Patent Publication Number: US-6988470-B2

Title: Swash plate combustion engine and method

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application is based on provisional patent application No. 60/434,565, filed on Dec. 18, 2002. The entire disclosure of the provisional application is considered to be part of the disclosure of the following application and is hereby incorporated by reference herein. 

   The present invention relates to improved swash plate combustion engines and related methods. 
   BACKGROUND 
   Swash plate engines with various features are known. For example, FIG. 1 of U.S. Pat. No. 5,437,251 to Anglim et al. is understood to disclose pistons at opposite ends of an engine housing which drive respective swash plate assemblies to in turn rotate an output shaft. Anglim mentions that a cylinder head can have threads used in linearly adjusting the position of the cylinder head relative to the piston heads to achieve variable compression in a combustion envelope between the piston head and the linearly-adjustable cylinder head. Rotating cam members of respective swash plate assemblies are shown supported at equal, but opposite, angles from perpendicular with respect to the power output shaft of the engine. This provides counterbalanced reciprocative travel of pistons. The rotatable cam members are each understood to be maintained at a fixed angle relative to the output shaft by a structure including a starter gear which interconnects the rotatable members. A pinch plate guide prevents rotation of non-rotatable or pinch plate portions of the swash plate assemblies. In the form shown in FIG. 1 of this patent, the pinch plate guide for each swash plate assembly comprises guide rods extending radially outwardly from the pinch plates into sliding contact with guide slots and guide members attached to the engine housing. These guide rods prevent rotation of the non-rotatable members of the swash plate assemblies. These non-rotatable members are driven by the reciprocating pistons such that the non-rotatable members reciprocate and drive the rotatable members of the swash plate assemblies and thus the output shaft. 
   U.S. Pat. No. 4,174,684 to Roseby et al. is understood to disclose a variable stroke internal combustion engine which includes first and second swash plate assemblies with rotatable members which are interconnected by a sliding bar. A crank arm coupled to the sliding bar can shift the position of the sliding bar to adjust the angle of the swash plate assemblies to adjust the engine stroke. The crank is actuated by a link coupled to an actuating mechanism such as a hydraulic piston, a screw or other actuating means. In the embodiment of FIG. 2 of this patent, the two swash plate assemblies are maintained by the sliding bar in what appears to be substantially parallel positions. In Roseby, a carrier has a central plate portion which is positioned between and separates the two swash plate assemblies. 
   Another example of a swash plate engine is disclosed in U.S. Pat. No. 3,319,874 to Welsh et al. In this patent, reciprocating pistons drive a first member of a swash plate assembly. The first member in one embodiment is restrained against rotation by an arm which extends through a ball of a ball and socket carried by a support block which reciprocably slides in a channel of the housing as the pistons move. Reciprocating motion of a first non-rotatable member of the swash plate assembly drives a rotatable member of the swash plate assembly and an output shaft. The rotatable member is coupled by a fixed link to a collar. In one example, a hydraulically actuated piston, acting through linkages, shifts the angle of the swash plate assembly to thereby vary the stroke of the engine. This hydraulically actuated piston is shown at the opposite end of the engine housing from the cylinders and thus adds to the overall length of the engine. 
   Although a number of swash plate engines are known, a need exists for an improved swash plate combustion engine and related methods. The present invention is related to new and unobvious swash plate combustion engine improvements alone and in various combinations and sub-combinations with one another as set forth in the claims below. It is a not a requirement that all of the disadvantages, or any one or more specific disadvantages, of known swash plate engines be overcome for a swash plate engine to fall within the inventive concepts set forth herein and in the claims below. 
   SUMMARY 
   An internal combustion engine in accordance with one embodiment comprises an engine housing. At least one cylinder, and more typically a plurality of cylinders, is/are positioned within the engine housing. The at least one cylinder has a longitudinal cylinder axis extending in a first direction. A piston is positioned within the at least one cylinder for reciprocation therein. One such reciprocatable piston is associated with and positioned within each of the respective cylinders in embodiments where a plurality of cylinders are provided. A rotatable output member is rotatably coupled to the housing for rotation about a first axis. Desirably, first and second swash plate assemblies are positioned within the engine housing. The first swash plate assembly comprises a first member and a second member. The first member is rotatably coupled to the second member for rotation relative to the second member and about the first axis. The second member is coupled to the housing such that the second member is restrained against rotation. The first member may be pivotally coupled to the output member for pivoting about a second axis which is transverse to the first axis. A piston rod is pivotally coupled to the piston and also pivotally coupled to the second member. The piston rod reciprocates with the reciprocal movement of the piston. Reciprocal movement of the piston results in reciprocal movement of the second member and rotation of the first rotatable member and output member about the first axis. The second swash plate assembly comprises a third rotatable member and a fourth member. The third member is rotatably coupled to the fourth member for rotation relative to the fourth member and about the first axis. The fourth member is also coupled to the housing such that the fourth member is restrained against rotation. The third member may also be pivotally coupled to the output member for pivoting about a third axis which is transverse to the first axis. The third member rotates with the rotation of the output member and with the rotation of the first member. Rotational movement of the third member results in reciprocal movement of the fourth member. Desirably, the reciprocal movement of the fourth member counterbalances the reciprocal movement of the second member. 
   In desirable embodiments, the second and third axes are parallel to one another and are in a common plane. Desirably, the first axis about which the output member rotates may also be in this common plane. 
   Throughout this description, the term “coupling” encompasses both direct connection of one member to another as well as indirect connection of one member to another through one or more intervening components. 
   In accordance with one alternative embodiment, the cylinders of the engine are all positioned adjacent to the same end portion of the housing and at the same side of the swash plate assemblies. This results in a more compact engine construction in comparison to a less desirable embodiment in which the swash plate assemblies are positioned between respective sets of cylinders adjacent the opposite end portions of the housing. 
   In a desirable embodiment, the first and second swash plate assemblies are positioned and coupled to one another such that the second and fourth members reciprocate relative to one another in opposite directions with the rotation of the first and third members. As a result, the swash plate assemblies at least partially counterbalance or vibration balance the operation of one another. 
   As an aspect of an embodiment, the second swash plate member may be coupled to the housing to restrain the second member against rotation. In one specific embodiment, a piston rod confining member is provided and is coupled to the housing. The piston rod confining member is configured to slidably engage the piston rod to permit reciprocal movement of the piston rod while restricting rotation of the piston rod about the first axis. This restricts the second member against rotation about the first axis as a result of the coupling of the second member to the piston rod. The fourth member in one specific embodiment is restrained to reciprocate without rotation about the first axis by a track and track follower mechanism. The track may be coupled to the housing with the track follower engaging and traveling along the track. The orientation of the track permits reciprocation of the fourth member without rotation. The track follower may comprise a rolling track follower which rotatably engages the track. In an alternative embodiment, the track follower comprises a slide member which slidably engages the track. The track may comprise a channel with spaced apart track follower engaging wall surfaces positioned for engagement by the track follower. A similar track and track follower arrangement may be used to couple the second member to the housing to restrict the second member against rotation, although this is less desirable. Other mechanisms may be used for directly or indirectly coupling of the second and fourth members to the housing to restrict the second and fourth members against rotation about the first axis. 
   Respective sets of bearings may be used to rotatably couple the first member to the second member and the third member to the fourth member. In specific examples, ball bearings or conical barrel bearings are used for this purpose. To facilitate installation of these bearings, in one embodiment, at least one of the first and second members and at least one of the third and fourth members may comprise a plurality of interconnected sections. The first member may comprise first and second annular sections which are sandwiched together and interconnected to comprise the first member. In a desirable embodiment, the first and second annular sections each define a portion of a first annular rotating surface. In addition, the second member comprises a second annular rotating surface which faces the first annular rotating surface. A first set of bearings is positioned between the first and second annular rotating surfaces in this embodiment. In addition, the fourth member may comprise at least first and second sections which each define a portion of a fourth annular rotating surface. The sections of the fourth member may be ring sections which are interconnected to comprise an annular fourth member with the fourth annular rotation surface. The third member may comprise a third annular rotation surface which faces the fourth annular rotating surface. A second set of bearings may be positioned between the third and fourth annular rotating surfaces. 
   Bearings may be used to couple the piston rod to the reciprocating swash plate member or members. Universal joints may be used for this purpose in one specific example. As another specific example, coupling members may be pivotally connected to the reciprocating swash plate member or members and project outwardly therefrom. A respective piston rod may be pivotally connected to each projecting coupling member portion. In variable stroke engine embodiments, bearings, such as tilt bearings may be used to pivotally couple the respective first and third members to the output member such that the first and third members pivot about the respective second and third axis. This allows the adjustment of the angles of the swash plate assemblies to vary the stroke of the engine, as explained below. 
   In one exemplary embodiment, a plurality of cylinders are provided. A respective piston and piston rod is associated with each cylinder. Although not required in all embodiments, the cylinders may be positioned closer to one end portion of the housing than any of the swash plate assemblies. This results in a more compact engine in comparison to an engine with cylinders at both sides of swash plate assemblies. The piston rods may each be coupled to the same reciprocating member of one swash plate assembly. Reciprocation of the pistons causes a reciprocation of the respective second and fourth members and results in rotation of the respective first and third members and the rotation of the output member about the first axis. 
   In an embodiment, a first set of bearings may pivotally couple the first member to the output member, a second set of bearings may pivotally couple the third member to the output member, a third set of bearings may rotatably couple the first member to the second member and a fourth set of bearings may rotatably couple the third member to the fourth member. A pressurized lubricating fluid supply in communication through a lubricating fluid passageway with each of the first, second, third and fourth bearings may be provided and be operable to provide lubricating fluid to such bearings. 
   In accordance with certain embodiments, the number of cylinders included in the engine, the firing order of such number of cylinders, and the swash plate rotation angle through which the first member rotates between firing of one cylinder and the next cylinder to fire are in accordance with the following table: 
                                           Swash Plate       Number of Cylinders   Firing Order   Rotation Angle                                            1   1      720°       2   1, 2, 1      360°       3   1, 3, 2, 1      240°       5   1, 3, 5, 2, 4, 1      144°       7   1, 3, 5, 7, 2, 4, 6, 1   102.857°       9   1, 3, 5, 7, 9, 2, 4, 6, 8, 1      80°       11   1, 3, 5, 7, 9, 11, 2, 4, 6, 8, 10, 1    65.454°                    
In the above table, the first member rotates 720 degrees during a complete firing cycle. In a specifically desirable embodiment, the engine includes five cylinders which fire in the following sequence: 1, 3, 5, 2, 4, and 1 and wherein the first member rotates through 144 degrees between the firing of one cylinder and the next cylinder to fire. The third member similarly rotates through 144 degrees between firing of one cylinder and the next cylinder to fire.
 
   The swash plate assemblies may be spaced apart sufficiently that they move along paths of travel that do not intersect one another. However, in desirable embodiments, which result in a more compact engine, the first and second swash plate assemblies may be configured such that the second and fourth members travel past one another as the engine operates during at least certain engine operating conditions. For example, at least one of the first and second swash plate assemblies may define an interior swash plate passageway. The other of the first and second swash plate assemblies is sized and positioned to reciprocate at least partially through the interior swash plate passageway as the second and fourth members reciprocate at least during certain operating positions of the first and second swash plate assemblies. For example, the reciprocating member of first swash plate assembly may swing through an interior swash plate passageway defined by the second swash plate assembly as the engine operates. 
   In embodiments where there are two swash plate assemblies, the interior swash plate passageway may be defined by either the first or second swash plate assemblies. In embodiments where the swash plate passageway is defined by the second swash plate assembly, the first swash plate assembly is sized and positioned such that the reciprocating member of the first swash plate assembly reciprocates at least partially through the interior swash plate passageway as the second and fourth members reciprocate, at least during certain operating positions of the first and second swash plate assemblies. Alternatively, in embodiments where the first swash plate assembly defines the interior swash plate passageway, the second swash plate assembly is sized and positioned such that the reciprocating member of the second swash plate assembly reciprocates at least partially through the interior swash plate passageway of the first swash plate assembly as the second and fourth members reciprocate, at least during certain operating positions of the first and second swash plate assemblies. 
   The first member may comprise a first annular rotation surface and the second member may comprise a second annular rotation surface. The first annular rotation surface rotates relative to the second annular rotation surface as the first member rotates relative to the second member. In addition, the third member may comprise a third annular rotation surface and the fourth member may comprise a fourth annular rotation surface. The third annular rotation surface rotates relative to the fourth annular rotation surface as the third member rotates relative to the fourth member. The first annular rotation surface may face outwardly with the second annular rotation surface facing inwardly. In addition, in this example, at least a major portion, and more desirably substantially all, of the first member may be positioned inwardly of the first annular rotation surface and at least a major portion, and more desirably substantially all, of the second member may be positioned outwardly of the second annular rotation surface. By a major portion in this description it is meant at least 50 percent. The term “substantially all” when used in this description means at least 80 percent. Alternatively, the first annular rotation surface may comprise a first inwardly facing surface and the second annular rotation surface may comprise a second outwardly facing surface. In this example, at least a major portion, and more desirably substantially all, of the first member may be positioned outwardly of the first annular rotation surface and at least a major portion, and more desirably substantially all, of the second member is positioned inwardly of the second annular rotation surface. Thus, in the first of these two examples, at least a major portion of the first member may rotate inwardly of the second member and in the second of these two examples, at least a major portion of the first member may rotate outwardly of the second member. In addition, in either of these two examples, a major portion of the third member may be rotating inwardly of the fourth member or alternatively outwardly of the fourth member. That is, the third annular rotation surface may comprise a third outwardly facing surface with the fourth annular rotation surface comprising a fourth inwardly facing surface. In this example, at least a major portion, and more desirably substantially all, of the third member may be positioned inwardly of the third annular rotation surface and at least a major portion, and more desirably substantially all, of the fourth member may be positioned outwardly of the fourth annular rotation surface. Alternatively, the third annular rotation surface may comprise a third inwardly facing surface and the fourth annular rotation surface may comprise a fourth outwardly facing surface. In this case, at least a major portion, and more desirably substantially all, of the third member may be positioned outwardly of the third annular rotation surface and at least a major portion, and more desirably substantially all, of the fourth member may be positioned inwardly of the fourth annular rotation surface. Thus, a major portion of the third member may rotate inwardly or alternatively outwardly of the fourth member. 
