US 3780996 A
Description (OCR text may contain errors)
United States Patent [191 Nutten SELF-PRIMING CARBURETOR  Inventor: Warren 1). Nutten, Grafton, Wis.
 Assignee: Tecumseh Products Company,
[221 Filed: Jan. 8, 1973 ] Appl. No.: 322,029
Related US. Application Data  Continuation of Ser, No. 40,563, May 26, 1970,
 References Cited UNITED STATES PATENTS 6/1957 Phillips 261/D1G. 68 l/l963 Phillips 261/D1G. 68
3,233,878 2/1966 Phillips 261/D1G. 68 1,334,411 3/1920 Spiller 261/72 R 1,345,297 6/1920 Weiland 261/72 R 3,133,129 5/1964 Phillips 261/69 A 3,323,504 6/1967 Jacobi 123/119 R 1,707,350 4/1929 Elliott 261/121 A 1,752,551 4/1930 Ensign et a1. 261/121 A 1,819,706 8/1931 Geiger 261/121 A 1,831,056 11/1931 Bicknell 261/34 A 2,656,167 10/1953 Phillips 261/D1G. 68 2,935,977 5/1960 Eberline 123/119 R 3,093,698 6/1963 Degenbardt 261/41 R 3,170,006 2/1965 Brown 26l/D1G. 68
[451 Dec. 25, 1973 3,198,187 8/1965 Bartholomew 261/34 A 3,231,250 1/1966 Kalert, Jr.... 261/41 D 3,265,050 8/1966 Tuckey 123/119 R OTHER PUBLICATIONS Primary Examiner-Tim R. Miles Attorney-William J. Waugaman et al.
 ABSTRACT A self-priming chokeless carburetor having only a throttle valve in the air-fuel mixture conduit supplemented by a relatively large priming well disposed serially between a fixed metering orifice communicating with the float bowl reservoir and the inlet to the main nozzle tube of the carburetor. When the engine is not running, the priming well fills with fuel to the level of the fuel in the float bowl. When the engine is cranked for starting, the supply of fuel in the well is drawn up the main nozzle to supply a rich mixture for starting. When the engine is running, the well cannot refill because fuel is aspirated from the well as fast as it is supplied thereto by the metering orifice.
452. 31!!! Qre riae iq s Enema AT REST PATENIEUDHZQIS 1915 3. 780.996
sum 10F 3 ENGINE AT REST RUN INVENTOR WARREN DANUTTEN START BY ATTORNEYS PATENIEUBEC2S ma 3. 780.996
sum 2 BF 3 INVENTOR WARREN D. NUTTEN ATTORNEYS mmgnum 25 ms SHEET 3 CF 3 FIG. 7 ENGINE AT RE w H; V Z
INVENTOR WARREN D. NUTTE MW ATTORNEYS SELF-PRIMING CARBURETOR This is a continuation of application Ser. No. 40,563, filed May 26, 1970, now abandoned.
This invention relates to internal combustion engine carburetion systems and more particularly to a carburotor for small internal combustion engines, such as those used on rotary lawnmowers.
An object of the present invention is to provide a carburetor which provides reliable starting within a rela-.
tively wide temperature range without the necessity of a choke valve, accelerator pump, and/or supplemental fuel pressurizing system such as a priming bulb or crank-case pressure boosting system.
Another object is to provide an engine carburetion system utilizing a carburetor of the above character which provides very smooth engine performance over a given engine speed range in an engine that is operating against a relatively constant load, such as the rotary blade of a rotary lawnmower, and which is adapted for governor operation of the throttle of the carburetor.
A further object is to provide an engine carburetion system of the above character which reduces the possibility of engine flooding under hot start conditions.
A still further object is to provide a carburetor of the above character which is extremely simple in construction, efficient in operation, economical to manufacture, provides uniform performance over extended intermittent operating conditions despite gradual clogging of the carburetor air filter, which does not require nor in fact permit adjustment of the carburetor air-fuel mixture setting nor choke manipulation and hence is less likely to be disabled, maladjusted or improperly operated.
Other objects, features and advantages of the present invention will become apparent from the following detailed description and accompanying drawings wherein:
FIG. 1 is a vertical sectional view taken along the axis of the air-fuel mixture conduit of one embodiment of a carburetor constructed in accordance with the pres ent invention. 7
FIGS. 2 and 3 are fragmentary vertical sectional views of a portion of the structure illustrated in FIG. 1 but enlarged thereover and respectively illustrating the fuel aspirating conditions when the engine is being cranked for starting and when the engine is running.
