It's magic!

Bill

Bill

Touchdown! Greaser!
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:confused:

I asked this question at the RDU BBQ, and everyone looked at each other, shrugged their shoulders, and said, "It's magic!"

Setup:

On my IO-360 engine, at shutdown, I set 1000rpm with the throttle, do a mag ground check, then pull mixture to idle cutoff, wait for the prop to stop moving, and turn off the master/mags. Normal stuff.

And, the hot start procedure, which works 95% of the time, is to TOUCH NOTHING, crank until it fires, and then advance the mixture.

So, where does that combustable mixture come from for the "crank until it fires" portion of the startup? I don't use boost pump, I don't even crack the mixture from idle cutoff, but yet it fires.

And everyone on the ramp said it was magic.
 
I think it works, Bill, because the mechanical fuel pump provides enough fuel pressure even if you don't have the electric pump on.
 
RotaryWingBob said:
I think it works, Bill, because the mechanical fuel pump provides enough fuel pressure even if you don't have the electric pump on.
Click to expand...

Yes, but the mixture is still at idle cutoff, the same setting that stopped a running engine, yet there is mixture enough to start a non-running engine?

If, at idle cutoff, enough fuel gets by to start a non-running engine, I would think a running engine would continue to run, or at least, cough and sputter.
 
Bill Jennings said:
Yes, but the mixture is still at idle cutoff, the same setting that stopped a running engine, yet there is mixture enough to start a non-running engine?

If, at idle cutoff, enough fuel gets by to start a non-running engine, I would think a running engine would continue to run, or at least, cough and sputter.
Click to expand...

I missed that part in your post, Bill. On a hot start, I've always advanced the mixture until the engine starts.

It must be magic!
 
Bill Jennings said:
:confused:

I asked this question at the RDU BBQ, and everyone looked at each other, shrugged their shoulders, and said, "It's magic!"

Setup:

On my IO-360 engine, at shutdown, I set 1000rpm with the throttle, do a mag ground check, then pull mixture to idle cutoff, wait for the prop to stop moving, and turn off the master/mags. Normal stuff.

And, the hot start procedure, which works 95% of the time, is to TOUCH NOTHING, crank until it fires, and then advance the mixture.

So, where does that combustable mixture come from for the "crank until it fires" portion of the startup? I don't use boost pump, I don't even crack the mixture from idle cutoff, but yet it fires.

And everyone on the ramp said it was magic.
Click to expand...

Well Bill, I obviously wasn't part of the conversation, as I'd a' dun' told ya'! ;)

Here's what's going on: when you pull the mixture to idle cutoff, the fuel flow out of your fuel servo is cut - in a perfectly operating system - to ZERO. The engine, at low idle RPM, simply draws what's left in the lines and fuel distributor (fuel spider) until it simply doesn't have enough fuel left to support combustion. Don't confuse this with the notion that ALL of the fuel got consumed, because it DIDN'T! The engine dies, you walk away, and come out to start it up again XX minutes later. The small amount of residual fuel present in those lines and spider, including the small amount that has dripped out the injector nozzles into your intake pipes/plenum and residing in vaporous form, is all that it takes to get your engine to fire during that notorious hot start. A very precise ratio of fuel/air must be met for the mixture to fire. You prime a hot IO-360 and you're gonna' crank that sucker until you achieve said mixture before you acheive combustion. Hopefully your starter isn't a pile of metal shavings by then. . . :eek:

I've got a few good articles on the subject, but can't do attachments. . .
 
It's not magic. The plane, being a good boy, knows whether your want the engine to be running or not and does what you want. :D

You might as well ask how the dog can follow complicated instructions.
 
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Fuel injected engines have fuel lines that run from the fuel spider to each cylinder. these lines run over the top of the engine and are full of fuel at shut down. this fuel will evaporate during shut down and fill the induction system with gasses that is drawn into the cylinder during start.

Remember fuel injectors are not a valve, they squirt fuel all the time the engine is running. to vary fuel mixture, you vary fuel pressure. Zero pressure engine stops. but leaves the injector line fuel of fuel. when it evaporates it expands, the only place it can go is out the injector.
 
