Compression ratios

Don't some of the new electronic ignition setups provide for knock sensors and auto-retard? If so, tossing that on top may also be a good idea if you're worried about running mogas.
 
Ted DuPuis said:
Any of you using a modern engine to make an estimate for our engines, please stop. It is an invalid comparison.
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Not so invalid, considering that the engine Tom is talking about is an O-200 with port EFI, and SDS.
 
bluerooster said:
Not so invalid, considering that the engine Tom is talking about is an O-200 with port EFI, and SDS.
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But it still has a big open chamber without swirl / tumble and, I suspect, without very much squish (if any). Lack of charge motion and cylinder volume will be bigger factors than how the fuel is delivered.
 
Knock sensors have been around quite a while. They protect the engine when low-octane fuels are used, though I sometimes wonder how much money is being saved by buying regular grade fuel and having retarded timing to prevent knock and end up with less power for a given gallon of fuel, which means less mileage.

I think knock sensors are also part of the emissions control stuff.
 
Capt. Geoffrey Thorpe said:
But it still has a big open chamber without swirl / tumble and, I suspect, without very much squish (if any). Lack of charge motion and cylinder volume will be bigger factors than how the fuel is delivered.
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Exactly.

ktup-flyer said:
My LS3 is 10.7:1 and runs on 93. I wouldn't exactly call it modern
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It's actually quite modern compared to our engines. The fact that it's pushrod gives it an older valvetrain technology, but there's nothing wrong with pushrods. They have their advantages, just like overhead cams.
 
A puppy dies every time someone calls any abnormal combustion event "detonation". Detonation never happens inside an internal combustion engine.
There, said it again.

Airplane engines were designed 50 years ago on old "tried and tested" engine design methods (heck, they even thought detonation could happen inside an engine back then!), comparing any car engine designed since the 60's to them is just silly. They are extremely awful engines producing crazy amounts of mechanical and thermal stress for very low outputs, while often requiring high grade fuels to run with any sort of reliability.
Modern aftermarket EFIs are all crap, but I'd love to have one on a test stand and calibrate it, just to see how far off the variables would be from a quick'n'dirty GTPower simulation.

Calibrating knock sensors is something I'd say 98% of experimental builders could not do with any sort of acceptable accuracy, unless someone actually destroys a few cases/heads and starts experimenting with it. Anyone who's scoped an unfiltered knock sensor signal knows how difficult it is to pick up anything meaningful from it. And the filtering algorithms are quite complicated. (And don't even let me get started with knocklink etc)...
 
So, every engine text book that explains detonation and has photographs of damaged pistons is wrong? And, we should call them what they are, reciprocating engines. Internal combustion engines is too vague, and includes rotary, turbine, and others.
 
GlennAB1 said:
So, every engine text book that explains detonation and has photographs of damaged pistons is wrong? And, we should call them what they are, reciprocating engines. Internal combustion engines is too vague, and includes rotary, turbine, and others.
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Yes, they are wrong. Newer books don't use that term anymore(unless author is dead/lazy/wont care). Detonation never occurs. Yes, engine can and will be damaged because of abnormal combustion, but it is not because of detonation.
The process is deflagration, never detonation. The flame front never exceeds speed of sound. Massive difference for people who work with this stuff for living.
And no, I mean any internal combustion engine, reciprocating engine would be too specific. Rotary/Z/diesel etc.
 
Since the damage caused by detonation, preignition, and I guess, deflagration too, is basically the same, does it matter to differentiate, Really? And, you don't include Gas turbine, jet engine, or rocket? They are all internal combustion engines.
 
GlennAB1 said:
Since the damage caused by detonation, preignition, and I guess, deflagration too, is basically the same, does it matter to differentiate, Really? And, you don't include Gas turbine, jet engine, or rocket? They are all internal combustion engines.
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It does matter to differentiate. The damage mechanism from knocking combustion is completely different than what detonation would cause. Detonation does not happen there so hard to say what it would do. Not melted pistons I would imagine.
The damage from knock is a process where a few things happen before the thermal load on pistons(usually) gets too high.
To fully understand what happens inside a combustion chamber, we need to understand what processes happen there. Otherwise we might as well say we are boiling fuel, which is just as inaccurate. Detonation means combustion, where the flame kernel expands in supersonic speeds. This never happens inside an xyz engine (I'm actually not sure if even jet engines or gas turbines "detonate"?)
Since this sort of combustion doesn't happen, we shouldn't use that term to describe the abnormal combustion processes you describe (knocking/pre-ignition).
 
For our purposes I understand "detonation" to be a timing event that leads to excessive/premature cylinder pressures too early in the compression stroke. Pre-ignition used to be the more correct label. A fast flame from low octane fuel is often credited. I've never experienced it in an airplane.

I just ordered a brand new EX aircraft engine with 8.9:1 and the manufacturer's instructions specify 100LL only. Maybe because of the cylinder volume? I never thought about it before but cylinder volume may have a place in the fuel choice discussion. I have no reason to screw with it. Avgas consistency and stability are far superior to any car gas I can buy. This engine uses modern fuel injection and electronic ignition, by the way.
 
