Oscillating piston apparatus

Abstract

A substantially closed cylinder containing a compressible fluid, such as air, and a free piston for reciprocation within the cylinder, and a number of elongated passageways, each having an end opening into the cylinder and an opposite closed end. The passageways are heated along their lengths. The fin-shaped open end portions and the cylinder wall are cooled. Piston reciprocation is effected by the force of heated expanding gas moving from the closed ends of the passageways to drive the piston in one direction as the gas cools in the region between the piston end and the cooled open ends of the passageways. The piston compresses the gas at the opposite piston face, which gas in turn drives the piston back after the force of the compressed gas exceeds the force of the cooling gas to regularly repeat such cycle.

What is claimed is: 
     
       1. An oscillating piston apparatus comprising a cylinder, a free piston in the cylinder, said .[.cyliner.]. .Iadd.cylinder .Iaddend.having a side wall with a port therein, a rebound chamber containing compressible fluid for reversing the motion of the piston.[...]..Iadd., .Iaddend.said rebound chamber having as a moving wall portion a face of the piston, means including said rebound chamber for sustaining oscillatory motion of the piston in the cylinder and means for controlling the location of the center of oscillation of the piston in the cylinder, said controlling means including said rebound chamber and a passageway communicating with the cylinder via the port, said passageway by-passing a portion, and only a portion.Iadd., .Iaddend.of the axial length of the cylinder, said passageway having a fluid flow impedance which is substantially the same for fluid flow in either direction through the passageway, wherein said port, said by-passed portion of the cylinder, and an unbypassed portion of the cylinder are all at least partially traversed by the piston. 
     
     
       2. The apparatus of claim 1 wherein the free piston is of substantially integral construction. 
     
     
       3. The apparatus of claim 1 wherein the cross-sectional dimensions of the free piston are substantially the same throughout substantially all of its length. 
     
     
       4. The apparatus of claim 1 wherein the means for sustaining includes means for alternately heating and cooling the fluid. 
     
     
       5. The apparatus of claim 1 wherein the means for sustaining includes heated passageway means communicating with said cylinder. 
     
     
       6. The apparatus of claim 5 further including means for heating said heated passageway means. 
     
     
       7. The apparatus of claim 5 wherein said heated passageway means includes at least one passageway having a characteristic length and breadth which are each substantially greater than its characteristic width. 
     
     
       8. The apparatus of claim 1 further including another rebound chamber for reversing the motion of the piston, wherein the two rebound chambers are gaseous chambers acting as compression springs on opposite faces of the piston. 
     
     
       9. The apparatus of claim 1 further including a second port in the cylinder side wall, said passageway further communicating with the cylinder via said second port, wherein the two ports are located at different axial positions in the cylinder side wall, and the axial length of the by-passed cylinder portion is determined primarily by the axial separation and size of said ports. 
     
     
       10. The apparatus of claim 1 wherein the axial length of said by-passed cylinder portion is less than the axial length of the piston side wall. 
     
     
       11. The apparatus of claim 1 wherein the passageway is of integral construction. 
     
     
       12. The apparatus of claim 1 wherein said controlling means comprises groove means in the cylinder wall in said by-passed portion of the cylinder. 
     
     
       13. The apparatus of claim 1 wherein said passageway has no moving parts. 
     
     
       14. The apparatus of claim 1 wherein the controlling means has no moving parts. 
     
     
       15. The apparatus of claim 1 wherein the sustaining means has no moving parts other than the piston itself. 
     
     
       16. The apparatus of claim 1 wherein the controlling means is of integral construction. 
     
     
       17. An oscillating piston apparatus comprising two cylinders, a free piston in each cylinder, each of said cylinders having a side wall with a port therein, means for sustaining oscillatory motion of each piston in its cylinder, and means for controlling the locations of the centers of oscillation of the pistons in their cylinders, said controlling means including: a common chamber for the two cylinders, said common chamber containing compressible fluid and having as moving wall portions one face of each piston, a separate rebound chamber for each piston, said rebound chamber containing compressible fluid and having as a moving wall portion the opposite face of the piston, and a fluid passageway for each cylinder communicating with the respective cylinder via the respective port, said passageway by-passing a portion, and only a portion, of the axial length of the cylinder, said passageway having a fluid flow impedance which is substantially the same for fluid flow in either direction through the passageway; whereby the center of oscillation of each piston is located near the mid-point of the by-passed portion of its cylinder. 
     
