DESCRIPTION
EXHAUST GAS PURIFICATION SYSTEM FOR INTERNAL COMBUSTION ENGINE

TECHNICAL FIELD
[0001] The present invention relates to a diesel engine
exhaust gas purification system, and more particularly to the
control of temperature at the inlet of a diesel particulate '
filter (hereinafter, DPF) that collects particulate matter
(hereinafter, PM) contained in the exhaust gas, during
regeneration of the DPF.
BACKGROUND ART
[0002] PM reduction is as important as NOx reduction in
exhaust gas regulations of diesel engines. DPF is known as an
I
effective technique in this regard.
#
DPF is a PM collecting device that uses a filter. As the
PM continues to accumulate in the DPF m engine operating
i
conditions with low exhaust gas temperatures, forced
regeneration is carried out' wherein the temperature is
forcibly raised to burn the PM. ' i
[0003] Common means of raising the temperature include *
delaying the fuel injection timing, post-injection, and intake
throttling, which all encompass the problem of adversely
affecting the- fuel economy. On the other hand, higher
temperatures mean a quick and efficient forced regeneration of
DPF with a smaller decrease of fuel economy since the higher
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the temperature, the higher the speed of burning the PM. '*
^ However, if the DPF temperature is too high, the PM burns
rapidly and the l3PF temperature rises quickly, which may
damage the DPF or» deteriorate the catalyst carried in the DPF.
Temperature control is therefore necessary, to maintain the
DPF temperature at a level suitable for the regeneration, so
as to prevent a drop in the fuel economy and ensure safe
regeneration of the DPF.
[0004] There is Japanese Patent Application Laid-open No.
2005-320962 (Patent Document 1) as an example of temperature
raising control in forced regeneration of DPF. The Patent
Document 1 describes a process of temperature control during
regeneration of DPF wherein an optimal feedback gain in
accordance with the operating condition is used to achieve
I
both of stability and responsivity of temperature feedback
control to raise the temperature to a target level. '•
There is a time delay between variable manipulation to
raise the temperature and a change in the exhaust gas ^
temperature. The time delay of control targets also varies
depending on the changes in the operating condition. For i
V
example, an increase in the exhaust gas flow rate increases *
the heat transfer coefficient and decreases the time delay,
while a decrease in the exhaust gas flow rate increases the
time delay between changes in variable and changes in exhaust
gas temperature as well as the time constant, whereby the time
delay is increased.
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Described in the document is a corrective actio'n performed
^,n consideration of the time delay to make the temperature
closer to the tai?get level quickly, wherein the operating
condition is detected to determine the current time delay from
the relationship between the exhaust gas flow rate and the
time delay that is known from the operating condition, an
optimal feedback gain is calculated in accordance therewith, •
and the temperature raising variables are corrected using this
feedback gain.
[0005] Patent Document 1: Japanese Patent Application Laidopen
No. 2005-320962
[0006] Patent Document 1 describes making the temperature
closer to a target level quickly by correcting a feedback gain
as described above. However, it is not possible to improve the
I
stability of control of the DPF inlet temperature by a
corrective action using only a feedback gain, particularly <•
when the exhaust gas flow rate has decreased, since, in such a
condition, the time delay between changes in variable (post- i
injection amount) and changes in exhaust gas temperature is
increased, and so is the time constant, because of which the ,
exhaust gas temperature control performance is deteriorated,' ^
i.e., it takes long until a change appears in the DPF inlet
temperature even when, for example, the post-injection amount
is excessive.-
[0007] If the temperature is controlled properly by
feedforward control at various operating condition points, the
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feedback variables will be zero in a steady state ^nd the '*
^^roblem associated with the feedback control described above
will not arise. 'In a small general-purpose engine, however, in
which the rpm and> the load change independently in use, it is
difficult to set the feedforward variables properly in all
operating conditions.
If, for example, the flow rate of the exhaust gas has
dropped largely in a short time*, and the flow rate remains low
after that, the DPF inlet temperature will rise, and when this
phenomenon appears, it is difficult to solve it by improvement
of the gain in the feedback control, or by optimization of
controlled variables in the feedforward control.
DISCLOSURE OF THE INVENTION
I
[0008] Accordingly, the present invention was made in view of
these problems, and it is an object of the invention to ,.
provide an exhaust gas purification system for an internal
combustion engine capable of stable control to keep the DPF ,'
inlet temperature at a targ'et level even when the flow rate of
exhaust gas remains low after a sudden drop in the flow rate. ,
[0009] To solve the problems described above, the present ' *
invention provides an exhaust gas purification system for an
internal combustion engine that includes a diesel oxide
catalyst (DOCJ and a diesel particulate filter (DPF) for
collecting particulate matter (PM) in exhaust gas in an
exhaust gas passage and that treats the PM collected in the
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•' -^ •p.
