U.S. patent application number 12/871064 was filed with the patent office on 2011-03-03 for exhaust gas purification apparatus.
This patent application is currently assigned to KABUSHIKI KAISHA TOYOTA JIDOSHOKKI. Invention is credited to Yoshifumi Kato.
Application Number | 20110047991 12/871064 |
Document ID | / |
Family ID | 43067043 |
Filed Date | 2011-03-03 |
United States Patent
Application |
20110047991 |
Kind Code |
A1 |
Kato; Yoshifumi |
March 3, 2011 |
EXHAUST GAS PURIFICATION APPARATUS
Abstract
An exhaust gas purification apparatus includes a first oxidation
catalyst provided in a passage through which exhaust gas flows, a
particulate matter collecting device provided downstream of the
first oxidation catalyst, a selective catalytic reduction catalyst
integrally formed with the particulate matter collecting device and
having ammonia adsorption property, a second oxidation catalyst
integrally formed with the particulate matter collecting device and
oxidizing ammonia at a predetermined temperature or higher and a
urea water supply device provided upstream of the selective
catalytic reduction catalyst for supplying urea water. The urea
water supply device supplies urea water only when a temperature of
the second oxidation catalyst is below the predetermined
temperature.
Inventors: |
Kato; Yoshifumi; (Aichi-ken,
JP) |
Assignee: |
KABUSHIKI KAISHA TOYOTA
JIDOSHOKKI
Kariya-shi
JP
|
Family ID: |
43067043 |
Appl. No.: |
12/871064 |
Filed: |
August 30, 2010 |
Current U.S.
Class: |
60/297 ;
60/303 |
Current CPC
Class: |
Y02A 50/20 20180101;
F01N 13/0097 20140603; F01N 2900/1602 20130101; F01N 2560/06
20130101; Y02T 10/12 20130101; Y02T 10/24 20130101; F01N 2610/02
20130101; F01N 3/208 20130101; F01N 2570/10 20130101; Y02A 50/2344
20180101; F01N 2570/14 20130101; F01N 13/0093 20140601; F01N 3/035
20130101 |
Class at
Publication: |
60/297 ;
60/303 |
International
Class: |
F01N 3/035 20060101
F01N003/035; F01N 3/10 20060101 F01N003/10 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 2, 2009 |
JP |
P2009-202823 |
Claims
1. An exhaust gas purification apparatus comprising: a first
oxidation catalyst provided in a passage through which exhaust gas
flows; a particulate matter collecting device provided downstream
of the first oxidation catalyst; a selective catalytic reduction
catalyst integrally formed with the particulate matter collecting
device and having ammonia adsorption property; a second oxidation
catalyst integrally formed with the particulate matter collecting
device and oxidizing ammonia at a predetermined temperature or
higher; and a urea water supply device provided upstream of the
selective catalytic reduction catalyst for supplying urea water,
wherein the urea water supply device supplies urea water only when
a temperature of the second oxidation catalyst is below the
predetermined temperature.
2. The exhaust gas purification apparatus according to claim 1,
further comprising: an exhaust gas temperature sensor for detecting
a temperature of exhaust gas, wherein the temperature of exhaust
gas detected by the exhaust gas temperature sensor is utilized as
the temperature of the second oxidation catalyst.
3. The exhaust gas purification apparatus according to claim 1,
wherein the urea water supply device supplies urea water upstream
of the first oxidation catalyst.
4. The exhaust gas purification apparatus according to claim 1,
wherein the second oxidation catalyst decreases combustion
temperature of the particulate matter captured by the particulate
matter collecting device to a temperature between 400 and
650.degree. C.
5. The exhaust gas purification apparatus according to claim 1,
further comprising: a casing housing the first oxidation catalyst,
particulate matter collecting device, the selective catalytic
reduction catalyst, the second oxidation catalyst and the urea
water supply device.
6. The exhaust gas purification apparatus according to claim 1,
characterized in that the exhaust gas purification apparatus is
mounted to an engine assembly.
7. The exhaust gas purification apparatus according to claim 1,
further comprising: a mixer provided on upstream end face of the
particulate matter collecting device for dispersing substances in
exhaust gas flowed through the Mixer.
8. The exhaust gas purification apparatus according to claim 1,
wherein the particulate matter collecting device, the selective
catalytic reduction catalyst and the second oxidation catalyst are
integrally formed in a manner that the selective catalytic
reduction catalyst and the second oxidation catalyst are coated in
either order on the particulate matter collecting device.
