US2954155A - Compressor - Google Patents

Description

Sept. 27, 1960 E. RAY ETAL 2,954,155

COMPRESSOR Filed Sept. 17, 1958 5 Sheets-Sheet 2 v. lo Q MHz "5 27223?"E 5%?) 5 0/70 arre a /40 g United States Patent COMPRESSOR Filed Sept. 17, 1958 Ser. No. 761,579

7 Claims. (Cl. 23053) This invention relates to a compressor, and more particularly to a compressor of the free piston type having an air motor piston operating in conjunction with a compressor piston so that expansion of engine compression bleed air in the air motor performs work which is absorbed by the compressor in compressing air to a desired pressure.

Heretofore, various hydraulic systems as well as electrical systems and low or high pressure pneumatic systems have been used in air vehicles for the secondary power which is necessary to assist or replace the pilot in such functions as flight control, dive brakes, landing gear, exhaust nozzle, inlet nozzle, thrust reverser actuation, and the like. At flight speeds of Mach 3 and higher hydraulic systems are unsatisfactory since they are unable to withstand the resultant temperatures of from 700 F. to 1200 F. and higher. Thus, such systems experience a coke-out of carbon-like deposits at these temperatures which tend to clog filters and stop valves and parts. Also, they develop alcohols, raisingthe vapor pressure and lowering the flash point and viscosity, and become increasingly corrosive on materials in the system as well as losing their lubricity and evolving gases. To prevent such effects, cooler means must be incorporated in the system which will prevent temperatures from exceeding approximately 350 F., thermostat means being utilized to maintain a lower temperature limit of 100 F. This relatively narrow temperature range reduces temperature dependent variations in hydraulic functions. Where a silicate ester base fluid has been used, other precautions have been required, such as sealing out atmospheric air to avoid moisture and hydrolysis and the provision of a nitrogen atmosphere in the ground test and test benches. And because the bulk modulus of hydraulic fluids varies greatly, designing a hydraulic system which is stable over the complete temperature range is exceedingly ditficult.

Other problems associated with hydraulic system operation at elevated temperatures include rapid deterioration of the fluid, increased danger of cavitation and increased fire hazard due to the possibility of leaks.

In contrast to the difliculties encountered with respect to hydraulic systems, air is extremely stable with temperature. The bulk modulus of air is fairly constant over a wide temperature range, and other advantages include the unlimited availabilityof air, the lack of fire hazard,

ready storage of energy, the possibility of small transmitting lines and the elimination of return lines afiording a decrease in system weight, the cleanliness of the system, the lack of adverse effects from nuclear radiation and the use of decompressed air for cooling certain components.

The increase in aerodynamic loads and in response requirements resulting from increasedflight velocities increases the power requirements for actuation. Thus, high pressure pneumatic systems are more desirable, so as to keep space and weight requirements of actuators and associated valving and ducting within reasonable limits. For example, it has been determined for a typical r 2,954,155 Patented Se pt.27, 1969 "ice application that a high pressure compressor bleed system may be substantially only 42% as heavy as a low pressure compressor system. Also, the response characteristics of a servo system are largely dependent on the bulk modulus of the power medium, and since this is a direct function of the pressure of the air, it is apparent that high pressures will be more effective than low pressures.

