U.S. patent application number 11/997691 was filed with the patent office on 2010-09-02 for exhaust gas purification device.
This patent application is currently assigned to MITSUBISHI FUSO TRUCK AND BUS CORPORATION. Invention is credited to Nobuhiro Kondo, Minehiro Murata, Yoshinaka Takeda.
Application Number | 20100218486 11/997691 |
Document ID | / |
Family ID | 37708759 |
Filed Date | 2010-09-02 |
United States Patent
Application |
20100218486 |
Kind Code |
A1 |
Murata; Minehiro ; et
al. |
September 2, 2010 |
EXHAUST GAS PURIFICATION DEVICE
Abstract
An exhaust gas purification device includes a NO.sub.x
adsorption catalyst (36) and HC supply means (48) for adding HC to
the exhaust gas flowing to the NO.sub.x adsorption catalyst (36).
The HC supply means (48) is controlled first to supply an amount of
HC required to keep the temperature of the NO.sub.x adsorption
catalyst (36) at a second temperature which is derived by
subtracting a temperature rise caused by rich spike from a first
temperature predetermined as a temperature necessary for S purge,
thereby increasing the temperature of the NO.sub.x adsorption
catalyst (36), and then to additionally perform the rich spike for
the S purge while continuing the HC supply.
Inventors: |
Murata; Minehiro;
(Kawasaki-shi, JP) ; Takeda; Yoshinaka;
(Kawasaki-shi, JP) ; Kondo; Nobuhiro;
(Kawasaki-shi, JP) |
Correspondence
Address: |
ROSSI, KIMMS & McDOWELL LLP.
20609 Gordon Park Square, Suite 150
Ashburn
VA
20147
US
|
Assignee: |
MITSUBISHI FUSO TRUCK AND BUS
CORPORATION
Kawasaki-shi, Kanagawa
JP
|
Family ID: |
37708759 |
Appl. No.: |
11/997691 |
Filed: |
August 1, 2005 |
PCT Filed: |
August 1, 2005 |
PCT NO: |
PCT/JP2006/315193 |
371 Date: |
February 1, 2008 |
Current U.S.
Class: |
60/286 ; 60/299;
60/303 |
Current CPC
Class: |
Y02T 10/12 20130101;
F02B 29/0406 20130101; B01D 53/9431 20130101; F02M 26/05 20160201;
F01N 3/2033 20130101; F02D 9/04 20130101; F01N 3/0885 20130101;
F01N 3/0821 20130101; F01N 2260/14 20130101; F01N 13/0097 20140603;
F02B 37/00 20130101; B01D 2251/208 20130101; F02M 26/10 20160201;
F02M 26/15 20160201; F01N 2410/12 20130101; Y02T 10/26
20130101 |
Class at
Publication: |
60/286 ; 60/299;
60/303 |
International
Class: |
F01N 9/00 20060101
F01N009/00; F01N 3/10 20060101 F01N003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 4, 2005 |
JP |
2005-226596 |
Claims
1. An exhaust gas purification device comprising: a NO.sub.x
adsorption catalyst arranged in an exhaust passage of an engine,
for adsorbing, in an oxidizing atmosphere, NO.sub.x contained in an
exhaust gas and for releasing and reducing, in a reducing
atmosphere, the adsorbed NO.sub.x; HC supply means for adding HC to
the exhaust gas flowing to the NO.sub.x adsorption catalyst; and
control means for causing the HC supply means to supply HC to raise
temperature of the NO.sub.x adsorption catalyst and also causing
the HC supply means to supply HC by rich spike to create the
reducing atmosphere and thus carry out S purge of the NO.sub.x
adsorption catalyst, wherein the control means controls the HC
supply means by first causing the HC supply means to supply an
amount of HC required to raise the temperature of the NO.sub.x
adsorption catalyst to a second temperature which is derived by
subtracting a temperature rise caused by the rich spike from a
first temperature predetermined as a temperature necessary for the
S purge, to raise the temperature of the NO.sub.x adsorption
catalyst, and then additionally performing the rich spike while
continuing the HC supply.
2. The exhaust gas purification device according to claim 1,
wherein the control means controls the HC supply means such that
the S purge by means of the rich spike is started after the
temperature of the NO.sub.x adsorption catalyst is kept at the
second temperature over a predetermined time.
3. The exhaust gas purification device according to claim 1,
wherein, if the temperature of the NO.sub.x adsorption catalyst
becomes higher than the second temperature while the temperature of
the NO.sub.x adsorption catalyst is being raised to the second
temperature by the HC supply from the HC supply means, the control
means decreases the amount of the HC supply.
4. The exhaust gas purification device according to claim 1,
wherein, if the temperature of the NO.sub.x adsorption catalyst
becomes higher than the first temperature while HC is being
supplied by the rich spike to create the reducing atmosphere around
the NO.sub.x adsorption catalyst, the control means decreases the
amount of the HC supply.
Description
TECHNICAL FIELD
[0001] The present invention relates to exhaust gas purification
devices for purifying exhaust gas emitted from an engine, and more
particularly, to an exhaust gas purification device provided with a
NO.sub.x adsorption catalyst.
BACKGROUND ART
[0002] An exhaust gas purification device has been known wherein a
NO.sub.x adsorption catalyst which, in an oxidizing atmosphere,
adsorbs NO.sub.x (nitrogen oxides) contained in the exhaust gas
and, in a reducing atmosphere, releases the adsorbed NO.sub.x for
reduction, is arranged in the exhaust passage of the engine to
remove NO.sub.x from the exhaust gas.
[0003] Meanwhile, fuel and lubricating oil for engines contain
sulfur components, and the sulfur components are also emitted, in
the form of SO.sub.x (sulfur oxides), together with the exhaust gas
from the engine. The SO.sub.x also is adsorbed to the NO.sub.x
adsorption catalyst through a mechanism similar to the NO.sub.x
adsorption mechanism, and as the SO.sub.x adsorption amount
increases, lowering of the NO.sub.x adsorption capacity, or the
so-called sulfur poisoning, occurs.
