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Claims  |
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What is claim is:
1. A turbocharger control system for a turbocharged internal combustion engine equipped with an exhaust-gas recirculation control system recycling part of inert exhaust gas back
through the engine under predetermined operating conditions of the engine, comprising:
a variable-displacement turbocharger variably adjusting a turbo-charging state; and
a turbocharger control unit correcting a controlled quantity of said variable-displacement turbocharger in response to correction made to a desired EGR rate for the exhaust-gas recirculation control system.
2. A turbocharger control system for a turbocharged internal combustion engine equipped with an exhaust-gas recirculation control system recycling part of inert exhaust gas back through the engine under predetermined operating conditions of the
engine, comprising:
a variable-displacement turbocharger variably adjusting a turbo-charging state;
sensors detecting operating conditions of the engine;
a turbocharger control unit configured to be electronically connected to said sensors and said variable-displacement turbocharger for automatically regulating the turbo-charging state; said turbocharger control unit comprising
(1) a desired EGR rate arithmetic-calculation section arithmetically calculating a desired EGR rate (MEGRM) as a function of a first predetermined engine operating condition of the operating conditions detected by said sensors,
(2) a desired turbo-charging controlled quantity arithmetic-calculation section arithmetically calculating a desired turbo-charging controlled quantity of said variable-displacement turbocharger as a function of the first predetermined engine
operating condition,
(3) a desired EGR-rate correction section correcting the desired EGR rate (MEGRM), and
(4) a desired turbo-charging controlled quantity correction section correcting the desired turbo-charging controlled quantity in response to correction made to the desired EGR rate by said desired EGR-rate correction section.
3. The turbocharger control system as claimed in claim 2, wherein the correction, which is made to the desired turbo-charging controlled quantity in response to the correction made to the desired EGR rate, is executed as a correcting action made
to a feedforward controlled quantity.
4. The turbocharger control system as claimed in claim 3, which further comprises an air-flow meter detecting an actual intake-air flow rate (QAVNT) and a desired turbo-charging state arithmetic-calculation section estimating a desired value
(QCSSP2) of the turbo-charging state, and wherein feedback control is executed so that feedback correction based on comparison of the desired value (QCSSP2) of the turbo-charging state with the actual intake-air flow rate (QAVNT) is made with respect to
the feedforward controlled quantity under a predetermined operating condition of the engine.
5. The turbocharger control system as claimed in claim 4, wherein, when the exhaust-gas recirculation control system executes EGR control, the feedback control of the variable-displacement turbocharger is inhibited, and open-loop feedforward
control is executed on the basis of only the feedforward controlled quantity.
6. The turbocharger control system as claimed in claim 2, wherein said desired turbo-charging controlled quantity correction section comprises a converting section converting the correction made to the desired EGR rate into a correction value of
an intake-air flow rate of fresh air induced into the engine, and an arithmetic-calculation section arithmetically calculating a correction value of the variable-displacement turbocharger, based on the correction value of the intake-air flow rate, and
whereby said desired turbo-charging controlled quantity correction section corrects the desired turbo-charging controlled quantity by the correction value of the variable-displacement turbocharger.
7. The turbocharger control system as claimed in claim 2, wherein said desired EGR-rate correction section corrects the desired EGR rate (MEGRM) as a function of a second predetermined engine operating condition of the operating conditions
detected by said sensors, except the first predetermined engine operating condition.
8. The turbocharger control system as claimed in claim 2, wherein the first predetermined engine operating condition includes at least one of engine speed and engine load, and the second predetermined engine operating condition includes at least
one of engine temperature, atmospheric pressure, and intake-air temperature.
9. In an internal combustion engine equipped with an exhaust-gas recirculation control system recycling part of inert exhaust gas back through the engine under predetermined operating conditions of the engine, and a variable-displacement
turbocharger variably adjusting a turbo-charging state, comprising:
a sensor means for detecting operating conditions of the engine;
a turbocharger control means configured to be electronically connected to said sensor means and said variable-displacement turbocharger for automatically regulating the turbo-charging state; said turbocharger control means comprising
(1) a desired EGR rate arithmetic-calculation means for arithmetically calculating a desired EGR rate (MEGRM) as a function of a first predetermined engine operating condition of the operating conditions detected by said sensors,
(2) a desired turbo-charging controlled quantity arithmetic-calculation means for arithmetically calculating a desired turbo-charging controlled quantity of said variable-displacement turbocharger as a function of the first predetermined engine
operating condition,
(3) a desired EGR-rate correction means for correcting the desired EGR rate (MEGRM), and
(4) a desired turbo-charging controlled quantity correction means for correcting the desired turbo-charging controlled quantity in response to correction made to the desired EGR rate by said desired EGR-rate correction section.
