Abstract:
An engine having a cylinder with a bore opposed pistons disposed for reciprocation in the bore includes compliant members that allow for angular adjustment of piston structure with respect to the cylinder in response to an imbalance in forces coupled to the pistons by connecting rods.

Description:
PRIORITY 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/378,959, filed Mar. 17, 2006, which claims priority under 35 U.S.C. § 119 as a Continuation-in-Part of PCT Patent Application PCT/US2005/020553, filed Jun. 10, 2005, entitled “Improved Two-Cycle, Opposed Piston, Internal-combustion Engine”, the US national phase of which is U.S. patent application Ser. No. 11/629,136, and also claims priority under 35 U.S.C. § 120 as a continuation-in-part of U.S. patent application Ser. No. 10/865,707, filed Jun. 10, 2004, for “Two-Cycle, Opposed Piston, Internal-Combustion Engine,” now U.S. Pat. No. 7,156,056, the disclosures of both of which are incorporated by reference in their entirety. 
       RELATED APPLICATIONS 
       [0002]    The following co-pending applications, all commonly assigned to the assignee of this application, contain subject matter related to the subject matter of this application. 
         [0003]    U.S. patent application Ser. No. 10/865,707, filed Jun. 10, 2004 for “Two Cycle, Opposed Piston Internal Combustion Engine”, published as US/2005/0274332 on Dec. 29, 2005, now U.S. Pat. No. 7,156,056, issued Jan. 2, 2007; 
         [0004]    PCT application US2005/020553, filed Jun. 10, 2005 for “Improved Two Cycle, Opposed Piston Internal Combustion Engine”, published as WO/2005/124124 on Dec. 15, 2005; 
         [0005]    U.S. patent application Ser. No. 11/095,250, filed Mar. 31, 2005 for “Opposed Piston, Homogeneous Charge, Pilot Ignition Engine”, published as US/2006/0219213 on Oct. 5, 2006; 
         [0006]    PCT application US/2006/011886, filed Mar. 30, 2006 for “Opposed Piston, Homogeneous Charge, Pilot Ignition Engine”, published as WO/2006/105390 on Oct. 5, 2006; 
         [0007]    U.S. patent application Ser. No. 11/097,909, filed Apr. 1, 2005 for “Common Rail Fuel Injection System With Accumulator Injectors”, published as US/2006/0219220 on Oct. 5, 2006, now U.S. Pat. No. 7,270,108; 
         [0008]    PCT application US06/012353, filed Mar. 30, 2006 “Common Rail Fuel Injection System With Accumulator Injectors”, published as WO/2006/107892 on Oct. 12, 2006; 
         [0009]    U.S. patent application Ser. No. 11/378,959, filed Mar. 17, 2006 for “Opposed Piston Engine”, published as US/2006/0157003 on Jul. 20, 2006; and 
         [0010]    U.S. patent application Ser. No. 11/512,942, filed Aug. 29, 2006, for “Two Stroke, Opposed Piston Internal Combustion Engine”, divisional of 10/865,707; 
         [0011]    U.S. patent application Ser. No. 11/629,136, filed Dec. 8, 2006, for “Improved Two Cycle, Opposed Piston Internal Combustion Engine”, CIP of 10/865,707; and 
         [0012]    U.S. patent application Ser. No. 11/642,140, filed Dec. 20, 2006, for “Two Cycle, Opposed Piston Internal Combustion Engine”, continuation of Ser. No. 10/865,707. 
     
    
     BACKGROUND 
       [0013]    The invention concerns an internal-combustion engine. More particularly, the invention concerns a two-cycle, opposed piston engine. 
         [0014]    The opposed piston engine was invented by Hugo Junkers around the end of the nineteenth century. Junkers&#39; engine uses two pistons disposed crown-to-crown in a common cylinder having inlet and exhaust ports near bottom-dead-center of each piston, with the pistons serving as the valves for the ports. The engine has two crankshafts, one disposed at each end of the cylinder. The crankshafts, which rotate in the same direction, are linked by connecting rods to respective pistons. Wristpins within the pistons link the rods to the pistons. The crankshafts are geared together to control phasing of the ports and to provide engine output. In a typical Junkers engine, a supercharger is driven from the intake crankshaft, and its associated compressor is used to scavenge the cylinders and leave a fresh charge of air each revolution of the engine. Optionally, a turbo-supercharger may also be used. The advantages of Junkers&#39; opposed piston engine over traditional two-cycle and four-cycle engines include superior scavenging, reduced parts count and increased reliability, high thermal efficiency, and high power density. In 1936, the Junkers Jumo airplane engines, the most successful diesel engines to that date, were able to achieve a power density that has not been matched by any diesel engine since. According to C. F. Taylor ( The Internal - Combustion Engine in Theory and Practice: Volume II, revised edition ; MIT Press, Cambridge, Mass., 1985): “The now obsolete Junkers aircraft Diesel engine still holds the record for specific output of Diesel engines in actual service (Volume I, FIG. 13-11).” 
         [0015]    Nevertheless, Junkers&#39; basic design contains a number of deficiencies. The engine is tall and requires a long gear train to couple the outputs of the two crankshafts to an output drive. Each piston is connected to a crankshaft by a rod that extends from the piston. The connecting rods are massive to accommodate the high compressive forces between the pistons and crankshafts. These compressive forces, coupled with oscillatory motion of the wristpins and piston heating, cause early failure of the wristpins. The compressive force exerted on each piston by its connecting rod at an angle to the axis of the piston produces a radially-directed force (a side force) between the piston and cylinder bore. The friction generated by this side force is mitigated by a lubricant film between the cylinder and piston, but the film ruptures beyond a certain temperature and side force. Since the temperature of the cylinder/piston interface is principally determined by the heat of combustion, the breakdown temperature of the lubricant imposes a limit on the engine combustion temperature, which, in turn, limits the brake mean effective pressure (BMEP, an indicator of engine power) achievable by the engine. One crankshaft is connected only to exhaust-side pistons, and the other only to inlet-side pistons. In the Jumo engine the exhaust side pistons account for up to 70% of the torque, and the exhaust side crankshaft bears the heavier torque burden. The combination of the torque imbalance, the wide separation of the crankshafts, and the length of the gear train produces torsional resonance effects (vibration) in the gear train. A massive engine block is required to constrain the highly repulsive forces exerted by the pistons on the crankshafts during combustion, which literally try to blow the engine apart. 
         [0016]    In an opposed piston engine described in Bird&#39;s U.K. Patent 558,115, counter-rotating crankshafts are located beside the cylinders such that their axes of rotation lie in a plane that intersects the cylinders and is normal to the axes of the cylinder bores. The side-mounted crankshafts are closer together than in the Jumo engines, thereby reducing the height of Bird&#39;s engine as compared with that of the Jumo engines. Bird&#39;s crankshafts are coupled by a shorter gear train that requires four gears, compared with five for the Jumo engine. The pistons and crankshafts in Bird&#39;s engine are connected by rods that extend from each piston along the sides of the cylinders, at acute angles to the sides of the cylinders, to each of the crankshafts. In this arrangement, the rods are mainly under tensile force, which removes the repulsive forces on the crankshafts and yields a substantial weight reduction because a less massive rod structure is required for a rod loaded with a mainly tensile force than for a rod under a mainly compressive load of the same magnitude. Bird&#39;s proposed engine has torsional balance brought by connecting each piston to both crankshafts. This torsional balance, the proximity of the crankshafts, and the reduced length of the gear train produce good torsional stability. To balance dynamic engine forces, each piston is connected by one set of rods to one crankshaft and by another set of rods to the other crankshaft. Piston load balancing substantially reduces the side forces that operate between the pistons and the internal bores of the cylinders. However, even with these improvements, traditional engine construction and conventional cooling prevent Bird&#39;s proposed engine from reaching its full potential for simplification and power-to-weight ratio (“PWR”, which is measured in horsepower per pound, hp/lb). 