   In accordance with one embodiment, a link or other coupling member or assembly may be utilized to couple the rotatable first member of the first swash plate assembly to the rotatable third member of the second swash plate assembly. This coupling assembly may comprise first, second and third elements. In one specific example, the first element may pivotally couple the rotatable first member of the first swash plate assembly to the second element at a first location positioned at one side of a plane bisecting the first axis. In addition, in this example, the third element may pivotally couple the rotatable third member of the second swash plate assembly to the second element at a second location at the other side of the plane bisecting the first axis. Desirably, the first and second locations are opposite to one another. In addition, the second element desirably rotates about the first axis with the rotation of the rotatable first and third swash plate assembly members. 
   In accordance with certain embodiments, the housing may comprise a valve cover portion, a cylinder head portion, a cylinder case portion, a swash plate case portion and an output member supporting portion. A plurality of cylinders having respective bores may be positioned within the cylinder case portion. Each of the bores has a bore diameter and a longitudinal cylinder axis. At least one combustion air intake port is provided in communication with each cylinder and at least one exhaust gas port is provided in communication with each cylinder. A respective air intake valve for each air intake port of each cylinder is provided and is selectively operable to open and close the associated air intake port. A respective exhaust valve for each exhaust gas port of each cylinder is provided and is selectively operable to open and close the associated exhaust gas port. The air intake valve or valves associated with each cylinder are opened to permit the ingress of combustion air into the associated cylinder and closed during combustion of an air-fuel mixture within the associated cylinder. The exhaust valve or valves associated with each cylinder are opened to permit the exhaust of combustion gases from the associated cylinder and through the associated exhaust gas port following combustion of the air-fuel mixture within the associated cylinder. A valve actuator is positioned within the valve cover portion of the housing and is operable to selectively open and close the air intake and exhaust valves. A respective piston is positioned within each cylinder and driven along the longitudinal cylinder axis of the associated cylinder in one direction in response to combustion of the air-fuel mixture in the associated cylinder. A respective piston rod is pivotally coupled to each piston. Respective first and second swash plate assemblies are positioned within the swash plate case portion of the housing. The first swash plate assembly comprises a first rotatable member for rotating about a first axis and relative to a second member. In this example, the piston rods are pivotally coupled to the second member. The engine also comprises an output member coupled to the output shaft supporting portion of the housing and which is rotatable about a first axis. The first member is desirably coupled to the output member for pivoting about a second pivot axis which is transverse to and which desirably is perpendicular to the first axis. The first member is drivenly coupled to the output member such that rotation of the first member rotates the output member. The second member is coupled to the housing to prevent rotation of the second member relative to the first member while permitting rotation of the first member relative to the second member. The second member is reciprocated by pistons when the pistons are driven to thereby cause rotation of the first member and rotation of the output member. In this embodiment, a second swash plate assembly is positioned within the swash plate case portion of the housing and comprises respective third and fourth members. The third member is rotatable relative to the fourth member and the fourth member is coupled to the housing so as to prevent the fourth member from rotating while permitting the third member to rotate relative to the fourth member. The third member is desirably coupled to the output member for pivoting about a third pivot axis which is transverse to and which desirably is perpendicular to the first axis. The first and third members are coupled together such that they rotate together. In addition, the second swash plate assembly is desirably oriented relative to the first swash plate assembly such that, as the second member of the first swash plate assembly reciprocates in a first direction, the fourth member of the second swash plate assembly reciprocates in a direction which is opposite to the first direction. 
   In a desirable optional configuration, the exhaust gas ports are shorter than the air intake ports to reduce the heating of the engine which is caused by hot exhaust gas exiting the engine through the exhaust gas ports. The air intake ports and exhaust gas ports, in one alternative embodiment, exit from the cylinder head portion in directions extending generally radially outwardly from the first axis. An exhaust gas port or ports for each cylinder may communicate with the cylinder at a location which is positioned radially outwardly from the first axis relative to the location where the air intake port or ports communicate with the cylinder. 
   In certain embodiments, respective portions of the housing may be interconnected discrete components. However, selected portions of the housing may be of a single monolithic one-piece construction. For example, selected components may be machined together, and more desirably cast together, as a unit. Thus, the cylinder head portion and cylinder case portion may be formed as single monolithic one-piece construction. Alternatively, the cylinder case portion and swash plate case portion may be formed of a single monolithic one-piece construction. The output member support portion may also be of a one-piece monolithic construction with the swash plate case portion. In addition, the longitudinal axes of the respective cylinders in plural cylinder engines may be parallel to one another, but this is not required. In addition, the longitudinal axes of the respective cylinders in plural cylinder engine embodiments may be positioned at a common distance or radius from the first axis about which the output member rotates. The longitudinal cylinder axis of each of the respective cylinders may be at an acute angle relative to the first axis about which the output member rotates. The acute angle in certain embodiments may be no greater than thirty degrees. 
   The cylinders may be of a monolithic one-piece construction with casting being a desirable method of forming the cylinders. The cylinders may have a gap between the cylinders such that cooling fluid may pass through the gap. The gap may be formed, for example, by machining or during casting if the cylinders are cast. Alternatively, the cylinders may have no gap between them. 
   Any suitable valve actuator mechanism for operating air intake and exhaust gas valves may be used. As a specific example, one form of a valve actuator may comprise a cam body supported for rotation about a cam body axis aligned with the first axis about which the output member rotates. The cam body may comprise at least one cam projecting from the cam body and at least one cam follower. The at least one cam and at least one cam follower are operable to open and close respective air intake and exhaust valves of the engine as the cam body rotates. The cam body may in one form comprise a cam disk with an outer periphery. The cam may comprise at least one projection extending outwardly from the outer periphery of the cam disk with the cam follower being engaged by the cam to operate the at least one of the air intake and exhaust valves. The cam body may comprise first and second major surfaces with the second major surface being positioned adjacent to the cylinders and the first major surface being positioned further from the cylinders than the second major surface. The cam may comprise at least one projection extending from the first surface and away from the second surface. Alternatively, the cam body may comprise a cam supporting projection spaced from the cam body axis and extending from the second major surface and away from the first major surface. The at least one cam may project radially inwardly from the cam supporting projection and toward the cam body axis. 
   The number of cams provided on the cam body and the rate of rotation of the cam body relative to the output member, as well as the direction of rotation of the cam body, may be varied depending upon the number of cylinders included in the engine. 
   In one example, for a one cylinder engine, the cam body may be rotated at one-half the speed of the output member and in either direction (the same or the opposite direction) relative to the direction of rotation of the output member. In this example, a first cam may be provided on the cam body in a position to selectively open and close the air intake valve for the cylinder and a second cam may be provided on the cam body in a position to selectively open and close the exhaust gas valve for the cylinder. 
   As another specific example, for a two cylinder engine with two associated pistons, the cam body may be rotated at one-half the speed of the output member and in either direction of rotation relative to the direction of rotation of the output member (in the same direction as the direction of rotation of the output member or a direction opposite to the direction of the rotation of the output member). In this example, a first cam may be provided on the cam body in a position to selectively open and close the air intake valves of both cylinders and a second cam may be provided on the cam body in a position to selectively open and close the exhaust valves of both cylinders. 
   As another example, for a three cylinder engine the cam body may be rotated at one-half the speed of the output member and in a direction which is opposite to the direction of rotation of the output member. The cam body, in this example, may include a first cam in position to selectively open and close the air intake valves of the three cylinders and a second cam in a position to selectively open and close the exhaust valves of the three cylinders. 
   As yet another example, the engine may consist of five cylinders. The cam body in this example may be rotated at a rate which is one-fourth of the rate of rotation of the output member and in a direction which is opposite to the direction of rotation of the output member. The cam body, in this example, may include a first set of two cams spaced 180 degrees apart from one another on the cam body in a position to selectively open and close the air intake valves of the five cylinders and a second set of two cams spaced 180 degrees apart on the cam body in a position to selectively open and close the exhaust valves of the five cylinders. 
   As a further example, in the case of a seven cylinder engine, the cam body may be rotated at a speed which is one-fourth the speed of rotation of the output member and in a direction which is the same direction as the direction of rotation of the output member. The cam body, in this example, may include a first set of four cams spaced 90 degrees apart on the cam body in a position to selectively open and close the air intake valves of the seven cylinders and a second set of four cams spaced 90 degrees apart on the cam body in a position to selectively open and close the exhaust valves of the seven cylinders. 
   The engine may be oriented horizontally with an oil pan positioned below the engine housing and coupled to the housing for collecting oil which is pumped to lubricate components of the engine within the housing, for example at least within the swash plate case portion, the cylinder head portion, and the cylinder case portion of the housing. 
   As mentioned above, the rotatable first and third members of the respective swash plate assemblies are desirably coupled together. In addition, as mentioned above, a coupling assembly which in one example is comprised of first, second and third coupling elements may be used for this purpose. In this example, the second coupling element may comprise a first collar portion having a longitudinal axis aligned with the first axis. A coupler such as a second collar may be slidably and drivenly coupled to the first collar portion such that the second collar rotates with the first collar portion and thereby with the second coupling element. The first rotatable member of the first swash plate assembly may be pivoted to the second collar for pivoting about a second pivot axis which is transverse to, and desirably perpendicular to, the first axis about which the output member rotates. Another coupler, such as a third collar in this embodiment, is provided and desirably surrounds a portion of the second collar. The third rotatable member of the second swash plate assembly may be pivoted to the third collar for pivoting about a third pivot axis which is transverse to, and desirably perpendicular to, the first axis. The second and third pivot axes are desirably parallel to one another. In addition, in one embodiment, the second and third pivot axes may be aligned with one another with the reciprocating portion of one of the swash plate assemblies reciprocating within the other of the swash plate assemblies to provide an extremely compact engine. The second and third collars may be shifted axially along the first axis, and desirably together to adjust the angle of the first and second swash plate assemblies relative to the first axis to thereby vary the stroke of the engine. The first collar portion and second collar may, in one specific example of an approach which allows axial movement of these components, be splined together by splines which extend in a direction parallel to the first axis such that the first collar portion and second collar may be moved relative to one another in a direction parallel to the first axis while remaining drivenly coupled together. 
   As one aspect of an embodiment, a first rotatable member of the first swash plate assembly may be coupled to the output member at a first location and the third rotatable member of the second swash plate assembly may be coupled to the output member at a second location with the first and second locations being positioned 180 degrees apart about the output member. 
   As an aspect of an embodiment, the reciprocating portion of a counterbalancing or second swash plate assembly may be comprised of a material which is heavier than the material comprising the reciprocating portion of the first swash plate assembly. As a result, the size of the second swash plate assembly may be reduced while still providing the desirable counterbalancing effect. 
   In accordance with other embodiments, a first swash plate assembly may be pivotally coupled to an output member for pivoting about a second axis which is perpendicular to the first axis about which the output member rotates. When the first swash plate assembly is in a first position, the first swash plate assembly defines a first plane at a first angle of inclination relative to a second plane perpendicular to the first axis and which intersects the second axis. The engine may comprise a mechanism operable to change the first angle of inclination and to shift the location of the second axis in a direction along the first axis to thereby vary the stroke of the engine. 
   A variable engine stroke adjuster may be included as an aspect of an embodiment and may be coupled to at least the first swash plate assembly to vary the tilt of the first swash plate assembly about the second axis and relative to the first axis so as to adjust the stroke of the engine. As a desirable aspect of an embodiment, the variable stroke adjuster may be operable to adjust the tilt of the first swash plate assembly so as to provide a minimum engine displacement for certain engine operating conditions, such as idle, and a maximum engine displacement for certain engine operating conditions, such as full power, which results in a stroke to bore rate ratio which is greater than one. More desirably, the variable engine stroke adjuster is coupled to both first and the second swash plate assemblies and is operable to vary the tilt of the second swash plate assembly relative to the first axis in the opposite direction from the change in tilt of the first swash plate assembly. 
   In accordance with an embodiment, each piston cylinder of the engine comprises a cylinder head portion and a cylinder wall portion. In addition, each piston comprises a piston head surface adjacent to the cylinder head portion of the associated cylinder in which the piston travels. Each piston repeatedly travels during a piston stroke between a top dead center position in which the piston head surface is closest to the cylinder head portion and a bottom dead center position in which the piston head surface is furthest from the cylinder head portion. In this example, the term “combustion chamber” is defined as the volume of the cylinder between the cylinder head portion and piston head surface when the piston head surface is in the top dead center position. In this embodiment, the piston, or pistons in plural cylinder embodiments, is/are coupled to a reciprocating member of a swash plate assembly such that the volume of the combustion chamber associated with the piston increases as the length of the piston stroke increases and decreases as the length of the piston stroke decreases. The term “combustion ratio” is defined as the ratio of the volume of the combustion chamber to the volume of the portion of the cylinder through which each piston travels between the top dead center position and the bottom dead center position. In a desirable embodiment, the combustion ratio is substantially constant as the stroke of the piston is varied. By substantially constant, it is meant that the combustion ratio is within plus or minus ten percent of a value for the ratio. As a specific example, the combustion ratio is about 1 to 10 for a gasoline combustion engine and 1 to 15–17 for a diesel combustion engine. 
   The engine may comprise a diesel fuel engine, wherein diesel fuel is injected into the compressed combustion air in the combustion chamber when the piston is at the top dead center position for combusting in the combustion chamber to drive the associated piston. Desirably, the quantity of diesel fuel injected into the combustion chamber is reduced with a reduction of the stroke or displacement. Alternatively, the engine may comprise a gasoline engine, wherein gasoline and combustion air is delivered as an air fuel mixture to the combustion chamber for combustion therein to drive the associated piston. Desirably, the quantity of gasoline and combustion air mixture delivered to the combustion chamber is reduced with a reduction in the volume of the stroke or displacement. As another alternative, the engine may comprise a direct injection engine which has a gasoline fuel supply which is delivered in a similar manner as fuel in a diesel fuel engine. 
   The angle of the swash plate assembly may be varied in response to at least one vehicle parameter (thus, in response to one or more such parameters). For example, the vehicle parameters may be selected from the group comprising a vehicle throttle pedal position, engine torque, engine horsepower requirements and/or to optimize fuel consumption efficiency for a given engine horsepower or torque. The engine may have a piston stroke to bore ratio which is less than one under certain engine operating conditions and which is greater than one under other engine operating conditions. For example, at highway cruising speed on flat ground, or under other conditions where the load on the engine is reduced, the stroke of the engine may be reduced. As a result, less fuel is required to operate the engine and greater fuel efficiency is achieved. 
