Patent Publication Number: US-6904876-B1

Title: Sodium cooled pistons for a free piston engine

Description:
BACKGROUND OF INVENTION 
   The present invention relates to free piston engines. 
   Conventionally, internal combustion engines have operated with the motion of the pistons mechanically fixed. For example, a conventional internal combustion engine for a motor vehicle includes a crankshaft and connecting rod assemblies that mechanically determine the motion of each piston within its respective cylinder. This type of engine is desirable because the position of each piston is know for any given point in the engine cycle, which simplifies timing and operation of the engine. While these conventional types of engines have seen great improvements in efficiency in recent years, due to the nature of the engines, that efficiency is still limited. In particular, the power density is limited because the mechanically fixed motion of the pistons fixes the compression ratio. Moreover, all of the moving parts that direct the movement of the pistons (and camshafts and engine valves as well) create a great deal of friction, which takes energy from the engine itself to overcome. The resulting lower power density means that the engine will be larger and heavier than is desired. Also, the flexibility in the engine design and packaging is limited because of all of the mechanical connections that must be made. 
   Consequently, is desirable, for environmental and other reasons, to have an engine with a higher power density than these conventional engines. The advantages of lighter relative weight, smaller package size, and improved fuel efficiency can be a great advantage in both vehicle and stationary power production applications. 
   Another type of internal combustion engine is a free piston engine. This is an engine where the movement of the pistons in the cylinders is not mechanically fixed. The movement is controlled by the balance of forces acting on each piston at any given time. Since the motion is not fixed, the engines can have variable compression ratios, which allow for more flexibility in designing the engine&#39;s operating parameters. Also, since there are no conventional crankshafts and rods attached to the crankshaft, which reduces piston side force, there is generally less friction produced during engine operation. However, these types of engines have not come into common use because, with free pistons, the complexity of engine operation is greatly increased. 
   One concern, in particular, is assuring sufficient heat transfer from each piston to its cylinder wall. Without this, there may be locations of overheating on the free piston assembly. Crankshaft engines inherently induce a side loading of the pistons, which is reacted against the cylinder walls. The contact induced by this side loading allows for significant heat transfer from each piston to its cylinder wall. But in a free piston engine, it is undesirable and unnecessary that there be side loading, thus eliminating the contact between the piston skirt and the cylinder wall. While this reduces the friction between the piston and cylinder and reduces the amount of lubrication oil needed on the cylinder walls, it also reduces the contact area for transferring heat. The ability to adequately cool a piston is especially important for engine configurations where there is a piston that operates adjacent to that cylinder&#39;s exhaust port. 
   SUMMARY OF INVENTION 
   In its embodiments, the present invention contemplates a piston assembly for use in a cylindrical combustion cylinder of an engine where the combustion cylinder is centered about an axis of motion. The piston assembly preferably includes a main body having a head portion, an opposed rear portion, and a cylindrical side wall extending therebetween, with the head portion adapted to be oriented generally normal to the axis of motion and the cylindrical side wall adapted to be generally centered about and extending in the direction of the axis of motion, and with the main body including at least one cooling bore contained therein and extending generally from adjacent to the head portion to adjacent to the rear portion; and a liquid sodium compound contained within and filling a portion of the at least one cooling bore. 
   In its embodiments, the present invention also preferably contemplates a free piston engine including an energy generation and control assembly having a first side and a second side in opposed relation to the first side; a first combustion cylinder assembly located adjacent to the first side of the energy generation and control assembly and including a first cylinder liner that defines a first engine cylinder, which is centered about an axis of motion; and a second combustion cylinder assembly located adjacent to the second side of the energy generation and control assembly and including a second cylinder liner that defines a second engine cylinder, which is centered about the axis of motion. The free piston engine also preferably includes an inner piston assembly including a first inner piston having a first main body with a first head portion, an opposed, first rear portion, and a first cylindrical side wall extending therebetween, with the first head portion oriented generally normal to the axis of motion and the first cylindrical side wall generally centered about and extending in the direction of the axis of motion, and with the first main body including at least one cooling bore contained therein and extending generally from adjacent to the first head portion to adjacent to the first rear portion; a second inner piston having a second main body with a second head portion, an opposed, second rear portion, and a second cylindrical side wall extending therebetween, with the second heat portion oriented generally normal to the axis of motion and the second cylindrical side wall generally centered about and extending in the direction of the axis of motion, and with the second main body including at least one cooling bore contained therein and extending generally from adjacent to the second head portion to adjacent to the second rear portion; and a push rod having a first end affixed to the first inner piston and a second end affixed to the second inner piston and a middle portion operatively engaging the energy generation and control assembly; and a liquid sodium compound contained within and filling a portion of each of the at least one cooling bores in the first main body and a portion of each of the at least one cooling bores in the second main body. 
   An advantage of an embodiment of the present invention is that a free piston engine, with an inherent ability to more easily vary the an opposed piston, opposed cylinder (OPOC) configuration of a free piston engine allows for a more inherently balanced free piston engine, while also being conducive for effective homogeneous charge, combustion ignition (HCCI) engine operation. Such an engine can operate with relatively few major moving parts, generally having less overall friction to overcome during engine operation than a crank engine. 
   Another advantage of an embodiment of the present invention is that the side of the free piston does not react load against the cylinder wall, thus reducing the friction between the piston and the cylinder wall. Moreover, since the side of the piston does not react a load against the cylinder wall, less lubricating oil is required along the cylinder wall. 
   A further advantage of an embodiment of the present invention is that, the sodium compound in the bores will assist in better transferring heat from the piston head to the piston rings as well as better equalizing the heat transfer through each of the rings, thus improving overall heat transfer from the piston to the wall of the engine cylinder. 

   
     BRIEF DESCRIPTION OF DRAWINGS 
       FIG. 1  is a perspective view of an opposed piston, opposed cylinder, free piston engine with hydraulic control and output, in accordance with the present invention. 
       FIG. 2  is an end view of the engine of  FIG. 1 . 
       FIGS. 3A and 3B  are a top, plan view of the engine of  FIG. 1 . 
       FIGS. 4A and 4B  are a side view of the engine of  FIG. 1 . 
       FIG. 5A  is a sectional view of the engine taken along line  5 A— 5 A in  FIG. 3A . 
       FIG. 5B  is a sectional view of the engine taken along line  5 B— 5 B in  FIG. 3B . 
       FIG. 6A  is a sectional view of the engine taken along line  6 A— 6 A in  FIG. 4A . 
       FIG. 6B  is a section view of the engine taken along line  6 B— 6 B in  FIG. 4B . 
       FIG. 7  is a perspective view of a portion of the engine of  FIG. 1 ; and, more specifically, a perspective view of the top of a hydraulic pump block assembly and inner piston assembly. 
       FIG. 8  is a perspective view similar to  FIG. 7 , but viewing the bottom of the hydraulic pump block assembly and inner piston assembly. 
       FIG. 9  is a perspective view of a cylinder liner of the engine of  FIG. 1 . 
       FIG. 10  is a schematic view of the hydraulic circuit of the engine of  FIG. 1 . 
       FIG. 11  is a schematic view of some of the electronic circuit employed with the engine of  FIG. 1 . 
       FIG. 12  is a perspective view of the inner piston assembly of the engine of  FIG. 1 , but without the piston rings shown for clarity in illustrating the cooling bores. 
       FIG. 13  is a partial section cut, taken along line  13 — 13  in  FIG. 12 . 
       FIG. 14  is partial section cut, taken along line  14 — 14  in  FIG. 12 . 
   

   DETAILED DESCRIPTION 
     FIGS. 1–14  illustrate an opposed piston, opposed cylinder, hydraulic, free piston engine  10 . The engine  10  includes a hydraulic pump block assembly  12 , with a first piston/cylinder assembly  14  extending therefrom, and a second piston/cylinder assembly  16  extending from the hydraulic pump block assembly  12  in the opposite direction so they are in line. The timing of the first piston/cylinder assembly  14  is opposite to the timing of the second piston/cylinder assembly  16 . Thus, when one is at top dead center, the other is at bottom dead center. Moreover, the motion is along or parallel to a single axis of motion. This configuration of free piston engine allows for a more inherently balanced engine. 
