US2281045A

US2281045A - Pump

Info

Publication number: US2281045A
Application number: US125129A
Authority: US
Inventors: Outin Jean
Original assignee: Precision Mecanique SA
Current assignee: Precision Mecanique SA
Priority date: 1936-02-19
Filing date: 1937-02-10
Publication date: 1942-04-28
Anticipated expiration: 1959-04-28
Also published as: BE419922A; DE736380C; GB490547A; DE707046C; GB496967A; FR814889A; FR822725A; FR814513A

Description

'April 2s, y1942. J l OWN l2,281,045I

PUMP

'Filed Feb. 1o, 1937 4 sheets-sheet 1 fia/wey:

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Y mgm Patented pr. 28, 1942 PUWIP Jean Outin, Paris, France, assignor to Socit La Precision Mecanique, Paris (Seine), France, a

society of France Application February 10, 1937, Serial No. 125,129 In France February 19, 1936 (Cl. BB3-139) l made according to another embodiment of the 13 Claims.

The present invention relates to methods and devices for adjusting the feed characteristics of piston pumps, and especially fuel injection pumps for internal combustion or explosion engines.

The object of the present invention is to provide a method and device of the type above referred to which is better adapted to meet the requirements of practice than the other devices for the same purpose used up to the present time, especially with a view to simplifying the construction of the pump. i The essential feature of the present invention consists in providing a pump, with at least two pistons one of which is the driving piston whereas the other one receives its movement from the first one through a hydraulic transmission means, the pressure and volume characteristics of which are capable of varying with the speed of the driving piston, and the whole being such that the movement of the second piston, which conditions the law of discharge of the liquid by the pump, can be modified as a function of said characteristics.

i Another feature of the present invention consists, in said piston pumps, in conditioning the amount of liquid fed for eachdelivery stroke of the piston, in accordance with the speed characteristic of said piston during the preceding delivery stroke thereof.

A third feature of the present invention consists in interposing in the clearance space, that is to say the delivery part of the pump, a plunger element adapted to permit of taking off, from the volume that would be really delivered by the main piston, a certain volume corresponding to a vacuum produced by said plunger element when it comes back to the lower end of its travel.

Other features of the present invention will result from the following detailed description of some specific embodiments thereof.

Preferred embodiments of the present invention will be hereinafter described, with reference to the accompanying drawings, given merely by way of example, and in which:

Figs. l, 2, and 3 show in diagrammatic sectional view a governor device adapted to be fitted to an injection pump of an existing type, these three figures coresponding to three different embodiments of the invention, respectively;

Figs. 4, 5, and 6 inclusive show, in diagrammatic sectional view, three other respective embodiments of a fuel injection pump made according to the present invention;

Fig. 7 is a diagrammatic axial sectional view of a system analogous to that of Figs. and 6,

present invention;

Fig. 8 is a section on the line 8-8 of Fig. 7;

Figs. 9 and 10 are views, analogous to Figs. 7 and 8', showing a portion of a system made according to a modification;

Fig. 11 is an axial view of a system of the same kind made according to another embodiment;

Fig, l2 is a partial view of a modification of laid system, this modification being characterized by another feature of the present invention;

Fig. 13 shows, also in section along an axis, a pump made according to another embodiment of the present invention.

In the following description, it will be supposed that the invention is applied to the construction of a pump for the injection of fuel into an engine.

Existing pumps of this kind include a piston moving in e. cylinder and coacting with at least one inlet orifice provided in this cylinder. In a system of this kind, delivery takes place as soon as the piston has covered said orifice. The volume that is injected is then equal to that of the portion of the cylinder corresponding to the displacement of the piston during this discharge stroke. This delivery ceases as soon as the piston has produced a discharge by again uncovering said orice (or a special discharge orifice). During the backward movement of the piston, the same phenomena occur in the reverse direction, that is to say a vacuum is produced in the cylinder between the time when said orifice is covered and the time when it is uncovered; this uncovering taking place, the liquid then fills up the Whole of the vacuum produced. It will be understood that the volume to be filled, and therefore the volume that is injected, is always substantially the same, Whatever be the speed ofworking of the pump and therefore of the engine.

In pumps of this kind, the variation of the amount of fuel that is injected is generally obtained by providing the piston or the cylinder with inclined ridges, in such manner that the relative angular displacements imparted from the outside to one of these two elements (piston or` cylinder) permit of varying the delivery or the filling stroke.

