Patent GB-975945-A (GB)
Title: Charge forming and fuel feeding apparatus for internal combustion engines
IPC: F02M17/04
Filed: 1960-12-12
Abstract: 975,945. Carburetters or charge forming devices. TILLOTSON MANUFACTURING CO. Dec. 12, 1960 [May 2, 1960], No. 34902/60. Heading F1B. A carburetter or charge forming device includes a body having a mixing passage with a throttle valve provided therein, a fuel chamber formed in the body in close proximity to the mixing passage, a main orifice in the mixing passage, a fuel inlet leading into the fuel chamber, a fuel inlet valve member controlling the flow of fuel into the fuel chamber, a fuel metering passage communicating between the fuel chamber and the main orifice, a flexible diaphragm forming a wall of the fuel chamber and subject to flexure under the influence of aspiration in the mixing chamber induced by the associated engine and means operative between the centre of the diaphragm and the valve adapted to directly engage the valve in such manner that opening and closing movement of the valve corresponds in magnitude directly to the aspiration induced movement of the diaphram. In initiating the operation of the charge forming apparatus or carburetter illustrated in Figs. 1 to 9, the choke valve 22 is first moved to a closed or substantially closed position and the throttle valve 62 moved to an open or near open position. The engine is then cranked, setting up reduced pressure in the mixing passage 12 which is effective on the main orifice 55 to communicate the reduced pressure to the fuel chamber 38. Reduction in the pressure in the fuel chamber 38 causes flexure of the diaphragm 40 upwardly as viewed in Figs. 5, 6 and 9, elevating the valve member 86 through engagement of its strut 88 with the member 54 at the central region of the diaphragm, moving the valve away from its seat 82. The fuel in the supply ducts 76 and 77 flows along the facets of the valve body 84 through the port in the annular valve seat 82 into the fuel chamber 38. The fuel is then conveyed under the influence of reduced pressure through the restricted channel 112 past the needle valve 110 into the channel 57 through the restricted passage 56 and out of the orifice 55 into the mixing passage 12. The liquid fuel thus delivered into the Venturi 16 of the mixing passage 12 is there mixed with incoming air admitted through the inlet 14 of the carburetter to form a combustible fuel and air mixture which is delivered into the engine crankcase 30. The high speed fuel adjusting means 110 is regulated by the knurled member 104 to provide for the amount of fuel required for high speed engine operation. During the period of high speed engine operation with the throttle 62 in open or near open position, the orifices 60 and 64 of the secondary system may or may not deliver additional fuel into the mixing passage dependent upon the extent of opening of the throttle 62. When the throttle 62 is substantially closed in engine idling position viz. the position shown in Fig. 8, the engine idling orifice 60 is supplied with fuel from the chamber 38 through the arrangement shown in Fig. 7. When the throttle 62 is in a partial open position or slightly away from idle position, the low speed orifice 64 comes into operation and supplies fuel in addition to the engine idling orifice 60. The fuel supplied to the supplemental chamber 65 for discharge through the secondary orifices 60 and 64 is conveyed to the supplemental chamber through the following channels viz. the bore 124 shown in Fig. 7 past the needle valve 128 through angularly arranged duct 130 and auxiliary chamber 65 to the secondary orifices. The fuel supplied to the secondary system, comprising the idle and low speed orifices 60 and 64, is metered or regulated by adjusting the needle valve 128 by manipulating the knurled member 123 shown in Fig. 7. It is found that during engine operation, a balance is maintained whereby the rate of flow of liquid fuel past the inlet valve 86, shown in Figs. 6 and 9, is substantially equal to the rate of delivery of the fuel from the diaphragm chamber 38 into the mixing passage 12 through one or more of the orifices or outlets 55, 60 and 64. It is essential, in order to secure satisfactory operation of the engine at idle and low speeds, to prevent reverse flow of air or air bleeding through the main orifice 55 of air from the mixing passage into the fuel to be discharged through the engine idling orifice 60, the low speed orifice 64 or both depending upon the relative position of the throttle 62 and the load on the engine. Ball check valves or other mechanical valve means have been heretofore utilized to prevent air from the mixing passage bleeding back through the secondary fuel system when the idling or low speed fuel delivery arrangement is in operation. A capillary or liquid seal system utilizing the capillary effect of the