U.S. patent application number 09/901180 was filed with the patent office on 2002-01-03 for method of reducing nox emissions using a fluid-cooled injector.
Invention is credited to Czarnik, Richard J., DiCarlo, Jeffrey P., Knapper, Curtis J., Simard, Thomas C., Tarabulski, Theodore J..
Application Number | 20020001554 09/901180 |
Document ID | / |
Family ID | 22593890 |
Filed Date | 2002-01-03 |
United States Patent
Application |
20020001554 |
Kind Code |
A1 |
Czarnik, Richard J. ; et
al. |
January 3, 2002 |
Method of reducing NOx emissions using a fluid-cooled injector
Abstract
A method for reducing emissions of oxides of nitrogen from a
combustion process using a temperature sensitive liquid reagent
injected into a stream of exhaust gases from the combustion process
and passing the exhaust gases and the reagent through a catalytic
reactor which reduces the oxides of nitrogen in the presence of the
reagent is disclosed. The steps of the method include providing an
injector having an orifice for atomizing the liquid reagent;
positioning a portion of the injector having the orifice within the
stream of exhaust gases; cooling the injector by continuously
circulating the reagent therethrough, thereby keeping both the
injector and the reagent within the injector below a critical
temperature at which the reagent will solidify; and injecting a
portion of the reagent into the exhaust stream upstream of the
reactor.
Inventors: |
Czarnik, Richard J.;
(Easthampton, MA) ; DiCarlo, Jeffrey P.; (Holyoke,
MA) ; Knapper, Curtis J.; (New Fairfield, CT)
; Simard, Thomas C.; (Agawam, MA) ; Tarabulski,
Theodore J.; (Brewster, NY) |
Correspondence
Address: |
Synnestvedt & Lechner LLP
Suite 2600
1101 Market Street
Philadelphia
PA
19107-2950
US
|
Family ID: |
22593890 |
Appl. No.: |
09/901180 |
Filed: |
July 9, 2001 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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09901180 |
Jul 9, 2001 |
|
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|
09164304 |
Oct 1, 1998 |
|
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|
6279603 |
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Current U.S.
Class: |
423/235 |
Current CPC
Class: |
B01D 53/8625 20130101;
B01D 53/90 20130101; Y02T 10/12 20130101; F01N 3/2066 20130101;
F01N 2260/024 20130101; F01N 2610/02 20130101; A61P 5/06 20180101;
F01N 2610/1453 20130101; F01N 2610/11 20130101; Y10T 137/6579
20150401; Y02T 10/24 20130101; Y02C 20/10 20130101; Y10T 137/6552
20150401 |
Class at
Publication: |
423/235 |
International
Class: |
B01D 053/56 |
Claims
What is claimed is:
1. A method of reducing emissions of oxides of nitrogen from a
combustion process using a temperature sensitive liquid reagent
injected into a stream of exhaust gases from said combustion
process and passing said exhaust gases and said reagent through a
catalytic reactor which reduces the oxides of nitrogen in the
presence of the reagent, said method comprising the steps of:
providing an injector having an orifice for atomizing said liquid
reagent; positioning a portion of said injector having said orifice
within said stream of exhaust gases; cooling said injector by
continuously circulating said reagent therethrough, thereby keeping
both said injector and said reagent within said injector below a
critical temperature at which said reagent will solidify; and
injecting a portion of said reagent into said exhaust stream
upstream of said reactor.
2. A method according to claim 1, wherein said reagent is an
aqueous urea solution.
3. A method according to claim 2, wherein said urea has a
concentration between about 25% and about 35%.
4. A method according to claim 1, further comprising the steps of
providing a surface facing said orifice within said exhaust gas
stream, and further atomizing said reagent injected into said
exhaust gas stream by impinging said reagent onto said surface.
5. A method according to claim 1, wherein said combustion process
occurs within an internal combustion engine.
6. A method according to claim 5, wherein said engine is a diesel
engine.
7. A method according to claim 6, wherein said reagent is injected
into said stream of exhaust gases in proportion to selected engine
operating parameters.