The object of the present invention is to arrange a pump of this kind in Asuch manner that it can automatically adjust, as a function of the working speed (and preferably according to a law which can be modified at will in accordance with the load of the engine), the feed characteristics of the pump, and this otherwise than through the known means, acting from the out-' side on the piston or cylinder. I-Iowev'er, it should be well understood that these known means can be utilized in supplement.

This automatic regulation can for instance act either on the adjustment of the beginning of the injection, thus permitting to modify the lead of the injection as a function of the speed and eventually to reduce the feed of the liquid delivered during the delay of ignition, or on the total amount of liquid delivered during each stroke.

According to the essential feature of the present invention, the pump is provided, in addition with its main piston, with at least one auxiliary piston the movement of which is imparted thereto by the rst piston, through a hydraulic transmission the pressure and volume characteristics of which are capable of varying in accordance with the velocity of the main piston, the Whole being so devised that the movement of the secondV piston, which movement determines the deliveryl from the pump to the engine, can be modified as a function of said characteristics.

I will ,now` describe several arrangements relating more especially to the means for effecting the auto-regulation of the volume of fuel delivered during `the injection.

According to one of these arrangements, I make use of an auxiliary piston (or other equivalent element) which receives the pressure from the main piston through aliquid, in combination with means, such as a leak or a throttled part, for varying the pressure or volume characteristics of said liquid as a function of the speed, and I utilize the displacements of this auxiliary piston for controlling at least one discharge orifice which is thus uncovered more or less early according to the value of the speed. Preferably, and as it will be assumed in the following description, said liquid will be the fuel itself.

In Fig. 1, I have shown a system of this kind capable of being adapted to any pump. The auxiliary piston 2lb, iitted in a bore IiiZ, receives, at III! the fuel delivered by piston 2id, note Fig. l1, of said pump.

The fuel, atitl, can flow through a passage lilla, which forms a by-pass with respect to piston 2lb, and which permits of directly deliv ering the liquid to the outlet conduit 9. In this passage ldlc, there is provided the above mentioned throttled part 33, which is for instance controlled by a screw IBG. Finally, piston 2lb controls a discharge conduit It'I.

In this case, the eect of throttle 33 is to modify the relative positions of pistons Zia and 2lb. This result is obtained, when the fuel is discharged by 2id, owing to the fact that said liquid iiows in conduit IIiIa with a speed proportional to that of piston Zia, and therefore of the engine. The throttle portions produce, between theopposite sides of piston 2lb, a difference of pressure proportional to said speed, whenthe corresponding force is capable of overcoming the resistance of spring 29, pist-on Zib leaves stop H14 and comes to uncover the orice IM, which produces the discharge of a certain amount of fuel.

The rate of feedof said fuel through the conduit leading to the injector therefore decreases when the speed increases- Fig. 2 shows a modification which operates as the structure shown in Fig. l. Piston 2lb has a neck portion which forms an annular channel Ill. This channel communicates with the upper end of the piston through bore Ita in the piston 2lb. As piston 2lb rises and channel 08 comes opposite orifice IEI, liquid is discharged from the chamber containing spring 2o through bore Ia, channel I, and oriiice itil, thus achieving a change in the rate of flow of liquid through conduit 9 in response to changes in speed of a piston operating in channel ISI.

Fig. 3 shows another structure operating in a somewhat similar manner. As the bottom of piston 2lb uncovers orifice I Gl, a discharge through orifice iii? will take place from the chamber ft2. Communication between chamber m2v and the chamber containing spring 29 is given by passage 33 in the side of piston E ib.

In Fig. 4, the auto-regulating device has been shown as being made as a part of the pump itself, orifices Ill? and 2t communicating with a feed chamber. The device functions as that of Fig. 1, the only dierence being that fuel exhausted through orifice IZI'I is conducted directly to the feed chamber.

According to another embodiment of the invention, auto-regulation is effected in such manner that the amount of fuel that is delivered for each injection stroke of the piston is determined volumetrically by the speed character-Y istic of said piston during the preceding delivery stroke.

For this purpose, it is still advantageous to have a Xed or adjustable throttle part to coact with an auxiliary piston acting as a vrelay with reference to the main piston, the whole being, for instance, such that this relay piston, same as in the embodiments of Figs, l to Ll, is displaced only when said throttle part produces, under the effect of the speed increase and as a result of the drop of pressure through said throttle part, an overpressure against the auX- iliary piston sufiicient for raising said piston, during the delivery stroke, and which has, on the contrary, a tendancy, as soon as the delivery ceases, to come back toward its initial position, thus producing a vacuum which is to be nlled up during the next delivery stroke, before the active injection stroke can start.

