Patent Abstract:
This invention relates to internal combustion engines with cylinders arranged parallel to the main shaft and where reciprocating movements of the pistons are converted to rotation by means of a Z-crank mechanism and motion converter, or conversely to systems such as pumps and compressors wherein rotation of the Z-crank and motion converter produces reciprocating motions of the pistons. The motion converter is prevented from rotation by a reaction control shaft or by a gear train. Connecting rods are prevented from rotating about their long axes. Double-ended configurations can be either opposed cylinder or opposed piston, and may include multiple pairs of pistons with each pair in a common cylinder. The Z-crank may be moved axially for the purpose of varying the compression ratio. Variation of the compression ratio is controlled by an engine control unit and is adjusted to optimize engine performance under varying loads and other conditions.

Full Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
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   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
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   REFERENCE TO SEQUENCE LISTING, A TABLE, OR A COMPUTER PROGRAM LISTING COMPACT DISK APPENDIX 
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   BACKGROUND OF THE INVENTION 
   The following disclosure relates generally to machines and apparatuses having axial piston arrangements and, more particularly, to apparatuses and methods for converting reciprocating linear motion of one or more pistons into rotary motion of an associated shaft oriented in parallel to the piston motion. 
   Various apparatuses are known that convert movement of a working fluid within a changeable cylinder volume into rotary motion of an input/output shaft. Conventional internal combustion engines, compressors, and pumps are just a few of such apparatuses. In conventional arrangements, the pistons are connected via connecting rods to a crankshaft that rotates on an axis oriented perpendicular to the direction of travel of the piston. 
   The theoretical advantages of the axial piston arrangement have been well understood for many years, but no prior effort has succeeded in the marketplace. The primary difficulty in implementing an axial piston engine is in the means provided for preventing rotation of the motion converter, or as commonly referred to, the “wobble plate.” 
   BRIEF SUMMARY OF THE INVENTION 
   It is an object of the invention to reduce friction losses in internal combustion engines and the like. 
   Another object of the invention to provide for variable compression ratio in internal combustion engines. 
   A further object of the invention is to provide a piston motion that is harmonic in nature and can be readily balanced and thereby reduce vibration. 
   It is an additional object of the invention to provide an improved means for preventing the rotation of the motion converter in an axial piston machine. 
   Another object of the invention is to provide a means for preventing the rotation of the connecting rods in an axial piston machine. 
   Yet another object of the invention is to provide for a one-piece or rigidly attached piston and connecting rod in an axial piston machine. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is an isometric view of an axial piston apparatus configured in accordance with an embodiment of the invention. 
       FIG. 2  is an isometric view of the axial piston apparatus of  FIG. 1  with various portions removed for purposes of clarity. 
       FIG. 3  illustrates a side elevation view and a top plan view of the axial piston apparatus of  FIG. 2 . 
       FIG. 4  is an exploded isometric view of the motion converter/Z-crank/reaction control shaft assembly of  FIGS. 1–3  configured in accordance with embodiments of the invention. 
       FIG. 5  is an isometric view of the Z-crank of  FIG. 4  configured in accordance with an embodiment of the invention. 
       FIG. 6  is an exploded isometric view of the motion converter and the Z-crank of  FIGS. 4 and 5  configured in accordance with embodiments of the invention. 
       FIG. 7  is a partially exploded isometric view of the reaction control shaft of  FIGS. 1–4  configured in accordance with an embodiment of the invention. 
       FIG. 8  is a partially cutaway isometric view of an axial piston apparatus having an anti-rotation gear train configured in accordance with another embodiment of the invention. 
       FIG. 9  is a side elevational view of the axial piston apparatus of  FIG. 8  with portions removed for purposes of clarity in accordance with an embodiment of the invention. 
       FIG. 10  is an isometric view of the axial piston apparatus of  FIG. 9  configured in accordance with an embodiment of the invention. 
       FIG. 11  is a top view of the axial piston apparatus of  FIG. 9  configured in accordance with an embodiment of the invention. 
       FIG. 12  is an exploded isometric view of a piston/connecting rod assembly configured in accordance with an embodiment of the invention. 
       FIG. 13  is an isometric view of an axial piston apparatus configured in accordance with yet another embodiment of the invention. 
       FIG. 14  is an exploded isometric view of a one-piece piston/connecting rod assembly configured in accordance with another embodiment of the invention. 
       FIG. 15  is an isometric view of an axial piston apparatus having opposed cylinders facing outwardly from each other in a back-to-back arrangement in accordance with an embodiment of the invention. 
