US2978986A - Free piston engine - Google Patents

Description

April 1l, 1961 F. B. CARDER ET AL FREE PISTON ENGINE 7 Sheets-Sheet 1 Filed Sept. 28. 1956 l QN Q .om

F. B. cARDl-:R ET AL 2,978,986

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April 1l, 1961 F. B. CARDER ET AL 2,978,986

FREE PIsToN ENGINE 7 Sheets-Sheet 6 Filed Sept. 28, 1956 IN V EN TORS'.

uw @E .Apl11,1961 F.B.cARDER ETAL FREE PIsToN ENGINE 7 sheets-shet 7 Filed Sept. 28. 1956 United States FREE PISTON ENGINE Filed Sept. 28, 1956, Ser. No. 612,740

22 Claims. (Cl. 103--27) This invention relates to internal combustion power plants, and more particularly to a free piston internal combustion engine provided with a hydraulic-pneumatic operating system, and wherein the operation of the free piston internal combustion engine is directly controlled by the operation of the hydraulic-pneumatic system.

In accordance with the invention, there is provided a free piston internal combustion engine wherein the engine, or power, pistons are provided with auxiliary pumping pistons, for pumping hydraulic fluid cyclically into the circuits. On the power stroke of the power pistons, the pump pistons pump the working ui'd into a high pressure accumulator in which the Work developed during the power stroke, producing the outboard movement arent of all the pistons, is absorbed, in part, by the compression of the gas in the pneumatic portion of the accumulator and, in part, by the external, useful output work produced by a hydraulic motor connected to the hydraulic half of the high pressure accumulator. On the inboard, or inward, movement, or compression stroke, of the engine pistons, the power required for compressing air in the engine cylinder is furnished by the expansion of the compressed gases in the pneumatic accumulator, and as the result thereof, the engine pistons are moved towards each other and the `air therebetween is compressed, ready for the next cycle.

This application is a continuation-in-part application of the earlier parent application Serial #445,706 tiled July 26, 1954, now abandoned, entitled Free Piston Engine.

The power plant embodying the invention comprises generally three basic components, namely, an internal combustion free piston engine directly actuating a hydraulic pump, a hydraulic-pneumatic high pressure accumulator to receive and store the energy from the engine in a form of fluid under pressure, and a liquid-actuated, energy consuming system comprising one or more motors, turbines, piston actuators, or the like, which utilize the energy owing from the pump and the high pressure accumulator, liquid under pressure acting through the pump pistons to return the engine pistons on their compression strokes.

In general, in the preferred forms of the invention, there is provided a diesel two-stroke cycle internal combustion engine. This engine comprises two opposed free pistons movable in a common cylinder and having a common combustion chamber. Seavenging air is admitted into the cylinder when the pistons reach an extended, or an outward, position and uncover the intake and exhaust portswhereby scavenging air is admitted to assist in the removal of the exhaust gases through the uncovered exhaust gas ports.

The engine pistons are provided with hydraulic pump pistons formed integrally thereon in tandem, or the equivalent, which convert the engines mechanical energy into potential energy stored in the high pressure accumulator n ICC able hydraulic fluid through a check valve into the accumulator which produces compression of gas in the accumulator, in the illustrated embodiment of the invention the accumulator being a suitable type of hydraulicpneumatic accumulator in which liquid is separated from gas by means of a ilexible diaphragm. The accumulator then discharges its stored potential energy through the expansion of gases and expulsion of the hydraulic fluid to any suitable power consuming system, such as a hydraulic motor, at a rate determined by the external load requirements.

The engine, or the power, pistons are also provided with return or bounce pistons which may be attached in tandem with Ithe engine pistons. The bounce pistons may also utilize the rear surface of the engine pistons. The bounce pistons are usedl to return the engine pistons to their inward position and thus produce a compression stroke. This is accomplished by utilizing some of the potential energy stored within the accumulator'system by means of a suitable pressure control valve which also serves to stop and start the engine in response to changes of pressure in the accumulator system. Dynamic balance and synchronization of the engine pistons may be maintained by means of suitable conventional connecting linkage, interconnecting the two free pistons.

It is an object of the invention to provide a novel free piston engine,` a control system for such engine, and a hydraulic-pneumatic accumulator associated therewith, in which a minimum number of moving parts and bearings are required, as compared with a conventional internal combustion engine employing connecting rods and a crank shaft.

Another object of the invention is to provide a power plant in which combustion energy is converted into hydraulic energy for delivery to any type and number of external hydraulically actuated devices with a simplified control mechanism which avoids the need for a separate hydraulic pump and the customary complex interconnections between the power pistons in the engine and the pump plungers in the hydraulic pump.

It is a further object of the invention to provide a novel power plant embodying a free piston engine and a hydraulic-pneumatic accumulator in which the functioning of the engine is controlled by a valve arranged to start the engine when the pressure of the working uid is below a predetermined minimum pressure, to maintain it in operation until a predetermined maximum pressure has been attained and to stop it when that maximum pressure is reached, whereby the engine operates always at a high load factor with consequent high efliciency.

With these and other objects not specilically mentioned herein in view, the invention consists in certain improvernents which will be apparent from the following description ofthe specific embodiments illustrated in the accompanying drawings and from the claims.

In the drawings:

Figure l is the schematic diagram of a preferred form of the invention;

Figure 2 is a side view, partly in section, of control valve;

Figure 3 is a side view, partly in section, of a modified form of the control valve;

Figure 4 is the schematic diagram, similar to Figure l, illustrating an engine having a separate accumulator for producing piston bounce, and using the control valve shown in Figure 3;

Figure 5 is the schematic diagram of the engine in which one set of the bounce and pumping is performed by hydraulic pistons and the return of the power pistons is effected by the pressure of a low pressure accumulator;

Figure 6 is a longitudinal sectional view through the power piston and its hydraulic attachments illustrating dit agrammatically'a modication of the piston and pump arrangement;

Figure 7 is a transverse sectional view on line 7 7, Figure 6;

Figure .8 is a sectional view similar to Figure 6 of another alternative modification ofthe piston and Vpump arrangement;

AFigure 9 isa ,transverse sectional view on line 9 9, Figure 8.

Figure l is a side view, partly 'in section, of a modilied form of the control valve suitable for application to engines of high power. l Referring Vto the drawings, and more particularly to Fig. l, the free piston engine is'provided with a cylinder 2i)` in which are located for movement to and from each other power pistons 22, `each ofzwhich is provided with a return, or bounce, piston 36 and a pump piston 32, shown in tandem arrangement and forming an integral part of piston 22. As shown in Figs. 6 to 9 inclusive, the bounce piston and the pump piston yportions may be telescoped within the skirts of pistons 22. The bounce pistons 30 move the power pistons k22 through a compression stroke, and the pump pistons 32 pump hydraulic fluid during the power stroke. Y

Cylinder 26 is providedwith conventional intake ports 34 and exhaust ports 36, and a suitable conventional type of diesel fuel injector 38. When pistons 22 in cylinder 20 uncover exhaust ports 36, exhaust gases ow out through exhaust ports as compressed air enters ports 34. The cylinder heads 23 have vents as indicated at 25 to obtain free travel of the power pistons.

