The maximum efficiency of the carnot engine only depends on two factors: 1 - The temperature of the hot reservoir (TH) 2 - The temperature of the cold reservoir (TC) And is given by (TH - TC) / TH or 1 - TC / TH So by that we can see the maximum efficiency (100%) would be when the difference of temperatures between the two reservoirs is infinite.
ec=Th-Tc/Th
The conceptual value of the Carnot cycle is that it establishes the maximum possible efficiency for an engine cycle operating between TH and TC. It is not a practical engine cycle because the heat transfer into the engine in the isothermal process is too slow to be of practical value. As Schroeder puts it "So don't bother installing a Carnot engine in your car; while it would increase your gas mileage, you would be passed on the highway by pedestrians."