U.S. patent application number 10/087762 was filed with the patent office on 2002-09-19 for fuel vapor emission control device for an engine.
This patent application is currently assigned to NISSAN MOTOR CO., LTD.. Invention is credited to Hayashi, Takane, Kakizaki, Shigeaki, Kobayashi, Masato, Tsuyuki, Takeshi.
Application Number | 20020129802 10/087762 |
Document ID | / |
Family ID | 26611207 |
Filed Date | 2002-09-19 |
United States Patent
Application |
20020129802 |
Kind Code |
A1 |
Hayashi, Takane ; et
al. |
September 19, 2002 |
FUEL VAPOR EMISSION CONTROL DEVICE FOR AN ENGINE
Abstract
A fuel vapor emission control device of an engine 10 includes a
canister 4 which adsorbs fuel vapor generated in a fuel tank 1, and
a purge valve 11 which opens and closes a pipe 6 connecting the
canister 4 and an intake passage 8 of the engine 10. A controller
21 sets a target purge rate according to a difference between a
target air-fuel ratio feedback deviation and an actual air-fuel
ratio feedback deviation during purge, and drives the purge valve
11 so that the target purge rate is achieved.
Inventors: |
Hayashi, Takane;
(Yokohama-shi, JP) ; Tsuyuki, Takeshi;
(Yokohama-shi, JP) ; Kakizaki, Shigeaki;
(Yokohama-shi, JP) ; Kobayashi, Masato;
(Yokohama-shi, JP) |
Correspondence
Address: |
FOLEY AND LARDNER
SUITE 500
3000 K STREET NW
WASHINGTON
DC
20007
US
|
Assignee: |
NISSAN MOTOR CO., LTD.
|
Family ID: |
26611207 |
Appl. No.: |
10/087762 |
Filed: |
March 5, 2002 |
Current U.S.
Class: |
123/698 ;
123/520 |
Current CPC
Class: |
F02D 41/0045 20130101;
Y02T 10/44 20130101; F02D 41/40 20130101; Y02T 10/40 20130101; F02M
25/089 20130101; F02D 41/0032 20130101; F02D 41/0042 20130101 |
Class at
Publication: |
123/698 ;
123/520 |
International
Class: |
F02M 025/08 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 14, 2001 |
JP |
2001-071564 |
Mar 14, 2001 |
JP |
2001-071562 |
Claims
What is claimed is:
1. A fuel vapor emission control device of an engine which is
air-fuel ratio feedback controlled, comprising: a canister which
adsorbs fuel vapor generated in a fuel tank of the engine, a purge
passage which connects the canister and an intake passage of the
engine, a purge valve which opens and closes the purge passage, and
a controller functioning to: compute a target air-fuel ratio
feedback deviation, which is the deviation between a target value
of the air-fuel ratio feedback correction coefficient and a basic
value of the air-fuel ratio feedback correction coefficient,
compute an actual air-fuel ratio feedback deviation which is the
deviation between an actual air-fuel ratio feedback correction
coefficient and the basic value of the air-fuel ratio feedback
correction coefficient, set a target purge rate according to the
difference between the target air-fuel ratio feedback deviation and
the actual air-fuel ratio feedback deviation, and drive the purge
valve so that the purge rate becomes the target purge rate.
2. The fuel vapor emission control device as defined in claim 1,
wherein the controller further functions to: set a larger target
purge rate variation amount, the larger the difference between the
target air-fuel ratio feedback deviation and the actual air-fuel
ratio feedback deviation becomes, and set the target purge rate by
adding the target purge rate variation amount to the immediately
preceding value of the target purge rate.
3. The fuel vapor emission control device as defined in claim 2,
wherein the controller further functions to: set a different value
to the target purge rate variation amount depending on whether the
difference between the deviations is positive or negative, even
when the magnitude of the difference between the deviations is
identical.
4. The fuel vapor emission control device as defined in claim 1,
wherein the controller further functions to: perform purge by
fixing the value of the purge rate to less than 1 percent until the
gas between the canister and the purge valve is purged.
5. The fuel vapor emission control device as defined in claim 1,
wherein the controller further functions to: compute an initial
value of the fuel amount adsorbed by the canister, and when the
initial value of the adsorbed fuel amount has computed, drive the
purge valve so that the purge rate becomes the target purge rate,
compute a fuel injection pulse width so that the air-fuel ratio of
the engine is a target air-fuel ratio, compute the fuel amount
desorbed from the canister using a canister model, correct the fuel
injection pulse width based on the computed desorbed fuel amount
such that the air-fuel ratio fluctuation of the engine due to
performing purge at the target purge rate is reduced, and drive a
fuel injector of the engine at the fuel injection pulse width after
correction, and the canister model comprises: an equation which
computes the fuel amount adsorbed by the canister based on the
immediately preceding value of the fuel amount adsorbed by the
canister and the immediately preceding value of the fuel amount
desorbed from the canister, and an equation which computes the fuel
amount desorbed from the canister based on the computed adsorbed
amount and the target purge rate.
6. The fuel vapor emission control device as defined in claim 5,
wherein the controller further functions to: estimate the fuel
amount desorbed from the canister by considering that the air-fuel
ratio fluctuation of the engine is due to purge, and compute the
initial value of the adsorbed fuel amount by inverse computation
from the estimated desorbed amount.
7. A fuel vapor emission control device of an engine which is
air-fuel ratio feedback controlled, comprising: a canister which
adsorbs fuel vapor generated in a fuel tank of the engine, a purge
passage which connects the canister and an intake passage of the
engine, a purge valve which opens and closes the purge passage,
means for computing a target air-fuel ratio feedback deviation,
which is the deviation between a target value of the air-fuel ratio
feedback correction coefficient and a basic value of the air-fuel
ratio feedback correction coefficient, means for computing an
actual air-fuel ratio feedback deviation which is the deviation
between an actual air-fuel ratio feedback correction coefficient
and the basic value of the air-fuel ratio feedback correction
coefficient, means for setting a target purge rate according to the
difference between the target air-fuel ratio feedback deviation and
the actual air-fuel ratio feedback deviation, and means for driving
the purge valve so that the purge rate becomes the target purge
rate.