DPF to regenerate the DPF, the system including: a>'
^regeneration control unit controlling a temperature raising
unit, when the Pisi' has accumulated more than a predetermined
amount, to heat up the DPF to around a predetermined target
temperature to burn off the accumulated PM, the regeneration
control unit including a feedforward controller outputting a
basic variable for the temperature raising unit based on an '
operating condition of the internal combustion engine, a
feedback controller outputting a correcting variable for
achieving the target temperature of the DPF, and a variable
adding unit adding the correcting variable output from the
feedback controller to the basic variable output from the
feedforward controller to compute a manipulated variable. The
system further includes at least one of an integrator resetter
I
resetting an integral value of an integrator forming the
feedback controller when a sudden drop in exhaust gas flow '•
rate is detected based on a monitored exhaust gas flow rate or
a control value calculated from the exhaust gas flow rate, and .
a basic variable calculating unit calculating the basic
variable to be output from the feedforward controller based oni
> , •
the exhaust gas flow rate or a control value calculated from
the exhaust.gas flow rate.
[0010] According to the invention, there is provided an
integrator resetter that resets the integral value of the
integrator forming the feedback controller when a sudden drop
in the flow rate of exhaust gas is detected based on the flow
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rate of exhaust gas passing through the DPF or a cdntrol value
.calculatfed from the exhaust gas flow rate, so that the late
post-injection ariiount, which is the manipulated variable to
raise the temperature, is prevented from being adversely
affected by the integral value remaining in the integrator in
the PID controller. As a result, the DPF inlet temperature can
be kept at around the target level even when the flow rate of
exhaust gas has dropped suddenlV-
[0011] The feedforward controller outputs a basic variable for
the temperature raising unit based on the operating condition
of the internal combustion engine. In an operating condition
in which the flow rate of the exhaust gas has decreased
suddenly and remains low for a while, in the conventional
technique, when there was (accumulated) still an integral
I
value before the sudden drop of the exhaust gas flow rate in
the integrator of the feedback controller, the response of the
DPF inlet temperature was slow (deadtime was long)
particularly when the flow rate was decreasing, so that it
would take time to output accumulated integral value, during
which the integral values are added as correcting variables, ,
whereby the DPF inlet temperature was raised, resulting in a' *
loss of the controllability.
In the present invention., as there is provided an
integrator resetter that resets the integral value of the
integrator, such loss of controllability of the DPF inlet
temperature caused by a remaining integral value is prevented.
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[0012] Alternatively, there is provided a basic variable.
^palculating unit calculating a basic variable of the
feedforward controller based on the flow rate of exhaust gas
or a control valufe calculated from the exhaust gas flow rate.
Namely, there is provided a basic variable calculating unit
calculating a basic variable by using an equation, more
specifically, a transfer function that models the temperature
rising characteristics of the exhaust gas in the DOC, so that
proper basic variables can be obtained under various operating
conditions.
Therefore, as compared to using a map prepared based on
various operating conditions beforehand, the feedforward
manipulated variables can be properly determined under various
operating conditions of a small general-purpose engine, in
I
.which the rpm and the load independently change in use, and
thus the controllability of the DPF inlet temperature can b^'
improved,
[0013] In the present invention, preferably, it may be
determined that there has been a sudden drop in the flow rate
of exhaust gas when any of the following applies: '
(1) The rate of decrease of the exhaust gas flow rate is
not higher..than a threshold;
(2) The exhaust gas flow rate has decreased to a threshold
or below; or ".
(3) The rate of decrease of the exhaust gas flow rate is
not higher than a threshold as well as the exhaust gas flow
7
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rate has decreased to less than a threshold. ;'
^ By monitoring the exhaust gas flow rate as well as the rate
of decrease in the flow rate, the integrator value is
prevented from being reset more than necessary, despite
frequent sudden drops in the exhaust gas flow rate during
transient operation. This prevents a loss of controllability
of the DPF inlet temperature during transient operation when '
the engine rpm and the engine load continuously change.
Or,
(4) The flow rate of the exhaust gas remains not higher
than the threshold for a certain period of time or longer.
With the additional condition, wherein the flow rate of
exhaust gas remains less than the threshold for more than a
certain period of time, unnecessary resetting of the integral
I
value during transient operation is prevented even more
reliably. '•
[0014] In the present invention, preferably, the integral
value of the integrator of the PID controller forming the >
feedback controller may be "reset when the integral value is,
positive. I
V
\. ' I ^
By resetting the integral value only when it is positive,
an unintended increase in the DPF inlet temperature caused by
a resetting action can be prevented. Namely, it is for
preventing an- unintended increase in the DPF inlet temperature,
which would occur if the integral value of the integrator of
the feedback controller is reset when the integral value is
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negative.
