U.S. patent application number 09/242496 was filed with the patent office on 2001-11-08 for exhaust emission control catalyst for diesel engines.
Invention is credited to FUJIMOTO, YOSHIO, ITO, KAZUHIRO, OHASHI, NOBUMOTO, TAKESHIMA, SHINICHI, TANAKA, TOSHIAKI.
Application Number | 20010039242 09/242496 |
Document ID | / |
Family ID | 16646109 |
Filed Date | 2001-11-08 |
United States Patent
Application |
20010039242 |
Kind Code |
A1 |
TANAKA, TOSHIAKI ; et
al. |
November 8, 2001 |
EXHAUST EMISSION CONTROL CATALYST FOR DIESEL ENGINES
Abstract
An exhaust purification catalyst for a compression ignition
engine comprises a carrier substrate and a metal-carrying layer
which carries catalytic metal formed on the surface of the carrier
substrate, and is characterized in that a diffusion restriction
layer having a thickness smaller than 170 .mu.m is provided on the
surface of the metal-carrying layer opposite to the carrier
substrate.
Inventors: |
TANAKA, TOSHIAKI; (SHIZUOKA,
JP) ; FUJIMOTO, YOSHIO; (SHIZUOKA, JP) ;
OHASHI, NOBUMOTO; (SHIZUOKA, JP) ; TAKESHIMA,
SHINICHI; (SHIZUOKA, JP) ; ITO, KAZUHIRO;
(SHIZUOKA, JP) |
Correspondence
Address: |
EDWARD W GREASON
KENYON & KENYON
ONE BROADWAY
NEW YORK
NY
10004
|
Family ID: |
16646109 |
Appl. No.: |
09/242496 |
Filed: |
February 12, 1999 |
PCT Filed: |
August 13, 1997 |
PCT NO: |
PCT/JP97/02826 |
Current U.S.
Class: |
502/344 ;
428/402; 502/327; 502/340; 502/341 |
Current CPC
Class: |
Y10T 428/2982 20150115;
B01J 35/04 20130101; B01J 37/0244 20130101; B01J 37/0246 20130101;
B01D 53/944 20130101; B01J 33/00 20130101; B01J 23/40 20130101;
B01D 53/94 20130101 |
Class at
Publication: |
502/344 ;
502/340; 502/341; 502/327; 428/402 |
International
Class: |
B01J 023/04; B01J
023/02 |
Foreign Application Data
Date |
Code |
Application Number |
Aug 13, 1996 |
JP |
8-213853 |
Claims
1. (Cancelled)
2. An exhaust purification catalyst for a compression ignition
engine comprising a carrier substrate, a porous metal-carrying
layer which carries catalytic metal formed on the surface of said
carrier substrate and characterized in that a porous diffusion
restriction layer which includes pores having sizes smaller than
those of said metal-carrying layer is provided on the surface of
said metal-carrying layer opposite to said carrier substrate.
3. (Amended) An exhaust purification catalyst for a compression
ignition engine according to claim 2, wherein a catalytic metal,
which has an oxidation power lower than that of said catalytic
metal carried on said metal-carrying layer, is carried on said
diffusion restriction layer.
4. (Amended) An exhaust purification catalyst for a compression
ignition engine according to claim 2, wherein a catalytic metal
which traps SO.sub.2 is carried on said diffusion restriction
layer.
5. (Amended) An exhaust purification catalyst for a compression
ignition engine according to claim 2, wherein a diffusion
restriction layer is provided between said carrier substrate and
said metal-carrying layer.
6. (Amended) An exhaust purification catalyst for a compression
ignition engine according to claim 2, wherein an adsorbing layer is
provided for adsorbing hydrocarbon between said carrier substrate
and said metal-carrying layer.
7. (Amended) An exhaust purification catalyst for a compression
ignition engine according to claim 2, wherein a portion having a
predetermined thickness of said diffusion restriction layer which
extends from the surface of said diffusion restriction layer
opposite to said metal-carrying layer into said diffusion
restriction layer comprises a thermal deterioration restriction
layer which has thermal resistance.