Description
FIELD OF THE INVENTION
[0001] This invention relates to a fuel vapor emission control.
BACKGROUND OF THE INVENTION
[0002] An engine comprises a fuel vapor emission control device
wherein the fuel vapor generated in a fuel tank when the engine has
stopped is first adsorbed by activated carbon in a canister, the
fuel adsorbed by the activated carbon is desorbed using a manifold
vacuum under a predetermined running condition after the engine
starts, and the fuel vapor is led into an intake passage downstream
of a throttle valve to be burnt.
[0003] JP-A-H6-264832 published by the Japanese Patent Office in
1994 discloses a technique wherein, in a fuel vapor emission
control device, to perform an effective purge rate setting and
maintain a large purge flowrate, the purge rate is set according to
an integrated flowrate. The purge rate is the ratio of the purge
flowrate and intake air flowrate. Specifically, as the integrated
value of the purge flow rate increases, the purge rate is
increased.
SUMMARY OF THE INVENTION
[0004] However, in this aforesaid prior art, the purge rate
relative to the integrated purge flowrate was set to its optimum
value in the state where fuel is adsorbed up to the maximum
capacity by the canister (full charge state), to prevent oversupply
of fuel vapor due to purge. As a result, when the canister is fully
charged, the optimum purge rate can be set, but when the adsorption
amount of the canister is low, the optimum purge rate cannot be
set. In other words, when the canister adsorption amount is small,
in the prior art, a small purge rate was set based on the full
charge state regardless of the fact that a large purge rate could
actually be set in practice. Further, as a fixed purge rate was
always set relative to the integrated purge flowrate, it cannot be
dealt with when a purge gas of higher concentration than that
expected was supplied.
[0005] It is therefore an object of this invention to perform a
large amount of purge and set an optimum purge rate in a fuel vapor
emission control device without worsening the combustion stability
of the engine and increasing exhaust gas emissions.
[0006] In order to achieve above object, this invention provides a
fuel vapor emission control device of an engine which is air-fuel
ratio feedback controlled, comprising a canister which adsorbs fuel
vapor generated in a fuel tank of the engine, a purge passage which
connects the canister and an intake passage of the engine, a purge
valve which opens and closes the purge passage, and a controller
functioning to compute a target air-fuel ratio feedback deviation,
which is the deviation between a target value of the air-fuel ratio
feedback correction coefficient and a basic value of the air-fuel
ratio feedback correction coefficient, compute an actual air-fuel
ratio feedback deviation which is the deviation between an actual
air-fuel ratio feedback correction coefficient and the basic value
of the air-fuel ratio feedback correction coefficient, set a target
purge rate according to the difference between the target air-fuel
ratio feedback deviation and the actual air-fuel ratio feedback
deviation, and drive the purge valve so that the purge rate becomes
the target purge rate.
[0007] The details as well as other features and advantages of this
invention are set forth in the remainder of the specification and
are shown in the accompanying drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0008] FIG. 1 is an overall schematic diagram of a fuel vapor
emission control device.
[0009] FIG. 2 is a block diagram showing the purge control
performed by a controller.
[0010] FIG. 3 is a flowchart showing the details of the purge
control.
[0011] FIG. 4 is a block diagram showing the details of processing
which determines whether or not correction processing is
possible.
[0012] FIG. 5 is a flowchart showing the details of the delay
correction.
[0013] FIG. 6 is a table specifying a relation between an intake
air flowrate and a dead time.
[0014] FIG. 7 is a table specifying a relation between the intake
air flowrate and a diffusion coefficient.
[0015] FIG. 8 is a diagram showing the concept of dead time
processing in the delay correction.
[0016] FIG. 9 is a flowchart showing the details of correction
processing.
[0017] FIG. 10 is a flowchart showing the details of the
computation of a fuel desorption amount based on a canister
model.
[0018] FIG. 11 is a block diagram showing the construction of the
canister model.
[0019] FIG. 12 is a flowchart showing the details of a target purge
rate setting.
[0020] FIG. 13 is a map specifying a relation of a purge air-fuel
ratio error relative to intake air flowrate and purge rate.
[0021] FIG. 14 is a table specifying a relation of the purge
air-fuel ratio error to the purge air-fuel ratio.
[0022] FIG. 15 is a flowchart showing a target purge rate setting
processing until the canister model starts up.
[0023] FIG. 16 is a table specifying the relation between a
difference of air-fuel ratio feedback deviations (=target air-fuel
ratio feedback deviation-actual air-fuel feedback deviation), and
purge rate variation amount.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0024] Referring to FIG. 1 of the drawings, a fuel vapor emission
control device of an engine 10, comprises a canister 4, a pipe 2
which connects the canister 4 to a fuel tank 1, and a pipe 6 (purge
passage) which connects the canister 4 to an intake passage 8
downstream of a throttle valve 7, and processes fuel vapor
generated in the fuel tank 1.
[0025] A vacuum cut valve 3 which opens when the pressure in the
fuel tank 1 drops below atmospheric and a bypass valve 14 are
provided in parallel in the pipe 2. A purge valve 11 which opens
when fuel adsorbed by a fuel adsorbent (activated carbon) 4a in the
canister 4 is desorbed, and a pressure sensor 13 which measures the
pressure in the pipe 6, are provided in the pipe 6. The canister 4
has an air port 5. The air port 5 is opened and closed by a drain
cut valve 12.
[0026] Fuel vapor generated in the fuel tank 1 is led to the
canister 4 via the pipe 2, the fuel component alone is adsorbed by
the activated carbon 4a in the canister 4, and the remaining air is
discharged to the outside via the air port 5. To process the fuel
adsorbed by the activated carbon 4a, the purge valve 11 is opened,
and air is drawn into the canister 4 from the air port 5 making use
of the manifold vacuum which develops downstream of the throttle
valve 7. In this way, the fuel which was adsorbed by the activated
carbon 4a is desorbed by air, and is introduced to the intake
passage 8 of the engine 10 together with the air via the pipe 6
(referred to as purge control).
[0027] A controller 21 drives fuel injectors 15 with a pulse width
corresponding to the fuel amount required to realize the target
air-fuel ratio (usually, the stoichiometric air-fuel ratio)
according to the intake air amount detected by an air flowmeter 9.
