Abstract:
A split-cycle engine includes a crankshaft rotatable about a crankshaft axis. A compression piston is slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through intake and compression strokes during a single rotation of the crankshaft. An expansion piston is slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through expansion and exhaust strokes during a single rotation of the crankshaft. A crossover passage interconnects the expansion and compression cylinders. The crossover passage includes crossover compression (XovrC) and crossover expansion (XovrE) valves defining a pressure chamber therebetween. At least one of the XovrC and XovrE valves is a balanced valve. A fluid pressure balancer biases the valve for balancing fluid pressures acting against the valve in both opening and closing directions, reducing the forces required in actuating the valve.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/218,757 filed Jul. 17, 2008 which claims the benefit of U.S. Provisional Patent Application No. 60/964,525 filed Aug. 13, 2007. 
     
    
     TECHNICAL FIELD 
       [0002]    This invention relates to pressure balanced valves for engines and particularly, though not exclusively, for use in split-cycle engines between the crossover passages and the expansion cylinders. 
       BACKGROUND OF THE INVENTION 
       [0003]    The term split-cycle engine as used in the present application may not have yet received a fixed meaning commonly known to those skilled in the engine art. Accordingly, for purposes of clarity, the following definition is offered for the term “split-cycle engine” as may be applied to engines disclosed in the prior art and as referred to in the present application. 
         [0004]    A split-cycle engine as referred to herein comprises: 
         [0005]    a crankshaft rotatable about a crankshaft axis; 
         [0006]    an expansion (power) piston slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft; 
         [0007]    a compression piston slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft; and 
         [0008]    a crossover passage interconnecting the expansion and compression cylinders, the crossover passage including a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween. 
         [0009]    Referring to the prior art,  FIG. 1  shows a split-cycle engine  1  having separate compression  2  and expansion  3  (combustion) cylinders connected by a pressurized crossover passage  4 . Another example is disclosed in U.S. Pat. No. 6,543,225 to Scuderi, filed on Jul. 20, 2001 and assigned to the assignee of the present invention, which is herein incorporated by reference in its entirety (the &#39;225 Pat.).  FIG. 1  (as well as the &#39;225 Pat.) illustrates inwardly opening poppet valves for the compression cylinder inlet valve  5 , the XovrE valve  6  and the exhaust valve  7 . The XovrC valve  8  is illustrated as a check valve but could be of any other suitable type, including an inwardly opening poppet valve similar to the other valves that move towards the piston when opening. 
       SUMMARY OF THE INVENTION 
       [0010]    The present invention provides various embodiments of outwardly opening pressure balanced valves that may be used in one or more locations of the split-cycle engine cylinders such as for the XovrC and XovrE valves, as well as for other uses. Outwardly opening valves move away from the piston and/or the cylinder when opening. In a split-cycle engine, they may assist in maximizing the compression and expansion ratios by reducing piston to head clearance volumes. In selected embodiments, the pressure balanced valves provide means for reducing the forces required in actuating the valves, particularly the cracking pressure and force to be overcome upon initial opening of the valve, when the crossover passage pressures are high and the compression or expansion cylinder pressures are low. 
         [0011]    In accordance with the present invention, a split-cycle engine includes a crankshaft rotatable about a crankshaft axis. A compression piston is slidably received within a compression cylinder and operatively connected to the crankshaft such that the compression piston reciprocates through an intake stroke and a compression stroke during a single rotation of the crankshaft. An expansion piston is slidably received within an expansion cylinder and operatively connected to the crankshaft such that the expansion piston reciprocates through an expansion stroke and an exhaust stroke during a single rotation of the crankshaft. A crossover passage interconnects the expansion and compression cylinders. The crossover passage includes a crossover compression (XovrC) valve and a crossover expansion (XovrE) valve defining a pressure chamber therebetween. At least one of the XovrC valve and the XovrE valve is a balanced valve. A fluid pressure balancer biases the valve for balancing fluid pressures acting against the valve in both opening and closing directions, reducing the forces required in actuating the valve. 