8. A method of reducing emissions of oxides of nitrogen from a
combustion process using a liquid reagent injected through an
injector into a stream of exhaust gases from said combustion
process, wherein at least a portion of said injector being
positioned within said stream of exhaust gases, said method
comprising the steps of: (1) continuously circulating said reagent
through said injector to keep both said injector and said reagent
within said injector below a critical temperature; (2) injecting at
least a portion of said reagent through said injector into said
exhaust stream; and (3) passing said exhaust gases and said reagent
injected therein through a catalytic reactor to reduce the oxides
of nitrogen.
9. A method according to claim 8, wherein said reagent is an
aqueous urea solution.
10. A method according to claim 8, wherein said urea has a
concentration between about 25% and about 35%.
11. A method according to claim 8, wherein said injector has an
orifice for atomizing said liquid reagent, and further comprising
the steps of providing a surface facing said orifice within said
exhaust gas stream, and further atomizing said reagent injected
into said exhaust gas stream by impinging said reagent onto said
surface.
12. A method according to claim 8, wherein said combustion process
occurs within an internal combustion engine.
13. A method according to claim 8, wherein said engine is a diesel
engine.
14. A method according to claim 8, wherein said reagent is injected
into said stream of exhaust gases in proportion to selected engine
operating parameters.
15. A method according to claim 9, wherein said critical
temperature is between about 95.degree. C. and about 140.degree. C.
Description
RELATED APPLICATION
[0001] This is a divisional application based on U.S. application
Ser. No. 09/164,304, filed Oct. 1, 1998.
FIELD OF INVENTION
[0002] This invention relates to methods for reducing NO.sub.x
emissions from internal combustion engines and especially to
methods using fluid-cooled injectors wherein the fluid is a liquid
reagent and a portion of the reagent is injected as an atomized
liquid reagent into the exhaust gas stream of an internal
combustion engine.
BACKGROUND OF INVENTION
[0003] Improved fuel efficiency for vehicles having internal
combustion engines can be achieved by using diesel engines or
gasoline engines operated with an excess of oxygen over the amount
necessary for complete combustion of the fuel. Such engines are
said to run "lean" or on a "lean mixture". The increase in fuel
economy, however, is offset by undesired pollution emissions,
specifically in the form of oxides of nitrogen (NO.sub.x).
[0004] One method used to reduce NO.sub.x emissions from internal
combustion engines is known as selective catalytic reduction (SCR).
SCR, when used, for example, to reduce NO.sub.x emissions from a
diesel engine, involves injecting an atomized reagent into the
exhaust stream of the engine in relation to one or more selected
engine operational parameters, such as exhaust gas temperature,
engine rpm or engine load as measured by engine fuel flow, turbo
boost pressure or exhaust NO.sub.x mass flow. The reagent/exhaust
gas mixture is passed through a reactor containing a catalyst, such
as, for example, activated carbon or metals, such as platinum,
vanadium or tungsten, which are capable of reducing the NO.sub.x
concentration in the presence of the reagent. An SCR system of this
type is disclosed in U.S. patent application Ser. No. 08/831,209,
hereby incorporated by reference.
[0005] An aqueous solution of urea is known to be an effective
reagent in SCR systems for diesel engines but suffers several
disadvantages. Urea is highly corrosive and tends to attack
mechanical components of the SCR system, such as the injectors used
to inject the urea mixture into the exhaust gas stream. Urea also
tends to solidify upon prolonged exposure to elevated temperatures,
such as encountered in diesel exhaust systems. Solidified urea
tends to accumulate in the narrow passageways and orifice openings
typically found in injectors. The solidified urea fouls moving
parts of the injector and clogs any openings, thus, rendering the
injector unusable.
[0006] Furthermore, if the urea mixture is not finely atomized,
urea deposits will form in the catalytic reactor, inhibiting the
action of the catalyst and thereby reducing the SCR system
effectiveness. High injection pressures are one way of dealing with
the problem of insufficient atomization of the urea mixture, but
high injection pressures often result in over-penetration of the
injector spray plume into the exhaust stream, causing the plume to
impinge on the inner surface of the exhaust pipe opposite the
injector. Over-penetration leads to inefficient use of the urea
mixture and reduces the range over which the vehicle can operate
with reduced NO.sub.x emissions. Like fuel for the vehicle, only a
finite amount of aqueous urea can be carried and what is carried
should be used efficiently to maximize vehicle range and reduce the
need for frequent fill ups of the reagent.
