U.S. patent application number 11/719373 was filed with the patent office on 2009-06-11 for exhaust emission control device.
This patent application is currently assigned to HINO MOTORS LTD.. Invention is credited to Takatoshi Furukawa, Shinya Sato.
Application Number | 20090145114 11/719373 |
Document ID | / |
Family ID | 36407170 |
Filed Date | 2009-06-11 |
United States Patent
Application |
20090145114 |
Kind Code |
A1 |
Sato; Shinya ; et
al. |
June 11, 2009 |
EXHAUST EMISSION CONTROL DEVICE
Abstract
Even in a vehicle with travel pattern of continuing operational
status with low exhaust temperature, a satisfactory NO.sub.x
reduction effect can be attained even at exhaust temperature lower
than that required conventionally therefor. In an exhaust emission
control device with selective reduction catalyst 10 incorporated in
an exhaust pipe 9, ammonia being added upstream of the catalyst 10
for reduction and purification of NO.sub.x, the device comprises an
ammonia generator 12 with a vessel 15 for holding urea water 23a
and with an electrode 16 for generation of ammonia 13a, 13b through
action of plasma on the urea water 23a in the vessel, the ammonia
13a, 13b generated in the generator 12 being fed upstream of the
catalyst 10.
Inventors: |
Sato; Shinya; (Tokyo,
JP) ; Furukawa; Takatoshi; (Tokyo, JP) |
Correspondence
Address: |
OBLON, SPIVAK, MCCLELLAND MAIER & NEUSTADT, P.C.
1940 DUKE STREET
ALEXANDRIA
VA
22314
US
|
Assignee: |
HINO MOTORS LTD.
TOKYO
JP
|
Family ID: |
36407170 |
Appl. No.: |
11/719373 |
Filed: |
November 17, 2005 |
PCT Filed: |
November 17, 2005 |
PCT NO: |
PCT/JP05/21108 |
371 Date: |
May 15, 2007 |
Current U.S.
Class: |
60/286 ;
60/297 |
Current CPC
Class: |
F02B 37/00 20130101;
F01N 2610/06 20130101; C01C 1/086 20130101; B01D 2259/818 20130101;
B01D 53/9431 20130101; B01D 2259/12 20130101; F01N 3/2066 20130101;
Y02A 50/2325 20180101; B01J 2219/0894 20130101; B01D 2251/2062
20130101; F01N 2610/02 20130101; F01N 2610/14 20130101; Y02A 50/20
20180101; B01D 2251/2067 20130101; B01J 2219/00006 20130101; Y02T
10/12 20130101; Y02T 10/24 20130101; F01N 3/36 20130101; F01N
2240/25 20130101; F01N 2240/28 20130101 |
Class at
Publication: |
60/286 ;
60/297 |
International
Class: |
F01N 9/00 20060101
F01N009/00 |
Foreign Application Data
Date |
Code |
Application Number |
Nov 18, 2004 |
JP |
2004-334578 |
Claims
1. An exhaust emission control device with a selective reduction
catalyst incorporated in an exhaust pipe, ammonia being added
upstream of the catalyst so as to reduce and purify NO.sub.x, said
exhaust emission control device comprising an ammonia generator
with a vessel for holding urea water and with an electrode for
generation of ammonia through action of plasma on the urea water in
the vessel, the ammonia generated in the ammonia generator being
fed upstream of the catalyst.
2. An exhaust emission control device as claimed in claim 1,
wherein dielectric pellets are charged in the urea water in the
vessel.
3. An exhaust emission control device as claimed in claim 1,
wherein ammonia gas is taken out from the ammonia generator.
4. An exhaust emission control device as claimed in claim 2,
wherein ammonia gas is taken out from the ammonia generator.
5. An exhaust emission control device as claimed in claim 1,
wherein ammonia water is taken out from the ammonia generator.
6. An exhaust emission control device as claimed in claim 2,
wherein ammonia water is taken out from the ammonia generator.
7. An exhaust emission control device as claimed in any one of
claims 1-6 further comprising a pH meter for detecting
concentration of ammonia taken out from the vessel and a controller
for outputting a command on an amount of ammonia to be fed upstream
of the catalyst on the basis of the detected value from the pH
meter.
Description
TECHNICAL FIELD
[0001] The present invention relates to an exhaust emission control
device applied to an engine such as diesel engine.
