U.S. patent application number 10/969847 was filed with the patent office on 2005-05-26 for exhaust gas purifying method and exhaust gas purifying system.
This patent application is currently assigned to Isuzu Motors Limited. Invention is credited to Gabe, Masashi, Nagaoka, Daiji.
Application Number | 20050109022 10/969847 |
Document ID | / |
Family ID | 34463763 |
Filed Date | 2005-05-26 |
United States Patent
Application |
20050109022 |
Kind Code |
A1 |
Nagaoka, Daiji ; et
al. |
May 26, 2005 |
Exhaust gas purifying method and exhaust gas purifying system
Abstract
In an exhaust gas purifying system (1) for applying NOx
purification by a NOx occlusion reduction type catalyst (42) and PM
purification by a DPF (41) to the exhaust gas of an internal
combustion engine, when it is judged that both regeneration of the
DPF (41) and sulfur purge of the NOx occlusion reduction type
catalyst (42) are necessary, the DPF regeneration control for
raising the temperature of the DPF (41) is performed and the sulfur
purge control for decreasing the oxygen concentration of the
exhaust gas flowing into the NOx occlusion reduction type catalyst
(42) is intermittently repeated. Thereby, it is possible to
efficiently purge the sulfur accumulated in the NOx occlusion
reduction type catalyst while preventing deterioration of fuel
efficiency and discharge of NOx, HC, and CO into atmospheric
air.
Inventors: |
Nagaoka, Daiji;
(Fujisawa-shi, JP) ; Gabe, Masashi; (Fujisawa-shi,
JP) |
Correspondence
Address: |
STAAS & HALSEY LLP
SUITE 700
1201 NEW YORK AVENUE, N.W.
WASHINGTON
DC
20005
US
|
Assignee: |
Isuzu Motors Limited
Tokyo
JP
|
Family ID: |
34463763 |
Appl. No.: |
10/969847 |
Filed: |
October 22, 2004 |
Current U.S.
Class: |
60/297 ; 60/276;
60/295; 60/301 |
Current CPC
Class: |
B01D 53/9409 20130101;
Y02T 10/12 20130101; F02D 41/029 20130101; F02D 41/0275 20130101;
F02D 2200/0818 20130101; Y02T 10/26 20130101; F02D 41/0245
20130101; B01D 53/9495 20130101; F02D 41/028 20130101 |
Class at
Publication: |
060/297 ;
060/276; 060/295; 060/301 |
International
Class: |
F01N 003/00; F01N
003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 21, 2003 |
JP |
JP2003-392528 |
Claims
What is claimed is:
1. An exhaust gas purifying method using an exhaust gas purifying
system having a DPF and an NOx occlusion reduction type catalyst in
order from the upstream side in an exhaust passage of an engine and
a control means; said control means comprising; an exhaust gas
component detection means to detect the oxygen concentration and
NOx concentration of the exhaust gas passing through the NOx
occlusion reduction type catalyst; a DPF regeneration control means
for controlling regeneration of the DPF; a NOx catalyst
regeneration control means for recovering the NOx occlusion ability
of the NOx occlusion reduction type catalyst; and a sulfur purge
control means for controlling the sulfur purge of the NOx occlusion
reduction type catalyst; wherein, when it is judged that both
regeneration of the DPF and sulfur purge of the NOx occlusion
reduction type catalyst are necessary, the DPF regeneration control
for raising the temperature of the DPF is performed and the sulfur
purge control to decrease the oxygen concentration in the exhaust
gas flowing into the NOx occlusion reduction type catalyst is
intermittently repeated.
2. The exhaust gas purifying method according to claim 1, wherein
the air-fuel ratio state of the exhaust gas flowing into the NOx
occlusion reduction type catalyst is brought into a stoichiometric
air-fuel ratio state in the sulfur purge control.
3. An exhaust gas purifying system having a DPF and a NOx occlusion
reduction arranged in order from the upstream side in an exhaust
passage of an engine, and a control means; said control means
comprising; an exhaust gas component detection means to detect the
oxygen concentration and NOx concentration of the exhaust gas
passing through the NOx occlusion reduction type catalyst: a DPF
regeneration control means for controlling regeneration of the DPF;
a NOx catalyst regeneration control means for recovering the NOx
occlusion ability of the NOx occlusion reduction type catalyst; and
a sulfur purge control means for controlling the sulfur purge of
the NOx occlusion reduction type catalyst; wherein when it is
judged that both regeneration of the DPF and sulfur purge of the
NOx occlusion reduction type catalyst are necessary, said control
means performs the DPF regeneration control for raising the
temperature of the DPF and executes intermittently the sulfur purge
control to decrease the oxygen concentration in the exhaust gas
flowing into the NOx occlusion reduction type catalyst,.
4. The exhaust gas purifying system according to claim 3, wherein
the air-fuel ratio state of the exhaust gas flowing into the NOx
occlusion reduction type catalyst is brought into a stoichiometric
air-fuel ratio state in the sulfur purge control.
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an exhaust gas purifying
method and an exhaust gas purifying system for purifying NOx by a
NOx occlusion reduction type catalyst and purifying PM by a
DPF.
