US3542346A - Carburetor fast idle cam construction - Google Patents

Description

United States Patent Timothy R. Keenon Dearborn Heights, Michigan 765,510

Oct. 7, 1968 Nov. 24, 1970 Ford Motor Company Dearborn, Michigan a corporation of Delaware Inventor Appl. No. Filed Patented Assignee CARBURETOR FAST IDLE CAM CONSTRUCTION 6 Claims, 3 Drawing Figs.

[56] References Cited UNITED STATES PATENTS 2,092,946 9/1937 Wolfard 261/52X 2,102,428 12/1937 Macauley etal 261/39(.2)UX 2,189,219 2/1940 Olson 261/39(.2)UX 2,943,848 7/1960 Gordon etal 261/39(.2)UX

Primary Examiner-Tim R. Miles Attorney--.Iohn R. Faulkner and Robert E. McCollum ABSTRACT: A motor vehicle carburetor includes, in addition to the automatic choke thermostatic spring, a thermally sensitive element on the fast idle cam face to progressively vary the engine idle r.p.m. as a function of ambient temperature changes and, ofwhich the following is a specification:

CARBURETOR FAST IDLE CAM CONSTRUCTION This invention relates, in general, to a carburetor for a motor vehicle engine. More particularly, it relates to a fast idle cam for use as part of a carburetor automatic choke to control the idle speed of the engine during cold weather starts.

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 generally provided to lessen the air intake during cold starting and preengine warmup to insure a richer mixture.

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 progressively opens it as the temperature returns towards a chosen level. A fast idle cam generally rotates with the choke, the cam having a number of steps of decreasing radius. The steps are adapted to be engaged by an adjustable screw or similar stop secured to the carburetor throttle shaft so that the throttle valve initially will be cracked open a predetermined degree to assure engine start up during cold starts. The choke valve subsequently moves progressively towards an open position by seating successively against decreasing radius steps, as the cam is rotated byv the thermostatic choke element, to progressively decrease the engine idle speed until the chosen warmup idle speed is reached.

In substantially all cases, the fast idle cam generally has a series of steps on the periphery of the cam of progressively decreasing radius, and a weighted opposite portion, The high step provides the greatest opening of the throttle valve and thereby the highest idle rpm. The throttle valve return spring forces the throttle idle adjustment screw against this cam during cranking of the engine when the greatest amount of friction in the engine must be overcome. Once the engine is running, a lower (but higher than normal) idle r.p.m. generally is sufficient. i

A disadvantage of having a construction of the above type, however, is that each step is a compromise of a plurality of idle speeds to cover a wide range ofambient temperature changes. That is, any one step usually provides a fixed idle r.p.m.; however, the engine friction and lubricant viscosity may change considerably within this range, rendering it more desirable to progressively increase or decrease the idle speed r.p.m.

Accordingly, the invention relates to the construction of a fast idle cam in which the portion engaged by the throttle valve stop contains a thermally sensitive member that expands or contracts radially in response to changes in ambient temperature to automatically vary the position of the throttle valve and, therefore, the idlespeed r.p.m. of the engine.

It is a primary object of the invention, therefore, to provide a carburetor fast idle cam that includes a thermostatic element that automatically varies the idle speed position of the throttle valve as a function of the changes in ambient temperature conditions, to prevent stalling of the engine during cold temperature operating conditions.

Other objects, features, and advantages of the invention will become more apparent upon reference to the succeeding detailed description thereof, and to the drawings illustrating a preferred embodiment thereof, wherein;

FIG. 1 is a crosssectional view through a portion of an internal combustion engine carburetor;

FIG. 2 is a perspective elevational view of a portion of the choke mechanism for the carburetor shown in FIG. 1; and,

FIG. 3 is an enlarged view ofa detail in FIGS. land 2.

FIG. 1 is a cross-sectional view obtained by passing a plane through approximately one-half of a known type of four bar- 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. The

passages are each formed with a main venturi section 22 containing a booster venturi 24 suitably mounted for cooperation therewith, by means not shown.

