Patent Publication Number: US-11655756-B2

Title: Single air supply using hollow piston rod

Description:
RELATED APPLICATIONS 
     This application is a continuation application of U.S. patent application Ser. No. 17/195,818, filed Mar. 9, 2021, which is a continuation of U.S. patent application Ser. No. 16/831,920, filed Mar. 27, 2020, which is a continuation of and claims the benefit of priority from U.S. patent application Ser. No. 16/207,479, filed Dec. 3, 2018 and the contents of all of which is incorporated herein by reference in their entirety. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to the field of internal combustion engines, and more particularly to the field of internal combustion engines having a free piston. 
     BACKGROUND 
     Internal combustion engines are known. The most common types of piston engines are two-stroke engines and four-stroke engines. These types of engines include a relatively large number of parts, and require numerous auxiliary systems, e.g., lubricant systems, cooling systems, intake and exhaust valve control systems, and the like, for proper functioning. 
     SUMMARY 
     Some embodiments may relate to a linear reciprocating engine. The linear reciprocating engine may include an internal combustion engine. The internal combustion engine may include a cylinder having a first combustion chamber at a first end thereof and a second combustion chamber at an opposing second end thereof, a piston slidably mounted within the cylinder, and a piston rod having a passageway extending through the piston into both combustion chambers. There may be provided at least one first opening in a first side of the piston rod configured to move into and out of the first combustion chamber to selectively communicate gas to the first combustion chamber, and at least one second opening in a second side of the piston rod configured to move into and out of the second combustion chamber to selectively communicate gas to the second combustion chamber. The piston may be slidable between a first position where the first opening is outside the first combustion chamber and the second opening is inside the second combustion chamber, and a second position where the first opening is inside the first combustion chamber and the second opening is outside the second combustion chamber. 
     According to some embodiments, an engine may be provided that enables gas to be communicated to each of two combustion chambers at separate times. Gas may be constantly supplied to flow through the passageway in the piston rod, while the gas is allowed to enter the cylinder only when an opening among the first openings and second opening is in communication with the cylinder. The first opening may include one or more openings, and the second opening may include one or more openings. Gas may travel through the piston rod to supply both of the combustion chambers appropriately during the various phases of the stroke of the engine. 
     Exemplary advantages and effects of the present invention will become apparent from the following description taken in conjunction with the accompanying drawings wherein certain embodiments are set forth by way of illustration and example. The examples described herein are just a few exemplary aspects of the disclosure. It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the invention. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG.  1    is a perspective view of a free piston engine, in accordance with embodiments of the present disclosure; 
         FIG.  2    is a perspective partial cross-sectional view of the engine of  FIG.  1    with the piston at top dead center on a right side of the cylinder, in accordance with embodiments of the present disclosure; 
         FIG.  3    is a perspective cross-sectional view of the engine of  FIG.  1   , in accordance with embodiments of the present disclosure; 
         FIGS.  4 A- 4 C  are views of a piston kit, in accordance with embodiments of the present disclosure; 
         FIG.  5    is a cross-sectional view of the engine of  FIG.  1    with the piston at top dead center on the right side of the cylinder, in accordance with embodiments of the present disclosure; 
         FIG.  6    is a cross-sectional view of the engine of  FIG.  1    with the piston in an expansion portion of a stroke from the right side of the cylinder to a left side of the cylinder, in accordance with embodiments of the present disclosure; 
         FIG.  7    is a cross-sectional view of the engine of  FIG.  1    with the piston at the end of the expansion portion of the stroke from the right side of the cylinder to the left side of the cylinder, in accordance with embodiments of the present disclosure; 
         FIG.  8    is a cross-sectional view of the engine of  FIG.  1    with the piston in a momentum portion of the stroke, in an early stage of compressing gasses on the left side of the cylinder, in accordance with embodiments of the present disclosure; 
         FIG.  9    is a cross-sectional view of the engine of  FIG.  1    as compression continues on the left side of the cylinder beyond the compression illustrated in  FIG.  8   , in accordance with embodiments of the present disclosure; 
         FIG.  10    is a cross-sectional view of the engine of  FIG.  1    with the piston at top dead center on left side of the cylinder, in accordance with embodiments of the present disclosure; 
         FIG.  11    is a cross-sectional view of the engine of  FIG.  1    with the piston in an expansion portion of a stroke from the left side of the cylinder to the right side of the cylinder, in accordance with embodiments of the present disclosure; 
         FIG.  12    is a cross-sectional view of the engine of  FIG.  1    with the piston at the end of the expansion portion of the stroke from the left side of the cylinder to the right side of the cylinder, in accordance with embodiments of the present disclosure; 
         FIG.  13    is a cross-sectional view of the engine of  FIG.  1    in a momentum portion of the stroke, in an early stage of compressing gasses on the right side of the cylinder, in accordance with embodiments of the present disclosure; 
         FIG.  14    is a cross-sectional view of the engine of  FIG.  1    as compression continues on the right side of the cylinder beyond the compression illustrated in  FIG.  13   , in accordance with embodiments of the present disclosure; and 
         FIG.  15    is a cross-sectional view of an engine with the piston at top dead center on the left side of the cylinder, in accordance with embodiments of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to internal combustion engines. While the present disclosure provides examples of free piston engines, it should be noted that aspects of the disclosure in their broadest sense, are not limited to a free piston engine. Rather, it is contemplated that the forgoing principles may be applied to other internal combustion engines as well. 
     As used herein, unless specifically stated otherwise, the term “or” encompasses all possible combinations, except where infeasible. For example, if it is stated that a component includes A or B, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or A and B. As a second example, if it is stated that a component includes A, B, or C, then, unless specifically stated otherwise or infeasible, the component may include A, or B, or C, or A and B, or A and C, or B and C, or A and B and C. 
     An internal combustion engine in accordance with the present disclosure may include an engine block. The term “engine block,” also used synonymously with the term “cylinder block,” may include an integrated structure that includes at least one cylinder housing a piston. In the case of a free piston engine block, the engine block may include a single cylinder, or it may include multiple cylinders. 
     In accordance with the present disclosure, a cylinder may define at least one combustion chamber in the engine block. In some internal combustion engines in accordance with the present disclosure, a combustion chamber may be located on a single side of a cylinder within an engine block. In some internal combustion engines in accordance with the present disclosure, the internal combustion engine may include two combustion chambers, one on each side of a cylinder within an engine block. 
     Embodiments of the present disclosure may further include a piston in the cylinder. In accordance with some embodiments of the disclosure used in a free piston engine, the piston may include two heads on opposite sides. In some embodiments, the piston may be considered to be “slidably mounted” in the cylinder. This refers to the fact that the piston may slide through a plurality of positions in the cylinder from one side of the cylinder to the other. While the present disclosure describes piston examples, the invention in its broadest sense is not limited to a particular piston configuration or construction. 
       FIG.  1    illustrates an exemplary embodiment of a free piston engine  10  according to the present disclosure. Free piston engine  10 , which is sometimes referred to herein simply as an engine, is one example of an internal combustion engine. Free piston engine  10  includes an engine block  8 . A cylinder  12  defining at least one combustion chamber may be included in engine block  8  and may have a central, longitudinal axis A, and, as shown in  FIG.  2   , a double-faced piston  50  mounted in cylinder  12 . Piston  50  may be configured to travel in a first stroke from a first end of the cylinder to an opposite second end of the cylinder, and in a second stroke from the second end of the cylinder back to the first end of the cylinder.  FIG.  2    illustrates a cutaway view showing a perspective partial cross-sectional view of the engine of  FIG.  1   .  FIG.  3    illustrates a perspective cross-sectional view, including a cross-sectional view of piston  50 .  FIGS.  5 - 10    illustrate an exemplary movement of piston  50  from a first end of the cylinder to a second end of the cylinder. At least one piston rod portion may be connected to the piston rod and may extend from a location within the at least one combustion chamber to an area external to the cylinder. As used herein, the term “piston rod portion” includes any portion of a rod or shaft, extending from a piston. In some embodiments, a piston rod portion may be a portion of a monolithic structure that makes up the piston, as well as other components. In some embodiments, a piston rod portion may be a portion of a piston rod that extends from only one face of a piston. 
     By way of example, as shown in  FIGS.  2  and  3   , a piston rod  40  may include a first piston rod portion  42  and a second piston rod portion  43 . First piston rod portion  42  may extend from one face of piston  50 . First piston rod portion  42  may extend from a location within the at least one combustion chamber to an area  65  external to the cylinder. Similarly, second piston rod portion  43  may extend from an opposite face of piston  50 , to another area  67  external to cylinder  12 . A piston kit may include first piston rod portion  42 , second piston rod portion  43 , and piston  50 . Piston rod portions  42  and  43  may be monolithic with each other, or may be completely separate structures, each extending from an opposite side or face of piston  50 . For example, piston kit  56  may be formed from a single piece of material. Piston rod  40  may be monolithic with piston  50 . In some embodiments, first piston rod portion  42  and second piston rod portion  43  may be monolithic with each other while being separate from piston  50 . 
     Cylinder  12  may include a first combustion chamber  71  and a second combustion chamber  73  (see  FIG.  7   ). Piston  50  may be configured to slide along axis A. In various positions within cylinder  12 , there may be a fluid communication path that connects first combustion chamber  71  with an air supply, or that connects second combustion chamber  73  with an air supply. 