O n the power stroke of pistons 22, hydraulic tluid is forced by pump pistons 32 through conduits SS, check valve 66 and conduit 59 into the high pressure accumulator 50. Hydraulic fluid is also forced by the -bounce pistons 30 through conduits 64, control valve 66 and conduit 68 into either accumulator 50, motor 78, or both of them, the amount of. iluid supplied to accumulator 50, depending on the load carried by motor 78. The larger the load, the larger percentage of fluid iiows directly to motor 78 rather than accumulator 50.

Scavenging of cylinder 26 and introduction of freshV air into the cylinder is accomplished by means of a dynamic compressor 40 which is driven by any suitable hydraulic motor 44 actuated by hydraulic fluid conveyed through a pipe 52 connected to the high pressure accumulator 50. Fluid at reduced pressure flows from motor 44 through a conduit 54 into a low pressure accumulator 56. Alternatively, scavenging pistons of known types may be substituted for blower 4t). Accumulators Si) and 56 may be of the diaphragm type, shown in Fig. 1, or of any other known type.

Check valve 6) permits flow of fluid from conduit 53 to conduits 59 and 68 as long as the fluid pressure is higher in conduit 5S than in conduit 59. A manually operated shut-olf valve 72 is connected in conduit 68 which is used to disconnect the high pressure accumulator 50 from the entire system when there is a long shutoif period. High pressure iiuid thus can ybe retained in accumulator Si) while the pressure in the remaining portion of the system drops because of unavoidable leakage. Since long shut-off periods are quite common, it is also customary to relieve the entire system, with the exception of the accumulators, of the entire pressure. This is accomplished by providing an additional shut-off valve for the low pressure accumulator (not shown) and providing an additional valve and connections for venting the remaining part of the system to an atmospheric sump, such as sump 5 or 5'. Such Venting also prevents undesired further motion of motor 78 and pistons 32 while pressure in the system would be dropping to ambient pressure. The venting systems of the above type are well-known in the art, do not constitute a part of this invention, and, therefore, need no further description. Conduit 88 connects conduit 59 with a valve 66 which is connected to the bounce pistons 30 through conduits 64, and conduit 58 is connected to conduit 74 and the low pressure accumulator 56 through a check valve 76. The arrow shows the permitted direction of flow through the check valves. Check valve 76 permits ow of fluid yfrom low pressure accumulator 56 to cylinders 28 through conduits 58 when the pump pistons 32 travel inwardly, i.e. when pistons 32 have their intake strokes, and pressure in duct 58 is lower than in accumulator 56. Check valve 76 is of a pressure shut-off type, welll-known in the art, in which the valve may be held on its seat, flow in rboth directions being thus blocked, by pressure applied through a piston connected to duct 87 leading from control valve 66.

The liquid-operated motor 78 maybe of variable or constant displacement type, or may lbe replaced by any hydraulic power consuming system comprising any number of motors, turbines, piston actuators or the like. Hydraulically operated motors and turbines are also well known in theart and need no detailed description or illustration. Motor 78 discharges the working uid into the low pressure accumulator 56.

Referring now to Fig. 2, it illustrates the construction and connections of the control valve 66 on an enlarged scale. Valve 66 contains a composite piston and poppet valve comprising a poppet head Si), a stem 113, a spool valve 110, a stem 112, apiston 114 and a push rod 130. Poppet head tis adapted to openl and close a pilot connection from duct 88 to duct 64 through pilot passages 89and 120. Piston 114 is in communication at all times on its upper surface with duct 88 through a pilot passage 121; piston 114 is also subjected to an upward force produced by a spring 82' which is mounted between a washer 168 and the enlarged end 131 of the push-rod 130. The force exerted by spring 82 .may be manually adjusted by a wing nut 16. and an adjusting screw 162 which is threaded through a xed support 104. A lock nut 103 secures screw 102' against undesired turning.

As indicated in Fig. 2, piston 114 connects the pilot passage 124 with the high pressure pilot passage 121 when piston 114 is in its lower position. When piston 114 is in its upper position, the pilot passage, or duct, 124 becomes disconnected from the high pressure pilot passage, or duct, 121 and duct 88. Piston 114 is in its upper position within its cylinder 125 only when Athe upward force exerted by spring v82 overcomes ,the downward force exerted by the pressure of the working fluid on the face, or head, 132 of piston 114 and the upper face, or the upper surface, of the poppet valve 80. The upper position of piston 114 also corresponds to the open position of the spool valve 116 which results in the connection of duct S8 to duct 64. With Ithe valve 66 thus in open position, the diesel engine is started because of the pressure exerted by the working iluid on the bounce pistons. Opening of valve 66 must also produce an immediate release of the check valve 76 from its closed position, so that this check valve, upon its release, can respond freely to the differential pressure which may exist between duct 74 and duct 5S; when pressure in duct 74 is higher than in duct S8, valve 76 permits the free ow of the hydraulic fluid from the hydraulic chamber of the low pressure accumulator S6 to the pumping pistons 32 during the suction stroke of these pistons. Such release of valve 76 by valve 66, upon the opening of valve 66, is accomplished by venting ducts 87 and 124 through a duct 96 which is connected to an atmospheric sump 5 or 5. This at once releases the hydraulic pressure exerted on that piston (not shown) which keeps Valve 76 closed as long as valve 66 is closed as will be mentioned more in detail later. Such locking ofthe check valve 76 is necessary for preventing the fluid from the low pressure acoumulator 56, which is under pressure of several atmospheres, from producing a partial compression stroke or moving of the composite pistons toward the center of cylinder 20 during long shutdown periods. The hydraulic chamber in the low pressure accumulator 56 must have a positive pressure of the order of several atmospheres so as to produce proper, quick iilling of the pumping cylinders 28 when pistons 32 perform their suction stroke. This positive pressure obviously should be kept as low as possible, and still avoid cavitation, since it is subtracted from the pressure in the high pressure accumulator 50 and thus reduces the useful hydraulic head available across the hydraulic motor 7 8.

