This invention relates to a method of operating the scavenging valves of two-stroke cycle marine internal combustion engines, and consists essentially in the use of means for driving the scavenging pumps from auxiliary engines normally employed solely for purposes independent of the propulsion of the vessel, such as operating the loading and unloading plant of a vessel, whereby such engines are utilised to assist in the propulsion of the vessel, and the useful propulsive output of the propelling engines is increased without any corresponding addition to or increase in the power plant of the vessel.