热效率 Thermal efficiency
（重定向自Thermodynamic efficiency）
![一系统的输出能量恒小于其输入能量](/Images/godic/202502/16/Efficiency_diagram_by_Zureks.svg5138.png")
在热力学中，热效率（![\eta_{th} \,](/Images/godic/202502/16/7a19ba3fdf776ed7bf92a998cb72996c5138.png")
）是一个无因次的物理量，指的就是热机的效率，也就是热机运作过程中，热机可以产生机械能作功之能量与热源可提供的总能量的比。
![\eta_{th} \equiv \frac{W_{out}}{Q_{in}}](/Images/godic/202502/16/13783df6e7c11433c8b9392175f88e3a5138.png")
其中
![Q_{in}](/Images/godic/202502/16/5989d34677ddf859c44500915c53b9b25139.png")
为燃料总能量![W_{out}](/Images/godic/202502/16/f7cd85505f5a299e710d26d749b4a6085139.png")
为产生机械能作功
根据热力学第一定律，热机的热效率恒小于等于100%。
热泵及制冷设备也有类似的无因次物理量，称为性能系数（COP），是输入功和提供热能（或抽出热能）的比例，不过能效可能大于100%，和一般对效率的认知不同。
![Output (mechanical) energy is always lower than input energy](/Images/godic/202502/16/Efficiency_diagram_by_Zureks.svg5139.png")
![La energía producida o energía salida del sistema (Esal) es siempre menor que la energía suministrada o energía entrada al sistema (Eent).](/Images/godic/202502/16/Efficiency_Diagram_es.svg5139.png")
![\eta_{th} \,](/Images/godic/202502/16/7a19ba3fdf776ed7bf92a998cb72996c5139.png")
) is a dimensionless performance measure of a device that uses thermal energy, such as an internal combustion engine, a steam turbine or a steam engine, a boiler, a furnace, or a refrigerator for example. For a power cycle, thermal efficiency indicates the extent to which the energy added by heat is converted to net work output. In the case of a refrigeration or heat pump cycle, thermal efficiency indicates the extent to which the energy added by work is converted to net heat output.