US2192512A - Pump - Google Patents

Description

Mara, s; 1940.

F. E. TWISS Filed Jan. 7. 1937 2 Sheets-Sheet I Jrtentar, Franlfl Fwals Patented Mar. 5, 1940 PUMP Frank E. Twiss, Taunton,

Mass, assignor to F. E. Twiss 00., Inc., Boston, Mass., a corporation of Massachusetts Application January I, 1937, Serial No. 119,455 16 Claims. (01.. 103-42) This invention pertains to pumps, for instance pumps such as are designed to supply liquid fuel to the carburetor of an internal combustion engine. Such a pump, of the magnetically actuated, high speed vibratory type is illustrated in patent to Twiss No. 2,043,270, dated June 9, 1936. Certain improvements in the patented pump are disclosed in a copending application for Letters Patent Serial No. 68,649, filed May 13, 1936, said application disclosing, inter alia, a by-pass arrangement whereby the delivery of fuel to the carburetor is automatically stopped when the fuel level at the carburetor reaches a predetermined point, even though the pump continue to operate, thereby to prevent overflow of or excess fuel at the carburetor. The by-pass arrangement of the aforesaid copending application includes a spring loaded relief valve which is designed to respond to a fluid pressure at the discharge of the pump corresponding to the predetermined desired maximum level at the carburetor,-the valve gradually opening in response to rise in pressure thereby to permit the fuel from the delivery of the pump to by-pass around the pump mechanism. While such prior arrangement is operative and useful for the intended purpose, the ordinary spring-loaded valve tends to open and close gradually and lifts against a constantly increasing resistance. Moreover, when the difference in level between the carburetor and pump is small, such a simple spring-loaded valve can not readily be made sufiiciently delicate to respond to the very slight pressure differential available for moving it.

Objects of the present invention are to provide an improved by-pass arrangement for pumps of the class described, including a controlling valve which is normally substantially balanced and thus very sensitively responsive to pressure I differential but which will remain open without substantial vibration or chatter until normal pressure conditions are restored and will then close suddenly and completely.- A further object is to provide an improved by-pass arrangement of simple construction readily capable of appli- '"cation to pumps of the type referred to without I requiring any change in the pumping mechahisnr' 'per' se and which is durable and cheap to sq'i an- 'th' -'c'abect of the invention is' 'to proar'rangement capable of installatio'ri'attl'ieir'itake of the pump, if desired, so that, even if the by-pass valve should leak after a long period of use or for other reason, the

level at the carburetor will not be abnormally raised.

Other objects of the invention and advantages thereof will be pointed out in the following more detailed description and by reference'to the ac- 5 companying drawings, wherein:

Fig.' l is a fragmentary diagrammatic elevation illustrating a pump of the type referred to, associated with a fuel tank and carburetor;

Fig. 2 is an elevation, to large scale and partly 1 in vertical-section, of an improved piston valve useful in the improved by-pass arrangement of the present invention;

Fig. 3 is a view, to larger scale, of one end of the valve of Fig. 2; 1

Fig. 4 is a view, to large scale, of the opposite end of the valve of Fig. 2;

Fig. 5 is a fragmentary diametrical vertical section, to large scale, through a pump, such as that of Fig. 1, having the present improvements embodied therein, the by-pass arrangement being shown as it appears when the pump is operating inthe normal manner to deliver fuel to the carburetor;

Fig. 6 is a fragmentary section, similar to Fig.

' 5, showing the by-pass arrangement as it appears when the pressure at the fuel level at the carburetor has reached such a point that no further delivery of fuel is desired; j

Fig. 'l is a view of one end of a controlling valve of modified construction designed for installation at the inlet side of the pump;

Fig. 8 is a perspective view, partly broken away and in section, of the valve of Fig. '7;

Fig. 9 is a view of the opposite end of the valve t Fig. 10 is a fragmentary vertical section through a pump such as that of Fig. 1, but employing the valve of Figs. 7, 8 and 9,-the controlling valve being shown in the normal posi- 40 tion, that is to say, with the pump delivering to the carburetor; and

Fig. 11 is a view similar to Fig. 10 but showing the controlling valve in position to block further intake of fuel, thereby preventing delivery of fuel to the carburetor.

