US2809022A - Carburetors - Google Patents

Description

M. E. CHANDLER Oct. 8, 1957 CARBURETORS 3 Sheets-Sheet l Filed Feb.' 29, 1952 Oct. '8, 1957 M. E. CHANDLER 2,809,022

CARBURETORS Filed Feb. 29, 1952 s sheets-sheet 2 CARBURETORS Filed Feb. 29, 1952 3 Sheets-Sheet 3 QQ @fum una .QQ hn QN .nw u

.Q .u uw@ uns Sv @s QW@ s um@ INVENTO R MZ/IEI @naef ATTORNEY United States Patent() CARBURETORS Application February 29, 1952, Serial No. 274,185 s Claims. (ci. 261-39) This invention pertains to carburetors for internal combustion engines of the type shown in U. S. Patent No. 2,432,274, issued December 9, 1947, and more particularly has reference to means for regulating the fuel-to-air ratio in the idling range of the engine.

In carburetors of the type mentioned, the fuel ow is controlled by an orifice, fixed in size during part of the operating range, but varied manually in the idling range and automatically in the higher power range of the engine. The metering head across the orice is controlled by a valve which is actuated by balancing the pressure differential of the air owing through the main air passage, against the pressure differential of the fuel flowing through the metering jet system, of the carburetor. At low air flows which obtain under idling conditions, the air pressure differential set up in the main air passage becomes so weak and erratic as to not constitute a reliable means for regulating the fuel flow, in accordance with the principle of balancing air flow pressure differential against fuel ow pressure diiferential. In order to overcome this difculty, it has heretofore been customary to provide a spring which biases the fuel valve, in opposition to fuel ilow differential, and becomes the controlling force acting of said valve at the low fuel flows which occur during idling operation of the engine. The fuel ow pressure differential is held approximately constant by said spring and the rate of fuel flow under idling conditions is regulated by an idle valve which is connected to the main throttle valve of the carburetor, so as to vary the opening of the idle valve in accordance with the position of the throttle valve, in the idling range of the engine, as is disclosed in U. S. Patent No. 2,432,274, cited above.

While such an arrangement improved the idling operation of the carburetor over previous constructions, it is attended with certain inherent disadvantages and limitations Which cause a lack of stability and repeatability in the metering of the fuel at idling air iiows. This is due primarily to the fact that, in carburetors of the type mentioned above, the main fuel valve is basically a pressure reducing valve which varies the unmetered fuel pressure as a function of air flow, and has to control fuel flows which vary from a minimum of about 25 pounds per hour (idling) to a maximum of about 2500 pounds per hour. This is a ratio of variation of approximately 100 to l-an extremely wide range of operation for precise control, even Where all the forces involved are steady.

Since the main fuel valve must be made large enough` to pass the maximum fuel flow required for the maximum power output of the engine, it has a certain inherent inertia and friction which reduce its ability to move with sufficient precision and steadiness to satisfactorily controlk the relatively small fuel flows required for idling operation. Because of these difficulties, which are inherent in any carburetor wherein the main fuel valve is employed as one of the elements to control the idle fuel flow, such carburetors have proven incapable of'maintaining the.

stability and repeatability of idle fuel metering that is now demanded for satisfactory engine operation.

In addition to the foregoing inadequacies of prior art carburetors to meet present requirements of stability and repeatability of fuel metering under idling conditions, it has been found desirable in aircraft carburetors to vary the idling fuel/air mixture in accordance with engine temperature and barometric pressure, in order tov obtain the best engine operation when the aircraft is flying at varying altitudes and when coming in for a landing. Since aircraft carburetors heretofore in use provide an idling fuel/ air mixture, based only on normal engine operating temperatures and under air pressure differentials that occur at sea level barometric pressures, such carburetors are incapable of giving the desired idling fuel/ air mixtures at other engine temperatures and ight altitudes.

An object of this invention is to improve carburetor operation under idling conditions by providing an additional, very small, fuel pressure regulating valve, in parallel with the main fuel pressure regulating valve, and arranged so that it maintains during idling operation a substantially constant fuel metering head in excess of that which would be produced by the main valve acting alone, or a metering head that is a maximum at the lowest idling fuel flow and decreases with increase in idling fuel flow in a selected manner, so as to blend with the metering head of the main fuel ow at the point of maximum idling fuel ilow.