At this time, the controller 21 detects the air-fuel ratio after
combustion by an oxygen sensor 18 attached to an exhaust passage
17, and corrects the fuel injection amount according to the
deviation from the target air-fuel ratio (hereafter, referred to as
air-fuel ratio feedback control). In this air-fuel ratio feedback
control, the deviation between the target air-fuel ratio and the
actual air-fuel ratio detected by the oxygen sensor 18 is reflected
in an air-fuel ratio feedback correction coefficient .alpha..
[0028] During purge control, the controller 21 maintains the
combustion stability of the engine, sets a high target purge rate
(ratio of purge flowrate to intake air flowrate) as far as possible
to the extent that exhaust gas emissions are not increased, and
drives the purge valve 11 so that the target purge rate is
realized. During purge control, fuel and air in the purge gas are
supplied to the engine 10, so the controller 21 corrects the fuel
injection amount according to the purge rate and purge air-fuel
ratio, and suppresses fluctuation of the air-fuel ratio of the
engine 10.
[0029] FIG. 2 is a block diagram showing the purge control
performed by a controller 2 1.
[0030] Each block will now be described. A block B1 computes a
maximum purge rate which can be set in the present running region
based on performance characteristics of the parts related to purge
control, and sets the target purge rate to follow this maximum
purge rate. However, sharp variations of purge rate lead to
fluctuation of the air-fuel ratio of the engine 10 and give rise to
increased emissions, therefore the variation amount of the purge
rate is limited so as not to exceed a predetermined amount (purge
rate variation amount limit) so that the purge rate does not vary
sharply. A block B2 computes a duty ratio of the purge valve 11
required to realize this target purge rate. A block B3 drives the
purge valve 11 at the duty ratio computed by the block B2.
[0031] A block B4 computes a fuel amount desorbed from the canister
4 when purge is performed at the above target purge rate using a
physical model (hereafter, referred to as a canister model)
described later. A block B5 computes a correction coefficient FHOS
of the fuel injection pulse width so that the air-fuel ratio
fluctuation due to purge is reduced based on the estimated
desorption amount. A block B6 performs a delay correction,
comprising a dead time correction and a diffusion processing, on
the correction coefficient FHOS. A block B7 performs a correction
of the fuel injection pulse width set according to the running
condition based on the correction coefficient FHOS after the delay
correction. A block B8 drives the fuel injectors 15 to perform fuel
injection at the fuel injection pulse width after the delay
correction.
[0032] The canister model represents the desorption characteristics
of the canister 4 to a high degree of precision, but as it is an
approximate model, values computed by using it (desorption amount,
adsorption amount, etc.) deviates to some extent from the actual
values. Also, the canister model computes the fuel amount desorbed
from the canister 4 using the computation result on the immediately
preceding occasion as described later, so the deviation between the
computed values and actual values increases as the operating time
of the model becomes longer due to the accumulation of the error.
Hence, to correct this deviation and maintain a high computational
precision of the model, when it is determined by the block B9 that
it is possible to perform a correction, the controller 21 corrects
the value of the fuel adsorption amount of the canister 4 which is
one of the internal parameters of the canister model in a block
B10.
[0033] Specifically, under the condition where all of the air-fuel
ratio fluctuation may be considered to be due to purge, the block
B9 determines that it is possible to perform correction processing.
The air-fuel ratio fluctuation is absorbed by air-fuel ratio
feedback control, and it appears as a fluctuation of the air-fuel
ratio feedback correction coefficient .alpha.. When it is
determined that the correction processing can be performed, the
block B10 estimates the amount of fuel desorbed from the canister 4
from the air-fuel ratio fluctuation (fluctuation of the air-fuel
ratio feedback correction coefficient .alpha.) at that time, and
computes the adsorption amount (fuel amount in the canister 4) by
inverse computation from the estimated desorption amount. The value
of the adsorption amount in the canister model is then corrected by
this value.
[0034] The specific details of the control performed by the
controller 21 will now be described.
[0035] FIG. 3 is a flowchart showing the purge control performed by
the controller 21, which is performed repeatedly when purge is
performed. In this control, the fuel injection amount (fuel
injection pulse width) is corrected according to the fuel amount
supplied from the canister 4 to the engine 10 by purge, and
air-fuel ratio fluctuation due to purge is suppressed.
[0036] In a step S1, it is determined whether it is possible to
perform correction of the value of the adsorption amount which is
an internal parameter of the canister model. When there is little
disturbance of the air-fuel ratio due to factors other than purge,
and the effect on the air-fuel ratio feedback correction
coefficient .alpha. due to purge is relatively large, i.e., when it
can be considered that the deviation from the target value of the
air-fuel ratio feedback correction coefficient .alpha. is
effectively due entirely to the effect of purge, it is determined
that correction processing can be performed.
[0037] Specifically, when the "steady state conditions", the "purge
valve precision condition" and the "purge effect conditions" shown
in FIG. 4 are all satisfied, it is determined that correction
processing can be performed. When any of these conditions is not
satisfied, it is determined that correction processing cannot be
performed. This correction processing corresponds to the processing
of the block B9 in FIG. 2.
[0038] As shown in FIG. 4, the "steady state conditions" are an
ignition condition (engine 10 does not misfire), fuel cut condition
(engine 10 does not perform fuel cut), blowby gas condition (blowby
gas is absent), EGR condition (exhaust gas recirculation rate is
constant), throttle opening area and engine rotation speed
condition (throttle opening area and engine rotation speed are
constant), target air-fuel ratio condition (target air-fuel ratio
is constant) and purge rate condition (purge rate is constant).
When it is determined that all of these conditions are satisfied
and there is little disturbance of the air-fuel ratio except due to
purge, it is determined that steady state conditions are
satisfied.
[0039] As the "purge valve precision condition", a purge flowrate
condition (purge flowrate is greater than a predetermined amount)
is set. When the purge flowrate is small, the control precision of
the purge flowrate falls and the computational precision in the
correction processing described later falls, so when the purge
flowrate is less than the predetermined amount, it is determined
that the purge valve precision condition is not satisfied. The
predetermined value may for example be set to 15 L/min.
