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womble (Mechanical) (OP)
24 Sep 02 13:30
Hi all,

Could anyone point me to some formulea or freeware to help me work out the dynamic compression ratio of a normally aspirated engine.
I have details on the bore, stroke, chamber size, camshaft duration and opening/closing points. Is this enough information?
Basically I'd like to do a comparison of dynamic compression ratio's between a stock engine and one with higher static compression and longer duration camshaft. I know I could get a comparison by turning them over and measuring the peak cylinder pressures but I'd like to be able to work it out beforehand if possible.

Cheers.
ivymike (Mechanical)
24 Sep 02 14:12
If you define dynamic compression ratio as the ratio of cylinder volumes between IVC and TDC, then you do have sufficient information to calculate it.

First, calculate the TDC cylinder volume (probably what you've labeled chamber volume)

Then calculate the piston position at the IVC angle, and the swept volume between that position and TDC.

CR.dyn = (Vol.tdc + Vol.IVC_swept) / Vol.tdc

womble (Mechanical) (OP)
24 Sep 02 16:55
Sorry ivymike, but im not familiar with the term IVC. Could please enlighten me?
How much do you know on runner lengths and plenum volume sizing for N/A engines?
ivymike (Mechanical)
24 Sep 02 17:10
intake valve closing; next to nothing.
Chumley (Automotive)
24 Sep 02 19:45
Womble:

See this site.  There are thousands of technical papers
from the 1920's and 1930's from MIT, Langley Research, and many other labs that pioneered the initial fundamental design work for spark ingition piston engines.  Many of the papers have performance charts, theory, photos(not good quality) and reams of tech talk on runner length, static and dynamic compression, detonation, turbo charging, supercharging, exotic fuels, and yes, even cold air intakes and water injection.(which by about 1940 and after thousands of hours of testing, they knew was a waste of time for all but the most specialized applications)  Check mostly the papers from 1920's and 30's.  If you get much later they have more turbine and rocket theory.  Beautiful girls, colorful  marketing broshures, and easy reading,  NOT, but lots of straight data that the engineers actually used to design and build some screaming reliable 3 bar boosted turbosupercharged fighter plane engines, YEP, its there, and it's free for the reading courtesy of our grandparents tax dollars.  Rant off!

http://naca.larc.nasa.gov/

Chumley   
tbuelna (Aerospace)
25 Sep 02 22:59
womble:

here's a website with a dynamic compression ratio calculator that works fairly well:

http://www.rbracing-rsr.com/comprAdvHD.htm

regards,

terry
womble (Mechanical) (OP)
27 Sep 02 16:50
Thanks chaps!
ProgressiveRacing (Automotive)
11 Nov 02 14:54
Hi guys,
This is an older thread but I've got a question about it. In many of the DCR calculations I've seen (probably for sake of simplicity) there were no altitude, temperature, or boost pressure variables.
I'm curious as to how one goes about working a formula with those variables in it. Don't realy want a program/calculator, just want to work through the math for my own understanding.

Thanks in advance!
SWB (Automotive)
11 Nov 02 22:00
My question is, we light the charge off prior to TDC, why do we pretend (in our calculations) that this doesn't happen? "Real" pre-combustion-compression ends when the spark plug fires. The longer you wait, the more compression you have. Naturally there is a compromise point, but that is not the subject.

Anyone want to take a stab at that one?

Sean
ivymike (Mechanical)
11 Nov 02 22:53
In which calculations are we pretending this?


SWB (Automotive)
12 Nov 02 20:40
Ivymike,

Take the formula you stated in your first post.

"CR.dyn = (Vol.tdc + Vol.IVC_swept) / Vol.tdc"

This is great, but prior to the piston making it to TDC the spark plug will fire. At that point, you not only have the piston compressing the gas, but the now burning and rapidly expanding gas compressing that which has not yet burnt. See my point? I.E. the negative portion of the "power stroke" has already begun and the compression stroke has in some respects, ended.

What that means, is that your swept volume is going to start at ivc and end at the ignition point. This could me your compression ratio ends up being reaaaaaaly low?

Sean
ivymike (Mechanical)
12 Nov 02 23:16
Sure, if you want to calculate something other than what I was calculating... I wasn't aware that there was any assumption that firing happened after compression when calculating compression ratio - I thought that compression ratio was just a useful number for comparing a particular geometric parameter between engines.  Next thing I know, we'll be worrying about cylinder pressure not staying at BMEP over the whole expansion stroke, and not being zero over the other strokes...



ivymike (Mechanical)
12 Nov 02 23:25
If you want to figure the effects of spark timing and combustion duration on engine performance, use some fancy software like WAVE.  If you want to do some quick hand calculations to give you an idea of how geometric changes to an engine might affect performance, do the standard calculations and compare the results to what has been observed previously.

SWB (Automotive)
13 Nov 02 21:54
I hear you.

Sean

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