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SESK: Stuff Everyone Should Know · sesk.info/eng-heat-engine-efficiency

How is a heat engine's thermal efficiency defined?

Efficiency is net work output divided by heat input.

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  1. Heat rejected divided by work
  2. Work output divided by heat input ✔
  3. Heat input divided by temperature

Why: A heat engine absorbs heat Qh, produces work W, and rejects the remainder Qc. Its efficiency is W/Qh, equal to 1 - Qc/Qh.

Huh, didn't know that: For a complete cycle, internal energy returns to its starting value, so W = Qh - Qc.

The dad joke
The heat engine asked what it got out of all that input.
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References

Answer: Work output divided by heat input. A heat engine absorbs heat Qh, produces work W, and rejects the remainder Qc. Its efficiency is W/Qh, equal to 1 - Qc/Qh.

  1. OpenStax University Physics Volume 2, Heat Engines openstax.org/books/university-physics-volume-2/pages/4-2-heat-engines
    The most important measure of a heat engine is its efficiency, which is simply what we get out divided by what we put in
    Checked 2026-10-10.

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