         [0017]    Bird&#39;s engine uses an engine block in which cylinders, cylinder intake and exhaust manifolds, cylinder cooling jackets and engine bearings are cast in a large, heavy unit serving as the primary structural element of the engine. Thermal and mechanical stresses transmitted through the engine block and uneven heating during engine operation cause non-uniform cylindrical distortion of the cylinders. The piston crowns bear extremely high temperatures during combustion and become distent radially as a result. The cooling system of Bird&#39;s engine provides liquid coolant through the cylinder jackets in the engine block, but the system is not adapted to mitigate the non-uniform distortion of the cylinders or to prevent expansion of the piston crowns. As a consequence, close tolerances cannot be maintained between cylinders and pistons without a high risk of engine damage or early engine failure. Of course, without close tolerances, it is difficult to provide an effective seal between cylinders and pistons to limit blowby (the escape of gasses past the piston) during engine operation, without the use of piston rings. A rigid piston structure in which connecting rods are coupled with wrist pins mounted to piston skirts over-constrains the pistons during operation of the engine. This over-constraint prevents any part of a piston from repositioning with respect to the axis of an associated cylinder in response to an imbalance of forces coupled to the piston through the connecting rods. 
         [0018]    A two-stroke, opposed-piston engine with side-mounted, counter-rotating crankshafts is described in PCT Patent Application PCT/US2005/020553. In this engine, the working elements (cylinders, pistons, linkages, crankshafts, etc.) are received upon a frame of passive structural elements fitted together to support the working elements. The frame bears the stresses and forces of engine operation, including compressive forces between the crankshafts. In contrast with the Junkers and Bird engines, the cylinders are not cast in an engine block, nor are they formed with other passive structural elements. Consequently, the cylinders are not passive structural elements of the engine. Thus, with the exception of combustion chamber forces, the cylinders are decoupled from the mechanical and thermal stresses of an engine block and are essentially only pressure vessels. Tailored application of liquid coolant to each cylinder of the engine compensates for asymmetrical heating of the cylinders, while the symmetrical application of liquid coolant to the interior surface of each piston crown maintains the shape of piston crowns during engine operation. A single intermediate gear between the two crankshafts shortens the gear train and substantially reduces torsional resonances between the crankshafts, as compared with Bird&#39;s engine. 
         [0019]    The engine described in PCT Patent Application PCT/US2005/020553 also includes a compliant member that allows for angular adjustment of piston structure with respect to the cylinder in response to an imbalance in forces coupled to the piston by the connecting rods. In this regard, an axially-centered tubular rod is mounted in the piston, and the connecting rods are linked to wrist pins attached to the rod. Piston compliance is realized in the innate flexibility of the tubular rod. Elimination of wrist pins from skirt mountings permits reduction of skirt mass and piston weight. 
         [0020]    Further benefits to the engine described in PCT Patent Application PCT/US2005/020553 have resulted from additional embodiments of a compliant piston structure including a compliance boot acting between the piston crown and an axially-centered rod mounted in the piston. A single wristpin mounted on an axially-centered piston rod, externally to the piston, couples the piston with associated connecting rods that run between the piston rod and the crankshafts of the engine. 
       SUMMARY 
       [0021]    A two-cycle, opposed piston engine includes a cylinder with a liner having a bore and a pair of opposed pistons disposed to reciprocate in the bore. Compliant members allow for angular adjustment of piston structure with respect to the cylinder in response to an imbalance in forces coupled to the pistons by connecting rods. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    The below-described drawings, which are not necessarily to scale, illustrate principles and examples discussed in the following detailed description. 
           [0023]      FIGS. 1A-1D  illustrate the structure of a cylinder used in an opposed piston internal-combustion engine 
           [0024]      FIG. 2  is a curve illustrating time-averaged cylinder heat flux measured in an axial direction during operation of an opposed piston engine. 
           [0025]      FIG. 3A  is a side perspective view of a piston and a piston rod with an attached wristpin.  FIG. 3B  is a side perspective view of the piston, with the skirt removed, and with the piston rod and wristpin attached thereto.  FIG. 3C  is a side perspective view of the piston rod that attaches to the piston.  FIG. 3D  is an exploded assembly view of the piston, with the skirt removed, and with the piston rod and wristpin associated therewith.  FIG. 3E  is an enlarged side sectional view of an upper portion of the piston with the skirt partially cut away.  FIG. 3F  is a magnified view of a portion of the piston crown showing an assembly detail. 
           [0026]      FIG. 4A  is a side view of an opposed piston engine showing a cylinder in which the pistons at top dead center are coupled by primarily tensile-loaded connecting rods to two crankshafts, with the view cut away to show a piston cooling structure. FIG.  4 B is a perspective view of an end of a piston and connecting rods in the engine of  FIG. 4A , with crankshafts removed. 
           [0027]      FIGS. 5A-5E  are perspective views of a multiple-cylinder implementation of the opposed piston engine showing assembly details at various stages of assembly. 
           [0028]      FIGS. 6A and 6B  are schematic diagrams of supply systems useable to control the application of liquid coolant to a cylinder and opposed pistons of the opposed piston engine. 
           [0029]      FIG. 7  is a schematic diagram of intake and exhaust gas flow in the opposed piston engine. 
           [0030]      FIGS. 8A-8F  illustrate applications of the opposed piston engine. 
       
    
    
     DETAILED DESCRIPTION 
       [0031]      FIGS. 1A-1D  illustrate a cylinder  1100  useable in an opposed piston internal-combustion engine. The cylinder  1100  has four parts: a cylinder liner  1102  formed as an open cylindrical tube with a cylindrical bore  1103 , an exhaust manifold  1104 , an inlet manifold  1106 , and a cylinder sleeve  1140 . Preferably, the cylinder  1100  is made from aluminum, such as a high-temperature aluminum alloy, and it may be cast or assembled by fixing the manifolds  1104  and  1106  to the cylinder sleeve  1140  and then fixing that subassembly to the outer surface of the cylinder liner  1102 . The longitudinal axis A c  of the cylinder liner  1102  is also the longitudinal axis of the cylinder  1100 . 
         [0032]    As best seen in  FIG. 1A , the cylinder liner  1102  has an exhaust port  1105  constituted of a series of circumferentially-spaced openings  1108  near an exhaust end  1109  of the cylinder liner  1102 . The cylinder liner  1102  also has an inlet port  1107  constituted of a series of circumferentially-spaced openings  1110  near an inlet end  1112 . Combustion gases spiraling toward the exhaust end  1109  of the cylinder liner  1102  are diverted at least generally out of the cylinder liner  1102  into the exhaust manifold  1104  shown in  FIG. 1C . Each opening  1110  of the inlet port  1107  has a ramped upstream end  1110   r  at which pressurized air flowing into the inlet port  1107  through the inlet manifold  1106  is diverted into the bore  1103  in a spiral direction toward the exhaust end  1109 . At a central portion  1114  of the cylinder liner  1102 , a number of threaded openings  1116  are provided in a circumferential sequence. At least one of the openings  1116  receives a fuel injector, and at least one other of the openings  1116  receives a sensor for sensing engine operating conditions such as pressure or temperature. In the cylinder liner  1102  shown, there may be, for example, two openings  1116  for receiving fuel injectors, one opening  1116  for receiving a pressure sensor, and one opening  1116  for receiving a temperature sensor. 
         [0033]    The curve  1200  of  FIG. 2  illustrates average heat flux measured across a longitudinal trace on the inside wall of a cylinder having a construction like that of the cylinder  1100  during engine operation. As the curve  1200  shows, the cylinder liner is non-uniformly heated with respect to its longitudinal axis. The cylinder liner has its greatest heat load in its central portion, where combustion occurs. Also, the end portion of the cylinder liner with the exhaust port experiences a greater heat load than the end portion with the inlet port. Thus, in order to minimize non-uniformities in the temperature of the cylinder and resulting cylindrical non-uniformity of the cylinder bore, the cylinder is cooled in a tailored manner that accommodates the non-uniform ways its portions are heated during engine operation. That is to say, a system for cooling a cylinder such as the cylinder  1100  provides a greater cooling capacity to the portion of the cylinder from near its axial center to the exhaust end than the portion from near its axial center to the inlet end, and provides the highest cooling capacity to the central portion of the cylinder. 
