Patent Publication Number: US-8967097-B2

Title: Variable stroke mechanism for internal combustion engine

Description:
BACKGROUND OF THE INVENTION 
     The present invention relates broadly to internal combustion engines and, more particularly, to so-called “variable stroke” engines wherein a mechanical connecting arrangement between reciprocating pistons and an engine crankshaft varies the extent of piston movement during the overall operational cycle of the engine. Generally, such mechanisms have the purpose of increasing the efficiency of internal combustion engines by achieving an effectively larger mechanical crank arm during the expansion stroke and an effectively shorter mechanical crank arm during the intake stroke. 
     Conventional internal combustion engines operate according to a repeating sequence of movements typically referred to as an intake stroke, a compression stroke, an expansion stroke, and an exhaust stroke. In this sense, “stroke” describes the reciprocating movements of a drive piston as it travels back and forth through a cylindrical combustion chamber in an engine housing or “block.” The term “cycle” may also sometimes be used interchangeably with the term “stroke.” Hence, an engine operating according to the above-described manner is commonly referred to as a four-cycle or four-stroke engine, indicating that to complete a full power cycle the piston must reciprocate four times in the cylinder. “Cycle” is also used to describe the complete power cycle of the engine. This usage of terminology is consistent within and well understood to those skilled in the art. 
     As indicated, various designs have been advanced to cause an engine piston to travel a longer or shorter distance during the intake, compression, expansion, or exhaust strokes or during any combination of them, and to modify the piston velocity in some portion of its travel. For example, the so-called top or bottom dead center position of the piston has been shifted up or down for every revolution or every two revolutions. All of these conditions are different versions of a variable stroke engine. Chadbourne U.S. Pat. No. 1,326,129 and Clarke U.S. Pat. No. 4,044,629 describe an extended expansion stroke. A practical application of an extended expansion stroke is the Millenia model automobile manufactured by Mazda, which utilizes a so-called Miller-cycle engine of the type designed in 1947 by U.S. engineer Ralph Miller. Miller&#39;s engines have been used for some time in ships and stationary power plants. The engineering goal is to reduce the engine&#39;s compression ratio without interfering with the power generating expansion stroke. In the Miller-cycle engine, the piston rises one-fifth of its stroke before the air intake valve is closed. After combustion occurs at the top of the stroke, the expanding gases push the piston all the way down to the bottom of the stroke, so the expansion ratio is not affected. 
     During the first half of the twentieth century, it was generally accepted among persons skilled in the art of internal combustion engines that the combustion products inside an engine cylinder had to be removed as completely as possible during the exhaust stoke following each expansion stoke and preceding the suceeding intake stroke. Many different patents propose differing ways to obtain a larger exhaust stroke. See, e.g., Hulse U.S. Pat. No. 1,326,733; Svete U.S. Pat. No. 2,394,269; Cady U.S. Pat. No. 1,786,423; Tucker U.S. Pat. No. 1,964,096; and Austin U.S. Pat. No. 1,278,563. Chadbourne U.S. Pat. No. 1,326,129 and Clarke U.S. Pat. No. 4,044,629 also refer to a larger exhaust and expansion stroke. However, due to emission regulations implemented in the latter part of the twentieth century, new engine designs have been advanced wherein a portion of exhaust gas is recirculated or retained in the combustion chambers as a means of reducing the atmospheric emission of NOx (oxides of nitrogen) caused by the oxidation of nitrogen in the combustion chamber. This is accomplished by allowing intake manifold vacuum to draw exhaust gas into the intake manifold through an EGR (exhaust gas recirculation) valve. 
     Others have used variable stroke designs to modify the engine compression ratio. A lot of work has been done, especially in Europe and Japan, to achieve a so-called variable compression ratio by means of an arrangement that varies the position of the piston relative to the head of the cylinder. 