         [0012]    In the following disclosed exemplary embodiments, the invention includes outwardly opening XovrE valves that control the timing of charge air and/or fuel flow from a crossover passage into an expansion cylinder of a split-cycle engine. 
         [0013]    Referring to  FIGS. 2-3 , a first exemplary embodiment includes a spring seated poppet valve, having a disc shaped poppet head mounted on an end of a valve stem, which is actuated by a cam and rocker mechanism. The valve head has upper and lower surfaces (faces). The upper surface may also be referred to as an inner surface because it faces into the crossover passage, and the lower surface may also be referred to as an outer surface because it faces away from and is disposed outside of the crossover passage. A balance piston is also mounted on a medial section of the stem of the poppet valve to form a poppet valve assembly. The balance piston is reciprocable in an enclosed balance chamber controlled by timed valves to allow entry of crossover passage pressure into the balance chamber beneath the balance piston to balance crossover passage pressure against the upper surface (face) of the poppet valve head prior to valve opening. After opening, the balance chamber is cut off from the crossover passage and the balance pressure is exhausted to atmosphere. The poppet valve is then balanced by equal pressures on both sides of the head while the valve is open. 
         [0014]    Referring to  FIGS. 4-5 , a second exemplary embodiment is similar to the first except that a relocated air spring is substituted for the coil spring of the first embodiment. However, other types of springs or seating devices may be employed in variations of these embodiments if desired. 
         [0015]    Referring to  FIGS. 6-8 , a third embodiment includes a spring seated piston valve (piston valve assembly) having a cylindrical shaped piston head mounted on an end of a valve stem, which is actuated by a cam and rocker mechanism. The piston head is received in a cylinder recess, which forms a balance chamber between the head and the end of the recess. The piston head replaces both the poppet valve head and the balance piston of the first embodiment. Control valves vent recess pressure before opening. In direct contrast to the poppet head of the poppet valve, the piston head is not subject to excessive cracking force upon initially being opened because the crossover passage pressure is applied only radially around the uniform cylindrical body of the piston head. After opening, the control valves connect crossover passage pressure with the balance chamber in order to balance the crossover passage pressure acting on the lower face of the piston head. 
         [0016]    Referring to  FIGS. 9-11 , a fourth embodiment varies from the third embodiment in the use of alternative balanced piston valve actuators, such as electric, hydraulic pneumatic or mechanical. Also, control ports in the piston valve head and the engine substitute for control valves in controlling venting and admission of crossover passage pressure to the balance chamber. 
         [0017]    Referring to  FIGS. 12-14 , in a fifth embodiment, a balance piston and poppet valve head are substituted for the piston head of the third embodiment. Control valves vent the balance chamber above the balance piston in order to equalize crossover passage pressure acting on both the lower surface of the balance piston and the upper surface of the poppet valve head when the poppet valve is closed or beginning to open. The control valves open the balance chamber to the crossover passage to balance crossover passage pressure acting on the lower surface of the poppet valve head when the poppet valve is fully open. 
         [0018]    Referring to  FIGS. 15-17 , in a sixth embodiment, a balance piston and poppet valve head on a valve stem are substituted for the piston head of the fourth embodiment. Ports in the balance piston and engine work in the same manner as those in the fourth embodiment. While a mechanical cam, rocker arm and spring actuating mechanism is shown, any other suitable actuating mechanism may be substituted. 
         [0019]    Referring to  FIGS. 18-22 , a seventh embodiment discloses several variations which could use either a poppet valve assembly (a poppet valve having a poppet head and stem, which is combined with a balance piston) or a piston valve, however only poppet valve assemblies are illustrated. The variations all include a common feature of a balance port disposed in the engine, which provides fluid communication between the combustion chamber of the expansion cylinder and a balance chamber located above the balance piston. 
