[Foosman(5)]

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We are all familiar with compression ratios, but I wonder if there is a relationshipto cylinder pressure, psi. General specsusually calls out an acceptable minimum compression measurementsprobably for service limits and acceptable ring blow by.

But, if a compression ratio is a constant, then why wouldthe pressure not be the same for all engines with the same compression ratio?

Here are a some basic info.

Atmospheric Pressure = 14.7 psi (STP)

i.e. Compression Ratio = 10:1 (constant)

Intake Valve Close= 30 BBDC (air inertia and volumetric efficiency

irrelevant at cranking speed)

PV=nRT (carnot - ottocycle)

I can see how temperature may affect pressure (like a diesel ignition), but I would like to discard heat-transfer lossdue toshort time intervaleven though the speed is slow.

Need for someone to do their thermodynamics's homework.

To me, the compression check psi numbers should be fairly standard, but are they?Why can't there be a psi table for8:1. 8.5:1, 9:1, 10:1, 12:1. Also, is there a"rule-of-thumb"?

 

[Trucknchuck]

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You must also take into account volume as the compression ratio is only one part of the equation. The total volume will affect the psi so that a ratio of say 10:1 will differ in a smaller cubic displacement as compaired to a large displacement..

cc's in the head also affect the psi even if the ratio is the same. so I do not belive you can come up with a psi to fit each displacement to ratio.

The differance in each chamber on the same head can make a change in psi on the same motor, same bank. Unless you cc the chambers to be the same, which in most motors are diffrerant even from the factory, there will be a change in the psi but not the ratio.

 

[exavid]

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I believe you'll find valve timing will be one of the variables that affect actual psi developed in cranking an engine. Compression ratio could be used to calculate the maximum possible pressure developed in a perfect engine with no friction, leakage, and a constant specified cranking speed. Valve timing and overlap will effect the amount of air sucked into the cylinder as will any restriction in the intake system. You'll never have full atmospheric pressure in the cylinder because the engine has to develop vacuum to suck the air in. With a normally aspirated engine that little bit of loss sucking in air will result in a loss of developed pressure. Cranking speed will make a difference too since there's always some air leakage in compressing the air in the cylinder. Faster cranking speed would allow less time for the leak down resulting in higher readings in psi. The other variables of atmospheric pressure at the time of testing and ambient temperature and dew point or humidity will also effect the measurement. There's really no practical way to determine the air pressure developed via compression in an engine except to measure it with fixed parameters in a new broken in engine.

 

[Foosman(5)]

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I just realized I made a bad assumption.

Intake Valve Close= 30 BBDC (air inertia and volumetric efficiency

irrelevant at cranking speed)

Actually, the intake valve closes ~30 ABDC. This is to take advance of the inertia ram effect.With the piston on its way back up, the it would have travelled 30 degrees, and (cos30) 86 remaining distance and displacement. So that's a potential 13 percent loss in volume that would also affect compression.

Various engines have different cam profiles and durations. So the intake valve closing would not be constant for engines for compression calculations.

 

[Grumpyguy]

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Not being a very technical sort of person I may be way of base here. Is compression ratio calculated from the actual pressures achieved, or is it simply figured from the dimensions of the engine?

 

[exavid]

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Compression ratio is a calculated value made by measuring or calculating the volume of the cylinder with valves closed. The volume with the piston at bottom dead center compared to the volume with the piston at TDC.

 

[Trucknchuck]

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It is bore x stroke + head cc - deck height. if you want the excact formula I can get my old note book out to get it for you as it involves calcules. I had it as I use to build vw buggie motors for the dunes and the us sand drags. In fact we (the shop I worked at) still hold a nat. record for a carburated vw dragster since 1989 for et at 3.47. Its walters vw out of banning,ca. its in hot vw's mag. fast for 300 ft of wet sand.

some folks call head cc as cylinderhead chamber size