U.S. patent application number 11/918472 was filed with the patent office on 2009-03-12 for device for cleaning exhaust gas of internal combustion engine.
This patent application is currently assigned to TOYOTA JIDOSHA KABUSHIKI KAISHA. Invention is credited to Hirohito Hirata, Masaya Ibe, Yoshihiko Itoh, Masaru Kakinohana, Yuji Sakakibara.
Application Number | 20090064664 11/918472 |
Document ID | / |
Family ID | 37865130 |
Filed Date | 2009-03-12 |
United States Patent
Application |
20090064664 |
Kind Code |
A1 |
Hirata; Hirohito ; et
al. |
March 12, 2009 |
Device for Cleaning Exhaust Gas of Internal Combustion Engine
Abstract
A device for cleaning exhaust gas of an internal combustion
engine according to the present invention includes a device (30)
for collecting particulate matter from exhaust gas in an exhaust
gas passage (15), ozone feeding device (40) capable of feeding
ozone to the device (30) for collecting particulate matter from
upstream thereof, and an NOx catalyst (20) disposed upstream from
the ozone feeding device (40), for cleaning NOx in the exhaust gas.
Since NOx is preliminarily removed by the NOx catalyst (20) at
apposition upstream from the ozone feeding device (40), the
consumption of ozone due to the reaction with NOx is prevented,
whereby it is possible to effectively use ozone for the purpose of
oxidizing and removing PM in the device (30) for collecting
particulate matter.
Inventors: |
Hirata; Hirohito;
(Shizuoka-ken, JP) ; Kakinohana; Masaru;
(Shizuoka-ken, JP) ; Ibe; Masaya; (Shizuoka-ken,
JP) ; Sakakibara; Yuji; (Aichi-ken, JP) ;
Itoh; Yoshihiko; (Aichi-ken, JP) |
Correspondence
Address: |
FINNEGAN, HENDERSON, FARABOW, GARRETT & DUNNER;LLP
901 NEW YORK AVENUE, NW
WASHINGTON
DC
20001-4413
US
|
Assignee: |
TOYOTA JIDOSHA KABUSHIKI
KAISHA
Aichi
JP
|
Family ID: |
37865130 |
Appl. No.: |
11/918472 |
Filed: |
September 15, 2006 |
PCT Filed: |
September 15, 2006 |
PCT NO: |
PCT/JP2006/318801 |
371 Date: |
October 15, 2007 |
Current U.S.
Class: |
60/286 ;
60/297 |
Current CPC
Class: |
F01N 2240/38 20130101;
Y02T 10/40 20130101; F01N 2610/146 20130101; F01N 3/103 20130101;
F01N 3/106 20130101; F01N 9/00 20130101; F01N 2900/1402 20130101;
F01N 13/009 20140601; B01D 2251/104 20130101; F01N 3/035 20130101;
B01D 53/9431 20130101; F01N 2900/0408 20130101; Y02T 10/47
20130101; Y02A 50/20 20180101; F01N 2560/026 20130101; F01N 2560/06
20130101; F01N 3/023 20130101; F01N 13/0097 20140603; F01N
2900/1602 20130101; F01N 2570/14 20130101; F01N 3/206 20130101;
B01D 53/944 20130101; F01N 2610/08 20130101; F01N 2610/14 20130101;
F01N 2250/14 20130101; F01N 3/0821 20130101; F01N 3/0842 20130101;
F01N 2250/02 20130101; Y02A 50/2344 20180101 |
Class at
Publication: |
60/286 ;
60/297 |
International
Class: |
F01N 9/00 20060101
F01N009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Sep 16, 2005 |
JP |
2005-270888 |
Claims
1. A device for cleaning exhaust gas of an internal combustion
engine, comprising a device for collecting particulate matter from
exhaust gas in an exhaust gas passage, ozone feeding means capable
of feeding ozone to said device for collecting particulate matter
from upstream thereof, and an NOx catalyst disposed upstream from
said ozone feeding means, for cleaning NOx in the exhaust gas.
2. The device for cleaning exhaust gas of the internal combustion
engine as defined by claim 1, further comprising a separate ozone
feeding means capable of feeding ozone to said NOx catalyst from
upstream thereof.
3. The device for cleaning exhaust gas of the internal combustion
engine as defined by claim 2, further comprising means for
detecting the temperature of exhaust gas flowing into said NOx
catalyst or that of a floor of the NOx catalyst, and means for
executing the feeding of ozone from the separate ozone feeding
means if the detected temperature of the exhaust gas or that of the
floor of said NOx catalyst is lower than a predetermined value.
4. The device for cleaning exhaust gas of the internal combustion
engine as defined by claim 2, further comprising means disposed at
a position upstream from said NOx catalyst or between said NOx
catalyst and said ozone feeding means, for detecting the
concentration of NOx in the exhaust gas, and means for controlling
an amount of ozone fed from said separate ozone feeding means in
accordance with the detected concentration of NOx.
5. The device for cleaning exhaust gas of the internal combustion
engine as defined by claim 1, further comprising an oxidation
catalyst for oxidizing unburned component in the exhaust gas,
disposed between said NOx catalyst and said ozone feeding
means.
6. The device for cleaning exhaust gas of the internal combustion
engine as defined by claim 1, further comprising an oxidation
catalyst disposed at a position upstream from said NOx catalyst,
for oxidizing unburned component in the exhaust gas.
7. The device for cleaning exhaust gas of the internal combustion
engine as defined by claim 1, wherein said NOx catalyst is of a
storage reduction type or a selective catalytic reduction type.
8. The device for cleaning exhaust gas of the internal combustion
engine as defined by claim 1, further comprising means for
generating ozone from gas outside said exhaust gas passage, wherein
said ozone feeding means supplies ozone generated in said ozone
generating means to said exhaust gas passage.
9. The device for cleaning exhaust gas of the internal combustion
engine as defined by claim 2, further comprising means for
generating ozone from gas outside said exhaust gas passage, and a
flow rate control unit for dividing ozone delivered from said ozone
generating means at a predetermined dividing ratio and feeding the
same to said ozone feeding means and said separate ozone feeding
means.
10. The device for cleaning exhaust gas of the internal combustion
engine as defined by claim 1, wherein said internal combustion
engine is a compressive ignition type internal combustion engine or
a direct injection and spark ignition type internal combustion
engine.
Description
TECHNICAL FIELD
[0001] The present invention relates to a device for cleaning
exhaust gas of an internal combustion engine, particularly to a
device for cleaning exhaust gas exhausted from a diesel engine by
collecting and oxidizing particulate matter in the exhaust gas.
BACKGROUND ART
[0002] Generally, it has been known that in exhaust gas exhausted
from a diesel engine, particulate matter (referred to as PM
hereinafter) mainly composed of carbon is contained and causes the
atmospheric contamination. Accordingly, many devices and/or methods
have been proposed for collecting such particulate matter and
removing the same from the exhaust gas.
[0003] For example, there are proposals in that a temperature of a
diesel particulate filter (DPF) is made to rise by forcibly
injecting fuel so that the collected PM is oxidized and burnt, or
that NO.sub.2 is generated from NO in the exhaust gas so that PM is
oxidized by NO.sub.2 (see, for example, Japanese Patent Laid-Open
No. 2002-531762), or that PM is oxidized by using catalyzed DPF
(see, for example, Japanese Patent Laid-Open Nos. 6-272541 and
9-125931). However, there are problems in such proposals that if
the fuel is forcibly injected, the fuel consumption becomes worse
and DPF may be broken due to the rapid temperature rise of PM, that
since the oxidation rate of PM caused by NO.sub.2 is insufficient
in the proposal described in Japanese Patent Laid-Open No.
2002-531762, it is difficult to completely oxidize PM exhausted
from the engine and remove the same, and that since both the
catalyst and PM are solid in the proposal described in Japanese
Patent Laid-Open Nos. 6-272541 and 9-125931, the both are not into
desirably contact with each other to result in the insufficient
oxidation reaction.
[0004] Accordingly, a technology has recently be disclosed (for
example, in Japanese Patent Laid-Open No. 2005-502823) in that
ozone O.sub.3 having a larger oxidation capacity than NO.sub.2 is
used for oxidizing PM. In a method and a device for the post
treatment of exhaust gas from the diesel engine described in
Japanese Patent Laid-Open No. 2005-502823, a device for generating
ozone O.sub.3 or nitrogen dioxide NO.sub.2 as oxidizing agent from
the exhaust gas by plasma is provided upstream from the particulate
filter, and in accordance with the temperature of the exhaust gas,
ozone is selectively used if the temperature is low and nitrogen
dioxide is selectively used if the temperature is high so that soot
collected by the particulate filter is oxidized and removed.
