U.S. patent application number 12/083873 was filed with the patent office on 2009-02-26 for exhaust purification system of internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Takamitsu Asanuma, Shinya Hirota, Shunsuke Toshioka.
Application Number | 20090049826 12/083873 |
Document ID | / |
Family ID | 38723446 |
Filed Date | 2009-02-26 |
United States Patent
Application |
20090049826 |
Kind Code |
A1 |
Toshioka; Shunsuke ; et
al. |
February 26, 2009 |
Exhaust Purification System of Internal Combustion Engine
Abstract
An NOx adsorbent is arranged in an exhaust passage of an
internal combustion engine, a fuel addition valve (28) is arranged
in the exhaust passage upstream of the NOx adsorbent, and, when the
NOx adsorbent should be made to release the NOx, the fuel addition
valve (28) adds fuel to the NOx adsorbent in the required fuel
addition amount to make the air-fuel ratio of the exhaust gas
flowing into the NOx adsorbent temporarily rich. In this case, the
required fuel addition amount is added divided into a plurality of
operations. The fuel addition rate of the fuel addition valve (28)
is detected and the overall addition time from the start of the
initial divided addition to the end of the final divided addition
is corrected in accordance with the fuel addition rate. Further,
the divided addition time, interval, or number of divided additions
is corrected so that the amount of fuel actually added from the
fuel addition valve (28) is maintained at the required fuel
addition amount.
Inventors: |
Toshioka; Shunsuke;
(Susono-shi, JP) ; Hirota; Shinya; (Susono-shi,
JP) ; Asanuma; Takamitsu; (Susono-shi, JP) |
Correspondence
Address: |
OLIFF & BERRIDGE, PLC
P.O. BOX 320850
ALEXANDRIA
VA
22320-4850
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
TOYOTA-SHI
JP
|
Family ID: |
38723446 |
Appl. No.: |
12/083873 |
Filed: |
May 24, 2007 |
PCT Filed: |
May 24, 2007 |
PCT NO: |
PCT/JP2007/061030 |
371 Date: |
April 21, 2008 |
Current U.S.
Class: |
60/286 |
Current CPC
Class: |
F01N 3/0871 20130101;
F01N 2560/026 20130101; F01N 2610/146 20130101; F01N 2560/025
20130101; Y02T 10/40 20130101; F01N 11/00 20130101; F01N 2560/06
20130101; F01N 2610/03 20130101; Y02T 10/47 20130101; F01N 2550/05
20130101; F01N 3/0842 20130101; F01N 3/0821 20130101; F01N 13/0097
20140603 |
Class at
Publication: |
60/286 |
International
Class: |
F01N 3/08 20060101
F01N003/08 |
Foreign Application Data
Date |
Code |
Application Number |
May 24, 2006 |
JP |
2006-144073 |
Claims
1. An exhaust purification system of an internal combustion engine
provided with: an NOx adsorbent arranged in an engine exhaust
passage, said NOx adsorbent absorbing NOx in exhaust gas when the
inflowing exhaust gas has a lean air-fuel ratio and releasing the
absorbed NOx when the inflowing exhaust gas has a rich air-fuel
ratio, a fuel addition valve arranged in the engine exhaust passage
upstream of said NOx adsorbent, an addition controlling means for
adding fuel from the fuel addition valve to the NOx adsorbent in
the required fuel addition amount when the NOx adsorbent should be
made to release the NOx so that the air-fuel ratio of the exhaust
gas flowing into the NOx adsorbent becomes temporarily rich, said
addition controlling means performing divided addition adding said
required fuel addition amount of fuel divided into a plurality of
operations, a detecting means for detecting a fuel addition rate of
the fuel addition valve or the amount of fluctuation of the fuel
addition rate with respect to a regular value, and a correcting
means for correcting the overall addition time from a start of an
initial divided addition to an end of a final divided addition to
shorten it and correcting control parameters of divided addition in
accordance with said detected fuel addition rate or amount of
fluctuation of the fuel addition rate so that the amount of fuel
actually added from the fuel addition valve is maintained at the
required fuel addition amount.
2. An exhaust purification system of an internal combustion engine
as set forth in claim 1, wherein said control parameters of divided
addition are a divided addition time and an interval from a
previous divided addition to a next divided addition or number of
divided additions.
3. An exhaust purification system of an internal combustion engine
as set forth in claim 1, wherein said correcting means corrects the
overall addition time so as to become shorter the smaller said fuel
addition rate or the larger the drop of said fuel addition
rate.
4. An exhaust purification system of an internal combustion engine
as set forth in claim 1, wherein the system is further provided
with an NOx purification rate detecting means for detecting an NOx
purification rate of the NOx adsorbent and said correcting means
corrects the overall addition time to shorten it or corrects it to
extend it so that said detected NOx purification rate matches a
target value.
5. An exhaust purification system of an internal combustion engine
as set forth in claim 1, wherein the system is further provided
with an HC amount detecting means for detecting an amount of HC in
the exhaust gas flowing out from the NOx adsorbent when fuel is
added and said correcting means corrects the overall addition time
to shorten it or corrects it to extend it so that said detected HC
amount matches a target amount.
6. An exhaust purification system of an internal combustion engine
as set forth in claim 1, wherein the system is further provided
with an NOx purification rate detecting means for detecting an NOx
purification rate of the NOx adsorbent and a determining means for
determining that the fuel addition valve is broken when the drop in
the detected NOx purification rate is larger than an allowable
value or said detected NOx purification rate is lower than an
allowable limit even if correcting the overall addition time to
shorten it and correct the control parameters of the divided
addition.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust purification
system of an internal combustion engine.