         [0020]    Referring to  FIG. 18 , in a first variation the balance port is open at all times. The poppet valve assembly remains balanced in either the open or closed position. Any suitable form of balance valve actuation may be utilized. 
         [0021]    Referring to  FIG. 19 , in a second variation, the balance port includes a control valve, which may be closed during combustion, to prevent gas flow into the balance chamber during combustion. 
         [0022]    Referring to  FIGS. 20-22 , in a third variation, a first balance port is disposed between the crossover passage and the balance chamber. A second balance port, similar to those of the first and second variations, is disposed between the combustion (expansion) chamber and the balance chamber. Control valves close the first balance port and open the second balance port during the engine piston exhaust stroke and through initial opening (cracking) of the engine valve. At or near top dead center and during the combustion and expansion stroke, the control valves open the first balance port and close the second balance port. 
         [0023]    Thus, during the engine exhaust stroke and when the poppet valve is cracking open, the poppet valve assembly is balanced by crossover passage pressure on the inner faces of the valve head and balance piston and by exhaust pressure on their outer faces, so that opening of the poppet valve is not impeded by an unbalanced high cracking pressure. When the poppet valve is fully open, crossover passage pressure communicates with inner and outer faces of both the balance piston and the valve head, thereby fully pressure balancing the valve assembly. When the poppet valve is closed on the expansion stroke, crossover passage pressure in the balance chamber assists in holding the poppet valve closed during combustion. Although mechanical valve actuation is illustrated, any suitable form of valve actuation may be utilized. 
         [0024]    Referring to  FIG. 23 , in an eighth embodiment a balance port is integrally incorporated into the poppet valve assembly itself, as opposed to the seventh embodiment where the balance port is separately incorporated into the engine. Specifically, the balance port extends from the expansion (combustion) chamber through the center of the valve head and axially into the valve stem. The port continues beyond the crossover passage and is connected with the balance chamber by lateral openings in the valve stem. Thus, the balance port tends to equalize the balance chamber and expansion chamber pressures at all times. Though the eighth embodiment only illustrates a poppet valve assembly, a piston valve assembly may also be utilized. 
         [0025]    These and other features and advantages of the invention will be more fully understood from the following detailed description of the invention taken together with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0026]    In the drawings: 
           [0027]      FIG. 1  is a schematic cross-sectional view of a prior art split-cycle engine; 
           [0028]      FIGS. 2 and 3  are schematic cross-sectional views of a first exemplary embodiment of the invention with a balanced poppet valve assembly, which is shown respectively in open and closed operating positions; 
           [0029]      FIGS. 4 and 5  are schematic cross-sectional views similar to  FIGS. 2 and 3  but showing a second exemplary embodiment of the invention with an air spring and balanced poppet valve assembly, which is shown respectively in similar operating positions to the first embodiment; 
           [0030]      FIGS. 6 ,  7  and  8  are schematic cross-sectional views showing a third exemplary embodiment of the invention with an alternative balanced cylindrical piston valve, which is shown respectively in initial opening (cracking), fully open and closed positions; 
           [0031]      FIGS. 9 ,  10  and  11  are schematic cross-sectional views showing a fourth exemplary embodiment of the invention with an alternative actuator and a balanced cylindrical piston valve with a control port in the piston head, which is shown respectively in initial opening (cracking), fully open and closed positions; 
           [0032]      FIGS. 12 ,  13  and  14  are schematic cross-sectional views showing a fifth exemplary embodiment of the invention including a poppet valve assembly having valved balance ports, the poppet valve assembly being shown respectively in initial opening, fully open and closed positions; 
           [0033]      FIGS. 15 ,  16  and  17  are schematic cross-sectional views showing a sixth exemplary embodiment of the invention including a poppet valve assembly having balance ports in the balance piston of the poppet valve assembly and in the engine, the poppet valve assembly being shown respectively in initial opening, fully open and closed positions; 
