Question

what is the main difference between an internal combustion engine and an external combustion engine

Answer 1

this is the difference between a interal combustion and external

Answer 2

Heat engines can be classified as:

Internal combustion (IC) engine and External combustion (EC) engine which can be further divided into reciprocating IC & EC engines and rotary IC & EC engines.
Now , let's discuss about IC and EC engines one by one.

External combustion engines are those in which combustion takes place outside the engine . Steam engine and steam turbines are the common example
For eg. In steam engine or steam turbine , heat generated due to combustion of fuel is employed to generate high pressure steam which is used as the working fluid in reciprocating engine or turbine.

Internal combustion engines are the petrol or diesel engines which we encounter in our daily life , the products of combustion are generated by air fuel combustion inside engine cylinder and these are our working fluid also .
Hence , combustion takes place inside the engine and products of combustion are working fluids also .