       FIG. 16  illustrates a side elevation view and a top view of the axial piston apparatus of  FIG. 15  in accordance with an embodiment of the invention. 
       FIG. 17  is an isometric view of an axial piston apparatus having opposed pistons facing toward each other in pairs sharing a common cylinder in accordance with an embodiment of the invention. 
       FIG. 18  illustrates a side elevation view and a top view of the axial piston apparatus of  FIG. 17 . 
   

   DETAILED DESCRIPTION 
   The following disclosure is directed to apparatuses and methods for converting reciprocal linear motion of one or more pistons into rotary motion of an output power shaft whose rotational axis is parallel to ther motions of the pistons or, conversely, for converting rotary motion of a similarly configured input shaft into reciprocal linear motion of one or more pistons. Various embodiments of the invention can be applied to internal combustion engines, external combustion engines, air compressors, air motors, liquid fluid pumps, and the like where movement of a working fluid within a volume-changing cylinder results from/in rotary motion of an input/output shaft. In contrast to conventional engines, compressors, and pumps where the crankshaft&#39;s rotational axis is perpendicular to the motions of the pistons, an axial piston apparatus configured in accordance with embodiments of the present invention can have one or more cylinders aligned in parallel with the rotational axis of the input/output shaft. As described in greater detail below, such a configuration can further include the capability to dynamically vary the compression ratio in the cylinders to alter the performance characteristics of the apparatus. 
   Certain embodiments of the apparatuses and methods described herein are described in the context of fluid pumps, fluid compressors, and internal combustion engines of both two- and four-stroke cycle designs. Accordingly, in these embodiments, the invention can include one or more features often associated with internal combustion engines, fluid pumps, or compressors such as fuel delivery systems, ignition systems, and/or various other engine/pump control functions. Because the basic structures and functions often associated with internal combustion engines, fluid pumps, fluid compressors and the like are known to those of ordinary skill in the relevant art, they have not been shown or described in detail here to avoid unnecessarily obscuring the described embodiments of the invention. 
   Certain specific details are set forth in the following description and in  FIGS. 1–18  provide a thorough understanding of various embodiments of the invention. Those of ordinary skill in the relevant art will understand, however, that the invention may have additional embodiments that may be practiced without several of the details described below. In addition, some well-known structures and systems often associated with engines, pumps, and compressors have not been shown or described in detail here to avoid unnecessarily obscuring the description of the various embodiments of the invention. 
   In the drawings, identical reference numbers identify identical or at least generally similar elements. To facilitate the discussion of any particular element, the most significant digit or digits in any reference number refers to the figure in which that element is first introduced. For example, element  130  is first introduced and discussed in reference to  FIG. 1 . In addition, any dimensions, angles and other specifications shown in the figures are merely illustrative of particular embodiments of the invention. Accordingly, other embodiments of the invention can have other dimensions, angles and specifications without departing from the spirit or scope of the present disclosure. 
     FIG. 1  is an isometric view of an axial piston apparatus  100  configured in accordance with an embodiment of the invention. For ease of reference, the phrase “axial piston apparatus” will be understood to include engines, pumps, compressors, etc. having the piston arrangement more or less as depicted, unless specifically identified otherwise. In one aspect of this embodiment, the apparatus  100  includes one or more cylinders  110  aligned in parallel with a rotational axis  131  of a Z-crank  130 . Although the illustrated embodiment depicts three cylinders  110 , in other embodiments, the engine  100  can include more or fewer cylinders  110  without departing from the spirit or scope of the present disclosure. As discussed in greater detail below, in those embodiments in which a four-stroke combustion process is utilized, it may be advantageous for the apparatus  100  to include an odd number of cylinders  110 . In contrast, those embodiments of the apparatus  100  utilizing a two-stroke combustion process may include an odd or even number of cylinders  110 . 
   In another aspect of this embodiment, pistons  112  reciprocate back and forth within the cylinders  110  parallel to the Z-crank rotational axis  131 . The pistons  112  are connected via connecting rods  114  to a “wobble-plate” or motion converter  120 . As described in greater detail below, the motion converter  120  is rotatably attached to the Z-crank  130  about a nutation axis  133  such that the Z-crank  130  is free to rotate with respect to the motion converter  120  about the nutation axis  133 . Accordingly, reciprocating motion of the pistons  112  in the cylinders  110  causes the motion converter  120  to nutate or wobble (but not rotate) relative to the Z-crank rotational axis  131 . 