     
       18. The apparatus of claim 17 wherein the means for sustaining includes means for synchronizing the oscillatory motion of the pistons. 
     
     
       19. The apparatus of claim 17 wherein the means for sustaining includes means for maintaining synchronous and opposite oscillatory motion of the pistons. 
     
     
       20. The apparatus of claim 17 wherein the means for sustaining includes means for alternately heating and cooling the fluid. 
     
     
       21. The apparatus of claim 17 wherein the means for sustaining includes heated passageway means for repeatedly heating the fluid in the common chamber. 
     
     
       22. The apparatus of claim 17 wherein the passageway for each cylinder consists of a passageway in said portion of the cylinder wall. 
     
     
       23. The apparatus of claim 17 wherein said controlling means for each cylinder comprises groove means in the cylinder wall in said by-passed portion of the cylinder. 
     
     
       24. The apparatus of claim 17 wherein said controlling means for each cylinder includes by-pass passageway means having no moving parts. 
     
     
       25. The apparatus of claim 17 wherein the free pistons are each of substantially integral construction. 
     
     
       26. The apparatus of claim 17 wherein the cross-sectional dimensions of each piston are substantially the same throughout substantially all of the piston length. 
     
     
       27. The apparatus of claim 17 wherein the axial length of said by-passed cylinder portion is less than the axial length of the piston side wall. 
     
     
       28. The apparatus of claim 17 wherein the sustaining means has no moving parts other than the two free pistons. 
     
     
       29. A naturally resonant oscillatory device comprising a chamber containing compressible fluid, said chamber having structure forming at least one peripheral wall portion susceptible to being oscillated at a natural resonant frequency of oscillation so as to cyclically decrease and increase the volume of the chamber, said chamber being substantially sealed during at least a substantial portion of the oscillation cycle, said chamber having fluid passageway means communicating with the at least one wall portion, means for heating said fluid passageway means, means for sustaining oscillatory motion of the at least one wall portion of the chamber so as to alternately decrease and increase the volume of the chamber, means including the oscillatory motion of the wall portion for repeatedly inducing a flow of cool fluid into said heated passageway means; said heated passageway means being designed in accordance with the frequency of oscillation to: (a) readily admit said cool fluid, (b) heat substantially all of said admitted fluid, (c) heat fluid in the passageway means as the oscillating wall portion moves in a direction to increase the volume of the chamber during said portion of the cycle, and (d) eject heated compressible fluid from the passageway means into a region of the chamber external to the passageway means as the oscillating wall portion moves in a direction to increase the volume of the chamber during said portion of the cycle; said cool fluid flow inducing means further including means for cooling fluid ejected from said heated passageway means, said means for sustaining including: (a) the heated fluid passageway means, (b) the flow inducing means including the motion of said wall portion and the means for cooling ejected fluid, (c) inertia of the structure, and (d) spring action of fluid compressed by the oscillating wall portion; wherein the energy for sustaining said oscillation is derived primarily from said heating and said cooling; wherein a net flow of fluid is induced into the heated passageway means while the volume of the chamber is decreasing during said portion of the cycle, said net flow being primarily and directly responsive to pressure variations of the fluid in the chamber resulting primarily and directly from changes in the chamber volume caused by the oscillating portion. 
     
     
       30. The device of claim 29 wherein said means for cooling said ejected fluid primarily comprises cooling of the ejected fluid by cool wall surfaces of the chamber external to said heated passageway means. 
     
     
       31. The device of claim 29 wherein said heating means includes means for heating said fluid passageway means substantially independently of the instantaneous phase of said at least one oscillating peripheral wall portion. 
     
     
       32. The device of claim 29 wherein said at least one oscillating peripheral wall portion includes two peripheral wall portions oscillating substantially in synchronism so as to, substantially with the same phase, cyclically decrease and increase the volume of the chamber. 
     
     
       33. The device of claim 32 wherein each of the wall portions is a free piston oscillating in a cylinder. 
     
     
       34. The device of claim 33 further including groove means for each cylinder bypassing only a portion of the cylinder for positioning the center of oscillation of the free piston. 
     
     
       35. The device of claim 32 wherein said cooling of the ejected fluid primarily includes cooling of the ejected fluid by cool walls of the chamber external to the heated passageway means. 
     
