14

Apparently jet engines can take several seconds, up to ten or fifteen seconds, to go from full power to idle.

Why is this? Can't a simple fuel valve shut off fuel flow? Or reduce it to whatever flow rate corresponds to idle? Either of these, if done with a valve, should happen almost instantly as far as I can tell.

This would surely cut thrust, regardless if the engine still spins for a while. Is there something dangerous about this idea? To me it seems like a fuel valve can close and open again as many times as it wants.

Note: I'm interested in turbojets and turbofans. And I care about throttling down only.

DrZ214
  • 17,711
  • 17
  • 99
  • 207
  • 2
    To prevent compressor surge. See (maybe duplicate): Why do turbine engines take so long to spool up?. – mins Oct 01 '15 at 06:57
  • 5
    Inertia. But thrust will drop immediately if the fuel supply is cut. No heating, no thrust. – Peter Kämpf Oct 01 '15 at 06:57
  • 1
    @mins I read that but I don't see how a surge could happen when throttling down suddenly. Maybe the opposite could happen, a sudden drop in pressure, but that doesn't sound dangerous like a spike in pressure is. – DrZ214 Oct 01 '15 at 07:02
  • See 8.2.6 Emergency shutdown of Gas Turbine Performance. – mins Oct 01 '15 at 07:08
  • Maybe just inertia of the rotating turbine, compressor disks – Andrius Oct 01 '15 at 08:15
  • If you drop the fuel flow down in a massive step, say from 100% thrust to idle, then you run the risk of the residual air flow through the burners being far too much for the sustained burning at the lower fuel flow rate, leaning the mixture out and causing the engine to flame out. You can't apply brakes to the fan or core, so the reduction in air flow has to happen naturally through loss of inertia in those moving components. – Moo Oct 01 '15 at 15:09
  • @PeterKämpf Can you explain "inertia" any better? Note I'm interested in cutting thrust immediately, not necessarily cutting speed. I know the airplane's inertia will keep it flying forward for a while. I'm sure the engine will still spin too because of rotational inertia. But your last sentences seem to agree that we can cut thrust right away despite all that. – DrZ214 Oct 01 '15 at 17:19
  • @Moo That's okay. Flame out will cut thrust too. All I care about is cutting thrust, either to idle or completely zero. Of course it would be nice if the engine is undamaged and restartable in the normal way, even if normal restart procedures can take a while. As a curious aside, however, I can't see why it would flame out in your scenario. An overabundance of oxygen mixed with fuel, however small, should not cause a fire to die. As you probably guessed I don't know much about the gas dynamics in engines so hopefully someone could explain why the flame would die. – DrZ214 Oct 01 '15 at 17:22
  • @DrZ214 the combustion would die because the reactions between the fuel and the oxygen become too spread out, meaning you either have to have a constant ignition source or the combustion ceases because it can't propagate. A flame is basically a chain reaction, with the energy released by one fuel-oxidiser reaction triggering a reaction in a neighbouring fuel-oxidiser pair, but move those pairs far enough apart and the energy is no longer enough to trigger the reaction. This is why fire spreads where there is both a fuel source and an oxidiser. – Moo Oct 02 '15 at 08:36
  • 5
    @DrZ214 of course, all of this is ignoring that in modern high bypass engines, the bulk of thrust (like 80% or more) is generated by the movement of air by the fan at the front - to cut that thrust to zero immediately, you need to stop that fan or gear the fan blades so they can be feathered. Stopping the fan means a hefty brake (those fans weigh tonnes and have a high RPM) and feathering the fans means more weight and complexity. Airlines don't want either of those, so it hasn't ever been a problem that has needed to be solved. – Moo Oct 02 '15 at 08:39
  • @Moo Okay then it sounds like what I want is a thrust reverser, or maybe a thrust deflector to vector the air in two opposite sideways directions, or even some kind of bleed air opening. PS your comment would make a great answer. – DrZ214 Oct 02 '15 at 08:51
  • @DrZ214 most jet engines have thrust reversers already :) Often they are used to enhance braking performance on landing, but they are also used to deflect spool-down thrust on landing as well when deployed at engine idle. – Moo Oct 02 '15 at 08:56
  • I guess the shut down must be done in a controlled fashion to accomodate for differential cooling patterns of each component. The blades cool at a rate way different than the engine nacelle and casing. – Raj Oct 14 '15 at 01:49
  • 2
    @PeterKämpf: While inertia will keep the shaft & everything attached spinning for a short while, the air passing through the engine does serve as a massive brake. Obvious, from an energy point of view : the spinning engine contains only a fixed amount of energy, and accelerating air through the engine drains that energy. High RPM means a lot of energy, but also a lot of air being accelerated, trust produced while the engine spools down, and thus energy carried away. – MSalters Oct 26 '15 at 15:31
  • @Moo another way to stop the fan quickly is to use the electric generator in reverse. Large commercial jet engines have a generator that supplies electric power to the cabin. If I'm not mistaken, it is mechanically simple to reverse this process (take electric power to brake the spinning turbine). But then the only question is, where will you get the power from to do that (since we just shut off the engines)? – DrZ214 Oct 26 '15 at 19:37
  • @DrZ214 even more important than where the energy comes from would be whether the generators can support the load - you can't simply turn a generator into a motor as they are designed for different loads, and as it will already be doing electrical energy production then regenerative braking is out as it won't add anything to it. Also bear in mind that not all engines may have generators. – Moo Oct 27 '15 at 11:41

3 Answers3

8

The simple answer is rotational inertia, once you get a heavy mass spinning fast that heavy mass will have a tendency to continue spinning fast unless something else is acted upon the object. Piston engines have a great deal of friction between the piston walls and pistons (how they hold the explosion in) this helps them slow down faster than a jet in which the turbine is made to easily handle this thus the joints are very well made and very free moving. While cutting fuel will keep the engine from continuing to generate thrust the turbine will continue to spin until a force causes it to stop (drag and friction really).

Dave
  • 101,073
  • 5
  • 220
  • 364
6

Jet engines have little mechanical margin, and operate at high temperatures. That means all components expand by more than the mechanical margins when heated. This is no problem when everything expands and contracts together in a controlled fashion. This may be counterintuitive but is easily demonstrated. Cut a circular hole in a metal plate, and heat up the plate. The hole will grow with the same expansion rate as the surrounding material.

Now what happens if we suddenly stop the fuel flow? The compressive heating of air flowing into the engine doesn't suddenly stop, but all the air flowing through the engine does cool it. This means that the turbine in the back does cool as relatively cool air flows through it. Not only does this cause thermal stresses, but the turbine will shrink.

MSalters
  • 1,840
  • 12
  • 14
  • Are you saying that even if the throttle control is pulled to idle, some ECU will continue to feed fuel into the engine to avoid rapid cooling? – feetwet Apr 21 '18 at 16:12
2

Basically it is that the more weight the moving object has (the turbine in this case) the more time it takes to slow down because of the energy stored in a moving object. It is the same in all object which have a large mass. In an automobile, there is the flywheel which stores some of the energy the engine produces so that a little hiccup in the pistons would not interrupt the spinning. With the jet engine, the large mass of the turbine also keeps the increase or decrease of speed at a constant rate so that the engine would not produce "jerks" of power when the throttle is changed.

SMS von der Tann
  • 11,974
  • 10
  • 68
  • 118