
excessive degree by utilizing the effects of cooling by fuel in the
air-fuel mixture supplied to the engine, when the load on the engine is
high. To carry out this air-fuel ratio control method, the following
techniques have conventionally been proposed:
(1) A desired exhaust gas temperature is set based on the amount of intake
air, engine rotational speed, and engine coolant temperature, and the
supply air-fuel ratio is controlled such that the actual exhaust gas
temperature becomes equal to the set desired exhaust gas temperature
(Japanese Provisional Patent Publication (Kokai) No. 60-90940).
(2) An exhaust gas temperature is estimated based on the amount of intake
air or engine rotational speed and the supply air-fuel ratio is enriched
to a greater extent as the estimated exhaust gas temperature is higher
(Japanese Patent Publication (Kokoku) No. 62-54977).
(3) The temperature of a catalytic converter provided in an internal
combustion engine is estimated based on the amount of intake air and the
supply air-fuel ratio, whereby the supply air-fuel ratio is controlled so
as to prevent an excessive rise in the temperature of the catalytic
converter (Japanese Provisional Patent Publication (Kokai) No. 62-203965).
(4) An engine temperature is estimated based on engine rotational speed,
load on the engine, and the supply air-fuel ratio, and enriching of the
supply air-fuel ratio is controlled depending on the estimated engine
temperature (Japanese Provisional Patent Publication (Kokai) No. 3-18643).
Further, to determine whether an oxygen concentration sensor arranged in an
exhaust passage of the engine is activated or not, the following technique
has also been proposed:
(5) The temperature of the oxygen concentration sensor is estimated based
on the amount of intake air and outside air temperature (Japanese
Provisional Patent Publication (Kokai) No. 1-219340).
According to the above techniques (2) to (5). the exhaust gas temperature
or the temperature of an engine component part is estimated based on
engine operating parameters, such as the amount of intake air and engine
rotational speed, but the actual exhaust gas temperature is not detected.
Therefore, there is a possibility of an estimated value of the exhaust gas
temperature becoming largely different from an actual value of same. To
overcome this disadvantage, it is required to make wider the engine
operating region in which the supply air-fuel ratio should be enriched
(hereinafter referred to as "the high load enriching region"), i.e. set a
reference temperature for determining whether the supply air-fuel ratio
should be enriched to a lower value. As a result, there can be cases where
the air-fuel ratio is unnecessarily enriched, which results in degradation
of fuel consumption and exhaust emission characteristics.
Further, according to the above technique (1), the temperature of the
exhaust system is determined only by detecting the exhaust gas
temperature, and the desired exhaust gas temperature is set based on the
amount of intake air, engine rotational speed, intake air temperature, and
engine coolant temperature to control the supply air-fuel ratio such that
the detected exhaust gas temperature becomes equal to the desired exhaust
gas temperature. However, the temperature of engine component parts, which
may rise to an excessive degree, varies not only by the exhaust gas
temperature but also by the volume of hot exhaust gases. More
specifically, even if the exhaust gas temperature remains unchanged, the
rate of rise in the temperature of engine component parts tends to be
lower when the volume of exhaust gases (which may be determined by the
engine rotational speed and engine load) is smaller than when the volume
of exhaust gases is larger. In the case of a three-way catalyst, for
example, it has been found that even if the exhaust gas temperature
remains unchanged, when the engine is in a high load and high engine
rotational speed condition, in which the volume of exhaust gases is
larger, the rate of thermal conduction to the three-way catalyst tends to
increase due to the flow of an increased volume of hot exhaust gases to
cause the temperature of the three-way catalyst to rise at a higher rate.
On the other hand, when the volume of exhaust gases is smaller, the
temperature of the three-way catalyst rises at a lower rate in spite of
the presence of the flow of hot exhaust gases. Further, it has also been
found that due to difference in thermal capacity between engine component
parts, the amount of thermal conduction therethrough varies with the
engine component parts, which causes the temperatures of the engine
component parts to rise at different rates as time elapses. Therefore,
there is room for improvement of this technique (1) concerning fuel
consumption and exhaust emission characteristics.
SUMMARY OF THE INVENTION
It is an object of the invention to provide an air-fuel ratio control
method for an internal combustion engine which is capable of improving the
accuracy of estimated temperatures of component parts of the engine, and
reducing fuel consumption and emission of CO when the engine is in a high
load operating condition.
