U.S. patent application number 12/850080 was filed with the patent office on 2011-02-10 for exhaust gas purifying apparatus.
This patent application is currently assigned to KABUSHIKI KAISHA TOYOTA JIDOSHOKKI. Invention is credited to Yoshifumi KATO.
Application Number | 20110030350 12/850080 |
Document ID | / |
Family ID | 42985543 |
Filed Date | 2011-02-10 |
United States Patent
Application |
20110030350 |
Kind Code |
A1 |
KATO; Yoshifumi |
February 10, 2011 |
EXHAUST GAS PURIFYING APPARATUS
Abstract
An exhaust gas purification apparatus includes an oxidation
catalyst provided in a passage through which exhaust gas flows, a
urea decomposition accelerator, a selective catalytic reduction
catalyst provided downstream of the urea decomposition accelerator
and a urea water supplying device for supplying urea water to the
urea decomposition accelerator. The urea decomposition accelerator
is provided downstream end surface of the oxidation catalyst and
has at least one of hydrophilic function and hydrolytic catalytic
function.
Inventors: |
KATO; Yoshifumi; (Aichi-ken,
JP) |
Correspondence
Address: |
SUGHRUE MION, PLLC
2100 PENNSYLVANIA AVENUE, N.W., SUITE 800
WASHINGTON
DC
20037
US
|
Assignee: |
KABUSHIKI KAISHA TOYOTA
JIDOSHOKKI
Kariya-shi
JP
|
Family ID: |
42985543 |
Appl. No.: |
12/850080 |
Filed: |
August 4, 2010 |
Current U.S.
Class: |
60/286 ; 60/297;
60/299; 60/303 |
Current CPC
Class: |
F01N 3/2073 20130101;
F01N 3/103 20130101; F01N 3/105 20130101; F01N 2240/40 20130101;
F01N 2510/00 20130101; F01N 13/009 20140601; F01N 13/0097
20140603 |
Class at
Publication: |
60/286 ; 60/299;
60/303; 60/297 |
International
Class: |
F01N 9/00 20060101
F01N009/00; F01N 3/10 20060101 F01N003/10; F01N 3/035 20060101
F01N003/035 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 5, 2009 |
JP |
P2009-182741 |
Claims
1. An exhaust gas purification apparatus comprising: an oxidation
catalyst provided in a passage through which exhaust gas flows; a
urea decomposition accelerator, wherein the urea decomposition
accelerator is provided downstream end surface of the oxidation
catalyst and has at least one of a hydrophilic function and a
hydrolytic catalytic function; a selective catalytic reduction
catalyst provided downstream of the urea decomposition accelerator;
and a urea water supplying device for supplying urea water to the
urea decomposition accelerator.
2. The exhaust gas purification apparatus according to claim 1,
wherein the urea water supplying device supplies urea water toward
downstream surface of the urea decomposition accelerator.
3. The exhaust gas purification apparatus according to claim 1,
wherein the urea water supplying device is an injection valve
provided at a position that is between the urea decomposition
accelerator and the selective catalytic reduction catalyst and
closer to the urea decomposition accelerator than the selective
catalytic reduction catalyst.
4. The exhaust gas purification apparatus according to claim 1,
further comprising: a particulate matter collecting device for
capturing particulate matter contained in the exhaust gas, wherein
the particulate matter collecting device is formed integrally with
the selective catalytic reduction catalyst.
5. The exhaust gas purification apparatus according to claim 4,
wherein the particulate matter collecting device is provided
downstream of the selective catalytic reduction catalyst.
6. The exhaust gas purification apparatus according to claim 4,
further comprising: a mixer provided on upstream end surface of the
particulate matter collecting device or the selective catalytic
reduction catalyst for distributing substances in the exhaust gas
over the end surface of the particulate matter collecting device or
the selective catalytic reduction catalyst.
7. The exhaust gas purification apparatus according to claim 1,
further comprising: a casing housing the oxidation catalyst, the
urea decomposition accelerator, the selective catalytic reduction
catalyst and the urea water supplying device.
8. The exhaust emission purification apparatus according to claim
1, further comprising: an exhaust gas temperature sensor provided
upstream of the oxidation catalyst for detecting a temperature of
the exhaust gas; a first NOx sensor provided upstream of the
oxidation catalyst for detecting NOx concentration; a second NOx
sensor provided downstream of the selective catalytic reduction
catalyst for detecting NOx concentration; and a dosing control unit
electrically connected to the first and the second NOx sensors, the
exhaust gas temperature sensor and the urea water supplying device,
wherein, when the temperature detected by the exhaust gas
temperature sensor is as high as a temperature at which the
selective catalytic reduction catalyst is activated, the dosing
control unit activates the urea water supplying device to supply
urea water, and when the temperature detected by the exhaust gas
temperature sensor is under the temperature at which the selective
catalytic reduction catalyst is activated, the dosing control unit
activates the urea water supplying device to stop supplying urea
water, wherein the dosing control unit controls supply quantity of
urea water based on NOx concentrations detected by the first and
the second NOx sensors.
