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A typical jet engine consists of an intake or fan section that accelerates air like a propeller. While a lot of the accelerated air bypasses the core of the engine, some of the accelerated air enters the compressor section of the engine where it is compressed or squeezed by blades spinning at a high rate of speed.
After entering the combustion chamber, the compressed air is then sprayed with fuel and ignited by igniter plugs. The expanding gases are directed reward to the turbine section of the engine. The spinning turbines extract enough energy to, in turn, drive the compressor section of the engine and also power various engine accessories. The combination of primary exhaust from the combustion process and the bypassed air routed around the engine from the fan section, create the total propulsion.
While the basics of jet engine operation are relatively simple, just how do you start the engine to accelerate a large amount of air to get the process moving?
Smaller jet engines are started using electrical power, either from a battery or an external power unit. Upon initiating the start, generally with the push of a button, the battery powers the starter motor which begins rotating the engine just like a piston-powered airplane or your car. Once the engine rotation reaches a minimum speed, ignition is introduced to begin the combustion process. As the engine continues to accelerate, the start sequence is terminated.
Larger jet engines require a dedicated source of air to initiate engine rotation (battery power is not enough) which is generally provided by an auxiliary power unit (APU). Certain ground support equipment (huffer cart) can also provide the necessary air for engine start in the case of an inoperative APU. The APU is a smaller turbine engine that is started using a traditional battery start as described above. Once the APU is up and running, when the main engine start sequence is initiated, air is extracted from the APU to drive an air turbine starter to rotate the engine and begin the start sequence.
Watch the electric engine start of a Pratt & Whitney, JT15D-4B turbofan engine on a Cessna Citation.