Patent Publication Number: US-7721641-B2

Title: Air compression apparatus and method of use

Description:
RELATED APPLICATIONS 
   This application claims priority to and is entitled to the filing date of U.S. Provisional application Ser. No. 60/573,250 filed May 21, 2004, and entitled “Multi-Stage Compressor with Integrated Internal Breathing,” and U.S. Provisional application Ser. No. 60/652,694 filed Feb. 14, 2005, and entitled “Compressor with Variable-Speed Pressure Stroke.” The contents of the aforementioned applications are incorporated herein by reference. 

   INCORPORATION BY REFERENCE 
   Applicant hereby incorporates herein by reference any and all U.S. patents and U.S. patent applications cited or referred to in this application. 
   TECHNICAL FIELD 
   Aspects of this invention relate generally to air compression systems, and more particularly to an apparatus and method for compressing air introduced into a cylinder through a hollow piston rod. 
   BACKGROUND ART 
   The following art defines the present state of this field: 
   Great Britain Patent No. GB 1043195 to Grant describes a reciprocating piston compressor or air motor having a plurality e.g. four cylinders extending radially from an axial valve chamber housing four angularly spaced ports and in which is rotatably mounted an axially adjustable tubular cylindrical distributing valve provided in a central portion with a suction port and a delivery port and adapted to be brought into sequential communication with each valve chamber port, the outer surface of the valve body is provided with a groove which at or immediately prior to opening of delivery port serves to connect the valve chamber port to an annular chamber bounded in part by the drive end of the valve body and the pressure therein acts against the discharge pressure in an annular chamber at the other end of said valve body and the resulting axial displacement of the valve controls the time of opening of the valve ports according to whether the pressure in one chamber is below or above that in another chamber. The valve portion comprises concentric tubes connected by webs and through which the suction port extends whilst the delivery port extends through the outer tube only. An axial extension tube provides air inlet means to said suction port. Each of the four valve chamber ports are roughly triangular and have a side parallel to the valve axis, a side normal to the axis and the third side has two portions of differing slopes which register with portions of the leading edge of the inlet port and with the leading edge of the delivery port. Lubricant is admitted to a bore leading to grooves and cooling water admitted through a pipe traverses a jacket surrounding the valve and a space round each cylinder. The pistons are each secured to a cross-head connected together in diametrically opposed pairs by the outside member whilst adjacent pistons are connected by connecting members and the cross-heads are reciprocated by two eccentric rings each rotatable within a slide block and having secured thereto a dished disc. The latter are secured together at their peripheries by bars and have balancing weights. 
   Great Britain Patent No. GB 1259755 to Sulzer Brothers Ltd. describes a compressor wherein a piston reciprocates in a cylinder without normally making physical contact with the cylinder, the piston being provided with a split ring having longitudinal grooves in its periphery. The ring may be of P.T.F.E. and acts to guide the piston in the event of abnormal operation causing the piston to approach the cylinder. During normal operation gas escaping past labyrinth seals or labyrinths formed in the periphery of the piston, acts on a conical ring to centre the piston. Radial holes pass through the ring and open into the grooves thereby to provide pressure equalization between the inside and outside of the ring. The piston may be double or, as shown, single acting and driven by a piston rod which extends through a cylinder seal for connection to a cross-head. 
   U.S. Pat. No. 4,373,876 to Nemoto describes a compressor having a pair of parallel, double-headed pistons reciprocally mounted in respective cylinder chambers in a compressor housing. The pistons are mounted on a crankshaft via Scotch-yoke-type sliders slidably engaged in the respective pistons for reciprocating movement in a direction normal to the piston axis. The sliders convert the rotation of the crankshaft into linear reciprocation of the pistons. The dimensions of these sliders are determined in relation to the other parts of the compressor so that, during the assemblage of the compressor, the sliders may be mounted in position by being passed over the opposite end portions of the crankshaft following the mounting of the pistons and crankshaft within the housing. 
   U.S. Pat. No. 5,050,892 to Kawai, et al. describes a piston for a compressor comprising a ring groove on the outer circumferential surface of the piston, and a discontinuous ring seal member with opposite split ends made of a plastic material and fitted in the ring groove. The ring member having an outer surface comprising a main sealing portion having an axially uniform shape and an outwardly circumferentially projecting flexible lip portion. Also, the inner surface of the ring member comprises an inner bearing portion able to come into contact with a first portion of a bottom surface of the ring groove such that the flexible lip portion of the outer surface is brought into contact with a cylinder wall of the cylinder bore and preflexed inwardly. An inner pressure receiving portion is formed adjacent to the inner bearing portion to receive pressure from the compression chamber, to further flex the flexible lip portion upon a compression stroke of the compressor and thereby allow the ring member to expand and the main sealing portion to come into contact with the cylinder wall of the cylinder bore. 
   Japanese Patent Application Publication No. JP 1985/0079585 to Michio, et al. describes a displacer rod bearing body, provided at its upper and lower parts with rod pin mounting parts, and reciprocatively slides a displacer rod bearing surface around a cross rod pin of a cross head. A displacer rod, secured to a displacer, is rotatably supported to an upper rod pin of the bearing body, and a compressor for the displacer is rotatably supported to a lower rod pin. 
   U.S. Pat. No. 5,467,687 to Habegger describes a piston compressor having at least one cylinder and a piston guided therein in a contact-free manner, which is connected via a piston rod to a crosshead. The piston rod consists of a pipe extending between the crosshead and the piston. In this pipe extends a tension rod, which can be extended by means of a hydraulic stretching device and under prestressing pulls the crosshead and the piston towards the pipe. 
   U.S. Pat. No. 6,132,181 to McCabe describes a windmill having a plurality of radially extending blades, each being an aerodynamic-shaped airfoil having a cross-section which is essentially an inverted pan-shape with an intermediate section, a leading edge into the wind, and a trailing edge which has a flange doubled back toward the leading edge and an end cap. The blade is of substantial uniform thickness. An air compressor and generator are driven by the windmill. The compressor is connected to a storage tank which is connected to the intake of a second compressor. 
   U.S. Patent Application Publication No. US 2002/0061251 to McCabe describes a windmill compressor apparatus having multiple double acting piston/cylinders actuated by the windmill. The windmill additionally has multiple pairs of blades to enhance power output and lift. 
   U.S. Pat. No. 6,655,935 to Bennitt, et al. describes a gas compressor and method according to which a plurality of inlet valve assemblies are angularly spaced around a bore. A piston reciprocates in the bore to draw the fluid from the valve assemblies during movement of the piston unit in one direction and compress the fluid during movement of the piston unit in the other direction and the valve assemblies prevent fluid flow from the bore to the valve assemblies during the movement of the piston in the other direction. A discharge valve is associated with the piston to permit the discharge of the compressed fluid from the bore. 
   U.S. Pat. No. 6,776,589 to Tomell et al. describes a reciprocating piston compressor having a suction muffler and a pair of discharge mufflers to attenuate noise created by the primary pumping frequency in the primary pumping pulse. The suction muffler is disposed along a suction tube extending between the motor cap and the cylinder head of the compressor. The discharge mufflers are positioned in series within the compressor to receive discharge gases from the compression mechanism and are spaced one quarter of a wavelength from each other so as to sequentially diminish the problematic or noisy frequencies created during compressor operation. The motor/compressor assembly including the motor and compression mechanism is mounted to the interior surface of the compressor housing by spring mounts. These mounted are secured to the housing to define the position of the nodes and anti-nodes of the frequency created in the housing to reduce noise produced by natural frequencies during compressor operation. 
   The prior art described above teaches single and double-acting air cylinders, but does not teach introducing air into an air cylinder through a hollow piston rod and applying varied speed and pressure to the piston body attached to the piston rod corresponding to the compressive work being done by the piston during its stroke. Aspects of the present invention fulfill this need and provide further related advantages as described in the following disclosure. 
   DISCLOSURE OF INVENTION 
   Aspects of the present invention teach certain benefits in construction and use which give rise to the exemplary advantages described below. 
   An air compression apparatus has a frame and a tank and a motor mounted to the frame. A drive mechanism is operably connected to the motor and at least one piston assembly is operably connected to the drive mechanism and configured to move within a respective cylinder mounted to the frame. The piston assembly includes: (1) a piston body sealingly and slidably installed within the cylinder so as to form an upper chamber above the piston body and a lower chamber below the piston body, the piston body being further formed with a cavity in communication with at least the lower chamber; (2) a piston rod having a hollow bore communicating between a drive end and a piston end, the drive end being connected to the drive mechanism such that the hollow bore is in communication with ambient air, the piston rod passing through the cylinder and the upper chamber so as to be connected at the opposite piston end to the piston body, the piston rod having at least one opening formed therein substantially at the piston end such that the hollow bore is in communication with the cavity; and (3) a lower piston valve installed on the piston body so as to selectively seal the lower chamber from the cavity. In use, upward travel of the piston body as caused by the drive mechanism acting through the piston rod opens the lower piston valve and allows ambient air to be drawn through the hollow bore, the at least one opening, and the cavity into the lower chamber, and downward travel of the piston body as caused by the drive mechanism acting through the piston rod closes the lower piston valve so as to compress the air within the lower chamber. 
   An aspect of the present invention may then be generally described as an improved air compression system where ambient air is introduced into a cylinder through a hollow piston rod so as to improve the air flow through the cylinder, resulting in more efficient and quiet operation. 
   A further aspect of the present invention may be generally described as single-acting or double-acting air compression cylinders each configured with a piston body having a cavity that is selectively sealed by one or more valves opening to allow the passage of ambient air through the hollow piston rod into a chamber within the cylinder above or below the piston body and alternately closing to compress the air within such chamber, further improving the efficiency of the air compression system. 
   A still further aspect of the present invention may be generally described as a drive mechanism for oscillating the piston body within each cylinder such that relatively greater force is applied to the piston body through the piston rod during peak air compression while relatively less force is applied to the piston body through the piston rod during most of the air gathering through the hollow piston rod, resulting is further improvements in operation of the air compression system. 
   Other features and advantages of aspects of the present invention will become apparent from the following more detailed description, taken in conjunction with the accompanying drawings, which illustrate, by way of example, the principles of aspects of the invention. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
     The accompanying drawings illustrate aspects of the present invention. In such drawings: 
       FIG. 1  is a perspective view, partially in section, of an exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 2  is an enlarged perspective view thereof taken from circle “ 2 ” of  FIG. 1 ; 
       FIG. 3  is a front view of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 4  is a reduced scale front view thereof in a first position of operation; 
       FIG. 5  is a reduced scale front view thereof in a second position of operation; 
       FIG. 6  is a reduced scale front view thereof in a third position of operation; 
       FIG. 7  is front view of an alternative exemplary embodiment of the air compression apparatus of the present invention in a first position of operation; 
       FIG. 8  is a front view thereof in a second position of operation; 
       FIG. 9  is a front view thereof in a third position of operation; 
       FIG. 10  is a front view thereof in a fourth position of operation; 
       FIG. 11  is a front view thereof in a fifth position of operation; 
       FIG. 12  is a front view thereof in a sixth position of operation; 
       FIG. 13  is a front view of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 14  is a front view of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 15  is a front view of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 16  is a front view, partially in section, of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 17  is a side view thereof; 
       FIG. 18  is a front view, partially in section, of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 19  is an enlarged scale sectional view taken from circle “ 19 ” of  FIG. 18 ; 
       FIG. 20  is a sectional view thereof in a first mode of operation; 
       FIG. 21  is a sectional view thereof in a second mode of operation; 
       FIG. 22  is a front view, partially in section, of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 23  is an enlarged scale sectional view taken from circle “ 23 ” of  FIG. 22 ; 
       FIG. 24  is a sectional view thereof in a first mode of operation; 
       FIG. 25  is a sectional view thereof in a second mode of operation; 
       FIG. 26  is a sectional view thereof in a third mode of operation; 
       FIG. 27  is a sectional view thereof in a fourth mode of operation; 
       FIG. 28  is partial sectional front view of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 29  is an top view thereof taken along line “ 29 - 29 ” of  FIG. 28 ; 
       FIG. 30  is a reduced scale sectional view thereof in a first mode of operation; 
       FIG. 31  is a reduced scale sectional view thereof in a second mode of operation; 
       FIG. 32  is a partial sectional front view of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 33  is a reduced scale top view thereof taken along line “ 33 - 33 ” of  FIG. 32 ; 
       FIG. 34  is a reduced scale sectional view thereof in a first mode of operation; 
       FIG. 35  is a reduced scale sectional view thereof in a second mode of operation; 
       FIG. 36  is a partial sectional front view of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 37  is a reduced scale top view thereof taken along line “ 37 - 37 ” of  FIG. 36 ; 
       FIG. 38  is a partial sectional front view of an alternative exemplary embodiment of the air compression apparatus of the present invention in a first mode of operation; 
       FIG. 39  is a reduced scale top view thereof taken along line “ 39 - 39 ” of  FIG. 38 ; 
       FIG. 40  is an enlarged scale partial sectional front view thereof in a second mode of operation; 
       FIG. 41  is a partial sectional front view of an alternative exemplary embodiment of the air compression apparatus of the present invention in a first mode of operation; 
       FIG. 42  is a reduced scale top view thereof taken along line “ 42 - 42 ” of  FIG. 41 ; 
       FIG. 43  is a partial sectional front view thereof in a second mode of operation; 
       FIG. 44  is a partial sectional front view of an alternative exemplary embodiment of the air compression apparatus of the present invention in a first mode of operation; 
       FIG. 45  is a top view thereof taken along line “ 45 - 45 ” of  FIG. 44 ; 
       FIG. 46  is a partial sectional front view thereof in a second mode of operation; 
       FIG. 47  is a partial perspective view of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 48  is a sectional view thereof taken along line “ 48 - 48 ” of  FIG. 47 ; 
       FIG. 49  is a left side view of an alternative exemplary embodiment of the air compression apparatus of the present invention; 
       FIG. 50  is a front view thereof; and 
       FIG. 51  is a right side view thereof. 
   

   MODES FOR CARRYING OUT THE INVENTION 
   The above described drawing figures illustrate aspects of the invention in at least one of its exemplary embodiments, which are further defined in detail in the following modes. 
   The subject of this patent application is an improved air compression apparatus, where “air” as used throughout is to be understood to mean and apply to any compressible medium, whether gas or liquid. The air compression apparatus described herein is an assembly made up in part of one or more cylinders, each containing a piston which is driven by a rod connected to a crank. The connection between the rod and the crank mechanism can take many forms depending on the design and application, but is typically achieved by attaching the free end of the rod to a flywheel, pivoting arm, or cam follower arrangement so that the cylinder moves relative to the crank in a manner that manipulates the velocity of travel of the piston and thereby increases the leverage exerted against the compressed air when the piston is approaching its top and bottom positions, or highest points of compression. It will be appreciated by those skilled in the art that while the general structure and operation of the improved air compressor of the present invention is shown and described herein in various exemplary embodiments, the invention is not so limited. Rather, a key inventive aspect of the improved compressor that transcends any particular design and construction is the principle that a relatively longer or larger volume working stroke of each piston combined with a coordinated variance in the speed of the piston during its stroke produces smoother and more efficient compression. Such relatively longer or larger volume stroke and/or speed variance of each piston is achieved in each of the exemplary embodiments of the present invention described hereinafter, the descriptions of which will further inform those skilled in the art of the novel principles of operation and structure of the air compression apparatus and provide a context for greater appreciation of its benefits. Specifically, embodiments are shown and described as having relatively smaller diameter, longer stroke cylinder configurations for smooth air gathering and compression at relatively lower speeds and as having relatively larger diameter, shorter stroke cylinder configurations that are able to operate efficiently at relatively higher speeds as compared to the longer stroke cylinder configurations due to reduced inertial effects and the like. Accordingly, numerous other designs and constructions are possible without departing from the spirit and scope of the invention. 
   With respect to the cylinder, a further key aspect of the invention that transcends any particular design and construction is that ambient air may be admitted through a hollow tube, which also acts as the piston rod, and then through a valve at the bottom of the piston itself into the bottom chamber of the cylinder during the upward stroke of the piston. This air is then compressed during the downward stroke of the piston. In some embodiments, the air so compressed in the bottom chamber is next transferred to the top chamber of the cylinder, above the piston, and further compressed as the piston moves upward in the cylinder. Or in other embodiments, the compressed air in the bottom chamber may be fed directly to the pressure holding tank and the top chamber may be fed ambient air through a valve at the top of the piston while the piston is on its downward stroke. The ambient air in the top chamber would then be compressed on the piston&#39;s upward stroke, while at the same time additional ambient air is again fed into the bottom chamber to be compressed on the downward stroke. In either case, the air compressed in the top chamber may then be transferred to the pressure holding tank, just as was the air from the bottom chamber during the previous phase of the cycle. The valve configurations and the locations of both the inlets and outlets for the two chambers of each cylinder may vary depending on the design and application, exemplary ones of which are described further below. In any such cylinder design, depending on the particular embodiment of the compressor, the air compressed in a first cylinder may be transferred to further cylinders for additional stages of compression. The additional cylinders may be connected to the same drive mechanism as the first cylinder or to a separate drive mechanism. It will be appreciated that by compressing air on the upstroke and the down stroke in each cylinder, the useful work done by the piston is effectively doubled for the same work by the motor in cycling the piston through its stroke. Moreover, by introducing ambient air into the cylinder&#39;s top and bottom chambers in alternating fashion through the piston rod itself and valves on the respective top and bottom sides of the piston, the air is caused to move through the cylinder at all stages of compression in a more laminar fashion. These effects coupled with the relatively longer or larger volume stroke and intermittent speed of the piston thus enable the air to effectively be “squeezed” rather than “slammed,” providing numerous additional benefits in terms of the performance, cost, and maintenance of the cylinders and the rest of the compressor. These and other advantages of the present invention will be further apparent with reference to the following more detailed description and the accompanying drawing figures. First described below are various embodiments of the drive mechanism and overall compressor structure with general reference to the operation of the piston itself, with further more detailed descriptions of the design and operation of various exemplary piston configurations then following. 