To attain the above object, the invention provides an air-fuel ratio
control method for an internal combustion engine including an exhaust
passage, an exhaust gas ingredient concentration sensor arranged in the
exhaust passage for detecting concentration of an exhaust gas ingredient,
and at least one component part temperature of which is to be controlled,
wherein the air-fuel ratio of an air-fuel mixture supplied to the engine is
feedback-controlled to a predetermined value in response to an output from
the exhaust gas ingredient concentration sensor, and when it is determined
that the engine is in a predetermined high load operating condition and at
the same time the at least one component part of the engine is in a
predetermined high temperature state, the air-fuel ratio of the air-fuel
mixture is inhibited from being feedback-controlled but enriched instead.
The air-fuel ratio control method according to the invention is
characterized by comprising the steps of:
(1) detecting a temperature of exhaust gases emitted from the engine;
(2) detecting rotational speed of the engine;
(3) detecting load on the engine;
(4) estimating temperature of the at least one component part of the engine
based on the temperature of exhaust gases, engine rotational speed, and
engine load, detected at the above steps (1) to (3); and
(5) determining that the at least one component part of the engine is in
the predetermined high temperature state when the estimated temperature of
any one of the at least one component part of the engine is higher than a
corresponding predetermined value.
Preferably, the estimated temperature T of the at least one component part
of the engine is calculated by the use of the following equation:
T=TE.times.KNE.times.KPB
where TE represents the detected temperature of exhaust gases, KNE
represents an engine rotational speed-dependent correction coefficient set
according to the detected engine rotational speed, and KPB represents an
engine load-dependent correction coefficient set according to the detected
engine load.
More preferably, the engine rotational speed-dependent correction
coefficient KNE is set to a larger value as the engine rotational speed is
higher, and the engine load-dependent correction coefficient KPB is set to
a larger value as the engine load is higher.
Preferably, the estimated temperature of the at least one component part of
the engine is obtained by averaging a plurality of values of the estimated
temperature, and the speed of the averaging is changed depending on the
engine load.
More preferably, the engine load is intake pipe absolute pressure corrected
according to intake air temperature.
Preferably, the air-fuel ratio of an air-fuel mixture supplied to the
engine is controlled to a first value richer than a stoichiometric
air-fuel ratio after it was determined that the at least one component
part of the engine is in the predetermined high temperature state and
before a predetermined time period elapses thereafter, and the air-fuel
ratio of the air-fuel mixture is controlled to a second value richer than
the first value after the predetermined time period elapses.
More preferably, the enriching of the air-fuel ratio of the air-fuel
mixture is carried out by multiplying a basic amount of fuel supplied to
the engine, which is determined according to the engine rotational speed
detected and intake pipe pressure, by a predetermined enriching
coefficient, the predetermined enriching coefficient being determined
based on the engine rotational speed detected and the intake pipe
pressure.
Further preferably, the predetermined enriching coefficient is set to a
value read from a map set according to the engine rotational speed
detected and the intake pipe pressure before the predetermined time period
elapses to thereby control the air-fuel ratio of the air-fuel mixture to
the first value, the predetermined enriching coefficient being set to a
value obtained by multiplying the value read from the map by an enriching
coefficient after the predetermined time period has elapsed to thereby
control the air-fuel ratio of the air-fuel mixture to the second value.
