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Claims  |
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What is claimed is:
1. A system for estimating an exhaust gas recirculation rate for an
internal combustion engine, said engine having a passage connecting an
exhaust pipe to an intake pipe for recirculating a portion of exhaust
gases in said exhaust pipe to a combustion chamber through said intake
pipe, and a valve provided at said passage for regulating an amount or a
flow rate of said exhaust gas to be recirculated in response to a command
value for valve lift amount, comprising:
engine operating condition detecting means for detecting operating
conditions of said engine at least including engine speed and engine load;
basic exhaust gas recirculation rate determining means for determining a
basic exhaust gas recirculation rate (1-KEGRMAP) at least based on said
detected engine speed and engine load;
first flow rate estimating means for estimating a first flow rate (QACT) of
said exhaust gas passing through said valve based on flow rate
characteristics of said valve including a valve lifting amount (LACT);
second flow rate estimating means for estimating a second flow rate (QCMD)
of said exhaust gas to be recirculated passing through said valve based on
flow rate characteristics of said valve including said command value for
valve lifting amount (LCMD);
net exhaust gas recirculation rate estimating means for estimating a net
exhaust gas recirculation rate based on said basic exhaust gas
recirculation rate (1-KEGRMAP), said first flow rate (QACT) and said
second flow rate (QCMD).
2. A system according to claim 1, further including:
ignition timing means for determining an ignition timing of said engine
based on said estimated net exhaust gas recirculation rate.
3. A system according to claim 1, wherein said first and second flow rate
estimating means estimates said first and second flow rates (QACT, QCMD)
based on a ratio between an upstream pressure and a downstream pressure
acting on said valve and valve lifting amount (LACT, LCMD).
4. A system according to claim 3, wherein said first and second flow rate
estimating means estimates said first and second flow rates (QACT, QCMD)
based on a ratio (PBA/PA) between pressure (PBA) in said intake pipe and
atmospheric pressure (PA) and valve lifting amount (LACT, LCMD).
5. A system according to claim 1, further including:
comparing means for comparing at least one of said command value for valve
lifting amount (LCMD) and said second flow rate (QCMD) with a limit value
(LCMDLL, QCMDLL); and
keeping means for keeping at least one of said command value for valve
lifting amount (LCMD) and said second flow rate (QCMD) to a predetermined
value (LCMDn-1, QCMDn-1) when at least one of said command value for valve
lifting amount (LCMD) and said second flow rate (QCMD) is found to be less
than said limit value (LCMDLL, QCMDLL).
6. A system according to claim 5, wherein said predetermined value is a
value (LCMDn-1, QCMDn-1) that has been determined at least one period
earlier.
7. A system according to claim 1, wherein said net exhaust gas
recirculation rate estimating means estimates said net rate of exhaust gas
recirculated based on said basic exhaust gas recirculation rate
(1-KEGRMAP) and a ratio (QACT/QCMD) of said first flow rate (QACT) and
said second flow rate (QCMD).
8. A system according to claim 7, further including:
comparing means for comparing at least one of said command value for valve
lifting amount (LCMD) and said second flow rate (QCMD) with a limit value
(LCMDLL, QCMDLL); and
keeping means for keeping at least one of said command value for valve
lifting amount (LCMD) and said second flow rate (QCMD) to a predetermined
value (LCMDn-1, QCMDn-1) when at least one of said command value for valve
lifting amount LCMD and said second flow rate (QCMD) is found to be less
than said limit value (LCMDLL, QCMDLL).
9. A system according to claim 7, further including:
delay time determining means for determining a delay time (.tau.) until
exhaust gas that passed through said valve enters said combustion chamber
of said engine; and
said net exhaust gas recirculating means estimates said net rate of exhaust
gas recirculated at each predetermined cycle consecutively and selects one
among said net rates that is estimated at a cycle corresponding to said
delay time (.tau.).