   Additionally, the following description discloses an engine that not only stores energy produced by the engine in the form of pressurized fluid, but also employs some of this pressurized fluid to start and, at times, assist in controlling the engine operation and maintaining the engine balance. 
   The first piston/cylinder assembly  14  includes a first cylinder jacket  18 , which mounts to the hydraulic pump block assembly  12 . The first cylinder jacket  18  includes a first exhaust gas scroll  20 , which is located adjacent to the hydraulic pump block assembly  12 . The interior of the first exhaust gas scroll  20  defines an inner exhaust channel  22  that extends circumferentially around the first cylinder jacket  18  and radially outward to a first exhaust flange  24 . The exhaust flange  24  is adapted to connect to an exhaust system (not shown) for carrying away the exhaust during engine operation. The exhaust system can be any type desired so long as it adequately treats and carries away the exhaust gasses. It may, for example, include an exhaust manifold, a muffler, a catalytic converter, a turbocharger, or a combination of these and possibly other components. 
   The first cylinder jacket  18  also has a coolant inlet  26 , which is located adjacent to the hydraulic pump block assembly  12 , and extends into a generally circumferentially extending coolant passage  28 . The coolant inlet  26  connects to a coolant cooling system (not shown), which can include, for example, a heat exchanger, such as a radiator, for removing heat from the engine coolant, a water pump for pumping the coolant through the cooling system, a temperature sensor and flow control valve for maintaining the coolant in a desired temperature range, coolant lines extending between the components, or a combination of these and possibly other components. The cooling system can be any type of engine cooling system desired so long as it removes the appropriate amount of heat from the engine. 
   At the opposite end of the first cylinder jacket  18  from the exhaust gas scroll  20  is a circumferentially extending air intake annulus  30 , the interior of which defines an intake channel  31 . Adjacent to the air intake annulus  30 , the first cylinder jacket  18  forms a fuel injector boss  32 , within which a first fuel injector  34  is mounted. The first fuel injector  34  is electrically connected to an electronic controller  35 , which provides a signal for determining the timing and duration of fuel injector opening. The first fuel injector  34  also connects to a fuel injector rail  37 , which supplies fuel from a fuel system  39  (only shown schematically). The fuel system  39  may include, for example, a fuel tank, fuel pump, fuel lines leading to the fuel rail, or a combination of these and possibly other components. Any type of fuel system that can provide an adequate amount of fuel under the desired pressure to the fuel injector  34  is generally acceptable. Preferably, the fuel injector rail  37  also includes a fuel pressure sensor  41  that is electrically connected to the controller  35 . The controller  35  is preferably powered by an electrical system with a battery (not shown), an electric generator or alternator, which is preferably powered by energy output from the engine  10 , or some other adequate supply of electrical power. Also, while the controller  35  is referred to in the singular herein, it may include multiple electronic processors in communication with one another, if so desired. 
   About mid-way between the first exhaust gas scroll  20  and the intake annulus  30 , the first cylinder jacket  18  forms a pressure sensor mounting boss  36 , within which is mounted a first cylinder pressure sensor  38 . The first cylinder pressure sensor  38  is preferably electrically connected to the controller  35 . Both the fuel injector boss  32  and the sensor mounting boss  36  extend through the first cylinder jacket  18  to a main bore  40  that extends the length of the first cylinder jacket  18 . The coolant passage  28 , inner exhaust channel  22  and the air intake annulus  30  are all open into the main bore  40  as well. 
   The first piston/cylinder assembly  14  also includes a first cylinder liner  42 , which extends through and is preferably press fit into the main bore  40  of the first cylinder jacket  18 . The first cylinder liner  42  includes a cylindrical shaped main bore extending therethrough that defines the first engine cylinder  44 . The central axis of the first engine cylinder is preferably along the axis of motion. The first cylinder liner  42  also includes a series of circumferentially spaced exhaust ports  46 , which extend between and connect the first engine cylinder  44  and the inner exhaust channel  22  of the first cylinder jacket  18 . 
   Adjacent to the exhaust ports  46 , the first cylinder liner  42  abuts the coolant passage  28  in the first cylinder jacket  18 . This coolant passage  28  connects to a series of spaced, helical ribs  48  that extend radially outward from the first cylinder liner  42  and abut the main bore  40  of the first cylinder jacket  18 , forming a series of cylinder coolant passages  50 . Within these ribs  48 , a cylinder pressure tap boss  52  extends from the first engine cylinder  44  to the sensor mounting boss  36  on the first cylinder jacket  18 . This allows the first cylinder pressure sensor  38  to be exposed to the first engine cylinder  44 , while sealing the sensor  38  from the engine coolant. 
   A fuel injector bore  54  aligns with the fuel injector boss  32  and extends through the ribs  48  to the first engine cylinder  44 . This allows the first fuel injector  34  to inject fuel directly into the first engine cylinder  44 . 
   The first cylinder liner  42  also has a series of circumferentially spaced air intake ports  56 , aligned with the air intake annulus  30  of the first cylinder jacket  18 , and opening into the first cylinder  44 . Adjacent to the air intake ports  56 , is a series of spaced oil mist holes  58  located circumferentially around the first cylinder liner  42 . 
   The first piston/cylinder assembly  14  also includes a first air belt  60 . The air belt  60  is mounted about the first cylinder liner  42 , abutting the first cylinder jacket  18  at the location of the air intake annulus  30 . An oil inlet tube  62  projects from and extends through the first air belt  60 , connecting to an oil mist annulus  64 . The oil mist annulus  64  abuts and extends circumferentially around the first cylinder liner  42  at the location of the oil mist holes  58 . The oil inlet tube  62  preferably connects to an oil mister (not shown), which has an inlet connected to a source of oil, and provides a mixture of oil and air to the oil mist annulus  64 . The source of oil may be a part of an oil supply system (not shown). The oil supply system may include, for example, an oil pump, an oil filter, an oil cooler, an oil sump, oil lines to transfer the oil through the system, or a combination of these and possibly other components. The oil supply system can be any such system that can cooperate with the engine components to adequately filter and supply lubrication oil to the engine while it is operating. 
   Also abutting and extending circumferentially around the first cylinder liner  42  is a coolant annulus  66 . The coolant annulus  66  connects to the cylinder coolant passages  50  and also to a coolant outlet  68  extending from the first air belt  60 . This coolant outlet  68  connects to the coolant cooling system (not shown), which was discussed above. The first air belt  60  also has a pair of pull rod passages  70  and an intake air passage  72  that are in communication with the air intake annulus  30  of the first cylinder jacket  18 . 
   The first piston/cylinder assembly  14  also incorporates a first scavenge pump  74 . The scavenge pump  74  includes a scavenge pump housing  76  that mounts to the first air belt  60 , and around the end of the first cylinder liner  42 . The scavenge pump housing  76  has a main pumping chamber  78 , with inlet ports  80  leading to an inlet chamber  82  and outlet ports  84  leading to an outlet chamber  86 . The main pumping chamber  78  is cylindrical in shape, with a generally elliptical cross section. 
   Mounted to the inlet chamber  82  is an inlet reed valve assembly  88  and a scavenge pump inlet cover  90 . The inlet cover  90  includes an air inlet  92 , which preferably connects to an air intake system (not shown). The air intake system may include, for example, an intake manifold that preferably receives air from some type of a turbocharger or mechanical supercharger, an air throttling valve, a mass air flow sensor, an ambient air temperature sensor, an air filter, or a combination of these and possibly other components. The air intake system may be any such system that supplies a desired volume of air at a desired pressure to the air inlet  92  for the particular engine operating conditions. 
   Reed valves  94  in the inlet reed valve assembly  88  are oriented to allow air flow into the inlet chamber  82  from the inlet cover  90 , but prevent air flow in the opposite direction. An outlet reed valve assembly  89  and scavenge pump outlet cover  91  are mounted to the outlet chamber  86 . The outlet cover  91  includes an air intake passage  93  that leads from the outlet reed valve assembly  89  to the air intake channel  31  of the first cylinder jacket  18  via the intake air passage  72  in the first air belt  60 . Reed valves  95  in the outlet reed valve assembly  89  are oriented to allow airflow out of the outlet chamber  86  to the air intake passage  93 , but prevent airflow in the opposite direction. 