Referring to Figs. 1 to 6 of the drawings, the fuel supply tank is indicated at T, the pump at P and the carburetor at C,-the fuel flowing from the tank to the pump through the pipe I, and flowing from the pump to the carburetor through the pipe 2.

The pump P may be of the general type disclosed in the patent above referred to or inmy aforementioned copending apflkatiom-com- 56 with a priming valve I I,

Y which it communicates prising a casing 3 having therein a pump cylinder 4 in which a piston 5 is reciprocated very rapidly, and with a short stroke by magnetic means indicated generally at 6. The casing is provided with an inlet duct or passage I which communicates with the supply pipe I and which is always in communication with an inlet chamber 8 in the piston 5, the piston having a valve device 9 which icontrols the delivery of fluid from the inlet chamber 8. The casing has an outlet duct or passage I which is always in communication with an outlet chamber in the piston, said outlet chamber being furnished with a valve device I2 which automatically controls the flow of fluid from the pump cylinder into the outlet duct or passage Ill. The outlet duct or e I0 merges with a delivery oriflce I 2 in the wall of the casing, and this orifice is preferably provided such as disclosed in my' copending application Serial No. 119,456, flled on even date herewith, patented June 14, 1938, No. 2,120,448.

In accordance with the present invention, a vertical bore I5. within the thickness of the casing diametrically intersects the outlet orifice I3.- One end of this bore I communicates with a y-p ss passage I6 (which may be a cored opening in the casing which extends around to a point directly above the inlet passage I with by means of a'vertical bore- I5, which enters the inlet opening 'I at the Point n.

The bore I5 constitutes a cylindrical guideway for a piston valve I8 .(Figsrz, 3 and 4). This piston valve comprises a cylindrical by-pass closing head I 9 and a cylindricaLactuating head 20, united by a stemportion 2I of substantially smaller diameter than the heads. The heads are designed to slide in contact with the walls of the bore l5, and are preferably provided with elongate peripherally spaced guide flns 22 and 23, respectively. Preferably, the extreme lower or outer end 24 of the piston, as viewed in' Fig. 2,

is of a diameter approximating that of the stem 2|. The peripheral surface of the stem gradu- I ally increases in diameter, as shown at 25, until it merges with the full diameter of the head l9. Likewise the valve gradually increases. in diameter from the end 24, as shown at 26, until it attains the full diameter of the head 20. The stem portion 2| is preferably provided withlanelongate bore 21 leading from the end of the head I9, such bore being designed to receive a very light, elongate and open coiled spring 28, one end of which rests upon, the bottom of this, bore 21, and whose other end is positioned by embracing a pin 29 projecting from a disk II seated on an annular shoulder at the end of the bore I5. An axial boss 20', of lesser diameter than the head, projects from. the head 20 orthe piston valve, and its lower flat end" normally contacts the end of the bore I5. Preferably, there is a slight clearance, for exfimple, 0.005", between the head 20 and the wall of the bore, so that fluid at outlet pressure leaks into the space below the head; I In the normal operation of the pump, the

,valve II occupies the position indicatedin Fig. 5, its lower head 20 being disposed just within the lower part of the bore ,I5. In this position 'the pressure of the fluid-flowing through the out-.

glet passage I0 and the orifice It acts with sublstantially equal'force downwardly and upwardly against the inner or--adjacent surfaces of the heads-20 and II respectively. but the'upper end arcane oi the piston is exposed, bymeans oi the bypass passage I6, to the lesser pressure of the inlet I. That portion of the outer or lower end surfaceof the head 20 which is exposed within the lower end of the bore I5 receives outlet pressure by reason of leakage past the head 20 but the end of the boss 30-.is normally protected mally opposite and very nearly equal, though slightly less, than the force exerted by the spring,-the spring exerting a very'slight preponderance of force tending to hold the valve in by-pass closing position, as shown in Fig. 5.