FIG. 4 is a vertical sectional view taken on the line 4-4 of FIG. 1, a portion of the carburetor being shown in front elevation to better illustrate the internal air vent system for the float bowl of the carburetor.
FIG. 5 is a horizontal sectional view taken on the line 5-5 of FIG. 4 but reduced in scale therefrom and having a portion broken away to better illustrate detail.
FIG. 6 is a top plan view of the float bowl stud of the carburetor, enlarged in scale over FIG. 4 and showing the stud separately.
FIGS. 7, 8, 9 and are fragmentary vertical sectional views similar to FIGS. 2 and 3 but illustrating another embodiment of a carburetor in accordance with the present invention, FIG. 7 illustrating the fuel delivery system when the engine is at rest, FIG. 8 at start up, FIG. 9 when the engine is running or under heavy load, and FIG. 10 when the engine is idling or under light load.
Referring in more detail to the accompanying drawings, FIGS. 1-6 inclusive illustrate one exemplary embodiment of a carburetor 20 constructed in accordance with the present invention and particularly adapted for use with a single cylinder four-stroke or two-stroke cycle internal combustion engine (not shown) having a given displacement, horsepower and operational speed range, as set forth by way of example in more detail hereinafter.
Carburetor 20 has the usual cast body 22 with an airfuel mixture conduit defined by a horizontal entrance bore 24, a central venturi 26 and an exit bore 28. A conventional air filter 30 is secured to the inlet end of bore 24 for filtering all air entering the carburetor. The outlet end of bore 28 communicates with an intake bore 32 of the engine 34, the engine likewise being conventional and therefore not illustrated in detail.
The bottom of the carburetor body or housing 22 has a circular flange portion 36 carrying an annular gasket 38 for receiving, in sealed relation, a carburetor float bowl 40. This float bowl surrounds a depending column portion or post 42 integral with the main carburetor casting. Post 42 extends down into bowl 40 and serves as an additional support for the bowl. A mounting stud 44 threads into a threaded counterbore 46 in post 42 to thereby clamp a raised central portion 48 of the bottom wall 50 of the bowl between a gasket 52 and the bottom end surface of post 42.
Referring to FIG. 4, fuel is gravity or pressure fed from a fuel tank via a hose line (not shown) to an inlet fitting 56 secured to body 22. Fitting 56 communicates with bowl 40 via a fuel passage 57 controlled by a conventional inlet needle valve 58. Valve 58 is biased upwardly toward closed position against a seat ring 59 by an arm 60 secured to an annular float 62, the float being pivotally suspended on a pin 64. The float and needle valve are adjusted to maintain a body 66 (FIG. 1) of liquid fuel in bowl 40 to a given level 68. A conventional dump valve 70 (FIG. 1) is provided to drain fuel from bowl 40 when the engine is to be inoperative for an extended period of time.
As best seen in FIGS. 4 and 6, stud 44 has a blind bore 72 coaxial with bore 46 and opening upwardly thereto which receives the open lower end 74 of a straight hollow tube 76 which forms the main (and sole) nozzle of carburetor 20. Tube 76 is secured by press fit in a bore 78 coaxial with bore 46 and has its open upper end 80 projecting into venturi 26. A slightly tapered bore 82 extends coaxially between bores 46 and 78 and defines therewith a comparatively large priming well 83. Well 83 thus constitutes an annular space surrounding tube 76 and extending from level 68 to the bottom of cavity 72 and also includes the interior of tube 76 between level 68 and lower end 74. Well 83 is located concentrically between tube 76 and the fuel reservoir 84 defined between post 42 and bowl 40.
Liquid fuel is admitted to well 83 via a calibrated fixed restriction orifice comprising radial passage 86 (FIGS. 1, 4 and 6) drilled through the shank 88 of stud 44 between the head 90 and threads 92 of the stud. The exterior end of passage 86 communicates with an annular chamber formed between a counterbore 87 in the lower end of post 42 and shank 88. Communication between this chamber in counterbore 87 and reservoir 84 is established by a pair of diametrically opposite milled slots 89 formed in the lower end surface of post 42, as best seen in FIGS. 1 and 5. Passage 86 is made as small as practical, on the order of 0.028 inch in diameter, and is calibrated to provide a fixed metering restriction to limit the maximum flow of liquid fuel into the priming well, depending upon the fuel-air ratio desired in the particular carburetor and engine under consideration, as will be better understood hereinafter. Likewise, the volumetric capacity of priming well 83 is precisely calibrated for a given carburetor and engine combination and may be, for example, in the order of 1.5 cubic centimeters in the case of one working example of a carburetor constructed pursuant to FIGS. 1-6 inclusive as specified in more detail hereinafter.
