jet engine

[fruitsalad]

how does a jet engine start up from cold as unlike a motor vehicle it does not have a starter motor, or so im told.

[Clanad]

It does have a starter of sorts, except that it is driven by compressed air. You may have noticed, while sitting at the gate getting ready for departure, the overhead air vents (called eyeball vents) produce a nice stream of cool air. You may also be observant enough to recognize that, just before the engines are started, there's no longer any air coming from these vents. The reason is that the air from the on-board Auxilary Power Unit (APU) or an air hose hooked to the plane from the gate is shut off so that full air pressure can be diverted to the starter mechanism. It resembles a regular starter on your car with the difference being it's size. It's much larger, but still drives the engine to required starting RPM's (actually measured in per cent since it rapidly becomes such a large number)... after the compressed air is no longer needed it's re-diverted back to the cabin for passenger comfort... By the way, starters on small turbo-prop and reciprocating powered airplane engines are exactly like the one on your on your car...

[Quiznite]

A good example is people who build model / homemade JET ENGINE's often use a leaf blower to get the thing moving.

[sarah_louise]

As long as the engine is spinning and low pressure fuel gets to the engine driven high pressure fuel pumps and then the nozzles in the combustion chambers and the high energy igniters are pressed the engine will start.
Various methods have been used. Cartridge starter, Electric Starters, Air Starter as mentioned and you will be reassured to know that just the forward speed of the aircraft through the air will rotate it enough if it went out.

[chakka35]

The APU is usually a small jet-engine in its own right, mounted high in the tail. It's what makes the jet noise as you approach the aircraft on boarding, and before the main power units have started. So if the APU produces the compressed air to start the main engines (and other services within the cabin before that) what starts the APU?
I may be out-of date but in the Trident (where the designers refused to allow the extra weight of a compressed-air cylinder) they used a squirt from the on-board oxygen bottles.

[Clanad]

The APU, although a small turbine engine, as chakka35 says, is small enough to be started by a conventional electric motor. On arriving at a cold and dark aircraft for the first flight of the day, the aircraft battery is turned on, checked for voltage , brought on line and then used to start the APU. The APU's are sometimes used by home builders of experimental aircraft to power them as jet engines for flight...