           [0034]      FIG. 18  is a schematic cross-sectional view of a first variation of a seventh exemplary embodiment of the invention wherein the first variation has, in common with the other variations, a poppet valve assembly and a separate balance port connected between a balance chamber and the combustion (expansion) chamber; 
           [0035]      FIG. 19  is a schematic cross-sectional view showing a second variation of the seventh exemplary embodiment of the invention wherein the balance port includes a control valve, which may be closed during combustion and/or expansion in the combustion chamber; 
           [0036]      FIGS. 20 ,  21  and  22  are schematic cross-sectional views showing a third variation of the seventh exemplary embodiment of the invention including a first valved balance port between the crossover passage and the balance chamber and a second valved balance port between the combustion chamber and the balance chamber, the poppet valve assembly being shown respectively in initial opening, fully open and closed positions; and 
           [0037]      FIG. 23  is a schematic cross-sectional view of an eighth exemplary embodiment of the invention of a poppet valve assembly having an integral balance port which extends axially through the valve stem and head of the poppet valve. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0038]    Referring first to  FIGS. 2 and 3 , numeral  10  generally indicates a first embodiment of a pertinent portion of a split-cycle engine of a type shown, for example, in the U.S. Pat. No. 6,542,225 previously noted. The exemplary engine  10  is shown schematically and not limited thereby as to general construction. Like reference numerals indicate like or similar components throughout the various embodiments. 
         [0039]    Engine  10  includes a combustion (expansion) cylinder  12  in which a power (expansion) piston  14  and connecting rod  16  are reciprocably connected to an output member, such as a crankshaft, not shown. A variable volume between the piston  14  and the closed end  18  of the cylinder  12  forms a combustion (expansion) chamber  20 . The combustion chamber communicates through an opening in a valve seat  22  with a crossover passage  24 , which stores and carries pressurized air from the compressor cylinder, not shown, for delivery to the combustion chamber  20 . The crossover passage may be kept at a variable but elevated pressure. 
         [0040]    In accordance with the invention, the valve seat  22  may be angled outwardly to provide for engagement by a disc shaped poppet head  26  of an outwardly opening poppet valve  28  having a stem  30 . The poppet head  26  is reciprocable within the crossover passage  24  and controls access to the combustion chamber  20 , cutting off air and/or fuel flow to the combustion chamber  20  when the head  26  is seated on the valve seat  22 . The valve head  26  has an upper surface (face)  61  and a lower surface (face)  62 . The upper surface  61  may also be referred to as an inner surface because it faces into the crossover passage  24 , while the lower surface  62  may also be referred to as an outer surface because it faces away from and is disposed outside of the crossover passage  24 . 
         [0041]    The poppet valve  28  is actuated by any suitable actuating mechanism  32 , mechanical, electrical, hydraulic pneumatic or combination thereof, as desired. The valve actuating mechanism  32  is represented in  FIGS. 2 and 3  by a cam  34  carried on a camshaft  35  and driving a pivotable rocker arm  36  that engages an actuator fitting  38  on the valve stem  30 . The fitting  38  also acts as a retainer for a valve spring  40  that engages a fixed member  42  of the engine and urges the valve  28  in a closing direction. 
         [0042]    A balance piston  44  (e.g., “fluid pressure balancer”) is carried on the valve stem  30  between the valve head  26  and the actuator fitting  38 . The valve head  26 , stem  30  and balance piston  44  may be referred to as a poppet valve assembly  46 . The balance piston  44  is reciprocable within an enclosed separate balance cylinder  48  of the engine spaced above the crossover passage  24 . The portion of the balance cylinder  48  below the balance piston  44  may be referred to as a balance chamber  50 . The balance chamber communicates with the crossover passage  24  by a first balance port  52  controlled by a first control valve  54  (V 1 ), such as a solenoid valve or other suitable valve. A second balance port  56  controlled by a second control valve  58  (V 2 ) communicates the balance chamber  50  with external ambient pressure. The poppet valve assembly  46 , actuating mechanism  32  and the associated balance chamber  50 , ports  52 ,  56  and valves  54  (V 1 ) and  58  (V 2 ) may be referred to as the balanced valve apparatus  60 . 