     
       36. The device of claim 35 wherein said cool walls primarily include walls of the chamber proximate the oscillating portions and the faces of the oscillating portions. 
     
     
       37. The device of claim 32 wherein the means for heating the passageway means includes for heating the passageway means substantially independently of the instantaneous phases of the oscillating wall portions. 
     
     
       38. The device of claim 32 wherein said wall portions communicate both with each other and with said heated passageway means via a fluid flow connecting means, wherein said means for sustaining oscillation includes cooling of said ejected fluid by cool walls of the connecting means proximate the oscillating portions. 
     
     
       39. The device of claim 29 wherein said heated passageway means includes an elongated passageway having an average length substantially greater than its average width. 
     
     
       40. The device of claim 29 wherein said chamber is substantially sealed during substantially all of the oscillatory cycle. 
     
     
       41. The device of claim 29 wherein said heating means includes means for heating said passageway means substantially independently of said natural resonant frequency of oscillation. 
     
     
       42. The device of claim 29 wherein said passageway means comprises an elongated passageway having an average length and an average breadth each of which is substantially greater than the average width of the passageway. 
     
     
       43. The device of claim 29 wherein there is an increase in effective exposure of a cool surface to fluid in the chamber as the wall portion moves in a direction to increase the chamber volume, whereby said means for cooling the ejected fluid includes cooling of the ejected fluid by said variably exposed cool surface. 
     
     
       44. The device of claim 29 wherein said heating of fluid in said passageway means and said cooling of ejected fluid each primarily comprises thermal transfer between the fluid and walls of the chamber. 
     
     
       45. The device of claim 29 wherein the means for sustaining oscillation includes cooling of the ejected fluid by cool wall surfaces of the chamber proximate the oscillating portion. 
     
     
       46. The device of claim 45 wherein said cool wall surfaces include an exposed face of the oscillating portion. 
     
     
       47. The device of claim 29 wherein the heated passageway means includes an elongated passageway having a characteristic passageway width selected in accordance with the oscillatory frequency to augment said oscillation. 
     
     
       48. The device of claim 29 wherein the heated passageway means includes a multiplicity of heated elongated passageways. 
     
     
       49. The device of claim 29 wherein the passageway means is formed to mate with an electric bulb which provides heat for heating the passageway means. 
     
     
       50. The device of claim 29 wherein said inducing of cool fluid into said heated passageway means for said heating and ejecting of said fluid takes place primarily while said chamber volume is decreasing during said portion of the cycle. 
     
     
       51. The device of claim 29 wherein said inducing of cool fluid into said heated passageway means for said heating and ejecting of said fluid takes place substantially entirely while said chamber volume is decreasing during said portion of the cycle. 
     
     
       52. The device of claim 29 wherein said wall portion comprises a free piston oscillating in .[.the.]. .Iadd.a .Iaddend.cylinder. 
     
     
       53. The device of claim 52 further including cylinder bypass means for positioning the center of oscillation of the free piston in the cylinder. 
     
     
       54. The device of claim 53 wherein said bypass means bypasses only a portion of the cylinder, said center of oscillation being positioned near the mid-point of the bypassed portion. 
     
     
       55. The device of claim 54 wherein the bypass means includes a bypass passageway bypassing said bypassed cylinder portion and having substantially equal fluid flow impedance in either direction through the passageway. 
     
     
       56. The device of claim 55 wherein said passageway is of integral construction. 
     
     
       57. The device of claim 55 wherein said passageway is integral with the cylinder. 
     
     
       58. The device of claim 55 wherein said bypass means comprises groove means in the cylinder sidewall. 
     
     
       59. The device of claim 55 wherein said bypass means comprises means for forming the cylinder side wall to provide a lower impedance to fluid flow between the piston and cylinder side walls in the bypassed portion of the cylinder than in portions of the cylinder beyond the bypassed portion. 
     