     Reference is now made to  FIG.  5   , which illustrates a side cross-sectional view of engine  10 . An area external to cylinder  12  (e.g., areas  65  and  67 ) may include a vestibule at each end of the cylinder that may be a space configured for supplying gas, such as air, to each of the combustion chambers at the opposite ends of the cylinder from one or more sources of the gases external to the cylinder. For example,  FIG.  5    shows a first vestibule  30  and a second vestibule  31 . First vestibule  30  is exterior to cylinder  12  and thus exterior to first combustion chamber  71 , and second vestibule  31  is exterior to cylinder  12  and thus exterior to second combustion chamber  73 . The vestibules may be contained by a portion of the structure forming engine block  8  or may be formed by a separate structure connected to engine block  8 . The vestibules may be connected to an inlet manifold (not shown) that supplies gas. A passageway  46  that may be a part of piston rod  40  may be configured to deliver gas from first vestibule  30  to cylinder  12 , including first combustion chamber  71 . In some embodiments, passageway  46  may be configured to deliver gas from second vestibule  31  to cylinder  12 , including second combustion chamber  73 . 
     Cylinder  12  may include a peripheral cylinder wall  13  and exhaust ports  18  in peripheral cylinder wall  13 . In some embodiments, exhaust ports  18  may consist of a single port. Exhaust ports  18  may be connected to an exhaust manifold configured for receiving exhaust gases or other gases from the combustion chambers and directing the gases away from the cylinder for exhaust aftertreatment. In the manner discussed above, for example, a passageway of the piston rod may be configured to introduce gas into a combustion chamber from a location outside the cylinder. Also, gases may exit the cylinder through an exhaust port, such as exhaust ports  18 . In an embodiment, areas  65  and  67  external to cylinder  12  may simply refer to any region on an opposite side of a cylinder head  14 ,  15  from cylinder  12 , regardless of whether the region is in direct contact with a cylinder head. In some embodiments, other ports may be provided to introduce gases from a manifold or other source located alongside the cylinder, rather than at ends of the cylinder. Thus, in a general sense, locations outside the cylinder may be at the ends of the cylinder, alongside the cylinder, or a combination of both, for example. 
     In accordance with embodiments of the disclosure, a piston rod may include a passageway configured to communicate gas flow between at least one combustion chamber and an area external to the cylinder. As used herein, the term “passageway” can be defined by any structure or void capable of communicating gas flow. It may include, for example, a channel or conduit completely or partially contained within at least part of the piston rod portion. 
     For example, in some exemplary embodiments of an engine according to the disclosure, the passageway in the piston rod may render piston rod  40 , including piston rod portions  42  and  43 , at least partially hollow. In some embodiments, the passageway may extend completely through piston  50 . An opening may be formed in one or each piston rod portion that may be in fluid communication with the passageway of the piston rod, to thereby permit fluid to enter or exit the passageway through the opening. It is understood that a “fluid” may include gas, such as air. As shown in  FIG.  2   , first piston rod portion  42  may include opening  45 . Opening  45  may be arranged at the end of first piston rod portion  42  opposite piston  50 , which may be an open end. Piston rod portion  43  may also include an opening  47  (see  FIG.  3   ). In some embodiments, opening  47  may be occluded. For example, piston rod portion  43  may include a plug  49  that may be screwed into an open end of piston rod portion  43  opposite piston  50 . 
       FIG.  4 A  shows a cross-sectional view of piston kit  56 . Piston kit  56  may include one piston, such as piston  50 , and a piston rod  40  that may include two piston rod portions, such as first piston rod portion  42  and second piston rod portion  43 . Piston kit  56  may include passageway  46  extending through piston  50  into both combustion chambers  71  and  73 . That is, piston kit  56  may include a passageway that extends through piston  50  and further extends beyond a first face of piston  50  and extends beyond a second face of piston  50  opposite to the first face. Furthermore, piston kit  56  may include plug  49 . Plug  49  may be configured to occlude one end of piston rod  40 . For example,  FIG.  4 A  shows plug  49  screwed into threads in second piston rod portion  43  to occlude opening  47 . Plug  49  may seal opening  47  in an air-tight manner. Therefore, air introduced into piston rod  40  through opening  45  may be forced to exit piston rod  40  through either first opening  44  or second opening  48 . First opening  44 , second opening  48 , and opening  45  may be in fluid communication through passageway  46 . 
       FIG.  4 B  shows another view of piston kit  56 . Components of piston kit  56  may be fastened to one another in a fixed manner. Piston  50  and piston rod  40  may be formed separately from one another and may be fastened by a fastener. For example, screws  59  may be provided that fasten first piston rod portion  42 , piston  50 , and second piston rod portion  43  to one another. Furthermore, in some embodiments, first opening  44  and second opening  48  may each include a set of holes aligned at an axial position of piston rod  40 . For example, as shown in  FIG.  4 B , second opening  48   b  on second piston rod  43  may include a row of holes aligned with plane XY that is perpendicular to axis A. The holes may extend completely through the wall of second piston rod  43  so that gases may be communicated between passageway  46  and areas external to piston rod  40 . 
       FIG.  4 C  is a cross-sectional view of second piston rod portion  43  taken at plane XY. In accordance with some embodiments, the holes may form an angle with a radial direction of piston rod  40 . For example, the axis of a hole of second opening  48   b  may form an angle θ with the radial direction of piston rod  40 . Sidewalls of holes may extend in a direction parallel to an axis, the axis being at an angle to the radial direction of piston rod  40 . Axes of other holes among the holes of second opening  48   b  may form the same or different angles with the radial direction of piston rod  40 . In some embodiments, angle θ may be less than or equal to 45 degrees. In some embodiments, angle θ may be in a range of 5 to 25 degrees. Openings having angled sidewalls may be useful for imparting or encouraging swirl on gas flows travelling through piston rod  40  and through second opening  48   b . In some embodiments, there may be swirl vanes incorporated within piston rod  40 . In addition to a circular hole, various other shapes may be employed for second opening  48   b.    
     By way of example with reference to  FIG.  3   , each piston rod portion  42  and  43  may include a space  53 ,  55 , respectively (e.g., hollowed out internal portions of piston rod portions  42  and  43 ), forming at least a part of a conduit configured to communicate gas flow between an interior of cylinder  12  and an area external to cylinder  12 , such as areas  65  or  67 . Piston  50  may also include a space  54  that forms a part of the conduit configured to communicate gas flow between the interior of cylinder  12  and external areas. Hollowed out regions may, for example, be a bore through a core of a piston rod portion or a piston. Spaces  53 ,  54 , and  55  may be contiguous. Passageway  46  may include spaces  53 ,  54 , and  55 . 
     As illustrated in  FIG.  5   , first combustion chamber  71  may be defined by a region between a side of piston  50  facing first head  14 , and first head  14  of cylinder  12 . Meanwhile, second combustion chamber  73  may be defined by a region between an opposite side of piston  50  that faces second head  15 , and second head  15  of cylinder  12 . Likewise, as illustrated in  FIG.  10   , second combustion chamber  73  at that time may be defined by a region between a side of piston  50  facing second head  15 , and second head  15  of cylinder  12 . Meanwhile, first combustion chamber  71  may be defined by a region between the side of piston  50  facing first head  14 , and first head  14  of cylinder  12 . For example, at a top dead center position at a respective end of cylinder  12 , the combustion chamber at that time may be defined as the clearance volume. Similarly, the combustion chamber on the opposite side of the cylinder may be defined as the remaining open volume in cylinder  12 . Of course, it is to be understood that each combustion chamber is a variable region that includes a swept volume on each side of the piston, and which is compressed as the piston moves from one end of the cylinder to the opposite end of the cylinder. A swept volume may be defined as the volume displaced by piston  50  during at least a part of its reciprocating motion in cylinder  12 . Total volume of a cylinder may equal swept volume plus clearance volume. 
     Piston rod  40  may include at least one first opening, such as first opening  44  in a first side of piston rod  40 , and at least one second opening, such as second opening  48  in a second side of piston rod  40 , the second side being opposite the first side. In one exemplary embodiment, as shown in  FIG.  5   , for example, first opening  44  may include one or more ports in piston rod  40 . First opening  44  may be configured to serve as an inlet for conveying gas into cylinder  12  via passageway  46 . While an exemplary embodiment shown in  FIG.  5    shows first opening  44  including a plurality of circular holes, a variety of shapes and arrangements may be used. For example, first opening  44  may include elongated slots, grooves, openings having angled sidewalls, and the like. As discussed above,  FIG.  4 C , which is a cross-sectional view of the exemplary second piston rod portion  43  shown in  FIG.  4 B , shows second opening  48   b  including a plurality of holes having angled sidewalls. In the case of angled sidewalls, an axis of openings may be angled with respect to a radial direction of piston rod  40 . For example, as shown in  FIG.  4 C , an axis of a hole of second opening  48   b  may form an angle greater than 0 relative to a radially normal direction of second piston rod portion  43 . Openings including angled sidewalls may be useful to impart swirl onto fluid flows entering cylinder  12  and may affect flow characteristics. First opening  44  and second opening  48  may be similarly formed. 
     A wall thickness of piston rod  40  may be varied along axis A. For example, as shown in  FIG.  5   , piston rod  40  may have thicker sidewalls in a region of first opening  44  or second opening  48 . Sidewalls that are thicker in a region of the openings as compared to other parts of piston rod  40  may be advantageous for alleviating stress concentrators at or near the openings. Furthermore, thicker sidewalls may improve fatigue strength of piston rod  40  without substantially increasing weight. 
     Piston rod  40 , together with piston  50 , may be configured to move in a linear reciprocating motion in cylinder  12 . Piston  50  may be configured to slide within cylinder  12  past a plurality of positions. Due to back and forth motion of piston rod  40 , first opening  44  and second opening  48  may selectively communicate fluid flow from outside cylinder  12  to inside cylinder  12 . First opening  44  may be arranged on piston rod  40  such that first opening  44  is configured to move into and out of first combustion chamber  71  to selectively communicate gas to first combustion chamber  71 . Similarly, second opening  48  may be arranged on piston rod  40  such that second opening  48  is configured to move into and out of second combustion chamber  73  to selectively communicate gas to second combustion chamber  73 . 