   The second piston/cylinder assembly  114  includes a second cylinder jacket  118 , which mounts to the hydraulic pump block assembly  12 . The second cylinder jacket  118  includes a second exhaust gas scroll  120  that is located adjacent to the hydraulic pump block assembly  12 . The interior of the second exhaust gas scroll  120  defines an inner exhaust channel  122  that extends circumferentially around the second cylinder jacket  118  and radially outward to a second exhaust flange  124 . The exhaust flange  124  is adapted to connect to the exhaust system (not shown), discussed briefly above. The second cylinder jacket  118  also has a coolant inlet  126 , which is located adjacent to the hydraulic pump block assembly  12 , and extends into a generally circumferentially extending coolant passage  128 . The coolant inlet  126  connects to the coolant cooling system (not shown). 
   At the opposite end of the second cylinder jacket  118  from the exhaust gas scroll  120  is a circumferentially extending air intake annulus  130 , the interior of which defines an intake channel  131 . Adjacent to the air intake annulus  130 , the second cylinder jacket  118  forms a fuel injector boss  132 , within which a second fuel injector  134  is mounted. The second fuel injector  134  is electrically connected to the electronic controller  35 , which provides a signal for controlling the timing and duration of fuel injector opening. The second fuel injector  134  also connects to the fuel injector rail  37 , which supplies fuel from the fuel system  39 . The fuel system  39  may include, for example, a fuel tank, fuel pump and fuel lines leading to the fuel rail. Preferably, the fuel injector rail  37  also includes a fuel pressure sensor  141  that is electrically connected to the controller  35 . 
   About mid-way between the second exhaust gas scroll  120  and the intake annulus  130 , the second cylinder jacket  118  forms a pressure sensor mounting boss  136 , within which is mounted a second cylinder pressure sensor  138 . Both the fuel injector boss  132  and the sensor mounting boss  136  extend through the second cylinder jacket  118  to a main bore  140  that extends the length of the second cylinder jacket  118 . The coolant passage  128 , inner exhaust channel  122  and the air intake annulus  130  are all open into the main bore  140  as well. 
   The second piston/cylinder assembly  114  also includes a second cylinder liner  142 , which extends through and is preferably press fit in main bore  140  of the second cylinder jacket  118 . The second cylinder liner  142  includes a cylindrical shaped main bore extending therethrough that defines the second engine cylinder  144 . The central axis of the second engine cylinder  144  is preferably along the axis of motion. The second cylinder liner  142  also includes a series of circumferentially spaced exhaust ports  146 , which extend between and connect the second engine cylinder  144  and the inner exhaust channel  122  of the second cylinder jacket  18 . 
   Adjacent to the exhaust ports  146 , the second cylinder liner  142  abuts the coolant passage  128  in the second cylinder jacket  118 . This coolant passage  128  connects to a series of spaced, helical ribs  148  that extend from the second cylinder liner  142  and abut the main bore  140  of the second cylinder jacket  118  to form a series of cylinder coolant passages  150 . Within these ribs  148 , a cylinder pressure tap boss  152  extends from the second engine cylinder  144  to the sensor mounting boss  136  on the second cylinder jacket  118 . This allows the second cylinder pressure sensor  138  to be exposed to the second engine cylinder  144 , while sealing the sensor  138  from the engine coolant. 
   A fuel injector bore aligns with the fuel injector boss  132  and extends through the ribs  148  to the second engine cylinder  144 . This allows the second fuel injector  134  to extend through to the second engine cylinder  144  and inject fuel therein. 
   The second cylinder liner  142  also has a series of circumferentially spaced air intake ports  156 , aligned with the air intake annulus  130  of the second cylinder jacket  118  and opening into the second engine cylinder  144 . Adjacent to the air intake ports  156 , is a series of spaced oil mist holes  158 , which are located circumferentially around the second cylinder liner  142 . 
   The second piston/cylinder assembly  114  also includes a second air belt  160 . The air belt  160  is mounted about the second cylinder liner  142 , abutting the second cylinder jacket  118  at the location of the air intake annulus  130 . An oil inlet tube  162  projects from and extends through the second air belt  160 , connecting to an oil mist annulus  164 . The oil mist annulus  164  abuts and extends circumferentially around the second cylinder liner  142  at the location of the oil mist holes  158 . The oil inlet tube  162  preferably connects to the oil mister (not shown), in order to provide an oil and air mixture to the oil mist annulus  164 . 
   Also abutting and extending circumferentially around the second cylinder liner  142  is a coolant annulus  166 . The coolant annulus  166  connects to the cylinder coolant passages  150  and also to a coolant outlet  168  extending from the second air belt  160 . This coolant outlet  168  connects to the coolant cooling system (not shown), discussed above. The second air belt  160  also has a pair of pull rod passages  170  and an intake air passage  172  that are in communication with the air intake annulus  130  of the second cylinder jacket  118 . 
   The second piston/cylinder assembly  114  also incorporates a second scavenge pump  174 . The scavenge pump  174  includes a scavenge pump housing  176  that mounts to the second air belt  160  and around the end of the second cylinder liner  142 . The scavenge pump housing  176  has a main pumping chamber  178 , with inlet ports  180  leading to an inlet chamber  182  and outlet ports  184  leading to an outlet chamber  186 . The main pumping chamber  178  is cylindrical in shape, with a generally elliptical cross section. Mounted to the inlet chamber  182  is an inlet reed valve assembly  188  and a scavenge pump inlet cover  190 . The inlet cover  190  includes an air inlet  192 , which preferably connects to the inlet manifold (not shown) that preferably receives air from some type of a supercharger or turbocharger (not shown). Reed valves  194  in the inlet reed valve assembly  188  are oriented to allow air flow into the inlet chamber  182  from the inlet cover  190 , but prevent air flow in the opposite direction. 
   An outlet reed valve assembly  189  and scavenge pump outlet cover  191  are mounted to the outlet chamber  186 . The outlet cover  191  includes an air intake passage  193  that leads from the outlet reed valve assembly  189  to the air intake channel  131  of the second cylinder jacket  118  via the intake air passage  172  in the second air belt  160 . Reed valves  195  in the outlet reed valve assembly  189  are oriented to allow air flow out of the outlet chamber  186  to the air intake passage  193 , but prevent air flow in the opposite direction. 
   Contained within the two piston/cylinder assemblies  14  and  16  are two piston assemblies—an inner piston assembly  200  and an outer piston assembly  250 . The inner piston assembly  200  has a first inner piston  202  that is mounted within the first engine cylinder  44 , with the head  210  of the first inner piston  202  facing away from the hydraulic pump block assembly  12 , and the rear  211  facing toward the hydraulic pump block assembly  12 . The first inner piston  202  mounts within the first engine cylinder  44  with a small clearance between its outer diameter and the wall of the first engine cylinder  44 . Accordingly, the first inner piston  202  also preferably includes three ring grooves about its periphery, with the first groove receiving a first compression ring  204 , the second receiving a second compression ring  206  and the third receiving an oil control ring  208 . All three of the rings  204 ,  206 , and  208  are sized to seal against the wall of the first engine cylinder  44 . 
   The first inner piston  202  preferably includes a first set of spaced, generally axially extending cooling bores  212 —extending from the rear  211  of the piston  202  toward the head  210  in a direction generally parallel to the axis of motion. Each bore  212  is partially filled with a sodium compound  215  and has a cap  214  for sealing the sodium compound  215  in the bore  212 . The sodium compound is preferably a liquid that is the same as or similar to the sodium compounds used to cool exhaust valves in some high performance engines. Also, preferably, two of the caps  219  are modified to also receive and retain guide rods (discussed below). The first inner piston  220  also preferably includes a second set of cooling bores  213  interleaved with the first set of cooling bores  212 . The second set of cooling bores  213  are preferably oriented radially inward as they extend from the rear  211  of the piston  202  toward the head  210 . Each bore is partially filed with a sodium compound  217  and has one of the caps  214  for sealing the sodium compound  217  in the bore  213 . By alternating the orientation of the second set of cooling bores  213  relative to the first set of cooling bores  212 , it is believed that heat can be better drawn from all portions of the head  210 —both radially outer and radially inner portions. However, as an alternative, both sets of cooling bores  212  and  213  can have the same orientation in the piston  202 , if so desired. 