10. A method for controlling a variable-displacement turbocharger employed in an internal combustion engine, wherein the engine includes an exhaust-gas recirculation control system recycling part of inert exhaust gas back through the engine and
having sensors detecting operating conditions of the engine, and an electronic control unit configured to be electronically connected to the sensors and the variable-displacement turbocharger for automatically regulating a turbo-charging state of the
variable-displacement turbocharger, the method comprising:
correcting a controlled quantity of the variable-displacement turbocharger in response to correction made to a desired EGR rate for the exhaust-gas recirculation control system.
11. A method for controlling a variable-displacement turbocharger employed in an internal combustion engine, wherein the engine includes an exhaust-gas recirculation control system recycling part of inert exhaust gas back through the engine and
having sensors detecting operating conditions of the engine, an air-flow meter detecting an actual intake-air flow rate (QAVNT), and an electronic control unit configured to be electronically connected to the sensors, the air-flow meter and the
variable-displacement turbocharger for automatically regulating a turbo-charging state of the variable-displacement turbocharger, the method comprising:
arithmetically calculating a desired EGR rate (MEGRM) as a function of a first predetermined engine operating condition of the operating conditions detected by the sensors,
arithmetically calculating a desired turbo-charging controlled quantity of the variable-displacement turbocharger as a function of the first predetermined engine operating condition,
correcting the desired EGR rate (MEGRM),
correcting the desired turbo-charging controlled quantity in response to correction made to the desired EGR rate, said correction, which is made to the desired turbo-charging controlled quantity in response to the correction made to the desired
EGR rate, is executed as a correcting action made to a feedforward controlled quantity,
estimating a desired value (QCSSP2) of the turbo-charging state,
arithmetically calculating a feedback correction value (DUTS) by comparing the desired value (QCSSP2) of the turbo-charging state with the actual intake-air flow rate (QAVNT),
executing feedback correction with respect to the feedforward controlled quantity, using the feedback correction value (DUTS) under a predetermined operating condition of the engine.
12. The method as claimed in claim 11, which further comprises inhibiting the feedback control of the variable-displacement turbocharger and executing open-loop feedforward control on the basis of only the feedforward controlled quantity, when
the exhaust-gas recirculation control system executes EGR control.
13. The method as claimed in claim 11, which further comprises converting the correction made to the desired EGR rate into a correction value of an intake-air flow rate of fresh air induced into the engine, and arithmetically calculating a
correction value of the variable-displacement turbocharger on the basis of the correction value of the intake-air flow rate, so that the desired turbo-charging controlled quantity is corrected by the correction value of the variable-displacement
turbocharger.
14. The method as claimed in claim 11, which further comprises correcting the desired EGR rate (MEGRM) as a function of a second predetermined engine operating condition of the operating conditions detected by the sensors, except the first
predetermined engine operating condition. |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to the improvements of a turbo-charger control system for a turbocharged internal combustion engine equipped with an exhaust-gas recirculation control system, specifically to techniques for cross-correlation between
exhaust-gas recirculation (EGR) control and turbocharger control during EGR addition particularly on diesel engines equipped with a variable displacement turbocharger and an exhaust-gas recirculation system.
2. Description of the Prior Art
A more earlier-model-internal combustion engine is often equipped with a variable displacement (often called as a "variable nozzle") turbocharger in which an inlet opening area (or an opening size) between respective two adjacent vanes or blades
of a turbine wheel is variably controlled depending on engine operating conditions. One such variable nozzle control device for a variable-nozzle turbocharger has been disclosed in Japanese Patent Provisional Publication No. 58-176417. On the other
hand, almost all of automotive internal combustion engines have exhaust-gas recirculation systems, which are used to reduce nitrogen oxide (NO.sub.x) emissions from exhaust gases of the engine by way of the fall of combustion temperature, caused by
recycling of some of the inert exhaust gas back through an intake manifold. One such EGR system has been disclosed in Japanese Patent Provisional Publication No. 60-230555. On one hand, EGR is useful to decrease the formation of NO.sub.x. On the other
hand, undesiredly excessive EGR deteriorates combustion, thus dropping engine power output, and also reducing driveability of the vehicle. In recent years, it is desirable to achieve high-precision EGR control in due consideration of various factors,
namely engine temperature (during cold engine start, during warming-up period, or in the engine warmed-up state), the reduced effective opening of an EGR control valve occurring owing to lubricating oil (engine oil) or a product of combustion (carbon
deposits) adhered to the EGR control valve, changes in an intake-air flow rate occurring due to changes in air density, arising from changes in environment from low-land driving to high-land driving under constant engine speed and load, and the delay in
boost pressure (often called "turbo-lag") on turbo-charged engines in a transient state, such as in a transition from normal-straight ahead driving to heavy vehicle acceleration.