Air flow through passages 20 is controlled in part by a choke valve 28 fixedly mounted on a shaft 30 rotatably mounted on side portions of the carburetor air born, as shown. Flow of fuel and air through each passage 20 is controlled by a conventional throttle valve 36 fixed to a shaft 38 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.

The rotative position of choke valve 28 is controlled in a known manner 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 flange. 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 end 59 of a coiled thermostatic spring element 60. The opposite 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 and 70, passage 68 being connected to an exhaust manifold heat stove, for example Cap 66 is secured to housing 42 by suitable 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 ambient temperature conditions of the air entering tube 68, or, if there is no flow, the temperature of the air within chamber 74. Accordingly, changes in ambient 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.

The leg 56 of spring lever 54 is pivotally fixed to the rod 76 of a piston 78. The latter is movably mounted in a bore 79 in housing 42. The under surface of piston 78 is acted upon by vacuum in a passage 80 that is connected to the carburetor main induction passages 20 by a port 82 that is located just slightly below throttle valve 36. Piston 78, therefore, is always subject to the vacuum existing in the intake manifold passage portion 18.

As is known, a cold weather start of a motor vehicle requires a richer mixture thana warmed 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. Once the engine does start, however, then the choke valve should be opened slightly to lean the mixture to prevent engine flooding as a result of an excess of fuel.

The known choke mechanisms described automatically accomplish the action described. That is, on cold weather starts, the temperature of the air in chamber 74 will be low so that spring element 60 will contract and rotate shaft 44 and choke valve 28 to a closed or nearly closed position, as desired. Upon cranking the engine, vacuum in passage 80 will not be sufficient to move piston 78 to open the choke valve. Accordingly, the engine will be started with a rich mixture. As soon as the engine is running, high vacuum in passage 30 now moves piston 78 downwardly and rotates shaft 44 a slight amount so that choke valve 28 is slightly opened so that less fuel is admitted to induction passage 20. Shortly thereafter, the exhaust manifold stove air in line 68 will become progressively warmer and cause choke element 60 to unwind and rotate shaft 44 and choke valve 28 to a more open position. Further details of construction and operation are not given since they are known and believed to be unnecessary for an understanding of the invention.

Turning now to the invention, during the engine warmup period, as started previously, the throttle valve must be open slightly to provide a fast idle speed sufficient to prevent stalling of the engine that may result from the increased friction and viscosity of the lubricating fluid in a cold engine. Accordingly, as best seen in FIG. 2, and separately in FIG. 3, a fast idle cam 84 is fixed on choke shaft 44. it includes a cylindrical hub 86 with a segment cam plate 88 projecting radially from one side, and a weighted lever segment 90 projecting from the opposite side. The peripheral portion of segment 88 includes a high step or engine starting cam surface 92, and a contoured step 94 ofprogressively changing radius.

Overlying cam face 94 is a bimetallic spring element 96 that is secured at one end as shown to cam lever portion 88. The bimetallic element is similar to coiled thermostatic spring 60. It consists of two pieces of metal having different coefficients of expansion so as to cause, in a known manner, arcuate contraction as the temperature increases, and a straightening out upon decrease in temperatures, to move away from face 94.

As best seen in FIG. 2, a throttle stop or screw 98 is adjustably mounted on a lever 100 fixed to throttle valve shaft 38 (also FIG. 1). The end of screw 98 is biased, by the throttle valve return spring, not shown, against the thermostatic spring element 96, during idle,to control the idle speed position of the throttle valve.

In operation, for a cold weather start, the upper part 102 of the two piece thermostatic spring element 96 will contract at a faster rate than the lower part 103 so as to straighten out element 96 and cause it to move radially outwardly from cam face 94. The cold ambient temperature will have also cause choke thermostatic spring element 60 to have rotated shaft 44 to a position closing or nearly closing choke valve 28. This positions the fast idle cam 84 so that screw 98 will be against the high step cam-surface 92 for engine starting purposes. Accordingly, during cranking of the engine for starting, the throttle valves 36 will be opened an extent determined by the count erclockwise movement of screw 98. As soon as the engine is started, the highidle vacuum acting against piston 78 is sufficient to rotate shaft 44 against the spring element 60 to rotate the fast idle cam clockwise. This latter rotation is sufficient to move the throttle valve adjusting screw 98 off the high cam face 92 and onto the end portion of the face 102 of the thermostatic spring element 96. Accordingly, the running condition of the engine will find the throttle valves at a lesser opening, thereby causing a lower idle r.p.m. of the engine.

