Patent Publication Number: US-6213064-B1

Title: Double throw engine

Description:
FIELD OF THE INVENTION 
     This invention relates to an engine, and in particular to an improved form of engine that is better balanced than the prior art. The invention may be applied to an internal combustion engine, hydraulic or pneumatic pumps and/or motors, a compressor and the like. 
     BACKGROUND OF THE INVENTION AND PRIOR ART 
     A problem with known engines, be they IC engines, hydraulic pumps, compressors and the like, that have pistons moving in cylinders, is that a consequence of the piston being driven from a rotating crank is that there are lateral forces that act cyclically on the walls of the cylinder as the piston goes through one cycle of operation. The presence of these lateral forces places a number of restrictions on the design of the cylinders since they must be designed in such a way as to overcome these problems. 
     SUMMARY OF THE INVENTION 
     According to the present invention there is provided an engine comprising at least two pairs of pistons and cylinders, said pairs being disposed along mutually orthogonal first and second axes, said pairs being driven by respective first and second cranks, said first and second cranks rotating about a primary crank, said primary crank in turn rotating about a third axis orthogonal to said first and second axes, said first and second cranks having a radius of throw from said primary crank equal to the radius of throw of said primary crank from said third axis. 
     Preferably a counter-balancing weight is provided opposite said primary crank. 
     The engine may be a four-cylinder engine having two pairs of orthogonally disposed cylinders, or may be an eight-cylinder engine, having four pairs of cylinders and wherein the cylinders are divided into two groups of four disposed in parallel planes, each plane comprising two mutually orthogonal pairs. 
     A major advantage of the present invention is that by selecting the correct counter-balancing weight all lateral forces on the cylinders may be eliminated or at least substantially reduced and this permits the use of alternative materials for the cylinder construction. Preferably therefore the cylinders are formed of a ceramic material. 
     In a particularly preferred embodiment the ceramic cylinders are pre-stressed. This may be achieved by forming the external surface of each cylinder with an at least partially tapering portion, and providing an annular surrounding member having an inner surface tapering in the opposite sense to the external surface of the cylinder, and means being provided for urging said surrounding member such that said tapering surfaces are brought together to generate a radially inwardly directed force. Preferably the urging means comprises spring means. 
     Viewed from another aspect the present invention provides an engine having four pairs of pistons and cylinders, said four pairs being grouped into two groups of two pairs in each group, the piston and cylinder pairs in each group being disposed on mutually orthogonal first and second axes and being driven by first and second cranks respectively, said first and second cranks rotating about a primary crank, said primary crank rotating about a third axis orthogonal to the first and second axes and comprising three interconnecting sections, junctions between said three sections defining spaces for receiving the respective first and second cranks of said two groups of pistons and cylinders, each said first and second crank having a radius of throw from said primary crank equal to the radius of throw of said primary crank from said third axis, and a counterbalancing weight being provided at each junction opposite said primary crank. 
     Viewed from still another aspect the present invention provides a cylinder for an engine wherein said cylinder is formed of ceramic material, and wherein said cylinder has an external surface having two portions tapering in opposite senses, and wherein an annular surrounding member having an internal surface tapering in an opposite sense to a first of said two portion-surrounds said first of said two portions, and wherein urging means acts upon said surrounding member whereby the tapering internal surface of said surrounding member and said first tapering portion of said external surface of said cylinder are brought together to generate a radially inwardly directed force, and wherein an annular locking member surrounds said second tapering portion of the external surface of said cylinder and having an internal tapering surface of opposite sense to the said second tapering portion. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     Some embodiments of the invention will now be described by way of example and with reference to the accompanying drawings, in which: 
     FIG. 1 is a plan view through part of an engine according to an embodiment of the invention showing the plane in which the pistons reciprocate, 
     FIG. 2 is a view of first and second cranks, 
     FIG. 3 shows the location of the counter-balancing weight receiving chamber, 
     FIG. 4 shows the crank assembly, 
     FIGS. 5 to  12  show the relative positions of the cranks during one firing cycle, 
     FIG. 13 is cross-section through a cylinder, 
     FIG. 14 is a section through an embodiment of the invention in the form of an eight-cylinder engine, and 
     FIG. 15 schematically illustrates the cranks for the purposes of explanation. 
     FIG. 16 is an isometric perspective view showing the relationship of the cranks 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     Referring firstly to FIG. 1 there is shown a first embodiment of the present invention showing a double throw engine having two mutually perpendicular piston pairs. Since the invention can be applied to a large number of different applications, such as an IC engine, or a hydraulic motor, pump, compressor or the like, for clarity the description here will be limited to the piston, cylinder, crank construction. The remaining parts of the engine, e.g. the exhaust and inlet design, are conventional. 
     The two pairs of pistons do not however lie in the same plane but in two parallel planes, a first plane comprising pistons  1 , 2  being above a second plane in which are located pistons  3 , 4 . In the following description for convenience reference shall be made to just one piston pair, but unless otherwise stated it should be understood that the description applies to both pairs equally. 
