US2419679A - Carburetor - Google Patents

Description

Patented Apr. 29, 1947 UNITED STATES PATENT ortica Application December No. 468,667

2 claims. 1

The present invention relates to the carburetion of internal combustion engines and speciilcally to the carburetion of such engines when equipped with supercharging means.

Among the objects of the invention is increased efficiency of such engines due to an increase of both power and economy of fuel.

Another object is to permit use of a smaller carburetor and smaller jets to improve metering and engine performance at low speeds and during idling.

Another object is to reduce the effect of airhorn Idisturbances and the effect of fuel level changes in the flow bowl. A

Another object is the reduction of vaporization in the float bowl.

Another object is to enable the delivery of a very rich mixture to the air entering the supercharger so that a small amount of heating will sutlice to prevent icing of the primary tube.

Still other objects and advantages will be apparent from the following description and the accompanying drawings in which the singlev figure of the drawing is a diagrammatic showing of the carburetor and its connection to the supercharger l and to an engine as represented by a single 'cylinder.

In the drawings, `an engine cylinder is indicated at A with its intake valve open at AI receiving fuel mixture through conduit C from the supercharger blower D. The intake side of blower D is connected to the mixing chamber E of the carburetor proper into which air from the outside is drawn through the air-horn F, and into which a rich fuel-air mixture flows from the primary carburetor H.

This latter consists of a venturi HI into which fuel enters through the jet H2 `from the fuel bowl H3, the flow of air through the venturi HI being controlled by a primary throttle H4.

The flow of air through the air-horn F is controlled by means of a main throttle Fl and the two throttles H4 and Fi are both actuated by a single control rod T so connected as to leave the throttle FI closed until throttle H4 is wide open.

This throttle arrangement, as shown, consists of a lever H5 actuated by a portion HI of the slidable rod T which moves the lever Hl to full open position against the action of a spring TI l. and maintains the throttle open during further movement of the rod. When the throttle H4 has been fully opened, further movement of the rod 'I causes a pin F2 to act on the forked lever Fl,

2 which is fixed to throttle Fi, to open the latter.

Since one of the chief purposes of the present invention is to increase the metering head in the primary carburetor, the following .structure is added to that already described. A conduit K is connected to the pressure side of the supercharger and leads to small chest Kl connected through a valved aperture K2 with a continuation K3 leading into the fuel chamber H3. The aperture K2 is controlled by a valve plate L pressed against its seat-to close the aperture by a-spring LI.

The valve L is moved to open the aperture by the further movement of throttle rod T beyond the full open position of throttle H4.. In other words, as the throttle FI begins to open, the aperture K2 also opens. f p

The conduit K3 leads into the bowl H3 through a restricted portion K4 in order to limit the amount of pressure in the bowl and such pressure may be more accurately regulated by the use of a bleed conduit M leading from the bowl H3 to the air-horn F, this conduit also being restricted as at MI. j

Fuel may be supplied to the bowl H3 through a suitable inlet I, with the inflow controlled in any suitable fashion to maintain a level above the inner end of jet H2 and below the outlet therefrom.

The primary purpose of this system is to increase the carburetor metering head to a value greater than that used in the conventional carburetors now on supercharged engines. Increasing the head by means of supercharging pressure produces the advantages of high metering head,

without the loss due to high air resistance as in the case of conventional carburetors where metering is accomplished by suction or resistance at the intake oi the engine or blower. An inspection of computations of this principle, shows metering heads 'as high as 8" Hg with practically no restriction to the blower entrance.

Full supercharger pressure might be used on the float bowl, but this would require higher fuel supply pressures and more diiiicult sealing of the fuel system. It is therefore indicated that the fuel bowl pressure be just high enough to give the most desirable metering.

The operation of the carburetor is as follows: The carburetor is composed of a Venturi tube H and a fuel iet H2 receiving fuel from a fuel reservoir H3. All fuel is'delivered kthru this jet. This Venturi tube permits passage of all of 'the air and fuel at intermediate and lower 'part throttle speeds but essentially none of the air but all o! the fuel at full throttle. 'I'he amount of mixture flow at these part-throttle speeds isv controlled by throttle H4 up to the limit of air dow that can be delivered through the passageway of throttle H4.

At full throttle an additional air inlet IE' is open when throttle Fl is also open. This air inlet is very large thus imposing essentially no resistance to air iiow at full throttle. Throttle Fi is positioned by lever F3 which in turn is p0- sitioned by rod T. Rod T also controls position of throttle H4 and valve L. After throttle H4 has been fully opened, throttle Fi is progressively opened to give greater air iiow until full throttle position has been reached.