[0002] Legal restriction on discharge quantities of particulate
matter (hereafter referred to as PM) and NOx (nitrogen oxide) is
enforced year by year together with legal restriction on discharge
quantities of CO (carbon monoxide) and HC (carbon hydride). Thus,
only improvement of an engine cannot manage a restriction value for
the enforcement of the restriction. Therefore, techniques are
adopted which reduces these matters discharged from an engine by
mounting an exhaust gas control system.
[0003] A filter referred to as a diesel particulate filter
(hereafter referred to as DPF) is developed for the PM and many NOx
purifying catalysts are developed for NOx.
[0004] When purifying PM and NOx simultaneously, it is impossible
to avoid the NOx flowing out by the DPF alone and to burn a soot
component in PM by the NOx occlusion reduction type catalyst alone.
Therefore, as disclosed in Japanese Patent Laid-Open No.
1997-53442, it is required to combine the catalyst with a DPF or
integrate the NOx purifying ability of the NOx occlusion reduction
type catalyst with the PM purifying ability of the DPF. Moreover,
it is required to combine both in order to purify the NOx generated
in the time of regeneration of the DPF.
[0005] This DPF frequently uses a wall-flow-type ceramic honeycomb
structure mainly containing cordierite and silicon carbide. The
wall-flow-type DPF is formed by having a plurality of cells
(through-holes) divided by porous partitions. Moreover, the DPF is
alternately closed like a checkered pattern at the exhaust gas
inlet-side end and exhaust gas exit-side end. Furthermore, the
exhaust gas passes through the porous partitions of the cells when
it moves from a cell whose upstream side is opened and whose
downstream side is closed to the next cell whose upstream side is
closed and whose downstream side is opened. When the exhaust gas
passes through the porous partitions, PM in the exhaust gas is
caught in the partition portion and the exhaust gas is
purified.
[0006] However, when the PM is accumulated in the porous partitions
of the cells, clogging occurs. Therefore, the ventilation
resistance increases, and the collection efficiency of the PM is
deteriorated. Therefore, to remove the PM accumulated in the porous
partitions of the DPF by burning, when it is judged that the
quantity of the collected PM exceeds a predetermined accumulated
quantity, the temperature of the DPF is raised to the PM combustion
start temperature or higher to remove the PM by burning.
[0007] The method for raising the temperature of a DPF to remove PM
by burning includes an exhaust gas temperature raising method by
performing post injection in the control of injecting fuel into a
cylinder, an exhaust gas temperature raising method by directly
injecting hydrocarbon (HC) into an exhaust passage, and a
current-carrying-heating method using an electric heater set in a
DPF.
[0008] Furthermore, in order to remove PM by burning even in a
state where the exhaust gas temperature is comparatively low, a
continuously regenerating type DPF is developed and proposed which
is constituted by combining an oxidation catalyst or the like with
the DPF. The DPF can remove the PM by burning at a comparatively
low temperature. However, in a state where an exhaust gas
temperature is low and clogging of the DPF progresses, exhaust gas
temperature raising control such as an intake-air throttling is
performed to temporarily raise the exhaust gas temperature in order
to remove the collected PM by burning.
[0009] A NOx occlusion reduction type catalyst is one of the NOx
purifying catalysts. This catalyst shows a NOx occlusion ability,
and a NOx release and purification ability of depending on the 02
(oxygen) concentration in the exhaust gas. In the NOx occlusion
reduction type catalyst, a catalyst metal having an ability to
oxidize NOx and a NOx occlusion material having an ability to
occlude NOx are supported on a porous catalyst coat layer such as
alumina (Al.sub.2O.sub.3). The catalyst metal is formed by platinum
(Pt), palladium (Pd) and so on. The NOx occlusion material is
formed by any one or several of alkaline metals, alkaline earth
metals, rare earths and the like. As alkaline metals includes
sodium (Na), potassium (K), cesium (Cs), and so on. The
alkaline-earth metals include calcium (Ca), barium (Ba) and so on.
The rare-earths include yttrium (Y), lanthanum (La) and so on.
[0010] First, in the case of an exhaust gas condition in which
O.sub.2 (oxygen) concentration in the exhaust gas is high (lean
air-fuel ratio (air-fuel ratio) state) as in the normal operational
state of a diesel engine, lean-burn gasoline engine, NO (nitrogen
monoxide) is oxidized by O.sub.2 contained in the exhaust gas as a
result of the oxidizing ability of the catalyst metal to become
NO.sub.2 (nitrogen dioxide). Since the NO.sub.2 is occluded by the
NOx occlusion material in the form of nitrate, the exhaust gas is
thus purified.
[0011] However, when this occlusion of the NOx continues, the NOx
occlusion material such as barium is changed to nitrate.
Accordingly, the NOx occlusion material is gradually saturated to
lose the ability for occluding NOx. To avoid such situation,
over-rich combustion is performed by changing operation conditions
of the engine to generate the exhaust gas (rich spike gas) having a
low O.sub.2 concentration, high CO concentration, and high exhaust
gas temperature and supply the exhaust gas to the catalyst.
[0012] In the rich/air ratio state of the exhaust gas, the NOx
occlusion material changed to nitrate by occluding NO.sub.2
releases the occluded NO.sub.2 and returns to the original
substance such as barium. Because O.sub.2 is not present in this
exhaust gas, the released NO.sub.2 is reduced on the catalyst metal
by using CO, HC, and H.sub.2 in the exhaust gas as reducers. Thus,
the NOx is converted into N.sub.2, H.sub.2O, and CO.sub.2, and
purified.