However, if the fuel level in the carburetor reaches a predetermined abnormal point, with a corresponding abnormal pressure in the outlet oriflce I2, the differential pressure, available at the upper and lower ends of the piston valve I 8 causes the latter to rise slightly in opposition to the spring 28. As soon as the piston valve rises, even slightly, full fluid pressure acts'upwardly'upon the entire area of the lower end of the head 20, thus snapping the piston valve upwardly to the position shown in Fig. 6. Since the head 20 does not leave the bore I5 when the valve is fully open, and as there is a body of fluid trapped beneath it in the end of bore I5, the valve is.restrained against vibration or chatter in response to the rapidly reciprocating piston. When the valve isin this position, pressure fluid from the outlet .oriflce I2 is permitted to flow through the bore I5 and between the flns 22 of the piston valve, and thence through the by-pass passage I6 back to the inlet duct or passage I. Thus a continued operation of the pump merely circulates fluid through the delivery or outlet passage III, the bore I 5, and the by-passpassage' I6 to the inlet, without further delivery of fluid through the outlet orifice I3. However, when the pressure at the oriflce I3 drops slightly, corresponding to a dropping to normal level at the carburetor, the spring 25 reasserts itself,

pushing the piston I! back to its normal posilight so that it exerts but little pressure,-the

valve being very nearly balanced under normal conditions, and as this spring may be made quite long and with an open' coil, its resistance to upward movement of the valve need not increase abnormally as the valve rises. Furthermore, and more important, the lift of the valve, after its flrst initiation, takes place suddenly and with a-snap action so that the by-pass is opened to its full extent at once rather than by a gradual and'slow action, such as commonly takes place in prior constructions. The. cessation of fuel delivery to the carburetor is thus substantially instantaneous so that there is no danger of overflow at the carburetor due to lag of the controlling device.

Referringnow to Figs. 7 to 11 inclusive, wherein a modifled form of the device is illustrated, like numerals have been applied to parts corresponding to those of the preceding res. In this arrangement the by-pass passage I6 is connected to the outlet duct or passage III by means of a relatively small vertical bore l3, but, at the diametrically opposite side of the pump casing, a vertical, valve guiding bore 33 is arranged to intersect the inlet duct or passage I and to connect the by-pass passage 16 with the inlet.

' end and opening at its peripheral surface. Preferably the lower head 36 is furnished with a longitudinal slot 39 which receives a rib 40 projecting from the inner wall of the bore 33, the rib and recess 39 preventing the piston from turning and keeping the recess 38 always at the same side as the inner portion 1 of the inlet passage 1. Preferably, the lower head 36 is furnished with an upwardly directed bore ll which receives the upper end of a light, open-coiled spring 42', whose lower end rests upon the bottom of the bore 33 and which, like the spring 28, is designed to restore the valve to by-pass-closing position when the outlet pressure returns to normal. The upper head 35 of the piston is preferably furnished with an upwardly directed boss 35* of a diameter substantially less than that of the head,

.such boss normally engaging a plate 35 seated against a shoulder at the upper end of the bore 33.

Normally the piston valve 34 is held in the upper inoperative position shown in Fig. 10, by the spring 42 just as the valve [8 is normally held in closed position by spring 28. In this position the boss 35- engages the plate 35' so that the effective area ofthe piston which is exposed to the pressure fluid in the upper end of the bore 33 is that represented by the diiference between the areas of the piston head 35 and the boss 35 The pressure acting upon this differential area is that of the pressure fluid at the outlet duct Ill of the pump, such pressure fluid acting through the bore Hi and the by-pass passage l6. Normally, the force exerted by the spring 42 (as in the case slight preponderance of downward pressure normally acting upon the differential area of the piston head, as compared with the normal upward pressure acting on the lower head 35, but when the pressure at the outlet orifice rises to a predetermined slight extent, the piston is moved downwardly very operation of the pump merely circulates the fluid jfi-pass passage, while at the same time the inlet 1 through the pump chambers and around the bysurface which is normally of the spring 28 acting on valve I8) is just suflicient to overcome the.

slightly in opposition tov the action of the spring. The slightest movement of the piston downwardly removes the upper suris entirely cut of! from the pump by the interposition rangement, even though the piston head 35 should not flt tight or be completely leakproof in the bore 33, there is substantially no danger that continued operation of the pump will build up undue pressure in the delivery duct l0.