10. A system according to claim 1, further including:
delay time determining means for determining a delay time (.tau.) until
exhaust gas that passed through said valve enters said combustion chamber
of said engine; and
said net exhaust gas recirculating means estimates said net rate of exhaust
gas recirculated at each predetermined cycle consecutively and selects one
among said net rates that is estimated at a cycle corresponding to said
delay time (.tau.).
11. A system according to claim 10, further including:
storing means for storing said net rates consecutively; and
said net exhaust gas recirculating means select one from among said stored
net rates that is estimated at a cycle corresponding to said delay time
(.tau.).
12. A system according to claim 10, wherein said delay time (.tau.) is
determined based on said operating conditions of said engine.
13. A system according to claim 10, wherein said delay time (.tau.) is a
fixed value.
14. A system according to claim 1, further including:
fuel injection correction coefficient determining means for determining a
fuel injection correction coefficient KEGRN based on said estimated net
exhaust gas recirculation rate; and
fuel injection amount correcting means for correcting a fuel injection
amount based on said fuel injection correction coefficient.
15. A system according to claim 14, wherein said net exhaust gas
recirculation rate estimating means estimates said net rate of exhaust gas
recirculated based on said basic exhaust gas recirculation rate
(1-KEGRMAP) and a ratio (QACT/QCMD) of said first flow rate (QACT) and
said second flow rate (QCMD).
16. A system according to claim 14, further including:
direct ratio determining means for determining a direct ratio (EA)
indicative of a ratio of an amount of exhaust gas that enters said
combustion chamber during a period (n) to a whole amount of exhaust gasses
(gt(n)) passing through said valve during said period (n);
carry-off ratio determining means for determining a carry-off ratio (EB)
indicative of exhaust gas that enters said combustion chamber during said
period (n) to exhaust gases (gc(n-m)) that passed said valve by a time (m)
period earlier and that remain in a space before said combustion chamber;
exhaust gas amount determining means for determining a whole amount of
exhaust gas (gin) that enters said combustion chamber during said period
(n) based on said direct ratio (EA) and said carry-off ratio (EB); and
said fuel injection correction coefficient determining means determines
said fuel injection correction coefficient (KEGRN) based on said estimated
net exhaust gas recirculation rate and said whole amount of exhaust gas
(gin).
17. A system according to claim 14, further including:
ignition timing means for determining an ignition timing of said engine
based on said estimated net exhaust gas recirculation rate.
18. A system according to claim 14, further including:
comparing means for comparing at least one of said command value for valve
lifting amount (LCMD) and said second flow rate (QCMD) with a limit value
(LCMDLL, QCMDLL); and
keeping means for keeping at least one of said command value for valve
lifting amount (LCMD) and said second flow rate (QCMD) to a predetermined
value (LCMDn-1, QCMDn-1) when at least one of said command value for valve
lifting amount (LCMD) and said second flow rate (QCMD) is found to be less
than said limit value (LCMDLL, QCMDLL).
19. A system according to claim 18, wherein said predetermined value is a
value (LCMDn-1, QCMDn-1) that has been determined at least one period
earlier.
20. A system according to claim 14, further including:
delay time determining means for determining a delay time (.tau.) until
exhaust gas passing through said valve enters said combustion chamber of
said engine; and
said fuel injection correction coefficient determining means determines
said fuel injection correction coefficient (KEGRN) at each predetermined
cycle consecutively and selects one from among said fuel injection
correction coefficients that is determined at a cycle corresponding to
said delay time (.tau.).
21. A system according to claim 20, further including:
storing means for storing said fuel injection correction coefficients
(KEGRN) consecutively; and
said fuel injection correction coefficient determining means select one
from among said stored coefficients that is estimated at a cycle
corresponding to said delay time (.tau.).
22. A system according to claim 14, wherein said first and second flow rate
estimating means estimates said first and second flow rates (QACT, QCMD)
based on a ratio between an upstream pressure and a downstream pressure
acting on said valve and valve lifting amount (LACT, LCMD).