9. The exhaust emission purification apparatus according to claim
1, wherein the predetermined temperature is a temperature, at which
the second oxidation catalyst is activated.
10. The exhaust gas purification apparatus according to claim 1,
wherein the selective catalytic reduction catalyst is made of a
zeolite replaced by a metal, and the second oxidation catalyst is
Ag/CeO2 catalyst.
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 SCR (selective catalytic reduction) 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 exhaust gas are
provided upstream of the SCR catalyst. The oxidation catalyst
oxidizes hydrocarbons (HC) and carbon monoxide (CO) in exhaust gas
into water (H2O) and carbon dioxide (CO2) and also promotes the
oxidation of nitrogen oxide (NO) into nitrogen dioxide (NO2).
[0004] A DPF (diesel particulate filter) is also provided in the
exhaust passage between the engine and the muffler for reducing
particulate matter (PM) such as carbon in 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, the urea SCR system is required to be downsized
and also to accomplish the NOx reduction efficiently for the usage
of urea water.
[0005] Published Japanese Translation 2006-519331 of PCT
International Publication discloses an exhaust gas purification
apparatus that includes a platinum-containing precatalyst having a
function of filtering PM in exhaust gas, an SCR catalyst provided
downstream of the platinum-containing precatalyst, a first supply
device provided upstream of the platinum-containing precatalyst for
supplying ammonia or urea and a second supply device provided
between the platinum-containing precatalyst and the SCR catalyst
for supplying ammonia or urea. The platinum-containing precatalyst
functions as a reduction catalyst under a temperature that is below
about 250.degree. C. and also as an oxidation catalyst under a
temperature that is about 250.degree. C. or higher. The SCR
catalyst is activated as a reduction catalyst under a temperature
that is about 250.degree. C. or higher.
[0006] In the exhaust gas purification apparatus according to the
above Published Japanese Translation, when the temperature of
exhaust gas is under T1 that is between 220.degree. C. and
270.degree. C., ammonia or urea is supplied from the first supply
device and then, ammonia or ammonia generated by the hydrolysis of
urea in the platinum-containing precatalyst reduces NOx in the
exhaust gas. When the temperature of exhaust gas exceeds T1,
ammonia or urea is supplied from the second supply device and then,
ammonia or ammonia generated in the SCR catalyst by the hydrolysis
of urea reduces NOx in exhaust gas. Oxidation of ammonia in the
platinum-containing precatalyst is prevented by supplying ammonia
or urea from the second supply device. Thus, the exhaust gas
purification apparatus uses the supplied ammonia or urea
efficiently for the NOx reduction.
[0007] However, when urea is supplied from either of the first
supply device and the second supply device in the exhaust gas
purification apparatus of the above Published Japanese Translation,
the time for which the supplied urea stays upstream of the
platinum-containing precatalyst or the SCR catalyst before reaching
the precatalyst or the catalyst should be long for ensuring the
time that is long enough for urea to be hydrolyzed into ammonia.
The distances between the first supply device and the
platinum-containing precatalyst and between the second supply
device and the SCR catalyst, respectively, should be long enough
for the hydrolysis of urea. Therefore, there has been problems in
that the apparatus increases its length and it is difficult to make
it small.
[0008] The present invention is directed to providing an exhaust
gas purification apparatus that improves the efficiency of NOx
reduction relative to the urea water usage and makes possible
downsizing of the apparatus.
SUMMARY OF THE INVENTION
[0009] An exhaust gas purification apparatus includes a first
oxidation catalyst provided in a passage through which exhaust gas
flows, a particulate matter collecting device provided downstream
of the first oxidation catalyst, a SCR catalyst integrally formed
with the particulate matter collecting device and having ammonia
adsorption property, a second oxidation catalyst integrally formed
with the particulate matter collecting device and oxidizing ammonia
at a predetermined temperature or higher and a urea water supply
device provided upstream of the SCR catalyst for supplying urea
water. The urea water supply device supplies urea water only when a
temperature of the second oxidation catalyst is below the
predetermined temperature.
