US2720890A - Control of vacuum in internal combustion engine - Google Patents

Description

Oct. 18, 1955 F. H; STROUD 2,720,890

CONTROL OF VACUUM IN INTERNAL COMBUSTION ENGINE Filed June 7, 1949 2 Sheets-Sheet l Zmventor Oct. 18, 1955 F. H. STROUD 2,720,890

CONTROL OF VACUUM IN INTERNAL COMBUSTION ENGINE Filed June 7, 1949 2 Sheets-Sheet 2 Zmvento'r atent filice 2,720,890 Patented Oct. 18, 1955 CONTROL @F VACUUI /i IN INTERNAL CGMBUSTHUN ENGINE Francis E. Stroud, Shelton, Wash. Application June '7, 1949, Serial No. 97,532 Claims. (Cl. 137-486) This invention relates to control of vacuum in internal combustion engines, and has for its general object the provision of a governing device effective to substantially eliminate excess vacuum, and which is to say to eliminate conditions of negative pressure over and above normal vacuum. Normal vacuum, in a gasoline-fueled internal combustion engine, is developed when the engine is idling and will normally fall Within a range of to 22 inches of mercury measured with a vacuum gauge. An excess of Vacuum occurs as the result of closing the throttle when the engine is turning over at a speed above idling speed, vacuum then reaching 24 to 26 inches. This condition of excess vacuum therefor obtains whenever a vehicle is coasting against the motor, and occurs both 011 level stretches and down-hill grades. Although the rise in vacuum which takes place is only a few inches, it should be borne in mind that a conversion of inches of mercury to pounds pressure per square inch, applied to the area of a cylinder wall represented by the product of the cylinders circumference multiplied by the length of the stroke, results in a pulling force of several hundred pounds. More exactly expressed, and taking a Dodge engine for example, a developed vacuum of inches exerts on each of the six cylinder walls a pulling force of somewhat more than a quarter ton, to-wit: 548 pounds.

Excess vacuum obtaining in the combustion chamber of an engine contributes to high operating costs and shortened engine life in consequence of drawing oil off the cylinder walls into the combustion space and additionally sucking raw gasoline from the idling jets into the cylinders. Aside from the raw gasoline diluting the lubricating oil, the gasoline and oil mixture within the combustion chamber is incapable of supporting combustion and burns only partially with the result that harmful gums and carbon deposits are produced, fouling the plugs and rings as well as the engines Valves. The imperfect combustion of the sucked oil and gasoline is readily apparent by noting the smoke and fumes contained in the exhaust from any automotive vehicle after the same has coasted under engine load and is again accelerated by an opening of the throttle. There have been heretofore devised quite a number of devices intended to control the vacuum in a gasolinefueled internal combustion engine and all of which, as with my device, operate on the principle of a relief valve. Many of these prior devices are extremely complicated while others are fairly simple but in no instance of which I am aware has there been a relief valve for this purpose which could be adjusted with any fine degree of accuracy in a manner to properly adapt itself to the widely different conditions of vacuum which obtain in the engines of various cars, trucks and busses. The present invention has its particular object the provision of a vacuumcontrolling valve structure of precision manufacture which admits of being easily and quickly set so as to adapt itself with an unusually fine degree of exactness to the particular condition of substantially any engine to which it may be applied.

It is a yet further object to provide a vacuum-control structure which is exceptionally compact, which admits of being quickly installed, and which requires, for the installation, only a single opening tapped into the intake manifold, an opening, moreover, which is found to commonly exist in the intake manifold of substantially every engine.

The invention has the still further and particular object of providing a vacuum-control device embodying a perfected mounting bracket enabling the devise to be applied in confined spaces devoid of any appreciable swing radius, and with the bracket being so associated with the device proper as to enable the latter to be removed and reinstalled, where occasion therefor requires, with a minimum of time and effort.