   The operation of the swash plate engine may be controlled in accordance with a wide variety of methods. As a specific example, for a diesel engine, under idle conditions, the engine stroke may be maintained at a level which is greater than the minimum engine stroke with the fuel supply reduced. When the fuel accelerator pedal is depressed, the engine is more responsive because the stroke has not been reduced to a minimum stroke. Under coasting conditions (e.g., when a vehicle is coasting and no engine braking is desired), the fuel supply may be reduced, for example to zero and the stroke reduced toward its minimum (e.g., toward or at zero displacement) level. Under an engine braking condition (e.g., a truck is traveling downhill and it is desired to have the engine assist in braking the vehicle), the stroke may be set at a high level, for example at or toward the maximum stroke with the fuel reduced to zero. A direct injection gasoline engine may be operated, for example, in the same manner. For a gasoline engine of the type with an air throttle which regulates the supply of combustion air to the engine, under idle conditions, the engine stroke may be maintained at a level which is greater than the minimum engine stroke with the combustion air supply and fuel supply both being reduced, for example by the throttle. This improves engine responsiveness in comparison to the case if the displacement had been reduced toward or to the minimum level. In this case, the fuel and combustion air supply is increased when the engine is operated at above idle conditions. Under coasting conditions, the engine displacement is reduced (e.g., toward or at the minimum, such as zero displacement) with the combustion air supply and fuel supply reduced (e.g., toward or at a minimal level or totally closed off). This increases engine fuel efficiency under these conditions. Under engine braking conditions, the engine displacement may be set at a high level (e.g., at or toward the maximum displacement level), the engine fuel may be reduced (e.g., toward a minimum fuel level or shut off), and the air supply may be maintained at a high level. Again, other engine control approaches may also be used. 
   In one specific form of variable stroke adjuster, an engine stroke varying cylinder and piston is positioned at least partially in the center of a plurality of cylinders of the engine. More desirably, the engine stroke varying cylinder and piston may be positioned entirely between the engine cylinders. The engine stroke varying piston may be coupled to the housing and is positioned within the engine stroke varying cylinder. Delivery of operating fluid to the stroke varying cylinder at one side of the stroke varying piston moves a first output shaft section of the output member in a first direction along the first axis. Delivery of operating fluid to the stroke varying cylinder at the opposite side of the stroke varying piston moves the first output shaft section in a second direction opposite to the first direction along the first axis. The first section of the output member is correspondingly shifted relative to a second shaft section of the output member. Swash plate assemblies in this embodiment are coupled to the first output shaft section such that movement of the first output shaft section changes the angle of tilt of the swash plate assemblies relative to the first axis about which the output member rotates. As a result, the stroke of the piston or pistons of the engine is increased or decreased. 
   In another form, a drive mechanism such as at least one adjustment gear is drivenly coupled to the first section and rotatable in a first direction to shift the first section in a first direction along the first axis. The adjustment gear is rotatable in a second direction opposite to the first direction to shift the first gear in a second direction opposite to the first direction. Rotation of the adjustment gear shifts the first section in either the first or second direction depending upon the direction of rotation of the adjustment gear to thereby adjust the angle of the swash plate assemblies to vary the stroke of the engine. An endless ball bearing track may be used to couple the first section to the housing. 
   Other mechanisms may be used to adjust the swash plate angle of at least one of the swash plate assemblies to vary the stroke of the engine. Desirably, the angle of the counterbalancing swash plate assembly is also adjusted in a direction opposite to the adjustment of the other swash plate assembly to enhance the counterbalancing function performed by the counterbalancing swash plate assembly. 
   As an engine operates, a piston travels within its associated cylinder between a top dead center position and a bottom dead center position and back to the top dead center position during a piston stroke. A piston tends to exert a force or ride against a first portion of the associated cylinder (one portion of the cylinder wall) during one portion of the piston stroke and against a second portion of the cylinder (a second portion of the cylinder wall) during another portion of the piston stroke. As an aspect of an embodiment, desirably the geometries of coupling of one or more pistons to the swash plate assembly or assemblies is such that each such piston shifts from exerting a force against a first portion of the cylinder to exerting a force against a second portion of the cylinder when the piston is in either the top dead center position or the bottom dead center position. In this embodiment, the shifting of forces between sections of the cylinder wall thus takes place desirably only when the piston is changing its direction of motion as it passes through the top dead center and bottom dead center positions. 
   It should be again noted that the present invention is directed to new and non-obvious aspects of a swash plate combustion engine both alone and in various combinations and sub-combinations with one another as set forth in claims below. In addition, the embodiments described herein are provided as examples with the invention not being limited to the described embodiments. 

   
     DETAILED DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a vertical sectional view through a first embodiment of a swash plate engine. 
       FIG. 2  is a view of one form of a cylinder case portion of an engine usable in the embodiment of  FIG. 1 , looking from the right toward the left in  FIG. 1 , and showing a portion of one form of a piston rod structure positioned in one of the cylinders. 
       FIG. 3  is a view, looking from the right toward the left in  FIG. 1 , of one form of a first swash plate assembly usable in the embodiment of  FIG. 1 . 
       FIG. 4  is a vertical sectional view through the first swash plate assembly of  FIG. 3 , taken along lines  4 — 4  of  FIG. 3 . 
       FIG. 5  is a view, partially in section, of one form of a connector usable for mounting a piston rod to an associated piston rod coupler receiving projection of the swash plate assembly of  FIG. 3 . 
       FIG. 6  is a view, looking from the right toward the left in  FIG. 1 , of a second or counterbalancing swash plate assembly usable in the  FIG. 1  engine, with a portion thereof shown broken away. 
       FIG. 7  is a sectional view of the swash plate assembly of  FIG. 6  taken along lines  7 — 7  of  FIG. 6 . 
       FIG. 8  is a side elevational view of one form of piston rod usable for coupling a piston of the  FIG. 1  embodiment to a swash plate assembly. 
       FIG. 9  is a vertical sectional view through the piston rod of  FIG. 8  illustrating one form of a slide guide which restricts the piston rod against rotation about the longitudinal axis of the engine of  FIG. 1 . 
       FIG. 10  is a cross-sectional view of the piston rod of  FIG. 8 , taken along lines  10 — 10  of  FIG. 9 . 
       FIG. 11  is a side view of one form of a collar or coupling mechanism for coupling the first swash plate assembly to an engine output member, such as an output shaft. 
       FIG. 12  is a vertical sectional view of the coupler of  FIG. 11 , taken along lines  12 — 12  of  FIG. 11 . 
       FIG. 13  is a side view of a form of second coupler or collar usable for coupling a second swash plate assembly of the  FIG. 1  embodiment to the output member. 
       FIG. 14  is a vertical sectional view of the coupler of  FIG. 13 , taken along lines  14 — 14  of  FIG. 13 . 
       FIG. 15  is a first view, looking from the left to the right in  FIG. 1 , which is partially broken away, of a portion of a form of coupling member usable to interconnect the rotating members of the two swash plates of the  FIG. 1  engine. 
       FIG. 16  is a vertical sectional view through the coupling member of  FIG. 15 , taken along lines  16 — 16  of  FIG. 15 . 
       FIG. 17  is a side view, looking from the right to the left in  FIG. 1 , of one form of cam body with cams for operating the air intake and exhaust valves of the engine of  FIG. 1 . 
       FIG. 18  is a sectional view through a portion of the cam body of  FIG. 17 , taken along lines  18 — 18  of  FIG. 17 . 
       FIG. 19  is a sectional view of a portion of the cam body of  FIG. 17 , taken along lines  19 — 19  of  FIG. 17 . 
       FIG. 20  is a view of a form of cylinder case portion of a housing in which the cylinders are formed together as a unit, as by casting, and illustrating optional coolant fluid flow passageways (in dashed lines) extending between the respective cylinders. 
       FIG. 21  is a sectional view through an alternative form of cylinder case portion in which the cylinders have longitudinal axes which are at an acute angle relative to an axis of the engine. 
       FIG. 22  is a sectional view through a portion of an embodiment in which a cylinder head portion and a cylinder case portion of an engine housing are formed together, as by casting, as a single piece monolithic element and which also schematically illustrates the positioning of respective air intake and exhaust gas valves. 
       FIG. 23  is a vertical sectional view through an engine housing having a valve cover portion, a cylinder head portion, a cylinder case portion, a swash plate case portion and an output member support portion, and also illustrating the positioning of an oil pan relative to the housing. 
       FIGS. 24–26  schematically illustrate exemplary bearing arrangements for coupling a rotatable portion of a swash plate assembly to a reciprocating portion of a swash plate assembly. 
       FIG. 27  schematically illustrates a construction in which a ball bearing structure is utilized for coupling a piston rod to a reciprocating disk portion of a swash plate assembly. 
       FIG. 28  schematically illustrates an embodiment in which a universal joint is used to couple a piston rod to a reciprocating member of a swash plate assembly. 
       FIG. 28A  is an end schematic view of the  FIG. 28  construction looking down from the top of  FIG. 28 . 
       FIG. 29  schematically illustrates one form of a guide member positioned to slidably engage a piston rod with the piston rod being coupled to a reciprocating portion of a swash plate assembly, the guide member restricting the reciprocating portion of the swash plate assembly against rotation. 
       FIG. 30  schematically illustrates a form of swash plate assembly having a reciprocating portion formed of plural ring sections, in this case two such sections, and a single piece rotating swash plate assembly member. 
       FIG. 31  is a schematic sectional view of the swash plate assembly of  FIG. 30  taken along lines  31 — 31  of  FIG. 30 . 
       FIG. 32  schematically illustrates a swash plate assembly with a lubricating fluid supply for delivering lubricating fluid to the bearings of the assembly. 
       FIG. 33  schematically illustrates a construction in which a rotating track follower travels within a track to restrict a reciprocating portion of a swash plate assembly against rotation. 
       FIG. 34  schematically illustrates an embodiment of a swash plate assembly in which a reciprocating portion of the swash plate assembly is positioned inside a rotary portion of a swash plate assembly. 
       FIG. 35  schematically illustrates a swash plate assembly similar to  FIG. 34  wherein the rotary portion of the swash plate assembly is formed of plural annular pieces which are interconnected in face-to-face relationship. 
       FIG. 36  is a schematic embodiment which is similar to  FIG. 35  in which the rotary portion of the swash plate assembly is formed of plural interconnected ring sections. 
       FIG. 37  schematically illustrates an embodiment of a swash plate assembly in which a piston rod is pivoted to a projecting element which is coupled by a bearing, such as a universal bearing, to a reciprocating portion of a swash plate assembly to thereby couple the piston rod to the swash plate assembly at an off-center location. 
       FIG. 38  schematically illustrates one form of a mechanism which may be used to vary the angle of a swash plate assembly to thereby vary the piston stroke or engine displacement. 
       FIG. 39  schematically illustrates an engine construction which, like the embodiment of  FIG. 1 , provides a substantially equal combustion ratio for various swash plate assembly angles. 
       FIG. 40  schematically illustrates an alternative mechanism for varying the angle of a swash plate assembly. 
       FIG. 41  schematically illustrates a swash plate engine embodiment in which a counterbalancing swash plate assembly is positioned far enough away from a driven swash plate assembly such that the reciprocating portions of the two swash plates do not pass through or interfere with the motion of one another during operation of the engine. 
       FIG. 42  schematically illustrates a swash plate engine in which, during at least one operating position of the engine, first and second swash plate assemblies have rotating members which are in a common plane. 
       FIG. 43  schematically illustrates a portion of a swash plate engine in which a counterbalancing swash plate assembly is positioned inwardly of a driven swash plate assembly. 
       FIG. 44  schematically illustrates a swash plate engine having plural swash plate assemblies which are pivotally supported by a common shaft with links which couple rotating members of each swash plate assembly to an output member or shaft. 
       FIGS. 45–47  schematically illustrate exemplary cam body constructions for use in operating air intake and exhaust valves of a swash plate engine. 
       FIG. 48  schematically illustrates a form of a control mechanism usable in certain embodiments for controlling the angle of a swash plate assembly to vary the engine stroke in response to at least one vehicle parameter. 
   

   DETAILED DESCRIPTION 
   With reference to  FIG. 1 , one form of an internal combustion engine is illustrated which comprises at least first and second swash plate assemblies wherein one of the swash plate assemblies is a counterbalancing swash plate assembly. The engine of  FIG. 1  comprises a housing  10  which may comprise a plurality of housing sections. In the  FIG. 1  form of housing  10 , the housing comprises a valve cover portion  16  within which valves and a valve actuator mechanism is positioned. The housing  10  also comprises an engine head portion  18  comprising the head or heads to one or more cylinders included within the engine and further defining air intake and air exhaust gas passageways leading to the cylinders and which are opened and closed by respective intake and exhaust valves as described below. The illustrated housing  10  further comprises a cylinder case portion  20  within which at least one cylinder is positioned. The  FIG. 1  engine comprises a five cylinder engine for illustration. In the embodiment of  FIG. 1 , the longitudinal axes of the respective cylinders are indicated by  22  and  24  (for two cylinders shown therein). Longitudinal axes  22 , 24  extend in a first direction and, in the  FIG. 1  embodiment, the first direction is parallel to a longitudinal axis  26  of the engine. The housing  10  also comprises a swash plate engine housing portion  28  within which swash plate assemblies of the engine are positioned. The engine housing of  FIG. 10  also comprises an output support portion  30  positioned to support an output member such as an output shaft  50  of the engine. 
   The various housing components may comprise separate elements which are interconnected, such as by bolts or other fasteners, with respective gaskets or seals between the housing sections. Alternatively, and as explained in greater detail below, a plurality of the housing sections may be formed of a single monolithic one-piece construction, such as being cast together. In the embodiment of  FIG. 1 , the swash plate engine containing portion  28  and output member support portion  30  are illustrated as being of a one-piece integrated monolithic construction. In contrast, the valve cover portion  16 , the cylinder head portion  18  and the cylinder case portion  20  in  FIG. 1  are illustrated as separate components which are interconnected with the other components of the housing to form the overall housing  10 . 