     In accordance with some embodiments of the disclosure, sliding action of the piston may enable gases to be introduced into cylinder  12 , while gases on opposite sides of piston  50  may be prevented from being exchanged with one another. For example, a piston ring circumscribing piston  50  may prevent leakage of compressed gases past piston  50 . 
     In some embodiments, the cylinder head on each side of the engine block may include (e.g., be connected to or integrally formed with) an intake manifold (not shown). In some embodiments, only one cylinder head may include an intake manifold. Passageway  46  may be configured to communicate gas flow between first combustion chamber  71  and the intake manifold at the first end of cylinder  12  when piston  50  is in a first position. Furthermore, passageway  46  may be configured to communicate gas flow between second combustion chamber  73  and the intake manifold when piston  50  is in a second position. Thus, for example, with reference to  FIG.  5   , gases from area  65  may enter cylinder  12  as second opening  48  bridges cylinder head  14 . With reference to  FIG.  10   , gas from area  65  may enter cylinder  12  as first opening  44  bridges cylinder head  14 . 
     A cylinder in accordance with embodiments of the disclosure may be closed at both ends. For example, cylinder  12  of engine  10  may be closed at both ends thereof by cylinder heads  14  and  15 , which may be connected to the cylinder  12  by a plurality of fasteners. As used herein, the term “closed” does not require complete closure. For example, despite that the cylinder heads may have openings therein through which piston rod  40  passes, the cylinder heads are still considered “closed” within the meaning of this disclosure. 
     In some embodiments, a peripheral portion of cylinder  12  may be provided with cooling fins (not shown). Alternative configurations of the engine  10  may include other external or internal features that assist with the cooling of the cylinder, such as water passageways formed internally within the cylinder walls or jacketing at least portions of the cylinder walls for water cooling, and other configurations of cooling fins or other conductive or convective heat transfer enhancement features positioned along the exterior of a cylinder peripheral wall to facilitate fluid cooling of the cylinder. Engine block  8  may include fluid passage  21  that may be used for circulating cooling water to a peripheral sidewall of cylinder  12 . Fluid passage  21  may communicate with fluid port  5  (see  FIG.  1   ). Engine block  8  may further include fluid passage  22  that may be used for circulating cooling water to cylinder heads  14  and  15 . Fluid passage  22  may communicate with fluid port  6 . A temperature of cooling water in fluid passage  22  may be greater than that of fluid passage  21 . 
     In accordance with exemplary embodiments of the disclosure, peripheral wall  13  of cylinder  12  may include at least one exhaust port between ends of cylinder  12 . By way of example only, cylinder  12  may include an exhaust port  18  in a peripheral side wall of cylinder  12  between first cylinder head  14  and second cylinder head  15 , the first and second cylinder heads  14 ,  15  being positioned at ends of the cylinder. In the exemplary embodiment illustrated in  FIGS.  5 - 14   , a plurality of distributed exhaust ports  18  may be spaced about the circumference of cylinder  12  at or approximately near a midpoint of cylinder  12  between the opposite ends of the cylinder. Exhaust ports  18  may be of any suitable size, shape, and distribution so as to accomplish the function of exhausting gases from the cylinder. One or more of the exhaust ports may, for example, be located in an axial central region of the cylinder peripheral wall, as illustrated in the figures. Although an exemplary embodiment shown in the figures is configured symmetrically, with exhaust ports  18  located midway between the opposite ends of cylinder  12 , alternative embodiments may position the exhaust ports at one or more radial planes intersecting the cylinder peripheral wall at locations other than the exact midway point between cylinder heads  14  and  15 . 
     In accordance with some exemplary embodiments of the disclosure, one or more exhaust ports  18  may be configured to communicate gas flow between the first combustion chamber and outside the cylinder when piston  50  is on the second combustion chamber side of the one or more exhaust ports  18 , and may be configured to communicate gas flow between the second combustion chamber and outside the cylinder when the piston is on the first combustion chamber side of the one or more exhaust ports  18 . By way of example only, this can occur when, as illustrated in  FIG.  8   , piston  50  is located to the left of one or more exhaust ports  18 , enabling conveyance of gas flow through the one or more exhaust ports  18  from the combustion chamber to the right of piston  50 . The one or more exhaust ports  18  enable gas flow to a location “outside” the combustion chamber. That outside location may be on the side of the cylinder as illustrated, or conduits  19  associated with the engine might deliver the gases to other locations. In some embodiments, conduits  19  may be connected to an exhaust manifold (not shown). 
     With reference to  FIG.  5   , an inlet manifold, which may include vestibules for communicating gases, such as air, may be connected to or formed integrally with each of cylinder heads  14 ,  15  at opposite ends of cylinder  12 . For example, first vestibule  30  may be formed at a first end on a side of cylinder  12 , and second vestibule  31  may be formed at a second end on the opposite side of cylinder  12 . Each of vestibules  30  and  31  may include a piston rod opening, such as opening  28 , which is axially aligned with axis A, and side opening  33 , or other openings. Side opening  33  may be positioned at a distal end of the inlet manifold, as shown, or at any location along the outer periphery of the inlet manifold. As shown in  FIG.  5   , side opening  33  may be sealed. Furthermore, first vestibule  30  and second vestibule  31  may each include a bushing  41 . Bushings  41  may be provided for aligning, supporting, guiding, and sealing (by, e.g., means of a dedicated seal) an end of first and second piston rod portions  42 ,  43  while allowing first and second piston rod portions  42 ,  43  to slide in and out of a respective opening  28 . In some embodiments, side opening  33  may be opened to allow inlet air to enter. 
     In some embodiments, an inlet chamber  32  may be provided for allowing inlet air to enter engine  10 . For example, as illustrated in  FIG.  5   , inlet chamber  32  may be arranged adjacent to first vestibule  30 . Passageway  46  may be configured to deliver gas from inlet chamber  32  to cylinder  12 . Gas may be delivered from inlet chamber  32  to second combustion chamber  73  in the first position of piston  50 , as shown in  FIG.  5   , and gas may be delivered from inlet chamber  32  to first combustion chamber  71  in the second position of piston  50 , as shown in  FIG.  10   . Inlet chamber  32  may include inlet opening  29 . Inlet opening  29  may be configured to direct inlet gases, such as air, into the inlet manifold in a direction along axis A. Inlet opening  29  may be configured to direct gases into passageway  46  substantially through opening  45  in first piston rod portion  42 . Although the inlet manifold of the exemplary embodiment shown in  FIG.  5    is illustrated as having a substantially cylindrically-shaped configuration, alternative embodiments may provide one or more inlet manifolds with other shaped profiles or cross sections, or may incorporate the inlet manifolds at least partially within cylinder heads  14 ,  15  as one or more internal passageways defined within each of the cylinder heads at each end of cylinder  12 . Inlet opening  29  may include a substantially cylindrical member connected to inlet chamber  32  along axis A, among other configurations. 
     Supplying air from one end of engine  10  through inlet chamber  32  may provide a number of benefits. For example, air may be directed to flow into engine  10  substantially in a direction parallel to axis A, which may be in a longitudinal direction of engine  10 . Piston rod  40  may include opening  45  and passageway  46 , which is arranged to extend substantially along axis A, and thus, air may flow through piston rod  40  with less turbulence, which may reduce pressure losses. Furthermore, regions of stagnation may be minimized. As compared with providing air entering through side opening  33 , for example, providing inlet chamber  32  may result in improved flow characteristics. 
     Air may be introduced into engine  10  through a vestibule among first vestibule  30  and second vestibule  31 . Air may be configured to flow from area  65  or area  67  (external to cylinder  12 ) to an interior of cylinder  12 , including first combustion chamber  71  or second combustion chamber  73 . Air may be introduced into engine  10  through inlet chamber  32  via a vestibule among first vestibule  30  and second vestibule  31 . For example, when piston  50  is in the first position, air may travel through inlet opening  29 , and then air may be in communication with area  65 , passageway  46 , and second combustion chamber  73 , as shown in  FIG.  5   . Also, when piston  50  is in the second position, air may travel through inlet opening  29 , and then air may be in communication with area  65 , first combustion chamber  71 , passageway  46 , and area  67 . 
     Engine  10  may include a first isolation area on one side of cylinder  12  and a second isolation area on an opposite side of cylinder  12 . The first isolation area may include area  65 , and the second isolation area may include area  67 . The first and second isolation areas may be configured to isolate non-active piston rod parts during alternate cylinder charges. For example, when piston  50  is in the first position, air may travel from inlet opening  29 , through passageway  46 , and into second combustion chamber  73 , as shown in  FIG.  5   . In the position as shown in  FIG.  5   , air from inlet  29  may not be in communication with area  67 . Thus, area  67  may isolate a portion of piston rod  40 . A non-active part of piston rod  40  may refer to a portion through which gases do not travel to reach a combustion chamber. 