     
       60. A naturally resonant oscillatory device comprising a variable volume chamber, said chamber having structure forming at least one wall portion susceptible to being oscillated at a natural resonant frequency of oscillation so as to alternately decrease and increase the volume of the chamber, said chamber maintained in a substantially closed condition during at least a portion of the oscillation cycle, said chamber having fluid passageway means communicating with the at least one wall portion, means for heating said fluid passageway means, means for sustaining oscillatory motion of the at least one wall portion of the chamber so as to alternately decrease and increase said chamber volume, means including the oscillatory motion of the wall portion for cyclically inducing a flow of cool fluid into said heated passageway means; said heated passageway means having a geometry and an average passageway width selected in accordance with the frequency of oscillation to readily admit said cool fluid and to heat by thermal transfer means substantially all of said admitted fluid so as to eject heated compressible fluid from the heated passageway means during said portion of the cycle as the oscillating wall portion moves in the same general direction as the ejected fluid and to heat fluid in the heated passageway means during said portion of the cycle as the oscillating wall portion moves in the same general direction as the .[.ejectef.]. .Iadd.ejected .Iaddend.fluid; said means for sustaining including: the heating of the fluid by said passageway means, the means for heating said passageway means, said flow inducing means, inertia of the structure and spring action of fluid compressed by the oscillating wall portion; wherein a net flow of fluid is induced into the heated passageway means while the fluid is being compressed toward the heated passageway means by the oscillating portion during said portion of the cycle, said net flow being primarily and directly responsive to pressure variations of the fluid in the chamber resulting primarily and directly from changes in the chamber volume caused by the oscillating portion. 
     
     
       61. The device of claim 60 wherein said cool fluid inducing means includes thermal transfer means for cooling said ejected fluid. 
     
     
       62. The device of claim 61 wherein said cooling means primarily includes cooling of the ejected fluid by cool walls of the chamber external to the heated passageway means. 
     
     
       63. The device of claim 61 wherein said cooling means includes cooling of the ejected fluid by cool walls of the chamber cyclically varied in effective exposure to the fluid by the oscillating portion. 
     
     
       64. The device of claim 61 wherein said cooling provides the primary cooling for sustaining said oscillation. 
     
     
       65. The device of claim 60 wherein the cool fluid inducing means primarily includes cooling of the ejected fluid by cool walls of the chamber proximate the oscillating portion. 
     
     
       66. The device of claim 60 wherein the cool fluid inducing means includes means for cooling the induced fluid. 
     
     
       67. The device of claim 66 wherein the energy for sustaining said oscillation is derived primarily from said heating and said cooling. 
     
     
       68. The device of claim 60 wherein the heat energy for sustaining said oscillation is provided primarily by said heating. 
     
     
       69. The device of claim 60 wherein said fluid passageway means is heated substantially in dependently of the instantaneous phase of the oscillating portion. 
     
     
       70. The device of claim 69 wherein the at least one oscillating peripheral wall portion includes two peripheral wall portions oscillating substantially in synchronism so as to substantially together alternately decrease and increase said chamber volume. 
     
     
       71. The device of claim 70 wherein each of the wall portions is a free piston oscillating in a cylinder. 
     
     
       72. The device of claim 71 wherein the heated passageway means is formed to be heated by an electric light bulb which provides sufficient heat energy for sustaining said oscillation while providing light for illumination of the surroundings. 
     
     
       73. The device of claim 72 further including groove means in the inside surface of the side-wall of each of said cylinders for controlling the center of oscillation of each piston in its cylinder. 
     
     
       74. The device of claim 70 wherein said wall portions communicate both with each other and with said heated passageway means via a fluid connecting means, and wherein said means for sustaining oscillation includes cooling of said induced fluid by cool walls of the connecting means proximate the oscillating portions. 
     
     
       75. The device of claim 69 wherein said heated passageway means includes an elongated passageway having an average length and an average breadth each of which is substantially greater than the average width of the passageway. 
     
     
       76. The device of claim 69 wherein said chamber is substantially sealed during substantially all of the oscillatory cycle. 
     
     
       77. The device of claim 69 wherein the heated passageway means includes a multiplicity of elongated heated passageways. 
     
     
       78. The device of claim 69 wherein said inducing of cool fluid into said passageway means for said heating and ejecting of said fluid takes place substantially entirely while said chamber volume is decreasing during said portion of the cycle. 
     
     
       79. The device of claim 60 wherein said passageway means is heated substantially independently of said natural resonant frequency of oscillation. 
     
     
       80. The device of claim .[.50.]. .Iadd.60 .Iaddend.wherein said ejected heated fluid is derived primarily from cool fluid induced into said heated passageway means while said chamber volume decreases during said portion of the cycle. 
     