   At least one cylinder, as previously mentioned, is included within the engine housing in the case of a single cylinder engine. For plural cylinder engines, a plurality of cylinders are provided. A respective reciprocatable piston is positioned within each of the cylinders included in the engine for reciprocation therein. In the engine of  FIG. 1 , a piston  36  is positioned for reciprocation within a cylinder  38  which has longitudinal axis  22 . In addition, a piston  40  is positioned for reciprocation within a cylinder  42  which has the longitudinal axis  24 . 
   A rotatable output member is coupled to the housing and rotatable about a first axis. In the embodiment of  FIG. 1 , the rotatable output member comprises an output assembly rotatably coupled by bearings  46  to a bearing retaining portion  48  of the housing portion  30 . The illustrated output assembly includes a first output shaft section  50  supported by the bearings  46  for rotation about an output axis which, in  FIG. 1 , corresponds to the longitudinal axis  26  of the engine. The axis about which the output member  50  rotates may be defined as a first axis regardless of whether it is coextensive or aligned with the longitudinal axis of the engine. The output assembly of  FIG. 1  also comprises a second output shaft section  52  which is coupled to output shaft section  50 , such as by a bolt  54 , such that the shaft sections  50 , 52  rotate together. The rotating output sections  50 , 52  may be coupled in any convenient manner to a drive axle or other power utilization apparatus of the engine. For example, a fly wheel with a gear  58  for coupling to an engine starter motor may be fastened, such as by bolts, to output section  50  for use as a power output device. 
   The engine of  FIG. 1  comprises at least first and second swash plate assemblies with two such assemblies being indicated respectively at  60  and  70  in  FIG. 1 . Each of these assemblies  60 , 70  is shown positioned within the swash plate portion  28  of housing  10 . In the  FIG. 1  embodiment, each of the first and second swash plate assemblies  60 , 70  are positioned closer to an end portion  72  of the engine than the engine cylinders (e.g., cylinders  38 , 42  and the other cylinders of the engine). Thus, all of the cylinders are at the same side of the swash plate assemblies. This is a desirable option as it reduces the overall length of the engine in comparison to an embodiment wherein some cylinders are disposed at one side of the swash plate assemblies and other cylinders are disposed at the opposite side of the swash plate assemblies. 
   Swash plate assembly  60  comprises a first member  62  and a second member  64  (which members may take forms other than those shown in  FIG. 1 ). The first member  62  is rotatably coupled to the second member  64 , such as by bearings  66 . Bearings  66  may comprise, for example, ball bearings or conical barrel bearings. The bearings may also be friction bearings in the form of bearing surfaces that slide in contact with one another. These friction bearings may be lubricated using a pressure lubrication system, such as in the form described in an embodiment below. As a result, the first member  62  is rotatable relative to the second member  64  and about the first axis, in this case axis  26 . In addition, as explained in greater detail below, the second member  64  is coupled to the housing  10  so that the second member is restrained against rotation. However, the second member is capable of reciprocation, and as it reciprocates it drives the first member in rotation. The first member may be pivotally coupled to the output member for pivoting about a second axis which is transverse to the first axis  26 . In the embodiment of  FIG. 1 , this second axis is indicated at  68 , is perpendicular to axis  26 , and extends into the page in  FIG. 1 . As explained below, the engine of  FIG. 1  has variable stroke capabilities with the stroke being varied by varying the angle of the first swash plate assembly relative to the first axis. In a less desirable embodiment, wherein the variable stroke feature is eliminated, the first member need not be pivoted to the output member. As a result of the connection of the first member to the output member, the first member  62  is rotated to thereby rotate and drive the output member as the second member  64  is reciprocated. In the  FIG. 1  embodiment, the first member  62  is pivoted to a coupling element such as a portion of a collar assembly indicated generally at  74 . The collar assembly in the  FIG. 1  embodiment is coupled to the output section  52 . For example, collar assembly  74  and output section  52  may have splines indicated at  76  which are aligned with axis  26 . This permits collar assembly  74  to shift axially relative to section  52  with these components being drivingly interconnected. 
   A piston rod  80  is pivotally coupled to piston  36  and also pivotally coupled to the second member  64 . Similarly, a piston rod  82  is pivotally coupled to piston  40  and to the second member  64 . In the same manner, each piston of the engine is coupled by an associated piston rod to the second member. The piston rods reciprocate with the reciprocal movement of the piston. Reciprocal movement of the pistons and piston rods results in reciprocal movement of the second member  64 . This reciprocal movement of the second member causes rotation of the first rotatable member  62  and the output section  50  about the first axis. In  FIG. 1 , the first swash plate assembly  60  is shown generally aligned in a first plane  86  which is at a first angle α with respect to the axis  26 . 
   The second swash plate assembly is shown in solid lines in  FIG. 1  generally aligned with a plane  88  which is at an angle β relative to the first axis. As a result, the swash plate assemblies  60  and  70  are generally at angles which are opposite to one another such that the reciprocating movements of the swash plate assemblies are counterbalanced by one another. The second swash plate assembly  70  comprises a third rotatable member  90  and a fourth member  92 . The third member  90  is rotatably coupled to the fourth member  92  for rotation relative to the fourth member and about the first axis, in this case axis  26 . The fourth member  92  is coupled to the housing  10 , such as explained below, so that the fourth member is restrained against rotation. The third member  90  may also be pivotally coupled to the output member for pivoting about a third axis which is transverse to the first axis. In this example, member  90  is pivotally coupled to section  50 , in this case via components  74  and  96 , for pivoting about an axis  94  which is not only transverse to the axis  26 , but in this example is perpendicular to axis  26  and extends into the page of  FIG. 1 . As a specific example, in  FIG. 1  the member  90  is pivoted to a collar assembly  96  carried by collar assembly  74 . The term collar in this description encompasses a component which at least partially surrounds another component as well as a component which entirely surrounds another component such as shown by the specific embodiment of  FIG. 1 . In addition, the rotatable member  90  is coupled by a coupling assembly to the rotatable member  62  such that member  90  and member  62  rotate together and about the axis  26 . Rotational movement of the third member results in reciprocal movement of the fourth member  92 . Although not required, in  FIG. 1  the axes  68  and  94  are spaced apart from one another. In addition, these two axes lie in a common plane which also contains the axis  26 . That is, axes  68  and  94  in the  FIG. 1  embodiment are parallel to one another. 
   With the construction shown in  FIG. 1 , as the second member  64  reciprocates due to the reciprocation motion of the pistons, the member  92  also reciprocates. Because of the respective oppositely angled inclinations of the swash plate assemblies  60  and  70 , the reciprocating members of these two swash plate assemblies, namely members  64  and  92 , reciprocate in opposite directions to counterbalance one another. Bearings, such as ball bearings or conical barrel bearings  93 , or friction bearings, may also rotatably interconnect members  90 , 92  of the swash plate assembly  70 . 
   A variety of alternative constructions may be utilized to restrict the motion of members  64  and  92  of the respective swash plate assemblies to reciprocation without rotation about the axis  26 . 
   For example, at least one piston rod motion confining member coupled to the housing (such as coupling members  116 , mounted to cylinder case portion  20 , two of which are numbered in  FIG. 2 , with piston rod engaging surfaces  119 , 121 ) and to a piston rod (e.g., to rod  82 ) may be used to engage the piston rod (e.g., to engage guide surfaces  120 , 122  of piston rod  80 ,  FIG. 2 ) to limit the motion of the piston rod to reciprocation without rotation about the first axis. Since the second member  60  is coupled to the housing by the piston rod motion confining member, and because the piston rod is thereby restricted against rotation about the first axis  26 , the second member  64  is also restricted against rotation about axis  26  and thus its motion is limited to reciprocation. The piston rod engaging guide restricts the piston rod against rotation about the first axis and thereby confines the second member  64  to reciprocation without rotation about the first axis. 
   Consider FIGS.  2  and  8 – 10 .  FIG. 2  illustrates the cylinder case portion  20  of the engine of  FIG. 1  having five cylinders  38 ,  110 ,  112 ,  42  and  114 . The respective cylinders in this embodiment are shown interconnected by coupling or sliding friction members, two of which are indicated at  116 . The underside of piston  36  (looking from the right in  FIG. 1 ) and cylinder  38  is illustrated in  FIG. 2 . A specific exemplary form of a piston rod  80  is also shown in this figure. Piston rod  80  comprises sliding member engaging surfaces  120 , 122  which are positioned to slide against respective portions (e.g., surfaces  119 , 121 ) of the members  116 . The engaging surfaces  120 , 122  are opposed from one another along a line  123  which is perpendicular to a line  125  through the axis  26 . In  FIG. 2 , axis  26  extends into the page. With this construction, the piston rod  80  is restricted against rotational movement about the axis  26 . However, the piston rod  80  may travel in directions along line  125 . Correspondingly, member  64  of the first swash plate assembly  60  ( FIG. 1 ) is also restrained against such rotational movement and is restricted to reciprocation.  FIGS. 8–10  illustrate the piston rod  80  of the  FIG. 2  form in greater detail. The surface  122  of piston rod  82  is also shown in  FIG. 1 . More specifically, the form of piston rod  80  shown in  FIGS. 8–10 , as best seen in  FIG. 9 , is comprised of first and second separate piston rod sections  81 , 83  which abut one another at respective ends  115 , 115   a  thereof and which are spaced apart from one another at the respective opposite ends  117 , 117   a  thereof. A pin receiving opening  191  is defined through ends  115 , 115   a . Opening  191  is aligned with an opening  89  through piston  36 . The piston rod sections  81 , 83  are connected, in this example, to piston  36  by inserting a retaining pin  95  through openings  89 , 91 . Retainers, such as snap rings  97 , 99  disposed in respective grooves at the respective ends of opening  89 , hold pin  95  in position. Ends  117 , 117   a  have respective openings  85 , 87  aligned along an axis which is parallel to the axis of openings  89 , 91 . Projections  220 , 221  of a coupler  194  (described below) are respectively disposed within the respective openings  85 , 87  so that the coupler  194  is captured by the piston rod  80  and is pivotal about the axis defined by openings  85 , 87 . Coupler  194  has an opening  191  for receiving a projection (e.g., a post  190  FIGS.  1 , 5 ) from the reciprocating swash plate member  64  ( FIG. 1 ). This latter exemplary coupling approach is described in greater detail below. 
     FIG. 29  illustrates an alternative form of piston rod confining member. In the embodiment of  FIG. 29 , a piston rod guide channel or slot  130  is defined by members  132 , 134  which are rigidly coupled to the housing, such as to cylinder case portion  20  or swash plate receiving portion  28 . The piston rod is slidably received within the guide channel  130  with the channel being oriented to restrict piston rod  80  against rotation about axis  26  while permitting reciprocation of the piston rod therein. Although only one piston rod is shown in  FIG. 29 , similar guide channels may be provided for each of the other piston rods in a plural cylinder engine construction. Collar  74  may shift axially, as indicated by respective arrows  135 , 137 , as the swash plate assembly angle is varied. Alternatively, selected one or more piston rods may be restricted against rotary motion with the mechanical interconnection of the components thereby restricting all of the piston rods to reciprocation without rotation. 
   As another option, a rotating restriction assembly such as a track and track follower assembly may be used to restrict the motion of reciprocating members of the swash plate assembly to reciprocation without rotation. This construction may be utilized for each or for only one of the swash plate assemblies. For example, in  FIG. 1 , a track and track follower mechanism is used only for the counterbalancing swash plate assembly  70 . In this example, with reference first to  FIG. 1 , the housing (in this case the swash plate receiving portion of the housing  28 ) comprises a channel or track, one wall of which is indicated at  140  in  FIG. 1  with the base of the track indicated at  142 . The illustrated track is generally arcuate in shape and is mounted to the interior wall of housing section  28 . A track follower, such as indicated at  144 , is coupled to the reciprocating member  92  of swash plate assembly  70 . The track follower  144  is positioned to engage the track. The track and track follower cooperate to confine the motion of the fourth member  92  to reciprocal motion without rotation about the first axis. Although other track follower mechanisms may be used, in the  FIG. 1  form, a slide shoe  146 , which have a durable low friction material surface such as of a conventional copper-bronze alloy, is coupled by a bearing  148 , such as a needle bearing or other suitable bearing, to a projecting support  150  extending from the perimeter of reciprocating member  92  of the swash plate assembly  70 . This same low friction material may be used for other low friction surfaces mentioned in this description, although such surfaces are not limited to this material. An alternative construction is indicated schematically in  FIG. 33 . In the  FIG. 33  construction, the track wall is indicated at  140 ′ and the track base is indicated at  142 ′. The track follower is shown at  144 ′ and comprises a rotating or roller member  146 ′ coupled by a bearing  148 ′ to a support structure  150 ′. The structure  150 ′ is coupled to the reciprocating member  64  of the swash plate assembly  60 . In the example of  FIG. 33 , piston rod confining elements which restrict the rotation of the piston rod about the first axis  26  may be eliminated. The use of a prime designation (′) in this description in connection with a number indicates that the element corresponds to a previously described similar element designated by the same number without the prime (′). Referring again to  FIG. 1 , the track follower in the  FIG. 1  embodiment thus comprises a slide member which slides in engagement with a channel having low friction track follower engaging wall surfaces. 
   An exemplary first swash plate assembly  60  is shown in FIGS.  1  and  3 – 5 . More specifically, as illustrated in  FIGS. 3 and 4 , the rotatable or first member  62  of swash plate assembly  60  comprises an annular structure with a central opening  160  and also comprises an outwardly facing annular surface  162  ( FIG. 4 ). Member  62  has outwardly projecting bearing capture portions  164 , 166  with respective outwardly facing surfaces  168 , 170  in which the bearings  66 , in this case conical bearings, are seated. The reciprocating member  64  of swash plate assembly  60  has an inwardly facing annular surface  172  ( FIG. 4 ). The illustrated member  64  has an inwardly projecting central section  174  of generally trapezoidal cross-section. Section  174  has side surfaces  176 , 178  which are also inwardly facing and opposite to the respective surfaces  168 , 170 . The bearings  66  are positioned between these opposed surfaces. Projecting portions  180 , 182 , extend in a direction perpendicular to a plane  86 . The plane  86  in this example bisects the swash plate assembly  60 . Portions  180 , 182  assist in retaining the bearings  66  in place. 