The head space 100 of priming well 83 (FIG. 1) is pressurized via a restricted air passage 102 which opens into a much larger counterbore 104 having its outlet positioned at the junction of venturi 26 and bore 24 and facing the carburetor inlet. Head space 106 in bowl 40 also is vented to bore 24 by an angled slot 108 (FIGS. 4 and 5) formed in the side wall of a cavity 110 which is closed at its lower end by a welch plug 112. A passage 114 extends from the upper end of cavity 110 and communicates at its other end with a Pitot tube 116 (FIG. 4) which has its free open end 118 positioned in bore 24 closely adjacent the outlet end 120 of filter 30 and generally facing the carburetor inlet.
Carburetor has only one air flow regulating valve in the fuel-air mixture conduit 24-28; namely, a throttle valve comprising a butterfly valve plate 122 secured by a screw 124 to a flatted portion of a rotatable throttle shaft 126. A throttle control level 128 is secured to the upper end of shaft 126 for connection to the usual engine speed governing linkage. In the closed position of the throttle, plate 122 contacts the wall of bore 28 as shown in FIG. 1. In carburetor 20, valve plate 122 is located in bore 28 approximately midway between jet 80 and the outlet of bore 28. Thus, it will be noted that carburetor 20 lacks the usual choke valve and also does away with idle jet and associated idle system fuel feeding and metering passages and/or needle valve.
In the operation of carburetor 20, assuming first that engine 34 is at rest, liquid fuel will be admitted to bowl 40 and will rise to the level 68 as shown in FIG. 1. As reservoir 84 fills, liquid will flow from reservoir 84 via slots 89 and metering passage 86 into well 83 to thereby fill cavity 72 and bore 46 and will rise part way up in bore 82 to the level 68, the lower portion of tube 76 likewise filling to this level. Hence a charge ofliquid fuel is now available between restriction 86 and the upper end outlet 80 of tube 76 which, due to the relatively large diameter of tube 76, has a comparatively unrestricted flow path up into venturi 26, the principal restriction to such flow being gravity acting on fuel lifted the vertical distance between level 68 and the upper end 80 of tube 76.
To start engine 34, the same is manually or power cranked, as by use of the usual starter pull rope, a windup spring recoil starter, or a starter motor. As soon as the engine piston is reciprocated on its initial intake stroke, air will be drawn through filter 30 and the airfuel mixture conduit 24-26-28 of the carburetor. This air flow creates a pressure differential between the inlet 104 of the air bleed passage I02 and the outlet 80 of the main jet. This pressure differential is communicated to the surface 68 of the liquid in well 83 surrounding tube 76 and is sufficient to force the liquid up tube 76 and into the venturi as shown in FIG. 2. The liquid emerges as a relatively heavy or dense gusher 132 which is caught by the airstream and partially broken up and evaporated to be carried as liquid and vapor with the air into the intake manifold of the engine. The initial fuel-air charge will be relatively rich because no air mixes with the liquid fuel prior to its leaving jet tube 76 during the time that the liquid fuel level is dropping from level 68 down to the lower end 74 of tube 76.
The volume of liquid fuel provided in the priming well 83 (including that within tube 76 from level 68 down to the lower end 74 of the tube) is calculated to provide a proper air-fuel ratio for starting of the engine, preferably during the first several starting attempts, taking into account the ambient temperature. It is to be understood that the rate of replenishment of the well via orifice 86 is such that very little fuel enters the well through orifice 86 during the starting attempt. Hence the well is substantially exhausted as the engine is cranked and coasts by flywheel momentum through several revolutions in response to each starting attempt.
Once the engine starts to run under its own power, it will accelerate rapidly due to the throttle valve 122 being held wide open for start-up. As the engine accelerates, the air flow through the carburetor conduit 24-28 increases substantially, thereby greatly increasing the lifting force exerted on any liquid entering well 83 via orifice 86. As shown in FIG. 3, the suction exerted at this time is sufficient to evacuate the surplus liquid fuel from well 83 so that the liquid fuel aspirated up tube 76 flows at a rate determined solely by the metering capacity of the fixed restriction 86, which in turn is sized to provide just enough liquid fuel to form a lean ratio of fuel to air in the mixture conduit properly apportioned to give optimum performance at the design running speed or speeds of the engine. Thus, when the engine is running the liquid emerges from orifice 86 to be caught in the airstream in cavity 72 and sucked up tube 76.
