Note: Descriptions are shown in the official language in which they were submitted.
FUEL METERING METHOD AND APPARATUS
FIELD OF THE INVENTION
The present invention relates to fuel metering for engines
using carburetors, and more particularly to a small diaphragm-
type or float-bowl-type carburetor for small internal combustion
engines such as used in portable tools such as chain saws, in
lawn mowers and other power lawn and garden equipment and in
small off-road sport vehicles, etc.
BACKGROUND OF THE INVENTION
The fuel flow in diaphragm-type carburetors is dependent
upon the pressure differential existing between the carburetor
venturi and atmosphere. The venturi pressure depends upon
engine design characteristics and operating conditions. A
diaphragm-type carburetor generally comprises a mixture conduit
in which fuel is mixed with air for delivery to the intake
manifold of an engine, a fuel chamber closed by a diaphragm and
communicating ~hrough a nozzle with the mixture conduit for
delivery of fuel thereto, and valve means controlled by the
diaphragm for controlling delivery of fuel from a fuel tank to
the fuel chamber. An air filter is provided for cleaning air
entering the mixture conduit. The mixture conduit is formed
with a restriction, e.g., a venturi. With the engine in
operation, air flows through the mixture conduit, a pressure
-2- ~ 2 3 ~ 7 ~
drop occurs across the venturi (i.e. a partial vacuum is created
in the venturi), and pressure on the outside of the diaphragm
causes the diaphragm to flex inwardly and effect delivery of
fuel through the nozzle, which is usually located at the throat
of the venturi where the pressure drop is at maximum, and this
diaphragm flexure effects opening of the valve means for delivery
of fuel to the fuel chamber.
It is well known that intake tuning of engines often has
an adverse affect on the fuel metering characteristics of the
carburetor, particularly with respect to single cylinder engines
operable over a relatively wide speed range. Engine intake
tuning can cause the carburetor to deliver fuel in an incorrect
ratio to the air flow due to unsteady air flow through the
carburetor and to the effect of the moving pressure wave forms
in the manifold and carburetor bore. These wave forms are
created by the opening and closing of the engine intake valve(s)
or port(s), and travel at the speed of sound, their behavior
being well known in the art.
The effect of the mGving pressure wave forms on the fuel
delivered from the nozzle of the carburetor has long been a
source of problems for the carburetor design engineer. The
wave effect is superimposed on the normal vacuum caused by the
engine intake air stream flow through the venturi. This in
turn causes the nozzle to deliver fuel in a manner which is not
fully responsive to the vacuum caused by the air flow. The
~3~ 2 ~ 2 ~
carburetor will function properly when the pressure drop P
across the main jet (some distance from the nozzle outlet in
the venturi) is proportional to the density and the square of
the velocity of the air flow, i.e., aP = ~ . The tuning waves
are superimposed on the venturi pressure drop and adversely
effect fuel metering otherwise designed to follow this
relationship. More particularly, this tuning wave is imposed
on the fuel delivery side of the main jet and adversely affects
the desired design value of the pressure drop a P, across the
main nozzle-fuel controlling restriction.
It is believed that such tuning waves, i.e., air pressure
waves generated in the carburetor mixture conduit by the sudden
opening and closing of the engine intake port, are responsible
for such well known problems as fuel "spit back" under certain
engine operating conditions as well as certain abnormal and
less well recognized deviations in the desiredengine-carburetor
performancecurves plotting fuel-air ratio against engine speed,
(e.g., undesirable "rich or lean spots" in the performance
curves) and related plots of such parameters as specific fuel
consumption, engine power output, exhaust constituents, etc.
Another well known problem adversely affecting the desired
or design fuel metering characteristics of a carburetor, whether
of the diaphragm or float type, is the gradual clogging by dirt,
dust and/or other solid air borne particles of the engine air
-4- 2~ 3~ ~
intake filter customarily disposed in front of the air entrance
to the carburetor mixture conduit.
OBJECTS OF THE INVENTION
Accordingly, it is a principal object of this invention
to provide a fuel metering system, apparatus and method,
particularly for a carburetor of the class described, which
functions effectively to better maintain a predetermined air
and fuel ratio by canceling or modulating the adverse effect
of the aforementioned moving pressure tuning wave forms operable
upon the pressure drop across the main jet or other controlling
fuel restriction feeding into the carburetor venturi.
Another object is to provide an improved carburetor
incorporating the aforementioned system which is also operable
as a vent system for the "dry" side of the diaphragm chamber
which functions effectively to prevent or reduce adverse effects
of air filter clogging relative to the maintenance of a
predetermined air and fuel ratio.