         [0043]      FIG. 2  of the drawings illustrates the position of the apparatus  60  when the poppet valve assembly  46  is open. The engine power piston  14  is beginning to descend while a pressurized air charge is forced through the valve seat  22  into the combustion chamber  20 . Since the valve head  26  is open to crossover passage pressure on both upper  61  and lower  62  faces, the first control valve  54  (V 1 ) is closed and the second control valve  58  (V 2 ) is open, venting the balance chamber  50  to ambient pressure. 
         [0044]      FIG. 3  shows the apparatus  60  positions when the poppet valve assembly  46  is closed. Second control valve  58  (V 2 ) is closed and first control valve  54  (V 1 ) is open, supplying crossover passage pressure to the balance chamber  50  so that the crossover pressure on the valve head  26  will be balanced. The opening force applied by the actuating mechanism  12  to (initially) crack open the valve head  26  is thus reduced. 
         [0045]      FIGS. 4 and 5  of the drawings illustrate a second embodiment of engine  68  and balanced valve apparatus  70  similar to the first embodiment of  FIGS. 2 and 3 . The balanced valve apparatus  70  of the second embodiment differs from the balanced valve apparatus  60  of the first embodiment in the substitution of an air spring  74  (within valve actuating mechanism  72 ) in place of the coil spring  40  shown in  FIGS. 2 and 3 . The air spring  74  is also relocated to engage the rocker arm  36  directly opposite its engagement with the cam  34 , but its function is the same. It should be understood that any other suitable spring or actuating mechanism could be used for operating the balanced valve assembly  46  of the invention if desired. Also, an air spring could be used in place of other springs in any form of mechanical actuating mechanism. 
         [0046]      FIGS. 6 ,  7  and  8  illustrate a third embodiment of engine  78  having a balanced valve apparatus  80 . The valve apparatus  80  includes a piston valve (piston valve assembly)  84 , having a cylindrical shaped piston head  82  mounted on a stem  83 . The piston valve  84  replaces the poppet valve assembly  46  of the first embodiment. The piston valve  84  is shown with the mechanical actuating mechanism  32 , but is not limited thereto. The piston valve  84  is reciprocable in a cylindrical recess  86  open to the crossover passage  24  but separated therefrom by the piston head  82 . 
         [0047]    The piston head  82  may be hollow to minimize its mass. A chamfer  88  on the lower periphery of the piston head  82  is adapted to seat on the valve seat  22 . The top (inner face)  90  of the piston head  82  and the end  92  of the recess  86  form a balance chamber  94  (e.g., “fluid pressure balancer”). A first balance port  96  controlled by a first control valve (V 1 )  98  communicates the balance chamber  94  with the crossover passage  24 . A second balance port  100  controlled by a second control valve (V 2 )  102  communicates the balance chamber with ambient pressure. 
         [0048]    In operation, when the engine power piston  14  is ascending during its exhaust stroke and discharging through an exhaust valve, not shown, the piston valve  84  is closed (seated on the valve seat). When the piston valve  84  is seated, the pressure in the crossover passage  24  can act only radially on the cylindrical outer surface of the piston head  82 . Since there is no vertical component to the crossover passage  24  pressure acting on the piston head  82 , the crossover passage pressure does not contribute to the cracking force that must be overcome upon initial opening of the head. 
         [0049]    Also during the power piston&#39;s  14  exhaust stroke, valve (V 1 )  98  is closed and valve (V 2 )  102  is open. Thus ambient pressure in the balance chamber  94  essentially balances exhaust pressure in the engine combustion chamber  20 . Therefore, when the piston valve  84  is cracked open (begins to open), as shown in  FIG. 6 , the actuating mechanism  32  can open the piston valve  84  by overcoming only the spring  40  seating force. 