BACKGROUND ART
[0002] Conventionally some diesel engines have been provided with
selective reduction catalyst incorporated in an exhaust pipe
through which exhaust gas flows, said catalyst having a
characteristic of selectively reacting NO.sub.x with a reducing
agent even in the presence of oxygen. A required amount of reducing
agent is added upstream of the selective reduction catalyst and is
reacted with NO.sub.x (nitrogen oxides) in exhaust gas on the
catalyst to thereby reduce a concentration of the discharged
NO.sub.x.
[0003] Meanwhile, it is well known in a field of industrial flue
gas denitration in a plant or the like that ammonia (NH.sub.3) has
effectiveness as a reducing agent for reduction and purification of
NO.sub.x. However, in a field of automobile, guaranteed safety is
hard to obtain with respect to travel with ammonia itself being
loaded, so that researches have been made nowadays on use of
nontoxic urea water as reducing agent (see, for example, Reference
1).
[0004] More specifically, when added to the exhaust gas upstream of
the selective reduction catalyst, the urea water is pyrolytically
decomposed by heat of the exhaust gas into ammonia and carbon
dioxide according to the following equation, and NO.sub.x in the
exhaust gas on the catalyst is satisfactorily reduced and purified
by the ammonia generated.
(NH.sub.2).sub.2CO+H.sub.2O.fwdarw.2NH.sub.3+CO.sub.2 [0005]
[Reference 1] JP 2002-161732A
SUMMARY OF THE INVENTION
Problems to be Solved by the Invention
[0006] It has been experimentally ascertained that, by adding
ammonia to selective reduction catalyst in such kind of exhaust
emission control device, NO.sub.x reduction effect will be
obtained, provided that exhaust temperature exceeds about
140.degree. C.; however, pyrolytical decomposition of urea water
into ammonia and carbon dioxide requires exhaust temperature of at
least about 170-180.degree. C. Thus, if an operating status with
exhaust temperature of lower than about 200.degree. C. continues
(generally speaking, low-load operational areas are areas with low
exhaust temperature), there is a problem that NO.sub.x reduction
ratio is hardly to enhance since decomposition of urea water into
ammonia does not proceed well. For example, in a vehicle such as
city shuttle-bus with travel pattern of almost always traveling on
congested roads, an operation with more than required exhaust
temperature does not continue for a long time, and operational
transitions occur with no chance of NO.sub.x reduction ratio being
enhanced, failing in obtaining satisfactory NO.sub.x reduction
effect.
[0007] The invention was made in view of the above and has its
object to provide an exhaust emission control device which can
obtain satisfactory NO.sub.x reduction effect even at exhaust
temperature lower than that required conventionally therefor and
even in a vehicle with travel pattern of continuing operational
status with low exhaust temperature, which can effectively generate
ammonia from urea water and which can enhance controllability in
adding ammonia to the exhaust gas.
Means or Measures for Solving the Problems
[0008] The invention is directed to an exhaust emission control
device with selective reduction catalyst incorporated in an exhaust
pipe, ammonia being added upstream of the catalyst so as to reduce
and purify NO.sub.x, said exhaust emission control device
comprising an ammonia generator with a vessel for holding urea
water and with an electrode for generation of ammonia through
action of plasma on the urea water in the vessel, the ammonia
generated in the ammonia generator being fed upstream of the
catalyst.
[0009] According to the above means, the ammonia generated through
action of the plasma on the urea water in the ammonia generator is
fed upstream of the selective reduction catalyst, so that a
required amount of ammonia can be surely added to the exhaust gas
even in an operational status with low exhaust temperature to
thereby be effectively reacted with NO.sub.x in the exhaust gas on
the selective reduction catalyst; as a result, NO.sub.x in the
exhaust gas is satisfactorily reduced and purified even at exhaust
temperature lower than that required conventionally therefor.
Generation of ammonia can be easily and rapidly adjusted since the
ammonia is generated through action of plasma on the urea water;
and response in feeding the ammonia can be enhanced since the
generated ammonia is added to the exhaust gas.
[0010] It is preferable in the exhaust emission control device that
dielectric pellets are charged in the urea water in the vessel.