BACKGROUND ART
[0002] Known is an internal combustion engine arranging in an
engine exhaust passage an NOx adsorbent absorbing NOx in exhaust
gas when the air-flow ratio of the inflowing exhaust gas is lean
and releasing the adsorbed NOx when the air-fuel ratio of the
inflowing exhaust gas becomes rich, arranging a fuel addition valve
in the engine exhaust passage upstream of the NOx adsorbent, adding
fuel to the NOx adsorbent in the required fuel addition amount so
that the air-fuel ratio of the exhaust gas flowing into the NOx
adsorbent becomes temporarily rich when the NOx adsorbent should be
made to release NOx, and adding the required fuel addition amount
divided into a plurality of operations. In this internal combustion
engine, the NOx generated when fuel is burned under a lean air-fuel
ratio is absorbed by the NOx adsorbent. On the other hand, if the
NOx absorption ability of the NOx adsorbent approaches saturation,
the air-fuel ratio of the exhaust gas is temporarily made rich and
thereby NOx is released from the NOx adsorbent and reduced.
[0003] However, if the abnormality occurs of the port of the fuel
addition valve becoming clogged by so-called deposits, the fuel
addition rate of the fuel addition valve will fall and the amount
of fuel actually supplied from the fuel addition valve will be
insufficient compared with the required fuel addition amount, so
sufficient release and reduction of NOx will become difficult.
[0004] Therefore, there is known an internal combustion engine
designed so that when an abnormality occurs in the fuel addition
valve, the divided addition time is corrected to extend it and the
amount of fuel actually added from the fuel addition valve is
maintained at the required fuel addition amount (see Japanese
Patent Publication (A) No. 2002-242663).
[0005] However, if correcting the divided addition time to extend
it, the overall addition time from the start of the initial divided
addition to the end of the final divided addition becomes longer.
At this time, the amount of fuel added from the fuel addition valve
is maintained at the required fuel addition amount, so the degree
of richness of the air-fuel ratio of the exhaust gas flowing in
when fuel is added becomes smaller and the NOx is liable to be
unable to be sufficiently released and reduced. That is, when
correcting the divided injection time to extend it, it is necessary
to correct the overall addition time to shorten it so as to
reliably release and reduce the NOx.
[0006] The above Japanese Patent Publication (A) No. 2002-242663
describes that when correcting the divided injection time to extend
it, the overall addition time not be allowed to become
unnecessarily long (see Japanese Patent Publication (A) No.
2002-242663, [0026] etc.), but does not describe to correct the
overall addition time to shorten it or how to correct it to shorten
it.
DISCLOSURE OF THE INVENTION
[0007] Therefore, an object of the present invention is to provide
an exhaust purification system of an internal combustion engine
able to reliably release and reduce NOx even when the fuel addition
rate of a fuel addition valve fluctuates from the regular
value.
[0008] According to the present invention, there is provided an
exhaust purification system of an internal combustion engine
provided with an NOx adsorbent arranged in an engine exhaust
passage, the NOx adsorbent absorbing NOx in exhaust gas when the
inflowing exhaust gas has a lean air-fuel ratio and releasing the
absorbed NOx when the inflowing exhaust gas has a rich air-fuel
ratio, a fuel addition valve arranged in the engine exhaust passage
upstream of the NOx adsorbent, an addition controlling means for
adding fuel from the fuel addition valve to the NOx adsorbent in
the required fuel addition amount when the NOx adsorbent should be
made to release the NOx so that the air-fuel ratio of the exhaust
gas flowing into the NOx adsorbent becomes temporarily rich, the
addition controlling means performing divided addition adding the
required fuel addition amount of fuel divided into a plurality of
operations, a detecting means for detecting a fuel addition rate of
the fuel addition valve or the amount of fluctuation of the fuel
addition rate with respect to a regular value, and a correcting
means for correcting the overall addition time from a start of an
initial divided addition to an end of a final divided addition to
shorten it and correcting control parameters of divided addition in
accordance with the detected fuel addition rate or amount of
fluctuation of the fuel addition rate so that the amount of fuel
actually added from the fuel addition valve is maintained at the
required fuel addition amount.
BRIEF DESCRIPTION OF THE DRAWINGS
[0009] FIG. 1 is an overview of a compression ignition type
internal combustion engine, FIG. 2 is a side cross-sectional view
of an NOx storing reduction catalyst, FIGS. 3A and 3B are
cross-sectional views of a surface part of a catalyst carrier,
FIGS. 4A and 4B are views showing the structure of a particulate
filter, FIG. 5 is a time chart for explaining the NOx release
control, FIG. 6 is a view showing a map of an NOx absorption amount
dNOx per unit time, FIG. 7 is a view showing a map of the required
fuel addition amount Q, FIG. 8 is a time chart for explaining a
fuel addition parameter, FIG. 9 is a view showing the relationship
between an overall addition time tALL and NOx purification rate
EFF, FIG. 10 is a view showing the relationship of the overall
addition time tALL and NOx purification rate EFF, FIG. 11 is a view
showing a map of an overall addition time optimum value tAM, FIG.
12 is a view showing a map of an overall addition time optimum
value tAM, FIGS. 13A, 13B, and 13C are time charts for explaining
fuel addition when correcting the overall addition time tALL to
shorten it, FIG. 14 is a flow chart for NOx release control, FIG.
15 is a view showing another embodiment of a compression ignition
type internal combustion engine, FIG. 16 is a time chart for
explaining another embodiment according to the present invention,
FIG. 17 is a flow chart for NOx release control of another
embodiment according to the present invention, FIG. 18 is a view
showing still another embodiment according to the present
invention, FIG. 19 is a view showing the relationship between the
overall addition time tALL and exhausted HC amount, FIG. 20 is a
time chart for explaining still another embodiment according to the
present invention, and FIG. 21 is a flow chart for NOx release
control of still another embodiment according to the present
invention.
