Nope. It is easy to make an efficient heat engine if temperature differential is so huge that it is sufficient for a steam turbine. And efficiency of a turbine

Nope. It is easy to make an efficient heat engine if temperature differential is so huge that it is sufficient for a steam turbine. And efficiency of a turbine will be nowhere like Carnot efficiency. Stirling engines work on much lower temperature difference, providing efficiencies dead close to Carnot levels (which is still, on that temperature difference, quite low).