Scientific research programmes of our Institute include development and construction of an effective Stirling engine and studies of a Stirling heat pump. According to this planned Stirling engine programme, a four-cylinder, double-action Stirling engine was designed, constructed and explored experimentally, with a regenerator having not less than 98% effectiveness.Theoretical calculations and analysis of published data on Stirling engines led to an understanding of why engines with piston swept volume greater than 100-150 cubic centimeters have comparatively low thermal efficiency. The relation between engine thermal efficiency and F/V, that is, cylinder inner heat exchange area to cylinder working fluid volume ratio has been established.With the purpose of determining the influence of inner heat exchangers upon engine thermal efficiency and to evaluate the acceptable range of ratio F/V we constructed and tested the above mentioned four-cylinder double-acting Stirling engine. Engine indicated thermal efficiency, when tested, was over 0.52 and F/V 150 m-1, and as a consequence specific power was low. Working fluids He, CO2, and N were used and power diagrams were obtained.