Al~traet--Low-grade energy Rankine-cycle engines are widely used nowadays for power generation from heat recovered or collected at low temperatures (e.g. from waste exhaust gases of internal combustion engines, thermal power end industrial plants, solar collectors, as well as from geothermal sources). The heat input to these engines occurs at low temperature, therefore lower overall efficiencies are expected. To maximize these efficiencies some performance factors (such as evaporator and condenser saturation temperatures, the isentropic efficiency of the expander, the magnitude of the superheat and the working fluid and Rankine-cycle engine type) affecting the overall efliciencies of Rankine-cycle engines, using refrigerants 113 and 114 as working fluids, are studied. The analysis of the calculations shows that the overall efficiencies of Rankine-cycle engines can be increased with the increase of the saturation temperature of the evaporator and the isentropic efficiency of the expander, the reduction of the saturation temperature of the condenser, using refrigerant 113 as working fluid in the cycle instead of R-114, admitting dry saturated vapour of the working fluid to the expander instead of superheated vapour and, finally, using regenerative Rankine-cycle engines instead of simple Rankine-cycle engines. h qr qx°a, qxso