     Air may be supplied to engine  10  through a single air supply. The air supply may be flow-connected to passageway  46  in piston rod  40  such that gas flow is communicated between first combustion chamber  71  and the air supply when piston  50  is in the first position. Furthermore, passageway  46  may be configured to communicate gas flow between second combustion chamber  73  and the air supply when piston  50  is in a second position. It is understood that air may be introduced into engine  10  by openings other than inlet opening  29 . For example, air may be introduced via one or more of side openings  33 . Air may be introduced through side opening  33  in first vestibule  30  while other openings (such as inlet opening  29 ) are sealed off. Thus, gas may be delivered from first vestibule  30  to cylinder  12 . Alternatively, air may be introduced through side opening  33  in second vestibule  31  while other openings are sealed off. Side opening  33  may be configured such that air is directed to flow into engine  10  substantially in a direction perpendicular to axis A. Side opening  33  may be configured to direct gases into passageway  46  substantially through first opening  44 , or through second opening  48 . Configuring openings to introduce air into engine  10  by way of, e.g., inlet opening  29 , or side opening  33 , may allow design flexibility in consideration of packaging constraints. For example, in some embodiments, when air is introduced through inlet opening  29 , engine  10  may be packaged into a long, thin space. In other embodiments, when air is introduced through side opening  33 , engine  10  may be packaged into a short, compact space. 
     In some embodiments, both first vestibule  30  and second vestibule  31  may be configured to supply air to engine  10 . For example, both side opening  33 , provided on first vestibule  30 , and side opening  33 , provided on second vestibule  31 , may be opened. The single air supply may supply air to both first vestibule  30  and second vestibule  31 . 
     Each of the cylinder heads  14 ,  15  may further include an injector  34  (see  FIG.  1   ) that opens into an annular or toroidal-shaped recess  36  formed in or contiguous with a flame face of a fire deck of each cylinder head at each end of the cylinder  12  in facing relationship with the combustion chambers at each end of cylinder  12  (see  FIG.  3   ). Recess  36  may be toroidal and may impart swirl flow to fuel gas injected by injectors  34  to facilitate more complete combustion of the gases within the combustion chambers. Each of cylinder heads  14 ,  15  may also include a cavity for accommodating and mounting a spark plugs  38 . Each of cylinder heads  14 ,  15  may also include a cavity for accommodating and mounting bushings  41  for aligning, supporting, guiding, and sealing (by, e.g., means of a dedicated seal) a respective one of piston rod portions  42 ,  43  that is supported by, and passes through each of the cylinder heads  14 ,  15  at opposite ends of the cylinder  12 . This is one example of how piston rod portions may extend from faces of a double-faced piston through a combustion chamber. Regardless of the particular details of any aperture through which the piston rods may extend at ends of the cylinder, a piston rod that extends to at least an end of the cylinder is said to extend through a combustion chamber within the meaning of this disclosure. 
     A double-faced piston consistent with embodiments of the present disclosure may be configured to travel in a first stroke from a first end of the cylinder to an opposite second end of the cylinder, and in a second stroke from the second end of the cylinder back to the first end. This length of travel is illustrated, by way of example, in  FIGS.  5 - 14   , where  FIG.  5    represents the beginning of a first stroke,  FIG.  10    represents the end of the first stroke, which may also be the beginning of a second stroke, and  FIGS.  6 - 9    represent exemplary intermediate positions.  FIG.  10    may represent the beginning of the second stroke,  FIG.  14    may represent the end of the second stroke, and  FIGS.  10 - 12    may represent some exemplary intermediate positions. 
     Piston  50  may be slidable between a plurality of positions throughout cylinder  12 . For example, piston  50  may be slidable between a first position and a second position. The first position may be a position where first opening  44  is outside cylinder  12  and second opening  48  is inside cylinder  12 . At the first position, second opening  48  may be inside the second combustion chamber of cylinder  12 . The second position may be a position where first opening  44  is inside cylinder  12  and second opening  48  is outside cylinder  12 . At the second position, first opening  44  may be inside the first combustion chamber of cylinder  12 . The first position may correspond to the beginning of a first stroke of engine, and the second position may correspond to the end of the first stroke. 
     According to various exemplary embodiments of the present disclosure, the piston may be sized relative to the cylinder to enable an expansion stroke portion of each stroke wherein the piston travels under gas expansion pressure, and a momentum stroke portion of each stroke for the remainder of the stroke following the expansion stroke portion. The expansion stroke portion of each of the first and second strokes of the piston is the portion of travel when the piston directly moves under the expansion pressure of combustion. For example, the expansion portion of a stroke may be defined as the portion from a combustion position of the piston at each end of the cylinder to the point at which exhaust gases may be exchanged between the combustion chamber in which ignition of combustion gases (including air and fuel) has just occurred and an area external to the cylinder. In some embodiments, the termination of the expansion stroke portion may coincide with a position where the piston begins to expose an exhaust port. 
     At the combustion position of the piston during each stroke, a clearance volume may remain between each of the opposite faces of the piston and a respective end of the cylinder as closed off by the cylinder heads  14 ,  15 . The combustion gases that are introduced into the combustion chamber before the piston reaches the combustion position may be compressed into the remaining clearance volume on that side of the piston between the piston face and the fire deck of the cylinder head. The compressed gasses in the clearance volume may be compressed into such a small volume that gas pressure prevents piston  50  from contacting a respective one of cylinder heads  14 ,  15 . The compressed gases, which in some embodiments may include a fuel/air mixture, may be ignited by either a spark, or by self-ignition resulting at least in part from the compression of the combustion gases. 
     The combustion position may be a point along axis A corresponding to the beginning of a combustion event in cylinder  12 . The combustion position may be a point where a predetermined compression ratio of gases in a combustion chamber is reached. For example, the combustion position may be a point where a compression ratio of a combustion chamber reaches 10:1. Combustion may be initiated at the combustion position by activating spark plug  38 . The combustion position may be a point where piston  50  changes direction. The combustion position may be a zero-speed position of piston  50 . In some embodiments, engine  10  may be configured so that piston  50  decelerates to zero-speed at the moment that combustion is initiated. The combustion position may correspond to the first position mentioned above, which may correspond to the beginning of the first stroke of engine  10 . In some embodiments, the combustion position may be a fixed position. However, it will be understood that the combustion position may be a variable position that may be determined, for example, by when spark plug  38  is activated, or when auto-ignition is configured to occur. 
     The expansion stroke portion of each stroke occurs after the ignition of the compressed combustion gases as chemical energy from the combustion in each combustion chamber is converted into kinetic energy (e.g., mechanical work) of the piston. Simultaneously with the expansion stroke portion of each stroke on one side of the piston, gas flow may occur for at least a portion of the expansion stroke portion between the combustion chamber on the opposite side of the piston and the intake manifold at the opposite end of the cylinder, as well as through exhaust ports. 
     In some embodiments, useful work may be extracted from engine  10  by, for example, mechanically coupling one end of piston rod  40  to an output. Second piston rod portion  43  may be connected to an apparatus at an end opposite piston  50  that may be configured to convert reciprocating linear motion to useful work. For example, a linear actuator may be coupled to second piston rod portion  43  at opening  47  (see, e.g.,  FIG.  5   ). Plug  49  may be integral with an actuator configured to convert linear motion to work. Such an actuator may include a generator, for example. 
     Referring back to  FIG.  5   , at the beginning of an expansion stroke portion of a stroke where piston  50  moves from the right end of cylinder  12  to the left end, piston  50  may be in a first position. From this first position, gas flow may occur between the combustion chamber on the left side of piston  50  and inlet chamber  32 , which may be on the right side of cylinder  12 . Also from this first position, gas flow may also occur between the combustion chamber on the left side of piston  50  and conduits  19  leading to an outside of cylinder  12  through one or more exhaust ports  18 . The communication of gases between the combustion chamber on the left side of piston  50  and conduits  19  may continue until the left face of piston  50  has moved past the one or more exhaust ports  18 , acting as an exhaust valve and shutting off communication between the left combustion chamber and the one or more exhaust ports  18 . Additionally, before piston  50  has closed off the one or more exhaust ports  18 , second opening  48  closest to the left face of the piston may have moved outside of the left combustion chamber, thereby closing off communication of gases between inlet chamber  32  and the left combustion chamber through passageway  46  of piston rod  40 . Thus, piston rod  40  may be configured such that a gas intake phase in a combustion chamber of cylinder  12  ends before an exhaust phase. A length from a proximal face of piston  50  to a proximal end of second opening  48  may be set such that fluid communication between second opening  48  and interior of cylinder  12  is stopped before fluid communication between exhaust ports  18  and interior of cylinder  12 . For example, a length from the left face of piston  50  to the right end of second opening  48  may be set to be greater than a length of piston travel in the expansion phase. The length from the left face of piston  50  to the right end of second opening  48  may be greater than the length of piston travel in the expansion phase by at least a quarter of the length of piston  50 . Lengths may be set to control, for example, a pressure buildup in passageway  46 , as will be discussed later. 
     As shown in  FIG.  6   , piston  50  may move to a further position. At the position shown in  FIG.  6   , second opening  48  is no longer inside cylinder  12 , and thus, fluid communication between passageway  46  of piston rod  40  and the interior of cylinder  12  via second opening  48  is stopped. 
     According to some embodiments, a length (in axial direction A) of piston  50 , a length of cylinder  12 , a location of exhaust ports  18 , and a location of first and second openings  44 ,  48  in first and second piston rod portions  42 ,  43  may be arranged such that when piston  50  is in a combustion phase in the first combustion chamber, piston  50  blocks exhaust ports  18  from communicating with the first combustion chamber and first opening  44  in first piston rod portion  42  is outside of the first combustion chamber, while simultaneously exhaust ports  18  are in fluid communication with the second combustion chamber, and second opening  48  in second piston rod portion  43  is within the second combustion chamber. This may be accomplished by various alternative structures. By way of example only with reference to the figures, the length of piston  50 , the length of cylinder  12 , the location of exhaust ports  18 , and the location of openings  44  and  48  in each of first and second piston rod portions  42 ,  43  extending from opposite faces of piston  50  may be arranged such that when piston  50  is in a combustion phase in a first combustion chamber on one side of piston  50 , piston  50  blocks exhaust ports  18  from communicating with the first combustion chamber. First opening  44  to the one side of piston  50  remains outside of the first combustion chamber, thereby preventing communication of gases between inlet chamber  32  on that one side of piston  50  and the first combustion chamber. 