It is a still further and important object to provide a device for the described purpose in which the relief valve contained therein is subject to little if any wear and requires no lubrication in order to assure free-working operation.

With the foregoing and still additional objects and advantages in view and which will appear and be understood in the course of the following description and claims, the invention consists in the novel construction and in the adaptation and combination of parts hereinafter described and claimed.

In the accompanying drawings:

Figure l is an elevational view illustrating the present device in operating position upon the intake manifold of a gasoline-fueled internal combustion engine, the engine being shown somewhat schematically.

Fig. 2 is a longitudinal vertical sectional view of the device incorporating a fragmentary showing of the intake manifold. While the relief valve is indicated as being in its closed position, directional arrows are applied to the view to indicate the course taken by vacuumrelieving atmospheric air when the valve is opened.

Fig. 3 is a front elevational view of the device.

Fig. 4 is a fragmentary side elevational view of the mounting bracket; and

Fig. 5 is an exploded side elevational view of the various parts which compose the device.

According to the present invention, and referring to said drawings, there is provided a casing 10 bored from one end for the greater part of the length to produce a cylinder 11 and having at the inner end of this cylinder a partition 12 formed centrally with an inlet port 13. Internal threads are provided at the outer end of the bore, and formed in the side wall more or less intermediate the threads and the partition is an outlet port 14 which, for a purpose to be hereinafter described, tapers inwardly. The egress side of the inlet port has its perimeter bevelled or otherwise suitably faced to operate as a valve seat. The casing is prolonged somewhat beyond the partition and within this prolongation, which terminates in an annular lip 15, there is provided a recess of relatively shallow depth accommodating the press-fit engagement of a screen 16. A cap 17 is snap-fitted in the socket formed by said lip, and this cap contains a body 18 of steel wool and presents openings 19 in its head Wall for admission of air, the purpose of the wool being that of a silencer to minimize noise incident to sucking of air.

Working within the inner length of the cylinder, and which is to say within that portion of the same proximate to the inlet port, is a piston formed with a hollow body section 20 which fits rather loosely within the cylinder and having the frontal end necked down as at 21, the nose of this neck serving the function of a valve by fitting against the valve seat. The end wall which occurs between the neck and the body section is provided at spaced intervals of the circumference with openings 22 giving access to the interior of the body section. The body section provides in its perimeter a spirally developed groove 23 running the full length thereof, and at the rear end presents a centrally apertured wall 24, preferably conical in shape.

There is received in the open rear end of the casing an interfitting set of three co-axially disposed controls comprised of an outer gland-like collar 25, an intermediate tube 26, and an inner rod 27, the mounting being one in which the collar finds thread engagement with the internal threads of the casing and the tube and the rod thread in turn the former into the collar and the latter into the tube. Each said control piece is of a length to assure a material portion of its threads being exposed, and terminates in an enlarged knurled head, as 30, 31 and 32, and received upon these exposed ends are respective jam-nuts, as 33, 34 and 35, which similarly present knurled surfaces.

The collar element presents at its inner end a counterbore 36, and taking a bearing purchase by one of its ends against the floor of this counter-bore is a compression spring 37 bearing by its other end against the rear face of the piston. The tube 26, inwardly of its external threads, is necked down to a diameter very slightly less than the center opening of the pistons rear wall 24, and such neck, designated 38, is prolonged in a degree such as to extend through said center opening into the interior of the piston. The frontal end of this tube operates as a stop limiting opening movement of the piston, and within the length of its neck there are provided two sets of axially separated ports, one said set, designated by 40, being located in immediate proximity of the front-end limit and serving to communicatively connect the interior of the tube with the interior of the piston and the other set 41 being removed forwardly therefrom to open into the cylinder in forwardly spaced relation beyond the travel limit prescribed for the piston. Each said set of ports is indicated as comprising two pairs of diametrically opposite wall openings with one pair being longitudinally offset from the other.

