We have seen that when mechanical work is done on a system, its internal energy increases (remember, we assume that the system does not have any systematic motion). The reverse process in which mechanical work is obtained at the expense of internal energy is also possible.

Heat engine is a device by which a system is made to undergo a cyclic process that results in the conversion of heat to work. The basic activity of a heat engine is shown in figure (12.9). It takes some heat from bodies at higher temperature, converts a part of it into mechanical work, and delivers the rest to bodies at lower temperature.

The substance inside the engine comes back to its original state. A process in which the final state of a system is the same as its initial state is called a cyclic process. An engine works in a cyclic process.

Efficiency Of a Heat Engine

Suppose an engine takes an amount \(Q_1\) of heat from high-temperature bodies, converts a part \(W\) of it into work, and rejects an amount \(Q_2\) of heat to low-temperature bodies. If the final state of the substance inside the engine is the same as the initial state, there is no change in its internal energy.

The efficiency of the engine is defined as
\(
\eta=\frac{\text { work done by the engine }}{\text { heat supplied to it }}=\frac{W}{Q_1} \dots(12.18)
\)

In a cycle, a certain amount of heat \(\left(Q_2\right)\) may also be rejected to the environment. Then, according to the First Law of Thermodynamics, over one complete cycle,
\(
W=Q_1-Q_2 \dots(12.19)
\)
i.e.,
\(
\eta=1-\frac{Q_2}{Q_1} \dots(12.20)
\)

For \(Q_2=0, \eta=1\), 1. e., the engine will have \(100 \%\) efficiency in converting heat into work.Such an ideal engine with \(\eta=1\) is never possible, even if we can eliminate various kinds of losses associated with actual heat engines. It turns out that there is a fundamental limit on the efficiency of a heat engine set by an independent principle of nature, called the Second Law of Thermodynamics. The mechanism of conversion of heat into work varles for different heat engines (for example, steam engine, internal combustion engine, and petrol engine).