The transition from the rich mixture at start-up as shown in FIG. 2 to the leaner running mixture shown in FIG. 3 is not a completely abrupt one because there is an intermediate stage occurring between the point in time when the level of fuel in well 83 has dropped flush with the lower end 74 of tube 76 (FIG. 2) and the time when the well has been evacuated to the condition as shown in FIG. 3. Once the level of the fuel drops below end 74, air entering the priming well head space via restriction 102 can enter tube 76. When this occurs there is a gross mixture of air bubbles and liquid fuel as well as droplets of fuel travelling up tube 76, which leans out the resultant fuel-air mixture in tube 76 until it becomes an emulsion or mist when the running condition is ultimately reached as shown in FIG. 3. However, well 83 is never fully evacuated; rather, the level of liquid in the well drops only far enough to allow air to enter tube 76 via its open lower end 74. Thus, the term emptied as used herein is to be understood in this context. The action which occurs during running can be likened to that which occurs when sucking soda pop through a straw from a nearly empty pop bottle; i.e., wherein the familiar gurgling sound signals that the bottle is almost dry and a good deal of air is being aspirated along with the liquid.
As fuel is withdrawn from float bowl 40 into jet tube 76, the air pressure in the bowl head space 106 is maintained at a value corresponding to that in inlet bore 24 of the carburetor by Pitot tube 116. Thus, despite any gradual reduction in the rate of air flow through the carburetor resulting from an increasing flow resistance offered by filter 30 should it gradually become clogged with the dirt particles, the pressure differential under running conditions between the surface 68 of the liquid in the float bowl and jet outlet 80 tends to remain relatively constant for any given engine speed. This internal venting of the float bowl is an important feature in conjunction with the use of the fixed metering orifice 86 because it helps insure that the fuel-air ratio of the carburetor under running conditions remains at the optimum design value. Therefore, optimum engine performance can be obtained over a relatively wide speed range without the provision of an adjustable orifice controlled by the usual threaded needle valve. In addition, there is less chance for dirt particles to be entrained in the liquid fuel supply system since the venting air for the float bowl is taken from the filtered airstream in the mixture conduit of the carburetor.
Engine speed and power control is obtained during running of the engine by the usual speed responsive governor mechanism connected to the throttle arm 128. Preferably the governor and throttle are designed to control the speed of the engine within a given range of say 2,600 r.p.m. up to 3,100 r.p.m. in the case of the fixed speed carburetor 20, which is a sufficient throttle variation for handling the normal grass cutting load variations encountered in rotary lawnmower applications.
When engine 34 is shut down and comes to a halt, the priming well 83 will refil to the level 68 shown in FIG. 1 within a matter of approximately one second, in the example given hereinafter. Then if the engine is immediately cranked again to start it, there will be another starting charge delivered to the engine as described previously. Although theengine now may be hot, under ambient temperature conditions of under say 90-l00F. it will not flood because the throttle now is wide open and there is no choke to restrict air flow through the carburetor; consequently, the resultant airfuel ratio is not too rich so as to prevent firing of the engine. Also, regardless of how long the engine is continuously cranked to start it, it will receive only one initial priming charge per each cranking effort, as contrasted with a carburetor having a choke which will continuously and cumulatively produce an overrich mixture so long as the choke is held closed. Even when the engine is repetitively and intermittently hand cranked for starting, it normally will not be flooded out so long as the engine drive train is connected to and/or has sufficient rotational inertia so that the engine spins through a minimum of say six revolutions on each pull of the starter cord. The reason for this anti-flooding characteristic is that the priming well 83 will be emptied in only two or three revolutions of the engine and hence the subsequent revolutions will induce a lean mixture which helps purge the engine of any overrich charge before it is stopped and the next cranking effort is applied. However, since well 83 requires only a second or two to refill when the engine is at rest, there will be another starting charge available in the time it takes to rewind the pull cord or reset the recoil starter on the lawnmower engine. On the other hand, should the engine be continually (and improperly) cranked by a starter motor, it will receive only one starting charge and thereafter the continuous cranking will not only not produce flooding but will rather pump the engine dry again.
Volume of starting charge 0.75 cc Diameter of venturi 7/16 in. Diameter of orifice 86 0.028 in. Length of orifice 86 0.048 in. Length of tube 76 1 7/16 in. Internal diameter of tube 76 0.063 in. Vertical distance from liquid level 68 to bottom end 74 of tube 76 7/16 in. Diameter of air bleed orifice 102 0.03] in.