9. The exhaust gas purification apparatus according to claim 1,
wherein the exhaust gas purification apparatus is fixed to an
engine assembly.
10. The exhaust gas purification apparatus according to claim 1,
wherein the urea decomposition accelerator is formed by coating the
downstream end surface of the oxidation catalyst with a material
that has a hydrolytic catalytic function and a hydrophilic
function.
11. The exhaust gas purification apparatus according to claim 10,
wherein the material includes at least one of silica (SiO2),
alumina (Al2O3), ceria (CeO2), titania (TiO2) and tungsten oxide
(WO3).
Description
BACKGROUND OF THE INVENTION
[0001] The present invention relates to an exhaust gas purification
apparatus and, more specifically, to an exhaust gas purification
apparatus having a urea selective catalytic reduction (hereinafter
referred to merely as SCR) system for reducing nitrogen oxides
(NOx) in exhaust gas emitted from a diesel engine.
[0002] The urea SCR system has been developed for reducing NOx in
exhaust gas emitted from a diesel engine. The urea SCR system
employs an SCR catalyst for converting NOx into nitrogen (N2) and
water (H2O) by chemical reaction between NOx and ammonia (NH3)
generated by hydrolysis of urea water.
[0003] The SCR catalyst is provided in the exhaust passage between
the engine and the muffler. Furthermore, an oxidation catalyst and
an injection valve for injecting urea water into the exhaust gas
are provided upstream of the SCR catalyst. The oxidation catalyst
oxidizes hydrocarbons (HC) and carbon monoxide (CO) in the exhaust
gas into water (H2O) and carbon dioxide (CO2) and also promotes the
oxidation of nitrogen oxide (NO) into nitrogen dioxide (NO2).
Another oxidation catalyst is provided downstream of the SCR
catalyst for promoting the oxidation of ammonia unreacted with NOx
so as to prevent emission of the unreacted ammonia into the
atmosphere.
[0004] A diesel particulate filter (hereinafter referred to merely
as DPF) is also provided in the exhaust passage between the engine
and the muffler for reducing particulate matter (PM) such as carbon
in the exhaust gas. The exhaust gas purification apparatus
including the urea SCR system and the DPF has many components
provided between the engine and the muffler and requires a large
space for mounting of such components to a vehicle. Therefore, a
downsized urea SCR system has been proposed for facilitating the
installation of the system in the vehicle.
[0005] Published Japanese Translation 2001-511494 of PCT
International Publication discloses an exhaust gas purification
apparatus that includes a mixing device functioning as a
gas-guiding device, an injection device for injecting urea water as
a reducing agent and a catalytic device provided downstream of the
injecting device and including a hydrolytic catalytic module and an
SCR catalytic module. The hydrolytic catalytic module is provided
upstream of the SCR catalytic module in the catalytic device. The
same Published Japanese Translation discloses another exhaust gas
purification device in which a second mixing device is provided
between the injecting device and the catalytic device.
[0006] The exhaust gas purification apparatus of the above
Published Japanese Translation has accomplished the improvement of
the efficiency of chemical reaction by the catalytic module in the
catalytic device by ensuring uniform distribution of the reducing
agent in the exhaust gas with the aid of the exhaust gas flow
caused by the mixing device for reducing NOx in the exhaust gas
effectively. In addition, the exhaust gas purification apparatus
achieves reduction of the distance between the injection device and
the catalytic device and of the structural space of the apparatus.
However, for ensuring the time that is long enough for urea water
to be hydrolyzed, the distance that the injected urea water moves
before reaching catalytic device should be long so that the time
for urea water or the reducing agent to stay upstream of the
catalytic device is long enough for the hydrolysis of urea water.
When the structural space of the exhaust gas purification apparatus
is reduced, urea water is supplied to the SCR catalytic module
without being hydrolyzed sufficiently into ammonia. The exhaust gas
purification apparatus improves the efficiency of hydrolysis by
providing the hydrolytic catalytic module in the catalytic
device.
[0007] However, when the structural space of the exhaust gas
purification apparatus is reduced, it is difficult for the
apparatus to provide a distance between the hydrolytic catalytic
module and the SCR catalytic module that is long enough to ensure
the reaction time for urea water to be hydrolyzed. Therefore, if
the structural space of the exhaust gas purification apparatus
attempted to be reduced, the quantity of unreacted urea water
supplied to the SCR catalytic module without being hydrolyzed into
ammonia increases, with the result that the efficiency of the
reduction of NOx in comparison to urea water usage
deteriorates.