7 High pressure pneumatic servos can be both stable and light; Thus, an engine bleed air pneumatic system for high speed flight is appreciably heavier than a high pressure pneumatic system with a compressor. Similarly, high pressure hydraulic systems are also heavier than high pressure pneumatic systems. The latter weight dif ference is the result of the weight of the hydraulicfiuid and the larger pressure lines and larger return lines. However, although it has been shown that high pressure pneumatic components are eflicient at temperatures of up to 1000 F. or more, there has a yet been no compressed air system which could make such a high pressure system operative. 7 The present invention provides a type of'compressor which is suitable for use in high performance high speed air vehicles, as well as in more conventional applications. The compressor essentially consists of an air motor piston attached to a compressor piston, the expansion of engine compressor bleed air in'the air motor performing work which is absorbed by the compressor in compressing air to the desired pressure, as stated. Some of the applications'ofsuch a unit are high speed long range chemically fueled bombers, interceptor and nuclear powered air craft. A further potential use is a compact pressure amplifier for missile systems. The compressorof the in vention utilizes a free piston construction which is advantageous in low specific speed applications because of its high efliciency as a positive displacement device and its inherent simplicity. The device also eliminates the undesirable flow' and pressure characterisfics of centrifugal compressors wherein leakage,'for low specific speed "applications, may amount to 50% of the required flow and thus drastically reduce the overall compressor efficiency. Because the load on mating surfaces of the free piston compressor is reduced, the need for the lubricating systems which represent a fundamental limitation in the use at high temperatures of conventional compressors having connecting rods andcrankshafts is also reduced. Unbalanced loads .are eliminated by air cushions in the compressor, and the bleed air can be used not only for performing the compression work but also for cooling, since the air motor exhaust is at a low temperature due to the expansion-process therein. Elimination of the connecting rod crank structure of conventional systems means that the cylinder walls are not subjected to side thrust. The use of opposing pistons re ducesvibration to a minimum and heavy mountings are unnecessary. The consequent space and weight reduction amounts to substantially 50% as compared with conven-' tional compressors. Because the unit is driven by engine compressor bleed air, the free piston compressor can be located remotely from the main engine so that space can be utilized which is not at a premium and whereambient temperatures are minimized. Also,the free piston unit requires fewer parts than conventional systems, thereby decreasing production costsand increasing reliability.

Accordingly it is an object of the present invention to provide. a free piston engine compressor which is adapted to use bleed air from the aircraft engine, this air being expanded in an air motor to perform work which is absorbedby the compressor in compressing air to a desired pressure. g A further object of the invention is to provide a system. wherein the exhaust air of the air motor which is cooled by its expansion affords a self-cooling action for the compressor.

Another object of the invention is to provide a system as described wherein frictional losses are reduced to a minimum by the absence of side thrust such as is encountered in engines using crankshaft machinery and wherein higher temperatures can be tolerated than in such machinery as the result of the thermal characteristics of the air utilized therein and the elimination of conventional lubricating means, though conventional lubrication may be used.

Another object of the invention is to provide a free piston compressor wherein the external forces are inherently balanced so that vibrations are at a minimum and heavy mountings are unnecessary.

' Another object of the invention .is to provide a free piston compressor as described which affords great savi n'gs in space and reduction of weight as compared with conventional designs and which permits a strategic location of the unit in a position where space is readily available and where the ambient temperatures are relatively low.

Yet another object of the invention is to provide a compressor unit as described in which the number of parts is reduced to a minimum and operating efficiency and reliability are greatly increased as compared with conventional systems.

Further objects and advantages of the invention will become apparent as the description proceeds in accordance with the drawings in which:

Figure 1 is a top plan view of a compressor according tothe present invention;

Figure 2 is a vertical sectional view of the compressor, partly broken away and taken through lines 2-2 of Figure 1;

Figure 3 is a vertical sectional view taken through lines 3+3 of Figure 1;

Figure 4' is a side elevational view of the air motor piston and piston structure for the first and fourth stages of the compressor;

Figure 5 is a showing of the piston structure for the second and third stages of the compressor; and

Figure 6 is a schematic showing of a cooling system for the compressor.