BEST MODE FOR CARRYING OUT THE INVENTION
[0010] FIG. 1 shows the case of application of the present
invention to a compression ignition type internal combustion
engine. However, the present invention can also be applied to a
spark ignition type internal combustion engine.
[0011] Referring to FIG. 1, 1 shows an engine body, 2 a combustion
chamber of each cylinder, 3 an electronic control type fuel
injector for injecting fuel into each combustion chamber 2, 4 an
intake manifold, and 5 an exhaust manifold. The intake manifold 4
is connected through an intake duct 6 to an outlet of a compressor
7a of an exhaust turbocharger 7, while an inlet of the compressor
7a is connected through an air flow meter 8 to an air cleaner 9.
Inside the intake duct 6 is arranged a throttle valve 10. Further,
around the intake duct 6 is arranged a cooling device 11 for
cooling the intake air flowing inside the intake duct 6. In the
embodiment shown in FIG. 1, the engine cooling water is guided into
the cooling device 11 where the engine cooling water is used to
cool the intake air. On the other hand, the exhaust manifold 5 is
connected to the inlet of the exhaust turbine 7b of the exhaust
turbocharger 7, while the outlet of the exhaust turbine 7b is
connected to an exhaust post-treatment device 20.
[0012] The exhaust manifold 5 and the intake manifold 4 are
connected with each other through an exhaust gas recirculation
(hereinafter referred to as an "EGR") passage 12. Inside the EGR
passage 12, an electrical control type EGR control valve 13 is
arranged. Further, around the EGR passage 12, a cooling device 14
is arranged for cooling the EGR gas flowing through the inside of
the EGR passage 12. In the embodiment shown in FIG. 1, engine
cooling water is guided into the cooling device 14 where the engine
cooling water is used to cool the EGR gas. On the other hand, the
fuel injectors 3 are connected through fuel feed pipes 15 to a
common rail 16. This common rail 16 is supplied with fuel from an
electronic control type variable discharge fuel pump 17. The fuel
supplied into the common rail 16 is supplied through the fuel feed
pipes 15 to the fuel injectors 3.
[0013] The exhaust post-treatment device 20 is provided with an
exhaust pipe 21 connected to an outlet of an exhaust turbine 7b, a
catalytic converter 22 connected to the exhaust pipe 21, and an
exhaust pipe 23 connected to the catalytic converter 22. Inside the
catalytic converter 22 are arranged, in order from the upstream
side, an NOx storing reduction catalyst 24 and particulate filter
25. Further, the exhaust pipe 23 is provided with a temperature
sensor 26 for detecting the temperature of the exhaust gas
exhausted from the catalytic converter 22 and an air-fuel ratio
sensor 27 for detecting the air-fuel ratio of the exhaust gas
exhausted from the catalytic converter 22. The temperature of the
exhaust gas exhausted from the catalytic converter 22 expresses the
temperature of the NOx storing reduction catalyst 24 and
particulate filter 25.
[0014] On the other hand, as shown in FIG. 1, the exhaust manifold
5 has a fuel addition valve 28 attached to it. This fuel addition
valve 28 is supplied with fuel from the common rail 16. Fuel is
added from the fuel addition valve 28 to the inside of the exhaust
manifold 5. In this embodiment according to the present invention,
this fuel is comprised of diesel oil. Note that the fuel addition
valve 28 can also be attached to the exhaust pipe 21.
[0015] The electronic control unit 30 is comprised of a digital
computer and is provided with a ROM (read only memory) 32, RAM
(random access memory) 33, CPU (microprocessor) 34, input port 35,
and output port 36 connected with each other by a bi-directional
bus 31. The output signals of the air flow meter 8, temperature
sensor 26, and air-fuel ratio sensor 27 are input through the
corresponding AD converters 37 to the input port 35. Further, the
accelerator pedal 39 has connected to it a load sensor 50
generating an output voltage proportional to the amount of
depression L of the accelerator pedal 39. The output voltage of the
load sensor 40 is input through the corresponding AD converter 37
to the input port 35. Further, the input port 35 has a crank angle
sensor 41 connected to it generating an output pulse each time the
crankshaft rotates by for example 15.degree.. The CPU 34 calculates
the engine speed N based on the output pulse of the crank angle
sensor 41. On the other hand, the output port 36 is connected
through the corresponding drive circuits 38 to the fuel injectors
3, throttle valve 10 drive device, EGR control valve 13, fuel pump
17, and fuel addition valve 28.
[0016] FIG. 2 shows the structure of the NOx storing reduction
catalyst 24. In the embodiment shown in FIG. 2, the NOx storing
reduction catalyst 24 forms a honeycomb structure and is provided
with a plurality of exhaust gas flow passages 61 separated from
each other by thin partition walls 60. On the two side surfaces of
each partition wall 60, for example, a catalyst carrier comprised
of alumina is carried. FIGS. 3A and 3B schematically show
cross-sections of the surface parts of this catalyst carrier 65. As
shown in FIGS. 3A and 3B, the catalyst carrier 65 has a precious
metal catalyst 66 carried dispersed on its surface. Further, the
catalyst carrier 65 has a layer of an NOx adsorbent 67 formed on
its surface.
[0017] In the embodiment according to the present invention, as the
precious metal catalyst 66, platinum Pt is used. As the ingredient
forming the NO.sub.x adsorbent 67, for example, at least one
ingredient selected from potassium K, sodium Na, cesium Cs, or
another such alkali metal, barium Ba, calcium Ca, or another such
alkali earth, and lanthanum La, yttrium Y, or another such rare
earth is used.
[0018] If the ratio of the air and fuel (hydrocarbons) supplied
inside the engine intake passage, combustion chambers 2, and
exhaust passage upstream of the NOx storing reduction catalyst 24
is referred to as the "air-fuel ratio of the exhaust gas", the
NO.sub.x adsorbent 67 absorbs the NO.sub.x when the air-fuel ratio
of the exhaust gas is lean and releases the absorbed NO.sub.x when
the oxygen concentration in the exhaust gas falls--in an "NO.sub.x
absorption/release action".
