Thermal efficiency measures how much of the heat burned in an engine becomes useful work. Car engines manage 25–40%, modern combined-cycle power plants over 60%.
How to use the Thermal Efficiency Calculator
- Engine
- Heat input Q_in
- Work output W
- Heat rejected Q_out
- Hot temperature
- Cold temperature
Results update as you type, every field has its own unit menu, and everything is calculated in your browser.
How it works
Choose irreversible (real data) or reversible (temperatures) and enter the values.
Tips
- Use absolute temperature (K) in gas laws and efficiency formulas.
- Temperature differences are the same in K and °C.
- Heat = mass × specific heat × temperature change when no phase change occurs.
- Phase changes need latent heat at a constant temperature.
Common mistakes to avoid
- Using °C in pV = nRT.
- Forgetting latent heat during melting or boiling.
- Confusing heat capacity (J/K) with specific heat (J/(kg·K)).
Inputs explained
| Field | What to enter |
|---|---|
| Engine | Choose one: Real engine – heat and work, Real engine – heat in and out, Reversible (Carnot) |
| Heat input Q_in | Units: J, kJ, MJ, kWh, cal, kcal … |
| Work output W | Units: J, kJ, MJ, kWh, cal, kcal … |
| Heat rejected Q_out | Units: J, kJ, MJ, kWh, cal, kcal … |
| Hot temperature | Units: °C, °F, K |
| Cold temperature | Units: °C, °F, K |
Example values
How the main result changes when Heat input Q_in changes and everything else stays at the example values above:
| Heat input Q_in (J) | Thermal efficiency (%) |
|---|---|
| 50 | 70 |
| 75 | 46.6667 |
| 100 | 35 |
| 125 | 28 |
| 150 | 23.3333 |
| 200 | 17.5 |
Formula
η = W/Q_in = 1 − Q_out/Q_in · Reversible: η_max = 1 − T_c/T_hWorked example
Inputs — Engine: Real engine – heat and work; Heat input Q_in: 100 J; Work output W: 35 J.
Results — Thermal efficiency = 35 %.
Key terms
- Heat engine
- Device converting heat into work.
Frequently asked questions
Why is thermal efficiency always below 100%?
The second law of thermodynamics requires heat rejection.
What is a typical car engine efficiency?
About 25–35% in normal driving.
How do combined-cycle plants reach 60%?
Exhaust heat from a gas turbine drives a steam turbine.
What limits the efficiency?
The temperatures available and irreversibilities.
What formula does the Thermal Efficiency Calculator use?
η = W/Q_in = 1 − Q_out/Q_in · Reversible: η_max = 1 − T_c/T_h.