23. A system according to claim 22, wherein said first and second flow rate
estimating means estimates said first and second flow rates (QACT, QCMD)
based on a ratio (PBA/PA) between pressure (PBA) in said intake pipe and
atmospheric pressure (PA) and valve lifting amount (LACT, LCMD).
24. A system according to claim 22 further including:
direct ratio determining means for determining a direct ratio (EA)
indicative of a ratio of an amount of exhaust gas that enters said
combustion chamber during a period (n) to a whole amount of exhaust gasses
(gt(n)) passing through said valve during said period (n);
carry-off ratio determining means for determining a carry-off ratio (EB)
indicative of exhaust gas that enters said combustion chamber during said
period (n) to exhaust gases (gc(n-m)) that passed said valve by a time (m)
period earlier and that remain in a space before said combustion chamber;
exhaust gas amount determining means for determining a whole amount of
exhaust gas (gin) that enters said combustion chamber during said period
(n) based on said direct ration (EA) and said carry-off ratio (EB); and
said fuel injection correction coefficient determining means determines
said fuel injection correction coefficient (KEGRN) based on said estimated
net exhaust gas recirculation rate and said whole amount of exhaust gas
(gin).
25. A system according to claim 14, further including:
delay time determining means for determining a delay time (.tau.) until
exhaust gas that passed through said valve enters said combustion chamber
of said engine; and
said net exhaust gas recirculating means estimates said net rate of exhaust
gas recirculated at each predetermined cycle consecutively and selects one
among said net rates that is estimated at a cycle corresponding to said
delay time (.tau.).
26. A system according to claim 25, further including:
storing means for storing said net rates consecutively; and
said net exhaust gas recirculating means select one from among said stored
net rates that is estimated at a cycle corresponding to said delay time
(.tau.).
27. A system according to claim 25, wherein said delay time (.tau.) is
determined based on said operating conditions of said engine.
28. A system according to claim 25, wherein said delay time (.tau.) is a
fixed value. |
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Claims |
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Description |
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BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention relates to an EGR (exhaust gas recirculation) rate
estimation system for an internal combustion engine, and more particularly
to a system for estimating a rate of recirculated exhaust gas that enters
the combustion chamber in the intake air supplied to the engine.
2. Description of the Prior Art
It is known in automotive engineering to connect the intake system and the
exhaust system of an engine for recycling a portion of the exhaust gases
to the intake system in order to reduce the formation of nitrogen oxides
(NOx) and at the same time, to enhance fuel economy. The passage
connecting the exhaust system to the intake system is provided with a
valve for regulating the amount or flow rate of recirculated exhaust gas.
The valve is hereinafter referred to as the "EGR control valve". In order
to control the amount or flow rate of exhaust gas to be recirculated, a
command value for the EGR control valve lifting amount is determined based
on engine operating parameters at least including engine speed and engine
load. In the vacuum-operated EGR control valve, the negative pressure in
the intake system is introduced and exerts a negative pressure on the
valve to open it in response to the command value, whereas the valve is
operated to open via a motor when the valve is of the power-operated type.
Here, the term "lifting" or "lift" is used to mean the opening area of the
EGR control valve.
As illustrated in FIG. 18, the actual behavior of the EGR control valve
lags behind the time that the command value is issued. Namely, there is a
response delay between the actual valve lifting and the command value to
do so. The lag time is constant in the motor-operated valve, but varies
with engine operating conditions in the vacuum-operated valve. Moreover,
it takes additional time for the exhaust gas passing through the valve to
enter the combustion chamber. Therefore, in order to carry out EGR control
properly, it becomes necessary to estimate accurately the amount or flow
rate of the recirculated exhaust gas. The amount or flow rate of
recirculated exhaust gas is generally estimated as a rate in the mass or
volume of intake air or mixture. The rate is referred to as "EGR rate" or
"exhaust gas recirculation rate" in the specification, which will be
explained in detail later.