[0013] The NOx occlusion reduction type catalyst has a problem in
that sulfur (sulfur component) in fuel is accumulated in the NOx
occlusion material, and the NOx purifying efficiency is
deteriorated as the operation of the engine continues. Therefore,
as disclosed in Japanese Patent Laid-Open No. 2000-192811, it is
required to perform sulfur purge control (desulfurization control)
by keeping the exhaust gas flowing into the catalyst in the
condition of a high temperature and a rich air-fuel ratio
atmosphere. Though different depending on the types of the catalyst
to be used, the temperature of the exhaust gas is set higher than
approximately 600.degree. C. to 650.degree. C.
[0014] In the case of the sulfur purge control of a diesel engine,
the rich air-fuel ratio state is realized by reducing the exhaust
gas volume through intake-air throttling or through a large
quantity of EGR as well as by a post injection or by directly
adding light oil to an exhaust pipe. Sulfur purge is accelerated by
bringing the exhaust gas into the rich air-fuel ratio state and
raising the temperature of the catalyst by the oxidation activation
reaction heat of the catalyst.
[0015] However, the sulfur purge for recovering the NOx occluding
ability of the catalyst by increasing the sulfur purge quantity has
the following problems.
[0016] Because the oxygen concentration in the exhaust gas is very
low under a rich air-fuel ratio state, the time required to raise
the temperature of the catalyst up to the temperature at which the
sulfur purge can be realized becomes very long. Therefore, the fuel
consumption amount during that time is increased, and the fuel
efficiency is deteriorated. Moreover, the denser is an air-fuel
ratio sate of the exhaust gas, the more a sulfur purge quantity
increases. However, when performing such a dense rich air-fuel
ratio state operation, there are problems that the fuel efficiency
is extremely deteriorated, and that exhaust gas components such as
HC, CO are generated in a large quantity. Moreover, when the oxygen
concentration in exhaust gas becomes 0% at a high temperature, a
problem occurs that hydrogen generated by a hydrogen generation
reaction is combined with sulfur to change harmful hydrogen sulfide
(H.sub.2S).
[0017] The sulfur purge has the two problems; the first problem is
to easily discharge purged sulfur in large quantity in an initial
period of the sulfur purge control, and the second problem is one
that a long time holding of the air-fuel ratio state of the exhaust
gas in a stoichiometric air-fuel ratio state is difficult in the
diesel engine.
[0018] Therefore, sulfur purge control sets a rich air-fuel ratio
state when a catalyst temperature rises to a temperature at which
sulfur separation can be made or higher. However, it is preferable
to set the rich air-fuel ratio state in a minimum time.
BRIEF SUMMARY OF THE INVENTION
[0019] It is an object of the present invention to provide an
exhaust gas purifying method and an exhaust gas purifying system
capable of efficiently purging the sulfur accumulated in a NOx
occlusion reduction type catalyst while preventing the fuel
efficiency from deteriorating and NOx, HC, and CO from being
discharged into atmosphere, in an exhaust gas purifying system
constituted by combining the NOx purifying ability of a NOx
occlusion reduction type catalyst with the PM purifying ability of
a DPF.
[0020] In order to achieve the above described object, the exhaust
gas purifying method of the invention using an exhaust gas
purifying system having a DPF and a NOx occlusion reduction type
catalyst arranged in order from the upstream side in an exhaust
passage of an engine, and the control means which is comprising an
exhaust gas component detection means to detect the oxygen
concentration and NOx concentration of the exhaust gas passing
through the NOx occlusion reduction type catalyst, a DPF
regeneration control means for controlling regeneration of the DPF,
a NOx catalyst regeneration control means for recovering the NOx
occlusion ability of the NOx occlusion reduction type catalyst, and
a sulfur purge control means for controlling the sulfur purge of
the NOx occlusion reduction type catalyst, wherein, when it is
judged that both regeneration of the DPF and sulfur purge of the
NOx occlusion reduction type catalyst are necessary, the DPF
regeneration control for raising the temperature of the DPF is
performed and the sulfur purge control to decrease the oxygen
concentration in the exhaust gas flowing into the NOx occlusion
reduction type catalyst is intermittently repeated.
[0021] This sulfur purge control is not set to the one-time period
of sulfur purge control to be continued until the sulfur purge is
completed. But the one-time period of the sulfur purge control is
set to one several-th of the time required to complete the sulfur
purge (e.g. 180 s to 300 s) or 2 s to 60 s and the short period
sulfur purge is repeatedly performed until the sulfur purge is
completed at predetermined time intervals (equal intervals or
unequal intervals). It is allowed to set the predetermined time
interval to a constant time. For example, it is allowed not to
perform the sulfur purge control when the temperature of a NOx
occlusion reduction type catalyst is equal to or lower than a
predetermined temperature but to change the predetermined time
interval in accordance with the temperature of the NOx occlusion
reduction type catalyst when the temperature of it is higher than
the predetermined temperature. It is possible to predetermine the
one-time period of the sulfur purge by considering the conditions
in which sulfur purge can be efficiently performed. These
conditions are previously obtained through experiments and the
like.