Valve 66 functions to open and connect duct 88 to duct 64 whenever the pressure in duct 88 falls below the predetermined minimum value p2 established bythe adjustment of spring 82. Valve 66 then closes again whenever the pressure reaches `a predetermined maximum value p1. This maximum value p1 is `also dependent on the force exerted by spring 82. However, the diierence between the maximum and the minimum pressures p1 and p2, i.e; (p1-p2) is independent of the spring tension and is established, or determined, by the effective areas of the poppet head 80 and piston 114. The effective area A1 of piston 114 is area 132 which is equal to the total area of the piston head minus the area of stem 112. This is due to the fact that high pressure from duct 88 is exerted only on one side of poppet head 80 when valve 66 is closed (this area will be designated as A2) but on Aboth sides when it is open. Neglecting the minor section area of stem 113 (the cross-sectional area of vstem 113 will be designated as A4; therefore g area of poppet SO-l-area of piston 114 p2 area of piston 114 1 areaofpoppetSO, =1+g pgarea of piston 114 p2 A1 Thus, for example, if the area of piston 114 is four times the area of poppet 80, the pressure ratio is 1%. As a further example, then, if the minimum pressure p2 established by the spring adjustment is 3,000 pounds per square inch, the maximum pressure p1, i.e. that which closes the valve, is 3,000 l%=3,750=p1 pounds per square inch. To prevent blocking of spool valve 110 by air or liquid trapped in spaces 119 and 121, vents 123 and 125 are provided which may be open to the atmosphere or externally interconnected.

The following equations may be Written in View of the above discussion:

Valve 66 opens when Izrl-PzA zS 1 (l) where S1 is the force exented by spring 82.

Valve 66 closes when land from S(1) and (2), (assuming (l) `and (2) are equal to S1 if the frictional forces are neglected) 6 and the difference p1- pz are the functions of the geometry of the pilot valve -113-114-112 and are independent of S1.`

In order to maintain hydraulic pressure in chamber 12 during long shutotlc periods, the disclosed system includes a positive displacement pump 17 driven by a motor 18 connected to `a source of current 19 through a pressure-sensitive switch 1. Switch 1 is connected to chamber 12 through a pipe 2 which closes switch 1 when pressure in chamber 12 falls below a predetermined value. Pump 17 is connected to chamber 12 through a pipe 3 and a check valve 4; it is `also connected to an atmospheric sump 5 through a pipe 6. When pressure in chamber 12 is maintained in the above manner during long shut-off periods, the entire engine is started by merely opening the shut-off valve 72. Fluid under high pressure travels through the ducts 68, 59 and 88, valve 66 and duct 64 to the bounce pistons 30 and bounce chambers 7 in cylinders 31 making all the pistons to travel inwardly, thus executing the compression stroke. It should be noted here that the shut-oil valve 53 is also opened at this time :for starting hydraulic motor or turbine 44 and compressor 40.

The op-eration of the engine illustrated in Figures l and 2 is as follows:

1t will be assumed here that the pressure in chamber 12 has been originally created by means of pump 17 which is operated from the external source of power 19. Chamber 12 is filled with the working fluid from an atmospheric sump 5 through pipes 6 and 3, line 3 being provided with a check valve 4 which allows the fluid to How only in one direction from pump 17 to chamber 12. Chamber 12 is also provided with an outgoing pipe or line 129 and a manually operated valve 130 which connects chamber l12 to the atmospheric sump 5. Valve 130 is opened at the time chamber 12 is being initially filled with the Working hydraulic huid. Line 129 and valve 130 permit the escape of the trapped air within chamber 12 and the remaining portion of the system. Similar pressurizing system is provided for pressurizing the low pressure accumulator 56. This portion of the pressurizing system has the same numerals as the pressurizing system for accumulator 50, but the numerals are primed for ditferentiating the two systems. Pipe lines are also provided (not illustrated) for the entire disclosed hydraulic system for initially removing any trapped air from the system. Since this feature of the power plant is known to the prior art and does not constitute a part of the invention no further description of the air esc-ape system is needed here.

After the hydraulic fluid reaches a predetermined pressure within chamber `12, switch 1 is actuated through pipe 2` and motor -18 is disconnected from source 19 with the result that pump 17 is stopped. It will be assumed here at this time that the remaining portion of the hydraulic system also has been lled properly with the hydraulic fluid without any trapped air and that accumulator S6 was also lled and pressurized to the desired pressure. The power plant at this stage may be started by merely opening the normally closed valve 72 which at once admits Ithe hydraulic fluid under high pressure from chamber 12 to the ducts 68, S9, 88, S9 and 121 (see Fig. 2 for ducts S9 and 121).

When the pressure in accumulator 50 falls below the minimum pressure established by the adjustment of spring 82, valve 66 opens. Pressure from accumulator 51B is then transmitted through ducts 68, 59, 88, cylinder 119 and duct 64 to the heads of the bounce pistons 30. The power pistons 22 are thus urged inward, closing ott exhaust port 36 and inlet port 34 and compressing the air in cylinder 20. As pistons 22 approach each other, fuel is injected at 3S and is ignited by the heat of compression. As the pistons 22 move inward, they carry with them pump plungers 32 which thus allow pump cylinders 28 to be filled with fluid from low pressure accumulator 56 owing through duct 74, check valve 76 and ducts 58,

The pressure of the gases in the combustion space between piston 22 brings them to rest and then, after ignition and burning of fuel, hot gases force them outwardly. The outward 'motion reverses the ow'from the bounce pistons 30, forcing fluid back through valve 66 into high pressure accumulator 56. Simultaneously, pump plungers 32 force the fluid from cylinders 28 through duc-ts 58, check valve 69, yducts 59 and 63 and into accumulator S0. It power is being used by motor 78, a portion or all of the uid from cylinders 2S and 31 may go directly to motor 7S instead of accumulator 56, the apportionment of the ow depending on the demand of the motor in relation to the supply of the pump elements. Fluid passing through motor 7S continues into low pressure accumulator 56 where it is ready to repeat its ow circuit.

As pistons 22 approach the outer ends of their stroke, exhaust port 36 and intake port 34 are uncovered, allowing air from blower 46 to scavenge the exhaust gases and to replace them with fresh air. The combined pressures on pistons 36 and 32 bring the pistons 22 to rest in their initial position, whereupon the cycle described above is repeated.

If the pressure in accumulator 56 rises suiiiciently during a power stroke to shut off valve 66 before pistons 22 have reached their extreme outer position, the piston inertia raises the pressure in cylinders 31 above that in accumulator 56. This pressure differential acting on poppet head S6 holds spool valve 111i` open until the bounce pistons 3i) come to rest. Thus assurance is provided that valve` 66 will close only when pistons 22 have completed a power stroke.

If provisions were not made to guard against it, after the stopping of the engine by the closing of valve 66, pistons 22 would creep inward and thus prevent development of the necessary compression for a` new start when valve 66 reopens. This action is due to the fact that, although bounce pistons 39 are shut off from pressure by valve 66, pump pistons 32 are not cut off from the low pressure accumulator' 56 in which the uid is at a pressure high enough to overcome the restraint of pistons 30 and thus cause pistons 22 to move inward.

