DESCRIPTION
EXHAUST GAS EMISSION CONTROL SYSTEM FOR DIESEL ENGINE
TECHNICAL FIELD
[0001]
This invention relates to an exhaust gas emission control
system for use in a diesel engine, and in particular to regeneration
control of a diesel particulate filter (hereafter, abbreviated as
DPF) for collecting a particulate matter (hereafter, abbreviated
as PM) contained in exhaust gas.
BACKGROUND ART
[0002]
In the scheme of emission control for diesel engines,
I
reduction of PM is as important as reduction of NOx. DPFs are known
#
as effective means to reduce PM. c.
A DPF is a PM collecting device using a filter. When an engine
is operating in a state where exhaust gas temperature is low, PM|
continues to accumulate on thfe DPF. Therefore, forced regeneration
is performed to burn the PM (or soot in the PM) by forcibly raising
the'temperature. ' ^
[0003]
In the DPF forced regeneration, late post-injection in which
fuel is injected into cylinders (injection timing is so late that
the fuel is not burned in the cylinders) is performed, and oxidation
reaction is caused by an oxidation catalyst (hereafter, abbreviated
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' ^ ^•
as DOC) arranged upstream of the DPF, so that heat obtained by tfhis
reaction'is utilized to keep the exhaust gas temperature m the
DPF high enough ('600 to 650°C) to burn the soot deposited on the
DPF. '• -
[0004]
In general, a late post-injection amount is subjected to
feedback control such as PID control so that the temperature in
the DPF is controlled at a target temperature. The target
temperature is determined based on a DPF inlet gas temperature,
a DPF outlet gas temperature, or a DPF internal temperature (these
temperatures are referred to as the DPF temperature).
[0005]
However, when the DPF temperature is controlled at a fixed
value of the target temperature, problems as follows may be induced.
I
When the target temperature is set high, the temperature may
#
possibly rise excessively when the soot deposited on the DPF'-is
burned. For example, when the engine is idling, a state called
"drop to idle" is produced, in which temperature tends to rise ^
excessively. If a large amount of soot is deposited in this "drop
to idle" state, the DPF internal temperature tends to rise rapidly
and excessively.
[0006]
When a critical soot deposition amount is defined as a soot
deposition amount at which a DPF catalyst reaches a temperature
(about 800 to 900°C) at which it degrades in the "drop to idle" state,
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^the DPF inlet temperature and the critical soot deposition amount
assume a relationship as shown in Fig. 14. It can be seen, from
this relationship, that as the soot deposition amount beconies
greater, the DPF target temperature must be set to a lower value.
*
Particularly, in an initial stage of DPF regeneration in which
the soot deposition amount is large, the risk of excessive rise
of DPF is increased if the DPF temperature is high.
[0007]
In contrast, if the target temperature is set low, the time
required for regeneration is increased, and hence the risk is
increased that late post-injected fuel falls from cylinder inner
walls into an oil pan, causing oil dilution. Fig. 15 shows time
variation of soot deposition amount during a regeneration process.
It can be seen that the regeneration time becomes longer as the
I
DPF regeneration temperature becomes lower.
[0008]
It is known that the DPF target temperature can be controlled
by varying the same based on some parameter. For example, Japanesei
Patent Application Publication No. 2007-239470 (Patent Document
1) describes that a target value of DPF inlet temperature is ,
determined based on any one of soot deposition amount, change rate^
of soot deposition amount, DPF temperature, change rate of DPF
temperature and the like.
[00091 • - .;
Japanese Patent Application Publication No. 2009-138702
(Patent Document 2) describes that a target value of DPF inlet
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temperature is set according to the time elapsed from the start
of forced regeneration of DPF, and the target value is set lower
as the time elapsed from the start of forced regeneration of DPF
becomes shorter. 'Furthermore, Japanese Patent Application
Publication No. 2010-071203 (Patent Document 3) is also known as
document disclosing a technique to set the DPF inlet temperature
target value.
[0010]
Patent Document 1: Japanese Patent Application Publication
No. 2007-239470
Patent Document 2: Japanese Patent Application Publication
No. 2009-138702
Patent Document 3: Japanese Patent Application Publication
No. 2010-071203
I
. [0011]
According to the technologies described in Patent Documei'hts
1 to 3 in which regeneration conditions such as late post-injection
i'
amount are controlled so that the DPF temperature becomes a target
DPF temperature, excessive rise of temperature is prevented.by
setting the target DPF temperature' lower as the time elapsed from
the start of forced regeneration becomes shorter.
[0012]
However, when the control is performed based on a DPF
temperature, izhe control cannot cope with significant change in
temperature characteristics of the soot regeneration amount. This
means that, the DPF internal temperature is raised by burning soot
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• ^.
i « • (by regeneration with O2) . ' . •
Therefore, a relationship between soot regeneration amount
and DPF temperature when the soot deposition amount is fixed is
represented by a trend as shown in Fig. 16. As shown in Fig. 16,
the soot regeneration amount changes with respect to the DPF
temperature not linearly but exponentially. Therefore, the soot
regeneration amount is increased significantly as the DPF
temperature rises, whereby the risk that rapid and excessive rise
of temperature occurs is increased. For example, the soot
regeneration rate when the DPF temperature is 630°C is about twice
as high as when it is 600°C.