[0022] Thereby, it is avoided that a lot of sulfur is released and
discharged at the beginning of sulfur purge control. Moreover, the
time for a stoichiometric air-fuel ratio state which is difficult
to keep for a long time in the case of a diesel engine, is
decreased. Furthermore, it is possible to avoid a lot of sulfur
from being temporarily produced in accordance with the intermittent
sulfur purge control. Furthermore, the slip (discharge to
atmospheric air) of HC, CO, H.sub.2S and the deterioration of
drivability are constrained.
[0023] Furthermore, it is preferable to determine the air-fuel
ratio state of exhaust gas in the sulfur purge control in
accordance with the air-fuel ratio state of the exhaust gas flowing
into the NOx occlusion reduction type catalyst. It is assumed that
the air-fuel ratio state of the exhaust gas is a rich air-fuel
ratio or preferably, a stoichiometric air-fuel ratio (theoretical
air-fuel ratio).
[0024] That is, in the sulfur purge control, the air-fuel ratio
state of the exhaust gas flowing into the NOx occlusion reduction
type catalyst is brought into a stoichiometric air-fuel ratio
state.
[0025] Moreover, an exhaust gas purifying system for achieving the
above described object having a DPF and a NOx occlusion reduction
type catalyst arranged in order from the upstream side in an
exhaust passage of an engine, and the control means which is
comprised of an exhaust gas component detection means to detect the
oxygen concentration and NOx concentration of the exhaust gas
passing through the NOx occlusion reduction type catalyst, a DPF
regeneration control means for controlling regeneration of the DPF,
a NOx catalyst regeneration control means for recovering the NOx
occlusion ability of the NOx occlusion reduction type catalyst, and
a sulfur purge control means for controlling the sulfur purge of
the NOx occlusion reduction type catalyst, wherein when it is
judged that both regeneration of the DPF and sulfur purge of the
NOx occlusion reduction type catalyst are necessary, the control
means executes intermittently the sulfur purge control to decrease
the oxygen concentration in the exhaust gas flowing into the NOx
occlusion reduction type catalyst, performing DPF regeneration
control for raising the temperature of the DPF.
[0026] The air-fuel ratio state of the exhaust gas flowing into the
NOx occlusion reduction type catalyst is preferably brought into a
stoichiometric air-fuel ratio state in the sulfur purge
control.
[0027] According to an exhaust gas purifying method and an exhaust
gas purifying system of the present invention, DPF regeneration
according to forcible combustion of PM and sulfur purge of a NOx
occlusion reduction type catalyst are simultaneously performed in
an exhaust gas purifying system used by combining a DPF with a NOx
occlusion reduction type catalyst. Therefore, it is possible to
raise the temperature of the NOx occlusion reduction type catalyst
by using the heat generated due to forcible combustion of the PM.
Therefore, it is possible to minimize the deterioration of fuel
efficiency.
[0028] Moreover, by intermittently repeating the sulfur purge
control, it is possible to prevent a lot of sulfur from being
released and discharged at the time of the sulfur purge control and
eliminate the necessity for keeping the air-fuel ratio state of
exhaust gas in a stoichiometric air-fuel ratio state for a long
time.
[0029] Furthermore, because the sulfur purge control for bringing
the air-fuel ratio state of exhaust gas into a stoichiometric
air-fuel ratio state is intermittently performed, it is possible to
efficiently perform sulfur purge while restraining slip (discharge
to atmospheric air) of HC, CO, H.sub.2S, and the like while
preventing the drivability form deterioration.
BRIEF DESCRIPTION OF THE DRAWINGS
[0030] FIG. 1 shows a constitution of an exhaust gas purifying
system of an embodiment of the present invention;
[0031] FIG. 2 shows a constitution of an exhaust gas purifying
apparatus of the first embodiment of the present invention;
[0032] FIG. 3 shows a constitution of an exhaust gas purifying
apparatus of the second embodiment of the present invention;
[0033] FIG. 4 shows a constitution of an exhaust gas purifying
apparatus of the third embodiment of the present invention;
[0034] FIG. 5 shows a constitution of an exhaust gas purifying
apparatus of the fourth embodiment of the present invention;
[0035] FIG. 6 shows a control flow for a sulfur purge of an exhaust
gas purifying method of an embodiment of the present invention;
and
[0036] FIG. 7 shows a time series of the excess air factor,
differential pressure between the front and the rear of a DPF, the
temperature of a NOx occlusion reduction type catalyst converter, a
NOx concentration, and a SO.sub.2 concentration of an embodiment
using a control flow for a sulfur purge of an exhaust gas purifying
method of an embodiment of the present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0037] An exhaust gas purifying method and an exhaust gas purifying
systems of embodiments of the present invention are described below
by referring to the accompanying drawings.
[0038] FIG. 1 shows a constitution of the exhaust gas purifying
system 1 of the embodiment. The exhaust gas purifying system 1 is
constituted an exhaust gas purifying apparatus 40A constituted by
arranging an oxidation catalyst (DOC) 41a, a DPF 41b, and a NOx
occlusion reduction type catalyst converter 42 in order from the
upstream side of an exhaust passage 20 in an engine (internal
combustion engine) E. Moreover, a continuously regenerating type
DPF 41 is constituted of the upstream-side oxidation catalyst 41a
and the downstream-side DPF 41b.