[0019] That is, explaining the case of using barium Ba as the
ingredient forming the NO.sub.x adsorbent 67 as an example, when
the air-fuel ratio of the exhaust gas is lean, that is, when the
oxygen concentration in the exhaust gas is high, the NO contained
in the exhaust gas, as shown in FIG. 3A, is oxidized on the
platinum Pt 66 and becomes NO.sub.2, next this is absorbed in the
NO.sub.x adsorbent 67 and, while bonding with the barium oxide BaO,
diffuses in the form of nitric acid ions NO.sub.3.sup.- inside the
NO.sub.x adsorbent 67. In this way, the NO.sub.x is absorbed inside
the NO.sub.x adsorbent 67. So long as the oxygen concentration in
the exhaust gas is high, NO.sub.2 is produced on the surface of the
platinum Pt 66. So long as the NO.sub.x adsorption ability of the
NO.sub.x adsorbent 67 is not saturated, the NO.sub.2 is absorbed in
the NO.sub.x adsorbent 67 and nitric acid ions NO.sub.3.sup.- are
produced.
[0020] As opposed to this, if the air-fuel ratio of the exhaust gas
is made rich or the stoichiometric air-fuel ratio, the oxygen
concentration in the exhaust gas falls, so the reaction proceeds in
the opposite direction (NO.sub.3.sup.-.fwdarw.NO.sub.2) and
therefore, as shown in FIG. 3B, the nitric acid ions NO.sub.3.sup.-
in the NO.sub.x adsorbent 67 are released in the form of NO.sub.2
from the NO.sub.x adsorbent 67. Next, the released NO.sub.x is
reduced by the unburned HC and CO contained in the exhaust gas.
[0021] In this way, when the air-fuel ratio of the exhaust gas is
lean, that is, when combustion is performed under a lean air-fuel
ratio, the NO.sub.x in the exhaust gas is absorbed in the NO.sub.x
adsorbent 67. However, when combustion continues under a lean
air-fuel ratio, during that time the NO.sub.x adsorption ability of
the NO.sub.x adsorbent 67 ends up becoming saturated and therefore
the NO.sub.x adsorbent 67 ends up no longer being able to absorb
the NO.sub.x. Therefore, in the embodiment according to the present
invention, before the adsorption ability of the NO.sub.x adsorbent
67 becomes saturated, fuel is supplied from the fuel addition valve
28 so as to temporarily make the air-fuel ratio of the exhaust gas
rich and thereby make the NO.sub.x be released from the NO.sub.x
adsorbent 67.
[0022] FIGS. 4A and 4B show the structure of the particulate filter
25. Note that FIG. 4A shows a front view of the particulate filter
25, while FIG. 4B shows a side cross-sectional view of the
particulate filter 25. As shown in FIGS. 4A and 4B, the particulate
filter 25 forms a honeycomb structure and is provided with a
plurality of exhaust flow passages 70, 71 extending in parallel
with each other. These exhaust flow passages are comprised of
exhaust gas inflow passages 70 with downstream ends closed by plugs
72 and exhaust gas outflow passages 71 with upstream ends closed by
plugs 73. Note that the hatched parts in FIG. 4A show the plugs 73.
Therefore, the exhaust gas inflow passages 70 and exhaust gas
outflow passages 71 are alternately arranged via thin partition
walls 74. In other words, the exhaust gas inflow passages 70 and
exhaust gas outflow passages 71 are arranged so that each exhaust
gas inflow passage 70 is surrounded by four exhaust gas outflow
passages 71 and each exhaust gas outflow passage 71 is surrounded
by four exhaust gas inflow passages 70.
[0023] The particulate filter 25 is for example formed from a
porous material such as cordierite. Therefore, the exhaust gas
flowing into the exhaust gas inflow passage 70, as shown by the
arrows in FIG. 4B, passes through the surrounding partition walls
74 and flows out into the adjoining exhaust gas outflow passages
71.
[0024] In the embodiment according to the present invention, the
peripheral walls of the exhaust gas inflow passages 70 and exhaust
gas outflow passages 71, that is, the two side surfaces of the
partition walls 74 and the inside walls of the fine holes in the
partition walls 74, carry, for example, a catalyst carrier
comprised of alumina. On the surface of the catalyst carrier 65, as
shown in FIGS. 3A and 3B, a precious metal catalyst 66 comprised of
platinum Pt is carried diffused in it and a layer of an NO.sub.x
adsorbent 67 is formed.
[0025] Therefore, when the fuel is burned under a lean air-fuel
ratio, the NOx in the exhaust gas is also absorbed in the NOx
adsorbent 67 on the particulate filter 25. The NOx absorbed in this
NOx adsorbent 67 is released by the fuel addition valve 28 adding
fuel.
[0026] On the other hand, the particulate matter contained in the
exhaust gas is trapped on the particulate filter 25 and is
successively oxidized. However, if the trapped amount of
particulate matter becomes greater than the amount of oxidized
particulate matter, the particulate matter is gradually deposited
on the particulate filter 25. In this case, if the deposited amount
of the particulate matter increases, a drop in the engine output
ends up being invited. Therefore, when the deposited amount of the
particulate matter increases, the deposited particulate matter must
be removed. In this case, if raising the temperature of the
particulate filter 25 under an excess of air to about 600.degree.
C., the deposited particulate matter will be oxidized and
removed.