Thus, when the control is performed based on DPF temperature,
the soot regeneration amount cannot be detected correctly, which
has a risk of excessive rise of temperature.
I
[0013]
The soot regeneration amount and the soot'regeneration rate
can be calculated by equations (1) and (2) below. The relationship
i
between soot regeneration amount and DPF temperature as shown in
Fig. 16 can be obtained from these equations (1) and (2).
>.• Soot regeneration amount [g/s] = soot regeneration rate [l/sj
X soot deposition amount [g] (1)
Soot regeneration rate [1/s] = A x exp(-B/RT) x QOa^ (2)
where A, B, and y each denote a constant, R denotes a gas constant,
T denotes a DPF temperature [K] , and QO2 denotes an O2 flow rate
[g/s].
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DISCLOSURE OF THE INVENTION
[0014]
When the control is performed based on DPF temperature, as
described above, the soot regeneration amount cannot be detected
correctly, and the situation in which the temperature is raised
by burning of soot cannot be grasped correctly, which has a risk
of excessive rise of temperature.
The invention has been made in view of these problems, and
an object of the invention is to provide an exhaust gas emission
control system for a diesel engine capable of preventing excessive
rise of temperature and oil dilution, by setting a target soot
regeneration amount, and directly controlling a soot regeneration
amount is to be the target soot regeneration amount so that the
I
. regeneration temperature and the regeneration time are optimized.
[0015]
V
In order to solve the aforementioned problems, the invention
i
provides an exhaust gas emission control system for a diesel engine'
as an internal combustion engine, comprising, in an exhaust,
passage, an oxidation catalyst (DbC) and a diesel particulate '
filter (DPF) for collecting soot m exhaust gas so that the soot
collected by the DPF is regenerated, the exhaust gas emission
control system further including: a regeneration control device
which-controls.a heating unit, when an amount of accumulated soot
exceeds a predetermined value, to heat the DPF up to near a
predetermined target temperature so as to burn and remove the
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•' -^ •?.
accumulated soot, wherein the regeneration control device ha's a
late post-injection control unit which injects fuel into a
combustion chamber at a timing not contributing to combustion, and
the late post-inj Miction control unit feed-back controls a late
post-injection amount such that a regeneration amount of the soot
regenerated by the DPF becomes a target soot regeneration amount.
[0016] - •
According to this invention, the late post-injection amount
is controlled based on a soot regeneration amount. This solves the
problem that the DPF temperature is raised rapidly and excessively
by significant increase of the soot regeneration amount.
In this manner, the target soot regeneration amount is set
and the late post-injection amount is controlled so that the soot
regeneration amount becomes the target soot regeneration amount,
I
whereby the temperature control for the DPF can be optimized. As
a result, the risk of excessive rise of temperature and oil dilutdon
can be minimized.
[0017] '
According to the invention, the late post-injection control
unit may preferably control based on the target soot regeneration
amount set to a fixed value..
In this manner, the soot regeneration amount is controlled
constant with the target soot regeneration amount set to a fixed
value, whereb-y the risk that the soot is rapidly burned can be
minimized, and hence the excessive rise of temperature can be
prevented.
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[0018] • . "
When the PF regeneration progresses and the deposition amount
is decreased, the soot regeneration amount will be reduced and the
regeneration time'will be prolonged if the DPF temperature is
controlled constant, possibly resulting in increase of the risk
of oil dilution. According to the invention, however, the soot
regeneration amount is kept constant even after the DPF
regeneration has progressed. Therefore, the regeneration time can
be shortened and the oil dilution can be suppressed.
[0019]
In the invention, the late post-injection control unit may
preferably vary the target soot regeneration amount according to
a regeneration elapsed time after the start of late post-injection,
such that the target soot regeneration amount is set to a small
I
value directly after the start of regeneration, then the set value
is increased as the regeneration progresses, and the target sDot
regeneration amount is set to a small value again at the end of
i
the regeneration. ^
In this manner, there is still deposited a large amount, of
soot directly after the start of regeneration, and hence the risk
y
of excessive rise of temperature m the DPF can be minimized by
lowering the target soot regeneration amount.
The target soot regeneration amount is increased as the
regeneration processes, whereby the regeneration time can be
shortened and the risk of oil dilution can be reduced.
In a final stage of the regeneration, the target soot
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regeneration amount is again reduced so that the soot iregeneratidn
amount is prevented from becoming too large and the risk of excessive
rise of temperature can be reduced.
[0020] - -
In the invention, the late post-injection control unit may
preferably vary the target soot regeneration amount according to
an amount of the acciomulated soot after the start of late
post-injection, such that the target soot regeneration amount is
set to a small value directly after the start of regeneration, then
the set value is increased as the regeneration progresses, and the
target soot regeneration amount is set to a small value again at
the end of the regeneration.