Moreover, since the recycled exhaust gas becomes a disturbance in the fuel
metering or air/fuel ratio control in an engine, it is proposed in
Japanese Laid-open Patent Application Sho 60(1985)-169,641 to determine a
fuel injection correction coefficient in response to the EGR rate and to
correct the fuel injection amount in a decreasing direction. The prior art
system takes into account the fact that the lag time varies with the
engine operating conditions in the vacuum-operated EGR control valve, and
it delays switching the correction coefficient for a period determined by
the engine operating conditions. Another reference, Japanese Laid-open
Patent Application Sho 59(1984)-192,838 teaches to change the value of a
fuel injection correction coefficient gradually.
Since, however, the behavior of the exhaust gas is more complicated, the
assignee proposed earlier in Japanese Laid-open Patent Application Hei
5(1993)-118,239 (filed in the United States and patented under the number
of U.S. Pat. No. 5,215,061) to establish a model describing the behavior
of the exhaust gas. In this earlier proposed system, the exhaust gas
behavior caused by valve lifting is expressed by an equation and the net
EGR rate is determined by this equation. Specifically, the earlier system
used a concept of two kinds of ratios named "direct ratio" and "carry-off
ratio". The first ratio corresponds to a fraction, to the exhaust gases
passing through the EGR control valve during the control cycle n, of the
exhaust gas that enters the combustion chamber during the cycle n, while
the second ratio corresponds to a fraction, to the exhaust gases which
passed through the valve by a time m cycles earlier (m.gtoreq.1), but that
remained in a space before the combustion chamber, of the exhaust gas that
enters the combustion chamber during the cycle n. The earlier system
estimates the net EGR rate by the direct ratio and the carry-off ratio.
In addition, the assignee proposed in Japanese Patent Application No. Hei
5(1993)-296,049 (filed in the United States on Oct. 31, 1994 under the
number of 08/331746 still pending) another system to estimate the net EGR
rate in such a manner that:
Net EGR rate=EGR rate at a steady-state.times.(Actual valve lifting
amount/Command value for valve lifting amount)
The system configuration is less complicated than that proposed in the
publication 5-118,239, since the net EGR rate is estimated from the ratio
between the command value and the actual value of valve lifting. (In order
to distinguish the EGR rate at a steady-state, the EGR rate is sometimes
referred to as the "net" EGR rate.)
However, the amount or flow rate of recirculated exhaust gas depends not
only on the amount of valve lifting (the opening area of the valve), but
also on engine operating conditions. In other words, the amount or flow
rate of recirculated exhaust gas varies with the engine operating
conditions even when the amount of valve lifting remains unchanged. The
estimation accuracy of the system is therefore not always satisfactory. As
a result, the correction coefficient for the fuel metering or air/fuel
ratio control calculated on the basis of the estimated net EGR rate is not
always proper.
Furthermore, Japanese Laid-open Patent Application No. Hei 4(1992)-311,643
discloses a system for estimating the partial pressures of air and exhaust
gas in the intake pipe respectively and based on the estimated values and
additionally on the engine speed, to calculate the amount of air entering
the combustion chamber. The prior art system needs, however, to determine
the amount of exhaust gas recycled in the intake pipe as well as the
intake air temperature and the volume of a space (the so-called "chamber")
before the combustion chamber and hence requires complicated calculations.
It is quite difficult to accurately measure the recycled exhaust gas flow
rate due to the delay in the recycled exhaust gas etc. and the
calculations are subject to uncertainties.
A first object of the invention is therefore to provide an EGR rate
estimation system for an internal combustion engine which is simple in
configuration without requiring complicated calculation, but that is able
to estimate the exhaust gas recirculate rate with high accuracy, while
reducing calculation uncertainties as much as possible.
