Magneto timing

Heywood, "Internal Combustion Engine Fundimental" Pg 451.
 

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Clark1961 said:
You've neglected to address my point about the speed of travel of the pressure front which is at the heart of the matter. If the pressure front hasn't disturbed the unburnt fuel/air mixture then it won't detonate. Advanced timing leads to flame front impingement, not detonation.
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Oh Boy....:mad2::mad2::rolleyes2:
 
Clark1961 said:
You've neglected to address my point about the speed of travel of the pressure front which is at the heart of the matter. If the pressure front hasn't disturbed the unburnt fuel/air mixture then it won't detonate. Advanced timing leads to flame front impingement, not detonation.
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Here's what I said in that post, if you'll go back and read it again:

"The pressure front can easily outrun the flame front, considering that the high temperature caused by compression increases the speed of sound enormously."

Dan
 
Dan Thomas said:
....the high temperature caused by compression increases the speed of sound enormously.

Dan
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Not to involve myself in this debate but I'm pretty sure you meant to say pressure rather than temperature.
 
Silvaire said:
Not to involve myself in this debate but I'm pretty sure you meant to say pressure rather than temperature.
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Speed of sound is proportionate to temp not pressure, that's why the speed of sound increases above 30k~ feet, even though the atmospheric pressure continues to decrease


-VanDy
 
VanDy said:
Speed of sound is proportionate to temp not pressure, that's why the speed of sound increases above 30k~ feet, even though the atmospheric pressure continues to decrease


-VanDy
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Sorry, its actually around 60k where it starts to increase again.
 
azure said:
What is the definition of detonation?
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The non-engine definition is combustion that propagates at a rate faster than the speed of sound.

For engines, detonation and knock are often used interchangeably ("The Internal Combustion Engine" Taylor and Taylor, 1961). And, it refers to combustion that is happening in advance of the flame front arrival - see the chart I posted a few posts back.
 
Capt. Geoffrey Thorpe said:
The non-engine definition is combustion that propagates at a rate faster than the speed of sound.
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Yes, that's what I was getting at. Since the definition I knew about involves supersonic flame front propagation, it seemed strange to read about the pressure front outrunning the flame front in the context of detonation.

For engines, detonation and knock are often used interchangeably ("The Internal Combustion Engine" Taylor and Taylor, 1961). And, it refers to combustion that is happening in advance of the flame front arrival - see the chart I posted a few posts back.
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Okay, thanks.
 
Dan Thomas said:
Here's what I said in that post, if you'll go back and read it again:

"The pressure front can easily outrun the flame front, considering that the high temperature caused by compression increases the speed of sound enormously."

Dan
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So you've claimed. Got anything to back it up?
 
azure said:
Yes, that's what I was getting at. Since the definition I knew about involves supersonic flame front propagation, it seemed strange to read about the pressure front outrunning the flame front in the context of detonation.
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Because of the turbulence / bulk motion in the chamber (how much depends on the cylinder design) the "normal" flame front is faster than the laminar flame speed but it is still relatively slow. The unburned mixture in the end gas region ends up getting compressed and that's where the spontaneous widespread combustion can happen (ahead of the flame front). Early spark timing, hot spots, etc. can get the combustion started earlier (or at multiple points) and result in higher pressure / temperatures in the end gas region which makes it more prone to knock.
 
Clark1961 said:
So you've claimed. Got anything to back it up?
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Numerous references, including these:

http://books.google.ca/books?id=mX1...nation in internal combustion engines&f=false

http://www.scribd.com/doc/58932213/40/DETONATION-IN-IC-ENGINES

http://books.google.ca/books?id=Utx...nation in internal combustion engines&f=false

If the normal flame front is around 100 FPS, it's well under the speed of sound and even at normal pressures and temperatures the pressure wave would outrun it. Anybody should be able to see that. Speed of sound at 59°F is around 1000 FPS. Raise the temperature and it goes way up. That unburned fuel ahead of the flame is getting compressed mightily and under the right conditions it will autoignite.