In Figs. 5 and 6, I have shown, by way of example, a system according to the invention.

Piston Zia. is fitted with distributing surfaces such that said surfaces 29?; and Zli coact with an inlet orifice 2t provided in cylinder Zil, a channel 208 connecting said surfaces ft2 and 2&3 together.

The delivery in this cylinder 255 toward conduit 9, leading to the injector (or other distribution member), past valve 20'1, takes place through a throttled passage 33, such as above mentioned, that is to say provided in a conduit Ima constituting a by-pass with respect to a relay piston 2lb capable of moving in a cylinderv 209 which is for instance in line with cylinder Zii, and of the same diameter or of a different diameter. This piston 2Ib is preferably subjected to the action of a spring 29, possibly adjustable and which tends to bring back piston 2 Ib toward piston 2 Ia.

This throttle passage 33, which is preferably adjustable, is for instance materialized, as shown by Figs. 5 and 6, by a needle valve It.

Although many other arrangements of such an apparatus are possible, as it will `be hereinafter explained, it is possible now to explain the working of such a system:

When the pump is working at low speeds, the fuel, during the delivery, passes through throttled passagef33 and, if the latter is suitably calculated,'account being taken of the force `exerted by spring 29, the whole can be such that piston 2lb remains in its lower position. The whole `of the fuel passes through conduit lilla, and the Volume that is injected for each piston stroke remains constant.

.Due to an acceleration of the engine, the pressure through passage 33 decreases, and consequently, the` pressure on the front side of piston 2lb, that is to say the pressure acting on the end of said piston that is located opposite cylinder 2'05, increases together with the speed. It is possible to calculate the whole in such manner that, from a certain speed, piston 2lb is lifted.

This piston will be lifted to a degree which varies `in accordance with the speed of revolution of the pump, that is to say the displacement of piston 2lb will be greater as the engine is Working at a higher speed.

But, at the end of each injection, piston 2lb is no longer acted upon by anything else than its spring` 29, since discharge is then taking place in cylinder 265. Therefore, said piston suddenly comes back to its initial position and creates behind itself, that is to say in cylinder 205, a partial vacuum the importance of which depends upon the position reached by the piston 2 Ib at the end of the injection. During the period of time between two successive injections, that is to say during the suction stroke of the piston, this partial vacuum can be filled up only through throttle part 33,

so that this vacuum will be practically main- 1 tained until a new delivery of fuel to cylinder 205 takes place.

It follows that, at the time of the following injection, the fuel discharged by piston 2m must first ll up the vacuum in question, whereby the volume of uid injected through conduit 9 will be accordingly reduced.

Finally, the volume injected into the engine will be equal to the difference between, on` the one hand, the volume displaced by the main piston 2 la between the beginning of the delivery and the discharge, and, on the other hand, the volume absorbed by the vacuum created above piston 2 lb.

This last mentioned volume will be chiefly a function of the displacements of piston 2 lb at the time of` the preceding injection, and it will also depend on the time of opening of the inlet ori iceZS.

Any injection is thus determined by the speed of the piston during the preceding injection and it is clear that (in the vicinity of a given adjustment) any acceleration produces a reduction of the injection, whereas, on the contrary, a deceleration produces an increase of the injection, which conditions are those for regulating the running of an engine.

. The principle having now been stated, it is now possible to examine many possible variations:

In particular, concerning the throttled passage 33, it can be made in Various manners, it being well understood that it is advantageous (in the preceding arrangement as well as in those of 1 to 4, all of which include a throttled passage or a leak) to make this throttle passage `33 easily adjustable. As a matter of fact, since, owing to said throttled passage, I obtain an auto-regulation for a given working speed, it will be easy, by modifying the s1ze of this throttled passage, to obtain all possible load conditions, the variation of section of such a throttled passage being, for instance, obtained by meansof the usual throttle pedal.

For instance, this throttled passage can be provided at the inlet of the conduit such as lola and controlled by piston Zia, which is provided, for this purpose, for instance, with a lateral passage 80 .coacting with the inlet of said conduit (Figs. 7 and 8). In a modification, piston 21a, is provided with an annular groove 2|3 in communication with cylinder 255 through a channel 2|3 (Figs. 9 and 10) the wholepermitting to vary the section or the useful length of the throttled passage through a rotation imparted to `piston 2id. In still another modification, the throttled passage is provided in piston 2lb itself, for instance in the form of at least one channel provided in said piston, or at least one external longitudinal groove, such as 33 in Fig. 3. Another embodiment of such an arrangement is described further on with reference to Fig. 11. Of course, still other arrangementsfare possible.