As mentioned above, the recirculated exhaust gas will be a disturbance for
carrying out the fuel metering or air/fuel ratio control in an engine.
A second object of the invention is therefore to provide an EGR rate
estimation system for an internal combustion engine which enables to
estimate the exhaust gas reticulation rate with high accuracy, thereby
enhancing the accuracy of the fuel metering or air/fuel ratio control in
an engine.
Moreover, when a command value for valve lifting amount is made zero to
discontinue the EGR operation, the amount of actual valve lifting does not
immediately decrease to zero due to a response delay in the operation of
the EGR control valve. The exhaust gas continues to pass through the valve
all the while, although the amount or flow rate of exhaust gas passing
therethrough is quite small. In addition, when a command value for valve
lifting amount becomes zero, there may arise a problem which could make
the EGR rate estimation difficult.
A third object of the invention is therefore to provide an EGR rate
estimation system for an internal combustion engine which is able to
estimate the exhaust gas recirculation rate correctly, accounting for the
delay in the valve operation and without causing any difficulty in the
estimation when a command value for valve lifting amount is made zero.
Apart from the above, some engines have a recirculation passage connecting
the exhaust system to the intake system at a position relatively farther
from the combustion chamber such that a transport delay of the recycled
exhaust gas could occur. The transport delay affects the EGR rate
estimation.
A fourth object of the invention is therefore to provide an EGR rate
estimation system for an internal combustion engine which can estimate the
exhaust gas recirculation rate accurately when the exhaust gas transport
delay could occur.
As mentioned repeatedly, the recirculated exhaust gas will be a disturbance
for carrying out the fuel metering or air/fuel ratio control and the
transport delay could degrade the accuracy of such control.
A fifth object of the invention is therefore to provide an EGR rate
estimation system for an internal combustion engine which can estimate the
exhaust gas recirculation rate when the transport delay could occur,
thereby enhancing the accuracy of the fuel metering or air/fuel ratio
control in an engine.
Aside from the above, the recycled exhaust gas will degrade the
ignitability of the mixture in the combustion chamber. Thus, the recycled
exhaust gas not only affects the fuel metering or air/fuel ratio control,
but also affects the ignition timing control in an engine.
A sixth object of the invention is therefore to provide an EGR rate
estimation system for an internal combustion engine which can estimate the
exhaust gas recirculation rate accurately, thereby enabling the ignition
timing under EGR operation to be determined properly.
SUMMARY OF THE INVENTION
This invention achieves these objects by providing a system for estimating
an exhaust gas recirculation rate for an internal combustion engine, said
engine having a passage connecting an exhaust pipe to an intake pipe for
recirculating a portion of exhaust gases in said exhaust pipe to a
combustion chamber through said intake pipe, and a valve provided at said
passage for regulating an amount or a flow rate of said exhaust gas to be
recirculated in response to a command value for valve lift amount,
comprising engine operating condition detecting means for detecting
operating conditions of said engine at least including engine speed and
engine load, basic exhaust gas recirculation rate determining means for
determining a basic exhaust gas recirculation rate (1-KEGRMAP) at least
based on said detected engine speed and engine load, first flow rate
estimating means for estimating a first flow rate QACT of said exhaust gas
passing through said valve based on flow rate characteristics of said
valve including a valve lifting amount LACT, and second flow rate
estimating means for estimating a second flow rate QCMD of said exhaust
gas to be recirculated passing through said valve based on flow rate
characteristics of said valve including said command value for valve
lifting amount LCMD, net exhaust gas recirculation rate estimating means
for estimating a net exhaust gas recirculation rate based on said basic
exhaust gas recirculation rate (1-KEGPRMAP), said first flow rate QACT and
said second flow rate QCMD.
