U.S. patent application number 09/728477 was filed with the patent office on 2001-08-09 for valve timing control for engine.
Invention is credited to Arai, Masahiro, Kawasaki, Takao.
Application Number | 20010011541 09/728477 |
Document ID | / |
Family ID | 18367541 |
Filed Date | 2001-08-09 |
United States Patent
Application |
20010011541 |
Kind Code |
A1 |
Kawasaki, Takao ; et
al. |
August 9, 2001 |
Valve timing control for engine
Abstract
A control system controls an intake air quantity to an engine by
controlling an intake valve closing timing, instead of, or in
cooperation with, a throttle valve. A controller estimates a
quantity of internal EGR gases remaining in a cylinder, and
controls the intake valve closing timing to a desired target timing
determined in due consideration of the estimated internal EGR
quantity. The estimated internal EGR quantity is a sum of a base
quantity and an increase correction quantity determined in
accordance with a valve overlap period, its middle angular position
in crankshaft rotation and intake pressure. The target intake valve
closing timing is determined in accordance with a target intake air
quantity and the estimated internal EGR quantity.
Inventors: |
Kawasaki, Takao; (Kanagawa,
JP) ; Arai, Masahiro; (Yokohama, JP) |
Correspondence
Address: |
Richard L. Schwaab
FOLEY & LARDNER
Washington Harbour
3000 K Street, N.W., Suite 500
Washington
DC
20007-5109
US
|
Family ID: |
18367541 |
Appl. No.: |
09/728477 |
Filed: |
December 4, 2000 |
Current U.S.
Class: |
123/568.14 ;
123/90.11; 123/90.15 |
Current CPC
Class: |
F02D 2200/0406 20130101;
Y02T 10/40 20130101; F02D 2041/002 20130101; F02D 41/0002 20130101;
F02D 41/0062 20130101; F02M 26/01 20160201; F02D 2013/0292
20130101; F02D 13/0215 20130101; F02D 37/02 20130101; F02D 2041/001
20130101; F02D 13/0253 20130101; F02D 13/0261 20130101; Y02T 10/12
20130101; F01L 9/20 20210101 |
Class at
Publication: |
123/568.14 ;
123/90.11; 123/90.15 |
International
Class: |
F02M 025/07; F01L
001/34 |
Foreign Application Data
Date |
Code |
Application Number |
Dec 3, 1999 |
JP |
11-344216 |
Claims
What is claimed is:
1. A control apparatus for controlling an intake air quantity to
the engine by varying an intake valve closing timing of the engine,
the control apparatus comprising: a controller configured to
calculate a target air quantity in accordance with an engine
operating state, to calculate an estimated internal EGR quantity of
the engine in accordance with the engine operating state, to
calculate a target intake valve closing timing in accordance with
the target air quantity and the estimated internal EGR quantity,
and to control an actual intake air quantity to the engine by
controlling an actual intake valve closing timing of the engine to
achieve the target intake valve closing timing.
2. The control apparatus as claimed in claim 1, wherein the
controller is configured to calculate the estimated internal EGR
quantity in accordance with a target exhaust valve closing timing
for the engine and an engine speed of the engine.
3. The control apparatus as claimed in claim 2, wherein the
controller is configured to calculate a base internal EGR quantity
in accordance with the target exhaust valve closing timing and the
engine speed, and to determine the estimated internal EGR quantity
by modifying the base internal EGR quantity with an overlap
correction quantity determined in accordance with a valve overlap
condition of the engine.
4. The control apparatus as claimed in claim 3, wherein the
controller is configured to determine a valve overlap quantity in
accordance with an interval between a target intake valve opening
timing and the target exhaust valve closing timing, and to
determine the estimated internal EGR quantity by addition of the
overlap correction quantity to the base internal EGR quantity.
5. The control apparatus as claimed in claim 4, wherein the
controller is configured to increase the base internal EGR quantity
with increase in an interval from one of an exhaust top dead center
and the target exhaust valve closing timing to the other.
