Cylinder Pressures

Supe

MalibuRacing Junkie
May 21, 2003
15,116
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36
Charlotte, NC
Posted for JP86SS:

"With all the discussions about the filling and exhaust cycling rates and CFM, there are rarely any results that indicate the differences in cylinder pressures at combustion time.
I'm not talking about static comp ratios but rather the pressure in the cylinders WITH mixtures at different AFRs and DCRs.
Obviously there must be a difference would have the same torque.
Ex: What is the difference in combustion pressure between a motor with 9:1 static CR and a 7.5 DCR and a fire breathing 13.5:1 at a 8.5 DCR? :x
I'm really interested in differences between the setups and potential pressure differences.
Gas, Alky or Nitro?
Not considering boosted apps, just N/A for now.
TIA"
 

Goob

Top Fueler
Jun 6, 2003
3,641
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Indianapolis
Ummm, all I know about it is that a tuned up Cup engine experiences 3200-3500 p.s.i. measured at the combustion chamber, it goes up with detonation or preignition, or poor tuning, and of course also goes down when HP goes down for other extremes of the tune up.

I also know that excessive detonation and cylinder pressures encountered in the Busch series 9:1 engine experiment were so severe that 9:1 specific head castings were made with thicker decks to withstand the pressures and heat of the unleaded 9:1 engines.

I don't think you'll find much hard fact information about this, as it's real expensive propietary information. It was about 1990 before someone in the engine building industry found a pressure sensing transducer to withstand the combustion chamber and get some real time measurements. That was when the Cup engine types started grinding camshafts with a different lobe for every valve, and other tricks, to balance the cylinder pressures/mixtures/EGT's and actual HP production in each cylinder under load. Once all things were evened out, it almost eliminated engine failures in the Cup series.
The Dorton brothers were way ahead of the pack on these types of engine development.
Randy was the first to track duty cycles of parts on an individual part basis to develop practical use limits on their parts, and verify improvements of parts changes.

Obviously when you can peak the cylinder pressures at the optimum time of the crankshaft rotation, more power will result at the flywheel.

Nitro engines run 55-80 degrees advance on the timing at different points of the run, because of the slow burn nature of Nitro, in a forced induction environment.

I dunno what they run for compression and timing lead on the N/A Nitro engines.

Not sure I understand the question fully, sorry I can't help.
 

JP86SS

Amateur Racer
Sep 26, 2004
113
0
0
N.E. Ohio
www.wideopenwest.com
My main reason started in thinking about head gasket failures and o ringing them etc.
I search and like you said I didn't find allot of info on just how much pressure is in the cylinder duringthe power stroke. If you could combine the multitude of variables there should be a pressure value that is borderline for head gasket yeild, cylinder wall stress that could be used when figuring out a build.
Just got me thinking and figured I'd spark the conversation and see what knowledge was out there on it.
 

CutlassRacer

MalibuRacing Junkie
Dec 18, 2004
5,402
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Gainesville, FL
www.facebook.com
Like Goob said, that stuff is really available to shops that have an almost infinite budget to test and research. I would love to have the facilities to study stuff like that.

Maybe search around for some high end engine discussion messag boards? If you find anything post up a link so we can read about it too :)
 

JP86SS

Amateur Racer
Sep 26, 2004
113
0
0
N.E. Ohio
www.wideopenwest.com
I did find a couple of papers on the subject.
I'll post up more info as I come across it. Most information I'm finding is over the top analytically and will take some thought to process in my pea brain.
Anyway, here's one of them.
http://www.vehicular.isy.liu.se/~larer/ISIS/Doc/EriAnd_2002_SAE.pdf
And another interesting one
http://me.engin.umich.edu/autolab/Publications/P2004_05.htm
 

JP86SS

Amateur Racer
Sep 26, 2004
113
0
0
N.E. Ohio
www.wideopenwest.com
I saw this in a PHR article

"A rule of thumb for typical production engines is that combustion pressure is equal to the CR times 100. This tells us that, from a 10:1 engine, we would expect to see about 1,000 psi of peak combustion pressure. For a well-developed high-performance engine, combustion pressures can be as much as the CR times 120."

Just thought i'd fill in since I've rarely heard any discussions on it.
The whole article on compression is here:
http://www.popularhotrodding.com/enginemasters/articles/hardcore/0606em_understanding_compression_ratio/index.html
Jp
 

78 Elky

Frequent Racer
May 27, 2003
372
0
0
Silver Lake, WI 53170
Here is a website that offers some combustion data of different drag engines: http://www.tfxengine.com/NitroEngineData.html

Under "Data" in the menu bar you can select from 4 different engine data sets.

While cylinder pressure is what is measured, it is the thermodynamics that are calculated from the curve that are important. Things such as 0-10% burn duration tells you information about ignition delay. A long 0-10% burn duration might tell you that the spark plug is not placed in an appropriate region of the mixture to initiate combustion.

10-90% burn duration tells you how fast the fuel/mixture is burning. Things such as egr content in the charge, cylinder pressure at start of combustion (boost, high CR, etc), air/fuel ratio, turbulence level in the combustion chamber during combustion are just some of the things that can affect this value.

Regardless of the above, for maximum power the location of the 50% burn point should fall about 10-15 crank angle degrees after TDC for maximum power.

Another important piece of data is the variation in cylinder pressure over a sample of say 100 cycles. A lower variation means the combustion processes are more stable.

There is a whole lot of specialized equipment that goes into collecting this data. A typical pressure transducer to measure pressure is about 3-5k and the charge amplifiers for each transducer are another 1k. Then there are the software packages that collect and analyze the data. AVL is the industry standard for cylinder pressure monitoring and there high speed data aquisition equipment starts at about 40k.
 

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