In this manner, the target soot regeneration amount is varied
according to an amount of the soot accumulated after the start of
I
late post-injection, whereby the same effect can be obtained as
when the target soot regeneration amount is varied according.to
a regeneration time elapsed after the start of late post-injection',
and the risk of excessive rise of temperature and oil dilution can
be reduced.
[0021]
V
In the invention, the target soot regeneration amount may*
preferably_,be varied in multiple, stages of two or more stages or
in a continuous fashion.
-Specifically, the target soot regeneration amount can be
varied according to a regeneration time elapsed after the start
of late post-injection and the target soot regeneration amount can
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i « • be varied according to a soot deposition amount after the start
of late post-injection, in multiple stages of two or more stages
or in a continuous fashion, whereby it is made possible to set a
target regeneratibn"soot amount appropriately according to
progress of regeneration after the start of late post-injection.
[0022]
In the invention, a rate limiter may preferably beprovid'ed
so that the soot regeneration amount varies slowly toward the target
soot regeneration amount directly after the start of late
post-injection.
In this manner, a rate limiter is provided, that is, a limit
is established on the rate of increase of soot regeneration amount,
whereby the overshoot of the soot regeneration amount directly
after the start of regeneration can be prevented and the excessive
I
. rise of temperature can be prevented.
[0023]
In the invention, an upper limit may preferably be established
for the target soot regeneration amount, the upper limit being '
obtained based on an upper limit for the temperature of the DPF.
In this manner, an upper limit established for the target
>,•
soot regeneration amount is obtained based on an upper limit beyond
which catalyst degradation occurs in the DPF, whereby degradation
of the DPF due to excessive rise of temperature can be prevented.
[0024] -
Further, the target soot regeneration amount upper limit may
preferably be set to a fixed value that is preliminarily obtained
10
•
•
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by calculation or test, or set to such a value that' the DPF''
temperature that is detected is increased to near a critical
temperature at which degradation of the catalyst of the DPF begins.
In this manner,' when an upper limit of the target soot amount
is set while monitoring the DPF temperature, the target soot amount
is set to a temperature at a very edge of the range where thermal
deterioration of the DPF will not occur. This makes it -possible
to perform regeneration at a hig*h temperature but not so high as
to result in excessive rise of temperature, and thus the
regeneration efficiency can be improved. Consequently, not only
deterioration of the DPF due to excessive rise of temperature can
be prevented but also oil dilution can be minimized.
[0025]
The exhaust gas emission control system according to the
I
invention is provided with a regeneration control device which
*
controls a heating unit, when an amount of accumulated soot exceieds
a predetermined value, to heat the DPF up to near a predetermined
target temperature so as to burn and remove the accumulated soot,i
and the regeneration control device has a late post-injection
control unit which injects fuel into'a combustion chamber at a timing
not contributing to combustion. The late post-injection control'
unit feed-back controls a late post-injection amount such that a
regeneration amount of the soot regenerated by the DPF becomes a
target soot regeneration amount. In other words, the late
post-injection control unit controls the late post-injection
amount based on a soot regeneration amount. Therefore, the
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invention is able to solve the problem of rapid and excessive tise
of DPF temperature due to significant increase of the soot
regeneration amount. Further, by setting the target soot
regeneration amouht'in this manner to control the late
post-injection amount such that the soot regeneration amount
becomes the target soot regeneration amount, the DPF temperature
control can be optimized and, as a result, the risk of excessive
rise of temperature and oil dilution can be minimized.
BRIEF DESCRIPTION OF THE DRAWINGS
[0026]
Fig. 1 is an overall configuration diagram of an exhaust gas
emission control system for a diesel engine according to the
invention;
t
Fig. 2 is a configuration block diagram of a soot deposition
amount estimation unit; '•
Fig. 3 is a configuration block diagram of a soot emission
amount calculation unit forming the soot deposition amount ^
estimation unit;
Fig. 4 is a configuration block diagram of a soot regeneration
> , • .
rate calculation unit forming the soot deposition amount estimation
unit;
Fig. 5 is a configuration block diagram showing a late
post-injection control unit according to a first embodiment;
Fig. 6 is a flowchart showing a-setting logic for setting
a target soot regeneration amount;
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^ Fig. 7 is a configuration block diagram according to a second
embodiment;
\ Fig. 8 is a'control flowchart according to the second
i
embodiment; •> -
Fig. 9 is an explanatory diagram showing a modification of
the second embodiment;
Fig. 10 is a configuration block diagram according t-o a thitd
embodiment;
Fig. 11 is an explanatory diagram showing a changing state
of a soot regeneration amount according to the third embodiment;
Fig. 12 is an explanatory diagram showing a modification of
the third embodiment;
Fig. 13 is a configuration block diagram according to a fourth
embodiment;
I
Fig. 14 is an explanatory diagram showing a relationship
between DPF inlet control temperature and critical soot deposition
amount;
Fig. 15 is an explanatory diagram showing a relationship between regeneration time and soot deposition amount; and
Fig. 16 is an explanatory diagram showing a relationship!
> ,•
between DPF temperature and soot regeneration amount.