     The compression ratio is the ratio between capacity of the cylinder and capacity of the combustion chamber; in other words, the air-fuel mixture that goes into the cylinder during the intake stroke is then compressed as many times as the compression ratio value. Generally, the higher the compression ratio, the higher the engine efficiency. Some limitations such as mixture pre-ignition, knocking, engine temperature, and even engine construction exist. Since the compression ratio is one of the main factors affecting the engine efficiency, it is desirable to optimize it for different operating conditions (speed rate, load, acceleration, etc.). Schechter U.S. Pat. No. 5,165,368 describes a representative example of such an engine 
     A variable piston stroke application has also been utilized to optimize the pressure acting on the piston. For this purpose, the piston speed is decreased, relative to the speed of a conventional piston, near the top dead center to maximize the combustion process and the resulting forces acting on the piston. Schaal et al U.S. Pat. No. 5,158,047, Williams U.S. Pat. No. 5,060,603, and McWhorter U.S. Pat. Nos. 3,686,972; 3,861,239; and 4,152,955 are representative of this concept. 
     More recently, U.S. Pat. No. 5,927,236, discloses an internal combustion engine design wherein a gear set arrangement is utilized to connect the crankshaft and the piston connecting rods of the engine via offset bearing surfaces to accomplish a variation of the length of piston stroke over a complete engine power cycle. In particular this design seeks to increase the piston stroke via an increased effective crank arm length during the expansion portion of the power cycle to increase the torque output, and to reduce the stroke and piston velocity during the intake or admission and exhaust portions of the cycle to increase volumetric efficiency, all thereby enhancing the thermoefficiency of the engine. 
     SUMMARY OF THE INVENTION 
     It is fundamentally an object of the present invention to further advance the invention of U.S. Pat. No. 5,927,236 described above. 
     More particularly, it is an object of the present invention to achieve further enhancements in torque output, horsepower output, fuel efficiency, volumetric efficiency, and emissions reduction in internal combustion engines utilizing improvements in the variable stroke mechanism of the design of U.S. Pat. No. 5,927,236. 
     Toward those ends, the present invention provides an improved internal combustion engine operable according to a four stroke cycle wherein the piston reciprocates within the combustion chamber through an intake stroke moving in a first direction, a compression stroke moving in a second direction, an expansion stroke moving in the first direction, and an exhaust stroke moving in the second direction. The position of the piston at the conclusion of the compression stroke and at the beginning of the expansion stroke is defined as top dead center. Basically, the engine of the present invention comprises an engine block defining at least one combustion chamber, a crankshaft rotatably mounted to the engine block, a piston disposed within the combustion chamber for reciprocation along a chamber axis, and a connecting rod pivotably mounted to the piston. 
     According to the present invention, a mechanism for producing a variable stroke length in an internal combustion engine utilizes a mechanical gearing assembly to connect the connecting rod rotatably to the crankshaft for translating the reciprocating motion of the piston into rotational motion of the crankshaft. Basically, the mechanical assembly comprises a gear set including at least a first gear member non-rotatably mounted to the engine block and a second gear member meshingly engaged with the first gear member. The second gear member has a first bearing surface to which the connecting rod is mounted and a second bearing surface mounted to the crankshaft for rotation of the second gear member with the crankshaft. The second bearing surface is offset from the crankshaft axis to move in a circular path about the crankshaft axis for imposing a uniform mechanical crank arm on the crankshaft throughout the four stroke cycle of the engine. The first and second bearing surfaces are spaced from one another by an offset distance causing the first bearing surface to move alternatingly in inner and outer elliptical paths about the crankshaft for imposing a variable cam arm on the crankshaft. Thus, the sum of the crank arm and the cam arm cause the length of the piston reciprocation to vary throughout the four stroke cycle of the engine. 
     In accordance with one aspect of the present invention, the inner and outer elliptical paths of the first bearing surface intersect at a point at or closely adjacent to the combustion chamber axis for causing the crank arm and the cam arm to cooperatively produce a positive torque on the crankshaft at the top dead center position of the piston. For example, in one embodiment of the invention, the inner and outer elliptical paths of the first bearing surface intersect at a point coinciding with the combustion chamber axis. Alternatively, the inner and outer elliptical paths of the first bearing surface may intersect at a point spaced a predetermined number of degrees, preferably less than or equal to approximately twenty-five degrees (25°), in advance of the combustion chamber axis in the direction of rotation of the crankshaft, for causing the position of the piston at the conclusion of the exhaust stroke and at the beginning of the intake stroke to be at a predetermined spacing below the top dead center position, thus permitting the combustion chamber to retain a predetermined volume of exhaust when beginning the subsequent intake stroke. 
