A method for operating an opposed-piston internal combustion engine cycle for enabling highly efficient stratospheric flight, comprising:
- (i) Providing an internal combustion engine in operation having a plurality of cylinders disposed symmetrically about a main shaft, each cylinder enclosing a pair of first and second opposed pistons, each of said pistons connected to a piston rod, wherein each piston translates back and forth along the long axis of the cylinder and wherein the combined motion of the piston pair is synchronized to allow the cyclical translation of the first piston to be in phase with that of the second piston, whereby each piston reaches their respective top dead center and bottom center positions at substantially the same time, and whereby the volume of the cylinder is defined by the product of the distance between the top surfaces of the first and second pistons and the cross sectional area of the cylinder, the maximal volume V0 defined where the distance between the top surfaces of the first and second pistons is the largest in the engine cycle, the first and second pistons being at the bottom dead center position, an intermediate volume V1 defined where the compression stroke of the piston pair begins, V1<V0, and the minimal volume V2, V2<V1, defined where the distance between the top surfaces of the first and second pistons is the smallest in the engine cycle and being the terminal volume of the compression stroke and the beginning volume of the expansion stroke, the first and second pistons being at the top dead center position, and each cylinder in the plurality of cylinders having at least one intake port extending through the opposed cylinder wall and disposed near the bottom dead center positions of each piston pair, and at least one exhaust port extending through the opposed cylinder wall and disposed near the top dead center positions of each piston pair, the intake and exhaust ports allowing gaseous communication between the internal volume of the cylinders and the external environment of the cylinders;
- (ii) Providing a gaseous working fluid having a temperature T1 at or below ambient temperature and pressure P1 substantially at ambient pressure, to an engine compartment housing the engine;
- (iii) Opening one or more exhaust ports disposed in the wall of each cylinder substantially near the end of the expansion phase of the previous engine cycle wherein the volume of the cylinder is substantially near V0 and the pressure of the exhaust gases within the cylinder P4 is greater than the gaseous working fluid pressure P1 on the exterior of the cylinder wall, and whereby the exhaust gases expand and exit through said open exhaust ports, and wherein V0 is the volume at which the pistons are at the bottom dead center position;
- (iv) Opening one or more intake ports disposed in the wall of each cylinder in succession to the opening of the one or more exhaust ports, whereby exhaust gases from the previous engine cycle are purged from the cylinder by means of a flow created by a pressure differential between the open intake and exhaust ports, and working fluid flows at ambient pressure into the cylinder through the open intake ports;
- (v) Closing the open exhaust ports and returning the cylinder volume to V1 from V0 by engaging the pistons in a partial compression stroke, wherein the working fluid is maintained at a constant pressure by means of the open exhaust ports;
- (vi) Closing the open intake ports and compressing the working fluid in the cylinders, by moving the pistons towards top dead center from cylinder volume V1 to a final volume V2, whereby V2 is substantially less than V1 and the compression ratio V1/V2 is 50:1 or greater, and wherein the volume V2 defines a combustion volume residing between the top surfaces of the pistons when both opposed pistons are at the top dead center position, whereby the value of V2 is such that the heat transfer surface area to volume ratio A/V2, wherein A is the total heat transfer surface area, comprising the intervening cylinder wall surface defined by the cylinder bore diameter and the distance between the insulated top surfaces of the opposed pistons, to volume V2, is at an optimal value in order to minimize combustion energy losses resulting from convective and conductive heat transfer to said wall surface;
- (vii) Metering a fuel charge of liquid hydrogen into the combustion volume V2 wherein the fuel charge is measured to maintain an equivalence ratio phi between 0.4 and 1.0;
- (viii) Metering a charge of water vapor into the combustion volume V2 to provide a fluid means of partially absorbing the heat released from combustion of the fuel;
- (ix) Combusting the fuel charge in the combustion volume V2 by spark ignition wherein the pistons are substantially stationary at the top dead center position for the duration of the combustion process and whereby spark formation and ensuing combustion is uniform in the cross section of V2;
- (x) Expanding the cylinder volume from V2 to V0 at a high initial velocity, wherein the pistons are propelled freely by the forces derived from the instantaneous pressure of the exhaust gases, from the top dead center position at V2 toward the bottom center position at V0 without mechanical constraint for an initial portion of the expansion stroke before mechanically engaging a coupling mechanism converting linear motion to rotational motion;
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- (xi) Coupling the pistons to a cylindrical end-cam for imparting rotation thereto, whereby the cylindrical end-cam comprises a central axis and a cam profile having a compression segment setting the length of the compression stroke, the compression segment followed by a dwell segment setting the dwell time of the pistons at top dead center, the dwell segment followed by a power drive segment setting the length of the expansion stroke, said power drive segment having a first portion and a second portion that is contiguous with said first portion, wherein the first portion of the power drive segment is substantially parallel to the central axis of the cylindrical end-cam to provide for rapid piston acceleration in a first portion of the expansion stroke, and the second portion of the power drive segment possessing a curvature whereby the slope of the power drive segment smoothly transitions from being substantially parallel to the central axis of the cylindrical end-cam to being substantially perpendicular to the central axis to gradually engage the piston force for converting the linear piston motion to rotary motion of the engine in a second portion of the expansion stroke, said power drive segment and said compression segment configured such that the expansion stroke length of the piston is longer than the compression stroke length of the piston.