[0008] The present invention is directed to providing an exhaust
gas purification apparatus for improving the efficiency of the
reduction of NOx in comparison to urea water usage.
SUMMARY OF THE INVENTION
[0009] An exhaust gas purification apparatus includes an oxidation
catalyst provided in a passage through which exhaust gas flows, a
urea decomposition accelerator, a selective catalytic reduction
catalyst provided downstream of the urea decomposition accelerator
and a urea water supplying device for supplying urea water to the
urea decomposition accelerator. The urea decomposition accelerator
is provided downstream end surface of the oxidation catalyst and
has at least one of hydrophilic function and hydrolytic catalytic
function.
[0010] Other aspects and advantages of the invention will become
apparent from the following description, taken in conjunction with
the accompanying drawings, illustrating by way of example the
principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0011] The features of the present invention that are believed to
be novel are set forth with particularity in the appended claims.
The invention together with objects and advantages thereof, may
best be understood by reference to the following description of the
presently preferred embodiments together with the accompanying
drawings in which:
[0012] FIG. 1 is a schematic view showing an exhaust gas
purification apparatus according to a first embodiment of the
present invention and its associated components;
[0013] FIG. 2 is a schematic cross sectional view of the exhaust
gas purification apparatus of FIG. 1; and
[0014] FIG. 3 is a schematic cross sectional view of an exhaust gas
purification apparatus according to a second embodiment of the
present invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0015] The following will describe the embodiments of the exhaust
gas purification apparatus according to the present invention with
reference to FIGS. 1 through 3. Referring to FIGS. 1 and 2 showing
the first embodiment, the exhaust gas purification apparatus which
is designated generally by 101 and its associated components will
be described. The exhaust gas purification apparatus 101 is
employed in a vehicle equipped with a diesel engine.
[0016] Referring to FIG. 1, an engine assembly including an engine
1 and the exhaust gas purification apparatus 101 is designated
generally by reference numeral 10. The engine 1 has a plurality of
cylinders 1A each having an intake port 1B to which an intake
manifold 4 is connected for distributing intake air to the
respective cylinders 1A. The intake manifold 4 has an inlet 4A to
which an engine intake pipe 3 is connected and the engine intake
pipe 3 is further connected to a compressor housing 8A of a
turbocharger 8. The compressor housing 8A is connected to an intake
pipe 2 through which outside air is introduced.
[0017] On the other hand, an exhaust manifold 5 is connected to a
plurality of exhaust ports 1C of the engine 1 for collecting
exhaust gas emitted from the respective exhaust ports 1C. An outlet
5A of the exhaust manifold 5 is connected to a turbine housing 8B
of the turbocharger 8, to which the exhaust gas purification
apparatus 101 having a substantially cylindrical shape is connected
and disposed adjacent to a lateral side of the engine 1. The
exhaust gas purification apparatus 101 is connected to an exhaust
pipe 6, the downstream end of which is further connected to a
muffler 7. The intake pipe 2, the turbocharger 8, the engine intake
pipe 3 and the intake manifold 4 cooperate to form an intake system
of the vehicle, while the exhaust manifold 5, the turbocharger 8,
the exhaust gas purification apparatus 101, the exhaust pipe 6 and
the muffler 7 cooperates to form an exhaust system of the vehicle.
The engine 1, the engine intake pipe 3, the intake manifold 4, the
exhaust manifold 5 and the turbocharger 8 cooperate to form the
aforementioned engine assembly 10.
[0018] Referring to FIG. 2, the exhaust gas purification apparatus
101 includes a casing 11 having a substantially cylindrical shape.
The casing 11 has an upstream end face 11A to which the outlet 8B2
of the turbine housing 8B of the turbocharger 8 is connected and a
downstream end face 11B to which the upstream end 6A of the exhaust
pipe 6 is connected. The casing 11 communicates internally with the
turbine housing 8B and the exhaust pipe 6.
[0019] The cylindrical casing 11 houses therein an oxidation
catalyst layer 12 supporting an oxidation catalyst and a diesel
particulate filter (DPF) 14 as a particulate matter collecting
device disposed downstream of the oxidation catalyst layer 12 with
respect to the flow of exhaust gas in the casing 11. The oxidation
catalyst layer 12 and the DPF 14 are made in the form of a layer
extending perpendicular to the axis of a cylindrical portion 11C of
the casing 11 over the entire radial dimension of the interior of
the cylindrical portion 11C. The oxidation catalyst layer 12 and
the DPF 14 are disposed spaced apart each other thereby to form
therebetween a space 16.