   Referring to  FIGS. 1 and 2 , there is shown a first exemplary embodiment of an improved air compression apparatus embodying the principles of the present invention. In this exemplary embodiment, the compressor  100  is an assembly comprised essentially of the following major parts: a pressure tank  102 , a motor  104 , a belt or geared speed reduction or drive mechanism  110  to reduce the number of revolutions per minute of a flywheel  120 , a crankpin  122  attached to the flywheel  120 , an intake block  126  rotatably attached to the crankpin  122 , a cylinder  130 , a piston assembly  140  moving within the cylinder  130 , a valve mechanism (not shown) integrated with the piston assembly  140  to control the passage of air flowing into the cylinder  130 , check valves  180  at the top and bottom of the cylinder  130  to control the passage of air to the pressure tank  102 , a hollow tube  170  rigidly attached to the intake block  126  at one end and the piston  140  at the opposite end and acting as a piston rod, a gland (not shown) at the top of the cylinder  130  to provide an airtight seal about the outside surface of the hollow piston rod  170 , a pivot arm  150  pivotably attached to both the base of the cylinder  130  and some distance away to a shaft  152  rigidly mounted to the compressor&#39;s frame  106 , and a guide bar  154  rigidly attached to the pivot arm  150 , which moves in response to movement of the crankpin  122  through a bearing  124  on the end of the crankpin  122  located within a slot  156  in the guide bar  154  and so causes the cylinder  130  to move in an oscillating fashion, shifting both vertically and horizontally, as the top of the cylinder  130  follows the crankpin  122  through connection of the pivot rod  170  to the intake block  126  and the bottom of the cylinder  130  shifts in response to movement of the pivot arm  150  in connection with the movement of the guide bar  154 , more about which will be said below. Additional minor parts may include tubing, bearings, screws, nuts, bolts, washers, clips, bushings, springs, retainers, connectors, filters, and other small parts as necessary to hold the major parts in proper relationship to each other and to provide for efficient movement of the various moving parts. 
   Regarding movement of the cylinder  130  in response to the cooperative movement of the flywheel  120 , the guide bar  154  and the pivot arm  150 , it will be appreciated that during use the cylinder  130  is effectively caused to move dynamically, both vertically and laterally, rather than being static or even pivoted about a single fixed point. As the motor  104  drives the flywheel  120  on its shaft  125 , the flywheel  120  in turn moves the crankpin  122  radially. Because the crankpin  122  is configured such that its free end is positioned within a slot  156  in the guide bar  154 , preferably through a roller bearing  122  or the like, movement of the flywheel  120  results in corresponding movement of the guide bar  154 . This movement of the guide bar  154  then translates to movement of the lower end of the cylinder  130 , again, both vertically and laterally, as the pivot arm  150  to which the guide bar  154  is rigidly affixed pivots about the shaft  152  rigidly mounted to the compressor&#39;s frame  106 , thereby causing the cylinder  130  to pivot about the pivot pin  158  installed in the pivot arm  150  offset from the pivot shaft  152 . At the same time, the radial movement of the flywheel  120 , and thus the crankpin  122 , also results in vertical and lateral movement of the piston rod  170 , and corresponding oscillation of the top end of the cylinder  130 , through rigid connection of the piston rod  170  to the intake block  126  and connection of the intake block  126  to the crankpin  122 . Accordingly, it will be appreciated by those skilled in the art that the oscillating movement of the cylinder  130  is caused by the corresponding movement of the guide bar  154  as driven by the crankpin through the rotation of the flywheel  120 . As such, both ends of the cylinder are effectively dynamically floating within the exemplary compressor mechanism, whereby the cylinder is articulated with little or no lateral forces acting on the piston rod during its operation, or as it cycles through its strokes. Put another way, the guide bar is configured to absorb most or all of the lateral forces resulting from the driving movement of the flywheel and crankpin, so that the only forces effectively acting on the piston rod during all phases of the compressor&#39;s operation are along the piston rod&#39;s axis so as to move the hollow piston rod up and down within the cylinder, with effectively no side load on the piston or piston rod during operation of the compressor. It will be further appreciated, then, that such construction and operation greatly reduces the wear of the piston itself, the gland sealing the top of the cylinder about the piston rod, and the other moving parts in the assembly, minimizes the heat build up in the cylinder, and practically eliminates the debris entering the air stream within the cylinder. The amount of debris may be further reduced by the selection and use of self-lubricating materials so as to eliminate lubricants from within the inner workings of at least the moving parts of the mechanism that directly contact the air stream. By way of example, the gland through which the piston rod operates is preferably a bronze bushing, the ring or rings about the circumference of the piston may be made of Teflon®, and the piston rod itself may be constructed of a highly polished steel, and the inside wall of the cylinder may be carbon coated. It will be appreciated, though, that numerous other such materials now known or later developed may be employed in the present invention. In turn, this reduced wear on the piston and other such moving parts results in increased efficiency, longer life, and less down-time and repair costs for the compressor as well as improved cleanliness of the compressed air produced. The geometry of the guide bar and pivot arm is merely exemplary, as is the distance from the pivot shaft to the point where the cylinder is pivotably mounted to the pivot arm, such variables being capable of virtually an infinite number of combinations to produce different performance values of the compressor depending on the application. Furthermore, the slot may be varied in shape utilizing various curves or angles, as explained more fully below with respect to an alternative embodiment, to more precisely control the extent and timing of the oscillations of the cylinder relative to the crank, such motion, again, acting to gear the effective speed of the piston relative to the cylinder and thereby to increase or decrease the effective amount of leverage applied by the motor against the compressed load of air within the cylinder. Similarly, the guide bar itself may be generally linear, or the free end thereof and, accordingly, the slot, may be slightly cocked to further achieve the desired variable speed of the piston while at the same time causing increased leverage to be applied to the compressed air through the piston, including helping the piston and cylinder to slow down at the apex of the flywheel where the most compressive work is being done. Relatedly, while the crankpin is shown as being mounted on the flywheel so as to extend perpendicularly therefrom, it may also be mounted at varying angles to the flywheel and include an additional pivot arm at the free end of the crankpin, between the intake block and the guide bar slot, in order to provide further or exaggerated attenuation and variable-speed effects of the piston rod, as, for example, in high pressure applications. Whether the crankpin is generally perpendicular to the flywheel, and thus the guide bar, or at some other angle, it is also contemplated that the bearing or other such device at the end of the crankpin or secondary pivot arm be captured within the slot through low friction discs, such as Teflon®, having a diameter larger than the width of the slot and mounted to the crankpin itself on opposite sides of the guide bar. It is further contemplated that a Teflon® or other such sleeve be installed within the slot in the guide bar to further reduce friction during operation of the compressor. It will thus be appreciated that a virtually infinite number of geometrical and mechanical variations on the exemplary embodiment of the compressor shown and described can be employed without departing from the spirit and scope of the invention. 
   In terms of the other structural elements of the exemplary compressor design of the present invention, a vertical pressure tank  102  may generally be employed, as illustrated in the accompanying drawings. The size and orientation of the tank  102 , the flywheel  120 , and the one or more cylinders  130 , and, in turn, the stroke length of each of the cylinders, will essentially dictate the other geometrical and mechanical considerations, including the size and shape of a protective housing (not shown) positioned about the working parts of the compressor  100 . The tubing  182  between the one or more cylinders and the tank is preferably flexible so as to accommodate the oscillation of each cylinder  130  during operation, though other types of rigid and semi-rigid tubing with rotating connectors may also be possible. Persons acquainted with the art will understand that various embodiments may employ variations in the configuration of the assembly within the scope of this invention. Some embodiments may employ a single piston or further pairs of pistons, driven by the same crank or by a further crank or cranks in a parallel structure, for additional compression. In some embodiments some or all of the moving parts that come in contact with the compressed air may be constructed of self-lubricating material, such as Teflon® piston rings or carbon composites, so that no oil is introduced into the air stream and further minimizing debris. Most embodiments of the compressor design will employ extended length, relatively small diameter cylinders, on the order of 1¾ to 2 inches (4.5 to 5 cm), with the crank driving the pistons through a relatively long stroke, on the order of 8 inches (20 cm), at relatively low revolutions per minute, on the order of 150 to 200 rpm, though it will be appreciated by those skilled in the art that numerous other cylinder and piston geometries and crank speeds may be employed depending on the application without departing from the spirit and scope of the invention. It will be further appreciated that the exemplary structure providing for variable rate of leverage against the compressed load of air enables a higher output of compressed air with less demand of power from the motor, as well as no need for means of heat dissipation due to the low friction, low speed, smooth operation of the one or more pistons. An exemplary motor that may be installed in the air compression apparatus of the present invention is a single phase, 6 hp electric motor rated at 3450 rpm at 120 volts and 60 cycles, though it will be appreciated that numerous power sources both now known and later developed may be employed without departing from the spirit and scope of the invention. In any event, the resulting compressor invention is also then generally characterized by a relatively low manufacturing cost, reduced maintenance and longer life through such benefits as reduced wear on the moving parts and even load on the drive motor during operation, and relatively cleaner compressed air output, higher pressure capability, quieter operation, and improved overall efficiency. 
   In another exemplary embodiment the pivot arm and guide bar may be replaced by a cam and cam follower or a yoke arrangement (not shown) at the shaft  125  holding the crank  120 , along with a drive rod attached to a pivot shaft (not shown) at the top of the cylinder. In this embodiment, as the crank turns, the cam or yoke mechanism drives the drive rod, which moves the cylinder up and down relative to the position of the crank, such motion acting to alter the effective motion of the piston relative to the cylinder and thereby to increase or decrease the effective amount of leverage applied by the apparatus against the compressed load of air within the cylinder. 
   In use, the drive mechanism  110  reduces the rotational speed of the motor shaft  108  to the desired rotational speed for the crank  120  so as to drive the piston  140  at the desired reduced number of strokes per minute. The rotational motion of the crankpin  122 , connected to the piston rod  170  through the intake block  126  and moving in a slot  156  in the guide bar  154 , causes a lateral oscillating motion of the cylinder  130 , as described above. In addition to the cylinder&#39;s lateral movement, the cylinder is caused to oscillate vertically relative to the crank  120  as the crank rotates, either by attachment to a pivot arm  150  offset a distance from the pivot shaft  152 , or by a cam or yoke arrangement (not shown) with a rod attached both to the cam or yoke and to the pivot point of the cylinder. The vertical oscillating motion of the cylinder assembly  130  relative to the crank  120  causes a controlled variation in the speed of the piston  140  relative to the cylinder  130  and to the compressed air load within the cylinder, providing for a controlled variation in the leverage applied by the crank  120  against the compressed air load. As the piston  140  is retracted toward the top of the cylinder  130  during part of the rotation of the crank  120 , the valve (not shown) at the bottom of the piston  140  is pulled open by the action of a vacuum created in the bottom chamber of the cylinder  130 , so that ambient air then passes through the hollow piston rod  170  and open valve into the bottom chamber. When the piston  140  has reached the top of its stroke, the valve at the bottom of the piston is closed, and the air in the bottom chamber is compressed by the downward movement of the piston  140  and driven through a check valve  180  into the pressure tank  102  or into the chamber in the cylinder  130  above the piston  140 . During the downward travel of the piston  140 , a valve  142  at the top of the piston admits air through the hollow piston rod  170  into the upper chamber. As the piston  140  moves upward, new air is drawn into the lower chamber and the air in the upper chamber is compressed and passed either into the pressure storage tank  102  or into another cylinder (not shown) for further compression in a similar manner. Based on this operation of an exemplary embodiment of the compressor, it will be appreciated that the mechanism is capable of effectively producing a variable rate of compression in four general phases. In a first phase, say, when the piston  140  is retracted toward the top of the cylinder  130  on its upstroke, as when the crankpin  122  is moving toward the top, or apex, of the flywheel  120  in a counter-clockwise direction through the effective quadrant of the flywheel between 3:00 and 12:00, or between ninety and zero degrees, the flywheel  120 , and thus the crankpin  122 , the piston rod  170 , and the piston  140  itself, is beginning to slow down as the piston  140  is nearing the top of its stroke. This slow-down enables the motor  104  to apply increased torque with relatively less additional work by the motor due to the cooperation of the reduction mechanism  110  and the other mechanical structure and principles at work in driving the flywheel  120 , thereby yielding a nice, smooth “squeezing” of the air during the final part of the upstroke compression in the upper chamber of the cylinder  130 . Essentially at the apex, the air in the upper chamber has reached its maximum compression for the cylinder  130  and is discharged through the upper chamber&#39;s check valve  180  as described above. Then, once the crankpin  122  has passed beyond the apex and is moving through roughly the second quadrant of the flywheel  120  between the 12:00 and 9:00 positions, a second phase of operation is begun wherein the flywheel  120 , and thus the crankpin  122 , the piston rod  170 , and the piston  140  itself, is speeding back up as the relatively easier, initial work of compression is being done in the lower chamber and ambient air is being introduced into the evacuated upper chamber as the piston  140  is on its down stroke. Next, a third phase of operation is initiated as the crankpin  122  continues to move counter-clockwise and enters the third quadrant of the flywheel  120  between 9:00 and 6:00 where, similar to the first phase, as the piston  140  is advanced toward the bottom of the cylinder  130  on its down stroke, the flywheel  120 , and thus the crankpin  122 , the piston rod  170 , and the piston  140  itself, is beginning to again slow down as the piston  140  is nearing the bottom of its stroke. Once more, this slow-down results in greater torque applied by the motor  104  and reduction mechanism  110  without a significant increase in the load on the motor as it drives the flywheel  120 , resulting in a smooth and efficient “squeezing” of the air during the final part of the down stroke compression in the bottom chamber of the cylinder  130 . When the air has reached its maximum compression in the lower chamber, it is then discharged through a check valve  180  or passed into the upper chamber for further compression on the piston&#39;s upstroke, as described above. Finally, once the crankpin  122  has moved counterclockwise into the fourth quadrant of the flywheel  120  between 6:00 and 3:00, the fourth phase of operation analogous to the opposite second phase is begun wherein the flywheel  120 , and thus the crankpin  122 , the piston rod  170 , and the piston  140  itself, is again speeding back up as the relatively easier, initial work of compression is being done now in the upper chamber and ambient air is once more being introduced into the evacuated lower chamber as the piston  140  continues on its upstroke. This four-phase, intermittent speed and pressure cycle is simply repeated to efficiently compress air from ambient conditions to a desired higher pressure. It will be appreciated by those skilled in the art that the drive mechanism and the other geometry of the compressor can be just as easily set up so that the flywheel effectively turns clockwise. As such, the descriptions of the operation of the flywheel throughout are to be understood as being merely exemplary. Once again, further speed and pressure variance during the cycle is achieved by the simultaneous, coordinated, dynamic movement of the cylinder  130  itself through its pivoted connection on the pivot arm  150  linkage within the mechanism. With reference to the preceding general description of the operation of an exemplary compressor through these four phases, then, it is to be understood that each of the angular positions about the flywheel referred to are for explanation of the principles of operation of the present invention only and that the exact positions and transitions of each of the four general phases of operation are not so limited, such positions and transitions being dictated by and varying with the particular application and the geometrical and mechanical design and orientation of the moving structural elements of a particular version of the compressor of the present invention. In the context of the operation of a compressor having a flywheel, it will be further appreciated that the flywheel is essentially a gear that is part of an overall reduction mechanism along with a motor  104 , a drive pulley  112  installed on the motor shaft  108  so as to be substantially coplanar with the flywheel  120 , a belt  114  or the like engaging the drive pulley  112  and the flywheel  120 , and one or more tensioners  116  or pulleys to take the slack out of the belt  114  during operation. In an exemplary embodiment of the compressor wherein the piston has a ten-inch stroke, driving the flywheel at an average speed of about 150 rpm would be typical, though numerous speeds are possible, again, depending on the application and, accordingly, the stroke required. Thus, the flywheel&#39;s operation, at least in this embodiment, is not as much a factor of its inertia as its rotational speed and torque translating to the axial forces acting along the piston rod so as to move the piston up or down within the cylinder. Moreover, because the majority of the moving parts are preferably constructed of aluminum or lightweight plastic, there is very little inertial effect, particularly at such relatively low rpm, such that the compressor operates with very little shaking or noise. Noise may be additionally reduced by mounting the motor on a resilient support to dampen vibration. Further, because the motor works hardest when it needs to during the final portion of each compression stroke or phase and works less when it doesn&#39;t need to, as when the piston has completed its up or down stroke and has started back in the opposite direction, it will be appreciated that the power requirements of the motor and the wear and tear on the motor are greatly reduced in the compressor design of the present invention. 
   Turning to  FIGS. 3-6 , there is shown an alternative embodiment of the compressor  200  of the present invention wherein the slot  256  in the guide bar  254  is “S-shaped” and the guide bar itself has a slightly different profile. As shown, the remaining structure of the compressor is essentially the same as that of the above-described exemplary embodiment, including a flywheel  220  with crankpin  222 , an intake block  226  connected between the crankpin  222  and the top of the piston rod  270 , a pivot arm  250  pivotally connected to both the frame  206  of the compressor and, at some distance away, the bottom end of the cylinder, and a guide bar  254  rigidly mounted to the pivot arm  250  and at its opposite free end dynamically linked to the crankpin  222  through location of a bearing  224  or the like of the crankpin within the slot  256  formed in the guide bar  254 . The S-shaped slot then further accentuates the principle at work in the previously described exemplary embodiment of the invention. Particularly, with reference to  FIGS. 4-6 , it will be appreciated by those skilled in the art that the curvature of the S-shaped slot  256  and the resulting accentuated movement of the guide bar  254 , and thus the cylinder  230 , as the guide bar  254  follows the crankpin  222  through the travel of the crankpin&#39;s bearing  224  within the slot  256  furthers the advantages achieved through the compressor design of the present invention of dynamically shifting the cylinder  230  and varying the speed of the piston (not shown) therein accordingly throughout the cycle. This is further evident with reference to the drawing figures, which indicate that while the guide bar  254  is rigidly attached to the pivot arm  250  at the bottom of the cylinder  230  and travels with the cylinder through its lateral oscillations, it does not necessarily do so identically. This is true of each of the embodiments, but is exaggerated through the use of an S-shaped slot  256  or the like. That is, as the flywheel  220  rotates, at some points during the cycle the cylinder  230  will essentially be “ahead” of the guide bar  254 , as, for example, in a first phase shown in  FIG. 4 , while at other times the cylinder  230  will essentially “lag” behind the guide bar  254 , as in a third phase shown in  FIG. 6 . It will be appreciated that the net effect of the cylinder&#39;s leading and following the guide bar as described and shown is greater attenuation, or more extreme oscillation, of the cylinder within the same basic geometry and overall movement of the flywheel and guide bar, such as, for example, in a typical eight-inch stroke configuration. It will also be appreciated with reference to  FIGS. 4-6  that pivot arm  250  pivots about the pivot shaft  252  as the guide arm  254  rigidly mounted to the pivot arm  252  follows the crankpin  222 . Accordingly, the relative movement of the cylinder  230  is caused by its pivotable connection effectively at its upper end with the crankpin  222  through the piston rod  270  and intake block  226  and effectively at its lower end with a pivot pin  258  mounted to the pivot arm  250 . With respect to the S-shaped slot alternative embodiment, then, as for other embodiments, it is to be understood that numerous modifications to the size and shape of the slot and the other components of the compressor are possible without departing from the spirit and scope of the invention. 
