Note: Descriptions are shown in the official language in which they were submitted.
This invention relates, in general, to a
carburetor for a motor vehicle engine. More particularly,
it relates to an automatic choke ~o provide cold weather
starts of an engine, while at the same time minimizing the
output of undesirable emissions.
As ambient temperature drops, friction within
the engine and the viscosity of the lubricants increase
significantly. Therefore, at low temperatures, the speeds
at which an engine normally would idle must be increased to
prevent stalling. Accordingly, a choke mechanism is provided
to richen the fuel/air mixture supplied to the engine during
cold starting and engine warm-up.
Generally, the choke apparatus includes a coiled
thermostatic spring that operatively rotates the choke valve
towards a closed or nearly shut position with decreasing
temperatures, and permits the progressive opening of it as
the temperature returns towards a chosen level. A manifold
suction responsive device generally overrides the coil
spring force and cracks open the choke a predetermined angle
of say twenty degrees, for example, when the engine starts,
to provide a leaner running mixture. The choke action provides
a richer than normal mixture so that sufficient fuel can be
vaporized to permit smooth starting and running of the engine
during cold weather.
The above construction has the disadvantage of
having a fixed pulldown regardless of the ambient temperatures.
That is, the vacuum servo usually operates against a stop so
that as soon as engine operating vacuum is obtained, the
choke valve is opened to a fixed angular position of say, in
this case, twenty degrees. If the outside ambient temperature
is say 50F, the best mixture for cold running conditions may
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be obtained if the choke valve were opened only to an
angle of, say ten or fifteen degrees. That is, at twenty
degrees, the mixture may be too lean, and the engine might
stall after starting.
This invention is directed to a choke construction
wherein the choke pulldown rate is modified in response to
ambient temperature conditions so that the changeover from
engine cranking to cold weather running air/fuel mixture
richness more accurately reflects actual operating conditions,
and thereby reduces the output of emissions of unburned
hydrocarbons and carbon monoxides due to nonburning of
too rich or too lean a mixture.
The aut`omatic choke system of this invention
for use with a carburetor having an air/fuel induction
passage and an unbalance mounted, air movable, choke valve
mounted for variable movement across the passage to control
airflow through the passage generally includes first engine
temperature responsive spring means operably connected to
the choke valve urging the choke valve to a closed position
with a force increasing as a function of decreases in the
temperature ofthe spring means from a first predetermined
level, a choke pulldown servo sensitive to engine manifold
vacuum for operatively moving the choke valve towards an
open position in opposition to the spring means, and second
ambient t~rature -responsive bimetallir coil spring means
between the servo and the choke valve for controlling the
opening of the choke valve as a function of ambient tempera-
ture changes from a second predetermined level.
In our copending Canadian application Serial No.
206,186 filed August 2, 1974 out of which his application
is divided, there is described a choke mechanism of the same
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general type. The parent application is specifically
directed to the structure and operation of the choke pulldown
servo whereas the present application is specifically direc-
ted to the structure and operation of the second ambient
temperature responsive bimetallic coil spring means located
between the servo and the choke valve.
In accordance with this invention, the second
ambient temperature responsive spring means has an inner
end secured to a shaft and an outer end movable circumferen-
tially in response to the coiling and uncoiling of the
second spring in response to ambient temperature changes,
means connecting the outer end ofthe second spring means to
the pulldown servo and a lever fixed to the shaft. The
lever has first and second legs extending in opposite
directions with one leg extending in a direction to be
engaged at times and moved by abutment means on the servo
upon operation of the servo and the second leg being movable
in a path containing the choke valve and moving alternately
in response either to movement of the lever by the abutment
means or in response to ambient temperature changes effecting
rotation of the shaft to operatively engage the choke valve
to move it to an open position. The degree of pulldown
opening of the choke valve varies as a function of the
ambient temperature level.
An automatic choke constructed in this way
will provide good cold weather starting characteristics
and yet decrease to a minimum the output of undesirable smog
producing elements.
The present invention is described further, by
way of illustration, with reference to the accompanying
drawings, in which:
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Figure 1 is a cross-sectional elevational view
of a portion of a two-barrel carburetor showing a conventional
choke construction;
` Figure 2 is a reduced size top plan view of a
carburetor embodying the invention;
Figure 3 is a side elevational view of a portion
of Figure 2;
Figure 4 is an enlarged cross-sectional view
taken on a plane indicated by and viewed in the direction
of the arrows 4-4 of Figure 2;
Figure 5 is an enlarged view of a portion of
Figure 2; and,
Figures 6 and 7 are enlarged end and side views
of Figure 2.