Governed speed range of engine under running condition 3 I00 to 3400 r.p.m.
(under no load conditions) Number of starting pulls at minimum ambient temperature of 40F. to start engine 4 Number of starting pulls to start engine at maximum ambient temperature of F. I Number of revolutions per pull 4 It is to be understood that the carburetor of FIGS. 1-6 inclusive is intended primarily for use on rotary lawnmower engines which normally are operated only in a relatively warm ambient temperature range of approximately 40F. to 120F. Such engines normally have a fixed loadconsisting of the rotary lawnmower blade which is cooperative with the present invention in that it insures that one pull of the starting cord on the engine will provide from three to six revolutions of the engine. With this parameter at hand, the starting charge is calibrated so that the priming well will be emptied in one pull of the cord at average cranking speed. Accordingly, the priming charge is optimized for hot start conditions. Additional engine priming is obtained at lower temperature conditions by giving the engine one, two or three additional pulls, as the case may be. The fixed speed carburetor 20 provides very good results for engines designed for applications having a generally constant or a very narrow speed range, as is the case with many appliances, such as rotary lawnmowers, garden sprayers and the like having a blade or other fixed load driven by the engine.
It is also to be understood that the throttle setting should be at full or almost full effective open position during starting in order to provide sufficient lifting capacity in the carburetor to evacuate the charge from the priming well. Hence when the engine is being started, it draws in a full complement of air because there is no restriction to air flow. The mixture thus is enriched only by the amount of the charge drawn up from well 83.
However, when the engine-carburetion system of the present invention is intended for engines driving appliances over a greater speed range than that described above, it is preferred to employ the modified carburetor 200 as shown in FIGS. 7-10 inclusive. In these figures like reference numerals are applied to parts described previously and their description not repeated, and corresponding elements are designated by a like reference numeral raisd by a prime suffix. Carburetor 200 is similar to carburetor 20 described previously except that it has a venturi 26 which is slightly reduced 7 in diameter to provide additional lifting capacity when employed with the same filter 30 and engine 34 described previously. Also, instead of metering orifice 86 being located in stud 44, a small radial passage 86' is drilled through the wall of post 42' at a point vertically between the lower end 74' of the main jet tube 76' and the liquid level 68 maintained in bowl 40. However, this relocation of the fuel metering orifice 86' is optional relative to the fuel-air rationing of the carburetor, i.e., orifice 86 may also be used in the modified carburetor 200, and orifice 86' may be used in carburetor 20.
One significant difference in carburetor 200 is that it has a small radial passage 202 formed in tube 76 which is located a predetermined distance below level 68. Another difference between carburetors 200 and is the provision of a hole 204 (FIG. 8) in throttle plate 122' which has a diameter calibrated to provide a minimum air flow through the fuel-air mixture conduit 24-28 of carburetor 200 even when the plate 122' is rotated to closed position as shown in FIG. 8. In lieu of hole 204, similar results could be obtained by notching plate 122, removing a chordal portion, or providing a stop limiting full closure of plate 122. However, hole 204 is preferred because its size is more readily controlled in manufacturing and it cannot get out of adjustment in the use of the carburetor.
In the operation of carburetor 200, assuming the engine is at rest as shown in FIG. 7, the priming well 83 is filled with liquid fuel from reservoir 66 via the fixed metering orifice 86' until the level of the fuel in the well rises to the level 68 of the fuel in bowl 40. This filling may take a matter of say 1 to 4 seconds.
To start the engine, the same is given a pull with the starter rope, or the wound recoil starter is engaged and released, to thereby crank the engine over for several revolutions, for example, three to six revolutions. The reciprocation of the piston through its first two or three intake strokes draws air through carburetor 200 at a minimum flow rate as indicated in FIG. 8. The pressure differential imposed on the priming charge of liquid fuel in well 83 causes the liquid within tube 76' to be forced up the tube to thereby deliver from outlet 80 an initial'spurt of liquid without entrained air until the level of the liquid in well 83 drops to the elevation of bleed hole 202. At this point a restricted quantity of air is admitted from the priming well head space 100 via orifice 202 to thereby partially intermingle with and air emulsify the liquid being raised in the tube. Hence as shown in FIG. 8, the column of liquid is broken up into small droplets as it is drawn from the elevation of orifice 202 up to outlet 80. The resultant emulsion 204, being less dense than the solid" column of liquid 206 below orifice 202, is more readily lifted by the smaller pressure differential available under engine cranking conditions. Also, because of air bleed 202, even though the lower end 74' of the tube 76' remains submerged in a body of liquid fuel in well 83, carburetor 200 is capable of delivering liquid fuel from the priming well over a wider range of engine speeds; i.e., from running speed down to a condition of lower air velocity through the mixture conduit of the carburetor corresponding to a fast idle" engine speed.