   Referring now to  FIGS. 7-12 , there is shown in six phases of operation yet another exemplary embodiment of the compressor  300  of the present invention wherein the flywheel  320  is “lobed,” or roughly elliptical in shape. The elliptical flywheel  320  is formed with an outer rim  329  defining the flywheel&#39;s elliptical profile as having a major diameter and a minor diameter. In the exemplary embodiment, opposing spokes  328  are formed substantially along the major and minor diameters so as to connect a hub  327  rotatably installed on the flywheel shaft  324  to the outer rim  329 , though it will be appreciated that this is not necessary and so is merely exemplary. As shown, much of the remaining structure of the compressor  300  is like that of the above-described exemplary embodiments, including the installation of a crankpin  322  on the flywheel  320  and an intake block  326  connected between the crankpin  322  and the top of the piston rod  370 . As explained more fully below, the crankpin  322  is mounted on the flywheel  320  within a first quadrant defined as an arcuate segment of the flywheel  320  between the major diameter and the minor diameter, or between the 12:00 and 3:00 positions as the flywheel is oriented with its major diameter substantially horizontal. For clarity and ease of explanation, and as an alternative embodiment of the present invention, the exemplary lobed flywheel does not include a pivot arm pivotally connected to both the frame of the compressor and the bottom end of the cylinder or a guide bar rigidly mounted to the pivot arm and at its opposite free end dynamically linked to the crankpin, though it will be appreciated that this structure, or any other such structure such as, for example, a cam or yoke arrangement, and its resultant advantages through articulating the cylinder both horizontally and vertically may also be employed in this lobed flywheel compressor design. Rather, the cylinder  330  is pivotally installed at its bottom end to a pivot pin  358  mounted to the frame  306  of the compressor  300 . Generally, with respect to the lobed flywheel configuration, it will be appreciated that the variation of speed and torque achieved as the flywheel  320  is driven by the motor  304  operating at a constant speed, and the resulting variation in the speed and pressure of the piston itself (not shown) through the linkage of the piston rod  370  to the flywheel  320  through the crankpin  322 , again produces smooth and efficient air compression. Particularly, in the first phase shown in  FIG. 7 , when the piston is retracted toward the top of the cylinder on its upstroke, as when the crankpin  322  is moving toward the top, or apex, of its travel on the flywheel  320  in a counterclockwise direction, the flywheel  320 , and thus the crankpin  322 , the piston rod  370 , and the piston itself, is beginning to slow down as the piston is nearing the top of its stroke. Specifically, at about this position in the cycle the lobed flywheel is positioned radially such that its major axis is roughly horizontal. Because the overall geometry is set up in this exemplary embodiment such that the belt  314  driving the flywheel  320  is substantially vertical when the flywheel is in this position, it will be appreciated that at this stage in the cycle the motor  304  is acting through the largest radial distance with respect to the axis of the flywheel  320  so as to apply the largest amount of torque and turn the flywheel  320  effectively at or near its slowest speed. Accordingly, the compressor geometry is configured such that at this stage in the flywheel&#39;s rotation, the piston is at or near the top of its stroke so that this slow-down and the resulting increased torque applied by the motor and reduction mechanism in driving the flywheel produces a nice, smooth “squeezing” of the air during the final part of the upstroke compression in the upper chamber of the cylinder. As with the other exemplary embodiments of the compressor of the present invention, it will be appreciated that the motor is able to provide increased torque, and thus increased pressure through the piston rod to the piston, without doing an appreciable amount of additional work. Therefore, again, the geometrical and mechanical relationships set up in the compressor help or enable the motor to do more work with less effort, and hence to operate more efficiently. Right at the peak of the movement of the piston rod  370 , as in the second phase of movement shown in  FIG. 8 , the air in the upper chamber has reached its maximum compression for the cylinder and is discharged through the upper chamber&#39;s check valve as previously described. Then, once the crankpin  322  has passed beyond this apex point and is beginning to move the piston through its down stroke, as in the third phase of operation shown in  FIG. 9 , the flywheel  320 , and thus the crankpin  322 , the piston rod  370 , and the piston itself, is speeding back up as the relatively easier, initial work of compression is being done in the lower chamber and ambient air is being introduced into the evacuated upper chamber as the piston is on its down stroke, again, more about which is said below. It will be appreciated that this increased speed and reduced torque is achieved as the effective or working diameter of the flywheel  320  is gradually reduced by shifting from the lobed flywheel&#39;s major diameter toward its minor diameter during its rotation; that is, as the working diameter becomes relatively smaller, the flywheel turns faster at a lower torque. As shown in  FIG. 10 , then, during an intermediate fourth phase of the operation of the exemplary lobed flywheel compressor embodiment, the flywheel  320  is continuing its counterclockwise rotation as its effective diameter decreases until the point shown where the minor diameter of the flywheel is generally horizontal. As such, this would effectively be the smallest working diameter of the flywheel  320 , or the point at which speed is roughly greatest and torque is roughly least. This is acceptable and, in fact, desirable during this phase as no real work is yet needed in essentially “gathering” the ambient air. Transitioning from this fourth phase to the position of the flywheel  320  indicated in  FIG. 11  results in the flywheel slowing down, similar to the first phase of  FIG. 7 , as its working diameter again shifts back toward the major diameter of the lobed flywheel. Thus, as the piston is now advanced toward the bottom of the cylinder on its down stroke, the flywheel  320 , and thus the crankpin  322 , the piston rod  370 , and the piston itself, is beginning to again slow down as the piston is nearing the bottom of its stroke. Once more, this slow-down results in greater torque applied by the motor and reduction mechanism in driving the flywheel, and ultimately the piston, at a relatively slower speed, so as to again produce a smooth “squeezing” of the air during the final part of the down stroke compression in the bottom chamber of the cylinder. When the air has reached its maximum compression in the lower chamber, basically at the position of the piston in the fifth phase shown in  FIG. 11 , it is then discharged through a check valve or passed into the upper chamber for further compression on the piston&#39;s upstroke, as described above. Finally, once the crankpin  322  has moved counterclockwise beyond this lowest position in the direction shown in the sixth phase of  FIG. 12 , the flywheel  320 , and thus the crankpin  322 , the piston rod  370 , and the piston itself, is again speeding back up as the flywheel  320  is once more rotating in orientation toward its minimum working diameter as the relatively easier, initial work of compression is being done now in the upper chamber and ambient air is once more being introduced into the evacuated lower chamber as the piston continues on its upstroke. This alternative intermittent speed and pressure cycle is simply repeated to again efficiently compress air from ambient conditions to a desired higher pressure. Once more, further speed and pressure variance during the cycle may be achieved by the simultaneous, coordinated, dynamic movement of the cylinder body itself through its pivoted connection on a pivot arm linkage within the mechanism and corresponding attenuation through a guide arm working in concert with the crankpin, or through other such structure, as explained above with respect to other exemplary embodiments of the present invention. With reference to the preceding general description of the operation of the alternative exemplary lobed flywheel compressor through its various phases, then, it is to be understood that each of the positions about the flywheel referred to or shown are for explanation of the principles of operation of the present invention only and that the exact positions and transitions of each of the phases of operation are not so limited, such positions and transitions being dictated by and varying with the particular application and the geometrical and mechanical design and orientation of the moving structural elements of any particular version of the compressor of the present invention, particularly in the event that a guide bar and pivot arm mechanism or other such structure is added to the structure shown. A double tensioner configuration involving a tensioner pulley  316  and an idler pulley  317  as shown may be employed so as to take slack variation out of the belt  314  or other such drive means during all phases of operation of the lobed flywheel design as above described. 
   Referring to  FIG. 13 , another exemplary embodiment of the air compression apparatus  400  of the present invention is shown wherein the flywheel  420  is again roughly elliptical in shape, formed with an outer rim  429  defining the flywheel&#39;s elliptical profile as having a major diameter and a minor diameter. In this exemplary embodiment, opposing spokes  428  are formed substantially along the major diameter while one spoke  417  is formed along the minor diameter so as to so as to connect the hub  427  rotatably installed on the flywheel shaft  424  to the outer rim  429 . A radially-outwardly projecting fastening plate  419  to which the crankpin  422  is mounted is formed on the flywheel outer rim  429  laterally offset from the drive belt  414 . A fourth spoke  418  is formed on the flywheel  420  offset from the minor diameter so as to also connect the hub  427  to the outer rim  429  so as to be substantially continuous with the fastening plate  419  and give support thereto, though it will again be appreciated that the structure and arrangement of any of the spokes is merely exemplary and that numerous other arrangements are possible without departing from the spirit and scope of the invention. With continued reference to  FIG. 13 , much of the remaining structure of the compressor  400  is like that of the above-described exemplary embodiments, including the installation of the crankpin  422  on the flywheel  420  and an intake block  426  connected between the crankpin  422  and the top of the piston rod  470  to facilitate passage of ambient air into the hollow piston rod as explained in more detail below. Similar to the embodiment of  FIGS. 7-12 , specifically, the fastening plate  419 , and thus the crankpin  422 , is mounted on the flywheel  420  substantially within a first quadrant defined as an arcuate segment of the flywheel  420  between the major diameter and the minor diameter. The cylinder  430  is again shown as pivoting about a pivot pin  458  mounted to the frame  406  of the compressor  400 . Once more, as a further alternative embodiment of the present invention, the elliptical flywheel compressor  400  may also include a pivot arm pivotally connected to both the frame of the compressor and the bottom end of the cylinder, a guide bar rigidly mounted to the pivot arm and at its opposite free end dynamically linked to the crankpin, or a cam or yoke arrangement so as to further articulate the cylinder both horizontally and vertically. A motor  404  having a drive pulley  412  installed on its shaft again cooperate with a tensioner pulley  416  and an idler pulley  417  to positively drive the elliptical flywheel  420  through the drive belt  414  during operation of the compressor  400 . As compared to the elliptical flywheel  320  of  FIGS. 7-12 , it will be appreciated that the ratio of the major diameter to the minor diameter in the present exemplary embodiment is essentially greater, resulting in relatively greater speed and torque variance during operation of the compressor  400  based on the working diameters of the flywheel  430  alone during its rotation. Once more, it will be appreciated by those skilled in the art that numerous configurations of the flywheel, elliptical or otherwise, may be employed in the compressor to suit particular applications and performance criteria without departing from the spirit or scope of the present invention. 
   Turning to  FIG. 14 , there is shown yet another exemplary embodiment of the air compression apparatus  500  of the present invention wherein the flywheel  520  is roughly elliptical in shape, again formed with an outer rim  529  defining the flywheel&#39;s elliptical profile as having a major diameter and a minor diameter. In this exemplary embodiment, opposing spokes  528  are formed substantially along the major diameter while one spoke  518  is formed along the minor diameter so as to connect the hub  527  to the outer rim  529 . A fourth spoke  519  is formed on the flywheel  520  offset from the minor diameter so as to also connect the hub  527  to the outer rim  529  and to extend radially substantially within a first quadrant defined as an arcuate segment of the flywheel  520  between the major diameter and the minor diameter. As shown, the crankpin  522  is mounted on the fourth spoke  519  so as to again position the crankpin  522  within the first quadrant, or out of phase with both the major and minor axes of the elliptical flywheel  520 . It will again be appreciated that the structure and arrangement of any of the spokes and even the precise location of the crankpin  522  on the flywheel  520  are merely exemplary and that numerous other arrangements are possible without departing from the spirit and scope of the invention. With continued reference to  FIG. 14 , two masses  515  are symmetrically located within the outer rim  529  substantially along the major diameter to add inertial effect to the flywheel  520 . Other locations and types and sizes of such weights are possible. Much of the remaining structure of the exemplary compressor  500  is like that of the above-described exemplary embodiments, including the installation of the crankpin  522  on the flywheel  520  and an intake block  526  connected between the crankpin  522  and the top of the piston rod  570  to facilitate passage of ambient air into the hollow piston rod as further explained below. The cylinder  530  is again shown as pivoting about a pivot pin  558  mounted to the frame  506  of the compressor  500 , though the cylinder is  530  is depicted as being relatively shorter and larger in diameter than the other cylinders shown and described above. More about this particular cylinder structure and operation is said below, but it will be appreciated that in such flywheel or crank-driven compressors, the effective stroke length is essentially dictated by the location of the crank pin on the crank and the degree of actuation of the cylinder body. Here, it will be appreciated that the crankpin  522  is shown positioned on the spoke  519  of the flywheel  520  a relatively short distance from the hub  527 , and hence the flywheel shaft (not shown). In the exemplary embodiment, the cylinder has a diameter of roughly 3¼ to 3½ inches (8¼ to 9 cm) and the radial location of the crankpin  522  translates to an approximately 1½ to 2 inch (4 to 5 cm) stroke. It will be appreciated by those skilled in the art that such a cylinder arrangement may be driven at relatively higher speeds, on the order of 500 to 700 rpm, for example, due to the reduced inertial effects resulting from essentially reduced attenuation of the cylinder and piston assembly. Once more, though not shown, it will be appreciated that as a further alternative embodiment of the present invention, the elliptical flywheel compressor may also include a pivot arm pivotally connected to both the frame of the compressor and the bottom end of the cylinder, a guide bar rigidly mounted to the pivot arm and at its opposite free end dynamically linked to the crankpin, or a cam or yoke arrangement so as to further articulate the cylinder both horizontally and vertically so as to potentially increase the stroke length. A motor  504  having a drive pulley  512  installed on the motor shaft  508  again cooperates with a tensioner pulley  516  and an idler pulley  517  to positively drive the elliptical flywheel  520  through the drive belt  514  during operation of the compressor  500 . As compared to the elliptical flywheel  520  of  FIGS. 7-12 , it will be appreciated that the ratio of the major diameter to the minor diameter in the present exemplary embodiment is essentially less, resulting in relatively less speed and torque variance during operation of the compressor  500 , which effect it will be appreciated is offset due to the increased inertial effects caused, in part, by the addition of symmetrical masses  515  to the flywheel  520  and the increased speed at which the flywheel may potentially be driven. Once more, it will be appreciated by those skilled in the art that numerous configurations of the flywheel, elliptical or otherwise, may be employed in the compressor in combination with various cylinder arrangements to suit particular applications and performance criteria without departing from the spirit or scope of the present invention. 
   Turning now to  FIG. 15 , there is shown a still further alternative embodiment of the air compression apparatus  600  of the present invention wherein the variable speed and pressure of the piston is achieved through a chain drive and cam follower mechanism. Two gears or sprockets  620 ,  621  operate in tandem to drive a chain or belt  614  to which a cam follower  622  is connected along a substantially oval path. In a preferred embodiment, the sprockets comprise a driving sprocket  620  and an idler sprocket  621  in spaced apart relationship such that the centers of the sprockets define a centerline parallel to and offset from the axis of the cylinder  630 . The cam follower  622  is located and travels within a slot  656  formed in a track arm  654  that is rigidly connected to the intake block  626  at an intermediate point along its length and substantially at a free end to a sliding bushing  652  operating along a fixed guide rod  650  secured between opposite attachment blocks  651 . Preferably, the guide rod is parallel to and offset from the centerline of the sprockets  620 ,  621  opposite the cylinder  630 . The intake block  626  is rigidly connected to the hollow piston rod  670  as in the other exemplary embodiments of the invention and is again formed with at least one passage (not shown) to allow ambient air to pass into the piston rod  670 , whereby the piston rod  670  is effectively rigidly attached to the track arm  654 . The generally diagonal or angled orientation of the track arm  654  relative to the substantially vertically oriented members of the assembly such as the piston rod  670  and guide rod  650 , preferably at an acute angle of between zero and ninety degrees relative to the guide rod, serves to provide increased pressure on the piston (not shown) during the high compression phase of operation, as explained more fully below. Both the guide rod  650  and the one or more cylinders are mounted to the compressor&#39;s frame  606  or pressure tank (not shown) using conventional attachment blocks or the like, though it is to be understood that the cylinder may also be pivotally or dynamically affixed in any of the exemplary ways shown and described in connection with the other exemplary embodiments of the present invention or using any other such means now known or later developed in the art. The drive mechanism, including the sprockets  620 ,  621  are also preferably installed on the frame  606  or the tank. Relatedly, while the inlet and outlet valves to the cylinder and, accordingly, the tubing leading to the tank, are not shown, it will be appreciated that they can be installed in numerous ways without departing from the spirit and scope of the invention. Though the chain drive, cylinder, and guide rod are effectively oriented vertically, it will also be appreciated that virtually any spatial orientation of these and the other components of the alternative chain drive compressor design are possible. As described more fully below, the substantially oval path of the chain drive coupled with the diagonal slot and its orientation relative to the cylinder results in the desired varied speed and pressure of the piston. 
