Source : Webster's Revised Unabridged Dictionary (1913)

Scavenging \Scav"eng*ing\, p. pr. & vb. n. of {Scavenge}. Hence,
   n. (Internal-combustion Engines)
   Act or process of expelling the exhaust gases from the
   cylinder by some special means, as, in many four-cycle
   engines, by utilizing the momentum of the exhaust gases in a
   long exhaust pipe.

Source : WordNet®

scavenging
     adj 1: searching for and salvaging anything useful from discarded
            material; "scavenging larvae"
     2: searching for and feeding on decaying matter; "scavenging
        hyenas and vultures"