     According to another aspect of the present invention, the first and second bearing surfaces of the second gear member are selectively configured and dimensioned to achieve a predetermined ratio of the length of the cam arm to the length of the crank arm. Preferably, the cam arm length is at least approximately 20% of the crank arm length and may be up to approximately 100% of the crank arm length. 
     According to a related aspect of the present invention, the ratio of the length of the cam arm to the length of the crank arm is selected to achieve a predetermined volumetric capacity in the combustion chamber at the conclusion of the intake stroke. 
     The first gear member may preferably be a pinion gear and the second gear member may preferably include a crown gear portion having gear teeth formed about a radially inward surface of an annular body to mesh with the pinion gear to move thereabout in the manner of a planetary gear. The second gear member also preferably includes a bearing portion which projects outwardly from the crown gear portion and has the first bearing surface formed on an outer surface of the bearing portion and the second bearing surface formed on an inner surface of the bearing portion. In this manner, the connecting rod may rotate on the first bearing surface and the second bearing surface may rotate on the crankshaft. 
     The present invention is adaptable to substantially any internal combustion engine, and may preferably be incorporated in a multi-cylinder engine having a plurality of combustion chambers and a plurality of gear sets corresponding in number to less than or equal to the number of combustion chambers. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a perspective view of an improved internal combustion engine equipped with a mechanical assembly for producing a variable length of piston stroke according to a preferred embodiment of the present invention; 
         FIG. 2  is a perspective view of the variable stroke mechanism according to the embodiment of  FIG. 1 ; 
         FIG. 3  is a cutaway diagrammatic sectional view of the internal combustion engine and variable stroke mechanism of  FIGS. 1 and 2 ; 
         FIG. 4  is a second cutaway view of the variable stroke mechanism illustrated in  FIG. 3 ; 
         FIG. 5  is a diagrammatic view of the expansion stroke of an internal combustion engine equipped with a variable stroke mechanism according to a preferred embodiment of the present invention; 
         FIG. 6  is a diagrammatic view of the exhaust stroke of an internal combustion engine according to the embodiment of  FIG. 5 ; 
         FIG. 7  is a diagrammatic view of the intake stroke of an internal combustion engine according to the embodiment of  FIG. 5 ; 
         FIG. 8  is a diagrammatic view of the compression stroke of an internal combustion engine according to the embodiment of  FIG. 5 ; 
         FIG. 9  is a diagrammatic view comparing the embodiment of the present invention shown in  FIGS. 5-8  against the invention of U.S. Pat. No. 5,927,236; 
         FIG. 10  is a diagrammatic view similar to  FIGS. 5-8 , depicting an alternative embodiment of a variable stroke mechanism for an internal combustion engine according to the present invention; 
         FIG. 11  is a graph depicting comparative torque curves for an internal combustion engine equipped with a variable stroke mechanism according to the present invention and for a conventional four-stroke internal combustion engine; 
         FIG. 12  schematically depicts the differing paths traced by the cam arm and the crank arm in differing embodiments of the variable stroke mechanism of the present invention utilizing respectively differing selective variations in ratio of the length of the cam arm to the length of the crank arm; and 
         FIG. 13  is a chart compiling comparative data for differing selective variations in the ratio of the length of the cam arm to the length of the crank arm in an embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Turning now to the accompanying drawings and, initially to  FIGS. 1-4 , an improved internal combustion engine according to a preferred embodiment of the present invention is illustrated generally at  10  and includes a conventional engine block  12 . It should be noted that the engine block  12  is only shown partially and diagrammatically as a support for the mechanical assembly according to the present invention. Further, for illustrative purposes, the engine is shown only as a two-cylinder engine. Nevertheless, it will be readily appreciated by those skilled in the art that the mechanism of the present invention may be adapted in varied configurations for virtually any multi-cylinder engine. 
     A generally conventional crankshaft  16  is provided with a crankshaft bearing surface  17  at the junction of the crankshaft  16  and the engine block  12 . A bearing cap  19 , formed as a form fitting archway having two bolt openings formed therein and a curved bearing surface  21  formed as an underside thereof, is mounted to the engine block  12  with preformed openings in the engine block and the bearing cap  19  in registry to accept conventional bolts to affix the bearing cap  19  to the engine block  12  to retain the crankshaft  16  in place. 