     Pistons  1 , 2  (and equivalently pistons  3 , 4 ) are formed integrally with and extend in opposite directions from a central piston body  5  and each piston comprises a piston shaft  6  with a piston head  7  at a distal end thereof. The piston body  5  is provided with four guide means  8 , for example guide wheels, at each comer of the body  5 —two on each side of the piston body  5 . The guide wheels  8  are adapted to engage guide rails  9  formed on the interior of an engine block  10  so as to allow the piston body  5  and pistons  1 , 2  to reciprocate. It will be understood that pistons  1 , 2  are therefore always exactly 180° out of phase. 
     Pistons  1 , 2 , 3 , 4  reciprocate within respective cylinders  11  which will be described in greater detail below. In the meantime it is sufficient to note that the cylinders  11  are secured to the engine block  10  and guard plates  12  are provided at the ends of the guide rails to prevent lubricating fluid from leaking out of the guide rails  9  and engine block  10 . 
     Formed in the centre of the piston body  5  is a circular crank receiving aperture  13  within which is located a first crank  14  to be described further below. Pin roller bearings are disposed between aperture  13  and crank  14  to permit rotation of the crank  14  within the aperture  13 . Crank  14  is in turn provided with an aperture  15  in which is received a crank pin  16 . Again bearings  17  or the like are provided to permit relative rotation of the crank  14  about the pin  16 . 
     First crank  14  is one part of an integrally formed double-crank as shown in FIG. 2 the other part of the double crank is a second crank  18  which is identical to the first crank  14 . The first and second cranks  14 , 18  are, however, offset by equal but opposite amounts with respect to a common crank axis  19  as shown in FIG.  2 —that is to say they have the same radius of throw, but out of phase with each other. It will be understood that the second crank  18  is received within a crank aperture formed in the piston body of the second pair of pistons  3 , 4  in a manner corresponding identically to the crank  14  and the first piston pair  1 , 2 . 
     As is also shown in FIG. 4 also fitted to the crank pin bearing first and second cranks  14 , 18  is a counter-balancing weight  20  which is adapted to rotate relative to the first and second cranks  14 , 18 . The counter-balancing weight is received within a chamber  21  formed above the two superimposed piston bodies  5  (FIG.  3 ), the chamber being sized sufficiently to allow rotation of the counter-balancing weight  20 . 
     The crank pin  16  is formed with an integral primary crank  22  and FIG. 4 shows the assembly of the first and second cranks  14 , 18  located on the crank pin  16  bearing the primary crank  22 . FIG. 16 shows the relationship between the first and second crank  14 ,  18  and the primary crank  22 . The following figures illustrate the relative positions of the first and second cranks  14 , 18  and the primary crank  22 . In these figures the axis x-x corresponds to the axis of reciprocation of pistons  1 , 2 , while the axis y-y corresponds to the axis of reciprocation of pistons  3 , 4 , it is orthogonal to axis x-x. 
     If the engine is a two stroke four cylinder engine, the firing sequence of pistons  1 - 4  will be  4 , 1 , 3 , 2 . With this firing sequence the primary crank  22  will rotate in a clockwise direction as viewed in FIG. 1, while the first and second cranks  14 , 18  will rotate in an antic-clockwise direction. FIG. 5 shows the position with the primary crank  22  and the second crank  18  at twelve o&#39;clock, and the first crank  14  at six o&#39;clock (the positions of first and second cranks  14 , 18  being described with reference to primary crank  22 ). The subsequent Figures show the positions of the cranks during one complete cycle. Upon rotation into the position of FIG. 6 the primary crank  22  is at a position corresponding to half-past one, crank  14  is at half-past four, crank  18  is at half-past ten. In FIG. 7 primary crank  22  has advanced to three o&#39;clock, crank  14  is also at three o&#39;clock, while crank  18  is at nine o&#39;clock. In FIG. 8 primary crank  22  is at half-past four, first crank  14  is at half-past one, second crank  18  is at half-past seven. In FIG. 9 primary crank  22  is at six o&#39;clock, crank  14  is at twelve o&#39;clock, and crank  18  is at six o&#39;clock. In FIG. 10 the primary crank  22  is at half-past seven, first crank  14  is at half-past ten, second crank  18  is at half-past four. In FIG. 11 primary crank  22  is now at nine o&#39;clock, first crank  14  is also at nine o&#39;clock while second crank  18  is at three o&#39;clock. Lastly in FIG. 12 primary crank  22  has advanced to half-past ten, first crank  14  is at half-past seven and second crank  18  is at half-past one. This completes one cycle. 
     It will be seen from FIGS. 5 to  12  that the first and second cranks  14 , 18  rotate in an opposite sense from the primary crank  22 . The first and second cranks  14 , 18  each have the same radius of throw as the primary crank  22  and rotate at the same angular velocity. This means that the pistons  1 , 2  and  3 , 4  reciprocate along their axes harmonically. Furthermore because the first  14  and second  18  cranks are at 180° with respect to each other, their rotations balance each other out. 
     In the engine of the present invention one advantage is that while the linear velocities of the cranks  14 , 18  along the x-x and y-y axes are variables depending on the angular position of the primary crank  22 , provided that the angular velocity of crank  22  is constant the sum of the kinetic energies of the pistons  1 , 2  and  3 , 4  is constant. Similarly although the linear accelerations and decelerations of the cranks  14 , 18  along the x-x and y-y axes are dependent on the angular position of primary crank  22 , again provided that the angular velocity of the primary crank  22  is constant the sum of the acceleration vectors of the pistons  1 , 2  and  3 , 4  corresponds to a constant centrifugal force acting through the centre O towards the primary crank  22  and which can be finely balanced by a counterweight. 
     This can be seen by the following analysis, which is best understood with reference to FIG. 15 which schematically shows the positions of cranks  14  (&lt;X&gt;),  18  (&lt;Y&gt;) and  22  (&lt;P&gt;) and in which the radii of throw of the cranks r=ØP=PX=PY, mass of &lt;X&gt; Mx=mass of &lt;Y&gt; My=K 2 , and angular velocity=dθ/dt=K 1 . &lt;P&gt; is rotating about Ø (the z-axis) in a clockwise direction at an angular velocity of K 1  while &lt;X&gt; and &lt;Y&gt; rotate about &lt;P&gt; in a anti-clockwise direction with the same angular velocity. With this situation the following conclusions can be drawn about the position, velocity, kinetic energy and acceleration of the pistons as they move along the X and Y axes. 
     