[0005] In this regard, in the method and the device for the post
treatment of the exhaust gas from the diesel engine disclosed in
Japanese Patent Laid-Open No. 2005-502823, the capacity for
oxidizing and removing PM is highly evaluated since ozone O.sub.3
having a larger oxidation capacity than that of NO.sub.2 is used.
However, in the technology disclosed in Japanese Patent Laid-Open
No. 2005-502823, ozone is generated from oxygen that is one of
components of the exhaust gas by using plasma and introduced into
the particulate filter together with the exhaust gas containing
NO.sub.x or others whereby an amount of ozone thus generated is
insufficient. Also, there is a risk in that the ozone having a
large oxidation capacity may react with NOx or others in the
exhaust gas and is consumed prior to being introduced into the
particulate filter, resulting in the reduction of the amount of
ozone usable for oxidizing and removing PM, which is problematic
since the cleaning efficiency becomes lower and the oxidation rate
of PM decreases.
DISCLOSURE OF THE INVENTION
[0006] An object of the present invention is to provide a device
for cleaning exhaust gas of an internal combustion engine wherein
ozone is effectively usable when PM is oxidized and removed by
using ozone.
[0007] To achieve the above-mentioned object, the device for
cleaning exhaust gas of an internal combustion engine according to
the present invention comprises a device for collecting particulate
matter from exhaust gas in an exhaust gas passage, ozone feeding
means capable of feeding ozone to said device for collecting
particulate matter from upstream thereof, and a NOx catalyst
disposed upstream from said ozone feeding means, for cleaning NOx
in the exhaust gas.
[0008] According to this inventive device for cleaning exhaust gas
of an internal combustion engine, since the NOx catalyst is
disposed upstream from the ozone feeding means, it is possible to
preliminarily remove NOx in the exhaust gas by the NOx catalyst at
a position upstream from the ozone feeding means. Thereby, NOx is
not substantially contained in the exhaust gas at an ozone-feeding
position, whereby ozone is prevented from being consumed by NOx in
the exhaust gas, and thus a larger amount of ozone is usable for
oxidizing and removing PM in the device for collecting particulate
matter. As a result, it is possible to effectively use ozone and to
improve the PM cleaning efficiency by ozone.
[0009] Preferably, the inventive device further comprises a
separate ozone feeding means capable of feeding ozone to said NOx
catalyst from upstream thereof.
[0010] The NOx catalyst does not effectively operate when the
temperature of exhaust gas or a catalyst floor is low. Accordingly,
in such a case, NOx is not completely cleaned by the NOx catalyst
but discharged therefrom in the downstream direction, whereby the
discharged NOx reacts with ozone fed from the ozone feeding means
and consumes the latter. According to this preferable aspect, since
ozone is fed to the NOx catalyst from the separate ozone feeding
means, the cleaning of NOx by the NOx catalyst is facilitated.
Accordingly, even if the temperature is as low as the NOx catalyst
does not effectively function, NOx is prevented from being
discharged from the NOx catalyst, and ozone is not uselessly
consumed but effectively used for removing PM.
[0011] Preferably, the inventive device further comprises means for
detecting the temperature of exhaust gas flowing into said NOx
catalyst or that of a floor of the NOx catalyst, and means for
executing the feeding of ozone from the separate ozone feeding
means if the detected temperature of the exhaust gas or that of the
floor of said NOx catalyst is lower than a predetermined value.
[0012] Thereby, it is possible to feed ozone to the NOx catalyst
only when the temperature is as low as the NOx catalyst does not
effectively operate, whereby ozone is effectively usable.
[0013] Or, the inventive device further comprises means disposed at
a position upstream from said NOx catalyst or between said NOx
catalyst and said ozone feeding means, for detecting the
concentration of NOx in the exhaust gas, and means for controlling
an amount of ozone fed from said separate ozone feeding means in
accordance with the detected concentration of NOx.
[0014] According to this aspect, it is possible to determine
whether NOx is cleaned or not in the NOx catalyst and the degree of
the cleaning thereof based on the detected NOx concentration, and
to effectively use ozone by controlling a feeding amount of ozone
in accordance with the detected NOx concentration.
[0015] Preferably, the inventive device further comprises an
oxidation catalyst for oxidizing unburned component in the exhaust
gas, disposed between said NOx catalyst and said ozone feeding
means.
[0016] When the unburned component (HC, CO or others) are
discharged from the NOx catalyst, the unburned component reacts
with ozone fed from the ozone feeding means and uselessly consume
the latter. According to this preferable aspect, it is possible to
oxidize and clean the unburned component discharged from the NOx
catalyst by the oxidation catalyst. Thereby, the consumption of
ozone due to the reaction with the unburned component is avoided,
and ozone is effectively usable.
[0017] Or, preferably, the inventive device further comprises an
oxidation catalyst disposed at a position upstream from said NOx
catalyst, for oxidizing unburned component in the exhaust gas.
Thereby, the unburned component is cleaned at a position upstream
from the ozone feeding means, and the reaction of ozone with the
unburned component is avoidable.
[0018] Preferably, the NOx catalyst is of a storage reduction type
or a selective catalytic reduction type.
[0019] Also, preferably, the inventive device further comprises
means for generating ozone from gas outside said exhaust gas
passage, wherein said ozone feeding means supplies ozone generated
in said ozone generating means to said exhaust gas passage.
[0020] For example, if a plasma system using a high voltage as the
ozone generating means is employed, the ozone generating efficiency
is higher when a high temperature raw material gas is used rather
than when a low temperature raw material gas is used. According to
this preferable aspect, since ozone is generated by using gas
outside the exhaust gas passage, it is possible to improve the
ozone generating efficiency in comparison with a case wherein ozone
is generated from a high temperature exhaust gas as described in
Japanese Patent Laid-Open No. 2005-502823.
[0021] According to the present invention, an excellent effect is
exhibited wherein ozone is effectively usable when PM is oxidized
and cleaned by using ozone.
BRIEF DESCRIPTION OF THE DRAWINGS
[0022] FIG. 1 diagrammatically illustrates a system of a device for
cleaning exhaust gas of an internal combustion engine according to
a first embodiment of the present invention;
[0023] FIG. 2 is a sectional view illustrating a wall flow type
honeycomb structure of DPF;
[0024] FIGS. 3A and 3B are diagrammatic illustrations,
respectively, for describing a mechanism for absorbing and
releasing NOx in a storage reduction type NOx catalyst;
[0025] FIG. 4 is a diagrammatic illustration of a selective
catalytic reduction type NOx catalyst;
[0026] FIG. 5 is a graph showing a temperature window of the
selective catalytic reduction type NOx catalyst;
[0027] FIG. 6 illustrates an overall structure of a system for
carrying out tests in relation to the first embodiment of the
present invention;
[0028] FIG. 7 illustrates a detail of a region VII in FIG. 6;
[0029] FIG. 8 is a graph showing test results when the storage
reduction type NOx catalyst was used;
[0030] FIG. 9 is a graph showing test results when the selective
catalytic reduction type NOx catalyst was used;
[0031] FIG. 10 diagrammatically illustrates a system of a device
for cleaning exhaust gas of an internal combustion engine according
to a second embodiment of the present invention;
[0032] FIG. 11 illustrates an overall structure of a system for
carrying out tests in relation to the second embodiment of the
present invention;
[0033] FIG. 12 illustrates a detail of a region XII in FIG. 11;
[0034] FIG. 13 is a graph showing test results when the storage
reduction type NO.sub.x catalyst was used;
[0035] FIG. 14 is a graph showing test results when the selective
catalytic reduction type NO.sub.x catalyst was used;
[0036] FIG. 15 diagrammatically illustrates a system of a device
for cleaning exhaust gas of an internal combustion engine according
to a third embodiment of the present invention;
[0037] FIG. 16 illustrates part of a system of a device for
carrying out tests in relation to the third embodiment of the
present invention, corresponding to the detail of the region VII in
FIG. 6;
[0038] FIG. 17 illustrates part of a system of a device for
carrying out tests in relation to the third embodiment of the
present invention, corresponding to the detail of the region VII in
FIG. 6; and
[0039] FIG. 18 is a graph showing test results when the selective
catalytic reduction type NOx catalyst was used.
BEST MODE FOR CARRYING OUT THE INVENTION
[0040] Preferred embodiments of the present invention will be
described below with reference to the attached drawings.