^[0015] in the present invention, preferably, the basic
variable calculating unit may calculate the basic variable of
the feedcontroller by using a preset equation of a transfer
function modeling the temperature rising characteristics of
the exhaust gas in the DOC in use of a deviation of a measured
DOC inlet temperature from the target DPF inlet temperature, '
and a control gain calculated b'ased on the exhaust gas flow
rate.
[0016] Namely, the temperature rising characteristics of the
exhaust gas in the DOC are modeled by a primary transfer
function, and a late post-injection amount that can achieve a
target DPF inlet temperature is obtained through calculation
as the basic variable of the feedforward controller.
I
[0017] More specifically, the late post-injection amount Z, or
a basic variable, is determined using a relational expression
of the primary transfer function Z = K/(l + as)e, wherein e is
the deviation of a measured DOC inlet temperature from the
target DPF inlet temperature, a is the time constant parameter
and Bv is the control gain determined from the flow rate of '
exhaust gas.
The smaller the design parameter (adjusting parameter) a is
set, the higher the sensitivity of the output will be relative
to changes in temperature deviation e and K, and the larger a
is set, the lower the responsivity.
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[0018] The late post-injection amount, which is thfe basic ''
variable; is calculated using a control gain, which is a
control value obt!ained from the exhaust gas flow rate, instead
of setting proper* basic variables by the feedforward
controller at various operating condition points, so that, as
compared to using a map prepared based on various operating
conditions beforehand, the feedforward variables can be
properly determined under various operating conditions.
[0019] Since the late post-injection amount, which is the
basic variable, is determined based on the deviation of the
measured DOC inlet temperature from the target value of the
DPF inlet temperature, the integrator of the PID controller
does not output a large value, i.e., a large deviation from
the target DPF inlet temperature is unlikely to occur, so that
I
a loss of the controllability of the DPF inlet temperature is
prevented under an operating condition in which the flow rate
of exhaust gas remains low after a drop in the exhaust gas
flow rate in a short time. [0020] In the present inverttion, preferably, the manipulated'
variable of the temperature raising unit may represent an ,
> •
amount of late post-injection that is performed in a period ' *
after a main injection and does not directly contribute to
combustion, after activation of the DOC.
The manipulated variable of the temperature raising unit
should preferably represent an amount, of late post-injection
that is performed in a period after a main injection and does
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•' • -> -f.
not directly contribute to combustion, after activation of the
DOC. . '
[0021] The late post-injection in the present invention shall
be described here^ .
Main injection is done to bring about main combustion in
the combustion chamber. Early post-injection refers to an
injection of fuel in a smaller amount than that of the, main '.
injection performed immediately after the main injection when
the pressure inside the cylinder is still highT This early
post-injection raises the temperature of the exhaust gas, and
the hot exhaust gas flowing into the DOC activates the DOC. A
second post-injection is performed after that, when the crank
angle is near the bottom dead center after the early postinjection.
This second post-injection is called late postinjection,
which does not contribute to the combust?ion inside
the combustion ch'amber, so that the fuel is discharged from,,
the combustion chamber into the exhaust gas passage in the '
exhaust stroke. This fuel discharged from the combustion '
chamber reacts in the already activated DOC, and the heat thus
generated by oxidation further raises the exhaust gas
temperature to a level of about 600°C necessary for the ' '
regeneration of the DPF, to promote burning of the PM.
[0022] According to the present invention, there is provided
an integrator, resetter that resets the integral value of the
integrator forming the feedback controller when a sudden drop
in the flow rate of exhaust gas is detected based on the flow
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rate of exhaust gas passing through the DPF or a control value
calculated from the exhaust gas flow rate, so that the late
post-injection ainpunt, which is the manipulated variable to
raise the temperature, is prevented from being adversely
»
affected by the integral value remaining in the integrator in
the PID controller. As a result, the DPF inlet temperature can
be kept at around the target level even when the flow rate of
exhaust gas has dropped suddenly.