As respects the control rod 27, the internal threads in which the same works, and namely the internal threads of the tube 26, terminate short of the rearwardly located ports 41 and leave a smooth bore throughout substantially the full length of the tubes neck portion 38. The rod is itself necked down at the front end to establish a close fit within this bore and projects into the latter in a degree sutficient to lap any given portion of the ports 41.

In applying the present invention to the fuel system of an internal combustion engine, the procedure is to install the device such as to bring the outlet port 14 into communication with the intake manifold, and engineered into the present invention is an especially advantageous mounting enabling the device to be installed with unusual ease and expedition and, should any need arise for a later servicing of the unit, permitting the device to be quickly and easily removed and re-installed. For this purpose there is provided a center-bored stud-fitting formed intermediate the length with a hexagonal head 42 and presenting upon the opposite ends tapered nipples 43 and 44, the nipple 43 being smooth faced and properly dimensioned to find a friction fit with the inwardly tapering outlet port 14 of the casing 10, and the nipple 44 being threaded and operating to engage either directly through a tapped opening in the wall of the intake manifold M or indirectly through a T-fitting or the like which serves by its remaining branch to accommodate another pipe or pipes made subject to engine vacuum. Used with this stud-fitting is a bracket composed of two complementing clamp pieces 45 and 46, each being in the nature of a saddle strap with the one closely overlying the casing and the other depending by its bend somewhat below the latter. The pendant portion of such clamp-piece is flattened off, as at 49, to admit of the hex-head 42 being firmly seated against the inner face thereof and a hole pierces this flat for accommodating the threaded nipple 44. At one side of the bracket, the two clamp pieces are caused to be connected by hooking an out-turned terminal 47 of the one in a mating slot 48 of the other, and at the other side separated ears 51-52, accommodating a nutted bolt 53, permit the clamp pieces to be drawn together and pressure thereby exerted through the flat 49 upon the shoulder of the hex-head 42. It becomes an easy matter to install the device, the procedure being one of first disconnecting the clamping cap 45 from the base strap 46, disengaging the casing from the twin-nipple fitting, and then, with the flat 49 of the base strap in position under the shoulder of the hex-head 42, tightly screwing the threaded end 44 of the fitting into the tapped opening which leads into the intake manifold M. It will be noted that the fitting can be applied in a rather confined spot inasmuch as the casing has been removed and swing radius is thus no problem, the strap 46 being loose on the nipple 44.

After securing the fitting in its said position, the casing is mounted thereon by inserting the exposed nipple 43 into the outlet port 14, and the bracket parts are then reassembled by hooking the terminal 47 of the top strap into the slot 48 of the bottom strap and applying the nutted bolt 53, the operation of drawing up on this bolt acting through the top strap to subject the casing to clamping pressure as the lower strap takes a purchase under the shoulder of the anchored fitting, hence firmly setting the nipple 43 in the tapered bore of the outlet port 14.

From the foregoing description it will be understood that the collar 25, the tube 26, and the rod 27 are each individually adjustable in an axial direction. Adjustment given to the collar acts to govern the effective spring load resisting opening movement of the air-admission valve and determines the vacuum pull necessary to unseat the valve. The adjustment given to the tube 26 determines the degree to which the valve opens. Excepting in unusual circumstances, the setting given this member is such as simply will permit the piston to be retracted in a degree assuring full flow capacity to the inlet port. This setting will thus be the same in the large majority of installations. The adjustability of the third said element, and namely the rod 27, serves the important end of accurately regulating the volume of air which engine vacuum will draw into the valve chamber and hence, by governing the vacuum differential, controls the point at which the relief valve closes.