First Embodiment
[0041] FIG. 1 diagrammatically illustrates a system of a device for
cleaning exhaust gas of an internal combustion engine according to
a first embodiment of the present invention, wherein reference
numeral 10 denotes a compression ignition type internal combustion
engine; i.e., a diesel engine, 11 denotes an intake manifold
communicated with an intake port, 12 denotes an exhaust manifold
communicated with an exhaust port, and 13 denotes a combustion
chamber. According to this embodiment, fuel supplied from a fuel
tank not shown to a high pressure pump 17 is sent thereby to a
common rail 18 and stored there under a high pressure. The highly
pressed fuel in the common rail 18 is directly injected from a fuel
injection valve 14 to the combustion chamber 13. Exhaust gas from
the diesel engine 10 flows via the exhaust manifold 12 and a
turbocharger 19 and reaches an exhaust gas passage 15 provided
downstream thereof, wherein the exhaust gas is cleaned and
discharged to an outer air. In this regard, types of the diesel
engine should not be limited to those having such a common rail
type fuel injection device, but any other exhaust gas cleaning
devices such as an EGR device may be optionally included.
[0042] In the exhaust gas passage 15, the NOx catalyst 20 for
cleaning NOx in the exhaust gas and a diesel particulate filter
(hereinafter referred to as DPF) 30 are arranged in series
sequentially from the upstream side. Between the NOx catalyst 20
and DPF 30; in other words, on the downstream side of the NOx
catalyst 20 and the on the upstream side of DPF 30, ozone feeding
nozzles 40 capable of feeding ozone (O.sub.3) to DPF 30 is
disposed. An ozone generator 41 is coupled to the ozone feeding
nozzle 40 so that the ozone generated by the ozone generator 41 is
supplied to the ozone feeding nozzle 40 via an ozone feeding
passage 42 and injected from the ozone feeding nozzles 40 into the
exhaust gas passage 15 toward DPF 30 disposed on the downstream
side.
[0043] DPF 30 is supported within a metallic case 31 of a generally
cylindrical shape having truncated conical opposite ends via a
supporting member not shown. The supporting member is insulative,
heat-durable and shock-absorbable, and made, for example, of
alumina mat.
[0044] As shown in FIG. 2, DPF 30 is a so-called wall-fall type
provided with a honeycomb structure 32 made of porous ceramics
wherein the honeycomb structure 32 is formed of ceramic material
such as cordierite, silica or alumina. The exhaust gas flows from a
left side to a right side as shown by an arrow in the drawing. In
the honeycomb structure 32, a first passage 34 having a closing
plug 33 at the upstream end and a second passage 36 having a
closing plug 35 at the downstream end are alternately arranged to
form a honeycomb shape. These passages 34 and 36 are called as a
cell, respectively, and disposed parallel to the flowing direction
of the exhaust gas. When the exhaust gas flows from the left side
to the right side in the drawing, the exhaust gas flows from the
second passage 36 into the first passage 34 through a flow passage
wall 37 formed of porous ceramics, and flows downstream. At that
time, PM in the exhaust gas is collected by the porous ceramics and
prevented from being discharged to an outer air. A filter wherein
PM is filtrated and collected when the exhaust gas passes the flow
passage wall as described above is called as a wall-flow type.
[0045] The ozone generator 41 may be of any types including one
wherein zone is generated while air or oxygen is fed as a raw
material into a discharge tube capable of applying high voltage.
Here, air or oxygen used as the raw material is different from that
disclosed in Japanese Patent Laid-Open No. 2005-502823, but a gas
taken from other areas than the exhaust gas passage 15, such as
that contained in outer air. In the ozone generator 41, the
efficiency for generating ozone is higher in a case wherein
high-temperature gas is used as raw material than in a case wherein
low-temperature gas is used. Accordingly, by generating ozone with
gas existing other than in the exhaust gas passage, it is possible
to improve the efficiency for generating ozone in comparison with
the case disclosed in Japanese Patent Laid-Open No.
2005-502823.
[0046] While the ozone feeding nozzle 40 will be described later in
more detail, it is disposed directly upstream from DPF 30 so that
the ozone is fed therefrom to DPF 30 not to be uselessly consumed
by the reaction thereof with NOx or components remaining unburned
in the exhaust gas (such as CO or HC). Also, to allow the ozone to
be evenly fed to all over the upstream end surface of DPF 30, a
plurality of ozone feeding ports 43 are arranged to cover a total
diameter of the upstream end surface of DPF 30. The ozone feeding
nozzle 40 extends in the diametrical direction of the casing 31 and
fixed thereto. In this regard, various configurations are possible
other than the ozone feeding nozzle 40 described above. For
example, if only one ozone feeding port is provided, a distance
between the ozone feeding port to the upstream end surface of DPF
is preferably prolonged so that the ozone is evenly fed all over
the upstream end surface.
[0047] The NOx catalyst 20 is supported within a generally
cylindrical metallic casing 21 having truncated conical opposite
ends via a supporting member not shown in the same manner as DPF
30. The supporting member is insulative, heat-durable and
shock-absorbable, and made, for example, of alumina mat.
[0048] The NOx catalyst 20 is preferably either a storage reduction
type NOx catalyst (NSR) or a selective catalytic reduction type NOx
catalyst (SCR).
[0049] In a case of the storage reduction type NOx catalyst, the
NOx catalyst 20 is structured by carrying, on a surface of a
substrate made of oxide such as alumina Al.sub.2O.sub.3, precious
metal such as platinum Pt as a catalytic component, and an NOx
absorbent component. The NOx absorbent component is at least one
selected from a group consisting of alkali metal such as potassium
K, sodium Na, lithium Li or cesium Cs, alkaline earth such as
barium Ba or calcium Ca, and rare earth such as lanthanum La or
yttrium.
[0050] The storage reduction type NOx catalyst 20 operates to
absorb NOx when an air-fuel ratio of the exhaust gas incoming into
the catalyst is leaner than a predetermined value (typically a
theoretical air-fuel ratio) and release NO.sub.x when the
concentration of oxygen in the exhaust gas is lowered. In this
embodiment, since the diesel engine is used, the air-fuel ratio of
the exhaust gas is lean in the usual state, whereby the NOx
catalyst 20 absorbs NOx contained in the exhaust gas.
Alternatively, if reducing agent is fed on the upstream side of NOx
catalyst 20 to make the air-fuel ratio of the exhaust gas incoming
into the catalyst to be rich, the NOx catalyst 20 releases the
absorbed NOx. The NOx thus released is reduced and cleaned by the
reaction with the reducing agent.
[0051] It is thought that the absorption/release and the
reduction/cleaning of NOx is carried out based on a mechanism
described below with reference to FIGS. 3A and 3B. This mechanism
will be described in an example wherein a storage reduction type
NOx catalyst carrying platinum Pt and potassium K on the surface of
a substrate formed of alumina Al.sub.2O.sub.3 is used. In this
regard, the same mechanism is obtainable even if other precious
metal, alkali metal or rare earth is used.
[0052] First, as shown in FIG. 3A, when the incoming exhaust gas
becomes lean, the concentrations of oxygen and NOx in the incoming
exhaust gas increases, and such oxygen O.sub.2 becomes a shape of
O.sub.2.sup.-- or O.sup.2- and is adheres to the surface of
platinum. On the other hand, NO in the incoming exhaust gas reacts
with O.sub.2.sup.- or O.sup.2- on the surface of platinum Pt to be
NO.sub.2 (2NO+O.sub.2.fwdarw.2NO.sub.2). Then the generated
NO.sub.2 is absorbed by potassium K that is an absorbent component,
as nitrate; i.e., potassium nitrate KNO.sub.3. As far as the oxygen
concentration is high in the incoming exhaust gas, NO.sub.2 is
generated on the surface of platinum Pt, and NO.sub.2 is absorbed
by K as far as the absorbent capacity of NOx has not been
saturated. Contrarily, when the oxygen concentration is lowered to
decrease the generated amount of NO.sub.2, the reaction proceeds in
the counter direction (NO.sub.3.fwdarw.NO.sub.2), whereby potassium
nitrate KNO.sub.3 in K is released as NO.sub.2 from the absorbent.
That is, when the oxygen concentration in the incoming exhaust gas
is lowered, NOx is released from K. As a degree of the lean state
in the incoming exhaust gas becomes lower, the oxygen concentration
in the incoming exhaust gas is lowered, whereby if the degree of
the lean state is made to be lower, NOx is released from K.
[0053] On the other hand, if the air-fuel ratio is made to be rich,
HC and CO in the incoming exhaust gas are reacted with oxygen
O.sub.2.sup.- or O.sup.2- on platinum Pt and oxidized. Also, if the
air-fuel ratio of the incoming exhaust gas is made to be rich, the
oxygen concentration in the incoming exhaust gas is extremely
lowered, whereby NO.sub.2 is released from K and, as shown in FIG.