   In a further aspect of this embodiment, the apparatus  100  also includes a reaction control shaft  150  slidably engaging the motion converter  130 . As explained in greater detail below, the reaction control shaft  150  restricts rotational movement of the motion converter  130  while allowing the motion converter  130  to nutate relative to the Z-crank rotational axis  131 . The reaction control shaft  150  is configured to accommodate this nutation by rotating about an axis  151  as the Z-crank  130  rotates about its rotational axis  131 . A gear train  160  controls motion of the reaction control shaft  150  relative to the Z-crank  130 . 
   In operation, reciprocating motion of the pistons  112  within the cylinders  110  causes the motion converter  120  to nutate relative to the Z-crank rotational axis  131 . Although the motion converter  120  nutates, it does not rotate a significant amount. Nutation of the motion converter  120  causes the Z-crank  130  to rotate relative to the motion converter  120  about the nutation axis  133 . Such motion also causes the Z-crank  130  to rotate about the Z-crank axis  131 . While the Z-crank  130  rotates, the reaction control shaft  150  also rotates about its axis  151  (e.g., at twice the Z-crank rotational speed) to accommodate the nutational movement of the motion converter  120  while restricting rotational movement of the motion converter  120 . 
   Accordingly, in an internal combustion engine embodiment, combustion of fuel gases in the cylinders  110  can impart linear motion to the pistons  112  which in turn causes the motion converter  120  to wobble or nutate relative to the Z-crank rotational axis  131  providing rotational shaft-power at the Z-crank  130 . This shaft-power can be utilized for any one of many applications including propelling air, land, and sea vehicles. Alternatively, when used as a pump or air compressor, shaft-power can be applied to the Z-crank  130  causing it to rotate about the Z-crank rotational axis  131  and thereby nutate the motion converter  120 . Nutation of the motion converter  120  in turn causes axial motion of the pistons  112  in the cylinders  110 . Such motion can be used to pump water, air or another fluid to or from a reservoir or source (not shown) for many applications. 
   In yet another aspect of this invention, the axial arrangement of the cylinders  110  relative to the Z-crank rotational axis  131  can advantageously facilitate compression ratio changes within the cylinders  110 . For example, in one embodiment the apparatus  100  can include a support plate  140  that provides rotational support to the Z-crank  130  and the reaction control shaft  150 . In the illustrated embodiment, the support plate  140  can be axially movable relative to the cylinders  110  back and forth parallel to the Z-crank rotational axis  131 . Accordingly, as the support plate  140  moves toward the cylinders  110 , the clearance between the top of the pistons  112  and the top of the combustion chamber within the cylinders  110  is reduced. As a result, such movement of the support plate  140  causes the compression ratio within the cylinders  110  to increase. Similarly, movement of the support plate  140  away from the cylinders  110  causes the compression ratio within the cylinders  110  to decrease. As will be appreciated by those of ordinary skill in the relevant art, controlling the compression ratio within the cylinders  110  in the foregoing manner can advantageously be used to alter or optimize various performance aspects of the axial piston apparatus  100 . 
   In one aspect of this embodiment, the axial piston apparatus  100  can include an actuator  142  operably connected to the support plate  140 , and an engine control unit  144  (“ECU”  144 ) that provides control inputs to the actuator  142 . In one embodiment, the actuator  142  can include a hydraulic actuator configured to move the support plate  140  back and forth relative to the cylinders  110 . In other embodiments, other types of mechanical, hydraulic, pneumatic and other types of actuators can be used to move the support plate  140  in response to inputs from the ECU  144 . The ECU  144  of the illustrated embodiment can include one or more facilities for receiving engine operating information and outputting control signals to the actuator  142 . For example, in one embodiment, the ECU can include a processor and a controller. In other embodiments, the ECU can include other functionalities. In yet another embodiment, the ECU  144  may be at least substantially similar to ECUs for controlling conventional internal combustion engines. In this embodiment, however, the ECU  144 , in addition to controlling engine functions such as fuel intake, ignition timing, and/or valve timing, can provide additional output signals to control the actuator  142  and move the support plate  140  in response to one or more of the engine operating parameters. In a further aspect of this embodiment, one or more engine sensors  146  can provide engine operating parameter input to the ECU  144 . Such engine sensors can include, for example, airflow rate, combustion and/or exhaust temperatures, throttle position, vehicle speed, etc. 