Another object is to provide a carburetor in which the fuel metering head, during the idling regime, is held substantially constant, or is varied in a selected manner, by a very small fuel pressure, regulating valve in parallel with a main fuel pressure regulating valve, and the idling fuel flow is controlled by a throttle-operated, idle fuel valve in the conventional manner; and when the air ow through the carburetor requires a fuel metering head in excess of that produced by the small parallel valve, said valve will close and the main fuel regulating valve will take over the fuel metering in the normal manner.

Still another object is to provide a carburetor wherein the idling fuel/air mixture is varied in accordance with engine temperature and/or barometric pressure.

With these and other objects in view, which may be incident to my improvements, my invention consists in the combination and arrangement of elements hereinafter described and illustrated in the accompanying drawings, in which:

Figure 1 shows, somewhat diagrammatically, a carburetor for an aircraft type internal combustion engine, including an idling mixture control system constructed in accordance with my invention;

Figure 2, is a View, similar to Figure l, of a modified form of my invention wherein the idle fuel/ air mixture isr varied in accordance with engine temperature and barometric pressure; and

Figure 3 is a graphical illustration of the idling fuel/ air metering characteristics which are obtained with the carburetors shown in Figures l and 2.

In Figures l and 2, elements of the carburetor which are the same in both species are indicated by the same reference numeral.

Figure 1 Referring lirst to Figure 1, there is shown a carburetor cases a second supercharger is used upstream from thel entrance 12.

Airowing `through the venturi 14 creates an air press sure differential between the entrance and throat of the venturi, which differential varies with the velocity of the air flowing. This air pressure differential is utilized to create a flow of air through a secondary air conduit'connecting the entrance and throat of the. venturi. A plurality of impact tubes 22 are provided in the entrance 12, with their ends open to receive the impact ofthe en- Valve 38 is positioned by a bellows 44 located in the.

chamber 40. The bellows 44 is sealedso as to expand and contract with variations in` pressure in chamber, 40. The bellows 44-is preferably partially filled with nitrogen or other suitable fluid havinganappreciable coefficient of thermal expansion, `so that it also expands and Vcontracts withvariations in the air temperature. Since the.

bellows 44responds to both temperature and pressure, it

may be said to respond,V to air density. The valvet38-isl contoured with relation to.the response characteristics ofy thebellows 44 so that the ypressure differential appearing across the fixed restriction 32 in the fuel meter 30 is an accurate measureof the mass rate ofair ow. passingV through` the venturi. In other words, theV bellows operated valve 38 compensates for changes in density of:-

the air passingthrough theYventuri.

-Fuel enters the carburetor from a pump(notshown)- and flows through a conduit 50,` pasta valve 52 and expansible chamber 53 inthe fuel meter 30,throughv a conduit 54, to a mixturek control56 including a disc valve 58 fixedon ashaft 60. From the mixture control\56,-A the fuel flows through one or both. of a pair of parallel-- conduits 62 and 64 into ajetsystem66. From the jetv system 66, the fuel flows through an idle valve 68, a conduit 70, a pressure regulator 72 and a conduit 74- to the fuel discharge nozzle 18.

The fuel meter 30 includes a chamber 76 in addition to the expansible chambers 28, 34 yand 53 previously described. Chambers 76 and 28 areseparated by.-a flexible diaphragmV 78, chambers 28y and 34 are separated-byl a flexible diaphragmASO; and chambers 34 and 53 aresep-y aratedby a flexible diaphragm 82. The three diaphragms 78,V 80,.and 82, areattached at their centers to the stem 0f the valve 52, which is balanced against inlet pressure."