In order to maintain constant mixture ratios of air to fuel as additional air is admitted thru throttle Fi, added fuel flow is produced by increasing the pressure in fuel reservoir H3 by admitting supercharger pressure thru orifice K4, connecting tubes K3, valve opening K2 and tube K to supercharger outlet C. Further control of pressure in fuel reservoir H3 is accomplished thru tube M and orice Ml. This inlet provides an atmospheric pressure connection when valve L is closed. Valve L is large in opening area compared with orifice K. Valve L is opened simultaneously with the cracking of throttle Fl. Both actions are accomplished by the movement of rod T. Thus rod T becomes the main engine throttle control.

By design, the air flow resistance thru- Venturi tube H and passageway past wide open throttle H5 to entrance E is selected so that this resistance equals the supercharger pressure rise from E to C. Under these conditions throttle Fi is fully closed: This relationship is essentially iixed lbeing independent of supercharger speed since the supercharger pressure rises as the square of the speed, and quantity of air delivered is proportioned to supercharger speed. This same air ows past throttle Hd and hence the resistance thru this passageway varies as the square of the air flow and therefore as the square or the supercharger speed.

in order-to obtain a smooth metering curve, valve L is opened under the above mentioned conditions. Then pressure at C is equal 'to pressure Ki, K2, K3, Kt, H3 and Fl--this being atmospheric pressure.

By the proper'selection of orice sizes of Kd and Mi, the pressure in H3 is thus regulated and a smooth metering curve of the desired shape is obtained.

Since supercharger pressure at C for a particular engine is determined by air flow characl teristics in the engine, including valve resistance, it is evident that the metering is largely determined by engine characteristics.

.As an illustration, the following example is given:

inthe case here considered, the pounds of air per minute were chosen, simply as a basis for computation and are not meant to represent a particular carburetor or engine. Full blower pressure is 33.3"v Hg but this is reduced by means of bleeds' so that at 20 pounds of air per minute to the engine, thepressure on ther'uel bowl is 8" Hg. This is still many times the conventional metering head.v a; I

A superchargedfour cycle engine has high pressures in theI intake' manifoldV at'high operat- 'ing speeds andy loads-but high vacuum at low poor metering, if at all usable. To overcome this diiiiculty. a primary section of this carburetor is in principle conventional, and serves all the conventional requirements of fuel supply, idle, acceleration, altitude, temperature control oi' mixture, etc. One feature of the carburetor is in the possibility of the choice of the resistance is made to iustequal the `blower pressure rise at a point where the primary throttle is wide open and the main throttle is still closed. In other words, at this point the blower just overcomes the primary resistance, and referring to the di.. agram, pressure at C will equal pressure at F.

To determine the resistance of the primary carburetor it is necessary to calculate the blower pressure rise under the above conditions. To simplify, assume an engine unsupercharged, with atmospheric pressurev at 30" Hg absolute. E valve resistance were 4" Hg, then absolute pressure at A would be 26" Hg. To supercharge this engine and raise the air consumption from l0 to 20 lbs. per minute would require 52" Hg at A. This air flow would raise the valve resistance from 4" to 11.3". It does not rise as the square of the air ow since the density also doubles and the ow is proportional to the square root of the ratio of the densities.

Under the fully supercharged condition, the main air throttle is wide open and since it is very large, the pressure beyond it at E would be practically atmospheric. The pressure at blower outlet C would be the sum of A and B or 63.3" Hg, B being the drop in pressure past the valve A or a rise of 33.3 throughthe blower. The ratio of outlet to inlet density oi' 2.1 to l will remain fairly constant for a fixed blower R. P. ivi.

, sure drop is then determined as 30"-14" or i6" speeds and,U -loads.;31.This range ofpressure, if

impressed on the oat bowl, would give extremely` For this engine at l0 lbs. of air per minute, unsupercharged, the primary carburetor would have a 16" Hg resistance. The metering head may be derived from a venturi or an orince that lowers this to a fraction of 16" or as a choice say 2" Hg suction metering head for the above conditions. The computations for metering head follow that of conventional carburatore in which the metering head would be proportional to the square of the air iiow, giving .5" at 5 lbs. air and .02" Hg at l. lb. A conventional idling system would be required to handle low flow rates. The density of air at-A would be greater than iigures shown at low flows, due to residual charge. This does not dier from conventional practice.