[0007] Additionally, aqueous urea is a poor lubricant. This
characteristic adversely affects moving parts within the injector
and requires that special fits, clearances and tolerances be
employed between relatively moving parts within an injector.
SUMMARY AND OBJECTS OF INVENTION
[0008] The method according to the invention concerns reducing
emissions of oxides of nitrogen from a combustion process using a
temperature sensitive liquid reagent injected into a stream of
exhaust gases from the combustion process and passing the exhaust
gases and the reagent through a catalytic reactor which reduces the
oxides of nitrogen in the presence of the reagent. The steps of the
method include providing an injector having an orifice for
atomizing the liquid reagent; positioning a portion of the injector
having the orifice within the stream of exhaust gases; cooling the
injector by continuously circulating the reagent therethrough,
thereby keeping both the injector and the reagent within the
injector below a critical temperature at which the reagent will
solidify; and injecting a portion of the reagent into the exhaust
stream upstream of the reactor.
[0009] The reagent is preferably an aqueous urea solution which is
injected into the stream of exhaust gases in proportion to selected
engine operating parameters. Preferably the urea has a
concentration between about 25% and about 35%. The reagent is
circulated continuously at a rate which will keep its temperature
below about 140.degree. C. and preferably below about 95.degree.
C.
[0010] The invention also provides an injector for delivery of a
fluid into a stream of hot gas, the injector being designed to
operate effectively with a corrosive, temperature-sensitive
reagent, such as aqueous urea. When used in the method according to
the invention for reducing NO.sub.x emissions, the injector is
mounted on an exhaust conduit of an internal combustion engine
where it injects the reagent into the exhaust gas stream.
[0011] The injector comprises a valve and a means for actuating the
valve between a closed position and an open position. Acceptable
actuating means include, for example, a solenoid-type actuator.
Preferably, the components of the valve exposed to extreme heat or
corrosive reagents like urea are made of a corrosion resistant
material such as stainless steel.
[0012] The valve includes an orifice through which the reagent is
expelled when the valve is in the open position. Regardless of the
state of the valve (i.e., open or closed), the reagent is
continuously circulated through it when the system is in operation,
at least a portion of the circulating reagent being expelled when
the valve is opened. The circulation of the reagent cools the valve
and minimizes the dwell time of the reagent within the valve,
thereby minimizing exposure of the reagent to heat and the creation
of urea deposits. Thus, aqueous urea, for example, can be
effectively used with such an injector without the characteristic
fouling and clogging of the injector. Means independent of the
valve actuating means are provided for continuously circulating the
reagent through the valve, as described in detail below.
[0013] Preferably, the valve comprises a valve body which has an
elongated cylindrical chamber therein in fluid communication with
the orifice. A valve seat is positioned within the chamber
surrounding the orifice. An elongated valve plunger is slidably
mounted within the chamber. One end of the plunger is sealingly
interengagable with the valve seat to close the orifice. The
plunger is connected with the actuating means and is movable from
the closed position where the plunger end sealingly engages the
valve seat and the open position where the plunger end is removed
from sealing interengagement with the valve seat to open the
orifice.
[0014] The means for independently circulating fluid through the
valve comprises a portion of the plunger which is arranged adjacent
to the plunger end. This portion of the plunger has a diameter less
than the chamber diameter and forms an annular fluid space or
passageway within the valve adjacent to the valve seat and the
orifice. The annular passageway, thus, allows for both the
continuous circulation of fluid through the valve and the expelling
of a portion of the fluid through the orifice when the valve is in
the open position.
[0015] Preferably, the independent fluid circulating means further
comprises a fluid inlet and a fluid outlet arranged within the
valve body in fluid communication with the annular passageway.
Fluid, such as the aqueous urea reagent, is supplied from a
reservoir and flows into the valve through the inlet, continues
through the annular passageway and exits the valve via the outlet,
thereby cooling the injector. When the valve is opened by the
actuator, the valve plunger is moved to the open position, and a
portion of the fluid is expelled from the chamber through the
orifice.