As the ambient temperature in chamber 74 increases due to the increase in temperature in line 68, continued clockwise rotation of the choke thermostatic spring lever 54 and shaft 44 and fast idle cam 84 by the choke spring element 60 will progressively rotate the throttle valve shaft 38 clockwise to progressively further decrease the engine idle speed.

Similarly, if the ambient temperature should decrease, then, when the fast idle cam is reset by an opening and return movement of the throttle valve by the operator, the throttle valve adjusting screw 98 will rest on a portion of the thermostatic spring element 96 closer to the high cam step face 92.

It will be clear from the foregoing, therefore, that the invention provides a fast idle cam with a thermally sensitive cam surface so that the engine idle speed will vary as a function of the changes in ambient temperature so that stalling of the engine during warmup will not occur even though friction forces and viscosity of the lubricants increase considerably with decreases in temperature. Therefore, it will be seen that the invention provides an advantage over the conventional choke mechanism combined with a fast idle cam in that it provides a progressively changing engine idle speed to suit the temperature conditions rather than a conventionally fixed series of ranges or curvature progressively changing radius.

While the invention has illustrated the fast idle cam thermostatic element as being attached to one part of the fast idle cam face, it will beclear that it could also be attached to the high cam step face, or alternately to the high step face alone, as desired, to establish a finer control during cranking of the engine, without departing from the scope of the invention.

While the invention has been illustrated 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.

1 claim:

l. A control means for controlling the idle speed of an internal combustion engine having a carburetor throttle valve variably movable between positions opening and closing an air-fuel mixture conduit ofsaid carburetor, said control means including abutment means operably fixed to said throttle valve, and stop means positioned in the path of movement of said abutment means and valve in one direction, said stop means including thermally sensitive means directly engaged by said throttle valve abutment means upon abutment thereon by said stop means and movable in response to ambient temperature changes to vary the stopped position of said abutment means, said stop means having a cam profiled face engagable by said abutment means, said face including said thermally sensitive means comprising a bimetal member deformable in response to temperature changes to vary the profile of said face.

2. A control means as in'claim 1, said cam face having a plurality of stepped parts.

3. A control as in claim 1, including means mounting said stop means for a rotatable movement, and thermostatic spring means connected to said stop means for rotation thereof to different positions in response to temperature changes to vary the attitude of said cam face relative to the point of abutment thereofwith said abutment means.

4. An engine idle speed control means for controlling the idle speed position of the throttle valve of a carburetor having a main induction passage for the flow of an airfuel mixture therethrough, a choke valve movably mounted in said passage adjacent one end, a throttle valve mounted in said passage and rotatable away from and towards an idle speed rest position and being axially spaced from said choke valve, thermally sensitive means movable in response to ambient temperature changes and operatively connected to said choke valve for movement at times thereof, and further means between said thermally sensitive means'and said throttle valve to control the idle speed rest position of said throttle valve, said further means including adjustable abutment means operably fixed to said throttle valve for rotation therewith, and fast idle cam means operably connected to and rotatable with said choke valve, said cam means having a cam face portion abuttable at times by said abutment means to arrest the movement of said throttle valve in one direction and define said idle speed rest position, said portion including a thermally sensitive part movable in response to ambient temperature changes to vary the cam profile of said portion and vary the rest position of said throttle valve.

5. An engine speedcontrol means as in claim 4, said thermally sensitive part comprising a bimetallic member fixed to said face portion abuttable by said abutment means and expandable and contractible away from said face portion in response to predetermined temperature changes.

6. An engine speed control means as in claim 5, said bimetallic member having a progressively changing slope.

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