         [0034]    With reference to  FIG. 1A , there also exists a potential for a circumferential temperature variation in the cylinder wall even with uniform heat flux if the available cooling is not uniform around the circumference. Non-uniform cooling also occurs in the central portion  1114  due to the sequence of openings  1116 . To maintain circumferential temperature uniformity, and thus cylindrical uniformity, in the central portion  1114 , the cooling adjacent to these openings  1116  subsumes the cooling that would have occurred had the openings not been present. 
         [0035]    To provide a tailored cooling capacity that meets these goals, a number of grooves or channels are provided on the outside surface  1120  of the cylinder liner  1102 . Referring to  FIGS. 1A ,  1 B, and  1 D, a first group  1122  of interlaced grooves  1123  spiral around the outside surface  1120  from the central portion  1114  toward the exhaust port  1105 , and a second group  1126  of interlaced grooves  1127  spiral around the outside surface  1120  from the central portion  1114  toward the inlet port  1107 . Each groove of these two groups originates in or near the central portion  1114 , follows a spiral path around the outside surface  1120 , and terminates near a respective port  1105 ,  1107  in a drilled radial section. The drilled radial section of each groove communicates with a drilled axial channel extending longitudinally within the cylinder liner  1102  through an edge of the cylinder liner  1102 . One such axial channel, indicated by reference numeral  1129  in  FIG. 1A , communicates through a drilled radial section  1130  with an end  1127   e  of a groove  1127  and penetrates the edge  1131  through a hole  1133 . This enables a stream of liquid coolant to flow from the beginning of a groove in or near the central portion  1144 , along the spiral of the groove toward a respective end of the cylinder liner  1102 , through a channel in the cylinder liner, and out of a hole in an edge of the cylinder liner  1102 . Each group  1122 ,  1126  of grooves conducts an aggregate flow of liquid coolant from the central portion  1114  to an end portion of the cylinder liner  1102 , enabling cooling of the respective corresponding portion of the cylinder liner, and thereby, of the cylinder  1100  itself. There is a pitch, or spacing, (which may be constant or varying) between the grooves of each group and the pitch for the grooves of the group  1122  extending from the central portion  1114  toward the exhaust end  1109  is less than the pitch for the group  1126  of grooves extending from the central portion  1114  toward the inlet end  1112 . As a result, more liquid coolant contacts the cylinder liner portion over a larger surface area including the exhaust port  1105  than the cylinder liner portion including the inlet port  1107 , thereby providing greater cooling capacity for the cylinder liner portion that includes the exhaust port  1105 . The coolant is also the coolest, and therefore has the greatest heat exchange capacity, as it enters the grooves near the central portion  1114  of the cylinder liner  1102  where the cooling requirements are the greatest. Furthermore, the grooves may have a variable cross-sectional area along their length that affects the local flow velocity of the coolant within the grooves and therefore the local rate of heat removal. Thus, the cooling capacity of the spiral grooves is settable over a wide range by varying any or all of the number of interlaced grooves, the length of the grooves, the pitch of the grooves, the cross-sectional area along the length of the grooves and the coolant flow rate into the channels. 
         [0036]    Still referring to  FIGS. 1A ,  1 B, and  1 D, a third group of grooves  1135  extend around the outside surface  1120  in the central portion  1114  of the cylinder liner  1102 , with each groove  1135  extending between two of the openings  1116  in the central portion. Each groove  1135  has an elongated portion  1137  that extends in an arc on the circumference of the cylinder liner  1102 , and cross portions  1138  at the opposed ends of the elongated portion  1137 . Each cross portion  1138  is transverse to the elongate portion  1137  so that each of the grooves  1135  has the shape of an I. As best seen in  FIG. 1A , each cross portion  1138  is positioned immediately adjacent an opening  1116 . In operation, liquid coolant introduced into each groove  1135  at the center of its elongate portion  1137  flows through the elongate portion  1137  toward each cross portion  1138  and then is exhausted from holes  1147  (best seen in  FIG. 1B ) in the cylinder sleeve  1140  at either end of each cross portion  1138 . Thus, liquid coolant flowing in each groove  1135  has an extended flow path at each end  1138  of the groove, near an opening  1116 . Consequently, each groove  1135  provides an enhanced capacity for cooling at the hottest parts of the central portion  1114 , near the openings  1116 . The cooling capacity provided for the central portion  1114  varies with the circumferential distance to the nearest opening  1116  in the central portion. The cooling in the grooves  1135  is a very effective, localized method for removing heat from the area of the openings  1116  that is not accessible to cooling by the group of spiral grooves  1122 ,  1126 . The effectiveness of heat removal in the central section  1114  is due to a stagnation flow pattern of the coolant occurring in the zone where the coolant flows to and touches the center of each end  1138  before flowing to the tips of the end. 
         [0037]    Assembly details of the cylinder  1100  are seen in  FIGS. 1B-1D . The tubular cylinder sleeve  1140  is received on the surface  1120  of the cylinder liner  1102 , centered on the central portion  1114  and extending to and meeting the exhaust and inlet manifolds  1104  and  1106 . The manifolds  1104 ,  1106  may be welded to the cylinder sleeve  1140  at the seams  1141  between the cylinder sleeve and the exhaust and inlet manifolds  1104  and  1106 . Such welds  1141   w  are best seen in  FIG. 1D . Alternatively, the manifolds  1104  and  1106  may be individually cast with respective portions of the cylinder sleeve  1140  and fixed to each other and to the cylinder liner  1102  by welding. Together, the exhaust and inlet manifolds  1104  and  1106  and the cylinder sleeve  1140  cover the grooves  1123 ,  1127 , and  1135 , confining the flow of liquid coolant in the grooves. As best seen in  FIG. 1B , the cylinder sleeve  1140  includes pipes  1142 ,  1144 , and  1145 . Each pipe  1142  is positioned over the beginning of a respective groove  1123  near the central portion  1114 ; each pipe  1144  is positioned over the beginning of a respective groove  1127  near the central portion  1114 ; and each pipe  1145  is positioned over the center of the elongate portion  1137  of a respective groove  1135 . Liquid coolant flows into grooves  1123  and  1127  through pipes  1142  and  1144 , near or at the central portion  1114  of the cylinder liner  1102 , and flows in streams through the grooves and the drilled channels  1129 , and out of the holes  1133  in the end edges  1131  of the cylinder liner  1102 . Liquid coolant flows into the grooves  1135  through pipes  1145 , and flows in streams through the elongate portions  1137 , to the ends  1138 . Holes  1147  provided through the cylinder sleeve  1140  are positioned at the tips of the ends  1138  to permit liquid coolant to flow out of the grooves  1135 . As best seen in  FIG. 1C , the pipes  1142 ,  1144  and  1145  receive couplings  1148  mounted on liquid coolant supply lines  1149  that connect to a liquid coolant supply system as explained below. Three liquid coolant supply circuits may be provided in a liquid coolant supply system to supply liquid coolant for the three groups of grooves. Each circuit is connected to a respective group of grooves by way of the pipes that communicate with the grooves to input liquid coolant at a desired pressure and flow rate for the group of grooves. In these figures, no lines are provided to conduct liquid coolant flowing out of the grooves on the outside surface  1120  of the cylinder liner  1102 . The liquid coolant may be collected by a sump in the engine. In this case, the liquid coolant is expelled through the holes  1133  at each end edge  1131  of the cylinder liner  1302 . Some portion of the liquid coolant will fall from the holes  1133  onto the outside skirt surfaces of the opposed pistons (not shown in  FIGS. 1A-1D ) as they reciprocate in the bore  1103 , thereby cooling and lubricating those surfaces during engine operation. Alternatively, the liquid coolant flowing out of the ends of grooves on the cylinder  1100  may be conducted in liquid coolant return lines connected by conventional fittings to the holes  1133  and  1147  for collection and recirculation of the liquid coolant as explained below. 