It is also possible, in order to increase the sensitiveness of the system, to cause the conduit such as Illia to open at El along the path of piston 2lb (Figs. 4, '7 and 8). During the action of the latter, I thus increase the drop of pressure, and therefore the action upon said piston.

As shown in Figs. '7 and 8 the piston Zli is movable to close off entirely the conduit mia,

so that at the end of the injection stroke the piston returns and produces a vacuum which is lled by fuel available for the next injection.

It should be noted that, for the various kinds of pumps that have been mentioned, it is not necessary that the throttled passage that acts during the injection should be the same as that which acts at the time of the filling, the latter being even possibly dispensed with.

As a matter of fact, it may be advantageous, in some cases, especially with a View to increasing the sensitiveness of the apparatus, to provide distinct calibrated orices and to arrange the parts in such manner that distribution is ensured by the driving piston 21a, or by the relay piston 2lb, or again by both simultaneously, the latter arrangement being for instance carried out as illustrated in Fig. 13.

According to this embodiment, I provide four calibrated conduits E, E', E and E" (fixed, adjustable, or automatic, such as Valves) opening, at different levels, into the pump body.

I provide in piston lla a conduit 203 which connects the upper iace of said piston with the annular space provided under the latter.

I provide in piston 2lb a conduit 2l5 connecting the upper face of this piston with the conduit E for the upper position of said piston.

Such a device Works in the following manner:

For example, the arrangements `of Figs. l to 4, with a discharge channel I El?, can be combined with the arrangements of Figs. 5 to 13, as shown at E01 in Fig- 13.

Whatever be the embodiment that is chosen, it is possible to devise a pump of the self-regulating type, that is to say which permits an automatic reduction of the rate of feed as a function of the speed, which ensures a regulation of theA speed of revolution of the engine.

`Such a pump can also be utilized, in particular, for the following purposes:

a. ,For imposing an upper limit to thespeed;

Furthermore, it shouldbe well understood that the rinvention can be applied to existing pumps.

` In this case, I will provide adapting -devices which,

being fitted to said pumps, permit them to benefit the advantages of the invention.

`Such adapting devices will be particularly useful `for constituting automatic overload relays, in combination with engines to which it has been found that it is useful to supply a lar-ge feed of fuel at low starting speeds, and a smaller feed at other speeds; Y

An adapting device of this kind would for instance consist of a cylinder block 209 (Fig. 1) which would be fitted upon the body Zl of a normal pump, and'more especially above cylinder 205. In this cylinder 209, is mounted a piston .2 Ib provided with grooves 2H and further fitted, if necessary, with a projection ZIB which does not fit in a uidtight manner in cylinder 209, the whole being subjected to the action of a spring 29.

In such an apparatus, the play existing around piston 2H] (and possibly grooves 2H) performs the same function as throttled passage 33. At

low working speeds, the leakage that occurs around Isaid part 28 is suicient for feeding the engine, without piston 2 ib.

On the contrary, as soon as the engine has reached its idling speed or any higher speed, piston Z ib assumes .the whole possible amplitude of its movement and thus eliminates a volume of fuel substantially equal to the volume of part 2 i B, this volumebeing equal to the desired overload.

Suchl an apparatus therefore eliminates the overload from a certain speed. But it can also be devised in such manner that it eliminates nearly the whole of the injection above a speed which may, for instance, be the upper limit speed of the engine. Above this speed the engine is but little fed with fuel and it cannot race.

Of course, instead of having such an apparatus acting between two speeds very close to each other, it Imay also be devised in such manner as gradually to eliminate `the overload, for instance between the idling speed and the upper limit speed, or between any two speeds, which permits, in particular, of correcting defective feed curves of or obtaining any desired curve.

Finally, the invention also permits of ensuring the gradual placing of the pump under pressure and it would extend to any analogous arrangement permitting to obtain the same result, and, especially, to :the arrangement in which part 2l8 is tted in a iiuidtight manner in its cylinder, as shown by Fig. l2, the .parts being arranged in such manner that the lower face of 2&8 is fully cleared.