A still further object is to provide a fuel metering system
method and apparatus of the aforementioned character which may
also be applied to float feed carburetors in a manner similar
to the application thereof to diaphragm carburetors.
Yet another object of the present invention is to provide
a fuel metering system and method of the aforementioned character
in which the effect of the aforementioned tuning waves is
utilized to advantage to modulate the fuel metering system in
- 2~28~7
a favorable manner to produce varying effects such as a lean
mixture for an economy range and/or a rich mixture for a power
range.
SUMMARY OF THE INVENTION
Briefly, the objects of the invention are accomplished by
providing a diaphragm carburetor with a vent passageway system
operable to sense dynamic as well as static components of the
engine tuning pressure wave imposed on the air stream in the
carburetor venturi and to route the same to the underside ("dry
side") of the diaphragm in such a manner that the pressure wave
will be better imposed on both sides of the diaphragm, thus
essentially canceling the adverse effect of the pressure wave
and leaving only the desired ~ P pressure drop operable
between the diaphragm fuel chamber and venturi main nozzle.
This is accomplished by communicating one end of the vent
passageway with the dry side of the diaphragm chamber and the
other end of the vent passageway, via a special air pressure
sensing pitot tube, with the carburetor venturi, the pitot tube
being oriented and located in a predetermined manner tnerein
relative to the intake air flow stream and main fuel nozzle
outlet so that the pitot tube and the fuel nozzle are exposed
to the same tuning pressure wave at the same instant of time.
Alternatively, the vent passageway pitot tube and the main fuel
nozzle opening are offset slightly from one another in the
venturi passage by a predetermined leading or lagging amount
-6-
in the direction of wave propagation in the carburetor throttle
bore to thereby introduce a leading or lagging wave impingement
relationship between these two venturi openings. This creates
a predetermined phase shift in the affect of the pressure wave
upon the metering pressure drop so as to modify it in a favorable
manner to thereby produce a leaner mixture for an economy range
of the engine or a richer mixture for a power range of the
engine, depending upon the direction and amount of the
predetermined offset spacing between the pitot tube and main
jet fuel nozzle.
Similarly, in a float-bowl-type carburetor, the
aforementioned one end of the vent passageway communicates with
the head space of the fuel sump or well in the float bowl, the
surface of the fuel and the float therein being considered the
equivalent of the diaphragm, and the bowl headspace being treated
as the "dry side n chamber.
BRIEF DESCRIPTION OF THE DRAWINGS
Other ob]ects as well as features and advantages of the
present invention will be more fully understood from the
following detailed description of exemplary but preferred
embodiments shown by way of example in the accompanying drawings,
which are to scale unless otherwise stated, wherein:
FIG. 1 is a side elevational view of a small, compact
diaphragm carburetor designed for a chain saw engine application
~7~ ~3577
incorporating the improved fuel metering system in a first
embodiment of the present invention, the intended direction of
engine intake air flow through the carburetor being indicated
by the arrow A/F.
FIG. 2 is a cross sectional view taken on the line 2-2 of
FIG. 1.
FIG. 3 is a bottom view of the carburetor of FIG. 1 and
rotated 90~ from the orientation in FIG. 1.
FIG. 4 is a fragmentary cross-sectional view taken on the
line 4-4 of FIG. 3, and is an inverted view relative to FIGS. 1,
2 and 5.
FIG. 5 is a elevational view of the carburetor of FIG. 1
looking into the choke end of the mixing passage and having a
portion broken away and shown in cross section to better
illustrate detail.
FIG. 6 iS a fragmentary cross-sectional view taken on the
lines 6-6 of FIG. 5.
FIG. 7 is a view taken on the line 7-7 of FIG. 5 illustrating
the ~ottom of the carburetor body with the bottom pla~e removed.
FIG. 8 iS a fragmentary cross-sectional view taken on the
line 8-8 of FIG. 7.
FIG. 9 iS a fragmentary bottom plan view of the bottom
cover assembly of the carburetor shown by itself.
FIG. 10 is a fragmentary cross-sectional view taken on the
line 10-10 of FIG. 9.
-8- ~ 7 ~
,
FIG. 11 is a fragmentary top plan view of a portion of a
se'aling gasket of the bottom cover assembly taken on the line
11-11 of FIG. 10.
FIG. 12 iS a side elevational view of a small, compact
float bowl carburetor designed for a twelve horsepower lawn and
garden appliance engine application incorporating the improved
fuel metering system in a second embodiment of the present
invention, the intended direction of engine intake air flow
through the carburetor being indicated by the arrow A/F.