[0040] As the "purge effect conditions", a purge execution
condition (purge is being performed), purge air-fuel ratio
condition (air-fuel ratio of purge gas is richer than a
predetermined air-fuel ratio, e.g., the variation amount of .alpha.
per 1% of the purge rate is higher than 1%) and a purge rate
condition (purge rate is greater than a predetermined value, e.g.,
the purge rate is higher than 3%. This predetermined value is set
so that the variation amount of the air-fuel ratio lies within the
target range for the system and the frequency of correction becomes
maximum) are set. When it is determined that all these conditions
are satisfied and the effect of purge on the air-fuel ratio is
relatively large, it is determined that the purge effect conditions
are satisfied.
[0041] Thus, when it is determined in the step S1 that correction
processing can be performed, the routine proceeds to a step S3 and
correction processing is performed. In the correction processing,
the fuel amount desorbed from the canister 4 is estimated from the
variation of the air-fuel ratio feedback correction coefficient
.alpha.. Also, the fuel amount which was adsorbed by the canister 4
is computed by inverse computation from the estimated desorption
amount, and the value of the adsorption amount which is an internal
parameter of the canister model is corrected by the value of the
adsorption amount calculated by this inverse computation (described
in detail later).
[0042] On the other hand, when it is determined in the step S1 that
correction processing cannot be performed, the routine proceeds to
a step S2. In the step S2, it is determined whether or not
correction processing has been performed previously. The reason why
this determination is performed is because, when correction
processing has not even been performed once, the initial value
(initial adsorption amount) required to make the canister model
function does not yet exist and purge control based on the canister
model cannot be performed. If the result of the determination shows
that correction processing has previously been performed, even only
once, the routine proceeds to a step S4, and if correction
processing has not been performed even once, this routine is
terminated. However, when correction processing has not been
performed even once, it does not mean that purge is not performed.
In this case, purge is performed by a purge control (boot-up
control shown in FIG. 15) which does not use the canister model,
described later.
[0043] In the step S4, the fuel desorption amount from the canister
4 is computed using the canister model. Specifically, the amount of
fuel desorbed from the canister 4 is computed according to the
flowchart shown in FIG. 10 (described in detail later).
[0044] In a step S5, a purge correction coefficient FHOS is
computed based on the desorption amount and the intake air
flowrate. The purge correction coefficient FHOS is computed
corresponding to the air-fuel ratio fluctuation (variation of the
air-fuel ratio feedback correction coefficient .alpha.) predicted
from the desorption amount supplied to the engine 10 computed by
the canister model. Specifically, for example, when the desorption
amount from the canister 4 increases and the fuel amount supplied
to the engine 10 increases, the air-fuel ratio of the engine 10
shifts to rich, and it may be expected that if it were attempted to
restore this, the air-fuel ratio feedback correction coefficient
.alpha. would vary to the small side. Hence, the purge correction
coefficient FHOS is computed as a small value in order to
correspondingly reduce the fuel injection amount in advance. The
computed correction coefficient FHOS is sequentially stored in a
predetermined data storage area (FIG. 8) of the controller 21.
[0045] In a step S6, a delay correction comprising a dead time
correction and diffusion processing is applied to the purge
correction coefficient FHOS. The reason for the dead time
correction is that there is a delay from when the purge valve 11
opens to when the purge gas reaches the cylinders of the engine 10
depending on the displacement velocity of the purge gas and the
distance between the purge valve 11 and cylinders of the engine 10.
Further, the reason for performing the diffusion processing is that
the fuel desorbed from the canister 4 diffuses during its passage
to the cylinders of the engine 10.
[0046] FIG. 5 is a flowchart showing the details of the delay
correction. This corresponds to the processing of the block B6 in
FIG. 2.
[0047] According to this, firstly, in a step S21, an intake air
flowrate is detected from the output of the air flow meter 9. In
steps S22, S23, a dead time and a diffusion coefficient are found
by looking up tables shown in FIG. 6, FIG. 7 respectively. The
intake air flow velocity increases the larger the intake air
flowrate, so a small value is set to the dead time. Also, when the
intake air flowrate increases and the intake air flow velocity
increases, the rate at which the desorbed fuel diffuses also
increases, so a large value is set to the diffusion
coefficient.
[0048] In a step S24, a displacement velocity equivalent value of
the purge gas is computed from the dead time. This purge gas
displacement velocity equivalent value is computed by inversing the
dead time found in the step S22.
[0049] In a step S25, the purge correction coefficients FHOS stored
in the data storage area (FIG. 8) of the controller 21, which
corresponds to the distance between the purge valve 11 and
cylinders of the engine 10, are read. In a step S26, the data is
shifted to the cylinder side by an amount equivalent to the
displacement velocity equivalent value of the purge gas. In a step
S27, the average value of the data which has overflowed from the
data storage area due to this shift is calculated.
[0050] In a step S28, diffusion processing is performed on the
average value of the overflowed data calculated in the step S27
using the diffusion coefficient found in the step S22. This
diffusion processing is generally diffusion processing using a
first order lag, the degree of diffusion increasing the smaller the
diffusion coefficient becomes.
[0051] FIG. 8 shows the concept of the dead time correction in the
delay correction. In the figure, the black points show data before
the data shift, and the white circles show data after the data
shift.
[0052] As shown in FIG. 8, the data storage area corresponding to
the distance from the purge valve 11 to the cylinders of the engine
10 is provided in the memory of the controller 21. The correction
coefficient FHOS computed according to the fuel amount desorbed
from the canister 4 is sequentially stored in the storage area. In
the dead time correction, this data is shifted to the cylinder side
by an amount corresponding to the displacement velocity of the
purge gas (=inverse of dead time). The correction coefficients
which have overflowed from the data storage area due to this data
shift are regarded as correction coefficients corresponding to the
purge gas which has reached and is supplied to the cylinders. By
performing dead time processing by this data shift, the computed
correction coefficients of the fuel pulse width can all be used for
correction of fuel injection even if the dead time varies
discontinuously. This avoids ignoring values and avoids the
repeated use of certain values for fuel injection correction when
the dead time varies. The value obtained by applying diffusion
processing to the average value of this overflowed data is then
used to correct a fuel injection pulse width Ti described later. In
this way, the delay of the desorbed fuel in reaching the cylinder
is reflected with high precision in the fuel injection amount
correction by means of simple processing.