[0004] To make the catalyst recover from such sulfur poisoning,
that is, to carry out S purge, a technique has been known in which
HC (hydrocarbon) is intermittently added to the exhaust gas so as
to raise the temperature of the NO.sub.x adsorption catalyst and
also to create a reducing atmosphere (hereinafter such addition of
HC is referred to as the "rich spike"). The S purge is disclosed,
for example, in Unexamined Japanese Patent Publication No.
2004-251172 (hereinafter referred to as "Patent Document 1").
[0005] When performing the S purge, it is necessary that the
temperature of the NO.sub.x adsorption catalyst should be raised to
a high temperature in the vicinity of 700.degree. C., for example,
and also that a reducing atmosphere should be created around the
NO.sub.x adsorption catalyst. By merely carrying out the rich spike
to supply HC, however, it is difficult to stably carry out the
temperature increase as well as the creation of reducing
atmosphere. Especially, in diesel engines and lean-burn engines, a
large amount of oxygen is contained in the exhaust gas, and when
the S purge is initiated by performing the rich spike to supply HC
to the exhaust gas, the HC rapidly reacts with oxygen locally in
the exhaust gas, giving rise to the problem that the temperature of
the NO.sub.x adsorption catalyst rises to an excessive level.
[0006] If the amount of HC supplied by carrying out the rich spike
is decreased in order to prevent such excessive temperature rise,
it takes a longer time to raise the temperature of the NO.sub.x
adsorption catalyst to a level where the S purge can be performed,
delaying the start of the S purge. A problem also arises in that
even after the S purge is initiated, a satisfactory reducing
atmosphere fails to be created around the NO.sub.x adsorption
catalyst, requiring a long time to complete the S purge.
[0007] In the exhaust gas purification device disclosed in Patent
Document 1, the NO.sub.x adsorption catalyst is carried on a
particulate filter. When the S purge is restarted after being
suspended, the temperature of the particulate filter is raised to
regenerate the filter and then the temperature is further increased
to carry out the S purge. Also in such cases where the temperature
of the NO.sub.x adsorption catalyst is raised stepwise, the exhaust
temperature needs to be increased to about 600.degree. C. in order
to regenerate the particulate filter. If the rich spike for the S
purge is initiated in such a state, the HC supplied by the rich
spike rapidly burns on the high-temperature catalyst, causing
excessive temperature rise of the catalyst and also making it
difficult to keep the amount of HC in the exhaust gas at a suitable
level necessary for the S purge.
[0008] Thus, with the conventional exhaust gas purification device,
it is difficult to stably carry out both the temperature increase
of the NO.sub.x adsorption catalyst and the creation of reducing
atmosphere for the S purge.
DISCLOSURE OF THE INVENTION
[0009] The present invention was made to solve the above problems,
and an object thereof is to provide an exhaust gas purification
device capable of stable and efficient recovery of a NO.sub.x
adsorption catalyst from sulfur poisoning, without causing
excessive temperature rise of the NO.sub.x adsorption catalyst.
[0010] To achieve the object, the present invention provides an
exhaust gas purification device comprising: a NO.sub.x adsorption
catalyst arranged in an exhaust passage of an engine, for
adsorbing, in an oxidizing atmosphere, NO.sub.x contained in an
exhaust gas and for releasing and reducing, in a reducing
atmosphere, the adsorbed NO.sub.x; HC supply means for adding HC to
the exhaust gas flowing to the NO.sub.x adsorption catalyst; and
control means for causing the HC supply means to supply HC to raise
temperature of the NO.sub.x adsorption catalyst and also causing
the HC supply means to supply HC by rich spike to create the
reducing atmosphere and thus carry out S purge of the NO.sub.x
adsorption catalyst, wherein the control means controls the HC
supply means by first causing the HC supply means to supply an
amount of HC required to raise the temperature of the NO.sub.x
adsorption catalyst to a second temperature which is derived by
subtracting a temperature rise caused by the rich spike from a
first temperature predetermined as a temperature necessary for the
S purge, to raise the temperature of the NO.sub.x adsorption
catalyst, and then additionally performing the rich spike while
continuing the HC supply.
[0011] In the exhaust gas purification device of the present
invention, prior to the start of the S purge of the NO.sub.x
adsorption catalyst by means of the rich spike, an amount of HC
required to maintain the NO.sub.x adsorption catalyst at the second
temperature, which is derived by subtracting the temperature rise
caused by the rich spike for creating a reducing atmosphere around
the NO.sub.x adsorption catalyst from the first temperature
predetermined as a temperature for recovering the catalyst from
sulfur poisoning, is added to the exhaust gas flowing to the
NO.sub.x adsorption catalyst. Accordingly, even in a situation
where the HC supply for creating a reducing atmosphere around the
NO.sub.x adsorption catalyst has just been started and thus the
exhaust gas still has a large content of oxygen, the temperature of
the NO.sub.x adsorption catalyst is prevented from rising to an
excessive level due to the reaction of the supplied HC with
oxygen.
[0012] Also, the S purge is carried out by additionally supplying
an amount of HC necessary for the rich spike to create a reducing
atmosphere around the NO.sub.x adsorption catalyst while the amount
of HC required to raise the temperature of the NO.sub.x adsorption
catalyst to the second temperature, which is obtained by
subtracting the temperature rise caused by the rich spike from the
first temperature, is continuously supplied. Consequently, during
the execution of the S purge by means of the rich spike, the
temperature of the NO.sub.x adsorption catalyst can be easily kept
at the first temperature, which is the temperature necessary for
the catalyst to recover from sulfur poisoning.
BRIEF DESCRIPTION OF THE DRAWINGS
[0013] FIG. 1 shows an entire configuration of an exhaust gas
purification device according to one embodiment of the present
invention;
[0014] FIG. 2 is a flowchart illustrating S purge control executed
in the exhaust gas purification device of FIG. 1; and
[0015] FIG. 3 shows how an HC supply amount, an excess air ratio of
exhaust gas flowing to a NO.sub.x adsorption catalyst, and an
outlet-side exhaust temperature of the NO.sub.x adsorption catalyst
change with the lapse of time during the execution of the S purge
control shown in FIG. 2.