3B, is reacted with unburned HC and CO via platinum Pt to be
reduced and cleaned, resulting in N.sub.2 and O.sub.2. When
NO.sub.2 disappears from the surface of platinum Pt in such a
manner, NO.sub.2 is sequentially released from K. Accordingly, if
the air-fuel ratio becomes rich in the incoming exhaust gas, NOx is
released from K in the short time duration.
[0054] Any kinds of reducing agent may be used in this operation,
provided it generates reduction components such as hydrogen
carbonate HC or carbon monoxide CO in the exhaust gas; examples
thereof may be of a gas form, such as hydrogen or carbon monoxide,
hydrocarbon in a liquid or gas form, such as propane, propylene or
butane, or liquid fuel such as gasoline, gas oil or kerosene. In
this embodiment, gas oil that is a fuel for the diesel engine is
used as a reducing agent for the purpose of avoiding the troubles
of storage or replenishment. The gas oil used as the reducing agent
may be supplied to the NOx catalyst 20 by, for example, injecting
the gas oil from a reducing agent injection valve separately
provided in the exhaust gas passage 15 at a position upstream from
the NOx catalyst 20 or injecting the gas oil from the fuel
injection valve 14 into the combustion chamber 13 at a final stage
of an expansion cycle or during an exhaust cycle which is called as
a post injection. In this connection, such the supply of reducing
agent for the purpose of releasing and reducing NOx in the NOx
catalyst 20 is called as a rich spike.
[0055] Next, in a case of the selective catalytic reduction type
Nox catalyst, as shown in FIG. 4, the NOx catalyst 20 may be, for
example, that carrying precious metal such as Pt on the surface of
a substrate made of zeolite, that carrying transition metal such as
Cu on the surface of the substrate by the ion exchange or that
carrying titania/vanadium catalyst
(V.sub.2O.sub.5/WO.sub.3/TiO.sub.2) on the surface of the
substrate. In such a selective catalytic reduction type NOx
catalyst, under the condition wherein the air-fuel ratio of the
incoming exhaust gas is lean, HC and NO are steadily and
simultaneously reacted to be N.sub.2, O.sub.2 or H.sub.2O and
cleaned. In this regard, for cleaning NOx, the existence of HC is
necessary. Since unburned HC is always contained in the exhaust gas
even if the air-fuel ratio is lean, it is possible to reduce and
clean NOx by using this. Also, the rich spike may be carried out to
supply the reducing agent as in the storage reduction type NOx
catalyst. In such a case, ammonia or urea may be used as reducing
agent other than those described before.
[0056] A drawback of such selective catalytic reduction NOx
catalyst is in that a temperature window through which the catalyst
becomes active is relatively narrow. That is, as shown in FIG. 5
wherein the relationship between the temperature of the incoming
exhaust gas or that of a catalytic floor and the cleaning degree of
NOx is shown, a high NOx cleaning degree is obtained only within a
relatively narrow temperature range of .DELTA.T and extremely drops
on the outside of this temperature range. On the other hand, the
storage reduction type NOx catalyst has a temperature window wider
than that of the selective catalytic reduction type NOx catalyst,
which is more advantageous than the former.
[0057] Returning to FIG. 1, according to this embodiment, means is
provided for detecting an amount of PM collected by DPF 30 or a
blockage degree of DPF by the collected PM. That is, exhaust gas
pressure sensors 51 and 52 for detecting the pressure of the
exhaust gas are provided in the exhaust gas passage 15 on the
upstream and downstream sides from DPF 30, respectively, and
connected to ECU 100 which is control means. ECU 100 determines the
amount of PM collected by DPF 30 or the blockage degree thereof
based on the difference between the exhaust gas pressures detected
by the upstream side and downstream side exhaust gas pressure
sensors 51 and 52, respectively.
[0058] In this embodiment, while the upstream side exhaust gas
pressure sensor 51 is disposed at a position downstream from the
NOx catalyst 20 as well as upstream from the ozone feeding nozzle
40, it may be disposed on the downstream from the ozone feeding
nozzle 40. Also, while the amount of PM or the blockage degree is
detected based on the pressure difference between the upstream and
downstream sides of DPF 30 in this embodiment, the amount of the
collected PM or the blockage degree may be detected solely by one
exhaust gas sensor disposed upstream from DPF 30. Further, the
blockage degree may be detected by the integration with time of
soot signals generated from a soot sensor disposed upstream from
DPF. Similarly, it is obtained by estimating engine characteristic
map data reserved in ECU and integrating the same with time.
[0059] Also, according to this embodiment, means is provided for
detecting the exhaust gas temperature flowing into DPF 30 or the
floor temperature of DPF. That is, a temperature sensor 53 is
provided directly upstream from DPF 30, and ECU 100 calculates the
exhaust gas temperature at a position directly upstream from DPF 30
based on a signal detected by the temperature sensor 53. The
temperature sensor 53 detects the temperature of the exhaust gas at
a position between the ozone feeding nozzle 40 and DPF 30. In this
connection, a temperature detecting section of the temperature
sensor 53 (a tip end in a case of a thermopile) is preferably
located in the vicinity of a center of the upstream end surface of
DPF 30. A temperature detection part of the temperature sensor 53
may be embedded in the interior of DPF 30 for the purpose of
detecting the inner floor temperature of DPF 30.
[0060] Also, in this embodiment, means is provided for detecting
the air-fuel ratio of the exhaust gas flowing into DPF 30. That is,
an air-fuel ratio sensor 54 is provided at a position downstream
from NOx catalyst 20 and upstream from DPF 30, so that ECU 100 is
capable of calculating the air-fuel ratio of the exhaust gas based
on a detected signal issued from the air-fuel ratio sensor 54. In
this embodiment, the air-fuel ratio sensor 54 detects the air-fuel
ratio of the exhaust gas on the upstream side from the ozone
feeding nozzle 40. These sensors 51, 52, 53 and 54 are all attached
to the casing 31.
[0061] Now, in this embodiment, since the NOx catalyst 20, the
ozone feeding nozzle 40 and DPF 30 are sequentially arranged in the
exhaust gas passage 15 from upstream, the following effects are
obtainable. That is, since the NOx catalyst 20 is disposed upstream
from the ozone feeding nozzle 40, it is possible to preliminarily
remove NOx from the exhaust gas by the NOx catalyst 20. Thereby,
the supplied ozone is prevented from being uselessly consumed due
to the reaction with NOx in the exhaust gas. Thus, it is possible
to use a more amount of ozone for the purpose of oxidizing and
removing PM collected by DPF 30. Accordingly, the PM cleaning
efficiency by ozone is improvable. In this connection, the removal
of PM collected by DPF 30 by means of the oxidation is referred to
as regeneration, and DPF 30 recovers its inherent performance by
the regeneration.
[0062] Here, the reactive consumption of NOx and ozone will be
described in more detail. If is assumed that Ozone O.sub.3 is
reacted with NOx, particularly NO in the exhaust gas, the reactive
formula is as follows:
NO+O.sub.3.fwdarw.NO.sub.2+O.sub.2 (1)
NO.sub.2 generated by this reaction is further reacted with ozone
O.sub.3 as follows:
NO.sub.2+O.sub.3.fwdarw.NO.sub.3+O.sub.2 (2)
NO.sub.3 thus generated is decomposed as follows:
2NO.sub.3.fwdarw.2NO.sub.2+O.sub.2 (3)
[0063] As seen in the formula (1), ozone O.sub.3 is consumed by the
oxidation of NO, and as seen in the formula (2), ozone O.sub.3 is
consumed by the oxidation of NO.sub.2. And, as seen in the formula
(3), NO.sub.2 on the right side becomes NO.sub.2 on the left side
of the formula (2) and consumes ozone O.sub.3 for the purpose of
oxidizing this NO.sub.2 on the left side of the formula (2).
[0064] In such a manner, NOx and ozone repeat the chain reaction.
Accordingly, even if ozone is fed to DPF 30 directly before the
same, much ozone is consumed for the purpose of oxidizing and
decomposing NOx when NOx is contained in the exhaust gas at this
position, whereby an amount of ozone given to DPF 30 is
significantly decreased. Since electric power is required for
generating ozone by the ozone generator 41, such useless
consumption of ozone results in the useless consumption of electric
power as well as the worsening of fuel consumption.
[0065] Contrarily, if the NOx catalyst 20 is disposed upstream from
the ozone feeding nozzle 40 and DPF 30, as in this embodiment,
since it is possible to feed ozone to the exhaust gas after NOx is
removed therefrom by the NOx catalyst 20, the supplied ozone is
prevented from being consumed by the reaction with NOx, instead
effectively usable for oxidizing PM in DPF 30 and removing the
same.