The above and other objects, features, and advantages of the invention will
be more apparent from the ensuing detailed description taken in
conjunction with the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a schematic diagram of the whole arrangement of a fuel supply
control system for an internal combustion engine to which is applied the
control method according to the invention;
FIGS. 2a and 2b are flowcharts of a program for performing calculation of
estimated temperatures of component parts of the engine;
FIG. 3 is a flowchart of a subroutine for correcting the intake pipe
absolute pressure depending on intake air temperature;
FIG. 4 is a diagram showing a table for calculating an intake air
temperature-dependent correction coefficient (KTEXG);
FIG. 5 is a flowchart of a subroutine for calculating an estimated
temperature value (TCAT) of a three-way catalyst;
FIG. 6a is a diagram showing a table for calculating a coefficient (KNCAT)
for correcting an exhaust gas temperature value;
FIG. 6b is a diagram showing a table for calculating a coefficient (KPBCAT)
for correcting an exhaust gas temperature value;
FIG. 6c is a diagram showing a table for calculating an averaging
coefficient (TREFO);
FIG. 7 is a flowchart of a subroutine for calculating an estimated
temperature value (TEXM) of an exhaust pipe;
FIG. 8a is a diagram showing a table for calculating a coefficient (KNEXM)
for correcting an exhaust gas temperature value;
FIG. 8b is a diagram showing a table for calculating a coefficient (KPBEXM)
for correcting an exhaust gas temperature value;
FIG. 8c is a diagram showing a table for calculating a coefficient (KVEXM)
for correcting an exhaust gas temperature value;
FIG. 9 is a flowchart of a subroutine for calculating an estimated
temperature value (TPIS) of pistons;
FIG. 1Oa is a diagram showing a table for calculating a coefficient (KNPIS)
for correcting an exhaust gas temperature value;
FIG. 1Ob is a diagram showing a table for calculating a coefficient
(KPBPIS) for correcting an exhaust gas temperature value;
FIG. 1Oc is a diagram showing a table for calculating a variable (DTPIS)
for correcting an exhaust gas temperature value;
FIG. 11 is a flowchart of a subroutine for calculating an estimated
temperature value (TEXV) of exhaust valves;
FIG. 12a is a diagram showing a table for calculating a coefficient (KNEXV)
for correcting an exhaust gas temperature value;
FIG. 12b is a diagram showing a table for calculating a coefficient
(KPBEXV) for correcting an exhaust gas temperature value;
FIG. 12c is a diagram showing a table for calculating a variable (DTEXV)
for correcting an exhaust gas temperature value;
FIG. 13 is a flowchart of a subroutine for setting a high temperature flag
(FXAVE);
FIGS. 14a and 14b are a flowchart of a program for calculating a high load
enriching coefficient (KWOT);
FIG. 15 is a diagram showing a table for calculating reference values
(PBWOT1, PBWOT2) for determining whether the engine is in a high load
operating condition;
FIG. 16 is a diagram showing a table for calculating an enriching
coefficient (XWOTR) and a leaning coefficient (XWOTL); and
FIG. 17 is a diagram showing a table for calculating an engine coolant
temperature-dependent enriching coefficient (XWOTTW).
DETAILED DESCRIPTION
The method according to 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 a fuel
supply control system which is adapted to carry out the control method of
this invention. In the figure, reference numeral 1 designates an internal
combustion engine. In an intake pipe 2 of the engine 1, there is arranged
a throttle body 3 accommodating a throttle valve 3' therein. A throttle
valve opening (.theta.TH) sensor 4 is connected to the throttle valve 3'
for generating an electric signal indicative of the sensed throttle valve
opening and supplying same to an electronic control unit (hereinafter
referred to as "the ECU") 5.
Fuel injection valves 6 ar each provided for each cylinder, not shown, and
arranged in the intake pipe 2 between the engine 1 and the throttle valve
3, and at a location slightly upstream of an intake valve, 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.
Further, an intake pipe absolute pressure (PBA) sensor 8 is provided in
communication with the interior of the intake pipe 2 via a conduit 7 at a
location immediately downstream of the throttle valve 3' for supplying an
electric signal indicative of the sensed absolute pressure to the ECU 5.
An intake temperature (TA) sensor 9 is inserted into the intake pipe 2 at
a location downstream of the intake pipe absolute pressure sensor 8 for
supplying an electric signal indicative of the sensed intake temperature
TA to the ECU 5.
An engine coolant temperature (TW) sensor 10, which may be formed of a
thermistor or the like, is mounted in the cylinder block of the engine 1
for supplying an electric signal indicative of the sensed engine coolant
temperature TW to the ECU 5. An engine rotational speed (NE) sensor 11 and
a cylinder-discriminating (CYL) sensor 12 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 11 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 12
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.
A three-way catalyst 14 is arranged within an exhaust pipe 13 connected to
the cylinder block of the engine 1 for purifying noxious components such
as HC, CO and NO.sub.X. An O.sub.2 sensor 15 as an exhaust gas ingredient
concentration sensor is mounted in the exhaust pipe 13 at a location
upstream of the three-way catalyst 14, for detecting the oxygen
concentration in the exhaust gases and supplying an electric signal
indicative of a detected value of the oxygen concentration to the ECU 5.