   In a further aspect of this embodiment, a variable compression axial piston engine in accordance with the present invention can be utilized to optimize engine performance to suit different operating conditions. For example, when the axial piston engine is operated at idle speeds, the compression in the combustion chambers can be reduced to enhance fuel efficiency. Alternatively, at higher RPMs, the compression within the combustion chambers can be increased. In other embodiments, the variable compression aspects of the present invention can be utilized in other ways to increase efficiency or performance. 
     FIG. 2  is an isometric view of the axial piston apparatus  100  of  FIG. 1  with the cylinders and housing removed for purposes of clarity. In one aspect of this embodiment, the connecting rods  114  are double-articulating connecting rods that can accommodate rotational movement about two axes at each end. For example, an upper wrist pin  218  joining the “small end” of the connecting rod  114  to the piston  112  is configured to gimbal or rotate in at least two axes with respect to the connecting rod  114 . Similarly, a lower wrist pin  216  joining the “big-end” of the connecting rod  114  to the motion converter  120  is also able to gimbal or rotate about at least two axes with respect to the motion converter  120 . Details of the connecting rod attachments will be described more fully below, as will an alternate embodiment of the invention wherein the connecting rods  114  are at least substantially fixed relative to the pistons  112 . In this alternate embodiment, the pistons  112  are at least partially spherically shaped, as shown in crossisection  1312  to accommodate minor tilting motions of the connecting rods  114 . 
   The gear train  160  introduced above with reference to  FIG. 1  is shown to good advantage in  FIG. 2 . In another aspect of this embodiment, the gear train  160  includes a Z-crank gear  262  rotatably coupled to a reaction control shaft gear  266  via an idler gear  264 . Both the idler gear  264  and the reaction control shaft gear  266  can have one-half as many teeth as the Z-crank gear  262 . Accordingly, this gear arrangement will cause the reaction control shaft  150  to rotate at twice the speed of the Z-crank  130 . As explained in greater detail below, in one aspect of this embodiment, this speed is necessary so that an offset portion  351  of the reaction control shaft  150  that guides the motion converter  120  will complete two orbits about its rotational axis as the Z-crank  130  completes one full rotation and the motion converter  120  completes one full nutation. In other embodiments, other gear arrangements can be used to provide the requisite timing between the Z-crank  130  and the reaction control shaft  150  without departing from the spirit or scope of the present invention. 
     FIG. 3  includes side elevation and top plan views of the axial piston apparatus  100  of  FIG. 2 .  FIG. 3  illustrates how fore and aft motion of the support plate  140  changes the axial position of the pistons  112  relative to the cylinders  110  (not shown) thereby changing the compression ratio in the cylinders  110 . In one aspect of this embodiment, the axial piston apparatus  100  includes a reaction control bearing  352  slidably and rotatably positioned on an offset bearing surface  351  of the reaction control shaft  150 . As described in greater detail below, the reaction control bearing  352  allows the motion converter  120  to nutate about the Z-crank rotational axis  131  while restricting rotational motion of the motion converter  120 . The reaction control bearing  352  further allows the motion converter  120  to travel back and forth along the offset bearing surface  351  as the motion converter  120  nutates. The reaction control bearing  352  can be configured to rotate relative to the offset bearing surface  351  to accommodate rotation of the reaction control shaft  150  about its rotational axis  151 . 
     FIG. 4  is an exploded isometric view of the motion converter/Z-crank/reaction control shaft assembly of  FIGS. 1–3  configured in accordance with embodiments of the invention. In one aspect of this embodiment, the Z-crank assembly  130  includes a motion connection throw or bearing surface  432  configured to receive the motion converter  120 . As explained above, the bearing surface  432  is aligned with the nutation axes  133 . The Z-crank assembly  130  can further include fore and aft bearing surfaces  434  and  435  for rotationally supporting the Z-crank  130  relative to the housing of the axial piston apparatus  100  ( FIG. 1 ). The fore and aft bearing surfaces  434  and  435  can be suitably supported in bearings to permit free rotation of the Z-crank  130  about the Z-crank rotational axis  131 . As illustrated, the Z-crank rotational axis  131  intersects the nutational axis  133  at a location that is at least approximately centered on the motion converter bearing surface  432 . Although the forward bearing surface  434  appears relatively short in  FIG. 4 , in other embodiments, the Z-crank  130  can extend further forward from the forward bearing surface  434  and provide rotational surfaces for actuating other mechanisms related to the axial piston apparatus  100 . For example, as explained in greater detail below, in one embodiment the Z-crank  130  can be extended forward from the forward bearing surface  434  to provide camshaft lobes for actuating poppet-valves or other fluid control valves associated with combustion or pump processes. 