[0016] In order to provide additional heat protection for the
injector, a heat shield is preferably interposed between the valve
and the stream of hot gas. The heat shield has an aperture which is
aligned with the orifice. The heat shield aperture allows fluid
expelled from the valve to pass through the heat shield and into
the hot gas stream. The heat shield preferably comprises a metal
plate and a layer of insulating material interposed between the
plate and the valve. The heat shield aperture passes through both
the layer of insulating material, as well as the metal plate.
[0017] To improve atomization of liquid reagents, especially at
relatively low injection pressures, an atomizing hook is preferably
mounted on the valve. The atomizing hook has an end surface which
is positioned in a spaced apart relation with the orifice. Liquid
reagent expelled through the orifice impinges on the hook end
surface where further atomization of the reagent occurs. The shape
and position of the hook end surface directly affect the dispersion
characteristics of the injected reagent.
[0018] It is an object of the invention to provide a method for
reducing NO.sub.x emissions from a combustion process by injecting
a temperature sensitive liquid reagent into a stream of exhaust
gases from the combustion process.
[0019] It is another object of the invention to provide a method
which uses urea as the liquid reagent.
[0020] It is still another object of the invention to provide a
method which uses aqueous urea at relatively high
concentrations.
[0021] It is yet another object of the invention to provide a
method which uses an injector to inject the reagent into the
exhaust stream.
[0022] It is again another object of the invention to provide a
method wherein the reagent is continuously circulated through the
injector to keep both the reagent and the injector below a
temperature at which the urea will solidify.
[0023] It is an object of the invention to provide an injector for
injecting a fluid into a stream of hot gas.
[0024] It is another object of the invention to provide an injector
useable with corrosive liquids such as aqueous urea.
[0025] It is yet another object of the invention to provide an
injector in which aqueous urea will not solidify when the injector
is exposed to heat.
[0026] It is still another object of the invention to provide an
injector which achieves fine atomization of liquid reagents at
relatively low injection pressures.
[0027] It is a further object of the invention to provide an
injector wherein a portion of the fluid being injected is also
continuously circulated through the injector to cool the
injector.
[0028] It is yet a further object of the invention to provide an
injector wherein the dwell time of the fluid within the injector is
minimized.
[0029] It is still a further object of the invention to provide an
injector useable in a pollution control system for reducing
NO.sub.x emissions of internal combustion engines.
[0030] These and other objects will become apparent from a
consideration of the following drawings and detailed description of
the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 shows a schematic diagram of a pollution emission
control system using an injector according to the invention;
[0032] FIG. 2 shows a longitudinal cross-sectional view of an
injector according to the invention; and
[0033] FIG. 3 shows a side view of the valve body of the injector
according to the invention.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
[0034] FIG. 1 illustrates a pollution control system as might be
used to reduce NO.sub.x emissions from the exhaust of a diesel
engine 3. The system includes an engine exhaust conduit 4 in fluid
communication with a catalytic reactor 5, a reagent reservoir 6
holding reagent 7, a central processing unit 8 and an injector 10.
Injector 10 is mounted on exhaust conduit 4 and fed reagent, for
example, a solution of aqueous urea via supply line 9 extending
from reservoir 7 to the injector. A pump 11 is used to pump the
reagent to the injector at a predetermined pressure. Reagent 7 is
circulated back to the reservoir via return line 12, the
circulation of the reagent being shown by the arrows 7a.
[0035] In operation, signals 13, representing engine operational
parameters such as exhaust gas temperature, engine speed and fuel
flow rate are monitored by central processing unit 8. In response
to these signals and preprogrammed algorithms, central processing
unit 8 sends control signals 14 and 15 to injector 10 and pump 11
respectively, the control signals commanding pump 11 to circulate
reagent and injector 10 to inject or cease injecting reagent into
exhaust gases 16 within the exhaust conduit 4. The reagent is
atomized upon injection into the conduit and forms a mixture with
the exhaust gases. This mixture enters the catalytic reactor 5
which contains a catalyst, such as activated carbon, or metals,
such as platinum, tungsten or vanadium, which reduces NO.sub.x in
the exhaust gases in the presence of the reagent. The exhaust exits
the conduit 4 and passes to the atmosphere.
[0036] During system operation, regardless of whether or not the
injector is releasing reagent into the exhaust gases 16, reagent 7
is circulated continuously between the reservoir 6 and the injector
10 to cool the injector and minimize the dwell time of the reagent
in the injector so that the reagent remains cool. Continuous
reagent circulation is necessary for temperature-sensitive
reagents, such as aqueous urea, which tend to solidify upon
exposure to elevated temperatures of 300.degree. C. to 650.degree.