Chamber 76 is connected through a conduitV 84`to the conduitr-in the fuel line on'the downstream side of the jet system 66. Chamber 53'is, ofcourse, supplied-with fuel at the pressure existing on the upstream side ot' thel It-` may, therefore, be seenV that the valve 52 jet system. Y is positioned in accordance with the balance between a downwardly-acting force which is a measureof the rate of ow ofair to theengine, and an upwardly acting force which is'a measure of the rate of flow of fuel to the engine. The downwardly acting force is the ydifference between the pressures in chambers 28 and 34, which difference is appliedto the diaphragm 80. The upwardly acting force is the difference between the pressures in chambers 53 and 76, which is the fuel pressure differential across the jet system. Whenever the fuel flow is disproportionateuwith respect to the air flow, the valve 52 is movedrto-correct the fuel ow and maintain the same fuel-to-air ratio as before. The foregoing statement, with respect to the constant fuel-airratio, assumes that the cross-sectional area in the jet system open to the flow of fuel therethrough4 is a constant.

matically under certain conditions.

Fuel entering-.the jet systern Y66 vthroughonduitA 62 This, however, isnot strictly constant, but is varied manually and auto-k 4 may pass through a restriction 86 directly to the outlet of the jet system. It may also pass through a passage 88 controlled by an enrichment valve 90 biased to closed position by a spring 92. Fuel passing through the conduit 64 flows through a fixed restriction 94. The opening movement of valve 90 is limited by a stop 96.

When the disc valve 58 of the mixture control 56 is in the position shown in full lines in the drawing, which is termed the lean position, fuel may enter the jet system only through the conduit 62'. With the valve S8 in its rich position, as shown in dotted lines on the drawing, fuel may enter the jet system through bothr conduits 62 and 64.

When the valve 58 is in its lean position, a first predetermined relationship is established between the fuel pressure differential across the jet system and the rate of flow of fuel. When the valve 58 is in its rich position, a second suchA relationship is established,` such that a greater' fuel flow is obtained for each-value of fuel pressure differential.v Since the fuel meter 30 establishes a fixed relationship betweenthe mass air flow and the fuel pressure differential, it may be seen that movement of valve 58 between its -lean and rich positions selects respectively smaller and greater fuel-air ratios.

The enrichment valve 904 opensunder high fuel pressure differential conditions, which correspond generally to conditions ofhigh power output, to increase the fuelair ratio. It should be noted that the'operation of valve 90` is the same whether the: mixture control is in its lean or rich position.- The stop 9,4 limits the enrichment effect which is produced'by valve'90.

The idle valve mechanism' includes a valve member 102 movable bymeans of' an arm 104 connected through alink 106 to anarmf108 on-the shaft 110 of the throttle 16.. A .spring 112 in fuel meter 30'biases the valve 52 toward open position, in'opposition to a spring 114 which biasesfsaid valve toward `'closed position. These springs and valve 102 cooperate with an idle fuelv pressure regulatingzvalve Vmechanism ;(described hereinbelow) to control the :fuel ow tothe engine under conditions of light load.

The pressure regulator 72.is` provided topreventvariations in pressureat the fuellnozzle 18 fromaffecting the flow through; the jet system. The regulator 72includes a -valve 116.biasedtoclosed.position by a spring 118. A diaphragm 120' separates a` pain of 'expansiblel chambers 122.and.124.. The chamber 124 receivesfuel at the pressure' on the downstream:v side of' the jet system, Ywhile chamber122 isv vented to -ventring-24 through conduit 126 and conduit 26. The effect of this arrangement is to maintain# a.-subs'tantial'lyI constantY pressure on the downstrearnside' of the-Yjetl 'systemi Idle fuel-pressureV regulatingA valvemechanism 33E? comprisesa casing'132,divided1by a flexible diaphragm 134 into two expansib'le' chambers 136 and .lit-lof which theformer is-connected .bya conduit lto chamber 76,

, and the-latter is connectedb'y a conduit 142.10 chamber 53, in fuelmeter 302 Fixedlymounted-'in casing 132 isa sleeve 144 lwhichis provided-'with a sleeve valve 146,1

attached at its\.left=.end todiaphragrn V131i. A port 148 in' sleeve valve 145 lis: adapted to varying the opening-of a -port" (in sleeve 144) which isl connected-to fuel inlet conduit `50' by av conduit 1152y having al restriction 154.,v AV spring 156interposed-between diaphragm 134 and the left endwall of' casing 132,'5biases valve 146mwardopen position, invoppositionto the fuel pressure differentialyactingon'diaphragm' 134, which consists of the difference between the unmetered fuel pressure 1) transmittedfromfchamber- 53y to chamberl 138, and the metered fuel pressure p3, ,transmitted from chamber 76 to chamber136l on diaphragm 1341's greater than the force Vof4 spring 156,"valve 146' is'moved tothe` left', elosing'port '150;'

whichreduceslthe ow'of fuel (under inletv pressure p1) fuel valve 56 if it were acting alone.

