Air start system

An air start system is a power source used to provide the initial rotation to start large diesel and gas turbine engines.

Compared to a gasoline ("petrol") engine, diesel engines have very high compression ratios to provide for reliable and complete ignition of the fuel without spark plugs. An electric starter powerful enough to turn a large diesel engine would itself be so large as to be impractical, thus the need for an alternative system. When starting the engine, compressed air is admitted to whichever cylinder has a piston just over top dead center, forcing wit downard. As the engine starts to turn the air start valve on the next cylinder in line opens to continue the rotation. As this goes on, fuel is injected into the cylinders, the engine is then under way and the air is cut off. To further complicate matters, a large engine is usually "blown over" first with zero fuel settings and the indicator cocks open, to prove that the engine is clear of any water build up and that everything is free to turn. After a successful blow ahead and a blow astern, the indicator cocks are closed on all the cylinders, and then the engine can be started on fuel.