6. The control apparatus as claimed in claim 4, wherein the
controller is configured to increase the base internal EGR quantity
as the engine speed increases when the target exhaust valve closing
timing is before an exhaust top dead center.
7. The control apparatus as claimed in claim 4, wherein the
controller is configured to decrease the base internal EGR quantity
as the engine speed increases when the target exhaust valve closing
timing is after an exhaust top dead center.
8. The control apparatus as claimed in claim 4, wherein the
controller is configured to increase the estimated internal EGR
quantity by increasing the overlap correction quantity with
increase in the valve overlap quantity representing a valve overlap
period.
9. The control apparatus as claimed in claim 4, wherein the
controller is configured to calculate the estimated internal EGR
quantity by decreasing the overlap correction quantity with
increase in a retard of the target exhaust valve closing timing
from an exhaust top dead center when the target exhaust valve
closing timing is after the exhaust top dead center.
10. The control apparatus as claimed in claim 8, wherein the
controller is configured to increase the estimated internal EGR
quantity by increasing the overlap correction quantity with
increase in an absolute value of an intake pressure on a negative
pressure side.
11. The control apparatus as claimed in claim 8, wherein the
controller is configured to calculate the overlap correction
quantity by modifying a base correction quantity determined by the
valve overlap quantity, with an intake pressure modification
quantity determined in accordance with an intake pressure and the
target exhaust valve closing timing.
12. The control apparatus as claimed in claim 11, wherein the
controller is configured to determine the valve overlap quantity by
converting a valve overlap angular interval expressed as an angular
distance in crankshaft rotation to a valve overlap time period.
13. The control apparatus as claimed in claim 11, wherein the
controller is configured to determine an intermediate quantity in
accordance with the valve overlap quantity, to set the base
correction quantity equal to the intermediate quantity when the
target exhaust valve closing timing is before an exhaust top dead
center, and to determine the base correction quantity by
subtraction from the intermediate quantity, of a subtrahend
proportional to a retard quantity of the exhaust valve closing
timing with respect to the exhaust top dead center when the target
exhaust valve closing timing is after the exhaust top dead
center.
14. The control apparatus as claimed in claim 11, wherein the
controller is configured to determine a modification coefficient,
as the modification quantity, in accordance with the intake
pressure, the target exhaust valve closing timing and the valve
overlap quantity; and wherein the controller is configured to
calculate the overlap correction quantity by multiplying the base
correction quantity by the modification coefficient.
15. The control apparatus as claimed in claim 11, wherein the
controller is configured to increase the modification quantity as
an absolute value of the intake pressure increases on a negative
side, and to increase the modification quantity in accordance with
a retard quantity of the exhaust valve closing timing from an
exhaust top dead center when the exhaust valve closing timing is
after the exhaust top dead center and the absolute value of the
intake pressure is higher than a predetermined level.
16. The control apparatus as claimed in claim 4, wherein the
controller is programmed to determine the estimated internal EGR
quantity by addition, to the base internal EGR quantity, of an
overlap increase correction quantity which is increased as the
valve overlap quantity increases; and wherein the controller is
programmed to modify the base internal EGR quantity with the
overlap increase correction quantity when the target exhaust valve
closing timing is after the target intake valve opening timing, and
to set the estimated internal EGR quantity equal to the base
internal EGR quantity when the target exhaust valve closing timing
is not after the target intake valve opening timing.
17. The control apparatus as claimed in claim 16, wherein the
controller is programmed to increase the base internal EGR quantity
with increase in an advance of the exhaust valve closing timing
from the exhaust top dead center when the exhaust valve closing
timing is before the exhaust top dead center, and to increase the
base internal EGR quantity with increase in a retard of the exhaust
valve closing timing from the exhaust top dead center when the
exhaust valve closing timing is after the exhaust top dead center;
and wherein the controller is programmed to determine the overlap
increase correction quantity in accordance with the valve overlap
quantity, the target exhaust valve closing timing and an intake
pressure controlled by a throttle valve.