8. An exhaust purification catalyst for a compression ignition
engine according to claim 7, wherein said thermal deterioration
restriction layer is made of alumina, and the remaining diffusion
restriction layer is made of titania.
9. (Added) An exhaust purification catalyst for a compression
ignition engine according to claim 2, wherein the thickness of said
diffusion restriction layer is equal to or lower than 170 .mu.m.
Description
TECHNICAL FIELD
[0001] The invention is related to an exhaust purification catalyst
for a compression ignition engine.
BACKGROUND ART
[0002] An exhaust purification catalyst is known for purifying HC,
CO and SOF included in an exhaust gas discharged from a compression
ignition engine (Japanese Unexamined Patent Publication No.
5-57191). The exhaust purification catalyst comprises a carrier
substrate, and an activated alumina layer carries catalytic metal,
for purifying HC, CO and SOF, on a surface of the carrier
substrate. Further, SO.sub.2 is included in the exhaust gas, and
toxic SO.sub.3 is produced when SO.sub.2 reaches the catalytic
metal and is oxidized. Thus, according to the above exhaust
purification catalyst, in order to prevent SO.sub.2 from reaching
the catalytic metal, a trap layer for trapping SO.sub.2 is provided
upstream of the activated alumina layer. The trap layer is made of
an alumina layer including, for example, Mn oxide therein.
[0003] In the above exhaust purification catalyst, the amount of
SO.sub.2 trapped by the trap layer is limited. Further, there is a
possibility that the trap layer may become unable to trap SO.sub.2
due to the deterioration thereof. Therefore, there is a problem
that it easily becomes impossible to prevent SO.sub.2 from reaching
the catalytic metal. Thus, the object of the invention is to
prevent SO.sub.2 from reaching the metal-carrying layer.
DISCLOSURE OF INVENTION
[0004] In the first invention, an exhaust purification catalyst for
a compression ignition engine comprises a carrier substrate and a
metal-carrying layer which carries catalytic metal formed on the
surface of the carrier substrate and is characterized in that a
diffusion restriction layer having a thickness less than 170 .mu.m
is provided on the surface of the metal-carrying layer opposite to
the carrier substrate. The diffusion restriction layer provided on
the surface of the metal-carrying layer prevents substances
included in the exhaust gas from diffusing into the metal-carrying
layer.
[0005] In the second invention, an exhaust purification catalyst
for a compression ignition engine comprises a carrier substrate and
a porous metal-carrying layer which carries catalytic metal formed
on the surface of the carrier substrate and is characterized in
that a porous diffusion restriction layer which includes pores
having sizes smaller than those of the metal-carrying layer is
provided on the surface of the metal-carrying layer opposite to the
carrier substrate. The diffusion restriction layer which has pores
having sizes smaller than those of the metal-carrying layer
prevents substances included in the exhaust gas from diffusing into
the metal-carrying layer.
[0006] In the third invention according to the first or second
invention, catalytic metal which has an oxidation power lower than
that of the catalytic metal carried on the metal-carrying layer is
carried on the diffusion restriction layer.
[0007] Therefore, the substances included in the exhaust gas are
oxidized in the diffusion restriction layer.
[0008] In the fourth invention according to the first or second
invention, a catalytic metal which traps SO.sub.2 is carried on the
diffusion restriction layer. SO.sub.2 included in the exhaust gas
is trapped by the diffusion restriction layer so that SO.sub.2 is
further restricted from diffusing into the metal-carrying
layer.
[0009] In the fifth invention according to the first or second
invention, a diffusion restriction layer is provided between the
carrier substrate and the metal-carrying layer. Therefore, the
chance that substances included in the exhaust gas stay in the
vicinity of the metal-carrying layer is increased.
[0010] In the sixth invention according to the first or second
invention, an adsorbing layer is provided for adsorbing hydrocarbon
between the carrier substrate and the metal-carrying layer.
Therefore, the chance that HC included in the exhaust gas stays in
the vicinity of the metal-carrying layer is increased.
