Source : Webster's Revised Unabridged Dictionary (1913)

Diesel engine \Die"sel en`gine\ or motor \mo`tor\ [After Dr.
   Rudolf Diesel, of Munich, the inventor.]
   A type of internal-combustion engine in which the air drawn
   in by the suction stroke is so highly compressed that the
   heat generated ignites the fuel (usually crude oil), the fuel
   being automatically sprayed into the cylinder under pressure.
   The Diesel engine has a very high thermal efficiency.

Source : WordNet®

diesel engine
     n : an internal-combustion engine that burns heavy oil [syn: {diesel},
          {diesel motor}]