YWith such an arrangemenhwhatever be the speed 4characteristic of piston Zia, injection can start only when said lower `face of 2 i8 is cleared, and the -device will deliver into conduit 9 only the difference between the volume delivered by piston 2 la and the volume absorbed by the vacuums that remain in the apparatus, as a consequence of part 2i8 to its initial position, when the main body is being filled. As a consequence the pressure will be established only when piston 21a has moved a certain distance beyond orice 2d, which involves a better fluidtightness K and a reduction of .the wear of the distribution surfaces of the pump. i

Another application of the invention consists in making use of the above arrangements for dosing very small rates of feed. This is delicate, while it is much less difdcuit in the case of more important rates of feed. An apparatus according to the present invention, since it permits of eliminating a iixed amount, can therefore be utilized in combination with a pump of average capacity so as to obtain, by taking off said fixed amount, the desired very low delivery. l

In each application of the invention, many variable quantities are available for eiiecting the desired adjustment, to wit, in particular:

The section of piston Zia and of piston 2lb;

The importance of throttled passages such as 33, especially of the part 2 i3;

The strength and iiexibility of spring 29;

The height and shape of part N8, when such a part is employed;

The shape of grooves 2 H, etc.

Furthermore, by tting a same pump body with a plurality of these adapting devices arranged to correspond to different respective functions, this pump body might be caused to benefit by all the advantages of these adapting devices. But it is also possible to combine in a single adapting device one or several functions by suitably devising therein the various adjustment means that are available.

Furthermore, it should be well understood that the usual adjustment means (by rotation of piston Zia and of oblique working surfaces such as 22@ in Fig. 5) can also be employed.

In a likewise manner, the above arrangements, owing to which the control organ of the pump no longer controls directly .the rate of feed thereof but the speed of the engine that is fed therethrough, might be applied to multi-cylinder pumps in which it might be advantageous to eliminate a certain number of cylinders successively and automatically, either close to the upper limit speed for eliminating any danger of racing of the engin-e, or at idling speed, so as to stabilize said idling working, by making use of a certain number of cylinders as brakes or as pumps (intended to feed the brakes of a vehicle for instance), the cylinders that are not eliminated therefore necessitating injections which are more important or better adapted to the -system of injection.

The control member determining the desired speed conditions would then act upon devices analogous to those above described, which devices would then be devised in such manner as to give, according to the cylinders that are considered, suitable respective delivery curves as aiunction of the speed.

For this purpose, for instance, it would suflice, for the diierent pumps, to act in a suitable manner upon the throttled passages such as 33, or E, E', E, E", etc., forv instance in such manner. that, at idling speed, it is iirst possible to eliminate the feed to a certain number of cylinders and to leave it only for the remaining cylinder or cylinders.

Of course, the specific embodiments above described are not the only possible ones. For instance the relay pistons might be replaced by an elastic membrane or any other element capable of permitting, especially in arrangements such as those described by Figs. 5 to 13, of modifying the volume of the clearance space. AlsoV the pump control system might be constituted by two members acting in a different manner on equiva` lent elements.

In a general manner, while I have, in the above description, disclosed what I deem to be practical and efficient embodiments of the present invention, it should be Well understood that I do not wish to be limited thereto as there might be changes made in the arrangement, disposition and form of the parts without departing from the principle of the present invention as comprehended within the scope of the appended claims.

What I claim is: 1. `A pump comprising a cylinder, an intake port in a wall of said cylinder for supplying uid to said cylinder from a source of fluid supply, an exhaust port remote from said intake port in a wall of said'cylinder, first piston means in said cylinder for forcing fluid from said intake port to said exhaust port and being movable to cover and uncover successively said intake port, a second piston movably mounted and spaced from said rst piston in said cylinder between said intake port and said exhaust port and movable by fluid displaced by said iirst piston, conduit means communicating with said cylinder adjacent said intake port at one end of said second piston and adjacent said exhaust port at an opposite end of said second piston for by-passing said second piston, means for limiting the movement of said second piston toward said first piston, and a second exhaust port in a wall of said cylinder adjacent said second piston and corncylinder when uncovered by movement of said second piston.

2. A pump as in claim l further comprising means for throttling said conduit means.

3. A pump as in claim l further comprising adjustable valve means for throttling said conduit means.

4- A pump comprising two communicating cylinder elements, a driving piston in one of said cylinder elements, a second piston movable in the other cylinder element, said cylinder elements being adapted to hold uid both between said pistons and on the side of said second piston that is farther from said first piston, an intake port in the wall of the cylinder element corresponding to said driving piston and controlled thereby a delivery conduit communicating with said other cylinder element on said side of the second piston, a delivery Valve between said other cylinder element and said conduit, an abutment for limiting the displacement of said second piston toward said first piston7 elastic means yieldingly urging said second piston against said abutment, a bypass passage permanently interconnecting the portion of said cylinder elements located between said pistons with the delivery side of said second mentioned piston, and means for throttling said passage.