Such charging of the dielectric pellets in the urea water brings
about generation of plasma on surfaces of the pellets, thereby
further effectively enhancing the action of generating ammonia from
the urea water.
[0011] In the exhaust emission control device, ammonia gas may be
taken out from the ammonia generator. Addition of such ammonia gas
to the exhaust gas causes no trouble of lowering the exhaust
temperature, so that NO.sub.x reduction effect of the selective
reduction catalyst in an operational status with low exhaust
temperature can be further enhanced.
[0012] In the exhaust emission control device, ammonia water may be
taken out from the ammonia generator. Addition of such ammonia
water to the exhaust gas substantially causes no trouble of
lowering the exhaust temperature, though subtle heat may be taken
upon evaporation of the water. Thus, NO.sub.x reduction effect of
the selective reduction catalyst in an operational status with low
exhaust temperature can be highly maintained.
[0013] In the exhaust emission control device, a pH meter may be
arranged which detects concentration of ammonia taken out from the
vessel and a controller may be arranged which outputs a command on
amount of ammonia to be fed upstream of the selective reduction
catalyst on the basis of detected value from the pH meter, whereby
actual amount of ammonia to be fed to the exhaust gas can be
controlled with high response.
EFFECTS OF THE INVENTION
[0014] The above-mentioned exhaust emission control device of the
invention has effects and advantages. Ammonia is effectively
generated through action of plasma on urea water in an ammonia
generator and is fed upstream of the selective reduction catalyst
so that, unlike the conventional supply of urea water, a required
amount of ammonia can be surely added to exhaust gas without
lowering in temperature of the exhaust gas; thus even in an
operational status with low exhaust temperature, NO.sub.x can be
effectively reduced by the selective reduction catalyst. Because of
ammonia being generated through action of the plasma on the urea
water, the generation of the ammonia can be easily and rapidly
adjusted; because of the generated ammonia being added to the
exhaust gas, response in feeding the ammonia to the exhaust gas can
be enhanced.
BRIEF DESCRIPTION OF THE DRAWINGS
[0015] FIG. 1 A schematic overall diagram showing an exhaust pipe
pathway of an engine to which an exhaust emission control device of
the invention is applied.
[0016] FIG. 2 A schematic diagram showing an embodiment to take out
ammonia gas from the ammonia generator shown in FIG. 1.
[0017] FIG. 3 A perspective view showing a case where a plurality
of vessels each similar to that of FIG. 2 are provided in module
structure.
[0018] FIG. 4 A schematic diagram of an embodiment to take out
ammonia water from the ammonia generator of FIG. 2.
[0019] FIG. 5 A schematic diagram of an embodiment to take out
ammonia gas from an ammonia generator different from that shown in
FIG. 2.
[0020] FIG. 6 A schematic diagram of an embodiment to take out
ammonia water from the ammonia generator of FIG. 5.
[0021] FIG. 7 A graph showing relationship between exhaust
temperature and NO.sub.x reduction ratio.
EXPLANATION OF THE REFERENCE NUMERALS
[0022] 9 exhaust pipe [0023] 10 selective reduction catalyst [0024]
12 ammonia generator [0025] 13 ammonia [0026] 13a ammonia gas
(ammonia) [0027] 13b ammonia water (ammonia) [0028] 15 vessel
[0029] 16 electrode [0030] 17 casing [0031] 19a dielectric pellet
[0032] 23a urea water [0033] 32 controller [0034] 37 pH meter
[0035] 38 detected pH value [0036] 39 ammonia feed command [0037]
40 vessel [0038] 41 electrode plate
BEST MODE FOR CARRYING OUT THE INVENTION
[0039] Embodiments of the invention will be described in
conjunction with drawings.
[0040] FIG. 1 is a schematic overall diagram showing an exhaust
pipe pathway of an engine to which an exhaust emission control
device of the invention is applied. In FIG. 1, reference numeral 1
designates an engine such as diesel engine, the engine 1
illustrated having a turbocharger 2 with a compressor 2a to which
air 4 from an air cleaner 3 is fed through an intake pipe 5. The
air 4 thus pressurized in the compressor 2a is further fed to an
intercooler 6 where it is cooled. The cooled air 4 from the
intercooler 6 is guided to an intake manifold (not shown) from
which it is guided to respective cylinders of the engine 1.