   The inner piston assembly  200  further includes a second inner piston  220  that is mounted within the second engine cylinder  144 , with the head  222  of the second inner piston  220  facing away from the hydraulic pump block assembly  12  and the rear  223  facing toward the hydraulic pump block assembly  12 . The second inner piston  220  mounts within the second engine cylinder  144  with a small clearance between its outer diameter and the wall of the second engine cylinder  144 . Accordingly, the second inner piston  220  also preferably includes three ring grooves about its periphery, with the first groove receiving a first compression ring  224 , the second receiving a second compression ring  226  and the third receiving an oil control ring  228 . All three of the rings  224 ,  226 , and  228  are sized to press and seal against the wall of the second engine cylinder  144 . 
   The second inner piston  220  also preferably includes a first set of spaced, generally axially extending cooling bores  230 —extending from the rear  223  of the inner piston  220  toward the head  222 . Each bore  230  is preferably partially filled with the sodium compound and has a cap  232  for sealing the sodium compound in the cooling bore  230 . Again it is preferred to have a second set of cooling bores  231  interleaved with the first set of cooling bores  230 , with the second set of cooling bores  231  oriented radially inward as they extend from the rear  223  to the head  222  of the second inner piston  220 . 
   The first inner piston  202  includes a centrally located, axially extending bore  216  therethrough that receives a fastener  218 , and the second inner piston  220  also includes a centrally located, axially extending bore  234  therethrough that receives a fastener  236 . The fasteners  218  and  236  are each threaded to respective ends of a push rod  240 , which extends through the hydraulic pump block assembly  12 . The push rod  240 , being fixed to each inner piston  202  and  220 , causes the two pistons  202  and  220  to move in unison, preferably along the axis of motion. The push rod  240  also includes an enlarged diameter region, which forms an inner plunger  242 . The inner plunger  242  is located midway between the two pistons  202  and  220 . The purpose of the inner plunger  242  will be discussed below with reference to the hydraulic pump block assembly  12 . 
   The inner piston assembly  200  also preferably includes a first guide rod  244  and a second guide rod  245 , with each extending through the hydraulic pump block assembly  12  to connect between the rear faces  211  and  223  of the first and second inner pistons  202  and  220 . The guide rods  244  and  245  keep the inner piston assembly  200  from rotating during engine operation. Also, preferably, at least one, and more preferably, both of the guide rods  244  and  245  include position sensor indices that can be employed to determine the axial position of the inner piston assembly  200  during engine operation. Such indices may take the form of a first set of copper rings  246  fixed around the first guide rod  244 . The second guide rod  245  also preferably includes indices, such as a second set of cooper rings  247 . The second guide rod  245  can then be employed as part of a position calibration sensor for assuring that the position sensor on the first guide rod  244  is reading the axial position of the inner piston assembly  200  accurately. 
   The outer piston assembly  250  has a first outer piston  252  that is mounted within the first engine cylinder  44 , with the head  254  of the first outer piston  252  facing toward the head  210  of the first inner piston  202 , and the rear  256  facing toward the first scavenge pump main chamber  78 . The first outer piston  252  mounts within the first engine cylinder  44  with a small clearance between its outer diameter and the wall of the first engine cylinder  44 . Accordingly, the first outer piston  252  also preferably includes three ring grooves about its periphery, with the first groove receiving a first compression ring  258 , the second receiving a second compression ring  260  and the third receiving an oil control ring  262 . All three of the rings  258 ,  260 , and  262  are sized to seal against the wall of the first engine cylinder  44 . 
   Mounted on the rear  256  of the first outer piston  252  is a first piston bridge  264 . The first piston bridge  264  moves with and essentially forms a portion of the first outer piston  252 . The first piston bridge  264  includes an outer, generally elliptical shaped portion  266  that is in sliding contact with and seals against the wall of the main pumping chamber  78  of the first scavenge pump  74 . The minor diameter of the elliptical portion  266  is preferably slightly smaller than the diameter of the head  254  of the first outer piston  252 , while the major diameter of the elliptical portion  266  is significantly larger than the diameter of the head  254 . A first pull rod boss  268  and a second pull rod boss  269  are located along the major diameter of the elliptical portion  266 , radially outward of the outer diameter of the first outer piston  252 . 
   A guide post boss  270  is located in the center of the first piston bridge  264 , centered on the axis of motion for the first outer piston  252 . A first guide post  271  is fixed to and extends from the first scavenge pump housing  76 . The first guide post  271  has a generally cylindrical outer surface that is centered about an extends parallel to the axis of motion. This outer surface just slips within the guide post boss  270  in order to allow the guide post boss  270  to telescopically slide along the first guide post  271 . Since the first guide post  271  is fixed, its position can be located accurately relative to the first engine cylinder  44 . The first guide post  271 , then, will allow for very accurate orientation of the first piston bridge  264  and hence the first outer piston  252  relative to the first engine cylinder  44 . 
   The guide post boss  270 , then, will slide on the guide post  271  during engine operation, maintaining proper orientation of the first outer piston  252  as it reciprocates in the first engine cylinder  44  so the only the piston rings  258 ,  260  and  262  are in contact with the wall of the first engine cylinder  44 . This generates only a relatively small amount of friction since generally only the piston rings  258 ,  260 , and  262  and guide post boss  270  are in sliding contact with other surfaces, while the outer surface of the first outer piston  252  moves without being in contact with the wall of the first engine cylinder  44 . 
   The outer piston assembly  250  also has a second outer piston  275  that is mounted within the second engine cylinder  144 , with the head  276  of the second outer piston  275  facing toward the head  222  of the second inner piston  220 , and the rear  277  facing toward the second scavenge pump main chamber  178 . The second outer piston  275  mounts within the second engine cylinder  144  with a small clearance between its outer diameter and the wall of the second engine cylinder  144 . Accordingly, the second outer piston  275  also preferably includes three ring grooves about its periphery, with the first groove receiving a first compression ring  278 , the second receiving a second compression ring  279  and the third receiving an oil control ring  280 . All three of the rings  278 ,  279 , and  280  are sized to seal against the wall of the second engine cylinder  144 . While the first outer piston  252  and second outer piston  275  are shown without sodium cooling channels, channels can be employed similar to the way they are employed with the inner pistons, if so desired. 
   Mounted on the rear  277  of the second outer piston  275  is a second piston bridge  282 . The second piston bridge  282  includes an outer, generally elliptical shaped portion  283  that is in sliding contact with and seals against the wall of the main pumping chamber  178  of the second scavenge pump  174 . The minor diameter of the elliptical portion  283  is preferably slightly smaller than the diameter of the head  276  of the second outer piston  275 , while the major diameter of the elliptical portion  283  is significantly larger than the diameter of the head  276 . A first pull rod boss  284  and a second pull rod boss  285  are located along the major diameter of the elliptical portion  283 , radially outward of the outer diameter of the second outer piston  275 . 
   A guide post boss  286  is located in the center of the second piston bridge  282 . A second guide post  287  is fixed to and extends from the second scavenge pump housing  176 . The second guide post  287  has a generally cylindrical outer surface that is centered about and extends parallel to the axis of motion. The outer surface slips within the guide post boss  286 . With the second guide post  287  being fixed relative to the second engine cylinder  144 , it will accurately align the second piston bridge  282  and hence the second outer piston  275  relative to the second engine cylinder  144 . The guide post boss  286 , then, will slide on the guide post  287  during engine operation, maintaining proper orientation of the second outer piston  275  as it reciprocates in the second engine cylinder  144 , so that the piston rings  278 ,  279  and  280  are in contact with the wall of the second engine cylinder  144 . Again, the friction will be minimized, while also allowing for proper guiding of the engine piston. 
   The second guide post  287  also forms part of a position sensor assembly  288 . The position sensor assembly  288  includes a sensor rod  289 , which has at least one index location  290 , affixed to and slidable with the second outer piston  275 . A sensor  291  mounts about the sensor rod  289  and extends through the second scavenge pump housing  176 , where an electrical connector  292  will connect the sensor  291  to the electronic controller  35 . The controller  35  can use the output from the sensor  291  to determine the position and velocity of the outer piston assembly  250 . 