When the pressure drops at the delivery duct 10, the spring 42 immediately becomes effective to lift the piston valve, and as soon as the valve reaches the position of Fig. 10, it is substantially balanced by the pressures acting at the exposed surfaces of its heads, so that it remains safely in this inoperative position until the pressure at the outlet again rises to a definite point.

In the above description, for convenience, such terms p; ar p p; bottom, etc. have been employed, but without intent thereby to impose any limitation upon the position in which the pump is used, since it is of the piston head 35. With this arcapable of efiective operation in substantiallyany position.

While certain desirable embodiments of the invention have been shown by way of example, it is to be understood that the invention is not necessarily limited to these particular embodiments but is regarded asbroadly inclusive of any equivalent arrangements within the terms of the appended claims.

I claim:

1. A fuel pump of the vibratory type comprising a casing having therein a cylinder and a reciprocatory piston, the casing having an inlet duct leading to the cylinder and an outlet orifice leading therefrom and aby-pass passage leading around the cylinder from the outlet orifice to the inlet duct, and a sliding, pressure-actuated valve for controlling the inlet to said by-pass passage, means normally holding the valve in bypass-closing position, said valve comprising an actuating head having an area of its end surface which is not normally exposed to the outlet pressure, said surface being exposed to outlet pressure at the initiation of the opening movement of the valve, whereby pressure acting onsaid surface suddenly completes such movement, the actuating head also having an area of its end exposed to outlet pressure and which is so proportioned as to initiate opening movement of the valve when the outlet pressure exceeds a predetermined value.

2. A fuel pump of the vibratory type comprising a casing having therein a cylinder and a reciprocatory piston, the casing having an inlet duct leading to the cylinder, an outlet duct leading therefronuand a by-pass .passage leading around the cylinder from the outlet orifice to the inlet duct, the casing having a cylindrical bore closed at one end and diametrically intersecting the outlet duct, and a piston valve sliding in said bore, said valve includinga by-pass-closing head,

and a second head axially spaced therefrom, the heads being of substantially the same diameter, the by-pass-closing head having a tapering portion which merges with the stem at the inner end of the head, and the second head having a portion which tapers toward its outer end surface, the tapering portions of the two heads having peripherally spaced guide fins for contact with the inner wall of the bore, and a spring tending to move the valve to by-pass closing position, the inner ends of the heads being of substantially equal effective areas and being exposed to the fluid pressure in the outlet duct, and means for act upon a portion at least of the area of the second head, the outer end surface of the bypass-closing head being normally exposed to inlet pressure, the normally exposed areas of the two heads being so proportioned that at normal out-, let pressure the valve is nearly balanced, but when exposed to abnormally high outlet pressure moves toward by-pass opening position.

3. A fuel pump of the vibratory type comprising a casing having. therein a cylinder and a reclprocatory piston, the casing having an inlet duct leading to thecylinder, an outlet duct leading therefrom, and a by-pass passage leading around the cylinder from the outlet duct to the inlet duct, the casing having a cylindrical bore closed at one end and diametrically intersecting said outlet duct, and a piston valve sliding in said bore, said valve including a by-pass-closing head and a second head axially spaced therefrom, and a stem uniting said heads, the heads being of substantially the same diameter, the valve having therein an elongated axial socket extending inwardly from the outer end of one of the heads, and a light open coiled spring seated in the socket and having its outer end abutting one end wall of the valve-guiding bore, the inner ends of the heads being of the same effective area and being exposed to the fluid pressure in the outlet duct, means for conducting pressure fluid from the outlet duct to act upon a portion-at least of the area of the second head, the outer end surface of the by-pass-closing head being normally.

exposed to inlet pressure, the normally exposed areas of the outer end'surfaces of the two heads being so proportioned, that at normal outlet pressure the valve is nearly balanced, but when exposed to abnormally high, outlet pressure moves toward by-pass opening position,