SUMMARY OF THE INVENTION
Heretofore, there is no technique for cross-correlation between exhaust-gas recirculation (EGR) control made to an EGR system and turbosupercharger control (simply turbocharger control) made to a variable-displacement turbocharger during EGR
addition. Gasoline engines utilize a comparatively small amount of EGR, so that a rate of exhaust gas recirculated (simply an EGR rate) is properly regulated within a comparatively small EGR-rate range. A turbo-supercharging state of the gasoline
engine is not so affected by exhaust-gas recirculation (EGR). In contrast to the above, on turbo-charged diesel engines, where an EGR rate is widely regulated from a predetermined minimum EGR rate (with an EGR control valve closed) to a predetermined
maximum EGR rate at which the quantity of EGR is nearly equal to the actually-induced fresh air flow rate (the actual intake-air flow rate), the exhaust-gas flow rate remarkably varies owing to changes in the EGR rate. For example, a greatly-increased
EGR rate largely reduces the exhaust-gas flow rate, thereby reducing the energy (in exhaust-gas flow) input to the turbine wheel. Such a greatly-increased EGR has a great influence (i.e., a remarkable fall in boost pressure) on the turbo-charging state. In this case, it is desirable to increase the controlled quantity (the controlled variable) of a fluid-flow throttling degree or a fluid-flow restricting degree of the inlet side of the turbine wheel of the turbocharger, (in other words, to decrease the
opening size of the inlet side of the turbine wheel), so as to rise the boost pressure provided by the turbocharger. Generally, a desired EGR rate is preset or preprogrammed for every engine operating ranges different from each other depending on engine
speed and load. As regards variations in the desired EGR rate predetermined or preset for every operating ranges, it is possible to preset a desired controlled quantity of turbo-charging operation of the turbocharger, simply a desired turbo-charging
controlled quantity, corresponding to a preset desired EGR rate for each engine operating range. However, there is an increased tendency of undesirable rise in boost pressure during transient-state running (rather than during steady-state running), for
example, in a transient state from cold-engine acceleration to completion of warm up of the engine. When input information data (engine speed, engine load, engine temperature, and the like) are within respective EGR permission zones or ranges during
cold diesel-engine acceleration operation or during starting and warming up from cold, the EGR rate is decreasingly corrected or EGR is almost stopped. In such a transient state where there is an increased tendency of switching between the EGR
permission mode and the EGR inhibition mode, assuming that the turbocharger control is executed in accordance with a relatively great EGR rate suitable for an engine warmed-up state and thus the controlled quantity of the fluid-flow restricting degree of
the inlet side of the turbocharger is regulated in a direction rising the boost pressure, there is a tendency for the boost pressure to excessively rise. Thereafter, as the engine/vehicle accelerates, the input informational data such as the engine
temperature (or engine coolant temperature), the engine speed, the engine load, and the like will be out of the EGR permission zones and be transferred to an EGR inhibition zone where an EGR mode is inhibited. Under such an EGR inhibition, if the
controlled quantity of the fluid-flow restricting degree of the inlet side of the turbocharger remains kept great, the boost pressure tends to further rise owing to the increased exhaust-gas flow. Furthermore, there is a problem of mutual interference
(undesirable system hunting) between the EGR control system and the turbocharger control system, when feedback control is made with regard to the turbocharger during execution of the EGR control or during EGR addition. A certain method for providing
stable system control and for avoiding the undesirable system hunting is to execute feedback control for the turbocharger only during inhibition of EGR (or only in a non-EGR region). However, if feedback correction for the turbo-charging controlled
quantity is executed from a time when shifting to the non-EGR region during the transient operating state, and then the turbocharger control system begins to decreasingly correct boost pressure, there may be a comparatively great delay of response time.
Owing to the delay of response time, the rise in boost pressure cannot be timely adequately suppressed. As discussed above, in the previously-noted transient state, it is difficult to attain satisfactory cross-correlation control between the EGR control
system and the variable-displacement turbocharger during EGR addition by way of only feedback control.
Accordingly, it is an object of the invention to provide a turbocharger control system for a turbocharged internal combustion engine equipped with an exhaust-gas recirculation system which avoids the aforementioned disadvantages of the prior art.
