What is primarily burned in an external combustion engine?

In an external combustion engine, the combustion of fuel occurs outside the engine itself, typically in a separate chamber or boiler. This design allows for a variety of fuels to be burned, including wood, coal, oil, or gases. The heat generated from burning the fuel is used to produce steam or heat air, which then drives the engine's pistons or turbines.

The primary characteristic of external combustion engines is that they do not rely on the internal combustion of the fuel within the engine’s cylinders to create power. Instead, the fuel is burned in a controlled environment that is separate from the working parts of the engine, which can enhance efficiency and allow for the use of different fuel types.

While options such as high-efficiency diesel, gasoline, and compressed natural gas are associated with internal combustion processes where the fuel is directly combusted within the engine, the answer emphasizes that in the context of external combustion, fuel being burned outside in a different location aligns with the operation of this engine type. Therefore, the choice highlighting fuel in an external box accurately represents the functioning of an external combustion engine.