   The outer piston assembly  250  also includes a first pull rod  293  and a second pull rod  294 . The first pull rod  293  connects between the first pull rod boss  268  on the first piston bridge  264  and the first pull rod boss  284  on the second piston bridge  282 . Since the bridges  264  and  282  are elliptical, the first pull rod  293  can couple them together and allow for movement parallel to the axis of motion without interfering with the operation of the engine cylinders. 
   The first pull rod  293  includes an enlarged diameter region, which forms a first outer plunger  295 . The first outer plunger  295  is located in the hydraulic pump block assembly  12  mid-way between the first piston-bridge  264  and the second piston-bridge  282 . A first pull rod sleeve  272  extends about the first pull rod  293  between the hydraulic pump block assembly  12  and the first cylinder jacket  18 , and a second pull rod sleeve  273  extends about the first pull rod  293  between the hydraulic pump block assembly  12  and the second cylinder jacket  118 . The pull rod sleeves  272  and  273  assure that the first pull rod  293  is entirely enclosed by engine components, thus preventing contaminants from contacting and interfering with the operation of the first pull rod  293 . 
   The second pull rod  294  connects between the second pull rod boss  269  on the first piston bridge  264  and the second pull rod boss  285  on the second piston bridge  282 . The second pull rod  294  includes an enlarged diameter region, which forms a second outer plunger  296 . The second outer plunger  296  is located in the hydraulic pump block assembly  12  mid-way between the first piston-bridge  264  and the second piston-bridge  282 . A third pull rod sleeve  274  extends about the second pull rod  294  between the hydraulic pump block assembly  12  and the first cylinder jacket  18 , and preferably a position sensing pull rod sleeve  281  extends about the second pull rod  294  between the hydraulic pump block assembly  12  and the second cylinder jacket  118 . The pull rod sleeves  274  and  281  assure that the second pull rod  294  is entirely enclosed by engine components, thus preventing contaminants from contacting and interfering with the operation of the second pull rod  294 . 
   Additionally, the second pull rod  294  preferably includes spaced copper rings  298  mounted thereon and located within the position sensing pull rod sleeve  281 . The position sensing pull rod sleeve  281  preferably includes a sensor assembly  297  located in close proximity to the copper rings  298 . The sensor assembly  297  is then connected to the controller  35 , and will detect the position of the copper rings  298 . The controller  35  can then use the output from the sensor assembly  29  to calibrate the other sensor  291 , thus assuring an accurate measurement of the position and velocity of the outer piston assembly  250 . 
   It is preferable for the engine  10  to be balanced in order to assure optimal operating characteristics. For the engine to be balanced, the total mass of the outer piston assembly  250 —that is, all of the parts that move with the outer pistons  252  and  275 —must equal the total mass of the inner piston assembly  200 —that is, all of the parts that move with the inner pistons  202  and  220 . Also, preferably, for a balanced engine, the hydraulic area of the inner plunger  242  of the push rod  240  is equal to the sum of the hydraulic areas of the outer plungers  295  and  296  of the pull rods  292  and  294 —with the hydraulic area of the first outer plunger  295  being equal to the hydraulic area of the second outer plunger  296 . Accordingly, the materials for the different components in the piston assemblies  200  and  250  are chosen to assure adequate thermal and strength characteristics while also balancing the masses of the assemblies. For example, the inner pistons  202  and  220 , and the push rod  240  may be made of cast iron, the pull rods  293  and  294  also made of cast iron, while the outer pistons  252  and  275  are made of aluminum and the elliptical shaped bridges  264  and  282  are made of steel. Although, other suitable materials may be employed, if desired. 
   As discussed above, the hydraulic pump block assembly  12  mounts between the first piston/cylinder assembly  14  and the second piston/cylinder assembly  16 . It includes a pump block  302 , preferably made of steel, through which various hydraulic porting and passages, coolant passages and lubrication oil sump and passages are formed. 
   The pump block  302  includes a push rod bore  304  through which the push rod  240  extends. The inner plunger  242  seals circumferentially around the push rod bore  304 . Both ends of the central bore  304  also seal against the push rod  240 —one end employing a seal plug  309  to create the seal. These seals form an inner pumping chamber  306  on one side of the inner plunger  242  and an inner coupler-pumping chamber  308  on the other side of the inner plunger  242 . 
   The pump block  302  also includes a first pull rod bore  310  through which the first pull rod  293  extends, and a second pull rod bore  312  through which the second pull rod  294  extends. The first outer plunger  295  seals circumferentially around the first pull rod bore  310  and the second outer plunger  296  seals circumferentially around the second pull rod bore  312 . The first pull rod bore  310  is shaped to seal, at each end, against the first pull rod  293 , with a seal plug  311  again employed at one end for sealing. The pull rod bore  310 , in conjunction with the first pull rod  293 , forms a first outer pumping chamber  314  on one side of the first outer plunger  295 , and a first outer coupler pumping chamber  316  on the other side of the first outer plunger  295 . The second pull rod bore  312  is shaped to seal, at each end, against the second pull rod  294 , with a seal plug  313  again employed at one end for sealing. The second pull rod bore  312 , in conjunction with the second pull rod  294 , forms a second outer pumping chamber  318  on one side of the second outer plunger  296 , and a second outer coupler pumping chamber  320  on the other side of the second outer plunger  296 . 
   The inner coupler-pumping chamber  308  and the first outer coupler pumping chambers  316  are connected with a first cross connecting passage  322 . In addition, the inner coupler pumping chamber  308  and the second outer coupler pumping chamber  320  are connected with a second cross connecting passage  323 . Consequently, the three-coupler pumping chambers  308 ,  316  and  320  are always in open fluid communication with each other. 
   A low-pressure passage  324 , with a restriction  326 , leads from the second cross connecting passage  323  to a first coupler adjustment valve  328 . The first coupler adjustment valve  328  is connected to the low-pressure reservoir  330  side of the hydraulic system  329 . It can be switched between a position that allows fluid flow from the second cross connecting passage  323  to the low pressure reservoir  330 , and a position that blocks such fluid flow. A high-pressure passage  332 , with a restriction  334 , leads from the first cross connecting passage  322  to a second coupler adjustment valve  336 . The second coupler adjustment valve  336  is connected to the high-pressure reservoir  338  side of the hydraulic system  329 . It can be switched between a position that allows fluid flow from the high pressure reservoir  338  to the first cross connecting passage  322 , and a position that blocks such fluid flow. The first and second coupler adjustment valves  328  and  336  are electrically connected to and operated by the electronic controller  35 . 
   A resonator passage  340  extends between the second cross connecting passage  323  and a Helmholtz resonator  342 , which is mounted on the pump block  302 . The Helmholtz resonator  342  is tuned to damp pulsations that occur as the fluid flows back and forth between the coupler pumping chambers  308 ,  316  and  320  through the cross connecting passages  322  and  323 . The Helmholtz resonator  342  may be eliminated from the engine  10 , if so desired. 
   These cross connecting passages  322  and  323 , together with the hydraulic components connected to them, form a hydraulic circuit that hydraulically couples the movement of the inner piston assembly  200  with the outer piston assembly  250 . Since, with the coupler adjustment valves  328  and  336  closed, the volume in the coupler pumping chambers  308 ,  316  and  320 , and the cross connecting passages  322  and  323 , is filled with an essentially incompressible liquid (such as hydraulic oil), this volume will remain constant. Also, as noted above, the inner plunger  242  of the push rod  240  is sized to displace twice the volume of fluid (per amount of linear movement) as each of the outer plungers  295  and  296  of the pull rods  293  and  294 , respectively. Consequently, if the inner piston assembly  200  moves one millimeter to the right, displacing fluid out of the inner coupler pumping chamber  308 , then the outer piston assembly  250  must move one millimeter to the left, in order to receive that amount of fluid in the two outer coupler pumping chambers  316  and  320 . This assures that, even though the motions of the inner piston assembly  200  and the outer piston assembly  250  are not mechanically fixed, they will move in virtually exact opposition to each other. Consequently, the top dead center and bottom dead center positions for the two piston assemblies  200  and  250  are reached simultaneously. 
   The first and second coupler adjustment valves  328  and  336  allow for the addition or removal of some of the fluid from the couplers should leakage around any seals change the volume of the fluid retained in the couplers. While this hydraulic system for coupling the piston assemblies  200  and  250  has been described, other mechanisms for assuring that the piston assemblies  200  and  250  move opposed to one another may be employed if so desired. 