from fuel inlet 50, through conduit 142 into chamber 53, and from thence through conduit 54, jet system 66, -idle valve 102, pressure regulator 72 and conduit 74 to discharge nozzle 18. This reduction of fuel flow into Ichamber 53 tends to lower the pressure p2 therein and in chamber 138, and such decrease in pressure (acting on diaphragm 134), causes spring 156 to move valve 146 to the right, thereby increasing the opening of port 1150, and raising the pressure p2 in chambers 138 and 53. Accordingly, as long as main fuel valve 52 remains closed, the fuel pressure upstream of idle valve 68 is held substantially constant, or may be varied in a selected manner, by the operation of valve 146. Since the metered fuel pressure p3, downstream of idle valve 102, is held substantially constant by pressure regulator 72, the metering head across idle valve 102 may be held substantially constant, and the rate of fuel flow to the engine is proportional to the opening of contoured idle fuel valve 102, which in turn is proportional to the opening of the throttle 16 in the idling range.

In a carburetor, such as described above, the fuel flow is proportional to the square root of the metering head p2 3 which is a straight line function of the air flow through the carburetor, as indicated by line A in Figure 3. In the idling range of engine operation, which lies in the region of from 0 to 10 percent of maximum air and fuel How, the air pressure differential set up by venturi 14 becomes very small and erratic (unsteady), so that it is no longer an accurate measure of the air flow. Therefore, in order to obtain an approximately constant metering head across the idle fuel valve, it has heretofore been customary to employ a spring (such as 112 in Figure l) with a setting such that it controls the main fuel valve (such as 52 in Figure 1), when the air pressure differential becomes so weak that its action on said valve is uncertain, as in the idling range. While such an arrangement improves the idle metering head, it does not control it with a sufficient degree of accuracy and reliability to uniformly produce satisfactory idling operation of the engine, owing to the inertia and extremely small movements of the main fuel valve under idling conditions.

To overcome this diiculty I provide a Very small, fuel pressure regulating valve 146, in parallel with the main fuel regulating valve 52, and arranged so that it maintains (during idling operation) an idle fuel metering head across the idle valve 102 in excess of that which would be produced by the main fuel valve 52 acting alone. To this end, springs 112 and 114 are adjusted so that when the air flow reaches a selected point, corresponding to the upper limit of the idling range (indicated at O in Figure 3), main fuel valve 52 closes, and sleeve valve 1446 opens. Conversely, when the rate of air flow increases beyond the upper limit of the idling range, valve 146 closes port 150, and main fuel valve commences to open, thus taking over the regulation of the fuel pressure metering head across jet system 66.

When main fuel valve is closed and Valve 146 is open,

i102 higher than that which would be produced by main Thus, if optimum idling operation of the engine is obtained by a fuel/air mixture resulting from a rate of fuel How of say 200 pounds per hour (equal to 8 percent of maximum fuel and air flow), springs 112 and 114 are set so as to close main fuel valve 52 when the air ow is reduced to 8 percent of its maximum. Then spring 156 is set so as to cause valve 146 to produce a metering head across idle valve 102 such as to obtain a fuel flow of 200 pounds per hour at that point (indicated by O in Figure 3). Within the idling range, the pressure p, upstream of idle fuel valve 102 may be held substantially constant by the operation of valve 146, or said pressure may be varied,

in a selected manner, such as to blend with the metering head of the main fuel flow at the point of maximum idling fuel oW, as indicated by the line B in Figure 3. Within the idling range, the rate of fuel flow is controlled by the pressure p2 upstream of contoured idle valve 102, and the movement of said valve which is proportional to the movement of throttle 16.

