What is the formula of thermal efficiency?

Hint:The measure of the Performance of the power cycle or heat engine is known as thermal efficiency.
Thermal efficiency is the ratio of the work to the heat input at a high temperature.
Thermal efficiency represents the fraction of heat converted to the work.

Complete step-by-step solution:
Thermal efficiency is the measure of thermal energy like an internal combustion engine, steam turbine, or steam engine. For a heat engine, the heat is converted into secondary energy.
The thermal efficiency formula can be written as,
\[{\eta _{th}} = \dfrac{W}{{{Q_H}}}\]
Here,
    ${Q_H}$is the heat
   $W$ is the work
   ${\eta _{th}}$ is thermal efficiency.
From the first law of thermodynamics, we can understand that the energy output cannot exceed the input.
From the second law of thermodynamics, it cannot be equal in a non-ideal process.
There are two types of thermal efficiency, they are indicated thermal efficiency and brake thermal efficiency. Therefore we can write the formula of thermal efficiency as follows,
\[{\eta _{th}} = \dfrac{W}{{{Q_H}}} = \dfrac{{{Q_H} - {Q_c}}}{{{Q_H}}} = 1 - \dfrac{{{Q_c}}}{{{Q_H}}}\]
To find the efficiency in percent we have to multiply the above equation by\[100\] .
Thermal efficiency has the most important features in the performance of the aircraft engine. Theoretically, heat and work are interchangeable; it gives the relation between each other. Therefore heat measures in the work unit, for example, ft-lb.

Note:The thermal efficiency represents the fraction of heat.
The Otto cycle thermal efficiency is the function of the compression ratio. It is used in spark engines.
The thermal efficiency is work done by the heat engine to the heat consumed by the engine.