MooneyDude said:
Well Bill, I obviously wasn't part of the conversation, as I'd a' dun' told ya'! ;)

Here's what's going on: when you pull the mixture to idle cutoff, the fuel flow out of your fuel servo is cut - in a perfectly operating system - to ZERO. The engine, at low idle RPM, simply draws what's left in the lines and fuel distributor (fuel spider) until it simply doesn't have enough fuel left to support combustion. Don't confuse this with the notion that ALL of the fuel got consumed, because it DIDN'T! The engine dies, you walk away, and come out to start it up again XX minutes later. The small amount of residual fuel present in those lines and spider, including the small amount that has dripped out the injector nozzles into your intake pipes/plenum and residing in vaporous form, is all that it takes to get your engine to fire during that notorious hot start. A very precise ratio of fuel/air must be met for the mixture to fire. You prime a hot IO-360 and you're gonna' crank that sucker until you achieve said mixture before you acheive combustion. Hopefully your starter isn't a pile of metal shavings by then. . . :eek:

I've got a few good articles on the subject, but can't do attachments. . .
Click to expand...

until tom gets his 5 posts and can post attachments, i vote for magic :D

btw, welcome to PoA tom!
 
Bill, I'm surprised at you. The idle cutoff does nothing to your checking account, which as any aircraft owner knows is REALLY what makes an airplane fly.
 
MooneyDude said:
Here's what's going on: when you pull the mixture to idle cutoff, the fuel flow out of your fuel servo is cut - in a perfectly operating system - to ZERO.

True

The engine, at low idle RPM, simply draws what's left in the lines and fuel distributor (fuel spider)

Not True, it can not get the fuel out of the lines from the spider because the fuel servo end is closed, nothing will flow, until the fuel is heated to the boiling point by heat from the engine as it cools. then it enters the induction system as a gas, and stays there unti the engine is cranked.


I've got a few good articles on the subject, but can't do attachments. . .
Click to expand...

Try starting the engine immediately after shut down, it will just sputter, as the fuel evaporates and enters the induction system.

This is a vary dangerious situation when you have a hot mag.
 
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mikea said:
It's not magic. The plane, being a good boy, knows whether your want to engine to be running or not and does what you want. :D

You might as well ask how the dog can follow complicated instructions.
Click to expand...

*snork*
 
NC19143 said:
Fuel injected engines have fuel lines that run from the fuel spider to each cylinder. these lines run over the top of the engine and are full of fuel at shut down. this fuel will evaporate during shut down and fill the induction system with gasses that is drawn into the cylinder during start.

Remember fuel injectors are not a valve, they squirt fuel all the time the engine is running. to vary fuel mixture, you vary fuel pressure. Zero pressure engine stops. but leaves the injector line fuel of fuel. when it evaporates it expands, the only place it can go is out the injector.
Click to expand...

Thanks, Tom, that makes sense. I'm thinking of injector as a car injector, which is an electrically controlled solenoid valve. I guess the aircraft injector is just a machined oriface with fuel pressure behind it.
 
Ken Ibold said:
Bill, I'm surprised at you. The idle cutoff does nothing to your checking account, which as any aircraft owner knows is REALLY what makes an airplane fly.
Click to expand...

Well, then my plane really does fly on magic!!! ;)
 
NC19143 said:
Try starting the engine immediately after shut down, it will just sputter, as the fuel evaporates and enters the induction system.

This is a vary dangerious situation when you have a hot mag.
Click to expand...

But oh, fine Sir, thoust art failing to consider the fact that the injector nozzles have AIR BLEED holes in them, and that in addition to helping atomize the fuel spray, this is precisely what they're for! As I said, not ALL fuel will get consumed / be drawn out of the lines when you go to idle cutoff - just enough to bring the F/A ratio down to a non-sustainable-for-combustion mixture as the engine winds down. If the air bleed holes were not in the injectors, what you say would be true - for the same reason a straw immersed in water holds a column of water when you put your finger on top and withdraw it. . . Why do you think that Lycoming has you tighten the injectors up such that you ensure the letter stamp on the hex is on the bottom/lower side when at full torque? Reason is so that the air bleed hole, which is opposite the letter stamp, gets to become the high point of the injector body due to the way it's angled, and thus allow air in and allow fuel to dribble out of the lines by gravity.
 
Just seeing this thread for the first time. Two comments. One, my hot start procedure is as Bill first described. Seems to work every time for me. I used to advance the mixture as I cranked, but experience has shown me that it is better not to.
Second point is the ground mag check. Why? Best place for mag check is just before you start your decent while still at cruise power and LOP. That gives you a real check of the ignition system health. I'm not sure what you'd learn on the ground before shutdown.
 
Lance F said:
Second point is the ground mag check. Why? Best place for mag check is just before you start your decent while still at cruise power and LOP. That gives you a real check of the ignition system health. I'm not sure what you'd learn on the ground before shutdown.
Click to expand...

I think we got caught up on words, here. :D

By mag ground check, I mean checking to verify the mags are grounded when the ignition switch is in the off position, not checking the left or right mags while ON the ground.
 
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