Referring now to Figures 1 and 2, a compressor 10 is disclosed in accordance with the present invention, which is adapted for use in a high performance air vehicle operating at high flight speeds and at high altitudes. In the embodiment shown, it is adapted to maintain an outlet pressure of substantially 5000 p.s.i. with the first stage inlet pressure varying from p.s.i. to 180 p.s.i. The delivery of the compressor is at a maximum in the neighborhood of 180 p.s.i. for the inlet condition since this corresponds to the flight condition where the speed is highest and the pneumatic system requirement is greatest. The air compressor inlet upper temperature is set at 250 F. and the case temperature at 275 F. to minimize the need for lubrication. These temperatures are maintained by a combination of the cooling effects of ram air, the air motor exhaust, and evaporative spray cooling. Oil impregnated piston rings are utilized in cylinder bores which are preferably chromium plated, and dieseling problems caused by auto ignition of lubricating oil are eliminated because the chamber temperatures are kept substantially below the level of 500 F. at which such phenomena occur. The compressor provides an internal balance of work during each cycle as well as a balance of work for each stroke, and the clearance and area ratios of the pistons are determined in accordance with these prerequisites. Likewise, the pressure ratios in the four stages of compression hereinafter described are determined in accordance with the clearance and area ratios and as a function of the compressor inlet pressure. The balance of forces on the piston assemblies of the compressor reduces vibration and strain on the system structure and affords a lighter and smaller assembly than would otherwise be available. In part this is achieved by providing for substantially equal gas forces on the two separate piston assemblies of the compressor. A synchronizing linkage is utilized between these assemblies to insure that the piston motions are in phase and that valve timing and actuation are provided. This synchronizing linkage also compensates for any differences in friction forces in the two 'pistonassemblies. The air motor forces and bounce chamber forces in the cornpresscr are substantially equal and, accordingly, equal compression forces are required for the piston assemblies to product a balance of the gas forces thereon. A balancing of compressive forces over a wide range of design points compensates for different pressure ratios at the respective stages and at every inlet condition, the outlet being constant at 5000 p.s.i. as stated. An optimum selection of stage clearances and successive stage area ratios is therefore utilized. Since piston moition in the free piston compressor corresponds to that in a system composed of a mass isolated between opposed springs, the compressor piston assemblies oscillate at the natural frequency of thesys tem this frequency being a function of the mass of the piston assemblies and of the gas utilized in the chambers.

The cooling demand is below that of conventional systems because of the elimination of metal to metal loads and the consequent reduction in friction heating, as well as the flexibility as to location of the compressor whereby the high pressure bleed air may be directed to some remote low ambient temperature location. Also important in this regard is the fact that the air motor exhaust is utilized as a heat sink, the air motor exhaust being capable of handling substantially a fourth of the cooling load, exclusive of cooling afforded by ram air. Further cooling at high temperatures may be provided by evaporation cooling of the spray type. The resultant increase in efficiency is seen from the fact that an engine driven compressor of the type indicated requires substantially .43 lb. of water per pound of air delivered while the present invention requires only .28 lb. of water per pound of air delivered to be used at maximum temperature conditions. To further assist in cooling the air-motor air, a ram air heat exchanger structure is utilized.

Hard, wear-resistant alloys such as tool steels, sintered carbide and Stellite, which will withstand high temperatures and which have a high compressive strength, are preferably used in the construction of the compressor, and such structural members as are not subject to wear may be formed from a suitable conventional high tem perature alloy such as stainless steel, while aluminum may be used where temperatures are not excessive. Poppet type valves are utilized for the air motor valving and these valves may be cam operated from a crankshaft driven by the synchronizing mechanism. The close correlation of the valving with respect to piston position thus obtained is important in order to afford a high degree of accuracy for the cycle. However, the pressures in the cylinder may also be used to operate the valves in accordance with the scope of the invention so that the valves are opened and closed as a function of the stroke, to which the pressure is related. Such control eliminates the need for cam shaft operation.

A As seen in Figures 1-5, the compressor 10 has a casing 12 in which air motor piston and compressor piston 14 and 16 are slidably received. The pistons 34 and 16 each have enlarged, dished ends 18 and 19 and shaft sections 20 and 21 which cooperate with the casing .12 to form air motor chambers, such as the chamber 22, which communicate with bleed air inlets, such as the inlets 24 and 25, the piston ends 18 and 1? each providing a bounce chamber such as chamber 26 in cooperation with the end walls 27 and 28 so as to substantially reduce the. number of bores required in each d O the device. Bleed airat substantially 1200.