BEST MODE FOR CARRYING OUT THE INVENTION
[0027]- -
The invention will be described in detail based on exemplary
embodiments shown in the drawings. It should be understood,
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• ' • -^ '^-
however, that the scope of the invention is not limited to dimensions, •
materials, shapes, and relative arrangement of components
described in these embodiments unless otherwise particularlydescribed.
••
[0028]
Referring to Fig. 1, an overall configuration of an exhaust
gas emission control system for a diesel engine according to the
invention will be described.
As shown in Fig. 1, a diesel engine (hereafter, referred to
as the engine) 1 has an exhaust passage 3 in which an exhaust gas
post-processing device 11 is provided. The exhaust gas
post-processing device 11 is composed of a DOC 7 and a DPF 9 arranged
downstream of the DOC 7 to collect PM.
There is also arranged, in the exhaust passage 3, an exhaust
I
gas turbocharger 17 having an exhaust gas turbine 13 and a compressor
15 that is coaxially driven with the exhaust gas turbine 13. 'Air
discharged from the compressor 15 of the exhaust gas turbocharger
17 passes through the air supply passage 19 and enters an inter
cooler 21, where the air is cooled. After that, the air supply flow
rate is controlled by an air throttle valve 23, and then the air
passes through an intake manifold 25 and then an intake port, and
is introduced into a combustion chamber via an intake valve of the
engine 1.
[0029], -
The engine 1 further has a fuel- injector (not shown) for
injecting fuel into the combustion chamber while controlling the
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• ' • • -. ' ^ -
injection timing, injection amount, and injection pressure of fuel,
and this fuel injector is connected to a regeneration control device
(ECU) 29 via a cohnection terminal 27.
An EGR (exhaus't gas recirculation) passage 33 is branched
out from midway of the exhaust passage 3 or an exhaust manifold
31 so that part of the exhaust gas is introduced into a downstream
region of the air throttle valve 23 via an EGR valve 35.
[0030] '
Combustion gas produced by combustion in the combustion
chamber of the engine 1, that is, exhaust gas 37 passes through
the exhaust manifold 31 and the exhaust passage 3, and drives the
exhaust gas turbine 13 of the exhaust gas turbocharger 17 to serve
as a power source of the compressor 15. The exhaust gas 37 then
passes through the exhaust passage 3 and flows into the exhaust
I
gas post-processing device 11.
The regeneration control device 29 for the DPF 9 receives
signals from a DPF inlet temperature sensor 39, a DPF outlet
,1
temperature sensor 41, a DOC inlet temperature sensor 43, an ain
flow meter 45, and an intake temperature sensor 47.
Further, the regeneration control device (ECU) 29 receives
signals from the EGR valve 35, the air throttle valve 23, an engine^
speed sensor. 4 9, and an intake manifold pressure sensor 51 and intake
manifold temperature sensor 53 in the intake manifold 25, as well
as a fuel inj-ection amount signal 55 from the fuel injector.
There are provided, in the regeneration control device 29,
a storage unit for storing various types of map data, a timer for
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• ' -^ '^.
measuring time elapsed from start of a late post fuel injection,
and the like.
[0031]
When the deposition amount of PM deposited on the DPF 9 exceeds
a predetermined value, the regeneration control device 29 control
the heating unit to raise the inlet temperature of the DPF 9 to
near the target set temperature (610 to 650°C) so that the deposited
PM is burned and removed.
Burning and removal of the PM by the regeneration control
device 29 will be described schematically below.
[0032]
Once forced regeneration is started according to a
determination based on conditions for starting the forced
regeneration, such as travel distance, engine operating hours,
I
. total fuel consumption, estimated soot deposition amount, and the
like, DOC temperature rising control is performed to activate'the
DOC 7. In this DOC temperature rising control, the opening of the
air throttle valve 23 is narrowed to reduce the amount of air flowing'
into the combustion chamber, whereby the exhaust gas temperature
is raised. Further, directly after a main injection, a first '
post-injection is performed by early post-injection to inject a
smaller amount of fuel than the main injection in a state in which
the pressure in the cylinders is still high. The exhaust gas
temperature i's raised by this early post-injection without
affecting the engine output, and the DOC 7 is activated by this
high-temperature exhaust gas flowing into the DOC 7. Thus, any
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Aunburned fuel in the exhaust gas is oxidized with activation' of
W
the DOC 7, and the exhaust gas temperature is raised by the oxidation
heat generated during the oxidation.
[0033] ^ ' •
It is then determined whether or not the DOC inlet temperature
has reached a predetermined temperature or whether or not the DPF
inlet temperature has reached a predetermined temperature, and if
it is determined that either the DOC inlet temperature or the DPF
inlet temperature has exceeded the predetermined temperature, the
inlet temperature of the DPF 9 is raised further more by late
post-injection. The term "late post-injection" means a second
post-injection performed after an early post-injection in a state
where the crank angle approaches near the bottom dead center. This
late post-injection causes the fuel to flow out of the combustion
I
.chamber to the exhaust passage 3 when an exhaust valve is open.
This discharged fuel is caused to react in the already activated
I.
