Freiburger, DCR, and testing.....

[Eric68]

Re: Freiburger, DCR, and testing.....

How are these calculators getting such high cranking pressures? What are they multiplying by? ??? I had figured lower pressures like David. I can't dispute real numbers that Eric got but I sure would like to know how Joe got so close to actual values. Help . . . ;D

Early intake closing timing + medium static compression ratios = high cranking pressures. If you go bigger with the cam the cranking pressure will come down, if you go smaller it will go up. Of course going higher with the static compression ratio will increase the cranking pressure too. The calculator is just using math to quantify this.

A DCR calculator is just taking a known volume of air and calculating how much it will be compressed. It is using the swept volume, chamber volume, and intake valve closing point to determine the compression ratio. The fancier calculators are adjusting the "known volume of air" for density altitude and VE and converting the output to cranking pressure.

I have an actual DCR calculator formula somewhere and if I can find it I will post it, but it a big messy formula that is a pain to work through by hand.

[MadmanMark]

Re: Freiburger, DCR, and testing.....

Here's something pretty simple I threw together with excel for my own entertainment. You have to enter when your intake closes for the dcr, which makes the calculation pretty simple:

http://marknkelley.home.att.net/SCR_DCR_calculator.xls

Calculating DCR shouldn't really be contentious, even though there may be different ways to calculate it, the result should always be the same (unless it factors in real dynamics like rpm, altitude, etc).

[Eric68]

Re: Freiburger, DCR, and testing.....

Mark, you are a madman ;) Cell B22 explains exactly why I've never tried putting a DCR formula into a spreadsheet! :o

=(B5*(PI()*((B6/2)^2)*(B7-((SQRT(B16^2-((B7/2)^2*(SIN(B15*PI()/180))^2))-(B7/2)*COS(B15*PI()/180)+B7/2-B16))))+B5*(PI()*((B6/2)^2)*(B11)+B8*(B9/2)^2*PI()+B10*(1/2.54)^3+B12/(2.54^3)))/(B5*(PI()*((B6/2)^2)*(B11)+B8*(B9/2)^2*PI()+B10*(1/2.54)^3+B12/(2.54^3)))

[Saltfever]

Re: Freiburger, DCR, and testing.....

Assuming non-adiabatic compression, if you compress one atmosphere (14.7 psi) 10X, how come you don't get 147 psi? At cranking speed VE should be 100%. And if you just use the static CR you don't have to account for any cam event. The result should be the highest "theoretical" cylinder pressure. However, the cylinder pressure listed in prior posts are even higher than the calculated static CR! ???

If you use cam closing points, with a even shorter stroke the CR should be even lower. However, the measured cylinder pressures listed above came out higher. I missing sumptin ???


Mark, thanks for the equation. It will take me a week but I'm going to walk through it.

[joe_rocket45]

Re: Freiburger, DCR, and testing.....

heat is part

[Eric68]

Re: Freiburger, DCR, and testing.....

Assuming non-adiabatic compression, if you compress one atmosphere (14.7 psi) 10X, how come you don't get 147 psi? At cranking speed VE should be 100%. And if you just use the static CR you don't have to account for any cam event. The result should be the highest "theoretical" cylinder pressure. However, the cylinder pressure listed in prior posts are even higher than the calculated static CR! ???

If you use cam closing points, with a even shorter stroke the CR should be even lower. However, the measured cylinder pressures listed above came out higher. I missing sumptin ???


Mark, thanks for the equation. It will take me a week but I'm going to walk through it.

Interesting question Saltfever! I like this discussion ;D

Correct me if I am wrong here, but I do not believe there is any such thing as non-adiabatic compression in an internal combustion engine. The mechanism for keeping temperature constant during compression just isn't there -- so as the piston rises, the force required to push the piston up is turned into an equivalent amount of heat in the gas being compressed and as a result, pressure not only rises from compression but from the added heat.

Here's a little something I dug up.

Compression in which no heat is added to or subtracted from the air and
the internal energy of the air is increased by an amount equivalent to the
external work done on the air. The increase in temperature of the air
during adiabatic compression tends to increase the pressure on account of
the decrease in volume alone; therefore, the pressure during adiabatic
compression rises faster than the volume diminishes. Lewis

[Saltfever]

Re: Freiburger, DCR, and testing.....

True, Eric. That is why I said non-adiabatic, in order to keep the calcs simple. I guess I'll have to dig out my old High School Chemistry book and walk through some of the gas-law problems. I'm sure adiabatic compression is the culprit, as you pointed out.