[0023] Alternatively, there is provided a b^l^ variable
calculating unit calculating a basic variable of the
feedforward controller based on the flow rate of exhaust gas
passing through the DPF or a control value calculated from the
exhaust gas flow rate, so that proper basic variables can be
obtained through calculation under various operating
conditions. Therefore, as compared to using a map {prepared
based on various 'operating conditions beforehand, the
feedforward manipulated variables can be properly determined *•
under various operating conditions of a small general-purpose "'
engine, in which the rpm and the load independently change inuse,
and thus the controllability of the DPF inlet temperature
can>be improved. • [0024] As the controllability of the DPF inlet temperature is
improved, the target temperature at the DPF inlet can be set
several tens of °C higher without the possibility of the DPF
reaching the temperature at which the catalyst held therein is
degraded. Thus thermal degradation of the catalyst held in the
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DPF is prevented, whereby the durability of the DPF is
^improved:
As the target*' temperature at the DPF inlet can be set
higher, the time for controlling the DPF regeneration is
reduced, whereby the problem of oil dilution caused by late
post-injection at the time of regeneration is resolved.
BRIEF DESCRIPTIOfi OF THE DRAWINGS
[0025] FIG. 1 is a schematic configuration diagram of a diesel
engine exhaust gas purification system according to one
embodiment of the present invention;
FIG. 2 is a configuration block diagram showing a first
embodiment of a regeneration control unit;
FIG. 3 is a configuration block diagram showing a second
I
embodiment of the regeneration control unit;
*
FIG. 4 is a control flowchart of the first embodiment; i-
FIG. 5(a) is a subroutine flowchart showing one of the
steps of the flowchart of FIG. 4 in detail;
FIG. 5(b) is a subroutine flowchart showing one of the
steps of the flowchart of FIG. 4 in detail; ,
TIG. 5(c) is a subroutine flowchart showing one of the ' *
steps of the flowchart of FIG. 4 in detail;
FIG. 5(d) is a subroutine flowchart showing one of the
steps of the flowchart of FIG. 4 in detail;
FIG. 6 is a control flowchart of the second embodiment;
FIG. 7(a) and FIG. 7(b) are subroutine flowcharts showing
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steps of the flowchart of FIG. 6 in detail;
^ FIG. '8 is a diagram for explaining the confirmation test
results of the first, embodiment; and
FIG. 9 is a characteristics chart for explaining a gain K
map.
BEST MODE FOR CARRYING OUT THE INVENTION
[0026] The illustrated embodimefnts of the present invention
will be hereinafter described in detail. It should be noted
that, unless otherwise particularly specified, the sizes,
materials, shapes, and relative arrangement or the like of
constituent components described in these embodiments are not
intended to limit the scope of this invention.
[0027] The overall configuration of the diesel engine exhaust
gas purification system according to the present invention
will be described' with reference to FIG. 1. ,.,
As shown in FIG. 1, an exhaust gas passage 3 of a diesel '
engine (hereinafter referred to as engine) 1 includes an exhaust gas after treatment' system 9 including a DOC
(oxidation catalyst) 5 and a DPF (particulate filter) 7 that
collects soot downstream of the DOC 5. <; ' In the exhaust gas passage 3 is also disposed an exhaust
gas turbocharger 13 having an exhaust gas turbine 11a and a
compressor lib coaxially driven with the turbine. Intake air
discharged from the compressor lib of the exhaust gas
turbocharger 13 flows through an intake air passage 15 and
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enters an intercooler 17 to be cooled. An intake throttle '*
valve 19'Controls the amount of intake air flowinq
therethrough, and after that, the air flows from an intake
manifold 21 into combustion chambers through intake ports of
respective cylinders via intake valves of the engine 1.
[0028] Although not shown, the engine 1 includes a common rail
fuel injection system that controls the timing of injection, •
and the amount and pressure of fuel to be injected into the
combustion chambers. The common rail fuel injection system
feeds fuel at a predetermined controlled pressure to the fuel
injection valves 22 of the respective cylinders at a
predetermined timing of injection.
[0029] An exhaust gas recirculation (EGR) passage 25
bifurcates from midway of the exhaust gas passage 3 or an
I
exhaust manifold 23 so as to introduce part of the exhaust gas
*
to a portion downstream of the intake throttle valve 19 ,.
through an EGR cooler 27 and an EGR valve 29. '
[0030] Combustion gas or exhaust gas 31 produced in the '
combustion chambers of the fengine 1 flows through the exhaust"
manifold 23 that connects to each'of the exhaust ports of the ,
y
cylinders and through the exhaust passage 3, and spins the ' *
exhaust gas turbine 11a of the exhaust gas turbocharger 13 to
drive the compressor lib, after which it flows into the
exhaust gas after treatment system 9 through the exhaust gas
passage 3.
[0031] The DPF 7 is disposed downstream of the DOC 5. The
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regeneration control unit 33 of the DPF 7 receives-signal c
^ inputs from an air flowmeter 35 that detects the amount of air
flowing into the rcompressor lib, a DOC inlet temperature
sensor 37, and a pPF inlet temperature sensor 39.