It is another object of the invention to provide a turbocharger control system for an internal combustion engine equipped with a variable-displacement turbocharger and an exhaust-gas recirculation system, which is capable of providing a proper
turbo-charging performance irrespective of in a transient operating state or in a steady operating state, by controlling the variable-displacement turbocharger in correlation with EGR control.
In order to accomplish the aforementioned and other objects of the present invention, a turbocharger control system for a turbocharged internal combustion engine equipped with an exhaust-gas recirculation control system recycling part of inert
exhaust gas back through the engine under predetermined operating conditions of the engine, comprises a variable-displacement turbocharger variably adjusting a turbo-charging state, and a turbocharger control unit correcting a controlled quantity of the
variable-displacement turbocharger in response to correction made to a desired EGR rate for the exhaust-gas recirculation control system.
According to another aspect of the invention, a turbocharger control system for a turbocharged internal combustion engine equipped with an exhaust-gas recirculation control system recycling part of inert exhaust gas back through the engine under
predetermined operating conditions of the engine, comprises a variable-displacement turbocharger variably adjusting a turbo-charging state, sensors detecting operating conditions of the engine, a turbocharger control unit configured to be electronically
connected to the sensors and the variable-displacement turbocharger for automatically regulating the turbo-charging state, the turbocharger control unit comprising a desired EGR rate arithmetic-calculation section arithmetically calculating a desired EGR
rate as a function of a first predetermined engine operating condition of the operating conditions detected by the sensors, a desired turbo-charging controlled quantity arithmetic-calculation section arithmetically calculating a desired turbo-charging
controlled quantity of the variable-displacement turbocharger as a function of the first predetermined engine operating condition, a desired EGR-rate correction section correcting the desired EGR rate, and a desired turbo-charging controlled quantity
correction section correcting the desired turbo-charging controlled quantity in response to correction made to the desired EGR rate by the desired EGR-rate correction section.
According a further aspect of the invention, a method for controlling a variable-displacement turbocharger employed in an internal combustion engine, wherein the engine includes an exhaust-gas recirculation control system recycling part of inert
exhaust gas back through the engine and having sensors detecting operating conditions of the engine, and an electronic control unit configured to be electronically connected to the sensors and the variable-displacement turbocharger for automatically
regulating a turbo-charging state of the variable-displacement turbocharger, the method comprises correcting a controlled quantity of the variable-displacement turbocharger in response to correction made to a desired EGR rate for the exhaust-gas
recirculation control system.
According to a still further aspect of the invention, a method for controlling a variable-displacement turbocharger employed in an internal combustion engine, wherein the engine includes an exhaust-gas recirculation control system recycling part
of inert exhaust gas back through the engine and having sensors detecting operating conditions of the engine, an air-flow meter detecting an actual intake-air flow rate, and an electronic control unit configured to be electronically connected to the
sensors, the air-flow meter and the variable-displacement turbocharger for automatically regulating a turbo-charging state of the variable-displacement turbocharger, the method comprising arithmetically calculating a desired EGR rate as a function of a
first predetermined engine operating condition of the operating conditions detected by the sensors, arithmetically calculating a desired turbo-charging controlled quantity of the variable-displacement turbocharger as a function of the first predetermined
engine operating condition, correcting the desired EGR rate, correcting the desired turbo-charging controlled quantity in response to correction made to the desired EGR rate, the correction, which is made to the desired turbo-charging controlled quantity
in response to the correction made to the desired EGR rate, is executed as a correcting action made to a feedforward controlled quantity, estimating a desired value of the turbo-charging state, arithmetically calculating a feedback correction value by
comparing the desired value of the turbo-charging state with the actual intake-air flow rate, executing feedback correction with respect to the feedforward controlled quantity, using the feedback correction value (DUTS) under a predetermined operating
condition of the engine.
BRIEF DESCRIPTION OF THE DRAWINGS
FIG. 1 is a system diagram illustrating one embodiment of a turbocharger control system of the invention.
FIG. 2 is a block diagram illustrating control and signal data flow during the automatic variable-nozzle turbocharger control of the embodiment.
FIG. 3 is a flow chart illustrating arithmetic calculation of a final desired intake-air flow rate (QCSSP2), necessary for the turbocharger control.
FIG. 4 shows a former stage of an arithmetic-calculation routine for a basic duty-cycle value necessary to control the inlet opening of a movablevane turbine of a variable nozzle turbocharger electronically connected to an electronic control unit
(ECU) shown in FIG. 1.
FIG. 5 is a latter stage of the arithmetic-calculation routine for the basic duty-cycle value, following the flow chart shown in FIG. 4.
FIG. 6 is a former stage of an arithmetic-calculation routine for a final duty-cycle value (LADUTY) output from the ECU.