Returning to the action occurring at start-up, and assuming the throttle is wide open, the initial cranking of the engine is sufficient to evacuate most of the charge in well 83 so that the liquid feeding conditions in the j 7 well progress from the conditions shown in FIG. 7 to the. iqllgwia d i srits ias.
FIG. 8. Once the engine starts and accelerates to running speed, as shown in FIG. 9, the suction differential on tube 76 is sufi'tcient to suck up all of the liquid fuel delivered from the fixed metering jet 86 as fast as it is delivered into well 83, the air-fuel ratio at this time thus being under the sole control of the liquid metering orifice 86 and air metering orifice 102. The resultant airfuel mixture at outlet 28 is thus leaned down to the optimum for maximum design engine running speed.
If the throttle valve 122' is then rotated back to idle position as shown in FIG. 10, air flow through the carburetor conduit is reduced to a value determined by the flow capacity of hole 204. This minimum air flow is designed to create a pressure differential acting between air bleed 102 and jet outlet sufficient to lift fuel from well 83 with the assistance of the emulsifying effect of orifice 202. However, the fuel-air ratio at this time will still be leaner than that produced upon initial starting due to orifice 202 being located below the normal level 68 of liquid in the bowl. That is, because the demand for fuel is less at idle condition, the feed of fuel via orifice 86 tends to catch up with the rate which it is sucked up tube 76, thus allowing fuel to collect in the lower portion of well 83 until it overflows the lower end of tube 76. Therefore the mixture 204 delivered from jet 80' under idle conditions (FIG. 10) is richer than the mixture 206 aspirated up tube 76 under running conditions (FIG. 9).
Moreover, when throttle plate 122 is reopened to increase the speed of the engine, the increasing draft through the carburetor will cause the liquid level in well 83 to once again drop below the lower end 74' of the tube, whereupon an additional emulsifying effect occurs in the well between orifice 86 and inlet 74' cumulative to that caused within tube 76' by air bleed 202. A transition thus occurs so that there is a constant change in the fuel-air ratio as the throttle is opened corresponding to the requirements of the engine in accelerating up to running speed.
When the engine is shut down, well 83 again refills to the level shown in FIG. 7. When the engine again is cranked, a fixed amount of priming charge will be delivered to the engine in response to the initial reciprocation of the piston through the first two or three intake strokes, similar to carburetor 20 described previously except that the mass flow rate of fuel is prolonged somewhat by the emulsifying stage introduced by orifice 202. Thus, if cranking is continued, the fuel-air ratio will rapidly lean out and flooding will not occur even under hot start conditions.
By way of example, an engine-carburetion system constructed in accordance with the modification of FIGS. 7-10 inclusive provided successful results using Governed speed range of engine under running conditions (no load) Idle speed Number of starting pulls at minimum ambient temperature of 40F. to
start engine Number of starting pulls to start 2000 r.p.m.
engine at maximum ambient temperature of l20F.
Revolutions per pull 2700-3400 r.p.m.
From the foregoing description it will now be apparent that an improved carburetor and enginecarburetion system has been provided which amply fulfills the objects stated previously. In addition, by providing an oversized fuel well designed to meet the requirements of a given engine, one which is considerably larger than the normal accelerating well provided in conventional carburetors, and by properly matching the capacity of the fuel well and the lifting capabilities of the carburetor to the engine, the carburetor is capable of purging the priming well of fuel at cranking speeds without the use of a choking device and reliable starting is obtained at ambient temperatures ranging from about 40F. to 120F. This simplifies the carburetor and thus considerably reduces manufacturing costs and servicing expenses. Moreover, it also greatly facilitates starting and operating of the engine by inexperienced operators who quite often are incapable of properly manipulating a choke and thus either tend to flood the engine or do not choke enough at the proper time.
Operation of the engine also is simplified and made more fool-proof because there is no variable fuel metering orifice to adjust, such as those found on conventional carburetors; i.e., although carburetors and 200 could have adjustable orifices 86 and 86', these orifices preferably are fixed and hence do not have a high-speed needle valve fuel feed to the main jet nor a low-speed needle valve in a fuel feed to an idle jet which the novice operator may and often does maladjust. Elimination of the idle system as well as these high and low speed needle valves further reduces the cost of manufacturing and servicing the carburetor.