C. as would be experienced in an engine exhaust system. It has been
found to be important to keep the urea mixture below 140.degree. C.
and preferably in a lower operating range between 5.degree. C. and
95.degree. C. to provide a margin of safety ensuring that
solidification of the urea is prevented. Solidified urea, if
allowed to form, would foul the moving parts and openings of the
injector, eventually rendering the injector useless. In the case of
a 310-horsepower diesel engine with a baseline NO.sub.x emissions
level of 8 grams/bHp-hr at full load, circulation rates of aqueous
urea between 0.5 gallons per minute and 0.75 gallons per minute
through an injector according to the invention have been found to
effectively cool the aqueous urea and prevent solidification. It
will be recognized that flow rates will depend on engine size and
NO.sub.x levels. It is an advantage of the invention that more
concentrated solutions can be utilized, i.e., 25-35%, because
throughout the system, the solution is not subject to conditions
which would cause significant hydrolysis or solubility
problems.
[0037] FIG. 2 shows a cross-sectional view of the preferred
embodiment of the injector 10 according to the invention. The
injector is shown mounted on an exhaust gas conduit 4, only
partially depicted, in cross-section. Injector 10 comprises a valve
body 18 having an elongated cylindrical chamber 20 disposed
therein. Chamber 20 is in fluid communication with an orifice 22
which opens onto the exhaust gases within conduit 4. Surrounding
orifice 22 is a valve seat 24 which can have any practical shape
but is preferably conical. A valve member in the form of an
elongated valve plunger 26 is slidably mounted within chamber 20.
Valve plunger 26 has an end 28 formed to sealingly interengage
valve seat 24, as seen in FIG. 2, thereby closing orifice 22 from
fluid communication with chamber 20.
[0038] Valve plunger 26 is movable within the chamber between the
closed position shown in FIG. 2 and an open position wherein end 28
is removed from sealing interengagement with valve seat 24. In the
open position, orifice 22 is opened to fluid communication with
chamber 20.
[0039] Together, the chamber 20 and the valve plunger 26 provide a
means for circulating fluid, such as the reagent, through the valve
for cooling the valve and for minimizing the dwell time of the
reagent within the valve. The circulating means comprises an
annular fluid passageway 30 formed between the relatively larger
inner diameter of chamber 20 and the relatively smaller outer
diameter of a section 32 of the valve plunger 26. Preferably,
plunger section 32 is arranged adjacent to plunger end 28 and close
to valve seat 24 and orifice 22. Positioning fluid passageway 30
close to the orifice allows the circulating fluid to directly cool
an otherwise hot part of the valve body most sensitive to the
adverse effects of heat. Thus, for example, aqueous urea, when used
with this cooled valve, will not solidify anywhere within chamber
20. If allowed to solidify, the urea could prevent plunger 26 from
seating properly or could cause the plunger to seize in either the
open or closed position and/or the orifice 22 could become clogged.
By directly cooling this region of the valve, however, the
detrimental effects of elevated temperature on the reagent, the
moving parts, and the openings of the valve are avoided.
[0040] As seen in FIG. 2, plunger 26 further comprises a guide
section 33 disposed adjacent to section 32 of the valve plunger.
Guide section 33 preferably has a polygonal cross-section formed by
a plurality of flats 33a intersecting at a plurality of corners
33b. Flats 33a provide fluid circulation spaces adjacent to the
chamber 20 and augment the cooling function of the fluid passageway
30. The flats also provide space for any debris formed within or
brought into chamber 20 to wash out of the chamber with the
circulating fluid.
[0041] The corners 33b of the guide section 33 provide a
stabilizing and a guiding function for plunger 26. The corners are
toleranced to ride close to or in light contact with the wall of
chamber 20 to provide support points which guide the plunger within
the chamber to ensure proper seating of plunger end 28.