   In operation, then, as the chain drive  614  moves, whether clockwise or counterclockwise as driven by the pair of sprockets  620 ,  621 , the cam follower  622  operates within the slot  656  of the track arm  654  so as to effectively shift the track arm  654  up and down vertically, resulting in varied speed and pressure of the piston rod  670  through its rigid connection to the track arm  654  via the intake block  626 . It is assumed for the purpose of the following more detailed explanation that the chain drive  614  is being driven clockwise and that the cylinder employed is “double-acting” as described elsewhere herein. In a first phase of operation wherein the cam follower  622  is positioned adjacent the upper drive sprocket  620  so that it is entering effectively a first quadrant between the 9:00 and 12:00 positions, or between two hundred seventy and three hundred sixty degrees, it will be appreciated that the piston is being pulled upwardly, or is on its upstroke, as the cam follower  622  continues in a clockwise direction on the chain drive  614  such that the piston is nearing the top of its stroke, or the maximum compression of the air in the cylinder&#39;s upper chamber. At this time, the speed of the piston is also slowing down as the cam follower  622  is moving on the chain  614  around the circumference of the upper sprocket  620  so as to shift toward increased horizontal displacement, as opposed to vertical displacement, which, in turn, results in reduced vertical displacement of the track arm  654  and, hence, the intake block  626 , the piston rod  670 , and the piston itself. Accordingly, it will be further appreciated that while the movement of the piston is slowing, the effective force on the piston is increasing due to the leverage effect achieved through the cam follower  622  moving more and more along the slot  656 , rather than against it, so as to take advantage of the fundamental “ramp” device known and used in various mechanical arts. As such, the track arm mechanism  654  enables the cam follower  622  to do more work in lifting the piston during its final phase of compression with the same effort, or, put another way, to apply more force without appreciably any more work by the motor (not shown) driving the chain drive  614  through the pair of sprockets  620 ,  621 . It will be appreciated by those skilled in the art that numerous other configurations of the track arm, both in terms of its orientation and the size and shape of its slot, taking advantage of and even further exploiting the effect of this mechanical principle are possible without departing from the spirit and scope of the invention. During this first phase of operation, then, the resulting slow-down of the piston while at the same time increasing the force it is applying to the column of air in the cylinder&#39;s upper chamber again results in a nice, smooth “squeezing” of the air during the final part of the piston&#39;s upstroke. When the cam follower  622  reaches the apex of its vertical travel around the upper sprocket  620 , or about the 12:00 position, the air in the upper chamber has reached its maximum compression for this cylinder and is discharged through the upper chamber&#39;s check valve as described herein elsewhere in connection with other exemplary embodiments of the present invention. Then, in a second phase of operation, once the cam follower  622  has passed beyond the apex and is moving through the second quadrant of the upper sprocket  620  roughly between the 12:00 and 3:00 positions, it is shifting back to increased vertical displacement as its horizontal displacement effectively about the radius of the upper sprocket  620  is completed. This increasing vertical displacement yields a corresponding increasing vertical displacement and speed of the track arm  654 . Accordingly, the intake block  626 , the piston rod  670 , and the piston itself are speeding back up as the relatively easier, initial work of compression is being done in the lower chamber of the cylinder  630  and ambient air is being introduced, or “gathered,” into the evacuated upper chamber as the piston is on its down stroke. This low-work, “air-gathering” second phase continues as the cam follower  622  travels the substantially linear section of the chain  614  effectively between opposite tangential points on the right sides of the respective upper and lower sprockets  620 ,  621 . Next, a third phase of operation is initiated as the cam follower  622  arrives at roughly the 3:00 position on the lower idler sprocket  621  and so enters what is effectively the third quadrant of the chain drive  614 , between the lower sprocket&#39;s 3:00 and 6:00 positions. In this third phase, then, analogous to the first phase, the piston is now being pushed downwardly as the cam follower  622  continues in a clockwise direction on the chain drive  614  such that the piston is nearing the bottom of its stroke, or the maximum compression of the air in the cylinder&#39;s lower chamber. Once more, during this phase, the speed of the piston is also slowing down as the cam follower  622  is moving on the chain  614  around the circumference of the lower sprocket  621  so as to shift toward increased horizontal displacement, as opposed to vertical displacement, again resulting in reduced vertical displacement of the track arm  654  and, hence, the intake block  626 , the piston rod  670 , and the piston itself. Again, while the movement of the piston is slowing, the effective force on the piston is increasing due to the leverage effect achieved through the cam follower  622  moving effectively along a mechanical ramp formed by the slot  656 , enabling the cam follower  622  to do more work in pushing the piston downward during its final phase of compression with the same essential effort by the motor, resulting in a smooth and efficient “squeezing” of the air during the final part of the down stroke compression in the bottom chamber of the cylinder  630 . When the air has reached its maximum compression in the lower chamber, it is then discharged through a check valve or passed into the upper chamber for further compression on the piston&#39;s upstroke, as described previously with other embodiments. Finally, in a fourth basic phase of operation analogous to the above-described second phase, once the cam follower  622  has passed beyond the low-point of the lower sprocket  621 , or roughly the 6:00 position, and is moving through effectively the fourth quadrant of the chain drive  614  between roughly the 6:00 and 9:00 positions on the lower sprocket  621 , the cam follower  622  is shifting back to increased vertical displacement as its horizontal displacement effectively about the radius of the lower sprocket  621  is completed. Once again, this increasing vertical displacement yields a corresponding increasing vertical displacement and speed of the track arm  654 , and, hence, the intake block  626 , the piston rod  670 , and the piston itself are speeding back up as the relatively easier, initial work of compression is being done in the cylinder&#39;s upper chamber and ambient air is being “gathered” into the now evacuated lower chamber as the piston is again on its upstroke. This low-work, “air-gathering” fourth phase continues as the cam follower  622  travels the substantially linear section of the chain  614  effectively between opposite tangential points, or 9:00 positions, on the left sides of the respective upper and lower sprockets  620 ,  621 . This four-phase, intermittent speed and pressure cycle is simply repeated to efficiently compress air from ambient conditions to a desired higher pressure. Once again, further speed and pressure variance during the cycle may be achieved by the simultaneous, coordinated movement of the cylinder body itself through a pivoted or dynamic connection to the mechanism rather than the rigid connection shown. 
   With reference to the preceding general description of the operation of an exemplary chain drive compressor  600  of the present invention through four basic phases, then, it is to be understood that each of the geometrical and mechanical elements and features discussed are for explanation of the principles of operation only and that the invention is not so limited. Rather, it will be appreciated that numerous changes to the geometry shown and described are possible without departing from the spirit and scope of the invention. For example, it is to be understood that though it is preferable to have the axis of the piston rod substantially aligned vertically over the centerline of the dual-sprocket chain drive so as to get essentially the same work of compression on both the upstroke and down stroke of the piston, this is not necessary and, depending on the application, may be less desirable in view of other design considerations. One instance where this may be desirable would be the use of the chain drive and track arm to operate two cylinders simultaneously in parallel, each offset vertically from the centerline of the chain drive on opposite sides. Or, as a further exemplary alternative, a second cylinder can be actuated by the single chain drive and track arm by extending co-linearly with, but in the opposite direction from, the first cylinder shown. In this embodiment, both cylinders could operate effectively along the centerline of the chain drive and could even share a common intake block. Whether one or more cylinders are driven, a single guide rod offset to one side of the chain drive, as shown, or a second guide rod offset on the opposite side of the chain drive to provide additional lateral stability may also be employed. Additionally, it will be appreciated by those skilled in the art that the chain drive embodiment of the compressor of the present invention may be particularly suited to high volume or high pressure contexts due to the relative ease with which the size or stroke of the one or more cylinders can be increased, and may be so modified accordingly. That is, a longer-stroke piston can be driven by the chain drive compressor by simply increasing the length of the guide rod or rods and the effective length of the chain drive, as by moving the sprockets further apart or even adding additional sprockets, pulleys, tensioners, tracks or the like to stabilize the linear sections of the chain or belt between the upper and lower sprockets. Additional, spaced-apart sliding bushings on each of the guide rods and rigidly connected to the track arm could be used to further stabilize the mechanism in such longer-stroke applications. The increased stroke also effectively increases the accuracy or precision of the derived air pressure due to the increased stroke ratio, or the total length the piston travels, and thus the volume of air compressed, compared to the length of the high-compression phase at or near the completion of the up and down strokes. It will be further appreciated that this increase in piston stroke length, and hence capacity of the compressor, is attainable by effectively increasing only the length of the mechanism, not its width or depth to any real extent. However, as a further example of alternative embodiments for the chain drive compressor design, larger or smaller sprockets can also be employed as needed based on the application and pressure requirements. Ultimately, movement of the chain  614  about the sprockets  620 ,  621  translates into oscillating linear movement of the track arm  654  and simultaneous axial displacement of the piston body (not shown) within the cylinder  630  as acted on by the piston rod  670  rigidly mounted to the track arm  654  through the intake block  626 . Accordingly, it is to be understood that the various embodiments of the chain drive compressor are merely exemplary, and that numerous other configurations may be employed without departing from the spirit or scope of the invention. 
   Referring to  FIGS. 16 and 17 , another alternative air compressor apparatus  700  of the present invention is shown as generally having two cylinders  730 ,  731  installed on a frame  706  in a substantially aligned offset arrangement. The first cylinder  730  is formed with a first lower cylinder wall  732  and has a first piston body  740  sealingly and slidably installed therein so as to form a first upper chamber  734  above the first piston body  740  and a first lower chamber  736  below the first piston body  740 . The second cylinder  731  is formed with a second lower cylinder wall  733  and has a second piston body  741  sealingly and slidably installed therein so as to form a second upper chamber  735  above the second piston body  741  and a second lower chamber  737  below the second piston body  741 . A first piston rod  770  and a second piston rod  771  are rigidly connected at respective adjacent ends to the drive mechanism  710 . The first piston rod  770  has a first hollow bore (not shown) and at least one first breathing hole  774  communicating between the first hollow bore and the ambient air. The first piston rod  770  passes through the first cylinder  730  and the first upper chamber  734  and is connected at a first piston end opposite the drive mechanism  710  to the first piston body  740  so that the first hollow bore selectively communicates with the first lower chamber  736 . Similarly, the second piston rod  771  has a second hollow bore  773  and at least one second breathing hole  775  communicating between the second hollow bore  773  and the ambient air. The second piston rod  771  passes through the second cylinder  731  and the second upper chamber  735  and is connected at a second piston end opposite the drive mechanism  710  to the second piston body  741  so that the second hollow bore  773  selectively communicates with the second lower chamber  737 . At least one first escape passage  738  is formed within the first cylinder  730  so as to selectively communicate between the first upper chamber  734  and the first lower chamber  736 , the first escape passage  738  having a first longitudinal length greater than the thickness of the first piston body  740 . Likewise, at least one second escape passage  739  is formed within the second cylinder  731  so as to selectively communicate between the second upper chamber  735  and the second lower chamber  737 , the second escape passage  739  having a second longitudinal length greater than the thickness of the second piston body  741 . A first lower piston valve  742  is installed on the first piston body  740  so as to selectively seal the first lower chamber  736  from the first hollow bore. A second lower piston valve  743  is installed on the second piston body  741  so as to selectively seal the second lower chamber  737  from the second hollow bore  773 . A first check valve  783  is installed in the first cylinder  730  so as to communicate with the first upper chamber  734  and a second check valve  784  is installed in the second cylinder  731  so as to communicate with the second upper chamber  735 . Similarly, a first one-way valve  780  is installed in the first cylinder  730  in fluid communication with the first upper chamber  734  and a second one-way valve  781  is installed in the second cylinder  731  in fluid communication with the second upper chamber  735 . More about the operation of these valves is said below with respect to the operation of the compressor  700 . Air lines  782  are then connected to the first and second one-way valves  780 ,  781 , whereby movement of the drive mechanism  710  effectively in a first direction acts on the first piston rod  770  to cause the first piston body  740  to travel toward the first lower chamber  736 , drawing ambient air into the first upper chamber  734  through the first check valve  783  while closing the first lower piston valve  742  and compressing the air in the first lower chamber  736  until the first piston body  740  nears the first lower cylinder wall  732  such that the at least one first escape passage  738  is temporarily no longer sealed by the first piston body  740  so as to allow the compressed air to pass from the first lower chamber  736  through the at least one first escape passage  738  and into the first upper chamber  734 , where the compressed air then mixes with the ambient air for further compression when the piston  740  begins its travel in the opposite direction. Simultaneously, movement of the drive mechanism  710  in the first direction acts on the second piston rod  771  to cause the second piston body  741  to travel toward the second upper chamber  735 , closing the second check valve  784  and further compressing the air in the second upper chamber  735  while opening the second lower piston valve  743  to allow ambient air to be drawn through the at least one second breathing hole  775  and the second hollow bore  773  into the second lower chamber  737 . Similarly, movement of the drive mechanism  710  in an opposite second direction acts on the first piston rod  770  to cause the first piston body  740  to travel toward the first upper chamber  734 , closing the first check valve  783  and further compressing the air in the first upper chamber  734  while opening the first lower piston valve  742  to allow ambient air to be drawn through the at least one first breathing hole  774  and the first hollow bore into the first lower chamber  736 . Simultaneously, movement of the drive mechanism  710  in the second direction acts on the second piston rod  771  to cause the second piston body  741  to travel toward the second lower chamber  737 , drawing ambient air into the second upper chamber  735  through the second check valve  784  while closing the second lower piston valve  743  and compressing the air in the second lower chamber  737  until the second piston body  741  nears the second lower cylinder wall  733  such that the at least one second escape passage  739  is temporarily no longer sealed by the second piston body  741  so as to allow the compressed air to pass from the second lower chamber  737  through the at least one second escape passage  739  and into the second upper chamber  735  to mix with the ambient air for further compression when the piston  741  begins its travel again in the first direction. It will be appreciated by those skilled in the art that while a standard check valve is employed in this exemplary embodiment for the purpose of introducing ambient air into the first and second upper chambers of the respective cylinders, upper piston valves as disclosed herein allowing for ambient air to be introduced through the hollow piston rods into the upper chambers as the pistons travel toward the lower chambers may also be employed. 
   As best shown in  FIG. 17 , the drive mechanism  710  comprises a piston rod mounting block  726  mounted to the respective adjacent ends of the first and second piston rods  770 ,  771  so as to rigidly support the first and second piston rods  770 ,  771  in a substantially coaxial arrangement. The first and second breathing holes  774 ,  775  are positioned along the respective first and second piston rods  770 ,  771  so as to be clear of the piston rod mounting block  726 . A yoke block  754  is rigidly mounted to the piston rod mounting block  726 . The yoke block  754  is formed with an outwardly-opening yoke channel  756  at an angle between zero and ninety degrees relative to the piston rod mounting block  726 , the operation of which is explained below. A cam pulley  720  is mounted to the frame (not shown) so as to rotate about a cam pulley shaft (not shown), the cam pulley having a cam follower  722  projecting therefrom offset from the cam pulley shaft and oriented so as to extend into and engage the yoke channel  756 . A drive pulley  712  is installed on a drive shaft  708  of the motor  704  so as to be substantially coplanar with the cam pulley  720 , and a drive belt  714  is then configured to engage the drive pulley  708  and the cam pulley  720  so that torque from the motor  704  is transmitted to the cam pulley  720  through the drive belt  714 , whereby rotational movement of the cam pulley  720  translates into oscillating linear movement of the piston rod mounting block  726  and simultaneous axial displacement of the first and second piston bodies  740 ,  741  within the respective first and second cylinders  730 ,  731  as acted on by the respective first and second piston rods  770 ,  771  rigidly mounted within the piston rod mounting block  726 , as explained more fully below. 
   In operation, then, as the cam pulley  720  rotates, whether clockwise or counterclockwise as driven by the motor  704  and drive pulley  712  through the belt  714 , the cam follower  722  operates within the yoke channel  756  of the yoke block  754  so as to effectively shift the piston rod mounting block  726  up and down vertically, resulting in varied speed and pressure of the respective piston rods  770 ,  771  through their rigid connection to the piston rod mounting block  726 . For the purposes of the following explanation, it is assumed that the cam pulley  720  is rotating counterclockwise as viewed from the front as shown in  FIG. 16 . In a first phase of operation of the compressor  700  the cam follower  722  is positioned within the yoke channel  756  at a location effectively within a first and fourth quadrant of the cam pulley  720  between the 6:00 and 12:00 positions, or between zero and one hundred eighty degrees, it will be appreciated that the piston rod mounting block  726  is being pulled upwardly, such that the first piston body  740  is on its upstroke and the second piston body  741  is on its down stroke, whereby the first lower piston valve  742  is closed so as to compress the air in the first lower chamber  736  while an effective vacuum is created in the first upper chamber  734  so as to pull ambient air in through the first check valve  783 . At the same time, the second lower piston valve  743  is opened so as to draw ambient air into the second lower chamber  737  while compressing the air in the second upper chamber  735 . As the cam pulley  720  continues its counterclockwise rotation the cam follower  722  continues to engage the yoke channel  756  and shift the piston rod mounting block  726  further upward, continuing the compression in the first lower chamber  736  and the second upper chamber  735 . This continues until the first piston body  740  nears the first lower cylinder wall  732 , at which time the speed of the piston rod mounting block  726  is slowing down as the cam follower  722  is continuing its arcuate path as it moves with the cam pulley  720  such that the cam follower  722  is shifting toward increased horizontal displacement, as opposed to vertical displacement, which, in turn, results in reduced vertical displacement of the yoke block  754  and, hence, the piston rod mounting block  726 , the piston rods  770 ,  771 , and the pistons  740 ,  741  themselves. Accordingly, it will be appreciated that while the movement of the pistons  740 ,  741  is slowing, the effective force on the pistons is increasing due to the leverage effect achieved through the cam follower  722  moving more and more along the slot  756 , rather than against it, so as to take advantage of the fundamental “ramp” device, again, known and used in various mechanical arts. As such, the yoke block  754  enables the cam follower  722  to do more work in lifting the pistons during their final phase of compression with the same effort, or, put another way, to apply more force without appreciably any more work by the motor  704  driving the cam pulley  720 . It will be further appreciated by those skilled in the art that numerous other configurations of the yoke block, both in terms of its orientation and the size and shape of its slot, taking advantage of and even further exploiting the effect of this mechanical principle are possible without departing from the spirit and scope of the invention. During this first phase of operation, then, the resulting slow-down of the pistons  740 ,  741  while at the same time increasing the force they are applying to the columns of air in the respective first lower chamber  736  and second upper chamber  735  again results in a nice, smooth “squeezing” of the air during the final part of the pistons&#39; stroke. When the cam follower  722  reaches the apex of its vertical travel on the cam pulley  720 , or about the 12:00 position, the air in the first lower chamber  736  has reached its maximum compression for this chamber and at that time passes through the exposed first escape passage  738  and into the first upper chamber  734  for further compression when the piston body  740  starts in the opposite direction as explained below. At the same time, the air in the second upper chamber  735  has also reached its maximum compression for this cylinder  731  and is then discharged through the one-way valve  781 . In a second phase of operation, once the cam follower  722  has passed beyond the apex and is moving through the second and third quadrants between the 12:00 and 6:00 positions, or between zero and one hundred eighty degrees, it will be appreciated that the piston rod mounting block  726  is now being pulled downwardly through the cam follower&#39;s engagement with the yoke channel  756  of the yoke block  754 , such that the first piston body  740  is on its down stroke and the second piston body  741  is on its upstroke, whereby the first lower piston valve  742  is opened so as to draw ambient air into the first lower chamber  735  while compressing the air in the first upper chamber  734  and the second lower piston valve  743  is closed so as to compress the air in the second lower chamber  737  while an effective vacuum is created in the second upper chamber  735  so as to pull ambient air in through the second check valve  784 . As the cam pulley  720  continues its counterclockwise rotation the cam follower  722  continues to engage the yoke channel  756  and shift the piston rod mounting block  726  further downward, continuing the compression in the first upper chamber  734  and the second lower chamber  737  and drawing ambient air into the first lower chamber  736  and second upper chamber  735 . This continues until the second piston body  741  nears the second lower cylinder wall  733 , at which time, the speed of the piston rod mounting block  726  is slowing down as the cam follower  722  is continuing its arcuate path as it moves with the cam pulley  720  such that the cam follower  722  is again shifting toward increased horizontal displacement, as opposed to vertical displacement, which, in turn, results in reduced vertical displacement of the yoke block  754  and, hence, the piston rod mounting block  726 , the piston rods  770 ,  771 , and the pistons  740 ,  741  themselves. Accordingly, it will be appreciated that while the movement of the pistons  740 ,  741  is slowing, the effective force on the pistons is again increasing due to the leverage effect achieved through the cam follower  722  moving more and more along the slot  756 , rather than against it. As such, the yoke block  754  enables the cam follower  722  to do more work in pushing the pistons during their final phase of compression with the same effort, or, put another way, to apply more force without appreciably any more work by the motor  704  driving the cam pulley  720 . During this second phase of operation, then, the resulting slow-down of the pistons  740 ,  741  while at the same time increasing the force they are applying to the columns of air in the respective first upper chamber  734  and second lower chamber  737  again results in a nice, smooth “squeezing” of the air during the final part of the pistons&#39; stroke. When the cam follower  722  reaches the low point of its vertical travel on the cam pulley  720 , or about the 6:00 position, the air in the first upper chamber  734  has reached its maximum compression for this cylinder  730  and is then discharged through the one-way valve  780 . At the same time, the air in the second lower chamber  737  has also reached its maximum compression for this chamber and at that time passes through the exposed second escape passage  739  and into the second upper chamber  735  to mix with the ambient air therein for further compression when the piston body  741  starts in the opposite direction as explained above when the cam follower  722  moves past the low point and back into the first phase of operation. This two-stage, intermittent speed and pressure cycle is simply repeated to efficiently compress air from ambient conditions to a desired higher pressure. Once again, further speed and pressure variance during the cycle may be achieved by the simultaneous, coordinated movement of the cylinders themselves through a pivoted or dynamic connection to the mechanism rather than the rigid connection shown. It will be appreciated by those skilled in the art that the structure and geometry shown is merely exemplary and that numerous other configurations can be practiced without departing from the spirit and scope of the invention. 