         [0038]    As seen in  FIGS. 1C and 1D , the exhaust and inlet manifolds  1104  and  1106  have respective internal annular volutes  1150  and  1152  that communicate with the exhaust and inlet ports  1105  and  1107 , respectively. Preferably each of the volutes  1150  and  1152  has the shape of a scroll in order to induce swirling of gasses flowing therethrough, while controlling turbulent mixing. Swirling the pressurized air facilitates scavenging and enhances combustion efficiency. Ducts  1153  and  1154  connect the exhaust and inlet manifolds  1104  and  1106  to a system for discharging exhaust gasses from and providing charge air to an opposed piston engine as described below. 
         [0039]    As seen in  FIGS. 1B-1D , the cylinder sleeve  1140  includes one or more openings  1156 , each aligned with a corresponding threaded opening  1116  in the cylinder liner  1102 . One or more fuel injectors  1158 , each threaded at its nozzle end, are mounted to the cylinder  1100  by being threaded into openings  1116 . Each fuel injector  1158  is coupled at  1159  to a high-pressure fuel line  1160  and may be provided with fuel by a system as described below. 
         [0040]    An annular groove is provided near each end of the cylinder liner  1102 , in the bore  1103 , for seating an O-ring. One such O-ring  1163  is visible through the inlet end  1112  in  FIGS. 1A and 1B , and both O-rings are visible in  FIG. 1D . The O-rings  1163  are provided to contact and wipe excess lubricant from the exterior surfaces of the skirts of opposed pistons (not seen in these figures) that move in the bore  1103 . The O-rings are preferably made of a resilient fluoro-elastomer material. 
         [0041]    With reference to  FIGS. 1C and 1D , the cylinder  1100  is provided with mounting brackets  1164  mounted to the outside surface of the cylinder sleeve  1140  that are received in a frame (not shown in these figures) when the cylinder  1100  is assembled into an opposed piston engine. The mounting brackets  1164  are shown mounted to the external surface of the cylinder sleeve  1140  by adjustable constricting clamps  1165 , although this is not meant to be limiting. The mounting brackets  1164  may be welded to the cylinder sleeve  1140 , or may be individually cast with respective portions of the cylinder sleeve  1140 , which are fixed to each other and to the cylinder liner  1102  by welding. 
         [0042]      FIGS. 3A through 3E  illustrate a piston  1300  useable in an opposed piston internal-combustion engine. The piston  1300  is preferably ringless, although this is not intended to exclude the use of piston rings on the piston  1300 , if required. Referring to  FIGS. 3A and 3B , the piston  1300  includes a cylindrical section  1302  with piston crown  1308  at one end. The cylindrical section  1302  has an open end  1309  opposite the crown  1308 . The portion of the cylindrical section  1302  extending from the crown  1308  to the open end  1309  forms the piston skirt  1310 . The longitudinal axis A p  of the cylindrical section  1302  is also the longitudinal axis of the piston  1300 . A piston rod  1330 , preferably a tubular rod, shown in  FIG. 3C , is attached to the piston  1300 . The piston rod  1330  includes a shaft  1332 , a central bore  1332 , a disc-shaped end section  1334 , and a threaded end section  1335 . Manifestly, the piston rod  1330  has a cylindrical cross-sectional shape. This is not intended to limit the construction of the piston rod, when other cross-sectional shapes may be used. 
         [0043]    With reference to the view of the piston  1300  (with the skirt  1310  removed) illustrated in  FIG. 3B , the crown  1308  is formed on a crown piece  1308   a . A crown backing piece  1308   b , complementary to the crown piece  1308   a , is joined to the crown piece  1308   a  by screws  1321 . Together, the crown pieces  1308   a  and  1308   b  form ribs  1322 . The ribs  1322  serve as load-bearing elements and define coolant flow passages  1329  therebetween for piston cooling, described later. Preferably, the ribs  1322  and the flow passages  1329  are evenly spaced circumferentially about the longitudinal axis A p . Preferably, the ribs, and therefore the passages, exhibit rotational symmetry around the longitudinal axis A p , and impart such symmetry to the internal portion of the piston, under the crown  1308 . The ribs  1322  extend radially toward the inner surface of the piston skirt  1310 , abut the back surface of the crown  1308 , and also extend longitudinally within the piston  1300  from a back surface of the crown toward the open end  1309 . The ribs  1322  transfer the axial loads exerted on the crown  1308  during engine operation to other elements of the piston  1300 . The precise shape, extent, and number, of ribs  1322  may vary, for example, according to engine design and operating specifications. Preferably, the crown pieces  1308   a  and  1308   b  constitute a single crown unit, with the skirt  1310  formed as a single cylindrical unit and joined to the crown unit. The crown unit and skirt may be assembled from machined parts or made by casting and/or machining high-temperature aluminum, steel alloy, or iron, and then joined by brazing, welding or threading. In this example, the skirt  1310  is threaded to the crown  1308  at  1325  as best seen in  FIG. 3B . 
         [0044]    With further reference to  FIGS. 3A and 3B , a single wristpin  1342 , external to the piston  1300 , is retained on the threaded end section  1335  of the piston rod  1330  by a stop  1343  and a bored threaded nut  1344 , between spacers  1345  and  1346 . If desired for increased precision of the spacing between the crown  1308  and the rotational axis of the wristpin  1342 , a machined shim  1349  may be provided between the spacer  1345  and the stop  1343 . 
         [0045]    Benefits are realized by allowing the structure supporting the piston  1300  to deform elastically in some manner during engine operation for the purpose of regaining and/or maintaining axial alignment between the piston  1300  and the cylinder as the piston reciprocates in the bore of the cylinder. Such deformation may be referred to as “compliance”. Compliance is provided by retaining the disc-shaped end section  1334  of the piston rod  1330  in a compliance boot  1336  that permits limited movement between the disc-shaped end  1334  of the piston rod  1330  and the crown  1308  by resiliently deforming in response to off-axial force acting on the piston rod  1330 . 
         [0046]      FIG. 3D  is an exploded view of the piston  1300  depicting further details of elements of the piston and an assembly sequence thereof. As seen in  FIG. 3D , the crown backing piece  1308   b  includes a central annulus with a bore  1328  to be centered on the longitudinal axis A p , behind the crown  1308 . The crown backing piece  1308   b  is secured to the crown piece  1308   a  by two screws  1321  extending through each rib  1322 . The disc-shaped end section  1334  of the piston rod  1330  is retained in a compliance boot  1336  constituted of a resilient material, for example a fluoro-elastomer material. The compliance boot  1336  is itself contained in the compliance container  1337 . The compliance container  1337  is a cylindrical enclosure assembled from a first piece  1337   a  and a second piece  1337   b . Threaded screws  1341  join the first and second pieces  1337   a  and  1337   b . The compliance boot  1336  may be constituted of a single molded piece, or it may be assembled from molded parts. Preferably, although without limitation, the compliance boot  1336  is assembled around the disc-shaped end section  1334  of the piston rod  1330  using two flat fluoro-elastomeric discs  1338  and  1339  and a fluoro-elastomeric ring  1340 . The disc  1338  is positioned between the first piece  1337   a  and the disc-shaped end section  1334 ; the ring  1340  is received around the perimeter of the disc-shaped end section  1334 ; and the disc  1339  is positioned between the disc-shaped end section  1334  and the second piece  1337   b . The disc  1338  has a central opening  1338   o  and four through holes  1338   t . The opening  1338   o  and the through holes  1338   t  may be lined with thin metal (preferably brass) backing rings. The disc  1339  has a central opening  1339   o  and four through holes  1339   t . The opening  1339   o  is large enough to clear the stop  1343  so that the disc  1339  may be received over the shaft  1332  of the piston rod  1330 . The opening  1339   o  and the through holes  1339   t  may be lined with thin metal (preferably brass) backing rings. The backing rings of the through holes  1338   t ,  1339   t  are not shown; the backing rings  1338   ob ,  1339   ob  for the openings  1338   o ,  13389   o  are seen in  FIG. 3E . With further reference to  FIGS. 3D and 3E , the first piece  1337   a  of the compliance container has a lipped central opening  1337   ao , and the second piece  1337   b  has a lipped central opening  1337   bo . The opening  1337   bo  is large enough to clear the stop  1343  so that the second piece  1337   b  may be received over the shaft  1331  of the piston rod  1330 . The compliance container  1337  encloses and retains the compliance boot  1336 , with the disc-shaped end section  1334  of the piston rod retained within the boot. A metal disc  1348  with a central opening  1348   o  and through holes  1348   t  is disposed in the compliance container  1337  between the disc  1338  and the piece  1337   a . Before the compliance container  1337  is assembled, the first piece  1337   a  is secured to the bottom of the crown backing piece  1308   b  by threaded screws  1333  that extend through through holes  1337   ai  in the first piece  1337   a  and that are retained in corresponding threaded holes (not seen) in the crown backing piece  1308   b . The compliance container  1337  is then assembled around the compliance boot  1336  by provision of elongate threaded screws  1341  that extend through through holes  1337   bt ,  1339   t ,  1334   t ,  1338   t , and  1348   t , and are retained in threaded through holes  1337  at. 