FIG. 13 is a cross-sectional view taken on the line 13-13
of FIG. 12 but enlarged double size thereover.
FIG. 14 is a cross-sectional view taken on the line 14-14
of FIG. 12 but enlarged double size thereover.
FIG. 15 is an end elevational view of the carburetor of
FIG. 12 looking into the choke end of the mixing passage.
FIG. 16 is a cross-sectional view taken on the line 16-16
of FIG. 15 but enlarged double size thereover.
FIG. 17 iS a side elevational view of the carburetor of
FIG. 12, shcwing the side opposite that of FIG. 12.
FIG. 18 iS an end elevational view of the carburetor of
FIG. 12, looking into the throttle end of the mixing passage.
FIGS. 19, 20 and 21 are fragmentary cross-sectional views
taken on the line 19-19 of FIG. 17 but enlarged four times
thereover and respectively illustrating the orientation of the
choke plate, choke shaft and modified shunt or bypass passageway
relative to one another with the choke fully closed ( FIG. 19),
with the choke positioned 15~ before full closure (FIG. 20) and
with the choke positioned in fully opened condition (FIG. 21)
respectively.
FIG. 22 is a top plan view of the carburetor of FIG. 12
rotated 90~ about the carburetor axis from the illustration of
FIG. 17.
FIG. 23 is a cross-sectional view taken on the line 23-23
of FIG. 22 but enlarged double sized thereover.
FIG. 24 is a cross-sectional view taken on the line 24-24
of FIG. 22 but enlarged double size thereover.
2 ~ 7 g ~ r~
--10--
DETAIL~D DESCRIPTION
Referring in more detail to the accompanying drawings,
FIG. 1 illustrates by way of example a diaphragm carburetor 20
designed for use with a chain saw engine and incorporating the
best mode presently known for carrying out the fuel metering
system of the invention. Except where indicated hereinafter,
carburetor 20 is of known design embodying conventional but
state of the art constructional features. Carburetor 20 has a
main body 22 with a top cover or cap plate 24 secured to its
upper surface and a bottom cover assembly 26 secured to its
under surface. From the side view of FIG. 1, the upper ends of
the choke shaft 28 and throttle shaft 30 may be seen, as may
the throttle stop arm 32, throttle stop adjustment screw 34,
high speed needle valve 36 and low speed needle valve 38.
As best seen in FIGS. 2 and 5, carburetor 20 has a mixture
conduit or bore 40 and a venturi constriction 42 disposed within
bore 40. A choke valve 44 is mounted on choke shaft 28 and
disposed in the entrance (in the choke bore) to bore 40 upstream
of venturi 42, while a throttle valve (not shown) is provided
on throttle shaft 30 so as to be disposed in bore 40 (in the
throttle bore) downstream of venturi 42.
Fuel may be supplied to the carburetor by a pump (not
shown) which may be formed by components disposed in and between
cover 24 and body 22, as will be understood by those skilled in
the art. The fuel pump discharge is connected through a passage
20285~7
- 1 1-
50 and filter screen 52 with a passage 54 leading to the metering chamber 56 of the
carburetor. A needle valve 58 in passage 54 is controlled by a diaphragm 60 disposed
between the metering or "wet" chamber 56 and a vent or "dry" chamber 62. Diaphragm 60
is connected with valve 58 by a lever 64 which is biased by a spring 66 in a direction to
move valve 58 toward closed position.
The main fuel nozz;le outlet 70 opens into venturi 42 of the c~bulctor and, in
accordance with conventional practice, has its axis oriented perpendicular to the axis of
venturi 42 and direction of engine intake air flow in bore 40. Hence primarily or only static
pressure of the air flow is sensed by nozzle 70. Nozzle 70 is connected via the main fuel
0 metering system, to metering chamber 56 through a fuel passageway network, including the
adjustable high speed needle valve 36 which controls flow through a passageway 72 leading,
via such network, to the fuel well 74 feeding, via capillary seal screen 75, nozzle outlet 70.
Preferably high speed needle valve 36 is a temperature compen~lin~ needle valve which
serves as the main fuel metering restriction, and is constructed in accordance with the
disclosure and claims of the Woody and Swanson United States Patent No. 4,759,883, issued
July 26, 1988 and assigned to the assignee of record herein.
In accordance with a principal feature of the present invention, call,urelor 20
is provided with a vent passageway
,'~
-12- 2~8577
system operable to modify and/or cancel the effects of tuning
pressure waves transiting venturi 42 which otherwise would
adversely affect the metering function of needle valve 36 and
associated passageway 72.