   In another aspect of this embodiment, the motion converter  120  has a centerbore  422  including one or more bearings (e.g., needle bearings) configured to rotatably receive the Z-crank bearing surface  432 . The motion converter  120  can further include a reaction control bearing bore  424  radially offset from the centerbore  422  and configured to rotatably receive the reaction control bearing  352 . The reaction control bearing  352  can similarly include a control shaft bore  454  configured to slidably and rotatably receive the offset bearing surface  351  of the reaction control shaft  150 . The reaction control shaft gear  266  is fixed to one end of the reaction control shaft  150  and is configured to be operably engaged with the Z-crank gear  262  fixed on the Z-crank  130  proximate to the aft bearing surface  435 . 
     FIG. 5  is an isometric view of the Z-crank  130  configured in accordance with an embodiment of the invention. In one aspect of this embodiment, the Z-crank  130  can include a forward splined portion  531  positioned proximate to the forward bearing surface  434 , and an aft splined portion  532  positioned proximate to the aft bearing surface  435 . The splined portions illustrated in  FIG. 5  can be utilized to accommodate axial movement of the Z-crank  130  relative to other parts that engage with the splined portions. For example, referring to  FIG. 3  above, axial movement of the support plate  140  causes the Z-crank  130  to move fore and aft along its rotational axis  131 . If the Z-crank aft splines  532  are engaged with, for example, a rotational member or other coupling that is axially (but not rotationally) fixed relative to the Z-crank  130 , then the aft splined portion  532  permits the Z-crank to move fore and aft relative to such a fixed coupling. Similarly, if the forward splined portion  531  is engaged with another rotational member that is also axially fixed relative to the Z-crank  130 , then the forward splined portion  531  accommodates the relative axial movement between the Z-crank  130  and the forward member. Thus, as the Z-crank/motion converter assembly moves fore and aft along the rotational axis  131  of the Z-crank  130 , the splined portions on the forward and aft end of the Z-crank  130  can accommodate the relative axial motion between the Z-crank and any mating features. In other embodiments, other features can be utilized to accommodate the relative motion of the Z-crank/motion converter assembly as the Z-crank moves fore and aft to change the compression ratio in the cylinders  110  ( FIG. 1 ). 
   In yet another aspect of this embodiment, the Z-crank  130  can include a counter-weight  534  laterally offset from the Z-crank rotational axis  131 . If required or desirable, the counter-weight  534  can be used to dynamically balance the motion converter/Z-crank assembly. 
     FIG. 6  illustrates exploded isometric views of the motion converter  120  and the Z-crank  130  configured in accordance with embodiments of the invention. The embodiments illustrated in  FIG. 6  are merely representative and, accordingly, and are not intended to limit the present invention to the configurations shown. Accordingly, in other embodiments, other components can be utilized to construct and practice the motion converter  120  and the Z-crank  130  of the present invention. In the illustrated embodiment, the Z-crank  130  can include an upper portion  634  mated to a lower portion  636  with a taper pin  637 . Prior to mating, the upper Z-crank portion  634  can receive a thrust bearing  638  and can be inserted through the motion converter bore  422 . After the upper Z-crank portion  634  is inserted through the motion converter bore  422 , it can receive another thrust bearing  638  and be inserted into the lower Z-crank portion  636 , thereby rotatably capturing the motion converter  120  on the Z-crank  130 . 
   In another aspect of this embodiment, the motion converter  120  can include needle bearings  628  received in the motion converter bore  422 . The needle bearings  628  facilitate rotational motion of the Z-crank  130  relative to the motion converter  120 . In other embodiments, other bearings in other configurations can be used to provide rotational freedom of the Z-crank  130  relative to the motion converter  120 . 
     FIG. 7  is a partially exploded isometric view of the reaction control shaft  150  shown in  FIGS. 1–4  above. In one aspect of this embodiment as mentioned above, the reaction control shaft gear  266  can be fixedly attached to a lower end of the reaction control shaft  150  to control the rotational motion of the reaction control shaft  150  about its rotational axis  151 . As shown to good effect in  FIG. 7 , the offset bearing surface  351  is cylindrical in cross-section and has a centerline axis  751  that is offset relative to the rotational axis  151  of the reaction control shaft  150 . In one aspect of this embodiment, this offset is necessary to facilitate the nutational motion of the motion converter  120 . In another aspect of this embodiment, the reaction control shaft  150  can include counter-weights  756  which can be machined or otherwise conformed to rotationally balance the reaction control shaft  150  about its rotational axis  151 . 