     Simultaneously, exhaust ports  18  are in fluid communication with the second combustion chamber on the opposite side of piston  50 , and second opening  48  in second piston rod portion  43  may be located within the second combustion chamber. Similarly, when piston  50  is in another combustion stage in the second combustion chamber on the opposite side, piston  50  blocks exhaust ports  18  from communicating with the second combustion chamber. Second opening  48  to the second side of piston  50  remains outside of the second combustion chamber, thereby preventing communication of gases between inlet chamber  32  on the second side of piston  50  and the second combustion chamber. Simultaneously, exhaust ports  18  are in fluid communication with the first combustion chamber on the first side of piston  50 , and first opening  44  in first piston rod portion  42  may be located within the first combustion chamber. 
     According to some embodiments, the length of piston  50 , a length of cylinder  12 , a location of exhaust ports  18 , and a location of first and second openings  44 ,  48  in first and second piston rod portions  42 ,  43  may be arranged such that when piston  50  continues to move through the first stroke, the combustion phase ends (concurrent with exhaust phase beginning) before first opening  44  enters cylinder  12 .  FIG.  7    illustrates a position where combustion on the right side of piston  50  may end while an exhaust phase may begin. The precise location where combustion ends and exhaust begins on right side of piston  50  may correspond to a position where the right face of piston  50  reaches the right edge of exhaust ports  18 . At this position, first opening  44  is outside cylinder  12 . 
     Following an expansion stroke portion (also called a combustion phase), piston  50  may continue to move in a momentum stroke portion for a remainder of the stroke. The momentum stroke portion of each stroke encompasses the remaining portion of the stroke following the expansion stroke portion. In accordance with embodiments of the disclosure, substantially the entire momentum stroke portion of the second stroke on the second combustion chamber side of piston  50  may coincide with compression of gases in the first combustion chamber. That is, the momentum that follows an expansion portion of the stroke in one combustion chamber may be used to compress gasses in the other combustion chamber. This may be made possible by an engine structure where an end of an expansion in one combustion chamber may correspond with a position different from the combustion position in an opposing combustion chamber. Such an engine design may enable further piston travel following an expansion portion of the stroke. In some embodiments, the further piston travel during the momentum portion of the stroke may be at least a width of the piston. A “width” of the piston may be synonymous with a length of the piston in the direction of axis A. In some embodiments the further piston travel may be multiple times a width of the piston. In other embodiments, the further piston travel may be a fraction of the width of the piston, for example at least a half a width of the piston. In yet other embodiments, the further travel may be at least a quarter a width of the piston. Further travel of piston  50  beyond at least one of exhaust ports  18  may be referred to as piston overshoot. 
     During the momentum stroke portion of each stroke, gases may be exchanged between the combustion chamber where ignition of combustion gases has just occurred and an area external to cylinder  12 . The exchange of gases may occur through exhaust ports formed in peripheral wall  13  of cylinder  12 . The exchange of gases may be aided by introduction of air into cylinder  12  through a passageway in the piston rod portion connected to the piston and extending from a location within the at least one combustion chamber to an area external to the cylinder. By way of one example with reference to  FIGS.  5 - 10   , the positions of piston  50  and piston rod portions  42 ,  43  are shown during a first stroke from the far-right position of piston  50  in cylinder  12 , as in  FIG.  5   , to the far-left position of piston  50  in cylinder  12 , as in  FIG.  10   .  FIGS.  10 - 14    show the positions of piston  50  and piston rod portions  42 ,  43  during a second stroke from the far-left position of piston  50  in cylinder  12 , as in  FIG.  10   , to the far-right position of piston  50  in cylinder  12 , as in  FIG.  14   . The far-left and far-right positions of piston  50  in cylinder  12  may be referred to as the combustion position for the stroke in which the combustion gases have been compressed and ignition of the gases at the beginning of a combustion phase is occurring. When piston  50  is in the far-right position, as in  FIG.  5   , and ignition is occurring for the combustion gases that have been compressed into a clearance volume between the right face of piston  50  and cylinder head  14  at the right end of cylinder  12 , piston  50  is at the combustion position for the stroke from the right end to the left end of cylinder  12  as viewed in  FIGS.  5 - 10   . Similarly, when piston  50  is in the far left position of  FIG.  10    and ignition is occurring for the combustion gases that have been compressed into a clearance volume between the left face of piston  50  and cylinder head  15  at the left end of cylinder  12 , piston  50  is at the combustion position for the stroke from the left end to the right end of cylinder  12  as viewed in  FIGS.  10 - 14   . 
       FIG.  5    illustrates a position where piston kit  56  is at a starting point of a first stroke that may be defined as a stroke from the right end of cylinder  12  to the left end of cylinder  12 . Components of piston kit  56 , including first piston rod portion  42 , piston  50 , and second piston rod portion  43  may be joined together such that piston kit  56  travels as a unit. In the position shown in  FIG.  5   , a combustion phase may begin in first combustion chamber  71  on the right side of piston  50 . Meanwhile, on the opposite side of piston  50 , gas may be introduced into cylinder  12  through a flow path including passageway  46  in piston rod  40 . For example, gas may be introduced into inlet chamber  32  via inlet opening  29 . The gas may be air. The gas may be pressurized relative to ambient atmospheric pressure. First piston rod portion  42  includes opening  45 , and thus, the gas from inlet chamber  32  is in fluid communication with passageway  46  of piston rod  40 . Furthermore, because opening  47  in second piston rod portion  43  may be occluded by plug  49  (or by virtue of second vestibule  31  being sealed off in an air-tight manner, for example), gas is forced into cylinder  12  via second opening  48 . In this position, cylinder  12  on the left side of piston  50  may begin to fill with gas. Because exhaust ports  18  are open, some gas may escape. 
     As combustion begins, piston  50  will move to the left. As shown in  FIG.  6   , piston  50  continues to move from the combustion position for the stroke from the right end of cylinder  12  to the left end of cylinder  12 .  FIG.  6    illustrates a position where second opening  48  in second piston rod portion  43  reaches cylinder head  15 . Accordingly, introduction of gas into cylinder  12  stops. 
       FIG.  7    illustrates a position where the combustion phase in first combustion chamber  71  on the right side of piston  50  may end and where a compression phase in second combustion chamber  73  on the left side of piston  50  may begin. The combustion phase may end when piston  50  begins to expose exhaust ports  18  on the right side of piston  50 . At the same time, an exhaust phase on the right side of piston  50  may begin. Thus, the exhaust phase may begin when first combustion chamber  71  becomes open. The compression phase may begin when piston  50  completely covers exhaust ports  18  on the left side of piston  50 . Thus, the compression phase may begin when second combustion chamber  73  becomes sealed. A width of piston  50  may be equal to that of exhaust ports  18 . Although in some embodiments combustion on the right side of piston  50  may end substantially simultaneously with compression beginning on the left side of piston  50 , the present disclosure is not so limited. Some embodiments may allow combustion and compression phases on opposite sides of piston  50  to begin at different times. For example, a width of piston  50  may be greater than a width of exhaust ports  18 , and thus, compression may begin while the combustion phase on the opposite side of piston  50  is still occurring. Similarly, a width of piston  50  may be less than a width of exhaust ports  18 , and thus, the combustion phase may end before the compression phase begins on the opposite side of piston  50 . 
     As piston  50  continues to move to the left, piston  50  may reach a position where piston  50  has just passed the centrally located exhaust ports  18 , as shown in  FIG.  8   . At this point, exhaust ports  18  are fully exposed on the right side of piston  50 . Thus, first combustion chamber  71  on the right side of piston  50  is in fluid communication with exhaust ports  18  and exhaust gases from the combustion exit the combustion chamber. Therefore, the expansion stroke portion of the stroke has ended, and the piston is continuing to travel toward the left end of cylinder  12  in the momentum stroke portion as a result of inertia remaining after the end of the expansion stroke portion. 
     As shown in  FIGS.  8  and  9   , piston  50 , first piston rod portion  42  on the right side of piston  50 , and exhaust ports  18  may be configured such that piston  50  passes all of exhaust ports  18  as the piston moves from the right end of cylinder  12  toward the left end of cylinder  12  before first opening  44  in first piston rod portion  42  enters first combustion chamber  71  on the right side of piston  50 . As shown in  FIG.  9   , piston  50  has moved completely to the left of exhaust ports  18  by the time first opening  44  in the right piston rod portion  42  begins to enter first combustion chamber  71  on the right side of piston  50  to permit gas flow between first combustion chamber  71  and first opening  44 . This relative sizing and spacing of the various components may allow exhaust gases generated in first combustion chamber  71  to begin exiting from the exhaust ports  18  before fresh pre-compressed air or other gases are introduced into first combustion chamber  71  through first piston rod portion  42  on the right side of piston  50 . In various alternative embodiments, the precise placement of openings through piston rod portions  42 ,  43  relative to the opposite faces of piston  50  may be varied such that the closest inlet port to each face of the piston enters the respective combustion chamber on the same side of the piston shortly after the face of the piston has passed the near edge of the exhaust ports, thereby allowing exhaust gases to begin exiting the respective combustion chamber a short time before introduction of the fresh pre-compressed air or other gases (see, e.g.,  FIGS.  9  and  14   ). 