[0020] The oxidation catalyst layer 12 supports thereon the
oxidation catalyst for oxidizing hydrocarbons (HC) and carbon
monoxide (CO) into water (H2O) and carbon dioxide (CO2) and also
promoting the oxidation of nitrogen monoxide (NO) into nitrogen
dioxide (NO2). The oxidation catalyst of the oxidation catalyst
layer 12 uses material such as platinum (Pt), palladium (Pd),
rhodium (Rh), silver (Ag), iron (Fe), copper (Cu), nickel (Ni),
gold (Au) or a mixture of two or more of these materials.
[0021] The DPF 14 is made of a porous material such as ceramic for
capturing particulate matter (PM) contained in the exhaust gas. The
DPF 14 has an (urea) SCR catalyst 15 as a selective catalytic
reduction catalyst supported thereon, e.g., by coating. The
selective catalytic reduction catalyst serves to promote the
chemical reaction selectively among specific chemical substances.
The SCR catalyst 15 catalyzes the reaction between nitrogen oxide
(NOx) and ammonia (NH3) thereby to reduce NOx into nitrogen (N2)
and water (H2O). Material of the SCR catalyst 15 includes an oxide
of zirconium (Zr), titanium (Ti), silicon (Si), cerium (Ce), or
tungsten (W), a complex of these oxides and a ZSM-5 type zeolite
partially replaced by a metal such as iron (Fe) and copper
(Cu).
[0022] The oxidation catalyst layer 12 supports on at least a part
of the downstream end surface 12B thereof with regard to the flow
of exhaust gas, i.e., on the surface thereof facing the DPF 14, a
hydrophilic layer 13 having a hydrophilic function and forming the
urea decomposition accelerator of the invention. The hydrophilic
layer 13 is formed by coating the end surface 12B of the oxidation
catalyst layer 12 with a catalytic material that has a hydrolytic
catalytic function for accelerating the hydrolysis and a
hydrophilic function. This catalytic material having the hydrolytic
catalytic function and the hydrophilic function includes a metal
oxide such as silica (SiO2), alumina (Al2O3), ceria (CeO2), titania
(TiO2), tungsten oxide (WO3) and the like. Material forming the
hydrophilic layer 13 is made of a single metal oxide or a
combination of the above metal oxides. The performance for
hydrolysis of the hydrophilic layer 13 can be improved by adding
silver (Ag) or platinum (Pt) other than the above metal oxides to
the material forming the hydrophilic layer 13.
[0023] An injection valve 18 that is an electromagnetic valve is
provided in the cylindrical portion 11C of the casing 11 at a
position between the oxidation catalyst layer 12 (or the
hydrophilic layer 13) and the DPF 14 (or the SCR catalyst 15).
Specifically, the position is closer to the oxidation catalyst
layer 12 (or the hydrophilic layer 13) than the DPF 14 (or the SCR
catalyst 15). The injection valve 18 forms a urea water supplying
device of the invention. The injection valve 18 is connected to a
urea water tank 19 provided in a vehicle (not shown) and operable
to inject urea water into the space 16 of the casing 11. The
injection valve 18 is provided at a position that is adjacent to
and immediately downstream of the hydrophilic layer 13 so that urea
water is injected by the injection valve 18 toward the downstream
end surface 12B of the oxidation catalyst layer 12, i.e., the
downstream surface 13B of the hydrophilic layer 13. The injection
valve 18 is electrically connected to a dosing control unit (DCU)
30 that controls the opening and closing operation of the injection
valve 18. The urea water tank 19 has an electric pump for supplying
urea water to the injection valve 18. The electric pump is
electrically connected to the DCU 30 and the pump operation is
controlled by the DCU 30.
[0024] A cylindrically-shaped mixer 17 is provided on the upstream
end surface 14A of the DPF 14 for distributing substances in the
exhaust gas uniformly over the end surface 14A. The mixer 17 has a
structure that is similar to that disclosed in Published Japanese
Translation H06-509020 of PCT international publication or Japanese
Patent Application Publication 2006-9608. The mixer disclosed in
Published Japanese Translation H06-509020 is made in the form of a
lattice that divides the gas passage into plural cells so as to
cause the gas flowing through each cell to flow spirally and also
to flow toward the adjacent cell. This helps the substances in the
exhaust gas to spread uniformly in the whole passage. On the other
hand, the mixer disclosed in Japanese Patent Application
Publication 2006-9608 has plural plates each extending
perpendicularly to the direction of gas flow, which provides
serpentine gas passage serving to distribute the substances in the
gas uniformly.
[0025] Another oxidation catalyst layer 20 that supports oxidation
catalyst for oxidizing ammonia is provided in the exhaust pipe 6
downstream of the exhaust gas purification apparatus 101. Platinum
(Pt), palladium (Pd), silver (Ag), iron (Fe), copper (Cu), nickel
(Ni), gold (Au) or the like may be employed as the material of the
oxidation catalyst of the oxidation catalyst layer 20.
