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 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. Significant complexity is added to the engine by using an air-start system, as the cylinder head must have an extra valve in each cylinder to admit the air in for starting, plus the required control systems. This added complexity and cost limits the use of air-starters to very large and expensive reciprocating engines.
Another method of air-starting an internal combustion engine is by using compressed air or gas to drive a fluid motor in place of an electric motor. They can be used to start engines from 5 to 320 liters in size and if more starting power is necessary two or more motors can be used. Starters of this type are used in place of electric motors because of their lighter weight and higher reliability. They can also outlast an electric starter by a factor of three and are easier to rebuild.
An air-starter on a turbine engine would typically consist of a radial inward flow turbine, or axial flow turbine, which is connected to the High Pressure compressor spool through the accessory gearbox, plus the associated piping and valves. Compressed air is provided to the system by bleed air from the aircraft's auxiliary power unit or from an air compressor mounted on ground support equipment.
Compared to electric starters, air-starters have a higher power-to-weight ratio. Electric starters and their wiring can become excessively hot if it takes longer than expected to start the engine, while air-starters can be run as long as their air supply lasts. Turbine starters are much simpler and are a natural fit for turbine engines, and thus are used extensively on large turbofan engines used on commercial and military aircraft.
The air starter industry has been known to be extremely competitive thus creating questionable, and unscrupulous behavior among many suppliers.
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