BEST MODE OF CARRYING OUT THE INVENTION
[0016] An embodiment of the present invention will be described
below with reference to the accompanying drawings.
[0017] FIG. 1 shows a system configuration of a four-cylinder
diesel engine (hereinafter referred to as the "engine") to which an
exhaust gas purification device according to the embodiment of the
invention is applied. Referring first to FIG. 1, the construction
of the exhaust gas purification device according to the present
invention will be described.
[0018] The engine 1 has a high-pressure accumulator (hereinafter
referred to as the "common rail") 2 provided in common for the
cylinders. High-pressure light oil as fuel supplied from a fuel
injection pump (not shown) and stored in the common rail 2 is
supplied to injectors 4 associated with the respective cylinders
and injected from the injectors 4 into the corresponding
cylinders.
[0019] An intake passage 6 has a turbocharger 8 arranged therein.
Intake air introduced from an air cleaner, not shown, into the
intake passage 6 flows into a compressor 8a of the turbocharger 8,
and the intake air turbocharged by the compressor 8a is guided
through an intercooler 10 and an intake air control valve 12 to an
intake manifold 14. An intake air flow sensor 16 for detecting the
flow rate of intake air flowing into the engine 1 is arranged in
the intake passage 6 on the upstream side of the compressor 8a.
[0020] Exhaust ports (not shown), through which exhaust gas is
discharged from the respective cylinders of the engine 1, are
connected through an exhaust manifold 18 to an exhaust pipe
(exhaust passage) 20. The exhaust manifold 18 and the intake
manifold 14 communicate with each other through an EGR passage 24
provided with an EGR valve 22.
[0021] The exhaust pipe 20 extending from a turbine 8b of the
turbocharger 8 is connected through an exhaust throttle valve 26 to
an exhaust after-treatment device 28. The rotary shaft of the
turbine 8b is coupled to the rotary shaft of the compressor 8a, so
that as the exhaust gas flowing through the exhaust pipe 20 acts
upon the turbine 8b, the compressor 8a is driven by the turbine
8b.
[0022] The exhaust after-treatment device 28 comprises an
upstream-side casing 30 and a downstream-side casing 34 which is
located downstream of the upstream-side casing 30 and communicates
with same through a communication passage 32.
[0023] The upstream-side casing 30 contains a NO.sub.x adsorption
catalyst 36, as well as a particulate filter (hereinafter referred
to as the "filter") 38 arranged downstream of the NO.sub.x
adsorption catalyst 36.
[0024] The NO.sub.x adsorption catalyst 36 has the function of
adsorbing NO.sub.x contained in the exhaust gas when exposed to an
oxidizing atmosphere in which the oxygen concentration of the
introduced exhaust gas is high, and releasing and reducing the
adsorbed NO.sub.x when exposed to a reducing atmosphere in which
the oxygen concentration of the introduced exhaust gas is low and
reducing components such as HC and CO (carbon monoxide) are
contained in the exhaust gas.
[0025] The filter 38 comprises a honeycomb-type ceramic substrate
and includes a large number of passages communicating the upstream
and downstream sides of the filter with each other. The upstream-
and downstream-side openings of the passages are alternately closed
so that particulates contained in the exhaust gas may be arrested
by the filter, thereby purifying the exhaust gas emitted from the
engine 1.
[0026] NO.sub.x in the exhaust gas that failed to be adsorbed by
the NO.sub.x adsorption catalyst 36 because of the limit of its
NO.sub.x adsorption capacity flows into the filter 38 and acts as
an oxidizing agent for the particulates caught and deposited on the
filter 38. Namely, the NO.sub.x flowing into the filter 38 oxidizes
the particulates and removes them from the filter 38, thereby
continuously regenerating the filter 38. The resultant matter, that
is, N.sub.2, is discharged into the air.
[0027] An exhaust temperature sensor 40 for detecting the exhaust
temperature Tc at the outlet side of the NO.sub.x adsorption
catalyst 36 is arranged in the upstream-side casing 30 at a
location downstream of the catalyst 36. Also, an upstream pressure
sensor 42 is arranged upstream of the filter 38 to detect the
exhaust pressure on the upstream side of the filter 38, and a
downstream pressure sensor 44 is arranged downstream of the filter
38 to detect the exhaust pressure on the downstream side of the
filter 38.
[0028] The downstream-side casing 34 contains a post-stage
oxidation catalyst 46. The post-stage oxidation catalyst 46 has the
function of oxidizing HC and CO that remain in the exhaust gas
without being removed by the NO.sub.x adsorption catalyst 36. Also,
the post-stage oxidation catalyst 46 has the function of oxidizing
HC separated from the filter 38 due to increase in temperature
during forced regeneration, described later, of the filter 38, as
well as the function of oxidizing CO produced as a result of the
burning of particulates during the forced regeneration of the
filter 38 and allowing the resultant matter, namely, CO.sub.2, to
be discharged into the air.
[0029] At a portion of the exhaust pipe 20 between the exhaust
throttle valve 26 and the exhaust after-treatment device 28 is
arranged a fuel addition valve (HC supply means) 48 which is
adapted to be supplied with fuel from the fuel injection pump (not
shown) to inject fuel into the exhaust pipe 20. Thus, the fuel
addition valve 48 adds fuel to the exhaust gas flowing toward the
NO.sub.x adsorption catalyst 36, whereby a reducing atmosphere is
created around the catalyst 38. Consequently, the NO.sub.x adsorbed
to the NO.sub.x adsorption catalyst 38 is released and reduced.
[0030] Fuel injection from the fuel addition valve 48 into the
exhaust gas is also carried out during the forced regeneration of
the filter 38, as described later, in order to raise the
temperature of the filter 38.