[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 of an exhaust gas purification
apparatus according to an embodiment of the present invention and
its associated components; and
[0013] FIG. 2 is a schematic cross sectional view of the exhaust
gas purification apparatus of FIG. 1.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0014] The following will describe the embodiment of the exhaust
gas purification apparatus according to the present invention with
reference to the accompanying drawings. Referring to FIGS. 1 and 2
showing the embodiment, the exhaust gas purification apparatus
which is designated generally by reference numeral 101 and its
associated components will be described. The exhaust gas
purification apparatus 101 is employed in a vehicle equipped with a
diesel engine.
[0015] 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.
[0016] 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.
[0017] 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.
[0018] The cylindrical casing 11 houses therein a first oxidation
catalyst layer 12 supporting a first oxidation catalyst and a
diesel particulate filter (DPF) 13D forming a particulate matter
collecting device disposed downstream of the first oxidation
catalyst layer 12 with respect to the flow of exhaust gas in the
casing 11. The first oxidation catalyst layer 12 and the DPF 13D
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 first oxidation catalyst layer 12 and the DPF 13D are disposed
spaced apart from each other thereby to form therebetween a space
16.
[0019] The first oxidation catalyst layer 12 supports thereon the
first 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 first oxidation catalyst of the first oxidation
catalyst layer 12 uses material such as platinum (Pt), palladium
(Pd), rhodium (Rh), silver (Ag), iron (Fe), cupper (Cu), nickel
(Ni), gold (Au) or a mixture of two or more of these materials.
[0020] The DPF 13D is made of a porous material such as ceramic for
capturing particulate matter (PM) contained in exhaust gas. For
preventing the deterioration of the DPF 13D caused by the
accumulation of PM, PM accumulated in the DPF 13D needs to be
burned.
[0021] Furthermore, the DPF 13D has an (urea) SCR catalyst 14 as a
selective catalytic reduction catalyst supported thereon, e.g., by
coating. The DPF 13D and the SCR catalyst 14 cooperate to
integrally form a DPF 13 with catalyst.
[0022] The selective catalytic reduction catalyst serves to promote
the chemical reaction selectively among specific chemical
substances. Specifically, the urea SCR catalyst (hereinafter
referred to as SCR catalyst) catalyzes the reaction between
nitrogen oxide (NOx) and ammonia (NH3) thereby to reduce NOx into
nitrogen (N2) and water (H2O). Though the details will be described
later, the SCR catalyst 14 has the above-described function and
preferably has high ammonia adsorption property and also catalytic
property under a low temperature. The ammonia adsorption property
of the SCR catalyst 14 should preferably be over 20 mg/l, i.e.
capable of adsorbing more than 20 mg of ammonia per litter of base
material supporting the SCR catalyst 14, and the SCR catalyst 14
should preferably be activated catalytically at 150.degree. C. or
higher. The SCR catalyst 14 should preferably be made of a zeolite
replaced by a metal such as iron and the like. The SCR catalyst 14
being activated means rapid increase of the reduction rate of NOx
by ammonia.
[0023] The DPF 13D supporting the SCR catalyst 14 also has a second
oxidation catalyst 15 supported thereon, e.g., by coating, for
decreasing the combustion temperature of PM captured by the DPF
13D. The DPF 13D and the second oxidation catalyst 15 cooperate to
integrally form the DPF 13 with catalyst. Thus, the DPF 13 with
catalyst is formed by the DPF 13D, the SCR catalyst 14 and the
second oxidation catalyst 15. The second oxidation catalyst 15
should preferably decrease the PM combustion temperature to a
temperature between 400 and 650.degree. C.
[0024] The second oxidation catalyst 15 functions not only to
decrease the PM combustion temperature but also to oxidize and
decompose ammonia under a predetermined temperature Tp.degree. C.
or higher. However, the second oxidation catalyst 15 neither
oxidizes nor decomposes ammonia at a temperature below the
predetermined temperature Tp.degree. C. The predetermined
temperature Tp.degree. C. corresponds to a temperature at which the
second oxidation catalyst 15 is activated. The oxidation catalyst
being activated means that 50% of object substance for oxidation is
oxidized to a predetermined level. Since the temperature at which
the catalyst is activated depends on the proportion of component
materials of the catalyst and the concentration of the catalyst in
the region of the DPF where the catalyst is supported thereon, the
temperature Tp.degree. C. at which the second oxidation catalyst 15
is activated may be between 150 and 250.degree. C. The second
oxidation catalyst 15 may be Ag/CeO2 catalyst, i.e., silver (Ag)
supported on ceria (CeO2).