[0041] Exhaust gas discharged from the respective cylinders of the
engine 1 is fed via an exhaust manifold 8 to a turbine 2b of the
turbocharger 2. The exhaust gas 7 thus having driven the turbine 2b
is discharged via an exhaust pipe 9 to outside of the vehicle.
[0042] Incorporated in the exhaust pipe 9 through which the exhaust
gas 7 flows is a selective reduction catalyst 10 encased by a
casing 11. The selective reduction catalyst 10 is in the form of a
flow-through type honeycomb structure and has a feature capable of
selectively reacting NO.sub.x with ammonia even in the presence of
oxygen.
[0043] In the above construction, the exhaust pipe 9 is provided
with a spray nozzle 14 upstream of the casing 11, said nozzle
injecting ammonia 13 generated in an ammonia generator 12 to add
the same to the exhaust gas 7.
[0044] FIG. 2 shows an embodiment in which ammonia gas 13a is taken
out from the ammonia generator 12 of FIG. 1. In the ammonia
generator 12, as ammonia 13, the ammonia gas 13a is generated and
is fed to the exhaust gas 7 in the exhaust pipe 9.
[0045] In FIG. 2, reference numeral 15 denotes a vessel made from
heat-resisting and insulating material such as polyethylene
fluoride (e.g., Teflon (registered trademark)). Arranged centrally
in the vessel 15 is an electrode 16 with its lower end extending
adjacent to a bottom of the vessel 15 and with its upper end
projected out of and fixed to the vessel 15. The vessel 15 is
encased by a casing 17 made from electro-conductive material such
as iron, the casing 17 being connected to an earth 18.
[0046] In the embodiment shown in FIG. 2, arranged in the vessel 15
is wirework 19 made of stainless steel into which charged are
dielectric pellets 19a which in turn may be made from material with
high dielectric constant such as titania, barium titanate or
alumina. The wirework 19 is connected to the earth 18.
[0047] Inserted into and opened in the vessel 15 adjacent to the
bottom thereof is a lower end of a urea water feed pipe 20 which
serves to feed urea water 23a in a urea water tank 23 arranged
above the vessel 15 into the vessel 15 via a urea water feed valve
21.
[0048] The electrode 16 is connected with power wire 25 which in
turn is connected to a power source 24 such as battery. The power
wire 25 is provided with a controller 26 for control of voltage,
driving pulse and the like. Thus, energization of the electrode 16
by the power source 24 generates plasma between the electrode 16
and casing 17, such plasma acting on the urea water 23a for
decomposition into ammonia and carbon dioxide.
[0049] Opened to space 27 in the vessel 15 and above a liquid level
of the urea water 23a is an ammonia feed pipe 28 which is connected
via a pump 29 and an ammonia feed valve 30 to the spray nozzle 14.
Thus, in this embodiment, the ammonia gas 13a generated in the
space 27 of the vessel 15 is taken out through the ammonia feed
pipe 28 and fed to the spray nozzle 14. In this connection, if the
space 27 is low in volume, the ammonia gas 13a may have difficulty
to stably feed; therefore, as shown in FIG. 1, the ammonia feed
pipe 28 is preferably provided with an ammonia gas storage tank 31
for temporary storage of the ammonia gas 13a generated in the space
27.
[0050] In FIGS. 1 and 2, reference numeral 32 denotes a controller
into which inputted is a liquid level signal 34 from a liquid-level
meter 33 arranged in the vessel 15 for detection of a liquid level.
Thus, depending upon the liquid level signal 34 from the meter 33,
the controller 32 outputs a urea water feed command 35 to control
an opening degree of the urea water feed valve 21 so as to keep
constant an amount of urea water 23a in the vessel 15.
[0051] The controller 32 outputs an electricity control command 36
to control the controller 26 such that the electricity fed to the
electrode 16 has predetermined voltage and drive pulse.
[0052] Further inputted to the controller 32 is a detected pH value
38 from a pH meter 37 which detects pH of the urea water 23a in the
vessel 15 (pH adjacent to the liquid level of the urea water 23a).
