US3752133A

US3752133A - Multiple heat automatic choke

Info

Publication number: US3752133A
Application number: US00306776A
Authority: US
Inventors: D Irish; R Junttonen
Original assignee: Ford Motor Co
Current assignee: Ford Motor Co
Priority date: 1972-11-15
Filing date: 1972-11-15
Publication date: 1973-08-14
Anticipated expiration: 1990-08-14
Also published as: GB1419602A; CA972240A; JPS4980427A; DE2339955A1; JPS525647B2; DE2339955C2

Abstract

The carburetor has a conventional automatic choke construction heating a bimetallic coil by engine exhaust stove heat to slowly open the choke valve during cold weather starts; supplemental heat is provided by electrically controlled positive temperature coefficient heater devices, the smaller of which is operable at all times concurrent with the stove heat, the larger device being operable above a predetermined ambient temperature to move the choke valve open faster, to reduce emissions.

Description

United States Patent Irish et al. Aug. 14, 1973 [54] MULTIPLE HEAT AUTOMATIC CHOKE 3,699,937 10/1972 De Petris 123/119 F [7 5 Inventors: David C. Irish, Dearbom; Richard J. FOREIGN PATENTS QR APPL Junttonen, Farmington, both of l 4 B Mich 48024 $64,563 0/19 4 Great ntaln 261/39 E [73] ssign or otor C mpany, Deafbom, Primary Examiner-Wendell E. Burns Mich- A ttorney Keith L. Zerschling and Robert E. Mc- 22 Filed: Nov. 15, 1972 Comm.

21 A N I 1 ppl 306776 57 ABSTRACT 52 us. c1. 123/119 F, 261/39 B, 261/39 13 The *9? 3. a 51 m1. c1..... F02d 11/08, F02n 1/10, F02n 23/04 "g i if? F" 581 Field 61 seal-611 261/39 E, 39 B, 39 R; eat 8 y e c F Va J? 123/1 19 F weather starts; supplemental heat is provided by electrically controlled positive temperature coefficient 1561 2:2:22521321105131?02531;;1121;323:2122

UN TED STA Es PATENTS being operable above a predetermined ambient tem- 2,511,318 6/1950 Beard 261/119 F perature to move the choke valve open faster Q re- 3,040,181 6/1962 El1f 261/39 E duce emissions 3.29l,461 12/1966 Pope 261/39 E 12 Claims, 3 Drawing Figures MULTIPLE HEAT AUTOMATIC CHOKE This invention relates, in general, to a carburetor for a motor vehicle engine. More particularly, it relates to an automatic choke to control the idle speed of the engine during cold weather starts, 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 generally provided to lessen the air intake during cold starting and pre-engine 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 manifold suction responsive device generally cracks open the choke a predetermined amount when the engine starts. The choke action provides a rich mixture so that sufficient fuel can be vaporized to permit smooth starting and running of the engine.

The above construction, while generally satisfactory, is a compromise between good cold weather running conditions on one hand and low emission outputs on the other hand. The richer than normal mixture existing during the choking operation may result in higher emission output such as, for example, CO.

It is an object of this invention to provide an automatic choke construction that will provide good cold weather characteristics and yet reduce to a minimum the output of undesirable smog producing elements.

It is another object of the invention to provide an automatic choke construction that provides a leaner than conventional air/fuel mixture immediately after the engine starts by pulling open the choke valve faster than would be by conventional choke systems.

It is also an object of the invention to provide an automatic choke construction including positive temperature coefficient heater elements operable to shorten the length of time required for the normal operation of an automatic choke.