[0053] Returning now to FIG. 3, in a step S7, the fuel injection
pulse width (drive pulse width of fuel injectors 15) Ti is
computed. Specifically, a basic pulse width Tion is corrected by
the air-fuel ratio feedback correction coefficient .alpha. and the
purge correction coefficient FHOS, by the following equation (1)
and the injection pulse width Ti of the fuel injectors 15 is
computed.
Ti=Tion.times.FHOS.times..alpha..times.K+TB (1)
[0054] where Tion=basic pulse width
[0055] FHOS=purge correction coefficient (value after delay
correction)
[0056] .alpha.=air-fuel ratio feedback correction coefficient
[0057] K=fuel injector coefficient
[0058] TB=fuel injector ineffectual pulse width
[0059] The basic pulse width Tion is set according to the intake
air flowrate and number of cylinders so as to realize the target
air-fuel ratio. The air-fuel ratio feedback correction coefficient
.alpha. is set to 100% (=1) when the target air-fuel ratio
coincides with the air-fuel ratio detected by the oxygen sensor 18,
is set to a smaller value than 100% when the detected air-fuel
ratio is richer than the target air-fuel ratio, and is set to a
larger value than 100% when the detected air-fuel ratio is leaner
than the target air-fuel ratio. In this way, the fuel injection
amount is corrected so that the actual air-fuel ratio approaches
the target air-fuel ratio. The fuel injector coefficient K is the
inverse of the ratio of the injector injection amount to the basic
value for the same applied pulse time and the same differential
pressure, and makes it possible to obtain the same fuel injection
amount even using an injector having different injection
characteristics. The fuel injector ineffectual pulse width TB
corrects for the delay from when a drive voltage is applied to the
fuel injectors 15 to open the valve, and fuel is actually
injected.
[0060] Next, the correction processing performed in the step S3
will be described in detail. FIG. 9 is a flowchart showing the
details of this correction processing, and corresponds to the
processing of the block B9 in FIG. 2.
[0061] First, it is determined in a step S31 whether purge is being
performed. This is because the processing in steps S32, S33 assumes
that purge is being performed, and if these processing are
performed when purge is not being performed, an appropriate
correction can no longer be made. When it is determined that purge
is not being performed, the routine is terminated and correction
processing is not performed.
[0062] When it is determined that purge is being performed, the
routine proceeds to the step S32. In the step S32, the desorption
amount (mass) is computed by the following equation (2) from the
intake air weight found from the intake air flowrate and intake air
temperature, purge rate, purge correction coefficient FHOS and
air-fuel ratio feedback correction coefficient .alpha.:
Dg=K1.times.(1-DLT+K2.times.PR).times.Qg (2)
[0063] where Dg=desorption amount
[0064] DLT=total air-fuel ratio deviation
(=(.alpha..times.FHOS/100)-100%)
[0065] PR=purge rate
[0066] K1=coefficient (constant determined by properties of
desorbed fuel)
[0067] K2=coefficient (constant determined by properties of
air)
[0068] Qg=intake air weight
[0069] The coefficient K1 is a coefficient determined by the
composition (average molecular weight) of the desorbed fuel, is
found by experiment, and may be set to, for example, 15.2. The
coefficient K2 is a coefficient determined by the average molecular
weight, average density and average capacity per mole of air, is
found by calculation, and may be set to, for example, 1.00672.
Equation (2) is an equation for computing the fuel amount desorbed
from the canister 4, from the deviation in the air-fuel ratio
relative to the basic value (first term and second term on the
right-hand side), the purge rate at that time (third term on the
right-hand side) and the intake air weight. In other words, it is
considered that the deviation in the air-fuel ratio feedback
correction coefficient .alpha. relative to the basic value is
completely due to purge, and the desorption amount is estimated
from the deviation of the air-fuel ratio.
[0070] In the step S33, an adsorption amount Yr (mass) of the
canister 4 is computed by the following equation (3) from the
desorption amount computed in the step S32 and purge flowrate:
Yr=KD.times.Dg (1/n(T)) (3)
[0071] where n(T)=desorption index
[0072] KD=desorption coefficient
[0073] T=activated carbon temperature
[0074] The desorption index n(T) is 2.56 when for example the
activated carbon temperature is 20.degree. C. The desorption
coefficient KD is a coefficient to correct for the phenomenon that
the desorption (or adsorption) occurs in proportion to the
concentration difference, and is set so that the unit desorption
characteristic of the canister (variation in total weight of
canister relative to integrated purge amount) and the canister
model output coincide. The unit desorption characteristic is found
by experiment, and the desorption coefficient KD is found by
calculation from the unit desorption characteristic. Equation (3)
is an inverse computation to an equation (5) which is an equation
of the canister model described later.
[0075] In a step S34, a value Y of the adsorption amount used to
compute the desorption amount based on the canister model is
replaced by the adsorption amount Yr computed in the step S33. In
this way, the value of the adsorption amount used in the canister
model can be corrected to a precise value, and the computational
precision of subsequent desorption amounts can be improved.
[0076] The computation of the desorption amount based on the
canister model in the step S4 of FIG. 3 will now be described
referring to the flowchart shown in FIG. 10. This processing
corresponds to the processing of the block B4 of FIG. 2.
[0077] Firstly, in a step S4 1, the current value Y of the fuel
amount adsorbed by the canister 4 is computed by the following
equation (4).
[0078] [Adsorption amount computational equation]
Y=Yz-Dgz (4)
[0079] Yz=immediately preceding value of adsorption amount
[0080] Dgz=immediately preceding value of desorption amount
[0081] Equation (4) computes the current adsorption amount Y (mass)
by subtracting the desorption amount Dgz on the immediately
preceding occasion from the immediately preceding value Yz of the
adsorption amount. However, when the correction processing shown in
FIG. 9 is performed, the computation of equation (4) is not
performed, or the value computed in equation (4) is ignored, and
the adsorption amount Yr computed by correction processing in
subsequent computations is used as the adsorption amount Y.