[0011] In the seventh invention according to the first or second
invention, a portion having a predetermined thickness of the
diffusion restriction layer which extends from the surface of the
diffusion restriction layer opposite to the metal-carrying layer
into the diffusion restriction layer comprises a thermal
deterioration restriction layer which has thermal resistance.
[0012] In the eighth invention according to the seventh invention,
the thermal deterioration restriction layer is made of alumina, and
the remaining diffusion restriction layer is made of titania.
BRIEF DESCRIPTION OF DRAWINGS
[0013] FIG. 1 is a cross-sectional view of the first embodiment of
the exhaust purification catalyst;
[0014] FIG. 2 is a view showing the amount of the produced sulfide
and the percentages of the purification of HC and SOF in the
different thicknesses of the diffusion restriction layer;
[0015] FIG. 3 is a view showing a relationship between the
thickness of the diffusion restriction layer and the property
ratio;
[0016] FIG. 4 is a view showing a relationship between the
proportion of the thickness of the diffusion restriction layer and
the percentage of the oxidation reaction;
[0017] FIG. 5 is a view showing a relationship between the catalyst
temperature and the rates of the oxidation reactions of HC, CO, SOF
and SO.sub.2;
[0018] FIG. 6 is a view showing a relationship between the catalyst
temperature and the percentage of the oxidation reaction of
SO.sub.2;
[0019] FIG. 7 is a cross-sectional view of the second embodiment of
the exhaust purification catalyst;
[0020] FIG. 8 is a cross-sectional view of the third embodiment of
the exhaust purification catalyst;
[0021] FIG. 9 is a cross-sectional view of the first embodiment of
the engine comprising the exhaust purification catalyst according
to the third embodiment; and
[0022] FIG. 10 is a cross-sectional view of the fourth embodiment
of the exhaust purification catalyst.
BEST MODE FOR CARRYING OUT THE INVENTION
[0023] Embodiments according to the invention will be explained
below, referring to the drawings. FIG. 1 is a partial cross
sectional view of the first embodiment of the exhaust purification
catalyst.
[0024] In FIG. 1, 10 denotes a carrier substrate for carrying
layers which purify an exhaust gas. The substrate 10 is a monolith
type such as a foam filter or a honeycomb filter, or a pellet type.
The material of the substrate 10 is a ceramic such as cordierite or
a metal.
[0025] A metal-carrying layer 12 which carries catalytic metal for
oxidizing HC (hydrocarbon), CO (carbon monoxide) and SOF (soluble
organic substance) included in the exhaust gas is provided on the
surface of the carrier substrate 10. The metal-carrying layer 12 is
porous. The material of the metal-carrying layer 12 is selected
from, for example, alumina, titania, silica and zirconia. on the
other hand, the catalytic metal carried on the metal-carrying layer
12 is selected from, for example, Pt, Rh and Pd.
[0026] A diffusion restriction layer 14 which prevents SO.sub.2
(sulfur dioxide) included in the exhaust gas from reaching the
metal-carrying layer 12 is provided on the surface of the
metal-carrying layer 12 opposite to the carrier substrate 10.
[0027] The diffusion restriction layer 14 is porous. The material
of the diffusion restriction layer 14 is selected from, for
example, alumina, titania and silica.
[0028] FIG. 2 shows the amount of the produced sulfide and the
percentages of the purification of HC and SOF without any diffusion
restriction layer and with diffusion restriction layers which have
different thicknesses at the catalyst temperature of 550.degree.
C.
[0029] FIG. 3 shows a relationship between the thickness of the
diffusion restriction layer 14 and the property ratios calculated
on the basis of FIG. 2. In FIG. 3, the property includes the amount
of the produced sulfide, and the percentages of the purification of
HC SOF, the property ratios includes ratios of the property in the
catalyst provided with diffusion restriction layers which have
different thicknesses to the property in the catalyst provided with
no diffusion restriction layer. That is, in FIG. 3, the solid line
illustrates the ratio of the amount of the produced sulfide in case
that the diffusion restriction layer 14 is provided to that in case
that no diffusion restriction layer is provided, the dotted line
illustrates the ratio of the purification percentage of HC in case
of the diffusion restriction layer 14 is provided to that in case
that no diffusion restriction layer is provided, and the dotted
line including some points illustrates the ratio of the
purification percentage of SOF in case that the diffusion
restriction layer 14 is provided to that in case that no diffusion
restriction layer is provided.