     
       81. The device of claim 60 wherein said inducing of cool fluid into said passageway means for said heating and ejecting of said fluid takes place primarily while said chamber volume is decreasing during said portion of the cycle. 
     
     
       82. The device of claim 60 wherein the wall portion comprises a free piston oscillating in a cylinder. 
     
     
       83. The device of claim 82 further including means bypassing only a portion of the cylinder to position the center of oscillation of the free piston within the bypassing portion. 
     
     
       84. The device of claim 83 wherein the bypass means includes a bypass passageway bypassing said cylinder portion and having substantially equal impedance for fluid flow in either direction through the passageway. 
     
     
       85. The device of claim 83 wherein said bypass means comprises means for forming the cylinder sidewall to provide a greater mean separation between the piston and cylinder side walls in the bypassed cylinder portion than in a cylinder portion beyond the bypassed portion. 
     
     
       86. The apparatus of claim 1 wherein said passageway and said port comprise an enlargement of the inside diameter of the cylinder in a region of the cylinder side-wall within said bypassed portion. 
     
     
       87. The apparatus of claim 17 wherein said passageway and said port for each cylinder comprise an enlargement of the inside diameter of the cylinder in a region of the cylinder side-wall within said bypassed portion. 
     
     
       88. The device of claim 82 further including means bypassing only a portion of the cylinder to position the center of oscillation of the free piston near the bypassed portion. .Iadd. 89. In a free-piston device having at least one free-piston mounted for reciprocation in a cylinder, a gaseous fluid maintained in the cylinder at at least one end of the free-piston, the improvement comprising means associated with the cylindrical walls of the piston and cylinder for reducing the tendency of the piston to creep into or out of the working gas space of the cylinder as it reciprocates in the cylinder, wherein the cylinder has a snug piston receiving bore, and means for providing a by-pass for the fluid proximate a desired mid-point of the stroke of the piston in the cylinder, said by-pass including at least one gas passage of a length less than the length of the piston and extending axially along the piston cylinder interface in opposite directions from the desired mid-point of the stroke of the piston. .Iaddend. .Iadd. 90. The free piston device defined in claim 89 wherein the gas passage is provided in the cylindrical wall of the cylinder. .Iaddend..Iadd. 91. The device of claim 90 further including means for maintaining a gaseous fluid at the end of the free piston opposite from the one end. .Iaddend..Iadd. 92. The device of claim 89 further including means for maintaining a gaseous fluid at the end of the free piston opposite from the one end. .Iaddend. .Iadd. 93. An oscillating piston apparatus comprising a cylinder, a free piston in the cylinder, means for sustaining oscillatory motion in the piston in the cylinder, and means for controlling the location of the center of oscillation of the piston in the cylinder, said controlling means including a rebound chamber for reversing the motion of the piston and at least one passageway extending axially along the side wall of the cylinder to bypass a portion of the cylinder and having a fluid flow impedance which is substantially the same for fluid flow in each direction through the at least one passageway, wherein said portion of the cylinder is at least partially traversed by the piston, and wherein the length of said bypassed cylinder portion in a direction along the cylinder axis is less than the length of the piston side wall in said direction. .Iaddend. .Iadd. 94. The apparatus of claim 93 further including another rebound chamber for reversing the motion of the piston, wherein the two rebound chambers are gaseous chambers acting as compression springs on opposite faces of the piston. .Iaddend..Iadd. 95. The apparatus of claim 94 wherein the free piston is of substantially integral construction. .Iaddend. .Iadd. 96. An oscillating piston apparatus comprising a cylinder, a free piston in the cylinder, means for sustaining oscillatory motion in the piston in the cylinder, and means for controlling the location of the center of oscillation of the piston in the cylinder, said controlling means including a rebound chamber for reversing the motion of the piston and at least one passageway bypassing a portion of the cylinder and having a fluid flow impedance which is substantially the same for fluid flow in each direction through the at least one passageway, wherein said portion of the cylinder is at least partially traversed by the piston, and wherein the length of said bypassed cylinder portion in a direction along