This vent passageway interconnects a specially constructed
pressure sensing port in venturi 42 with the dry side chamber
62. Referring to FIGS. 5 and 6, a pitot tube protruberance 80
is cast integrally with the body 22 so as to project from venturi
42 into the air flow stream drawn through bore 40 by engine
intake suction. Projection 80 has a flared mouth 82 defined by
an integral hood 84 leading to a short entrance passage 86.
The axis of passage 86 is parallel to the axis of bore 40, and
thus, mouth 82 faces directly upstream relative to the air flow
through bore 40. Mouth 82 defines the end of the vent passageway
system communicating with the carburetor throat. Passage 86
constitutes a blind bore which is connected near its blind end to
a passage 88, the axis of which is disposed perpendicular to
the axis of bore 86. Passage 88 merges with a coaxial counterbore
90 closed at its Guter ei-ld by a press fit ball 91. Passage 90
is intersected by a short passage 92 (shown schematically in
phantom by dash lines in FIG. 5) which in turn opens to the
bottom face 94 of body 22, as best seen in FIG. 7. Passage 92
communicates with a passage 96 formed in bottom cover 26 (FIGS.
9, 10 and 11), and passage 96 is connected by a passage 98 and
100 to the dry side chamber 62. Passage 100 constitutes the
- -13- c~ w7
opposite end of the vent passageway system communicating with
the dry side of the diaphragm metering chamber 62.
In accordance with the present invention, it has been found
that the entrance 82 of the vent passageway communicating with
bore 40 should be located in the plane of the circle defined
by venturi 42, which plane also includes the outlet of nozzle
70. It has also been found that this entrance to the vent
passageway is preferably configured as a pitot tube, as shown
in FIGS. 5 and 6, having the flared mouth 82 and hood 84.
Moreover, the axis of the entrance passage 86 should preferably
be located within a zone in the aforementioned venturi plane
where the air stream velocity in bore 40 and passing through
venturi 42 is at a maximum under engine operating conditions
when choke valve 44 is fully opened.
Thus, as best seen in FIG. 5, when carburetor 20 is provided
with choke shaft 28 and associated choke valve plate 44, opening
86 is located generally in the center of the chordal segment
defined by the lower edge of shaft 28 and the portion of venturi
42 disposed between shaft 28 and nozzle 70~ The diametric~lly
opposed chordal segment in this plane, located on the other
side of shaft 28, could likewise be chosen for locating pitot
tube projection 80, but the head of the mounting stud 102 for
choke plate 44 offers a slight obstruction to this side of the
choke shaft 28 and hence the aforementioned chordal zone closer
to nozzle 70 is chosen as preferred in this particular
-14~ 4~ J
arrangement. If carburetor 20 were not provided with a choke
shaft 28 and associated choke valve 44, the axis of entrance 86
of the pitot tube 80 preferably would be disposed coincident
with the axis of venturi 42 (and bore 40), inasmuch as air
streamvelocitywould begreatestatthe center ofanunobstructed
venturi. Preferably, in the working embodiment illustrated
herein, the total axial length of interconnected passages 86,
88, 90, 92, 96, 98 and 100 is .336 inches and the diameter of
these passages ranges from about .049 to about .062 inches
respectively, the diameter of venturi 42 is .546 inches, the
diameter of shaft 28 is .186 inches, and the axis of passage 86
is located .095 inches from the nearest point on the surface
of venturi 42.
Inasmuch as carburetor 20 is provided with a choke valve
44, and pitot tube 80 is located directly downstream of valve
44, in accordance with another feature of the invention a shunt
passageway system is provided for producing a bypassing "shut-
off" of the effect of the vent passageway 82-100 when initiating
choke valve closure, i.e., after the ~irst 5~ of rotation of
choke shaft 28 from its fully opened position on FIG. 5 toward
its closed position. This "shut-off" shunt passageway comprises
a slot or flat 104 formed in shaft 28 (FIG. 4) which is
continuously open at one end to carburetor bore 40 and extends
axially of shaft 28 so as to extend into body 22 from bore 40
a sufficient distance to selectively register with a passageway
r"~
-15-
106 formed in body 22 in response to rotation of shaft 28. The
intersection of passage 106 with the bore 108 receiving choke
shaft 28 is located so as to be out of registry with slot 104
when choke valve 44 is in its fully opened position, but to
begin registration with slot 104 after the first 5~ of rotation
of shaft 28 from full open toward closed position, passage 106
and slot 104 being in full registration in the fully closed
position of choke valve 44.
Passage 106 opens at the bottom face 94 of body 22 (FIGS.