DOC 7 and heat of oxidation thus generated further raises the exhaust
gas temperature to a temperature required for regeneration of th©
DPF 9, for example, to 610 to 650°C to promote combustion of ,PM.
[003,4]
-The regeneration control device 29 further has a soot
deposition 'amount estimation unit 60 to be used in determination
on the conditions for starting forced regeneration of the DPF, and
a late post-rnjection control unit 62 for controlling the
aforementioned late post-injection amount.
The soot deposition amount estimation unit 60 will be
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^described with reference to Figs. 2 to 4. The soot deposition
amount estimation unit 60 calculates an amount of soot accumulated
on the DPF 9 always based on an operating state of the engine
regardless of whether or not DPF 9 is regenerated.
[0035]
As shown in Fig. 2, the soot deposition amount estimation
unit 60 has a soot emission amount calculation unit 64, and a so'ot
regeneration rate calculation unit 66. The soot emission amount
calculation unit 64 calculates a soot emission amount based on
detection signals or calculated values of engine speed, fuel
injection amount, and oxygen excess ratio. Specifically, as shown
in Fig. 3, a soot emission amount map 68 according to an air fuel
ratio A, is provided. It is determined by a transient state
determination unit 70 whether or not the state is a transient state
1
based on a change of air fuel ratio X. When it is determined that
Ithe
state is not a transient state, a soot emission amount is
calculated by using a soot emission amount map 72 using a base X
i'
along the route indicated by the symbol E. The transient state
determination unit 70 determines whether the state is transient
state F or steady state E based on a difference in moving average,
AX between previous and current values of air fuel ratio, and based
on a comparison result between AX and a threshold and a comparison
result between a current air fuel ratio X and a threshold.
[0036]
When it is determined that the state is a transient state,
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the soot emission amount map 68 according to the air fuel rstio
A, is used along the route indicated by the symbol F to calculate
*'.
a soot emission amount. When the state is the transient state, a
•\-
correction coefficient according to an air fuel ratio X is
calculated by a soot emission amount correction unit 74, and is
multiplied by the integrator 76 to correct the soot emission amount.
[0037] , '
The soot regeneration rate calculation unit 66 calculates
a soot regeneration rate based on calculated values obtained by
using detection signals of engine speed, fuel injection amount,
exhaust gas flow rate, DOC inlet temperature, DPF inlet temperature,
and DPF outlet temperature, and a calculated value pf exhaust O2
flow rate calculated by using a predetermined formula.
Specifically, as shown in Fig. 4, the soot regeneration rate
calculation unit 66 has a soot regeneration rate map with O2 80,
Ia
NO2 conversion ratio map 82, a NOx emission amount map 84, and
a soot regeneration rate map with NO2 86.
The soot regeneration rate map with O2 80 is principally used
for calculation in forced regeneration, while an arithmetic
expression may be used in place of the map. The following equation
(2) described before is used as the arithmetic expression.
Soot'lregeneration rate [1/s] = A x exp(-B/RT) x Q02^ (2)
where A, B, and y each denote a constant, R denotes a gas constant,
T denotes DPF temperature [K], and QOs^ denotes a O2 flow rate [g/s] .
[0038]
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^ A NOx emission amount according to an engine operating state
is calculated by the NOx emission amount map 84, a conversion ratio
from NOx to NO2 is calculated by the NO2 conversion ratio map 82,
and the conversion ratio to NO2 is multiplied by the NOx emission
amount by the integrator 88. A rate of soot regeneration caused
by O2 that is generated when NOx is converted into NO2 is calculated
by the soot regeneration rate map 86 with O2, and a result of this
calculation is added by an adder 90 to a calculated value from the
soot regeneration rate map 80 with O2/ and a result of the addition
is output.
[0039]
Returning to Fig, 2, the soot regeneration rate calculated
by the soot regeneration rate calculation unit 66 is multiplied
by the estimated soot deposition amount by the integrator 92 to
t
calculate a soot regeneration amount. This soot regeneration
amount is subtracted by the adder 94 from the soot emission amo'iint
calculated by the soot emission amount calculation unit 64, in other
words, the regeneration amount is subtracted from the emission '
amount so that a deposition amount is thereby estimated.
The output from the adder 94 is integrated by an integrator'
96 and an estimated soot deposition amount is calculated. The
estimated soot deposition amount- is divided by a DPF capacity by
a divider 98, and a division result is output as a deposition amount
per unit capacity.
[0040]
First Embodiment
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r
^ A first embodiment of the late post-injection;Control lihit
62 provided in the regeneration control device 2 9 to control the
late post-injection amount will be described with reference to Figs.
5 and 6. -. -
The late post-injection control unit 62 is characterized by
feedback controlling the late post-injection amount so that the
soot regeneration amount regenerated by the PPF 9 becomes-a target
soot regeneration amount.
[0041]
As shown in Fig. 5, the target soot regeneration amount is
set to a constant, whereas the soot regeneration amount is
calculated based on an actual engine operating state, and these
regeneration amounts are input to an adder 100 which calculates
a difference between them. The difference is subjected to feedback
I
PID arithmetic processing by a PID controller 102, and the result
is output with its upper limit restricted by a late post-injectdon
amount limiter 104.
