Compression ratios

Tom-D said:
= new era thinker, change the terms and definitions to fit your agenda.
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New era thinker - the definitions have always been the same, our knowledge has expanded so now we know the old term was incorrect.
 
GlennAB1 said:
Ok, the FAA wrote: "Most combustion that occurs is considered a deflagration" You agreed. And, you previously wrote: "All combustion is deflagration..." So, if the FAA was correct, as you agreed, what is there other than "most combustion"?
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Oh my... Ok, I fixed the comment. Do you now understand what I meant?
 
mtuomi said:
It is impossible, since the "sonic velocity" increases when temperature increases
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But the FAA wrote: "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". Wouldn't that put the shock wave in a considerably cooler area?
 
GlennAB1 said:
But the FAA wrote: "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". Wouldn't that put the shock wave in a considerably cooler area?
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Of course not? I find it hard to follow your logic here. The shock wave is a result of detonation, and detonation doesn't happen so the shock wave never forms.

The energy required to cause supersonic combustion in an engine simply isn't there. It is just like trying to push an aircraft past the transsonic range - you need a ton of energy to get through it, and fuel/air mixture simply doesn't burn that fast in those conditions, so it never occurs.
 
"Detonation" was a common theory for knock at one time, but the current understanding is auto ignition ahead of the flame front. This understand is built, for the most part, on high speed photography taken at various wavelengths using optically accessible engines. If you want to stop by my office, I can dig out some of my books with copies of the photographs.

FWIW, "Internal Combustion Engine Fundamentals" by John Heywood is a good reference. Heywood, at one time lead the Sloan Institute at MIT which was a position first held by C.F.Taylor who, among other things, was one of the chief designers of the Wright Whirlwind engines.
 
GlennAB1 said:
Except that it's an exception, not "usually taught", just like deflagration...
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What I meant was, that most of the time the reason why airplanes fly is explained incorrectly with OWTs about "longer way round the top" etc.
 
What is the definition of deflagration (and how is it different from detonation)? I can't find "deflagration" in my Amer Heritage dictionary or in Marks engineering handbook (circa 1987) or in my old combustion text.
 
nrpetersen said:
What is the definition of deflagration (and how is it different from detonation)? I can't find "deflagration" in my Amer Heritage dictionary or in Marks engineering handbook (circa 1987) or in my old combustion text.
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Exactly!
new era thinker
 
mtuomi said:
New era thinker - the definitions have always been the same, our knowledge has expanded so now we know the old term was incorrect.
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Funny.... it worked so well for how long??
 
Capt. Geoffrey Thorpe said:
"Detonation" was a common theory for knock at one time, but the current understanding is auto ignition ahead of the flame front.
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Otherwise known as spontaneous combustion, cause by a rise in pressure and temp prior to ignition
 
mtuomi said:
"Detonation" is not debated, it is a known fact that it does not happen.
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Oh Give me a freaking break.

You are probably not old enough to remember the mis fueling AD of the early 90s, but the fuel delivery company bought a bunch of new engines because of the FAA's worries over detonation of the engines that bought fuel from 4 airports in SoCal.
all you had to have to get a new -- NEW-- engine was a fueling slip from the pump of the 4 airports that had a 2% diesel mix in their tanks.

If you really don't believe detonation does not exist, ask Lycoming and TCM. they both have hot lines to their support techs.
 
I wouldn't rule out correct mag timing.....it's just a critical as the compression ratio.
 
Well there is an old saw that says - those who can do and those who can't hang around on the internet claiming to be experts.
I think it was Abe Lincoln who first said it.
As an old coot who has professionally melted/broken more pistons than any internet expert has ever seen I don't care if it is called deflagration or detonation or 'ohhh sheet', it is hard on the engine.
And it does not change the question that Tom-D posed. The answer is still, yes you can run a 9.5:1 static compression ratio air cooled engine on car gas but you have to be careful - actual 91 octane gas, conservative ignition timing, and don't let it get lean.
 
One problem is that different authorities are inconsistent with their terminology.
Taylor and Taylor, for example, (I have the 1961 edition) like the word "Detonation", but use it to describe auto ignition in the end gas regions and not an actual supersonic combustion front that the word would imply.

In any case, the phenomena happens well after top dead center unless you have a severe pre-ignition problem or you have drastically over advanced the spark (well beyond any reasonable value for MBT) and is due to auto ignition in the end gasses - a phenomena that is related to the time/temperature/pressure history in that region. As a flame front progresses through the cylinder, the gasses ahead of that front are compressed and heated, given time, various radicals are formed and will auto ignite causing an abnormal combustion event that I would refer to as knock. If this phenomena is severe enough, engine damage will occur.

Things that would prevent knock are changes to the fuel to include more aromatics and less long chain paraffins, reduction of compression ratio, retarding spark, or changes to the combustion chamber such as a smaller bore and increasing charge motion through tumble / swirl / squish. The O-200 has a large bore compared something like a Rotax, but a small bore compared to some other aircraft engines. This suggests that it could run a bit higher compression than some aircraft engines, but not nearly as high as the Rotax. In the end, however, the only way to really find out is to put it on a dyno.
 
Tom-D said:
If you really don't believe detonation does not exist, ask Lycoming and TCM. they both have hot lines to their support techs.
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Well, to be fair, those people really don't know what they're talking about when it comes to items like detonation.
 
Checkout_my_Six said:
Oh...no, make it stop. lol :D
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Don't get me wrong, they're good guys - but they only know what they know. Most of what they know is how to make your engine run right and fix stuff that's wrong with it, not anything regarding the more complex dynamics of operation.
 