18. The control apparatus as claimed in claim 1, wherein the
control apparatus further comprises a variable valve timing
actuator comprising a solenoid to vary the actual intake valve
closing timing in response to an electric control signal produced
by the controller, and a sensor system to sense engine operating
conditions to determine the engine operating state.
19. A control apparatus for controlling an intake air quantity to
an engine by controlling an intake valve closing timing of the
engine, the control apparatus comprising: means for determining an
estimated internal EGR quantity in accordance with an engine
operating state; means for determining a target intake valve
closing timing in accordance with at least the estimate internal
EGR quantity; and means for controlling the intake air quantity to
the engine by controlling an actual intake valve closing timing of
the engine to the target intake valve closing timing.
20. A control process for varying valve timings of intake and
exhaust valves of an engine, the control process comprising:
estimating an internal EGR quantity in accordance with an engine
operating state; and controlling an intake valve closing timing in
accordance with a required intake air quantity and the internal EGR
quantity.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to control technique for
controlling valve events of an engine with an actuator such as
solenoid actuator, and in particular to control of an intake valve
closing timing to vary an intake air quantity to obtain a desired
engine torque.
[0002] Unlike an engine of a conventional type controlling the
intake air quantity with a throttle valve, an engine of a new type
(proposed by a Published Japanese Patent Application, Kokai No.
H10(1998)-37727) is arranged to control the intake air quantity
mainly by controlling the intake valve closing timing.
[0003] An intake air control system of such a type for controlling
the intake air quantity by controlling the intake valve closing
timing achieves a target air quantity (required intake air
quantity) corresponding to required torque by controlling the
volume of cylinder intake air corresponding to an effective intake
stroke determined by the intake valve closing timing, at the intake
pressure that is held approximately constant at the atmospheric
pressure in the absence of a throttle valve, and that is determined
in dependence on the throttle opening when there is provided a
throttle valve.
SUMMARY OF THE INVENTION
[0004] To control the intake air quantity accurately by controlling
the intake valve closing angle, the intake valve is to be closed at
a piston position at which the cylinder contains an amount of fresh
air corresponding to the target air quantity in addition to an
amount of residual gases (internal EGR quantity) remaining in the
combustion chamber. The internal EGR quantity of residual gases is
influenced deeply by the existence or nonexistence of valve overlap
and the length of valve overlap. The valve overlap condition has a
great influence on the internal EGR quantity especially in a
solenoid type valve actuating system having such a high response
speed in opening and closing valves that the exhaust and intake
valves are both held approximately in the fully open state during a
valve overlap period. Accordingly, it has been found that simple
correction or compensation for the internal EGR quantity is not
always adequate for controlling intake air quantity and hence
engine torque accurately. Moreover, even by a system devised to set
target exhaust valve closing timing and target intake valve opening
timing to obtain a proper internal EGR quantity and to adjust the
target intake valve closing timing in conformity with the setting
of the valve timings, an adequate control is difficult since the
actual internal EGR quantity is affected by other factors besides
valve timings.
[0005] It is, therefore, an object of the present invention to
provide control apparatus and/or method capable of controlling the
intake valve closing timing accurately and thereby controlling the
intake air quantity and hence engine torque adequately.
[0006] According to the present invention, a control apparatus for
controlling an intake air quantity to the engine by varying an
intake valve closing timing of the engine comprises a controller
that is configured or programmed:
[0007] to calculate a target air quantity in accordance with an
engine operating state,
[0008] to calculate an estimated internal EGR quantity of the
engine in accordance with the engine operating state,
[0009] to calculate a target intake valve closing timing in
accordance with the target air quantity and the estimated internal
EGR quantity, and
[0010] to control an actual intake air quantity to the engine by
controlling an actual intake valve closing timing of the engine to
achieve the target intake valve closing timing.
[0011] A control apparatus according to one aspect of the invention
comprises: means for determining the estimated internal EGR
quantity; means for determine the target intake valve closing
timing in accordance with at least the estimate internal EGR
quantity; and means for controlling the intake air quantity to the
engine by controlling an actual intake valve closing timing of the
engine to the target intake valve closing timing.