   The hydraulic pump block assembly  12  also includes a pair of oil inlets  344  and  345  that extend through the pump block  302  to an oil sump  346  located on the underside of the pump block  302 . The oil sump  346  is open to various moving components in the pump block assembly  12  in order to allow for splash lubrication of the moving components—particularly the portion of the cylinder walls  44  and  144  along which the first and second inner pistons  202  and  220  slide. The oil sump  346  also includes an oil return outlet  348 . The oil inlets  344  and  345 , and the oil return outlet  348  are connected to the oil supply system (not shown). The oil sump  346  also allows for air to move back and forth behind the inner pistons  202  and  220  as they reciprocate during engine operation. 
   Two coolant inlets  350  are mounted on the bottom of the pump block  302 . The coolant inlets  350  connect to a series of coolant passages  352  that extend throughout the pump block  302 , which then connect to two coolant outlets  354  mounted on the top of the pump block  302 . The coolant inlets  350  and the coolant outlets  354  connect to the coolant cooling system (not shown). The coolant flowing through the pump block  302  will assure that the moving parts do not overheat during engine operation. 
   The hydraulic pump block assembly  12  also includes a low pressure rail  356 , mounted on top of the pump block  302 , that includes a low pressure rail port  358  connected through a hydraulic line to the low pressure reservoir  330 . The low pressure rail  356  opens to three sets of one-way low pressure check valves, an inner set  360 , a first outer set  362  and a second outer set  363 . The inner set of check valves  360  connects through a passage  364  to the inner pumping chamber  306 , with the valve set  360  only allowing fluid flow from the low pressure rail  356  to the inner pumping chamber  306 . The first outer set of check valves  362  connects through a passage  365  to the first outer pumping chamber  314 , with the valve set  362  only allowing fluid flow from the low pressure rail  356  to the first outer pumping chamber  314 . The second outer set of check vales  363  likewise connects through a passage  366  to the second outer pumping chamber  318 , with the valve set  363  only allowing fluid flow from the low pressure rail  356  to the second outer pumping chamber  318 . While the inner set of check valves  360  includes four individual valves and each of the outer sets of check valves  362  and  363  includes two valves, different numbers of individual valves can be employed, if so desired. But preferably, the inner set  360  provides for twice the valve open area as each of the outer sets  362  and  363  since the inner plunger  242  has twice the pumping capacity as either of the outer plungers  295  and  296 . 
   A high pressure rail  368  mounts to the bottom of the pump block  302  and includes a high pressure rail port  369  connected through a hydraulic line to the high pressure reservoir  338 . The high pressure rail  368  opens to three one-way high pressure check valves, an inner check valve  370 , a first outer check valve  371  and a second outer check valve  372 . The inner check valve  370  connects to the inner pumping chamber  306  via a fluid passage  373 , with the check valve  370  only allowing fluid flow from the inner pumping chamber  306  to the high pressure rail  368 . The first outer check valve  371  connects to the first outer pumping chamber  314  via a fluid passage  374 , with the check valve  371  only allowing fluid flow from the first outer pumping chamber  314  to the high pressure rail  368 . The second outer check valve  372  connects to the second outer pumping chamber  318  via a fluid passage  375 , with the check valve  372  only allowing fluid to flow from the second outer pumping chamber  318  to the high pressure rail  368 . Again, the inner check valve  370  preferably has twice the opening area as each of the outer check valves  371  and  372 . 
   The low pressure rail  356  preferably includes a pressure sensor  376  mounted therein for measuring the pressure of the fluid in the low-pressure rail  356 . The high-pressure rail  368  likewise preferably includes a pressure sensor  377  mounted therein for measuring the pressure of the fluid in the high-pressure rail  368 . Both of the pressure sensors  376  and  377  are electrically connected to the electronic controller  35 , for receiving and processing the pressure signals. 
   Mounted on top of the pump block  302 , adjacent to the low-pressure rail  356 , is a hydraulic starting and control valve  379 . This hydraulic starting and control valve  379  is only shown schematically herein, but is preferably a hydraulic valve such as, for example, a Moog hydraulic control valve part number 35-196-4000-I-4PC-2-VIT, made by Moog Inc. of East Aurora, N.Y. The control valve  379  engages four ports on the pump block  302 , a high pressure port  380 , a low pressure port  381 , an inner pumping chamber port  382  and an outer pumping chamber port  383 . The high-pressure port  380  is connected through a fluid passage to the high-pressure rail  368 , and the low-pressure port  381  is connected through a fluid passage to the low pressure rail  356 . The inner pumping chamber port  382  connects through a first starting/spilling fluid passage  384  to the inner pumping chamber  306 , while the outer pumping chamber port  383  connects through a second starting/spilling fluid passage  385  to the two outer pumping chambers  314  and  318 . 
   The control valve  379  can operate to hydraulically connect the high pressure port  380  with the inner pumping chamber port  382 , while at the same time connecting the low pressure port  381  with the outer pumping chamber port  383 . The control valve  379  can also operate to hydraulically connect the low pressure port  381  with the inner pumping chamber port  382 , while at the same time connecting the high pressure port  380  with the outer pumping chamber port  383 . Under a third operating condition, the control valve  379  will block the flow of hydraulic fluid between the high and low pressure ports  380  and  381  and both the inner and the outer pumping chamber ports  382  and  383 . The electronic controller  35  preferably controls which operating state the control valve  379  is in. 
   The hydraulic pump block assembly  12  may also include piston stoppers, which set a maximum distance at each end of travel for the pistons. These stops may be needed due to the fact that the piston motion is determined by a balance of the forces—rather than a fixed mechanical path—for a free piston engine. Piston stops for the inner piston assembly  200  preferably include radially stepped portions  388  spaced on either side of the inner plunger  242  of the push rod  240 , with matching stops  389  located at each end of the central bore  304 —on the pump block  302  and the seal plug  309 . The relative position of the stepped portions  388  to the stops  389  will determine the maximum travel of the inner piston assembly  200  in either direction. If the stepped portions  388  engage the stops  389 , the piston motion in that direction will stop. 
   Piston stops for the outer piston assembly  250  preferably include radially stepped portions  390  and  391  spaced on either side of the outer plungers  295  and  296  of the first and second pull rods  293  and  294 , respectively. The pump block  302  and seal plugs  311  and  313 , in a similar fashion to the inner piston assembly  200 , will include matching stops  392  and  393 , located on opposite ends of the first and second pull rod bores  310  and  312 , respectively. 
   As an alternative, the piston stops may be eliminated. With this configuration, the head  210  of the first inner piston  202  hitting the head  254  of the first outer piston  252  will act as a stop in one direction, while the head  222  of the second inner piston  220  hitting the head  276  of the second outer piston  275  will act as a stop in the other direction. While this may at first seem undesirable, the piston heads have relatively large surface areas for contact, and, the pressure within the cylinder where the pistons are acting as stops will rise dramatically just prior to collision, thus slowing the speed at impact. 
   The hydraulic pump block assembly  12  also preferably includes a pair of position sensors. A first position sensor  395  is mounted in the pump block  302  surrounding the portion of the first guide rod  244  that includes the first set of copper rings  246 . Preferably, a second position sensor  396  is mounted in the pump block  302  surrounding the portion of the second guide rod  245  that includes the second set of copper rings  247 . The position sensors  395  and  396  are electrically connected and provide position signals to the electronic controller  35 . With the sensor information from the first position sensor  395 , the electronic controller  35  can determine the position and velocity of the inner piston assembly  200 . The information from the second position sensor  396  is preferably used for calibration of the first position sensor  395 . 