Another object of the invention is to provide an automatic choke construction including a thermostatically controlled bimetal spring normally urging the choke valve closed with decreasing ambient temperature changes and opposed by a suction operated motor device that initially cracks open the choke valve to a predetermined amount permitting running operation during cold weather; engine exhaust manifold heat being directed to the spring coil to warm it; and a first supplemental heat source providing additional heat at all times to the coil spring in the event the exhaust manifold heat flow is low; and, a second supplemental heat source providing further additional heat to the coil at ambient temperatures above a predetermined level to cause the closing force on the choke valve to be removed earlier than would be were the supplemental heat sources not provided; the supplemental heat sources consisting of positive temperature coefficient semi-conductor heater elements whose internal resistances increase with increases in the heater internal temperature and decreases in current flow so as to be selflimited in temperature output thereby eliminating the need for thermostatic circuit breakers to prevent bimetal coil distortion above predetermined temperature levels.

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 cross-sectional elevational view of a portion of a four-barrel carburetor embodying the invention; and,

FIG. 2 is an enlarged view of a portion of the automatic choke shown in FIG. 1; and, 7

FIG. 3 is a graph plotting the changes in internal resistance of the heaters of this invention with changes in internal temperature.

FIG. 1 is obtained by passing a plane through approximately one-half of a known type of four-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 a 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.

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. 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 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 righthand end portion of shaft 44. The other legportion 58 of the lever is secured to the outer end 59 of a coiled bimetallic thermostatic spring element 60 through an arcuate slot 62 in an insulating gasket 64.

Leg 56 is also 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 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 8th. More specifically, hot air will flow into the area around the spring coil 60 through a hole 83 in gasket 64 and out through slot 62 to the bypass slots around piston 78.

As thus far described, the construction is conventional. 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 42. Accordingly, changes in ambient temperature will rotate the spring lever 54 to rotate shaft 4-4 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 known choke mechanism described automatically accomplishes the action described. That is, on cold weather starts, the temperature of the air in chamber 42 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, higher 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 fuel is admitted to induction passage 20. Shortly thereafter, the exhaust manifold stove air in line 68 will become progressively warmer and act on choke element 60 to cause it to unwind slowly and rotate shaft 44 and choke valve 28 to a more open position. It should be noted that the degree of warmth of the coil will depend upon the flow of hot air through tube 68, which in turn will vary as a function of the level of the vacuum in port 82. Further details of construction and operation are not given since they are known and believed to be unnecessary for an understanding of the invention.

As thus far described, the construction is conventional. Turning now to H6. 2, it will be seen that the thermostatic spring coil 60 is centrally staked to a metal post 84. The post is formed as an integral part of a thin metal, aluminum, for example, disc 85 that is approximately the diameter of coil 60. The disc constitutes a heat sink or transfer member to evenly radiate heat to the coil from a pair of

heater elements

86 and 87 to which it is secured.

Heater elements

86 and 87 are positive temperature coefficient (PTC) semiconductors in the shapes of flat ceramic discs fixed on disc 85. The element 86 has a central spring-leg type current carrying contact lug 88 projecting through an insulated cover of choke cap 90. The heat sink disc is grounded through the cover to the cast choke housing by extensions and ground terminals 92.

Lug 88 is normally spaced from a contact 93 fixed on a bimetallic thermal switch 94 that is sensitive to ambient temperature changes. The switch closes above 65: F, for example, to engage the contacts and conduct current to the heater from a terminal 96. The vehicle alternator could serve as a suitable source of electrical energy to the terminal, when the vehicle is running.

Heater element 87, on the other hand, is connected at all times, to be energized at all times when the vehicle is running, by an interconnection 97 to terminal 96. Heater disc 87, as will be seen, is smaller in size and, therefore, heat output capacity, than heater disc 86, for a purpose to be described. It is a characteristic of each of the PTC heaters 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 in H6. 3. When the PTC heaters are electrically energized, as by applying line voltage to terminal 96 from the alternator at all times for heater 87, and for heater 86 when switch 941 closes, the Joule heat causes rapid self-heating of the PTC elements. The hcater'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 coil 60, therefore, due to extreme temperature levels is thereby eliminated.

lln this instance, therefore, the PTC device provides heat to coil 60 that is supplemental to that provided by the primary exhaust manifold hot air system. There may be instances when the manifold heat flow is so low during cold weather operation due to low airflow in the carburetor that the coil 60 would not be warmed enough by primary manifold heat alone to provide the normal slow progressive heating of the coil 60. The continuous heating of the smaller heater 87 in this case provides the desired compensatory primary heat at all times below its output limit to cause the choke to operate in the desired manner. Subsequently, when the bimetal 94 closes and current passes through the larger PTC element 36, the internal temperature generated is transferred by conduction to coil 60 through the post 84 and by radiation to the coil from the heat sink 85, in the same manner as the heat of disc 87.