Signals from an engine rpm sensor 41 and an engine load
sensor 43 are also input to the regeneration control unit
(ECU) 33.
[0032] This regeneration control unit 33 controls a
temperature raising unit when the amount of"^Pf^accumulated in
the DPF 7 exceeds a predetermined level to raise the
temperature at the inlet of the DPF 7 to around a target level
(of about 600°C) to burn off the accumulated PM.
The outline of the control by the regeneration control unit
33 to burn off the PM will be described first.
I
When conditions for starting forced regeneration are met,
which is determined based on, for example, mileage, running,'
time of the engine, a total amount of fuel consumed, in the
case with a vehicle, and when the forced regeneration is >
started, the DOC temperatur'e raising control is executed to '
activate the DOC 5. This DOC temperature raising control ,
involves reducing the degree of opening of the intake throttle *
valve 19 to reduce the amount of air flowing into the
combustion chambers, so as to increase unburnt fuel in the
exhaust,gas. .A first post-injection is then performed, wherein
a smaller amount of fuel than the main injection is injected
immediately after the main injection when the pressure inside
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. ' • ^ ^.
the cylinders is still high, this early post-injection raising
^he exhaust gas temperature without affecting the engine
output. The high^^temperat.ure exhaust gas flows into and
activates the DOC, 5,- and as the unburnt fuel in the exhaust
gas is oxidized by the activated DOC 5, the exhaust gas
temperature is further raised by the heat generated from the
oxidation.
[0033] When the DOC inlet temperature is determined to have
reached and exceeded a predetermined level, a late postinjection
is carried out to further raise the inlet
temperature of the DPF 7. The late post-injection refers to a
second post-injection after the early post-injection mentioned
above, wherein fuel is injected when the crank angle is near
the bottom dead center. By this late post-injection, fuel
flows out from the combustion chambers to the exhaust gas
passage 3 when the exhaust valves are open. The discharged .,.,
fuel reacts at the activated DOC 5, so that the exhaust gas
temperature is further raised by the heat generated from the '
oxidation to achieve a temperature necessary for the
regeneration of the DPF 7, e.g. 600°C, to promote burning of ,
the'PM. ' '
[0034] (Fir^t Embodiment)
Next, a first embodiment- of the control of the amount of
late post-injection by the regeneration control unit 33 will
be described with reference to the control configuration block
diagram of FIG. 2.
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The regeneration control unit 33 controls the amount of)'the
late post-injection (manipulated variable) to constantly keep
the inlet temperature of the DPF 7 at the setpoint of about
600°C and for that purpose includes: A feedforward controller
47 that outputs a command indicative of a basic injection
amount (basic variable) of late post-injection based on a
feedforward control map 45 that defines basic injection
amounts preset in accordance wi'th the engine rpms and fuel
injection amounts (engine loads); a feedback controller 4 9
that outputs a command indicative of a correction amount of
late post-injection (correcting variable) for achieving the
target temperature of the DPF 7; and an injection amount
(manipulated variable) adding unit 51 (see FIG. 2) that
computes an amount of injection by adding the correction
t
amount output from the feedback controller 4 9 to the basic
injection amount output from the feedforward controller 47. /•
In the first embodiment, an integrator resetter 55 is
further provided for resetting the integral value of an i
integrator 53 that forms part of the feedback controller 49.
[0035] The feedforward controller 47 computes a basic ,
injection amount, which is a feedforward control command 57,' *
based on the feedforward control map 45 that defines basic
amounts of injection preset .in accordance with the engine rpms
and fuel inje.ction amounts (engine loads) that represent
various operating conditions of the engine, as described above.
[0036] The feedback controller 49, on the other hand, includes
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a target temperature setting unit 59 that sets an initial
target value of the DPF 7 inlet temperature at the start of
the control and a, target temperature thereafter. The feedback
controller inputs, a.target DPF 7 inlet temperature and the
measured DPF 7 inlet temperature to an adder-subtracter 61,
and performs feedback calculation at a PID controller 63 using
the deviation of the measured inlet temperature from the
target inlet temperature obtained as an output signal of the
adder-subtracter 61 to compute a correction amount of
injection as a feedback control command 65.
The PID controller 63 calculates the proportional element
(P) using a proportional gain Kp, the derivative element (D)
using a derivative gain Kd, and the integral element (I) using
an integral gain Ki, and the calculation results are all input
to an adder 67 so as to compute the feedback cont rJl command I
65. ' ,.,
[0037] The feedforward control command 57 and the feedback '
control command 65 are input to the adder (injection amount '
adding unit) 51, which outputs an addition command 69'. This •
addition command signal 69 is input to a command saturation
unit'71 to set a limit to the output signal for protection of the DPF 7. The signal that has passed through the command
saturation unit 71 is output as a late post-injection command
signal.