   The stationary member  64  of swash plate assembly  60  comprises a respective projection for coupling to an associated piston rod of the engine with one such projection being provided for coupling to each piston rod. Thus, in the  FIGS. 3 and 4  embodiments, for a five cylinder engine, there are five such projections equally spaced about the perimeter of member  64  and each designated by the number  190  in  FIGS. 3 and 4 . One such projection is also indicated at  190  in  FIG. 1 . The respective projections  190  are coupled to the piston rods. In the construction shown in  FIGS. 1 ,  3  and  4 , an offset coupling approach is used for coupling the respective piston rods to the projections  190 . More specifically, an annular coupler  194  ( FIG. 1 ) is rotatably coupled by bearings  196  to a respective post  190  with a similar coupler being provided for each of the posts or projections. The bearings  196  are positioned within an opening  191  defined by the coupler  194 .  FIG. 5  illustrates an exemplary coupler  194 . A washer  200 , with a low friction surface, is positioned between coupler  194  and a shelf portion  202  ( FIG. 4 ) of member  64 . A second washer  204  ( FIG. 5 ), with a low friction surface, is positioned between a retaining cap  206  and the coupler  194 . Cap  206  has a projecting portion  208  designed to fit within a recess  210  ( FIG. 4 ) in the distal end of the associated post or projection  190 . Cap  206  is fastened in place, such as by a bolt  212 , to secure the coupler  194  to the associated post  190 . A projecting piston rod receiving projection  220  (one being shown in  FIG. 1  and in  FIG. 5 ) extends outwardly from the side of coupler  194  and supports bearings  222 . A piston rod receiving projection  221  extends outwardly in a direction opposite to projection  220  and supports bearings  222 . Piston rod end  117  is pivotally coupled to projection  220  and piston rod end  117   a  is pivotally coupled to projection  221 . The piston rod ends are held in place in this construction, following their assembly onto coupler  194  because piston rod section ends  115 , 115   a  ( FIG. 9 ) are captured and held together by the associated piston and coupling pin (e.g.,  36 , 95  in  FIG. 9 ). Washers with low friction surfaces may be positioned between the piston rod end sections  117 , 117   a  and adjacent components. With the connection of the piston rod  80  to the reciprocating member  64  of swash plate assembly  60 , reciprocation of the piston rod causes a corresponding reciprocation of the member  64 . This in turn drives the rotatable member  62  of the swash plate assembly  60  in rotation about the axis  26 . 
   The rotating member  62  of swash plate assembly  60  (as best seen in  FIGS. 3 and 4 ), define respective openings  230 , 232  which are desirably of circular cross-section and have longitudinal axes which are aligned with the pivot axis  68 . Respective bearings, such as tilt bearings  234  and  236  ( FIG. 3 ), are received within the respective openings  230 , 232 . The bearings  234 , 236  pivotally couple the swash plate assembly  60  to the output member for pivoting about the pivot axis  68 . More specifically, bearings  234 , 236 , in the construction shown, pivot the swash plate assembly  60  to the collar assembly  74  ( FIG. 1 ) which is coupled to the output shaft or member as explained in greater detail below. 
   In the construction of  FIG. 1 , the rotating member  62  of swash plate assembly  60  is coupled to the rotating member  90  of swash plate assembly  70  by a rotating member coupling assembly. One form of such a coupling assembly is indicated generally at  250  in  FIG. 1 . A connector is provided on rotary member  62  for coupling to the coupling assembly. In the form shown, the connector comprises first and second spaced apart flanges  252 , 254  ( FIG. 3 ) which project from one major surface of rotary member  62 . Flanges  252 , 254  are each provided with a respective coupling pin receiving opening  256 , 258  for purposes explained below. In the construction shown in  FIGS. 3 and 4 , flanges  252 , 254  define a link receiving gap  260  therebetween. The gap  260  in this construction is centered on an axis  261  which is perpendicular to the pivot axis  68 . The axis  262  intersects the axis  68  at the location of output pivot axis  26 , which extends into the page in  FIG. 3 . 
   In the embodiment of  FIGS. 1 ,  3  and  4 , at least a major portion of the rotary member  62  is positioned inside the reciprocating member  64  of the swash plate assembly. Alternatively, a major portion of rotary member  62  may be positioned outside of member  64 . In this example, the piston rod connection posts would extend inwardly instead of outwardly. That is, depending upon the construction, at least a major portion, and some cases substantially all, of the rotary member  62  is positioned inside the reciprocating member  64 . Alternatively, the surface  162  ( FIG. 4 ) may be inwardly directed with a major portion, and in some cases substantially all, of the rotary member  62  being positioned outside the reciprocating member  64 . In the same manner, in swash plate assembly  70  ( FIG. 1 ), a major portion of reciprocating member  92 , and desirably substantially all of the reciprocating member, may be positioned either outwardly or inwardly of rotating member  90 , depending upon the construction. 
   Desirably, at least one of the members  62  and  64  are of a plural piece construction. This facilitates the assembly of the swash plate mechanism and the positioning of the bearings, if used, between the respective members  62  and  64 . For example, the rotating member  62  may be comprised of first and second sections  262 , 264  ( FIG. 4 ) which are sandwiched together and interconnected, such as by bolts, some of which are indicated at  266 , to comprise the first member. Thus, in the  FIG. 4  construction, the first member  62  is formed of two such sections placed together in face-to-face relationship. Each of these sections define a portion of the annular rotating surface  162 . As explained in greater detail below, desirably at least one of the third and fourth members  90 , 92  ( FIG. 1 ) of the second swash plate assembly are also comprised of a plurality of interconnected sections to facilitate the positioning of bearings between such members. 
   With reference to  FIGS. 6 and 7 , in the construction shown, the reciprocating member  92  comprises first and second ring sections  270 , 272  which are interconnected, such as by fasteners or bolts indicated at  274 , to complete the member  92 . Thus, each of the ring sections  270 , 272  define a portion of an annular rotating surface as described below. The member  90  rotates relative to the annular rotating surface. With reference to  FIGS. 6 and 7 , rotating member  90  in this example, comprises an annular member which defines an outwardly facing annular rotating surface  280 . Member  90  includes side leg portions  282 , 284  which define a portion of the surface  280  and which include inwardly directed distally positioned bearing retaining flanges indicated respectively at  286  and  288 . The reciprocating member  92  comprises an inwardly directed annular surface  290  which generally faces the surface  280 . Member  92  includes a central trapezoidal portion  292  with respective bearing engaging side surface  296 , 298 . The bearings  93  are positioned between leg portions  282 , 284  and the surfaces  296 , 298 . Thus, in the embodiment shown, at least a major portion and desirably substantially all of the rotary member  90  is positioned inwardly of the reciprocating member  92 . Also, at least a major portion and desirably substantially all of member  92  is outwardly of member  90 . In alternative constructions, at least a major portion of the member  92  may be positioned inwardly of the member  90  with the respective annular surfaces  280 , 290  then facing generally in the opposite directions. 
   The member  90 , in this example, comprises inwardly extending projections  300 , 302  ( FIG. 6 ) each of which defines a respective circular opening  304 , 306 . The openings  304 , 306  have longitudinal axes which are aligned with the pivot axis  68 . Respective pivot pins  308 , 310  are positioned within the respective openings  304 , 306 . Pins  308 , 310  may be retained in place by set screws, pins or other fasteners. Transversely extending openings (not numbered) are shown in members  300 , 308  and  302 , 310 , which may be used for this purpose. Alternatively, pins  308 , 310  may be pivoted to the respective projections  300 , 302 . Respective coupling members, such as flanges  312 , 314 , are mounted to rotating member  90  and project outwardly therefrom. Each member  312 , 314  includes a respective pin receiving opening  316 , 318 . The members  312 , 314  are used in coupling the rotary member  90  of swash plate assembly  70  to the rotary member  62  of swash plate assembly  60  (in the embodiment of  FIG. 1 , this coupling is accomplished by coupling assembly  250 ) as explained below so that these rotary members rotate together. The projections  312 , 314  are symmetric with respect to a line  320  which intersects the rotation restriction track follower  144 . Line  320  is perpendicular to the pivot axis  68 . Line  320  intersects the axis  68  at the location of pivot axis  26 , which extends into the page in  FIG. 6 . 
   As best seen in  FIG. 6 , the swash plate assembly  70  defines an interior passageway  322 . This passageway is also shown in  FIG. 1 . As the swash plate assembly  60  ( FIG. 1 ) is driven to rotate the output member  52  ( FIG. 1 ), the reciprocating member  64  of swash plate assembly  60  reciprocates back and forth. Similarly, because of the coupling of the rotary members  62  and  90  ( FIG. 1 ) together, as explained in greater detail below, at the same time the member  92  reciprocates in generally the opposite direction to the member  64 . The passageway  322  provides clearance such that the member  64  ( FIG. 1 ) and swash plate assembly  60  may pass through the passageway  322  of swash plate assembly  70  at least in part as the engine is driven. This occurs at least during certain operating positions of the engine of  FIG. 1 . For example, looking at the lower portion of the engine of  FIG. 1 , it is apparent that a portion of reciprocating member  64  (and thereby of swash plate assembly  60 ) has passed through the passageway  322 . That is, the portion of the member  64  (and thereby of swash plate assembly  60 ) at the lower portion of the engine is to the right of the adjacent portion of the reciprocating member  92  of swash plate assembly  70 . This construction allows for a more compact engine as the swash plate assemblies need not be spaced apart far enough to avoid traveling past one another during all engine operating positions. Alternatively, the pivot axes  68  and  94  may be spaced far enough apart that these reciprocating portions of the swash plate engine need not travel past one another, although this is less desirable. 
   Thus, in the  FIG. 1  construction, the counterbalancing swash plate assembly  70  passes within at least a portion of the driven swash plate assembly  60  during certain operating positions of the engine. In other constructions, as explained more fully below, the axes  68  and  94  may be aligned with one another. 
   Although the swash plate assemblies  60 , 70  may be mounted directly to the output shaft with or without a pivotal coupling to the output shaft, in the  FIG. 1  construction, intermediate couplers are utilized to interconnect the swash plate assemblies and the output member  50 . With reference to  FIGS. 11–14 , a first coupler comprises a collar mechanism  74  shown in  FIGS. 11 and 12 . The collar  74  comprises first and second outwardly projecting swash plate assembly supporting projections  330 , 332  which, in this example, are circular in cross-section. Projection  330  is coupled by bearing  232  ( FIG. 3 ) to the first swash plate assembly  60  ( FIG. 1 ). Projection  332  is coupled by bearing  230  ( FIG. 3 ) to the first swash plate assembly. Thus, collar assembly  74  supports the first swash plate assembly for pivoting about the pivot axis  68  as shown in  FIG. 11 . One end portion of collar assembly  74  defines an annular surface  334  which supports the collar assembly  96  as can be seen in  FIG. 1 . A lower portion  338  of the collar  74  shown in  FIG. 11  may be threaded to receive a keeper  340  ( FIG. 1 ) which retains the collar assembly  96  in place. A stop, such as an enlarged shelf portion  342  of collar  74  cooperates with keeper  340  to retain the collar  96  at the appropriate location on collar  74 . One or more splines  335  may be positioned on the surface  334  ( FIG. 12 ) of the collar  74 . These splines  335  extend axially in a direction parallel to axis  26 . Desirably, one or more axially extending mating splines  337 , projecting inwardly from the interior surface of collar  96 , interfit with splines  335  so that collars  74 , 96  rotate together. Other mechanisms may be used to couple components  74 , 96  together. The upper end portion  350  of collar  74  in the  FIG. 11  embodiment defines an interior chamber  352  ( FIG. 12 ) for use in one embodiment of a mechanism described below for varying the stroke or displacement of the engine. A central shaft receiving passageway  354  ( FIG. 12 ) is also provided within the interior of collar assembly  74 . The chamber  352  and opening  354  are aligned with the axis  26  in this embodiment. 
   The collar  96  is best understood with respect to  FIGS. 13 and 14 . The illustrated collar  96  includes a central opening  360  having a longitudinal axis which is aligned with the axis  26 . Opening  360  receives the surface  334  of the collar  74  with splines  335 , 337  in interfitting engagement. In addition, the collar  96 , in the form shown, includes first and second projections  361 , 363  each having a respective internal passageway  362 , 364  extending therethrough and communicating with the passageway  360 . The longitudinal axes of passageways  362 , 364  are aligned with the pivot axis  94 . In addition, passageway  362  includes an outer section  366  of an enlarged diameter for receiving a tilt bearing or other bearing or bushing for pivotally coupling the collar to the swash plate assembly  70 . In the same manner, passageway  364  includes an outer end portion with an enlarged passageway  368  for receiving a similar bearing or bushing. With reference to  FIGS. 6 and 13 , the pin  310  is received within passageway  366  with the bearing disposed between the pin and the wall of the passageway. Similarly, the pin  308  is received in the passageway  368  with the bearing disposed between the pin and wall of the passageway. As can be seen in  FIG. 14 , a recess  370  is provided in collar  96  to accommodate the inclination of the first swash plate assembly  60 . The operation of recess  370  to provide clearance for swash plate assembly  60  is shown in  FIG. 1 . 
   With reference to  FIGS. 1 ,  15  and  16 , and as previously mentioned, a mechanism is provided in the embodiment of  FIG. 1  for interconnecting the rotating member  62  of the first swash plate assembly  60  to the rotating member  90  of the second swash plate assembly  70 . A plurality of links or other coupling elements may be used for this coupling purpose. In one form specifically shown in  FIGS. 15 and 16 , the output section  52  comprises a portion of the coupling mechanism. Specifically, the illustrated output section  52  comprises a cylindrical or collar portion  380  projecting away from output section  50  in  FIG. 1 . The illustrated collar portion  380  comprises a right cylinder with a longitudinal axis which is coincident with the first axis  26 . Elongated splines  76  project outwardly from the outer surface of collar portion  380 . Splines  76  engage corresponding inwardly projecting splines of the collar  74  ( FIG. 1 ) to permit axial sliding motion of the collar  74  relative to collar portion  380  while maintaining these components drivenly connected together. That is, collar  74  rotates with the rotation of collar portion  380  and the output section  52 . The illustrated member  52  comprises first and second upwardly extending spaced apart legs  382 , 384 , each with a respective opening  386 , 388  extending therethrough. The leg  384  is visible in  FIG. 1 . Section  52  also comprises first and second projecting leg portions  390 , 392  positioned at the opposite side of a plane  394  bisecting member  52  from the projections  382 , 384 . Plane  394  also intersects the first axis  26 , which extends into the page in  FIG. 15 . Leg  390  terminates in a projection  395  of circular cross-section having an axis which extends in a direction parallel to the plane  394 . An enlarged shelf or stop  396  is positioned inwardly of the distal end of projection  395 . Leg  392  terminates in an outwardly extending projection  398  which is also of circular cross-section and which has an axis which is parallel to the plane  394 . The projection  398  extends in an opposite direction from projection  395 . An enlarged stop or shelf  400  is positioned inwardly of the distal end of projection  398 . The projection  398  is shown in  FIG. 1 . A link  402  ( FIG. 1 ) pivotally couples the projections  382 , 384  to the projections  256 , 258  ( FIG. 3 ) of the first swash plate assembly. Second links, one being indicated at  404  in  FIG. 1 , pivotally interconnect the respective projections  395 , 398  to the projections  312 , 314  ( FIG. 6 ) of the second swash plate assembly. As a result, the rotating members of the two swash plate assemblies are interconnected to rotate together with the rotation of section  52  and the output member  50 . 