BRIEF EXPLANATION OF THE DRAWINGS
These and other objects and advantages of the invention will be more
apparent from the following description and drawings, in which:
FIG. 1 is a schematic view showing the overall arrangement of an EGR rate
estimation system for an internal combustion engine according to the
invention;
FIG. 2 is a flowchart showing the operation of the EGR rate estimation
system for an internal combustion engine;
FIG. 3 is an explanatory view showing the flow rate characteristics of the
EGR control valve determined by the amount of valve lifting and the ratio
between upstream pressure (manifold absolute pressure) and downstream
pressure (atmospheric pressure);
FIG. 4 is an explanatory view showing the characteristics of mapped data of
a coefficient KEGRMAP;
FIG. 5 is an explanatory view showing the characteristics of mapped data of
a command value for valve lifting amount LCMD;
FIG. 6 is a flowchart showing the subroutine of the flowchart of FIG. 2 for
calculating a coefficient KEGRN;
FIG. 7 is a flowchart, similar to FIG. 6, but showing a second embodiment
of the invention;
FIG. 8 is a flowchart, similar to a portion of the flowchart of FIG. 2, but
showing a third embodiment of the invention;
FIG. 9 is a flowchart, similar to FIG. 2, but showing a fourth embodiment
of the invention;
FIG. 10 is a flowchart showing the subroutine of the flowchart of FIG. 9
for calculating the coefficient KEGRN;
FIG. 11 is an explanatory view showing the configuration of a ring buffer
used in the flowchart of FIG. 10;
FIG. 12 is an explanatory view showing the characteristics of mapped data
of a delay time .tau. used in the flowchart of FIG. 10;
FIG. 13 is a timing chart showing the calculation of the coefficient KEGRN
in the flowchart of FIG. 10;
FIG. 14 is a flowchart, similar to FIG. 10, but showing a fifth embodiment
of the invention;
FIG. 15 is a flowchart, similar to FIG. 10, but showing a sixth embodiment
of the invention;
FIG. 16 is a flowchart showing the determination of ignition timing
according to a seventh embodiment of the invention;
FIG. 17 is an explanatory view showing the relationship between mapped data
referred to in the flowchart of FIG. 16; and
FIG. 18 is a timing chart showing a delay in actual valve lifting to a
command value and another delay until exhaust gas enters the combustion
chamber of the engine.
DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS
Embodiments of the invention will now be explained with reference to the
drawings.
FIG. 1 shows the overall arrangement of the EGR rate estimation system for
an internal combustion engine according to the invention.
In the figure the engine is a four cylinder engine and main engine unit 1
of the engine is provided with an air intake pipe 2 which has a throttle
valve 3 at an appropriate location therewith. The throttle valve 3 is
connected with a throttle position sensor 4 (illustrated as ".theta.TH" in
the figure) which generates an output indicative of the position (opening
degree) of the throttle valve 3 and sends it to an electronic control unit
5 (illustrated and will be hereinafter referred to as "ECU").
The ECU 5 is comprised of a microcomputer made up of an input circuit 5a, a
central processing unit 5b, a memory 5c, and an output circuit 5d. The
input circuit 5a receives the output of the throttle position sensor 4 and
other outputs of some similar sensors explained later, shapes wave forms
of the outputs, converts the voltage levels of the outputs to
predetermined levels, and converts a digital value into an analog value,
if necessary. The central processing unit (CPU) 5b carries out various
calculations for the EGR rate estimation, fuel metering control and some
similar operations as will be explained later in accordance with programs
stored in the memory 5c. The CPU 5b also determines and outputs
manipulated variables to devices concerned through the output circuit 5d.
Each cylinder has a fuel injection valve 6 for injecting fuel in the
combustion chamber (not shown) installed in the vicinity of an intake port
(not shown) of the combustion chamber. The fuel injection valve 6 is
connected with a fuel pump (not shown) to be supplied with fuel and is, on
the other hand, electrically connected with the ECU 5 to be regulated in
its opening period, thereby defining the fuel injection amount. The intake
pipe 2 is provided with a manifold absolute pressure sensor 7 (illustrated
as "PBA") downstream of the throttle valve 3 which generates an output
indicative of the manifold absolute pressure PBA and an intake air
temperature sensor (illustrated as "TA") farther downstream of the
manifold absolute pressure sensor 7 which generates an output indicative
of the intake air temperature TA. These outputs are outputted to the ECU
5.
