Scavenge Air

Scavenge air is the pressurised fresh charge of air introduced into the cylinder of a two-stroke engine to sweep out exhaust gases after the power stroke and supply the oxygen needed for the next combustion event. In uniflow-scavenged engines — the type used in most large marine two-stroke designs — scavenge air enters through ports in the lower cylinder liner wall and exhaust exits through a mechanically driven exhaust valve in the cylinder head. Turbochargers driven by exhaust energy supply most of the scavenge air at full load, while electrically driven auxiliary blowers take over at low engine loads and during starting. Scavenge air temperature, pressure, and the condition of the scavenge receiver are key performance indicators for engine room watchkeepers. A scavenge fire, caused by ignition of accumulated oil and carbon deposits in the scavenge air receiver, is a serious hazard that generates dense smoke, high temperatures, and can lead to exhaust valve failure. Engineers monitor scavenge pressure differentials across the air cooler, drain accumulated oil from the receiver regularly, and keep the scavenge spaces clean as preventive measures. SOLAS II-2 fire protection requirements apply to the machinery space housing the scavenge receiver.