       
         
           
               
               
               
               
             
               
                   
                   
               
             
            
               
                   
                 Position 
                 &lt;Y&gt; = 2rcosθ 
                 &lt;X&gt; = 2rsinθ 
               
               
                   
                 Velocity 
                 dy/dθ = −2rsinθ 
                 dx/dθ = 2rcosθ 
               
               
                   
                   
                 dy/dt = dy/dθ.dθ/dt 
                 dx/dt = dx/dθ.dθ/dt 
               
               
                   
                   
                 dy/dt = −2rsinθ.K 1   
                 dx/dt = 2rcosθ.K 1   
               
               
                   
                   
               
            
           
         
       
     
     Thus the sum of the kinetic energy 
     
       
         ½ My ( dy/dt ) 2 +½ Mx ( dx/dt ) 2 =½ K   2 (2 r ) 2 (K 1 ) 2   
       
     
     which is a constant 
     The acceleration can be calculated as follows 
     
       
           d   2   y/dt   2 =−2 rK   1  cos θ  d   2   x/dt   2 =−2 rK   1  sin θ 
       
     
     and the vector sum of the acceleration along the X and Y axes is 2rK 1  in the direction of ØP. This corresponds to a constant centrifugal force in the direction of the Z-axis which can easily be balanced by a counter-balancing weight. 
     In summary it will be seen that the pistons are driven by the first and second cranks  14 , 18  which rotate about the primary crank  22 . At the same time the primary crank  22  rotates about the z-z axis in the opposite sense to the rotation of the first and second cranks about the primary crank. The throw of the first and second cranks from the primary crank is equal to the throw of the primary crank from the z-z axis. The counter-balancing weight is fixed opposite the primary crank and rotates therewith. 
     In a conventional internal combustion engine any lateral forces acting on the pistons are taken up by the guide rails guiding movement of the piston in the engine block and by the cylinder walls. In conventional engines these lateral forces can be substantial and therefore this imposes design constraints upon the construction of the engine block and the cylinders. In particular the cylinder walls have to be constructed from a material strong enough to bear these lateral forces. This is disadvantageous because it does not allow the use of ceramic materials in the construction of the cylinders. Ceramic materials have excellent wear characteristics and also have very good heat resistant properties, but they also tend to be brittle which means they are liable to crack or break under lateral forces. In the internal combustion engine of the present invention, however, the forces acting on the pistons can be finely balanced to remove or at least substantially reduce any such lateral forces and ceramic materials may be used in the cylinder construction. 
     Preferably, however, if a cylinder is to be constructed from ceramic materials it must be pre-stressed. One way of achieving this is to wind a steel wire around the ceramic cylinder. This has drawbacks, however, in that the tension in the wire reduces when it expands under the action of heat, and also in that the steel wire hinders the dissipation of heat by convection. 
     The present invention provides an alternative method for pre-stressing the ceramic cylinders. FIG. 13 shows an exemplary cylinder in cross-section. The cylinder is located between four rectangularly disposed shafts  30 . The shafts are provided with threaded end portions  31 , 32 ; threaded portions  31  fix the cylinder to the engine bock, while threaded portions  32  allow a cylinder end plate  33  to be located—the end plate  33  having threaded screw holes at its four corners to receive threaded end portions  32  of shafts  30 . 
     Cylinder end plate  33  has a stepped surface that defines by way of two stepped portions  34 , 35  the cylinder end surface  36 . The cylinder wall is defined by an annular cylinder wall portion  37  one end of which is received abutting against stepped portion  35  of the cylinder end plate  33 . The cylinder wall portion  37  has a smoothly cylindrical interior surface to define a space for sliding movement of the piston head. The exterior surface of the cylinder wall portion  37 , however, is formed with tapered surfaces  38 , 39  such that the thickness of the wall portion  37  increases away from the cylinder end plate  33  until it reaches a maximum and then decreases again. 