[0012] According to the present invention, a control process for
varying valve timings of intake and exhaust valves of an engine,
comprises: estimating an internal EGR quantity in accordance with
an engine operating state; and controlling an intake valve closing
timing in accordance with a required intake air quantity and the
internal EGR quantity.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 is a schematic view showing an engine provided with a
variable valve timing actuator and a control unit according to one
embodiment of the present invention.
[0014] FIG. 2 is a view for illustrating a solenoid drive device
for intake or exhaust valve employed in the system of FIG. 1.
[0015] FIG. 3 is a flowchart showing a routine for setting an
intake valve closing timing according to the embodiment.
[0016] FIG. 4 is a flowchart showing a subroutine for determining a
base internal EGR quantity in a non-overlap state according to the
embodiment.
[0017] FIG. 5 is a flowchart showing a subroutine for determining
an overlap correction quantity used to modify the base internal EGR
quantity.
[0018] FIG. 6 is a flowchart showing a subroutine for determining a
modification coefficient based on an intake pressure, for modifying
the overlap correction quantity.
[0019] FIG. 7 is a graph showing a characteristic of the base
internal EGR quantity.
[0020] FIG. 8 is a graph showing a characteristic of the overlap
correction quantity.
[0021] FIG. 9 is a graph for illustrating influence of the intake
pressure on the base correction quantity.
[0022] FIG. 10 is a graph showing a characteristic of a quantity
OLEGC0 with respect to a valve overlap time OLTIME, which can be
used in the subroutine of FIG. 5.
DETAILED DESCRIPTION OF THE INVENTION
[0023] FIG. 1 shows a control system according to one embodiment of
the present invention.
[0024] As shown in FIG. 1, an engine 1 has a plurality of cylinders
each having a combustion chamber 3 defined by a piston 2.
Combustion chamber 3 of each cylinder has a spark plug 4, and
intake and exhaust valves 5 and 6 of an electromagnetic drive type.
An intake passage 7 is for admitting air into each combustion
chamber 3, and an exhaust passage 8 conveys the exhaust gases away
from engine 1.
[0025] FIG. 2 schematically shows a solenoid (electromagnetic)
valve actuator for intake or exhaust valves 5 or 6. A valve element
20 is attached to one end of a valve stem 21. A plate-shaped
movable member 22 is attached to valve stem 21. Movable member 22
is normally held at a neutral position by springs 23 and 24. An
opening solenoid (electromagnetic coil) 25 is disposed below
movable member 22, and a closing solenoid (electromagnetic coil) 26
is above movable member 22.
[0026] Before a start of engine 1, movable member 22 is operated to
oscillate resonantly by energizing opening and closing solenoids 25
and 26 alternately. When the amplitude is increased sufficiently,
movable member 22 is attracted and held by one of the solenoids 25
and 26.
[0027] The valve is moved from a closed state to an open state by
deenergizing upper closing solenoid 26 attracting movable member
22, thereby allowing movable member 22 to move downwards by the
biasing force of spring 23, and attracting movable member 22 by
energizing lower opening solenoid 25 when movable member 22
approaches lower opening solenoid 25 sufficiently. Thus, valve
element 20 is lifted from a valve seat and the valve is opened.
[0028] The valve is moved from the open state to the closed state
by deenergizing lower opening solenoid 25 attracting movable
element 22, thereby allowing movable member 22 to move upwards by
the biasing force of spring 24, and attracting movable member 22 by
energizing upper closing solenoid 26 when movable member 22
approaches upper closing solenoid 25 sufficiently. Thus, valve
element 20 is seated on the valve seat and the valve is closed.
[0029] As shown in FIG. 1, there is provided, in intake passage 7,
an air flowmeter 14 for sensing an intake air quantity, and a
throttle valve 15 whose opening degree is controlled
electronically. A solenoid type fuel injector 9 is provided in the
intake port for each cylinder.
