The internal combustion engine works by changing heat energy into kinetic energy. There are many ways to do this, some better than others. But no matter how efficiently it is done, and no matter the size of the engine, the heat energy generated never completely changes into kinetic energy. Some energy is always lost.
This is certainly true in internal combustion engines where only about a third of the heat generated is transformed into the mechanical energy that moves the piston and turns the crankshaft. Another third goes out the exhaust, wasted. The rest tries to spread round the engine.
Heat always moves from areas of higher temperature to areas of lower temperature, which can be a problem. To control this movement, it is necessary to understand how heat is transferred.
Heat travels in just three ways.