CA1036443A - Altitude compensation in carburetor choke systems - Google Patents

Abstract

ALTITUDE COMPENSATION IN CARBURETOR
CHOKE SYSTEMS
ABSTRACT OF THE DISCLOSURE
In a carburetor having a choke mechanism including a thermostatically responsive bimetallic coiled spring that exerts an increasing closing force on the choke valve as the temperature decreases below the normal engine operating level, an intermittently operable heater device is provided adjacent the bimetallic spring to reduce the spring closing force on the choke valve when operable. The heater device is rendered operable by an engine vacuum responsive switch to compensate for increases in altitude which would tend to decrease manifold vacuum. The heater device is also and alternatively rendered operable by a temperature sensitive switch to achieve more rapid choke valve opening once a predetermined temperature level has been achieved.

Description

~L03~443 The present invention relates in general ~o a motor vehicle type carburetor. More particularly, it relates to a carburetor choke mechanism that provides the same cold engine operation regardless of changes in air density caused ; by changes in altitude.
This application is a division of our copending Canadian Application Serial No. 221,589 filed March 7, 1975.
Most commercial carburetor chokes do not provide means for compensating for changes in air density with changes in altitude. Accordingly, when a car equipped with a conven-~; tional choke is operated in mountainous regions or at any level above that at which it was calibrated, the less dense air provides a richer mixture induction into the engine. The engine accordingly may load under acceleration, because less air now is available to combine with the liquid fuel droplets, resulting in less power output. In order to obtain the same acceleration, as at lower altitutudes, therefore, the throttle plate is opened wider. This decays the level of manifold vacuum as compared to that at the calibrated level.
An alternate way of looking at the same problem is that at higher than sea level, or higher than the altitude at which the choke was calibrated, when the engine is accelerated, the air inducted is at less than atmospheric pressure.
Accordingly, the pressure differential between the carburetor inlet and the engine combustion chamber is now less than when measured at the lower altitude. This again results in lower manifold vacuum levels as the altitude increases.
Most commercial carburetors have choke mechanisms containing a thermostatically responsive bimetallic coiled spring that exerts an increasing closing force on the choke .