Since valve 146 is relatively very small, in comparison with main fuel valve, and is actuated by a relatively large diaphragm through a large operating range, this valve is highly sensitive and responds quickly and accurately to variations in fuel pressure differential p2----p,l and thereby holds said pressure differential extremely close to the selected value. Another factor which adds to the steadiness and accuracy of operation of valve 146 is that it is not subject to fluctuations in air pressure differential, as is main fuel valve 52, even during the idling region, in prior art carburetors. While the fluctuations of air pressure differential in the idling range are small in comparison in the force of the spring (such as 112) which controls the main fuel valve 52 during idling operation, they are appreciable and tend to upset the steadiness of the main fuel valve and thus adversely affect close control of the idle fuel flow.

From what has been disclosed above, it is apparent that the construction shown in Figure l provides a simple and very effective means of obtaining a close, stable and accurate control of the fuel flow in the idling regime.

F gure 2 Recent experience in the operation of aircraft has shown that, aside from accuracy and stability under idling conditions, a constant fuel/air mixture does not meet all requirements in the idling operation of the engine. Thus, when flying at altitude or when coming in for a landing the pressure drops in the carburetor and the cylinder head temperatures of the engine are different. Hence, good idling engine operation requires a different idling mixture. Since prior art carburetors can only give an idling mixture based on engine requirements when the engine is up to normal temperature, and under carburetor pressure drops that occur at sea level, such carburetors are incapable of meeting present requirements which demand -form of my invention, as illustrated in Figure 2.

Since most of the elements of the carburetor shown in Figure 2 are the same as in the carburetor shown in Figure l, and are indicated by the same reference numerals, these elements will not be further described. Referring to Figure 2, it will be noted that the carburetor here illustrated diers from that shown in Figure l, only in the additional mechanism that is added to the idle fuel pressure regulating mechanism 130, in order to make it responsive to engine cylinder head temperature and barometric pressure.

In the construction shown in Figure 2, the left end of casing 132 is provided with a cylindrical extension 160 in which is slidably mounted a cup piston 162 which forms the outer support for spring 156. Bearing against the outer face of piston 162 is the upper end of a lever 164, centrally pivoted to a fixed support 166, and having its lower end pivotally connected by a rod 168 to a diaphragm 170 that divides a casing 172 into two expansible chambers 174 and 176. Chamber 174 is connected by conduits 178, 126 and 26 to Vent ring 24, while chamber 176 is connected by conduits 180 and 182 to a ternperature responsive device 184, and is also connected by conduits 180 and 186 to the main air passage 17 below throttle 16. In chamber 176 is a spring 188 which biases diaphragm 170 to the right, in opposition pressure differandthe air pressure. inthe air passage 17 below throttle 16,V which. is-transmitted: by conduits 186 and 1,80 to chamber 176..

The. temperature responsive-device 184,` comprises a casing-.i196 having a transverse partition 192, providedl with aport194. The left end of. casing 196 is vented to the 'atmosphere through a conduit "197. Fixed to the left end ofcasing -196-is an exterio'rlyl threaded sleeve 198 which is threaded 'into a socket inA a cylinder headA 2001of the engine. Fixedly attached to the interiorof sleeve'198 is-a rod 202vwhich lterminates at itsright end in a contoured valve 204, adapted to varythe opening through port 194.' Rod 262 -is made of ya material having a much smaller. coefficient-'of thermal expansion than sleeve 19,8, so. that as thesleeve and-rod are heated by heat from engine cylinder head ),V sleeve'198-expands at a greater ratethanrodtlt) and withdraws the valve 264 from port 194',- thus increasing `the opening'of-said port. Conversely, a lowering of-the temperature of the engine cylinder head 200 causes a corresponding closure in port 194.