4 and 7 and 8), and is connected to passage 96 by a cross
passageway 110 formed in bottom cover 26 as well as in a sealing
gasket 112 (FIGS. 4 and 11) which may be formed integrally with
diaphragm 60. Cover 26 may be provided with a raised rib 114
(FIG. 10) to help seal this cross passageway 110 when gasket 112
is pressed against the upper face 116 of cover assembly 26.
Additionally, a depressed or recessed ledge 118 may be formed
in gasket 112 to cooperate with the rib 114 to further define
a sealing connection for cross passageway 110. It is also to
be noted that slot 104 in choke shaft 28 faces upstream relative
to air flow in bore 40 in the closed condition of choke shaft 28.
OPERATION
The system and method of the present invention will be
understood from the following description of the operation of
carburetor 20. At engine start-up, when choke valve plate 44
has been rotated by shaft 28 to fully closed position to induce
16
a high vacuum in bore 40 downstream of the choke plate to thereby
induce the appropriate start-up fuel flow rate via nozzle 70,
shunt passageway 104-110 is fully open and thereby communicates
the vent passageway 82-100 with atmospheric pressure upstream
of the closed choke plate (usually immediately behind the
carburetor air filter, not shown). Hence the pressure wave
transmissioncapability of theventpassageway 82-100 isrendered
inoperable, and the high vacuum conditions immediately behind
the choke plate in the vicinity of pitot tube 80 cannot be
communicated to the dry side chamber 62 because of the shunting
effect of the shunt passageway 104-110.
After the engine begins to run under its own power and as
choke 44 is rotated toward open position, shunt passageway 104-
110 is gradually closed off by the deregistration of slot 104
with passage 106 by the corresponding rotation of shaft 28 in
bore 108. Once choke 44 is fully open and the engine is running
without choking assistance shunt passageway 104-110 is fully
closed and hence vent passageway 82-100 becomes fully operable
to transmit pressure wave effects to the dry side chamber 62.
In the engine running mode with choke 44 open, the air
flow through the carburetor is not steady because there are
moving pressure wave forms generated in the manifold and
communicated by the intake air stream in carburetor bore 40.
These waves are formed from the opening and closing of the
engine intake valve(s) or port(s), and travel at the speed of
-17-
sound, their behavior being well known. The effect of these
waves on the fuel delivered from outlet of the nozzle 70 of the
carburetor has long been a source of problems. The wave effect
is superimposed on the normal vacuum caused by the velocity of
the air flow thro~gh venturi 42, and introduces an undesirable
pressure variation modulation in venturi 42. Thus, hitherto,
this in turn has caused nozzle 70 to deliver fuel in a manner
which is not in proper design response to the vacuum caused by
the air flow.
It is to be understood that, in the absence of such moving
pressure wave forms, the carburetor will function properly when
the static pressure drop ~ P across the main controlling
restriction formed in passage 72 by valve 36 (FIG. 5), located
some distance from the outlet of nozzle 70 in venturi 42, is
proportional to the density and the square of the velocity of
the air flow in accordance with the formula relationship: ~ P
=~ V2. However, when intake tuning pressure waves are present
in carburetor 20, the same would be superimposed on the pressure
drop ~ P and would adversely effect fuel metering but for the
corrective, canceling counter-or modulating effect of the vent
passageway system 82-100 of the present invention. It has been
found that vent passageway 82-100 is operable to route the
tuning pressure wave to the dry side chamber 62 of the carburetor
in such a manner that the pressure wave will be imposed on both
sides of diaphragm 60 to thereby counter-modulate or cancel the
'~ G ~ r~
--18--
adverse effect of the pressure wave communicated by nozzle 70
to the wet side chamber 56 of the carburetor, thereby leaving
only the desired static pressure drop P as established by
the predetermined design parameters engineered for the
particular carburetor and engine application.
As indicated previously, in order to accomplish this it
has been found critical to place the venturi-communicating
opening 82 of vent passageway 82-100 in the plane of venturi 42,
co-planar with nozzle 70, and preferably located in the zone
of maximum air flow velocity when choke valve 44 is open.
Although the theory of operation is not as yet completely
understood, it is believed that this relationship insures that
opening 82 and nozzle 70 will be exposed to the same pressure
wave at the same instant of time. In any event, this orientation
and location relationship, as well as the pitot tube
configuration of protuberance 80, have been found to be critical
to the ability of the vent passageway 82-100 to counter-modulate
or cancel the tuning waves, or to at least cancel or substantially
reduce the adverse effect of such tuning waves on the
predetermined fuel metering parameters desired for the
carburetor.
