When we use the warmth of the heater power source to power my engine is better to be well insulated from the environment and the connection of the unit with external heat exchanger and heat exchanger itself. So all the heat we can "catch" from the heater can turn into mechanical energy - ideally will have no heat losses due to heat exchange with the environment. In this way we can use a working substance having a boiling point higher than ambient temperature (e.g. 373K water BP. At ambient temperature 290K water is in the liquid state). Then with heater will heat up substance to a temperature higher than its boiling point . Initially, we will have a cold part at a temperature equal to the environment, but gradually cold part to warm to a temperature close to the boiling point of the working substance (water 373K). Depending on the temperature that we have achieved in the external heat exchanger by heating will have a temperature difference between the hot and cold part of the unit. If I heat the water to 600K will have available temperature difference of 227 degrees between the hot and cold part to be able to convert the heat from the heater into mechanical energy. Bearing in mind that the use of a closed cycle working substance heat starts at the boiling point (about 373K of water) and not by the ambient temperature (say 290K) which increases the efficiency of the unit.