As the aircraft -gains altitude and the baromctric pressure of the atmosphere decreases; agreater throttle opening is required to maintain the same engine speed, hence the pressure drop across the throttlel decreases with increasing altitude. Since the pressure differential acting on diaphragm 170 varies directly with the pressure drop across throttle 16,- iti follows `that said pressure differential will decrease with increasing altitude. As said pressure differential decreases the` thrust on spring 156 which causes-valve 146 to move to left, it reduces the opening of port 150. This resultsin a decrease in the pressure differential pg- 3 and a corresponding decrease in idle fuelfl'ow to the engine, as described hereinabove, and-indicated by lineC in Figure 3. Conversely, an increase in the pressure differential acting on diaphragm 170A results in anincreasein idle `fuel flow to the engine; sothat the idle mixture is varied inversely with altitude and directly with barometricpressure.-

If it shouldl be-desired to var-y the idle fuel ow in the oppositeA manner, i.- e., directly with altitude and inversely withabarometric pressure, this result can be obtained by simply connecting conduit 178- to chamber 176, and conduit 18) to chamber 174, and moving spring 188 to charnber 174 so as to oppose the reversed direction of the pressure differential acting on diaphragm 173.

As thecylinder head temperature of the engine increases, the length of sleeve 198 increases more than the length of rod 262, which causes valve 204, to increase the opening ofy port 194, thus permitting atmospheric air to bleedA into chamber 176 and` increase the pressure therein. The amount of this-increase of pressure in chamber 176 dependsalso upon the relative sizes of the restrictions in conduits 180, 182 and 186; the larger the s iZe of restrictionin 182 as comparedwith that in186, the greater the increase in pressure in chamber 176 for any given opening of port 194, and vice versa. The restriction in conduits 180 and186 also retard the transmission to chamber 176 ofY any sudden fluctuations of pressure in the main air passage 17, below throttle 16.

Conversely, a decrease in temperature of engine cylinder headY 209 decreases the pressure in chamber. 176 and thus increases theV pressure differential acting on diaphragm 170. And since, as shown above, any increase inthe pressure differential acting on the diaphragm 17? causes a corresponding increase in idle fuel flow (and vice versa), it follows that ,the idle mixture ratio is varied inversely with engine cylinder head temperature. If it should Vbe Adesired' to vary the idle fuel ow inthe 'oppositedirection, i. e., directly with engine cylinder headtemperature, this: result can be obtained by simply reversing-the arrangement ,of sleeve 198 and-rod 2m), so. that valve-204 would-close port 194, with increasing temperature of the engine cylinder head. To Vthis end rod 202 would be made of material having Va higher coefficient of thermal expansion than-that of the material of sleeve 19S.

From what has been' shown above, itis clear that the carburetor illustratcd'in -Figure 12,' operates inthe same manner as the carburetor-ofv Figure l, except that theV additional elements incorporatedV in Figure 2 modify the action of the idle fuel pressure regulating-valve mechanisrn 130,` so as to vary the idle fuely mixture with barometric pressure (altitude) and engine cylinder head temperatnre, in a selectedA manner in order to meet the requirement of optimum engine performance in the idlingrange of operation.

While I have shown and described the preferred embodiments of my invention, I -desire it to be understood that I do not limit myself to the precise details of construction disclosed by Wayv of illustration, as these may be changed and modified by those skilled in the art without departing from the spirit of my invention or'exceeding the scope of the appended claims.

I claim:

l. A carburetor for an internal combustion engine, comprising: an air supply throttle; means for supplying to said engine air and 'liquid fuel in selected mixture proportions throughout ther operating range of said engine above its idling range, including a metering orice, for metering said fuel supply in selected proportion to the rate of said air supply during said operating range; means, unaffected by variations in therate of air supply, for supplying fuel to said engine throughout its idling range, under a selected variable metering head which is governed by the pressure drop across saidorice and which causes said idle-fuel to flow at such rates as to form an idling mixture -of selected varied proportions richer than said operating range mixture proportions; and means for modifying said idling rates of fuel flow in accordance with the pressure drop across said throttle.

2. A carburetor for an internal combustion engine, comprising: an air supply throttle; means for supplying to said engineair and liquid fuel in selected mixture proportions throughout the operating range of said engine above its idling range, including a metering orifice, for metering said fuel supply in selected proportion to the rate of said air supply during said operating range; means, unaffected by variations in the rate of air supply, for supplying fuel to said engine throughout its idling range, under a selected variable metering head which is governed by the pressure drop across said orifice and which causes said idle fuel to ow at such rates as to form an idling mixture of selected varied proportions richer than said operating range mixture proportions; and means for moditying said idling rates of fuel ow in accordance with engine temperature.

