  Treating the black hole molecules as working substance and considering its
phase structure, we study the black hole heat engine by a charged anti-de
Sitter black hole. In the reduced temperature-entropy chart, it is found that
the work, heat, and efficiency of the engine are free of the black hole charge.
Applying the Rankine cycle with or without a back pressure mechanism to the
black hole heat engine, the compact formula for the efficiency is obtained. And
the heat, work and efficiency are worked out. The result shows that the black
hole engine working along the Rankine cycle with a back pressure mechanism has
a higher efficiency. This provides a novel and efficient mechanism to produce
the useful mechanical work, and such black hole heat engine may act as a
possible energy source for the high energy astrophysical phenomena near the
black hole.