Further, an exhaust gas temperature sensor 16 is mounted in the exhaust
pipe 13 at a location upstream of the O.sub.2 sensor 15 for supplying a
signal indicative of a detected value of the exhaust gas temperature to
the ECU 5. A vehicle speed (VSP) sensor 17, which is also connected to the
ECU 5, detects a travelling speed (vehicle speed VSP) of an automotive
vehicle on which the engine is installed, and supplies a signal indicative
of the vehicle speed VSP to the ECU 5.
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 referred to as "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 above-mentioned signals from the
sensors to determine operating conditions in which the engine 1 is
operating such as an air-fuel ratio feedback control region and open-loop
control regions, 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:
T.sub.OUT =Ti.times.K.sub.1 .times.KWOT.times.KTW.times.K.sub.O2 +K.sub.2(
1)
where Ti represents a basic fuel amount, more specifically a basic fuel
injection period, and is read from a Ti map set according to the engine
rotational speed NE and the intake pipe absolute pressure PBA and stored
in the memory means 5c. KWOT is a high load enriching coefficient for
enriching an air-fuel mixture supplied to the engine when the throttle
valve 3' is substantially fully open, and determined in such a manner as
described hereinbelow with reference to FIGS. 14a and 14b. KTW is an
engine coolant temperature-dependent fuel increasing coefficient for
enriching the air-fuel mixture when the engine coolant temperature TW is
equal to or lower than a predetermined value. K.sub.O2 is an air-fuel
ratio feedback correction coefficient which is set responsive to the
oxygen concentration in the exhaust gases when the engine is in a feedback
control region, and set, when the engine is not in the feedback control
region but in any of particular regions (open loop control regions), to a
value peculiar thereto. A state in which the engine is in a predetermined
high load operating condition and the temperature of any of predetermined
component parts of the engine is high is included in the open loop control
regions.
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 accelerability depending on engine
operating conditions.
The CPU 5b supplies the fuel injection valves 6 with driving signals based
on the fuel injection period T.sub.OUT calculated as above via the output
circuit 5d.
FIG 2 shows a program for calculating estimated temperature values of
component parts of the engine, i.e. the three-way catalyst 14, the exhaust
pipe 13, pistons in the cylinders, and the exhaust valves, and setting,
based on the estimated temperature values, first and second enriching
flags FHSFE1, FHSFE2 for enriching the supply air-fuel ratio. This program
is executed at constant time intervals (e.g. of 80 millisec.).
At a step S1, an output from the exhaust gas temperature sensor 16 is read.
The exhaust gas temperature sensor 16 used in the present embodiment is
comprised of a thermistor. The output from the exhaust gas temperature
sensor 16 is in a non-linear relationship to the exhaust gas temperature.
Therefore, the sensor output is converted into the exhaust gas temperature
(TE) by the use of a table stored in the memory means 5c. In performing
the conversion, linear interpolation is effected for values of the sensor
output other than the values of same stored beforehand.
At a step S2, the intake pipe absolute pressure PBA is corrected based on
the intake air temperature TA, by a subroutine shown in FIG. 3. More
specifically, an intake air temperature-dependent correction coefficient
KTAEXG is calculated according to the intake air temperature TA, and the
intake pipe absolute pressure PBA is multiplied by the coefficient KTAEXG
to obtain corrected intake pipe absolute pressure PBAEX. The correction
coefficient KTAEXG is read from a KTAEXG table in FIG. 4, in which
coefficient values KTAEXG0 to KTAEXG2 (e.g. 1.15, 1.0, 0.95) are set
corresponding to predetermined values TAEXG0 to TAEXG2 (e.g. -10.degree.
C., 30.degree. C., 50.degree. C.) of the intake air temperature TA. For
values other than the predetermined values TAEXG0 to TAEXG2 of the intake
air temperature TA, interpolation is carried out. Reading of values from
other tables, referred to hereinafter, is similarly carried out. The
intake air temperature-dependent correction of the intake pipe absolute
pressure PBA is performed in order to take into consideration variation in
charging efficiency caused by the intake air temperature TA. More
specifically, as the charging efficiency is lower, the weight of intake
air which is used in burning of the air-fuel mixture decreases, so that
combustion temperature tends to become lower accordingly. In view of this
tendency, the intake pipe absolute pressure PBA is corrected such that its
value decreases to such an extend as corresponds to an amount of decrease
in the weight of the intake air, whereby the accuracy of calculation of
estimated values of increased temperatures of component parts in the
exhaust system of the engine is improved.