When the PTC internal temperature reaches the switching temperature Ts, say 12] C, as seen in FIG. 3, the internal resistance becomes 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 elements by the current flow then is essentially balanced by the heat loss by the PTCs to the environment and to the bimetal post 84. Therefore, for all intents and purposes, the heat of the PTCs 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, heater 87 is energized to transfer a small heat output to disc 35 and coil 60, but heater element 86 is not. The bimetal switch contacts 94, 88 remain open, and the PTC heater 86 remains deenergized. Therefore, the choke hot air system supplemented by the small output heater 87 provide the only heat source for choking functions below 65 F. The bimetal coil 60 will unwind, therefore, only as a function of the increased heating by the hot air from passage 68 and heater 87.

Above 65 F, however, the conventional exhaust manifold stove heat system and heater 87 constitute the primary heat source, while the now energized larger PTC heater 86 acts as the supplemental source to rapidly permit the opening of the choke valve by airflow faster than were it being controlled by the primary heat source alone. This leans the fuel/air mixture earlier than with conventional choke arrangements, and lowers undesirable emission outputs.

With the above described two phase choke construction, therefore, it will be seen that it is possible to pro vide a reliable and accurate short duration choking effect thereby minimizing vehicle exhaust emission without jeopardizing the cold weather choking function.

We claim: 1. An automatic choke system for use with a carburetor having an air/fuel induction passage,

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, and first and second self-limiting output temperature heater devices operably associated with the choke f valve to effect opening of the choke valve by varying degrees in response to increases in temperature of the heater devices up to their individual output limits, the first heater device being operable at all times, and control means to render the second heater device operable above a predetermined ambient temperature level. 2. A choke system as in claim 1, the control means comprising a thermostatically responsive on-off switch means.

3. A choke system as in claim 1, the heater devices comprising positive temperature coefficient (PTC)elements characterized by increasing internal impedance with increasing temperatures up to their limits limiting further current flow and heat buildup.

4. 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,

and first normally operable and second intermittently operable self-limiting output temperature, positive temperature coefficient (PTC) heater devices located adjacent the spring means operable to transfer their heat output thereto when operable up to their limit 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, and other means to render the second heater device operable. 5. A choke system as in claim 4, the other means comprising temperature sensitive means rendering the second heater device operable above a predetermined ambient temperature level to begin transferring its heat output to the spring means.

6. A choke system as in claim 5 including a heat sink secured to the second heater device for uniformly radiating the heat of the second heater device to the spring means.

7. A choke system as in claim 5, the heater devices comprising positive temperature coefficient heater elements characterized by increasing internal impedance with increases in internal temperature up to their limits limiting furthercurrent flow and heat buildup.

8. A two phase automatic choke system for use with a carburetor having an air/fuel induction passage open at one end and adapted to be connected to an engine intake manifold at the other end for subjecting the passage to varying manifold vacuum, the passage having a throttle valve mounted for a variable movement between positions opening and closing the passage to control air/fuel flow through it,

the choke system including an unbalance mounted, air movable choke valve mounted anterior of the throttle valve for variable movement across the passage to control airflow towards the throttle valve,

first power means sensitive to engine manifold vacuum for moving the choke valve towards an open position in opposition to the spring means,

and means supplemental to the first power means for effecting subsequent movement of the choke valve towards a position more open than the position effected by the first power means,

the supplemental means including first and second electrically energized positive temperature coefficient (PTC) heater devices located adjacent the bimetallic spring means and operable to transfer their heat output to the spring means to reduce its choke valve closing force and permit opening of the choke valve by airflow through the passage against it, the first heater device being operable at all times,

and temperature responsive means operable above a predetermined ambient air temperature to energize the second heater device,

the internal resistance of the PTC heaters above a second predetermined temperature level restricting current flow and heat output to thereby eliminate the need for temperature responsive cut out switch means to deenergize the heater elements above the second predetermined temperature level.