Thus, depending upon the detected pH value 38 from the pH meter 37,
the controller 32 outputs ammonia feed command 39 to control an
opening degree of the ammonia feed valve 30 to control the flow
rate of the ammonia gas 13a fed to the spray nozzle 14. More
specifically, the controller 32 and an engine control computer
(ECU: Electronic Control Unit) (not shown) exchange data such as
revolution speed and load of the engine 1, detected temperatures of
inlet and outlet temperature sensors 42a and 42b for the selective
reduction catalyst 10 and intake air amount; on the basis of a
current operational status judged from such data, an amount of
NO.sub.x generated is presumed. An amount of the ammonia gas 13a to
match the presumed generation amount of NO.sub.x is calculated so
that the required amount of ammonia gas 13a is added to the exhaust
gas 7.
[0053] FIG. 3 shows an embodiment with a plurality of such ammonia
generators 12. Inner space of a casing 17 is partitioned in the
form of lattice by electrically conductive material, each
partitioned space of the casing 17 receiving the module-structured
ammonia generators 12 each constituted by the vessel 15. With the
plural ammonia generators 12 being arranged as shown in FIG. 3, the
urea water feed pipe 20 is branched into a plurality of branch
pipes 20a. Through a urea water feed valve 21 in each of the branch
pipes 20a, urea water 23a is fed to each of the vessels 15. To the
respective electrodes 16 of the vessels 15, energization is
effected by the power wire 25 in the same condition. The ammonia
gas 13a generated in the respective vessels 15 is taken out
together by the single ammonia feed pipe 28.
[0054] Next, mode of operation of the above embodiments will be
described.
[0055] As shown in FIG. 2, with a predetermined amount of urea
water 23a being fed to the vessel 15 in the ammonia generator 12,
electricity is fed from the power source 24 (battery) for control
of voltage, driving pulse and the like by the controller 26 to a
predetermined condition. Then, plasma is generated between the
electrode 16 and casing 17 and, by the action of the plasma
generated, the urea water 23a is decomposed as shown by
(NH.sub.2).sub.2CO+H.sub.2O.fwdarw.2NH.sub.3+CO.sub.2
into ammonia and carbon dioxide. As a result, the urea water 23a in
the vessel 15 is changed into ammonia water while the ammonia gas
13a is generated in the upper space 27 of the vessel 15, the
ammonia gas 13a containing carbon dioxide and evaporated water.
[0056] As mentioned above, in the ammonia generator 12, the plasma
acts on the urea water 23a for decomposition into ammonia gas 13a,
so that the ammonia gas 13a can be generated easily and
quickly.
[0057] Further, in this connection, when the dielectric pellets 19a
made of material with high dielectric constant such as titania,
barium titanate or alumina are charged in the urea water 23a in the
vessel 15, plasma is generated on respective surfaces of the
pellets 19a, which substantially enhance decomposition reaction of
the urea water 23a, resulting in effective generation of the
ammonia gas 13a. With the plural ammonia generators 12 being
arranged as shown in FIG. 3, the ammonia gas 13a can be generated
concurrently in large quantity for supply.
[0058] In the above, by driving the pump 29, the ammonia gas 13a
generated in the vessel 15 is taken out through the ammonia feed
pipe 28 and is injected by the spray nozzle 14 upstream of the
selective reduction catalyst 10 to be added to the exhaust gas 7 in
the exhaust pipe 9.
[0059] Then, the controller 32 and the engine control computer (not
shown) exchange data such as revolution speed and load of the
engine 1, detected temperatures by the inlet and outlet temperature
sensors 42a and 42b for the selective reduction catalyst 10 and
intake air amount to thereby detect the current operational status,
so that a generation amount of NO.sub.x is presumed on the basis of
the detected operational status. An amount of the ammonia gas 13a
to match the presumed generation amount of NO.sub.x is calculated
and the ammonia feed valve 30 is controlled by the ammonia feed
command 39 so as to feed the required amount of ammonia gas 13a.
Since the detected pH value 38 from the pH meter 37 in the vessel
15 is inputted into the controller 32, the controller 32 can
calculate ammonia concentration depending upon the detected pH
value 38 of the pH meter 37 to compensate, on the basis of such
ammonia concentration, the ammonia feed command 39 for control of
the opening degree of the ammonia feed valve 30.