[0026] An exhaust gas temperature sensor 52 is provided upstream of
the oxidation catalyst layer 12 and also downstream of the upstream
end face 11A of the casing 11 for detecting the temperature of
exhaust gas. The exhaust gas temperature sensor 52 is electrically
connected to the DCU 30 and sends detected temperature information
to the DCU 30. A first NOx sensor 51 is provided in the casing 11
at a position upstream of the exhaust gas temperature sensor 52 for
detecting the NOx concentration and a second NOx sensor 53 is
provided downstream of the downstream end face 11B of the casing
11, more specifically, at a position downstream of the oxidation
catalyst layer 20 in the exhaust pipe 6, for detecting the NOx
concentration. The first and the second NOx sensors 51, 53 are
electrically connected to the DCU 30 and send information about the
NOx concentration to the DCU 30. As described above, the exhaust
gas purification apparatus 101 having the SCR catalyst 15 and the
DPF 14 integrated together is mounted to the engine assembly 10 at
a position adjacent to the engine 1 (refer to FIG. 1).
[0027] The following will describe the operation of the exhaust gas
purification apparatus 101 according to the first embodiment and
its associated components with reference to FIGS. 1 and 2.
Referring to FIG. 1, while the engine 1 is running, outside air is
flowed into the compressor housing 8A of the turbocharger 8 through
the intake pipe 2. The air is pumped by a compressor wheel (not
shown) in the compressor housing 8A and flowed to the engine intake
pipe 3 under an increased pressure. The air is flowed into a
cylinder 1A in the engine 1 through the engine intake pipe 3 and
the intake manifold 4. Then, the air in the cylinder 1A is mixed
with fuel (light oil) supplied into the cylinder 1A and the fuel is
ignited spontaneously for combustion.
[0028] Exhaust gas produced by the combustion is discharged into
the exhaust manifold 5 through a plurality of exhaust ports 1C to
be collected by the exhaust manifold 5 and then flows into the
turbine housing 8B of the turbocharger 8. The exhaust gas flowing
through the turbine housing 8B increases rotation speed of the
turbine wheel (not shown) in the turbine housing 8B and the
compressor wheel connected to the turbine wheel and then is
discharged into the exhaust gas purification apparatus 101. After
flowing through the exhaust gas purification apparatus 101, the
exhaust gas flows through the oxidation catalyst layer 20, the
exhaust pipe 6 and the muffler 7 and then is discharged outside the
vehicle (not shown).
[0029] Referring to FIG. 2, all the exhaust gas flowed into the
exhaust gas purification apparatus 101 flows firstly through the
oxidation catalyst layer 12. While the exhaust gas flows through
the oxidation catalyst layer 12, hydrocarbons and carbon monoxide
in the exhaust gas are oxidized into carbon dioxide and water, and
part of NO is oxidized into NO2 that can be reduced easily. After
flowing through the oxidation catalyst layer 12, the exhaust gas
flows through the hydrophilic layer 13 and the mixer 17 and then
into the DPF 14 supporting the SCR catalyst 15. PM in the exhaust
gas is captured by the DPF 14.
[0030] Meanwhile, the DCU 30 activates the electric pump in the
urea water tank 19 and also opens the injection valve 18 for
injection of urea water from the injection valve 18 toward the
hydrophilic layer 13 located upstream of the space 16.
[0031] The injected urea water is adsorbed on the surface 13B of
the hydrophilic layer 13. Specifically, the urea water injected
onto the surface 13B of the hydrophilic layer 13 is dispersed in
radial directions of the cylindrical portion 11C of the casing 11
due to the hydrophilic property of the hydrophilic layer 13 and is
adsorbed uniformly on the surface 13B.
[0032] The oxidation catalyst layer 12 has therein the heat due to
the exhaust gas flowing therethrough and also the reaction heat due
to the oxidation of NO and the like in the exhaust gas. The urea
water adsorbed on the surface 13B of the hydrophilic layer 13 is
hydrolyzed into ammonia and carbon dioxide (CO2) by the heat that
the oxidation catalyst layer 12 has, the heat of the exhaust gas
flowing through the hydrophilic layer 13 and also the hydrolytic
catalytic function of the hydrophilic layer 13. The urea water is
then adsorbed uniformly on the surface 13B of the hydrophilic layer
13 and, therefore, the reaction time required for the hydrolysis
can be ensured, with the result that the urea water is hydrolyzed
into ammonia effectively. Furthermore, since the urea water is
dispersed and adsorbed uniformly on the surface 13B of the
hydrophilic layer 13, ammonia is generated uniformly on the surface
13B of the hydrophilic layer 13.