         [0050]    As shown in  FIG. 7 , the crossover passage pressure acts against the bottom (outer face)  106  of the piston head  82  when the piston valve  84  is fully open. Thus valve V 1  ( 98 ) is opened and valve V 2  ( 102 ) is closed to direct crossover passage pressure to the balance chamber  94 . Then the pressure on the piston valve  84  remains balanced until the piston valve is closed by the valve spring  40  as shown in  FIG. 8 . This continues through combustion and the expansion stroke while crossover passage pressure is maintained in the balance chamber  94 , assisting the valve spring  40  to hold the piston valve  84  closed against combustion and expansion pressures. 
         [0051]    During the following exhaust stroke, the piston valve  84  is again cracked open as shown in  FIG. 6  and the cycle is repeated. 
         [0052]      FIGS. 9 ,  10  and  11  illustrate a fourth embodiment of engine  108  and balanced valve apparatus  110  similar to those of the third embodiment of  FIGS. 6 ,  7  and  8 . They differ in that an alternative valve actuating mechanism  114  is shown as representative of any suitable type shown schematically, such as electro-magnetic, pneumatic, hydraulic, mechanical or a combination thereof. A piston valve  116 , having a stem  117  and a modified piston head  119 , is disposed in a cylindrical recess  124 , defining a balance chamber  94 . Pressure in the balance chamber  94  is controlled by balance ports  118  (P 1 ),  120  (P 2 ) and  122  (P 3 ). 
         [0053]    Ports P 1  and P 2  are disposed in the engine  108  and piston head  119  respectively, and connect when the piston valve  116  is fully open to communicate crossover passage  24  pressure to the balance chamber  94 . At this time port P 3 , disposed in the engine  108 , is blocked by the piston head  119  as shown in  FIG. 10 , maintaining the balance chamber pressure. When the piston valve is fully closed ( FIG. 11 ) or cracked open ( FIG. 9 ), ports P 1  and P 2  are misaligned and block air flow from the crossover passage  24  while port P 3  is open and vents the balance chamber  94  to ambient pressure. The arrangement of ports P 1  and P 2  may be varied such that ports P 1 , P 2  connect earlier and disconnect later to have a longer “balanced period.” 
         [0054]      FIGS. 12 ,  13  and  14  illustrate a fifth embodiment of engine  128  having a balanced valve apparatus  130 , which includes a poppet valve assembly  132  opened and closed by valve actuating mechanism  32 . The valve actuating mechanism  32  is mechanical, although it is not so limited. 
         [0055]    The poppet valve assembly  132  includes a poppet valve  133  having a poppet head  134  disposed on the lower end of a valve stem  135 . The poppet valve assembly  132  also includes a balance piston  136  mounted on a medial section of the stem  135  of the poppet valve  133 . The balance piston  136  has a lower surface (face)  131  and an upper surface (face)  137 . The lower surface  131  may also be referred to as an inner surface because it faces into the crossover passage  24 , and the upper surface  137  may also be referred to as an outer surface because it faces away from and is disposed outside of the crossover passage  24 . 
         [0056]    The balance chamber  94 , balance ports  96 ,  100  and control valves  98 ,  102  are similar and operate in a like manner as previously mentioned components having the same reference numbers. Accordingly, during the power piston&#39;s  14  exhaust stroke, valve (V 1 )  98  is closed and valve (V 2 )  102  is open. Thus, ambient pressure in the balance chamber  94  essentially balances exhaust pressure in the engine combustion chamber  20 . Additionally, the vertical component of the crossover passage  24  pressure acting downwardly upon the upper surface (inner face)  129  of poppet head  134  is balanced by the same crossover passage pressure acting upwardly upon the lower surface (inner face)  131  of balance piston  136 . Therefore, when the poppet valve assembly  132  is cracked open (begins to open), as shown in  FIG. 12 , the actuating mechanism  32  can open the poppet valve assembly  132  by overcoming only the spring  40  seating force. 