[0030] Referring to FIG. 3, each property ratio is decreased when
the thickness of the diffusion restriction layer 14 is decreased.
However, the degree of the decrease in the property ratio is
different for every property ratio. That is, the degree of the
decrease in the property ratio related to the amount of the
produced sulfide is greater than those related to other property
ratios. Therefore, in the exhaust purification catalyst according
to the first embodiment, the amount of the produced sulfide is
decreased while a decrease in the efficiency of purification of HC
and SOF is prevented.
[0031] FIG. 4 shows a relationship between the proportion of the
thickness of the diffusion restriction layer and the efficiency of
the oxidation reaction of SO.sub.2. The proportion of the thickness
of the diffusion restriction layer means the proportion of the
thickness of the diffusion restriction layer 14 to that of the
metal-carrying layer 12. Further, in FIG. 4, the proportion of the
thickness of the diffusion restriction layer is a proportion
relative to the about 170 .mu.m thickness of the metal-carrying
layer.
[0032] Referring to FIG. 4, the percentage of the oxidation
reaction of SO.sub.2 is decreased when the proportion of the
thickness of the diffusion restriction layer is increased. Also,
the percentages of the purification of HC, CO and SOF are decreased
when the proportion of the thickness of the diffusion restriction
layer is decreased. Further, the percentage of the oxidation
reaction of SO.sub.2 is about zero when the proportion of the
thickness of the diffusion restriction layer exceeds 100%, i.e.,
about 170 .mu.m. Therefore, preferably, in order to prevent the
decrease of the purification percentages of HC, CO and SOF and to
decrease the percentage of the oxidation reaction of SO.sub.2, the
thickness of the diffusion restriction layer is equal to or smaller
than 170 .mu.m and the proportion of the diffusion restriction
layer is 3% to 98%.
[0033] Further, the rate of change of the decrease of the
percentage of the oxidation reaction of SO.sub.2 in the range of
the proportion of the thickness between 3% and 20% is larger than
that in the range of the other proportion of the thickness.
Therefore, further preferably, in order to prevent the decrease in
the purification percentages of HC, CO and SOF and to decrease the
percentage of the oxidation reaction of SO.sub.2, the proportion of
the diffusion restriction layer is 3% to 20%.
[0034] The percentage of the oxidation reaction is increased to
produce a large amount of the toxic SO.sub.3 when the catalyst
temperature is increased. FIG. 5 shows a relationship between the
catalyst temperature and the percentage of the oxidation reaction
of SO.sub.2. The curve A denotes the percentage of the oxidation
reaction of SO.sub.2 in case that no diffusion restriction layer is
provided, and the curve B denotes the percentage of the oxidation
reaction of SO.sub.2 in case that the diffusion restriction layer
is provided. Referring to FIG. 5, in the exhaust purification
catalyst according to the first embodiment, SO.sub.2 is prevented
from reaching the metal-carrying layer so that the inclination of
the increasing of that the percentage of the oxidation reaction of
SO.sub.2 when the catalyst temperature is increased is smaller than
that in the exhaust purification catalyst provided with no
diffusion restriction layer.
[0035] The effect of preventing each compound, i.e., HC, CO, SOF
and SO.sub.2 from reaching the metal-carrying layer is different
for every compound. FIG. 6 shows a relationship between the
catalyst temperature and the rates of the oxidation reaction of HC,
CO, SOF and SO.sub.2. The curve C denotes the rate of the oxidation
reaction of HC, CO and SOF, and the curve D denotes the rate of the
oxidation reaction of SO.sub.2. Further, the dotted line denotes
the rate of the oxidation reaction in case that no diffusion
restriction layer is provided, and solid line denotes the rate of
the oxidation reaction in case that the diffusion restriction layer
is provided.