         [0047]    Deformation of the compliance boot  1336  is contained by the compliance container  1337 , the disc  1348 , and the backing rings in the discs  1338  and  1339 . The diameters of the through holes  1338   t  and  1339   t , with backing rings, are slightly larger than the diameter of the screws  1341  in order to provide space within which the compliance boot  1336  may resiliently deform. Preferably, although without limitation, the compliance boot  1336  allows pivotal movement of the piston rod  1330  with respect to a pivot point P on the axis Ap. 
         [0048]    With reference again to  FIG. 3D , the wristpin  1342  has a clearance hole  1347  so that the wristpin can be received on the threaded end  1335  of the piston rod  1330 , and mounted thereto to be external to the piston  1300 . Once the crown pieces  1308   a  and  1308   b , the compliance boot  1336  and the compliance container  1337  have been assembled, the shim  1349  (if used) and the spacer  1345  are received on the threaded end  1335  of the piston rod  1330 , against the stop  1343 , followed by the wristpin  1342 , the spacer  1346 , and the threaded nut  1344 . Although not shown in  FIG. 3D , one of three connecting rods is received on the wristpin  1342  prior to mounting the wristpin to the threaded end  1335 . This arrangement may be understood with reference to  FIG. 4B , where a centrally-mounted connecting rod  1447   a  with a forked end  1447   aw  having two laterally-spaced engaging arms with aligned openings is slidably received on the wristpin  1342 . The forked end  1447   aw  is positioned on the wristpin  1342  such that the clearance hole  1347 , threaded end  1335 , and threaded nut  1344  are centered between the engaging arms of the forked end  1447   aw.    
         [0049]    Referring to  FIG. 3E , it may be desirable that the piston  1300  be cooled to alleviate thermally-induced distortion during engine operation. Distortion of the piston results from thermal expansion, compression pressure, combustion pressure, inertial forces and blowby pressure. The greatest risk of thermal distortion occurs at the crown  1308 , especially adjacent to and at the corner  1312 . Without cooling, this portion of the piston  1300  may bulge during engine operation, giving the piston  1300  a mushroom or tulip shape and raising the risk of contact between the piston and the cylinder bore, if not controlled. The distortion may be eliminated, or at least substantially reduced, by maintaining as thin a cross section x-x (see  FIG. 3E ) as possible in the crown  1308  in order to minimize the thermal impedance where maximum heating occurs, while cooling the crown by application of one or more streams of liquid coolant on the back surface  1316  of the crown. Since the distortion is substantially uniform, such cooling may be tailored to the substantially symmetric heat distribution in and adjacent the crown  1308 . 
         [0050]    Referring again to  FIGS. 3B and 3E , the application of liquid coolant to the back surface  1316  of the crown  1308  may be understood. The bore  1328  in the crown backing piece  1308   b  transitions to the radially-distributed flow passages  1329 . Each flow passage  1329  is positioned between a respective pair of ribs  1322  and is axially inclined so as to transition from the bore  1328  at a slant along the back surface  1316  of the crown  1308 . Each flow passage  1329  extends toward the edge  1312  of the crown  1308 . Near the edge  1312  of the crown  1308 , each flow passage  1329  transitions in a sharp reverse curve  1329 ′ to be directed toward the open end of the piston  1300 . The bore  1328  of the piston rod  1330  constitutes a channel to deliver a stream of a liquid coolant to the crown  1308  by way of the flow passages  1329 . The bore  1332  of the piston rod  1330  communicates through the opening  1337   ao  in the compliance container  1337  and the bore  1328  in the crown backing piece  1308   b  with the flow passages  1329 . A stream S 1  of liquid coolant C introduced through the bore in the threaded nut  1344  received on the threaded end of the piston rod  1330  flows in a first direction along the axis of the piston  1300 , through the bores  1332  and  1328 . The stream S 1  impinges on the back surface  1316 , aligned with the center of the crown, and fans out into streams S 2  that pass through the flow passages  1329 , flowing in an inclined axial direction along the back surface  1316  of the crown  1308  toward the edge  1312 . Near the edge  1312  of the crown, the liquid coolant C flows out of the crown, along the skirt  1310  toward the open end  1309 . The viscosity and velocity of the coolant C and the number and dimensions of the flow passages  1329  may be varied to assure turbulence of the streams in the local flow of the coolant within the flow passages  1329  and along the back surface  1316 . As is known, turbulence enhances the capacity of the coolant to conduct heat away from the back surface  1316  and the sides of the flow passages  1329 . The flow rate of the coolant C is raised to a level to assure a high rate of heat removal from the crown  1308 . Thus, the cooling capacity of the flow passages  1329  is settable over a wide range by varying any or all of the number of passages, the dimensions of the passages, the axial orientation of the passages, and the viscosity and flow rate of the coolant C into the piston  1300 . Preferably, the coolant C flows out of the open end  1309  of the piston  1300  to be collected with liquid coolant flowing out of the cylinder  1100  by a sump. 
         [0051]    Thus, rotationally symmetrical delivery of streams of liquid coolant directed at the back surface  1316  of the crown  1308  assures uniform cooling of the crown during engine operation and eliminates, or substantially reduces, swelling of the crown and the portion of the skirt immediately adjacent the crown during engine operation. The shape of the piston  1300  is thereby substantially maintained, even at high BMEP. According to an exemplary piston design utilizing such streams to control thermal distortion, the differential expansion of the crown relative to the lower cylindrical portion of a 3.15 inches (8.0 cm) diameter piston can be maintained at less than 0.001 inch (0.025 mm). With effective cooling of the crown  1308 , it becomes less important to transfer heat through the piston skirt  1310 . As a result, the skirt  1310  may be made thinner than otherwise would be necessary, thereby lowering the mass of the piston. 
         [0052]    A two-cycle, opposed piston internal-combustion engine illustrated in  FIG. 4A  is now described. This description presumes a compression-ignition engine for the sake of illustration and example only. It could instead be a spark-ignited engine. The described engine is constituted of at least one cylinder with tailored cooling in which cylindrically non-uniform thermal distortion is eliminated or substantially reduced by application of streams of a liquid coolant in the manner described with respect to the cylinder  1100  illustrated in  FIGS. 1A-1D . A cylinder of this engine has a pair of opposed pistons, in each of which thermally-induced radial distention is eliminated or substantially reduced by application of one or more streams of a liquid coolant in the manner described with respect to the piston  1300  illustrated in  FIGS. 3A-3E . While the cylinder and pistons are separately cooled by application of a liquid coolant, tailored cooling of the cylinder together with symmetrical cooling of the pistons may be relied upon to cool these elements and to maintain mechanical clearance between them during engine operation, which may thereby eliminate the need for piston rings. 