3. A carburetor for an internal combustion engine, comprising: an air supply throttle; means for supplying to said engine air and liquid fuel in selected mixture proportions throughout the operating range of said engine above its idling range, including a metering orice, for metering said fuel supply in selected proportion to the rate of said air supply during saidoperating range; means, unaffected by variations in the rate of air supply, for supplying fuel to said engine throughout its idling range, under a selected variable metering head which is governed by the pressure drop across said orifice and which causes said idle fuel to flow at such rates as to form an idling mixture of selected varied proportions richer than said operating range mixture proportions; and means for modifying said idling rates of fuel ilow in accordance with barometric pressure and engine temperature.

4. A carburetor for an internal combustion engine, comprising: an air supply throttle; means for supplying to said engine air and liquid fuel in selected variable proportions throughout the operating range of said engine above its idling range, including a main fuel llow regulating valve, responsive to the relative rates of said air and fuel supplies, and an idle fuel pressure regulating valve, separate from but acting co-ordinately with the action of `said main valve, for supplying lidling fuel to said engine under a variable metering head such that its flow rate decreases from a maximum at lowest idling speed to a minimum `in idling mid-range, and then increases during the remainder of said idling range; and means for modifying the action of said idle fuel pressure regulating valve in accordance with the pressure drop across said throttle.

5. A carburetor for an internal combustion engine, comprising: an air supply throttle; means for supplying to said engine air and liquid fuel in selected variable proportions throughout the operating range of said engine above its idling range, including a main fuel flow regulating valve, responsive to the relative rates of said air and fuel supplies, and an idle fuel pressure regulating valve, separate from but acting co-ordinately with the action of said main valve, for supplying idling fuel to said engine under a variable metering head such that its ow rate decreases from a maximum at lowest idling speed to a minimum in idling mid-range, and then increases during the remainder of said idling range; and means for modifying the action of said idle fuel pressure regulating valve in accordance with engine temperature.

6. A carburetor for an internal combustion engine, comprising: an air supply throttle; means for supplying to said engine air and liquid fuel in selected variable proportions throughout the operating range of said engine above its idling range, including a main fuel flow regulating valve, responsive to the relative rates of said air and fuel supplies, and an idle fuel pressure regulating valve, separate from but acting co-ordinately with the action of said main valve, for supplying idling fuel to said engine under a variable metering head such that its ow rate decreases from a maximum `at lowest idling speed to a minimum in idling mid-range, and then increases during the remainder of said idling range; and means for modifying the action of said idle fuel pressure regulating valve in accordance with barometric pressure and engine temperature.

7. A carburetor for an internal combustion engine, comprising: an air supply throttle; means for supplying to said engine air and liquid fuel in selected variable proportions throughout the operating range of said engine above its idling range, including a main fuel ow regulating valve, responsive to the relative rates of said air and fuel supplies, and an idle fuel pressure regulating valve, separate from but acting co-ordinately with the action of said main valve, for supplying idling fuel to said engine under a variable metering head such that its :flow rate decreases from a maximum at lowest idling speed to a minimum in idling mid-range, and then increases during the remainder of said idling range; and means, responsive to engine temperature, for modifying the action of said idle fuel pressure regulating valve in accordance with said temperature.

8. A carburetor for an internal combustion engine, comprising: an air supply throttle; means for supplying to said engine air and liquid fuel in selected variable proportions throughout the operating range of said engine above its idling range, including a main fuel flow regulating valve, responsive to the relative rates of said air and fuel supplies, and an idle fuel pressure regulating valve, separate from but acting oo-ordinately with the action of said main valve, for supplying idling fuel to said engine under a variable metering head such that its ow rate decreases from a maximum at lowest idling speed to a minimum in idling mid-range, `and then ncreases during the remainder of said idling range; a valve, responsive to engine temperature, and a valve, responsive vto barometric pressure, for modifying the action of said idle fuel pressure regulating valve in accordance with variations in said temperature and barometric pressure.

References Cited in the le of this patent UNITED STATES PATENTS 2,232,201 Barnes Feb. 18, 1941 2,558,921 Barr July 3, 1951 2,621,910 Volz et al Dec. 16, 1952

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