When discussing engines, the ratio of output work to input work expressed as a percentage is called .

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Efficiency η is the ratio of input work over the output work. It is defined as:
η=work done/heat absorbed=(Q₂-Q₁)/Q₁ where Q₁ is the heat absorbed and Q₂-Q₁ is the work done.

This ratio is never greater or equal to 1, or 100% because the second law of thermodynamics doesn't allow it.

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