After installation, the device can be adjusted either with or Without a vacuum gauge although, perforce, the use of a gauge assures a more perfect setting. The motor should be warmed up to normal operating temperature, and is thereafter, while performing the adjustments, repeatedly accelerated and decelerated between a medium R. P. M. and an idling speed. After loosening the several jam-nuts the first adjustment is made upon the collar 25, the operator backing off this member until the relief valve (the spring-closing piston) opens when the throttle is first closed after having been accelerated. The next step, with the car idling, is to screw in the tube 26 as far as possible, and namely until the frontal end is brought to bear against the closed relief valve, and then back the same off approximately l /2 turns. With standard threads, this gives the valve a stroke travel of approximately As pre viously set forth, this setting is intended to give full-flow capacity to the opened inlet port, and the movement will suifice to this end when the device concerned has the diameter which I presently employ on structures engineered for passenger-vehicle usage. The rod 27 is next adjusted to govern the point at which the valve will close, a clockwise movement giving a later (lower R. P. M.) closing and a counterclockwise movement giving an earlier (higher R. P. M.) closing. It is desirable that this closing point correspond to a running speed of 12 to 15 miles per hour, and the opening point should correspond to a car speed of 18 to 20 miles per hour. These results can be arrived at by a final adjustment of the spring tension and the vacuum-differential after the first approximate setting has been made and the performance observed.

When the adjustments are final, the three jam-nuts 33, 34 and 35 are each set down firmly on the related adjustment member. Where a vacuum gauge is employed, the procedure above described is first followed and final adjustment made so that the relief valve opens at approximately 2 above the determined idling vacuum and closes at about 14" of vacuum. This adjustment usually corresponds to the car speeds recited. When properly adjusted, the valve will both open and close with a snap. Should any fluttering on the seat be evidenced, the valve may require either a greater spring tension or a reduction of the areal exposure of the ports 41, the latter end being accomplished by screwing in on the rod 27.

In order to assure maximum clarity in an understanding of the invention it is perhaps advisable to enlarge somewhat upon the operating principles concerned, in which connection it is here stated that the air-fiow capacity of the outlet port is or should be equal to or in excess of that of the inlet port, and the largest permissible opening of the ports 41 should not exceed the air-flow capacity of said inlet port. In conjunction with these expressed ratios, it should be borne in mind that a rise in vacuum within the intake manifold reflects an almost instant and corresponding pressure condition within that part of the cylinder which lies to the rear of the piston. There is, however, a momentary time lag before a like drop in pressure will obtain at the front end of the cylinder, due to the relative restrictions in the connecting passage-way through the tube 26, and the consequence is that there is set up a pressure differential favoring the front end of the cylinder with the result that this differential augments atmospheric pressure applied against the exposed face of the reduced neck 21, the two forces working in concert to push the piston rearwardly and open the inlet port. The significant accomplishment is that the piston instantly partakes of a snap opening and an immediate drop of several inches vacuum occurs, the effective surface to which atmospheric pressure is now applied being the full area of the pistons front face (less the area of the openings 22) plus the inner surface of the pistons rear wall 24, and which sutlices to establish a pressure differential upon opposite faces of the piston sufficient to counter the spring pressure as the vacuum progressively falls off to the given closing point. It is highly important, in order that the piston will maintain itself in open position as the vacuum drops, that a material differential obtain as between the atmospheric pressure at one side and the vacuum pressure at the other side of the piston, and the limited flow of vacuumdisplacing air which is permitted to flow through the tube accomplishes this end. It should be particularly noted that a decrease in the permitted flow of vacuumdisplacing air through the tube 26, obtained by screwing in the rod 27 to cut down the areal exposure of the ports 41, increases the vacuum differential and reflects itself in a later closing of the inlet port. Conversely, increasing the permitted air flow by backing off the rod 27 to enlarge the areal exposure of the ports 41 serves to decrease the pressure differential and causes the inlet valve to close earlier. Thus, the regulation given to the rod 27 governs the vacuum level at which the present device is inactivated. The arrangement is one in which the resistant pressure given to the spring determines the vacuum level at which the admission valve will open. The setting given to the rod, by regulating the amount of vacuum-displacing air permitted to enter the rear end of the cylinder and thus governing the pressure differential upon opposite sides of the piston, determines the vacuum level at which the admission valve will close.