Fortuitously, vent passageway 82-100 has been found to
also perform a second function, namely, it prevents the fuel/air
mixture from going over rich as the air filter located upstream
of the entrance to the carburetor bore becomes clogged with
--19--
2 ~ 7 7
dirt particles. Vent passageway 82-100 of the present invention
thus provides the further advantages of the full inside vent
as disclosed in the United States Brown patent 3,174,732 but
is believed to be operable in an improved manner thereover to
thereby better maintain a predetermined air/fuel ratio
regardless of air filter clogging.
The system of the invention is also believed to reduce or
eliminate undesirable carburetor performance characteristics
resulting from phase shifts between engine suction pulses and
tuning wave pulses otherwise occurring with changes in engine
speed.
It is also to be understood that the principles of the
present invention may be applied to a float bowl carburetor.
In such a carburetor the surface of the fuel in the bowl is
treatedas equivalent to the diaphragm 60, and the vent passageway
system in accordance with the invention is operable in a manner
similar to that described in conjunction with the hereinabove
disclosed diaphragm carburetor 20.
If desired, slightly shifting the location of the pi'~ot
tube 80 so as to dispose entrance 82 either slightly forwardly
or rearwardly (upstream or downstream relative to air flow) of
the plane of the venturi 42 will cause a given pressure wave
impingement on nozzle 70 to lead or lag impingement of this wave
at entrance 82. The resultant phase shift can thus be established
in accordance with an empirically predetermined dimensional
~ ~ 2 ~ e~! 7 ~
-20-
relationship relative to the fuel metering effect of diaphragm
60 so as to produce a lean mixture for an economy range or a
rich mixture for a power range, as will now be understood by
those skilled in the art in view of the foregoing disclosure.
Thus, it will be appreciated from the foregoing disclosure
that the preferred embodiments of the fuel metering system,
method and apparatus for internal combustion engines described
and/or illustrated herein amply fulfill the aforementioned
objects of the invention. However, it will be reali~ed that
further variations of the inventive concepts will occur from
the foregoing disclosure to those skilled in this art.
For example, the effect of the vent passageway 82-100 of
the present invention may be modified or modulated by venting
the dry side chamber 62 to atmosphere in a controlled manner.
As shown in FIGS. 2, 3 and 10, bottom cover assembly 26 may be
provided with a well 130 containing filter media 132, and an
annular row of slots 134 may be provided in the bottom of the
well 130 to communicate filter media 132 with atmosphere. A
cover 136 is seated over well 130 and is provided with spaced
ribs 137 to press down the filter media 132 so the same is held
spaced from the underside of cover 136. A restricted orifice
138 is provided in cover 136 communicating with the head space
above filter media 132. Orifice 138 is shown enlarged (not to
scale) but preferably has a diameter of .025 inches in a working
embodiment of carburetor 20 as determined by emperical testing.
-21- ~f~
Atmospheric bleed orifice 138 on the dry side chamber 62 can
thus be employed to modulate the effect of pressure wave
cancellation provided by vent passageway 82-100, as may be found
desirable for certain engine applications or for particular
operating conditions found desirable for given applications.
It is to be further understood that the vent passageway
system 82-100 of the present invention, because of the pitot
tube arrangement of the venturi end of the vent passageway
system, better measures or senses both dynamic and static
pressure conditions in venturi 42 of bore 40, rather than
primiarly static pressure conditions as is the case with prior
art vent passageway systems such as that disclosed in the
aforementioned Brown '732 patent as well as in the United States
Patents to Phillips 3,065,957; Brown et al 3,181,843 and Yagi
et al 4,494,504 (FIGS. 19 and 20).
Although the transit time of a pressure wave in air and
liquid is different, it also has been found that, within
reasonable limits, the ratio of the liquid path length from the
wet side chamber 55 ~o nozzle 70, to air path length, from dry
side chamber 62 to venturi 42 via vent passageway 82-100, can
be varied without significantly affecting the operation of the
vent passageway in canceling the adverse effect of intake tuning
moving pressure waves in the carburetor bore.
In addition, the communication of the sensed pressure wave
via the vent passageway system to the dry side chamber 62 has
-22- ~2~7;7
been found to be effectively transmitted to the wet chamber 56
via the diaphragm 60 without thereby adversely affecting the
diaphragm in performing its principal function of controlling,
via its associated lever linkage 64, the fuel inlet valve 58.