     The engine block is formed with two cylindrical bores  14  ( FIGS. 3 and 4 ) within which two conventional pistons  22  are disposed for reciprocating movement. Two identical conventional connecting rods  24  are mounted pivotably to the pistons  22  and are in turn mounted to the crankshaft  16  via the mechanical assembly of the present invention in a manner described in greater detail hereinafter. Conventional end caps  26  are attached to the connecting rods  24  to retain them in a rotatable manner in association with the crankshaft  16 . As will be seen in greater detail hereinafter, the connecting rods  24  are not mounted directly to the crankshaft  16  but are mounted to a bearing surface of the mechanism of the present invention. 
     The mechanical assembly of the present invention comprises a gear set  30  mounted to the crankshaft  16  in association with each piston and connecting rod assembly such that the gear sets  30  form the primary driving portion of the present invention. Each gear set  30  includes a first gear member  32 , preferably in the form of a pinion gear, in operational meshing engagement with a second gear member  36 , preferably in the form of a crown gear. Eccentric counterweights  20  are mounted to the crankshaft for balancing, as would be commonplace in a two cylinder engine. The gear sets  30  are configured as mirror images of one another that essentially divide the engine  10  into two minor-imaged halves, with the gear sets  30  in the center. 
     With reference to  FIG. 2 , the first gear member  32  is illustrated in the preferred form of a pinion gear having a cylindrical body with a row of teeth  34  formed circumferentially thereabout. The second gear member  36  is illustrated in the preferred form of a crown gear having a cup-like generally cylindrical gear body  37  with a continuous array of teeth  38  formed around a radially inward annular surface thereof. The two first gear members  32  are separated by and mounted to a cylindrical support member  35  which is fixed to the engine block by a clamping member  33  as seen in  FIG. 1 . Each first gear member  32  is thereby fixed in position within the engine block against rotation. The teeth of the second gear member  36  are meshingly engaged with the teeth of the first gear member  32  for rotation of the second gear member  36  about the first gear member  32  in a planetary manner. 
     Turning now to  FIGS. 3 and 4 , a more diagrammatic representation of the mechanism according to the present invention is provided. Hereagain, the engine block is illustrated only partially and schematically, as indicated generally at  12 , to depict one cylinder  14  through which a piston  22  moves. 
     The second gear member  36  includes a bearing member  48  in the form of an annular hub projecting in an axial direction outwardly from the exterior side of the cup-like gear body  37  opposite and linearly displaced from the interior gear teeth  38 . The bearing member  48  has a first outer annular bearing surface  40  formed circumferentially around the exterior surface thereof about which the connecting rod  24  is rotationally mounted, and a second inner bearing surface  42  formed about the radially inward surface of the bearing member  48 . The inner bearing surface  42  is cylindrical about a common central axis with the cup-like cylindrical gear body  37 . The outer bearing surface  40  is also cylindrical but is eccentric with respect to the axis of the inner bearing surface  42  and the gear body  37 , whereby the body of the bearing member  48  has a radially enlarged offset portion  44  between the bearing surfaces  40 ,  42  which defines a maximum offset distance  46  explained in greater detail hereinafter. The bearing member  48  defining the bearing surfaces  40 ,  42  may be formed integrally with the gear member  36 , but this is not a specific requirement. The only requirement is that the bearing member  48  must rotate unitarily with the gear member  36  and integral formation is the simplest way to achieve this result. 