4. A fuel pump of the vibratory type comprising a casing having therein a cylinder and a reclprocatory piston, the casing having an inlet duct leading to the cylinder, and an outlet duct leading therefrom, and aby-pass passage leading around the cylinder from the outlet to the inlet' duct, and a piston valve for controlling theflow of fluid through the by-pass passage, said valve being arranged to slide in a bore intersecting one of said ducts, said bore being substantially closed at one end, the valve including a by-pass-closing head having a substantial portion of its outer end surface normally exposed to the fluid pressure subsisting in the bypass passage, the valve also including a dashpot head which-is axially spaced from the first-named head and disposed within the closed end of the bore, the inner surfaces of the heads areas and exposed to the pressure of the fluid flowing through that duct which is intersected by the valve-receiving bore, the dashpot head sliding with a slight clearance in the closed end of the bore so that said closed end receives pres sure fluid by leakage past the dashpot head from the last-named duct, and means tendingto hold the valve in by-pass-closing position.

5. A fuel pump of the class described comprising a casing having. therein a cylinder and a reclprocatory piston, the casing having inlet and outlet ducts leading respectively to. and from the cylinder, and a by-pass passage leading around the cylinder from one of said ducts to the other, the casing having a cylinder bore closed at one end and transversely intersecting one of said ducts, the closed end of the bore normally communicating by means of arestricted passage with said latter duct, and a piston valve arranged to being of substantially equal efl'ective aioasis slide in said bore, spaced heads, one

pass passage, the other valve head being a dashpot head and disposed in the closed end of the bore, stantially equal effective areas and exposed to the pressure of the fluid flowing through that duct which is intersected by the valve-receiving bore, a boss projecting from one of said heads and having an end surface which normally closely contacts a flxed surface of the casing whereby said end surface of the boss is normally protected from the fluid pressure to which the remainder of the outer surface of said latter head is normally exposed, and a spring urging the valve toward by-pass-closing position, the parts being so designed and arranged that the algebraic sum'of the pressures on the normally exposed surfaces of the heads is slightly less than the force exerted by the spring and opposite thereto whereby the valve is nearly balanced, slight movement of the valve away from normal position separating the end surface of said boss from the surface with which it normally contacts, thereby increasing the efl'ective fluid pressure on the head from which the boss projects, thereby quickly completing the movement of the valve.

6. A fuel pump ofthe class described comprising a casing having therein a cylinder and a reclprocatory piston, the casing having inlet and outlet ducts leading respectively to and from the cylinder, and a by-pass passage leading around the cylinder from one of said ducts to the other, the casing having a cylindrical bore closed at one end and transversely intersecting one of said ducts, the closed end of the bore normally communicating by means of a restricted passage with said latter duct, and a piston valve arranged to slide in saidbore, said valve including axially spaced heads, one of which normally closes the by-pass passage and has a substantial portion at least of its outer surface normally exposed to the fluid pressure subsisting in the by-pass passage, the other valve head being a dashpot head and disposed in the closed end of the bore, the inner ends of the heads being of substantially equal effective areas and exposed to the pressure of the fluid flowing through that duct which is intersected by the valve-receivingbore, means normally protecting a portion of the outer end of ing the area of the second head which is exposed to-fluid pressure whereby'the valve is quickly moved to the limit of its path of travel.

'I. A fuel pump of the class described comprising a casing having therein a cylinder and a reciprqcatory piston, the casing having inlet and outlet ducts leading respectively to and from the cylinder, and a by-pass passage leading around the inner ends of the heads being of subthe cylinder from one of said ducts to the other,

the casing having a cylindrical bore closed at one end and transversely intersecting one of said ducts, and a piston valve arranged to slide in said of substantially equal effective areas and ex-.

posed to the pressure of the fluid flowing through that duct which is intersected by the valvereceiving bore, the-outer ends of the heads having areas respectively exposed to the fluid pressures which subsist in the by-pass passage and in the closed end of the bore, respectively, the closed end of the bore communicating by means of a leak passage with the last-named duct, said exposed areas of the outer ends of the heads being of such relative extents and so related to the pressures to which they are respectively exposed that the valve normally has a slight bias to move to by-pass-opening position, and means normally slightly overcoming such bias, the parts being so constructed and arranged that slight movement of the valve toward by-pass-opening position suddenly increases the area of one head which is exposed to pressure, thereby insuring quick movement of the valve to full open position.

