Question

Can a heat engine be $100\%$ efficient$?$

Answer 1

Heat engine:

1. A heat engine is a device that continuously converts thermal energy into other forms of energy like mechanical energy, electrical energy, etc.
2. The efficiency of the heat engine is defined as the net work done to the heat absorbed during a complete process.

Explanation

Efficiency, $\eta =\frac{Workdonebytheengine}{heatabsorbed}$

$\eta =\frac{W}{Q_1}=\frac{\left(Q_1-Q_2\right)}{Q_1}=1-\left(\frac{Q_2}{Q_1}\right)$

$Q_1$ is the amount of heat absorbed

$Q_2$ is the amount of heat rejected

$W$ is the work done by the system

Kelvin Planck's statement of the Second law of thermodynamics :

It states that “ It is impossible for any device that operates on a cycle to receive heat from a single reservoir and produce a net amount of work.”

Therefore, A heat engine can have 100% efficiency if no heat is rejected to the sink. The entire heat absorbed by the system gets converted into work but according to the second law of thermodynamics, this is impossible.
No, a heat engine can't be 100 % efficient