The accomplishments of the present device will perhaps be most fully appreciated by considering several tests conducted on vehicle engines. One of these tests was concerned with a 1949 Ford showing an idling vacuum level of 21.25 inches. Bearing in mind that the vacuum condition in an engine operating under load is always less than normal, and which is the natural consequence of the combustible mixture rushing from the carburetor through the open throttle into the intake manifold to displace the vacuum, the result of closing the throttle in an engine unequipped with compensating means is to effect an almost instant rise above the idling level. With the motor of this particular vehicle, and without the present device installed, the tests showed that a closing of the throttle at a vehicle speed of M. P. H. caused the vacuum to instantly rise to 23.75 inches, thereafter falling off somewhat as the R. P. M. of the engine dropped but not reaching the idling vacuum level until the vehicle had slowed down to 15 M. P. H. Very nearly the same degree of rise and fall, albeit sharper, was shown when the throttle was closed at 30 M. P. H., the peak vacuum in this instance being almost an identical figure. With the present device installed, the vacuum condition which followed a closing of the throttle at 50 M. P. H. was 22 inches, dropping to the idling level of 21.75 inches at 42 M. P. H. and thereafter progressively falling to 15 inches of vacuum at a speed of 15 M. P. H. Much the same result occurred upon a closing of the throttle at: a 30 M. P. H. speed, the drop in vacuum from an initial 22 inches being here somewhat more abrupt in that the vacuum level was reached at 28 M. P. H., the graphed drop as the car slowed to a speed of 15 M. P. H. showing rather a steeper declination than obtained where deacceleration occurred at 50 M. P. H.

A further laboratory test of considerable interest has been conducted by setting up machinery to duplicate the conditions of a prolonged coasting with an engine in gear, the engine being that of a Ford tractor. In this case the test engine (without the present device installed) was started, allowed to idle, and then overdriven by another engine at a R. P. M. of 1150, showing a vacuum of 23.5 inches. After 8 /2 hours of running and allowing suitable time for oil drip, oil consumption was carefully measured to note the effects of the excess vacuum. 0.68 pint of oil were consumed, the spark plugs were somewhat fouled, and collected in the trap at the end of the exhaust pipe was a considerable amount of mixed oil and gas. After installation of the present unit, and repeating the test under identical conditions, the vacuum level of the overdriven engine showed at 17 inches and the amount of oil consumed was of such an insignificant amount as to preclude measure. The spark plugs were in clean condition, and there was an entire absence of any oil deposit in the exhaust trap.

Rather exceptional results from the use of the device have been reported from substantially all users, indicative of which is a report of a truck operator making a check run of 2200 miles with the device installed on a White engine and comparing the same with previous cost figures of the same truck over the same route. The device showed an increase in gas mileage from 5 to 9 miles per gallon, and a reduction in the oil consumed from 17 to 4 quarts. The explanation for the saving in gas is that excess vacuum exerts a sucking influence on the idling jet and, particularly with a truck engine, ordinarily draws a rather large stream of gasoline into the intake manifold during periods of de-acceleration.

The device of the present invention is, to all intents and purposes, a safety relief valve, remaining closed during normal running of the engine and only opening when the throttle is closed and the developed excess of vacuum sucks the air-admission valve from its seat, whereupon suflicient air is permitted to enter the intake manifold to accomplish a reduction in vacuum to normal or lower. The air-admission valve is held open by pressure differential and closes when this differential is insufficient to overcome the tension of the spring. This closing is made to occur somewhat before normal idling speed is regained or the instant the throttle is again opened, both of which most usually reflect between 12 and 14 inches of vacuum. Once the present device is installed and properly adjusted in accordance with the condition of any particular engine it is thereafter substantially trouble-free and should require no further adjustment over long periods of usage.