[0038] Further, a PID auto-tuner 75 is provided for
automatically tuning the feedback controller 49. The auto-
19 •
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tuning is based on a deviation between the output signal of)«
the command saturation unit 71 and the output signal of the
adder 51, which is provided from an adder/subtractor 73. The
output signal from a calculation element 77 of the PID autotuner
75 is input to an adder-subtractor 78 to be input to the
integrator 53.
The PID auto-tuner 75 provided as an anti-windup measure',
(for preventing input saturation) of the feedback controller
4 9 prevents the integral value of the integfSt^r 53 of the PID
controller 63 in the feedback controller 4 9 from accumulating
while the command saturation unit 71 is limiting the command.
Thereby, the command following capability is improved when the
setpoint of the feedback control is changed.
[0039] In the first embodiment, there is further provided the
integrator resetter 55 that resets the integral va l«ue of the I
integrator 53. This integrator resetter 55 includes a sudden
drop determination unit 79 that determines whether or not the '
flow rate of exhaust gas has dropped suddenly, so that the ''
integral value of the integrator 53 is reset when this suddendrop
determination unit 7 9 detects a sudden drop in the
V
exhaust gas flow rate. • »
[0040] The control flow of this integrator resetter 55 will be
described with reference to FIG. 4. Step SI of FIG. 4 is shown
in detail in FIG. 5(a), step 32 is shown in detail in FIG.
5(b), step S3 is shown in detail in FIG. 5(c), and step S4 is
shown in detail in FIG. 5(d).
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First, the rate of change in the flow rate of exhaust gas
is determined at step SI in FIG. 4. This is done, as shown in
FIG. 5(a), by fiffst calculating the flow rate Gex of exhaust
gas at step Sll. ,The exhaust gas flow rate is calculated based
on a signal indicative of the flow rate of air Ga from the air
flowmeter 35, and a signal indicative of the fuel injection
command Gf from the common rail fuel injection system {not
shown), by the equati on Gex ~ Ga* + Gf. Next, the time I
derivative dGex/dt of the exhaust gas flow rate Gex is
calculated at step S12. Step S13 determines whether or not the
time derivative dGex/dt is less than a threshold Kl, and if yes.
Flag 1 is turned on at step S14, and if not. Flag 1 is turned
off at step S15, after which the process is returned.
[0041] Referring back to the flow of FIG. 4, the flow rate of
exhaust gas is determined at step S2. As shown in FIG. 5(b),
step 321 determines whether or not the exhaust gas flow rat^..
Gex is less than a threshold K2, and if yes. Flag 2 is turned '
on at step S22, and if not. Flag 1 is turned off at step S23, '
after which the process is returned. •
[0042] Next, 'the timer counts up at step S3 in the flow of FIG.^
4. -As shown in FIG. 5(c), step 331 determines whether or not <
Flag 1 is ON or Flag 3 is ON, and if yes, step 332 determines
whether or not Flag 2 is ON.. If "No" at step 331, Flag 3 is
turned off at^ step 335, and the timer is set to zero at step
336.
If step S32 determines that Flag 2 is ON, Flag 3 is turned
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on at step S33, and At (cyclic time for processing .-the
subroutine) is added to the timer at step S34.
[0043] Next, the'integral. value is reset at step S4 in the
flow of FIG. 4. ij.s-shown in FIG. 5(d), step S41 determines
whether or not the timer count exceeds a threshold K3, and if
not, the process is returned, and if yes, step S42 determines
whether or not the integral value exceeds a threshold K4. This
is done by setting K4 = 0, for fexample, if the integral value
is positive.
If the integral value is positive, the process goes to step
S43, where the integral value is reset to zero. The timer is
reset at step S44 and the process is returned.
[0044] As described above, the sudden drop determination unit
7 9 determines that there has been a sudden drop in the flow
I
rate of exhaust gas by determining the rate of change in the
flow rate of exhaust gas at step SI and further by determining
the flow rate of exhaust gas at step S2, so that, despite '
frequent sudden drops in the exhaust gas flow rate during transient operation, the integral value is prevented from
being reset more than necessary by thus monitoring the exhaust,
y
gas"flow rate as well as the rate of decrease in the flow ra'te;
This prevents a loss of controllability of the DPF inlet
temperature during transient operation when the engine rpm and
the engine load continuously change.
