CA1036442A - Temperature sensitive choke opening mechanism for carburetor automatic choke - Google Patents

Abstract

TEMPERATURE SENSITIVE CHOKE OPENING
MECHANISM FOR CARBURETOR AUTOMATIC CHOKE

ABSTRACT OF THE DISCLOSURE

The carburetor has a conventional automatic choke construction heating a bimetallic coil by engine exhaust stove heat to permit the slow opening of the choke valve.
during cold weather starts; a choke pull down servo sensitive to engine manifold vacuum for operatively moving the choke valve towards an open position in opposition to the coil, and a second bimetallic coil between the servo and the choke valve for controlling the opening of the choke valve as a function of ambient temperature changes.

Description

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 ~al364~Z
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 \
~(~1369~z 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 1~369~4Z
; 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, ~L~3S9~4z 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 ~1~36~4Z
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 ~03644Z
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 1~36~4Z
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.

Claims (3)

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

1. An automatic choke system for use with a car-buretor 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, first engine temperature responsive spring means operably connected to the choke valve urging the choke valve towards a closed position with a force increasing as a function of decreases in the temperature of the 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 temperature responsive bimetallic spring means between the servo and the choke valve for controlling the opening of the choke valve as a function of ambient temperature changes from a second predetermined level, the second ambient temperature responsive spring having an inner end secured to a shaft and an outer end movable circumferentially in response to the coiling and uncoiling of the second spring in response to ambient temperature changes, means connecting the outer end of the second spring means to the pulldown servo, and a lever fixed to the shaft, the lever having first and second legs extending in opposite directions, one leg extending in a direction to be engaged at times and moved by abutment means on the servo upon operation of the servo, 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 pulldown position, the degree of pulldown opening of the choke valve varying as a function of the ambient temperature level.

2. The choke system of claim 1 wherein said choke pulldown servo consists of first and second flexible annular diaphragms, lever arms having an operable connection to the choke valve and to the first diaphragm, first extendable spring means yieldably connecting the diaphragms, second spring means biasing the second diaphragm towards the first diaphragm in a choke closing direction and manifold vacuum means acting at times on the second diaphragm in opposition to the first and second spring means for moving the second diaphragm in a choke valve opening direction.

3. The choke system of claim 2, including delay means associated with the diaphragms controlling the rapidity of movement of the diaphragms in a choke valve opening direction to provide a controlled rate of opening.