It may be here pointed out that I have so engineered the device as to give the air-admission valve fair tolerance and enable the same to partake of what may be termed a floating movement within the cylinder. This perforce allows some air to traverse the outside surface of the valve. The spiral grooving provided in the perimeter suffices to clear the surface of any speck of dirt which might pass the filter screen and be drawn with this outer strata of air into the narrow interstice surrounding the valve. While material is of no particular import to the invention, the several parts of the device other than for the air-admission valve, which is of brass, are desirably made of aluminum, and it is a feature of the invention, important from the standpoint of making adjusting instructions easy tofollow, that each of the three jamnuts 33, 34 and 35 are suitably color-anodized to enable the same to be readily distinguished one from the other. The particular color applied is or may be a matter of preference, yellow, green and red, in the respective instance, being presently employed.

As more or less of a rsum generalizing on the foregoing, the device of the present invention automatically reduces excess vacuum during periods of de-acceleration occurring when changing gears, coming to a stop, going down hill, etc. The car, in coasting under load, is caused to be held back by compression rather than vacuum and hence eliminates the cause responsible for drawing oil past the rings and the valve guides, and the sucking of gasoline through the idling jet. This reflects itself in less carbon, grit, and sludge in the engine, and hence increases the engine life, lowers niaintainance costs, reduces the consumption of oil (upwards of 50%), saves an appreciable amount of previously wasted gasoline, and greately reduces smoke and fumes issuing from the exhaust pipe.

It is thought that the invention will have been clearly understood from the foregoing detailed description of my now preferred embodiment. Changes in the details of construction will suggest themselves and may be resorted to without departing from the spirit of the invention, wherefor it is my intention that no limitations be implied and that the hereto annexed claims be given the broadest interpretation to which the employed language fairly admits.

What I claim is:

1. A device for controlling excess vacuum in an internal combustion engine and comprising a casing bored from the rear end, having a centrally placed inlet port at the front end which is open to the atmosphere, and providing an outlet port in the side wall, the bore being internally threaded at the open rear end thereof and leaving a smooth-faced cylinder at the front end of the bore, means for mounting said casing, a hollow piston working in the cylinder between said inlet and outlet ports and formed with a reduced axially placed nose prolonged forwardly beyond the front Wall of the piston and arranged to normally seat against and close the inlet port, the piston having a central aperture in its rear wall and a severalty of circumferentially spaced apertures in its front wall, an internally threaded gland-like nut working in the internal threads of the bore, a spring engaged at one end by the nut and at the other end bearing against the piston to yieldingly urge the nose into its said portclosing position, a tubular piece threaded for endwise adjustment in the internal threads of the nut and extending forwardly therefrom with its front end fitting the front aperture of the piston and projecting therethrough into the hollow center of the piston, the endwise adjustment given to the tubular piece governing the limit of the pistons permitted rearward travel, said tubular piece being provided with two sets of axially separated wall openings one placed to communicate with the interior of the piston and the other placed to communicate with the interior of the cylinder at a point rearwardly removed beyond the rear travel limit of the piston, said tubular piece being internally threaded at its rear end, and a rod threaded for endwise adjustment in the internal threads of the tubular piece and extending forwardly therefrom to have the front end lap and cut off variable portions of the tubes rearwardly placed set of wall openings, the rear ends of the threadedly mounted nut, tube, and rod being each exposed for adjustment.

2. Structure according to claim 1 in which the exposed ends of each of the three threadedly mounted members, and namely the nut, the tubular piece, and the rod, are each fitted with a jam-nut, said jam-nuts being of anodized aluminum presenting distinguishing colors to facilitate adjustment.

3. Structure according to claim 1 in which the piston has a free-fitting engagement in the cylinder and provides in its perimeter a spirally developed groove of minor airfiow capacity.