[0039] The oxidation catalyst 41a is formed by a monolithic
catalyst having a lot of polygonal cells made of structural
material of cordierite, SiC (silicon carbide), or stainless steel.
A catalyst coat layer occupying the surface area is present in
inner walls of the cells to make the large surface support a
catalyst metal such as platinum or vanadium and the ability of
catalyst is generated. Thereby, it is possible to change NO in the
exhaust gas to NO.sub.2 by an oxidation reaction
(NO+O.fwdarw.NO.sub.2).
[0040] Moreover, the DPF 41b can be formed by a monolith-honeycomb
wall-flow filter obtained by alternately sealing inlets and exits
of porous-ceramic honeycomb channels or a felt-like filter obtained
by laminating inorganic fibers of alumina or the like at random.
Such filter collects the PM in the exhaust gas. By combining the PM
with the upstream front-stage oxidation catalyst 41a, the collected
PM is burned by NO.sub.2 having a high oxidative power and
removed.
[0041] The NOx occlusion reduction type catalyst converter 42 is
formed by a monolithic catalyst similarly to the oxidation catalyst
41a. A catalyst coat layer is formed on the support body such as
aluminum oxide or titanium oxide to make the catalyst coat layer
support a noble metal such as platinum and a NOx occlusion material
(NOx occlusion substance) such as barium.
[0042] The NOx occlusion reduction type catalyst converter 42
purifies the NOx in the exhaust gas by occluding the NOx in the
exhaust gas when oxygen concentration in the exhaust gas state is
high (lean air-fuel ratio state). The NOx occlusion reduction type
catalyst converter 42 releases the occluded NOx and reduces the
released NOx when oxygen concentration in the exhaust gas is low or
zero (rich air-fuel ratio state). Thereby, it is prevented that NOx
is discharged into the atmosphere.
[0043] The first temperature sensor 51 and the second temperature
sensor 52 are provided on the upstream side and the downstream side
of the DPF 41b respectively. Furthermore, the fist exhaust gas
concentration sensor 53 and the second exhaust gas concentration
sensor 54 are provided on the front and the rear of the NOx
occlusion reduction type catalyst converter 42, that is, nearby the
inlet and the exit of the exhaust gas purifying apparatus 40A in
FIG. 1. The exhaust gas concentration sensors 53 and 54 are the
integrator sensors in which a .lambda. (excess air factor) sensor,
a NOx concentration sensor, and O.sub.2 concentration sensor are
integrated. Moreover, to estimate a deposition quantity of PM, a
differential pressure sensor 55 for detecting an exhaust
differential pressure .DELTA.P between the front and the rear of
the DPF is provided with a conduction pipe connected to the front
and the rear of the DPF 41b (FIG. 1) or the front and the rear of
the exhaust gas purifying apparatus 40A (FIG. 2).
[0044] Output values of these sensors are input to a control unit
(ECU: engine control unit) 50 which performs the overall control of
operations of the engine E and performs the regeneration control of
the continuously regenerating type DPF 41 and the recovery control
of the NOx purification capacity of the NOx occlusion reduction
type catalyst converter 42. Moreover, a common-rail
electronic-control fuel-injection system for fuel injection of the
engine E, a throttle valve 15, an EGR valve 32, and the like are
controlled depending on control signals output from the control
unit 50.
[0045] The control unit 50 calculates a NOx purifying rate RNOx
(=1.0-CNOx2/CNOx1) based on the values CNOx1 and CNOx2 detected by
the first and second exhaust gas concentration sensors 53 and 54.
Furthermore, the PM accumulation quantity of the DPF 41b is
estimated based on the differential pressure .DELTA.P detected by
the differential pressure sensor 55 or the like.
[0046] In the exhaust gas purifying system 1, air A passes through
an air cleaner 11 and a mass air flow (MAF) sensor 12, a compressor
13a of a turbocharger 13, an intercooler 14, and a throttle valve
15 in an intake passage 10 and enters into the cylinder of the
engine through an intake manifold 16. The quantity of the air A is
adjusted by a throttle valve 15.
[0047] Moreover, the exhaust gas G generated in the cylinder flows
out from an exhaust manifold 21 and drives a turbine 13b of the
turbocharger 13 in an exhaust passage 20. And the exhaust gas G
passes through the exhaust gas purifying apparatus 40A and a
not-illustrated silencer to discharge into the atmosphere. Then,
the exhaust gas G becomes purified exhaust gas Gc in the exhaust
gas purifying apparatus 40A.
[0048] Furthermore, some of the exhaust gas G passes through an EGR
cooler 31 of an EGR passage 30 and an EGR valve 32 as ERG gas. This
gas is re-circulated into the intake manifold 16. The quantity of
EGR gas Ge is controlled by an EGR valve 32.
[0049] FIG. 2 shows the exhaust gas purifying apparatus 40A. FIGS.
3 and 4 show constitutions of the exhaust gas purifying apparatuses
40B and 40C of other embodiments. The exhaust gas purifying
apparatus 40B in FIG. 3 is constituted of the oxidation catalyst
41a and the DPF 43 supporting a NOx occlusion reduction type
catalyst. The exhaust gas purifying apparatus 40C in FIG. 4 is
constituted of the oxidation catalyst 41a and the DPF with the
catalyst 44 supporting the NOx occlusion reduction type catalyst.
