No real heat engine ever reaches its theoretical Carnot efficiency. According to the source, roughly what fraction of the Carnot maximum does a real engine's actual efficiency typically reach?
AAbout 0.3 of the Carnot maximum
BAbout 0.5 of the Carnot maximum
CAbout 0.9 of the Carnot maximum
DAbout 0.7 of the Carnot maximum
Answer & Solution
Correct answer: D. About 0.7 of the Carnot maximum
1. The Carnot efficiency represents an unreachable theoretical ceiling for any real heat engine operating between two given temperatures.
2. A rough rule of thumb is given for how close real engines actually get to that ceiling.
3. It states that an actual efficiency of about 0.7 of the Carnot maximum is usually the best that can be accomplished.
4. The other fractions listed, 0.3, 0.5 and 0.9, do not match this stated rule of thumb.
_Source: OpenStax College Physics (CC BY 4.0), Ch 15 "Thermodynamics", section 15.4 Carnot's Perfect Heat Engine: The Second Law of Thermodynamics Restated_
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