[0042] Immediately above guide section 33 is a reduced circular
cross-section 35 of plunger 26. Reduced section 35 provides an
annular space for fluid to flow into the chamber through an inlet,
described in detail below. Above the reduced section is a circular
guide section 37. Circular guide section 37 provides the main
guiding function for sliding motion of the plunger 26 within the
chamber 20. The tolerance between the circular guide section and
the chamber is sufficient to allow relative motion and lubrication
of the plunger while still guiding the plunger's motion and forming
a partial hydraulic seal between the plunger and the chamber.
[0043] Generally, the specific tolerances required at the various
sections between the valve plunger and the chamber will vary
according to the operating temperature, operating pressure, the
desired flow rate and circulation rate of the reagent, the
tribological properties of the reagent and the materials chosen for
the valve plunger and valve body. The tolerances for optimum
injector performance are best obtained experimentally by a few
field trials.
[0044] The cooling fluid is delivered to the annular fluid
passageway 30 through fluid inlet 34. Fluid inlet 34 is arranged
within valve body 18 in fluid communication with chamber 20 and is
externally connected to supply line 9 (FIG. 1). It is preferred
that the fluid inlet be positioned to deliver fluid to chamber 20
in a region removed from the valve seat 24 adjacent to reduced
section 25 and guide section 33, as shown in FIG. 2. Positioning
the fluid inlet upstream from the seat, as shown, allows the fluid
to contact valve plunger 26 over a substantial length before it
encounters the valve seat, thereby enhancing the cooling function
of the fluid. Fluid, such as reagent 7, is pumped via pump 11 at a
predetermined pressure into the fluid inlet 34 from which it flows
along valve plunger 26 into annular fluid passageway 30.
[0045] A fluid outlet 36 is provided to remove the fluid from the
annular fluid passageway. Fluid outlet 36 is arranged within valve
body 18 in fluid communication with chamber 20. Preferably, fluid
outlet 36 is positioned as shown in FIG. 2 for removing fluid from
chamber 20 in the region of the valve seat 24. Fluid outlet 36 is
externally connected to return line 12 (FIG. 1), thus permitting
the fluid (such as reagent 7) to circulate from reservoir 6,
through supply line 9, through fluid inlet 34, into annular fluid
passageway 30, through fluid outlet 36, through return line 12 and
back into reservoir 6. This circulation keeps critical regions of
the valve body 18 cool and minimizes the dwell time of the fluid in
the injector.
[0046] When the valve plunger 26 is moved from the closed position,
shown in FIG. 2, to an open position, plunger end 28 is removed
from sealing interengagement with seat 24. This action opens
orifice 22 and allows at least a portion of the circulating fluid
to be expelled through the orifice and into exhaust conduit 4. To
effect the opening and closing of the orifice, actuating means are
provided, preferably in the form of solenoid 38 mounted atop valve
body 18. Solenoid 38 has an armature 40 connected to valve plunger
26. When the solenoid is energized, the armature 40 is drawn
upward, thereby sliding valve plunger 26 within chamber 20 from the
closed position to the open position. The solenoid would be
energized, for example, in response to a signal 14 (see FIG. 1)
from central processing unit 8, which decides, based upon sensor
input signals 13 and its preprogrammed algorithms, when reagent is
needed for effective selective catalytic reduction of NO.sub.x
emissions in the exhaust stream.
[0047] As seen in FIG. 2, valve plunger 26 is biased in the closed
position by a biasing member, preferably in the form of a coil
spring 42 coaxially disposed about valve plunger 26. The valve
plunger has a shoulder 44 which serves as a lower spring seat
against which the spring can push to bias the valve plunger. An
upper plate 46 is fixed to the valve body 18 and serves as the
upper spring seat, as well as a stop to limit the upward travel of
the valve plunger.
[0048] Spring 42 is located within a spring chamber 48 which is
isolated from chamber 20 by seal 50. Seal 50 is preferably made of
carbon reinforced Teflon.RTM. or glass reinforced Teflon.RTM. and
prevents any corrosive reagent from entering the spring chamber and
possibly attacking or fouling the spring and the solenoid.
[0049] Injector 10 is shown mounted on exhaust conduit 4 by means
of sleeve 52 which is welded to an opening in the conduit by
weldment 54. Preferably, valve body 18 has external threads 19
which engage matching internal threads 53 in sleeve 52 to attach
the injector to the sleeve. In order to minimize conductive heat
transfer between the sleeve and the valve body, the external
threads 19 are not continuous around the circumference of valve
body 18 but interrupted or discontinuous, as seen in FIG. 3.