~(1364~3 valve as the temperature decreases below the normal engine operating level. This provides a richer mixture during cold engine operations, with the mixture being leaned progressively as the temperature increases. Most of the constructions slowly decrease the bimetal spring force and open the choke valve by flowing air warmed by the engine exhaust system past the coiled spring into the manifold.
It will be noted from the above, however, that since the manifold vacuum level lowers progressively as the altitude increases, the flow of hot air past the bimetallic spring will also decrease with altitude changes. During cold ambient driveaways, this can produce insufficient vacuum to pull enough hot air across the bimetal to insure maintaining the choke valve open. For example, if at the calibrated altitude, an acceleration is made that normally drops the net manifold vacuum force to say 2"Hg., higher altitude requiring opening the throttle plate wider to obtain the same acceleration may drop the manifold vacuum to a nearly atmospheric pressure level. Little or no pressure differential, therefore, will exist between the hot air source and the manifold vacuum to cause flow of warm air past the bimetal spring to warm it.
This results in a loading of the engine with a rich mixture to the point where it may stall because of the choke valve being in a position more closed than should be for the particu-lar temperature setting.
In accordance with the present invention, there is provided an automatic choke system 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, thermo-static spring means operably connected to the choke valve ~36~43 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 predetermined level, an intermit-tently operable heater device located adjacent the spring means operable to transfer its heat output to the spring means when operable to reduce the choke valve closing force of the spring means and permit opening of the choke valve by airflow through the passage against it, a source of electrical energy, circuit means connecting the source to the heater device, temperature sensitive switch means in the circuit means operable to break and make the circuit as a function of temperature changes from a predetermined level, and engine vacuum responsive means operable below a predetermined temperature to selectively render the heater device operable and inoperable in response to changes in engine vacuum, means mounting the vacuum responsive means to at times elec-trically connect the source of electrical energy to the heater device in another circuit that is parallel to the first-mentioned circuit connecting the source and heater device through the temperature sensitive switch means to permit connection of the source of electrical energy to the heater device when the temperature sensitive switch means is operable to break the first-mentioned circuit bypassing the temperature sensitive switch means.
By utilization of the engine vacuum responsive means to intermittently operate the heater device in response to changes in engine vacuum, choke opening may be varied in response to altitude of the vehicle.
The invention is described further, by way of illus-tration, with reference to the accompanying drawings, in which:
Figure 1 is a cross-sectional elevational view of ~36~43 a portion of a two-barrel carburetor embodying the invention;
Figure 2 is an enlarged view of a detail of Figure l; Figure 2a is a view corresponding to Figure 2 illustrating the parts in different operative positions; and, Figure 3 is a graph illustrating the operating characteristics of a detail shown in Figures 1 and 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. 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 unbalance mounted on a shaft 30 rotatably mounted on side portions of the carburetor air horn, as shown.
Flow of fuel and air through each passage 20 is controlled by a conventional throttle valve 36 (only one shown) 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 ~1~31~44;~
by ~ 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 ~4 ~or mounting the end o~ 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 thermostatic spring element 60 through an arcuate slot in an insulating gasket 64.
Leg 56 is also pivotally fixed to the rod 76 of a pi~ton 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 carburetor main induction passages 20 by a port 82 located just slightly below throttle valve 36. Piston 78, therefore, is always subjected to any vacuum existing in the intake manifold passage portion 18.
The casing 42 is provided with a hot air passage 68 connected to an exhaust manifold heat stove, for example. The cylinder in which piston 78 slides is provided with bypass slots, not shown, in a known manner so that the vacuum acting on the piston will cause a flow of the hot air from passage 68 to passage 80. More specifically, hot air will flow into ~364~3 the area round the spring coil 60 through a hole in g~sket 64 and out through the slot to the bypass slots around piston 78.
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. Accordinglyl 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.
As is known, a cold weather start of a motor vehicle requires a richer mixture than a 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 choke mechanism described automatically accom-plishes 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 soo~ as the engine is running, high vacuum in passage 80 moves piston 78 downwardly and rotates shaft 44 a slight amount so that choke valve 28 is slightly opened so that less ¦ 30 fuel is admitted to induction passage 20. Shortly thereafter, the exhaust manifold stove air in line 68 will become ~36443 progressively warmer and cause choke element 60 to unwind slowly and rokate 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.
Figure 2 shows thermostatic spring coil 60 centrally staked to a metal post 84. The post is formed as an integral part of an aluminum disc 85. The disc oonstitutes a heat sink or transfer member to evenly radiate heat to the coil from a heater element 86 to which it is secured.
Heater element 86 is a positive temperature coeffic-ient (PTC) semiconductor in the shape of a flat ceramic disc.
It is fixed on disc 85 and has a central spring-leg type current carrying contact lug 88. The lug projects through a hole in a wall 89 of an insulated cover or choke cap 90.