[0031] An ECU (control means) 50 is a control device for performing
integrated control including the operation control of the engine 1
and comprises a CPU, memories, timer-counters, etc. The ECU
calculates various control values and controls various devices in
accordance with the calculated control values.
[0032] To collect information necessary for various control
actions, the input side of the ECU 50 is connected with various
sensors including, besides the aforementioned intake air flow
sensor 16, exhaust temperature sensor 40, upstream pressure sensor
42 and downstream pressure sensor 44, a rotational speed sensor 52
for detecting the engine rotation speed, and an accelerator
position sensor 54 for detecting the amount of depression of the
accelerator pedal. The output side of the ECU 50 is connected to
various devices controlled in accordance with respective calculated
control values, such as the injectors 4 associated with the
respective cylinders, the intake air control valve 12, the EGR
valve 22, the exhaust throttle valve 26, and the fuel addition
valve 48.
[0033] The ECU 50 also takes charge of the calculation of a fuel
amount to be supplied to the individual cylinders of the engine 1,
as well as the control of fuel supply from the injectors 4
according to the calculated fuel supply amount. The fuel supply
amount (main injection amount) required to operate the engine 1 is
determined by being read out from a pre-stored map on the basis of
the engine rotation speed detected by the rotational speed sensor
52 and the accelerator position detected by the accelerator
position sensor 54. The amount of fuel supplied to the individual
cylinders is adjusted by means of the valve opening time of the
injectors 4. The injectors 4 are opened for a time period
corresponding to the determined fuel amount to effect the main
injection of fuel into the respective cylinders, thereby supplying
the engine 1 with the amount of fuel necessary for the engine
operation.
[0034] Also, the ECU 50 controls the forced regeneration of the
filter 38. The particulates deposited on the filter 38 are oxidized
and removed by the continuous regeneration of the filter 38
utilizing the reaction of the particulates with the NO.sub.2
flowing into the filter 38 through the NO.sub.x adsorption catalyst
36. However, it is sometimes the case that the continuous
regeneration fails to fully remove the deposited particulates by
oxidation. If the particulates are left unremoved, an excessive
amount of particulates is accumulated in the filter 38, possibly
clogging the filter 38. Accordingly, the filter 38 is forcedly
regenerated as needed depending on the degree of accumulation of
particulates in the filter 38.
[0035] Specifically, if it is judged based on, for example, the
detected values of the upstream pressure sensor 42, the downstream
pressure sensor 44 and the intake air flow sensor 16, that the
amount of particulates accumulated in the filter 38 has reached a
predetermined amount, forced regeneration control is initiated.
[0036] In the forced regeneration control, the intake air control
valve 12 and the exhaust throttle valve 26 are operated in the
closing direction to raise the exhaust temperature, and also fuel
is injected from the fuel addition valve 48 into the exhaust gas to
raise the temperature of the filter 38 up to a level at which the
particulates can be burned up. Specifically, the HC supplied from
the fuel addition valve 48 flows to the NO.sub.x adsorption
catalyst 36, where the exhaust gas is heated due to the oxidation
of the HC, and the resultant high-temperature exhaust gas flows
into the filter 38. The particulates accumulated in the filter 38
are burned by the high-temperature exhaust gas, whereby the filter
38 is forcedly regenerated.
[0037] In addition, the ECU 50 performs control to properly remove
NO.sub.x by means of the NO.sub.x adsorption catalyst 36. The
engine 1 is a diesel engine, and thus, lean-burn operation is
carried out in most of the engine operating region. Consequently,
the oxygen concentration of the exhaust gas is high and NO.sub.x
contained in the exhaust gas is adsorbed to the NO.sub.x adsorption
catalyst 36. If the NO.sub.x-adsorbing state of the NO.sub.x
adsorption catalyst 36 continues for a long time, the NO.sub.x
storage capacity of the NO.sub.x adsorption catalyst 36 becomes
saturated, causing the possibility that the NO.sub.x in the exhaust
gas fails to be adsorbed by the NO.sub.x adsorption catalyst 36 and
is emitted directly into the air.
[0038] To prevent the saturation of the NO.sub.x adsorption
capacity, the ECU 50 controls the fuel addition valve 48 so as to
inject fuel into, and hence add HC to the exhaust gas at
predetermined intervals of time, for example, thereby creating a
reducing atmosphere around the NO.sub.x adsorption catalyst 36 and
allowing the NO.sub.x adsorbed to the catalyst 36 to be released
and reduced.
[0039] The fuel and lubricating oil used in the engine 1 equipped
with the aforementioned exhaust gas purification device contain
sulfur components, and the sulfur components are discharged, in the
form of SO.sub.x, from the engine 1 together with the exhaust gas.
The SO.sub.x contained in the exhaust gas is adsorbed to the
NO.sub.x adsorption catalyst 36 by a mechanism similar to the
NO.sub.x adsorption mechanism, and as the SO.sub.x adsorption
amount increases, reduction of the NO.sub.x adsorption capacity of
the NO.sub.x adsorption catalyst 36, namely, sulfur poisoning,
occurs. Also in cases where the sulfur poisoning is left
unattended, the NO.sub.x removal efficiency of the NO.sub.x
adsorption catalyst 36 lowers, causing the possibility that the
NO.sub.x in the exhaust gas is emitted directly into the air
without being adsorbed by the NO.sub.x adsorption catalyst 36.
[0040] Accordingly, in the exhaust gas purification device provided
with the NO.sub.x adsorption catalyst 36, recovery of the catalyst
36 from the sulfur poisoning, that is, S purge, is carried out when
necessary. Specifically, the SO.sub.x adsorption amount of the
NO.sub.x adsorption catalyst 36 is estimated from the fuel
consumption amount and operation time of the engine 1, for example,
and if the estimated SO.sub.x adsorption amount is larger than a
predetermined value, the ECU 50 executes S purge control. In the S
purge control, the temperature of the NO.sub.x adsorption catalyst
36 is raised to about 700.degree. C., and it is also necessary that
a reducing atmosphere be created around the NO.sub.x adsorption
catalyst 36.