As indicated previously hereinabove, and as illustrated
in FIGS. 12 through 24, the foregoing principles of the invention
also may be applied to a float bowl carburetor. By way of
illustration and not by way of limitation, a prior art
commercially available float bowl carburetor 20' manufactured
and sold by Walbro Corporation of Cass City, Michigan, assignee
of the inventors herein, under Part No. LMKl for use on a Kohler
C.V. 12 engine is illustrated in FIGS. 12 through 24, wherein
the same has been modified to incorporate the vent passageway
system in accordance with the invention so as to be operable in
a manner similar to that described in conjunction with the
hereinabove disclosed diaphragm carburetor 20. For purposes
of brevity and to facilitate correlation of corresponding
structure and function, the float bowl carburetor 20' as shown
in FIGS. 12 through 24 is described in association with reference
numerals raised by a prime suffix applied to those elements
corresponding in structure and function to that described
previously in conjunction with carburetor 20, and their
description is not repeated inasmuch as the construction of the
float bowl carburetor 20' will be well understood by those
-23- ~ ~ 2 ~} ~i 7 ~
skilled in the art when viewing the illustrations of FIGS. 12
through 24.
As best seen in FIGS. 13, 15 and 16, float carburetor 20'
is provided with a vent passageway system of the present invention
operable to modify and/or cancel the effects of tuning pressure
waves transisting venturi 42' which otherwise would adversely
affect the metering function of fixed high speed jet 36', well
74' and main nozzle 70'. This vent passageway interconnects a
specially constructed pressure sensing port in venturi 42' with
the head space 62' of the float bowl 63. As will be understood
by those skilled in the art, bowl 63 contains liquid fuel in
the sump or well thereof in which an annular hollow float 60'
is partially submerged, and float bouyancy is operable via lever
64' to control inlet valve 58' in response to fuel sump surface
level variations.
Thus, as in the diaphragm carburetor 20, a pitot tube 80'
is provided so as to project from venturi 42' into the air flow
stream drawn through bore 40' by engine intake suction. Pitot
tube 80' in the embodiment illustrated in FIGS. 12-24 may be
cast integrally with carburetor body as in the carburetor 20.
However, in the embodiment illustrated in FIGS. 12-24, pitot
tube 80' is fabricated from separate parts including a
cylindrical brass plug 140 having a blind bore 142 drilled
therein so that the open end of bore 142 forms a mouth 82' of
the pitot tube 80'. A tube 144 is inserted at one end into a
~ ~ 2 ~ ~ 7 1
-24-
side opening drilled in plug 140, and is press fit into a drilled
passage 146 which extends perpendicularly to the carburetor
axis and which in turn is sealed at its outer end by a press
fit ball 148. Passage 146 intersects a larger diameter drilled
passage 150alreadyprovided in carburetor 20', and which extends
downwardly into a larger diameter counter bore 152 (FIG. 13)
and which may, if desired, be at its lower end closed by a welch
plug which seats at 154 in accordance with conventionalpractice.
When a welch plug is used, a notch 156 in the sidewall intended
to receive the welch plug communicates passage 150 with the
head space 62' of bowl 63. As shown in FIG. 16, a large diameter
passage 158 extends parallel to the carburetor axis and has its
mouth 160 located adjacent the choke end of the carburetor just
downstream from the air filter (not shown) normally provided
upstream of the entrance to carburetor mixing passage 40'. The
downstream end of passage 158 perpendicularly intersectspassage
150andtogethertherewith provides the main air pressure venting
system for the float bowl head space 62' in accordance with the
conventional practice.
In the adaption of the commercial float bowl carburetor
20' toaccommodate the principles of the invention as illustrated
herein, passage 158 is sealed by a plug 162 and not utilized.
However, in a float bowl carburetor originally designed to
incorporate the invention these pre-existing passages 158 and
150 would be eliminated in favor of a simplier passageway from
2 'L~ tf' 7
-25-
.
tube 144 to headspace 62'. Likewise, mouth 82' of pitot tube
80' would be designed to be flush with the mid-plane of venturi
42' and the centerline of nozzle 70', unless a phase shift
offset relationship as described previously, was desired.