The disc 85 is grounded through the cover to the cast choke housin~ by extensions and ground terminals 92. Lug 88 is adapted to be engaged at times by the face of a bimetallic thermal switch 94. The switch is of the overcenter spring type, and is sensitive to ambient temperature changes. The switch has a central hole through which projects a current carrying lug 96. The lug 96 is fixed to a spring leg type conductor 98 connected to an electrical spade-like terminal 100. The terminal would be connected to any suitable souxce ~ of electrical energy, such as the vehicle alternator, so that `~ current would be supplied as long as the vehicle is running.
The choke cap 90 also includes a vacuum switch 102 that is of the plug-in type. That is, switch 102 has a hollow housing 103, through one side of which projects a pair o~
current carrying contact prongs 104, 106. Prong lQ4 extends through an electrical socket 107 in cap 90 to a position 1~36~4~
adjacent the backside of con~act lug 96. It is spaced from the lug when the bimetal switch 94 is in the overcenter posi-tion shown, contacting lug 88. When bimetal switch 94 moves overcenter to its alternate position, lug 96 engages prong 104 to conduct current through it. Prong 106 on the other hand extends through an electrical socket 108 as shown into engagement with a current carrying connector 110 fixed to lug 88, to conduct current to heater 86 in a manner to be described.
The hollow interior of switch 102 is divided by an annular flexible diaphragm 111 into an air chamber 112 and a manifold vacuum chamber 113. A vacuum tube 114 leads into chamber 113 and is connected at its other end, not shown, to a suitable manifold vacuum port similar to carburetor port 82, for example.
Chamber 112 is vented to ambient pressure through a hole, not shown, in housing 102. Attached to diaphragm 111, on the air chamber side, is a contact bridge plate 116. A
preloaded spring 118, in vacuum chamber 113, biases the diaphragm 111 and contact plate 116 to a bridging position with respect to prongs 104 and 106. The force of spring 118 is chosen to be such as to require a net manifold vacuum force of at least as high as 2"Hg., for example, in chamber 113 before the diaphragm will be drawn rightwardly and unbridge contacts 104 and 106 to break the circuit and open the switch.
Returning now to heater element 86 per se, it is a characteristic of the PTC heater that its internal resistance varies directly with the skin temperature of the element, from a predetermined switch point Ts. The change in the internal resistance is not a linear function of the elements' internal temperature but varies in the manner shown more clearly 1~)36443 .
in Figure 3. When the PTC heater 86 is electrically energized, as by applying line voltage to its terminals from the alter-nator when either switch 94 or 102 closes, the Joule heat causes rapid self~heating of the PTC element. The heater resistance remains almost constant as it heats from room temperature. It increases as the PTC temperature nears the switching temperature Ts, or desired upper limit, at which point the resistance increases sharply, as shown. The electrical characteristics, of course, can be controlled by the chemical composition and process of making it.
It will be seen, therefore, that it is an inherent property of this semiconductor to obtain a very high impedance to current flow at high internal temperatures, and that the semiconductor has an ability to maintain a high maximum temperature. The need for a cut off thermostat to protect against distortion of the bimetallic spring 60, therefore, due to extreme temperature levels is thereby eliminated.
In this instance, therefore, the PTC device provides heat to bimetal spring 60 that is supplemental or substitutive to that provided by the primary exhaust manifold hot air system, depending upon whether switch 94 or 102 is closed.
When current passes through the PTC element, a change in the ~` internal temperature is noticed. This heat generated is transferred by conduction to spring 60 through the post 84 and by radiation to the spring from the heat sink 85.
When the PTC internal temperature reaches the switching temperature Ts, say 180F, as seen in Figure 3, the internal resistance is so high that the current flow is very low and essentially cut off. It will be seen, therefore, that the heat input to the PTC element by the current flow then is essentially balanced by the heat loss by the PTC to the environment and to the bimetal post 84. Therefore, for all intents and purposes, the heat of the PTC remains at a constant level.
The overall operation is believed to be clear from the above description and the drawings, and therefore will not be repeated in detail.
In brief, below an ambient temperature level of 65+F, as seen in Figure 2a, the bimetal switch 94 has flipped overcenter to a position disengaging the end of lug 96 from contact lug 88. The PTC heater 86, therefore, cannot be energized through switch 94 at this time. At this tempera-ture level, closing of the choke valve is needed to provide the proper enrichment. Therefore, only a slow unchoking coordinated with the engine temperature rise is desîred, which is normally provided by the slowly rising exhaust heated air passing through the choke housing. Therefore, it is not desired at this time to energize the PTC heater unless the hot air should decrease below a minimum flow level.
As stated previously, due to altitude changes, there are times when the manifold vacuum decreases to a level so low during vehicle accelerative movements that the pressure differential is insufficient to draw enough hot air through the choke housing to warm the bimetal to follow the schedule of unchoking desired. It is at this time, when manifold vacuum drops below the 2"Hg. level, for example, that the switch 102 closes by the contact plate 116 bridging the two prongs 104 and 106. This connects the alternator current to the PTC heater around the bimetal switch 94 to provide heat to the bimetal spring to compensate for that lost by the lack of heat flow from the exhaust hot air system. The closing of vacuum switch 102, therefore, makes up or compensates for the 64~L3 inability of the hot air system to ~unction as designed.
The vacuum switch there~ore bypasses the open bimetal switch 94 to separately and in parallel energize the PTC heater until such time as the manifold vacuum again rises over the