[0082] In a step S42, the desorption amount Dgk for the basic purge
flowrate is computed by the following equation (5):
[0083] [Desorption amount computation equation for basic purge
flowrate]
Dgk=(Y/KD) n(T) (5)
[0084] where Y=adsorption amount
[0085] KD=desorption constant
[0086] n(T)=desorption index
[0087] T=activated carbon temperature
[0088] Equation (5) applies the concept of the
adsorption/desorption phenomenon (Freundlich equation) to the
canister desorption. Thus, the fuel desorption characteristic of
the canister 4 can be suitably expressed. The Freundlich equation
is given in "Theory of Surfaces II", edited by Tsukada (Maruzen,
1995), p.25-p.27, p.108-p.115.
[0089] In a step S43, the desorption amount is computed by the
following equation (6):
[0090] [Desorption amount computation equation for purge
flowrate]
Dg=k.times.PQ.times.Dgk (6)
[0091] k=constant
[0092] PQ=purge flowrate (=purge rate.times.intake air
flowrate)
[0093] Dgk=desorption amount for basic flowrate.
[0094] Equation (6) is an equation which computes the desorption
amount by a linear approximation from the fact that the purge
flowrate and desorption amount are effectively in direct
proportion. The constant k is the inverse of the basic purge
flowrate. Here, the desorption amount for the basic flowrate is
calculated by equation (5), and the desorption amount is computed
by multiplying this by the purge flowrate in equation (6), but
equation (5) and (6) may be combined into one equation.
[0095] In a step S44, the activated carbon temperature T is
computed by the following equation (7): [Activated carbon
temperature computation equation]
T=Tz-Kt1.times.(YZ2-Yz)+Kt2.times.(Tz-Ta) (7)
[0096] Tz=immediately preceding value of activated carbon
temperature
[0097] Kt1=heat absorption coefficient
[0098] Yz2=value of adsorption amount two occasions previously
[0099] Yz=immediately preceding value of adsorption amount
[0100] Kt2=heat transfer coefficient
[0101] Ta=canister atmosphere temperature
[0102] Equation (7) comprises a past temperature (first term on
right-hand side), a temperature drop due to desorption (second term
on right-hand side), and a temperature rise due to heat transfer
(third term on right-hand side). The coefficient Kt1 is equal to
the temperature drop of the activated carbon when 1 g of adsorbed
material (fuel vapor) has desorbed from the adsorbent (activated
carbon), and corrects for the temperature drop of the activated
carbon due to desorption. The coefficient Kt2 is equal to the
temperature rise of the activated carbon due to heat transfer when
the temperature difference between the environment and the
activated carbon is 1.degree. C., and corrects for the temperature
rise of the activated carbon due to heat transfer. The coefficients
Kt1, Kt2 are found by experiment.
[0103] The reason why the activated carbon temperature T is
computed, is because the desorption index n(T) in equation (5) is
affected by the activated carbon temperature T and the desorption
characteristics vary. In particular, when the desorption amount is
large, the effect of computational error in the desorption amount
on the air-fuel ratio fluctuation is large and high computational
precision is required, but when the desorption amount is large, the
drop in the activated carbon temperature T is large, so the effect
on the desorption characteristics of the fuel in the canister 4 can
no longer be ignored.
[0104] The canister model comprises the above equations (4) to (7),
and if equations (5) and (6) are combined, it comprises three
equations. This may be shown graphically by FIG. 11. The canister
model comprises a block B41 which computes the adsorption amount, a
block B42 which computes the basic desorption amount, a block B43
which computes a desorption amount corresponding to the purge gas
flowrate, and a block B44 which computes the activated carbon
temperature. These blocks correspond respectively to equation (4)
through (7).
[0105] Next, the setting of the target purge rate will be
described.
[0106] FIG. 12 is a flowchart showing the setting of the target
purge rate. This corresponds to the processing of the block B5 in
FIG. 2. The purge valve 11 is driven at a duty ratio such that the
target purge rate set by this processing is achieved.
[0107] First, in a step S51, the air-fuel ratio (purge air-fuel
ratio) of the purge gas, is computed based on the desorption amount
computed based on the canister model and purge flowrate. In this
embodiment, the purge air-fuel ratio is computed based on the
desorption amount computed according to the canister model, so the
purge air-fuel ratio can be computed economically and precisely.
The purge air-fuel ratio may be detected by an HC sensor.
[0108] In a step S52, the error in the purge air-fuel ratio is
estimated from running conditions, for example, from parameters
such as the engine rotation speed, engine load and intake air
flowrate. The estimation of the purge air-fuel ratio error is for
example performed by looking up the table shown in FIG. 13, the
purge air-fuel ratio error increasing as the intake air flowrate
decreases or the purge rate decreases. Alternatively, the purge
air-fuel ratio error may be found by looking up a table specifying
the relation between the purge air-fuel ratio and the purge
air-fuel ratio error, as shown in FIG. 14. When the purge air-fuel
ratio error has been found, the routine proceeds to a step S53, and
the purge air-fuel ratio calculated in the step S51 is corrected
based on this error.
[0109] In a step S54, a purge rate variation amount limit is
computed based on the purge air-fuel ratio after error correction.
When the purge rate varies, the air-fuel ratio of the engine 10
varies, and the purge rate variation amount limit is computed so
that the air-fuel ratio fluctuation of the engine 10 is kept within
a tolerance width. The tolerance width of the air-fuel ratio
fluctuation is set to a width at which it can be absorbed by
performing air-fuel ratio feedback control which does not increase
exhaust emissions.
[0110] In a step S55, a purge rate upper limit PVMX specified by
the size of the purge valve 11 is computed. The reason for
calculating the purge rate upper limit PVMX is that, if the target
purge rate is set larger than the purge rate obtained at the
maximum opening of the purge valve 11, the actual purge rate and
target purge rate no longer coincide, the error in the computation
of FHOS increases so that the air-fuel ratio fluctuation increases,
and emissions increase. Specifically, if the purge valve size is
constant, the purge gas flowrate through the valve increases the
larger the differential pressure on either side of the purge valve,
so a large value is computed as the upper limit PVMX when the
differential pressure on either side of the purge valve is
large.