To avoid the `above described diiiiculty, check valve 76 may be of the pressure shut-otrc type, as described above, with duct S7 connecting it to control valve 66. When valve 66 closes, piston 114 opens duct S7 to the -high pressure, thus actuating the shut-off provision in valve 76. When valve 66 is open, duct 67 and passage, or port, 124 are vented through a drain, or low pressure, duct 96 under the relieved edge of piston 114, valve 76 being thus unblocked. in this manner, the creepage of the pistons is prevented.

The arrangement shown'in Figs. 3 and 4 employs a separate bounce pressure accumulator 152 to actuate the bounce pistons 36, the ilow from accumulator 152 being simply one of reciprocal interchange of fluid between the accumulator 152 and the cylinders 31 with no intermingling with the fluid circulating through cylinders 28, accumulator 50, motor 7 'il and accumula-tor 56. The valve arrangement of Fig. 2 requires some modiiication, as shown in Fig. 3, when applied to the system illustrated in Fig. 4.

The valve 166 diiers from valve 66 essentially in the omission of poppet head titl and passages S9 and 120, there being substituted therefor the mechanical detent arrangement shown in Fig. 3. For this purpose, the Valve stem 313 extends through the end of the casing of valve 166 and is notched at 326 and 327. Arranged for engagement with notches '326 and 327 are rollers 328 rotatably supported in forked lugs 336 of levers 332, pivoted at 334 on the'valve casing. The outer ends of levers 332 are urged together by tension spring 336, the tension of which may be adjusted and secured by means of screw 338, wing .nut 340 and lock nut 342. Duct 18S interconnects accumulator 152 and valve 166 while the other side of valve 166 connects with duct 64, as in` the arrangement of Fig. l. A by-pass line made up of ducts 167 and checkV valve 16S connects duct 64 with duct 169 and the bounce accumulator 152 to allow the direct flow of uid from cylinders 31 through ducts 64, 167 and check valve 168 into accumulator 152; flow in the reverse direction is prevented by check valve 168. ln Fig. 4, the pump cylinders 28 are connected to the high pressure accumulator 50 through the ducts 58 and 59. Duct 137 connects check valve 76 to valve 166 for the same reasons as in Fig. l. Also, pilot line 321 connects accumulator 5t) with cylinder 315 and the head of piston 314 which corresponds to piston 114 of Fig. 2. Duct 324 and duct 187 cooperate with piston 314 to control check valve 76, as do the corresponding parts in-Figs. l and 2. Cylinder 319 is provided with vents 323 and 32S to insure free travel of the spool valve 310.

The operation of the modification of the invention illustrated in Figs. 3 and 4 is as follows:

Accumulator 152 acts on pistons 36 precisely as a spring, and a mechanical spring could have been substituted for it if it were not for dimensional inconvenience. Continuing the mechanical analogy, valve 166 acts as a detent holding the hypothetical spring in compressed position when pressure in accumulator Sil has acted to move it into a blocking position.

The action of the pump plungers 32 is unchanged and is substantially that of a conventional hydraulic plunger pump. As to the operation of valve 166, ywhen pressure in accumulator 56 drops below the minimum predetermined value p2, the force of spring 162 which may be designated as a force S1 overcomes a force F1 produced by the pressure on piston 314 (F1=p2A1 in this case, where A1 is the eective area of the piston) and the holding force S2 of rollers 328 in their engagement with notch 327, and causes the valve assembly, comprising piston 311-4, stem 312, spool valve 310 and stem 313, to rise until notch 326 engages rollers 328, at which time communication from accumulator 152 to duct 64 is opened by spool valve 310. The pistons 30 are, therefore, forced inward to compress the air into the combustion space. The engine 10 then continues to run in the manner previously described untilpressure in accumulator 50 reaches the pre-v determined maximum value p1. Pressure on piston 314 then overcomes the combined force of spring 182 and roller 328, causing piston 314 to jump to its lowest position, which closes valve 166. Closing of valve 166 stops the engine.

By-passV duct 167 and check valve 16S serve to allow pistons 22 to complete their power stroke when valve 166 closes while they are still moving outward.

The arrangement of Figs. 3 and 4 has certain advantages over that of Figs. l and 2. Thus, detent spring 336with its adjusting provision, Wing nut 340, permits adjustment of the range of pressure, or the difference between pl and p2, i.e. [n1-p2, in addition to the adjustment of mean pressure,

Pri-P2 2 by wing nut 306, while in valve 66 of Fig. 2 the range of pressure (py-p2), is iixed -by the proportions of poppet head and piston 114.

Using the same terminology as in Equations l through 4, and designating the force exerted by spring 336 as being equal to S2, one obtains:

For closing valve 66:

9 Accordingly, by eliminating spring 336, S2 becomes equal to zero and p1=p2, while when spring 336 is tightened, -the difference between p1 and p2, i.e. 1l-p2, becomes larger.

Equations and 6 also indicate that p1 and p2 are equally affected by the adjustment of spring 182, p1 and p2 both increasing equal amounts when spring 182 is tightened, and vice-versa.

The use of the separate accumulator 152 for bounce makes it independent of the pressure liuctuation in accumulator Si), which is advantageous for some applications.

The Valve arrangement of Fig. 3 may be substituted for that of Fig. 2 in the form of the invention illustrated in Fig. l by simply replacing poppet head 80 and its connections with a mechanical detent similar to that of Fig. 3.

In the modification of the invention illustrated in Fig. 5 the power pistons S22., power cylinder 523, exhaust ports 536, inlet ports 534, scavenging blower 540, fuel injector 538, motor 544 and hydraulic lines 552 and 554 function in `the same manner as the corresponding parts in Fig. 1. The pump plungers 532, however, are attached directly to, or formed integral with, power pistons 522 the intermediate bounce pistons, numbered 30 in Fig. l, being eliminated in Fig. 5.

Pistons 532, sliding in cylinders 52S, alternately discharge uid into and receive fluid from duct 564, which is connected through duct 559 and check valve 565 to high pressure accumulator 550 and the input side of motor 578 and to low pressure accumulator 556. check valve 576 and duct 574 tothe discharge side of motor 578 and to low pressure accumulator 556. Manually operated shut-off valve 572 serves to retain pressure in accumulator SSil during extended periods of rest.

Regulator valve 566 is similar in construction to valve 366 illustrated in Fig. 3, described previously. In Fig. 5, valve 566 is connected to duct 574, which leads to the low pressure accumulator S56 and the low pressure side of motor 57S, instead of the high pressure accumulator and the motor inlet, as is the case for the arrangements illustrated in Figs. 1, 2 and 3 with respect to valves 66 and 366. Also, the necessity for blocking check valve 576 does not exist in this arrangement because, when valve 566 is closed, all communication to piston 532 is broken. Valve 566 is therefore modified from the arrangement of Fig. 3 by the omission of passages 324 Iand duct 187.