   Based on the foregoing, it will be appreciated that with respect to at least one exemplary embodiment, the air compression apparatus can be generally described as an improved multi-stage gas compressor. The principle at work in the exemplary embodiment compressor  700  described above and shown in  FIGS. 16 and 17  is an assembly made up in part of valved pistons moving within cylinders, each driven by a shaped path within a yoke. Passages in and around the pistons transfer the gas from one chamber to another in increasing stages of compression. Again, those skilled in the art will appreciate that numerous other mechanical arrangements are possible for achieving the multi-stage air compression described. The individual chambers within the system may be either dynamic or static. The volume of each dynamic chamber is less than that of the dynamic chamber preceding it in the compression cycle by a calculated amount in order to provide for a stepped increase in pressure from the supply or ambient pressure to the higher pressure in the external holding tank. The dynamic chambers also change in volume dynamically, in response to movement of the yoke, to enhance the movement of gas from one chamber to another and to provide for increased efficiency in the application of power from the motor. The static chambers provide holding and transitional space for the gas as it moves throughout the system. 
   In the preferred embodiment shown, two cylinders  730 ,  731  act in parallel, with both cylinders independently compressing gas into the external holding tank (not shown) through the air lines  782 . In another preferred embodiment (not shown), the cylinders act in series, with the second cylinder receiving compressed gas from the first cylinder and compressing it further. The compressor  700  is an assembly made up of the following major parts, depending on the particular embodiment: a case enclosing the whole assembly (not shown), including several chambers and sub-chambers connected by gas passages, a shaft  708  driven by a motor  704 , a yoke driver  720  either attached rigidly to the shaft or driven by a drive pulley  712  mounted on the shaft  708  through a belt  714 , a yoke  754 , a path  756  of particular shape and design within the yoke  754 , one or more track rollers  722  moving within the path  756  in the yoke  754 , a partly hollow piston rod  770 ,  771  attached rigidly to mounting block  726  attached rigidly to the yoke  754  so as to engage each track roller  722  through the yoke path  756 , a partly hollow piston  740 ,  741  rigidly attached to each piston rod  770 ,  771 , an inertial valve  742 ,  743  within each piston  740 ,  741 , a cylinder  730 ,  731  enclosing each piston  740 ,  741 , escape air passages  738 ,  739  connected at each cylinder  730 ,  731 , in some preferred embodiments a spring-loaded automatic check valve (not shown) at the entrance to each cylinder escape air passage  738 ,  739 , a gland encircling each piston rod  770 ,  771 , and a spring-loaded automatic check valve  780 ,  781  at the gas exit point of each sub-chamber  734 ,  735 . The gland may be comprised of a linear ball bearing in combination with a rod seal. Check valves or further piston inertial valves or the like may be employed in introducing ambient air into the upper chambers of each cylinder as explained elsewhere. Additional minor parts may include bearings, screws, clips, bushings, springs, retainers, connectors, tubing, filters and other small parts as necessary to hold the major parts in proper working relationship to each other, to provide for efficient movement of the various moving parts, and to provide for controlled passage of gas from one chamber to another. The path  756  within the yoke  754  may be shaped in any one of several different ways, depending on the particular embodiment. The purpose of the shaped path  756  is to apply a controlled amount of mechanical leverage to the piston  740 ,  741  proportional to the pressure applied to the piston  740 ,  741  by the compressed gas, as explained above. That is, the piston moves faster, with a lower degree of leverage, when the pressure is low, and slower, with a higher degree of leverage, when the pressure is high. This proportional variation in leverage, again, provides for more efficient utilization of the power drawn from the motor and for reduced vibration and heat. In some embodiments, the path in the yoke may be constructed so as to provide for a different rate and extent of piston travel in different cylinders. The piston rod  770 ,  771  is hollow from a point above the mounting block  726  to the hollow part of the piston  740 ,  741  and collects and transports the gas to be compressed by the piston to which it is connected. The piston  740 ,  741  has a hole extending from its top to the upper end of the piston rod  770 ,  771 . This hole in the piston  740 ,  741  is provided at the upper end with an inertial valve  742 ,  743  which opens to admit gas when the piston begins moving downward and closes to compress the gas when the piston begins moving upward. Controlled passage is provided for the gas compressed by the piston to escape from the lower chamber  736 ,  737  into the sub-chamber  734 ,  735 . The gas in the sub-chamber  734 ,  735  is further compressed as the piston  740 ,  741  moves downward in the respective cylinder  730 ,  731  as explained above. In one preferred embodiment, with the cylinders working in series, the gas compressed in the first sub-chamber is passed through a transition chamber to the hollow piston rod of the second cylinder where the compression cycle is repeated above and below the piston in order to achieve a higher pressure output. In another preferred embodiment as shown in  FIGS. 16 and 17 , with the two cylinders  730 ,  731  working in parallel, each cylinder takes in gas at ambient pressure and each of the two cylinders compresses gas independently, each expressing gas directly into the external holding tank, which results in a greater volume of gas being compressed to a relatively lower initial output pressure, depending, of course, on the geometries of the cylinders. In a preferred embodiment, the two pistons  740 ,  741 , with their connecting rods  770 ,  771  and the yoke  754 , form a rigid structure which moves as a single structural unit, so that little side load is present at the pistons. Other embodiments may employ further pairs of pistons, driven by the same yoke or by additional yokes in a parallel structure for additional compression. Preferably all the moving parts which come in contact with the gas are constructed of self-lubricating material so that no oil is introduced into the gas stream as it is being compressed. A further enhancement to address noise reduction during operation of the compressor is shown in  FIG. 16 . A woven or mesh sleeve  790  may be installed substantially concentrically within each hollow piston rod  770 ,  771  so as to essentially position its outer wall in contact or substantially adjacent to the inner wall of the piston rods  770 ,  771  so as to effectively interrupt its smooth surface. As such, it will be appreciated that the sleeve  790  will serve to dampen sound waves traveling up the hollow piston rods  770 ,  771  during operation, and thus further reduce noise. Those skilled in the art will appreciate that such a woven or mesh sleeve or any other such tubular member having desirable acoustic damping characteristics may be installed within the hollow piston rod of any variation of the present invention. 
   It will be appreciated by those skilled in the art that the various structural and geometrical configurations of the drive mechanism of the air compression apparatus of the present invention are merely exemplary and that numerous such drive systems can be employed in achieving variable-speed, variable-pressure actuation of the one or more pistons operably connected to the drive mechanism so as to yield efficient, clean, and quiet air compression as described herein. With respect to the drive mechanism alone, it will be appreciated, specifically, that efficiency gains are due, in part, to running the motor and crank, yoke or other drive linkage at a relatively slower average speed and at varied speed so that effectively lower speed and higher pressure are transmitted to the one or more pistons when they are doing the greatest amount of work in compressing the air or gas and higher speed and lower pressure are transmitted to the one or more pistons when they are doing less work. Relatedly, the relatively slow, variable speed of the moving parts results in improved power usage of the motor and less heat build up in the system, further improving the efficiency. Moreover, by each of the drive mechanisms shown and described serving to effectively apply pressure to the one or more pistons substantially along the respective piston rod, there is little to no side load on the pistons themselves as they move within the cylinder, further reducing heat build-up and also serving to reduce the wear on the moving parts and, thus, the amount of contaminants in the compressed air output. Accordingly, it is to be understood that numerous other designs of the drive mechanism beyond those exemplary embodiments shown and described are possible without departing from the spirit and scope of the invention. 
   The one or more cylinders employed in compressors according to the present invention may take on various configurations as well, again, depending on the application, numerous examples of which are described in more detail below. Several novel cylinder designs have been conceived, as shown in the drawings, capable of cooperating with the mechanical and operational advantages achieved through structure such as in the exemplary embodiments shown and described, which yield a relatively longer working stroke or larger compressed volume of each piston along with coordinated variance in the speed of the piston during its stroke, so as to ultimately produce smoother and more efficient compression. Specifically, an added operational benefit provided by the various pistons according to the present invention is the introduction of air into the cylinder through a hollow piston rod and valves above and below the piston itself, though it will be appreciated that a single valve either above or below the piston may be employed so as to form a single- or multi-stage cylinder, as described, for example, with respect to the embodiment of  FIGS. 16 and 17 . Where the cylinder is configured to be double-acting as by having valves on the top and bottom of the piston, for example, this results in compressing the air on both the upstroke and the down stroke in each cylinder, so as to effectively double the useful work done by the piston as it cycles through its stroke. This type of piston design also serves to move air through the cylinder at all stages of compression in a more laminar fashion. That is, it will be appreciated by those skilled in the art that introducing ambient air into the cylinder through the hollow piston rod and then through valves located effectively on or about the upper and lower surfaces of the piston enables the air to enter the respective chambers both immediately adjacent to the working surface of the piston and generally in the direction the piston will be traveling on its compression stroke. This results in the ambient air effectively being pushed along and squeezed toward its maximum compression, rather than being “slammed” or run into by the piston at some intermediate point in the stroke. Then, when the compressed air is to be evacuated from the cylinder, it is preferably done so at or near the “top,” or high compression section, of each chamber. In this way, the air never really has to reverse direction between the time it is introduced into each chamber and when it exits. It will be appreciated that these features translate to lower heat build-up and wear of the cylinder&#39;s internal moving parts and increases the efficiency in operation. Again, these effects coupled with the relatively larger volume and intermittent speed of the piston can further enable the air to effectively be “squeezed” rather than “slammed,” providing numerous additional benefits in terms of the performance, cost, and maintenance of the cylinders and the compressor. With respect to the valves and other parts of the cylinder, spring-loaded automatic check valves, which open and close in response to the direction and pressure of the air flow, are preferably provided at the air exit point of each chamber to prevent any backward movement of compressed air through the system. In an alternative preferred embodiment, breathing chambers are provided at the exit points of each chamber so effectively stage the compressed air as it evacuates the cylinder while still preventing backflow, yielding further benefits in operation as described below. The hollow piston rods are preferably made of a high-strength material, such as high-grade steel, polished smooth so as to move freely, with minimal friction and wear, through a gland. This gland provides a wall of separation between the air in the upper chamber and the ambient air by sealing about the outside surface of the piston rod. In some embodiments two or more cylinders may be provided in series, with the air being fed at increasing pressures from chamber to chamber, until the final chamber delivers the compressed air to the output pressure tank. Thus, persons familiar with the art may construct, within the principles of this invention, various embodiments applicable to high-volume or high-pressure air compression, encompassing a broad variety of specialty compressors for various types of applications. 
   Turning to  FIGS. 18-21 , there is shown a first exemplary embodiment air compression cylinder  230  of the present invention as potentially employed in at least compressor systems such as those shown and described with respect  FIGS. 1-13 , though it is noted that the embodiment of the cylinder  130  of  FIGS. 1 and 2  employs a slightly different intake block  126  than the intake block  226  shown in  FIG. 18 . Generally, the cylinder  230  has an annular wall  231 , an upper end  232  and an opposite lower end  233 . The upper and lower ends  232 ,  233  may be installed within the annular wall  231  by a fastener such as a machine screw, by welding, through a press- or interference-fit, or through any other such means now known or later developed in the art. Depending on the assembly technique, an o-ring may be seated within a circumferential groove formed about the upper and lower ends  232 ,  233  so as to positively seal the joint between the annular wall  231  and the respective upper and lower ends  232 ,  233 . Exit valves  280 ,  281  lead from the respective upper and lower ends  232 ,  233  to the air lines  282  and tank  202  ( FIG. 3 ). In the exemplary embodiment, upper and lower one-way valves  280 ,  281  are installed in the ends  232 ,  233  in fluid communication with the upper chamber and lower chambers  234 ,  235  so as to allow air flow therethrough only out of the cylinder  230  while preventing any backflow, as is known in the art. A piston assembly  240  is operably connected to the drive mechanism  210  ( FIG. 3 ) and configured to move within the cylinder  230  mounted to the frame  206  ( FIG. 3 ) as described above with respect to the numerous exemplary embodiments of the present invention. Turning to  FIG. 19 , the piston assembly  240  comprises a piston body  241  having an upper piston wall  244  and an offset lower piston wall  245  joined about an annular piston wall  246  so as to define at least one radially-outwardly-opening circumferential piston ring channel  260  in which at least one piston ring  262  is inserted so as to sealably and slidably contact the inside surface of the cylinder wall  231  during operation of the piston, more about which is said below. The upper and lower piston walls  244 ,  245  may be integral with the annular piston wall  246 , as shown in  FIG. 19 , or may be installed thereon as separate components, as shown in other exemplary embodiments of the invention, using any mechanical fastening technique, such as screws or other such fasteners, a weld, or a press-fit, both now known or later developed in the art. The piston body  241  so installed within the cylinder  230  thus forms an upper chamber  234  between the piston body  241  and the upper end  232  of the cylinder  230  and a lower chamber  235  between the piston body  241  and the lower end  233  of the cylinder  230 . The piston body is further formed with a cavity  247  substantially bounded by the upper and lower piston walls  244 ,  245  and the annular piston wall  246  so as to be in selective communication with at least the lower chamber  235 , though the cavity  247  is shown in the exemplary embodiment as selectively communicating with the upper and lower chambers  234 ,  235  in cooperation with the upper and lower piston valves  242 ,  243 , the operation of which are explained more fully below. Connected to the piston body  241  is a piston rod  270  having a hollow bore  273  communicating between a drive end and a piston end, the drive end being connected to the drive mechanism  210  such that the hollow bore  273  is in communication with ambient air. In the exemplary embodiment, this is accomplished by installing the drive end of the piston rod  270  within an intake block  226  such that the bore  273  is able to communicate with ambient air through an opening  227  formed in the intake block  226 . The piston rod  270  passes through the cylinder  230  at its upper end  232 , as through a gland (not shown) that sealingly and slidably engages the outside surface of the piston  270 , and then through the upper chamber  234  so as to be connected at the opposite piston end to the piston body  241 . The piston rod has at least one opening formed therein substantially at the piston end such that the hollow bore  273  is in communication with the cavity  247 . A lower piston valve  243  is installed on the piston body  241  so as to selectively seal the lower chamber  235  from the cavity  247 , while an upper piston valve  242  is installed adjacent to the piston body  241  so as to selectively seal the upper chamber  234  from the cavity  247 . In this way, when the air line  282  is connected to the cylinder  230  so as to communicate with both the upper chamber  234  and the lower chamber  235  through the respective upper and lower valves  280 ,  281 , it will be appreciated that upward travel of the piston body  241  as caused by the drive mechanism  210  ( FIG. 3 ) acting through the piston rod  270  closes the upper piston valve  242  so as to compress the air within the upper chamber  234  while opening the lower piston valve  243  to allow ambient air to enter the lower chamber  235  through the hollow bore  273  of the piston rod  270 , whereas downward travel of the piston body  241  as caused by the drive mechanism  210  acting through the piston rod  270  opens the upper piston valve  242  and allows ambient air to be drawn through the piston rod bore  273  into the upper chamber  234  while closing the lower piston valve  243  to compress the air in the lower chamber  235 . Specifically, in the exemplary embodiment of  FIGS. 18-21 , the cavity  247  comprises an upper piston bore  248  formed in the upper piston wall  244  in communication with a lower piston bore  249  formed in the lower piston wall  245 , the lower piston bore  249  having an internal diameter substantially equivalent to the external diameter of the piston rod  270  such that the piston rod  270  is seated within the lower piston bore  249  so as to communicate therewith through the hollow bore  273 . The upper piston bore  248  has an internal diameter greater than the external diameter of the piston rod  270 , so that the piston rod  270  is formed with one or more cross-holes  274  positioned therein so as to communicate between the hollow bore  273  and the upper piston bore  248  and thereby allow for communication between the upper and lower piston bores  248 ,  249  essentially through the hollow bore  273  of the piston rod  270 . Regarding the lower piston valve  243 , an outwardly-opening annular channel is formed in the lower piston wall  245  and a lower o-ring  266  is seated within the annular channel. Accordingly, in the exemplary embodiment, the lower piston valve  243  comprises a lower valve disk  267  movably mounted on the piston body  241  substantially adjacent to the lower piston wall  245  so as to selectively contact the o-ring  266  and seal the lower piston bore  249 , and thus the hollow bore  273  from the lower chamber  235 . Regarding the construction of the upper piston valve  242 , a collar  268  is slidably installed on the piston rod  270  and formed with a shoulder on its lower end substantially adjacent to the upper piston wall  244  on which an upper o-ring  269  is seated so as to selectively contact the upper piston wall  244  or an outwardly-opening countersink formed on the upper piston bore  248  so as to seal the upper piston bore  248  and, thus, seal the cavity  247  from the upper chamber  234 . A keeper ring, shoulder, or other such mechanical device may be installed on the piston rod  270  above the collar  268  so as to maintain the collar  268  along the piston rod  270  substantially adjacent to the piston body  241  during all stages of operation, as described below. 