the cylinder axis is less than the length of the piston side wall in said direction, wherein the cross-sectional dimensions of the free piston are substantially the same throughout substantially all of the length of the piston. .Iaddend. .Iadd. 97. The apparatus of claim 93 wherein the free piston is of substantially integral construction. .Iaddend..Iadd. 98. The apparatus of claim 93 wherein the cross-sectional dimensions of the free piston are substantially the same throughout substantially all of the length of the piston. .Iaddend. .Iadd. 99. In a free-piston engine or pump having at least one free-piston mounted for reciprocation in a cylinder, a pressurized gaseous fluid maintained in the cylinder at at least one end of the free-piston, the improvement comprising means associated with the cylindrical walls of the piston and the cylinder for reducing the tendency of the piston to creep into or out of the working gas space of the cylinder as it reciprocates in the cylinder, wherein the cylinder has a snug piston receiving bore and means extending axially on the cylinder wall providing at least one gas passage of a length less than the length of the piston and extending in opposite directions from the mid-point of the stroke of the piston. .Iaddend. .Iadd. 100. A method of reducing the tendency of a free-piston of a free-piston motion device to creep into or out of the working space of its cooperating cylinder comprising the steps: forming a bore in a cylinder to snugly receive a piston within the bore; and   forming an axial gas passage in the cylindrical surface of the cylinder of a length less than the length of the piston. .Iaddend. .Iadd. 101. An oscillating piston apparatus comprising a cylinder, a free piston in the cylinder between rebound chambers containing compressible fluid at opposite faces of the free piston, means for sustaining oscillation of the free piston in the cylinder, bypass means for controlling the location of the center of oscillation of the free piston in the cylinder, said bypass means comprising at least one passageway bypassing a portion of a side wall of the cylinder, said passageway being capable of carrying fluid in each direction according to the polarity of the instantaneous pressure differential across the passageway, said passageway having at least one port in the cylinder sidewall, said passageway configured to bypass a varying length of the piston sidewall as the piston oscillates in the cylinder, said length generally decreasing as the piston moves in a direction away from a preferred location for the center of oscillation of the piston, movement of the center of piston oscillation away from said preferred location in said direction causing a greater travel of the piston in said direction and a greater average impedance to fluid flow in the opposite direction along the piston sidewall, whereby there is a net fluid flow along the piston sidewall in said direction tending to move the center of piston oscillation in the opposite direction. .Iaddend..Iadd. 102. The apparatus of claim 101 wherein said bypass means comprises groove means in the cylinder sidewall, and said passageway is a groove in the cylinder wall. .Iaddend. .Iadd. 103. The apparatus of claim 101 wherein said bypass means is disposed solely exterior to the piston. .Iaddend..Iadd. 104. The apparatus of claim 101 wherein said passageway is formed in the sidewall of the cylinder. .Iaddend..Iadd. 105. The apparatus of claim 101 wherein the bypass means is formed solely in the sidewall of the cylinder. .Iaddend..Iadd. 106. The apparatus of claim 101 wherein said bypass means consists of substantially axial groove means in the cylinder sidewall. .Iaddend..Iadd. 107. The apparatus of claim 101 wherein said passageway is formed to have a fluid flow impedance which is substantially the same for fluid flow in either direction through said passageway. .Iaddend..Iadd. 108. The apparatus of claim 101 wherein said passageway is configured and disposed such that said passageway remains stationary relative to the cylinder throughout the oscillatory cycle. .Iaddend..Iadd. 109. The apparatus of claim 101 wherein the length of said port in a direction along the cylinder axis is a substantial fraction of the axial length of the piston sidewall, said piston sidewall blocking said port to a varying degree during the oscillatory cycle. .Iaddend..Iadd. 110. The apparatus of claim 101 wherein the length of said port in a direction along the cylinder axis is approximately equal to the axial length of the piston sidewall. .Iaddend..Iadd. 111. The apparatus of claim 101 wherein the means for sustaining includes means for alternately heating and cooling the fluid. .Iaddend..Iadd. 112. The apparatus of claim 101 wherein all surfaces of said bypass means remain stationary with respect to said cylinder throughout the oscillatory cycle. .Iaddend..Iadd. 113. The apparatus of claim 101 wherein the piston sidewall is