Another one for the pros, Head Questions
01-18-2006, 03:17 PM
#46
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Originally Posted by Geo
And therein lay the confusion here. The reduction in volume of gas is a reduction in VE. The CR is a static measure and remains the same always. The change in volume of gas in the cylinder (reduction in VE) is what changes the pressure in the cylinder, but CR is static and constant and VE changes the volume of gas and the pressure under compression. The volume of the cylinder didn't change, just the gas, ergo, VE.
01-18-2006, 07:22 PM
#47
Drifting
anyone having trouble with what skunk is referring to may find it cleared up a little bit if they read about the miller cycle engine; which exploits the characteristics of the open valve.
01-18-2006, 11:46 PM
#48
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Well, the 944S head is here. It looks to be the one I'm going to run. I'd like to think that the 16v cars that didn't survive weren't built as well as I plan to do this one. I would like to know why they don't survive? Heat is always an issue, it weakens metal significantly if not properly adressed. I'm not planning on just bolting it on and going with it. Porsche built one that made 420hp and finished a 24 hr race. I'd say there is hope. In every detail(besides bore size100mm vs 104mm) it appears to be the same as the S2 head. I have not cc'd the chamber yet, pistons will be custom so it's not a big deal. The plan is to go double adjustable pulleys, I hope to go with custom cams as well, we'll see. Details can be worked out, I beleive the hard parts have been selected. Thanks for the info guys, it's been a good learining expeience, I still have more reading to do.
01-21-2006, 06:40 PM
#49
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The VE vs CR vs ECR discussion is apples and oranges. Compression ratio is a primary design parameter set by geometry. Effective compression ratio is an index- it is a calculated number that helps predict how changes effect performance. Volumetric efficiency is a test result. It is how much air your motor moves compared to what some 17-year old claims his Civic pumps.
I think of VE as the engine designer's version of "Forty-two." It is, indeed the single, all encompassing measure of engine pumping performance. Why? because it is defined as such. Like Forty-two, it is pretty useless information unless you know what the question was. Even as an "ultimate answer" it is limited sinse it doesn't address combustion or friction. BMEP and BSFC are more inclusive since they consider thermal and mechanical efficiency.
Why do the texts stress VE? Mostly because the writers designed aircraft engines during WWII using slide rules and dynos. Aircraft motors run slowly at a fairly constant speed. In this environment, VE can be constant and adds value. Not so for road cars.
A couple more links:
http://www.personal.utulsa.edu/~kenneth-weston/
A PDF copy of Weston's "Energy Conversion", now out of print. I skimmed chapter six (on engines) and it seems reasonably well organized. It is reasonably accessable if you ignore anything Latin-looking.
http://naca.central.cranfield.ac.uk/report.php?NID=937
A UK mirror of a NACA site- 405.pdf is a 1932 study on valve timing for supercharged engines. Like our Skunkworks friend, he found increased overlap increased horsepower. However, the power required more boost, was at higher RPM, and used more fuel. Typical of the work of the time- long on data, short on theory.
http://www.vehicular.isy.liu.se/~lar...i_2001_SAE.pdf
A SAAB SAE paper on turbocharging. Talks about their variable wastegate. While most manufacturers use timing to control knock, Saab controlled boost using rapidly-cycled fast dumping waste gate. I think this paper is a "post mortem" for this design- it talks about how the boost pulses hosed up flow dynamics.
Finally, here is a quote from a U of Wisconsin student DK Trumpy's master's thesis, "The Preknock Kinetics of Ethane In a Spark-Ignited Engine."
"Chemical rate equations depend on temperature and specie concentrations. ...<>... The concentrations of 28 species are predicted by numerically integrating, with a predictor-corrector subroutine, the 28 differential equations resulting from a 75 reaction mechanism for the oxidation and decomposition of ethane. ...<>...
Arrhenius rate data and thermochemical data are extracted from the literature. Where data are not available, the specific heats are computed using Einstein functions to represent the vibrational contributions."
Says it all, doesn't it? ;-)
Abstract at:
http://www.erc.wisc.edu/publications...is_Trumpy.html
Have fun,
Bil
I think of VE as the engine designer's version of "Forty-two." It is, indeed the single, all encompassing measure of engine pumping performance. Why? because it is defined as such. Like Forty-two, it is pretty useless information unless you know what the question was. Even as an "ultimate answer" it is limited sinse it doesn't address combustion or friction. BMEP and BSFC are more inclusive since they consider thermal and mechanical efficiency.
Why do the texts stress VE? Mostly because the writers designed aircraft engines during WWII using slide rules and dynos. Aircraft motors run slowly at a fairly constant speed. In this environment, VE can be constant and adds value. Not so for road cars.
A couple more links:
http://www.personal.utulsa.edu/~kenneth-weston/
A PDF copy of Weston's "Energy Conversion", now out of print. I skimmed chapter six (on engines) and it seems reasonably well organized. It is reasonably accessable if you ignore anything Latin-looking.
http://naca.central.cranfield.ac.uk/report.php?NID=937
A UK mirror of a NACA site- 405.pdf is a 1932 study on valve timing for supercharged engines. Like our Skunkworks friend, he found increased overlap increased horsepower. However, the power required more boost, was at higher RPM, and used more fuel. Typical of the work of the time- long on data, short on theory.
http://www.vehicular.isy.liu.se/~lar...i_2001_SAE.pdf
A SAAB SAE paper on turbocharging. Talks about their variable wastegate. While most manufacturers use timing to control knock, Saab controlled boost using rapidly-cycled fast dumping waste gate. I think this paper is a "post mortem" for this design- it talks about how the boost pulses hosed up flow dynamics.
Finally, here is a quote from a U of Wisconsin student DK Trumpy's master's thesis, "The Preknock Kinetics of Ethane In a Spark-Ignited Engine."
"Chemical rate equations depend on temperature and specie concentrations. ...<>... The concentrations of 28 species are predicted by numerically integrating, with a predictor-corrector subroutine, the 28 differential equations resulting from a 75 reaction mechanism for the oxidation and decomposition of ethane. ...<>...
Arrhenius rate data and thermochemical data are extracted from the literature. Where data are not available, the specific heats are computed using Einstein functions to represent the vibrational contributions."
Says it all, doesn't it? ;-)
Abstract at:
http://www.erc.wisc.edu/publications...is_Trumpy.html
Have fun,
Bil
01-21-2006, 08:27 PM
#50
Race Director
Well, I see plenty of discussion of volumetric efficiency, but no mention of "effective compression ratio." Perhaps one needs ask why. I suppose I could have missed it, but I doubt it. Perhaps they are as ignorant as me.
01-23-2006, 07:59 AM
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Originally Posted by Bill.e1
Like our Skunkworks friend, he found increased overlap increased horsepower. However, the power required more boost, was at higher RPM, and used more fuel.
My opinion is that if you get knock with the fuel you run,at the boost needed for a desired HP,it's always better to change the pressure in the cylinder by lowering the ECR by means of cams instead of going to (much) lower ignition settings or lower static CR. Not by advancing/retarding the cam,but by having cams with the proper timing/duration/overlap etc made. If you have a hp increase by either advancing or retarding the cam already there,the cam you have is not optimal for your setup. 
Last edited by Skunk Workz; 01-23-2006 at 08:21 AM.