   Referring now to  FIGS. 20 and 21 , in operation, the piston body  241  is slidably moved up and down within the cylinder  230  during operation of the air compression apparatus of the present invention as described herein. In a first stage of operation as shown in  FIG. 20 , the piston assembly  240  including the piston body  241  and piston rod  270  is moving downwardly in the direction of arrows  201 . As such, the inertial and air pressure effects cooperate to close the lower piston valve  243  by causing the lower piston disk  267  to shift vertically upwardly into contact with the o-ring  266 , thereby sealing off the hollow bore  273  from the lower chamber  235 . As shown, a flat wave spring incorporated into the structure securing the lower piston disk  267  in place adjacent to the lower piston wall  245  may help bias the lower piston disk upwardly. A coil spring or other such structure now know or later developed in the art may be employed instead, or, as in other embodiments shown and described herein, no biasing means at all may be employed. Also during the first stage of operation, the upper piston valve  242  is opened by the inertial and air pressure effects again cooperating to lift the collar  268  to unseat the o-ring from the countersink formed about the upper piston bore  248 . It will be appreciated that the vacuum air pressure effect, specifically, is caused by the immediately preceding stage of operation during which high pressure compressed air was evacuated from the upper chamber  234 . Once the collar  268  has shifted upwardly as shown, inertial effects caused by the rapidly descending piston  241  work to maintain the collar&#39;s offset position with respect to the upper piston wall  244 . It will be further appreciated that the retaining ring  209  shown or other such structure serves to limit the movement of the collar  268  relative to the piston body  241  and keep it substantially adjacent to the upper piston wall  244 . In this stage, then, as shown by arrows  203 , ambient air passing through the hollow bore  273  of the piston rod  270  passes through the cross-holes  274 , the opening or upper bore  248  of the cavity  247 , and into the upper chamber  234 . At the same time, because the lower piston valve  243  is closed through the engagement of the lower piston disk  267  with the o-ring  266 , further downward travel of the piston body  241  serves to compress the air in the lower chamber  235 . It will be appreciated that the more that pressure builds up in the lower chamber  235 , the greater the seal between the lower piston disk  267  and the o-ring  266 , as the increasing pressure applies greater and greater upward force against the lower piston disk  267 . This process of introducing ambient air into the upper chamber  234  and compressing the air in the lower chamber  235  continues until the piston body  241  nears the bottom end  233  of the cylinder  230  as dictated by the structure and geometry of the driving mechanism  210  discussed above with respect to various exemplary embodiments. Once the piston body  241  has reached its lowest position within the cylinder  230 , it will again be appreciated that the air in the lower chamber  235  has effectively reached its maximum pressure and is at that time discharged from the lower chamber  235  as described elsewhere herein. At that point, the piston  241  then transitions to a second stage of operation during which it is traveling upwardly within the cylinder  230  as indicated by arrows  202  in  FIG. 21 . During this stage, it will again be appreciated that the inertial and air pressure effects cooperate to now close the upper piston valve  242  by causing the collar  268  to shift downwardly as the piston body  241  is moving rapidly upward, thereby seating the o-ring in the countersink formed about the upper piston bore  248  to seal off the hollow bore  273  from the upper chamber  234 . At the same time, the lower piston valve  243  is opened by the inertial and air pressure effects again cooperating to pull the lower piston disk  247  downwardly and space it from the o-ring  266 . It will be appreciated that the vacuum air pressure effect, specifically, is caused by the immediately preceding stage of operation during which high pressure compressed air was evacuated from the lower chamber  235 . Once the lower piston disk  267  has shifted downwardly as shown, inertial effects caused by the rapidly ascending piston  241  work to maintain the disk&#39;s offset position with respect to the lower piston wall  245  and the o-ring  266 , specifically. It will be further appreciated that the structure of the lower piston valve  243  serves to retain the lower piston disk substantially adjacent to the lower piston wall  245  and that while a rigid plate mounted through screws, pegs, or other such fasteners is shown, numerous other mechanical means, now known or later developed, for maintaining the position of the lower piston disk  267  relative to the lower piston wall  245  may be employed. In this second stage, then, as shown by arrows  204 , ambient air passing through the hollow bore  273  of the piston rod  270  passes out the end of the bore  273 , through the opening that is the lower bore  249  and between the lower piston disk  267  and the o-ring  266  into the lower chamber  235 . At the same time, because the upper piston valve  242  is closed through the engagement of the o-ring  269  on the collar  268  with the countersink of the upper bore  248  or with the upper piston wall  244  itself, further upward travel of the piston body  241  serves to compress the air in the upper chamber  234 . It will again be appreciated that the more that pressure builds up in the upper chamber  234 , the greater the seal between the countersink and the o-ring  269 , as the increasing pressure applies greater and greater downward force against the collar  268  as the piston  241  travels upward. This process of introducing ambient air into the lower chamber  235  and compressing the air in the upper chamber  234  continues until the piston body  241  nears the top end  232  of the cylinder  230  as dictated by the structure and geometry of the driving mechanism  210  discussed elsewhere. Once the piston body  241  has reached its highest position within the cylinder  230 , it will again be appreciated that the air in the upper chamber  234  has effectively reached its maximum pressure and is at that time discharged from the upper chamber  234  as described. At that point, the piston  241  then transitions back to the first stage of operation during which it is traveling downwardly within the cylinder  230  as shown in  FIG. 20 . Based on the foregoing description of the cylinder  230  in operation, it will be appreciated that the view shown in  FIG. 19  with both the upper and lower piston valves  242 ,  243  open is essentially a static view of the construction for explanatory purposes and does not necessarily reflect the positions of the moving parts of the assembly at any given stage of operation. It will also be appreciated that while the cavity  247  is shown as having an annular space between the opposite upper and lower bores  248 ,  249 , in this embodiment it is not necessary for the introduction of ambient air through the piston rod  270  to either the upper or lower chambers  234 ,  235 . As such, and for other reasons related to manufacturing and assembly, the piston body  241  could just as easily have been a solid, unitary construction with the upper and lower bores  248 ,  249  formed therethrough, though it will be appreciated by those skilled in the art that removal of material, and thus weight, from the piston  241  has other advantages during operation, particularly depending on the size of the piston and the speed at which it is moving. And whether the piston body  241  is of unitary or modular construction, it will also be appreciated that extending a portion of the annular piston wall  246  or the upper piston wall  244  radially inwardly so as to engage the outside surface of the piston rod  270  may be preferable in further supporting the piston rod within the piston body. Once more, it will be appreciated that the various components of the piston assembly, including the one or more components of the piston body and the piston rod itself, may be assembled together to effectively form a single rigid structure using techniques now know or later developed in the art. 
   Turning now to  FIGS. 22-27 , a further exemplary embodiment of the air compression apparatus of the present invention is shown. Generally, the cylinder  830  has an annular wall  831 , an upper end  832  and an opposite lower end  833 . The upper and lower ends  832 ,  833  may be installed within the annular wall  831  as described above. Exit valves  880 ,  881  lead from the respective upper and lower ends  832 ,  833 . A piston assembly  840  is operably connected to the drive mechanism and configured to move within the cylinder  830  as described previously. Turning to  FIG. 23 , the piston assembly  840  comprises a piston body  841  having an upper piston wall  844  and an offset lower piston wall  845  joined about an annular piston wall  846 . Once more, the upper and lower piston walls  844 ,  845  may be integral with the annular piston wall  846  or may be installed thereon using any mechanical fastening technique now known or later developed. The piston body  841  so installed within the cylinder  830  thus forms an upper chamber  834  between the piston body  841  and the upper end  832  of the cylinder  830  and a lower chamber  835  between the piston body  841  and the lower end  833  of the cylinder  830 . The piston body is further formed with a cavity  847  substantially bounded by the upper and lower piston walls  844 ,  845  and the annular piston wall  846  so as to preferably be in selective communication with both the upper and lower chambers  834 ,  835  in cooperation with the upper and lower piston valves  842 ,  843 , the operation of which are explained more fully below. Connected to the piston body  841  is a piston rod  870  having a hollow bore  873  communicating between a drive end and a piston end, the drive end being connected to a drive mechanism such that the hollow bore  873  is in communication with ambient air. The piston rod  870  passes through the cylinder  830  at its upper end  832 , as through a gland (not shown), and then through the upper chamber  834  so as to be connected at the opposite piston end to the piston body  841 . A lower piston valve  843  is installed on the piston body  841  so as to selectively seal the lower chamber  835  from the cavity  847 , while an upper piston valve  842  is installed adjacent to the piston body  841  so as to selectively seal the upper chamber  834  from the cavity  847 . The construction and operation of the upper and lower piston valves are in many respects the same as that disclosed with respect to the exemplary embodiment shown in  FIGS. 19-22 . Specifically, here, the cavity  847  again comprises an upper piston bore  848  formed in the upper piston wall  844  in communication with a lower piston bore  849  formed in the lower piston wall  845 , with the piston rod essentially seated within the lower piston bore  849  while freely communicating with the upper piston bore  848  through one or more cross-holes  874  formed in the piston rod  870 . In addition, an upper release valve  805  is installed within the piston body  841  offset from the cavity  847  so as to selectively communicate between the upper chamber  834  and the lower chamber  835 . The upper release valve  805  has an upwardly-projecting, spring-biased upper contact pin  807  configured to contact the surface of the upper end  832  after the piston body  841  has traveled sufficiently upwardly so as to effectively seal the upper exit bore  836 , whereby displacement of the upper contact pin  807  temporarily opens the upper release valve  805  and allows compressed air to pass from the upper chamber  834  through the upper release valve  805  and into the lower chamber  835 . Similarly, a lower release valve  806  is installed within the piston body  841  offset from the cavity  847  and from the upper release valve  805  so as to selectively communicate between the lower chamber  835  and the upper chamber  834 , the lower release valve  806  having a downwardly-projecting, spring-biased lower contact pin  808  configured to contact the surface of the lower cylinder end  833  after the piston body  841  has traveled sufficiently downwardly so as to seal the lower exit bore  837  and displace the lower contact pin  808  to temporarily open the lower release valve  806  and allow compressed air to pass from the lower chamber  835  through the lower release valve  806  and into the upper chamber  834 . 
   In operation, then, referring now to  FIGS. 24-27 , the piston body  841  is slidably moved up and down within the cylinder  830  during operation of the air compression apparatus of the present invention as described herein. In a first stage of operation as shown in  FIG. 24 , the piston assembly  840  including the piston body  841  and piston rod  870  is moving downwardly in the direction of arrows  801 . As such, the inertial and air pressure effects cooperate to close the lower piston valve  843  by causing the lower piston disk  867  to shift vertically upwardly into contact with the o-ring  866 , again, with or without the assistance of a biasing spring, thereby sealing off the hollow bore  873  from the lower chamber  835 . At the same time, the upper piston valve  842  is opened by the inertial and air pressure effects cooperating to lift the collar  868  to unseat the o-ring from the countersink formed about the upper piston bore  848 . Once the collar  868  has shifted upwardly as shown, inertial effects caused by the rapidly descending piston  841  work to maintain the collar&#39;s offset position with respect to the upper piston wall  844 . In this stage, then, as shown by arrows  803 , ambient air passing through the hollow bore  873  of the piston rod  870  passes through the cross-holes  874 , the opening or upper bore  848  of the cavity  847 , and into the upper chamber  834 . At the same time, because the lower piston valve  843  is closed through the engagement of the lower piston disk  867  with the o-ring  866 , further downward travel of the piston body  841  serves to compress the air in the lower chamber  835 . This process of introducing ambient air into the upper chamber  834  and compressing the air in the lower chamber  835  continues until the piston body  841  nears the bottom end  833  of the cylinder  830  as again dictated by the structure and geometry of the driving mechanism. Here, though, substantially at or near the low point of the piston&#39;s downward travel in the direction of arrows  801 , as shown in  FIG. 25 , a second stage of operation occurs wherein the lower end of the lower piston wall  845 , configured in the exemplary embodiment as a downwardly-projecting boss, just enters the lower exit bore  837 . Preferably, the outside diameter of the lower piston wall  845  is only slightly smaller than the inside diameter of the lower exit bore  837  so as to temporarily separate or seal off the exit bore from the lower piston chamber  835 . Just at or after that time, further downward travel of the piston body  841  causes the lower release valve  806  to be actuated as the lower contact pin  808  contacts the surface of the lower cylinder end  833 . It will be appreciated that the exact location of the lower piston valve  843  relative to the lower end  833  at this stage is not critical. The displacement of the lower contact pin  808  temporarily opens the lower release valve  806  and allows compressed air to pass from the lower chamber  835  through the lower release valve  806  and into the upper chamber  834 , as indicated by arrows  811 . Those skilled in the art will appreciate that the gust of compressed air into the upper chamber  834  will cooperate with the reversal of direction of the piston assembly  840  as it starts upward to close the upper piston valve  842  and hence begin the work of compression in the upper chamber  834 . Thus, once the piston body  841  has reached its lowest position within the cylinder  830 , the air in the lower chamber  835  has effectively reached its maximum pressure and is at that time either briefly introduced to the upper chamber  834  through the lower release valve  806  or discharged from the lower chamber  835  as described elsewhere herein. At that point, the piston  841  then transitions to a third stage of operation during which it is traveling upwardly within the cylinder  830  as indicated by arrows  802  in  FIG. 26 . During this third stage, it will again be appreciated that the inertial and air pressure effects cooperate to now close the upper piston valve  842  by causing the collar  868  to shift downwardly as the piston body  841  is moving rapidly upward, thereby seating the o-ring in the countersink formed about the upper piston bore  848  to seal off the hollow bore  873  from the upper chamber  834 . At the same time, the lower piston valve  843  is opened by the inertial and air pressure effects again cooperating to pull the lower piston disk  847  downwardly and space it from the o-ring  866 . It will be appreciated that during this intermediate third stage of upward travel of the piston  241 , the upper and lower release valves  805  and  806  remain closed. In this third stage, then, as shown by arrows  804 , ambient air passing through the hollow bore  873  of the piston rod  870  passes out the end of the bore  873 , through the lower bore  849  and between the lower piston disk  867  and the o-ring  866  into the lower chamber  835 . At the same time, because the upper piston valve  842  is closed through the engagement of the o-ring  869  on the collar  868  with the countersink of the upper bore  848 , further upward travel of the piston body  841  serves to compress the air in the upper chamber  834 . This process of introducing ambient air into the lower chamber  835  and compressing the air in the upper chamber  834  continues until the piston body  841  nears the top end  832  of the cylinder  830  as dictated by the structure and geometry of the driving mechanism. Here, again, substantially at or near the high point of the piston&#39;s upward travel in the direction of arrows  802 , as shown in  FIG. 27 , a fourth stage of operation occurs wherein the upper piston valve  868 , configured in the exemplary embodiment as an upwardly-projecting boss or collar, just enters the upper exit bore  836 . Preferably, the outside diameter of the collar  868  is only slightly smaller than the inside diameter of the upper exit bore  836  so as to temporarily separate or seal off the exit bore from the upper piston chamber  834 . Just at or after that time, further upward travel of the piston body  841  causes the upper release valve  805  to be actuated as the upper contact pin  806  contacts the surface of the upper cylinder end  832  after the piston body  841  has traveled sufficiently upwardly, again, so as to receive the upper piston valve  842  within the upper exit bore  836 . As such, the displacement of the upper contact pin  806  temporarily opens the upper release valve  805  and allows compressed air to pass from the upper chamber  834  through the upper release valve  805  and into the lower chamber  835 , as indicated by arrows  812 . Those skilled in the art will appreciate that the gust of compressed air into the lower chamber  835  will cooperate with the reversal of direction of the piston assembly  840  as it starts downward to again close the lower piston valve  843  and hence begin the work of compression in the lower chamber  835  during the first stage of operation described above with respect to  FIG. 24 . Thus, once the piston body  841  has reached its highest position within the cylinder  830 , the air in the upper chamber  834  has effectively reached its maximum pressure and is at that time either briefly introduced to the lower chamber  835  through the upper release valve  805  or discharged from the upper chamber  834  as described. At that point, the piston  841  then transitions back to the first stage of operation during which it is traveling downwardly within the cylinder  830  as indicated by arrows  801  in  FIG. 24 . It will be appreciated, then, that the upper and lower release valves  805 ,  806  in the alternative embodiment of  FIGS. 22-27  cooperate with the inertial and other air flow and pressure effects during operation to selectively close the respective lower and upper piston valves  843 ,  842  so as to enable compression of the air in the lower and upper chambers  835 ,  834 . Based on the foregoing description of the cylinder  830  in operation, it will be appreciated that the view shown in  FIG. 23  with both the upper and lower piston valves  842 ,  843  open is essentially a static view of the construction for explanatory purposes and does not necessarily reflect the positions of the moving parts of the assembly at any given stage of operation. 
   Turning now to  FIGS. 28-31 , there is shown yet another exemplary embodiment of the air compression apparatus of the present invention. A cylinder  930  has a piston assembly  940  inserted therein so as to sealably and slidably engage the inside surface of its annular wall  931 . The piston assembly  940  is operably connected to a drive mechanism so as to move up and down within the cylinder as previously described. Specifically, the piston assembly  940  comprises a piston body  941  having an upper piston wall  944  and an offset lower piston wall  945  joined about an annular piston wall  946 . In this exemplary embodiment, the annular piston wall  946  is further formed with a radially-outwardly-projecting circumferential rib  965  so as to define an upper piston ring channel  960  and a lower piston ring channel  961 . While the respective upper and lower channels  960 ,  961  are shown as being formed between the rib  965  and opposite radially outward flanges of the annular wall  946 , it will be appreciated that the piston body  941  could just as easily be constructed as shown in  FIGS. 19-27 , wherein the upper and lower piston ring channels would effectively be formed between the rib  965  and the upper and lower piston walls. In either construction, or such other construction as within the spirit and scope of the invention, an upper piston ring  962  is inserted within the upper piston ring channel  960  and a lower piston ring  963  is inserted within the lower piston ring channel  961  so as to cooperate to sealably and slidably contact the inside surface of the cylinder wall  931 . Again, the upper and lower piston walls  944 ,  945  may be integral with the annular piston wall  946  or may be installed thereon using any mechanical fastening technique now known or later developed in the art. The piston body  941  is further formed with a cavity  947  substantially bounded by the upper and lower piston walls  944 ,  945  and the annular piston wall  946 . Accordingly, the cavity  947  comprises an annular space substantially between the upper and lower piston walls  944 ,  945 . One or more upper breathing holes  948  are formed in the upper piston wall  944  so as to selectively communicate between the upper chamber  934  and the annular space, and one or more lower breathing holes  949  are formed in the lower piston wall  945  so as to selectively communicate between the lower chamber  935  and the annular space. While four round breathing holes are shown in the exemplary embodiment, it will be appreciated that the number, size, shape, and arrangement of the breathing holes may vary without departing from the spirit and scope of the invention, which can be said for the other embodiments of the present invention as well. The piston rod  970  is formed with cross-holes  974  and is connected to the piston body  941  such that its hollow bore  973  communicates with the annular space through the cross-holes  974 . An outwardly-opening lower annular channel is formed in the lower piston wall  945  about each lower breathing hole  949  with a lower o-ring  966  seated therein. As such, in the exemplary embodiment, the lower piston valve again comprises a lower valve disk  967  movably mounted on the piston body  941  substantially adjacent to the lower piston wall  945  so as to selectively contact each lower o-ring  966  and seal the lower breathing holes  949 . Similarly, an outwardly-opening upper annular channel is formed in the upper piston wall  944  about each upper breathing hole  948  with an upper o-ring  969  seated therein. Analogous to the lower piston valve, the upper piston valve comprises an upper valve disk  968  movably mounted on the piston body  941  substantially adjacent to the upper piston wall  944  so as to selectively contact each upper o-ring  969  and seal the upper breathing holes  948 . In the exemplary embodiment of  FIGS. 28-31 , the piston end of the pivot rod  970  is closed, as with a plug, and formed with an outwardly-opening threaded hole. A retainer having a threaded hole and an upwardly-facing shoulder is fastened to the bottom end of the piston rod  970  substantially abutting the lower piston wall  945  through a fastener screw. A similar retainer having a clearance hole for the piston rod  970  and a downwardly-facing shoulder is installed substantially abutting the upper piston wall  944  and held in place by a retaining ring  909  or the like fixed on the piston rod  970 . The upper and lower valve disks  968 ,  969  are thus retained adjacent to the respective upper and lower piston walls  944 ,  945  by the respective shoulders of the retainers while being free to shift vertically so as to selectively open and close the respective upper and lower piston valves during various stages of operation, as described more fully below. 