[0027] Therefore, in the embodiment according to the present
invention, when the amount of particulate matter deposited on the
particulate filter 25 exceeds the allowable amount, while
maintaining the air-fuel ratio of the exhaust gas flowing into the
particulate filter 25 lean, the fuel addition valve 28 adds fuel,
the heat of oxidation reaction of the added fuel is used to raise
the temperature of the particulate filter 25, and thereby the
deposited particulate matter is removed by oxidation.
[0028] Note that in FIG. 1, the NOx storing reduction catalyst 24
can be omitted. Further, as the particulate filter 25 in FIG. 1, a
particulate filter not carrying an NOx adsorbent 67 can be
used.
[0029] Now, in the embodiment according to the present invention,
as shown by X in FIG. 5, each time the NOx amount cumulative value
.SIGMA.NOx exceeds the allowable value MAX, fuel is added from the
fuel addition valve 28 and the air-fuel ratio of the exhaust gas
flowing into the NOx adsorbent 67 carried on the NOx storing
reduction catalyst 24 and on the particulate filter 25 is
temporarily switched to rich. As a result, NOx is released from the
NOx adsorbent 67 and reduced.
[0030] In this case, in the embodiment according to the present
invention, the NOx amount dNOx absorbed in the NOx adsorbent 67 per
unit time is stored as a function of the required torque TQ and
engine speed N in the form of a map as shown in FIG. 6 in advance
in the ROM 32. By cumulatively adding this NOx amount dNOx, the
cumulative value .SIGMA.NOx of the NOx amount absorbed in the NOx
adsorbent 67 is calculated.
[0031] On the other hand, for switching the air-fuel ratio of the
exhaust gas flowing into the NOx adsorbent 67 to rich, the fuel is
added from the fuel addition valve 28 by the required fuel addition
amount Q. This required fuel addition amount Q is for example
stored as a function of the intake air amount Ga and temperature Tc
of the NOx adsorbent 67 in the form of a map as shown in FIG. 7 in
advance in the ROM 32.
[0032] As will be understood from FIG. 5, in the embodiment
according to the present invention, the required fuel addition
amount Q is added divided into a plurality of operations. In the
example shown in FIG. 8, the divided addition of the time tDIV is
performed four times separated by the intervals tINT. Here, if the
number of divided additions is expressed by n (=2,3, . . . ), the
overall addition time tALL from the start of the initial divided
addition to the end of the final divided addition is expressed by
the following formula:
tALL=ntDIV+(n-1)tINT
[0033] In this case, the fact that if the overall addition time
tALL changes, even if maintaining the fuel addition amount
constant, the NOx purification rate EFF of the NOx adsorbent 67
changes was confirmed by the present inventors. This will be
explained while referring to FIG. 9. Here, if the amount of NOx
flowing into the NOx adsorbent 67 in the time from the end of the
previous addition of fuel for release and reduction of NOx to the
end of the next addition of fuel for release and reduction of NOx
is expressed as "Nin" and the amount of NOx flowing out from the
NOx adsorbent 67 as "Nout", the NOx purification rate EFF can for
example be expressed by (Nin-Nout)/Nin.
[0034] FIG. 9 shows the NOx purification rate EFF of the NOx
adsorbent 67 when holding the fuel addition amount constant and
changing the overall addition time tALL. Note that for example by
maintaining the divided addition time tDIV and number of divisions
n and changing the interval tINT, it is possible to maintain the
fuel addition amount constant and change the overall addition time
tALL.
[0035] As shown in FIG. 9, there is an optimum value tAM maximizing
the NOx purification rate EFF in the overall addition time tALL.
The NOx purification rate EFF falls as the overall addition time
tALL becomes shorter than or longer than the optimum value tAM.
This is considered to be because if the overall addition time tALL
becomes shorter, the time during which the air-fuel ratio of the
exhaust gas flowing into the NOx adsorbent 67 is held rich becomes
shorter and if the overall addition time tALL becomes longer, the
degree of richness of the air-fuel ratio of the exhaust gas flowing
into the NOx adsorbent 67 becomes smaller.
[0036] However, as explained at the start, if the port of the fuel
addition valve 28 is clogged by deposits mainly comprised of solid
carbon, the fuel addition amount per unit time of the fuel addition
valve 28, that is, the fuel addition rate q, falls from the value
when the fuel addition valve 28 is not clogged, that is, the
regular value qp. In this case, if correcting the divided addition
time tDIV to extend it, the amount of fuel actually added from the
fuel addition valve 28 can be maintained at the required fuel
addition amount Q. On top of this, if correcting the interval tINT
to shorten it to correct the overall addition time tALL to shorten
it to the optimum value tAMp of when the fuel addition rate q is
the regular value qp, it appears that the NOx purification rate EFF
can be maintained at the maximum.
[0037] However, the fact that when the fuel addition rate q falls
from the regular value qp, even if correcting the overall addition
time tALL to shorten it to the above-mentioned tAMp, the NOx
purification rate EFF cannot be maintained at the maximum and the
overall addition time tALL has to be further corrected to shorten
it was discovered by the present inventors. This will be explained
with reference to FIG. 10.
[0038] In FIG. 10, the curve P shows the NOx purification rate EFF
when the fuel addition rate q is the regular value qp, while the
curve C shows the NOx purification rate EFF when the fuel addition
rate q falls from the regular value qp. Note that in each case, the
amount of fuel actually added from the fuel addition valve 28 is
maintained at the required fuel addition amount Q.
[0039] As will be understood from FIG. 10, when the fuel addition
rate q falls from the regular value qp, the NOx purification rate
EFF shown by the curve C shifts in the direction where the overall
addition time tALL becomes shorter and the optimum value tAMc of
the overall addition time tALL for maximizing the NOx purification
rate EFF becomes shorter than the optimum value tAMp when the fuel
addition rate q is the regular value qp. This is believed to be
because of the following reasons. That is, if the fuel addition
rate q falls, the penetrating force of the added fuel falls, so the
added fuel depositing on the inside walls of the exhaust passage
increases. The added fuel deposited once on the inside wall
surfaces of the exhaust passage later evaporates, separates from
the inside wall surfaces of the exhaust passage, and then reaches
the NOx adsorbent 67. For this reason, the added fuel gradually
separates from the wall inside the exhaust passage and reaches the
NOx adsorbent 67. As a result, the degree of richness of the
air-fuel ratio of the exhaust gas flowing into the NOx adsorbent 67
becomes smaller.
