need thermo help to understand ping

[predator]

I was trying to totally disprove a post on this page and well ended up whipping my own ass...

http://www.ls1.com/forums/showthread.php?t=34406&page=3

Below you can see my attempt... my issue is i think i have the gas constant all messed up for air and i dont think i can use the noble gas equation (which im trying to do). Nor do i know how to find R...

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Here is what we have.

Outside Temp = 75 F (assumed) (Temps must be converted to Rankien so that's 534.7 R)
Outside Pressure = 14.7 psi (can skew slightly, but not enough to really affect things tooooooooo much)
R(Outside) = ??????????

Post boost temp = ?
Post boost pressure = 29.7 psi
R(Post Boost) = ??????????

Post Compression Pressure = 321.1 psi (that is 21.8:1 final compression ration using Final Compression Ratio (FCR) = [(Boost/ 14.7) +1] x CR)
Post Compression Temp = ?
R(Post Compression) = ??????????

So we start with PV = mRT

with a lil allgebra we get m = PV/RT

and since the mass out is equal to the mass in and volume doesnt change we can say:
P(Outside)/[R(Outside) x T(Outside)] = P(Post Boost)/[R(Post Boost) x T(Post Boost)

Therefore:
T(Post Boost) = [P(Post Boost) x R(Outside) x Temp(Outside)]/[P(Outside) x R(Post Boost)]

Here we get that T(Post Boost) = ????????

Now that we have that we can say that:
T(Post Compression) = [P(Post Compression) x R(Post Boost) x Temp(Post Boost)]/[P(Post Boost) x R(Post Compression)]

That gives us a Temp of ??????? at top dead center with no cooling. Since gasoline spontaneously combust at 495 F (954.7 R) and im going to assume that predetonation is cause by spontaneous combustion due to cylender temps.

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As you can tell i got lost... and to tell you the truth i dont even know if i was headed in the right direction... i dont even know if it is the heat from increased cylinder pressure that even causes detonation... just sounded logical...

i know there is sooooooo much more involved that that simple stuff... but i was simply trying to demonstrate that the heat produced by simply compressing the air would cuase such a rediculous combustion chamber temperature that the intercooling system to bring inlet air down would be in no way feasible.

i really hit a flaw when i plug in a 10:1 comp engine with no boost and get enough heat to combust gasoline...

someone explain this to me... both where my formulas are wrong and what all is involved im ping...

-me

[Sarge]

Your leaving out that gasoline is a coolant. You have the formula correct for compressing air...but I fail to see where you introduce the major cooling factor of vaporized gasoline which drops cylinder temps significantly. Dont forget gasoline is known as an aliphatic hydrocarbon ( I'll help you out...Google that)...your correct gas combust at around 495f but what your leaving out in that scenario is the time to combust. Dont forget your ideal ignition s 20 degrees before TDC...this allows the power of the "explosion" to have an effect on the process.....so your talking nano second here....and that pretty much eliminates time as an equation thus disallowing the 495F relevance. So what are you looking for? Specific. The forumula for the perfect combustion?

[predator]

no i just dont guess i understand ping completely... or thermo for that matter

I mean what causes it? my throughts were heat cause by the compression of the air fuel mixture... you just said that the mixture is lit b4 TDC and since maximum compression would happen at TDC... apparently my assumption of what causes it is wrong.

I know things like carbon build up and what not will cause it, but im really looking for what causes detonation in a boosted motor...

and what does octane do how does it stabilize fuel... does it raise the ignition temp allowing you to put more pressure (therfore more heat) onto the mixture?

and i really dont think my formula is right... let me punch out the numbers real fast...

if i stick with temp in Rankein and assume R is constant all the way across (which means i can negate it) I get...

Temp (post boost) = 620 F
Temp (post compression) = 6250 F

As you can see... something is F'ed up BAD... You say that fuel is a major cooling factor and i get that... as any liquid vaporizes it pulls heat fromt he air to do so... and the degree drop from liquid to gas pull hundreds if not thousands of time more heat from the air than does any just degree change (without phase change)... but i have an issue with the fact that even just the heat cause by the boost would blow an intake off when you sprayed fuel on it. its hot enough to ignite the fuel...

and with a few more lil calcs.... the air entering the chamber would have to be... -413.2 degrees F to keep the TDC temp below 500 F. No intercooler can do that... isnt that approaching absolute zero?

something is deffinately not right... I even tried it with no boost and 10:1 compression and im getting temps that would melt steel...

I hate thermo....

-me

[mistermike]

If the fuel / air mixture was igniting due to compression alone, it would be preignition and not ping (detonation) Detonation is, by definition, initiated by normal ignition, whereas preignition can occur well in advance of the actual spark, usually by a piece of glowing carbon or sharp edge of metal. Aside from the cooling effect of the fuel, there is also heat loss into the surrounding cylinder wall, valve, head, and piston surfaces. Also, a mixture containg vaporized fuel will transfer heat faster than air alone.

[predator]

isnt its PreINGnition

And im with you that the heat of combustion is passed either down the exhaust or into the surroundings, but the INTAKE air should be cooler than the engine... which is prolly 200ish degrees... so the surroundings of the air wont be taking way heat... actually adding heat would be more logical.

and yes after a supercharger you do use some form of cooling to get the engine temp cool enough for my formula to work... well you would have to do something that science has yet to do... and that would be acheive absolute zero...

i mean it was clealy stated that the final compression ratio is 21.8:1. hell if someone can find out what im doing wrong just see what inlet air temp would have to be to even allow fuel to be compressed to that degree. i mean we are talking like deisel compression ranges here. im still pretty sure that octane has to be EXTREME... and the inlet temps have to be LOW to even compress gasoline to such extremes withough ignition...

-me