Question

How is the efficiency of a heat engine determined?

Answer 1

All heat engines have this common way of working.

An amount of heat `Q_1` is supplied to it from a higher temperature reservoir called the source.

The engine uses an amount of this heat to perform work and rejects the remaining amount (we are considering non dissipative cycles for simplicity, for dissipative cycles a part of this energy is lost by dissipative means) to a reservoir at lower temperature called the sink.

If `Q_2` be the rejected heat, then `W = Q_1 - Q_2` is the amount of work performed.

The efficiency of the engine is then defined as,

`eta = W/Q_1 = (Q_1 - Q_2)/Q_1`

`implies eta = 1 - Q_2/Q_1`

From the Thermodynamics of the engine, `Q_1` and `Q_2` can be known (for example by considering pV or TS diagrams) and the efficiency is then readily calculated.