   In a further aspect of this embodiment, the reaction control bearing  454  includes a ball bearing  752  and a retaining ring  754 . The ball bearing  752  is received on the reaction control bearing  352  at an angle relative to the reaction control bearing bore  454 . In a further aspect of this embodiment, the angle of the ball bearing  752  accommodates the nutational movement of the motion converter  120  relative to the reaction control shaft  150  as the Z-crank  130  rotates. In addition, the ball bearing  752  allows the reaction control bearing  352  to rotate relative to the reaction control bearing bore  424  ( FIG. 4 ) of the motion converter  120 . This relationship between the ball bearing  752 , the reaction control shaft  150 , and the motion converter  120  can be seen with reference to  FIG. 3 . The retaining ring  754  can be threadably installed onto the reaction control bearing  352  to retain the ball bearing  752 . 
   Prior to assembly of the reaction control shaft  150  (for example, prior to installing the first counterweight  756 ), the bearing surface  351  of the reaction control shaft  150  is inserted through the reaction control bearing bore  454  of the reaction control bearing  352 . The first counterweight  755  can then be installed on the reaction control shaft  150 . 
   The foregoing discussion describes one embodiment of the present invention for restricting rotational movement of the motion converter  120  as it nutates relative to the Z-crank rotational axis  131  ( FIGS. 1–3 ). In other embodiments, other apparatuses and methods can be utilized to restrict this rotational movement without departing from the spirit or scope of the present invention. Specifically, other apparatuses and methods can be utilized to restrict this rotational movement while still enabling the variable compression features of the present invention. One such embodiment is described in greater detail below with reference to  FIG. 8  and on. 
     FIG. 8  is a partially cutaway isometric view of an axial piston apparatus  800  having an anti-rotation gear train  860  configured in accordance with another embodiment of the invention. Although the axial piston apparatus  800  of  FIG. 8  includes six pistons  812  and associated hardware, this number is in no way limiting and, in other embodiments, the axial piston apparatus  800  can include more or fewer pistons  812 . Similarly, although the illustrated embodiment may depict a two-stroke diesel engine configuration, in other embodiments, the anti-rotation gear train  860  and associated features can be utilized with other axial piston apparatuses (e.g., 4-stroke engine or pump apparatuses) configured in accordance with the present disclosure. In the illustrated embodiment, a forward splined portion  831  of a Z-crank  830  protrudes beyond an engine block or housing  801 . As discussed above, the forward splined portion  831  can be utilized to drive a camshaft for, among other things, actuating inlet poppet valves for providing fuel mixture to combustion chambers in the cylinders  810 . 
     FIG. 9  is a side elevation view of the axial piston apparatus  800  of  FIG. 8  with the housing  801  removed to better illustrate aspects of the anti-rotation gear train  860  configured in accordance with an embodiment of the invention. As shown in  FIG. 9 , the anti-rotation gear train  860  replaces the reaction control shaft  150  described above and serves the same function, namely, to restrict rotational movement of a motion converter  920 . 
   In one aspect of this embodiment, the anti-rotation gear train  860  (the “gear train  860 ”) includes a fixed gear  862 , a first planetary gear  864 , a second planetary gear  866 , and a motion converter gear  868 . The fixed gear  862  can be fixedly mounted to a lower portion of the Z-crank  830  and meshed with the first planetary gear  864 . In one embodiment, the fixed gear  862  and the planetary gear  864  can be straight gears. In other embodiments, these gears can have other configurations. In another aspect of this embodiment, the first planetary gear  864  can be fixedly mounted on a common shaft with the second planetary gear  866 . Accordingly, the first and second planetary gears  864  and  866  are fixed relative to each other and rotate about a common axis  835 . In a further aspect of this embodiment, the second planetary gear  866  can be beveled or tapered to mesh with the correspondingly tapered motion converter gear  868 . The motion converter gear  868  can be rotatably mounted (e.g., with needle or roller bearings) to a bearing surface  832  of the Z-crank  830 . Further, the motion converter gear  868  can be fixedly attached to the motion converter  920 . 