[0025] The SCR catalyst 14 and the second oxidation catalyst 15 may
be applied in either order to the DPF 13D to be supported thereon,
firstly the SCR catalyst 14 and then the second oxidation catalyst
15 or in the reverse order. Alternatively, a mixture of the SCR
catalyst 14 and the second oxidation catalyst 15 may be supported
on the DPF 13D. The DPF 13D and the SCR catalyst 14 may be formed
integrally in a manner that the SCR catalyst 14 is disposed in rear
of the DPF 13D and the DPF 13D supports thereon the second
oxidation catalyst 15.
[0026] An injection valve 18 that is an electromagnetic valve is
provided upstream of the first oxidation catalyst layer 12 in the
cylindrical portion 11C of the casing 11. The injection valve 18
forms the urea water supply device of the present 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 upstream of
the first oxidation catalyst layer 12 (SCR catalyst 14) in the
casing 11. The injection valve 18 is electrically connected to a
dosing control unit (DCU) 20 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 20 and the
pump operation is controlled by the DCU 20. The DCU 20 may be
provided separately or formed integrally with an ECU for the
vehicle.
[0027] A cylindrically-shaped mixer 17 is provided on the upstream
end face 13A of the DPF 13 with catalyst for distributing
substances in exhaust gas uniformly over the end face 13A. 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 exhaust gas to be dispersed 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.
[0028] An exhaust gas temperature sensor 31 is provided downstream
of the upstream end face 11A of the casing 11 for detecting the
temperature of exhaust gas. The exhaust gas temperature sensor 31
is electrically connected to the DCU 20 and sends detected
temperature information to the DCU 20. As described above, the
exhaust gas purification apparatus 101 includes the SCR catalyst 14
and the DPF 13D in a manner that they are integrally formed and is
fixed to the engine assembly 10 and disposed adjacent to the engine
1 (refer to FIG. 1).
[0029] The following will describe the operation of the exhaust gas
purification apparatus 101 according to the 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.
[0030] 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 the 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 exhaust pipe 6 and the muffler 7 and
then is discharged outside the vehicle (not shown).
[0031] Referring to FIG. 2, all the exhaust gas flowed into the
exhaust gas purification apparatus 101 flows firstly through the
first oxidation catalyst layer 12. While the exhaust gas passes
through the first 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 first oxidation catalyst layer
12, the exhaust gas flows through the mixer 17 and then into the
DPF 13 with catalyst. PM in the exhaust gas is captured by the DPF
13D of the DPF 13 with catalyst.
[0032] Meanwhile, the DCU 20 performs either one of the following
two operations described under (1) and (2) based on the temperature
information sent by the exhaust gas temperature sensor 31. [0033]
(1) When the temperature T of exhaust gas detected by the exhaust
gas temperature sensor 31 is below the predetermined temperature
Tp.degree. C. at which the second oxidation catalyst 15 is
activated:
[0034] It is noted the temperature T of exhaust gas detected by the
exhaust gas temperature sensor 31 may be regarded as the
temperature of the second oxidation catalyst 15 of the DPF 13 with
catalyst. Therefore, when the temperature T of exhaust gas detected
by the exhaust gas temperature sensor 31 shows Tp .degree. C. or
higher, it may be considered that the second oxidation catalyst 15
is under a temperature environment where the second oxidation
catalyst 15 can oxidize and decompose ammonia.
[0035] The following description will be made with the assumption
that the predetermined temperature Tp.degree. C. is 250.degree.
C.
[0036] When the temperature of exhaust gas is below 250.degree. C.,
the DCU 20 operates the electric pump in the urea water tank 19 and
also opens the injection valve 18. Then, urea water is injected
from the injection valve 18 upstream of the first oxidation
catalyst layer 12 in the casing 11.
[0037] The injected urea water is carried by the exhaust gas and
flows through the first oxidation catalyst layer 12. The first
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 other substances in exhaust gas. Therefore,
most of the urea water flowing through the first oxidation catalyst
layer 12 is hydrolyzed into ammonia and carbon dioxide by the heat
that the first oxidation catalyst layer 12 has and the heat of the
exhaust gas flowing through the first oxidation catalyst layer
12.