         [0053]    As shown in  FIG. 4A , the engine  1400  includes at least one cylinder  1100  with opposed pistons  1300 A and  1300 B disposed in it for reciprocating opposed motion toward and away from each other and the center of the cylinder  1100 . The longitudinal axis A c  of the cylinder is collinear with the longitudinal axes A p  of the pistons  1300 A and  1300 B. The pistons  1300 A and  1300 B are coupled to first and second side-mounted counter-rotating crankshafts  1430  and  1432  which, in turn, are coupled to a common output (not shown in this figure). A single wristpin  1342  is mounted to each of the pistons  1300 A and  1300 B by way of a piston rod  1330 . Each of the wristpins  1342  connects ends of a plurality of connecting rods  1447  to a respective one of the pistons  1300 A and  1300 B. The perspective of  FIG. 4A  illustrates only two connecting rods  1447  for each piston, but it is to be understood that one or more additional connecting rods are not visible. 
         [0054]    In  FIG. 4A , the two side-mounted crankshafts  1430  and  1432  are disposed with their axes parallel to each other and lying in a common plane that intersects the cylinder  1100  at or near its longitudinal center and that is perpendicular to the longitudinal axis A c  of the cylinder. The crankshafts rotate in opposite directions. The connecting rods  1447  are connected to crank throws on the crankshafts  1430  and  1432 . In plan, each connecting rod  1447  has an elongate straight section extending from a crankshaft toward a wristpin. At the end of the straight section, each connecting rod  1447  curves toward one of the wristpins  1342 . The curved shape of the connecting rods  1447  shortens the overall width of the engine, while providing clearance between the connecting rods and the ends of the pistons during engine operation. 
         [0055]      FIG. 4B  is a perspective view of an end of a piston  1300  in the engine  1400  with crankshafts removed to illustrate details of the connecting rods  1447 . As seen in  FIGS. 4A and 4B , each piston  1300 A and  1300 B has three connecting rods mounted to its single wristpin  1342 . As discussed above, a centrally-mounted connecting rod  1447   a  has a forked wristpin end  1447   aw  with two laterally-spaced engaging arms having aligned openings (not seen) that are mounted, with needle bearings  1436 , to the wristpin  1342 . The centrally-mounted connecting rod  1447   a  has a crankshaft end  1447   ac  with an opening for mounting, with roller bearings  1438 , to a crankshaft. Second, laterally mounted, connecting rods  1447   b  are mounted to a wristpin  1342 , outboard of a first connecting rod  1447   a , each between a respective end of the wristpin  1342  and one arm of the forked wristpin end  1447   aw . Each second connecting rod  1447   b  has a wristpin end  1447   bw  having an opening for mounting, with a needle bearing  1436 , to a wristpin  1342 . Each second connecting rod  1447   b  has a crankshaft end  1447   bc  with an opening for mounting, with a roller bearing  1438 , to a crankshaft. 
         [0056]    The geometric relationship between the connecting rods  1447 , wristpins  1342 , and crankshafts  1430 ,  1432  shown in  FIGS. 4A and 4B  keeps the connecting rods  1447  principally under tensile stress as the pistons  1300 A and  1300 B move in the cylinder  1100 , with a limited level of compressive stress resulting from inertial forces of the pistons at high engine speeds. This geometry eliminates or at least substantially reduces side forces between the pistons  1300 A and  1300 B and the bore of the cylinder  1100 . 
         [0057]    In  FIG. 4A , additional details and features of the cylinder  1100  and the pistons  1300 A and  1300 B are shown. In the cylinder  1100 , the exhaust port  1105  is covered by the exhaust manifold  1104  through which the products of combustion flow out of the cylinder  1100 . During high power operation of the engine  1400 , for example at BMEP=150 psi, the average external temperature of the exhaust manifold  1104  and the duct  1153  may reach or exceed 375° C., a high enough temperature to coke diesel fuel. The average temperature of the manifold  1104  and duct  1153  is reduced from the high initial exhaust gas temperature by the subsequent flow of scavenging air. Nevertheless, the exterior surfaces of exhaust manifold  1104  and the duct  1153  may be covered with an insulating coating such as a high temperature paint. Silicone-based compositions are useful for this purpose. One such composition is metal oxide filled paint with a thermal conductivity (K) of less than 1 W/meter-° K sold under the trade name Corr-Paint CP4040 by Aremco. Another suitable composition is a coating formulated by mixing sil-cell spherical microballoons sold by Eager Plastics, Inc. or microspheres of glass sold by Potters Europe with a silicone based binder system sold under the trade name Aremco 8080 by Aremco; this composition provides a coating having a thermal conductivity (K) of less than 0.36 W/meter-° K. Alternatively, or in addition, the interior surfaces of the exhaust manifold  1104  and duct  1153  may be coated with a ceramic material. It may also be desirable to apply a liquid coolant to the exterior surfaces of the exhaust manifold  1104  and duct  1153 , although this would make the energy removed from the exhaust gas unavailable for use in a turbocharger. 
         [0058]    Referring to  FIG. 4A , the cylinder  1100  also has an inlet port  1107  covered by the inlet manifold  1106  through which pressurized air flows into the cylinder  1100 . Because of their locations with respect to these ports, the pistons  1300 A and  1300 B may be respectively referred to as the “exhaust” and “inlet” pistons, and the ends of the cylinder  1100  are similarly named. 
         [0059]    The relation between piston length and the length of the cylinder, coupled with a phase difference between the pistons  1300 A and  1300 B as they traverse their bottom dead center positions, modulates port operations and sequences them correctly with piston events. Thus, a phase offset between the bottom dead center positions produces a sequence in which the exhaust port  1105  opens when the exhaust piston  1300 A moves near its bottom dead center position, then the inlet port  1107  opens when the inlet piston  1300 B moves near its bottom dead center position, following which the exhaust port closes after the exhaust piston moves away from its bottom dead center position, and then the inlet port  1107  closes after the inlet piston  1300 B moves away from its bottom dead center position. 
         [0060]    With reference to  FIG. 4A , two coolant reservoirs  1460 A and  1460 B are provided outboard of the open ends of the pistons  1300 A and  1300 B. Each reservoir has an elongate nozzle  1461  that is received in the threaded nut  1344  mounted on the threaded end  1335  of the piston rod  1330 . Liquid coolant for cooling the associated piston  1300 A or  1300 B is fed through the threaded nut  1344  from the reservoir  1460 A or  1460 B by way of the nozzle  1461 . The liquid coolant is thus fed at a constant pressure into the bore  1332  of a corresponding piston rod  1330 . The pressure forces liquid coolant out of the piston rod  1330  in one or more constantly-flowing streams directed onto the back surface of a crown  1308  through the flow passages  1329 . 
         [0061]    An opposed piston engine according to this specification has working elements (cylinders, pistons, linkages, crankshafts, etc.) received upon a structural unit in the form of a frame of passive structural elements fitted together to support the working elements. The frame bears the stresses and forces of engine operation, such as compressive forces between the crankshafts, and the cylinders are neither cast in a block nor formed with other passive structural elements. Each cylinder is supported in the engine frame and is thus decoupled from the mechanical and thermal stresses of an engine block. Hence, the cylinders  1100  are essentially only cooled pressure vessels. This engine construction, together with cooling of the cylinder  1100  and pistons  1300 A and  1300 B in the manner described above, eliminates non-uniform cylindrical distortion of the cylinder and swelling of the piston crowns, and permits the cylinder-piston interface to be very close-fitting. Advantageously, with tailored cooling, this characteristic affords the option of an engine design that may dispense with the need for piston rings. 
         [0062]      FIGS. 5A-5E  are side perspective views showing increasingly complete assembly of the opposed piston engine  1400  with side-mounted crankshafts based on the cylinder and piston constructions of  FIGS. 1A-1D  and  3 A- 3 C. The engine  1400  has two cylinders, although this is merely for the sake of illustration. In fact, it can be scaled to engines of any size and engines having one, two or three or more cylinders. In  FIG. 5A , the engine  1400  includes two cylinders  1100  having the construction illustrated in  FIGS. 1A-1D , with opposed pistons  1300 A and  1300 B disposed in it. The wristpins  1342  of the opposed pistons are visible in  FIG. 5A . Connecting rods  1447  are coupled to the wristpins  1342  and to the crankshafts  1430  and  1432 . The exhaust ducts  1153  are received in corresponding openings in an engine plate  1510 , and the inlet ducts are received in corresponding openings of an engine plate  1520 . At least one fuel injector  1158  injects fuel into the cylinder  1100 . Pipes  1142 ,  1144 , and  1145  conduct liquid coolant into respective groups of grooves on the outer surface of the cylinder  1100 . 
