How does the APU work?

Question

Exactly how does the APU help the engines during sartup on commercial aircraft?

Answer 1

For the majority of aircraft with APU's air is used to start the main engines.  This air can come from either the bleed air section of the APU (just downstream of the compressor) or from a load compressor attached to the front of the APU.

Either Bleed Air or LC air is sent to the main engine and used to turn an Air Turbine Starter.  Not direct injection to the turbine section.  The ATS acts like any other starter, it drives a geartrain to spool up the main engine.

The other type of APU uses a generator to supply electricity for Main engine start.l  In this case the starter is an accessory drive motor/generator that spools up the main to start and then generates power after reaching N1 light off.

More modern engines are becoming only electric machines. These APU's and Main engine TRU (transformer rectifier units) produce 400 hz ac power so that smaller conductors are routed throughout the AC instead of insulated Stainless ducts.  (which are bloody heavy)

The A380 APU now sports 2each 750 kw generators AND load compressor.  

wer

Answer 2

The APU is basically a small turbine engine that creates a couple things.  It can be used to supply electrical power to the aircraft to power the various systems.  It also creates compressed air which is used to start the compression cycle of the jet engines.  To make it simple - for a jet engine to start, the turbine sections need to start rotating to get air into the compressor.  This is accomplished by the APU directing compressed air into the engines.

Answer 3

Like barnstormer said....The APU actuallly provides air for the air turbine starter or the electric starter.

it is the main engine starter..  Think of a big  pneumatic wrench attached to the gearbox...actually sounds a lot like it too.

jj

Answer 4

to make it simple, the turbine in the apu provides electricity and hydraulics to the plane.  the air or exhaust that the apu makes air bleed downdrafts to the engines to make them spin and fire up for flight.

Answer 5

example from the Mi 24:

the APU in compressor mode produces compressed air. thist air is ducted to the small expansion turbine, that is rigidly linked to the engine gearbox - the similar box that includes the torque suppply for oil pumps and other engine supportive items. this turbine is being proppeled by the compressed air and produces  torque which happens to be trasnported to the compressor shaft. thus the compressed air decompresses and starts the compressor to spin. once the sufficient compression is reached, the ignition sequence starts and the engine propels itself. then, the compressed air from the PAU begins to be ejected into ambient air again, and the APU is ready to start another engine / i do not remember if the Mi 24 has the crossbleed/

it is similar like the starter of your car, except of that the car starter uses the electricity to provide torque, while this engine starter unit requires comprressed air.

However the key feature of the APU is still providing the electricity /or torque/ for aircraft systems to operate while engines are not running.

Answer 6

The APU on large commercial aircraft supply essential DC electrical power to the different systems associated with engine start, such as the the fuel and pneumatics used in engine start.  

Larger turbine engines use a pneumatic starter and the APU supplies the air for that starter.  The air supplied turns the starter which turns the engine for the start sequence.  

And no the APU supplied air does not turn the engine turbine shaft it turns the starter motor mounted on the accessory drive to turn the turbine shaft

Edit

From the Boeing 727 Flight Handbook

Bleed air from the APU compressor is ducted through an APU load control valve.  The load control valve will be signaled to open when the 2 left or 2 right bleed switch is placed to the open with the APU operating.  The APU bleed air is then available to the engine start valves and the right and left air conditioning pacts.  

Essential AC power can be obtained in the APU or external power position of the essential power selector switch while the aircraft is on the ground

Answer 7

The APU is a small turbine engine, typically mounted in the rear of a commercial airliner.  It turns a generator, which provides electrical power to the plane for interior lights, air conditioning, electronics and starting the main engines in a sequential manner.

Answer 8

APU when running gives both pneumatics and electrical to the aircraft on ground when engine not running. During engine start-up bleed air from APU goes to the engine start valve (open either manually or auto during start-up) and goes to the starter motor where a crankshaft is connected to the N2 compressor driving it with air power, at 46% RPM starter cuts-out automatically so as not to damage the starter motor (he will become the driven instead of the driver w/c could result to internal damage). The start valve closes as indicated by the light extuinguishing or in the Display Unit the starter valve is not in line anymore.

So far the B747-400 P&W P901A APU is the best for it cam start all 4 engines though it is seldom used but two engine starting is done all the time.