[0030] A control unit (C/U) 10 controls intake and exhaust valves 5
and 6, throttle valve 15, fuel injectors 9 and spark plugs 4, in
accordance with input information on engine operating conditions
collected by various sensors. A crank angle sensor 11 produces a
crank angle signal in synchronism with the engine revolution. An
engine speed (rpm) can be determined from the crank angle signal.
An accelerator pedal sensor 12 is arranged to sense an accelerator
opening (or accelerator pedal depression degree). The signals of
these sensors are inputted to control unit 10 serving as a main
component of a controller in the control system.
[0031] In accordance with the engine operating conditions such as
the accelerator opening (degree) and engine speed, this control
system controls the actual intake air quantity so as to achieve a
target torque mainly by controlling the closing timing of intake
valve 5. Moreover, to reduce exhaust emissions, especially the
amount of NOx, this control system further controls the closing
timing of exhaust valve 6, and the opening timing of intake valve 5
(and the opening timing of exhaust valve 6) to control the internal
EGR quantity to an adequate level in accordance with engine
operating conditions. In practice, however, the internal EGR
quantity is affected not only by these valve timings but other
factors as well. Therefore, this control system is arranged to
estimate the internal EGR quantity in accordance with engine
operating conditions, and to adjust the closing timing of intake
valve 5 (of each cylinder, to be exact) in accordance with the
estimated internal EGR quantity.
[0032] The control system determines the intake air quantity in
accordance with sensed values supplied from various sensors, and
controls the fuel injection quantity of fuel injector 9 (of each
cylinder, to be exact) in accordance with the intake air
quantity.
[0033] FIG. 3 shows a main routine for controlling the intake valve
closing timing according to the embodiment of the present
invention.
[0034] Step S1 is a step for reading the accelerator opening sensed
by accelerator pedal sensor 12, the engine speed sensed by crank
angle sensor 11 etc., and for calculating a target air quantity
FQH0EM adequate for a demanded torque.
[0035] Step S2 calculates a base internal EGR quantity EVEGR0
representing the internal EGR quantity in the non-overlap state in
which there is no valve overlap of intake and exhaust valves. This
calculation is carried out by following a subroutine shown in FIG.
4.
[0036] At steps S21 and S22, control unit 10 reads an engine speed
Ne and a target exhaust valve closing timing EVC. In accordance
with the values obtained at steps S21 and S22, control unit 10
calculates base internal EGR quantity EVEGR0, as EGR rate with
respect to target air quantity FQH0EM, at step S23, by lookup from
a map table prepared from characteristic data as shown in FIG. 7.
Base internal EGR quantity EVEGR0 is a quantity of burned gases
remaining in a cylinder in the non-overlap state in which there is
no overlap of the intake and exhaust valves. Accordingly, as the
cylinder volume determined by the piston position at the exhaust
valve closure timing EVC becomes smaller, base internal EGR
quantity EVEGR0 decreases. Base internal EGR quantity EVEGR0
becomes smallest at top dead center, as shown in FIG. 7. For the
same piston position, base internal EGR quantity EVEGR0 differs
between the condition before top dead center (BTDC) and the
condition after top dead center (ATDC). When EVC is before top dead
center, the exhaust valve is closed in the state the residual gas
mixture is somewhat compressed. When EVC is after top dead center,
on the other hand, the exhaust valve is closed in the state pulling
back the exhaust gases discharged into the exhaust passage, and,
due to a delay in return of the exhaust gases, the residual gas
quantity, that is the internal EGR quantity, becomes smaller.
Moreover, this tendency is attributable to the inertia, and
affected by engine speed Ne, too. As engine speed Ne becomes
higher, the inertia is increased, and hence the influence exerted
on the internal EGR quantity is increased. When EVC is after TDC,
the exhaust gas flow changes and hence the influence of the inertia
is great, so that the decrease of the internal EGR quantity due to
an increase of engine speed Ne becomes greater. At positions away
from top dead center, the piston speed is higher, and the amount of
change of the internal EGR quantity with respect to a change in
engine speed Ne increases.