         [0057]    As shown in  FIG. 13 , the crossover passage pressure acts upwardly against the lower surface (outer face)  139  of the poppet head  134  when the poppet valve assembly  132  is fully open. Thus valve V 1  ( 98 ) is opened and valve V 2  ( 102 ) is closed to direct crossover passage pressure to the balance chamber  94  and downwardly against the upper surface (outer face)  137  of the balance piston  136 . Then the pressure on the poppet valve assembly  132  remains balanced until the poppet valve assembly  132  is fully closed by the valve spring  40  as shown in  FIG. 14 . This continues through combustion and the expansion stroke while crossover passage pressure is maintained in the balance chamber  94 , assisting the valve spring  40  to hold the poppet valve assembly  132  closed against combustion and expansion pressures. 
         [0058]    During the following exhaust stroke, the poppet valve assembly  132  is again cracked open as shown in  FIG. 12  and the cycle is repeated. 
         [0059]      FIGS. 15 ,  16  and  17  illustrate a sixth embodiment of engine  138  having a balanced valve apparatus  140 , which includes a poppet valve assembly  142  opened and closed by valve actuating mechanism  32 . The valve actuating mechanism  32  is mechanical, although it is not so limited. 
         [0060]    The poppet valve assembly  142  includes a poppet valve  143  having a poppet head  141  disposed on the lower end of a valve stem  145 . The poppet valve assembly  142  also includes a balance piston  144  mounted on a medial section of the stem  145  of the poppet valve  143 . 
         [0061]    Port  120  (P 2 ) disposed in balance piston  144 , as well as ports  118  (P 1 ) and  122  (P 3 ) disposed in the engine  138 , are similar and operate in a like manner as previously mentioned components having the same reference numbers. Accordingly, ports P 1  and P 2  connect when the poppet valve assembly  142  is fully open to communicate crossover passage  24  pressure to the balance chamber  94 . At this time port P 3 , disposed in the engine  138 , is blocked by the balance piston  144  as shown in  FIG. 16 , maintaining the balance chamber pressure. When the poppet valve assembly  142  is fully closed ( FIG. 17 ) or cracked open ( FIG. 15 ), ports P 1  and P 2  are misaligned and block air flow from the crossover passage  24  while port P 3  is open and vents the balance chamber  94  to ambient pressure. At all times, crossover passage pressure is balanced against the inner faces  146  and  147  of the poppet head  141  and balance piston  144  respectively. 
         [0062]    Referring to  FIGS. 18-22 , a seventh embodiment of the invention includes three variations, all of which share a common feature of a balance port  152  disposed in an engine  148 , which provides fluid communication between a balance chamber  94  and a combustion chamber  20  of the engine  148 . Although all variations show a poppet valve assembly  149 , it should be noted that a single piston valve (such as piston valve  84  of the third embodiment) may also be used. 
         [0063]      FIG. 18  illustrates the first variation in which an engine  148  includes a balanced valve apparatus  150  with a poppet valve assembly  149 . The poppet valve assembly  149  includes a poppet valve  155  having a separate poppet head  157  and balance piston  159 . The poppet head  157  and balance piston  159  each include inner faces  151  and  153  respectively, both of which are open to the crossover passage  24 . When the poppet valve  155  is closed, the valve head  157  is seated on the valve seat  22  separating the crossover passage  24  from the engine combustion chamber  20 . The balance piston  159  forms a balance chamber  94  with an end  92  of a cylindrical recess  86 . 
         [0064]    In the first variation, a balance port  152  within the engine  148  provides fluid communication between the balance chamber  94  and the combustion chamber  20  in the expansion cylinder. Port  152  is always open to balance combustion chamber pressure on both the outer face  160  of poppet head  157  and the outer face  161  of balance piston  159 . Additionally, the downward vertical component of the crossover passage  24  pressure against the inner face  151  of poppet head  157  is always balanced against the upward vertical component of the crossover passage pressure against the inner face  153  of balance piston  159 . 