[0040] This being so, to make the degree of richness sufficiently
large, it is necessary to add the fuel in the required fuel
addition amount Q in a short time, that is, it is necessary to
further correct the overall addition time tALL to shorten it.
[0041] In this case, the optimum value tAM of the overall addition
time tALL when the fuel addition rate q falls from the regular
value qp, as shown in FIG. 11, becomes shorter the larger the drop
.DELTA.q (=qp-q) of the fuel addition rate q from the regular value
qp or, as shown in FIG. 12, becomes shorter the smaller the fuel
addition rate q itself.
[0042] Therefore, in the embodiment according to the present
invention, the fuel addition rate q or its drop .DELTA.q is
detected, the optimum value tAM of the overall addition time is
calculated in accordance with this fuel addition rate q or its drop
.DELTA.q, and the overall addition time tALL is set to this optimum
value tAM. On top of this, the divided addition time tDIV, interval
tINT, or number of divisions n is corrected in accordance with the
detected fuel addition rate q or its drop .DELTA.q so that the
amount of fuel actually added in this overall addition time tALL
matches with the required fuel addition amount Q.
[0043] For example, when the fuel addition rate q is the regular
value qp, the addition time required for adding the required fuel
addition amount Q is substantially (Q/qp), so the divided addition
time tDIVp when the fuel addition rate q is the regular value qp
can be found from the following formula:
tDIVp=(Q/qp)/n
[0044] This being so, the divided addition time tDIV required for
adding the required fuel addition amount Q at the time of the fuel
addition rate q can be found from the following formula:
tDIV=tDIVp(qp/q)
[0045] Therefore, the interval tINT required for performing the
divided addition of the divided addition time tDIV n number of
times in the overall addition time tAL can be found from the
following formula L:
tINT=(tALL-ntDIV)/(n-1)
[0046] Various methods are known for detecting the fuel addition
rate q or its drop .DELTA.q. For example, it is possible to detect
the fuel addition rate q or its drop .DELTA.q in accordance with
the extent of rise of the NOx adsorbent temperature Tc occurring
when actually adding fuel from the fuel addition valve 28. That is,
it is learned that at the time of fuel addition, when the rise in
the NOx adsorbent temperature Tc detected by the temperature sensor
26 (FIG. 1) is large, the fuel addition rate q is large, while when
the rise in temperature is small, the fuel addition rate q is
small.
[0047] In the embodiment according to the present invention, when
the fuel addition valve 28 adds fuel so as to release NOx from the
NOx adsorbent 67 and reduce it, the fuel addition rate q or its
addition amount .DELTA.q is detected based on the NOx adsorbent
temperature Tc. Based on this fuel addition rate q or its addition
amount .DELTA.q, the optimum value tAM of the overall addition time
tALL is calculated from the map of FIG. 11 or FIG. 12. At the fuel
addition for release and reduction of NOx performed next, the
overall addition time tALL is set to this optimum value tAM, the
required fuel addition amount Q is calculated, and the divided
addition time tDIV and interval tINT for making the amount of fuel
actually added during the overall addition time tALL match with the
required fuel addition amount Q are calculated using the fuel
addition rate q or its drop .DELTA.q.
[0048] FIG. 13A and 13B show an example of addition of fuel when
the fuel addition rate q falls below the regular value qp. In the
example shown in FIG. 13A, compared with the case where the fuel
addition rate q shown in FIG. 13C is the regular value qp, the
overall addition time tALL is corrected to shorten it, the divided
addition time tDIV is corrected to extend it, the interval tINT is
corrected to shorten it, and the number of divided additions n is
maintained. On the other hand, in the example shown in FIG. 13B,
compared with the case shown in FIG. 13C, the overall addition time
tALL is corrected to shorten it, the divided addition time tDIV is
corrected to extend it, the number of divided additions n is
reduced, and the interval tINT is also corrected to shorten it.
[0049] FIG. 14 shows the NOx release control routine.
[0050] Referring to FIG. 14, first, at step 100, the NOx amount
.SIGMA.NOx absorbed in the NOx adsorbent 67 is calculated. In the
embodiment according to the present invention, the NOx amount dNOx
absorbed per unit time is calculated from the map shown in FIG. 6,
and this dNOx is added to the NOx amount .SIGMA.NOx absorbed in the
NOx adsorbent 67. Next, at step 101, it is determined if the
absorbed NOx amount .SIGMA.NOX has exceeded the allowable value
MAX. When .SIGMA.NOX>MAX, the routine proceeds to step 102 where
the required fuel addition amount Q is calculated from the map of
FIG. 7. At the next step 103, the fuel addition parameter is
calculated. That is, the divided addition time tDIV, interval tINT,
and number of divided additions n required for addition of the
required fuel addition amount Q calculated at step 102 in the
overall addition time tALL preset at step 107 of the previous
processing cycle are calculated using the fuel addition rate q or
its drop .DELTA.q detected at step 106 of the previous processing
cycle. At the next step 104, fuel is added based on the fuel
addition parameter determined at step 103. At the next step 105,
the absorbed NOx amount .SIGMA.NOx is returned to zero. At the next
step 106, the fuel addition rate q or its drop .DELTA.q is
detected. At the next step 107, the overall addition time optimum
value tAM is calculated from the map of FIG. 11 or FIG. 12 based on
the fuel addition rate q or its drop .DELTA.q detected at step 106.