     Shortly after piston  50  has passed exhaust ports  18  during the momentum stroke portion of the stroke from the right end of cylinder  12  to the left end of cylinder  12 , as shown in  FIG.  9   , the edges of first opening  44  in first piston rod portion  42  closest to the right face of piston  50  start to enter first combustion chamber  71  on the right side of piston  50 . At this point, a scavenging phase may occur on the right side of piston  50  as a result of gases, such as fresh air, being introduced into first combustion chamber  71  through first opening  44  of first piston rod portion  42 . Scavenging may refer to the process of pushing exhausted gas including combustion products out of cylinder  12  and drawing in fresh air for the next cycle. A certain amount of scavenging may be desired so that the next cycle does not begin with a mix of exhaust gases rather than substantially clean air. First opening  44  may be configured such that when piston  50  is in the momentum stroke portion of the first stroke from the right end to the left end of cylinder  12 , gas flow may be continuously communicated between first combustion chamber  71  and an area external to cylinder  12 . In the exemplary embodiment shown in  FIG.  10   , fresh, pre-compressed air may be introduced into first combustion chamber  71  from inlet chamber  32  located opposite cylinder head  14  or integral with cylinder head  14  on the right end of cylinder  12 . Simultaneously, exhaust gases may be scavenged from first combustion chamber  71  by the incoming pre-compressed air or other gases and forced out through exhaust ports  18 . 
     Some aspects of the present disclosure may involve cylinder  12  and piston  50  being sized such that the expansion stroke portion of the first stroke on a first side of piston  50  as piston  50  moves from the first end of cylinder  12  to the second end of cylinder  12  coincides with at least one of a scavenging phase and a gas boost phase on a second side of piston  50 . A similar coincidence may occur in connection with the second stroke. By way of non-limiting example with reference to the figures, as piston  50  continues to move toward the left end of the cylinder, as shown in  FIGS.  9  and  10   , gas flow may be continuously communicated between first combustion chamber  71  and an area external to cylinder  12 . The continuous flow of pre-compressed air or other gases introduced from inlet chamber  32  into first combustion chamber  71  may assist with cooling of cylinder  12  as well as scavenging of exhaust gases from first combustion chamber  71  and boosting the gas pressure in first combustion chamber  71 . A similar coincidence is illustrated for the second stroke in  FIGS.  13  and  14   . In some embodiments, the coincidence of compression on one side of cylinder  12  with scavenging and gas boost on the other side of cylinder  12  may precisely correspond. In other embodiments compression on one side of cylinder  12  may substantially overlap with scavenging and gas boost on the other side of cylinder  12 . 
     Some aspects of the present disclosure may involve cylinder  12  and piston  50  being sized such that the momentum stroke portion of the first stroke on a first side of piston  50  as piston  50  moves from the first end of cylinder  12  to the second end of cylinder  12  coincides with a compression phase in the combustion chamber on a second side of piston  50 . By way of non-limiting example, simultaneously with the momentum stroke portion of the first stroke from the right end of cylinder  12  to the left end of cylinder  12 , after piston  50  has moved past exhaust ports  18  toward the left end of cylinder  12 , gases on the left side of piston  50  are compressed during a compression phase on the left side of piston  50 . When piston  50  is all the way to the left, as shown in  FIG.  10   , the combustion gases on the left side of piston  50  will have been compressed into the remaining clearance volume of second combustion chamber  73  and ignition may occur to begin the second stroke. 
     As best seen by way of non-limiting example in  FIGS.  5 - 14   , cylinder  12  and piston  50  may be sized such that a total distance piston  50  travels during the first stroke from the right end of cylinder  12  to the left end of cylinder  12 , or during the second stroke from the left end of cylinder  12  to the right end of cylinder  12 , may be substantially greater than a distance piston  50  travels during the expansion stroke portion of either stroke. In some exemplary embodiments, cylinder  12  and piston  50  may be sized such that the total distance piston  50  travels during each stroke from one end of cylinder  12  to the opposite end of cylinder  12  may exceed the distance piston  50  travels during the expansion stroke portion of the stroke by at least the length of piston  50  from one face to the opposite face. In other exemplary embodiments, cylinder  12  and piston  50  may be sized such that a total distance piston  50  travels in each stroke exceeds, by at least the length of piston  50 , a distance traveled by piston  50  during compression of gases on one side of piston  50 . The length of the piston  50  from one face to the opposite face in the exemplary embodiment shown in the figures may be less than ½ of a distance from at least one of cylinder heads  14 ,  15  to centrally located exhaust ports  18 . This configuration and relative sizing of the piston and cylinder may allow for a significantly greater length of the total stroke for the piston in each direction during which fresh pre-compressed air or other gases may be introduced into the cylinder for the purposes of scavenging exhaust gases and cooling the cylinder after each combustion occurs at opposite ends of the cylinder. 
     At the beginning of an expansion stroke portion of a stroke from the left end of cylinder  12  to the right end, as shown in  FIG.  10   , gas flow may occur between first combustion chamber  71  on the right side of piston  50  and inlet chamber  32  on the right side of cylinder  12 , and between first combustion chamber  71  on the right side of piston  50  and exhaust ports  18 . The communication of gases between first combustion chamber  71  and exhaust ports  18  may continue until the right face of piston  50  has moved past exhaust ports  18 , acting as an exhaust valve and shutting off communication between first combustion chamber  71  and exhaust ports  18 . Additionally, before piston  50  has closed off exhaust ports  18 , first opening  44  of first piston rod portion  42  may have moved outside of first combustion chamber  71 , thereby closing off communication of gases between inlet chamber  32  and first combustion chamber  71  through piston rod  40 . 
     The length of piston  50 , the length of cylinder  12 , the location of exhaust ports  18 , and the location of openings  44 ,  48  in each of the first and second piston rod portions  42 ,  43  extending from opposite faces of piston  50  may be arranged such that when piston  50  is in a combustion phase in second combustion chamber  73  on the left side of piston  50 , piston  50  blocks exhaust ports  18  from communicating with second combustion chamber  73 . Meanwhile, second opening  48  to the left side of piston  50  remains outside of second combustion chamber  73 , thereby preventing communication of gases between inlet chamber  32  and second combustion chamber  73 . Simultaneously, exhaust ports  18  are in fluid communication with first combustion chamber  71  on the right side of piston  50 , and first opening  44  in first piston rod portion  42  is located within first combustion chamber  71 . 
     The momentum stroke portion of each stroke may encompass the remaining portion of the stroke following the expansion stroke portion. During the momentum stroke portion of each stroke, gases may be exchanged between the combustion chamber where ignition of combustion gases has just occurred and an area external to the cylinder. The exchange of gases may occur through exhaust ports formed in the peripheral wall of the cylinder.  FIGS.  10 - 14    show the positions of piston  50  and piston rod portions  42 ,  43  during a second stroke from the far-left position of the piston in  FIG.  10    to the far-right position of the piston in  FIG.  14   . As discussed above, the far-left and far-right positions of piston  50  in cylinder  12  may be referred to as combustion positions for the respective strokes, in which the combustion gases have been compressed and ignition of the gases at the beginning of a combustion phase may be occurring. When piston  50  is in the far-left position as in  FIG.  10    and ignition is occurring for the combustion gases that have been compressed into a clearance volume between the left face of piston  50  and cylinder head  15  at the left end of cylinder  12 , piston  50  is at the combustion position for the stroke from the left end to the right end of cylinder  12 , as viewed in  FIGS.  10 - 14   . 
     As combustion begins in the second stroke, piston  50  will move to the right. As shown in  FIG.  11   , piston  50  continues to move from the combustion position for the stroke from the left end of cylinder  12  to the right end of cylinder  12 .  FIG.  11    illustrates a position where first opening  44  in first piston rod portion  42  reaches cylinder head  14 . Accordingly, introduction of gas into cylinder  12  stops. 
       FIG.  12    illustrates a position where the combustion phase in second combustion chamber  73  on the left side of piston  50  may end and where a compression phase in first combustion chamber  71  on the right side of piston  50  may begin. The combustion phase may end when piston  50  begins to expose exhaust ports on the left side of piston  50 . At the same time, an exhaust phase on the left side of piston  50  may begin. Thus, the exhaust phase may begin when second combustion chamber  73  becomes open. The compression phase may begin when piston  50  completely covers exhaust ports  18  on the right side of piston  50 . Thus, the compression phase may begin when first combustion chamber  71  becomes sealed. According to a ratio of width of piston  50  to that of exhaust ports  18 , the timing of the end of the combustion phase on the left side of piston  50  and beginning of compression on the right side of piston  50  may be adjusted. 
     As the piston continues to move to the right, piston  50  may reach a position where piston  50  passes the centrally located exhaust ports  18 , as shown in  FIG.  13   . At this point, exhaust ports  18  have been fully exposed on the left side of piston  50 . Thus, second combustion chamber  73  on the left side of piston  50  is in fluid communication with exhaust ports  18  and exhaust gases from the combustion that occurred on the left side of piston  50  during the expansion stroke portion of the second stroke may exit second combustion chamber  73 . Therefore, the expansion stroke portion of the second stroke has ended, and the piston is continuing to travel toward the right end of cylinder  12  in the momentum stroke portion as a result of inertia remaining after the end of the expansion stroke portion. 