[0066] More specifically, when the NOx catalyst 20 is of the
storage reduction type, NO on the left side of the formula (1)
becomes NO.sub.2 by the action of precious metal (Pt in the
illustrate embodiment) which is a reaction component, and the
generated NO.sub.2 is absorbed by K or others which is an
absorption component as described before with reference to FIGS. 3A
and 3B. Accordingly, NO and NO.sub.2 are not discharged from the
NOx catalyst 20 and the reaction of them with ozone is prevented.
On the other hand, if the NOx catalyst 20 is of a selective
catalytic reduction type, as described before with reference to
FIG. 4, the discharge of NO and NO.sub.2 from the NOx catalyst 20
is restricted. Thereby the reaction thereof with ozone is
prevented.
[0067] The timing for feeding ozone is first preferable when the
amount of PM collected by DPF 30 reaches a predetermined value or
more. Thus, ECU 100 executes to feed ozone by switch-on the ozone
generator 41 when the difference (Pu-Pl) between an upstream side
exhaust gas pressure Pu detected by the upstream exhaust pressure
sensor 51 and a downstream side exhaust gas pressure Pl detected by
the downstream exhaust pressure sensor 52 reaches a predetermined
value or more. Contrarily, if the difference (Pu-Pl) is lower than
the predetermined value, ECU 100 stops the feeding of ozone by
switching-off the ozone generator 41.
[0068] The timing for feeding ozone is secondarily preferable when
the temperature of exhaust gas flowing into DPF 30 or the floor
temperature of DPF 30 is within a proper range wherein the ozone is
effectively usable. The temperature range is, for example from 150
to 250.degree. C. Accordingly, ECU 100 executes the feeding of
ozone by switching-on the ozone generator 41 when the temperature
detected by the temperature sensor 53 is within this range.
Alternatively, if the detected temperature is not within such a
range, ECU 100 stops the feeding of ozone by switching-off the
ozone generator 41.
[0069] The timing for feeding ozone is thirdly preferable when
unfavorable components causing the reaction with zone are not
contained in the exhaust gas flowing into DPF 30. Such unfavorable
components are, for example, NOx described before and unburned HC
that may react with ozone to uselessly consume ozone as described
in more detail later. Whether or not such unfavorable components
are contained in the exhaust gas can be assumed by the air-fuel
ratio of the exhaust gas detected by the air-fuel ratio sensor 54.
Accordingly, if it is determined that unfavorable components are
contained based on the detected air-fuel ratio in the exhaust gas,
ECU 100 stops the supply of ozone by switching-off the ozone
generator 41. On the other hand, if it is determined that the
unfavorable components are not contained, ECU 100 executes the
supply of ozone by switching-on the ozone generator 41.
[0070] These three conditions may be optionally combined and
suitably coupled by the AND/OR relationship. According to this
embodiment, while ozone generated by switching-on the ozone
generator 41 is immediately fed, it may be possible to store the
preliminarily generated ozone and feed the same by switching a
valve.
[0071] Also, it is possible to pressurize ozone by a pump or a
compressor prior to being fed.
[0072] Test results using model gas in relation to the first
embodiment will be described below.
(I) Tests Wherein the Reduction Type Nox Catalyst is Used
(1) Test Equipment
[0073] FIG. 6 illustrates an overall structure of a test equipment,
and FIG. 7 illustrates a detail of an area VII of FIG. 6. Reference
numeral 61 denotes a plurality of gas bombs, each filled with raw
material gas for producing model gas similar to exhaust gas of the
diesel engine. The raw material gas referred to herein is N.sub.2,
O.sub.2, CO or others. Reference numeral 62 denotes a model gas
generator provided with a mass flow controller for mixing
predetermined amounts of the respective raw material gasses to
produce the model gas MG. The model gas MG passes NOx catalyst 64
disposed in an upstream quartz tube 65, then passes DPF 66 disposed
in a downstream quartz tube 65, and is discharged outside from an
exhaust gas duct not shown.
[0074] As shown in FIG. 6, gaseous oxygen O.sub.2 fed from an
oxygen bombs 67 is bifurcated, one of which is fed to an ozone
generator 69 after a flow rate thereof has been controlled by a
flow rate control unit 68. In the ozone generator 69, oxygen is
selectively and partially converted to ozone O.sub.3. Such oxygen
and ozone (or oxygen alone) reaches an ozone analyzer 70. Other of
the bifurcated oxygen is mixed with gas fed from the ozone
generator 69 after a flow rate thereof has been controlled by
another flow rate control unit 71, and reaches the ozone analyzer
70. In the ozone analyzer 70, the concentration of ozone in gas
flowing thereto; i.e., the concentration of ozone in the gas
supplied to DPF 66 is measured, and thereafter, the flow rate of
the fed gas is controlled by the flow rate control unit 71.
Redundant fed gas is discharged outside from a discharge duct not
shown, and the gas which flow rate has been controlled is mixed
with the model gas MG in a three-way elbow 72 located between the
upstream quartz tube 63 and the downstream quartz tube 65, and
thereafter, fed to DPF 66 together with the model gas MG.
[0075] On the outer circumferences of the upstream quartz tube 63
and the downstream quartz tube 65, electric heaters 73 and 74 are
provided, respectively, to control the temperature of the NOx
catalyst 64 and DPF 66. Also, temperature sensors 75 and 76 are
provided for measuring the temperatures at positions directly
upstream from the NOx catalyst 64 and DPF 66, respectively.
[0076] On the downstream side of DPF 66, an exhaust gas analyzer 77
for measuring the concentration of HC, CO and NOx, an exhaust gas
analyzer 78 for measuring the concentration of CO.sub.2 and an
ozone analyzer 79 for measuring the concentration of ozone are
disposed in series from upstream.
(2) Test Conditions
[0077] The electric heater 73, 74 were controlled so that the
temperature detected by the temperature sensors 75 and 76 became
250.degree. C. The composition of the model gas is 210 ppm of NO,
5% of O.sub.2, 3% of H.sub.2O except for the residual N.sub.2 in
the volume concentration. A flow rate of the model gas is 9.5
liters, and a pressure of the model gas is 0.4 MPa. The composition
of the fed gas is 20000 ppm of ozone O.sub.3 and the residual of
O.sub.2. In this regard, the ozone generator 69 is made ON so that
ozone can be supplied. If the ozone generator 69 is made off to
stop the feeding of ozone, the fed gas is solely O.sub.2. The flow
rate of the fed gas is 0.5 liters/min.
(3) Test Method
[0078] N.sub.2 is made to flow as model gas until the temperature
detected by the temperature sensors 75 and 76 reaches a constant
value (250.degree. C.), and after the temperature becomes constant,
NO and O.sub.2 are added to the model gas. Simultaneously
therewith, oxygen is introduced into the ozone generator 69. When
ozone is generated, the ozone generator 69 is made ON
simultaneously with the introduction of oxygen. An amount of
oxidation of PM (the oxidation rate) in DPF 66 was calculated by
the concentration of CO and CO.sub.2 detected by the exhaust gas
analyzers 77, 78. That is, a product of the flow rate of the model
gas, the detected volume concentration and the measurement time is
divided by a volume corresponding to 1 mol (for example, 22.4
liters) to obtain the number of moles during the measurement time,
and based on this number of moles, the amount of oxidation of PM
(oxidation rate) is calculated.
(4) Example and Comparative Examples
Example 1
[0079] The NOx catalyst 64 and DPF 66 of the following
specification were disposed, and the ozone generator 69 was
switched on to measure the oxidation rate of PM.
[0080] NOx Catalyst (of a Storage Reduction Type)
[0081] A honeycomb structure made of cordierite having a diameter
of 30 mm, a length of 25 mm, a cell wall thickness of 5 mil (milli
inch length, 1/1000 inch) and the number of cells of 400 cpsi
(cells per square inch) was coated with .gamma.-Al.sub.2O.sub.3. A
coated amount was 120 g/L (wherein a denominator L means per 1
liter of catalyst). Barium acetate was coated thereon with water
and calcined at 500.degree. C. for 2 hours. An amount of barium
acetate was 0.2 mol/L. This catalyst was immersed in aqueous
solution containing dinitrodiammine platinum so that Pt is carried
thereon, and after being dried, calcined at 450.degree. C. for one
hour. An amount of carried Pt was 2 g/L.
