Ok, back to the main topic. 

EXTERNAL COMBUSTION occur with no clear boundary and surrounding. 
Mean while, the combustion occur inside the system is called INTERNAL COMBUSTION. Fuel and oxygen supply to the system and a spark will make combustion start.

Reciprocating and gas turbine is an example of internal combustion engine.

Reciprocating engine widely use as world accelerate. Chain saw, motorcycle, car, lorry, bus is an example of reciprocating engine usage.

Image source: Unsplashed / 𝑨𝒄𝒕𝒊𝒏𝒈 𝑺𝒖𝒃 𝑳𝒕.𝑷𝒂𝒓𝒊𝒘𝒂𝒕 𝑻𝒉𝒂𝒊𝒏𝒂𝒑𝒓𝒆𝒂𝒘

Gas turbine mostly use as aircraft engine.

But, both of them at a large scale can be use to generate electricity.

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