[0038] After flowing through the first oxidation catalyst layer 12,
the exhaust gas containing urea water and ammonia flows through the
space 16 and then to the mixer 17. Urea water and ammonia are
flowed through the mixer 17 while being dispersed and then into the
DPF 13 with catalyst. Meanwhile, urea water in the exhaust gas that
is not hydrolyzed in the first oxidation catalyst layer 12 is
hydrolyzed into ammonia due to the heat of the exhaust gas before
reaching the DPF 13 with catalyst that is integrally formed with
the SCR catalyst 14. The time during which the urea water stays in
the first oxidation catalyst layer 12, the space 16 and the mixer
17 while flowing therethrough before reaching the SCR catalyst 14
satisfies the reaction time required for the hydrolysis of urea
water. Thus, the hydrolysis of urea water is accomplished with a
high efficiency.
[0039] As described above, since urea water injected from the
injection valve 18 is hydrolyzed not only in the first oxidation
catalyst layer 12 but also in the space 16 through which urea water
flows before reaching the DPF 13 with catalyst, urea water is
hydrolyzed into ammonia with a high efficiency. Therefore, the
distance between the first oxidation catalyst layer 12 and the DPF
13 with catalyst in the exhaust gas purification apparatus 101,
i.e., the distance of the space 16 can be shortened, thus making it
possible to construct the exhaust gas purification apparatus 101
small.
[0040] Ammonia contained in exhaust gas flowing into the DPF 13
with catalyst performs either one of the following two operations
(1A) and (1B) depending on the temperature condition of the SCR
catalyst 14 of the DPF 13 with catalyst. The temperature of the SCR
catalyst 14 is equivalent to the temperature of the second
oxidation catalyst 15 and, therefore, the temperature T of the
exhaust gas detected by the exhaust gas temperature sensor 31 is
regarded as the temperature of the SCR catalyst 14. [0041] (1A)
When the temperature of the SCR catalyst 14 is below temperature
Ts.degree. C. at which the SCR catalyst 14 is activated:
[0042] The following description will be made with the assumption
that the predetermined temperature Ts.degree. C. is 150.degree. C.
at which catalyst is generally activated. When the temperature of
the SCR catalyst 14 is below 150.degree. C., the SCR catalyst 14 is
not activated and, therefore, ammonia contained in the exhaust gas
flowing into the DPF 13 with catalyst does not reduce NOx
(including NO and NO2) contained in the exhaust gas by the
catalytic reaction of the SCR catalyst 14 but is adsorbed on the
SCR catalyst 14. After flowing through the DPF 13 with catalyst,
the exhaust gas from which harmful ammonia is removed is discharged
from the exhaust gas purification apparatus 101. Therefore, the use
of an SCR catalyst with high ammonia adsorption property is
preferable for preventing harmful ammonia from being discharged
outside the vehicle (not shown). [0043] (1B) When the temperature
of the SCR catalyst 14 is the temperature Ts.degree. C.
(150.degree. C.) at which the SCR catalyst 14 is activated, or
higher
[0044] Ammonia that is contained in exhaust gas flowing into the
DPF 13 with catalyst reduces NOx in the exhaust gas into N2 by the
catalytic reaction of the SCR catalyst 14. Residual ammonia that is
not used in the reduction of NOx is adsorbed on the SCR catalyst
14. Thus, the exhaust gas having reduced its NOx content and
removed harmful ammonia therefrom while flowing through the DPF 13
with catalyst is discharged from the exhaust gas purification
apparatus 101. The lower the temperature Ts.degree. C. at which the
SCR catalyst 14 is activated, the larger the temperature range in
which the SCR catalyst 14 can reduce NOx in exhaust gas. Therefore,
the temperature Ts.degree. C. at which the SCR catalyst 14 is
activated should preferably be low.
[0045] In either case (1A) or (1B), the catalytic temperature of
the second oxidation catalyst 15 of the DPF 13 with catalyst is
substantially equivalent to the temperature of the exhaust gas
detected by the exhaust gas temperature sensor 31, which is below
250.degree. C. that does not cause catalyst to oxidize and
decompose ammonia. Therefore, the second oxidation catalyst 15
neither oxidizes nor decomposes ammonia in exhaust gas flowing
through the DPF 13 with catalyst.