Preferably, the thread contact area is minimized by using
intermittent arcs of threads subtending angles on the order of
20.degree. arranged circumferentially around valve body 18, with
flat regions 21 arranged between each thread arc. The flats have an
across-the-flat dimension which is less than the root diameter of
the thread on valve body 18 and, therefore, make no contact with
sleeve 52.
[0050] In the configuration shown, hot exhaust gases within the
conduit are prevented from impinging directly upon the valve body
18 by the interposition of a heat shield 56 between the valve body
and the exhaust gases. Heat shield 56 includes an outer metal plate
58 and a layer of insulating material in the form of a thermal
gasket 60 interposed between outer plate 58 and valve body 18.
Preferably, outer plate 58 is made of stainless steel to resist the
corrosive environment within the exhaust conduit. Gasket 60 is
preferably made of a flexible graphite foil material whose low
thermal conductivity serves to isolate valve body 18 from outer
plate 58, reducing conductive heat transfer to the injector and
thereby helping to keep the fluid circulating within the valve
cool.
[0051] Heat shield 56 surrounds the orifice 22 and has an aperture
62 which passes through both the outer plate and the insulating
thermal gasket and permits fluid expelled from the injector to pass
through the heat shield and into the conduit. The heat shield has a
substantially planar surface which is preferably oriented
perpendicular to the jet of fluid expelled from the injector.
[0052] Further thermal protection for the injector is provided by a
radiant heat reflector 70 seen edge on in FIG. 2. Reflector 70 is
preferably a round disc of polished aluminum having an outer
diameter of sufficient extent such that the surface 70a of the disc
blocks radiant heat transfer from exhaust conduit 4 to parts of the
injector which have a direct line of sight to the conduit. The
reflector has a centrally positioned aperture 72 which fits around
valve body 18 and sits atop sleeve 52 to mount the reflector
between the exposed parts of the injector and the conduit 4.
Reflector 70 is retained in position by a nut 74 which threads onto
valve body 18.
[0053] It is desired to keep the injection pressure relatively low
to prevent the fluid jet or plume from the injector from
over-penetrating into the exhaust gas stream and impinging on the
sidewall of the conduit. Injection pressures within a range of 30
to 100 psi have been found to prevent over-penetration. An
injection pressure of 67 psi is preferred for the injector
according to the invention.
[0054] However, lower injection pressures might not atomize the
injected fluid to a sufficiently fine size for effective catalytic
reduction of the NO.sub.x. To assist dispersion and atomization of
the fluid within the conduit and yet maintain reasonably low
injection pressures, an atomization hook 64 is provided. It is an
advantage of the invention that no secondary atomization fluid is
required.
[0055] Hook 64 is mounted on the valve, preferably on the metal
plate 58 of heat shield 56 as seen in FIG. 2. Preferably, the hook
is made of stainless steel to withstand the corrosive environment
within the exhaust conduit. Mounting the hook on the heat shield
serves to thermally isolate the hook from the valve body 18.
Because the hook extends into the exhaust stream, it will be hot,
and being metal, it will tend to conduct heat readily. However, by
mounting the hook on the heat shield heat conducted by the hook
will be blocked by the thermal gasket 60, and heat transfer from
the hook to the valve body will be minimized by this preferred
mounting of the hook 64.
[0056] Hook 64 has an end surface 66 which is positioned in a
spaced-apart relation facing orifice 22. When the valve plunger 26
is actuated into its open position by solenoid 38, expelling fluid
at a predetermined pressure from orifice 22, the fluid jet will
impinge on end surface 66. This impingement will cause further
atomization of the fluid. The dispersion characteristics of the
fluid are a function of the shape of the end surface, which is
tuned to a particular size and shape of the exhaust stream to
ensure maximum dispersion and penetration of the fluid without
over-penetration.
[0057] An injector wherein critical valve components are directly
cooled by circulating fluid according to the invention provides a
component for a pollution control system which allows a corrosive
and heat-sensitive reagent, such as aqueous urea, to be effectively
employed to reduce NO.sub.x emissions and thereby ultimately attain
greater fuel efficiency without the adverse effects of increased
undesired emissions.
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