   An example of the operation of the gear train  860  will now be explained in accordance with an embodiment of the invention in which a combustion force F drives the pistons  812  to provide shaft-power output from the Z-crank  830 . In this embodiment, combustion gases move the pistons  812  causing the motion converter  920  to wobble or nutate relative to the Z-crank axis  931 . As the motion converter  920  nutates, it causes the Z-crank  830  to rotate about its rotational axis  931 . Simultaneously, however, the gear train  860  prevents the motion converter  920  from rotating relative to the nutational axis  833 . Rotation of the motion converter  920  is prevented by the motion converter gear  868  which is fixed relative to the motion converter  920  and engaged with the second planetary gear  866 . The second planetary gear  866  is fixed relative to the first planetary gear  864  which in turn meshes with the fixed gear  862 . In a further aspect of this embodiment, the ratio of the fixed gear  862  to the first planetary gear  864  should be equal to the ratio of the motion converter gear  868  to the second planetary gear  866 . When this ratio is met, the gear train  860  as illustrated in  FIG. 9  can at least substantially prevent significant rotation of the motion converter  920 . 
   If the motion converter  920  is allowed to rotate freely about the nutation axis  833  as the Z-crank  830  rotates, then the motion converter  920  cannot convert linear motion of the pistons  812  into torque at the Z-crank  830  nor, conversely, can the motion converter  920  convert torque from the Z-crank  830  into linear motion of the pistons  812 . Accordingly, in an ideal situation, the motion converter  920  will move in a purely nutational motion without any substantial rotation. 
     FIGS. 10 and 11  are isometric and top views, respectively, illustrating further aspects of the axial piston apparatus  800  discussed above with reference to  FIG. 9 . 
     FIG. 12  is an exploded isometric view of a piston/connecting rod assembly configured in accordance with an embodiment of the invention. In one aspect of this embodiment, the piston/connecting rod assembly shown in  FIG. 12  can be at least generally similar to the double-articulating piston/connecting rod assemblies described above with reference to  FIG. 2 . For example, the upper wrist pin  218  can be received in an upper trunnion  1201  which pivotally connects the upper end (i.e., the “small end”) of the connecting rod  114  to the piston  112 . Similarly, the lower wrist pin  216  can be received in a lower trunnion  1201  which pivotally connects the lower end (i.e., the “big end”) of the connecting rod  114  to a corresponding motion converter (e.g., the motion converter  120  or  920  described above). To accommodate rotation of the wrist pins about at least two axes, the trunnions  1201 ,  1202  can include a spherical surface and opposing trunnion pins. The spherical surface and opposing trunnion pins can be received within an interior portion of mating spherical shell bearings to accommodate rotation about a trunnion pin axis  1211  as well as rotation about a wrist pin axis  1213 . A key or similar feature can be used to register the spherical shell bearings in the corresponding ends of the connecting rod  114 . As will appreciated by those of ordinary skill in the relevant art, other methods and apparatuses can be utilized to pivotally connect the piston  112  to the connecting rod  14 , and the connecting rod  14  to a corresponding motion converter, in accordance with the present disclosure. The embodiment illustrated in  FIG. 12  represents only one such method. 
     FIG. 13  is an isometric view of an axial piston apparatus  1300  that is at least generally similar in structure and function to the axial piston apparatus  100  described above with reference to  FIG. 1 through 5 . In one aspect of this embodiment, however, the axial piston apparatus  1300  includes one-piece piston/connecting rod assemblies  1313 . The one-piece piston/connecting rod assemblies  1313  can include a piston portion  1312  and a connecting rod portion  1314 . The piston portion  1312  can have a spherical cross-section to accommodate slight angular motion of the connecting rod portion  1314  relative to the cylinder (not shown) resulting from the nutational movement of the motion converter  120 . Such one-piece piston/connecting rod assemblies  1313  may, in certain embodiments, reduce the overall cost of the axial piston apparatus  1300  relative to other configurations. As shown in  FIG. 14 , for example, the one-piece piston/connecting rod assembly  1313  necessarily has a lower part count than a piston assembly having the double-articulated connecting rod  114 . 
   Various aspects of the axial piston apparatuses described above can be combined to create engine and/or pump configurations in addition to those described above. For example, various dual-Z-crank configurations can be achieved in accordance with the present disclosure. Such dual-Z-crank configurations can include pistons facing towards each other in pairs sharing common cylinders. Alternatively, such configurations can include opposed cylinders facing outwardly relative to each other similar to two axial piston apparatuses positioned back-to-back. Such configurations may be advantageously self-counterbalancing and not require further counterbalancing via weights, etc. 