Since this device uses the same fuel jet system for both unsupercharged and supercharged engines, doubling the air owpwill require four times the metering head. Fuel flow is proportional to the square root of the metering head. Since 2'? Hg head was assumed for 10 lbs. per minute air now, 8" Hg will be required for proper meter ing to 20 lbs. of air. All other points may be calculated by the same relation, minor corrections may be required in the actual carburetor, as is the general case. A In the carburetor assumed in these calculations, the fuel metering is at less than atmospheric pressure. when the air volume is less than 10` lbs. per minute. Above this point the fuel bowl pressure is above atmospheric. The bowl pressure, is derived by bleeding 33.3" Hg from the outlet side of the blower down to 8" Hg. The

selection of the orifice sizes will be such as to accomplish these results and keep substantially the saine ratio for other initial pressures. For example, 33.3" Hg at 20 lbs. of air gave 8 head; then 16.3 Hg and 15 lbs.- air would give 3.9" pressure head. 'Howeven since at 15 ibs. of air per minute, the main throttle is partially closed, there will also be some vacuum head on the fuel orifice. The computed data shows that the heads from the two sources diier somewhat from the required head. The variation being a maximum of about 4.5% in rate of fuel flow.

To determine the head due to vacuum, it is necessary to determine the amount of throttling. Since the pressure C is determined as previously explained, and the ratio of inlet and outlet pressures of the blower for the same speed is fixed, then the absolute pressure at E would be 45.3/2.1. This is 22" absolute or 8" Hg vacuum at 15 lbs. air rate. At this how rate the main throttle (air only) is partly open and the primary throttle wide open. 'I'he primary carburetor having been designed to give 2" Hg vacuum metering head at 16" Hg depression will give 1 head at 8" depression since this ratio is closely iixed. The pressure metering head is 16.3 i: 8/33.3 or 3.9" Hg. The 8/33.3 being the ratio or' the bowl pressure to the above atmosphere pressure in the manifold. By similar computation, metering heads for all points between 10 and 20 lbs. of airv per minute can be determined.

The foregoing calculations were made on the basis of ilxed engine speeds but from them the trend of metering for ilxed throttle and varying engine speeds can be determined. For example, take lbs. of air per minute, with xed throttle, then cut engine speed to one half.v Assuming that supercharger blower is of the centrifugal type, then the pressure rise is proportional to I the square of the speed. The air consumption at ilxed throttle is approximately proportional to the engine speed or '7.5 lbs. in this case. The

pressure rise at C would be 4.1" Hg and the vacuum at E would be V4 of 8 or 2" Hg, which in turn would give a vacuum metering head of .24"` Hg and a pressure metering at H3 oi! V4 of 3.9"

. or .98 Hg. The total metering head would therefore be 1.23" Hg which is 1/4 of 4.9 and would give 'B the fuel now of that for 15 lbs. of air per minute. This indicates that the system is not greatly conscious" of enine Speed. All points for all speeds may likewise be determined. Since the device is neither conscious of engine speed or ythrottle position, the air-fuel ratio will remain Y. essentially nxed for all engine operating condiphragm pressure regulator could be `substitut/ed for the float chamber. This carburetor would -permit the advantages of anti-icing systems such as entering the fuel below the throttle or more directly upon the supercharger impeller. in this case the jet would discharge into a small tube and be conducted to the impeller. The air through the small tube would require some heat ing, but a very small amount compared to where all the air was to be heated. Several such .iets and tubes could be used where it might be desirable to improve distribution. Under supercharged conditions, the increased pressure on the fuel would lessen its tendency to produce vapor.

I claim:

1; In a carburetor for a supercharged internal combustion engine a primary carburetor comprising a fuel bowl, a Venturi tube, a fuel jet for supplying fuel from the fuel bowl to said venturi to incoming air and a throttle for controlling the passage of such fuel-air mixture to the super-s charger and thence to the en'gine, an air inlet for additional air leading to the mixture between said throttle and said supercharger, a throttle for controlling said air inlet, means for opening said secondthrottle when the iirst throttle is substantially wide open and means for applying superatmospheric pressure upon the fuel in said boyll when the second throttle opens.4

2. In a carburetor for supercharged internal combustion engines a fuel inlet, an air inlet for inducing fuel flow from said fuel inlet and producing fuel-air mixture, a conduit for leading said mixture to the inlet of the supercharger, a second air inlet opening to said conduit, throttles for each of said air inlets, means for actuating said throttles in succession, the throttle for thev second air inlet remaining closed until the other is substantially wide open, and means acting in conjunction with the second air inlet throttle to cause pressure to be applied increasingly to the fuel supply to thereby force fuel through said vfuel inlet, said second air inlet beingrelatively large whereby to reduce air ow through the nrst air inlet as the throttle for the said second air The following references are of record in the file oi this patent:

v Number UNITED STATES PATENTS

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