   The operation of the engine  10  will now be described. Since this engine  10  is a free piston engine, the piston motion is determined by a balance (equilibrium) of forces acting on the piston assemblies  200  and  250 . For example, the major forces are generally in-cylinder pressures of the opposed engine cylinders  44  and  144 , the friction created by the various moving parts, the air scavenging, the inertia of the moving piston assemblies  200  and  250 , and any loads caused by the plungers  242 ,  295  and  296 . Consequently, the piston assemblies  200  and  250  each must receive input forces at the appropriate time and amount in order to cause sustained reciprocal piston motion. This reciprocal motion must be sufficient to obtain the needed compression in the cylinders  44  and  144  for the combustion process. By employing inputs to control the motion of the piston assemblies  200  and  250 , especially near the end of travel for each stroke, the piston top dead center positions, and hence the compression ratio, can be controlled. Moreover, the ability to vary the compression ratio makes HCCI combustion much more feasible, since the compression ratio needed to cause combustion can vary based on engine operating conditions. Since the balance of forces must be precisely timed and controlled, the electronic controller  35  monitors and actuates the engine components that are critical for efficient and sustained engine operation. 
   Prior to engine start-up, the high-pressure reservoir  338  of the hydraulic system  329  retains a hydraulic fluid under a relatively high pressure, which may be, for example, 5,000 to 6,000 pounds per square inch (PSI). The low-pressure reservoir  330  of the hydraulic system  329  retains hydraulic fluid under a relatively low pressure, which may be, for example, 50 to 60 PSI. 
   Upon initiation of the engine starting process, the electronic controller  35  energizes the starting and control valve  379 , alternating between a first valve position with the high pressure port  380  open to the inner pumping chamber port  382  and the low pressure port  381  open to the outer pumping chamber port  383 , and a second valve position with the high pressure port  380  open to the outer pumping chamber port  383  and the low pressure port  381  open to the inner pumping chamber port  382 . 
   In the first valve position of the control valve  379 , fluid from the high pressure reservoir  338  will be pushed into the inner pumping chamber  306 , causing the inner plunger  242  of the push rod  240 , and hence the entire inner piston assembly  200 , to begin moving to the right (as illustrated in the figures herein). This will cause the fluid in the inner coupler pumping chamber  308  to be pushed through the first and second cross connecting passages  322  and  323  and into the first and second outer coupler pumping chambers  316  and  320 . This, in turn, will cause the first and second outer plungers  295  and  296  of the first and second pull rods  293  and  294 , respectively, and hence the entire outer piston assembly  250 , to begin moving to the left (as illustrated in the figures herein). As the outer piston assembly  250  moves to the left, fluid from the first and second outer pumping chambers  314  and  318  will be pushed through the control valve  379  and into the low pressure reservoir  330 . 
   This opposed movement of the two piston assemblies  200  and  250  will cause the first outer piston  252  and first inner piston  202  to simultaneously move apart toward their bottom dead center positions in the first engine cylinder  44 , while the second outer piston  275  and second inner piston  220  will move simultaneously at one another toward their top dead center positions in the second engine cylinder  144 . Both piston assemblies  200  and  250  move back and forth along a single, linear axis of motion. The single axis of motion extends through the center of the two engine cylinders  44  and  144 , as indicated by the double arrows shown in the engine cylinders  44  and  144  in  FIGS. 10 and 11 . 
   In the second valve position of the control valve  379 , fluid from the high pressure reservoir  338  will be pushed into the first and second outer pumping chambers  314  and  318 , causing the first and second outer plungers  295  and  296  of the first and second pull rods  293  and  294 , respectively, and hence the entire outer piston assembly  250 , to begin moving to the right. This will cause the fluid in the first and second outer coupler pumping chambers  316  and  320  to be pushed through the first and second cross connecting passages  322  and  323  and into the inner coupler pumping chamber  308 . This will, in turn, cause the inner plunger  242  of the push rod  240 , and hence the entire inner piston assembly  200 , to begin moving to the left. As the inner piston assembly  200  moves to the left, fluid from inner pumping chamber  306  will be pushed through the control valve  379  and into the low pressure reservoir  330 . 
   This opposed movement of the two piston assemblies  200  and  250  will cause the first outer piston  252  and first inner piston  202  to simultaneously move at one another toward their top dead center positions in the first engine cylinder  44 , while the second outer piston  275  and second inner piston  220  will move simultaneously away from one another toward their bottom dead center positions in the second engine cylinder  144 . 
   By precisely and rapidly switching between the three valve positions of the starting and control valve  379 , the piston assemblies  200  and  250  can be made to alternately switch between causing compression in the first engine cylinder  44  and causing compression in the second engine cylinder  144 . The electronic controller  35 , by monitoring the position sensors  288  and  395 , determines the position and velocity of both piston assemblies  200  and  250 . The position and velocity information is then employed by the controller  35  to determine the appropriate timing for the switching of the starting and control valve  379  in order cause the desired amount of compression ratio in the engine cylinders  44  and  144 . One can see from this discussion, then, that the starting and control valve  379  controls the movement of the piston assemblies  200  and  250  at engine start-up in a way that will cause the piston assemblies  200  and  250  to move as needed for engine operation. 
   The engine  10  operates as a two stroke engine, and without any separate valve system to open and close the intake and exhaust ports of the engine cylinders  44  and  144 . Thus, the compression, combustion (which includes ignition), expansion, and gas exchange (which includes intake and exhaust) of the fuel/air mixture is accomplished over two strokes of the pistons. This arrangement minimizes the number of moving parts as well as minimizing the total package size of the engine  10 . 
   The movement of the inner piston assembly  200  causes the inner pistons  202  and  220  to selectively block and open the exhaust ports  46  and  146  to the respective engine cylinders  44  and  144 . The movement of the outer piston assembly  250  causes the outer pistons  252  and  275  to selectively block and open the intake ports  56  and  156  to the respective engine cylinders  44  and  144 , as well as causing the piston bridges  264  and  282  to charge the intake air. The movement of the outer piston assembly  250  also causes the outer pistons  252  and  275  to selectively block and expose the fuel injectors  34  and  134 , respectively, to the engine cylinders  44  and  144 . Consequently, the motion of the inner and outer piston assemblies  200  and  250  caused by the starting and control valve  379  provides the movement needed to bring air charges into the engine cylinders  44  and  144 , allow for fuel to be supplied into the cylinders to mix with the charge air, and provide compression sufficient for combustion to occur. 
   Preferably, the combustion process under normal operating conditions is a homogeneous charge, compression ignition (HCCI) type, which takes advantage of the variable compression ratio capability of this engine  10  to allow for this very high efficiency type of combustion. The HCCI process employs a homogeneous air/fuel charge mixture that is auto-ignited due to a high compression ratio; that is, pre-mixed fuel/air charges are compression heated to the point of auto-ignition (also called spontaneous combustion). With the auto-ignition caused by the HCCI process, there are numerous ignition points throughout the fuel/air mixture to assure rapid combustion, which allows for low equivalence ratios (the ratio of the actual fuel-to-air ratio to the stoichiometric ratio) to be employed since no flame propagation is required. This results in improved thermal efficiency while reducing peak cylinder temperatures, significantly reducing the formation of oxides of nitrogen versus the more conventional types of internal combustion engines. Although, if so desired, spark plugs may be employed in each engine cylinder, with the engine operating as a spark ignition engine. 
   More specifically, the intake, compression, combustion and exhaust events will be described for the first engine cylinder  44  (being equally applicable to the second engine cylinder  144 ) during normal HCCI engine operation. The movement of the first outer piston  252  charges the intake air as well as determines the timing and duration of the air intake ports  56  and first fuel injector  34  being open to the first engine cylinder  44 . As the first outer piston  252  moves toward its top dead center position, the volume in the main pumping chamber  78  of the first scavenge pump  74  increases, causing air to be pulled in through the inlet reed valves  94 . 
   After top dead center—typically after a combustion event—the movement of the first outer piston  252  reduces volume in the main pumping chamber  78 , causing the air to be compressed and forced out through the outlet reed valves  95  and into the air intake passages  93  and  72  and the intake channel  31 . As the first outer piston  252  continues to move toward its bottom dead center position, it will expose the air intake ports  56 , allowing the compressed air to flow into the first engine cylinder  44  from the intake channel  31 . The first fuel injector  34  is also exposed to the first engine cylinder  44  at this time. The controller  35  will activate the first fuel injector  34 , causing fuel to be sprayed into the incoming air charge. The outer piston position sensor  291  is employed by the controller  35 , as well as the fuel pressure sensor  41 , in order to determine the timing and duration of fuel injector actuation. 