DPF
[0082] PM was collected on DPF made of cordierite (not coated with
catalyst) having a diameter of 30 mm, a length of 50 mm, a cell
wall thickness of 12 mil and the number of cells of 300 cpsi. PM
was collected by proving a container capable of arranging twelve
honeycomb structures made of cordierite having a length of 30 mm
and a length of 50 mm in parallel to each other in the exhaust pipe
for a diesel engine having a displacement volume of 2 liters,
through which passes exhaust gas under the driving conditions of
2000 rpm and 30 Nm for one hour. The tests were conducted while the
honeycomb structure collecting PM was disposed in the quartz tube
so that a surface carrying PM is opposed to the upstream side.
Comparative Example 1
[0083] DPF having the same structure as that in Example 1 was
solely disposed without providing the NOx catalyst on the upstream
side thereof, and the oxidation rate was measured while the ozone
generator 69 is made on.
Comparative Example 2
[0084] The NOX catalyst and DPF having the same structure as those
in Example 1 were disposed, and the oxidation rate was measured
while the ozone generator 69 was made off.
Comparative Example 3
[0085] DPF having the same structure as that in Example 1 was
solely disposed without providing the NOx catalyst on the upstream
from DPF, and the oxidation rate was measured while the ozone
generator 69 was made off.
(5) Test Results
[0086] FIG. 8 illustrates the PM oxidation rates in five minutes
after N.sub.2 is changed to the model gas (in other words, after
O.sub.2 is introduced into the ozone generator). In the drawing, a
unit g/hL of the PM oxidation rate on the vertical axis represents
grams of oxidized PM per one hour in DPF of 1 liter. In Comparative
examples 2 and 3, the oxidation of PM could not be confirmed. By
comparing Example 1 with Comparative example 1, it will be
understood that the NOx catalyst of the storage reduction type
absorbs NOx and the reaction of NOx with O.sub.3 is restricted on
the downstream side of the catalyst. By the comparison of Example 1
with Comparative examples 2 and 3, the effect of the addition of
ozone will be understood. That is, PM is not oxidized unless ozone
is added.
(II) Tests Wherein the Selective Catalytic Reduction Type Nox
Catalyst is Used
(1) Test Equipment
[0087] A test equipment was the same as that in the above-mentioned
tests (I).
(2) Test Conditions
[0088] Test conditions were the same as those in the
above-mentioned tests (I), except that the composition of model gas
represented by the volume concentration is NO of 210 ppm,
C.sub.3H.sub.6 of 500 ppm, O.sub.2 of 5%, H.sub.2O of 3% and the
residual N.sub.2. Propylene C.sub.3H.sub.6 is added herein since HC
is necessary for cleaning NOx in the NOx catalyst of a selective
catalytic reduction type. While several tens kinds of HC are
discharged from the engine, propylene C.sub.3H.sub.6 has the
largest concentration among them, whereby propylene C.sub.3H.sub.6
is used as a representative of HC.
(3) Test Method
[0089] N.sub.2 is made to flow as model gas until the temperature
detected by the temperature sensors 75 and 76 reaches a constant
value (250.degree. C.), and after the temperature becomes constant,
NO, C.sub.3H.sub.6 and O.sub.2 are added to the model gas.
Simultaneously therewith, oxygen is introduced into the ozone
generator 69. When ozone is generated, the ozone generator 69 is
made ON simultaneously with the introduction of oxygen. An amount
of oxidation of PM (the oxidation rate) in DPF 66 was calculated by
the concentration of CO and CO.sub.2 detected by the exhaust gas
analyzers 77, 78. At that time, by taking the concentrations of CO
and CO.sub.2 generated from C.sub.3H.sub.6 introduced into the
model gas into account, the carbon balance was calculated.
(4) Example and Comparative Examples
Example 2
[0090] The NOx catalyst 64 and DPF 66 of the following
specification were disposed, and the ozone generator 69 was
switched on to measure the oxidation rate of PM.
[0091] NOx catalyst (of a selective catalytic reduction type)
[0092] A honeycomb structure made of cordierite having a diameter
of 30 mm, a length of 25 mm, a cell wall thickness of 4 and the
number of cells of 400 cpsi was coated with zeolite of ZSM-5 type.
A coated amount was 120 g/L. Pt was carried thereon by using
aqueous solution containing dinitro-diamine platinum and after
being dried, calcined at 450.degree. C. for 1 hour. An amount of
carried Pt was 2 g/L.
DPF
[0093] DPF was the same as that in the above-mentioned tests
(I).
Comparative Example 4
[0094] DPF having the same structure as that in Example 2 was
solely disposed without providing the NOx catalyst on the upstream
from DPF, and the oxidation rate was measured while the ozone
generator 69 was made on.
Comparative Example 5
[0095] The NOX catalyst and DPF having the same structure as those
in Example 2 were disposed, and the oxidation rate was measured
while the ozone generator 69 was made off.
Comparative Example 6
[0096] DPF having the same structure as that in Example 2 was
solely disposed without providing the NOx catalyst on the upstream
from DPF, and the oxidation rate was measured while the ozone
generator 69 was made off.
(5) Test Results
[0097] FIG. 9 illustrates the PM oxidation rates in five minutes
after N.sub.2 is changed to the model gas (in other words, after
O.sub.2 is introduced into the ozone generator). In Comparative
examples 5 and 6, the oxidation of PM could not be confirmed. By
comparing Example 2 with Comparative example 4, it will be
understood that the NOx catalyst of the selective catalytic
reduction type reduces NOx and the reaction of NO with O.sub.3 is
restricted on the downstream side of the catalyst. In this
connection, in a case of this NOx catalyst of the selective
catalytic reduction type, it will be understood that excessive
C.sub.3H.sub.6 is not sufficiently cleaned, and C.sub.3H.sub.6
passing through the NOx catalyst reacts with ozone directly before
DPF to consume ozone, whereby ozone is not thoroughly used for
oxidizing PM. By the comparison of Example 2 with Comparative
examples 5 and 6, the effect of the addition of ozone will be
understood. That is, PM is not oxidized unless ozone is added.
[0098] As apparent from the above description, the present
invention is suitably applicable to a system wherein the
temperature of exhaust gas is relatively high and the effect of the
NOx catalyst is easily obtainable, namely, a system capable of
sufficiently cleaning NOx by the action of the NOx catalyst. A
typical example of such a system is a diesel engine for an
automobile. Also, it is possible to separately use the NOx catalyst
of a selective catalytic reduction type when the concentration of
reducing agent (HC) in the exhaust gas is high, and use that of a
storage reduction type in the other cases.
Second Embodiment
[0099] Next, a second embodiment according to the present invention
will be described with reference to the attached drawings. In this
regard, the same reference numerals are used in the drawings for
denoting the same parts as those in the first embodiment, and the
detailed explanation thereof will be eliminated.
[0100] FIG. 10 diagrammatically illustrates a system of a device
for cleaning exhaust gas of an internal combustion engine according
to the second embodiment. As illustrated, in the second embodiment,
second ozone feeding nozzles 90 are provided as separate means for
feeding ozone (O.sub.3) in an exhaust gas passage 15 on the
upstream side from NOx catalyst. The second ozone feeding nozzles
90 are of the same structure as the ozone feeding nozzles 40
disposed upstream from DPF 30. The second ozone feeding nozzles 90
and the ozone feeding nozzles 40 are connected to a flow rate
control unit 91, and the flow rate control unit 91 is connected to
the ozone generator 41. The flow rate control unit 91 distributes
ozone delivered from the ozone generator 41 to the ozone feeding
nozzles 40 and the second ozone feeding nozzles 90, respectively,
at a predetermined ratio. There is a case wherein ozone is supplied
to either one of them. The flow rate control unit 91 is connected
to ECU 100 and controlled thereby.
[0101] To determine an amount of ozone fed from the second ozone
feeding nozzles 90, a NOx sensor 92 is provided as means for
detecting the concentration of NOx relating to the NOx catalyst 20.
The NOx sensor 92 detects the concentration of NOx in the exhaust
gas in the exhaust gas passage 15. In this embodiment, the NOx
sensor 92 is disposed at a position downstream from the ozone
feeding nozzle 90 and upstream from the ozone feeding nozzle 40.
The NOx sensor, however, may be disposed at a position upstream
from the second ozone feeding nozzles 90. The NOx sensor 92 is
connected to ECU 100 and ECU 200 calculates the concentration of
NOx based on the output from the NOx sensor 92. The NOx sensor 92
and the second ozone feeding nozzles are attached to the casing
21.
[0102] In the first embodiment, there is a problem in that when the
temperature of exhaust gas or the floor temperature of the NOx
catalyst is low, the catalyst does not effectively operate, whereby
NOx is not cleaned in the NOx catalyst 20 to discharge NOx
downstream from the NOx catalyst 20. The discharged NOx reacts with
ozone fed from the ozone feeding nozzles 40 in the above-mentioned
manner and consumes ozone.
