A free-piston Stirling engine (FPSE) with a generalized displacer-drive configuration having the displacer sprung to both the engine casing and power piston is described, characterized and analyzed. The analysis includes motion of the moving components, power transmitted from the displacer to the piston and power produced by the gas in the work space, and the effect of the displacer-drive configuration to the engine efficiency. Results are presented, from testing a FPS engine which has a gas spring linking the displacer to the engine casing and the power piston magnetically coupled to a linear alternator. The analysis and analytical prediction of the engine characteristics showed close agreement with the test results.