25. i is cooled by a heatexchange structure 29 to approximately 730 F. in the embodiment shown and is introduced through the arcuate passages 24 and 25 through poppet valves 30 and 31, which are activated in accordance with synchronizing means as hereinafter set forth. Expansion of this air in the chamber 22 cools the air to approximately 310 F., as indicated schematically in Figure 6. The air is then transmitted by suitable means (not shown) to precoolers 32 and 33, which also are shown schematically in Figure 6, and introduced into a chamber 34'defined by the casing 12 and a relatively reduced portion 35 of the piston 14 adjacent the piston head 36, through a valve 37 which is spring biased to inlet pressure, conduit means 38 being' provided in a central core member 40 in communica tion with the precooler means 32 and 33'. The inner end of the reduced portion 35 has formed thereon a relatively small piston head 39 slidably received in a bore 41 in the central portion 40. The outlet for the first stage is controlled by avalve 42 which is spring biased to the discharge pressure produced by actuation of the piston 14 and the piston head 36, and a passage 44 in the member '40 communicates with an intercooler 46 and the inlet portion 48 for the second stage, suitable conduit means (not shown) being provided for this purpose. The right hand side of the compressor corresponds in essential features to the left hand side to provide a balanced and synchronized operation, as stated, piston 16 on the right side of the compressor having a relatively reduced portion 50 adjacent a piston head 51 which cooperates with the casing 12'to provide a second stage compressor chamber (not shown), the inner end of the reduced portion 50 having a piston head 52 formed thereon. The outlet for this stage is suitably valved in a similar manner to the outlet for stage one and the air is transmitted to the intercooler 46 prior to its introduction to the inlet of stage three by the movement of the piston 16 and its piston head 51.

Thus, the air is transmitted from an outlet 54 to an air inlet 56 and into a chamber defined by a core mem-' ber 58 corresponding to the member 40, and the piston head 52 of the piston 16. Upon inward movement of the piston head 52, the air is released through a suitable valve structure and conduit means '60 to the intercooler 46 and then through a suitable inlet in the casing 12 (not shown) and a spring biased valve 62 from a conduit 64 to a chamber 66 at the inner end of the member 40 and the bore '41 therein, the chamber 66 and bore 41 representing the fourth compression stage. Each of the piston portions of the piston 14 is provided with suitable oil impregnated rings 68', 70 and 72, and corresponding rings are provided for the piston 16, so that leakage is minimized, but the intercooler 4-6 maintains a moderate temperature so that any possibility of dieseling is avoided. Upon inward movement 'of the piston head 39 the compressed air is released through an outlet nozzle 74 threadedly engaged in the core member 40 and its extension 76 and thereupon transmitted through the orifice 78 by suitable means to an accumulator (not shown).

In order to synchronizethe movements of the pistons 14 and 16, a linkage means 80 is provided comprising a shaft 82. journaled in the casing 12 at right angles to the longitudinal axis of the casing, and within a pair of opposed journal plates. 84 and 86 secured to the casing portions 88 and 90 by suitable bolts 92. A pinion gear 94 is fixed upon the shaft 82 and is adapted to drive rack means such asthe rack96 seen in Figure 3 so as to synchronize the pistons 14 and 16 and compensate for any slight differences in force balance which might occur. A similar rack (not shown) is provided for the right side of the compressor. The poppet valves 30 and 31 are driven by cam means 08 and 100 formed integrally on the shaft 82 so that their action is also synchronized in accordance with the action of the pistons.