8. A fuel pump of-the vibratory type comprising a casing having therein a cylinder and a reciprocatory piston, the casing having an inlet duct leading to the cylinder, an outlet duct leading therefrom, and a by-pass passage leading around the cylinder from the outlet orifice to the inlet passage, the casing having a cylindrical bore closed at one end and transversely intersecting the outlet duct, and a valve sliding in said bore, said valve including a by-pass-closing head and an actuating head, the heads-being axially spaced and of substantially the same diameter, the inner ends of the heads being of substantially equal effective areas and exposed to the fluidpressure subsisting in the outlet duct, the outer end of the by-pass-closing head being exposed to inlet pressure, the actuating head sliding with slight clearance in the closed end of the bore, a portion only of the outer end surface of said actuating head normally engaging the end. wall of the bore, the remainder of said end surface being normally exposed to outlet pressure within said closed end of the bore, the relative areas of said portions of said end surface, with respect to the normal difference in inlet and outlet pressures, being such that the valve has a slight bias toward opening movement, and means normally slightly overcoming said bias whereby the valve is nearly balanced, the parts being so constructed and arranged that when the valve begins to open in response to abnormal outlet pressure substantially the entire outer end surface of the actuating head is exposed to outlet pressure, thereby suddenly move the valve to full open position.

9. A fuel pump of the vibratory type comprising a casing having therein a cylinder and a reciprocatory piston, the casing having an-inlet duct leading to the cylinder,'and an outlet orifice leading therefrom, and a by-pass passage leading around the cylinder from the outlet orifice to the inlet duct, and a piston valve for controlling the flow of fluid through the by-pass passage, said valve including a by-pass-closing head, an actuating head, and'a stem uniting said heads, the inner end surfaces of the heads being normally exposed to outlet pressure, the outer end surface of the by-pass-closing head normally being exposed to inlet pressure, means normally protecting a portion at least of the outer end surface of the actuating head from outlet pressure, a spring normally operative to hold the ranged as to admit outlet pressure to act upon substantially the entire area of the outer end surface of the actuating head when the valve begins to move toward by-pass-opening position, thereby suddenly to complete such movement.

10. A fuel pump of the vibratory type comprising a casing having therein a cylinder and a reciprocatory piston, the casing having an inlet duct leading to the cylinder, an outlet orifice leading therefrom, and a by-pass passage leading around the cylinder from the outlet orifice to the inlet duct, and a piston valve for controlling the how of fluid through the by-pass passage, said valve including a by-pass-closing head, an actuating head, and means rigidly uniting the heads in axially spaced relation, the inner ends of the heads being ofsubstantially equal effective areas and exposed to the fluid pressure subsisting in the outlet duct, the entire outer end surface of the by-pass-closing head normally being exposed to inlet pressure, the outer end surface of the actuating head comprising a portion which is normally exposed to outlet pressure and a portion which is normally protected from outlet pressure,

the areas of said portions being so related, with I reference to the normal difference in inlet and outlet pressures, that the algebraic sum of the leading therefrom, and a by-pass passage leading around the cylinder from the outlet orifice to the inlet duct, and a valve for controlling the flow of fluid through the by-pass passage, said valve head, and means rigidly uniting the heads, the inner end surfaces of the heads being normally exposed to outlet pressure, the outer end surface of the by-pass-closing head normally being exposed to inlet pressure, the outer end surface of v the actuating head comprising a portion which is 40 including a by-pass-closing head, an actuating normally exposed to outlet'pressure and a portion which normally abuts a protecting surface, the areas of said portions being so related with reference to the normal difference in inlet and outlet pressure as to cause the valve to have a slightbias toward opening, and a light coiled spring normally operative slightly to overcome said bias thereby nearly to balance the valve, the parts being so designed and arranged that upon slight opening of the valve in response to abnormal outlet pressure the entire area of the outer end surface of the actuating head is exposed to outlet pressure thereby suddenly moving the valve 1 to full open position. i

12. A fuel pump of the class described comprising a casing having therein a cylinder and a by-pass passage, a stem uniting the heads, the

head which normally closes the by-pass passage having a recess extending from its outer end to a point in its peripheral surface, balancing means normally holding the valve nearly balanced in bypass-closing position, the outer end surface of the by-pass-closing head comprising an area which is normally exposed to by-pass pressure and whichis so proportioned that when by-pass pressure becomes excessive the valve will begin to open, said end surface also comprising a normally protected area so arrangedvythat upon initiation of the opening movement of the valve it is' exposed to by-pass pressure, ..whereby\ the valve is quickly moved untilthe recess in th by-pass-closing head is positioned to connect theby-pass passage to the inlet duct.

