Can someone explain how compression is calculated?

[XH29N0G]

After seeing some of the posts over the past few days about calculating dynamic and static compression, as well as cranking PSI, I started wondering how the calculators worked (mathematically).  I can get a large part of the way there using geometry, but I cannot figure out how the cranking pressure is calculated.  Let me explain below.

I can calculate static compression by calculating the volume with the piston at the bottom and top of its travel.

I can calculate the dynamic compression using geometry (see picture below).

I know the cranking pressure should depend on the temperature of the air going in and also the temperature after compression.  This does not appear to depend entirely on adiabatic heating of the gas because that would make cranking pressures much higher than we measure with a compression gauge.  I am guessing that it depends in large part on the temperature of the engine block which would cool the mixture.  But it looks like there is more to it.  

I am guessing there is a trade off between the two and the calculators like that at Wallace Racing take this into account or use an empirical relationship.  

If someone can explain what is done, I would really appreciate it.  

[A383Wing]

way over my head....so I guess this would be no help, huh?

http://www.csgnetwork.com/compcalc.html

[XH29N0G]

way over my head....so I guess this would be no help, huh?

http://www.csgnetwork.com/compcalc.html

What I am trying to understand is how things like this work.  I figured out how to calculate the stuff on the one you posted.  If I run this one http://www.wallaceracing.com/dynamic-cr.php I also see a cranking PSI value come out of it.  That involves the behavior of gases when heated and compressed and I am looking to figure out how that is done. 

[Cooter]

Cylinder pressure and compression are dependant on camshaft specs. Too much overlap, and cylinder pressure will go away.
static compression ratios can be 'figured' but it is my opinion they are an approx.

[BSB67]

I doubt very much that there is a temperature change component in the cylinder pressure derived from dynamic compression calculators.  Actual atmospheric pressure should be used for the cylinder pressure but most just use altitude to get close.  The dynamic compression is just math, as you know.  Have you tried just plugging the numbers into the ideal gas law?
I'd start by holding temperature constant. My guess is that will get you close.   

And of course, the real unknown is when exactly does the piston really start to build cylinder pressure?  That is why the numbers from the calculators usually don't match real measured cylinder pressure numbers.  Some calculators use 0.050" numbers, some use 15° before actual seat timing, some use seat timing.  We know that the motor actually start to build cylinder pressure before the valve actually touches the seat, but the shape of the closing ramp near the seat will effect when the pressure will actually start to build.  And lets not forget how the health of the starter and battery will also effect the actual measured cylinder pressure.

FWIW, the calculators that I have used that seem to be closest to measured results are the ones that use 0.050" timing.  And of course there is no actual science there.  Probably empirically derived, or, more likely used out of convenience as it is on every cam card and the results are close enough.

[XH29N0G]

Thanks,  I think you are right, but P1V1=P2V2 doesn't exactly work because there is work done.

I have some more insight after doing some more calculations chancing across the web page at:

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

The description here says that the both the adiabatic index and mismatch between the actual point where the intake valve seals and the point reported for closing on the cam card.

If the point reported on the cam card were perfect, they would calculate

              P_gauge as P_atm x (DynamicCR)^1.4 - P_atm

The exponent (1.4) reflects/is the adiabatic index (heat capacity ratio).  

BUT Because the sealing point of the valve is further along than its reported closing point, this exponent is shifted in the calculators to something like 1.875. to compensate for a later actual closing.

When I use this value in the equation I just listed, I come within a PSI of the cranking pressure estimates given by the Wallace Racing calculator.  (This is without adding anything to the closing point listed for the cam.)

So it is empirical/approximate - both of you look correct.  

[BSB67]

FWIW, the Wallace calculator if the farthest off from real measured values.  It come nowhere close to real measured values.

For cranking compression the exponent is 1.1875 in that tech write-up for aggressive closing ramp, but the author uses 1.2 apparently for not aggressive lobes.   This is what he is using to fudge the actual point where pressure starts to build,  and acknowledges that the rate of closure in the last 15° makes a difference.

I did spend a minute using the gas law and held everything constant and that did not generate pressure values that matched real numbers either.  It is hard for me to imaging that the gas temperature changes a few hundred degrees F by simply cranking the engine over.  Maybe so.  Working backwards and plugging the temp change needed it actually come close to his adjusted 1.2 adiabatic exponent value (i.e. an adjustment of a fudge factor).

[XH29N0G]

I think we are on the same page - a fudge factor to fit it.  At least now I have an idea now where the magic of the calculators comes from.  



Next up.... how long will it take the gas molecules to exchange their energy with piston, cylinder, valves, and head...  

And most importantly.  Thanks for humoring me with my questions.

[BSB67]

Yeah, it was kinda fun.