     As will thus be understood with reference to  FIGS. 3 and 4 , three rotational axes are thereby defined by the present mechanical assembly. The crankshaft  16  rotates about a crankshaft axis  70  which, as seen in  FIGS. 3 and 4 , is coincident with the geometric axis of the first gear member  32 , i.e., the axis about which the gear member  32  would rotate if its mounting to the engine block permitted free rotation. The disposition of the first gear member  32  is adjustable to permit a few degrees of rotational adjustment about the crankshaft axis  70 . The second gear member  36 , including its integral bearing member  48 , rotates about an axis  72  which is parallel to but offset from the crankshaft axis  70  by a predetermined offset distance  50 . This crankshaft offset  50  is present in every crank-driven internal combustion engine and produces a non-varying mechanical crank arm acting on the crankshaft  16  through which the pumping reciprocation of the piston is translated into rotation of the crankshaft  16 . Due to the eccentric orientation of the bearing surface  40 , the connecting rod  24  rotates about a separate axis  74  which also extends parallel to the crankshaft axis  70  and the offset axis  72 . The distance between the offset axis  72  and the connecting rod axis  74  defines a maximum offset distance  46  which imposes a variable cam arm acting on the crankshaft  16 . In this manner, the maximum offset distance  46  combines with the crankshaft offset  50  to define a total effective crank length which, as will be explained in greater detail hereinafter, varies according to the varying cam arm throughout the engine operational cycle. 
     Those skilled in the art will appreciate that the engine has not been described as having any valve system, cooling system, ignition system, and the accompanying structural components to provide a fully operational internal combustion engine. The engine will necessarily include these systems and components, but such systems and components do not need to differ from the conventional components and systems in a standard internal combustion engine. Hence, such components are beyond the scope of and unnecessary to a description and understanding of the present invention, and are therefore omitted so that the present invention may be described with greater clarity. Any suitable valving system, cooling system, ignition system, and associated structural components will operate satisfactorily with the present invention and it should be noted that the present invention is adaptable to virtually any standard crank-driven internal combustion engines. 
     As in conventional internal combustion engines, detonation of an air fuel mixture in the combustion chamber portion of the cylinders  14  above the pistons  22  drives the pistons  22  downwardly and causes rotation of the crankshaft  16 . Multi-cylinder engines provide multiple piston/cylinder arrangements arranged for sequential detonation of the fuel air mixture in a predetermined sequence for smooth engine operation. Generally, the greater number of cylinders, the smoother the engine will operate. Although the present invention is shown as a two-cylinder engine, the present invention is fully adaptable to engines having virtually any number of cylinders. The present invention is equally adaptable to spark ignited engines, diesel and other compression ignited engines, and radial engines. 
     The improvement of the present invention acts to vary the effective stroke length, i.e., the extent of piston travel, throughout the four strokes of the complete engine operational power cycle. The engine according to the present invention operates according to a modified Atkinson cycle wherein a full cycle of engine operation is defined by four separate strokes: the intake, or admission, stroke; the compression stroke; the expansion, or power, stroke; and the emission, or exhaust stroke. During the intake stroke in a given cylinder, the associated intake valves for the cylinder are opened while the piston is drawn downwardly by rotation of the crankshaft to thereby accept the fuel air mixture into the combustion chamber. During the compression stroke, the intake valves are closed while the piston reciprocates upwardly to compress the fuel air mixture in the combustion chamber to a predetermined degree, and at a predetermined time the compressed fuel-air mixture is detonated, e.g., a spark plug associated with the cylinder is fired, thus initiating the expansion stroke by driving the piston downwardly again through the cylinder as gases produced by the combustion expand. At the completion of the expansion stroke, the exhaust valves for the cylinder are opened as the piston begins to reciprocate again in the upward direction in the cylinder to execute the exhaust or emission stroke, during which the piston drives the waste combustion gases from the combustion chamber through the exhaust valves, to prepare to repeat the four strokes or cycles of the four-cycle engine. The so-called stroke length is defined as the distance the piston travels within the combustion chamber during each of the four strokes of the operational cycle. In a conventional internal combustion engine, the stroke length is fixed and does not vary throughout the four operational strokes of the engine operational power cycle. 
     By contrast, the present invention acts to provide a variable stroke length. Since the teeth of the second gear member  36  are formed on a inner surface thereof, the second gear member  36  rotates planetarily about the first gear member  32  in the same direction as the crankshaft  16  rotates. The gearing between the first and second gear members  32 ,  36  is selected to provide a 1:2 gear ratio, whereby the bearing member  48 , and thereby its offset portion  44  and the maximum offset spacing  46 , rotate through a one-half revolution for every single rotation of the crankshaft  16 . The effect of this mechanical action is depicted in  FIGS. 5-8  each of which illustrates a single stroke of the engine&#39;s operational power cycle. 