Dan
 
Dan Thomas said:
Numerous references, including these:

http://books.google.ca/books?id=mX1...nation in internal combustion engines&f=false

http://www.scribd.com/doc/58932213/40/DETONATION-IN-IC-ENGINES

http://books.google.ca/books?id=Utx...nation in internal combustion engines&f=false

If the normal flame front is around 100 FPS, it's well under the speed of sound and even at normal pressures and temperatures the pressure wave would outrun it. Anybody should be able to see that. Speed of sound at 59°F is around 1000 FPS. Raise the temperature and it goes way up. That unburned fuel ahead of the flame is getting compressed mightily and under the right conditions it will autoignite.

Dan
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So the first reference was blank, the second was for diesel engines and the third did not provide hard data. Okay, we can agree to disagree. Your argument depends on acceptance of your claims without data to support. Since flame front speed depends on mixture your position is untenable.You are confusing impingement with detonation. No big deal. You are trained to believe one thing when the physics actually show something else is happening. That's okay. Pretty typical for the world we live ing.
 
Clark1961 said:
So the first reference was blank, the second was for diesel engines and the third did not provide hard data. Okay, we can agree to disagree. Your argument depends on acceptance of your claims without data to support. Since flame front speed depends on mixture your position is untenable.You are confusing impingement with detonation. No big deal. You are trained to believe one thing when the physics actually show something else is happening. That's okay. Pretty typical for the world we live ing.
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The first reference works just fine for me.

The second is for both diesel and gasoline engines; they talk about octane rating (not applicable to diesel) and types of spark ignition, also not applicable to diesel.

The third article makes this statement on pages 208-209:

"The advancing flame front compresses the charge farthest from the spark plug, thus raising its temperature....If the end charge reaches its auto-ignition temperature and remains for some time to complete the pre-flame reactions, the charge will auto-ignite leading to knocking combustion."

Does anyone else see that I'm out to lunch on this?

Dan
 
From a practical perspective as a pilot, there are many things that can influence and eventually lead to detonation. Those include (but aren't limited to):

-Spark advance
-Mixture setting
-Induction air temperature
-CHT
-Power setting

From a practical perspective, that means we also have a number of things to consider as to probability of detonation and how to get out of it. These no real point in arguing over the semantics of it. Oh wait, this is the internet...
 
Ted DuPuis said:
From a practical perspective as a pilot, there are many things that can influence and eventually lead to detonation. Those include (but aren't limited to):

-Spark advance
-Mixture setting
-Induction air temperature
-CHT
-Power setting

From a practical perspective, that means we also have a number of things to consider as to probability of detonation and how to get out of it. These no real point in arguing over the semantics of it. Oh wait, this is the internet...
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Don't forget boost pressure in the intake manifold.. That is why thousands of intelligent motor heads run this unit..... http://www.msdignition.com/Products...log/6462_-_MSD_6-BTM_Boost_Timing_Master.aspx

If advanced ignition timing didn't promote detonation then this product and several others would not exist..:no::nonod:
 
Dan Thomas said:
"The advancing flame front compresses the charge farthest from the spark plug, thus raising its temperature....If the end charge reaches its auto-ignition temperature and remains for some time to complete the pre-flame reactions, the charge will auto-ignite leading to knocking combustion."
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That's pretty much how it works. One could argue that it's not the flame front per se, but the release of energy at and behind the flame front that raises the temperatures / pressures throughout the cylinder. But close enough.

Things like heat release rates and mass fraction burned are pretty easy to observe in an engine if you plug in some pressure sensors (BTDT). Rasweiller and Withrow (I doubt that I spelled those correctly) worked out the basic methodology for doing the necessary calculations given the pressure / volume data at GM back in the 1950's.

Lots of work has been done with "optical" engines that use either a clear cylinder head or clear piston top and in "bombs" and rapid compression machines of various sorts.

Looking for a PHD topic? Still lots of work to be done...
 
