A balanced compound, regenerative cycle, external heat source engine is mad by using mechanically double acting rigidly affixed and self-aligned reciprocating opposed piston and cylinder structure in which substantially all of the energy created during reciprocating motion is converted or translated into rotational motion by the action of a scotch yoke type, transfer lubricated, sliding/roller bearing mechanism which acts on a single and centrally located crankshaft which is contained in a telescopic crankcase structure. A balanced compound engine structure may be constructed so that two or more subsystems are housed in one or more modules. Two or more power modules may then be coupled together and an engine power and speed control may be attained by varying the relative phase angle of the couple.