[0036] Referring to FIG. 6, the rate of the oxidation reaction of
HC, CO and SOF in the catalyst provided with the diffusion
restriction layer is smaller than that in the catalyst provided
with no diffusion restriction layer when the catalyst temperature
exceeds a predetermined temperature T.sub.1. Further, the
percentage of the oxidation reaction of SO.sub.2 in the catalyst
provided with the diffusion restriction layer is smaller than that
in the catalyst provided with no diffusion restriction layer when
the catalyst temperature exceeds a predetermined temperature
T.sub.2.
[0037] Further, the percentage of the decrease of the rate of the
oxidation reaction of HC, CO and SOF by the diffusion restriction
layer is smaller than the percentage of the decrease of the rate of
the oxidation reaction of SO.sub.2 by the diffusion restriction
layer. That is, the effect of the diffusion restriction layer to
prevent SO.sub.2 from diffusing is larger than that of the
diffusion restriction layer to prevent HC, CO and SOF. This is
caused since the characteristic of SO.sub.2 to adsorb on the
diffusion restriction layer is larger those of HC, CO and SOF, so
that SO.sub.2 rather than HC, CO and SOF cannot easily reach the
metal-carrying layer.
[0038] Therefore, in the catalyst provided with the diffusion
restriction layer, the rate of the oxidation reaction of SO.sub.2
is kept relatively small and the rates of the oxidation reaction of
HC, CO and SOF are kept relatively large when the catalyst
temperature is lower than the predetermined temperature T.sub.2,
and thus the oxidation of SO.sub.2 is prevented while HC, CO and
SOF are sufficiently purified. On the other hand, even when the
catalyst temperature is higher than the predetermined temperature
T.sub.2, the increase of the rate of the oxidation reaction of
SO.sub.2 which is largely increased in the range of the temperature
higher than the predetermined temperature T.sub.2 is largely
prevented while the rate of the oxidation reaction of HC, CO and
SOF is kept relatively large. Therefore, even when the catalyst
temperature is higher than the predetermined temperature T.sub.2,
the oxidation of SO.sub.2 is prevented while HC, CO and SOF are
sufficiently purified.
[0039] Therefore, according to the invention, the oxidation of
SO.sub.2 is prevented while HC, CO and SOF are sufficiently
purified regardless of the catalyst temperature.
[0040] HC, CO and SOF may be discharged from the catalyst to the
outside thereof without being purified in the metal-carrying layer
when the times that HC, CO and SOF remain in the metal-carrying
layer are short.
[0041] Therefore, the object of the exhaust purification catalyst
according to the second embodiment is to increase the time that HC,
CO and SOF remain in the metal-carrying layer.
[0042] FIG. 7 is a cross-sectional view of the exhaust purification
catalyst according to the second embodiment. In the exhaust
purification catalyst according to the second embodiment, a second
diffusion restriction layer 16 for preventing HC, CO and SOF from
diffusing is provided between the carrier substrate 10 and
metal-carrying layer 12. The second diffusion restriction layer 16
is porous. The material of the second diffusion restriction layer
16 is selected from alumina, titania, silica and zirconia.
[0043] In the exhaust purification catalyst according to the second
embodiment, the second diffusion restriction layer 16 slows HC, CO
and SOF which passes through the metal-carrying layer 12 without
being purified by the catalytic metal carried by the metal-carrying
layer 12 in vicinity of the metal-carrying layer 12. Therefore, the
chance that HC, CO and SOF are slowed in vicinity of the catalytic
metal is increased so that the purification percentages of HC, CO
and SOF are increased.
[0044] Alternatively, an adsorbing layer for adsorbing HC may be
provided as the second diffusion restriction layer 16 between the
carrier substrate 10 and the metal-carrying layer 12. The material
of the adsorbing layer is zeolite comprised of, for example,
alumina and silica. HC which passes through the metal-carrying
layer without being purified by the catalytic metal is adsorbed on
the adsorbing layer. Therefore, the chance of the staying of HC in
vicinity of the metal-carrying layer 12 is further increased so
that the purification percentage of HC is increased, compared with
that in the catalyst with the second diffusion restriction layer.