It will be noted from the foregoing and from the
illustrations of FIGS. 13, 15 and 16 that pitot tube 80' is
oriented and located in a manner quite similar to pitot tube 80
of carburetor 20. The mouth 82' of the entrance bore 142 of
pitot tube 80' is disposed a very short distance upstream of
the plane of the minor diameter of venturi 42' and just slightly
upstream from coplanar relationship with main nozzle 70' in
order to accomodate the physical limitations imposed by this
pre-existing carburetor design. Passage 142 like passage 86,
extends parallel to the main axis A of bore 40' and venturi
42'. Since carburetor 20' is provided with a choke plate 44'
mounted on choke shaft 28', similar to diaphragm carburetor 20,
pitot tube 80' is offset from axis A toward the side wall of
venturi 42' so as to be disposed of about halfway between axis
A and the associated side wall of venturi 42'. However, taken
vertically in the carburetor as seen in FIGS. 13 and 15, pitot
tube 80' is centered in horizontal alignment with axis A. In
any event, the entrance mouth 82' of pitot tube 80' is located
in the chordal space just downstream of choke plate 44' wherein
engine induced air flow velocity is maximized, taken into
consideration the presence of choke plate 44' and choke shaft
~ . 2 ~ i ~
-26-
28' and their obstructing effect relative to air flow through
carburetor bore 40' and venturi 42'.
Inasmuch as float bowl carburetor 20', like diaphragm
carburetor 20, is provided with a choke valve 44', and pitot
tube 80' is located directly downstream of choke valve 44',
carburetor 20' is also provided with a shunt passageway system
for producing a by-passing "shut-off" of the effect of the vent
passageway 142, 144, 150, 156 when initiating choke valve
closure. This "shut-off" shunt passageway comprises, as best
seen in FIGS. 19-21 and 24, a drilled passage 170 extending
from the front face of carburetor 20' parallel to axis A and
diametrically intersecting the bore 29 (FIG. 24) which receives
choke shaft 28' above choke plate 44'. The inner end of passage
170 terminates at an intersection with an angled drilled passage
172 which in turn intersects the upper end of passage 150. The
outer end of passage 170 is sealed by press fit ball 174, and
likewise the outer end of passage 172 is sealed by a press fit
ball 176. Another angled drilled passage 178 is provided
upstream of choke plate 44', near its upper edge, and which
opens into carburetor bore 40'. Passage 178 intersects passage
170 where both passages meet choke shaft bore 29. Choke shaft
28' is provided with a drilled cross passage 180 extending
diametrically of shaft 28' so as to be rotatable, in response
to choke shaft rotation, into and out of registry at its opposite
ends with passages 170 and 178.
-27-
Hence, as seen by comparing FIGS. 19 and 20, when choke
plate 44' is in fully closed position (FIG. 19), the upper end
space of passage 150 is in communication via passages 172, 170
and 178 with bore 40' upstream of the closed choke plate. As
choke plate 44' is rotated by shaft 28' from fully closed
position through an angle of 15~ the communication via passage
180 is gradually shut off. As choke shaft rotation continues
(clockwise as viewed in FIGS. 19-21) beyond 15~ from closing,
to the fully opened position of the choke plate 44' shown in
FIG. 21 deregistration of passage 180 with passage 170 shuts
off communication between passages 178 and passage 150.
From the previous description of diaphragm carburetor 20,
and choke shunt passageway 104-110 embodied therein, it will
be understood that the pressure wave transmission capability
of vent passageway 142, 144, 146 is rendered inoperable when
the choke valve is closed as in FIG. 19, and hence the high
vacuum conditions immediately behind choke plate 44' in the
vicinity of pitot tube 80' cannot be communicated to the air
chamber 62' of the float bowl 63' because of the shunting effect
of the shunt passageway 178, 180, 170. After the engine begins
to run under its own power and choke 44' is rotated toward open
position, shunt passageway 180 is gradually closed off by the
deregistration of passage 180 with passage 178 and 170 by the
corresponding rotation of shaft 28'. Once choke 44' is fully
open and the engine is running without choking, shunt passageway
2 ~ ?, ,~
-28-
178, 180, 170 is fully closed and hence vent passageway 142,
144, 146 becomes fully operable to transmitpressurewave effects
to the air chamber 62' of the float bowl 63.
As in carburetor 20, the effect of the vent passageway
142, 144, 150 may be modified or modulated in carburetor 20'
by venting the bowl headspace 62' to atmosphere in a controlled
manner. For this purpose, a restricted orifice 138' is provided
in the body of carburetor 20' communicating passage 150 with
atmosphere, in the manner of orifice 138 as described previously
in conjunction with carburetor 20.
It will thus be seen from the foregoing that the principles
of the invention, as described in detail and conjunction with
diaphragm carburetor 20, can also be readily embodied in a
float-controlled carburetor 20' by modifying the same in
essentiallythesame mannerandfor thesame purposesas described
in conjunction with diaphragm carburetor 20.
In view of the above, the invention should not be limited
to the preferred embodiments described and/or shown herein, but
can be modified in various ways within the scope of the appended
claims and applicable prior art.
