What is "efficiency of a heat engine"

efficience \ef*fi"cience\ ([e^]f*f[i^]sh"ens), efficiency \ef*fi"cien*cy\ ([e^]f*f[i^]sh"en*s[y^]), n. [L. efficientia.]

1. The quality of being efficient or producing an effect or effects; efficient power; effectual agency.

   The manner of this divine efficiency being far above us.
   --Hooker.

2. (Mech.) The ratio of useful work to energy expended.
   --Rankine.

   efficiency of a heat engine, the ratio of the work done by an engine, to the work due to the heat supplied to it.