[0037] Thus, base EGR quantity EVEGR0 is increased as the extent of
separation (such as an angular distance in terms of degrees in
crank shaft rotation) of EVC from exhaust TDC (that is, TDC on the
exhaust stroke) becomes larger. Base EGR quantity EVEGR0 is
increased with increase in engine speed Ne on the BTDC side, and
base EGR quantity EVEGR0 is decreased with increase in engine speed
Ne on the ATDC side. The rate of decrease of base EGR quantity
EVEGR0 with respect to increase in engine speed Ne on the ATDC side
is greater than the rate of increase of base EGR quantity EVEGR0
with respect to increase in engine speed Ne on the BTDC side.
[0038] Reverting to FIG. 3, step S3 is for examining whether there
is a valve overlap between target opening timing IVO of intake
valve 5 and target closing timing EVC of exhaust valve 6.
[0039] If there is no overlap, then control unit 10 proceeds to
step S4, and sets an overlap correction quantity OLEGR1 for
correcting the internal EGR quantity, to zero (OLEGR1=0).
Consequently, estimated internal EGR quantity EGRREM is set equal
to base internal EGR quantity EVRGR0 (EGRREM=EVRGR0) at a next step
S8.
[0040] If there is a valve overlap, control unit 10 proceeds from
step S3 to step S5, and calculates a base overlap correction
quantity OLEGR0 by a subroutine shown in FIG. 5.
[0041] At steps S31, S32 and S33, control unit 10 reads engine
speed Ne, target exhaust valve closing timing EVC of exhaust valve
6 and intake valve opening timing IVO of intake valve 5. At step
S34, control unit 10 converts a valve overlap quantity (expressed
in degrees of crankshaft rotation) to a valve overlap time OLTIME
by using the input values of Ne, EVC and IVO according to the
following equation.
OLTIME=(EVC-IVO)/Ne
[0042] Then, at step S35, control unit 10 determines an
intermediate quantity OLEGC0 from valve overlap time OLTIME by
lookup from a map table. Thus, step 35 determines a value of
quantity (intermediate quantity) OLEGC0 corresponding to the
current value of valve overlap time OLTIME.
[0043] In this example, the intermediate quantity OLEGC0 is
increased monotonically with increase in valve overlap time OLTIME,
as shown in FIG. 10.
[0044] At step S36, control unit 10 examines whether exhaust valve
closing timing EVC is on the BTDC side or on the ATDC side.
[0045] When EVC is before TDC, control unit 10 proceeds from step
S36 to step S37, and sets quantity OLEGC0 determined at step S35,
as base overlap (increase) correction quantity OLEGR0 directly
without modification (OLEGR0=OLEGC0).
[0046] Wh65 to step S38, and determines base overlap (increase)
correction quantity OLEGR0 by modifying the obtained value of
OLEGC0 in accordance with target closing timing EVC by the
following equation.
OLEGR0=OLEGC0-EVC(retard quantity after TDC).times.constant
[0047] The variation quantity (or increase quantity) of the
internal EGR between the base internal EGR quantity in the
non-overlap state and the internal EGR quantity in the valve
overlap state varies, even for the same valve overlap quantity (or
time), since the influence of back-flow differs in dependence on
exhaust valve closing timing EVC. FIG. 8 shows variation of the
increase quantity of the internal EGR quantity with respect to EVC
for different values of the overlap quantity (time) on the
condition that engine speed Ne is constant and the intake pressure
is constant.
[0048] When EVC is judged to be before TDC, the internal EGR
increase quantity (OLEGR0) is held approximately constant
irrespective of changes in EVC, as shown in FIG. 8. In the case of
EVC being before TDC, burned gases in the cylinder tend to be
sucked back into the intake port on the lower pressure side during
the valve overlap, rather than being expelled into the exhaust
port, and the backward flow becomes dominant. Then, the burned
gases of the back-flow to the intake port are sucked again into the
cylinder in the subsequent intake stroke, so that the EGR rate is
approximately constant. Therefore, the correction by EVC (the
advance quantity before top dead center) is not performed at step
S37.
