Starting and ignition - Air

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Air

Fig. 11-6 An air starter motor.

9. Air starting is used on most commercial and some military jet engines. It has many advantages over other starting systems, and is comparatively light, simple and economical to operate. 10. An air starter motor transmits power through a reduction gear and clutch to the starter output shaft which is connected to the engine. A typical air starter motor is shown in fig. 11-6.



11. The starter turbine is rotated by air taken from an external ground supply, an auxiliary power unit (A.P.U.) or as a cross-feed from a running engine. The air supply to the starter is controlled by an elec- trically operated control and pressure reducing valve that is opened when an engine start is selected and is automatically closed at a predetermined starter speed. The clutch also automatically disengages as the engine accelerates up to idling r.p.m. and the rotation of the starter ceases. A typical air starting system is shown in fig. 11-7.

Fig. 11-7 An air starting system.

12. A combustor starter is sometimes fitted to an engine incorporating an air starter and is used to supply power to the starter when an external supply of air is not available.  The starter unit has a small combustion chamber into which high pressure air, from an aircraft-mounted storage bottle, and fuel, from the engine fuel system, are introduced. Control valves regulate the air supply which pressurizes a fuel accumulator to give sufficient fuel pressure for atomization  and  also  activates  the  continuous ignition system. The fuel/air mixture is ignited in the combustion  chamber  and  the  resultant  gas  is directed  onto  the  turbine  of  the  air  starter.  An electrical circuit is provided to shut off the air supply  which in turn terminates the fuel and ignition systemson completion of the starting cycle.
13. Some turbo-jet engines are not fitted with starter motors, but use air impingement onto the turbine blades as a means of rotating the engine. The air is obtained from an external source, or from an engine that is running, and is directed through non-return valves and nozzles onto the turbine blades. A typical method of air impingement starting is shown in fig. 11-8.

Fig. 11-8 Air impingement starting.