[0060] Since the liquid level in the vessel 15 is gradually lowered
due to the fact that the ammonia gas 13a decomposed from the urea
water 23a is taken out as mentioned above and due to evaporation of
the water, the urea water feed valve 21 is controlled by the
controller 32 on the basis of the liquid level signal 34 from the
liquid level meter 33 so as to feed, to the vessel 15, the urea
water 23a adjusted to a predetermined concentration in the tank 23,
whereby the amount of the urea water 23a in the vessel 15 is kept
constant.
[0061] According to the above embodiment, irrespective of the
temperature of the exhaust gas 7, the ammonia generator 12
generates the ammonia gas 13a by the action of the plasma, the
ammonia gas 13a being injected into and fed to the exhaust gas 7 in
the exhaust pipe 9. As a result, in comparison with the
conventional feed of urea water, a required amount of ammonia can
be surely added to the exhaust gas 7 even if the temperature of the
exhaust gas 7 is low; thus, even in a vehicle with travel pattern
of continuing the operational status with low exhaust temperature,
a sufficient NO.sub.x reduction effect can be exhibited even at
exhaust temperature lower than that conventionally required
therefor. Since the ammonia gas 13a causes no problem of lowering
the exhaust temperature upon addition to the exhaust gas 7,
NO.sub.x reduction effect can be further highly maintained in the
operational status with low exhaust temperature.
[0062] In fact, according to experimental results effected by the
inventors as shown in the graph of FIG. 7 where comparison was made
between a case X of the above-mentioned embodiment of the invention
and a conventional case Y of urea water being added to exhaust gas
as it is, it has been actually ascertained that high NO.sub.x
reduction ratio can be obtained with temperature (the inlet exhaust
temperature of the selective reduction catalyst 10 being about
140.degree. C. or so) lower in the case X of the inventive
embodiment than in the conventional case Y.
[0063] Since the ammonia gas 13a is generated through the action of
the plasma on the urea water 23a, the generation of the ammonia gas
13a can be easily and rapidly adjusted. Since the generated ammonia
gas 13a is added to the exhaust gas 7, the amount of ammonia gas
13a to be fed to the exhaust gas 7 can be controlled with high
response.
[0064] FIG. 4 shows an embodiment where ammonia water 13b generated
through action of plasma on the urea water 23a in the vessel 15 in
the ammonia generator 12 of FIG. 2 is injected upstream of the
selective reduction catalyst 10. In this embodiment, the ammonia
feed pipe 28 is opened in the liquid adjacent to the liquid level
within the vessel 15. The urea water 23a fed by the urea water feed
pipe 20 to a position adjacent to the bottom of the vessel 15 flows
upward while decomposed into ammonia and carbon dioxide through the
plasma formed between the electrode 16 and casing 17. The ammonia
generated by decomposition dissolves in water so that ammonia water
13b exists in and especially at the upper part of the liquid in the
vessel 15. Thus, such ammonia water 13b is injected by the spray
nozzle 14 through the pump 29 and ammonia feed valve 30 into the
exhaust pipe 9 upstream of the selective reduction catalyst 10 for
mixture with the exhaust gas 7.
[0065] When the ammonia water 13b generated in the ammonia
generator 12 is added in this manner to the exhaust gas 7, ammonia
in the ammonia water 13b is reacted with NO.sub.x and NO.sub.x
reduction effect can be obtained just like the above. Even in a
vehicle with travel pattern of continuing operational status with
low exhaust temperature for a long time, a sufficient NO.sub.x
reduction effect can be obtained even at exhaust temperature lower
than that conventionally required therefor. More specifically, when
the ammonia water 13b is added to the exhaust gas 7, subtle heat
may be taken upon evaporation of the water; however, endotherm
required for evaporation of the water is lower than heat required
in a conventional pyrolytical decomposition of the urea water 23
into ammonia and carbon dioxide in utilization of heat of the
exhaust gas 7. Thus, lowering in temperature of the exhaust gas 7
is subtle; therefore, according to the invention, also in a case
where the ammonia water 13b is fed to the exhaust gas 7, the
NO.sub.x reduction effect can be highly maintained even in an
operational status with low exhaust temperature.
[0066] FIG. 5 is a schematic diagram showing an embodiment to take
out ammonia gas 13a from an ammonia generator 12 which is
structurally different from that of FIG. 2. This ammonia generator
12 is provided with a laterally elongated vessel 40 made from
heat-resisting and insulating material for holding urea water 23a.