Would it be correct to say

1) there is no flame front with detonation, and
2) there are multiple flame fronts with preignition (excepting dual ignition)
 
nrpetersen said:
Would it be correct to say

1) there is no flame front with detonation, and
2) there are multiple flame fronts with preignition (excepting dual ignition)
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With pre-ignition (ignition before the spark plug fires, typically from "hot spots"), you have the potential for multiple flame fronts from early ignition sources plus the plugs later on and you will often initiate "knock" in the end gas as well.
With the typical "knock" you get ignition points beyond the main flame front in the end gas regions which give rise to a rapid pressure increases and the audible knocking sound.
You wouldn't expect a supersonic flame front that is he hallmark of "detonation" from the gasoline / air mixture.
 

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Capt. Geoffrey Thorpe said:
With pre-ignition (ignition before the spark plug fires, typically from "hot spots"), you have the potential for multiple flame fronts from early ignition sources plus the plugs later on and you will often initiate "knock" in the end gas as well.
With the typical "knock" you get ignition points beyond the main flame front in the end gas regions which give rise to a rapid pressure increases and the audible knocking sound.
You wouldn't expect a supersonic flame front that is he hallmark of "detonation" from the gasoline / air mixture.
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Exactly. I know people might call my detonation-hate nitpicking, but the difference is important on understanding the combustion process.
 
mtuomi said:
Exactly. I know people might call my detonation-hate nitpicking, but the difference is important on understanding the combustion process.
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What really happens during a detonation event? The fuel air mixture actually explodes in the cylinder rising the pressure way above designed limits long before the piston can reach TDC and start down the bore to relieve it. it is why the case thru studs get stretched which allows the cases to fret, seams to leak, studs get stripped. and heads get cracked, pistons get shattered, rods get bent.

The EGT will actually go down, simply because the mixture is completely burned and the heat from it is dissipated into the cylinder rather than it being discharged in the exhaust.
 
My Rotax 912ULS has 10.5:1 compression and requires 91 octane or better. It has no variable timing or knock sensors.
 
mtuomi said:
The process is deflagration, never detonation. The flame front never exceeds speed of sound. Massive difference for people who work with this stuff for living.
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We argued about this once before. Call it what you want deflagration or whatever, but the speed of sound is dependent on temperature. You can't apply typical 1000 FPS speeds (sea level temp) to the enormous temps found in a cylinder. One of my texts says deflagration can occur at 5000 FPS compared to normal combustion of about 100 FPS.

The speed of sound in a gas at 1700°F is 1553 MPH, or 2278 FPS. That's a typical high EGT. The temperature ahead of the pressure wave could be considerably higher than that, and probably is, judging by the CHT rise during the detonation or deflagration event. In any case, it's far faster than normal combustion and is more than enough to cause the damage we see in engines that have been overboosted or otherwise mistreated.
 
mtuomi said:
Yes, they are wrong. Newer books don't use that term anymore(unless author is dead/lazy/wont care). Detonation never occurs. Yes, engine can and will be damaged because of abnormal combustion, but it is not because of detonation.
The process is deflagration, never detonation. The flame front never exceeds speed of sound. Massive difference for people who work with this stuff for living.
And no, I mean any internal combustion engine, reciprocating engine would be too specific. Rotary/Z/diesel etc.
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You need to contact the FAA and tell them their 2012 published, 8083-32 AMT Powerplant Handbook is wrong for having Detonation and not Deflagration in it.
 
Tom-D said:
What really happens during a detonation event? The fuel air mixture actually explodes in the cylinder rising the pressure way above designed limits long before the piston can reach TDC and start down the bore to relieve it. it is why the case thru studs get stretched which allows the cases to fret, seams to leak, studs get stripped. and heads get cracked, pistons get shattered, rods get bent.

The EGT will actually go down, simply because the mixture is completely burned and the heat from it is dissipated into the cylinder rather than it being discharged in the exhaust.
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Whaat? Nowhere even close.
 
GlennAB1 said:
You need to contact the FAA and tell them their 2012 published, 8083-32 AMT Powerplant Handbook is wrong for having Detonation and not Deflagration in it.
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Not the only place where they get it all wrong.
 
Dan Thomas said:
We argued about this once before. Call it what you want deflagration or whatever, but the speed of sound is dependent on temperature. You can't apply typical 1000 FPS speeds (sea level temp) to the enormous temps found in a cylinder. One of my texts says deflagration can occur at 5000 FPS compared to normal combustion of about 100 FPS.