[0033] Since the urea water dispersed and adsorbed on the
hydrolytic catalyst as described above is hydrolyzed, the
hydrolysis takes place with a high efficiency. Furthermore, the
urea water can make use of the heat of hot exhaust gas immediately
after being emitted from the turbocharger 8 of the engine 1.
Therefore, the urea water can easily ensure the heat and the
temperature required for the hydrolysis. Moreover, urea water is
injected and hydrolyzed into ammonia in the region that is
downstream of the oxidation catalyst layer 12 and, therefore, no
ammonia flows into the oxidation catalyst layer 12 and is oxidized
by the oxidation catalyst of the oxidation catalyst layer 12.
[0034] Ammonia generated on the hydrolysis is dispersed uniformly
in radial directions of the cylindrical portion 11C of the casing
11 and flows to the mixer 17 together with the exhaust gas. Ammonia
is further dispersed while flowing through the mixer 17 and then
flows into the DPF 14. Ammonia that is flowed into the DPF 14
together with the exhaust gas reduces NOx contained in exhaust gas
including NO and NO2 into N2 by the catalytic reaction of the SCR
catalyst 15. After being dispersed uniformly on the hydrophilic
layer 13, ammonia is dispersed again at the mixer 17 and then
supplied uniformly to the entire DPF 14 and the SCR catalyst 15,
thereby reducing NOx effectively at the SCR catalyst 15.
[0035] Unreacted ammonia which has not been used in the reduction
of NOx is discharged outside the exhaust gas purification apparatus
101 together with exhaust gas.
[0036] Therefore, the exhaust gas containing residual unreacted
ammonia and N2 after flowing through the DPF 14 where PM is removed
is discharged from the exhaust gas purification apparatus 101 into
the exhaust pipe 6. The exhaust gas thus discharged into the
exhaust pipe 6 flows through the oxidation catalyst layer 20
provided in the exhaust pipe 6 and then is discharged through the
muffler 7 outside the vehicle (not shown). Since the residual
ammonia in the exhaust gas is oxidized and decomposed while flowing
through the oxidation catalyst layer 20, no harmful ammonia is
discharged outside.
[0037] The catalyst has a characteristic that it activates the
catalytic action at a temperature more than a predetermined
temperature. The DCU 30 is operated to open the injection valve 18
when the temperature detected by the exhaust gas temperature sensor
52 is the predetermined temperature at which the SCR catalyst 15 is
activated or higher, and to close the injection valve 18 when the
detected temperature is under the predetermined temperature. Thus,
the DCU 30 determines whether or not NOx reduction should be
performed depending on the temperature detected by the exhaust gas
temperature sensor 52.
[0038] Furthermore, the DCU 30 controls the injection quantity of
urea water by adjusting the opening of the injection valve 18 based
on the NOx concentration detected by the first NOx sensor 51.
Similarly, the DCU 30 controls the injection quantity of urea water
by adjusting the opening of the injection valve 18 based on the NOx
concentration detected by the second NOx sensor 53, that is the NOx
concentration of exhaust gas after flowing through the SCR catalyst
15 and the oxidation catalyst layer 20. For example, when the NOx
concentration detected by the second NOx sensor 53 exceeds a
predetermined level, the DCU 30 increases the injection quantity of
urea water by opening the injection valve 18 further. Thus, the DCU
30 adjusts the supply quantity of urea water to the SCR catalyst
15, i.e., the supply quantity of ammonia, thereby controlling the
NOx reduction performance of the exhaust gas purification apparatus
101.
[0039] Referring to FIG. 1, the exhaust gas purification apparatus
101 is disposed adjacent to the engine 1 and, therefore, hot
exhaust gas immediately after being emitted from the engine 1 flows
into the exhaust gas purification apparatus 101 through the
turbocharger 8. Furthermore, the heat generated by the engine 1 is
imparted to the exhaust gas purification apparatus 101 located
adjacent to the engine 1 and transmitted inward through outer wall
of the casing 11.
[0040] Referring to FIG. 2, the oxidative catalyst layer 12, the
hydrophilic layer 13 and the DPF 14 supporting the SCR catalyst 15
all disposed inside the casing 11 are subject to the heat of the
hot exhaust gas and the heat imparted from the engine 1 and,
therefore, the temperature of the respective components tends to
increase. The temperature increasing rate of the respective
components, i.e., the oxidation catalyst of the oxidation catalyst
layer 12, the hydrophilic catalyst of the hydrophilic layer 13 and
the SCR catalyst 15 in the exhaust gas purification apparatus 101
during a cold start of the engine 1 is improved and the time
required for activating each catalyst is shortened. Eventually, the
performance of NOx reduction is improved.