[0103] Contrarily, according to the second embodiment, if the
temperature of exhaust gas or the floor temperature of the NOx
catalyst is low, it is possible to add ozone from the second ozone
feeding nozzles 90 to the exhaust gas to clean NOx by the NOx
catalyst. Thereby, even if the temperature is too low to
effectively function the NOx catalyst, NOx is prevented from being
discharged downstream from the NOx catalyst 20, whereby ozone fed
for the purpose of removing PM is inhibited from being uselessly
consumed, instead, effectively used for the removal of PM.
[0104] The mechanism for cleaning NOx in the NOx catalyst will be
described. First, if the NOx catalyst 20 is of a storage reduction
type, NO in NOx reacts with ozone as represented by the following
formula:
NO+O.sub.3.fwdarw.NO.sub.2+O.sub.2 (1)
[0105] NO.sub.2 thus generated is absorbed or trapped by NOx
absorbent component such as K. In this connection, NO.sub.2
generated by the reaction represented by the above-mentioned
formulas (2) and (3) is also absorbed by the NOx absorbent
component in the same manner. Accordingly, NOx is prevented from
being discharged from the NOx catalyst 20. Here, Pt or others is a
sole component not operated at a lower temperature whereby the
function of the NOx absorbent component is not damaged even at a
low temperature. Thereby, it is possible to absorb NO.sub.2 as
described above even at a low temperature.
[0106] When the NOx catalyst is of a selective catalytic reduction
type, NOx is reduced and cleaned by the reaction with NO.sub.2
generated by the reaction represented by the formulas (1) to
(3).
[0107] When ECU 100 determines during the oxidation removal of PM
that the concentration of NOx in the exhaust gas is a predetermined
value (hardly equal to zero) of more based on the output from the
NOx sensor 92, ECU controls the flow rate control unit 91 to feed
ozone from the second ozone feeding nozzles 90 as well as controls
the flow rate of ozone to a predetermined value in accordance with
the NOx concentrations. That is, if the NOx concentration in the
exhaust gas reaches the predetermined value or more, this means
that the temperature of exhaust gas or the catalyst floor is too
low to clean NOx in the NOx catalyst 20. Accordingly, as a
countermeasure thereto, ozone is fed to the upstream from the NOx
catalyst 20 to accelerate the cleaning of NOx by the NOx catalyst
20. At that time, an amount of ozone fed from the flow rate control
unit 90; i.e., the second ozone feeding nozzles 90 may be
controlled in a feedback manner. In this regard, when the NOx
sensor 92 is provided upstream from the NOx catalyst 20, ozone is
fed from the second ozone feeding nozzles 90 if the concentration
of NOx detected by the NOx sensor 92 reaches the predetermined
value or more at which NOx is removable by the NOx catalyst 20. Of
course, means for detecting the temperature of exhaust gas flowing
into the NOx catalyst or that of the catalyst floor may be provided
for the purpose of starting or stopping the supply of ozone from
the second ozone feeding nozzles 90 as well as controlling the
feeding rate of ozone in accordance with the temperature of exhaust
gas or that of the catalyst floor. In this case, it is preferable
that ozone is fed when the temperature of exhaust gas or that of
the catalyst floor is at a predetermined value or lower.
[0108] Since tests were conducted also in relation to the second
embodiment while using the model gas, which results were as
follows:
(I) Tests Wherein the Storage Reduction Type Nox Catalyst is
Used
(1) Test Equipment
[0109] FIG. 11 illustrates an overall structure of a test
equipment, and FIG. 12 illustrates a detail of an area XII of FIG.
11. The test equipment is the same as that in the first embodiment,
except for the following points. According to the test equipment in
the second embodiment, fed gas consisting of ozone and oxygen or
oxygen only is fed to a three-way elbow 10 disposed at a position
between the NOx catalyst 64 and the DPF 66 and another elbow 102
disposed at a position upstream from the NOx catalyst 64 at a
predetermined dividing ratio, and mixed with the model gas.
(2) Test Conditions
[0110] The test conditions are the same as those disclosed in the
item (I) (2) of the first embodiment except for the following
points. According to the second embodiment, the electric heaters 73
and 74 are controlled so that the temperature detected by the
temperature sensors 75 and 76 becomes 100.degree. C. A reason why
this target temperature is set lower than 250.degree. C. which is
the target temperature of the first embodiment is to test the
effect of the ozone supply to the NOx catalyst 64 at a low
temperature. A flow rate of the fed gas is 125 cc/min at a position
upstream from the NOx catalyst 64 and 375 cc/min at a position
between the NOx catalyst 64 and DPF 66. In this regard, in
Comparative example 10 described later, the fed gas is not fed to
the position upstream from the NOx catalyst 64, but is fed to the
position between the NOx catalyst 64 and DPF 66 at a flow rate of
500 cc/min.
(3) Test Method
[0111] The test method is the same as that described in the item
(I) (3) of the first embodiment.
(4) Example and Comparative Examples
Example 3
[0112] Example 3 is the same as Example 1 described in the item (I)
(4) of the first embodiment.
Comparative Example 7
[0113] Comparative example 7 is the same as Comparative example 1
described in the item (I) (4) of the first embodiment.
Comparative Example 8
[0114] Comparative example 8 is the same as Comparative example 2
described in the item (I) (4) of the first embodiment.
Comparative Example 9
[0115] Comparative example 9 is the same as Comparative example 3
described in the item (I) (4) of the first embodiment.
Comparative Example 10
[0116] The same NOx catalyst and DPF as those in Example 3 were
arranged and the oxidation rate of PM was measured in a state that
the ozone generator 69 is made on. In this case, as described
before, fed gas was not fed to upstream from the NOx catalyst 64
but was fed to a position between the NOx catalyst 64 and DPF 66 at
a rate of 500 cc/min.
(5) Test Results
[0117] FIG. 13 illustrates the comparison of the oxidation rates of
PM in five minutes between the respective example and comparative
examples after switching the model gas composition from N.sub.2
(after O.sub.2 is introduced into the ozone generator). As shown,
the oxidation of PM could not be confirmed in Comparative examples
8 and 9. By the comparison of Example 3 with Comparative example 7,
it is understood that the NOx catalyst of a storage reduction type
absorbs NOx to restrict the reaction of O.sub.3 with NO downstream
from this catalyst. That is, if ozone is not added, PM is not
oxidized. By the comparison of Example 3 with Comparative example
10, the effect of the addition of ozone to the upstream side of the
NOx catalyst will be apparent. Particularly, in spite of the total
flow rate of the fed gas; i.e., 500 cc/min in Example 3 is the same
as that in Comparative example 10, the oxidation rate of PM is
higher when ozone is fed to the upstream side of the NOx catalyst
than when it is not fed. That is, it is concluded that even if
ozone is partially consumed for the purpose of cleaning NOx, the
effect thereof is better than when more ozone is fed to DPF without
cleaning Nox.
(II) Tests Wherein the Selective Catalytic Reduction Type Nox
Catalyst is Used
(1) Test Equipment
[0118] The test equipment is the same as that of (I).
(2) Test Conditions
[0119] The test conditions are the same as those described in item
(II) (2) of the first embodiment, except for the following point.
That is, as in the same manner as the storage reduction type
catalyst, the temperature of the electric heaters 73 and 74
detected by the temperature sensor 75 and 76 is controlled at
100.degree. C. The flow rate of fed gas is 125 cc/min at a position
on the upstream side of the NOx catalyst 64 and 375 cc/min at a
position between the NOx catalyst 64 and DPF 66. In this regard, in
Comparative example 14 described later, the fed gas is not fed to
the upstream side of the NOx catalyst 64 but fed to the position
between the NOx catalyst 64 and DPF 66 at 500 cc/min.
(3) Test Method
[0120] The test method is the same as that described in item (II)
(2) of the first embodiment.
(4) Examples and Comparative Examples
Example 4
[0121] Example 4 is the same as Example 2 described in item (II)
(4) of the first embodiment.
Comparative Example 11
[0122] Comparative example 11 is the same as Comparative example 4
described in item (II) (4).
Comparative Example 12
[0123] Comparative example 12 is the same as Comparative example 5
described in item (II) (4).
Comparative Example 13
[0124] Comparative example 13 is the same as Comparative example 6
described in item (II) (4).
Comparative Example 14
[0125] DPF having the same structure as that in Example 4 was
solely disposed without providing the NOx catalyst on the upstream
from DPF, and the oxidation rate was measured while the ozone
generator 69 was made off.