Furthermore, it is determined at step S3 that there has
been a sudden drop in the exhaust gas flow rate when the
22
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• ' ^ r',
condition in which the exhaust gas flow rate is not higher "
^than the•threshold has continued for more than a certain
period of time. Namely, with the additional condition for the
determination, wherein the flow rate of exhaust gas remains
not higher than the threshold for a certain period of time or
longer as determined at step S41, unnecessary resetting of the
integral value during transient operation is prevented, even •
more reliably. *
[0045] FIG. 8 shows the confirmation test results. FIG. 8(a)
shows the flow rate of exhaust gas, which was decreased
stepwise from a steady state. FIG. 8(b) shows the changes in
the late post-injection amount at this time, while FIG. 8(c)
shows the changes in the temperature of the exhaust gas at the
DPF inlet. Dotted lines in FIGS. 8(b) and 8(c) represent a
I
conventional technique in which the integrator resetter 55 is
#
not provided, while solid lines represent the present ,,
invention having the integrator resetter 55.
FIG. 8(b) shows that the late post-injection amount is '
decreased with a faster response speed than the conventional "
- technique. FIG. 8(c) shows that the exhaust gas temperature at,
the"DPF inlet is prevented from overshooting. ' ^
[0046] As described above, the first embodiment includes the
integrator resetter 55 that .resets the integral value of the
integrator 53^ forming the feedback controller 4 9 when the
sudden drop determination unit 7 9 determines that the flow
rate of exhaust gas has dropped suddenly based on the flow
23
11-446PCT
rate of exhaust gas passing through the DPF 7 or the rate of
decrease-in the exhaust gas flow rate (control value) that is
the time derivatilye of the flow rate. Thereby, the late postinjection
amount,., which is the manipulated variable to raise
the temperature, is prevented from being adversely affected by
the integral value remaining in the integrator 53 in the PID
controller 63. As a result, the DPF inlet temperature can be.
reliably kept at around the target level even when the flow
rate of exhaust gas has dropped rapidly,
[0047] As the controllability of the DPF inlet temperature is
improved, the target temperature at the DPF inlet can be set
several tens of °C higher without the possibility of the DPF 7
reaching the temperature at which the catalyst held therein is
degraded. Thus thermal degradation of the catalyst held in the
I
DPF is prevented, whereby the durability of the DPF 7 is
improved. c-
As the target temperature at the DPF inlet can be set
higher, the time for controlling the DPF regeneration is ,
reduced, whereby the problem of oil dilution caused by late
post-injection at the time of regeneration is resolved. ,
[00'48] (Second Embodiment) ' *
Next, a second embodiment of the control of the amount of
late post-injection by the regeneration control unit 33 will
be described >?ith reference to the control configuration block
diagram of FIG. 3.
The feedforward controller 47 is the same as that of the
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11-446PCT
' •
' 'the fuel injection command Gf obtained from the common rail
fuel injection system (not shown) at step S122.
[0053] Next, at step S130 in the flow of FIG. 6, the exhaust
gas flow rate-input is filtered using a low path filter 89 and
by a primary delay process to remove noise. After that, at
• 26 ,
11-446PCT
,' -p.
step S140, the control gain K is determined. The G'ontrol gain
^ K is obtained from a gain K map 91 such as the one shown in
FIG. 9, whereby i' gain K is given relative to a certain
exhaust gas flow rate. This gain K map 91 is determined in
#
advance based on test data or through simulation and
calculation.
[0054] Step S150 computes a transfer function. The late postinjection
amount Z, which is thfe basic variable, is determined
using a relational expression of the primary transfer function
Z = K/(l + cys)e, wherein e is the deviation of a measured DOC
inlet temperature from the target DPF inlet temperature, a is
the design parameter (adjusting parameter), and K is the
control gain determined from the flow rate of exhaust gas. The
smaller CT is set, the higher the sensitivity of the output
I
will be relative to changes in temperature deviation e and K,
I -
and the larger a is set, the lower the responsivity.
After that, at step S160, the unit of the injection amount ,
calculated at step S150 is converted to compute a command
value, and the process is returned.