         [0063]      FIGS. 5B and 5C  show the engine  1400  without cylinders, pistons and reservoirs. The engine  1400  has a frame constituted of end plates  1522  and  1524 , and a middle plate  1526  positioned between the end plates  1522  and  1524 . Through holes  1528  are provided through the plates  1524  and  1526  for mounting cylinders to the frame. The plates  1522 ,  1524  and  1526  have bearings  1530 ′ for rotatably supporting the crankshaft  1430  and bearings  1532 ′ for rotatably supporting crankshaft  1432 . The end and middle plates  1522 ,  1524 , and  1526  are held together on one side by a number of engine plates including engine plate  1510  and counterpart engine plate  1511 , and on a second side by engine plate  1520  and counterpart engine plate  1521 . One reservoir  1460  (shown in  FIG. 4A ) is mounted to one side of the frame between engine plates  1520  and  1511 , the other to the other side of the frame between engine plates  1510  and  1521 . 
         [0064]    Continuing with the description of  FIGS. 5B and 5C , the gearbox  1570  houses an output gear train through which the opposing rotational motions of the crankshafts  1530  and  1532  are coupled to an output drive shaft. The ends of the crankshafts  1430  and  1432  extend into the gearbox  1570 . A gear wheel  1572  with a toothed outer rim is fixed to the end of the crankshaft  1430  and a gear wheel  1573  with a toothed outer rim is fixed to the end of the crankshaft  1432 . An output gear wheel  1575  has an annulus  1576  with a toothed inside circumference  1577  and a toothed outside circumference  1578 . As seen in these figures, the outer rim of the gear wheel  1572  engages the inside circumference  1577  of the gear wheel  1575  at one location and the outer rim of the gear wheel  1573  engages the outside circumference  1578  of the gear wheel  1575  at another location diametrically opposite the one location. The gear ratio between the inner gear  1572  and the inside circumference  1577  may be 33/65 with MOD 4 teeth on the inner gear and the inside circumference, while the gear ratio between the outer gear  1573  and the outside circumference  1578  may be 33/65 with MOD 5 teeth on the outer gear and the outside circumference. This arrangement of gears permits the opposing rotations of the crankshafts  1430  and  1432  to be translated into the continuous rotation of the output gear wheel  1575  with an odd number of gears (three, in this case), with a non-integral gear ratio, and without any intermediary belts, chains, or other torque transfer elements. The result is a simple output gear train, shorter than that of Bird&#39;s engine, in which the crankshafts are commonly coupled by a single gear (the gear wheel  1575 ), which reduces torsional resonances between the crankshafts, as compared with Bird&#39;s engine 
         [0065]    As seen in  FIGS. 5B and 5C , an axle plate  1581  is attached by threaded screws to the annulus  1576  and a cover  1582  is fastened by threaded screws to the end plate  1522 , over the gearbox  1570 . The axle plate  1581  has a central axle  1586 . The cover  1582  includes an output bearing  1585  that receives the axle  1586 , thus enabling the frame to support the output gear  1575  for rotation. The axle  1586  constitutes the output drive of the engine  1400 . It may be coupled to an intermediate transmission or directly to the driven component by one or more shafts, gears, belts, chains, cams or other suitable torque transfer element or system (not shown). 
         [0066]      FIG. 5D  shows the engine  1400  with two cylinders  1100  mounted to the end and middle plates  1524  and  1526  by threaded screws and/or bolts  1527  extending through holes  1528  in the plates  1524  and  1526  into threaded holes in the mounting brackets  1164 . The threaded screws  1527  provide for easy removal of cylinders from the engine  1400  for inspection, repair, or replacement of cylinders or pistons. The assembled engine  1400  is seen in  FIG. 5E , with reservoirs  1460 A and  1460 B mounted by threaded screws between the end plates  1522  and  1524 . The engine plates  1520 ,  1521 ,  1510 , and  1511 , reservoirs  1460 A and  1460 B, and cover plates  1580  are mounted by threaded screws and/or bolts to the end and middle plates  1522 ,  1524 , and  1526  of the frame. 
         [0067]    The frame parts for the engine  1400  are preferably made of high temperature aluminum alloy (such as 5454 aluminum) that is cast and/or machined as necessary for assembly and operation of the engine. Engine fuel and scavenge systems may be as described below. Preferably, the liquid coolant and the fuel used for the engine  1400  are diesel fuel that may also serve as a lubricant for the pistons and other engine elements. Preferably, engine operations are controlled by way of an engine control unit (ECU) with associated sensors and actuators, as needed. 
         [0068]    The mounting of auxiliary engine apparatus to the engine  1400  may be understood with reference to  FIG. 5E . For example, a turbocharger  1590  is mounted to the engine plate  1510  for ease of coupling to one or more exhaust ducts and a supercharger  1591  is mounted to the engine plate  1520  for ease of coupling to inlet ducts. A fuel injection pump  1593  is driven by a timing belt from the end of one of the crankshafts. Coolant, lubricant and scavenging pumps (not shown) are mounted to the back of the engine  1400  and are driven by the end of one of the crankshafts. The coolant pump provides liquid coolant to the pipes in the cylinder sleeve  1140  and to the reservoirs  1460 A and  1460 B. A sump pump  1594  is mounted to the bottom plate  1580 . Although not shown in these figures, the extensions of the crankshafts through the back plate  1524  may also be employed to drive vibration dampers and engine accessories. 
         [0069]    Control of the delivery of liquid coolant by a liquid coolant supply system  1600  useable in the second embodiment is illustrated in the schematic diagram of  FIG. 6A . The supply system  1600  includes a programmable engine control unit (ECU)  1601 . The ECU  1601  senses a temperature of the cylinder  1100  by way of a sensor  1610  threaded into one of the openings  1116  in the cylinder liner  1102 . The ECU  1601  also senses temperatures of the crowns of the pistons  1300 A and  1300 B by way of sensors  1611 A and  1611 B mounted in the pistons  1300 A and  1300 B. Other sensors (not all shown) may provide inputs indicative of various engine operating conditions to the ECU  1601 . In the supply system  1600 , a scavenge pump  1594  recovers coolant exhausted from the cylinder  1100  and pistons  1300 A and  1300 B and pumps the coolant through an air separator  1630  and a filter  1631  to a (dry) sump  1632 . 
         [0070]    As illustrated in  FIG. 6A , a cylinder coolant circuit pump  1634 A pumps coolant collected in the sump  1632  through a heat exchanger  1635 A and a bypass valve  1636 A and into a manifold  1638 A. Liquid coolant for provision to the grooves in the cylinder  1100  is maintained at a selected pressure in the manifold  1638 A by control of the bypass valve  1636 A by the ECU  1601  and a pressure sensor  1639 A in the manifold  1638 A. From the manifold  1638 A, the liquid coolant flows through proportional valves  1642 ,  1644 , and  1645  and into grooves on the outside surface of the cylinder  1100  via pipes  1142 ,  1144 , and  1145 , respectively. All of the valves  1636 A,  1642 ,  1644 , and  1645  are controlled by the ECU  1601 . 
         [0071]    As illustrated in  FIG. 6A , a piston coolant circuit pump  1634 B pumps coolant collected in the sump  1632  through a heat exchanger  1635 B and a bypass valve  1636 B into a manifold  1638 B. Liquid coolant for provision to the piston rods  1330  in the pistons  1300 A and  1300 B is maintained at a selected pressure in the manifold  1638 B by control of the bypass valve  1636 B by the ECU  1601  and a pressure sensor  1639 B in the manifold  1636 B. From the manifold  1638 B, the liquid coolant flows through proportional valves  1660 A and  1660 B into the reservoirs  1460 A and  1460 B and from the reservoirs, through the bores  1332  of the piston rods  1330  onto the back surfaces of the crowns in the pistons  1300 A and  1300 B. All of the valves  1636 B,  1660 A, and  1660 B are controlled by the ECU  1601 . 
