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Heat engines
Intermediate
Convert heat to work in a cycle.
η = W/Q_hA heat engine absorbs Q_h from a hot reservoir, converts part to work W and rejects Q_c: η = W/Q_h = 1 − Q_c/Q_h.
A power plant takes 1000 MJ and outputs 350 MJ electricity: η = 35%.
Efficiency is limited by temperatures.
In more detail
A heat engine converts heat into work by moving between a hot reservoir and a cold reservoir. Its efficiency is the work output divided by the heat input.
Real-world uses
- Car engines are heat engines that burn fuel to produce work.
- Steam turbines in power stations are heat engines.
- Power stations and car engines convert heat from burning fuel into work and waste the rest to the surroundings.
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Frequently asked questions
What is the efficiency of a car engine?
Typically 20 to 35%, limited by the second law.
What does the rest of the heat go to?
It is rejected to the environment as waste heat.
How do I calculate efficiency?
Divide the work output by the heat input from the hot reservoir, which always gives a value below one.
Related topics in Heat & Thermodynamics
Second law of thermodynamics · Kelvin–Planck statement · Clausius statement · Carnot engine · Carnot cycle · Refrigerators and heat pumps