What is a heat engine

A heat engine is a device that, by repeating a cycle, continuously converts heat into useful work. Its operation always requires two sources at different temperatures: it absorbs heat $Q_H$ from a hot source at temperature $T_H$, converts part of it into mechanical work $L$, and releases the rest $Q_C$ to a cold source at temperature $T_C$.

The physical intuition is that heat never converts entirely into work: part of it must always “slide” towards the cold. It is this spontaneous flow from hot to cold that the engine intercepts and partly diverts in the form of work, just as a water wheel intercepts water falling from a height.

Diagram of a heat engine: it absorbs heat $Q_H$ from the hot source, produces work $L$ and releases $Q_C$ to the cold source.

The efficiency $\eta$ measures what fraction of the absorbed heat is actually converted into work. Since over a full cycle the internal energy returns to its initial value, the first law imposes $L_\text{netto} = Q_H - |Q_C|$: everything that goes in and does not come out as released heat becomes work.

Principle — Efficiency

$\ev{\eta = \frac{L_\text{netto}}{Q_H} = 1 - \frac{|Q_C|}{Q_H}}$

Efficiency is therefore always less than $1$: it would equal $1$ only if $Q_C = 0$, i.e. if the engine released nothing to the cold. But this is precisely what the second law forbids.

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Topics: Macchine termiche Concepts: Rendimento Objects: Macchina termica

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