"RTG" is an acronym that stands for one of the following: "Radioisotope Thermionic Generator", "Radioactive Thermal Generator" or "Radioisotope Thermoelectric Generator".

A Stirling engine generator is less powerful than a combustion engine, but more ecologically sustainable. It operates on heat exchange. There are many types of Stirling engines.

Radioisotope thermoelectric generator

Stirling generators can be purchased from multiple online dealers that refurbish parts. The refurbished part is much cheaper price than a new stirling generator.

A Stirling engine generator falls into the category of external combustion engine. This type of engine converts heat energy to mechanical work by compressing and expanding air or other gas.