Why is a perfect heat engine impossible?

Heat Engine:

1. It is a system that converts all the heat energy into mechanical energy.
2. Some examples of heat engine are steam engine and car engine.
3. A perfect heat engine is one which works with $100\%$ efficiency, that is it converts all the heat energy in it to mechanical energy.

Second Law of Thermodynamics:

1. It states that “Heat always travels from a hotter body to a colder body.”

Perfect Heat Engine:

1. According to the second law of thermodynamics, there is always some loss of heat in a heat engine.
2. The heat loss can be due to friction between parts of the engine, heat escaping via radiation, etc.
3. It is impossible to achieve with $100\%$ thermal efficiency.

Hence, a perfect heat engine is impossible.