Question

Why efficiebcy of carnot's cycle is not 100%?

Answer 1

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.