[0049] When EVC is after TDC, the quantity of exhaust back-flow
into the intake port decreases because of downward movement of the
piston during the valve overlap. Moreover, as EVC is shifted away
from top dead center, the intake negative pressure in the cylinder
increase, and the pressure difference from the intake negative
pressure in the intake port decreases. Therefore, the tendency to
the exhaust back-flow decreases. In the overlap state, as compared
to the non-overlap state, the quantity of return flow of exhaust
gases from the exhaust port into the cylinder increases (, so that
the scavenging efficiency decreases) by transmission of the intake
negative pressure into the cylinder through the opening of the
intake valve, and the internal EGR quantity increases accordingly.
However, this increase quantity in the internal EGR quantity is
large when the overlap is around top dead center, but this increase
quantity decreases as the overlap shifts away from TDC. When the
overlap is near top dead center, the influence from the intake
negative pressure on the condition in the cylinder is great as
compared with the non-overlap state where the intake valve is not
opened, and therefore, the internal EGR increase quantity due to
the return of the exhaust gases is large. When, on the other hand,
the overlap is away from TDC, the difference between the intake
negative pressure increased by the downward movement of the piston
in the non-overlap state where the intake valve held closed during
this period, and the intake negative pressure transmitted from the
intake port into the cylinder during the overlap becomes smaller,
and hence the difference in the exhaust return flow quantity
between the overlap state and the non-overlap state becomes
smaller. Namely, the increase in the internal EGR quantity due to
the return-flow of the exhaust gases during the overlap decreases
as the overlap (or EVC) shifts away from TDC.
[0050] For the reasons above, when EVC is after TDC, the increase
(OLEGR0) of the internal EGR quantity due to overlap is decreased
as EVC shifts away from TDC, as shown in FIG. 8.
[0051] Therefore, step 38 performs the subtraction by the
subtrahend proportional to EVC (the retard quantity after TDC).
[0052] Reverting to FIG. 3, step S6 calculates an intake pressure
modification coefficient OLEGCB for modifying the thus-calculated
base overlap correction quantity OLEGR0 in accordance with the
intake pressure (boost pressure).
[0053] Base overlap correction quantity OLEGR0 is calculated as an
increase quantity of the internal EGR quantity due to a valve
overlap under the condition where the intake pressure is constant
(-50 mmHg). Even for the same overlap quantity (or time) and EVC,
the exhaust back-flow quantity is varied by a variation in the
intake pressure. This is taken into account by modification based
on the intake pressure. In the case where there is provided no
throttle valve, and the intake pressure is held approximately
constant at the atmospheric pressure, the modification can be
omitted. However, the modification based on the intake pressure is
needed or desired when the intake pressure is controlled at a
predetermined level by throttling the opening degree of throttle
valve 15, to supply the vacuum pressure for braking, or for the
suction of evaporative fuel vapor and blow-by gas into the intake
system, and the intake air quantity is controlled by controlling
the intake valve closing timing in consideration of the intake
pressure.
[0054] Intake pressure modification coefficient OLEGCB is
calculated by a subroutine shown in FIG. 6.
[0055] Step S41 reads the target intake pressure calculated by the
above-mentioned intake pressure control process of controlling the
intake pressure by controlling the throttle opening degree. Step
S42 reads target closing timing EVC of exhaust valve 6. In
accordance with the values of these input quantities, step S43
obtains a value of intake pressure modification coefficient OLEGCB
by lookup from a map table prepared on the basis of characteristic
data, as shown in FIG. 9. FIG. 9 shows a magnification (or
multiplying factor) which is a ratio of the internal EGR increase
quantity under the condition of intake pressure (negative pressure)
being equal to -100 mmHg or -300 mmHg to the internal EGR increase
quantity under the condition of intake pressure being equal to -50
mmHg. For each of the intake pressure conditions of -100 mmHg and
-300 mmHg, there are shown two characteristic lines in the two
cases of overlap quantity (crank angle) being equal 20.degree. and
40.degree.. As shown in FIG. 9, at an intake pressure of -100 mmHg,
the magnification remains constant at about 2 irrespective of
variation in EVC (the middle of the valve overlap period). At an
intake pressure of -300 mmHg, on the other hand, the magnification
is held constant at about 3.about.4 when EVC is before TDC, and the
magnification is increased linearly with increase in the extent of
separation of EVC from TDC when EVC is after TDC. When EVC is after
TDC at a constant intake pressure (-50 mmHg), as mentioned before,
the pressure difference in the cylinder due to the existence and
nonexistence of a valve overlap becomes smaller as EVC is shifted
away from TDC, so that the increase quantity of the exhaust
back-flow quantity tends to decrease. However, when the intake
negative pressure increases to -300 mmHg, the pressure difference
in the cylinder due to the existence and nonexistence of a valve
overlap becomes greater, so that the increase quantity is held at a
larger level, and hence the magnification is increased as compared
to the example of -50 mmHg.
