Jim Carriere wrote:
"Peter Maus" wrote in message
Student, and a beginner here...still learning about pistons,... but
I've seen this term before here...Please explain 'hot starts'.
It is a turbine only term, there is no piston engine equivalent.
Yes, I realize that. But it comes up so often in my reading. I'm
curious, and have not found reasonable explanations.
It is when
there is too much fuel in the engine or too little air moving through it.
Now, for the thorough explanation...
Here is a basic distinction between gas turbines and piston engines- piston
engines normally use the right amount of air for how much fuel they use.
Gas turbines almost always use excess air, sometimes two or three times as
much as necessary to burn the fuel. This extra air makes the flame
temperature several hundred degrees cooler.
Ok. I'm with you, there. Diesels are non stoichiometric as well.
Although I wouldn't want to fly one.
In general terms, here is how a gas turbine engine normally starts:
1)
First, a starter turns the compressor, accelerating it from zero speed.
2)
Ignitors (like spark plugs) fire whenever the starter is turning in most
engine designs, and usually once or two sparks per second.
3)
Next, above some designated rpm, an exact flow of fuel is introduced. This
sprays near the ignitors and usually the flame starts within a few seconds.
4)
After the flame starts, the rpm will accelerate even more than with the
starter only.
5)
Above some designated rpm, approximately idle speed, the starter is not
used. It may be disengaged, power removed from it, and either automatically
or by the pilot flipping a switch (depending on which design).
6)
The engine is now at idle speed (usually above 50% rpm) after a normal
start.
A hot start may occur for a few main reasons:
A weak starter may not turn the engine fast enough. Between (4) and (5),
both the flame and starter are required to accelerate the engine to idle
speed. With the starter only, the engine can turn up to around 25% of
maximum rpm. Without the starter but the flame only, the engine can run at
any speed as long as it does not drop below around 40-50% rpm. If it drops
below that it will not be able to recover speed eventually stop. So... if
the starter is weak, or it drops out at too low an rpm (5), then the engine
will not be able to reach idle speed. This is like a car that will almost
start but sputters and dies instead. The difference with the gas turbine is
that fuel will keep on coming and burning as a turbine winds down over the
next several seconds, and may then burn too hot. The engine may also "hang"
at an rpm below idle with the flame temperature otherwise acceptable, but
that is called a "hung start," not "hot start." Air starters may be weak
from too little air spinning them or from being worn out. Electric starters
may also wear out, or be weak from a weak aircraft battery just like a car.
The oil in the engine may also be cold and thick, which requires a stronger
starter.
The fuel may be turned on too early (3), before the engine is turning fast
enough and moving enough air through the engine. So not enough air means
too hot a flame.
The fuel may take too long ignite even if everything else is functioning
correctly (2,3,4). The unlit fuel can collect in the engine over several
seconds, then ignite all at once. So too much fuel means too hot a flame.
Sometimes the flame may go all the way out the exhaust pipe like the
Batmobile, this is referred to as "torching."
The engine may malfunction and spray use too much fuel during a start (3).
Lastly, the engine may still be hot if it was just running. In this case,
you simply have to run the starter for a little longer to blow fresh air
through the engine to cool it off before turning on the fuel.
So I hope that makes sense. I simplified most of the explanation, tried to
make all the theory relevant but provide some real day-to-day examples of
causes. Still, it is all more or less true for all gas turbine engines.
Thank you. You've been very helpful. This clarifies a lot.
Much obliged.
p
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