[0041] Thus, the exhaust gas purification apparatus 101 of the
present invention includes the oxidation catalyst layer 12 provided
in exhaust gas passage, the hydrophilic layer 13 that is provided
at least on the downstream end surface 12B of the oxidation
catalyst layer 12 and having at least one of the hydrophilic
function and the hydrolytic catalytic function, the SCR catalyst 15
provided downstream of the hydrophilic layer 13 and the injection
valve 18 for supplying urea water to the hydrophilic layer 13.
[0042] The urea water supplied to the hydrophilic layer 13 is
dispersed and adsorbed on the hydrophilic layer 13 by the
hydrophilic function and can make use of the heat generated by the
oxidation of NO to NO2 and the heat of the exhaust gas flowing
through the oxidation catalyst layer 12. Therefore, urea water is
hydrolyzed very efficiently and ammonia resulting from the
hydrolysis of the urea water is dispersed uniformly over the
hydrophilic layer 13. Thus, the hydrolysis of urea water into
ammonia is accomplished with high efficiency, which helps to
improve the reaction of the ammonia in the SCR catalyst 15. The
hydrolysis reaction of urea water supplied to the hydrophilic layer
13 is accelerated by the heat of the oxidation catalyst layer 12
and the heat due to the hydrolytic catalytic function of the
hydrophilic layer 13, thereby improving the efficiency of the
hydrolysis. Thus, the reduction of NOx in the exhaust gas
purification apparatus 101 using the urea water can be improved by
the hydrophilic function and the hydrolytic catalytic function of
the hydrophilic layer 13. Since the efficiency of hydrolysis of
urea water is improved by providing the hydrophilic layer 13, the
distance between the hydrophilic layer 13 and the SCR catalyst 15
can be shortened, thereby making it possible for the exhaust gas
purification apparatus 101 to be made small.
[0043] Since the injection valve 18 supplies urea water at a
position downstream of the hydrophilic layer 13, neither urea water
is supplied to the oxidation catalyst layer 12 nor ammonia produced
by the hydrolysis of urea water flows through the oxidation
catalyst layer 12. Therefore, the oxidation of ammonia into NOx by
the oxidation catalyst of the oxidation catalyst layer 12 can be
prevented. Due to the structure where the DPF 14 supports the SCR
catalyst 15, the SCR catalyst 15 and the DPF 14 are formed
integrally, thereby making it possible for the entire apparatus to
be formed small. Furthermore, since the oxidation catalyst layer
12, the hydrophilic layer 13, the SCR catalyst 15 formed integrally
with the DPF 14 and the injection valve 18 are all housed in the
single casing 11, the entire apparatus can be made still
smaller.
[0044] The exhaust gas purification apparatus 101 is mounted to the
engine assembly 10 and the hot exhaust gas emitted from the engine
assembly 10 is introduced into the exhaust gas purification
apparatus 101. The heat that the engine assembly 10 generates in
operation is transmitted inside the casing 11 of the exhaust gas
purification apparatus 101. Therefore, the time for the temperature
of the hydrophilic layer 13 to be increased to the level required
for the hydrolysis of urea water and also for the temperature of
the SCR catalyst 15 to the level required for activating the SCR
catalyst 15 during a cold start of the engine can be shortened,
with the result that the NOx reduction performance can be
improved.
[0045] The exhaust gas purification apparatus 102 according to a
second embodiment of FIG. 3 is made by modifying the DPF 14
supporting the SCR catalyst 15 of the exhaust gas purification
apparatus 101 according to the first embodiment. The following
description will use the same reference numerals for the common
elements or components in the first and the second embodiments, and
the description of such elements or components will be omitted.
[0046] Referring to FIG. 3, the oxidation catalyst layer 12 having
on the downstream side thereof the hydrophilic layer 13, an SCR
catalyst layer 25 supporting the SCR catalyst and the DPF 24 are
provided in this order in the downstream direction in the casing 11
of the exhaust gas purification apparatus 102. The oxidation
catalyst layer 12 and the SCR catalyst layer 25 are disposed across
the space 16 and the SCR catalyst layer 25 and the DPF 24 are
disposed adjacent to each other. The mixer 17 is provided on the
upstream end surface 25A of the SCR catalyst layer 25.
[0047] The exhaust gas introduced into the casing 11 flows through
the mixer 17 after flowing through the oxidation catalyst layer 12
and the hydrophilic layer 13. NOx contained exhaust gas is reduced
into N2 in the SCR catalyst layer 25, PM contained in exhaust gas
is captured in the DPF 24 and the resulting exhaust gas is
discharged outside the exhaust gas purification apparatus 102.
[0048] The rest of the structure and the operation of the exhaust
gas purification apparatus 102 according to the second embodiment
is the same as those of the exhaust gas purification apparatus 101
according to the first embodiment. The description of such
structure or operation will be omitted.
[0049] The exhaust gas purification apparatus 102 according to the
second embodiment offers the same advantageous effects as the
exhaust gas purification apparatus 101 according to the first
embodiment.