The DPF with the catalyst includes the DPF supporting an oxidation
catalyst and the DPF supporting an oxidation catalyst and the PM
oxidation catalyst.
[0050] The PM oxidation catalyst is made of the oxide of cerium
(Ce) or the like. In the case of the catalyst-carrying filter
carrying both of the PM oxidation catalyst and the oxidation
catalyst, PM is oxidized depending on a reaction
(4CeO.sub.2+C.fwdarw.2Ce.sub.2O.sub.3+CO.sub.2,
2Ce.sub.2O.sub.3+O.sub.2.fwdarw.4CeO.sub.2, or the like) using
O.sub.2 in the exhaust gas in the catalyst-carrying filter at a low
temperature (between 300.degree. C. and 600.degree. C.). The PM is
oxidized by O.sub.2 in the exhaust gas at a temperature higher than
the temperature (600.degree. C. or higher) at which the PM is
burned by O.sub.2 in the exhaust gas.
[0051] Moreover, there are apparatuses as an exhaust gas purifying
apparatus having no oxidation catalyst at the most upstream side,
such as the exhaust gas purifying apparatus constituted of a DPF
not having a catalyst but having only a filter and a NOx occlusion
reduction type catalyst converter; the exhaust gas purifying
apparatus constituted of a DPF with a catalyst carrying an
oxidation catalyst and a NOx occlusion reduction type catalyst
converter; and the exhaust gas purifying apparatus DPF with a
catalyst supporting both an oxidation catalyst and a PM oxidation
catalyst and a NOx occlusion reduction type catalyst converter.
[0052] In short, any exhaust gas purifying apparatus may be used as
the exhaust gas purifying apparatus of the present invention as
long as the apparatus performs NOx purification by the NOx
occlusion reduction type catalyst and PM purification by the DPF to
the exhaust gas of the engine.
[0053] Moreover, the control unit of the exhaust gas purifying
system 1 is built in the control unit 50 of the engine E. The
control unit 50 controls operations of the engine E and the exhaust
gas purifying system 1. As shown in FIG. 5, the control unit of the
exhaust gas purifying system 1 is constituted of a control means Cl
of the exhaust gas purifying system having an exhaust gas component
detecting means C10, a DPF control means C20, and a NOx occlusion
reduction type catalyst control means C30.
[0054] The exhaust gas component detecting means C10 is a means for
detecting the oxygen concentration (or excess air factor .lambda.)
and the NOx concentration in the exhaust gas and is constituted of
the first and second exhaust gas concentration sensors 53 and
54.
[0055] The DPF control means C20 is constituted of a PM
accumulation quantity calculating means C21, a DPF regeneration
start judgment means C22, and a DPF regeneration control means
C23.
[0056] In the DPF control means C20, is performed the following.
The PM accumulation quantity calculating means C21 calculates the
PM accumulation quantity of the DPF 41b based on the differential
pressure .DELTA.P detected by the differential pressure sensor 55,
or the like. The DPF regeneration start judgment means C22 judges
whether the clogging state of the DPF 41b exceeds a predetermined
clogging state depending on whether the PM accumulation quantity
exceeds a predetermined determination value. When DPF regeneration
start is judged, the DPF regeneration control means C23 raises an
exhaust gas temperature through post injection, EGR control, and
the like, and the DPF 41 is regenerated.
[0057] The NOx occlusion reduction type catalyst control means C30
is a means for regenerating the NOx occlusion reduction type
catalyst converter 42 and controlling a sulfur purge and is
constituted of a regeneration start judgment means of NOx catalyst
C31, a NOx catalyst regeneration control means C32, a sulfur purge
start judgment means C33, and a sulfur purge control means C34.
[0058] The NOx occlusion reduction type catalyst control means C30
calculates a NOx purification rate RNOx based on the NOx
concentration detected by the exhaust gas component detecting means
C10. Moreover, when the NOx purification rate RNOx becomes lower
than a predetermined determination value, the means C30 judges that
regeneration of the NOx catalyst is started. An exhaust gas state
is brought into a predetermined rich air-fuel ratio state and a
predetermined temperature range (between approximately 200.degree.
C. and 600.degree. C. though depending on a catalyst) by performing
post injection in the fuel injection control of the engine E, EGR
control, and intake-air throttling control by the NOx catalyst
regeneration control means C32. Thereby, the NOx purification
capacity, that is, the NOx occlusion capacity is recovered and the
NOx catalyst is regenerated. Moreover, the sulfur purge is
performed by the sulfur purge start judgment means C33 and the
sulfur purge control means C34.
[0059] In such exhaust gas purifying system 1, the exhaust gas
purifying method of NOx occlusion reduction type catalyst of the
present invention is performed in accordance with the sulfur purge
control flow shown in FIG. 6.
[0060] The control flow shown in FIG. 6 is a control flow relating
to the sulfur purge of the NOx occlusion reduction type catalyst.
The control flow is shown as a flow for being repeatedly called
from the control flow of the whole exhaust gas purifying system
together with the control flow relating to the regeneration of the
NOx occlusion capacity of the NOx occlusion reduction type catalyst
converter 42 or the regeneration control flow of the DPF 41b, or
the like. The above control flow is performed for judging the
necessity of the sulfur purge and the DPF regeneration, and if
required, the sulfur purge control is intermittently performed at
the same time as the regeneration control of the DPF.