The work output of the air motor of the compressor of the invention is equal to the work input of the compressor, and the work, therefore, in each instance, is a function of the inlet pressure and the stroke. The strokes of the air motor and compressor are equal so that con-' trol may he provided by varying the inlet pressure to the compressor as a function of the inlet pressure to the air motor. In the embodiment of the invention shown, these 'two inlet pressures are substantially equalized throughout the range of operating conditions. After the system demand is satisfied and the accumulator is charged, the compressor may be shut off by closing the air motor inlet. Thereafter, when pressure falls below a prescribed limit, the air motor inlet is opened and the accumulator thus recharged. Therefore, the control of the output may be made a function of system demand;

The bounce chamber pressure is controlled as a function of thecompressor inlet pressure in order to maintain the work balance between the pistons for the inward and outward strokes. Simple thermostatic controls (not shown) may be utilized to control the temperature throughout the cycle so that, for example, when the compressor bleed air temperature is above 550 a heat exchanger may be opened to ram air and them air inlet may be closed when the temperature falls below 520 F. During low speed operation the air motorexhaust may be discharged overboard and the compressor inlet cooled by ram air. When the compressor inlet reaches 210 F. thea-ir motor exhaust may be allowed to cool the compressor inlet air. As flight speeds increase, the air motor exhaust not being entirely sufficient to cool the'compressor inlet, ram air fiow-to the precooler, inter-coolers and aftercoolers is used until thecornpressor inlet reaches 240 F. and thereafter evaporative cooling continues until the compressor inlet tempe'rature is reduced to 230 F., whenjevaporative coolingmay be terminated and ram 'a'iradmitted; When the compressor inlet temperature falls to 200; F., the air motor exhaust may again be discharged overboard so that ram air absorbs the cooling load. In order to place the system in readiness fol-further operation after it hasbeen shut elf, a valve may be provided in theair motorinlet line which may be closed so as to leave the bounce chamber open to its scheduled inletpress-ure. s'ulting force unbalance will then force the piston assembly to stop at the inward extreme of its stroke.

The compressor of the 'invention'is adapted to operate in an ambient temperature of 700 F. with no lubrication but with some cooling. The absence of oil lubrication is made possible by the fact that substantially the only unbalanced load' of the system is the piston assembly itself, i.e., the weight of the assembly. Also, the load on the synchronizing mechanism may be reduced to a minimum by a method of balancing forces fora wide' range of Operating conditions while variations in friction force, valve loading and any residual unbalancing forces which may exist form the remainder of the load on the mechanism. i

Although the invention has been described in our application with respect to certain specific principles and details thereof, it will be apparent to those skilled in the art that modifications and variations may be'efliected without departing from the spirit an'd'scope of the in vention as set forth in the hereunto appended claims;

We claim as our invention: 7 I v 1. In a compressor for high speed aircraft and the;

like, a. free piston compression system compressing a. casing, air inlet means in said casing,,a first p1ston,sl-id-a-. blyand recipr'ocably disposed insald casing'having compressor chamber unit with said casing, 21 second piston having a relatively enlarged portion defining an air motor and a bounce chamber with said casing, a relatively reduced portion defining a second stage compressor chamber with said casing and a further reduced portion defining a third stage compressor chamber with said casing, said pistons being calibrated to provide a balance of forcestherebetween, air inlet means in said casing and conduit and valve means between each of said stages provided by said pistons and said casing.

2. A free piston compressor comprising a casing, a pair of force-balanced pistons in said casing in co-axially slidable and reciprocable relationship, a shaft journalled between said pistons in perpendicular relation thereto, a pinion onsaid shaft, rack means driven by said pinion and linking said pistons in synchronized relationship, cam means on said shaft, and inlet valve means in said casing, said valve means being operable by said cam means to meter gas into said casing in accordance with the movement of said pistons, one of said pistons having a relatively enlarged'portion defining a bounce chamber andairmotor with said casing, said valves affording in let of gas into said chamber, said piston having a relatively reduced portion defining a first stage compression chamber with said casing, and a further reduced portion defining a fourth stage compression chamber with said casing, the other of said pistons havin a relatively enlarged portion defining an air motor and a bounce chamber with said casing, a second relatively reduced portion defining a second stage compression chamber with said casing and a further reduced portion defining a third stage compression chamber with said casing, and valve and conduit means connecting said compressor chambers.

