Table of Contents
What Is Thermal Efficiency?
Thermal efficiency is the ratio of useful work output to heat input. No real engine can be 100% efficient due to the second law of thermodynamics. The Carnot efficiency sets the theoretical maximum for any heat engine between two temperature reservoirs. Modern combined-cycle gas turbines achieve about 60%, while car engines reach 20-35%.
Improving thermal efficiency reduces fuel consumption and emissions. The key strategies are increasing the hot source temperature, decreasing the cold sink temperature, and minimizing irreversibilities such as friction and heat leaks.
Formulas
η = W / QH
ηCarnot = 1 - TC/TH
Typical Efficiencies
| System | Efficiency |
|---|---|
| Combined-cycle gas turbine | 55-62% |
| Diesel engine | 35-45% |
| Gasoline engine | 20-35% |
| Nuclear plant | 33-37% |
| Coal plant | 33-40% |
FAQ
Can efficiency exceed Carnot?
No. Carnot is the absolute theoretical maximum. Real engines always fall short due to irreversibilities.