   Referring now to  FIGS. 30 and 31 , in operation, the piston body  941  is slidably moved up and down within the cylinder  930  during operation of the air compression apparatus of the present invention as described herein. In a first stage of operation as shown in  FIG. 30 , the piston body  941  as driven through the piston rod  970  is moving downwardly in the direction of arrows  901 . As such, the inertial and air pressure effects cooperate to close the lower piston valve by causing the lower piston disk  967  to shift vertically upwardly into contact with the lower o-rings  966 , thereby sealing off the cavity  947  and, effectively, the hollow bore  973  from the lower chamber  935 . At the same time, the upper piston valve is opened by the inertial and air pressure effects again cooperating to lift the upper valve disk  968  out of contact with the upper o-rings  969 . Once the upper valve disk  968  has shifted upwardly as shown, inertial effects caused by the rapidly descending piston  941  work to maintain the disk&#39;s offset position with respect to the upper piston wall  944 . It will be further appreciated that the retainer shown or other such structure serves to limit the movement of the upper valve disk  968  relative to the piston body  941  and keep it substantially adjacent to the upper piston wall  944 . In this stage, then, as shown by arrows  903 , ambient air passing through the hollow bore  973  of the piston rod  970  passes through the cross-holes  974 , the breathing holes  948  of the cavity  947 , and into the upper chamber  934 . At the same time, because the lower piston valve is closed through the engagement of the lower piston disk  967  with the o-rings  966 , further downward travel of the piston body  941  serves to compress the air in the lower chamber  935 . It will be appreciated that the more that pressure builds up in the lower chamber  935 , the greater the seal between the lower piston disk  967  and the o-rings  966  about the lower breathing holes  949 , as the increasing pressure applies greater and greater upward force against the lower piston disk  967 . This process of introducing ambient air into the upper chamber  934  and compressing the air in the lower chamber  935  continues until the piston body  941  reaches its lowest position within the cylinder  930 , at which point the compressed air in the lower chamber  935  is discharged as explained previously. At that point, the piston  941  then transitions to a second stage of operation during which it is traveling upwardly within the cylinder  930  as indicated by arrows  902  in  FIG. 31 . During this stage, it will again be appreciated that the inertial and air pressure effects cooperate to now close the upper piston valve by causing the upper valve disk  968  to shift downwardly as the piston body  941  is moving rapidly upward, thereby sealing against the upper o-rings  966  about the upper breathing hole  948  to seal off the hollow bore  973  from the upper chamber  934 . At the same time, the lower piston valve is opened by the inertial and air pressure effects again cooperating to pull the lower piston disk  947  downwardly and space it from the o-rings  966 . It will be appreciated that the vacuum air pressure effect, specifically, is caused by the immediately preceding stage of operation during which high pressure compressed air was evacuated from the lower chamber  935 . Once the lower piston disk  967  has shifted downwardly as shown, inertial effects caused by the rapidly ascending piston  941  work to maintain the disk&#39;s offset position with respect to the lower piston wall  945  and the o-rings  966 , specifically. It will be further appreciated that the structure of the lower piston valve shown as a retainer with a shoulder serves to retain the lower piston disk  967  substantially adjacent to the lower piston wall  945  and that while such a retainer is shown, numerous other mechanical means, now known or later developed, for maintaining the position of the lower piston disk  967  relative to the lower piston wall  945  may be employed. In this second stage, then, as shown by arrows  904 , ambient air pulled through the hollow bore  973  of the piston rod  970  passes through the cross-holes  974 , the cavity  947 , and the lower breathing holes  949  and then between the lower piston disk  967  and the o-rings  966  into the lower chamber  935 . At the same time, because the upper piston valve is closed through the contact between the upper piston disk  968  and the upper o-rings  969 , further upward travel of the piston body  941  serves to compress the air in the upper chamber  934 . It will again be appreciated that the more that pressure builds up in the upper chamber  934 , the greater the seal about the upper breathing holes  969 , as the increasing pressure applies greater and greater downward force against upper piston disk  968  as the piston  941  travels upward. This process of introducing ambient air into the lower chamber  935  and compressing the air in the upper chamber  934  continues until the piston body  941  reaches its highest position within the cylinder  930 , at which point it will again be appreciated that the air in the upper chamber  934  has effectively reached its maximum pressure and is at that time discharged. At that point, the piston  941  then transitions back to the first stage of operation during which it is traveling downwardly within the cylinder  930  as indicated in  FIG. 30 . 
   Turning to  FIGS. 32-35 , there is shown yet another exemplary embodiment of the air compression apparatus of the present invention involving a construction analogous to that of the previous embodiment of  FIG. 28-31 , with a few notable changes. Specifically, the piston assembly  1040  again comprises a piston body  1041  having an upper piston wall  1044  and an offset lower piston wall  1045  joined about an annular piston wall  1046 . Once more, at least two of these elements may be of a unitary construction, and any of them may be joined together using any means now known or later developed in the art. In this exemplary embodiment, the annular piston wall  1046  is further formed with a radially-outwardly-opening circumferential groove  1065  in which a piston o-ring  1066  is seated. The piston ring  1062  is then seated in the piston channel  1060  formed circumferentially about the annular piston wall  1046  between the radially outward edges of the upper and lower piston walls  1044 ,  1045  so as to cooperate with the piston o-ring  1066  to sealably and slidably contact the inside surface of the cylinder wall  1031 . A path for the ambient air being pulled through the hollow bore  1073  of the piston rod  1070  is formed generally as previously. Regarding the lower piston valve, however, in this exemplary embodiment, the lower valve disk  1067  is formed with two concentric upwardly-opening first and second annular channels  1005 , the channels being configured to define a seal area therebetween that is substantially adjacent to the lower breathing holes  1049 . A first lower o-ring  1011  is seated within the first annular channel  1005  and a second lower o-ring  1012  is seated within the second annular channel  1006 , the o-rings selectively contacting the lower piston wall  1045  so as to seal the lower breathing holes  1049 . Again, an end wall plug  1013  is installed within the hollow bore  1073  substantially at the end of the piston rod  1070  and formed with an outwardly-opening threaded hole configured to threadably receive a fastener  1007 . A sleeve is installed over the fastener  1007  to give the fastener something to tighten against so as to form a rigid connection of the lower piston wall  1045  to the piston rod  1070 . The lower valve disk is further formed with a clearance hole  1014  offset from and substantially concentric with the first and second annular channels  1005 ,  1006  such that the fastening screw  1007  and sleeve pass through the clearance hole  1014 . A similar clearance hole or a threaded hole is formed in the lower piston wall  1045  so as to allow the screw to be secured within the plug  1013 . Furthermore, a return spring  1008  may be positioned about the sleeve and threaded body of the screw  1007  between its head and the lower piston disk  1067  so as to bias the disk upwardly. 
   Referring now to  FIGS. 34 and 35 , in operation, the piston body  1041  is slidably moved up and down within the cylinder  1030  during operation of the air compression apparatus of the present invention as described herein. Once more, in a first stage of operation as shown in  FIG. 34 , the piston body  1041  as driven through the piston rod  1070  is moving downwardly in the direction of arrows  1001 . As such, the inertial and air pressure effects cooperate to close the lower piston valve by causing the lower piston disk  1067  to shift vertically upwardly so as to bring the first and second lower o-ring  1011 ,  1012  into contact with the lower piston wall  1045 , thereby sealing the lower breathing holes  1049  and, effectively, the hollow bore  1073  from the lower chamber  1035 . It will be further appreciated that the structure of the lower piston valve shown as including a fastener  1007  configured with return spring  1008  serves to further lift and bias the lower valve disk  1067  upwardly. At the same time, the upper piston valve is as before. In this stage, then, as shown by arrows  1003 , ambient air passing through the hollow bore  1073  of the piston rod  1070  passes into the upper chamber  1034 . At the same time, because the lower piston valve is closed, further downward travel of the piston body  1041  serves to compress the air in the lower chamber  1035 . This process of introducing ambient air into the upper chamber  1034  and compressing the air in the lower chamber  1035  continues until the piston body  1041  reaches its lowest position within the cylinder  1030 , at which point the compressed air in the lower chamber  1035  is discharged. At that point, the piston  1041  then transitions to a second stage of operation during which it is traveling upwardly within the cylinder  1030  as indicated by arrows  1002  in  FIG. 35 . During this stage, the upper piston valve is again closed as in previous embodiments, while the lower piston valve is opened by the inertial and air pressure effects again cooperating to pull the lower piston disk  1067  downwardly, even against the relatively light force of the return spring  1008 , so as to space the o-rings  1011 ,  1012  from the lower piston wall  1045  and allow air to flow through the lower breathing holes  1049 . It will be appreciated that the vacuum air pressure effect, specifically, is caused by the immediately preceding stage of operation during which relatively high pressure compressed air was evacuated from the lower chamber  1035 , which cooperates with inertia to help shift the lower valve disk  1067  downwardly against the resistance of the return spring  1008 . Again, though such a fastening and biasing structure is shown, it will be appreciated that numerous other mechanical means, now known or later developed, for maintaining the position of the lower piston disk  1067  relative to the lower piston wall  1045  may be employed. In this second stage, then, as shown by arrows  1004 , ambient air pulled through the hollow bore  1073  of the piston rod  1070  passes through the lower breathing holes  1049  and then between the lower piston wall  1045  and the lower piston disk  1067  and its o-rings  1011 ,  1012  into the lower chamber  1035 . At the same time, because the upper piston valve is closed, further upward travel of the piston body  1041  serves to compress the air in the upper chamber  1034 . This process of introducing ambient air into the lower chamber  1035  and compressing the air in the upper chamber  1034  continues until the piston body  1041  reaches its highest position within the cylinder  1030 , at which point it will again be appreciated that the air in the upper chamber  1034  has effectively reached its maximum pressure and is at that time discharged. At that point, the piston  1041  then transitions back to the first stage of operation during which it is traveling downwardly within the cylinder  1030  as indicated in  FIG. 34 . 
   Turning now to  FIGS. 36 and 37 , there is shown yet another exemplary embodiment of the air compression apparatus of the present invention involving a construction analogous to that of the previous embodiment of  FIG. 28-31 , with a few more notable changes. Specifically, the piston assembly  1140  again comprises a piston body  1141  of either unitary or modular construction having an upper piston wall  1144  and an offset lower piston wall  1145  joined about an annular piston wall  1146 . In this exemplary embodiment, the annular piston wall  1146  is again formed with a radially-outwardly-opening circumferential groove in which a piston o-ring is seated. Here, the piston ring  1162  is formed with one or more radially-outwardly-opening circumferential piston ring grooves  1163 . In operation, as the piston ring  1162  slidingly and sealingly engages the inside surface of the cylinder wall  1131 , the one or more grooves  1163  serve to lessen the overall frictional drag against the cylinder wall  1131  by reducing the overall contact area while effectively setting up improved sealing dynamics. That is, each of the circumferential peaks adjacent to the respective grooves  1163  is effectively a separate piston ring, whereby air attempting to pass by the entire piston ring  1162  must essentially overcome each such sub-piston ring. It will be appreciated that air doing so will then effectively gather in the groove beyond the compromised sub-piston ring before then “attempting” to breach the next sub-piston ring. Put another way, individual seal areas on the piston ring  1162  number one more than the number of grooves  1163 . For example, in the exemplary embodiment shown, four offset circumferential piston grooves  1163  are formed in the piston ring  1162 , so that effectively five peaks, or seals, must be passed to compromise the piston ring and allow unwanted air to move between chambers on opposite sides of the piston  1141 . It will be further appreciated that the radially-outward force applied to the back of the piston ring  1162  by the piston o-ring  1166  further improves the sealing performance. As a further improvement to the piston ring  1162 , a diagonal slit  1164  is formed in the piston ring  1162  rather than the conventional vertical slit. In this way, as pressure is applied to the piston ring  1162  from either direction as the piston  1141  is moving up or down in the cylinder  1130  and compressing air in the upper or lower chambers, the outward pressure on the piston ring  1141  as air attempts to get under and by it, though effectively slightly increasing the circumference of the piston ring, which can result, under normal circumstances, in slightly opening the vertical slit and allowing air to leak through, here only shifts one side of the diagonal slit  1164  with respect to the other while still keeping both sides of the slit in contact and not allowing any air to pass. To further facilitate this effect, the width of the piston ring  1162  in the vicinity of the slit  1164  can be slightly reduced to allow for this shifting along the slit to happen within the fixed piston channel. In order to accommodate the grooved piston ring  1162  of the present embodiment, it will be appreciated by those skilled in the art that the outside diameter of the annular piston wall  1146  may be reduced so as to effectively form a deeper piston ring channel. As best shown in  FIG. 37 , a further modification to the structure of the air compression apparatus of the present invention shown in the exemplary embodiment is also made with respect to the structure of the annular piston wall  1146 . Multiple radially-inwardly-projecting longitudinal fins  1109  are formed about the inside surface of the annular piston wall  1146 . It will be appreciated by those skilled in the art that such fins  1109  serve to reduce noise levels during operation of the air compression apparatus by effectively not allowing sound waves to bounce directly off the inside surface of the annular piston wall  1146  and back up the hollow piston rod  1170 . This effect, combined with the other improvements in the noise level of operation achieved, in part, as explained above, through the relatively slower speeds of operation and the relatively gentle “squeezing,” rather than “slamming,” of the air within the cylinder, serves to further improve the quietness of the air compression apparatus of the present invention. It is noted that even the direction of air movement as essentially always being into the hollow piston rod, particularly in the double-acting embodiments of the cylinder, and the length over which this happens further opposes the travel of shock or sound waves out of the piston rod during operation of the compressor. Moreover, those skilled in the art will appreciate that, as explained above with reference to the exemplary embodiment shown in  FIGS. 16 and 17 , the inclusion of a woven or mesh sleeve or other such acoustic sleeve or strip within the hollow piston rod serves to still further reduce the operational noise level of the air compression apparatus of the present invention. 
   Referring now to  FIGS. 38-48 , generally, the air compression apparatus of the present invention may have a cylinder formed at one or both ends with a breathing chamber, or a sub-chamber in which compressed air may be collected from the main upper or lower chamber in which the work of compression by the piston is accomplished in order to allow for more efficient transfer of the compressed air out of the cylinder and into a pressure tank. That is, it will be appreciated that the Bernoulli effect experienced when pushing compressed, or high pressure, air through a restriction, namely, the exit valve, can have a detrimental effect on the efficiency and quietness of a compressor&#39;s operation. As such, it is advantageous to effectively stage the compressed air in a sub-chamber, or breathing chamber, between the upper and lower chambers of the cylinder and the respective upper and lower exit ports. The principles of the present invention have thus been further applied to this problem to achieve yet another improvement to the overall operation of an air compression system. Accordingly, while the following exemplary embodiments show various means by which a breathing chamber can be constructed so that compressed air can selectively pass into the breathing chamber before going through the exit valve and through an air line to the tank, those skilled in the art will appreciate that numerous other constructions are possible without departing from the spirit and scope of the invention. Moreover, with respect to the very exemplary embodiments shown, it will be further appreciated that the sizes and proportions of the various components are also exemplary and may be varied to suit particular applications. 
   Turning first to  FIGS. 38-40 , an upper end  1232  of the cylinder  1230  is formed by an upper cylinder wall  1290  and an offset upper chamber plate  1291  sealably installed within the cylinder so as to form therebetween an upper breathing chamber  1292 . The upper chamber plate  1291  is formed with at least one selectively sealable upper breathing hole  1293  communicating between the upper chamber  1234  and the upper breathing chamber  1292 . The upper chamber plate  1291  is further formed with an upwardly-extending boss that can itself accommodate the piston rod  1270  or have a further tube installed therein. Either way, substantially axially aligned piston bores are formed in the upper cylinder wall  1290  and the upper chamber plate  1291  for the passage therethrough of the piston rod  1270 , whereby any such construction effectively serves as a gland through which the piston rod  1270  slidably operates. As previously, various combinations of such components may be unitary or modular in construction using techniques now known or later developed in the art. In the exemplary embodiment, an o-ring is seated on the upper end of the upwardly-extending boss formed on the upper chamber plate  1291  such that the upper cylinder wall  1290  sealably sits thereon, the assembly then being held in such arrangement within the cylinder wall  1231  by opposing retaining rings or other such structure now known or later developed. An upwardly-opening upper annular channel  1294  is formed in the upper chamber plate  1291  about each upper breathing hole  1293  with an upper o-ring  1295  seated therein, as best shown in  FIG. 39 . An upper chamber disk  1296  is movably mounted within the upper breathing chamber  1292  substantially adjacent to the upper chamber plate  1291  so as to selectively contact the upper o-rings  1295  and seal the upper breathing holes  1293 . Again, while four round breathing holes are shown in the exemplary embodiment, it will be appreciated that the number, size, shape, and arrangement of the breathing holes may vary without departing from the spirit and scope of the invention. The upwardly-projecting boss may be formed with a flange or have a retaining ring or the like installed thereon so as to limit the vertical displacement of the upper chamber disk  1296  during operation. It will be appreciated by those skilled in the art that with this basic construction, air will move from the upper chamber  1234  to the upper breathing chamber  1292  based on principles of fluid dynamics, whereby the air in the system will tend to move from areas of high pressure to areas of low pressure wherever possible. Accordingly, it will be further appreciated that where a standard connector  1280  is installed in the upper cylinder wall  1290  as shown or in the cylinder wall  1231  between the upper cylinder wall  1290  and the upper chamber plate  1291 , the pressure in the breathing chamber will at least tend toward the pressure in the line and, thus, the pressure in the tank, assuming that there is no check valve in the air line either. In this scenario, air compressed in the upper chamber  1234  will only be able to unseat the upper valve disk  1296  and move into the breathing chamber  1292  as shown by arrows  1201  in  FIG. 40 , when its pressure is greater than that of the tank. Otherwise, if the tank pressure is greater, no more air can enter the breathing chamber or the tank itself. It will be appreciated that where the tank pressure is greater, this pressure effectively acts downwardly on the upper chamber disk  1296  so as to force it into contact with the upper o-rings  1295 , as shown in  FIG. 38 , effectively sealing off the breathing chamber  1292  from the upper chamber  1234  until the pressure within the tank drops or the pressure within the upper chamber increases. 