     Turning now to  FIGS. 5-8 , the progression of the piston  22  and the associated geared mechanical assembly of the present invention connecting the piston  22  via the connecting rod  24  to the crankshaft  16  is schematically depicted in sequence through the expansion or power stroke, the exhaust stroke, the intake stroke, and the compression stroke. In each of  FIGS. 5-8 , the respective stroke is depicted at an initial starting position, an intermediate position, and a final position, designated by letters in sequence, with the final position of one stroke also constituting the initial position of the next following stroke under the same letter designation. Thus, the expansion or power stroke of the assembly is schematically depicted in  FIG. 5 , with the initial position of the piston  22  (commonly referred to as the “top dead center” position) illustrated at A, an intermediate driving position illustrated at B, and the final position illustrated at C.  FIG. 6  depicts at C, D and E the initial, intermediate and final positions of the assembly in the exhaust stroke following the expansion/power stroke.  FIG. 7  depicts at E, F and G the initial, intermediate and final positions of the assembly in the intake stroke following the exhaust stroke.  FIG. 8  depicts at G, H and A the initial, intermediate and final positions of the piston  22  in the compression stroke following the intake stroke. 
     In  FIGS. 5-8 , the longitudinal centerline axis of the cylinder  14  of the engine block  12  along which the piston  22  reciprocates is indicated at  102 , and the rotational axis of the crankshaft  16  is represented at  70 . A junction point representing the connection between the bearing member  48  and the connecting rod  24  coinciding with the axis  74  is schematically indicated at  52 , and a spaced apart junction point representing the connection between the bearing member  48  and the crankshaft  16  coinciding with the axis  72  is indicated at  54 , with the spacing between the respective junctions  52 ,  54  representing the maximum offset spacing  46 . The mechanical crank arm acting on the crankshaft  16  by means of the crankshaft offset is represented at  50  as extending between the crankshaft axis  70  and the junction  54  between the bearing member  48  and the crankshaft  16 , and the variable cam arm produced by the maximum offset distance is represented at  46  as extending between the junction  52  between the connecting rod  24  and the bearing member  48  and the junction  54  between the bearing member  48  and the crank shaft  16 . During rotation of the crankshaft  16  about its axis  70 , the bearing member/crankshaft junction  54  traces a circular path  56  concentric about the crankshaft axis  70  as the assembly rotates. Owing to the spacing of the axes  72 ,  74  by the offset  46 , the connecting rod/offset junction  52  traces two separate elliptical paths alternating between an outer elliptical path  58  and an inner elliptical path  60 . 
     As will be understood, but for the presence of the mechanical gear set assembly of the present invention, the connecting rod  24  would otherwise be joined to the crankshaft  16  at  54 , as in a conventional engine. Instead, as depicted in  FIG. 5 , the cam arm produced by the maximum offset  46  and the crank arm produced by the crankshaft offset  50  in the present mechanical gear set assembly together effectively increase the stroke length of the piston  22  after detonation as the expansion/power stroke progresses through the intermediate position of the piston  22  at B to the final position at C. Specifically, during the expansion stroke, the junction  52  between the bearing member  48  and the connecting rod  24  follows its outer eccentric path  58 , whereby as the assembly progresses through position B and then to position C in  FIG. 5 , the crankshaft  16  has rotated through a one-half revolution during which the cam arm produced by the maximum offset  46  has acted to add to the crankshaft offset  50  to increase the effective stroke length such that the expansion stroke has been completed with the effective total crank length at its maximum. This increase of the effective stroke length acts to increase the work performed by the engine  10  in the expansion stroke. At this point, the exhaust stroke begins. 
     As illustrated by piston positions C, D, and E in  FIG. 6 , during the subsequent exhaust stroke the crankshaft/offset junction  54  passes the offset/connecting rod junction  52  as the junctions  52 ,  54  progress in their respective paths such that, at the conclusion of the exhaust stroke, the junctions  52 ,  54  have essentially reversed their relative positions from the beginning of the expansion stroke in  FIG. 5A , whereby the total length of the piston stroke during the exhaust stroke is substantially the same as the piston stroke length of the expansion stroke. As a result, the piston fully evacuates combustion gases and by-products from the combustion chamber. 
