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.
[Webster 1913 Suppl.]