When the soot deposition amount is too small, the soot )
regeneration amount will be small and the late post-injection
amount will be too great, which ma]^ possibly cause excessive rise
of temperature. Therefore, a limiter is provided to limit the late*
post-injection amount. The limiter sets an upper limit for the
inlet temperature of the DPF" 9, for example, at 700°C. Since the
value to be set for the late post-injection amount differs depending
on an operating state (exhaust flow rate and exhaust temperature) ,
an upper limit is set by a late post-injection amount upper limit
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• ' -. ^.
>- I* -
map (using engine speed and fuel injection amount as parameters)
106.
[0042] ''
As shown in Fig. 6, a target soot regeneration amount setting
logic determines in step SI whether or not the late post-injection
control has been started. If the late post-injection control has
been started, the target soot regeneration amount set value is set
to the target soot regeneration amount m step S2, whereas if the
late post-injection control has not been started, the target soot
regeneration amount is Set in step S3 to a soot regeneration amount
that is calculated based on an actual engine operating state so
as to null the difference and to avoid accumulation of data in the
integrator in the PID controller 102.
The soot regeneration amount calculated based on an actual
I
engine operating state is provided by the regeneration amount
indicated in the portion Q for calculating a soot regeneration
amount as shown in the arithmetic operation of the soot regeneration
amount in Fig. 2. '
[0043] ^ , '
According to the first embodiment described above, the late
> , •
post-injection amount is controlled based on a soot regeneration
amount, which makes it possible to solve the problem that the DPF
temperature rapidly rises excessively due to substantial increase
of the-soot regeneration amount. Thus, a target soot regeneration
amount is set and the late post-injection amount is controlled so
that the soot regeneration amount becomes this target soot
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regeneration amount, whereby the control of the temperature of "the
'9
DPF 9 is optimized and as a result the risk of excessive rise of
temperature and oil dilution can be minimized.
[0044] ^' -
The target soot regeneration amount is set to a fixed value
to control the soot regeneration amount constant, whereby the risk
of rapid burning of the soot can be reduced and the excessive rise
of temperature can be prevented.
When the PF regeneration progresses further and the
deposition amount is reduced, the soot regeneration amount will
be decreased and the regeneration time will be increased if the
DPF temperature is set to a fixed value. This will increase the
risk of oil dilution. According to the embodiment, however, even
when the DPF regeneration progresses, the soot regeneration amount
I
is maintained constant, and hence the regeneration time is
shortened, whereby oil dilution can be suppressed. '•
[0045]
.1
Second Embodiment A second embodiment of the late post-injection control unit
62 will be described with referenbe to Figs. 7 to 9. i
The second embodiment is different from the first embodiment
only in the„setting of the target, soot regeneration amount, while
the other features are the same as those of the first embodiment.
Accordingly, -like components are assigned with like reference
numerals or symbols and description thereof will be omitted.
[0046]
23
11-444PCT
•' _ "?.
As shown in Fig. 7, the target soot regeneration amount is
changed according to regeneration time elapsed after the start of
late post-injection. The -target soot regeneration amount is
changed such that it'is set to a small value in a stage Ml directly
after the start of regeneration, whereas it is set to a greater
value than the one in the stage Ml in an intermediate stage M2 where
the regeneration has progressed. In a final stage M3, the target
soot regeneration amount is again set to a smaller value than the
one in the stage M2.
[0047]
Since the temperature of the DPF 9 is low and the soot
regeneration amount is small in the stage Ml directly after the
start of regeneration, the late post-injection amount possibly
becomes too large. In addition, since a large amount of soot has
I
likely been deposited directly after the start of regeneration,
the target soot regeneration amount is set to a small value.'-
V
In the final stage M3, toward the end of the regeneration,
the regeneration has progressed and the soot deposition amount has^
been decreased, whereby the soot regeneration amount becomes small
and hence the late post-injection amount becomes excessively large.i
This increases the risk that the upper limit of the late *
post-injection amount is continued to be updated, or the
temperature rises excessively. Therefore, the target soot
regeneration amount is set to a smaller value in the stage M3 than
in the stage M2 so that the target soot regeneration amount is
optimized toward the end of the regeneration.
24
,11-444PCT
[0048]
A specific target soot regeneration amount is obtained by
calculation using the equations (1) and (2) above based on the two
parameters of soot^deposition amount and DPF temperature for each
of the stage directly after the start of regeneration, the
intermediate stage, and the final stage.
[0049]
Next, a specific control flow will be described with reference
to Fig. 8.
In step Sll, it is determined whether or not a late post (LP)
injection control has been started. When it has been started, the
control proceeds to step S12, and it is determined whether or not
the regeneration time elapsed from the start of the late
post-injection control is equal to or more than the regeneration
I
elapsed time threshold. When the elapsed time is not more than the
*
threshold, the control proceeds to step S14, in which the target
soot regeneration amount is set to a set value for the first stage.
