Air start system

The air start system on a large slow speed diesel engine is used to initiate ignition and consists of the following components: a compressor, an air reservoir (large cylindrical tank), associated piping, a control valve (this is linked to the camshaft), and the air start valves.

When starting the engine, compressed air is admitted to whichever cylinder has a piston just over top dead center, forcing it downward. 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.

Compared to a gasoline (petrol) engine, diesels 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.