Question

Can we achieve 100% efficiency from an engine by making the processes reversible?

Answer 1

Never. This would go against the laws of thermodynamics. The highest efficiency possible in a given situation is the Carnot Efficiency. The equation is Hot Temperature minus Cold Temperature over Hot Temperature. These temperatures must be in kelvins to work in the equation. Hence the only way to get 100% efficiency is to have an engine run in an absolute zero vacuum, which we dont have the technology to produce, assuming its possible at all. Even in this case it's likely that maintaining the absolute zero temperature would take more energy than any engine can produce.