Why turbine but not piston?
 

Note: I am not sure if this is the correct forum, mods please move if belongs in another forum.

Can somebody please explain to me why a piston engine starts what I guess you call almost instantly, from 0 to 1000rpm... When a turbine has the ability to slowly spool up from zero to idle speed and why is this done in stages up to the idle rpm?

I asked my female instructor this question and she said not to ask a female such a technical question.

Thank you for any replies.

multi_engined, from your recent posts I take it you have a PPL or are just about to attain your PPL. Having said that, I am not gonna answer your first question about piston engines as you should already know why it goes from 0-1000 RPM almost instantaneously.

You also shouldn't worry too much about turbine engines now, when you study ATPL Systems, you will go over turbine engines so many times it will make your head hurt.

It is suffice to say though, without going into too much detail, that turbo-fan/prop spins because it is directly connected to the turbine at the back of the engine. The turbine at the back of the engine spins because hot air from the combustion chambers flow across the turbine disc thus causing it to rotate.

When the turbine engine is started, it takes time for the turbine (at the back) to accelarate from zero to idle because fuel isn't added to the combustion chambers instantly like it is in a piuston engine. Normal start-up should be no more than 25-30 secs, and if its any longer you probably are dealin with a hung start. Again there's a lot involved in turbine engines, and you'll appreciate the sophistication of the technology when you start studying your ATPL's.

Hope that somewhat answers your question.

Engines accelerate by increasing the fuel flow (effectively), but this must be matched to airflow. Put too much fuel into a turbine engine combustor when there isn't enough mass airflow through the engine and you overtemp or surge the engine. Thus, fuel flow has to wait for mass airflow to catch up, and this takes time.

To put it in a couple of words:
Piston>expansion in a closed chamber. the only way for the gas to expand is to displace the piston with no slip at all
Turbine> semi-open expansion chamber > through a turbine (air coupling, lots of slip)

Chalk and cheese!

A piston engine cannot idle at much less than about 600-800 rpm, unless specifically designed to do so. There is therefore no point in trying to take it slowly; if this were done there is a good chance that the engine could kick back, damaging the starting gear or the crankshaft, especially if there is a heavy propellor on the end. The starter motor is therefore designed (big current draw for a short time) to kick it straight over compression from where it should fire and go straight up to a reliable idle speed. Once the engine fires, the starter kicks out of engagement.

A turbine engine relies on a smooth, steady gas flow being established through the combustion chamber and the rest of the unit before fuel can be introduced, to prevent a hot start. The starter motor (often also the generator and thus remaining in mesh with the rest of the engine) is therefore designed to give a smooth, steady acceleration to a speed where initial combustion can take place and subsequently to continue to assist in the acceleration of the compressor towards idle speed, often close to 30,000rpm. Due to inertia, this cannot be done instantly.