Moreover, the main engine unit 1 is equipped with an engine coolant
temperature sensor 9 (illustrated as "TW") which generates and sends to
the ECU 5 an output indicative of the coolant water temperature TW. In
addition, the main engine unit 1 is provided at its crank shaft or cam
shaft (neither shown) with a crankshaft sensor 10 (illustrated as "CRK")
which generates an output at every predetermined crank angular positions
including piston top dead center (TDC) position, and a reference cylinder
position sensor 11 (illustrated as "CYL") which generates an output
indicative of a predetermined crank angular position CYL of a selected
cylinder from among the four cylinders. These sensor outputs are also sent
to the ECU and the output CRK is counted by a counter (not shown) to
detect the engine speed NE.
The main engine unit 1 has an exhaust pipe 13 and a catalytic converter 14
is provided at an appropriate location therewith to decrease HC and CO
emissions and NOx emissions, or all three of these exhaust pollutants. In
the exhaust pipe 13, an air/fuel ratio sensor 15 (illustrated as "LAF") is
installed upstream of the catalytic converter 14 and generates an output
indicative of the oxygen concentration in the exhaust gas. The output is
forwarded to the ECU 5 and is input in a circuit (not shown) where it is
subjected to appropriate linear processing to obtain an air/fuel ratio
which varies linearly with the oxygen concentration of the exhaust gas
over a broad range extending from the lean direction to the rich
direction.
Furthermore, an atmospheric pressure sensor 16 (illustrated as "PA") is
installed in the vicinity of the main engine unit 1 and generates an
output indicative of the atmospheric pressure PA at the place where the
engine is located. And a third temperature sensor 17 (illustrated as "TC")
is provided on the floor of the intake pipe 2 in the vicinity of the
intake port and generates an output indicative of the intake pipe floor
temperature TC. These sensor outputs are sent to the ECU 5.
Here, the engine is equipped with an EGR system 25. This will be explained
next.
The EGR system 25 has an EGR passage 18 which extends from the exhaust pipe
13 to the intake pipe 2 and is connected thereto at a position 18a. Here,
it is assumed that the distance between the position 18a and the
combustion chamber is relatively short and hence, it is not necessary to
take the transport delay of the recirculated exhaust gas into account.
A vacuum-operated EGR control valve 19 is provided in the passage 18 at an
appropriate location therewith. The EGR control valve 19 is generally
comprised of a valve member 19a for opening/closing the passage 18, a
diaphragm 19b connecting with the valve member 19a and a spring 19c urging
the diaphragm 19b and the valve member 19a in the closing direction
(downward in the figure).
The diaphragm 19b separates the inside of the valve 19 into two chambers
19d and 19e. The chamber 19d is connected with the intake pipe 2 via a
conduit 20 and receives the negative pressure therefrom. The conduit 20
has a solenoid valve 22 of the normally-closed type which regulates the
negative pressure to be introduced in the chamber 19d. The conduit 20 has
a second conduit 23 which is branched off at downstream of the solenoid 22
and is opened to the ambient atmosphere so that air is introduced to the
conduit 20 via an orifice provided at the second conduit 23 and then to
the chamber 19d. The other chamber 19e in the valve 19 is opened to the
ambient atmosphere. Thus, when the negative pressure regulated by the
solenoid valve 22 is applied to the chamber 19d, the valve member 19a is
lifted in the opening direction (upward in the figure) and exhaust gas
will be introduced to the intake pipe 2 by an amount corresponding to the
amount of valve lifting.