5. A pump as in claim 4, further comprising means for adjusting said throttling means.

6. A pump comprising two communicating i cylinder elements, a mechanically operated driving piston in one of said cylinder elements, a piston movable in the other cylinder element, an abutment limiting the displacement of said second piston toward said first mentioned piston, elastic means urging said second piston toward said first mentioned piston, a discharge orice normally covered by said second piston in said second named cylinder element adapted to be opened when said second piston moves away from saidV abutment, said cylinder elements being adapted to hold fluid both between said pistons and on the side of said second piston that is farther from said rst piston, a delivery conduit connected with said second named cylinderelement on said last mentioned side of said second piston, a by-pass passage permanently interconnecting the portion of said cylinder elements located between said pistons with the delivery side of the second named cylinder element, and means for throttling said passage.

7. A pump according to claim 6, said throttling means being adjustable to vary the section ,of said passage.

8. An adapting device for use with a pump hav-` ing a first cylinder and a reciprocating driving piston for pumping iiuid therein comprising a hollow structure having a second cylinder adapted to be fitted on the delivery end of said rst cylinder, piston means injsaid second cylinder movablev in response to uid pressure from said driving piston, and means for controlling linear movement of said second piston in response to changes in the iiuid delivery rate produced by velocity variations of said driving piston.

9. A pump structure comprising two communieating cylinder elements, a mechanically operated driving piston movable in a first cylinder element, said first cylinder element being provided with a liquid inlet port, means on said piston cooperating with said inlet port to control liquid now to said cylinder element, a second piston movable in a second cylinder element, an abutment limiting the displacement of said second piston toward said first piston, 'elastic means urging said second piston toward said abutment, a liquid delivery conduit communicating with said second cylinder element, the end portion of said second cylinder element adjacent said delivery conduit being enlarged, a projection on said second piston forming a plunger movable in said secend cylinder element and mounted for movement into said enlarged portion of said second cylinder, and said second piston having a passageway connecting the portions of said cylinder elements located between said pistons with the delivery side of said second cylinder element between said second piston and the projection thereof.

10. A fluid delivery pump comprising a cylinder having a fluid intake conduit and a fluid exhaust conduit, rst means for intermittently forcing iiuid from said inlet conduit through said cylinder to said exhaust conduit, means mounted within said cylinder for displacement in either direction between said Iii-st means and said exhaust conduit in response to nuid pressure from said rst means, a delivery Valve in said exhaust conduit, and means intermediate said first means and said valve for forming a differential in pressure on opposite sides of said intermediate means, whereby the degree of movement of said mounted means varies with the changes in the liuid delivery rate produced by velocity variations of said iirst means.

1l. A fluid delivery pump as in claim 10 further comprising second exhaust means for releasing uid from said cylinder in response to a predetermined degree of movement of said mounted means.

12. A pump comprising two communicating cylinder elements, adriving piston in one of said cylinder elements, a second piston movable in the other cylinder element, said cylinder elements being adapted to hold iiuid both between the pistons and on the side of said second piston that is farther from said irst piston, an intake port in the wall of the cylinder element corresponding to said driving piston and controlled thereby, a delivery conduit communicating with said other cylinder element on said side of the second piston, a delivery valve between said other cylinder element and said conduit, an abutment for limiting the displacement of said second piston toward said rst piston, elastic means yieldingly urging said second piston against said abutment, and a throttle passage permanently interconnecting the portion of said cylinder elements located between said pistons with the delivery side'of said second piston.

13. A pump comprising two communicating cylinder elements a drivingl piston. in one of said cylinder elements, a second piston movable in the other cylinder element, said cylinder elements being] adapted to hold iiuid both between the t' pistons and on. the side of said secondpiston that is farther from said rst piston, an intake port in the wall of the cylinder element corresponding to said driving piston and' controlledy thereby, a delivery conduit communicating with said other cylinder element on said side of the second piston, a delivery valve between said other cylinder element and said conduit, an abutment for limiting the displacement of said second piston toward said first piston, elastic means yieldingly urging said second piston against said abut-