     Surrounding the portion of the cylinder wall portion  37  closest to the cylinder end plate  33  is an annular pressure means comprising plate  40  similarly sized to cylinder end plate  33  and having four apertures corresponding to the positions of shafts  30  so as to allow plate  40  to slide to and fro along the shafts  30 . Plate  40  has an inner annular portion  41  having a tapered surface  42  complementary to surface  38  of cylinder wall portion  37  and being in close engagement therewith. Spring means  45  are provided between cylinder end plate  33  and plate  40  so as to urge plate  40  away from cylinder end plate  33 . Surrounding tapered portion  39  of cylinder wall portion  37  is a locking ring  43  having a tapered inner surface  44  complementary to tapered surface  39 . 
     It will thus be understood that the effect of the spring means is to urge the plate  40  in a direction such that the tapered inner surface  42  acts on tapered portion  38  of cylinder wall portion  37  so as to urge portion  38  inwardly. Thus an external pressure is provided around the periphery of the cylinder so as to prevent the ceramic cylinder from cracking under the internal pressure of combustion. Plate  40  and locking ring  43  are preferably both made of aluminium for better heat dissipation. It will also be understood that the springs are not in contact with the cylinder and so do not present any obstacle to heat dissipation. 
     The embodiment described above is a four-cylinder engine. However the invention is equally applicable to an engine having a greater number of cylinders and FIG. 14 shows a sectional view through an engine block for an eight-cylinder embodiment. In this embodiment the engine block may be regarded as comprising three sections: two end sections  100 , 101  and a middle section  102 . Extending through the engine block is a crankshaft that may also be regarded as being made up of three sections  103 , 104 , 105 . In FIG. 14 the three sections are shown as being slightly separated, but this is for clarity of illustration only and in reality the three sections  103 , 104 , 105  are connected together so that they rotate as one shaft. 
     Each crankshaft section  103 , 104 , 105  is adapted to rotate about a common axis  106 , and each crankshaft section is rotatably mounted within respective engine block sections  100 , 101 , 102  by two annular bearing sets  107  per engine block section. Each engine block section  100 , 101 , 012  is provided with annular bearing sets at each end of the crankshaft section  103 , 104 , 105  which in addition to rotatably supporting the crankshaft sections  103 , 104 , 105  define spaces therebetween which may be used for other components. For example a lubricating pump may be located in the space defined in engine block section  100 . 
     As can be seen from FIG. 14 crankshaft section  104  is formed with two projecting axles  108 , 109  extending from opposite ends of the section  104  and parallel to but displaced from the central axis of rotation  106  of the crank shaft sections  103 , 104 , 105 . Crank shaft sections  103 , 105  are provided with corresponding recesses  110 , 111  for locating the ends of the axles  108 , 109  such that the three crank shaft sections come together to form a single commonly rotating crank shaft. It will be seen, however, that the recesses are shallower than the length of the axles  108 , 109  such that when the axles  108 , 109  are received in the recesses  110 , 111  there exists a space surrounding the axles between the ends of the crank shaft sections. Two such spaces are defined: one between crank shaft sections  103  and  104 , and the other between sections  104  and  105 . Into each such space—and fitted over the respective axles  108 , 109 —are located first and second cranks corresponding to first and second cranks  14 , 18  in the first embodiment described above. Thus four pistons may be driven in their respective cylinders by cranks located between engine block sections  100 , 102  and another four may be driven by cranks located between engine block sections  102 , 101 . This engine therefore has two sets of four cylinders disposed in mutually parallel planes. 
     In this embodiment, in respect of the pistons located between engine block portions  100 , 102  and between portions  102 , 101 , the crank shaft portion  104  functions as the equivalent of the primary crank  22  of the first embodiment. At the same time since the first crank shaft portion  103  and third crank shaft portion each have a weight offset caused by the presence of recesses  110 , 111  and so these portions can function as the respective counter-balancing weights.