[0111] In a step S56, a purge rate upper limit TIMNMX based on the
characteristics of the injectors 15 is computed from the relation
between the minimum fuel injection pulse width determined according
to the characteristics of the fuel injectors 15, the immediately
preceding value of the target purge rate and the purge correction
coefficient. When the purge rate increases, the fuel amount
supplied to the engine 10 increases due to purge, and the fuel
injection pulse width is corrected to be shorter so as to decrease
the fuel injection amount from the fuel injectors 15
correspondingly. However, to maintain the injection precision of
the fuel injectors 15, the injection pulse width must be larger
than the predetermined minimum pulse width. In other words, to make
the fuel injection pulse width larger than the minimum pulse width,
the purge rate must be smaller than a certain value. Due to this
reason, the upper limit of the purge rate is specified by the
injection characteristics of the fuel injector 15.
[0112] In a step S57, assuming all possible running regions that
could be reached from the present running region, the minimum purge
rate from among these is predicted, and a purge rate upper limit
PRMNMX is computed from this minimum purge rate and the purge rate
variation amount limit. When for example the vehicle is accelerated
by depressing the accelerator pedal to the maximum, the target
purge rate is set to a very small value, but if the target purge
rate is set to a large value immediately before the accelerator
pedal depression is a maximum, the variation amount of the purge
rate is limited to below the variation amount limiting value, so
the purge rate can no longer follow the target purge rate. This
tracking delay increases emissions, so the purge rate upper limit
is specified also from the minimum possible purge rate so that this
tracking delay does not occur.
[0113] In a step S58, the air-fuel ratio feedback correction
coefficient .alpha. is monitored, and if it does not exceed a
predetermined value (e.g., 80%), the largest value of the purge
rate which makes the air-fuel ratio feedback correction coefficient
.alpha. higher than the predetermined value, is computed as a purge
rate upper limit ALPMX. The reason why this upper limit ALPMX is
provided is that, under fuel ratio feedback control, although the
air-fuel ratio feedback correction coefficient .alpha. is
controlled to be within 100.+-.25%, if the air-fuel ratio feedback
correction coefficient .alpha. is controlled under the
predetermined value and a large amount of purge is performed, the
air-fuel ratio is subject to disturbances other than purge and
tends to leave the above control range, so in such a case, it is
necessary to make the air-fuel ratio feedback correction
coefficient .alpha. larger than the predetermined value
immediately.
[0114] In a step S59, the smallest value of the above upper limits
PVMX, TIMNMX, PRMNMX, ALPMX is selected, and this value is set to
the maximum purge rate.
[0115] In steps S60, S61, the immediately preceding value of the
target purge rate is compared with the maximum purge rate. When the
immediately preceding value of the target purge rate is equal to
the maximum purge rate, the target purge rate is left at the
immediately preceding value (step S63), when the immediately
preceding value of the target purge rate is larger than the maximum
purge rate, the target purge rate is set to a value obtained by
subtracting the purge rate variation amount limit from the
immediately preceding value (step S64), and when the immediately
preceding value of the target purge rate is less than the maximum
purge rate, the target purge rate is set to a value obtained by
adding the purge rate variation amount limit to the immediately
preceding value (step S62).
[0116] Therefore, the target purge rate is set to follow the
maximum purge rate within the limits of the purge rate variation
amount limit, and the optimum purge rate which can achieve the
maximum purge without increasing exhaust gas emissions is set. When
the maximum purge rate is set, not only the upper limits PVMX,
TIMNMX, ALPMX determined by physical constraints, but also the
upper limit PRMNMX determined so that a shift to the maximum purge
rate can be performed without delay even if the running region
changes, are taken into consideration. Hence, the optimum purge
rate can be set to perform the maximum amount of purge without
increasing exhaust gas emissions even if the running conditions
change.
[0117] The processing of the aforesaid canister model cannot be
performed when correction processing has not yet been performed and
there is no initial value (initial adsorption amount) used by the
canister model. However, to achieve a large amount of purge, purge
must be performed even before the above correction processing.
Thus, a purge rate is set by the boot-up control shown in FIG. 15
instead of the above processing until correction processing is
performed, and purge is performed at the set purge rate. In boot-up
control, the air-fuel ratio fluctuation due to the purge is
absorbed by air-fuel ratio feedback control, and the fuel injection
amount is not corrected.
[0118] The processing shown in FIG. 15 will now be described.
First, in a step S71, the integrated purge flowrate (total purge
flowrate from the start of purge) is compared with the volumetric
capacity of the pipe 6 (precisely, the volumetric capacity of the
pipe 6 from the canister 4 to the purge valve 11). When the
integrated purge flowrate exceeds the purge pipe volumetric
capacity, the routine proceeds to a step S72, and when it does not
exceed the purge pipe volumetric capacity, the routine proceeds to
a step S75.
[0119] In the step S75, the target purge rate is set to an initial
purge rate. The initial purge rate is a small value less than 1%.
The reason why the initial purge rate is set to such a small value
is that, if the integrated purge flowrate does not reach the purge
pipe volumetric capacity, although the gas in the purge pipe is
supplied to the engine 10 before purge starts, the air-fuel ratio
of the gas in this purge pipe is unknown, and there is a
possibility that if the target purge rate setting shown in the step
S72 and subsequent steps were performed, the combustion stability
of the engine 10 would be impaired.
[0120] In other words, if low concentration purge gas present in
the pipe is supplied when purge starts and the air-fuel ratio
fluctuation due to this is small, it is determined that a large
amount of purge could be performed and a large purge rate is set.
If a large purge rate is set in this way, all the low concentration
gas in the pipe would be supplied, and when the high concentration
purge gas which was originally intended is supplied, a large amount
of desorbed fuel would suddenly be supplied which would lead to an
impairment of the combustion stability of the engine 10.