The modification illustrated in Fig. 5 operates as follows:

Starting with the position shown in Fig. 5, iiow from accumulator 550 through motor 578 into low pressure accumulator 556 reduces the pressure in accumulator 550 yand pilot line 521, thus allowing spring 582 to move spool valve 519 to its upper position with valve 566 opened for communication from accumulator 556 to pump cylinders 523. The areas of plungers 532 are considerably larger than the step areas of pistons 30 of Fig. l so that a Ibounce impulse is imparted to pistons 522 by the lower pressure of accumulator 556 as compared to that delivered in the anrangement of Fig. 1 by the high pressure of accumulator 50 to pistons 22. Pistons 522 are thus impelled inwardly, receive the combustion impulse from the fuel injection at 533, and are thereby impelled outwardly with increased energy. As the direction of motion reverses, check valve 576 closes, thus blocking return of uid through valve 566 to accumulator 556, while at the same time check valve 5611` opens and permits the fluid discharged from cylinders 528 to enter accumulator 550 and motor-578.

At the outer end of the stroke, check valve 560 closes and check valve 576 opens, the cycle described above being then repeated.

If the energy absorbed by motor 578 is less thanV that supplied by engine 510, the balance is 4absorbed by accumulator 550 in the form of raised pressure. When pressure in accumulator 550 reaches the maximum value for which valve 566 is adjusted, spring 582 is overpowered and spool valve 510 is moved down to the closed position shown, whereby the bounce pressure is removed from plungers 532 and the action of the engine arrested until pressure in accumulator S50 again falls to the minimum value for which valve 566 is adjusted. Since pressure in accumulator 550 can rise only during a power stroke the shutting of valve 566 can occur only while plungers 532 are moving outward. .lf the closing should occur before the end of the stroke, the pistons 522 will continue to move outwardly and will complete the power stroke, discharging fluid from cylinders 528 into accumulator 550 through check valve 560 in the normal manner. Consequently no by-pass connection for valve 566 is required7 as is the case with valve 66 and 366, to insure completion of the power stroke after shut-off.

The pressure in accumulator 556 must be maintained at a value sufficient to provide the required bounce pressure. This m-ay be assured by control of the amount of hydraulic fluid in the closed system in the manner described in lconnection with pumps 17 and 17 in Fig. l.

The details of several auxiliary systems and devices which are known in the art and are customary in the free piston engine and hydraulic arts have been omitted from the drawings `and descriptions for simplifying them. These include: means for synchronizing the pistons, piston seals, lubricating devices, cylinder cooling provisions, alternate scavenging syste-ms, supercharging provisions, fuel injection devices and regulators therefor, means for returning the pistons to their outer positions if they have gotten out of place while the engine is stopped, means for the initial pressure charging of the accumulators means for eliminating air from the hydraulic system, hydraulic packings, fluid coolers, provisions for collecting leaked fluid and restoring it to the system, and so forth. Such systems require no disclosure here because they are known to those skilled in the art :and do not constitute a part of this invention.

Fig. 5, when compared with Figs. 1 and 4, has the advantage of greater simplicity, with consequent savings in weight and space.

Figs. l, 4 and 5 contain simplified arrangements of the piston systems. However, when so arranged, the overall length of the engine and pump assembly becomes unduly great. In practice this difliculty may be overcome by locating the hydraulic elements within the piston skirts as in the two alternative modifications illustrated in Figs. 6 to 9 inclusive. In Figs. 6 and 7 the pump and bounce plungers are concentrically arranged while in Figs. S and 9 they `are disposed transversely of the interior of the piston skirt.

Piston 606 in cylinder 607 is shown at the end of the power stroke. A tube 604 is mounted centrally within the skirt of piston 606. A tube 611, secured to cylinder cover 612i, fits slidably within tube 604and tube 610 also secured to, or formed integrally with cover 612, fits slidably over tube 604. Tube 611 acts as the pump piston, fluid being forced on the power stroke through conical passage `605, the interior of tube 611 and duct 653 to a high pressure accumulator, such as 50 in Figs. 1 and 4.

Similarly, tube 604 acts as the bounce piston, in place of step piston 30 in Figs. 1 and 4, communicating through passages 602, 601 and duct 664 with the source of bounce pressure, such as accumulator 50 in Fig. 1 and accumulator 152 in Fig. 4. When the disposition of Figs. 6 and 7 is used with thearrangement of Fig. 5, the tube 610 and its connections are eliminated, tube 611 acting to provide both bouncing and pumping functions.

In the alternative arrangement of Figs. 8 and 9, there are three tubes 808, 810 and 81:1 attached to cylinder cover 814, the central tube 810 acting as bounce piston cated by the arrows.

ber 803 where it circulates on contact with the cylinder head, leaving through passage 807 in tube 811, as indi- This modification is adapted for use in the arrangements of Figs. 1 and 4 but requires a change in the duct connections shown there. Thus, refe-rring to Figs. 1 and 4, duct 58 is replaced by two ducts in parallel, one connecting ducts S50 with duct Si? .and the other ducts 857 with duct 74. Augmented provisions for cooling the hydraulic uid are then also necessary. Alternatively, cylinders 808 and 311 may act in parallel with fluid owing in and out of both, passages 857 and 058 being joined `and connected to ducts 50 of Figs. 1 and 4 without alteration. The cooling elect is then still ctfective due to the swirl of the entering and leaving iluid and the heat distribution is more uniform.

In engines of high power in which relatively large amounts of fluid are to be handled, the main valve, such as the spool valve 110 in Fig. 2, 310 in Fig. 3 and 510 in Fig. 5, may become so heavy as to make direct actuation inadvisable. In such cases, the invention contemplates the use of a small pilot valve which directs high pressure tluid to a piston arranged to actuate a large main v alve, multiplication of actuating force being thus attained.

An arrangement of control valve of the above described type is illustrated in Fig. 10. The space 1019 above piston 1014 is at all times connected through ducts 1022 and 1088 to the high pressure accumulator line, 59 in Figs. l and 4 and 559 in Fig. 5. Poppet head 1010 is attached through stern 1013 to piston 1014, the space 1011 above poppet 1010 being connected to the high pressure accumulator 50 through ducts 1.08.0 and 59. Flange 1018 on piston 1014 serves to limit its upward travel. Piston 1014 is urged upwards by spring 1082 which is retained by washer 1006 on adjusting screw 1002 threaded through supporting bracket 1004. The force exerted by spring 1002 may be adjusted by means of wing nut 100, whose setting is secured by lock nut 1005.

