Basically all engines are heat engines. They convert heat from a fuel into motion somehow. ‘Steam engines’ do this by boiling off steam, and they are *external combustion* engines. This makes them naturally very bulky, but potentially more efficient.
Petrol and diesel engines are *internal* combustion engines. This allows them to produce far more power, but typically at a somewhat lower efficiency. In these, the burning fuel expands instead of steam.
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