The second law of thermodynamics implies

Efficiency of an ideal heat engine $\mathrm{\eta }=\left(1-\frac{{\mathrm{T}}_2}{{\mathrm{T}}_1}\right)$will be 100%, if T₂ = 0 which is not possible to achieve.
Alternatively $\mathrm{\eta }=\frac{\mathrm{W}}{{\mathrm{Q}}_{\mathrm{A}}}=\frac{{\mathrm{Q}}_{\mathrm{A}}-{\mathrm{Q}}_{\mathrm{R}}}{{\mathrm{Q}}_{\mathrm{A}}}$
According to second Law of Thermodynamics, heat rejected (Q_R) cannot be zero.
Hence $\mathrm{\eta }$cannot be 100%.