13. A fuel pump of the class described comprising a casing having therein a cylinder and a reciprocatory piston, the casing having inlet and outlet ducts leading respectively to and from the cylinder, and a by-pass passage leading around the cylinder from one of said ducts to the other, the casing having a cylindrical bore closed at one end at least and transversely intersecting the inlet passage, and a piston valve arranged to slide in said bore, said valve including axially spaced heads, one of which normally closes-the by-pass passage and comprises an area'normally exposed to bypass pressure, the head which normally closes the by-pass passage having a recess extending from its outer end to a'point in its peripheral surface, said head also having a boss projecting from its end,the boss having a surface which normally engages the end wall of the bore and is thereby protected from by-pass pressure, and a spring urging the valve in the directionof said latter head, the outer end surface of the other head normally being exposed to inlet pressin'e, the parts being so designed and. ar-' ranged that the algebraic sum of the effective pressures acting upon normally exposed surfaces of the heads is slightly less and opposite to the force exerted by the spring, whereby the valve is normally nearly balanced but held in by-passclosing position, slight movement of the valve away from by-pass-closingposition in response to abnormally high by-pass pressure, removing a the boss from the'end wall of the bore, thereby exposing an additional area of said head to fluid pressure and unbalancing the valve, whereby it moves quickly to the other end of its path of movement, in which position the valve head 50 closes the outer portion of the inlet duct while the lateral end of the recess in the valve head registers with the inner end of the inlet duct. 14. Afuel pump comprising a pumping chamber, an inlet duct and an outlet duct, and a by- Pass passage leading from the outlet duct to the inlet duct, and a nearly balanced pressureactuated valve movable in a valve chamber and arcane normally closing said passage, said valve and sure, a change in the differential between said" pressures being suiiicient to cause a slight movement of the valve toward its open position and the uncovering of the additional area and its exposure to outlet pressure, whereby the valve is suddenly moved to its fully opened position.

15. A; fuel pump of the vibratory type comprising a casing having therein a cylinder and a reciprocatory piston, the casing having an inlet duct leading to the cylinder, an outlet duct leading therefrom, and a by-pass passage leading around the cylinder from the outlet duct to the inlet duct, the casing having a cylindrical bore closed at one end and diametricallyintersectingthe outlet duct, and a piston valve sliding in said bore, said valve including. 8 .by-pass-closing head,"

an actuating head, and means uniting said heads,

the inner ends of the heads being oi substantially equal eifective areas and exposed to the fluid pressure subsisting inthe outlet duct, and means for conducting pressure fluid from the outlet duct to act upon a part at least of the area of the outer end of the actuating head, the outer surface of the other head being normally exposed to inlet pressure, the normally exposed areas of the outer surfaces of the two heads being so proportioned that at normal outlet pressure the valve is substantially balanced but, when exposed to abnormally high outlet pressure, moves toward .by-pass-opening position.

16. A fuel pump of the vibratory type comprising a casing having therein a cylinder and a reciprocatory piston, the casing having an inlet duct leading to-the cylinder andan outlet duct leading therefrom and a by-pass passage leading around the-cylinder. from the outlet duct to the inlet duct, and a pressure-actuated, passage- 2 closing valve at one end of said by-pass passage, means normally holding said valve in by-passclosing position, the valve comprising a part which is normally protected from the discharge pressure but which is exposed to discharge pressure by slight opening movement of the valve, thereby suddenly to complete the opening movement, the valve valso having a part which is always exposed to charge pressure and which is so proportioned that the valve responds to abnormally high e pressure thereby to initiate opening of the valve.

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