[0061] When this control flow starts, the sulfur quantity occluded
in the catalyst 42 is calculated in accordance with the fuel
consumption and the sulfur quantity contained in fuel by the sulfur
purge start judgment means C33 and integrated to calculate an
accumulated sulfur quantity Ssp. Moreover, the accumulated PM
quantity PMst of the DPF 41b is calculated in accordance with the
pressure difference .DELTA.P detected by the PM accumulation
quantity calculating means C21.
[0062] Then, in the next step S12, it is judged by the sulfur purge
start judgment means C33 whether sulfur purge is necessary. In the
case of this judgment, it is judged that sulfur purge is necessary
when the accumulated sulfur quantity Ssp becomes larger than a
predetermined limit value Sso0.
[0063] Moreover, in step S12, it is judged by the DPF regeneration
start judgment means C22 whether DPF regeneration is necessary. In
the case of this judgment, when the accumulated PM quantity PMs
becomes larger than the predetermined judgment value PMs0 for
regeneration start, it is judged that DPF regeneration is
necessary.
[0064] When it is judged in this step S12 that sulfur purge is not
necessary or that DPF regeneration is not necessary, the DPF
regeneration control or sulfur purge control is not performed and
return is performed.
[0065] Moreover, when it is judged that sulfur purge is necessary
and DPF regeneration is also necessary, step S13 is started.
[0066] In the case of the exhaust gas temperature raising control
for regenerating the DPF in step S13, the exhaust gas temperature
is raised by performing post injection in accordance with the fuel
injection of an engine or cutting EGR and control is performed so
that the exhaust gas temperature flowing into the DPF 41b enters a
PM self-ignition region and temperature region free from abnormal
combustion (approx. 500.degree. C.). In this temperature control,
feedback control is performed so that the temperature of exhaust
gas flowing into the NOx occlusion reduction type catalyst
converter 42 becomes a temperature capable of performing sulfur
purge (approx. 600.degree. C. to 650.degree. C.) or higher while
monitoring the temperature detected by the temperature sensor 52
and adjusting the fuel quantity of post injection.
[0067] The PM accumulated in the DPF 41b is forcibly burned and
removed in accordance with the exhaust gas temperature rise.
Moreover, temperatures of the DPF 41b, the exhaust gas, and NOx
occlusion reduction type catalyst converter 42 are raised by the
combustion heat of the PM and the oxygen concentration of the
exhaust gas passing through the DPF 41b is decreased.
[0068] Furthermore, after performing the DPF regeneration control
in this step S13 for a predetermined time tdpf (e.g. 10 min to 15
min), it is judged in step S14 whether sulfur purge is completed.
When the sulfur purge is not completed in this judgment, step S15
is started to perform the sulfur purge control but when the sulfur
purge is completed, step S16 is started.
[0069] Whether the sulfur purge is completed in step S14 is judged
in accordance with whether a sulfur purge quantity integrated value
Spu calculated in the next step S15 exceeds an accumulated sulfur
quantity Ssp calculated in step S11 (or predetermined limit value
Ssp0). The sulfur purge quantity integrated value Spu is set to
zero as an initial value in step S11 or calculated in the following
step S15. Moreover, when the sulfur purge quantity integrated value
Spu exceeds the accumulated sulfur value Ssp (or predetermined
limit value Ssp0), it is judged that sulfur purge is completed and
step S16 is started without performing the sulfur purge control in
the next step S15.
[0070] Then, the sulfur purge control in step S15 performs feedback
control of multistage injection including pilot injection and post
injection so that a predetermined oxygen concentration (or
excessive air rate .lambda.) is obtained by monitoring an oxygen
concentration (or excessive air rate .lambda.) while controlling
post injection, intake air throttle, and EGR. The oxygen
concentration (or excessive air rate .lambda.) is an oxygen
concentration (or excessive air rate .lambda.) of exhaust gas
flowing into the NOx occlusion reduction type catalyst converter
42, which is a value detected by a second exhaust gas concentration
sensor 54. The oxygen concentration (or excessive air rate
.lambda.) of the control target is assumed as a rich air-fuel
ratio, preferably as a stoichiometric air-fuel ratio (theoretical
air-fuel ratio). The air-fuel ratio state of the exhaust gas
flowing into the NOx occlusion reduction type catalyst converter 42
is brought into a rich air-fuel ratio state, preferably to a
stoichiometric air-fuel ratio state by the air-fuel ratio control
to efficiently perform sulfur purge.
[0071] In the case of the air-fuel ratio control, oxygen is
consumed by oxidizing HC and CO by an upstream-side oxidation
catalyst 41a or PM by the DPF 41b. Therefore, it is not necessary
to realize a complete rich air-fuel ratio or complete
stoichiometric air-fuel ratio immediately after an exhaust manifold
21 of an engine E. That is, even if the excessive air ratio
.lambda. is a shallow rich state of 1.01 to 1.02, it is possible to
bring the NOx occlusion reduction type catalyst converter 42 into a
rich atmosphere or stoichiometric atmosphere in which sulfur can be
purged. Therefore, it is only necessary to perform control so that
the oxygen concentration (or excessive air rate .lambda.) detected
by a second exhaust gas concentration sensor 54 closest to the
upstream side of the NOx occlusion reduction type catalyst
converter 42 becomes a rich air-fuel ratio, preferably a
stoichiometric air-fuel ratio. Thereby, it is possible to restrain
deterioration of fuel efficiency, discharge quantity of HC or CO to
the atmospheric air, drivability (ride quality), and the like.