[0041] A routine for the S purge control is started when it is
concluded by an S purge control discrimination routine, not shown,
that the estimated SO.sub.x adsorption amount is larger than the
predetermined value and thus that the S purge is required. The S
purge control routine is executed at predetermined control
intervals, following the procedure shown in the flowchart of FIG.
2.
[0042] Upon start of the S purge control, it is first determined in
Step S2 whether or not the value of a flag F1 is equal to "1". The
flag F1 indicates whether to permit the HC supply by the rich spike
for the S purge, and the value "1" means that the rich spike is
permitted. The initial value of the flag F1 is "0"; therefore, in
the first control cycle just after the start of the S purge
control, the process proceeds to Step S4.
[0043] In Step S4, it is determined whether or not the value of a
flag Fa is "1". The flag Fa indicates whether a timer A, described
later, has started counting or not, and the value "1" means that
the timer A has started counting. The initial value of the flag Fa
is "0", and therefore, the process proceeds to Step S6.
[0044] In Step S6, the timer A is started, and then in Step S8, the
value of the flag Fa is set to "1" correspondingly with the start
of the timer A.
[0045] Subsequently, in Step S10, it is determined whether or not
the outlet-side exhaust temperature Tc of the NO.sub.x adsorption
catalyst 36, detected by the exhaust temperature sensor 40, is
higher than or equal to a predetermined temperature T2.
[0046] The predetermined temperature T2 is corresponding to an
outlet-side exhaust temperature of the NOx adsorption catalyst 36
in a condition where a temperature of the NO.sub.x adsorption
catalyst 36 is equal to a temperature (second temperature) which is
obtained by subtracting a temperature rise that is expected to be
caused by the HC supply for the rich spike from a temperature
(first temperature) necessary for the S purge. Accordingly, if it
is judged in Step S10 that the outlet-side exhaust temperature Tc
of the NO.sub.x adsorption catalyst 36 is higher than or equal to
the predetermined temperature T2, then it means that the
temperature of the NO.sub.x adsorption catalyst 36 is found to be
higher than or equal to the second temperature.
[0047] The first temperature represents a temperature necessary to
carry out the S purge and is set to a temperature higher by several
tens of degrees Celsius than a lower-limit temperature at and above
which the S purge of the NO.sub.x adsorption catalyst 36 can be
effected. The first and second temperatures are set to respective
appropriate values taking account of the characteristics of the
NO.sub.x adsorption catalyst 36 and engine 1. In this embodiment,
the first and second temperatures are set, for example, to
700.degree. C. and 500.degree. C., respectively.
[0048] In accordance with the result of the comparison in Step S10
between the outlet-side exhaust temperature Tc of the NO.sub.x
adsorption catalyst 36 detected by the exhaust temperature sensor
40 and the predetermined temperature T2, Step S12 is executed if
the temperature of the NO.sub.x adsorption catalyst 36 is judged to
be lower than the second temperature, or Step S14 is executed if
the temperature of the NO.sub.x adsorption catalyst 36 is judged to
be higher than or equal to the second temperature.
[0049] In Steps S12 and S14, the opening/closing of the fuel
addition valve 48 is controlled so that HC may be added to the
exhaust gas in an amount read out from a pre-stored map based on
the engine rotation speed detected by the rotational speed sensor
52, the amount of depression of the accelerator pedal detected by
the accelerator position sensor 54 and the like. The map stores HC
supply amounts required to raise the temperature of the NO.sub.x
adsorption catalyst 36 to the second temperature. Specifically,
where Step S12 is executed, a large-amount map storing relatively
large HC supply amounts is used because the temperature of the
NO.sub.x adsorption catalyst 36 is lower than the second
temperature. On the other hand, where Step S14 is executed, a
small-amount map storing relatively small HC supply amounts is used
since the temperature of the NO.sub.x adsorption catalyst 36 is
higher than or equal to the second temperature.
[0050] The process then proceeds to Step S16, where it is
determined whether or not the time to counted by the timer A,
started in Step S6, has reached a predetermined time t1. In the
initial stage after the start of the S purge control, the
predetermined time t1 is not yet reached by the time ta, so that
the present control cycle ends.
[0051] In the next control cycle, the value of the flag F1 is still
"0". Accordingly, the process proceeds from Step S2 to Step S4, and
since the value of the flag Fa has been set to "1", the process
proceeds from Step S4 directly to Step S10.
[0052] In Step S10, it is determined whether or not the outlet-side
exhaust temperature Tc of the NO.sub.x adsorption catalyst 36 is
higher than or equal to the predetermined temperature T2, as
mentioned above, to thereby determine whether or not the
temperature of the NO.sub.x adsorption catalyst 36 is higher than
or equal to the second temperature.
[0053] If it is judged that the temperature of the NO.sub.x
adsorption catalyst 36 is still lower than the second temperature,
the HC supply is carried out using the large-amount map, in Step
S12. On the other hand, if it is judged that the temperature of the
NO.sub.x adsorption catalyst 36 is higher than or equal to the
second temperature, the HC supply is performed using the
small-amount map, in Step S14.
[0054] In this manner, the HC supply is repeatedly performed in
Step S12 or S14 at the control intervals, so that the temperature
of the NO.sub.x adsorption catalyst 36 is elevated to the second
temperature or thereabout.
[0055] FIG. 3 shows changes with time of the amount of HC supplied
from the fuel addition valve 48, the excess air ratio of the
exhaust gas supplied to the NO.sub.x adsorption catalyst 36, and
the outlet-side exhaust temperature of the NO.sub.x adsorption
catalyst 36.
[0056] Upon start of the S purge control, the HC supply is
performed in Step S12 or S14 in FIG. 2. Since it takes some time
for the outlet-side exhaust temperature Tc of the NO.sub.x
adsorption catalyst 36 to reach the predetermined temperature T2,
the HC supply using the large-amount map is carried out in Step
S12.