Further, the adsorbing layer is porous so that the diffusion of CO
and SOF is prevented, and the purification percentages of CO and
SOF are kept substantially equal to that in the catalyst with the
second diffusion restriction layer.
[0045] Alternatively, in order to prevent SO.sub.2 from reaching
the metal-carrying layer 12, the average radius of the pores of the
diffusion restriction layer 14 provided in the outermost side
relative to the carrier substrate may be smaller than that of the
metal-carrying layer 12 to further increase the effect to prevent
SO.sub.2 from diffusing.
[0046] Alternatively, in order to prevent SO.sub.2 from reaching
the metal-carrying layer 12, a component which has a high affinity
for SO.sub.2 may be added to the diffusion restriction layer 14
provided in the outermost side relative to the carrier substrate to
trap SO.sub.2 in the diffusion restriction layer 14 to further
increase the effect to prevent SO.sub.2 from diffusing. The
component which has a high affinity for SO.sub.2 may be, for
example, a transition metal such as Zr, or a rare earth element, or
an alkaline metal, or an alkaline earth metal.
[0047] The layers of the exhaust purification catalyst are subject
to the heat of the exhaust gas. In particular, the diffusion
restriction layer of the exhaust purification catalyst according to
the above embodiments are largely subject to the heat of the
exhaust gas. Therefore, each layer of the exhaust purification
catalyst, in particular, the diffusion restriction layer, is
deteriorated by the heat of the exhaust gas.
[0048] Further, it is preferable to use titania which has
relatively large poisoning resistance against sulfur as the
material of the diffusion restriction layer, but titania has a
problem that it is easily deteriorated by heat.
[0049] Therefore, the object of the exhaust purification catalyst
according to the third embodiment is to prevent the heat
deterioration of each layer of the exhaust purification catalyst,
in particular, of the diffusion restriction layer.
[0050] FIG. 8 is a cross-sectional view of the exhaust purification
catalyst according to the third embodiment. In the exhaust
purification catalyst according to the third embodiment, in
addition to the exhaust purification catalyst according to the
first embodiment, a heat deterioration restriction layer 18 which
has a predetermined thickness for preventing the diffusion
restriction layer 14 from being deteriorated by the heat of the
exhaust gas is provided on the surface of the diffusion restriction
layer 14 which is located opposite to the metal-carrying layer 12.
For example, the material of the heat deterioration restriction
layer 18 can be alumina or zeolite which has large resistance
against heat. The heat deterioration restriction layer 18 is
porous. Therefore, the heat deterioration restriction layer 18
prevents SO.sub.2 from reaching the metal-carrying layer. That is,
according to the third embodiment, the diffusion restriction layer
14 and the heat deterioration restriction layer 18 serve as the
diffusion restriction layer for preventing SO.sub.2 from reaching
the metal-carrying layer. In other words, a portion which has a
predetermined thickness from the surface of the diffusion
restriction layer opposite to the metal-carrying layer toward the
interior of the diffusion restriction layer is the heat
deterioration restriction layer 18, and the remaining portion is
the diffusion restriction layer 14. The thickness of the diffusion
restriction layer 14 added by the heat deterioration restriction
layer 18 is smaller than about 170 .mu.m to not prevent HC, CO and
SOF from reaching the metal-carrying layer 12, preferably, several
.mu.m to several scores of .mu.m.
[0051] HC is adsorbed on the heat deterioration restriction layer
18 if the material of the heat deterioration restriction layer 18
is zeolite which has a property to adsorb HC. Therefore, if the
temperature of the exhaust purification catalyst does not reach a
catalyst activating temperature, HC is adsorbed on the heat
deterioration restriction layer 18 until the temperature of the
exhaust purification catalyst reaches the catalyst activating
temperature. Therefore, the purification percentage of HC in the
exhaust purification catalyst is increased. Further, the droplets
of the SOF vapors become gaseous after the SOF is adsorbed on the
heat deterioration restriction layer 18. Gaseous SOF can easily be
purified. Therefore, the purification percentage of SOF in the
exhaust purification catalyst is increased.