   The illustrated construction has a desirable geometry. That is, whether one or more pistons are included in the engine, such as a plurality of pistons as shown in the  FIG. 1  embodiment, a respective piston and piston rod is associated with each cylinder. Each piston repeatedly travels within its associated cylinder between a top dead center position and a bottom dead center position and back to the top dead center position during a piston stroke. During normal operation of an internal combustion engine, the piston exerts a force against a first portion of the wall of the associated cylinder during one portion of a piston stroke and against a second portion of the cylinder wall during another portion of a piston stroke. With the illustrated geometry, a swash plate engine is disclosed wherein each piston shifts from exerting a force against the first portion of the cylinder wall to the second portion of the cylinder wall when the piston is either in the top dead center or bottom dead center position. This improves the wear and reduces the noise of the engine and holds true in the illustrated construction in embodiments where the stroke of the engine is varied. It will be apparent to those of ordinary skill in the art that other geometries may be utilized which still achieve this desirable result. Although desirable, it is possible to construct an engine incorporating inventive features of this disclosure without this feature. As a desirable property of the engine of  FIG. 1 , the stroke of the engine may be varied, for example as the engine operates. In addition, as the stroke of the engine is varied, the extent of counterbalancing provided by the counterbalancing swash plate assembly may also be varied to provide improved counterbalancing benefits. 
   Referring again to  FIG. 1 , in this figure the respective swash plate assemblies  60  and  70  are shown in solid lines in the maximum displacement position or maximum stroke position of the engine. When in this position, the plane  86  defined by the first swash plate assembly is at an angle of inclination of α relative to the first axis. In addition, the plane  88  defined by the counterbalancing swash plate assembly  70  is at an angle β relative to the first axis about which the output member rotates, in this case axis  26 . In addition, α and β are, in the construction shown in  FIG. 1 , opposite to one another. A mechanism is desirably provided for changing the angle α to thereby vary the stroke of the engine. Desirably, the angle β is also changed in the opposite direction from the change in the angle α. By shifting the location of the pivot axis  68  along axis  26  toward at least one of the piston cylinders, the angle α increases toward 90 degrees. At the same time, in the construction shown in  FIG. 1 , because of the manner of coupling the second swash plate assembly  70  via collar  96  to collar  74 , the angel β also shifts in the opposite direction toward 90 degrees. When the engine is in its minimum stroke or displacement position, the plane  88  is shifted to the dashed line position shown by the number  88 ′ in  FIG. 1  and the second swash plate assembly  70  is also shifted to the dashed line position shown by the number  70 ′ in  FIG. 1 . In addition, the first swash plate assembly  60  has been shifted to the dashed line position shown by the number  60 ′ in  FIG. 1  with the plane  86  being shifted to the dashed line position indicated by the number  86 ′ in  FIG. 1 . This shifting of the angle of inclination of the swash plate assembly  60  to vary the stroke of the engine can be accomplished in any suitable manner. 
   In the specific approach shown in  FIG. 1 , a fluid actuated cylinder mechanism is utilized for accomplishing this stroke variation. In the embodiment of  FIG. 1 , the section  350  of collar  74  is mounted to a shaft  410  having a longitudinal axis aligned with the axis  26 . Shaft  410  has an enlarged head portion  412  slidable within the interior of a cylinder  414  which is rotatably coupled by bearings  416 , 418  to the engine housing. The cylinder  414  is also fastened, as by a bolt  420 , to a drive pulley  422  useful in driving other components of a vehicle (e.g., air conditioning, alternator, etc.) or of another apparatus in which the engine is used. The exterior surface of section  350  is rotatably coupled to a wall section  424  (which may be a low friction surface) of a portion of the cylinder case portion  20  of the housing. Wall section  424  defines a pocket  426  within which collar section  350  may slide. Thus, section  350  may move axially in the direction of axis  26  while rotating relative to the housing. A piston  428  fixedly mounted to the housing is disposed within the interior of chamber  352 . A cap  430  closes the end of the chamber  352 . A first fluid supply passageway  432  communicates with the interior chamber  352  at one side of piston  428 . A second fluid supply passageway  434  communicates with chamber  352  at the opposite side of piston  428 . Pressurized fluid from a source (not shown) is delivered through one of the passageways  432 , 434  while being bled from the other of the passageways to shift the collar section  350  in a first direction. Fluid is delivered to the opposite passageway while bled from the other passageway to shift the section  350  in the opposite direction. For example, by delivering fluid under pressure through line  432  to the chamber  352  at the left side of piston  428  in  FIG. 1 , while bleeding fluid through line  434 , the section  350  is shifted to the left with its maximum leftward shifted position being indicated by the dashed line  350 ′ in  FIG. 1 . The dashed line  412 ′ in  FIG. 1  indicates the leftwardmost position of enlarged head  412  of shaft  410 . As section  350  is shifted to the left in  FIG. 1 , the angle α increases towards 90 degrees and the angle β also increases towards 90 degrees, eventually reaching the dashed positions  60 ′ and  70 ′ shown in  FIG. 1 . The position of the swash plate assemblies  60 , 70  may be varied to any location intermediate the maximum and minimum displacement positions illustrated in  FIG. 1 . As explained below, the angle of the swash plate assembly may be varied in response to at least one vehicle parameter such as a vehicle throttle pedal position. 
   As another example, each engine cylinder has a bore. In the construction shown in  FIG. 1 , the ratio of the piston stroke to the bore may be less than one under certain engine operating conditions and greater than one under other engine operating conditions. For example, under high torque and/or high horsepower engine demand conditions, the ratio may be greater than one. As another example, the ratio may be greater than one under conditions where it is desirable to use the engine in braking the vehicle, such as when traveling downhill. As another example, the ratio may be greater than one at first vehicle speeds and less than one at other vehicle speeds. For example, at highway cruising speed on level roadways, the ratio may be less than one (e.g., at 55 miles per hour). Fuel throttle position is another vehicle parameter which may be sensed and used in controlling the ratio of piston stroke to bore. Less fuel is needed to power the engine at lower piston stroke to bore ratios and the engine fuel consumption is more efficient. Combinations of one or more of these and other vehicle parameters may be used in controlling the engine displacement. 
   As a more specific example, at low horsepower conditions where the engine is not being used in braking the vehicle, the stroke to bore ratio may be from 0.3 to 0.8 although the engine is not limited to this example. As another specific example, and without limiting the generality of the engine, the bore of a typical cylinder may be 80 mm. At engine idle condition, the engine may be adjusted to provide a 30 mm stroke. At highway speeds, the engine may be adjusted to provide a 60 mm stroke. At full load (high torque conditions), the engine may be adjusted to provide a stroke of 100 mm. 
   Each cylinder included in the engine comprises a cylinder head portion, such as indicated at  440  for cylinder  42  and a cylinder wall portion. The piston also comprises a piston head surface  442  (for piston  40  in  FIG. 1 ) which is adjacent to the cylinder head portion of the associated cylinder within which the piston travels. The piston repeatedly travels during a piston stroke between a top dead center position in which the piston head surface is closest to the cylinder head portion and a bottom dead center position in which the piston head surface is furthest from the cylinder head portion. The term “combustion chamber” is defined as the volume of the cylinder between the cylinder head portion and the piston head surface when the piston head surface is in the top dead center position. A piston stroke length adjuster, which may be in the form described above, is coupled to at least the first swash plate assembly (e.g.,  60 ) and is operable to vary the angle of the first swash plate assembly relative to the first axis  26  to vary the stroke of the piston. As previously mentioned, desirably the angle of the second swash plate assembly (e.g.,  70 ), the counterbalancing swash plate assembly in some constructions, is also simultaneously varied. In the construction shown in  FIG. 1 , the piston  40  is coupled to the first swash plate assembly  60  such that the combustion chamber volume associated with piston  40  increases as the length of the piston stroke increases and decreases as the length of the piston stroke decreases. This is true for the other cylinders in the plural cylinder engine of  FIG. 1 . That is, the volume of the combustion chamber associated with each piston increases as the length of the piston stroke increases and decreases as the length of the piston stroke decreases. In addition, in the  FIG. 1  construction, the term “combustion ratio” is defined as the ratio (V c +V H )/V c . In this formula, V c  is the volume of the combustion chamber. In addition, V H  is the volume of the portion of the cylinder through which the piston travels between the top dead center position and bottom dead center position (the volume swept by the piston during a stroke). In the construction shown in  FIG. 1 , the combustion ratio is substantially constant as the stroke of the piston is varied by varying the angular operating position of the swash plate assembly  60 . For example, the combustion ratio may be maintained substantially at 1 to (9–12) (desirably 1 to 10) for a gasoline engine and from 1 to (14–17) (desirably 1 to 16) for a diesel engine. Other engine geometries may be used to achieve this characteristic if desired in the particular engine construction. Alternatively, the engine may be designed such that the combustion ratio may be variable, such as being maintained substantially constant for a range of swash plate angles and gradually varied for other ranges of swash plate angles. As a specific example, the combustion ratio may be gradually increased at small swash plate angles for low engine load conditions (e.g., engine idle). 
   With further reference to  FIG. 1 , at least one combustion air intake port is provided in communication with each cylinder and at least one exhaust gas port is provided in communication with each cylinder. An air intake port  450  is shown for cylinder  42  in  FIG. 1  and an air exhaust gas port  452  is shown in  FIG. 1  for cylinder  38 . An air intake valve  454  is shown to selectively open and close air intake port  450 . Although not visible in  FIG. 1 , there may be two air inlet ports and two air inlet valves for each cylinder. An exhaust valve  456  is shown for selectively opening and closing the exhaust port for cylinder  42 . An exhaust valve  458  is shown in position to selectively open and close the exhaust port  452  for cylinder  38 . An air intake valve  460  is shown for selectively opening and closing an air intake port for the valve cylinder  38 . Under the control of a valve actuator, each air intake valve is operable to open to permit the ingress of combustion air into the associated cylinder and close during (a) combustion of an air-fuel mixture within the associated cylinder in the case of a gasoline engine; and (b) compression of air to cause combustion of injected fuel in the case of a diesel engine. In addition, each exhaust valve is opened to permit the exhaust of combustion gases from the associated cylinder and through the associated exhaust gas port following combustion of the air-fuel mixture within the associated cylinder. 
   Although not required, desirably the exhaust gas ports are shorter than the air intake ports. Consequently, the hot exhaust gases have less of an opportunity to transfer heat to the engine, thereby reducing the engine cooling requirements. In addition, in the embodiment of  FIG. 1 , the air intake ports and the exhaust gas ports exit from the cylinder head portion  18  in directions extending generally radially outwardly from the axis  26 . In addition, the exhaust gas port for each cylinder communicates with the cylinder at a location which is positioned radially outwardly from the axis  26  relative to the location where the air intake port communicates with the cylinder. 
   A valve actuator is positioned within the valve cover portion of the housing and operable to selectively open and close the air intake valves and the exhaust valves. Valve actuation is well known in the art and thus a commercially available valve actuator mechanism may be used. However, a desirable embodiment is illustrated in connection with  FIG. 1 . Since the mechanisms are the same, the same numbers will be used for the actuating mechanism shown in connection with cylinder  38  and cylinder  42 . The air intake and air exhaust valves are biased to a closed position. A rocker arm  470  is pivoted to a support coupled to the housing for pivoting about a pivot axis  472 . A first end portion  474  of the rocker arm is coupled to the exhaust valve  458 . A second end portion  476  of the rocker arm is positioned for engagement by a cam when the air intake valve(s) of the associated cylinder in  FIG. 1  are to be opened. In  FIG. 1 , in association with cylinder  38 , a first projection  477  (hidden by end portion  476 ) extends from end portion  476  to a position where it engages the upper end of valve  460 . A second projection, like projection  479  visible in  FIG. 1  for cylinder  42 , extends from end portion  476  into engagement with the other air intake valve  454  (shown for cylinder  42  but not shown for cylinder  38  in  FIG. 1 ) associated with cylinder  38 . As the rocker arm end portion  476  is engaged by a cam, the projections  477 , 479  are pivoted and open the air intake valves. A cam body  478  is supported for rotation about the perimeter of cylinder  414  and also about the axis  26 . Cam body  478  comprises respective cams positioned on the cam body, such as explained below, for operating the respective rocker arms to open and close the air intake and exhaust valves at desired times. A first cam follower  480 , comprising, in this example, a tapered roller rotatably coupled to a projecting end portion  476  is positioned to follow a track along one major surface of cam body  478 . As cam follower  480  engages a projecting cam, the end portion  476  is urged to the right in  FIG. 1  to open the air intake valves. Roller or cam follower  480  returns to the position shown in  FIG. 1  after the cam passes. A cam follower comprising a roller  482  pivotally coupled to an inwardly projecting portion of rocker arm  470  bears against the outer perimeter of the cam body  478 . As a cam along the outer perimeter of the cam body engages the roller  482 , the rocker arm is urged in a direction which pivots the end portion  474  to the right in  FIG. 1 , resulting in opening of the exhaust valve. The rocker arm returns to the position shown in  FIG. 1  following the passage of the cam. A suitable location of the cams on the cam body will become more apparent from the description below. 