This optimum value tAM is set as the overall addition time tALL for
the next fuel addition.
[0051] Note that it is also possible that when the detected fuel
addition rate q is larger than a threshold value or its drop
.DELTA.q is smaller than a threshold value, the overall addition
time tALL, divided addition time tDIV, interval tINT, or number of
divided additions n are not corrected and when the fuel addition
rate q becomes smaller than a threshold value or its drop .DELTA.q
becomes larger than a threshold value, the overall addition time
tALL etc. are corrected.
[0052] FIG. 15 shows another embodiment according to the present
invention. The embodiment shown in FIG. 15 differs in configuration
from the embodiment of FIG. 1 in the point that an NOx sensor 80 is
attached to the exhaust pipe 23 for detecting the NOx amount or NOx
concentration flowing out from the NOx adsorbent 67 and the point
that an alarm device 81 is provided for indicating a breakdown of
the fuel addition valve 28. The output signal of the NOx sensor 80
is input through the corresponding AD converter 37 to the input
port 35, the alarm device 81 is connected through the corresponding
drive device 38 to the output port 36, and control is performed
based on the output signal from the electronic control unit 30.
[0053] As explained above, when the fuel addition rate q of the
fuel addition valve 28 falls, if the overall addition time tALL is
held constant, the NOx purification rate of the NOx adsorbent 67
falls. Therefore, in another embodiment according to the present
invention, the NOx purification rate EFF of the NOx adsorbent 67 is
detected based on the output of the NOx sensor 80. The overall
addition time tALL is corrected so that this NOx purification rate
EFF is maintained at the maximum.
[0054] That is, deposits clog the fuel addition valve 28 along with
the elapse of time, therefore fuel addition rate q becomes smaller
along with the elapse of time and the fuel addition rate drop
.DELTA.q becomes greater along with the elapse of time. On the
other hand, as explained referring to FIG. 9 or FIG. 10, when the
fuel addition rate q falls, if correcting the overall addition time
tALL to shorten it, the NOx purification rate EFF can be increased.
Therefore, in another embodiment according to the present
invention, each time fuel is added to release and reduce NOx, the
overall addition time tALL is shortened.
[0055] However, as will be understood from FIG. 9, if the overall
addition time tALL becomes shorter than the optimum value tAM, the
shorter the overall addition time tALL becomes, the more the NOx
purification rate EFF drops. Therefore, in another embodiment
according to the present invention, when as a result of the
correction of the overall addition time tALL to shorten it, the
overall addition time tALL is corrected to shorten it so long as
the NOx purification rate EFF rises. When as a result of the
correction of the overall addition time tALL to shorten it, the NOx
purification rate EFF falls, the overall addition time tALL is
corrected to extend it. In this way, the overall addition time tALL
can be maintained at the optimum value tAM and the NOx purification
rate EFF can be maintained at the maximum.
[0056] In FIG. 16, X indicates the timing when fuel is added for
release and reduction of NOx. As shown in FIG. 16, the overall
addition time tALL is for example corrected to shorten it by a
small predetermined value .DELTA.Y. As a result, when the NOx
purification rate EFF rises, the overall addition time tALL is
further corrected to shorten it by .DELTA.Y. As opposed to this,
when as a result of the overall addition time tALL being corrected
to shorten it, the NOx purification rate EFF falls, the overall
addition time tALL is extended by .DELTA.Y, that is, is returned to
the original value. In this way, the NOx purification rate EFF is
maintained at the maximum.
[0057] For example, when the clogging of the fuel addition valve 28
becomes serious and the fuel addition rate q becomes considerably
small, even if correcting the overall addition time tALL to shorten
it, the NOx purification rate EFF can no longer be maintained at
the maximum and is liable to fall below the previous NOx
purification rate EFF0. Therefore, when the drop (=EFF0-EFF) of the
NOx purification rate EFF as a result of correction of the overall
addition time tALL to shorten it from the previous NOx purification
rate EFF0 is larger than the allowable value, it is determined that
the fuel addition valve 28 has broken and the alarm device 81 is
actuated. Note that when the NOx purification rate EFF is lower
than the allowable limit as a result of correction of the overall
addition time tALL to shorten it, it can be determined that the
fuel addition valve 28 is broken.
[0058] FIG. 17 shows the NOx release control routine of another
embodiment according to the present invention.
[0059] Referring to FIG. 17, first, at step 200, the NOx amount
.SIGMA.NOx absorbed in the NOx adsorbent 67 is calculated. Next, at
step 201, it is determined whether the absorbed NOx amount
.SIGMA.NOX has exceeded the allowable value MAX. When
.SIGMA.NOX>MAX, the routine proceeds to step 202 where the
required fuel addition amount Q is calculated from the map of FIG.
7. At the next step 203, the fuel addition parameter is calculated.
That is, the divided addition time tDIV, interval tINT, and number
of divided additions n required for addition of the required fuel
addition amount calculated at step 202 in the overall addition time
tALL preset at step 209 or step 212 of the previous processing
cycle are calculated using the fuel addition rate q or its drop
.DELTA.q detected at step 206 of the previous processing cycle. At
the next step 204, the fuel is added based on the fuel addition
parameter determined at step 203. At the next step 205, the
absorbed NOx amount .SIGMA.NOx is returned to zero. At the next
step 206, the fuel addition rate q or its drop .DELTA.q is
detected. At the next step 207, the NOx purification rate EFF of
the NOx adsorbent 67 is detected. At the next step 208, it is
determined if the current NOx purification rate EFF detected at
step 207 is higher than the previous NOx purification rate EFF0.