Referring again to FIG. 2, at a step S3, an estimated temperature value
(hereinafter simply referred to as the "catalyst temperature") TCAT of the
three-way catalyst 14 is calculated by a subroutine shown in FIG. 5.
In FIG. 5, at a step S31, it is determined whether or not the engine is in
a starting mode. If the answer to this question is affirmative (YES), i.e.
if the engine is in the starting mode, both the catalyst temperature TCAT
and an average value TCATave thereof are set to a predetermined initial
value TCATO (e.g. 400.degree. C.) at steps S32 and S33, and then the
program proceeds to a step S37.
If the answer to the question of the step S31 is negative (NO), i.e. if the
engine is not in the starting mode, correction coefficients KNCAT and
KPBCAT for converting the exhaust gas temperature TE detected into the
catalyst temperature TCAT are calculated at steps S34 and S35, and then
the catalyst temperature TCAT is obtained by multiplying the exhaust gas
temperature TE by these correction coefficients at a step S36.
KNCAT is an engine rotational speed-dependent correction coefficient set
according to the engine rotational speed. As shown in FIG. 6a, a value
thereof is read from a KNCAT table in which values KNCAT0 to KNCAT2 are
set corresponding to predetermined values NCAT0 and NCAT2 of the engine
rotational speed. The correction coefficient KNCAT assumes a larger value
as the engine rotational speed NE is higher, and if NE=NCAT1 (e.g. 3,000
rpm), KNCAT=1.0.
KPBCAT is an engine load-dependent correction coefficient which is set
according to the corrected intake pipe absolute pressure PBAEX. As shown
in FIG. 6b, a value thereof is read from a KPBCAT table in which values
KPBCAT0 to KPBCAT2 are set corresponding to predetermined values PBACAT0
to PBACAT2 of the corrected intake pipe absolute pressure PBAEX. The
correction coefficient KPBCAT assumes a larger value as the corrected
intake pipe absolute pressure PBAEX is higher, and if PBAEX=PBACAT1 (e.g.
510 mmHg), KPBCAT=1.0.
The reason for setting the correction coefficients KNCAT and KPBCAT such
that they increase as the engine rotational speed NE and the intake pipe
absolute pressure PBA increase is that the rate of thermal conduction
varies with the volume of exhaust gases, which causes a resulting
variation in the temperature of a component part (in this case the
three-way catalyst) of the engine. In estimating temperatures of the
exhaust pipe, the pistons in the cylinders, and the exhaust valves, which
will be described later, the exhaust gas temperature TE will be converted
into the respective temperatures in a similar manner, i.e. by correcting
the exhaust gas temperature TE according to the volume of exhaust gases
which is determined by the engine rotational speed NE and the engine load.
At a step S37, an average value TCATave of the catalyst temperature is
calculated by the use of the following equation (2):
##EQU1##
wherein (n) and (n-1) indicate that the values are obtained in the present
loop and the last loop, respectively. TREFO is an averaging coefficient
which determines the rate of contribution of a present value TCAT(n) of
the catalyst temperature to a present value TCATave(n) of the average
value. As TREFO increases, TCAT(n) contributes to TCATave(n) at a larger
rate, so that the averaging speed increases. In this connection, in the
present embodiment, the averaging coefficient TREFO is read from a TREFO
table, as shown in FIG. 6c, in which values thereof are set according to
the corrected intake pipe absolute pressure PBAEX.
In the TREFO table, predetermined values TREFOL and TREFOH (e.g. 87, 1190)
are set corresponding to predetermined values PBTREFL and PBTREFH (e.g.
150 mmHg, 480 mmHg) of the corrected intake pipe absolute pressure. The
averaging value TREFO increases as the engine load is higher. This takes
into consideration the fact that as the engine load is lower, the volume
of exhaust gases (the amount of exhaust gases emitted per unit time
period) is smaller, which results in a smaller rate of change in the
temperature of a component part. By thus setting the averaging coefficient
TREFO, it is possible to obtain a proper average value TCATave
corresponding to the engine load.
