Apparatuses and methods related to an engine for converting heat into mechanical output using a working fluid in a closed circulating system are disclosed. In some embodiments, the engine includes a pump to pressurize the working fluid, a regenerative heat exchanger to transfer heat from a first portion of the working fluid to a second portion, a heating device to heat the working fluid, and first and second scroll expanders to expand the working fluid and generate the mechanical output. Other embodiments may be described and claimed.