fuel in restricted passages or ducts is described in Specification 833,036. The charge forming device or carbureter of the invention utilizes the principles of that type of capillary or liquid seal. In the embodiment of the invention illustrated in Figs. 1 to 9, several zones or regions of fuel restriction are provided, which together cumulate an effective seal arrangement preventing back bleeding of air through the orifice 55. With reference to Fig. 5, the restricted passage 56 providing the outlet or orifice 55, is of a cross-sectional area to accommodate the passage of fuel sufficient to provide a proper fuel mixture at high engine speeds. This region provides an hydraulic seal or capillary seal tending to prevent reverse flow or back bleeding of air into the duct 57 shown in Fig. 6. The annular region surrounding the needle valve 110 in the bore or restricted passage 106 also provides a region of capaillary seal. It is found that the seal strength at the restriction provided by the needle valve 110 is highly efficient where the needle or tapered portion of the valve in the bore or restricted passage 106 is of a low included angle and the restricted passage 106 made of substantial length. Thus the needle 110 in adjusted position provides for an annular restriction of but a few thousandths of an inch surrounding the needle which, combined with a substantial length of the liquid column surrounding the needle, provides a high strength seal in addition to that provided by the restricted outlet passage 56. The passage 112 from the diaphragm chamber 38 to the bore 96 surrounding the tenon portion 108 of valve 110 is also of comparatively small cross-sectional area, being sufficient to satisfy the maximum mixture requirements of the engine at high speeds and hence the restriction 112 provides a further region of capillary or liquid seal assisting in preventing back bleeding of air through the orifice 55. It is found by test that the seal strength for carburetters, embodying a Venturi having a choke band or region up to approximately three-quarters of an inch, is maintained effective under a pressure which will support a column of water approximately one inch in height, as pointed out in Specification 833,036. The seal strength varies approximately inversely to the size of the Venturi and increasing the Venturi size tends to promote a seal of lesser strength. It is to be noted that for satisfactory operation the capillary seal strength of the seal regions, when the engine is operating in the low speed range, must be greater than the pneumatic force or aspiration required to cause flexure of the diaphragm. The carburetter or charge forming apparatus illustrated in Figs. 1 to 9 is of the so-called "free" position carburetter as it is adaptable for operation in extreme angular and inverted positions. When used in an inverted position, in the absence of a fuel pump or means for delivering fuel under low pressure other than gravity, the fuel tank must be maintained with the fuel supply level above the carburetter in order to assure operation. In the capillary seal arrangement, it should be noted that the idle fuel supply is at the inlet side of the high speed fuel restriction. Such arrangement provides ample supply of fuel to the idle system yet the capillary seal at the main nozzle and the high speed adjusting restriction prevents backbleeding of air through the nozzle 55. A pressure which will support a column of water three sixteenths of an inch in height effective on the diaphragm 40 is usually sufficient to overcome the spring 90, the weight of the valve 86 and the low hydraulic force on the valve 86 to open the valve to admit fuel to the chamber 38. It is found that the liquid or capillary seal is maintained effective to prevent back bleeding with various sizes of Venturi having a choke band or restricted region up to a diameter of approximately an inch. Fig. 15 shows a modification in which the fuel feeding means includes a diaphragm pumping or pulsing means for delivering fuel to the charge forming component enabling the positioning of a fuel supply tank below the charge forming apparatus. Fig. 22 is a sectional view illustrating a modified form of mixing passage embodying a multiple Venturi arrangement. Other modifications are described. The body may be formed of cast metal or moulded resinous plastic material. The closure member 44 may be of sheet metal or of moulded plastic material. The diaphragm 40 is preferably of fabric impregnated with a material such as synthetic rubber. The apparatus of the invention may be employed with two stroke cycle or four stroke cycle engines. The apparatus provides for delivering a fuel air mixture particularly for low horsepower engines of the character utilized for powering lawnmowers, chain saws, marine motors or wherever a high tilt tolerance charge forming apparatus is desirable.