In the above within the vessel 40, a plurality of electrodes 16 are
arranged in laterally spaced-apart relationship and are spaced at
their lower ends to the liquid level of the urea water 23a by a
predetermined distance. The respective electrodes 16 are connected
to power wire 25 which in turn is connected via a controller 26 to
a power source 24 such as battery. Arranged on a bottom of the
vessel 40 is an electrode plate 41 which is made from conductive
material and is connected to the earth 18. The vessel 40 is further
provided with a urea water feed pipe 20 similar to that in the
above-mentioned embodiment.
[0067] Further arranged in space 27 within the vessel 40 above the
liquid level of the urea water 23a is an ammonia feed pipe 28
similar to that in FIG. 2 embodiment. The ammonia feed pipe 28 is
connected to the spray nozzle 14 via a pump 29 and the ammonia feed
valve 30 of FIG. 2.
[0068] In FIG. 5 embodiment, only the urea water 23a is fed in the
vessel 40; however, alternatively, just like FIGS. 2 and 4,
dielectric pellets 19 made from material with high dielectric
constant such as titania, barium titanate or alumina may be charged
in the vessel 40 in addition to the urea water 23a.
[0069] In the FIG. 5 embodiment, through plasma generated by
energization of the electrodes 16 and electrode plate 41, the urea
water 23a is decomposed into ammonia and carbon dioxide, ammonia
gas 13a being generated in the upper space 27 of the vessel 15.
[0070] Thus, in the ammonia generator 12 of FIG. 5, irrespective of
temperature of the exhaust gas 7, the ammonia gas 13a can be
generated by the action of the plasma. When added to the exhaust
gas 7, the ammonia gas 13a is reacted with NO.sub.x to reduce
NO.sub.x; thus, even in a vehicle with travel pattern of continuing
operational status with low exhaust temperature for a long time, a
satisfactory NO.sub.x reduction effect can be obtained with exhaust
temperature lower than that conventionally required therefor.
[0071] FIG. 6 is a diagram showing an embodiment to take out
ammonia water 13b from the ammonia generator of FIG. 5. In this
embodiment, in order to inject the ammonia water 13b generated by
decomposition of the urea water 23a in the vessel 40 through plasma
into an exhaust pipe 9 upstream of selective reduction catalyst 10,
an ammonia feed pipe 28 is opened in the ammonia water 13b within
the vessel 40. More specifically, urea water 23a fed to a position
adjacent to the bottom of the vessel 40 by a urea water feed pipe
20 flows upward while gradually decomposed into ammonia and carbon
dioxide by the plasma formed between electrodes 16 and an electrode
plate 41, so that ammonia water 13b exits above within the vessel
40. Thus, such ammonia water 13b adjacent to the liquid level is
injected upstream of the selective reduction catalyst 10 via a pump
29 and the ammonia feed valve 30 of FIG. 2.
[0072] Since the ammonia water 13b generated in the ammonia
generator 12 is added in this manner to the exhaust gas 7, ammonia
in the ammonia water 13b is reacted with NO.sub.x to obtain
NO.sub.x reduction effect just like the above. Thus, even in a
vehicle with travel pattern of continuing operational status with
low exhaust temperature for a long time, a satisfactory NO.sub.x
reduction effect can be obtained with exhaust temperature lower
than that required conventionally therefor. More specifically, when
the ammonia water 13b is added to the exhaust gas 7, subtle heat
may be taken upon evaporation of the water; however, lowering in
temperature of the exhaust gas 7 is subtle in comparison with a
conventional pyrolytical decomposition of the urea water 23a into
ammonia and carbon dioxide in utilization of heat of the exhaust
gas 7. Thus, also in a case of feeding the ammonia water 13b, the
NO.sub.x reduction effect can be highly maintained even in an
operational status with low exhaust temperature.
[0073] It is to be understood that an exhaust emission control
device of the invention is not limited to the above embodiments and
that various changes and modifications may be made without
departing from the scope of the invention.
INDUSTRIAL APPLICABILITY
[0074] An exhaust emission control device of the invention can be
effectively utilized in effectively generating ammonia from urea
water and in enhancing controllability in ammonia addition for
obtaining a satisfactory NO.sub.x reduction effect even in a
vehicle with travel pattern of continuing operational status with
exhaust temperature lower than that required conventionally
therefor.
* * * * *