Due to the float bowl being internally vented from the filtered airstream within the carburetor, there is less likelihood of the fixed metering orifice 86 or 86' becoming clogged by dirt particles and fuel-air ratio produced by these fixed orifices remains within the design limits over prolonged periods of engine use. If desired, further filtering can be provided in carburetor 20 by placing a disc'like filter between the lower end of post 42 and the bottom wall 50 of carburetor 20, slots 89 being omitted in this instance, to thereby final filter the fuel being admitted to the annular bore 87 prior to delivery to the orifice 86. In carburetor 200 the optional but preferred location of orifice 86 in post 42' at a point well above the bottom of the bowl also reduces the chances of dirt clogging orifice 86.
The engine carburetion system of the present invention thus is particularly advantageous for small engines driving inexpensive warm weather appliances, such as rotary lawnmowers, where impending government safety standards restrict the blade tip speed. The enginecarburetion system of the present invention has been found to ideally meet these requirements; i.e., an engine as specified previously utilizing carburetor 200 driving a rotary lawnmower blade operates quite well in a range from 2,000 rpm. to 2,800 rpm. without any power loss at about mid range even though the venturi diameter was reduced to three-eighths inch as compared to a one-half inch diameter venturi of a conventional choke and throttle carburetor in order to provide lifting capacity for the priming charge under wide open throttle and hand-cranking conditions.
mlslaimam W Y 1. In combination, an internal combustion engine operable to run in a given speed range and in an ambient temperature environment of about 40 to F., a predetermined external load connected to said engine, a chokeless carburetor for automatically supplying a priming charge of fuel and air to said engine under open throttle conditions and in a given engine cranking speed range without the use of a choke, strangling or supplemental pressurizing fuel lifting means, said carburetor comprising an air-fuel mixture conduit having an air inlet and a fuel-air outlet connected to the fuel-air intake of said engine,
said carburetor having a float bowl type fuel supply, an oversize printing charge fuel well and a main fuel passage serially connecting said supply to said conduit for induction of fuel from said supply to said conduit between said inlet and outlet thereof solely due to the subatmospheric pressure created by the airstream drawn through said conduit directly from the inlet to the outlet thereof by the intake vacuum generated by said engine, a throttle valve in said conduit constituting the sole variable air restrictor in said conduit, said throttle having a wide open starting position presenting a minimum restriction to air flow through said conduit, an air bleed passage having an inlet opening to a source of air at a pressure greater than that at said main fuel passage outlet and having an outlet opening to said priming well, and a metering orifice connecting said fuel well to said fuel supply and operable to fill said fuel well to a given full level with a predetermined priming charge of liquid fuel from said supply only when said engine is stationary and to control the rate of liquid fuel supply to said well when the engine is running, said main fuel passage having an inlet in said well below said given level and an outlet opening to said conduit constantly exposed to the air flowing directly from the inlet to the outlet of said conduit past said passage outlet while said engine is being cranked at starting speed with said throttle at said open position, said conduit, passage and well being sized and oriented relative to one another and the engine such that when full said priming well and said passage between said inlet thereof and said given level has a volumetric capacity correlated with the air flow and aspirating characteristics of said conduit when said engine is cranked for starting with said throttle open to said starting position such that said well is substantially evacuated of the liquid fuel between said passage inlet and said given level during the induction of said priming charge from said well to said conduit via said passage by said subatmosphen'c pressure developed in said conduit in response to the engine being cranked from one to several revolutions at a start-up cranking speed in said given engine cranking speed range to thereby supply said priming charge to the engine as the sole source of fuel to enable starting of the engine while said throttle is maintained at said open position.
2. The combination as set forth in claim 1 wherein said fuel supply comprises a receptacle adjacent said fuel passage adapted to be connected to a source of liquid fuel, means for maintaining liquid fuel in said receptacle at said given level about said inlet of said main fuel passage, said priming well being located between said passage and said receptacle and extending adjacent said passage with at least a portion of said well being disposed above said inlet of said passage, an air bleed passage having an inlet opening to said conduit and facing said conduit inlet between said passage outlet and said conduit inlet, said air bleed passage having an outlet opening to said priming well above said given level, said meteringorifice connecting said receptacle with said priming well below said given level, and an air vent having an inlet opening to a source of air pressure greater that that at said passage outlet but not in excess of atmospheric pressure and having an outlet opening to the head space of said receptacle, said conduit having a venturi therein sized to have an air flow capacity correlated with the power requirement of said engine to enable the same to run in said given speed range but being restricted sufficiently to develop a suction for aspirating said prime charge at start-up whereby said air vent and air bleed passage together with said conduit venturi suction provide the sole pressure differential for supplying said priming charge from said well to said conduit.