9. A two phase automatic choke system for use with an internal combustion engine carburetor having an air/fuel induction passage open at one end and adapted to be connected to an engine intake manifold at the other end for subjecting the passage to varying manifold vacuum, the passage having a throttle valve rotatably mounted across the passage adjacent the lower end for a variable movement between positions opening and closing the passage to control air/fuel flow through it,

the choke system including an unbalance mounted,

air movable choke valve rotatably mounted across the passage adjacent the upper end for variable opening and closing movements to control airflow towards the throttle valve,

a thermostatically responsive spring coil operably connected to the choke valve and normally urging the choke valve towards a closed position with a force increasing with decreases in the temperature of the coil from a predetermined level,

first power means operably connected to the choke valve and sensitive to engine manifold vacuum for moving the choke valve from an initially closed position towards an open position in opposition'to the coil and in response to operation of the engine from a start to a running conditon,

the first power means including a vacuum operated movable piston means, and a first heat source transferring engine heat to the coil and comprising a hot air containing duct operably connected from the engine exhaust system to the coil for warming the coil to reduce its choke valve closing force, and supplemental second and third electrically controlled, temperature responsive heat meansin a parallel and series flow arrangement, respectively, with the first power means heat source and effecting subsequent movement of the choke valve towards a position more open than the position effected by the first power means alone,

the supplemental means including a source of electrical energy, first and second positive temperature coefiicient (PTC) heater device located adjacent the bimetallic spring means and operable to transfer their heat output to the coil to reduce its choke closing force and permit opening of the choke valve by airflow through the passage against it, means connecting the first heater device to the source at all times for concurrent operation with the first power means, and temperature responsive switch means operable above a predetermined ambient air temperature to connect the source to the second heater device to energize the second heater device, the internal resistance of the first and second heater devices above a predetermined heater temperature level increasing to a level restricting further heat buildup beyond a predetermined level.

10. A choke system as in claim 9 including a heat sink secured to the second PTC device for uniformly radiating the heat of the second PTC device to the coil, the second PTC device being larger than the first device and providing a greater heat output.

11. A choke system as in claim 10, the heat sink comprising a thin metallic disc secured to the coil mounting means between the second PTC device and coil.

12. A choke system as in claim 9, including a housing enclosing the PTC heater devices and coil, an insulating gasket dividing the housing into a first chamber and a second chamber, the first chamber having an inlet and outlet for the hot air duct, the second chamber containing the heater devices and coil and containing an aperture for flow of hot air through the gasket in a path to indirectly heat the coil, the PTC heater devices being located in the housing on the side of the coil opposite to the gasket to receive heat from a direction opposite to that from the gasket.

UNITED STATES PATENT AND TRADEMARK OFFICE Certificate Patent No. 3,752,133 Patented August 14, 1973 David C. Irish & Richard J. Junttonen Application having been made by David C. Irish and Richard J. Junttonen, the inventors named in the patent above identified, and Ford Motor Co., Dearborn, Mich., a corporation of Delaware, the assignee, for the issuance of a certificate under the provisions of Title 35, Section 256, of the United States Code, deleting the name of Richard J. Junttonen as a joint inventor, and a showing and proof of facts satisfying the requirements of the said section having been submitted, it is this 26th day of January 1982, certified that the name of the said Richard J. Junttonen is hereby deleted from the said patent as a joint inventor with the said David C. Irish.