[0052] Alternatively, a very small amount of the noble metals may
be carried on the heat deterioration restriction layer 18. The
noble metals oxidize HC, CO, SOF and SO.sub.2. However, the rates
of the oxidation reaction of HC, CO and SOF are larger than that of
SO.sub.2. Therefore, the production of the sulfide is prevented
while the purification percentages of HC, CO and SOF are
increased.
[0053] Therefore, according to the third embodiment, the
deterioration of each layer of the exhaust purification catalyst by
the heat of the exhaust gas is prevented. In particular, in the
exhaust purification catalyst according to the third embodiment,
the deterioration of the diffusion restriction layer by the heat of
the exhaust gas is prevented.
[0054] The purification percentage of the exhaust purification
catalyst is larger than a predetermined percentage when the
catalyst temperature is higher than a predetermined temperature
(the catalyst activating temperature). Therefore, in order to
increase the purification percentage of the exhaust purification
catalyst, it is necessary to rapidly increase the catalyst
temperature to the catalyst activating temperature and to keep the
catalyst temperature higher than the catalyst activating
temperature. If the exhaust purification catalyst is positioned
closer to the engine, the catalyst temperature is rapidly increased
to the catalyst activating temperature. However, if the heat
resisting property of the exhaust purification catalyst is low, the
exhaust purification catalyst is deteriorated by the heat of the
exhaust gas. Further, the rate of the oxidation reaction of
SO.sub.2, is increased when the catalyst temperature of the exhaust
purification catalyst is high, and thus there is a problem that a
large amount of the toxic SO.sub.3 is produced.
[0055] Therefore, the object of an engine according to the first
embodiment is to prevent the heat deterioration of the exhaust
purification catalyst and the oxidation of SO.sub.2, and to rapidly
increase the catalyst temperature of the exhaust purification
catalyst to the catalyst activating temperature.
[0056] FIG. 9 is a cross-sectional view of the engine according to
the first embodiment. In FIG. 9, 21 denotes an engine body.
Combustion chambers 22 are formed in the engine body 21. Piston 23
is positioned in each combustion chamber 22. Further, the engine
body 21 comprises fuel injectors 24 for injecting fuel into the
combustion chambers 22. Further, intake and exhaust ports 25 and 26
are formed in the engine body 21. An intake passage 27 is connected
to the intake port 25. Further, intake valves 28 are positioned in
the openings of the intake port 25 which opens to the combustion
chambers 22. On the other hand, an exhaust passage 29 is connected
to the exhaust port 26. Further, exhaust valves 30 are positioned
in the openings of the exhaust port 26 which opens to the
combustion chambers 22.
[0057] Further, an exhaust purification catalyst 31 is positioned
in the exhaust port 26 for purifying the exhaust gas. The exhaust
purification catalyst is that according to the third
embodiment.
[0058] The heat deterioration restriction layer 18 is provided in
the exhaust purification catalyst 31 for preventing the heat
deterioration of the diffusion restriction layer 14. Therefore, the
exhaust purification catalyst 31 can be positioned in the exhaust
port 25. Thus, the catalyst temperature of the exhaust purification
catalyst 31 can rapidly be increased to the catalyst activating
temperature. Further, the diffusion restriction layer 14 is
provided in the exhaust purification catalyst 31 for preventing
SO.sub.2 from reaching the metal-carrying layer 12. Therefore, even
if the catalyst temperature of the exhaust purification catalyst is
increased, SO.sub.2 is prevented from being oxidized in the
metal-carrying layer 12 to produce sulfur oxide.
[0059] Therefore, in the engine according to the first embodiment,
the heat deterioration of the exhaust purification catalyst and the
oxidation of SO.sub.2 are prevented while the catalyst temperature
of the exhaust purification catalyst is rapidly increased to the
catalyst activating temperature.
[0060] Further, the purification power of the exhaust purification
catalyst is decreased if SOF adsorbs on the exhaust purification
catalyst. However, in the engine according to the first embodiment,
the catalyst temperature of the exhaust purification catalyst is
kept high so that SOF is oxidized without being adsorbed on the
exhaust purification catalyst. Therefore, in the engine according
to the first embodiment, poisoning of the exhaust purification
catalyst by SOF is prevented.