Further, an amount of rise in the temperature of a component part of the
engine per unit time period is dependent not only on the engine rotational
speed and the engine load but also on the thermal capacity of the
component part, each component part of the engine having a thermal
capacity peculiar thereto. Therefore, the averaging coefficient TREF is
set for each component part, separately. In estimating the temperatures of
the exhaust pipe, the pistons in the cylinders, and the exhaust valves,
which will be described later, the averaging coefficient TREF is set for
each of the component parts in a similar manner according to an amount of
rise in the temperature of the component part of the engine per unit time
period.
Referring again to FIG. 2, at a step S4, an estimated temperature value
(herinafter simply referred to as the "exhaust pipe temperature") TEXM of
the exhaust pipe is calculated by a subroutine shown in FIG. 7.
Similarly to the program of FIG. 5, if the engine is in the starting mode
(the answer to the question of the step S41 is affirmative (YES)), both
the exhaust pipe temperature TEXM and an average value TEXMave thereof are
set to a predetermined initial value TEXMO (e.g. 400.degree. C.) at steps
S42 and S43. On the other hand, if the engine is not in the starting mode
(the answer to the question of the step S41 is negative (NO)), correction
coefficients KNEXM, KPBEXM, and KVEXM for converting the exhaust gas
temperature TE detected into the exhaust pipe temperature TEXM are
calculated at steps S44 to S46, and then the exhaust pipe temperature TEXM
is obtained by multiplying the exhaust gas temperature RE by these
coefficients at a step S47.
KNEXM and KPBEXM are engine rotational speed-dependent and engine
load-dependent correction coefficients for obtaining the exhaust pipe
temperature which correspond to the engine rotational speed-dependent
correction coefficient KNCAT and the load-dependent correction coefficient
KPBCAT. Values thereof are read from a KNEXM table shown in FIG. 8a and a
KPBEXM table shown in FIG. 8b. In the KNEXM table, similarly to the
aforementioned KNCAT table, values KNEXM0 to KNEXM2 are set corresponding
to predetermined values NEXM0 to NEXM2 of the engine rotational speed, and
if NE=NEXM1 (e.g. 3,500 rpm), KNEXM1=1.0. In the KPBEXM table, similarly
to the KPBCAT table, values KPBEXM0 to KPBEXM2 are set corresponding to
predetermined values PBAEXM0 to PBAEXM2 of the corrected exhaust pipe
absolute pressure PBAEX, and if PBAEX=PBAEXM1 (e.g. 510 mmHg),
KPBEXM1=1.0.
KVEXM is a vehicle speed-dependent correction coefficient which is set
according to the vehicle speed VSP. As shown in FIG. 8c, values KVEXM0 to
KVEXM2 are set corresponding to predetermined values VEXM0 to VEXM2 of the
vehicle speed. The coefficient KVEXM decreases as the vehicle speed VSP is
lower, and if VSP=VEXM1 (e.g. 120 km/h), KVEXM=1.0. This is to lower the
estimated temperature value of the exhaust pipe when the vehicle speed VSP
is higher, since the exhaust pipe of the engine is more cooled as the
vehicle travels at a higher speed.
At a step S48, an average value TEXave of the exhaust pipe temperature is
calculated by the use of the following equation (b 3):
##EQU2##
The equation (3) is similar to the equation (2), and where the averaging
coefficient TREF1 is set to a fixed value, e.g. 20.
Referring again to FIG. 2, at a step S5, an estimated temperature value
(hereinafter simply referred to as the "piston temperature") TPIS of the
pistons in the cylinders is calculated by a subroutine shown in FIG. 9.
In the program of FIG. 9, the piston temperature TPIS and an average value
TPISave thereof are calculated in a manner similar to those of the
programs described with reference to FIGS. 5 and 7. More specifically, if
the engine is in the starting mode (the answer to the question of a step
S51 is affirmative (YES)), both the piston temperature TPIS and the
average value TPISave thereof are set to their predetermined initial value
TPISO (e.g. 80.degree. C.) at steps S52 and S53. On the other hand, if the
engine is not in the starting mode (the answer to the question of th | | |