[0056] In this way, the late post-injection amount, which is
the bSsic variable, is calculated using a control gain, which
is a control value determined from the exhaust gas flow rate,
instead of calculating a basic injection amount as a
feedforward control command 57^ using a feedforward control map
45 that defines proper basic variables at various operating
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11-446PCT
• ' -^ ''-
condition points, as with the feedforward controller 47 of the
first embodiment. Therefore, as compared to using a map
prepared based oA various.operating conditions beforehand, the
feedforward variables can be properly determined under various
operating conditions of a small general-purpose engine, in
which the rpm and the load independently change in use, and
thus the controllability of the DPF inlet temperature can be -
improved. *
[0056] Since the late post-injection amount, which is the
basic variable, is determined based on the deviation of the
measured DOC inlet temperature from the target value of the
DPF inlet temperature, the integrator 53 of the FID controller
63 does not output a large value, i.e., a large deviation from
the target DPF inlet temperature is unlikely to occur, so that
I
a loss of the controllability the DPF inlet temperature is
prevented under an operating condition in which the flow rate
of exhaust gas remains low after a drop in the exhast gas flow'
rate in a short time. [0057] It goes without saying that the configurations of the'
first and second embodiments can be combined. In this case, ,
the'feedforward controller 81 may be configured as in the * *
second embodiment, and the integrator 53 of the PID controller
63 in the feedback controller 4 9 may include the integrator
resetter 55. .With such a configuration, the controllability of
the DPF- inlet temperature can be improved even more.
28
11-446PCT
INDUSTRIAL APPLICABILITY •'
[0058] The present invention enables stable control to keep
the DPF inlet teiAperature - at a target level even when the flow
rate of exhaust gas'remains low for a while after a drop in
9
»
the flow rate, and therefore can be suitably applied to a
diesel engine exhaust gas purification system.
I
I-
• '

11-446PCT
• ' -^ ^^
CLAIMS
1. An exhaust gas purification system for an internal
combustion engin^' that includes a diesel oxide catalyst (DOC)
and a diesel particulate filter (DPF) for collecting
particulate matter (PM) in exhaust gas in an exhaust gas
passage and that treats the PM collected in the DPF to
regenerate the DPF,
the system comprising: *
a regeneration control unit controlling a temperature
raising unit, when the PM has accumulated more than a
predetermined amount, to heat up the DPF to around a
predetermined target temperature and burn off the accumulated
PM,
the regeneration control unit including a feedforward
I
controller outputting a basic variable for the temperature
raising unit based on an operating condition of the internal'
combustion engine, a feedback controller outputting a
correcting variable for achieving the target temperature of i
the DPF, and a variable adding unit adding the correcting
variable output from the feedback controller to the basic ,
>. •
variable output from the feedforward controller to compute a' *
manipulated variable,
the system further comprising:
at least one of an integrator resetter resetting an
integral value of an integrator forming the feedback
controller when a sudden drop in exhaust gas flow rate is
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11-446PCT
• detected based on a monitored exhaust gas flow rate or -a ^-
^control value calculated from the exhaust gas flow rate, -and a .>
basic variable calculating unit calculating the basic variable
i', . . . • - •
to be output from the feedforward controller based on the
exhaust gas flow rate or a controJ. value calculated =.from the
exhaust gas flow rate. •
* ' . ' • •••
2. The exhaust gas purification system for an internal V
combustion engine according to claim 1, wherein the integrator
resetter determines that there has been a sy^dgji drop in the -
exhaust gas flow rate when the rate of change of exhaust gas
flow rate shows a decrease rate of less than a threshold.
3. The exhaust gas purification system for an internal
. 1
combustion engine according to claim 1, wherein the integrator
resetter determines that there has been a sudden drop in the
exhaust gas flow rate when the exhaust gas flow rate has >
.' decreased to less, than a threshold.
4. The exhaust gas purification system for an internal
combustion engine according to claim 1, wherein the integrator ,
resetter determines that there has been a sudden drop_in the
• ' •' * •
exhaust gas flow rate when the exhaust gas flow rate shows a
decrease rate of less than a threshold, as well as when the
exhaust gas flow rate has decreased to less than a threshold.
5. The'exhaust gas purifica'tion system for an internal
combustion engine according to" claim 3 or 4, wherein
determination is made that there has been a sudden drop in the
exhaust, gas flow rate when the exhaust gas flow rate remains
31 .
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less than the threshold for more than a certain period of time.
^ 6. The exhaust gas purification system for an internal
combustion engine according to claim,!, wherein th'e integral
value of the integrator forming the'feedback controller is
reset when the integral value is positive.
'7. The exhaust gas purification system for an internal v.
•( , ' • .A . - ^
•combustion engine according to claim 1, wherein the basic
variable calculating unit calculates, the basic variable by
iksing an equation of a preset transfer function modeling
temperature rising characteristics of the exhaust gas in the
DOC...in use of a deviation of a measured DOC inlet temperature
from the target DPF inlet temperature, and a control gain
calculated based on the exhaust gas flow rate.
8. The exhaust gas purification system for an internal
combustion engine according to any one of claims 1 to 7,
wherein the manipulated variable of the temperature raising
unit represents an amount of late post-injection that is
performed in a per±T5d after a main injection and that does not
directly contribute to combustion, after activation of the DOC.