Whether you are estimating on the spot or checking a result somebody else gave you, the thermal efficiency calculator is the quickest way to get from heat received (qin), heat rejected (qout), cold reservoir temperature (tc), hot reservoir temperature (th), heat received (qin) and heat rejected (qout) to thermal efficiency (ηth), thermal efficiency (ηth,rev) and net work output (wnet,out).
What Is the Thermal Efficiency Calculator?
This thermal efficiency calculator is for the kind of problem where you know the physical quantities but not the number you need. It turns heat received (qin), heat rejected (qout), cold reservoir temperature (tc), hot reservoir temperature (th), heat received (qin) and heat rejected (qout) into thermal efficiency (ηth), thermal efficiency (ηth,rev) and net work output (wnet,out).
Physics calculations go wrong in predictable ways, usually through mixing units or dropping a digit. Work through the result one step at a time and check that the figures are the size you would expect at each stage.
How the Thermal Efficiency Physics Works
The relationship is applied step by step, and each step is a standard piece of physics rather than a single opaque formula. That means you can check the intermediate values against what the theory predicts, which is usually faster than checking only the final answer.
Keep every value in the same unit system throughout the calculation. Mixing metres and feet part way through is the single most common reason a correct formula gives a wrong answer.
Units, Assumptions and the Limits of the Model
The calculation assumes ideal conditions: no air resistance, no heat loss, and dimensions that do not change while you work. Those assumptions are reasonable for most classroom and engineering estimates, and unreasonable for anything more precise.
Where a value is a rate, it has to be quoted for the same period as everything else. A per-hour figure cannot be combined with a per-day figure without converting one of them first.
A Worked Example
Working through an example, with heat received (qin) set to 10, heat rejected (qout) set to 5, cold reservoir temperature (tc) set to 2, the calculator returns thermal efficiency (ηth) of 0.5, and net work output (wnet,out) of 5, and thermal efficiency (ηth) (efficiency energy per mass) of 0.5. Substitute your own figures and the same steps produce the same kind of answer.
Frequently Asked Questions
What do I need to work out thermal efficiency (ηth)?
Enter heat received (qin), heat rejected (qout), cold reservoir temperature (tc), hot reservoir temperature (th), heat received (qin) and heat rejected (qout). The calculator then works through the relationship and reports thermal efficiency (ηth), thermal efficiency (ηth,rev) and net work output (wnet,out).
What does the thermal efficiency actually calculate?
It takes heat received (qin), heat rejected (qout), cold reservoir temperature (tc), hot reservoir temperature (th), heat received (qin) and heat rejected (qout) and converts them into thermal efficiency (ηth), thermal efficiency (ηth,rev) and net work output (wnet,out) using the standard relationship for this problem. Each step is applied in turn, so every output is traceable back to the numbers you typed in.
How should I enter heat received (qin)?
Use the value you actually have, in the unit shown next to the field. Enter every value in the same unit system. The calculator assumes the figures you supply are consistent, so it cannot correct a mistake that happens before the calculation starts.
Can I use this for a quick estimate as well as a precise answer?
Yes. The calculator is built for quick checks, so round your inputs to a sensible number of significant figures and the result will be just as useful. If you need more precision, keep the same method and add digits to the inputs rather than adjusting the method itself.
Why does a small change in one value move the answer so much?
That happens when an output depends on a ratio, because dividing by a small number magnifies any change in it. Check the units first, then check whether the value you entered is in the range the relationship was designed for.