So the next questions would be about the DCR calculators. Do they consider adiabatic compression in their results?

[Eric68]

Re: Freiburger, DCR, and testing.....

You got me on that one. I do not have the formula for a calculator that gives cylinder pressure, only for a DCR calculator that gives a ratio. In order to calculate cranking PSI it seems to me that you would first have to calculate the DCR in terms of a ratio, then multiply the ratio times atmospheric pressure and apply corrections for VE, adiabatic compression, etc. :-\

[joe_rocket45]

Re: Freiburger, DCR, and testing.....

there was a thread on speedtalk that brought this up, but finding it could be a long hard job...

[joe_rocket45]

Re: Freiburger, DCR, and testing.....

http://www.speedtalk.com/forum/viewtopic.php?t=2495&postdays=0&postorder=asc&start=15

From "Panic":

http://victorylibrary.com/mopar/cam-tech-c.htm

Using this, calculate the effective compression ratio (“CRE”); also lower - the combustion chamber volume is unchanged, but the cylinder volume is less. At cranking speed, the absolute cranking pressure (“CP”) is a function of the 1.2 power of the effective compression ratio (i.e., for 8:1 compression ratio, use 8^1.2) times atmospheric pressure (14.7 psi @ sea level, &c.). This adjustment (1.2 power) is a polytropic value used in preference to the traditional adiabatic value (1.4) for the ratio of variable heats for air and similar gases at the temperatures present. This compensates for the temperature rise caused by compression, as well as heat lost to the cylinder. 1.2 is not accurate in all cases, since the amount of heat lost will vary among engines based on design, size and materials used, but provides useful results for purposes of comparison.
To predict a pressure gauge reading subtract 14.7 (or the correct atmospheric pressure at test elevation) to compensate for the fact that a gauge in free air reads “0”, not 14.7 psi, even though atmospheric pressure is always present.

[MadmanMark]

Re: Freiburger, DCR, and testing.....

You got me on that one. I do not have the formula for a calculator that gives cylinder pressure, only for a DCR calculator that gives a ratio. In order to calculate cranking PSI it seems to me that you would first have to calculate the DCR in terms of a ratio, then multiply the ratio times atmospheric pressure and apply corrections for VE, adiabatic compression, etc. :-\

DCR is just a geometric ratio - head, intake, and carb flow are not considered. To get to cranking PSI, you start to get into fluid dynamics to find the bottlenecks in the system (good programs for this are usually more complicated simulations). Also, I don't think VE is ever 100% at cranking. VE changes with rpm and typically peaks near the torque peak. Some of the engine simulation programs (I've used Dyno2000) will give you VE, PSI, and other terms with respect to rpm. The problem with these programs is they are usually limited by the amount of input detail allowed. Usually you'll enter head flow at different lifts and all your cam timing events, but things like intake and carb flow are typically oversimplified.

And there's no way to keep temp constant during the combustion process (due to the explosion that pushes your crank down), so there's no non-adiabatic compression.

I threw the spreadsheet together after reviewing http://cochise.uia.net/pkelley2/DynamicCR.html. Someone had posted this during one of the past dcr discussions and I thought it did the best job of summarizing it. The link is a bit easier to follow than the excel formulas.

[JeffMcKC]

Re: Freiburger, DCR, and testing.....

Eric I think you will find the DCR on my motor with that cam is 8.8 and the cranking comp is only 157 now how is it on the high side of pump gas by formulas, yet it is in noway having a chance to detonate on it ?

[Eric68]

Re: Freiburger, DCR, and testing.....

Eric I think you will find the DCR on my motor with that cam is 8.8 and the cranking comp is only 157 now how is it on the high side of pump gas by formulas, yet it is in noway having a chance to detonate on it ?

Jeff,

I get 7.6:1 DCR and predict 174 psi cranking compression assuming 1000' elevation. I would think that you would be just fine on pump gas Jeff. Shoot, you could probably play with 87 octane. We both have the same static compression ratio, but you have 20* more cam!

[Freiburger]

Re: Freiburger, DCR, and testing.....

Since you're taking me to school, I still need to understand how "compression ratio," or even an output expressed as a ratio, is used to describe or measure the differences in pre-combustion cylinder pressure based on inlet events that happen prior to the intake valve closing. Really. I still see it as a breach of logic, so there's something here I'm obviously not seeing.

[JeffMcKC]

Re: Freiburger, DCR, and testing.....

Not for long Eric LOL : ;D