3. The combination as set forth in claim 2 wherein said metering orifice comprises a hole of uniform diameter providing a fixed restriction of fuel flow therethrough.
4. The combination as set forth in claim 2 wherein said passage has an air bleed connecting the interior thereof with said priming well at an elevation above said inlet of said passage.
5. The combination as set forth in claim 4 wherein said air bleed is disposed at an elevation below said given level.
6. The combination as set forth in claim 4 wherein said metering orifice communicates with said fuel receptacle and said priming well at an elevation above the bottom of said receptacle and below said given level.
7. The combination as set forth in claim 4 wherein said throttle valve has an opening therein calibrated to provide a minimum air flow opening in said mixture conduit when said throttle valve is in closed position in said mixture conduit to enable said engine to run at idle speed.
8. The combination as set forth in claim 7 wherein said throttle valve is located between said passage outlet and said conduit outlet.
9. The combination as set forth in claim 2 wherein said fuel receptacle comprises a float bowl and said fuel level maintaining means comprises a float bowl and associated fuel inlet valve mechanism whereby fuel is gravity fed from said receptacle to said well to replenish the same when the engine is stationary.
10. The combination as set forth in claim 2 wherein said mixture conduit inlet is connected to the outlet of an air filter whereby all of the air passing into said mixture conduit is drawn through said air filter, and
wherein said air vent inlet opens into said conduit adja-' cent to said conduit inlet and faces said filter outlet.
11. The combination as set forth in claim 2 wherein said engine comprises a single cylinder engine having means for governing the running speed of said engine to a speed in the range of about 2,700 to about 3,400 revolutions per minute to thereby define said given speed range, said engine having a displacement in the order of about seven to eleven cubic inches, and wherein said priming well and passage has a starting charge volume of about 0.75 cubic centimeter, said iposed above said inlet of said main fuel passage. said conduit venturi has a diameter in the range of about five-sixteenths to seven-sixteenths of an inch, said metering orifice comprises a restricted orifice having an effective diameter of about 0.028 inch and length of about 0.048 inch, and said air bleed passage has a restrictive orifice therein having an effective diameter ranging from about 0.023 to 0.031 inch.
12. The combination as set forth in claim 1 wherein said priming well. has at least a portion thereof dismetering orifice connecting said float bowl fuel supply with said priming well below said given level, and an air vent having an inlet opening to a source of air pressure greater than that at said main fuel passage outlet but not in excess of atmospheric pressure and having an outlet opening to a head space of said float bowl fuel supply, said conduit having a venturi therein sized to have an air flow capacity correlated with the power requirement of said engine to enable the same to run in said given speed range but being restricted sufficiently to develop a suction for aspirating said priming charge at start-up whereby said air vent and air bleed passage together with said conduit venturi suction provide the sole pressure differential for supplying said priming charge from said well to said conduit.
13. The combination as set forth in claim 12 wherein said main fuel passage has an air bleed connecting the interior thereof with said priming well at an elevation above said inlet of said main fuel passage.
14. The combination as set forth in claim 12 wherein said air bleed passage outlet is disposed at an elevation above said given level.
15. The combination as set forth in claim 14 wherein said fuel supply comprises a float bowl, fuel level maintaining means and associated fuel inlet valve mechanism for naintaining liquid fuel inlet valve said given level whereby fuel is gravity fed from said bowl to said well to replenish the same when the engine is stationary.
16. The combination as set forth in claim 15 wherein said mixture conduit inlet is connected to the outlet of an air filter whereby all of the air passing into said mixture conduit is drawn through said air filter, and wherein said air vent inlet and said air bleed passage inlet open into said conduit downstream of said filter outlet.
17. The combination as set forth in claim 15 wherein said engine comprises a single cylinder engine having governor means for governing the operation of said throttle valve such that the running speed of said engine ranges from about 2700 to about 3400 revolutions per minute to thereby define said given speed range, wherein said engine has a displacement in the order of about seven to eleven cubic inches, wherein said priming well and main fuel passage have a priming charge volume of about .75 cubic centimeter and wherein said conduit venturi has a diameter in the range of about five-sixteenths to seven-sixteenths of an. inch.