Referring to the drawings, Figure 1 is obtained
by passing a plane through approximately one-half of a known
type of two-barrel, downdraft type carburetor. The portion
of the carburetor shown includes an upper air horn section
12, an intermediate main body portion 14, and a throttle
valve flange section 16. The three carburetor sections are
secured together by suitable means, not shown, over an
intake manifold indicated partially at 18 leading to the
engine combustion chambers.
Main body portion 14 contains the usual air-fuel
mixture induction passages 20 having fresh air intakes at
the air horn ends, and connected to manifold 18 at the
opposite ends. Each of the passages is formed with a main
venturi section 22 containing a booster venturi 24 suitably
mounted for cooperation therewith, by means not shown.
Flow of fuel and air through each passage 20 is
controlled by a conventional throttle valve 26 fixed to a
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shaft 27 rotatably mounted in flange portion 16. The
throttle valves are rotated in a known manner by depression
of the vehicle accelerator pedal, and move from an idle
speed position essentially blocking flow through passage 20 to
a wide open position essentially at right angles to the position shown.
Airflow through passages 20 is controlled in
part by a choke valve 28 unbalance mounted on a shaft 30
rotatably mounted on side portions of the carburetor air
horn, as shown. The rotative position of choke valve 28
is controlled in part by a semiautomatically operating
choke mechanism 40. The latter includes a hollow housing
portion 42 that is formed as an extension of the carburetor
throttle fl~ange. The housing is apertured for supporting
rotatably one end of a choke lever operating shaft 44,
the opposite end being rotatably supported in a casting 46.
A bracket or lever portion 48 is fixed on the left end
portion of shaft 44 for mounting the end of a rod 52 that is
pivoted to choke valve shaft 30. It will be clear that
rotation of shaft 44 in either direction will correspondingly
rotate choke valve 28 to open or close the carburetor air
intake, as the case may be.
An essentially L~shaped thermostatic spring
lever 54 has one leg 56 fixedly secured to the opposite or
right-hand end portion of shaft 44. The other leg portion
58 of the lever is secured to the outer end 59 of a coiled
bimetallic temperature responsive spring element 60 through
an arcuate slot, not shown, in an insulating gasket 61. The
opposite inner end portion 62 of the spring is fixedly
secured on the end of a nipple 64 that is formed as an
integral portion of a choke cap 66 of heat insulating material.
Nipple 64 is bored as shown to provide hot air passages 68
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; and 70, passage 68 being connected to a conventional exhaust
manifold heat stove, for example. Cap 66 is secured to
housing 42 by suitabl~e means, such as the screw 72 shown,
and defines an air or fluid chamber 74 within the two.
As thus far described, it will be clear that the
thermostatic spring element 60 will contract or expand as
a function of the changes in engine operating temperature
conditions of the air entering -tube 68, or, if there is no
flow, the temperature of `the air wi~hin chamber 74. Accor-
dingly, changes in engine operating temperature will rotate
the spring lever 54 to rotate shaft 44 and choke valve 28
in one or the other directions, as the case may be.
Housing 42 contains a bore 79 that is acted upon
by vacuum in a passage 80. The passage is connected to the
carburetor main induction passages 20 by a port 82 that is
located just slightly below throttle valve 26. Charnber 74,
therefore, is always subject to the vacuum existing in the
intake manifold passage portion 18, to effect flow of the
heated air esserltially at atmospheric pressure through tube
68 into chamber 74.
As is known, a cold weather start of a motor
vehicle requires a richer mixture than a warm engine start
because considerably less fuel is vaporized. Therefore,
the choke valve is shut or nearly shut to increase the
pressure drop thereacross and draw in more fuel and less
air. Once the engine does start, however, then the choke
valve should be opened slightly to lean the mixture to
prevent engine flooding or stalling as a result of an excess
of fuel.
The choke mechanism shown on the right side in
Figure 1, supplemented by the choke mechanism 84 in Figure 2,
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automatically accomplishes the above action. Morespecifically, in Figure 2, a control lever 86 is fixed on
the left end 88 of choke shaft 30, and has a finger portion
90. The finger portion is adapted to abut and be moved at
times by the out-turned pin end 92 of one leg 94 of a bell-
crank lever 96. The lever 96 is fixed on the end of a shaft
98 which is rotatably supported in a pair of bushings 100
extending from a supporting bracket 102 shown in Figure 3.
The bracket has an L-shape, with ears 104 secured by bolts
105 to the top of the carburetor air horn surface, as best
seen in Figure 6.
The bellcrank 96 has a second leg 106 that is
adapted to be engaged by the end of a screw 108. The screw
is adjustably mounted in a finger-like or tab extension 110
of a yoke shaped member 112 loosely mounted on shaft 98.