2"Hg. level (when the hot air flow is sufficient) sufficient to overcome the force of spring 118 and unbridge contacts 104 and 106.
It will be clear that switch 102 will be so constructed and arranged as to be triggered into action whenever the mani-fold vacuum drops to a level indicating insufficient airflow through the choke housing. It will be clear of course that the particular level desired can be controlled by the choice of the preload of spring 118.
~bove 65F, use of the exhaust hot air alone to determine choke pull-off is not generally satisfactory as the rate of warming of the bimetal by this air alone is too slow, for good emission results.
Therefore, above 65F, the bimetal switch 94 moves overcenter to the position shown in Figure 2 engaging lugs 96 and 88 and energizing the PTC heater 86. The overcenter action permits the spring legs of contact ~8 to move to the left and disengage the current source from the vacuum switch prong 104. The bimetal coiled spring 60, therefore, will unwind as a function of both the exhaust manifold stove heat and the supplemental heat from the energized PTC element, which will permit the opening of the choke valve by airflow faster than were it being controlled by the primary heat source alone. It will also be noted that above 65F, the vacuum switch 102 is inoperative to supply curren`t to the PTC
element.

From the foregoing, it will be seen that the invention ~l~)36443 provides a simple plug-in type vacuum swi~ch that can be added to a conventional electric choke type carburetor construction to assure heating of the choke bimetal below a predetermined temperature level whenever the manifold vacuum drops below a predetermined level. Therefore, altitude changes producing lower manifold vacuum levels are compensated for by providing a substitutive heat source to the bimetal during low vacuum conditions that are indicative of insufficient hot airflow through the choke housing.
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 invencion.

Claims (4)

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 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, thermostatic 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 predetermined level, an intermittently operable heater device located adjacent the spring means operable to transfer its heat output to the spring means when operable to reduce the choke valve closing force of the spring means and permit opening of the choke valve by airflow through the passage against it, a source of electrical energy, circuit means connect-ing the source to the heater device, temperature sensitive switch means in the circuit means operable to break and make the circuit as a function of temperature changes from a predetermined level, and engine vacuum responsive means operable below a predetermined temperature to selectively render the heater device operable and inoperable in response to changes in engine vacuum, means mounting the vacuum responsive means to at times electrically connect the source of electrical energy to the heater device in another circuit that is parallel to the first-mentioned circuit connecting the source and heater device through the temperature sensitive switch means to permit connection of the source of electrical energy to the heater device when the temperature sensitive switch means is operable to break the first-mentioned circuit bypassing the temperature sensitive switch means.

2. The choke system of claim 1 wherein the vacuum responsive means comprises a spring-closed engine manifold vacuum-opened switch.

3. The choke system of claim 2 wherein said switch comprises a unitized removable plug-in prong type switch for easy disconnection of the switch from said another circuit, and the heater device includes a socket-type receptable for receiving the prongs of said latter switch.

4. The choke system of claim 1, wherein said heater device comprises a self-limiting output temperature positive temperature coefficient heater element characterized by increasing internal impedance with increases in internal temperature up to its limit limiting further current flow and heat build up, thereby eliminating the need for a thermostatic cut-off switch to prevent heat damage to the spring.