  A quantum heat engine of a specific type is studied. This engine contains a
single particle confined in the infinite square well potential with variable
width and consists of three processes: the isoenergetic process (which has no
classical analogs) as well as the isothermal and adiabatic processes. It is
found that the engine possesses exotic properties in its performance. The
efficiency takes the maximum value when the expansion ratio of the engine is
appropriately set, and, in addition, the lower the temperature is, the higher
the maximum efficiency becomes, highlighting aspects of the influence of
quantum effects on thermodynamics. A comment is also made on the relevance of
this engine to that of Carnot.