A oating piston 1084 is slidable in cylinder 1023 and normally engages the end of stem 1085 of main poppet V-alve 1086, which is guided by stem 1000 sliding in ixed guide 1009, the travel ibeing limited by Contact of the shoulder on stem 1090 against guide 1039. Spring 1030 is arranged to urge valve 1086 toward its seat. The poppet valve 1086 connects duct 1014 to duct 1064 through passage 1091 and cylinder '1023.

Chamber 1035 is connected by ducts 1020 and 1021 to the lower side of floating piston 1084 and also through duct 1021 and valve port 1092 to duct 1096. Connection through valve port 1092 takes place when poppet valve 1010 closes. Duct 1096 is connected at all times to the low pressure accumulator 56 through duct 74. Main duct 1014 is connected to the high pressure accumulator 50 through duct 'SQ in the modification of Figs. 1 and 4, but to the low pressure duct 574 through check valve 576 in the modification of Fig. 5. Main duct 1064 leads in all three modifications to the bounce lines 64 and 564, respectively.

While the pressure in duct 1008, acting on the top of puppet 1010 and piston 1014, exceeds the force of spring 1082, established by adjusting sci-ew 1002, puppet 1010 is held in the closed position, shown in Fig. l0, and duct 1021 is open to the low pressure lines through port 1092 and duct 1096. There being only low pressure under piston 1004, spring 1003 holds it in its low position, as shown, together with poppet 1086 and stem 1005. Passage 1091 is, therefore, closed by poppet valve 1006, the bounce action being thereby inhibited and the engine stopped.

When pressure in duct 1088 falls below the value. which can hold piston 1014 and poppet 1010 down against the force of spring 1082, the pilot valve assembly rises, closing port 1092 and opening communication between the chambers 1011 and 1035. High pressure fluid is thus admitted under piston 1084 which `thereupon rises carrying poppet valve 1086 with it, bounce communication from duct 1014 to duct 1064 being thereby established and the engine started.

It is readily seen that the pressure limits of valve 1066 are subject to the same considerations as those described for valve 66 of Fig. 2. The valve arrangement ofrFig. l0 Ialso allows the completion of a power stroke before stopping the engine, since, if the pilot Valve closes before. the end of a power stroke, valve 1086 opens, as a check valve, and stays open until the pistons 22 have come to rest; it then closes.

When it is to 'be used in the arrangements of Figs. 1 and 4, valve 1066 requires provision for blocking check valve 76. This is provided by passage 1024, duct 1037 and passage 1025 in piston 1014, which expose the blocking piston of valve 1076, or 376, Fig. 3, to high pressure when valve v1066 is closed, and vent it to low pressure line 1096 when valve 1066 is open. When the valve arrangement of Fig. l0 is applied to the system of Fig. 5, the check valve blocking provisions are omitted, as explained previously.

In all modications of the invention illustrated and described, when the scavenging arrangement exemplified by blower 40 and motor 44 of Fig. 1 is used, provision may be included `for stopping motor 44 when valve 66 closes. For this purpose,v the motor shut-oli valve 53 may be of the pressure actuated type, well known in the a1-t, connected to duct 87, (the connection being omitted from the drawings to avoid complexity). Alternatively, any of the valve arrangements known in the art may be arranged for actuation Jby the motion of any of the valve parts in the control valve 66, 266, 566 or 1066.

What is claimed as new is:

1. A free piston engine comprising a irst accumulator means, engine cylinder means; a pair or opposed engine piston means reciprocally mounted in said engine cylinder means including two opposed power pistons and two opposed pumping and bounce piston means; said pumping and bounce piston means maintaining said rst accumulator means lled with .a working hydraulic uid under pressure when said engine is in operation; a second low pressure accumulator connected to said pumping and bounce means and normally filled with said working uid; and control means for starting and stopping said engine in response to the pressure of said fluid in said iirst accumulator means, said control means being hydraulically connected to: said first accumulator means, said second low pressure accumulator and to said pumping and bounce piston means; said control means including a pilot piston hydraulically connected to said lirst accumulator means; a spring connected toi one end of said pilot piston to oppose the pressure exerted on said pilot piston by said fluid, said pilot piston being operated by and being responsive to pressure of the working fluid contained in said rst accumulator means, said pressure acting on one side, or head, of said pilot piston in the direction opposite to the direction of the pressure exerted on the other end of said pilot piston by said spring, and a valve closed and opened with the aid of said pilot piston and the fluid from said first accumulator means; said valve being hydraulically connected in the circuit of said pumping and bounce piston means, said valve becoming closed when pressure of the rfluid on said pilot piston reaches a maximum pressure p1, and becoming open when said last recited pressure drops to a minimum pressure p2.

2. The free piston engine as dened in claim l in which said control means also includes means for -adjusting the compression of said spring for simultaneously and equally adjusting the values of p1 and p2.

3. The free piston engine as defined in claim 1 in which said control means also includes means for increasing or decreasing the magnitude of the difference between p1 and p2.

4. The free piston engine as defined in claim 1 in which said control means includes first means for simultaneously and equally varying in the same direction the magnitudes of p1 and p2, and second means, independent of the first means, for increasing or decreasing the difference between pl and p2.

5. The free piston engine as defined in claim 1 in which said control means includes means for simultaneously increasing p1 and decreasing by an equal amount p2 for varying the magnitude of the dierence between p1 and p2 While maintaining p14-p2 constant.

6. The free piston engine as defined in claim 1 which includes a direct mechanical connection between said pilot piston and said valve, said valve being a spool valve, a cylinder for said spool Valve, said last-recited cylinder being hydraulically connected to said accumulator means on one side and to said pumping and bounce piston means on the other side; a poppet valve, a push-rod connected to said poppet Valve and to said spool valve, a duct hydraulically connecting the head side of said poppet valve to said accumulator means, and a second ducthydraulically connecting the push-rod side of the poppet valve to said pumping and bounce piston means.

7. A free piston engine including two opposed piston means, each piston means including a power piston, a bounce piston and a pumping piston; power, bounce and pump cylinders for the respective pistons, a high pressure accumulator, a rst duct interconnecting said pump cylinders and said accumulator, a first check valve in said first duct permitting flow of working liquid only from said pump cylinders to said high pressure accumulator, a second duct interconnecting said accumulator and said bounce cylinders, a control valve connected in series with said second duct, said control valve having hydraulically operated means hydraulically connected to said second duct for opening said control valve when pressure of the liquid in said high pressure accumulator reaches a predetermined minimum pressure p2 and for closing said control valve when said pressure reaches a maximum pressure p1, a low pressure accumulator, a third duct connecting said low pressure accumulator to said pump cylinders, and a second check valve in series with said third duct permitting flow of the liquid only from said low pressure accumulator to said pumping pistons through said third duct.