When it is determined in step S12 that the elapsed time is equaL
to or greater than the threshold, the control proceeds to step S13,
in which the target soot regeneration amount is set to a set value
for the second stage.
When.it is determined in step Sll that the late post (LP)
injection control has not been started, the control proceeds to
step S15, in which the target soot regeneration amount is set to
an actual soot regeneration amount.
[0050]
25
11-444PCT
• ' -. 'j'.
Although in the flowchart of Fig. 8, the switching is done
in two stages, the switching may be done in three stages as shown
in Fig. 7 or may be done in more stages. Furthermore, the target
soot regeneration*amount may be changed in a continuous fashion
instead of in multiple stages. In this case, the target soot
regeneration amount can be calculated by using a target soot
regeneration amount calculation curve 108 established according
»
to a regeneration elapsed time as shown in Fig. 9. Alternatively,
the target soot regeneration amount may be calculated by using a
linear arithmetic expression or the calculation curve thereof. In
this manner, an optimal target soot regeneration amount can be set
according to progress of regeneration after the start of late
post-injection.
[0051]
I
Still further, a soot deposition amount instead of
regeneration elapsed time can be used to determine whether or ftot
the target amount is to be switched. In this case, the estimated
i'
soot deposition amount (output value of the part indicated by the'
symbol S) shown in Fig. 2 may be used as the soot deposition amount
so that the switching is done according to a decrease of the soot
deposition amount.
[0052]
According to the second embodiment described above, since
a large amount of soot is still deposited directly after the start
of regeneration, the risk of excessive rise of DPF temperature can
be reduced by setting the target soot regeneration amount to a small
26
^1-444PCT
value.
Since the target soot regeneration amount is increased in
the intermediate stage of regeneration, the regeneration time can
be shortened and Hence the risk of oil dilution can be reduced.
t
In the final stage of regeneration, the target soot
regeneration amount is again reduced to prevent the soot
regeneration amount from becoming too large, whereby the risk of
the excessive rise of temperature can be reduced.
Consequently, the regeneration temperature and regeneration
time can be optimized to minimize the risk of excessive rise of
temperature and oil dilution.
[0053]
Third Embodiment
A third embodiment of the late post-injection control unit
I
62 will be described with reference to Figs. 10 to 12.
#
The third embodiment is characterized by a rate limiter '110
that is provided to ensure gradual change toward the target soot
regeneration amount directly after the start of late >
post-injection.
The other configuration features are the same as those of
the fi.rst embodiment. Therefore, like components are assigned with
like reference numerals or symbols and description thereof will
be omitted.
[0054} • -. ;
As shown in Fig. 10, a rate limiter 110 is provided in a circuit
in which a signal indicating a target soot regeneration amount is
27
, 11-444PCT
^ input to the adder 100. This improves the overshoot of soot
regeneration amount, and prevents the excessive rise of temperature.
In Fig. 11, the bold and thin two-dot chain lines represent cases
in which the rate ^limiter is not applied, while, the bold and thin
solid lines represent cases in which the rate limiter is applied.
As seen therefrom, the overshoot of soot regeneration amount can
be improved when the rate limiter is applied than when~the rate
limiter is not applied.
[0055]
The rate limiter may have an inclined section defined by
multiple stages.
Further, as shown in Fig. 12, an inclination setting map 112
may be provided by using a soot deposition amount (estimated soot
deposition amount (output value of the part indicated by the symbol
I
. S) in Fig. 2), so that the degree of inclination in the inclined
9
section of the rate limiter is changed according to the soot'
deposition amount.
<
By setting the degree of inclination according to a soot •
deposition amount, the problem of oil dilution can be avoided, that
is caused by prolonged processing time due to unnecessarily slow
change.
[0056]
Thus, the provision of the rate limiter 110 makes it possible
to limit the Trate of increase of the soot regeneration amount.
Accordingly, the overshoot of soot regeneration amount directly
after the start of regeneration and the excessive rise of
28
11-444PCT
^temperature can be prevented.
[0057]
Fourth Embodiment
A fourth embbdiment of the late post-injection control unit
62 will be described with reference to Fig. 13.
In the fourth embodiment, an upper limit is set for the target
soot regeneration amount. Like components to those of the first
embodiment are assigned with like reference numerals or symbols
and description thereof will be omitted.
[0058]
As shown in Fig. 13, a selector 113 is provided in a circuit
in which a signal indicating a target soot regeneration amount is
input to the adder 100 so that the selector 113 selects a smaller
one. Specifically, a signal from a target soot regeneration amount
I
upper limit setting unit 114 is input to the selector 113, which
selects a smaller one between a target soot regeneration amount
upper limit signal and a target soot regeneration amount signal,
{
and outputs the selected signal to the adder 100. >
[0059]
Further, as shown in Fig. 13', the target soot regeneration
amount upper limit setting unit 114 is provided with a change-over
switch 116... The change-over switch 116 receives a target soot
regeneration amount upper limit signal ul that is set as a fixed
value.- •
The target soot regeneration amount upper limit setting unit
114 is further provided with a soot regeneration rate calculation
29
11-444PCT
, ' _ •?-
unit 118 for calculating a soot regeneration rate. : In the soot
regeneration rate calculation unit 118, a soot regeneration rate
is calculated by using the equation (2) based on a DPF temperature
upper limit and arf O2 flow rate, and the integrator 120 multiplies
the calculated soot regeneration rate by the soot deposition amount
to provide a target soot regeneration amount.