     FIG. 15  is an isometric view of an axial piston apparatus  1500  having a first axial piston apparatus  1501  operably coupled to a second axial piston apparatus  1502  in a back-to-back relationship. In one aspect of this embodiment, the combined apparatuses include two Z-cranks which are coupled together and provide shaft-power output via an output gear  1530 . Various mechanical features of the axial piston apparatus  1500  illustrated in  FIG. 15  can be at least generally similar in structure and function to their corresponding counterparts of the axial piston apparatus  100  described above. In addition, however, the axial piston apparatus  1500  can include a Z-crank actuator to simultaneously (or independently) move the coupled Z-cranks back and forth relative to each other on their rotational axis. Such movement can vary the compression in one or both sets of cylinders (not shown) to provide the variable compression aspects of the invention described above. When two complete axial piston apparatuses are coupled back-to-back as illustrated in  FIG. 15 , the reaction forces of the two motion converters can cancel out. Accordingly, counterbalancing of such apparatuses may not be required when the two opposing Z-cranks are in directly opposing phases relative to each other. 
     FIG. 16  illustrates a side elevation view and a top view of the axial piston apparatus  1500  of  FIG. 15 . As shown in the side elevation view, the opposing Z-cranks  1530  are coupled together as are the corresponding reaction control shafts  1550 . In a further aspect of this embodiment, the opposed motion converters  1520  can be in phase for four-stroke engine applications and at least slightly out of phase for two-stroke engine applications and compressor or pump applications. Varying the phase for two-stroke engine applications and compressor or pump applications may be advantageous, in selected embodiments, to accommodate the intake port or outlet port timing arrangements in the cylinders of such applications. In other embodiments, however, the opposing motion converters  1520  can have other phase timings with respect to each other without departing from the spirit or scope of this disclosure. 
     FIG. 17  is an isometric view of an axial piston apparatus  1700  having an opposed piston configuration in accordance with yet another embodiment of the invention. In one aspect of this embodiment, opposing pistons  1712  linearly reciprocate in common cylinders (cylinders are not shown in  FIG. 17 ). The axial piston apparatus  1700  can have coupled Z-cranks  1730  and coupled reaction control shafts  1750  similar to the axial piston apparatus  1500  shown in  FIG. 15 . In the embodiment depicted in  FIG. 17 , however, the variable compression features described above can be implement by moving one or both of the opposing Z-cranks toward or away from each other to accordingly change the working volumes in the corresponding cylinders. In a further aspect of this embodiment, the axial piston apparatus  1700  can be configured as a two-stroke engine utilizing exhaust and intake ports instead of poppet-type valves. In this embodiment, one or more exhaust ports can be positioned toward one end of a cylinder and one or more intake ports can be positioned toward the other end. The opposed Z-cranks  1730  may then be configured to operate slightly out of phase so that the exhaust ports on one end are open before the intake ports open on the other end. Such sequential timing may be desirable to maintain the momentum and/or flow direction of the fluid moving into and out of the corresponding cylinder volume. In a further embodiment, such an engine configuration may be supercharged or turbocharged to provide additional advantages depending on the particular application. 
     FIG. 18  illustrates a side elevation view and a top view of the axial piston apparatus  1700  of  FIG. 17  to further illustrate aspects of this embodiment. 
   The foregoing description of the embodiments of the invention are not intended to be exhaustive or to limit the invention to the precise embodiments disclosed herein. While specific embodiments of, and examples for, the invention are described herein for illustrative purposes, various equivalent modifications are possible within the scope of the invention, as those of ordinary skill will recognize. For example, although certain functions may be described in the present disclosure in any particular order, and alternate embodiments, these functions can be performed in a different order or, alternatively, these functions may be performed substantially concurrently. In addition, the teachings of the present disclosure can be applied to other systems, not only the representative axial engine, compressor, pump systems described herein. Further, various aspects of the invention described herein can be combined to provide yet other embodiments. 
   Accordingly, aspects of the invention can be modified, if necessary or desirable, to employ the systems, functions, and concepts of conventional engine, pump and/or compressor apparatuses to provide yet further embodiments of the invention. These and other changes can be made to the invention in light of the above-detailed description. Accordingly, the actual scope of the invention encompasses the disclosed embodiments described above and all equivalent ways of practicing or implementing the invention. 
   Unless the context clearly requires otherwise, throughout this disclosure the words “comprise,” “comprising,” and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense, that is to say, in the sense of “including, but not limited to.” Words using the singular or plural number also include the plural or singular number, respectively. Additionally, the words “herein,” “above,” “below,” and words of similar import, when used in this application, shall refer to this application as a whole and not to any particular portions of this application. 
   The following examples represent additional embodiments of axial piston apparatuses configured in accordance with the present disclosure.

Technology Classification (CPC): 5