FIG. 7 is a latter stage of the arithmetic-calculation routine for the final duty-cycle value (LADUTY).
FIG. 8 is a schematic operational block diagram explaining the function and construction of the turbocharger control system and the EGR control system in the invention.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
Referring now to the drawings, particularly to FIG. 1, the turbocharger control system of the invention is exemplified in a turbocharged and intercooled diesel engine with a variable nozzle turbocharger (or a variable displacement turbocharger)
and an exhaust-gas recirculation (EGR) control system. The diesel engine 1 is electronically connected to an electronic concentrated engine control system (ECCS) or an electronic engine control unit (ECU) being capable of monitoring various
engine/vehicle parameters through a number of engine/vehicle sensors, to control various systems and devices, for example an electronic fuel injection control system, an electronic ignition system, the EGR control system and the variable displacement
turbocharger, and to ensure that the exhaust emissions and fuel economy standards are maintained. As shown in FIG. 1, reference sign 4 denotes a variable displacement turbocharger, often called a "variable nozzle turbocharger". The variable
displacement turbocharger 4 comprises a movable-vane turbine 4A having a plurality of movable turbine blades or vanes and a compressor pump 4B, to variably adjust its turbo-charging state. The turbine 4A is disposed in an exhaust gas passage, so that
the turbine wheel is spun by the exhaust gas, whereas the compressor pump 4B is disposed in an air-intake passage (or an induction passage) 3. The turbine wheel is the same shaft as the compressor-pump rotor, and so the compressor pump is driven in
synchronization with rotation of the turbine wheel so as to produce a high pressure on fresh air which is introduced through an air cleaner 9 into the intake passage 3. The super-charged air is directed into an intercooler 11 so as to aftercool the
super-charged air and thus enhance air density. The variable nozzle turbocharger 4 has a plurality of movable turbine blades or vanes for variably throttling or restricting the inlet opening area or the inlet opening size of the turbine 4A. In order to
increase or decrease boost pressure, the fluid-flow throttling degree or the fluid-flow restricting degree of each movable turbine vane is adjusted by means of an actuator 5 linked to the turbine vanes. Actually, the movable-turbine-vane actuator
permits each vane angle to be varied such that the exhaust gas strikes the turbine vanes or blades at a designated angle and velocity suited to each engine speed range. In the shown embodiment, the actuator 5 comprises a diaphragm type vacuum-operated
actuator. Although it is not clearly shown, the vacuum-operated actuator 5 consists of a diaphragm unit, a diaphragm chamber, and an operating rod mechanically linked to the movable turbine vanes. The stroke of the operating rod is controlled by way of
a controlled vacuum applied to the diaphragm chamber. The vacuum is produced by a vacuum pump 6, which is located near the engine cylinder block, and delivered through a vacuum line or a vacuum tube (not numbered) via a one-way check valve 7 to a vacuum
reservoir 8, and thus temporarily stored in the reservoir 8. The vacuum stored in the reservoir 8 is delivered to a duty-cycle controlled electromagnetic solenoid valve 10. The diaphragm chamber has a signal port or a controlled vacuum port connected
to an outlet port of the duty-cycle controlled electromagnetic solenoid valve 10, to introduce the controlled vacuum into the diaphragm chamber of the vacuum-operated actuator 5. As can be appreciated from the connection line between the duty-cycle
controlled valve 10 and the air cleaner 9, the duty-cycle controlled valve 10 has an atmospheric-pressure inlet port, such that the atmospheric-pressure inlet port (an air bleed) is cyclically opened and closed according to a controlled duty cycle or a
controlled duty factor which is determined by the ECU 14. The duty-cycle controlled valve 10 is provided to suitably dilute the incoming vacuum with the atmosphere. Thus, air of partial vacuum (the negative pressure) in the pressure chamber of the
duty-cycle controlled valve 10 is properly mixed with the atmosphere of a normal pressure, and thus the out-going vacuum (the controlled negative gauge pressure) from the duty-cycle controlled valve 10 can be properly regulated at a pressure level
somewhat higher than the incoming vacuum. The lower a reading of vacuum in the diaphragm chamber of the vacuum-operated actuator 5, the greater the stroke of the operating rod of the actuator 5. Actually, as the controlled duty-cycle value of the
pulsewidth modulated signal (the control signal) applied to the duty-cycle controlled valve 10 increases, the rate of atmospheric pressure supply decreases. This increases the vacuum delivered to the diaphragm chamber of the actuator 5, and as a result