   The cam body in  FIG. 1  for a five cylinder engine may be driven in the following manner in a specific example. A first gear  490  is coupled to member  414  such that gear  490  is driven in the same direction as the output shaft section  54  about the axis  26 . In the  FIG. 1  construction, an idler gear  492  is pivoted by a pin  494  to the housing section  16 . Idler gear  492  is coupled to gear  490  such that it is driven by gear  490 . Gear  492  engages a ring gear  496  and drives the ring gear in rotation. A coupling plate  498  carried by ring gear  496  extends radially inwardly from the ring gear and overlays a major surface of the cam body  478 . The coupling plate  498 is mounted to the cam body  478  such that rotation of the ring gear drives the cam body in rotation. In a specific example, the gears are selected such that the cam body is rotated at a rate which is one-fourth of the rate of rotation of the engine output section  50 . In addition, for this five cylinder engine, the cam body is rotated in a direction which is opposite to the direction of rotation of the output section  50 . Also, as explained below, the illustrated cam body comprises a first set of two cams spaced 180 degrees apart from one another on the cam body in position to selectively open and close the air intake valves and a second set of two cams spaced 180 degrees apart on the cam body and positioned to selectively open and close the exhaust valves. In embodiments where it is desired to drive the cam body in a direction which is the same direction as the direction of rotation of output section  50 , the gear  490  may be enlarged to engage the ring gear  496  (and/or the ring gear may corresponding be reduced in dimension) or, alternatively, an additional intermediate idler gear, such as gear  492 , may be positioned between gear  490  and the ring gear  496 . The gear sizes and gear design may be selected to achieve the desired rotating rate of the cam body relative to the engine output rotation and to achieve the desired direction of cam body rotation in relation to the engine output rotation. 
   Other configurations of cams and cam bodies as well as mechanisms for rotating the cam body may also be used. 
   An exemplary cam body for a five cylinder engine is illustrated in  FIGS. 17–19 . The illustrated cam body comprises first and second generally opposed major surfaces  500 , 502  and an outer periphery  504 . The direction of rotation of cam body  478  is indicated by arrow  506  in  FIG. 17 . First and second diametrically opposed exhaust valve operating cams  508 , 510  are shown projecting outwardly from the periphery of the cam body. In addition, air intake valve actuating cams  512 , 514  are shown projecting outwardly from the major surface  500  of the cam body. In the cam body of  FIG. 17 , I 0  is 10 degrees before TT; I c  is 20 degrees after BT, E 0  is 20 degrees before BT; and E c  is 10 degrees after TT. 
   Desirably, the number of cylinders included in the engine, the firing order for each such number of cylinders and the swash plate rotation angle through which the first member rotates between firing one cylinder and the next cylinder to fire of the engine are in accordance with the following table: 
                                           Swash Plate       Number of Cylinders   Firing Order   Rotation Angle                                            1   1      720°       2   1, 2, 1      360°       3   1, 3, 2, 1      240°       5   1, 3, 5, 2, 4, 1      144°       7   1, 3, 5, 7, 2, 4, 6, 1   102.857°       9   1, 3, 5, 7, 9, 2, 4, 6, 8, 1      80°       11   1, 3, 5, 7, 9, 11, 2, 4, 6, 8, 10, 1    65.454°                    
and wherein the first member rotates 720° during a complete firing cycle.
 
   Other configurations, although less desirable, may also be used. In the above table, an equal firing gap is assumed. 
   In the embodiment of  FIG. 17 , the cam body comprises a cam disk. In addition, the cam body comprises first and second major surfaces  500 , 502  as previously described. The surface  500  in the  FIG. 1  embodiment is positioned adjacent to the cylinders while the surface  502  is positioned furthest from the cylinders. The cam body in the  FIG. 1  embodiment thus comprises at least one projection extending from the first surface  500  and away from the second surface. The cam body  478  may take any suitable form.  FIGS. 45–47  show examples of alternative cam body constructions.  FIG. 45  illustrates a cam body with a cam  510 ′ projecting outwardly from the periphery of the cam body.  FIG. 46  shows a cam  510 ′ projecting outwardly from the periphery of the cam body; a cam  510 ″ projecting from major surface  500  of the body (thus cams  510 ′ and  510 ″ are similar to cams shown in  FIG. 17 ). Alternatively, the cam body of  FIG. 46  may have a cam  510 ′″ projecting from surface  502  and away from the surface  500 . In the  FIG. 47  construction, the cam body comprises a cam supporting projection  520  spaced from the axis  26  about which the cam body rotates. The illustrated projection  520  is annular and extends from the major surface  500  and away from major surface  502 . At least one cam, such as cam  522 , projects radially inwardly from the cam supporting projection  520  and toward the axis  26 .  FIGS. 45–47  are provided to illustrate examples of the wide variety of cams and cam body designs which may used in a swash plate engine. The invention is not limited to any particular valve actuator mechanism. 
   As specific desirable examples for a swash plate engine having a specified number of cylinders (a five cylinder engine example is not described below since an example of such an engine is set forth above), the following constructions may be employed. In these constructions, where plural cylinders are utilized, the cylinders are desirably spaced equally about the axis of the engine. The number of posts  190  (e.g., such as shown in  FIG. 1 ) or other connections provided for coupling piston rods to the swash plate assemblies match the number of cylinders and are positioned at symmetric locations about the driven swash plate assembly. 
   For a one cylinder engine, the cam body may be rotated at one-half of the speed of the output member (e.g., shaft section  50 ). The cam body may be rotated in either direction (the same or opposite) relative to the direction of rotation of the output member. In addition, a first cam is provided on the cam body in a position to selectively open and close the air intake valve and a second cam is provided on the cam body in a position to selectively open and close the exhaust valve. 
   For an engine consisting of two cylinders and two associated pistons, the cam body may be rotated at one-half the speed of the output member and in either direction relative to the direction of rotation of the output member. A first cam is provided on the cam body in a position to selectively open and close the intake valves and a second cam is provided on the cam body in a position to selectively open and close the exhaust valves. 
   For an engine consisting of three cylinders and three associated pistons, the cam body may be rotated at one-half the speed of the output member, the cam body being rotated in a direction which is opposite to the direction of rotation of the output member. The cam body includes a first cam in a position to selectively open and close intake valves and a second cam in position to selectively open and close exhaust valves. 
   For an engine consisting of seven cylinders and seven associated pistons, the cam body may be rotated at a rate which is one-fourth of the speed of rotation of the output member, the cam body being rotated in a direction which is the same as the direction of rotation of the output member, the cam body including a first set of four cams spaced 90 degrees apart from one another on the cam body in position to selectively open and close the air intake valves and a second set of four cams spaced 90 degrees apart on the cam body and positioned to selectively open and close the exhaust valves. 
   Referring again to  FIG. 1 , the illustrated engine in this figure has a longitudinal axis which is oriented horizontally. Although this orientation is not required, when employed the housing may comprise an oil pan  530  for collecting lubricating oil that flows downwardly through the engine. An oil pump  532  may be provided to take oil from the oil pan  530  and distribute it to various components of the engine within the at least the swash plate case portion, the cylinder head portion and the cylinder case portion of the housing. 
   In the cylinder case portion construction of  FIG. 2 , all of the cylinders are interconnected and desirably at least one coolant fluid flow passageway is provided between each of the adjacent cylinders of the engine. In the embodiment of  FIG. 20 , the illustrated cylinders are also coupled together. However, in the  FIG. 20  embodiment, the cylinders, as well as the central chamber defining portion  426 , if included, are of a monolithic one-piece construction which may be machined but more desirably is formed by casting all of the cylinders together as a unit. A coolant fluid flow passageway is desirably provided between each of the adjacent cylinders of the engine. Coolant fluid flow passageways are shown by dashed lines in  FIG. 20  with one of them being numbered at  540 . As a specific example, when formed by casting, the gap between the cylinders provided by the coolant fluid flow passageway may be about 6 mm in width, although this is variable. If the fluid flow passageways are formed by machining instead of during casting, typically a closer tolerance is more easily provided, such as a gap of 1–2 mm in width. This can result in an engine of a reduced overall dimension. 
   In the embodiment of  FIG. 1 , the longitudinal cylinder axis of each of the respective cylinders is parallel to one another and desirably positioned at a common radius from the first axis. However, different cylinders may be located at different radii from the first axis. In the embodiment of  FIG. 21 , a cylinder case portion  20  is provided with cylinders having respective longitudinal axes which are skewed with respect to one another. For example, the longitudinal axes of the respective cylinders may be at an acute angle, for example, of no greater than 30 degrees, relative to the first axis  26 . In  FIG. 21 , the axis of cylinder  38  and the axis of cylinder  42  are at an angle of θ from the axis  26 . 
   Desirably, the chamber  426  is positioned at least partially between the cylinders and most desirably the chamber  426  is positioned entirely between these cylinders. This is shown in both the  FIG. 1  and  FIG. 20  constructions. Although not required, this design results in a more compact engine construction. 
     FIG. 22  illustrates an engine housing construction which may be utilized in the  FIG. 1  embodiment wherein the cylinder head portion  18  and cylinder case portion  20  of the engine are of a single monolithic one-piece construction, for example formed by casting. In  FIG. 22 , intake and exhaust valves for the respective cylinders  38  and  42  are shown in schematic form. 
     FIG. 23  schematically illustrates an engine housing with housing components which are similar to those of  FIG. 1 . In the  FIG. 23  construction, various components may be formed as single piece monolithic elements, such as by casting. In  FIG. 23 , the swash plate case portion  28  and output member support portion  30  are of a one-piece monolithic construction. As an alternative, in  FIG. 23 , the swash plate case portion  28  and cylinder case portion  20  may be formed as a monolithic one-piece construction. In this case, the output member supporting portion  30  would typically be formed as a separate piece. 
     FIGS. 24 ,  25  and  26  illustrate exemplary bearing arrangements for interconnecting rotatable swash plate members to reciprocating swash plate members. Although these designs can be used in connection with swash plate assembly  70 , they are shown with respect to swash plate assembly  60  for convenience. In  FIG. 24 , ball bearings are used to rotatably couple rotary member  62  to reciprocating member  64  of the swash plate assembly  60 . These ball bearings are indicated schematically at  550 . In  FIG. 25 , needle or barrel bearings  552  are shown for this purpose. An alternative needle or barrel bearing construction is shown in  FIG. 26  with the bearings indicated by the number  554 . The bearing arrangement may also be friction surfaces which slide in contact with one another. 
   In  FIG. 27 , a roller bearing construction is utilized to interconnect the piston rod  80  to the post  190  of reciprocating member  64  of the swash plate assembly  60 . Lines  558  and  560  illustrate exemplary angles through which the plane  562  defined by the swash plate assembly may be pivoted. The collar  74  shifts axially in the  FIG. 27  embodiment in the directions indicated by respective arrows  563 , 565 , as the swash plate angle is varied. 
   In  FIGS. 28 and 28  A, a universal shaft coupling employing a universal bearing  564  is utilized to couple the piston rod  80  to the support post  190  of reciprocating member  64  of the swash plate assembly  60 . 
     FIGS. 30 and 31  illustrates a swash plate assembly  60  usable, for example, in embodiments where three piston rods are to be coupled to the first swash plate assembly. In this case, three of the coupling posts  190  are provided and project outwardly from the reciprocating member  64  of the swash plate assembly. In this example, the rotating member  62  of the swash plate assembly is of a single piece construction. In addition, the reciprocating member  64  is formed of plural ring sections  570 , 572 . These ring sections, which may be more than two such sections if desired, may be interconnected by fasteners, such as bolts  574 . Like the  FIG. 1  embodiment, the members  62  and  64  define respective annular rotating surfaces against with which bearings  66  may ride.  FIG. 31  should be compared with  FIG. 4  as this will assist in clarifying the understanding of the  FIG. 31  embodiment. 
   The embodiment of  FIG. 32  illustrates a mechanism which may be incorporated into any of the swash plate assemblies heretofore described for delivering lubricating fluid to the bearings, including friction bearings, of the swash plate assembly. In the  FIG. 32  embodiment, a source of pressurized lubricating fluid is coupled via a line  576  (shown schematically in  FIG. 32 ) to respective passageways  578  and  580 . The passageway  578  communicates with a respective tilt or other bearing which pivotally couples the swash plate assembly to the output member. The passageway  580  communicates with the bearings which couple the reciprocating member of the swash plate assembly to the rotary member of the swash plate assembly. Thus, passageway  580  communicates with the bearings  66  and passageway  578  is coupled to, for example, tilt bearings (e.g., bearings  234 , 236  as shown in  FIG. 3 ). 
     FIG. 34  illustrates an embodiment wherein the rotating member  62  of the swash plate assembly  60  is positioned generally outwardly of the reciprocating member  64  of the swash plate assembly. In the embodiment of  FIG. 34 , the annular rotating surface defined by member  62  is generally inwardly facing while the annular surface of member  64  is generally outwardly facing. Bearings (not shown in  FIG. 34 ) are typically positioned between members  64  and  62 , such as shown in the  FIG. 1  embodiment. Schematic coupling of piston rods  80 , 82  to respective post elements  190  of reciprocating member  64  are also shown in this figure. 
   In the embodiment of  FIG. 35 , which is like that of the embodiment of  FIG. 34 , the rotating member  62  is formed of plural sections which are typically each annular in cross-section. Section  62   a  is shown in face-to-face orientation with section  62   b  with these sections then being interconnected, such as by fasteners  586 , to hold sections  62   a  and  62   b  together. In  FIG. 36 , the sections  62   a  and  62   b  have been replaced by ring sections  62   a ′ and  62   b ′ which are each semi-circular in configuration and which are held together by fasteners, such as bolts  588 . 
   In the embodiment of  FIG. 37 , a universal bearing  594  pivotally connects a coupler  595  to the post  190  (a cap or other retainer being omitted from  FIG. 37 ). Coupler  595  includes a projection  596  which is pivoted by a pivot pin  592  to the piston rod  80 . This provides an alternative form of off-center coupling of the piston rod to the swash plate assembly. 