[0057] The HC thus supplied is oxidized on the NO.sub.x adsorption
catalyst 36, so that the temperature of the NO.sub.x adsorption
catalyst 36 rises. When the outlet-side exhaust temperature Tc of
the NO.sub.x adsorption catalyst 36 becomes equal to or higher than
the predetermined temperature T2, the HC supply is switched and HC
is supplied using the small-amount map in Step S14. Each time the
outlet-side exhaust temperature Tc of the NO.sub.x adsorption
catalyst 36 crosses the predetermined temperature T2 thereafter,
the HC supply is switched between the one using the large-amount
map (Step S12) and the one using the small-amount map (Step S14).
Supplying HC in this manner serves to keep the outlet-side exhaust
temperature Tc of the NO.sub.x adsorption catalyst 36 at around the
predetermined temperature T2.
[0058] At this time, the excess air ratio of the exhaust gas
flowing in the NO.sub.x adsorption catalyst 36 is lower than that
before the start of the control because of the HC supply carried
out in Step S12 or S14, and thus the oxygen concentration has
become low, but the catalyst is still in an oxidizing
atmosphere.
[0059] The timer A keeps counting while the temperature of the
NO.sub.x adsorption catalyst 36 is maintained at around the second
temperature, and if it is judged in Step S16 in FIG. 2 that the
time to counted by the timer A has reached the predetermined time
t1, the process proceeds to Step S18 to set the value "1" for the
flag F1, whereupon the control cycle ends.
[0060] Thus, before the predetermined time t1 elapses after the
start of the S purge control, only the HC supply in an amount
required to raise the temperature of the NO.sub.x adsorption
catalyst 36 to the second temperature is carried out in Step S12 or
S14, and the HC supply by the rich spike is not performed.
[0061] Immediately after the S purge control is initiated, the
oxygen concentration around the catalyst is high and the HC
supplied in Step S12 or S14 reacts with oxygen on the catalyst, so
that rapid temperature rise occurs locally in the NO.sub.x
adsorption catalyst 36. However, since HC is supplied at this time
only in an amount necessary to raise the temperature of the
NO.sub.x adsorption catalyst 36 to the second temperature and also
since the HC supply by the rich spike is not performed, the
temperature of the catalyst 36 does not rise to an excessive
level.
[0062] Uneven temperature distribution of the NO.sub.x adsorption
catalyst 36 caused by the local temperature rise becomes uniform
over the entire area with the lapse of time, and the predetermined
time t1 is set to be long enough to allow the temperature
distribution to become uniform. In this manner, before the
predetermined time t1 elapses after the start of the S purge
control, only the HC supply of Step S12 or S14 is carried out and
the HC supply by the rich spike is not performed, whereby the
temperature of the NO.sub.x adsorption catalyst 36 is raised to the
second temperature or thereabout while at the same time the uneven
temperature distribution of the catalyst 36 caused by local
temperature rise is made uniform over the entire area of the
NO.sub.x adsorption catalyst 36.
[0063] In the control cycle executed after the value of the flag F1
is set to "1" in Step S18 and thus the HC supply by the rich spike
is permitted, the process proceeds from Step S2 to Step S20.
[0064] In Step S20, it is determined whether or not the value of a
flag Fb is "1". The flag Fb indicates whether a timer B, described
later, has started counting or not, and the value "1" means that
the timer B has started counting. The initial value of the flag Fb
is "0", and accordingly, the process proceeds to Step S22.
[0065] In Step S22, the timer B is started, and then in Step S24,
the value of the flag Fb is set to "1" correspondingly with the
start of the timer B.
[0066] Subsequently, in Step S26, it is determined whether or not
the outlet-side exhaust temperature Tc of the NO.sub.x adsorption
catalyst 36, detected by the exhaust temperature sensor 40, is
higher than or equal to a predetermined temperature T1.
[0067] The predetermined temperature T1 is corresponding to an
outlet-side exhaust temperature of the NO.sub.x adsorption catalyst
36 in a condition where a temperature of the NO.sub.x adsorption
catalyst 36 is equal to the first temperature necessary to carry
out the S purge. Accordingly, if it is judged in Step S26 that the
outlet-side exhaust temperature Tc of the NO.sub.x adsorption
catalyst 36 is higher than or equal to the predetermined
temperature T1, then it means that the temperature of the NO.sub.x
adsorption catalyst 36 is found to be higher than or equal to the
first temperature.
[0068] In accordance with the result of the comparison in Step S26
between the outlet-side exhaust temperature Tc of the NO.sub.x
adsorption catalyst 36 detected by the exhaust temperature sensor
40 and the predetermined temperature T1, Step S28 is executed if
the temperature of the NO.sub.x adsorption catalyst 36 is judged to
be lower than the first temperature, or Step S30 is executed if the
temperature of the NO.sub.x adsorption catalyst 36 is judged to be
higher than or equal to the first temperature.
[0069] In Steps S28 and S30, the opening/closing of the fuel
addition valve 48 is controlled so that HC may be added to the
exhaust gas in an amount read out from the map, used in Step S12 or
S14, on the basis of the engine rotation speed detected by the
rotational speed sensor 52, the amount of depression of the
accelerator pedal detected by the accelerator position sensor 54
and the like. Specifically, where Step S28 is executed, the
large-amount map is used because the temperature of the NO.sub.x
adsorption catalyst 36 is lower than the first temperature. On the
other hand, where Step S30 is executed, the small-amount map is
used since the temperature of the NO.sub.x adsorption catalyst 36
is higher than or equal to the first temperature.
[0070] After the HC supply is thus carried out in Step S28 or S30,
HC is additionally supplied from the fuel addition valve 48 to
perform the rich spike, in Step S32. The HC supply by the rich
spike is executed in addition to the HC supply of Step S28 or S30,
by additionally opening the fuel addition valve 48 only for a
predetermined period. The intervals of the rich spike are varied in
accordance with the operating condition of the engine 1 and the
like.
[0071] In this embodiment, therefore, the HC supply by the rich
spike is not effected every time Step S32 is executed in the
individual control cycles, but is effected only when the current
control cycle matches the timing for performing the rich spike.