[0072] After performing the sulfur purge control for a
predetermined time tsp, the sulfur purge quantity integrated value
Spu is calculated from a previously input sulfur purge quantity map
and the like in accordance with a catalyst temperature Ts
calculated from temperatures detected by the first and second
temperature sensors 51 and 52, and engine speed Ne showing an
operation state and load Q of an engine. The catalyst temperature
Ts is calculated from temperatures detected by the first and second
temperature sensors 51 and 52. The sulfur purge quantity map is
brought into map data so that a sulfur purge quantity can be
calculated from the engine speed Ne and load Q by using the
catalyst temperature Ts and the like as a parameter. The sulfur
purge quantity map is determined by experimentally obtaining the
sulfur purge quantity for unit time by using catalyst temperature,
time, space speed (exhaust gas flow rate), and S/V ratio as
parameters and putting the data in order.
[0073] The predetermined time tsp for performing the sulfur purge
control is set to one several-th of the time required to complete
sulfur purge (such as 180 s to 300 s) or 2 s to 60 s. Thereby, it
is possible to prevent a lot of sulfur from being purged and
discharged at the beginning of the sulfur purge control. Moreover,
in the case of a diesel engine, it is possible to decrease the time
for the stoichiometric air-fuel ratio state which is difficult to
keep for a long time. The predetermined time tsp is a value set
earlier than the control, which is obtained from a condition in
which sulfur purge can be efficiently performed previously obtained
from experiments.
[0074] In the next step S16, the accumulated PM quantity PMs of the
DPF 41b is calculated from the pressure difference .DELTA.P
detected by a pressure difference sensor 55 and the like. Then, in
the next step S17, it is judged whether sulfur purge and DPF
regeneration are completed. This judgment is performed in
accordance with whether the sulfur purge quantity integrated value
Spu exceeds the accumulated sulfur quantity Ssp (or predetermined
limit value Ssp0) and whether the accumulated PM quantity PMs
becomes equal to or less than a predetermined judgment value PMs1
for completing regeneration.
[0075] When it is judged that neither sulfur purge nor DPF
regeneration is completed, step S13 is restarted but when it is
judged that sulfur purge and DPF regeneration are both completed,
return is performed.
[0076] Therefore, steps S13 to S17 are repeated until both sulfur
purge and DPF regeneration are completed. When the both are
completed, step S18 is started. In step S18, return control to a
normal lean state is performed and return is performed after the
return control.
[0077] According to the exhaust gas purifying method and exhaust
gas purifying system 1 having the above configuration, regeneration
of the DPF 41b according to forcible combustion of PM and sulfur
purge of the NOx occlusion reduction type catalyst 42 are
simultaneously performed. Therefore, because the temperature of the
NOx occlusion reduction type catalyst can be raised by using the
heat produced due to the forcible combustion of the PM. It is
possible to minimize deterioration of fuel efficiency.
[0078] Moreover, by intermittently repeating sulfur purge control,
it is possible to prevent a lot of sulfur from being purged and
discharged at the time of the sulfur purge control. Furthermore, it
is not necessary to keep the air-fuel ratio state of exhaust gas in
a rich air-fuel ratio state or stoichiometric air-fuel ratio state
for a long time.
[0079] Furthermore, because the sulfur purge control for bringing
the air-fuel ratio state of exhaust gas into a rich air-fuel ratio
state or stoichiometric air-fuel ratio state is intermittently
performed, it is possible to efficiently perform sulfur purge while
restraining deterioration of slip of HC, CO, H.sub.2S, and the like
and drivability.
[0080] Furthermore, by bringing the air-fuel ratio state of exhaust
gas into a stoichiometric air-fuel ratio state, the above advantage
can be further exhibited.
Embodiments
[0081] FIG. 7 shows the excessive air rate .lambda., pressure
difference .DELTA.P between front and rear of a DPF, catalyst
temperature (bed temperature of NOx occlusion reduction type
catalyst converter) Tn, and concentrations of NOx and SO.sub.2 at
the downstream side of an exhaust gas purifying system when
performing sulfur purge in accordance with the control flow shown
in FIG. 6 by using the exhaust gas purifying system shown in FIG.
2.
[0082] According to FIG. 6, by intermittently repeating the sulfur
purge control for decreasing the excessive air rate .lambda.
immediately before a NOx occlusion reduction type catalyst
converter to 1.01 to 1.02 while performing the regeneration control
of a DPF, it is found that the catalyst temperature Tn rises at the
time of the sulfur purge control and SO.sub.2 is discharged.
Moreover, the quantity of the SO.sub.2 to be discharged at one time
is decreased because the sulfur purge control is kept for a short
time.
[0083] Furthermore, because the pressure difference .DELTA.P
between front and rear of the DPF is lowered, it is found that
combustion of PM is progressed. In FIG. 6, the sulfur purge control
is performed for 1 min after a 3 min stop period and this cycle is
repeated. Furthermore, in the range of FIG. 6, neither sulfur purge
nor DPF regeneration is completed.
* * * * *