  In this work, the relationship between Carnot engine and Szil\'ard engine was
discussed. By defining the available information about the temperature
difference between two heat reservoirs, the Carnot engine was found to have a
same physical essence with Szil\'ard engine: lossless conversion of available
information. Thus, a generalized Carnot's theorem for wider scope of
application can be described as "all the available information is 100% coded
into work".