the fluid-flow restricting degree of the movable turbine vanes of the turbine 4A is increased. In other words, the inlet opening area of the turbine 4A is reduced. As described above, the increased duty-cycle value of the PWM signal applied to the
duty-cycle controlled valve 10 increases the fluid-flow restricting degree of the inlet side of the turbine, with the result that the exhaust-gas flow velocity increases and thus the boost pressure increases. Conversely, when the duty-cycle signal value
output to the duty-cycle controlled valve 10 decreases, the rate of atmospheric-pressure supply increases. This decreases the vacuum delivered to the diaphragm chamber of the actuator 5, thus decreasing the fluid-flow restricting degree of the movable
turbine vanes of the turbine 4A. As discussed above, the decreased duty-cycle signal value is utilized for increasing the opening of the inlet side of the movable-vane turbine 4A. The increased opening of the inlet side of the turbine 4A reduces the
exhaust-gas flow velocity, thereby reducing the boost pressure. As seen in FIG. 1, the EGR control system of the embodiment includes an exhaust gas recirculation (abbreviated simply as "EGR") passage 12 which connects the air-intake passage 3 of the
downstream side of the intercooler 11 with the exhaust-gas passage 2 of the engine 1. An EGR control valve device 13 is fluidly disposed in the middle of the EGR passage 12. The EGR control valve device 13 comprises an EGR control valve and an
EGR-valve actuator. The EGR passage 12 and the EGR control valve device 13 are provided to send some of the exhaust gas back through the intake manifold or the intake passage 3 of the downstream side of the intercooler 11, thereby reducing the
production of oxides of nitrogen (NO.sub.x) at the exhaust system. The EGR-valve actuator of the EGR valve device 13 is connected to the EGR control valve so as to adjust the opening of the EGR control valve. The EGR-valve actuator usually comprises a
stepper motor (also known as a "stepping motor" or a "step-servo motor"). The EGR-valve actuator (or the stepper motor) is connected via a signal line to the output interface or a drive circuit of the ECU 14, so that the angular steps or essentially
uniform angular movements of the EGR-valve actuator can be obtained electromagnetically depending on a control signal or a drive signal which is output from the output interface of the ECU 14 and indicative of a desired opening of the EGR control valve.
That is, the command value of the EGR-control-valve opening is arithmetically calculated as a desired number of angular steps of the step motor. By increasing the number of angular steps, the EGR control valve opening, simply the EGR valve opening, can
be controlled-substantially continually from the full-open position to the fully-closed position. Although it is not shown in FIG. 1, in a conventional manner, a throttle valve (not shown) is disposed in the air-intake passage 3 usually upstream of the
confluent point between the outlet port of the EGR passage 12 and the air-intake passage 3. The throttle valve is usually comprised of a butterfly valve. The opening and closing of the throttle valve can be electronically controlled in response to a
control signal from the ECU 14. The throttle valve is operated between an open mode position and a closed mode position. Also, the EGR control valve 13 and the electronically-controlled throttle valve (not shown) cooperate with each other to properly
regulate the quantity of exhaust gas recirculated so that the amount of NO.sub.x is reduced at various engine operating conditions, such as at high loads, at low speeds, during starting and warming up from cold, or in a transient state (in presence of
environmental variation) from low-land driving to high-land driving. For example, under a certain engine operating condition where a large amount of EGR is required, the throttle valve is shifted to a valve position as close to its closed position as
possible in order to produce a negative pressure in the air-intake pipe downstream of the throttle valve, whereas the EGR control valve is regulated or adjusted to a desired EGR control valve opening based on the certain engine operating condition.
Conversely, when less EGR is required or there is no necessity for EGR, the throttle valve is shifted to its full-open position corresponding to the open mode position. With the throttle valve maintained at the closed mode position, the differential
pressure between a pressure in the exhaust system (simply an exhaust pressure) and a pressure in the induction system including the intake manifold and the collector is enlarged to the maximum, thereby facilitating recirculation of the exhaust gas.
Details of the electronic control unit (ECU) 14 are described hereunder.