(5) Test Results
[0126] FIG. 14 illustrates the PM oxidation rates in five minutes
after N.sub.2 is changed to the model gas (in other words, after
O.sub.2 is introduced into the ozone generator). In Comparative
examples 12 and 13, the oxidation of PM could not be confirmed. By
comparing Example 4 with Comparative example 11, it will be
understood that the NOx catalyst of the selective catalytic
reduction type reduces NOx and the reaction of NO with O.sub.3 is
restricted on the downstream side of the catalyst. In this
connection, in a case of this NOx catalyst of the selective
catalytic reduction type, it will be understood that excessive
C.sub.3H.sub.6 is not sufficiently cleaned, and C.sub.3H.sub.6
passing through the NOx catalyst reacts with ozone directly before
DPF to consume ozone, whereby ozone is not thoroughly used for
oxidizing PM. By the comparison of Example 4 with Comparative
examples 12 and 13, the effect of the addition of ozone will be
understood. That is, PM is not oxidized unless ozone is added.
[0127] By the comparison of Example 4 with Comparative example 14,
the effect of the addition of ozone to the upstream side of the NOx
catalyst will be apparent. Particularly, it is concluded that even
if ozone is partially consumed for the purpose of cleaning NOx, the
effect thereof is better than when more ozone is fed to DPF without
cleaning Nox.
Third Embodiment
[0128] Next, a third embodiment according to the present invention
will be described with reference to the attached drawings. In this
regard, the same reference numerals are used in the drawings for
denoting the same parts as those in the first embodiment, and the
detailed explanation thereof will be eliminated.
[0129] FIG. 15 diagrammatically illustrates a system of a device
for cleaning exhaust gas of an internal combustion engine according
to the second embodiment. As illustrated, in the third embodiment,
an oxidation catalyst 110 is disposed in a casing 21 common to the
NOx catalyst 20.
[0130] In the first embodiment, as described before, when the NOx
catalyst 20 is of a storage reduction type, the rich spike (the
separate injection or the post injection of gas oil) is executed
for releasing or reduction-cleaning NOx stored in the NOx catalyst
20. Alternatively, if such rich spike is not executed, HC in the
exhaust gas passes through the NOx catalyst 20 with substantially
no interference. On the other hand, when the NOx catalyst 20 is of
a selective catalytic reduction type, HC in the exhaust gas
similarly passes through the NOx catalyst 20 with substantially no
interference unless the NOx catalyst 20 is within an active
temperature range, and even if it is within the active temperature
range and reducing agent (such as gas oil) is added to the exhaust
gas, excessive HC not cleaned by the NOx catalyst 20 is discharged
from the NOx catalyst 20. When HC is discharged from the NOx
catalyst 20 in such a manner, this HC reacts with ozone fed from
the ozone feeding nozzles 40 to uselessly consume ozone. That is,
ozone O.sub.3 partially oxidizes HC to generate HC oxide such as
CO, CO.sub.2 or H.sub.2O. If so, such an amount of consumed ozone
cannot be fed to DPF to reduce the PM oxidation efficiency.
[0131] Contrary to this, according to the third embodiment, since
the oxidation catalyst 110 is disposed at a position downstream
from the NOx catalyst 20 and upstream from the ozone feeding
nozzles 40, it is possible to oxidize HC discharged from the NOx
catalyst 20 and cleaned. Thereby, it is possible to restrict the
discharge of HC from the oxidation catalyst 110 and prevent ozone
from being consumed by the discharged HC, whereby the PM oxidation
efficiency is improved.
[0132] Here, the oxidation catalyst 110 is a catalyst for reacting
unburned components such as HC or CO with O.sub.2 to generate CO,
CO.sub.2, H.sub.2O or others, and is formed of Pt/CeO.sub.2,
Mn/CeO.sub.2, Ni/CeO.sub.2, Cu/CeO.sub.2 or others.
[0133] In the third embodiment, it is possible to combine
structures of the second embodiment. That is, the second ozone
feeding nozzles 90, the flow rate control unit 91 and the NOx
sensor 92 used in the second embodiment may be provided in the
third embodiment.
[0134] Tests were conducted also in this third embodiment by using
model gas, results of which are as follows.
(1) Test Equipment
[0135] An overall structure of the test equipment is the same as
that in the first embodiment shown in FIG. 6. In this connection,
details of the area VII in FIG. 6 are different from each other in
accordance with Example and Comparative examples described later.
As shown in FIG. 16, in Example 5 described later, the NOx catalyst
64 and the oxidation catalyst 120 are arranged in series in the
upstream quartz tube 63. For this purpose, the upstream quartz tube
63 is longer than that used in the first embodiment. In the
Comparative example 15 described later, as shown in FIG. 17, the
NOx catalyst is solely disposed in the upstream quartz tube 63. The
NOx catalyst 64 is of a selective catalytic reduction type in this
embodiment and that of a storage reduction type is not tested.
(2) Test Conditions
[0136] The test conditions are the same as those described in item
(II) (2) of the first embodiment.
(3) Test Method
[0137] The test method is the same as tat described in item (II)
(3) of the first embodiment.
(4) Example and Comparative Examples
Example 5
[0138] The NOx catalyst 64 and DPF 66 the same as those used in
Example 2 described in the item (II) (4) of the first embodiment. A
honeycomb structure is used as the oxidation catalyst, made of
cordierite having a diameter of 30 mm, a length of 25 mm, a cell
wall thickness of 4 mil and the number of cells of 400 cpsi was
coated with Ce--Zr composite oxide. A coated amount is 120 g/L. Pt
was carried thereon by using aqueous solution containing
dinitrodiamine platinum and, after being dried, calcined at
450.degree. C. for one hour. The amount of Pt tcarried thereon is 2
g/L.
Comparative Example 15
[0139] Comparative example 15 is the same as Example 2 described in
the item (II) (4) of the second embodiment.
(5) Test Results
[0140] FIG. 18 illustrates the PM oxidation rates in five minutes
after N.sub.2 is changed to the model gas (in other words, after
O.sub.2 is introduced into the ozone generator). According to this
result, it will be understood that by providing the oxidation
catalyst between the NOx catalyst and the ozone feeding nozzles,
the PM oxidation rate is improved.
[0141] As shown in FIG. 15, while the oxidation catalyst 110 is
disposed downstream from the NOx catalyst 20, the oxidation
catalyst 110 may be provided upstream from the NOx catalyst 20.
Thereby, it is possible to enhance the operation of the NOx
catalyst 20 since or CO in the exhaust gas is partially oxidized by
the oxidation catalyst 110 prior to being introduced into the NOx
catalyst 20. Particularly, since a large amount of HC is discharged
in the interval from the initiation of the staring stage of the
engine to the completion of the warming-up stage, the oxidation
catalyst may be disposed at a position as close as possible to the
engine body in the upstream direction to accelerate the activity of
the oxidation catalyst so that HC is positively cleaned thereby.
Even in the above-mentioned case wherein the oxidation catalyst 110
is disposed upstream from the NOx catalyst 20, since the object for
preliminarily remove HC by the oxidation catalyst 110 before the
supply of ozone is achievable, there may be cases wherein such a
reverse arrangement may be employed.
[0142] While the present invention has been described with
reference to the preferred embodiments, it is possible that the
present invention may include other embodiments. For example, while
the wall flow type DPF is employed as the PM collecting device in
the above-mentioned embodiments, many other filter structures may
be employed. For example, a straight flow type filter using static
electricity may be adopted, wherein a direct voltage is applied
between a pair of electrodes existing in the exhaust gas to
generate electric discharge so that PM is charged in minus and
attracted to the plus side or earth side electrode. Accordingly,
the PM collecting device is formed as an electrode on the plus side
or the earth side. Also, the shape or structure of the substrate
may be a plate, tube, pellet or mesh form other than the
above-mentioned honeycomb form.
[0143] The present invention may be applicable to all kinds of
internal combustion engines having a possibility for generating PM,
other than the diesel engine as a compressive ignition type
internal combustion engine, including, for example, a direct
injection spark ignition type internal combustion engine, more
concretely, a direct injection lean burn type gasoline engine. In
this engine, fuel directly injected into the combustion chamber is
not completely burned in a high load area wherein a large amount of
fuel is injected, whereby there is a possibility of generating PM.
The same effect and operation is expected as those described before
if the present invention is applied to such an engine.
[0144] The embodiment of the present invention should not be
limited to those described above, but includes all changes,
modifications or equivalents within a spirit or scope of the
present invention defined by the attached claims. Accordingly, the
present invention should not be limitative but applicable to any
other techniques included within a spirit of the present
invention.
INDUSTRIAL APPLICABILITY
[0145] The present invention is applicable to all internal
combustion engines having the possibility of generating particulate
matter.
* * * * *