   Turning to  FIGS. 41-43 , there is shown an alternative embodiment upper breathing chamber in connection with the air compression apparatus of the present invention. The upper end  1332  of the cylinder  1330  is again formed by an upper cylinder wall  1390  and an offset upper chamber plate  1391  sealably installed within the cylinder so as to form therebetween an upper breathing chamber  1392 . The upper chamber plate  1391  is formed with at least one selectively sealable upper breathing hole  1393  communicating between the upper chamber  1334  and the upper breathing chamber  1392 . The upper chamber plate  1391  is further formed with an upwardly-extending boss that can itself accommodate the piston rod  1370  or have a further tube installed therein. Either way, substantially axially aligned piston bores are formed in the upper cylinder wall  1390  and the upper chamber plate  1391  for the passage therethrough of the piston rod  1370 , whereby any such construction effectively serves as a gland through which the piston rod  1370  slidably operates. As previously, various combinations of such components may be unitary or modular in construction using techniques now known or later developed in the art. An o-ring is again seated on the upper end of the upwardly-extending boss formed on the upper chamber plate  1391  such that the upper cylinder wall  1390  sealably sits thereon, the assembly then being held in such arrangement within the cylinder wall  1331  by opposing retaining rings or other such structure now known or later developed. An upwardly-opening counterbore  1394  is formed in the upper chamber plate  1391  about each upper breathing hole  1393  with an upper o-ring  1395  seated therein, as best shown in  FIG. 42 . Also shown, an upwardly-opening circumferential channel  1397  is formed in the upper chamber plate so as to substantially connect the counterbores  1394 , of which there are four in the exemplary embodiment. As explained more fully below, the channel further enables air flow through the breathing holes  1393 . A ball  1396  is movably seated within each of the counterbores  1394  so as to selectively seal the breathing holes  1393  through contact with the respective o-rings  1395 . In an alternative embodiment, a gasket material is seated or pinched substantially at the base of each counterbore  1394 . It will be appreciated by those skilled in the art that with this basic alternative construction, air will move from the upper chamber  1334  to the upper breathing chamber  1392  again based on pressure differential. Accordingly, where no one-way valves are employed in the air lines, the pressure in the breathing chamber  1392  will tend toward the pressure in the tank. In this scenario, air compressed in the upper chamber  1334  will only be able to unseat the balls  1396  and move into the breathing chamber  1392  as shown by arrows  1301  in  FIG. 41 , when its pressure is greater than that of the tank. It will be appreciated that the balls  1396  will likely never be positioned spaced from the counterbores  1394  as shown, such that the balls in this location are merely exemplary and to facilitate viewing of the other features of the apparatus. It is further contemplated that a retaining disk or the like may be installed on the upper chamber plate  1391 , as in a notch on its boss, so as to effectively limit the vertical displacement of the balls in much the same way that a retaining ring or the like may limit the movement of the upper chamber disk  1296 . In any event, when the pressure in the upper chamber  1334  is greater than that of the breathing chamber, and thus, the tank, the balls  1396  will be unseated from the o-rings  1395  sufficiently to allow air to move from the upper chamber  1334  through the breathing holes  1393  and the counterbores  1394  and around the balls  1396  into the breathing chamber  1392 . Again, the circumferential channel  1397  further enables this breathing. Otherwise, if the tank pressure is greater, no more air can enter the breathing chamber or the tank itself. It will be appreciated that where the tank pressure is greater, this pressure effectively acts downwardly on the balls  1396  so as to force them into their respective counterbores  1394  and, thus, contact with the upper o-rings  1395 , as shown in  FIG. 43 , effectively sealing off the breathing chamber  1392  from the upper chamber  1334  until the pressure within the tank drops or the pressure within the upper chamber increases. 
   Turning now to  FIGS. 44-46 , a further exemplary embodiment of the air compression apparatus is shown directed to a lower breathing chamber configuration. A lower cylinder wall  1490  is sealably installed within the annular cylinder wall  1431  as by a screw fastener, though any assembly means now know or later developed may be employed. The lower cylinder wall  1490  is formed with an upwardly-projecting sidewall that extends into the cylinder and is configured to sealingly retain a lower chamber plate  1491  offset from the substantially horizontal base of the lower cylinder wall  1490  so as to form therebetween a lower breathing chamber  1492 . The lower chamber plate  1491  is formed with at least one selectively sealable lower breathing hole  1493  communicating between the lower chamber  1435  and the lower breathing chamber  1492 . A lower chamber disk  1496  is movably mounted within the lower breathing chamber  1492  substantially adjacent to the lower chamber plate  1491 . As best shown in  FIG. 45 , the lower chamber disk  1496  is formed with an upwardly-opening lower annular channel  1494  having a lower o-ring  1495  seated therein. The lower chamber disk  1496  may be further formed with at least one lower chamber passage  1497  radially-outwardly offset from the lower annular channel  1494 . While the passage  1497  is configured in the exemplary embodiment as an arrangement of holes, it will be appreciated that virtually any opening configuration that will allow air to flow through the lower breathing hole  1493  and around the lower chamber disk  1496  when it is shifted downwardly so as to space the o-ring  1495  from the lower chamber plate  1491  can be employed. It will be further appreciated that only a minimal amount of structure radially outward of the annular channel  1494  is required, primarily to stabilize the lower chamber disk  1496  laterally within the lower breathing chamber. As such, for example, spaced apart spines projecting radially outwardly from just beyond the annular channel  1494  could also be employed. A return spring  1408  is positioned substantially between the lower chamber disk  1496  and the lower cylinder wall  1490  so as to bias the lower chamber disk upwardly. In use, as with the upper breathing chamber exemplary embodiments shown and described, the pressure in the lower breathing chamber will at least tend toward the pressure in the line and, thus, the pressure in the tank, assuming that there is no check valve in the air line. A two-way, sealed connector  1480  is shown as connecting the air line  1482  to the lower cylinder wall  1490 , though it will be appreciated that any such connector now known or later developed in the art may be employed. Air compressed in the lower chamber  1435  will only be able to unseat the lower valve disk  1496  and move into the lower breathing chamber  1492  as shown by arrows  1401  in  FIG. 46  when its pressure is greater than that of the tank. In addition, the pressure in the lower chamber  1435  must also be able to overcome the force of the return spring  1408  biasing the lower valve disk  1496  upwardly. Otherwise, if the tank pressure is essentially greater, no more air can enter the lower breathing chamber or the tank itself. It will be appreciated that where the tank pressure is greater, this pressure effectively acts upwardly on the lower chamber disk  1496  so as to force its o-ring  1495  into contact with the lower chamber plate  1491 , as shown in  FIG. 44 , effectively sealing off the lower breathing chamber  1492  from the lower chamber  1435  until the pressure within the tank drops or the pressure within the lower chamber increases. 
   Referring to  FIGS. 47 and 48 , yet another alternative embodiment of the lower end  1532  of an air compression apparatus is shown as having an annular body configured with a circumferential o-ring for receipt within an annular cylinder wall as generally described above. The annular lower end  1532  includes a lower breathing chamber  1592  defined by the intersection of a substantially vertical, upwardly-opening counterbore  1593 , formed in what is essentially the lower chamber plate, and a substantially horizontal cross-hole  1594  configured for receipt of a connector (not shown). An upwardly-projecting support post  1595  is formed on what is essentially the lower cylinder wall so as to extend into the lower breathing chamber  1592  substantially coaxially with the counterbore  1593 . Though the lower end  1532  is shown as being formed of a unitary construction, it will be appreciated by those skilled in the art that it could also be modular and include such components as a lower cylinder wall, from which the support post extends, a lower chamber plate, either of which having a vertical annular wall configured to sealingly engage the other, whereby the size of the lower breathing chamber of the exemplary embodiment could be increased. A plug  1597  is threadably or otherwise installed in the counterbore  1593  having a downwardly-facing seat intersected by a breathing hole  1598 . A ball  1596  is movably inserted within the counterbore  1593  so as to selectively seal the at least one lower breathing hole  1598  and is biased upwardly by a return spring  1508  positioned about the support post  1595 . Thus, again, assuming that the pressure in the lower breathing chamber  1592  at any given time is roughly equivalent to the tank pressure, it will be appreciated that the ball  1596  will not be displaced so as to allow air to flow into the lower breathing chamber until the pressure in the lower chamber is greater than the tank pressure. When the tank pressure is greater, it cooperates with the return spring  1508  to bias the ball  1596  upwardly in sealing engagement with the plug  1597 . It will be appreciated by those skilled in the art that the embodiments of the upper and lower breathing chambers so shown and described are merely exemplary and that numerous other configurations are possible without departing from the spirit and scope of the invention. 
   Referring now to  FIGS. 49-51 , there is shown a still further exemplary embodiment of the air compression apparatus  1600  of the present invention essentially incorporating the principles of construction and use discussed above in a multi-cylinder arrangement. A tank  1602  is installed on a frame  1606  along with a motor  1604 . The motor is configured with a driving shaft  1608  and pulley  1612  arranged to turn a flywheel  1620  through a belt  1614  as above. Though a belt tensioner apparatus could again be provided to take up any slack in the belt  1614  during operation, it is not necessary because the flywheel is circular. Alternatively, the motor could be pivotally or dynamically mounted to the frame so as to allow some relative movement between the drive pulley and the flywheel to take care of any variance in tension. A flywheel crankpin  1622  is installed on the flywheel in a first position and pivotally connected to a flywheel intake block rigidly mounted to a first piston rod  1670  being driven within a first cylinder  1630  that is pivotally mounted at its base to the frame  1606  through a first pivot pin  1658 . First and second pillow block bearings  1603 ,  1604  are installed on the tank in an offset arrangement such that respective first and second through holes formed in the bearings  1603 ,  1604  are substantially aligned. A flywheel shaft  1625  rigidly mounted within the flywheel  1620  then rotatably passes through both block bearings  1603 ,  1604  so as to extend beyond the opposite side of the tank  1602 . A drive arm  1605  is rigidly mounted to the flywheel shaft  1625  opposite the flywheel  1620 . The drive arm  1605  has a drive arm crankpin  1623  installed thereon and is mounted on the flywheel shaft  1625  such that the drive arm crankpin  1623  is out of phase with the flywheel crankpin  1622 , as explained more fully below. A drive arm intake block  1627  is pivotally mounted on the drive arm crankpin  1623  which is then rigidly installed on a second piston rod  1671  of a second cylinder  1631  pivotally mounted on a second pivot pin  1659  installed on the frame  1606 . The first and second cylinders  1630 ,  1631  are, thus, pivotally installed on the frame  1606  in a substantially offset arrangement about the tank  1602 . The first cylinder has a first piston body sealingly and slidably installed therein so as to form a first upper chamber above the first piston body and a first lower chamber below the first piston body, the first piston body being further formed with a first cavity in communication with the first lower chamber. Likewise, the second cylinder has a second piston body sealingly and slidably installed therein so as to form a second upper chamber above the second piston body and a second lower chamber below the second piston body, the second piston body being further formed with a second cavity in communication with the second lower chamber. A first piston rod  1670  is rigidly connected to the flywheel intake block  1626  and a second piston rod  1671  is rigidly connected to the drive arm intake block  1627 , each having a hollow bore configured to communicate with the ambient air through the respective intake block. As in the other exemplary embodiments, the piston rods pass through the cylinders and the upper chambers so as to be connected to the respective pistons operating within the cylinders  1630 ,  1631 . Furthermore, at least lower piston valves are installed on the respective piston bodies so as to selectively seal the first lower chamber from the first cavity and the second lower chamber from the second cavity. In the exemplary embodiment, air lines (not shown) again connect the one or more outlets at least of the lower chambers of each cylinder to the tank, though it will be appreciated that the cylinders can each be connected to further cylinders or holding tanks in series for further compression. As such, in operation, rotation of the flywheel  1620  as driven by the motor  1604  acts on the first piston rod  1670  through the flywheel crankpin  1622  and the flywheel intake block  1626  to cause the first piston body to travel within the first cylinder  1630 , alternately opening the first lower piston valve to pull ambient air through the hollow piston rod into the first lower chamber and closing the first lower piston valve to compress the air in the first lower chamber. At the same time, rotation of the flywheel  1620  acts on the second piston rod  1671  through rotation of the flywheel shaft  1625  translating to rotation of the drive arm  1605  and radial movement of the drive arm crankpin  1623  and the drive arm intake block  1627  to cause the second piston body to travel within the second cylinder  1631 , alternately opening the second lower piston valve to pull ambient air through the second hollow piston rod  1671  into the second lower chamber and closing the second lower piston valve to compress the air in the second lower chamber. Preferably, the opening of the first lower piston valve is not concurrent with the opening of the second lower piston valve, and the closing of the first lower piston valve is not concurrent with the closing of the second lower piston valve. This is accomplished due to the flywheel crankpin  1622  and the drive arm crankpin  1623  being out of phase, as best seen in  FIGS. 50 and 51 . As a result, it will be appreciated by those skilled in the art that the higher torque output of the motor, as when a piston is nearing the top or bottom of its travel and essentially maximum compression is being done in the cylinder, is not demanded of both cylinders at the same time. Rather, when one cylinder is requiring more power, it is desirable that the other is doing the relatively easier work of gathering air. In an exemplary embodiment, the respective crankpins, and thus cylinders, may be approximately sixty or one hundred twenty degrees out of phase, though it will be further appreciated that numerous such arrangements may be optimal depending on the cylinder arrangement and application. It will be appreciated that cylinders of different size and stroke length can be employed in the same compressor, as when staging of the compression is to be accomplished, for example, which would further effect the kinematic arrangement. Moreover, other changes, such as the addition of a counterweight to the drive arm  1605  substantially opposite the drive arm crank pin  1623 , may be made to take further advantage of the inertial characteristics of the air compression apparatus of the present invention. 
   With all of the embodiments of the air compression apparatus of the present invention, o-rings and the like may be used liberally throughout the construction to provide seals between all mechanically joined components. An example of the kind of o-ring employed in the present invention is a Viton® o-ring having a temperature range of −10 to 400 degrees Fahrenheit (−23 to 204 degrees Celsius). Furthermore, it is to be understood that all o-rings are to be seated as by being mechanically trapped or press fit or otherwise secured so as to effectively remain in the positions shown, as by means now known or later developed in the art. This is to be particularly understood for those o-rings seated around breathing holes in many of the exemplary embodiments, such that even as sealing members are selectively shifted out of contact with the o-rings, they remain seated in their respective channels. The other components shown and described, except as otherwise mentioned, are primarily constructed of aluminum or steel. The gland sealing the piston rod is generally formed as is known in the art of bronze, though it will be appreciated that in the present invention the bushing is capable of being relatively longer due to the substantially coaxial travel of the piston assembly within the cylinder as described above. This increased length of the gland&#39;s bronze bushing results in, among other things, better mechanical support and sealing about the piston rod as well as relatively longer life. Moreover, it will be appreciated by those skilled in the art that numerous combinations of the structure and geometry of the drive mechanism and the cylinder arrangements shown and described can be practiced depending on the application and performance requirements. Drives and cylinders can be mixed and matched to suit particular needs, such that the embodiments shown are to be understood as merely exemplary. Particularly, the lengths and diameters of the cylinders and piston assemblies can vary widely from the geometries shown and described without departing from the spirit and scope of the invention. Specifically, while the hollow piston rod is shown and described herein as being tubular or annular, it will be appreciated that the rod can take a variety of configurations without departing from the invention. Again, the cylinders themselves can be arranged in parallel or in series, and the described advantages can be achieved using the disclosed drive mechanisms with virtually any cylinder arrangement now known or later developed, and need not be the novel cylinder design of the present invention whereby ambient air is introduced into the cylinder through the hollow piston rod. Or, advantages in construction and use can be achieved through the novel cylinder design of the present invention involving breathing through the hollow piston rod alone, again, whether the cylinder is single-acting or double-acting, single-staging or multi-staging, or actuated by a drive mechanism alone or along with other cylinders, and so need not involve any of the particular drive mechanisms disclosed to still derive the advantages of the cylinder construction described herein. Thus, while use of both the disclosed drive mechanisms and cylinders is preferable, it is not required and the invention is not so limited. 
   Accordingly, it will be appreciated by those skilled in the art that the present invention is not limited to any particular configuration of the compressor and its cylinder or cylinders, and that numerous such configurations are possible without departing from the spirit and scope of the invention. Therefore, aspects of the present invention may be more generally described as improved air compression providing for a relatively longer or larger-volume working stroke of each piston combined with a coordinated variance in the speed of the piston during its stroke to produce smoother and more efficient compression. The improved compressor may further consist, in part, of one or more pistons that compress the air both on the “upward” and “downward” strokes. In any such embodiments, a hollow rod is preferably attached to the piston and passed through a gland at the top end of the cylinder so as to provide a compressible space above the piston between the hollow rod and the wall of the cylinder, i.e., the upper chamber, and between the piston and the bottom of the cylinder, i.e., the lower chamber, such that the piston compresses air both on the “upstroke” and on the “down stroke.” In many of the exemplary embodiments, the cylinder is of extended length and the system operates at a relatively low number of strokes per minute so that a greater volume of air is compressed to a higher pressure with less physical motion of the parts and, thus, with increased potential for heat dissipation between strokes. Moreover, the improved breathing of the cylinder through the piston assembly through physically separating the chamber inlet and outlet locations, or placing the inlets and outlets on different surfaces, yields greatly improved air flow through the cylinder, which provides numerous advantages as described herein. Accordingly, the extended length or larger volume of the cylinder and the reduced and variable rate of motion of the piston within the cylinder of the typical embodiment of the compressor of the present invention along with the introduction of ambient air into the cylinder through a hollow piston rod provide for smooth compression and for less demand of power with a larger volume of compressed air per stroke, ultimately resulting in the compressor of the present invention operating more efficiently. Such other structure and resulting benefits of operation are possible without departing from the spirit and scope of the invention. 
   While aspects of the invention have been described with reference to at least one exemplary embodiment, it is to be clearly understood by those skilled in the art that the invention is not limited thereto. Rather, the scope of the invention is to be interpreted only in conjunction with the appended claims and it is made clear, here, that the inventor believes that the claimed subject matter is the invention.