4. A device for controlling excess vacuum in an internal combustion engine and comprising a cylinder member closed at the rear end, having a centrally placed airadmission port at the front end, and formed in the side wall with an outlet port, a hollow piston received in the cylinder for endwise sliding movement between said admission and outlet ports and formed with a reduced axially placed nose prolonged forwardly beyond the front wall of the piston and acting by forward movement of the piston to seat against and close said admission port, said piston having a central aperture in its rear wall and a plurality of circumferentially spaced apertures in the front wall, a spring bearing against the piston to yieldingly urge said nose into port-closing position, a tubular piece disposed axially within the cylinder with its front end fitting the rear aperture of the piston and projecting therethrough into the hollow center of the piston, said tubular piece having openings at the front end and in the side wall acting as respects the front opening to establish communication from the interior of the piston to the interior of the piece and as respects the side-Wall opening to establish communication from the interior of the piece to the outlet port, and a rod fitting closely within the rear end of the tubular piece and arranged for relative endwise adjustment to enable the front end of said rod to lap and cut off variable portions of said side-wall opening.

5. Structure according to claim 4 in which the piston is provided in its perimeter with a spirally developed groove of minor air-flow capacity.

6. A device for controlling excess vacuum in an internal combustion engine comprising a cylinder member having in the front wall a centrally placed air-admission port and having in the side Wall an outlet port whose flow capacity is at least equal to that of the admission port, a hollow piston movable in the cylinder between said ports, said piston providing front and rear walls with openings in each of said walls and having an axially placed nose prolonged forwardly beyond the front wall and acting by forward movement of the piston to seat upon and close said admission port, a spring yieldingly urging the piston into a position whereat the nose closes the admission port, said piston being retracted against the force of the spring by the influence of vacuum obtaining at the outlet port, and a tube fitting in said rear-wall opening of the piston and acting to conduct air from the interior of the piston into a rear part of the cylinder communicating with the outlet port and located rearwardly beyond the latter.

7. Structure according to claim 6 in which the flow capacity of the tube is adjustable within a given range having a ceiling limit no greater than the flow capacity of the admission port.

8. Structure according to claim 6 in which the tube is rigidly supported by the cylinder member with the front end fitting freely in the rear-wall opening of the piston and serving the added function of a stop limiting the retractive movement of the piston.

9. A device for controlling excess vacuum in an internal combustion engine and comprising, in combination: a cylinder member internally threaded at the rear end, providing an air-admission port at the front end, and having an outlet port in the side wall, a. valve mounted within the cylinder for movement between two positions one of which closes the admission opening and the other of which establishes communication between said admission and outlet openings, said valve being arranged to open under the influence of a vacuum condition obtaining at the outlet, a spring yieldingly urging the valve into port-closing position, and an assembly of three threadedly adjustable control elements threaded one within another and having the outer element of the three threaded in the internal threads of the cylinder member, one of said control elements bearing upon the end of the spring so as to tension the latter, a second of said control elements serving as a limit stop for the valve and hence governing the degree to which the valve may open, the third of said control elements governing the effective flow capacity or the communicating opening between the admission and the outlet openings.

10. The structure of claim 9 in which each of said control elements presents an end protruding for manual adjustment beyond the element in which the same is threaded, and having jam-nuts fitted on said protruding ends, said jam nuts being dilferently colored to facilitate adjustment.

References Cited in the file of this patent UNITED STATES PATENTS 1,120,118 Ashlock Dec. 8, 1914 1,146,931 Engel July 20, 1915 1,199,152 Bruce Sept. 26, 1916 1,228,157 Wyand May 29, 1917 1,302,844 Pepler May 6, 1919 1,303,817 Hugill May 13, 1919 1,421,135 Alton June 27, 1922 2,581,478 Gaddoni Jan. 8, 1952 2,588,493 De Stefano Mar. 11, 1952 2,601,563 Selwyn June 24, 1952 2,616,445 Gaddoni Nov. 4, 1952

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