When EFF>EFF0, next, the routine proceeds to step 209, where the
overall addition time tALL is shortened by the predetermined value
AY. Next, the routine proceeds to step 210, where the current NOx
purification rate EFF is made EFF0. As opposed to this, when
EFF<EFF0, the routine proceeds from step 208 to step 211, where
it is determined if the drop (=EFF0-EFF) of the current NOx
purification rate EFF from the previous NOx purification rate EFF0
is larger than an allowable value LMT. When EFF00-EFF<LMT, next,
the routine proceeds to step 212, where the overall addition time
tALL is extended by a predetermined value .DELTA.Y. Next, the
routine proceeds to step 210. As opposed to this, when
EFF0-EFF>LMT, next, the routine proceeds to step 213, where the
alarm device 82 is actuated.
[0060] FIG. 18 shows still another embodiment according to the
present invention. The embodiment shown in FIG. 18 differs in
configuration from the embodiment of FIG. 15 in the point of an HC
sensor 82 for detecting the amount of HC or HC concentration in the
exhaust gas flowing out from the NOx adsorbent 67 being attached to
the exhaust pipe 23. The output signal of the HC sensor 82 is input
through the corresponding AD converter 37 to the input port 35.
[0061] FIG. 19 shows the HC amount QHC exhausted from the NOx
adsorbent 67 when fuel is added when holding the fuel addition
amount constant and changing the overall addition time tALL. In
FIG. 19, the curve P shows the HC amount QHC exhausted when the
fuel addition rate q is the regular value qp, while curve C shows
the HC amount QHC exhausted when the fuel addition rate q falls
from the regular value qp.
[0062] Looking at for example the curve P of FIG. 19, as the
overall addition time tALL becomes shorter, the HC amount QHC
exhausted becomes greater, while as the overall addition time tALL
becomes longer, the HC amount QHC exhausted becomes smaller. On the
other hand, if the fuel addition rate q falls from the regular
value qp, the NOx purification rate EFF shown by the curve C shifts
in a direction where the overall addition time tALL becomes
shorter. This is because, as explained above, if the fuel addition
rate q falls, the degree of richness of the air-fuel ratio of the
exhaust gas flowing into the NOx adsorbent 67 becomes smaller.
[0063] This being so, it is learned that when the HC amount QHC
exhausted when adding fuel is small, the degree of richness of the
inflowing exhaust gas becomes smaller and the fuel addition rate q
falls.
[0064] Therefore, in still another embodiment according to the
present invention, the HC amount QHC exhausted when fuel is added
is detected by the HC sensor 82 and the overall addition time tALL
is corrected so that this exhausted HC amount QHC matches with the
target value QHCt. That is, in the example shown in FIG. 19, the
overall addition time tALL is corrected by being shortened from
tAMpp to tAMcc. As a result, a large amount of HC being exhausted
from the NOx adsorbent 67 can be prevented, the degree or richness
of the air-fuel ratio of the inflowing exhaust gas can be
maintained high, and the NOx can be released and reduced well.
[0065] In FIG. 20, X shows the timing for addition of fuel for
release and reduction of NOx. As shown in FIG. 20, when the
exhausted HC amount QHC is smaller than the target value QHCt, the
overall addition time tALL is corrected to be shortened by for
example a small predetermined value AZ. As opposed to this, if the
overall addition time tALL becomes larger than the target value
QHCt, the overall addition time tALL is extended by .DELTA.Z, that
is, is returned to the original value. In this way, the exhausted
HC amount QHC is maintained at the target value QHCt.
[0066] In this case, if setting the exhausted HC amount for
maximizing the NOx purification rate EFF at the target value QHCt,
it is possible to maintain the NOx purification rate EFF at the
maximum.
[0067] FIG. 21 shows the NOx release control routine of still
another embodiment according to the present invention.
[0068] If referring to FIG. 21, first, at step 300, the NOx amount
.SIGMA.NOx absorbed in the NOx adsorbent 67 is calculated. Next, at
step 301, it is determined if the absorbed NOx amount .SIGMA.NOX
has exceeded an allowable value MAX. When .SIGMA.NOX>MAX, the
routine proceeds to step 302 where the required fuel addition
amount Q is calculated from the map of FIG. 7. At the next step
303, the fuel addition parameter is calculated. That is, the
divided addition time tDIV, interval tINT, and number of divided
additions n required for addition of the required fuel addition
amount calculated at step 302 in the overall addition time tALL
preset at step 309 or step 310 of the previous processing cycle are
calculated using the fuel addition rate q or its drop .DELTA.q
detected at step 306 of the previous processing cycle. At the next
step 304, fuel is added based on the fuel addition parameter
determined at step 303. At the next step 305, the absorbed NOx
amount .SIGMA.NOx is returned to zero. At the next step 306, the
fuel addition rate q or its drop .DELTA.q is detected. At the next
step 307, the HC amount QHC exhausted from the NOx adsorbent 67 is
detected. At the next step 308, it is determined whether the
exhausted HC amount QHC detected at step 307 is smaller than the
target value QHCt. When QHC<QHCt, next, the routine proceeds to
step 309, where the overall addition time tALL is shortened by the
predetermined value .DELTA.Z. As opposed to this, when
QHC.gtoreq.QHCt, the routine proceeds from step 308 to step 310,
where the overall addition time tALL is extended by a predetermined
value .DELTA.Z.
[0069] Note that when the HC amount QHC exhausted when correcting
the overall addition time tALL to shorten it falls compared with
the HC amount QHC0 exhausted at the time of the previous fuel
addition and the drop at this time (=QHC0-QHC) becomes larger than
an allowable value, it may be determined that the fuel addition
valve 28 has broken down and an alarm device 81 may be actuated.
Alternatively, when the HC amount QHC exhausted when correcting the
overall addition time tALL to shorten it is smaller than the
allowable limit, it may be determined that the fuel addition valve
28 has broken down.
* * * * *