         [0072]    The ECU  1601  illustrated in  FIG. 6A  is programmed by mapping pre-calibrated values of cylinder and piston temperatures and other sensory data indicative of engine operating conditions to coolant pressures and flow rates for various engine operating loads. The ECU  1601  senses engine operating conditions and cylinder and piston temperatures, determines the current engine load and accesses and computes the required pressures and flow rates for the three circuits of the cylinder  1100  and the pistons  1300 A and  1300 B. The ECU  1601  then controls the valves  1636 A,  1642 ,  1644 , and  1645  to provide coolant to the coolant circuits of the cylinder  1100  as required at the current engine operating point. This control may be either open loop or closed loop. For example, at full engine power, using diesel fuel as the coolant, the pressure and flow rates provided to the pipes  1142  and  1144  may be less than 1 bar at 1 gallon per minute, and the pressure and flow rate provided to the pipes  1145  may be less than 1 bar at 4 gallon per minute. At the same time, the ECU  1601  also sets the valves  1636 B,  1660 A, and  1660 B to provide coolant to the coolant circuits of the pistons  1300 A and  1300 B as required to control thermal distortion of the crowns  1308  at the current engine operating point. For example, at full engine power, using diesel fuel as the coolant, the pressure and flow rates provided to the reservoirs  1460 A and  1460 B may be less than 3 bar at 15 gallons per minute per piston. 
         [0073]    Control of the delivery of liquid coolant by an alternate liquid coolant supply system  1650  is illustrated in the schematic diagram of  FIG. 6B . The system  1650  provides a first coolant (water, for example) to the cylinder  1100  and a second, different coolant (lubricant or diesel fuel, for example) to the pistons  1300 A and  1300 B. The supply system  1650  includes the programmable engine control unit (ECU)  1601  and the sensors  1610 ,  1611 A, and  1611 B in the cylinder  1100  and pistons  1300 A and  1300 B. The supply system  1650  utilizes liquid coolant return lines  1661  connected conventionally to the holes  1147  in the cylinder sleeve  1140  and the holes  1133  at the ends of the cylinder  1100 . The liquid coolant return lines  1661  converge into a return manifold  1662  that returns the first liquid coolant from the cylinder  1100  to a reservoir  1663 . 
         [0074]    As seen in  FIG. 6B , cylinder coolant circuit pump  1664  pumps the first liquid coolant collected in the reservoir  1663  through a heat exchanger  1665  and a bypass valve  1666  into a manifold  1667 . First liquid coolant for provision to the grooves in the cylinder  1100  is maintained at a selected pressure in the manifold  1667  by control of the bypass valve  1666  by the ECU  1601  and a pressure sensor  1669  in the manifold  1667 . From the manifold  1667 , the first liquid coolant flows through proportional valves  1672 ,  1674 , and  1675  into grooves on the outside surface of the cylinder  1100  through pipes  1142 ,  1144 , and  1145 , respectively. All of the valves  1666 ,  1672 ,  1674 , and  1675  are controlled by the ECU  1601 . 
         [0075]    The supply system  1650  shown in  FIG. 6B  also includes the piston coolant circuits of the supply system  1600 , which are constituted of the elements in sequence from the scavenge pump  1594  through the reservoirs  1460 A and  1460 B to deliver the second liquid coolant for cooling the pistons  1300 A and  1300 B as described above in connection with  FIG. 6A . As with the system  1600 , the second liquid coolant is streamed into the pistons  1300 A and  1300 B and recovered by the scavenge pump  1594 . 
         [0076]    The ECU  1601  illustrated in  FIG. 6B  is programmed and operates the supply system  1650  in the manner of the supply system  1600  to map pre-calibrated values of cylinder and piston temperatures and other sensory data indicative of engine operating conditions to first and second coolant pressures and flow rates for various engine operating loads, and to control the provision of the first and second liquid coolants at those pressure and flow rates to the cylinder  1100  and pistons  1300 A and  1300 B, respectively. 
         [0077]    It should be evident that the liquid coolant supply systems of  FIG. 6A  and  FIG. 6B  can control the cooling of the cylinder  1100  independently of the pistons  1300 A and  1300 B in response to engine operating conditions by varying the flow rates and pressures of the liquid coolant applied to the cylinder  1100  separately from the flow rates and pressures of the liquid coolant applied to the pistons  1300 A and  1300 B. Thus, the liquid coolant supply systems can maintain the cylinder  1100  at the same or different temperatures as the pistons  1300 A and  1300 B, and can vary those temperatures independently in response to changing engine operating conditions. Independent control of the temperatures of the cylinder  1100  and the pistons  1300 A and  1300 B enables the liquid coolant supply systems to maintain mechanical clearance or spacing between the bore  1103  of the cylinder  1100  and the outside diameters of the pistons  1300 A and  1300 B within a desired range as engine operating conditions vary. 
         [0078]    Fuel system embodiments for providing diesel fuel to the fuel injectors of an opposed piston engine such as that described herein are illustrated in FIGS. 9A-9C of the priority PCT Patent Application PCT/US2005/020553 (“the priority application”), which, as discussed above, is incorporated herein by reference. As is described in the cited passages, the liquid coolant provided to cool the cylinders and/or the pistons may be the diesel fuel also provided to power the opposed piston engine. 
         [0079]    A system for providing charge air to and discharging exhaust gasses from the opposed piston engine  1400  is illustrated in  FIG. 7 . The system may scale to serve one or more cylinders  1100 . In the system  1700 , an air inlet manifold line  1734  and an exhaust manifold line  1732  are respectively connected to the inlet ports  1107  and the exhaust ports  1105  of one or more cylinders  1100 . These manifold lines are preferably mounted outside the engine enclosure. The engine schematically illustrated in  FIG. 7  is a turbo-supercharged or supercharged engine. Thus, the manifold lines are connected to a turbo-supercharger  1736 . Specifically, the exhaust gases moving through the exhaust manifold line  1732  drive a turbine  1740  en route to an exhaust line  1738  to mechanically drive a compressor  1742 . The compressor  1742  draws air in on an air inlet line  1737  and pressurizes the intake air before directing the intake air to the inlet manifold line  1734  by way of an intercooler  1739 . A supercharger  1746  or equivalent device may be connected between the intercooler  1739  and the compressor  1742  and is mechanically driven to provide scavenge air for starting the engine. 
         [0080]    The uses and applications of an opposed-piston engine set forth in this specification are many fold. It can be scaled for any application using two-cycle engines, including two-cycle diesel engines. The engine can be installed in or mounted on a variety of powered vehicles, tools, devices, or other apparatus requiring the delivery of rotary power. See  FIGS. 8A-8F  for examples in this regard. In  FIG. 8A , this two-cycle opposed-piston engine  1400  is installed in a surface vehicle, which can include wheeled or tracked vehicles, such as automobiles, motorcycles, scooters, trucks, tanks, armored military vehicles, snow-mobiles, and all equivalent and similar instances. In  FIG. 8B , this engine is installed in a water-going vehicle such as a boat, hovercraft, submarine, personal water craft, and all equivalent and similar vehicles. In  FIG. 8C , this engine is installed in a fixed or rotary-wing aircraft. In  FIG. 8D , this engine is installed in a powered implement such as a lawnmower, edger, trimmer, leaf blower, snow blower, chain saw, and all equivalent and similar devices. In  FIG. 8E , this engine is installed in an electrical power generating device. In  FIG. 8F , the engine is installed in a pumping device. 
         [0081]    Although the invention has been described with reference to specific illustrations and examples, it should be understood that various modifications can be made without departing from the spirit of the principles of our engine. Accordingly, the invention is limited only by the following claims.