     As shown in  FIG.  13   , the piston  50 , second piston rod portion  43  on the left side of piston  50 , and exhaust ports  18  may be configured such that piston  50  passes all of exhaust ports  18  as the piston moves from the left end of cylinder  12  toward the right end of cylinder  12  before second opening  48  in second piston rod portion  43  enters second combustion  73  chamber on the left side of piston  50 . As shown in  FIG.  13   , piston  50  has moved completely to the right of exhaust ports  18  by the time second opening  48  in the left piston rod portion  42  begins to enter second combustion chamber  73  on the left side of piston  50  to permit gas flow between second combustion chamber  73  and second opening  48 . This relative sizing and spacing of the various components may allow exhaust gases generated in second combustion chamber  73  to begin exiting from exhaust ports  18  before fresh pre-compressed air or other gases are introduced into second combustion chamber  73  through second piston rod portion  43  on the left side of piston  50 . In various alternative embodiments, the precise placement of openings through piston rod portions  42 ,  43  relative to the opposite faces of piston  50  may be varied such that the closest inlet port to each face of the piston enters the respective combustion chamber on the same side of the piston shortly after the face of the piston has passed the near edge of the exhaust ports, thereby allowing exhaust gases to begin exiting the respective combustion chamber a short time before introduction of the fresh pre-compressed air or other gases. 
     Shortly after piston  50  has passed exhaust ports  18  during the momentum stroke portion of the stroke from the left end of cylinder  12  to the right end of cylinder  12 , as shown in  FIG.  13   , the edges of second opening  48  in second piston rod portion  43  closest to the left face of piston  50  start to enter second combustion chamber  73 . At this point, a scavenging phase may occur on the left side of piston  50  as a result of gases such as fresh air being introduced into second combustion chamber  73  through second opening  48  of second piston rod portion  43 . Second opening  48  may be configured such that when piston  50  is in the momentum stroke portion of the second stroke from the left end to the right end of cylinder  12 , gas flow may be continuously communicated between second combustion chamber  73  and an area external to cylinder  12 . In the exemplary embodiment shown in  FIG.  14   , fresh, pre-compressed air may be introduced into second combustion chamber  73  from inlet chamber  32 . Simultaneously, exhaust gases may be scavenged from second combustion chamber  73  by the incoming pre-compressed air or other gases and forced out through exhaust ports  18 . 
     As the piston continues to move toward the right end of the cylinder, as shown in  FIGS.  13  and  14   , gas flow may be continuously communicated between second combustion chamber  73  and an area external to the cylinder. The continuous flow of pre-compressed air or other gases introduced from inlet chamber  32  into second combustion chamber  73  may assist with cooling of cylinder  12  as well as scavenging of exhaust gases from second combustion chamber  73 , and boosting the gas pressure in second combustion chamber  73 . Simultaneously with the momentum stroke portion of the second stroke from the left end of the cylinder to the right end of the cylinder, after piston  50  has moved past the exhaust ports  18  toward the right end of cylinder  12 , gases on the right side of piston  50  are compressed during a compression phase on the right side of piston  50 . When the piston is all the way to the right, as shown in  FIG.  14   , the combustion gases on the right side of the piston will have been compressed into the remaining clearance volume of first combustion chamber  71  and ignition will occur to begin another stroke from the right end of the cylinder to the left end of the cylinder. Thereafter, further strokes may continue in this manner. 
     In accordance with some embodiments of the present disclosure, regardless of other particular structures in the engine, a cylinder and a double-faced piston may be sized such that a total distance the piston travels during a first stroke is substantially greater than a distance the piston travels during an expansion stroke portion of the first stroke. By way of example with reference to  FIGS.  5 - 10   , the total distance of piston travel may be measured from the combustion position on the right side of cylinder  12 , as illustrated in  FIG.  5   , for example, to the combustion position on the left side of cylinder  12 , as illustrated in  FIG.  10   , for example. This total distance traveled is substantially greater than the expansion portion of the stroke which occurs when, in the progression of  FIGS.  5 - 10   , piston  50  reaches at least one of exhaust ports  18 . In some embodiments, the end of the expansion stroke may be marked by other occurrences, such as the opening of a mechanical valve, or the cessation of expansion in some other manner. Regardless of how the expansion stroke portion ends, such embodiments are contemplated to be within the scope of this disclosure so long as the total distance of travel is substantially greater than the expansion portion alone. By way of non-limiting examples, the total distance may be considered substantially greater if the difference between the expansion portion of the stroke and a non-expansion portion of the stroke is either multiple times the width of the piston, the width of the piston, greater than three quarters the width of the piston, greater than half the width of the piston, or greater than a quarter width of the piston. Thus, for example, a double-faced piston may have an axial length from one face of the piston to an opposite face of the piston that is less than or equal to ½ of a distance from at least one of the first cylinder head and the second cylinder head to an exhaust port. 
     In some embodiments, a cylinder and a piston may be sized such that the total distance the piston travels during each stroke from one end of the cylinder to the opposite end of the cylinder may exceed the distance the piston travels during the expansion stroke portion of the stroke by at least the length of the piston from one face to the opposite face. In other exemplary embodiments, the cylinder and the piston may be sized such that a total distance the piston travels in each stroke exceeds, by at least the length of the piston, a distance traveled by the piston during compression of gases on one side of the piston. The length of piston  50  from one face to the opposite face in an exemplary embodiment shown in the figures may be less than ½ of a distance from at least one of cylinder heads  14 ,  15  to exhaust ports  18 . This configuration and relative sizing of the piston and cylinder may allow for a significantly greater length of the total stroke for the piston in each direction during which fresh pre-compressed air or other gases may be introduced into the cylinder for the purposes of scavenging exhaust gases and cooling the cylinder after each combustion occurs at opposite ends of the cylinder. 
     In some embodiments, a cylinder and a piston may be configured such that an amount of overshoot of the piston after the end of an expansion phase may be substantially greater than the length of a compression volume. The compression volume may correspond to the clearance volume in cylinder  12  as discussed above. For example, piston  50  and cylinder  12  may be sized such that a length that piston  50  travels in a momentum stroke portion is substantially greater than the length of the clearance volume between one side of piston  50  and the closest cylinder head  14 ,  15  at the combustion position. In some embodiments, an amount of overshoot is at least a quarter the length of piston  50 . Setting an amount of overshoot in this manner to be, for example, at least a quarter the length of piston  50 , may be useful to ensure a sufficient duration for scavenging to occur in cylinder  12 . 
     In accordance with some embodiments of the present disclosure, an internal combustion engine may include a piston kit being formed of an assembly of separate pieces, including a pair of piston rod portions and a piston comprising a disk. By way of example, and as shown in  FIG.  4 B , various embodiments of an engine according to the disclosure may include a double-faced piston, such as piston  50 , and one or more piston rods, such as first piston rod portion  42  and second piston rod portion  43 . Piston rod portions may extend from a center of the piston. For example, each of first piston rod portion  42  and second piston rod portion  43  may extend from the radial center of piston  50 . Piston  50 , first piston rod portion  42 , and second piston rod portion  43  may thus be coaxial. Because piston  50  moves linearly along axis A, and because mechanical load may be transferred through second piston rod portion  43  that may be connected to an actuator, load may be transferred along the same axis A. Thus, substantially all forces acting on piston  50  may act in a direction parallel to axis A. Furthermore, there are no side forces acting on piston  50 , i.e., forces acting in a direction perpendicular to axis A. Compared to a conventional engine with pistons connected to a crankshaft, and which experience lateral forces, piston  50  may avoid side forces acting in a direction different from the primary movement direction of the piston. Due to a lack of experiencing lateral forces, piston  50  may experience reduced stress and reduced accumulated heat, and thus, may have reduced need for cooling. In some embodiments, piston  50  may be regarded as a transverse stressless action piston. Furthermore, piston kit  56  may be substantially rotationally symmetric about axis A. Further still, in some embodiments, piston kit  56  may be symmetrical with respect to a median plane. The median plane may be a plane at an axial center of piston  50  that is perpendicular to axis A. 
     Engine  10  may be provided with a single air supply. The single air supply may be connected to an inlet chamber. For example, in the embodiment as shown in  FIGS.  5 - 14   , inlet chamber  32  includes inlet opening  29 , which may be connected to a source that supplies air. The source that supplies air may be the only air supply provided in engine  10 . It is understood that inlet chamber  32  may be provided on the left side of engine  10 , as well. For example, the configuration of engine  10  shown in  FIGS.  5 - 14    may be mirrored. Air may be supplied to inlet chamber  32  at a pressure that may be higher than atmospheric ambient pressure. When air is supplied in this manner, a design may be achieved that is compact. Furthermore, such a design may be less complex than one requiring separate air supplies. 
     When a single air supply is provided, inlet chamber  32  may be configured to permit fresh air to flow into both first combustion chamber  71  and second combustion chamber  73 . Furthermore, in some embodiments, first vestibule  30  and second vestibule  31  may be configured as isolation areas. An isolation area may be an area external to cylinder  12  that is configured to isolate non-active piston rod parts during alternate cylinder charges. For example, first vestibule  30  may act as a first isolation area on one side of cylinder  12  and second vestibule  31  may act as a second isolation area on an opposite side of cylinder  12 . 
     In some embodiments, multiple air supplies may be provided. For example, rather than providing inlet chamber  32 , engine  10  may include two air supplies, each configured to supply air to one of first vestibule  30  or second vestibule  31 . Engine  10  may include a first air supply that communicates with first vestibule  30  and a second air supply that communicates with second vestibule  31 , each through a respective side opening  33 . The two air supplies may be connected upstream of side openings  33 . Thus, a single air supply may be bifurcated to form multiple air supplies. In this embodiment, one or both of first piston rod portion  42  and second piston rod portion  43  may include occluded ends. In such cases, additional openings may be provided in first piston rod portion  42  and second piston rod portion  43 . For example, a further set of openings may be provided on first piston rod portion  42  that is spaced apart from first opening  44 . The further set of openings may be configured to communicate gas from first vestibule  30  while first opening is inside cylinder  12 . The further set of openings may be configured to be outside cylinder  12  when first opening  44  is inside cylinder  12 . A structure of the further set of opening may be similar to that of first openings  44 . Such further set of openings may be similarly provided in second piston rod portion  43 . 