[0046] Thus, ammonia generated on the hydrolysis of urea water
injected by the injection valve 18 is neither oxidized nor
decomposed by the second oxidation catalyst 15, but used for
reducing NOx in exhaust gas or adsorbed on the SCR catalyst 14 with
a high efficiency. [0047] (2) When the temperature T of the exhaust
gas detected by the exhaust gas temperature sensor 31 is the
predetermined temperature Tp.degree. C. (250.degree. C.), at which
the second oxidation catalyst 15 is activated, or higher:
[0048] The DCU 20 stops the operation of the electric pump in the
urea water tank 19 and also closes the injection valve 18 thereby
to stop the injection of urea water from the injection valve 18.
Therefore, exhaust gas introduced into the casing 11 containing
neither urea water nor ammonia generated by the hydrolysis of urea
water flows through the first oxidation catalyst layer 12 and the
mixer 17 and then into the DPF 13 with catalyst.
[0049] Meanwhile, the SCR catalyst 14 of the DPF 13 with catalyst
has a lot of ammonia that is generated and adsorbed on the SCR
catalyst 14 when the temperature T of the exhaust gas is below
250.degree. C. The SCR catalyst 14 is activated when the
temperature T of the exhaust gas is 250.degree. C. or higher.
Therefore, NOx in exhaust gas flowing into the DPF 13 with catalyst
is reduced by ammonia that is adsorbed on the SCR catalyst 14 under
the catalytic reaction of the SCR catalyst 14. Thus, the exhaust
gas having reduced its NOx content and removed harmful ammonia
therefrom while flowing through the DPF 13 with catalyst is
discharged from the exhaust gas purification apparatus 101.
[0050] The temperature at which the second oxidation catalyst 15 of
the DPF 13 with catalyst is activated is 250.degree. C. at which
the second oxidation catalyst 15 can oxidize and decompose ammonia,
or higher. However, exhaust gas flowing through the DPF 13 with
catalyst contains no ammonia, but ammonia is only adsorbed on the
SCR catalyst 14. Since the ammonia that is adsorbed on the SCR
catalyst 14 is used for reducing NOx as described before, the
ammonia is neither oxidized nor decomposed by the second oxidation
catalyst 15. Thus, when the exhaust gas temperature is 250.degree.
C. or higher, ammonia generated by the hydrolysis of urea water is
used for reducing NOx in the exhaust gas without being oxidized and
decomposed by the second oxidation catalyst 15.
[0051] 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.
[0052] Referring to FIG. 2, the first oxidation catalyst layer 12
and the DPF 13 with catalyst both 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
first oxidation catalyst layer 12 and the SCR catalyst 14 of the
DPF 13 with catalyst, 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 during such cold start of the
engine 1 is shortened. Eventually, the performance of NOx reduction
is improved.
[0053] Thus, the exhaust gas purification apparatus 101 according
to the present invention includes the first oxidation catalyst
layer 12 provided in exhaust gas passage, the DPF 13D provided
downstream of the first oxidation catalyst layer 12, the SCR
catalyst 14 that is integrally formed with the DPF 13D and can
adsorb ammonia, the second oxidation catalyst 15 that is integrally
formed with the DPF 13D and oxidizes ammonia at a predetermined
temperature or higher and the injection valve 18 provided upstream
of the SCR catalyst 14 for supplying urea water only when the
temperature of the second oxidation catalyst 15 is below a
predetermined temperature.
[0054] The exhaust gas purification apparatus 101, in which the SCR
catalyst 14 and the second oxidation catalyst 15 are integrally
formed with the DPF 13D, can be made small. Ammonia is generated on
the hydrolysis of urea water supplied by the injection valve 18
with the aid of the first oxidation catalyst layer 12 and the like.
However, since urea water is supplied below a predetermined
temperature at which the second oxidation catalyst 15 is not
activated for the oxidation, no generated ammonia is oxidized and
decomposed by the second oxidation catalyst 15. In other words,
when the temperature of the second oxidation catalyst 15 is below
the predetermined temperature at which the second oxidation
catalyst 15 is not activated for the oxidation, NOx in exhaust gas
is reduced under the catalytic reaction of the SCR catalyst 14 by
ammonia that is generated on the hydrolysis of urea water and
contained in exhaust gas. Residual ammonia that is not used for
reducing NOx is adsorbed on the SCR catalyst 14. On the other hand,
when the temperature of the second oxidation catalyst 15 is the
predetermined temperature or higher, no urea water is supplied and
NOx in exhaust gas is reduced by ammonia that is adsorbed on the
SCR catalyst 14 under the catalytic reaction of the SCR catalyst
14. Thus, ammonia generated on the hydrolysis of urea water is used
efficiently for reducing NOx, thereby improving the efficiency of
NOx reduction for the urea water usage.
