What is 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 called?

scavenging — 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's 1913.)