[0061] A exhaust turbine wheel 32 of a turbo charger is positioned
in the exhaust passage 29. The exhaust turbine wheel 32 absorbs the
heat energy so that the temperature of the exhaust gas downstream
of the exhaust turbine wheel 32 is lower than that upstream of the
exhaust turbine wheel 32. The temperature of the exhaust gas
upstream of the exhaust turbine wheel 32 is high for the exhaust
purification catalyst with no heat deterioration restriction layer
so that the exhaust purification catalyst is deteriorated by the
heat. Therefore, the exhaust purification catalyst with no
diffusion restriction layer should be positioned downstream of the
exhaust turbine wheel 32. However, the temperature of the exhaust
gas downstream of the exhaust turbine wheel 32 is low so that the
temperature of the exhaust purification catalyst is not increased
to the catalyst activating temperature.
[0062] On the other hand, in the engine according to the first
embodiment, the exhaust purification catalyst 31 can be positioned
upstream of the exhaust turbine wheel 32. That is, the exhaust
purification catalyst 31 can be positioned in the exhaust system
between the combustion chamber 22 and the exhaust turbine wheel 32.
Therefore, the temperature of the exhaust purification catalyst is
increased to the catalyst activating temperature.
[0063] It should be noted that the words "upstream" and
"downstream" are used along the flow of the exhaust gas, and the
phrase "exhaust system" means the exhaust passage or the exhaust
port.
[0064] Exhaust particulates such as SOF and soot are included in
the exhaust gas discharged from the compression ignition engine.
The size of the exhaust particulates becomes large when the exhaust
particulates flow downwardly. That is, the size of the exhaust
particulates is small at the upstream side. In the engine according
to the above first embodiment, the exhaust purification catalyst 31
is positioned in the exhaust port 26 relatively close to the
combustion chambers 22. Therefore, the size of the exhaust
particulate in the exhaust gas which passes through the exhaust
purification catalyst 31 is relatively small, so that there is a
possibility that the exhaust purification catalyst 31 cannot trap
the exhaust particulate to purify them. Therefore, the object of
the exhaust purification catalyst according to the fourth
embodiment is to purify the exhaust particulate even if the exhaust
purification catalyst is positioned in the exhaust system close to
the combustion chambers.
[0065] FIG. 10 is a cross-sectional view of the exhaust
purification catalyst according to the fourth embodiment. In the
exhaust purification catalyst according to the fourth embodiment, a
filter 35 for trapping the exhaust particulates is substantially
positioned in the central portion of the exhaust purification
catalyst 31 between inlet and outlet ends 33 and 34 of the exhaust
purification catalyst 31. The filter 35 is, for example, a foam
filter or a metallic non-woven fabric.
[0066] The filter 35 can trap a relatively small exhaust
particulate. The exhaust particulate trapped in the filter 35 is
burned by the heat generated from the exhaust gas and by the
purification reaction of the exhaust gas in the exhaust
purification catalyst 31 upstream of the filter 35. Therefore, in
the exhaust purification catalyst according to the fourth
embodiment, the exhaust particulate can be purified even if the
exhaust purification catalyst is positioned in the exhaust system
close to the combustion chambers.
[0067] Further, NO.sub.2 is produced in the exhaust purification
catalyst when the exhaust gas is purified. NO.sub.2 serves as a
catalyst for promoting the burning of the soot. Therefore, NO.sub.2
produced in the exhaust purification catalyst 31 upstream of the
filter 35 flows into the filter 35 so that the soot in the exhaust
particulates is easily burned. That is, the soot trapped in the
filter 35 is early burned. Therefore, the increase of the flow
resistance of the filter 35 is prevented, and the characteristics
of discharging the exhaust gas is kept high even if the filter 35
is positioned in the exhaust purification catalyst 31.
[0068] Alternatively, the filter 35 may carry a catalytic metal for
improving the characteristics of purifying the exhaust
particulates.
* * * * *