3; A compressor system for high speed aircraft comprising a free piston compressor having a pair of pistons, a casing for said pistons cooperating therewith to define an air motor andbounce chamber for each of said pistons, first and fourth compression stages for one of said pistons and second and third compression stages for the other of said pistons, inlet means for each of said pistons, and a cooling system comprising precooler means for said air motors, said air motors being adapted to aiford further cooling of air introduced into said casing through said inlet means, and intercooler means between each of said compressor stages, said compressor system having conduit and valve means between each of said compressor stages and the intercooler means associated therewith.

4. In a compressor having a pair of pistons and a casing slidably receiving said pistons, an air motor and bounce chamber, a first stage compression chamber and a fourth stage compression chamber defined by said casing and one of said pistons, an air motor and bounce chamber, a second stage compression chamber and a third stage compression chamber defined by the other of said pistons With said casing, said casing having valve and conduit means between each of said stages, inlet means for said air motor adapted to be operated synchronously with said pistons, and linkage means for said pistons comprising a pinion journalled therebetween and rack means for each of said pistons in engagement with said pinion whereby said pistons are maintained in balance during actuation thereof by said air motors.

5. In a compressor having a pair of pistons and a casing slidably receiving said pistons, an air motor and bounce chamber, a first stage compression chamber and a fourth stage compression chamber defined by said casing and one of said pistons, an air motor and bounce chamber, 21 second stage compression chamber and a third stage compression chamber defined by the other of said pistons with said casing, said casing having valve and 8 conduit means between each of said stages, inlet means for said air motor adapted to be operated synchronously with said pistons, and linkage means for said pistons comprising a pinion journalled therebetween and rack means for each of said pistons in engagement with said pinion whereby said pistons are maintained in balance during actuation "thereof by said air motors, sa i'd inlet means including an inlet passage, poppet valve means for each of said air motors, and cam means operable by said pinion means for opening and closing said poppet valve means in accordance with the operation of said pistons.

6. A compressor comprising a casing, a first piston defining a first piston portion of relatively large diameter having a dished construction at the outer face thereof, a second piston portion of relatively reduced diameter, a third piston portion of further reduced diameter, a portion between said first and second portions defining an air motor with said casing, said casing defining a bounce chamber with said first portion, and a portion between said second and third portions defining a first stage compression chamber with said casing, said casing defining a fourth stage compression chamber with said third piston portion, a second piston defining a first piston portion of relatively large diameter, a second piston portion of reduced diameter, a third piston portion of further reduced diameter, said casing defining a bounce chamber with said first piston portion of said second piston, a portion between said first and second piston portions of said second piston defining an air motor with said casing, and a portion between said second and third piston portions of said second piston defining a second stage compression chamber with said casing, said casing defining a third stage compression portion with said third piston portion of said second piston, inlet means for each of said air motors including valve means adapted to open and close in response to the movement of said pistons, said pistons being calibrated to afiord balanced reciprocating action therebetween, conduit means between each of said compression stage chambers, and valve means responsive to the pressure in each of said chambers for permitting flow of gas into each of said chambers successively in response to the reciprocating action of said pistons, said casing providing outlet means from said fourth stage compressor chamber.

7. In a compressor having a pair of pistons in a casing slidably receiving said pistons, an air motor and bounce chamber, a first stage compression chamber and a fourth stage compression chamber defined by said casing and one of said pistons, an air motor and bounce chamber, a second stage compression chamber and a third stage compression chamber defined by the other of said pistons with said casing, said casing having valve and conduit means between each of said stages, inlet means for said air motor adapted to be operated synchronously with said pistons and linkage means operatively connected to each of said pistons maintaining said pistons in balance during actuation thereof by said air motors.

References Cited in the file of this patent UNITED STATES PATENTS

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