   After reaching bottom dead center, the first outer piston  252  moves toward the top dead center position again. During this movement, the first outer piston  252  will close off the air intake ports  56  and the fuel injector bore  54  from the first engine cylinder  44 . The air/fuel charge is compressed as the first outer piston  252  continues to move toward the top dead center position. One will note that the first fuel injector  34  injects directly into the first engine cylinder  44 , yet it is not directly exposed t the combustion event since it is covered by the first outer piston  252  when the piston  252  is at or near top dead center. 
   The movement of the first inner piston  202  determines the timing and duration of the exhaust ports  46  being open to the first engine cylinder  44 . As the first inner piston  202  moves away from top dead center—typically after a combustion event—the piston  202  will move past the exhaust ports  46 , allowing the exhaust gases to flow out through the exhaust ports  46 . The exhaust gasses will then flow through the first exhaust gas scroll  20  and out through rest of the exhaust system (not shown). After bottom dead center, the first inner piston  202  moves toward top dead center and, part of the way through this stroke, will cover the exhaust ports  46 , effectively closing them. Any exhaust gasses that have not flowed out through the exhaust ports  46  at this time will remain in the cylinder  44  as internal exhaust gas recirculation (EGR) during the next combustion event. As the first inner piston  202  continues to move toward top dead center, the air/fuel charge is compressed. 
   As the first inner piston  202  reciprocates, the sodium compound  215  and  217  in the cooling bores  212  and  213 , respectively, is splashed back and forth. The significant heat increase in the first inner piston  202  will be at or near the head  210  since this is the face exposed to the combustion and is also near the exhaust ports  46 . Thus, as the sodium compound  215  and  217  moves near the head  210 , it will tend to absorb heat, while it will tend to give off heat as it moves toward the rear  211 . This heat redistribution will facilitate heat transfer to the rings  204 ,  206  and  208  as well as more equal heat transfer through all three piston rings  204 ,  206  and  208  to the wall of the first engine cylinder  44 . 
   Since the second engine cylinder  144  operates opposed to the first engine cylinder  44 , the combustion event in the first engine cylinder  44  will cause the first inner and outer pistons  202  and  252  to be driven apart while the combustion event in the second engine cylinder  144  will cause the first inner and outer pistons  202  and  252  to move toward one another (causing compression in the first cylinder  44 ), thereby continually perpetuating the engine operating cycle. The self-sustaining operation of the engine  10 , then, is maintained by controlling the fuel injection prior to each of the combustion events, taking into account the various operating conditions under which the engine  10  is operating at the time. The fuel injection control can be used to control the length of the piston stroke, which must be enough to obtain the compression ratio needed for combustion but avoid collisions with the piston stops. Of course, to allow for transient conditions, occasional non-combustion events, system imbalances, and other factors, the starting and control valve  379  can be employed at times, in combination with the fuel control, to correct the piston motion. This includes assuring not only the appropriate compression ratio is reached for the given engine operating conditions, but also that the auto-ignition occurs at or just after the top dead center positions in order to avoid wasting combustion energy changing the direction of the motion of the piston assemblies  200  and  250 . 
   During normal engine operation, as the combustion events cause the piston assemblies  200  and  250  to reciprocate, the push rod  240  and pull rods  293  and  294  will drive the plungers  242 ,  295 , and  296  back and forth in their respective bores  304 ,  310 , and  312 . As the inner piston assembly  200  moves to the right (as seen in the figures), movement of the inner plunger will cause the inner set of low pressure check valves  360  to open, allowing fluid from the low pressure rail  356  to be drawn into the inner pumping chamber  306 . The fluid leaving the low-pressure rail  356  is replenished from the low-pressure reservoir  330 . The amount of fluid maintained within the low pressure rail  356  and the ability of the low pressure reservoir  330  to refill the low pressure rail  356  must be sufficient to maintain the fluid flow through the sets of low pressure check valves. Otherwise, cavitation problems can occur. 
   At the same time, the outer piston assembly  250  moves to the left, with the outer plungers  295  and  296  causing the fluid in the first and second outer pumping chambers  314  and  318  to be pumped through the first and second outer high pressure check valves  371  and  372  to the high pressure rail  368 . This displaces fluid into the high pressure reservoir  338 . This fluid under pressure in the high-pressure reservoir  338  is then available as a stored energy source for the engine operation as well as driving other components and systems. Since the hydraulic fluid energy available is a function of the pressure level and the amount of hydraulic fluid flow, one can use the desired energy output when deciding upon the piston stroke, the piston frequency and/or the dimensions of the hydraulic fluid plungers when initially laying out the dimensions for the engine. For the piston frequency, generally, the higher the mass of the moving piston assemblies, the lower the optimal operating frequency of the engine. 
   During the engine stroke that causes the inner piston assembly  200  to move to the right, the inner plunger  242  pumps fluid from the inner coupler-pumping chamber  306  to the two outer coupler-pumping chambers  316  and  320 . As discussed above, this allows the two-piston assemblies  200  and  250  to maintain an opposed motion to one another. If the position sensors  288  and  395  detect that the two piston assemblies  200  and  250  are not centered appropriately in the engine cylinders, then one of the coupler adjustment valves  328  and  336  can be activated to correct for the offset. 
   During the following engine stroke, as the inner piston assembly  200  moves to the left, the fluid pressure created by the inner plunger  242  will open the inner high pressure check valve  370 , forcing fluid to flow to the high pressure rail  368  and on to the high pressure reservoir  338 . The outer piston assembly  250  simultaneously moves to the right, with the outer plungers  295  and  296  causing fluid to be drawn from the low pressure rail  356  through the first and second outer sets of low pressure check valves  362  and  363 . During this engine stroke, the outer plungers  295  and  296  also pump fluid from the outer coupler pumping chambers  316  and  320  to the inner coupler pumping chamber  306 . 
   Accordingly, since the inner piston assembly  200  and outer piston assembly  250  always move opposed to one another—and hence the inner plunger  242  always moves opposed to the two outer plungers  295  and  296 —each stroke of the engine provides only for either the inner plunger  242  or the outer plungers  295  an  296  to pump fluid to the high pressure reservoir  338 . The opposite stroke direction in each case will operate to pump fluid around in the coupling system. If, on the other hand, one desires to obtain pumping action into the high pressure reservoir in both directions for both the inner and outer plungers  242 ,  295  and  296 , then a different type of coupling system should be employed. 
   In addition to the operation of the subsystems that are internal to the engine, of course, the external systems will also function during engine operation as needed to maintain the operation of the engine  10 . Thus, the cooling system will pump coolant through the coolant passages  28 ,  50 ,  66 ,  128 ,  150 ,  166 , and  352  as needed in order to assure that engine components do not overheat. Also, the fuel system  39  will store and provide fuel to the fuel injectors  34  and  134  at the desired pressure. The electrical system will provide electrical power to the controller  35 , sensors and other components requiring electrical power to operate. The oil supply system will provide lubricating oil to the engine as needed for providing lubrication to certain components. And, the air intake system will provide air to the air inlets  92  and  192  as needed during engine operation. 
   Although the fluid employed for the energy storage medium and the control valve has been disclosed as hydraulic oil, other suitable fluids may also be employed if so desired. For example, the fluid may be a gas, with a pneumatic energy storage system for the reservoirs. The fluid may be a refrigerant that can be in the liquid or gaseous state. In both of these examples, since the fluid is no longer a liquid (being generally incompressible), the coupling system employed to assure the opposed motion of the two piston assemblies would also change. However, the OPOC free piston engine configuration, especially one employing HCCI combustion, can still be used to produce the energy stored in the fluid energy storage medium. 
   Moreover, while the exemplary embodiment of an OPOC free piston engine discussed in detail herein employs a hydraulic fluid as the energy storage and control medium, the OPOC free piston engine that may employ linear alternators for engine control and electrical energy production. The hydraulic pump block assembly would be replaced with a linear alternator assembly, with the pull and push rods forming a part of or driving linear alternator components. The piston/cylinder assemblies—including scavenge pumps—would operate to produce energy from combustion events to drive the linear alternators. So, HCCI combustion, with the desired high quantities of charge air, can still be employed with the OPOC free piston engine coupled to a linear alternator, as is preferred for maximizing the power density of the engine. 
   While certain embodiments of the present invention have been described in detail, those familiar with the art to which this invention relates will recognize various alternative designs and embodiments for practicing the invention as defined by the following claims.