[0072] Since the amount of HC required to raise the temperature of
the NO.sub.x adsorption catalyst 36 to the second temperature is
supplied in Step S28 or S30, the HC supply by the rich spike,
carried out in Step S32, allows the temperature of the NO.sub.x
adsorption catalyst 36 to be further increased above the second
temperature due to the oxidation of the additionally supplied HC
and also creates a reducing atmosphere around the NO.sub.x
adsorption catalyst 36.
[0073] The process then proceeds to Step S34, where it is
determined whether or not the time tb counted by the timer B,
started in Step S22, has reached a predetermined time t2. The
predetermined time tb is set to a time period within which the
NO.sub.x adsorption catalyst 36 can be fully recovered from sulfur
poisoning by carrying out the rich spike. While the predetermined
time t2 is not reached by the time tb counted by the timer B, the
control cycle ends after the execution of Step S34.
[0074] In the next and succeeding control cycles, the value of the
flag F1 still remains at "1", and accordingly, the process proceeds
to Step S20, where it is determined whether or not the value of the
flag Fb is "1". Since the value of the flag Fb was set to "1" in
Step S24 when the timer B was started, the process proceeds from
Step S20 directly to Step S26.
[0075] In Step S26, it is determined whether or not the outlet-side
exhaust temperature Tc of the NO.sub.x adsorption catalyst 36 is
higher than or equal to the predetermined temperature T1, as stated
above, to thereby determine whether or not the temperature of the
NO.sub.x adsorption catalyst 36 is higher than or equal to the
first temperature.
[0076] If it is judged that the temperature of the NO.sub.x
adsorption catalyst 36 is still lower than the first temperature,
the HC supply is carried out using the large-amount map, in Step
S28. On the other hand, if it is judged that the temperature of the
NO.sub.x adsorption catalyst 36 is higher than or equal to the
first temperature, the HC supply is performed using the
small-amount map, in Step S30.
[0077] Then, in Step S32, the HC supply by the rich spike is
carried out.
[0078] The amount of HC supplied in Step S28 or S30 is determined
from the map used in Step S12 or S14 and is set to an amount
required to raise the temperature of the NO.sub.x adsorption
catalyst 36 to the second temperature, as mentioned above. The
second temperature is derived by subtracting the temperature rise,
which is expected to be caused when the HC supply by the rich spike
is carried out, from the first temperature necessary for the S
purge. Consequently, by performing the HC supply in Step S28 or S30
as well as the HC supply by the rich spike in Step S32, it is
possible to maintain the temperature of the NO.sub.x adsorption
catalyst 36 at around the first temperature necessary for the S
purge.
[0079] In this case, the temperature of the NO.sub.x adsorption
catalyst 36 is adjusted by switching between the HC supply using
the large-amount map in Step S28 and the HC supply using the
small-amount map in Step S30, taking into account the temperature
rise of the NO.sub.x adsorption catalyst 36 caused by the rich
spike, as mentioned above. The temperature of the NO.sub.x
adsorption catalyst 36 can therefore be easily kept at the first
temperature.
[0080] FIG. 3 illustrates how the amount of HC supplied from the
fuel addition valve 48, the excess air ratio of the exhaust gas
supplied to the NO.sub.x adsorption catalyst 36 and the outlet-side
exhaust temperature of the NO.sub.x adsorption catalyst 36 change
with the lapse of time at this time.
[0081] Once the time ta counted by the timer A reaches the
predetermined time t1 after the start of the S purge control, the
HC supply is performed in Step S28 or S30 shown in the flowchart of
FIG. 2 and also the HC supply by the rich spike is carried out in
Step S32, so that the outlet-side exhaust temperature of the
NO.sub.x adsorption catalyst 36 further increases from the
predetermined temperature T2. However, the outlet-side exhaust
temperature Tc of the NO.sub.x adsorption catalyst 36 does not
instantly rise to the predetermined temperature T1 but remains
below T1 for some time, and therefore, the HC supply using the
large-amount map is performed in Step S28.
[0082] When the outlet-side exhaust temperature Tc of the NO.sub.x
adsorption catalyst 36 becomes equal to or higher than the
predetermined temperature T1, that is, when the temperature of the
NO.sub.x adsorption catalyst 36 becomes equal to or higher than the
first temperature, the HC supply is switched and HC is supplied
using the small-amount map, in Step S30. Each time the outlet-side
exhaust temperature Tc of the NO.sub.x adsorption catalyst 36
crosses the predetermined temperature T1 thereafter, that is, each
time the temperature of the NO.sub.x adsorption catalyst 36 crosses
the first temperature thereafter, the HC supply is switched between
the one using the large-amount map in Step S28 and the one using
the small-amount map in Step S30. Also, at this time, the HC supply
by the rich spike is carried out in Step S32, in addition to the HC
supply in Step S28 or S30. By supplying HC in this manner, it is
possible to keep the outlet-side exhaust temperature Tc of the
NO.sub.x adsorption catalyst 36 at around the predetermined
temperature T1, and thus to keep the temperature of the NO.sub.x
adsorption catalyst 36 at around the first temperature necessary
for the S purge.
[0083] When the rich spike is carried out, the excess air ratio of
the exhaust gas around the NO.sub.x adsorption catalyst 36
temporarily significantly drops because of the HC supplied by the
rich spike, and therefore, the oxygen concentration lowers,
creating a reducing atmosphere around the catalyst. As a result,
the SO.sub.x adsorbed to the NO.sub.x adsorption catalyst 36 is
released, so that the catalyst 36 recovers from sulfur
poisoning.
[0084] In this manner, the S purge of the NO.sub.x adsorption
catalyst is carried out by the rich spike in Step S32, and if the
time counted by the timer B reaches the predetermined time t2, it
is judged that the S purge of the NO.sub.x adsorption catalyst 36
is completed, whereupon the process proceeds to Step S36. In Step
S36, the values of the flags F1, Fa and Fb, all used for the S
purge control, are set to "0", and then in Step S38, the timers A
and B are reset, whereupon the control cycle ends. Also, this S
purge control routine is terminated by the S purge control
discrimination routine, not shown.