       FIG. 7  illustrates the intake stroke for drawing in the fuel/air mixture by the piston  22 , beginning at the final position E of the exhaust stroke of  FIG. 6 . During this portion of the operational cycle, the offset/connecting rod junction  52  traces its inner path  60 , causing the effective stroke length to be progressively reduced as the assembly advances to and through position F until, at position G at the conclusion of the intake stroke, the effective stroke length has reduced to its minimum length by an amount equal to the crankshaft offset  50  less the maximum offset  46 . From the initial position E to the final position G in  FIG. 7 , the crankshaft rotates through a one-half revolution. As a result of the reduction in the effective stroke length during the intake stroke, the amount of work that the engine must perform in retracting the piston to admit the fuel/air mixture into the combustion chamber  14  is correspondingly reduced achieving a commensurate reduction in fuel usage. The speed at which the piston  22  moves downwardly during the intake stroke is commensurately slower than during the preceding expansion and exhaust strokes. 
       FIG. 8  depicts the subsequent compression stroke beginning at position G at the conclusion of the intake stroke. As the assembly progresses through intermediate position H to return to starting position A to begin another expansion stroke, the offset/connecting rod junction  52  completes its movement through its inner path  60  and once again moves to its outer path  58 . 
     The present invention provides several advancements over the invention of U.S. Pat. No. 5,927,236. According to one aspect of the present invention, the gear set  30  is arranged such that the connection between the bearing member  48  of the second gear member  36  and the connecting rod  24  orient the outer and inner elliptical paths  58 ,  60  traced by the junction point  52  (i.e., the axis  74  for the connecting rod  24 ) to intersect at a point at or closely adjacent to the combustion chamber axis  102  which thereby causes the mechanical crank arm and the cam arm to cooperatively produce a positive torque on the crankshaft at the top dead center position of the piston. More specifically,  FIGS. 5-8  depict an embodiment of the present invention wherein the intersection of the elliptical outer and inner paths  58 ,  60  coincides with the axis  102  of the cylinder/combustion chamber. By contrast, as depicted in  FIG. 9 , in the preferred embodiment illustrated and described in U.S. Pat. No. 5,927,236, the intersection of the outer and inner paths  58 ,  60  is oriented ninety degrees (90°) in advance of the cylinder axis  102  as viewed in relation to the rotational direction of the crankshaft. 
     Advantageously, this modified orientation of the mechanical arrangement in the present invention produces an enlarged mechanical crank arm acting on the piston at its top dead center position over the preferred embodiment of U.S. Pat. No. 5,927,236, as is seen comparatively in  FIG. 9 , and commensurately improves the mechanical crank arm and attendant crankshaft torque over the full range of the four cycles of the engine. The enhancement of the crank arm is particularly remarkable as compared to a conventional engine without any variable stroke arrangement, as is illustrated in the graph of  FIG. 11 . In  FIG. 11 , curve  104  plots the mechanical crank arm, measured in millimeters, produced in a 1000 cc displacement four cylinder engine embodying the present invention against the crank angle of the engine over its four cycles or strokes, and curve  106  comparatively plots the mechanical crank arm produced in a conventional 1000 cc displacement four cylinder engine wherein the piston connecting rods are connected directly to the crankshaft without any mechanical or other arrangement to vary the piston stroke. 
     As the curve  106  illustrates, conventional engines suffer a significant amount of negative torque on the crankshaft during the compression stroke when approaching the expansion/power stroke, typically requiring that the combustion spark be fired as much as thirty five degrees (35°) in advance of the top dead center position. In such an engine, the piston must perform negative work to overcome the negative torque, which continues to prevail until a torque value of zero is reached at the top dead center (TDC), and no significant amount of positive torque is produced until about sixteen degrees (16°) after the top dead center position (ATDC). This is a primary reason that convention engines are generally incapable of idling at engine speeds lower than about 800 RPM. By contrast, with the arrangement of the present invention orienting the intersection of the outer and inner elliptical paths  58 ,  60  traced by the junction point  52  (i.e., the axis  74  for the connecting rod  24 ) at or closely adjacent to the combustion chamber axis  102 , the increased crank arm produces positive torque increasing from thirty five degrees) (35° in advance of the top dead center position to the top dead center position, and at sixteen degrees (16°) after the top dead center position, the torque produced in the present engine is more than double that of the conventional engine. 
