An engine is disclosed having cylinders slidably disposed in an engine block. The cylinders reciprocate, engaging movable pistons at each end. A piston and cylinder end define a combustion chamber. A centrally located crankshaft extracts work, and converts the reciprocating motion into rotary motion. Precompression plates fixed to the cylinder reciprocate in precompression chambers to pre-compress and inject an air/fuel mixture. An exhaust gas recovery system, including a catalytic converter in the precompression chamber, extracts work from energy in the exhaust gas stream to improve the engine efficiency. In one embodiment, a central piston assembly is slidable within the cylinder, and reciprocates relative to the cylinder, substantially in synchrony with the motion of the cylinder, thereby producing an effective stroke length that is greater than the stroke length of the cylinder alone. Offset piston journals engage a gliding block to couple the central piston assembly to the crankshaft.