[0085] As described above, when the S purge of the NO.sub.x
adsorption catalyst 36 is needed, the HC supply by the rich spike
is not instantly started, but HC is supplied first from the fuel
addition valve 48 in such an amount that the temperature of the
NO.sub.x adsorption catalyst 36 is raised to the second temperature
derived by subtracting the temperature rise, which is expected to
be caused by the HC supply by the rich spike, from the first
temperature necessary for the S purge. Accordingly, while the
oxygen concentration is still high immediately after the start of
the S purge control, the temperature of the NO.sub.x adsorption
catalyst 36 can be prevented from rising to an excessive level.
Also, the temperature of the NO.sub.x adsorption catalyst 36 does
not rise to a level far above the first temperature immediately
after the rich spike is initiated to produce a reducing atmosphere
around the NO.sub.x adsorption catalyst 36.
[0086] Since the temperature of the NO.sub.x adsorption catalyst 36
is prevented from rising to an excessive level, the HC supply
amount need not be restrained, unlike the case where the rich spike
is performed as soon as the S purge control is initiated, and the
NO.sub.x adsorption catalyst 36 can be brought earlier to a state
where the S purge can be executed.
[0087] Also, even if rapid temperature rise occurs locally in the
NO.sub.x adsorption catalyst due to oxygen present around the
catalyst at the start of the S purge control, such uneven
temperature distribution is made uniform over the entire area of
the NO.sub.x adsorption catalyst 36 because the HC supply by the
rich spike is not effected until the predetermined time t1 passes
after the start of the S purge control. Because of the uniform
temperature distribution, the temperature of the NO.sub.x
adsorption catalyst 36 does not rise to an excessive level when the
HC supply by the rich spike is carried out thereafter.
[0088] Further, after the HC supply by the rich spike is started,
the amount of HC required to raise the temperature of the NO.sub.x
adsorption catalyst 36 to the second temperature is continuously
supplied, and in addition to this HC supply, the HC supply by the
rich spike is effected. The second temperature is derived by
subtracting the temperature rise, which is expected to be caused by
the HC supply by the rich spike, from the first temperature
necessary for the S purge. Thus, by adjusting the amount of the HC
supply on which the amount of the HC supply by the rich spike is
added, it is possible to easily keep the temperature of the
NO.sub.x adsorption catalyst 36 at the first temperature necessary
for the S purge.
[0089] While the exhaust gas purification device according to the
embodiment of the present invention has been described, it is to be
noted that the invention is not limited to the foregoing embodiment
alone.
[0090] For example, in the above embodiment, the fuel addition
valve 48 is used as the HC supply means. Instead of using the fuel
addition valve 48, post-injection of fuel into the individual
cylinders of the engine 1 may be executed following the main
injection, thereby increasing the HC content in the exhaust gas. In
this case, the injectors 4 associated with the respective cylinders
serve as the HC supply means of the present invention.
[0091] Also, in the embodiment, whether the temperature of the
NO.sub.x adsorption catalyst 36 has reached the first/second
temperature or not is determined on the basis of the outlet-side
exhaust temperature Tc of the catalyst 36 detected by the exhaust
temperature sensor 40. Alternatively, a temperature sensor may be
provided on the carrier of the NO.sub.x adsorption catalyst 36 to
directly detect the temperature of the catalyst 36, or the
inlet-side exhaust temperature of the NO.sub.x adsorption catalyst
36 may be detected to estimate the temperature of the catalyst 36
from the detected temperature.
[0092] However, the foregoing embodiment, in which the temperature
of the NO.sub.x adsorption catalyst 36 is determined from the
outlet-side exhaust temperature Tc of the catalyst 36, is
advantageous in that the detected temperature is less affected by
local temperature variations that occur in other locations such as
at the inlet or in the interior of the NO.sub.x adsorption catalyst
36, in comparison with the case where the temperature sensor is
provided on the other position of the NO.sub.x adsorption catalyst
36.
[0093] Also, in the above embodiment, the S purge control is
started when the SO.sub.x adsorption amount of the NO.sub.x
adsorption catalyst 36, which is estimated from the fuel
consumption amount, operation time of the engine 1 and the like,
becomes equal to or larger than the predetermined value, but the
condition for starting the S purge control is not limited to such
condition alone. For example, the S purge control may be initiated
at predetermined intervals of time in the course of the operation
of the engine 1, or a NO.sub.x sensor may be arranged downstream of
the NO.sub.x adsorption catalyst 36 so that the S purge control may
be started when the NO.sub.x content in the exhaust gas, detected
by the NO.sub.x sensor, is equal to or larger than a predetermined
amount.
[0094] Furthermore, in the embodiment, the S purge control is
terminated on the basis of the time elapsed from the start of the
HC supply by the rich spike, but the condition for terminating the
S purge control is not limited to such condition alone. For
example, also in this case, a NO.sub.x sensor may be arranged
downstream of the NO.sub.x adsorption catalyst 36, and if the value
detected by the NO.sub.x sensor after the start of the S purge
control becomes equal to or lower than a predetermined value, the S
purge control may be terminated on the assumption that the NO.sub.x
adsorption catalyst 36 has recovered its NO.sub.x removal function.
Also, the time period upon lapse of which the S purge control is
terminated may be varied depending on the operating condition of
the engine 1.
[0095] In the foregoing embodiment, moreover, the exhaust
after-treatment device 28 is constituted by the upstream- and
downstream-side casings 30 and 34 separate from each other but may
comprise a single casing.
[0096] The exhaust gas purification device of the above embodiment
is applied to a diesel engine, but the engine type is not
particularly limited. The present invention is applicable to any
type of engine insofar as the engine is provided with a NO.sub.x
adsorption catalyst and HC supply means for supplying HC to the
NO.sub.x adsorption catalyst.
* * * * *