8. The lfree piston engine as defined in claim 7 in which said hydraulically operated means in said control valve includes a pilot cylinder and a pilot piston within said pilot cylinder, a fourth duct connecting the second check valve to said pilot cylinder, a fifth duct connecting said pilot cylinder to said high pressure accumulator, said pilot piston being Iresponsive to pressure of liquid in said high pressure accumulator, said pilot piston connecting said fourth duct to said iifth duct when said pressure is equal to said pressure p1, and hydraulically operated control means connected to said second check valve and to said fourth duct, said control means being responsive to pressure in said high pressure laccumulator when said pilot piston respondsvto pressure p1 and connects said fourth duct to said fifth duct and said high pressure accumulator for blocking the normal operation of said second check valve as long as said second check valve remains connected to said high pressure accumulator through said fourth and fifth ducts and said pilot cylinder.

9. The free piston engine as defined in claim 7 which also includes means for blocking the normal operation of said second check valve upon closing of said control valve.

10. The free piston engine as defined in claim 7 which includes a pilot cylinder hydraulicallyconnected to said high pressure accumulator, a pilot piston in said pilot cylinder, a spool valve operated by said pilot piston and a spring mechanically connected to said pilot piston for opposing the pressure of the liquid in said pilot cylinder on said pilot piston.

11. In a free piston engine having a cylinder means and power, bounce and pumping piston means mounted in said cylinder means, rst and second accumulator means, a control means for starting and stopping said engine, said control means including a hydraulic control valve hydraulically connected on one side to said .irst accumulator means and on the other side to said bounce means, a pilot cylinder and a pilot piston, said pilot cylinder being hydraulically connected to said iirst accumulator means, mechanical linkage between said pilot piston and said control valve for closing and opening said control valve, a biasing spring mechanically linked to said pilot piston for opposing pressure exerted on said pilot piston by hydraulic uid filling said first accumulator means and said pilot cylinder, and means for adjusting the pressure exerted by said spring on said pilot piston.

l2. In the free piston engine, as defined in claim 11 in which said control means also includes an additional means mechanically linked to said control Valve and to said pilot piston for making said Valve open when the pressure of said hydraulic fluid is equal to a minimum pressure p2 and close when said pressure is equal to a maximum pressure p1.

13. in the 'free piston engine defined in claim 12 in which said additional means comprises a stem connected to said control means, two spaced detents on said stem, two hinged jaws engaging said stem, a spring connected to said jaws for pressing said jaws against said stem, and means for varying the pressure of said spring against said jaws.

14. In the free piston engine as defined in claim 12 in which said additional means comprises a poppet valve having a head, a stem connecting said poppet valve to said control valve, and a duct connecting the head side of said puppet valve to said first accumulator means.

15. A free piston engine comprising an engine cylinder means including a power cylinder, a bounce cylinder and a pump cylinder; power, bounce and pump pistons within respective cylinders; a high pressure accumulator, a bounce accumulator and a low pressure accumulator, first duct means interconnecting said pump cylinder with said high and low pressure accumulators, first and second check valves in said first duct means, said first check valve permitting flow of working iuid from said pump cylinder to said high pressure accumulator only during the power stroke of said engine, and said second check valve permitting ow of said fluid from said low pressure accumulator to said pump cylinder only during the compression stroke of said engine, second duct means interconnecting said bounce cylinder with said bounce accumulator, a third `check Valve in said second duct means permitting fiow of fluid to said bounce accumulator from said bounce cylinder only during the power stroke of said engine, third duct means shunting said third check valve by being ccnnected on both sides of said third check Valve to said second duct means, and a control means connected in series with said third duct means, said control means including a valve, a pilot means hydraulically connected to said high pressure accumulator and mechanically connected to said valve, said pilot means being operated by fluid from said high pressure accumulato-r for closing and opening said valve, and additional means for making said pilot means close said valve only when the pressure of said fluid in said high pressure accumulator reaches a maximum pressure p1 and open said valve when said pressure reaches a minimum pressure p2.

1 6. The free'piston engine as defined in claim 15 in which said lcontrol means *also includes first means for adjusting the value of Pri-P2 Z and second means yfor decreasing or increasing the difference between p1 and p2 without altering 17. A free piston engine comprising a power cylinder and a dual purpose bounce-and-pump cylinder, a power piston within said power cylinder, a dual purpose bounceand-pump piston within said bounce-and-pump cylinder, said bounce-and-pump piston being mechanically connected to said power piston and operated by said power piston, a first high pressure accumulator, afrst duct connecting said first accumulator to said bounce-and-pump cylinder, a second low pressure accumulator, a second duct connecting said second accumulator to said bounceand-pump cylinder, a first check valve connected in series with said tirst duct, said first check valve having means permitting tlow of working fluid only from said bounceand-pump cylinder to said first accumulator during the power stroke of said power piston, -a second check valve connected in series with said second duct, said second check valve having means permitting tiow of working fluid only from said second accumulator to said bounce-andpump cylinder during the compression stroke of said power piston, a control valve connected in series with said second duct, a pilot cylinder hydraulically connected to said first accumulator, a pilot piston within said pilot cylinder, said pilot piston being mechanically connected to said control valve for closing and opening said valve in response to pressure of said fluid in said first accumulator, and means mechanically connected to said pilot piston for making said pilot piston close said valve only when said pressure in the high pressure accumulator is equal to a maximum pressure p1 and open said valve only when said pressure is equal to a minimum pressure p2.

18. A free piston engine including a power cylinder and a power piston within said cylinder, bounce-andpump piston means, bounce-and-pump cylinder means for said bounce-and-pump piston means, said bounce-andpump' piston means being mechanically connected to said. power piston, a high pressure accumulator, a first duct connecting said accumulator to said bounce-andpump cylinder means, a check valve in said first duct permitting flow of duid only in one direction from said bounce-and-pump cylinder means to said accumulator during the power stroke of said power piston, or pilot cylinder, a pilot piston within said pilot cylinder, a first vent passage in said pilot piston, a second duct connecting said pilot cylinder to said accumulator, said second duct conveying hydraulic uid under pressure from said accumulator to said pilot cylinder and said pilot piston for hydraulically actuating said pilotpiston in response to a variation in pressure of lsaid fluid in said high pressure accumulator, spring means mechanically connected to said pilot piston for opposing the pressure of said fluid on said pilot piston, a poppet valve mechanically connected to said pilot piston and actuated by said pilot piston; first and second hydraulic chambers, an orifice between said first and second chambers, said poppet valve closing and opening said orifice, a third duct connecting said rst chamber to said accumulator, a master cylinder, a master piston in said master cylinder, a second vent passage in said pilot cylinder, said second vent passage matching said first vent passage in said pilot piston when said pilot piston is in the depressed position, said depressed position closing said orifice by means of said poppet valve, said second vent passage also extending into and opening into the master cylinder for conveying iiuid to and from said master cylinder for-operating said master piston, a fourth duct connecting said second vent passage with said secondfchamber whereby compressed fluid from said accumulator is conveyed into said master cylinder through: said first, third and fourth ducts, said iirst and second chambers `and said orifice; a fifth duct, a master valve connected in series said fifth duct, one end of said fifth duct being connected to said high pressure accumulator and the other end of said `fifth duct being connected to said cylinder means; a low pressure accumulator, a sixth duct connecting said second vent passage to said low pressure accumulator, the fluid from said high pressure accumulator actuating said pilot piston so as to close said poppet valve and convey uid from said master cylinder through the first and second vent passages and through said sixth duct to said low pressure accumulator `for deenergizing said master piston and thereby closing said master valve for stopping said engine when the pressure of the fluid in said high pressure accumulator reaches maximum pressure p1; and for closing said first and vsecond vent passages, opening said poppet valve and actuating said master piston for opening said master valve when said pressure in the high pressure accumulator reaches kminimum pressure p2.