[0056] Thus, the control system of this example increases the
modification quantity OLEGCB as an absolute value of the intake
pressure increases on the negative side, and increases the
modification quantity OLEGCB in accordance with the retard quantity
or angle of the exhaust valve closing timing EVC from the exhaust
top dead center when the exhaust valve closing timing EVC is after
the exhaust top dead center and the absolute value of the intake
pressure is higher than a predetermined level.
[0057] Reverting to FIG. 3, step S7 calculates a final overlap
correction quantity OLEGR1 by multiplying base correction quantity
OLEGR0 calculated at step S5 by intake pressure modification
coefficient OLEGCB calculated at step S6
(OLEGR1=OLEGR0.times.OLEGCB).
[0058] Step S8 calculates estimated internal EGR quantity EGRREM by
addition of overlap correction quantity OLEGR1 calculated at step
S7, to base internal EGR quantity EVEGR0 calculated at step S2.
Thus, the internal EGR quantity is estimated by adding the
calculated overlap increase correction quantity OLEGR1 to base
internal EGR quantity EVEGR0.
EGRREM=EVEGR0+OLEGR1
[0059] Step S9 calculates a modified target air quantity HQH0FM by
modifying target air quantity FQH0EM calculated at step S1, in
accordance with estimated internal EGR quantity EGRREM.
HQH0FM=FQH0EM.times.(1+EGRREM)
[0060] This modification is not intended to change the target air
quantity per se, but it is a modification for convenience to take
account of changes in the intake valve closing timing required to
obtain the target air quantity (the quantity of fresh air) caused
by the internal EGR quantity. Thus, the target air quantity is
calculated as a total amount of gases in the cylinder by adding the
quantity of the internal EGR gas to the target air quantity.
[0061] Step S10 calculates target closing timing IVC of intake
valve 5 in accordance with modified target air quantity HQH0FM.
[0062] Therefore, the control unit delivers a valve control signal
corresponding to target intake valve closing timing IVC, to the
above-mentioned electromagnetic drive unit, and thereby controls
intake valve 5 so that intake valve 5 is closed at target intake
valve closing timing IVC.
[0063] This control system estimates the internal EGR quantity in
accordance with engine operating conditions accurately by taking
account of valve overlap period, and controls the actual intake
valve closing timing by modifying the target intake valve closing
timing in accordance with the estimated internal EGR quantity.
Therefore, the control system can obtain a quantity of fresh air
suited to the target intake air quantity and control the torque
accurately.
[0064] In the illustrated embodiment, engine 1 is a prime mover of
a motor vehicle, and control unit 10 includes, as a main component,
at least one onboard computer.
[0065] This application is based on a Japanese Patent Application
No. 11-344216. The entire contents of this Japanese Patent
Application No. 11-344216 with a filing date of Dec. 3, 1999 are
hereby incorporated by reference.
[0066] Although the invention has been described above by reference
to certain embodiments of the invention, the invention is not
limited to the embodiments described above. Modifications and
variations of the embodiments described above will occur to those
skilled in the art in light of the above teachings. The scope of
the invention is defined with reference to the following
claims.
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