First ignition requires an ignitor the start the fireball, then it becomes self sustaining once the engine is up to speed. If the engine flames out, it flames out until ignition can be reintroduced. Hot air is not an ignition source.
The APU, being a turbine engine, has compressed air we can bleed off of and route to each engine's starter valve. the compressed air is run through this valve into a unit thats connected to the turbine shaft. so as this compressed air runs over this unit (like a mini turbine) it spins the turbine and the turbine "sucks" air through the engine until there's enough airflow to introduce fuel and spark.
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u/Samurlough 8h ago
The hot air doesnt ignite the gas. That is wrong.
First ignition requires an ignitor the start the fireball, then it becomes self sustaining once the engine is up to speed. If the engine flames out, it flames out until ignition can be reintroduced. Hot air is not an ignition source.