It should be noted here that the opening area of the EGR control valve 19
is determined in terms of the amount of valve lifting, since the amount of
lifting in the EGR control valve used here is proportional to its opening
area. Accordingly when a different valve such as a linear solenoid is
used, another parameter will be used to determine the opening area.
The solenoid valve 22 is electrically connected with the ECU 5 and receives
a command value that corresponds to the amount of lifting (opening) of the
EGR control valve 19. A sensor 24 is installed at the EGR control valve 19
and generates an output indicative of the amount of stroke of the valve
member 19a, i.e., the actual amount of valve lifting and sends it to the
ECU 5.
Based on the detected parameters, the CPU 5b in the ECU 5 determines a fuel
injection amount in terms of the opening period of the injection valve 6
and ignition timing for the ignition system (not shown). The CPU 5b
further estimates the exhaust gas recirculation rate and based on the
estimated EGR rate corrects the fuel injection amount to be supplied to
the engine and determines the ignition timing to be supplied to the engine
in a manner explained later.
FIG. 2 is a flowchart showing the operation of the EGR rate estimation
system according to the invention.
Before starting the explanation of the flowchart, however, the EGR rate
estimation according to the invention will be briefly described.
Viewing the EGR control valve 19 alone, the amount or flow rate of exhaust
gas passing therethrough will be determined from its opening area (the
aforesaid amount of lifting) and the ratio between the upstream pressure
and downstream pressure at the valve. In other words, the amount or flow
rate of the mass of exhaust gas passing through the valve will be
determined from the flow rate characteristics of the valve, i.e.,
determined from the valve design specification.
Viewing therefore the EGR control valve 19 in the engine, it will be
possible to estimate the exhaust gas recirculation rate to a large degree
by detecting the amount of the EGR control valve lifting and the ratio
between the manifold absolute pressure PBA (negative pressure) in the
intake pipe 2 and the atmospheric pressure PA exerted through the conduit
23. (Although, in practice, the exhaust gas flow rate characteristics
change slightly with exhaust manifold pressure and exhaust gas
temperature, the change can be absorbed by the ratio between the gas flow
rates as explained later.) The invention is based on this concept and
estimates the EGR rate on the basis of the flow rate characteristics of
the valve.
The EGR rate will be classified into two kinds of rates, i.e., one at a
steady-state and the other at a transient state. Here, the steady-state is
a condition in which the EGR operation is stable and the transient state
is a condition in which the EGR operation is being started or terminated
so that the EGR operation is unstable.
The inventors consider that the EGR rate at a steady-state is a value where
the amount of actual valve lifting is equal to the command value for valve
lifting amount. On the other hand, the transient state is a condition in
which the amount of actual valve lifting is not equal to the command value
so that the EGR rate deviates from the EGR rate at a steady-state
(hereinafter referred to as "steady-state EGR rate) by the exhaust gas
flow rate corresponding to the discrepancy in the actual amount and the
command value, as illustrated in FIG. 3. (In the figure, the upstream
pressure is indicated by the manifold absolute pressure PBA and the
downstream pressure by the atmospheric pressure PA)
This will be summarized as follows:
At a steady-state,
command value=actual valve lifting amount, and
gas flow rate corresponding to actual valve lifting amount/gas flow rate
corresponding to command value=1.0
At a transient,
command value.noteq.actual valve lifting amount, and
gas flow rate corresponding to actual valve lifting amount/gas flow rate
corresponding to command value.noteq.1.0
As a result, it can be concluded that:
net EGR rate=(steady-state EGR rate).times.(ratio between gas flow rates).
Thus, by determining the steady-state EGR rate to a desired value with
respect to engine operating conditions at least including the engine speed
and the engine load, it becomes possible to estimate the exhaust gas
recirculation rate by multiplying the steady-state EGR rate by the ratio
between the gas flow rates corresponding to the actual valve lifting
amount and | | |