Ted DuPuis said:
Don't get me wrong, they're good guys - but they only know what they know. Most of what they know is how to make your engine run right and fix stuff that's wrong with it, not anything regarding the more complex dynamics of operation.
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When you mention detonation they will understand what it is and how to make your engine stop doing it.
 
Checkout_my_Six said:
yup....they're not engineers. ;)
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No they aren't. That's not an insult, just a statement. They've got a lot of good knowledge that the engineers don't have, too. When I was working at the leaky grey engine factory, I found a lot of the engineers ignored the mechanics on the floor who had some really good and valid points in many cases. I always listened to what they had to say.

Tom-D said:
When you mention detonation they will understand what it is and how to make your engine stop doing it.
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They will understand what you're telling them and they'll tell you to clean your injectors, check your spark plugs, check your mag timing, etc. My guess is most people who call saying their engines are detonating aren't correct in what they think is occurring, much less what is actually occurring. I remember my instructor (an A&P) telling me on my second lesson that his Comanche 180 would detonate in the climb because the CHTs were around 420F. No, it had nothing to do with detonation, it was that the baffling needed work and he probably needed to turn up the fuel flow.

What our friend is saying above has to do with the current knowledge of combustion (which he is correct in saying is hugely advanced vs. the 1940s) and the actual proper term for what is going on vs. what the term was thought to be. From the perspective of a pilot/mechanic/end user it's more or less semantics since what's important is the end effects.
 
Ted DuPuis said:
No they aren't. That's not an insult, just a statement. They've got a lot of good knowledge that the engineers don't have, too. When I was working at the leaky grey engine factory, I found a lot of the engineers ignored the mechanics on the floor who had some really good and valid points in many cases. I always listened to what they had to say.
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Now....go and make my leaky gold cylinders....more better. o_O
 
Ted DuPuis said:
My guess is most people who call saying their engines are detonating aren't correct in what they think is occurring, much less what is actually occurring. I remember my instructor (an A&P) telling me on my second lesson that his Comanche 180 would detonate in the climb because the CHTs were around 420F. No, it had nothing to do with detonation, it was that the baffling needed work and he probably needed to turn up the fuel flow.
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I think it's kind of like any time there is any kind of an electrical problem many people will say "must be a short" when it is hardly ever due to an actual short circuit...

When I was doing combustion analysis, some guys came in with a prototype vehicle that had a problem with "high speed detonation" or something like that. We instrumented the engine with in cylinder pressure sensors, etc. (takes several days), put the vehicle on the dyno, and were unable to detect any knock at all. Called in the engineer to let him drive and demonstrate the condition. He gets in while I'm standing next to the front wheels to listen to the engine (hood open). Well, they didn't have the proper duct to connect from the air cleaner to the engine that was installed so they used an over-sized length of dryer duct. Well, when he got on the gas the extra length of the duct allowed it to fold and collapse on itself causing a big power loss- at which point the driver calls out that it is detonating. I have him repeat a couple times, get a wire clipper and my pocket knife, and shorten up the duct so it wouldn't collapse on itself (while he waited in the car). "Try it now". "Hey! It's fixed! What did you do? There wasn't much else to say at that point.
 
Checkout_my_Six said:
Now....go and make my leaky gold cylinders....more better. o_O
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I've been out of that business for a few years now.

Capt. Geoffrey Thorpe said:
I think it's kind of like any time there is any kind of an electrical problem many people will say "must be a short" when it is hardly ever due to an actual short circuit...

When I was doing combustion analysis, some guys came in with a prototype vehicle that had a problem with "high speed detonation" or something like that. We instrumented the engine with in cylinder pressure sensors, etc. (takes several days), put the vehicle on the dyno, and were unable to detect any knock at all. Called in the engineer to let him drive and demonstrate the condition. He gets in while I'm standing next to the front wheels to listen to the engine (hood open). Well, they didn't have the proper duct to connect from the air cleaner to the engine that was installed so they used an over-sized length of dryer duct. Well, when he got on the gas the extra length of the duct allowed it to fold and collapse on itself causing a big power loss- at which point the driver calls out that it is detonating. I have him repeat a couple times, get a wire clipper and my pocket knife, and shorten up the duct so it wouldn't collapse on itself (while he waited in the car). "Try it now". "Hey! It's fixed! What did you do? There wasn't much else to say at that point.
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I've had a lot of similar situations. The first thing I want to do when I get told about a problem in a plane/car is to drive or fly it. We would always drive cars at the shop after the customer brought it in to diagnose the real problem. Sometimes they got it right, more often they got it wrong.
 
A change in terminology does not change the age old phenomena. you raise the temperature above the temperature required for ignition and the fuel will burn.
 
Tom-D said:
A change in terminology does not change the age old phenomena. you raise the temperature above the temperature required for ignition and the fuel will burn.
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Characterizing the actual phenomena is critical to avoiding the problem at the design stage. If you don't want to learn, why are you here?
 
Clark1961 said:
Characterizing the actual phenomena is critical to avoiding the problem at the design stage. If you don't want to learn, why are you here?
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Who said I didn't want to learn, or is that just your bias opinion?
 
Tom-D said:
A change in terminology does not change the age old phenomena. you raise the temperature above the temperature required for ignition and the fuel will burn.
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Except, knock is not just a "temperature required for ignition" phenomena.

It's related very much to time / temperature / pressure history along with fuel properties. If you can get the flame front to the walls fast enough the pre-combustion radicals don't have time to form and you don't get auto-ignition even if the temperature is the same. And, different fuel types break down at different rates - long straight chains tend to go faster than more compact arrangements - that's why different fuels have different "octane" ratings even though the ignition temperatures and burn rates are about the same.
 
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It is probably just a difference in engineering theory or terminology. Some turbine manufacturers use the term "surge" and some use "stall" for the same phenomenon.
 
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