[0050] When PM is burned in the DPF 24 in the exhaust gas
purification apparatus 102 for preventing the accumulation of PM,
the influence of the combustion heat on the SCR catalyst of the SCR
catalyst layer 25 is reduced as compared with the exhaust gas
purification apparatus 101 according to the first embodiment. The
exhaust gas purification apparatus 102 reduces the deterioration of
the catalytic function of the SCR catalyst layer 25 due to the heat
caused by burning PM and improves the durability of the SCR
catalyst layer 25.
[0051] The exhaust gas purification apparatuses 101 and 102
according to the first and the second embodiments are provided in
the engine assembly 10 having the turbocharger 8, respectively, but
the present invention is not limited to this structure. When the
engine assembly 10 dispenses with the turbocharger 8, the exhaust
gas purification apparatuses 101 and 102 may be directly connected
to the outlet 5A of the exhaust manifold 5, respectively. The
exhaust gas purification apparatuses 101 and 102 may be provided
spaced apart from the engine assembly 10, respectively.
[0052] In the second embodiment, the oxidation catalyst layer 12,
the SCR catalyst layer 25, the DPF 24 and the injection valve 18
are all provided in the casing 11 of the exhaust gas purification
apparatus 102, but the present invention is not limited to this
structure. For example, only the DPF 24 may be provided outside the
casing 11 separately from the other components.
[0053] In the first and second embodiments, the oxidation catalyst
layer 20 is provided separately from the exhaust gas purification
apparatuses 101, 102, but the present invention is not limited to
this structure. The oxidation catalyst layer 20 may be provided
inside the casing 11 downstream of the DPF 14, 24 of the exhaust
gas purification apparatuses 101, 102, respectively.
[0054] The injection valve 18 is provided downstream of the
oxidation catalyst layer 12 so as to supply urea water to the
hydrophilic layer 13 in the exhaust gas purification apparatuses
101, 102 according to the first and the second embodiments,
respectively, but the present invention is not limited to this
structure. The injection valve may be so arranged that urea water
is supplied toward the upstream side of the oxidation catalyst
layer 12. The supplied urea water can be hydrolyzed while flowing
through the oxidation catalyst layer 12 and, therefore, the
efficiency of the hydrolysis of urea water can be improved. Since
urea water is dispersed while flowing through the oxidation
catalyst layer 12, urea water can be dispersed and adsorbed more
uniformly on the hydrophilic layer 13. Accordingly, ammonia
produced by the hydrolysis of urea water can be dispersed and be
supplied to the SCR catalyst more uniformly. Thus, the efficiency
of the reduction of NOx by ammonia in the SCR catalyst is
improved.
[0055] Although the hydrophilic layer 13 is supported on part of
the downstream end surface 12B of the oxidation catalyst layer 12
in the first and the second embodiments, the hydrophilic layer 13
may be supported on the entire end surface 12B of the oxidation
catalyst layer 12.
[0056] The single hydrophilic layer 13 having the hydrolytic
catalytic function and the hydrophilic function is used as the urea
decomposition accelerator in the first and the second embodiments,
but the present invention is not limited to this structure. The
single hydrophilic layer serving as the urea decomposition
accelerator may be formed of two different layers, one layer of
which is a hydrophilic layer made of a material having only the
hydrophilic function and the other layer of which is a hydrolytic
catalytic layer made of a material having only the hydrolytic
catalytic function for accelerating the hydrolysis of urea water.
In this case, the hydrophilic layer should preferably be provided
downstream of the hydrolytic catalytic layer, that is on the side
facing the DPF 14 and 24 of the first and the second embodiments,
respectively.
[0057] The urea decomposition accelerator may be formed of only the
hydrophilic layer made of the material having the hydrophilic
function. Since urea water dispersed and adsorbed on the
hydrophilic layer can make use of the heat of the oxidation
catalyst layer 12, urea water is hydrolyzed efficiently and the
resulting ammonia is dispersed uniformly over the entire
hydrophilic layer. On the other hand, the urea decomposition
accelerator may be formed of only the hydrolytic catalytic layer
made of the material having the hydrolytic catalytic function. In
this case, urea water making use of the heat of the oxidation
catalyst layer 12 is subject to the hydrolytic catalytic function
by the hydrolytic catalytic layer and, therefore, the efficiency of
the hydrolysis can be improved.
[0058] The casing 11 of the exhaust gas purification apparatuses
101, 102 according to the first and the second embodiments,
respectively is cylindrically-shaped, but the casing 11 according
to the present invention is not limited to this shape. The casing
11 may be formed with a cross-section including a prism such as
quadratic prism, a sphere or an ellipsoid.
[0059] Furthermore, the mixer 17 may be dispensed with in the first
and the second embodiments.
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