The speed of sound in a gas at 1700°F is 1553 MPH, or 2278 FPS. That's a typical high EGT. The temperature ahead of the pressure wave could be considerably higher than that, and probably is, judging by the CHT rise during the detonation or deflagration event. In any case, it's far faster than normal combustion and is more than enough to cause the damage we see in engines that have been overboosted or otherwise mistreated.
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But the main point is, that since there is no detonation, the damage is caused by something completely different. The peak pressure has almost zero effect on the damage that happens in the combustion chamber, and why it damages pistons.
All combustion is deflagration, even normal combustion, so there is no deflagration "event". What matters really is how the multiple flame kernels behave, and how the "ripples" break the boundary layer in the cylinder walls and allow more heat to transfer to the mechanical parts inside the engine. That is why EGT drops and CHT increases normally.
Thinking it's an explosion that happens leads to these urban myths like what Tom-D just repeated. Understanding what the process is makes it easier to figure out what damage it causes, and why.
 
mtuomi said:
But the main point is, that since there is no detonation, the damage is caused by something completely different. The peak pressure has almost zero effect on the damage that happens in the combustion chamber, and why it damages pistons.
All combustion is deflagration, even normal combustion, so there is no deflagration "event". What matters really is how the multiple flame kernels behave, and how the "ripples" break the boundary layer in the cylinder walls and allow more heat to transfer to the mechanical parts inside the engine. That is why EGT drops and CHT increases normally.
Thinking it's an explosion that happens leads to these urban myths like what Tom-D just repeated. Understanding what the process is makes it easier to figure out what damage it causes, and why.
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OK. We need an online reference for this. Back it up.
 
GlennAB1 said:
But, you wrote: "Newer books don't use that term anymore"
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Yes, but FAA isn't the source where I would go to if I need to know more about engines. Read John Heywood's Internal Combustion Engine Fundamentals. Even though some of the stuff in it is debated, it is a pretty good basic source for this kind of stuff.
 
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Dan Thomas said:
OK. We need an online reference for this. Back it up.
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I wouldn't know where to go online to look for stuff like that. When you go to the doctor, do you ask for online sources to back up his opinions? I'm an engine calibration engineer, this is what I do for living.

Long story short:

Engine knocks - the pressure waves disrupt the protective boundary layer on cylinder walls - more heat transfers to the cylinder and piston skirts - piston melts - owner pays a lot of money.

No sound barriers were harmed during this process.
 
mtuomi said:
But the main point is, that since there is no detonation, the damage is caused by something completely different. The peak pressure has almost zero effect on the damage that happens in the combustion chamber, and why it damages pistons.
All combustion is deflagration, even normal combustion, so there is no deflagration "event". What matters really is how the multiple flame kernels behave, and how the "ripples" break the boundary layer in the cylinder walls and allow more heat to transfer to the mechanical parts inside the engine. That is why EGT drops and CHT increases normally.
Thinking it's an explosion that happens leads to these urban myths like what Tom-D just repeated. Understanding what the process is makes it easier to figure out what damage it causes, and why.
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From the FAA: DOT/FAA/TC-TT14/34
"Most combustion that occurs is considered a deflagration, which is when the flame front produced from the ignition propagates at a speed below the sonic velocity, or the speed of sound. This speed is 343 m/s or 767 mph in room temperature air at sea level [8]. If the flame speed is above the sonic velocity, it is considered a detonation. This induces a shock wave to form outside of the explosion, which causes a much more abrupt rise in pressure and temperature when compared to a deflagration."
 
Another internet expert, who has never designed or built big Radial engines like Curtiss-Wright or the P&W 2800/4360, both explain the detonation aspect in their operators manual. and the damage it can cause.
 
GlennAB1 said:
From the FAA: DOT/FAA/TC-TT14/34
"Most combustion that occurs is considered a deflagration, which is when the flame front produced from the ignition propagates at a speed below the sonic velocity, or the speed of sound. This speed is 343 m/s or 767 mph in room temperature air at sea level [8]. If the flame speed is above the sonic velocity, it is considered a detonation. This induces a shock wave to form outside of the explosion, which causes a much more abrupt rise in pressure and temperature when compared to a deflagration."
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This is mostly correct. Only that this process (detonation) never happens inside an engine that we are talking about.
It is impossible, since the "sonic velocity" increases when temperature increases -> it never "catches up". 30-40 years ago people thought this is what happened, and that's when they came up with the term, and it stuck.
 
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Tom-D said:
Another internet expert, who has never designed or built big Radial engines like Curtiss-Wright or the P&W 2800/4360, both explain the detonation aspect in their operators manual. and the damage it can cause.
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Nope, I've never built them, I'm the guy sitting behind a laptop modeling how the engine works, and telling how we can set it for optimal operation. I don't consider myself an internet expert. People pay me to fly around the world to be the local expert on them.
The people who wrote those operators manuals knew nothing compared to what we know nowadays. Yes they called it "detonation" but that is because they didn't know what it really was.
 
mtuomi said:
Nope, I've never built them, I'm the guy sitting behind a laptop modeling how the engine works, and telling how we can set it for optimal operation. I don't consider myself an internet expert. People pay me to fly around the world to be the local expert on them.
The people who wrote those operators manuals knew nothing compared to what we know nowadays. Yes they called it "detonation" but that is because they didn't know what it really was.
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= new era thinker, change the terms and definitions to fit your agenda.
 
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