Fred W. Sherling Associate Solicitor

Claims

1. An automatic choke system for use with a carburetor having an air/fuel induction passage, 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, and first and second self-limiting output temperature heater devices operably associated with the choke valve to effect opening of the choke valve by varying degrees in response to increases in temperature of the heater devices up to their individual output limits, the first heater device being operable at all times, and control means to render the second heater device operable above a predetermined ambient temperature level.

2. A choke system as in claim 1, the control means comprising a thermostatically responsive on-off switch means.

4. 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, and first normally operable and second intermittently operable self-limiting output temperature, positive temperature coefficient (PTC) heater devices located adjacent the spring means operable to transfer their heat output thereto when operable up to their limit 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, and other means to render the second heater device operable.

5. A choke system as in claim 4, the other means comprising temperature sensitive means rendering the second heater device operable above a predetermined ambient temperature level to begin transferring its heat output to the spring means.

7. A choke system as in claim 5, the heater devices comprising positive temperature coefficient heater elements characterized by increasing internal impedance with increases in internal temperature up to their limits limiting further current flow and heat buildup.

8. A two phase automatic choke system for use with a carburetor having an air/fuel induction passage open at one end and adapted to be connected to an engine intake manifold at the other end for subjecting the passage to varying manifold vacuum, the passage having a throttle valve mounted for a variable movement between positions opening and closing the passage to control air/fuel flow through it, the choke system including an unbalance mounted, air movable choke valve mounted anterior of the throttle valve for variable movement across the passage to control airflow towards the throttle valve, 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, first power means sensitive to engine manifold vacuum for moving the choke valve towards an open position in opposition to the spring means, and means supplemental to the first power means for effecting subsequent movement of the choke valve towards a position more open than the position effected by the first power means, the supplemental means including first and second electrically energized positive temperature coefficient (PTC) heater devices located adjacent the bimetallic spring means and operable to transfer their heat output to the spring means to reduce its choke valve closing force and permit opening of the choke valve by airflow through the passage against it, the first heater device being operable at all times, and temperature responsive means operable above a predetermined ambient air temperature to energize the second heater device, the internal resistance of the PTC heaters above a second predetermined temperature level restricting current flow and heat output to thereby eliminate the need for temperature responsive cut out switch means to deenergize the heater elements above the second predetermined temperature level.

9. A two phase automatic choke system for use with an internal combustion engine carburetor having an air/fuel induction passage open at one end and adapted to be connected to an engine intake manifold at the other end for subjecting the passage to varying manifold vacuum, the passage having a throttle valve rotatably mounted across the passage adjacent the lower end for a variable movement between positions opening and closing the passage to control air/fuel flow through it, the choke system including an unbalance mounted, air movable choke valve rotatably mounted across the passage adjacent the upper end for variable opening and closing movements to control airflow towards the throttle valve, a thermostatically responsive spring coil operably connected to the choke valve and normally urging the choke valve towards a closed position with a force increasing with decreases in the temperature of the coil from a predetermined level, first power means operably connected to the choke valve and sensitive to engine manifold vacuum for moving the choke valve from an initially closed position towards an open position in opposition to the coil and in response to operation of the engine from a start to a running conditon, the first power means including a vacuum operated movable piston means, and a first heat source transferring engine heat to the coil and comprising a hot air containing duct operably connected from the engine exhaust system to the coil for warming the coil to reduce its choke valve closing force, and supplemental second and third electrically controlled, temperature responsive heat means in a parallel and series flow arrangement, respectively, with the first power means heat source and effecting subsequent movement of the choke valve towards a position more open than the position effected by the first power means alone, the supplemental means including a source of electrical energy, first and second positive temperature coefficient (PTC) heater device located adjacent the bimetallic spring means and operable to transfer their heat output to the coil to reduce its choke closing force and permit opening of the choke valve by airflow through the passage against it, means connecting the first heater device to the source at all times for concurrent operation with the first power means, and temperature responsive switch means operable above a predetermined ambient air temperature to connect the source to the second heater device to energize the second heater device, the internal resistance of the first and second heater devices above a predetermined heater temperature level increasing to a level restricting further heAt buildup beyond a predetermined level.