You are correct. "Καύσιμα" in Greek literally means burnables but we use it for fuel. I think the whole think is "καύσιμα υλικά" or something, but over the years the second word was dropped. But I could be wrong, so don't take my word for it. And it's used for all kinds of fuel. For any kind of machine that needs some solid, liquid or gas to work, we use καύσιμα.