19. In a free piston engine, including a power piston, a `bounce piston and pump piston; power, bounce and pump cylinders for the respective pistons; a high pressure accumulator; al rst duct connecting said high pressure accumulator to said pump cylinder; a check valve connected in series with said first duct, said check valve permitting flow of working iiuid only in one direction from said pump cylinder to said high pressure accumulator; a low pressure accumulator; a second duct connecting said low pressure accumulator to said pump cylinder; a check valve connected `in series with said second duct, said check valve permitting flow of uid only in one direction from said `low .pressure accumulator to said pump cylinder; a third duct interconnecting high pressure accumulator with said bounce cylinder; and control means connected to said third duct, said control means including a main valve connected in series with said third d-uct; a master cylinder, a hydraulically actuated master piston within said master cylinder for opening and closing said main valve; and a pilot valve connected to said high pressure accumulator and actuated by the :fluid under pressure in Saidhigh pressure accumulator, said. pilot valve including a poppet valve and duct means for conveying fluid under pressure to said master cylinder and master piston when 'the pressure of said fiuid in said high pressure accumulator drops to a minimum pressure p2, a pilot piston hydraulically connected to said high pressure accumulator and mechanically connected to said. poppet Valve for opening land closing said poppet valve in response to the variation in pressure of fluid in said high pressure accumulator, and, additional duct means closed and opened by said pilot piston, said additional duct means and said pilot piston venting the uid from said master cylinder to said low pressure accumulator when the pressure in said high pressure accumulator reaches a maximum value p1.

20, A free piston engine comprising engine cylinder means; first and second opposed engine piston means reciprocally mounted in said cylinder means; each of said engine piston means including a power piston and additional piston means for pumping a working fiuid and for producing a compression stroke of said engine piston means; a first accumulator means hydraulically connected to said additional piston means; said additional piston means pumping said fiuid into said first accumulator means during the power stroke of said engine for converting the power generated during the power stroke into a potential energy of the fluid within said first accumulator means; a second Vlow pressure accumulator hydraulically connected to said additional piston means; and control means for starting and stopping said engine; said control means `being hydraulically connected to: said rst accumulator means, said second accumulatorand said additional piston means; said control means including a valve hydraulically connected on one side to said additional piston means and on the other side to one of said accumulators; a pilot cylinder hydraulically connected to said rst accumulator means, a pilot piston within said cylinder, the fluid from said rst accumulator means normally exerting a hydraulic pressure on said pilot piston, a spring mechanically connected to said pilot piston for opposing said hydraulic pressure, and means interconnecting said pilot piston and said valve 4for closing said valve when pressure of said fluid in said rst accumulator means and said pilot cylinder is equal to a maximum pressure p1 and for opening said valve when the last recited pressure is equal to a minimum pressure p2, whereby said engine is operated between the pressure limits p1 and p2.

2l. A free piston engine comprising engine cylinder means; rst and second opposed engine piston means reciprocally mounted in said cylinder means; each of said engine piston means including a power piston and additional piston means for pumping a working fluid and for producing a compression stroke of said engine piston means; a first accumulator means hydraulically connected to said additional piston means; said additional piston means pumping said fluid into said rst accumulator means during the power stroke of said engine for converting the power generated during the power stroke into a potential energy of the uid within said rst accumulator means; a second low pressure accumulator hydraulically connected to said additional piston means; and control means for starting and stopping said engine; said control means being hydraulically connected to: said iirst accumulator means, said second accumulator and said additional piston means; said control means including a Valve hydraulically connected on one side to said additional piston means and on the other side to one of said accumulators; a pilot cylinder hydraulically connected to said iirst accumulator means, a pilot piston within said cylinder, the uid from said rst accumulator means normally exerting a hydraulic pressure on said pilot piston, a spring mechanically connected to said pilot piston for opposing said hydraulic pressure, and means interconnecting said pilot piston and said valve for closing said valve when pressure of said uid in said rst accumulator means and said pilot cylinder is equal to a maximum pressure p1 and for opening said valve when the last recited pressure is equal to a minimum pressure pz, whereby said engine is operated between the pressure limits p1 and p2.

22. A free piston engine comprising an engine cylinder, a pair of opposed engine pistons yreciprocally mounted in said cylinder, each of said pistons including bounce and pumping piston means, accumulator means, first duct means interconnecting said accumulator means and said bounce and pumping piston means, said accumulator means containing liquid compressed by said piston means, said piston means receiving said compressed liquid from said accumulator means through said iirst duct means to cause a compression stroke of said engine pistons, an automatic control mechanism, second duct means hydraulically connected to said iirst duct means and to said control mechanism for operating said control mechanism by means of said compressed liquid for starting and stopping said engine in response to variation of pressure of said liquid in said accumulator means, said control mechanism including a pilot piston and a pilot cylinder hydraulically connected to said iirst duct means and said accumulator means through said second duct means, said pilot piston being constantly responsive to pressure of said liquid in said accumulator means, a valve connected in series with that portion of said first duct means which conveys said liquid to said bounce piston means, and additional means operatively interconnecting said valve with said pilot piston, said pilot piston closing said valve with the aid of said additional means when the pressure of said liquid in said accumulator means in equal to a maximum pressure p1 and opening said valve when the last recited pressure is equal to a minimum pressure p3.

References Cited in the iile of this patent UNITED STATES PATENTS 1,241,691 White a Oct. 2, 1917 1,920,104 Pescara July 25, 1933 2,178,310 Pescara Oct. 31, 1939 2,230,760 Pescara Feb. 4, 1941 2,410,001 Ashbaugh Oct. 29, 1946 2,434,280 Morain Ian. 13, 1948 2,434,877 Welsh et al. Ian. 20, 1948 2,600,251 Lewis et al. June 10, 1952 2,666,569 Bent Jan. 19, 1954

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