N801BH said:
Agreed.

I was thinking about us ground based vehicles who can dial in more boost, where as most aviation units "people proof " fixed boost levels:rolleyes:...
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I want my plane to be equipped with "Emergency power" rating. When you just need more boost.
 
this whole thread is pretty much a moot point, because you can't vary the timing enough on a mag to cause any of this. You only have a slot on the mag that only allows a little variation, and if you go too far advanced the engine will kick back on the starter and not start.

Want detonation ?? mis-fuel, add 10% diesel.
 
Ted DuPuis said:
I want my plane to be equipped with "Emergency power" rating. When you just need more boost.
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I want an aircraft engine that will keep its cylinders on with that feature.
 
Clark1961 said:
You've neglected to address my point about the speed of travel of the pressure front which is at the heart of the matter. If the pressure front hasn't disturbed the unburnt fuel/air mixture then it won't detonate. Advanced timing leads to flame front impingement, not detonation.
Click to expand...
whet he is talking about is pressure-induced detonation in the end gas (unburned area ahead of the flame front) which is a widely-known phenomonen in the automotive world and the primary effect that is regulated in real-time by knock control adjustment of timing
 
Tom-D said:
I want an aircraft engine that will keep its cylinders on with that feature.
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Like this one?
rolls_royce_merlin_66_by_namelessfaithlessgod-d2jyjax.jpg
 
JHW said:
i think you'll be hard pressed to find a civilian owner of a merlin outside of racing who's ever been into the 2nd supercharger
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I agree, but it is an example of something with emergency power.
 
Derated GTSIO-520s on the 310 with the ability to get full rated power should be sufficient for my liking.
 
While detonation can be controlled (or rather prevented) through timing, it is not a direct function of timing, it is a function of temperature and fuel qualities. That said with regards to your O-470 being timed at 24°btdc rather than 22°btdc, consider the factor that the 22° rating was developed for 80 octane fuel, where as you are likely running 100 octane fuel. The 24° setting will get you a cleaner, more thorough burn and get you more efficient use of your fuel dollar. Personally, with a low compression (below 8.5:1) ratio engine, I would personally not hesitate to experiment with up to 28°btdc timing when using 100LL. If I was running Regular MoGas, I'd keep 22°, Premium MoGas 24° would would not cause me undue concern.
 
JHW said:
whet he is talking about is pressure-induced detonation in the end gas (unburned area ahead of the flame front) which is a widely-known phenomonen in the automotive world and the primary effect that is regulated in real-time by knock control adjustment of timing
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Well then if it is widely know then it should be easy to document.
 
Dan Thomas said:
http://www.google.ca/url?sa=t&rct=j...=ocnmfCVDo_Uvu9h28gNSiQ&bvm=bv.48705608,d.cGE

http://z32.wikispaces.com/Ignition+Timing

http://www.cdxetextbook.com/fuelSys/efi/ecu/ECUsettings.html

http://www.avweb.com/news/pelican/182132-1.html

Every one of them talks about ignition timing as a factor.

Dan
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If that's your idea of documentation then good luck. The Pelican's Perch stuff is the closest to support and that should be telling for you. The rest is just repeating hear-say.

Good luck.
 
Dan Thomas said:
http://www.google.ca/url?sa=t&rct=j...=ocnmfCVDo_Uvu9h28gNSiQ&bvm=bv.48705608,d.cGE

http://z32.wikispaces.com/Ignition+Timing

http://www.cdxetextbook.com/fuelSys/efi/ecu/ECUsettings.html

http://www.avweb.com/news/pelican/182132-1.html

Every one of them talks about ignition timing as a factor.

Dan
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There are two pressures involved, compression pressure and combustion pressure. Timing is how we control the sum total of the two by regulating when the peak combustion pressure wave occurs within the compression pressure cycle. The quality of the fuel determines when the combustion peak occurs after ignition which is why we have to change our timing with our fuel supply when the 'octane rating' changes in order to get the most work done without getting into detonation.
 
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