         [0065]    When the poppet valve  155  is open, the pressures tend to be equal on all faces  151 ,  153 ,  160  and  161 . The poppet valve  155  is opened and closed by a generic valve actuating mechanism  114 . 
         [0066]      FIG. 19  illustrates the second variation, which includes a control valve  154  in the port  152 . The valve  154  may be closed during combustion in the combustion chamber  20  to avoid fouling the chamber with combustion products and to reduce the compression ratio during the combustion process. 
         [0067]      FIGS. 20 ,  21  and  22  illustrate the third variation in which balance port  152  and control valve  154  are retained and an additional balance port  156 , controlled by a another control valve  158  (V 1 ), is added between the crossover passage  24  and the balance chamber  94 . The control valve  154  is identified as V 2 . The valve V 1  ( 158 ) is closed during the power piston&#39;s  14  exhaust stroke and during cracking open (initial opening) of the engine poppet valve  155 . However, control valve V 1  ( 158 ) is open at or near top dead center of the power piston  14  and during the expansion stroke. Valve V 2  ( 154 ) is open during the exhaust stroke and during poppet valve cracking but closed when the poppet valve  155  is fully open and during the expansion stroke. 
         [0068]    The results are like those of the fifth embodiment. The poppet valve  155  is pressure balanced during the power piston&#39;s  14  exhaust stroke and during its cracking open (i.e., the beginning of the opening of the poppet valve). During these periods, the pressure in the balance chamber  94  is essentially balanced with the pressure in the engine combustion chamber  20 . Additionally, the vertical component of the crossover passage  24  pressure acting downwardly upon the upper surface (inner face) of the poppet head is balanced by the same crossover passage pressure acting upwardly upon the lower surface (inner face) of the balance piston. The valve  155  remains balanced during charging of the combustion chamber  20  with crossover passage  24  pressure. During this period, the crossover passage  24  pressure acts upwardly against the bottom surface (outer face) of the valve head, and the same pressure in the balance chamber  94  acts downwardly against the upper surface (outer face) of the balance piston. The crossover passage  24  pressure remains in the balance chamber  94  through the expansion stroke after the valve  155  has closed to assist in offsetting combustion pressure in the combustion chamber  20 . 
         [0069]      FIG. 23  illustrates the eighth embodiment in which an engine  168  includes a balanced valve apparatus  170  having a poppet valve assembly  172  that is actuated by a generic actuating mechanism  114 . For ease of manufacturing, the eighth embodiment includes a balance port  178  integrally incorporated into the poppet valve assembly  172 , as opposed to the seventh embodiment where the balance port is separately incorporated into the engine. Although this embodiment shows a poppet valve assembly  172 , it should be noted that a single piston valve (similar to piston valve  84 ) with an internal balance port may also be used. 
         [0070]    Poppet valve assembly  172  includes a poppet valve  173  having a generally disc shaped poppet head  174  mounted to a lower end of a valve stem  176 . The poppet valve assembly  172  also includes a balance piston  175  mounted to a medial section of the stem  176 . The balance piston  175  reciprocates in a cylindrical recess  177  and defines a balance chamber  182  above the balance piston  175 . 
         [0071]    Poppet valve assembly  172  includes the internal balance port  178 , which extends axially from the expansion chamber  20  through the valve head  174  and valve stem  176  above the balance piston  175 . Lateral openings  180  extend the internal balance port  178  to provide fluid communication with the balance chamber  182 . 
         [0072]    Accordingly, pressure in the expansion chamber  20  maintains a pressure balance on the outer faces  188  and  190  of poppet valve head  174  and the balance piston  175  respectively at all times. Additionally, pressure in the crossover passage  24  maintains a pressure balance on the inner faces  184  and  186  of the poppet valve head  174  and the balance piston  175  respectively at all times. 
         [0073]    Although the invention has been described by reference to specific embodiments, it should be understood that numerous changes may be made within the spirit and scope of the inventive concepts described. Accordingly, it is intended that the invention not be limited to the described embodiments, but that it have the full scope defined by the language of the following claims.