[0121] When the integrated purge flowrate exceeds the pipe
volumetric capacity, the routine proceeds to a step S72, and the
difference between the actual air-fuel ratio feed back deviation
and target air-fuel ratio feedback deviation is computed. The
target air-fuel ratio feedback deviation is the deviation between a
target value t.alpha. of the air-fuel ratio feedback correction
coefficient and the basic value (100%) of the air-fuel ratio
feedback correction coefficient (=.vertline.t.alpha.-100%). The
actual air-fuel ratio feedback deviation is the deviation between
the actual air-fuel ratio feedback correction coefficient .alpha.
and the basic value of the air-fuel ratio feedback correction
coefficient (=.vertline..alpha.-100.vertline.%).
[0122] For example, if the target value of the air-fuel ratio
feedback correction coefficient .alpha. is set to 80% in order to
maintain the air-fuel ratio fluctuation due to purge within a range
in which it can be fully absorbed by air-fuel ratio feedback
control, the target air-fuel ratio feedback deviation is set to
20%.
[0123] In a step S73, a purge rate variation amount is calculated
according to the difference between the target air-fuel ratio
feedback deviation and actual air-fuel ratio feedback deviation by
looking up a table shown in FIG. 16. The purge rate variation
amount is set to a value which becomes larger, the larger the
absolute value of the difference between the target air-fuel ratio
feedback deviation and actual air-fuel ratio feedback deviation, so
that convergence to the target value is enhanced. Also, different
values are set depending on the sign of the difference between the
target air-fuel ratio feedback deviation and actual air-fuel ratio
feedback deviation, even if the absolute value of the difference is
the same. The purge variation amount is set to a large value
(absolute value) when the difference between the air-fuel ratio
feedback deviations shifts to the negative side.
[0124] The reason why different characteristics are set to the
purge variation amount depending on the sign of difference between
the air-fuel ratio feedback deviations, is that when the difference
between the air-fuel ratio feedback deviations shifts to the
negative side, the air-fuel ratio feedback correction coefficient
.alpha. takes a smaller value than the target 80%, and compared to
the case where it is shifted to the positive side, it is a
disadvantageous state wherein the engine stability is adversely
affected by disturbances other than purge and emissions tend to
increase.
[0125] In other words, the reason for changing the characteristics
of the purge variation amount depending on the difference between
the air-fuel ratio feedback deviations is to rapidly return the
control point to the safe side from the viewpoint of engine
combustion stability and preventing emission increase.
[0126] After computing the purge variation amount as described
above, the routine proceeds to a step S74, and the purge variation
amount computed in the step S73 is added to the target purge rate
computed on the immediately preceding occasion this routine was
executed, so as to compute a new target purge rate. In a step S76,
the purge flowrate is calculated from the target purge rate and
intake air flowrate, and the value of the integrated purge flowrate
is updated.
[0127] Therefore, according to this processing, the optimum purge
rate can be set regardless of the adsorption state of the canister
4. Even if a purge of higher concentration than that envisaged is
supplied, the target purge rate is suitably updated according to
the air-fuel ratio fluctuation due to this and the optimum purge
rate is always set.
[0128] In this embodiment, the processing shown in FIG. 15 is
performed until the initial value of the canister model is computed
by correction processing. After correction processing is performed,
purge is performed based on the canister model, but purge may also
be performed at any time by the processing shown in FIG. 15.
[0129] Next, the overall operation according to the above control
will be described.
[0130] In the fuel vapor processing device according to this
invention, during purge, the target purge rate is set to as large a
value as possible to the extent that the engine combustion
stability does not decrease and emissions do not increase, and the
purge valve 11 is driven to achieve the target purge rate. During
purge, purge gas containing fuel which has desorbed from the
canister 4 is supplied to the engine 10, so the controller 21
predicts the air-fuel ratio fluctuation of the engine 10 due to the
supplied fuel resulting from purge by estimating the fuel amount
desorbed from the canister 4, and corrects the fuel injection pulse
width applied to the fuel injectors 15 so as to suppress this
air-fuel ratio fluctuation.
[0131] The desorbed fuel amount from the canister 4 is precisely
estimated in a short time using the canister model represented by
equation (4) to equation (7). Due to the canister model, if the
specifications of the canister are known, it is only necessary to
change the parameters of the model if the vehicle type or canister
used is changed, and it is not necessary to reconstruct the maps
and tables.
[0132] The canister model is an approximate model, and as the
desorption amount computed based on the model contains some errors,
these errors accumulate as the operating time of the model becomes
longer. For this reason, the controller 21 estimates the fuel
amount desorbed from the canister 4 from the variation of the
air-fuel ratio feedback correction coefficient .alpha., and
corrects the value of the adsorption amount which is an internal
parameter of the canister model using an adsorption amount obtained
by inverse computation from the estimated desorption amount. The
correction processing does not require any special sequence to be
performed, and it is unnecessary to reduce the purge rate to
perform the correction. Also, the correction processing is
performed when it is considered that disturbances of the air-fuel
ratio other than purge are small and the air-fuel ratio fluctuation
(variation of air-fuel ratio feedback coefficient .alpha.) is
almost entirely due to purge and the effect of purge on the
air-fuel ratio is relatively large, so a high correction precision
may be expected.
[0133] There is a delay from when the purge valve 11 is opened to
when the desorbed fuel reaches the cylinders of the engine 10, and
as the fuel also diffuses before it reaches the cylinders, the
correction of fuel injection pulse width is performed taking this
delay and diffusion into account.
[0134] Further, the purge control which uses the aforesaid canister
model cannot be implemented until the initial value of the
adsorption amount is calculated by correction processing, but until
the initial value of the canister model is computed, the target
purge rate is set according to the difference between the target
air-fuel ratio feedback deviation and the actual air-fuel ratio
feedback deviation, and the purge valve 11 is driven so that this
target purge rate is achieved. In this way, purge can be performed
even before the initial value is computed by correction processing,
and an effective purge can be performed in all running regions.
[0135] The entire contents of Japanese Patent Applications
P2001-71562 (filed Mar. 14, 2001) and P2001-71564 (filed Mar. 14,
2001) are incorporated herein by reference.
[0136] Although the invention has been described above by reference
to a certain embodiment of the invention, the invention is not
limited to the embodiment described above. Modifications and
variations of the embodiments described above will occur to those
skilled in the art, in the light of the above teachings. The scope
of the invention is defined with reference to the following
claims.
* * * * *