engine efficiency

heat engine

• efficiency $$\tag{1} \varepsilon = \frac{W _{\rm output}}{W _{\rm input}} = \frac{| W |}{| Q_H| }. $$
• Carnot engine $$\tag{2} \varepsilon_C = \frac{T_H - T_L}{T_H}. $$

refrigerator

• coefficient of performance $$\tag{3} K = \frac{W _{\rm output}}{W _{\rm input}}. $$
• Carnot refrigerator $$\tag{4} K_C = \frac{T_L}{T_H - T_L} $$