A pin 114 is anchored at one end in aligned holes in member
112, and projects through the eye 1~16 of a pulldown lever or
rod 118. The other end of pin 114 is slotted for receiving
the outer upturned end 120 of a bimetallic coil spring 122.
The spring is wound around shaft 98 and keyed at its inner
end to the shaft. The coil spring 122 in this case is
responsive to ambient temperature changes of the air surround-
ing the air horn section of the carburetor to coil or uncoil
the spring. The rod 118 moves down when vacuum is applied
causing pin 114, me~mber 112, tab 110 and screw 108 to contact
and move leg 106, rotate shaft 98, leg 94 so that pin end
92 contacts finger portion 90 and rotates shaft 88 opening
the choke valve plate 28. This provides minimum choke
plate opening at extremely cold temperatures. Conversely,
pin 114 contacts bimetal end 120 rotating bimetal 122 and
shaft 98 so that leg 94 actuates lever 86 to rotate shaft
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88 and open choke plate 28 a greater amount due to high
bimetal environment temperatures. In this case, the spring
122 has rotated leg 106 away from screw 108 so that the
pulldown by rod 118 has less effect on opening the valve.
In Figure 4 the pulldown rod 118 projects
sealingly through the housing 124 of a vacuum sensitive servo
or motor device 126. The servo contains a pair of spaced
annular flexible diaphragms 128 and 130 edge mounted in the
housing walls. An annular retainer assembly 132 is riveted
to each of the diaphragms, and are interconnected by an
extension spring 134. The retainer assembly for diaphragm
128 also is fixed to pulldown lever 118. The lower diaphragm
130 is biased upwardly by a compression spring 136.
The upper chamber 138 defined between the housing
and diaphragm 128 communicates with the atmosphere either
slowly through an orificed flow time delay device 140 in
a port 142, or rapidly past a ball check valve 144 engageable
with a seat 146. The central chamber 148 defined between
the diaphragms is essentially a dead air space. The lower
chamber 150 between diaphragm 130 and the housing is
connected to engine manifold vacuum through a side port
152. The port is connected by cast passages 154 and 156 to
a carburetor port 158. Port 158 opens into the induction
passage 20 at a point below the closed position of the
throttle valve, like port 82.
On cold weather starts, the temperature of the
air in chamber 74 will be low so that spring element 60
will contract and urge shaft 44 and choke valve 28 to an
essentially closed position, as desired. Initially, screw
108 will be adjusted to separate lever 96 from finger 110
to provide the desired initial preload by a predetermined
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unwinding of the spring 122. During cranking of the engine,
therefore, manifold vacuum in servo passage 154 will not
be sufficient to move the lower diaphragm 130 against the
force of coil spring 60 to open the choke valve. ~pring
134 may slightly extend. Accordingly, the engine will be
started with a rich mixture. As soon as the engine is
running, however, high vacuum in servo passage 154 acting
on diaphragm 130 will move it downwardly. The preload force
of ambient temperature responsive spring 122 will be chosen
so that at the lowest ambient temperature setting, say
-20F, for example, the engine running vacuum force on
diaphragm 130 will be sufficient to overcome the forces of
both coil springs 122 and 60 and springs 134 and 136 to
pull open the choke valve 28 a slight amount, say to an
angle of six degrees, for example. That is, the vacuum on
diaphragm 130 will extend spri~g 134 which will then pull
diaphragm 128 and lever 118 downwardly. This will rotate
finger 110 and screw 108 against bellcrank leg 106. The
opposite leg 94 will then pivot lever 86 and choke valve
28 open the six degrees. The extension of spring 134
permits the diaphragm ~8 to move down slowly as a function
of the orifice 140.
If the engine temperature warms faster than
ambient, the lessening closing force exerted by the coil
spring 60 will permit movement of the choke valve to a
greater opening by airflow against the choke valve. In
this case, lever 86 will rotate counterclockwise away from
the pin end 92 of lever 96. On the other hand, engine
starts with cold engines and with warmer than the coldest
setting ambient temperatures, the relative movement of spring
122 between the pin 114 and the slot in shaft 98 will rotate
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shaft 98 and cause lever 106 to rotate clockwise thereby
rotating lever 86 cc, thus opening choke plate 28 a greater
amount as a function of ambient temperature increases (20,
for example) then would normally be the case by the pulldown
servo 120 alone. The rate of pulldown, therefore, will
always be modified in accordance with ambient temperature
changes to permit a greater or less choke valve opening
as the ambient temperature changes.
While the invention has been shown and described
in its preferred embodiment, it will be clear to those
skilled in the arts to which it pertains, that many changes
and modifications may be made thereto without departing-
from the scope of the invention.
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