     As depicted in  FIG. 10 , alternative embodiments of the present invention are possible wherein the intersection of the elliptical outer and inner paths  58 ,  60  is within about twenty five degrees (25°) of the combustion chamber axis. Specifically,  FIG. 10  depicts an alternative embodiment wherein the mechanical arrangement of the present invention is configured to orient the intersection of the elliptical outer and inner paths  58 ,  60  at a point spaced a predetermined number of degrees less than or equal to approximately twenty five degrees (25°) in advance of the combustion chamber axis, as viewed in the direction of rotation of the crankshaft. Position A′ in  FIG. 10  depicts the piston and other associated mechanical components of the present invention at the initial outset of the expansion stroke, i.e., the top dead center position, and it can be seen that the arrangement still produces a significantly enlarged mechanical crank arm acting on the piston at such position as compared to U.S. Pat. No. 5,927,236. Furthermore, an additional advantage achieved by this arrangement is that the position of the piston at the conclusion of the exhaust stroke and at the beginning of the intake stroke is caused to be at a predetermined spacing below the top dead center position, as represented by position E′ in  FIG. 10 . The selective orientation of the mechanical arrangement in this manner permits the combustion chamber to retain a predetermined volume of exhaust gases when beginning the intake stroke which in turn contributes to the reduction of harmful emissions from the engine. 
     It has also been discovered that the selective variation of the ratio of the length of the cam arm  46  to the length of the crank arm  50  enables the volumetric capacity in the combustion chamber at the conclusion of the intake stroke and the related functional variable of the compression to expansion ratio to be selectively varied. For example, it is contemplated that the cam arm length may vary from at least approximately 20% of the crank arm length up to approximately 100% of the crank arm length. Such variations may be accomplished by varying the dimensions and eccentric relationship of the outer and inner bearing surfaces  40 ,  42  of the bearing member  48  to achiever differing selected crank arms  50  and differing selected cam arms  46 .  FIG. 12 , schematically depicts the relative changes and relative differences produced in the configuration (sizes and shapes) in the outer and inner elliptical paths  58 ,  60  traced by the junction point  52  (i.e., the axis  74  for the connecting rod  24 ) when selectively varying the cam arm to crank arm ratio in increments of twenty percent (20%). The chart of  FIG. 13  compiles comparative data for the attendant variables affected by such changes. In particular, the data compiled in this chart assumes a 1000 cc displacement engine, i.e., with a fixed 1000 cc admission cycle, and with a fixed compression ratio of 10:1 for an engine employing spark ignited combustion or a fixed 15:1 compression ratio for an engine employing compression to ignite combustion. Overall, the chart illustrates that increases in the cam arm to crank arm ratio achieve a substantial advantage over otherwise comparable 1000 cc displacement conventional engines in fuel consumption (miles traveled per gallon) and in torque. For example, with a cam arm to crank arm ratio of 70% in such a 1000 cc engine, the engine achieves an expansion capacity of 1739 cc, and an expansion ratio of 16.7:1 is achieved with a compression ratio of 10:1. In effect, this configuration of a 1000 cc engine according to the present invention will produce the torque and horsepower output of a 1739 cc engine but will consume only the same amount of fuel as a conventional 1000 cc engine. 
     It will therefore be readily understood by those persons skilled in the art that the present invention is susceptible of broad utility and application. Many embodiments and adaptations of the present invention other than those herein described, as well as many variations, modifications and equivalent arrangements will be apparent from or reasonably suggested by the present invention and the foregoing description thereof, without departing from the substance or scope of the present invention. Accordingly, while the present invention has been described herein in detail in relation to its preferred embodiment, it is to be understood that this disclosure is only illustrative and exemplary of the present invention and is made merely for purposes of providing a full and enabling disclosure of the invention. The foregoing disclosure is not intended or to be construed to limit the present invention or otherwise to exclude any such other embodiments, adaptations, variations, modifications and equivalent arrangements, the present invention being limited only by the claims appended hereto and the equivalents thereof.