A signal u2 indicating this target soot regeneration amount
is input to the change-over switch 116.
[0060]
By switching the change-over switch 116, the target soot
regeneration amount upper limit that is obtained based on an upper
limit for temperature of the DPF 9 can be set as a fixed value that
is preliminarily obtained by calculation or tests. Alternatively,
the target soot regeneration amount upper limit can be regulated
I
. and set while monitoring the DPF temperature such that DPF
#
temperature is caused to rise very close to the upper limit df a
temperature range in which catalyst degradation of the DPF will
not occur. '
[0061]
In other words, the target soot regeneration amount upper
> , •
' limit can be set to a value very close to the upper limit of a
temperature range where catalyst-degradation of the DPF will not
occur, by regulating the DPF-temperature upper limit as an input
signal to the- soot regeneration rate calculation unit 118 while
monitoring the DPF internal temperature.
[0062]
30
,11-444PCT
^ When the target soot regeneration amount upper:'liinit is'set
in this manner while monitoring the DPF temperature, the target
soot amount is set a temperature at a very edge of the range where
thermal deterioratio'n of the DPF will not occur. Therefore, the
regeneration can be performed at a high temperature but not so high
as to result in excessive rise of temperature and thus the
regeneration efficiency can be improved. Consequently, not only
»
deterioration of the DPF due to excessive rise of temperature can
be prevented but also oil dilution can be minimized.
[0063]
It should be understood that the third and fourth embodiments
can be combined with the first embodiment or the second embodiment
as required.
I
INDUSTRIAL APPLICABILITY
*
[0064]
The invention is suitably applicable to exhaust gas emission
I
control system for use in diesel engines since, according to the
invention, the DPF inlet temperature can be stably controlled at
a target temperature even when the exhaust gas flow rate continues
to be low after the exhaust gas flow rate is decreased.

11-444PCT
^ CLAIMS
1. An exhaust gas emission control system for a diesel engine
as an internal combustion"engine, comprising, in an exhaust
passage, an oxidation catalyst (DOC) and a diesel particulate
filter (DPF) for collecting soot in exhaust gas so that the soot
collected by the DPF is regenerated, the exhaust gas emission
control system further comprising:
a regeneration control device which controls a heating unit,
when an amount of accumulated soot exceeds a predetermined value,
to heat the DPF up to near a predetermined target temperature so
as to burn and remove the accumulated soot,
wherein the regeneration control device has a late
post-injection control unit which injects fuel into a combustion
chamber at a timing not contributing to combustion, and
I
the late post-injection control unit feed-back controls a
late post-injection amount such that a regeneration amount of'the
I,
soot regenerated by the DPF becomes a target soot regeneration
. • . .'
amount. '
2. The exhaust gas emission control system for a diesel '
• ) . •
, engine according to claim 1, wherein the late post-mjection
control unit controls based on the target soot regeneration amount
set to a fixed value.
3. The exhaust gas emission control system for a diesel
engine according to claim 1, wherein the late post-injection
32
11-444PCT
^control unit varies the target soot regeneration amount ac"cording
to a regeneration elapsed time after the start of late ' -
post-injection, s,uch that the target soot regeneration amount is
set to a small value directly after the start of regeneration, then
the set value is increased as the regeneration progresses, and the
target soot regeneration amount is set to a small value again at
the end of the regeneration.
»
4. The exhaust gas emission control sye^ftm for a diesel
engine according to claim 1, wherein the late post-injection
control unit varies the target soot regeneration amount according
to an amount of the accumulated soot after the start of late
post-injection, such that the target soot regeneration amount is
set to a small value directly after the start of regeneration, then
the set value is increased as the regeneration progresses, and the
target soot regeneration amount is set to a small value again at
• I -
the end of the regeneration. ,
5. The exhaust gas emission control system for -a diesel
engine according to claim 3 or 4, wherein the target soot
regeneration amount is varied in multiple stages of two or mo,re'
stages or in a continuous fashion.
6. The exhaust gas emission control system for a diesel
engine according to claim 1, wherein a rate limiter is provided
so that the soot regeneration amount varies slowly toward the target
33
11-444PCT
soot regeneration amount directly after the start of late
post-injection.
7. The exhaust gas emission control system for a diesel
engine according to claim 1, wherein an upper limit is established
* fpr the target soot regeneration amount, the upper limit being
obtained based on an upper limit for the temperature of the DPF.
8. The exhaust gas emission control system for a diesel
,engine according to claim 1, wherein the upper limit of the target
soot regeneration amount is set to a fixed value that is
preliminarily obtained by calculation or test, or set to such a
value that the DPF temperature that is detected is increased to
near a critical temperature at which degradation of the catalyst
of the DPF begins.