[0055] The exhaust gas temperature sensor 31 is provided in the
exhaust gas purification apparatus 101 for detecting the
temperature of exhaust gas flowing through the second oxidation
catalyst 15. Regarding the temperature detected by the exhaust gas
temperature sensor 31 as the temperature of the second oxidation
catalyst 15, controlling of the urea water supply from the
injection valve 18 can be made easily by using the temperature of
the second oxidation catalyst 15.
[0056] By supplying urea water from the injection valve 18 provided
upstream of the first oxidation catalyst layer 12, urea water
flowing through the first oxidation catalyst layer 12 can make use
of the heat that the first oxidation catalyst layer 12 has therein
such as the reaction heat due to the oxidation of nitrogen monoxide
in exhaust gas into nitrogen dioxide and the heat that the exhaust
gas has therein. Therefore, urea water flowing through the first
oxidation catalyst layer 12 can be hydrolyzed at a high efficiency.
Furthermore, by supplying urea water upstream of the first
oxidation catalyst layer 12, the time during which the supplied
urea water stays upstream of the SCR catalyst 14 before reaching
the SCR catalyst 14 is lengthened and. therefore, urea water can be
hydrolyzed efficiently before reaching the SCR catalyst 14, with
the result that NOx reduction performance of the exhaust gas
purification apparatus 101 is improved. Accordingly, the distance
between the first oxidation catalyst layer 12 and the DPF 13 with
catalyst including the SCR catalyst 14 can be shortened, thereby
making the exhaust gas purification apparatus 101 small.
[0057] Since the second oxidation catalyst 15 reduces the
combustion temperature of the DPF13 with catalyst during burning of
PM captured by the DPF 13D, the influence of the heat caused by
burning PM for regenerating the DPF 13D on the SCR catalyst 14 can
be reduced. Therefore, the catalytic function of the SCR catalyst
14 for reducing NOx in the exhaust gas purification apparatus 101,
i.e., the durability of the SCR catalyst 14, can be improved.
[0058] Furthermore, since the first oxidation catalyst layer 12,
the DPF 13 with catalyst (the DPF 13D, the SCR catalyst 14 and the
second oxidation catalyst 15) and the injection valve 18 are all
housed in the single casing 11, the exhaust gas purification
apparatus 101 can be made still smaller.
[0059] 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 generated by the engine assembly 10 in
operation is transmitted inside the casing 11 of the exhaust gas
purification apparatus 101. Therefore, the time for the temperature
of the exhaust gas purification apparatus 101 to be increased to
the level required for the hydrolysis of urea water and also for
the temperature of the SCR catalyst 14 to the level required for
activating the SCR catalyst 14 during a cold start of the engine
can be shortened, with the result that the NOx reduction
performance can be improved.
[0060] The exhaust gas purification apparatus 101 in the embodiment
is provided in the engine assembly 10 having the turbocharger 8,
but the present invention is not limited to this structure. When
the engine assembly 10 dispenses with the turbocharger 8, the
exhaust gas purification apparatus 101 may be directly connected to
the outlet 5A of the exhaust manifold 5. The exhaust gas
purification apparatus 101 may be provided spaced apart from the
engine assembly 10.
[0061] The injection valve 18 is provided upstream of the first
oxidation catalyst layer 12 so as to supply urea water upstream of
the first oxidation catalyst layer 12 in the exhaust gas
purification apparatus 101 according to the embodiment, but the
present invention is not limited to this structure. The injection
valve 18 may be so arranged that urea water is supplied toward the
downstream side of the first oxidation catalyst layer 12. This
structure prevents ammonia generated on the hydrolysis of urea
water from being oxidized and decomposed by the first oxidation
catalyst layer 12.
[0062] The casing 11 of the exhaust gas purification apparatuses
101 according to the embodiment is cylindrically-shaped, but the
casing 11 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.
[0063] The exhaust gas purification apparatus 101 according to the
embodiment includes the exhaust gas temperature sensor 31 provided
upstream of the end face 11A of the casing 11, but the structure is
not limited to this structure. The exhaust gas temperature sensor
31 may be provided at a position that is adjacent to and
immediately upstream or downstream of the second oxidation catalyst
15. The mixer 17 of the exhaust gas purification apparatus 101 may
be dispensed with in the embodiment.
* * * * *