Knowing the limits imposed by thermodynamics, the engineer Carnot designed the model of an engine which could comply with them...
It is a theoretical model, not a real one, but we can already see in it the fundamental characteristics of real heat engines: 
The system is based on the phenomena of the expansion and contraction of gases through heat gain and loss. 
The work of expanding and contracting the gases can be easily converted into the typical circular movement of the majority of engines.

The piston, the mechanism we see in the figure, is responsible for converting its displacement into a circular movement which can subsequently be converted to displace a vehicle, move a crane, a robot, etc.

The following visual will help you understand Carnot's design.