     Flow of gases within piston kit  56  may occur in different directions. In some embodiments, a passageway in a piston assembly may be configured to communicate gas flow in a first direction from a first side of the piston to a second side of the piston, and to communicate gas flow in a second direction from the second side of the piston to the first side of the piston. For example, passageway  46  may be provided in piston rod  40 , wherein passageway  46  is configured to allow gas flow in a first direction from area  67  on one side of piston  50  to the inside of cylinder  12  through second opening  48 . Piston  50  may be at the first position, for example at the combustion position on the right side of cylinder  12  at the time of supplying gas to cylinder  12  through second opening  48 . Passageway  46  may also be configured to allow gas flow in a second direction from area  65  on the opposite side of piston  50  to the inside of cylinder  12  through first opening  44 . Piston  50  may be at the second position, for example at the combustion position on the left side of cylinder  12  at the time of supplying gas to cylinder  12  through first opening  44 . 
     In some embodiments, flow of gases within piston kit  56  may be in the same direction. For example, when inlet chamber  32  is provided, gas may flow from inlet opening  29  through passageway  46  and into cylinder  12  via second opening  48  (see  FIG.  5   ). At another position of piston  50 , gas may flow from inlet opening  29  through passageway  46  and into cylinder  12  via first opening  44  (see  FIG.  10   ). 
       FIG.  15    illustrates an embodiment of engine  10  including chamber  39 . The embodiment of  FIG.  15    may be similar to that of  FIG.  5    except that occlusion may be accomplished by chamber  39 . Second piston rod portion  43  may be provided without plug  49 . Chamber  39  may be a structure that is attached to or integral with second vestibule  31 . Chamber  39  may be sealed in an airtight manner. When piston  50  is in the second position, as shown in  FIG.  15   , air may be introduced from inlet opening  29 . Because the left side of engine  10  is occluded, air that is inlet to piston rod  40  may travel through passageway  46  to area  67  but does not escape to areas outside engine  10 . Instead, air may be forced into cylinder  12  through first opening  44 . Thus, engine  10  may function in a similar manner to that as shown in  FIG.  5   . 
     A piston rod may be configured such that the piston assembly is slidable between a position where piston rod openings are blocked and a position where at least one of piston rod openings are opened. In some positions throughout the range of travel of piston  50 , there may be positions where none of first opening  44  or second opening  48  is in fluid communication with cylinder  12 . For example,  FIGS.  6 ,  7 ,  8 ,  9 ,  11 ,  12 , and  13    show instances where first opening  44  and second opening  48  have not yet entered cylinder  12 . A pressure buildup period may occur between, for example, the position of  FIG.  6    and the position of  FIG.  9   . In a first pressure buildup position, such as that shown in  FIG.  6   , gases introduced from inlet opening  29  may be in fluid communication with passageway  46 , but the gases may be unable to exit from engine  10 . Thus, pressure may begin building in regions in fluid communication with inlet chamber  32 , such as area  65 , area  67 , and inside passageway  46 . Upon reaching a second pressure buildup position, such as that shown in  FIG.  9   , an internal pressure in regions in fluid communication with inlet chamber  32  may rise to a predetermined level. Thereafter, when first opening  44  becomes exposed to the inside of cylinder  12 , pressure is released and high-pressure air may be delivered to the inside of cylinder  12 . Delivery of high-pressure air into cylinder  12  may increase the amount of work that engine  10  may output. For example, high-pressure air introduced into cylinder  12  may serve to further advance piston  50  toward the left side of cylinder  12 , as shown in the embodiment of  FIG.  9   . 
     An engine in accordance with exemplary embodiments of the disclosure may produce further benefits. For example, an engine may facilitate nearly continuous scavenging of hot exhaust gases from the cylinder while continuously supplying fresh air for combustion. The nearly continuously introduced fresh pre-compressed air may decrease the temperature within the cylinder and increase the engine efficiency and engine service life. 
     Various alterations and modifications may be made to the disclosed exemplary embodiments without departing from the spirit or scope of the disclosure. For example, the burned gases produced by the engine  10  may be used for driving a turbo charger. The compressed air introduced into the cylinder may be pressurized by an external compressor that is driven by the reciprocating piston rod portions extending from opposite ends of the cylinder. Other variations may include imparting a swirl effect to the gases introduced into the cylinder by changing the angle of the inlet ports and of the outlet ports so that gases are not directed radially into or out of the cylinder. 
     To expedite the foregoing portion of the disclosure, various combinations of elements are described together. It is to be understood that aspects of the disclosure in their broadest sense are not limited to the particular combinations previously described. Rather, embodiments of the invention, consistent with this disclosure, and as illustrated by way of example in the figures, may include one or more of the following listed features, either alone or in combination with any one or more of the following listed features, or in combination with the previously described features. 
     For example, there may be provided a linear reciprocating engine. The engine may include a cylinder having a first combustion chamber at a first end thereof and a second combustion chamber at an opposing second end thereof; a first cylinder head located at an end of the first combustion chamber; a second cylinder head located at an end of the second combustion chamber; a piston slidably mounted within the cylinder; and a piston rod including at least one piston rod portion extending through the first combustion chamber and the second combustion chamber, the at least one piston rod portion having at least one first port located on a first side of the piston and at least one second port located on a second side of the piston, opposite the first side of the piston. There may also be provided the following elements:
         wherein the at least one piston rod portion includes a passageway extending through the piston configured to communicate gas flow therethrough.   wherein the piston rod is slidable to a first position where the at least one first port and the at least one second port are blocked to enable a pressure build up in the passageway.   wherein the piston rod is slidable to a second position where the at least one second port is open to release pressurized air into the second combustion chamber.   wherein the piston rod is slidable to a third position where the at least one first port is open to release pressurized air into the first combustion chamber.       

     Furthermore, for example, there may be provided a linear reciprocating engine including a cylinder having a first combustion chamber at a first end thereof and a second combustion chamber at an opposing second end thereof; a vestibule located external to the first combustion chamber proximate the first end; a piston slidably mounted within the cylinder; a first piston rod portion extending from the piston through the first combustion chamber and into the vestibule, the first piston rod portion including a hollow tube portion having an inlet port and at least one first sidewall opening therein; and a second piston rod portion extending from the piston through the second combustion chamber, the second piston rod portion having a hollow tube portion and at least one second sidewall opening therein. There may also be provided the following elements:
         wherein the first piston rod portion is flow-connected to the second piston rod portion.   wherein the at least one first sidewall opening and the at least one second sidewall opening are positioned such that during a first stroke portion, the at least one inlet port is located in the vestibule to supply the first combustion chamber with air from the vestibule.   wherein during a second stroke portion the at least one inlet port is located in the vestibule to supply the second combustion chamber with air from the vestibule.   wherein the engine further includes a fresh air pump connected to the single air inlet.   wherein the engine further includes a pump for supplying pressurized air to the vestibule.   wherein the engine includes two combustion chambers   wherein the single air inlet and piston rod are configured to permit fresh air to flow into both combustion chambers.       

     Furthermore, for example, there may be provided a linear reciprocating engine including a cylinder having a first combustion chamber at a first end thereof and a second combustion chamber at an opposing second end thereof; a first vestibule located external to the first combustion chamber proximate the first end; a second vestibule located external to the second combustion chamber proximate the second end; a piston slidably mounted within the cylinder; a first piston rod portion extending from the piston through the first combustion chamber and into the first vestibule, the first piston rod portion having a first elongated passageway portion therethrough and at least one first port therein; and a second piston rod portion extending from the piston through the second combustion chamber and into the second vestibule, the second piston rod portion having a second elongated passageway portion therethrough and at least one second port therein. There may also be provided the following elements:
         wherein the first passageway portion flow connects to the second passageway portion.   wherein when the at least one first port is located in the first vestibule, the at least one second port is located in the second combustion chamber, thereby permitting flow communication between the first vestibule and the second combustion chamber.   wherein when the at least one second port is located in the second vestibule, the at least one first port is located in the first combustion chamber, thereby permitting flow communication between the second vestibule and the first combustion chamber.       

     Furthermore, for example, there may be provided an internal combustion engine including a cylinder having a first combustion chamber at a first end thereof and a second combustion chamber at an opposing second end thereof; a vestibule external to the combustion chamber; a piston slidably mounted within the cylinder; and a piston rod extending from the piston through the combustion chamber and into the vestibule. There may also be provided the following elements:
         wherein the piston rod includes an opening therethrough extending to the piston, and at least one port therein.   a port in the vestibule for supplying pressurized air to the vestibule to thereby enable, during a combustion stroke of the piston, pressurized air to move through the piston rod and cool the piston.       

     Furthermore, for example, there may be provided a linear reciprocating engine including a cylinder having a first combustion chamber at a first end thereof and a second combustion chamber at an opposing second end thereof, the cylinder having one or more side ports therein; a first cylinder head located at an end of the first combustion chamber; a second cylinder head located at an end of the second combustion chamber; a double-faced piston slidably mounted within the cylinder; a first piston rod portion extending from the piston through the first combustion chamber and into a first pressurizable vestibule, the first piston rod portion having a first elongated passageway portion therethrough and at least one first port therein; a second piston rod portion extending from the piston through the second combustion chamber and into a second pressurizable vestibule, the second piston rod portion having a second elongated passageway portion therethrough, the second elongated passageway portion being flow connected to the first elongated passageway and at least one second port therein. There may also be provided the following elements:
         wherein the at least one first port and the at least one second port are respectively arranged such that when the double-faced piston is located in a central position in the combustion chamber and the first vestibule and the second vestibule are pressurized, a static air flow condition exists in the first piston rod portion and the second piston rod portion.