As appreciated from the system diagram shown in FIG. 1, the ECU 14 comprises a microcomputer containing a memory (ROM, RAM), an input/output interface (or input interface circuitry and output interface circuitry), and a central processing unit
(CPU). The memory is generally designed to store informational data from the input and output interfaces, preprogrammed characteristic map data, and the results of ongoing arithmetic calculations. The input/output interface is the device that allows
data to be transferred between input and output devices, CPU and the memory. Output signals from the input/output interface are amplified to operate electrical loads, namely the EGR-valve actuator. Arithmetic and logic sections of the CPU perform
necessary arithmetic calculations shown in FIGS. 3 through 7. The output interface of the ECU 14 is connected to the throttle-valve actuator (not shown), the EGR-valve actuator, and the duty-cycle controlled valve 10 for the variable nozzle turbocharger
4. On the other hand, the input interface of the ECU 14 is connected to various engine/vehicle sensors, for receiving an engine-speed indicative signal N, a basic fuel-injection amount indicative signal Tp (regarded as an engine-load equivalent value),
an atmospheric-pressure indicative signal Pa from an atmospheric-pressure sensor (serving as an air-density sensor) 15, an engine temperature indicative signal Tw from an engine coolant temperature sensor (a water temperature sensor) 16, and an actual
intake-air flow rate indicative signal QAVNT from an air-flow meter 17. In the embodiment, although the atmospheric-pressure sensor 15 is used to detect a change in air density (changes in environment), an intake-air temperature sensor may be combined
with the atmospheric-pressure sensor to more precisely detect changes in environment. The air-flow meter 17 comprises a hot-wire mass air flow meter which is located in the air-intake passage just downstream of the air cleaner 9. The air-flow meter 17
is provided for detecting an actual flow rate QAVNT of fresh air passing through the air cleaner. The engine speed data N and the basic fuel-injection amount indicative input information data Tp (regarded as a representative value of engine load) are
generally used as fundamental engine operating parameters needed for determining the fuel-injection amount and fuel-injection timing. As described later, the input information data N and Tp are also used for determination of a basic desired intake-air
flow rate (QCSSP1), a desired EGR rate (MEGRM), correction or compensation for the desired EGR rate (MEGRM), and correction or compensation for the basic desired intake-air flow rate (QCSSP1), that is, determination and correction of the controlled
duty-cycle signal value of the duty-cycle controlled valve 10 (in other words, the controlled quantity of the fluid-flow restricting degree of the variable nozzle turbocharger 4). The atmospheric-pressure indicative signal Pa from the sensor 15 is used
to correct the controlled quantity of the fluid-flow restricting degree of the variable nozzle turbocharger 4. The engine temperature indicative signal Tw from the sensor 16 is used to correct the desired EGR rate. The actual intake-air flow rate
indicative signal QAVNT from the air-flow meter 17 is used for feedback control for the intake-air flow rate (containing a turbo-charging state).
Referring now to FIG. 2, there is shown the block diagram illustrating the fundamental control and signal data flow of the variable-nozzle turbocharger control of the embodiment. Briefly speaking, a basic duty-cycle value DUTB1 (serving as a
feed-forward controlled quantity) of the duty-cycle controlled valve 10 is arithmetically calculated. Additionally, a final desired intake-air flow rate QCSSP2 or a basic desired intake-air flow rate QCSSP1 is arithmetically calculated or estimated and
also an actual intake-air flow rate QAVNT is detected or measured by means of the air-flow meter 17. In a feedback-control inhibition zone or region (Tp.ltoreq.Tm) where feedback control is inhibited, the inlet opening area of the turbine 4A of the
variable nozzle turbocharger 4 is subjected to open-loop control (or feedforward control) according to which the duty-cycle value of the duty-cycle controlled valve 10 of the variable nozzle turbocharger 4 is based on the feedforward controlled quantity
DUTB1 (or the corrected basic duty-cycle value DUTB 2). Within a feedback-control permission zone (simply a feedback-control zone) (Tp>Tm) where feedback control is permitted, first, a feedback correction value DUTS is arithmetically calculated by
comparison (QCSSP2-QAVNT) between the final desired intake-air flow rate QCSSP2 and the actual intake-air flow rate QAVNT, and then the feedback control is executed on the basis of both the feedforward controlled quantity DUTB1 and the feedback
correction value DUTS. In order to enhance the accuracy of the variable nozzle turbocharger control, a first correcting operation (ADF1; ADF2) based on changes in atmospheric pressure (Pa) and a second correcting operation or a transient correction
(VNEGR2; VEGR) based on changes in the EGR rate are further utilized. In the embodiment, in due consideration of changes in the intake-air flow rate occurring owing to the EGR-rate correction (Z) made to a desired EGR rate, the previously-noted
feedforward controlled quantity DUTB1 is properly corrected as the corrected feedforward controlled quantity (or the corrected basic duty-cycle value DUTB2=DUTB1.times.ADF2.times.VEGR), and additionally the basic desired intake-air flow rate QCSSP1 is
properly corrected as the corrected intake-air flow rate QCSSP1A (=QCSSP1.times.ADF1.times.VNEGR2). Details of the operation of each of the blocks shown in FIG. 2 are described in detail in reference to the flow charts of FIGS. 3 through 7.