impervious to fluid flow through the sidewall. .Iaddend..Iadd. 114. The apparatus of claim 101 wherein the outer surface of the piston sidewall has the shape of a cylinder section. .Iaddend..Iadd. 115. The apparatus of claim 101 wherein the length of said bypass means in a direction along the cylinder axis is approximately equal to the axial length of the piston sidewall. .Iaddend..Iadd. 116. The apparatus of claim 101 wherein the axial length of said bypass means is slightly less than the axial length of the piston sidewall. .Iaddend..Iadd. 117. An oscillating piston apparatus comprising a cylinder, a free piston in the cylinder, means for maintaining a compressible fluid at at least one end of the free piston, means for sustaining oscillation of the free piston in the cylinder, and passageway means extending axially along an interface between the piston and cylinder for locating the center of piston oscillation proximate said passageway means. .Iaddend..Iadd. 118. The apparatus of claim 117 wherein said passageway means comprises groove means disposed along said interface. .Iaddend..Iadd. 119. The apparatus of claim 118 wherein said groove means has an orientation approximately parallel to the cylinder axis. .Iaddend..Iadd. 120. The apparatus of claim 118 wherein said groove means is disposed in the sidewall of the cylinder. .Iaddend..Iadd. 121. The apparatus of claim 117 wherein said passageway means consists of groove means in the sidewall of the cylinder. .Iaddend..Iadd. 122. The apparatus of claim 117 wherein said passageway means has substantially equal fluid flow impedances in opposite directions along said passageway means. .Iaddend..Iadd. 123. The apparatus of claim 117 wherein said passageway means is shorter in an axial direction than the expected extremes of travel of the piston. .Iaddend. .Iadd. 124. The apparatus of claim 117 wherein the length of said passageway means along the cylinder axis is approximately equal to the axial length of the piston sidewall. .Iaddend..Iadd. 125. The apparatus of claim 117 wherein the axial length of said passageway means is less than the axial length of the piston sidewall. .Iaddend..Iadd. 126. The apparatus of claim 117 wherein said passageway means bypasses a portion, and only a portion, of the axial length of the cylinder. .Iaddend..Iadd. 127. The apparatus of claim 117 wherein the piston sidewall is impervious to fluid flow through the sidewall. .Iaddend..Iadd. 128. The apparatus of claim 117 wherein the outer surface of the piston sidewall has the shape of a cylinder section. .Iaddend..Iadd. 129. The apparatus of claim 117 wherein said passageway means is disposed solely exterior to the piston. .Iaddend..Iadd. 130. The apparatus of claim 117 wherein all bounding surfaces of said passageway means are stationary relative to the cylinder. .Iaddend..Iadd. 131. The apparatus of claim 117 wherein the means for sustaining comprises means for alternately heating and cooling the fluid. .Iaddend. .Iadd. 132. The apparatus of claim 117 wherein said piston oscillates between rebound chambers containing compressible fluid at opposite ends of the piston. .Iaddend..Iadd. 133. An oscillating piston apparatus comprising a cylinder, a free piston in the cylinder between rebound chambers containing compressible fluid at opposite end faces of the free piston, means for sustaining oscillation of the free piston in the cylinder, cylinder bypass means for controlling the location of the center of oscillation of the free piston in the cylinder, wherein said bypass means includes at least one bypass passageway bypassing a portion of the cylinder proximate a desired location for the center of piston oscillation, said passageway being capable for carrying fluid in either direction through said passageway according to the polarity of the instantaneous pressure differential across the passageway, said bypass means in conjunction with the piston motion facilitating a gradual and automatic adjustment in the relative amounts of fluid in said rebound chambers tending to prevent drift of the piston center of oscillation away from said preferred location, wherein said bypass means is disposed solely exterior to the piston and includes at least one port along the piston-cylinder interface. .Iaddend..Iadd. 134. The apparatus of claim 133 wherein the fluid flow impedance of said passageway is substantially the same in either said direction. .Iaddend..Iadd. 135. The apparatus of claim 133 wherein said cylinder bypass means is disposed in the cylinder sidewall. .Iaddend. .Iadd. 136. The apparatus of claim 133 wherein the cylinder bypass means bypasses a portion, and only a portion, of the axial length of the cylinder. .Iaddend..Iadd. 137. The apparatus of claim 133 wherein the length of said passageway in a direction along the cylinder axis is approximately equal to the axial length of the piston sidewall. .Iaddend..Iadd. 138. The apparatus of claim 133 wherein the length of said