   An alternative mechanism for varying the angle a of swash plate assembly  60  is illustrated in  FIG. 38 . In the embodiment of  FIG. 38 , the output shaft  50 ′ includes an outwardly extending projection  600 . In addition, rotary member  62  comprises a projecting portion  602  which may be similar to projections  252 , 254  of  FIG. 3 . A link  604  interconnects projection  600  with projection  602 . The link  604  is pivotally coupled to each of these projections. Shaft portion  50 ′ is slidably coupled to support  74 ′ with these elements being drivenly interconnected, such as by splines. A counterbalancing swash plate assembly may also be mounted to member  74 ′. A hydraulic or mechanical mechanism may be used to axially shift member  74 ′ relative to output shaft member  50 ′. As axial shifting of member  74 ′ occurs, such as in the respective directions represented by double-headed arrow  606 , the angle α is adjusted to thereby vary the stroke of the engine. For example, the plane of swash plate assembly  60  may be shifted from location  86  to location  86 ′ in this figure. In this case, pivot axis  68  is shifted to the location  68 ′. In the position shown by location  86 , the engine is in a minimum or zero displacement position. In the position shown by location  86 ′, the engine in this example is shown shifted to a maximum displacement position. Thus,  FIG. 38  provides yet another example of a mechanism which may be used to vary the angle of the swash plate assembly to thereby vary the stroke of the engine. Swash plate assembly  60  is coupled to structure  74 ′ by a bearing, such as tilt bearing  230 ′, to permit this pivoting motion. 
     FIG. 39  is similar to  FIG. 38  except that  FIG. 39  schematically illustrates an embodiment having particularly desirable relationships between the various components. Again, a counterbalancing swash plate assembly may be included in the  FIG. 39  construction. As is the case of the  FIG. 38  construction, the position of swash plate assembly  60  shown in solid lines in  FIG. 39  corresponds to a zero displacement position. When the angle of the swash plate assembly has been shifted to angle α to position the plane at location  86 ′, the swash plate assembly is in its maximum displacement position. Increasing the swash plate angle increases the stroke. In a desirable construction, a swash plate engine maintains a substantially constant combustion ratio at minimum and maximum displacement. In a desirable geometry, the radius R 1  is less than the radius R 2 . That is, R 1  corresponds to the radius from axis  26  to the location where the piston rod  80  is pivotally coupled to the swash plate member  64 . In addition, R 2  is the radius from the axis  26  to the pivot axis location where the link  604  is pivoted to the extension  602 . In addition, R 3  is desirably less than R 2 . R 3  is the radius from the axis  26  to the pivot axis location where link  604  is pivoted to projection  600 . These dimension and geometries, as well as the dimensions of the combustion chamber, may be varied in different engines to accomplish a substantially equal combustion ratio for all engine displacements. Thus, a more or less equal combustion ratio is desirable in some cases for all engine displacements (swash plate angles). Alternatively, the engine may have a combustion ratio which may be variable in some engine embodiments, such as to improve fuel efficiency or to reduce undesirable emissions, under certain engine loads. 
     FIG. 40  illustrates an alternative mechanism for varying the engine stroke. In the embodiment of  FIG. 40 , the swash plate assemblies  60 , 70  are pivoted to a collar  610  which is slidably mounted to an output shaft section  50 ′ such that collar  610  is movable in the respective directions indicated by double arrow  612 . The swash plate assemblies may also be coupled together, that is the rotating members of each swash plate assembly may be interconnected so that such members rotate together (although this is not shown in  FIG. 40 ). Shaft section  50 ′ rotates about the axis  26  as indicated by arrows  613 . The collar  610  is coupled to a pin  614  which extends through shaft section  50 ′. Therefore, the collar  610  is also rotationally linked to the output shaft section  50 ′. An elongated slot  616  is provided in the shaft section to allow the collar  610  and pin  614  to move in the respective directions of arrow  612 . Movement of the collar adjusts the angles of the swash plate assemblies  60  and  70  to thereby vary the stroke of the engine as previously described. The pin  614  is connected to a shaft extension  618  positioned within the interior of output shaft section  50 ′. Shaft extension  618  has an enlarged head portion  620  which is rotatably coupled by bearings  622  to a second shaft section  624 . Thus, shaft extension  618  is supported for a rotation with the output shaft member  50 ′. The shaft section  624  is axially shiftable in directions indicated by arrows  626 . Shifting of shaft section  624  in either direction indicated by arrow  626  causes a corresponding movement of shaft extension  618  and of the collar  610  to vary the engine displacement. In the embodiment of  FIG. 40 , a mechanical mechanism is utilized to shift shaft section  624 . In this embodiment, a rack gear or other gear  628  is coupled to the exterior of shaft section  624 . Gear  628  is engaged by a gear  630  which is rotated in respective first and second directions to shift the shaft section  624  in the respective directions indicated by arrows  626 . Gear  630  may be driven in any convenient manner such as by an electric motor or a hydraulic motor. Gear  630  in combination with gear  628  rotates shaft section  624  respectively in either direction indicated by double-headed arrow  615 . A feedback loop may be included to provide an indication of the gear position. The housing, such as a portion of the cylinder case section of the housing indicated at  640 , is coupled to shaft section  624  to support the shaft section while permitting the movement of the shaft section in the respective directions of arrows  626 . In the embodiment shown in  FIG. 40 , an endless ball bearing track  642  is defined by the interior surface of housing section  640  and the exterior surface of shaft section  626 . As shaft section  624  is moved either of the directions of arrows  626 , the shaft section rotates as permitted by the ball bearing track  642 . As a result, the collar  610  is respectively slid in one of the directions indicated by double-headed arrow  612  corresponding to the direction of movement of shaft section  624  to thereby adjust the displacement of the engine. In the embodiment of  FIG. 41 , the respective swash plate assemblies may be interconnected and supported, for example, in the same manner as described above in connection with  FIG. 1 . However, in the  FIG. 41  embodiment, the locations of pivots  68 , 94  are far enough apart that the respective swash plate assemblies, that is the reciprocating members of the respective swash plate assemblies, may swing without either of the swash plate assemblies needing to pass through a portion the other of the swash plate assemblies. Again, desirably, the reciprocating portions of the swash plate assemblies are designed to swing opposite to one another for mass balancing purposes. In addition, the reciprocating member of the counterbalancing swash plate assembly, swash plate assembly  70  in  FIG. 41 , as well as in the other embodiments, may be entirely or at least partially comprised of a material of a greater density or weight than the reciprocating member of swash plate assembly  60 . As a result, the counterbalancing swash plate assembly may be of a reduced dimension while still providing the desired counterbalancing function. For example, the reciprocating member of swash plate assembly  60  may be comprised primarily of steel while the reciprocating member of swash plate assembly  70  may be comprised of steel with inserts such as lead balancing inserts  659 , which may, for example, be annular or of any other suitable configuration. 
   In the embodiment of  FIG. 42 , during one operating position of the engine, the driven swash plate assembly  60  and counterbalancing swash plate assembly  70  are aligned in a common plane indicated by the number  660  (a minimum engine displacement position). The angles of the respective swash plate assemblies may be varied, as indicated by dashed lines  662  for swash plate assembly  60  and  664  for swash plate assembly  70 , to increase the swash plate angles and correspondingly increase the engine displacement. In  FIG. 42 , at an increased engine displacement position, the pivots  68  and  94  have been shifted to respective locations  68 ′ and  94 ′. 
   In the embodiment of  FIG. 43 , the counterbalancing swash plate assembly  70  is shown positioned within a driven swash plate assembly  60 . The rotary members of the swash plate assemblies are desirably interconnected, such as described above in connection with  FIG. 1 . In the embodiment of  FIG. 43 , the annular rotating surface defined by the rotary member of swash plate assembly  60  is inwardly facing while the annular surface of the reciprocating member of swash plate assembly  60  is outwardly facing. 
     FIG. 44  illustrates a swash plate engine having first and second swash plate assemblies  60 , 70  arranged for counterbalancing purposes. In the embodiment of  FIG. 44 , each of the swash plate assemblies are pivotally mounted to a common shaft section  670  which is slidably and drivenly coupled to an output shaft section  50 ′ for rotation with the output shaft section. A first link  672  couples the rotating member of swash plate assembly  60  to a projection  674  from the shaft section  50 ′. In addition, a second link  676  couples the rotating member  90  of swash plate assembly  70  to a projection  678  of shaft section  50 ′. Projections  674  and  678  extend in opposite directions from the opposite sides of shaft section  50 ′. That is, the link  672  and the link  676  are coupled to the shaft section  50 ′ at positions located at 180 degrees apart from one another on opposite sides of the shaft section  50 ′. In the embodiment of  FIG. 44 , shifting of shaft section  670  axially in the respective directions indicated by double-headed arrow  680  varies the angles of the respective swash plate assemblies  60 , 70  to vary the engine displacement. Thus, in the example of  FIG. 44 , the rotary members of the swash plate assemblies are indirectly interconnected through the output shaft section  50 ′ and their respective links  672  and  676 . 
   An exemplary control mechanism for a swash plate engine of  FIG. 1  is illustrated in  FIG. 48 . In  FIG. 48 , an engine/vehicle parameter(s) sensor is indicated at  690 . This block schematically represents one or more parameters that are being sensed for use in controlling the stroke of the engine. For example, engine torque or horsepower requirements may be sensed in a conventional manner by one or more sensors  691 . As another example, the position of a throttle pedal of the vehicle may be sensed by a conventional throttle position sensor indicated at  692 . Other vehicle and engine parameters may also be used as a basis for controlling the engine. For example, a braking condition (service brake position) and/or brake temperature may be sensed by associated sensors  694  so that, for example, the engine displacement may be increased in the event the engine is to be used to assist in braking the vehicle. Engine speed may be sensed by sensor  694  and used as a control parameter. Also, fuel consumption may be sensed with the engine displacement being adjusted to improve fuel efficiency. A manually actuated adjustment control  695  may also be included to give the engine or vehicle operator some control over the engine displacement (e.g., to increase displacement under high torque or extreme braking conditions. A signal or signals corresponding to the sensed parameter or parameters is transmitted on a bus  697 , which may be the existing data bus of a vehicle, to an engine controller  696 . The engine controller  696  includes a signal path  690  for sending appropriate signals to an engine stroke adjuster  699 . The engine stroke adjuster comprises a mechanism for varying the angle of at least one swash plate assembly and more desirably the angle of at least a first swash plate assembly and a counterbalancing swash plate assembly with the angles being adjusted in opposite directions to enhance the counterbalancing features of the engine. Examples of the engine stroke adjuster have been previously described. A hydraulic cylinder activated mechanism, a mechanical mechanism and/or an electronically controlled mechanism may be used to cause the shifting of the angle of the desired swash plate assembly. The swash plate assembly is part of a swash plate engine  700 . Control signals to a hydraulic fluid control valve, to a gear adjustment motor or other control mechanism are delivered along a path  698  to the swash plate adjuster  699  to cause the swash plate engine  700  to adjust its stroke. Feedback may be provided to the engine controller for use in monitoring the engine stroke adjustment. 
   In the case of a gasoline fuel engine wherein gasoline fuel and combustion air is delivered as an air-fuel mixture to the combustion chamber for combustion therein to drive an associated piston, the engine controller  696  may send signals via a path  704  to a fuel injector or other fuel supply controller  706  to adjust the amount of fuel delivered to the swash plate engine. Typically, the quantity of fuel is reduced with a reduction in the volume of the engine displacement as a result of a change in the stroke of the swash plate engine. As a result, the amount of fuel that is delivered to the engine may be controlled to maximize fuel efficiency and/or exhaust gas consistency (which may be another control parameter). In the case of a gasoline engine, a combustion air throttle  710  may be used. In the event the engine displacement is reduced, the engine controller  696  may send a signal via a line  705  to the air supply throttle  710  to reduce the amount of air delivered to the swash plate engine in combination with the reduction in the gasoline supplied by fuel supply controller  706 . Conversely, if the engine displacement is increased, the engine controller may cause an increase in gasoline delivered to a gasoline swash plate engine via fuel supplier  706  together with an increase in the amount of air being delivered to the engine via air controller  710 . The use of an air throttle can increase the responsiveness of the engine and can assist in realizing a more optimum fuel consumption efficiency and/or a more optimum exhaust gas consistency. 
   In the case of a diesel fuel engine, an air throttle is typically omitted, but the quantity of injected fuel is typically reduced with engine displacement reductions and increased with engine displacement increases. The quantity of fuel may also be adjusted to increase fuel efficiency and/or exhaust gas consistency 
   The swash plate engine  700  may be adjusted to increase the stroke of the engine under high torque or heavy load requirements (e.g., during startup or climbing a hill) and/or during braking events while reducing the stroke under idle conditions and at less demanding times, such as when the vehicle is cruising at highway speed on flat ground. 
   The operation of the swash plate engine may be controlled in accordance with a wide variety of methods. As a specific example, for a diesel engine, under idle conditions, the engine stroke may be maintained at a level which is greater than the minimum engine stroke with the fuel supply reduced. When the fuel accelerator pedal is depressed, the engine is more responsive because the stroke has not been reduced to a minimum stroke. Under coasting conditions (e.g., when a vehicle is coasting and no engine braking is desired), the fuel supply is reduced to zero and the stroke reduced toward its minimum (e.g., toward or at zero displacement) level. Under an engine braking condition (e.g., a truck is traveling downhill and it is desired to have the engine assist in braking the vehicle), the stroke may be set at a high level, for example at or toward the maximum stroke with the fuel reduced to zero. A direct injection gasoline engine may be operated, for example, in the same manner. For a gasoline engine of the type with an air throttle which regulates the supply of combustion air to the engine, under idle conditions, the engine stroke may be maintained at a level which is greater than the minimum engine stroke with the combustion air supply and fuel supply both being reduced. This improves engine responsiveness in comparison to the case if the displacement had been reduced toward or to the minimum level. In this case, the fuel and combustion air supply is increased when the engine is operated at above idle conditions. Under coasting conditions, the engine displacement is reduced (e.g., toward or at the minimum, such as zero displacement) with the combustion air supply and fuel supply reduced (e.g., toward or at a minimal level or totally closed off). This increases engine fuel efficiency under these conditions. Under engine braking conditions, the engine displacement may be set at a high level (e.g., at or toward the maximum displacement level), the engine fuel may be reduced (e.g., toward a minimum fuel level or shut off), and the air supply may be maintained at a high level. Again, other engine control approaches may also be used. Having described the principles of my invention with reference to several embodiments, it should be apparent to those of ordinary skill in the art that the embodiments may be modified without departing from the principles of my invention. I claim all such embodiments as fall within the scope and spirit of the following claims.