Referring now to FIG. 3, there is shown the arithmetic-calculation routine for a desired intakeair flow rate (or a final intake-air flow rate QCSSP2). The routines shown in FIGS. 3-7 are cyclically executed as time-triggered interrupt routines
to be triggered every predetermined intervals such as several milliseconds or several 10 milliseconds.
In step S1, a basic desired intake-air flow rate (simply, a desired intake-air flow rate) QCSSP1 is arithmetically calculated on the basis of the engine speed indicative data N and the engine load indicative data (i.e., the basic fuel-injection
amount indicative data Tp) from a predetermined three-dimensional characteristic map illustrating the relationship among engine speed (N), engine load (Tp), and a basic desired intake-air flow rate (QCSSP1). The basic desired intake-air flow rate QCSSP1
is set or retrieved from the predetermined characteristic map, considering that EGR is performed at a desired EGR rate which is set at the same operating condition (the same engine speed and load) (see step S31of FIG. 5). In step S2, an
atmospheric-pressure-change dependent correction factor ADF1 for the basic desired intake-air flow rate QCSSP1 is retrieved on the basis of the atmospheric pressure indicative signal data Pa from the sensor 13 and the engine load indicative data Tp, from
a predetermined characteristic map. There is a decrease in air density, arising from change in environment from low-land driving to high-land driving. The previously-noted atmospheric-pressure-change dependent correction factor ADF1 for the basic
desired intake-air flow rate QCSSP1 will be referred to as a "first intake-air-quantity correction factor". The first intake-air-quantity correction factor ADF1 is required to compensate for an excessive rise in boost pressure at the high engine load
range, which excessive boost-pressure rise may occur if the same desired intake-air flow rate (calculated during the low-land driving) remains kept constant. In step S3, a basic value (MEGRM) of the desired EGR rate (simply the basic EGR rate) is
arithmetically calculated by way of retrieval from a preprogrammed three-dimensional characteristic map representative of the relationship among the engine speed N, the basic fuel-injection amount Tp (equivalent to the engine load), and the basic EGR
rate MEGRM. In step S4, a first EGR-rate correction factor KEGR1, which is used for primarily correcting or compensating for the basic EGR rate (MEGRM) on the basis of a predetermined operating parameter (such as the engine coolant temperature Tw)
except engine speed and load, is derived or retrieved as a function f(Tw) of the engine coolant temperature Tw from a predetermined or preprogrammed two-dimensional characteristic map. As is generally known, there is the problem of increased
cylinder-wall wear occurring owing to carbon deposits adhered to the engine cylinder wall by exhaust gas flow recirculated particularly at low engine temperatures. Thus, the water temperature (Tw) versus first-EGRrate-correction-factor (KEGR1)
characteristic map is designed to decrease the first correction factor KEGR1 as the water temperature Tw (regarded as the engine temperature) lowers, in such a manner as to set the first correction factor KEGR1 at a predetermined minimum value during the
start of the engine from cold, and to gradually rise the first correction factor KEGR1 up to 1.0 according to the increase of the water temperature, and to set the first correction factor KEGR1 at the maximum value (i. e., 1.0) after the engine has been
warmed up. In the shown embodiment, the first correction factor KEGR1 is defined as a coefficient with which the basic EGR rate MEGRM is multiplied, and thus the first correction factor KEGR1 is preprogrammed as a value less than or equal to "1". Also,
in the shown embodiment, although the engine coolant temperature Tw is used as the predetermined operating parameter except engine speed and load, which parameter is needed for determining the first EGR-rate correction factor KEGR1, the other operating
parameter such as fuel-injection timing, atmospheric pressure or the like may be used as the predetermined operating parameter except engine speed and load. The intake-air flow rate tends to vary owing to the EGR rate varied or primarily corrected or
affected by the first EGR-rate correction factor KEGR1. Thus, in step S5, a basic intake-air flow rate correction factor A is arithmetically calculated as a rate of change in a basic intake air flow rate (precisely a basic induced fresh-air flow rate
per cylinder) based on the engine speed (N) and load (Tp), under a particular condition where a totally-induced gas flow rate (the induced fresh-air flow rate plus quantity of exhaust gas recirculated) is kept constant. In the embodiment, the basic
intake-air flow rate correction factor A is arithmetically calculated as a function f(MEGRM, KEGR1) on the basis of both the basic EGR rate MEGRM and the first EGR-rate correction factor KEGR1, from the following first expression (1) or the following
second expression (2). When the EGR rate is defined as a ratio of the exhaust-gas-recirculation am | | |