passageway in a direction along the cylinder axis is less than the axial length of the piston sidewall. .Iaddend..Iadd. 139. The apparatus of claim 133 wherein the piston sidewall has a contiguous design such that fluid is prevented from flowing through the piston sidewall. .Iaddend..Iadd. 140. The apparatus of claim 133 wherein the shape of the outer surface of the piston sidewall is a cylinder section. .Iaddend..Iadd. 141. The apparatus of claim 133 wherein the means for sustaining includes means for repeatedly heating the fluid. .Iaddend..Iadd. 142. The apparatus of claim 133 wherein the length of said bypass means in a direction along the cylinder axis is less than the expected axial length of the piston cylinder interface. .Iaddend. .Iadd. 143. The apparatus of claim 133 wherein the axial length of said bypass means is approximately equal to the axial length of the piston sidewall. .Iaddend..Iadd. 144. The apparatus of claim 133 wherein the axial length of said bypass means is slightly less than the axial length of the piston sidewall. .Iaddend..Iadd. 145. The apparatus of claim 133 wherein said bypass means is of integral construction. .Iaddend..Iadd. 146. The apparatus of claim 133 wherein said bypass means is stationary relative to the cylinder. .Iaddend..Iadd. 147. The apparatus of claim 133 wherein said bypass means comprises groove means along the piston-cylinder interface. .Iaddend..Iadd. 148. The apparatus of claim 133 wherein, throughout the oscillatory cycle, said bypass means bypasses no more than a portion of the piston. .Iaddend..Iadd. 149. The apparatus of claim 133 wherein said bypass means comprises an enlargement of the inside diameter of the cylinder in a region of the piston-cylinder interface. .Iaddend..Iadd. 150. The apparatus of claim 133 wherein said bypass means consists of substantially axial groove means along the piston-cylinder interface. .Iaddend. .Iadd. 151. The apparatus of claim 150 wherein the axial length of said groove means is approximately equal to the axial length of the piston sidewall. .Iaddend..Iadd. 152. The apparatus of claim 150 wherein the axial length of said groove means is slightly less than the axial length of the piston sidewall. .Iaddend..Iadd. 153. In an oscillating piston apparatus having a cylinder, a means for sustaining oscillation of a free piston in the cylinder, and a rebound chamber containing compressible fluid at at least one end of the cylinder for reversing piston motion, the improvement comprising bypass means providing a limited bypass of a seal between side walls of the piston-cylinder for controlling the location of the center of oscillation of the piston in the cylinder, said bypass means being free of moving parts. .Iaddend..Iadd. 154. The apparatus of claim 153 wherein said bypass means is configured to bypass and thereby reduce the piston-cylinder seal primarily during a limited portion of the piston stroke while the piston is proximate a preferred position for its center of oscillation. .Iaddend..Iadd. 155. The apparatus of claim 156 wherein said bypass means is integral with the cylinder sidewall. .Iaddend..Iadd. 156. The apparatus of claim 154 wherein said bypass means comprises groove means in the cylinder sidewall. .Iaddend. .Iadd. 157. In an apparatus having a cylinder, a free piston susceptible to undergoing oscillation along the cylinder axis, a rebound chamber at at least one end of the cylinder for reversing piston motion, and a means for sustaining the oscillation of the free piston, the improvement comprising means for controlling the location of the center of oscillation of the piston in the cylinder, wherein said controlling means comprises an axially extending slight enlargement of a portion of an annular clearance space between sidewalls of the piston and cylinder, said enlargement facilitating a net leakage of fluid along the piston sidewall tending to move the center of oscillation axially toward a preferred location thereof. .Iaddend..Iadd. 158. The apparatus of claim 157 wherein said enlargement comprises groove means in said annular clearance space. .Iaddend..Iadd. 159. The apparatus of claim 157 wherein said enlargement comprises groove means in the cylinder sidewall. .Iaddend..Iadd. 160. In an oscillating free piston apparatus, the improvement comprising cylinder bypass means for controlling the location of the center of oscillation of a free piston oscillating in a cylinder of said apparatus, wherein the cylinder bypass means is free of moving parts. .Iaddend..Iadd. 161. The apparatus of claim 160 wherein said cylinder bypass means is designed so that the full axial length of the piston sidewall becomes available for contributing to the piston cylinder sliding seal if the piston moves sufficiently far in either direction from the mid-point of said cylinder bypass means. .Iaddend..Iadd. 162. The apparatus of claim 160 wherein said bypass means is groove means in the cylinder sidewall. .Iaddend.