An external combustion electric generator is a heated engine whereby an internal working fluid is heated by combustion of an external source through the engine wall or by the means of a heat exchanger.
When this fluid expands it then acts on the mechanism of the engine and causes motion and usable work and when the fluid has cooled off it is then compressed and reused or can also be dumped and the cycle of cool fluid in pulled in again.
When you talk about burning fuel you are referring to combustion working with an oxidizer which in turn supplies heat. The fluid used can be of any composition but the most common used by people is gas and another is single phase liquid.
The external combustion electric generator works by repeated heating and cooling. When the liquid is heated it causes the pressure to rise and in turn acts on the power piston which produces a power stroke.
When the liquid has cooled down then the pressure drops the pistons then don’t need to work so hard to recompress the liquid on the return strokes. The fluids that are working between the hot and cold heat exchangers flow cyclically.
Electric external combustion generators cannot start instantly they first need to warm up but the warm up time is relatively short. These generators can be simple as well as complicated such as a gas generator which adds weight and then increases the cost and can also create some bad complications.
External combustion electric generators are eco friendly and with the power and versatility it has it will run virtually everything from your garden equipment to trucks and cars and if big enough will even run locomotives. This is definitely going to become a future alternative that will be used by everyone from home owners to large companies.