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Compression ratio converted to PSI

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Brad in WI

09-06-2006 02:14:17

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How does a person convert a compression ratio of 14.2:1 over to a PSI reading. I would like to know the formula as I have several tractors that I want to check and all have different ratios. I have not checked it yet just want to get an idea if I am in the ballpark when I get a chance to check it out. Thanks
Brad

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Maybe,,,

09-06-2006 16:33:36

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Re: Compression ratio converted to PSI in reply to Brad in WI, 09-06-2006 02:14:17

Just a guess,,,,,

100 psi 5 to 1
120 psi 6 to 1
140 psi 7 to 1
160 psi 8 to 1
180 psi 9 to 1
200 psi 10 to 1
220 psi 11 to 1
240 psi 12 to 1
260 psi 13 to 1
280 psi 14 to 1
300 psi 15 to 1

Thats just a guestimated pressure to compression ratio at cranking speed. There is no real way to make it exact, unelss you spec the engine out at 14.2 to 1 etc etc then run a compression test to get a relative psi number at cranking speed.

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Apples

09-06-2006 12:52:15

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Re: Compression ratio converted to PSI in reply to Brad in WI, 09-06-2006 02:14:17

and oranges.

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Jerry/MT

09-06-2006 12:15:54

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Re: Compression ratio converted to PSI in reply to Brad in WI, 09-06-2006 02:14:17

Compression ratio is actually the ratio of the volume of the cylinder at BDC divided by the volume of the cylinder at TDC. I can give you an equation for the ideal compression pressue but it will calculate to high for several reasons. The main reason is that the equation is for a constant mass in the cylinder. In reality you don't have that as the valves don't closed til part way through the compression stroke, so the mass isn't constant. There is also some leakage past the piston and then the fuel get's injected causing some evaporation and a change in the gas properties and the mass. Having said that pcomp = 14.7 x (V1/V2)^1.4 ~ psia, where V1/V2 is the compression ratio. So if the compression ratio is 16, and you're at std day conditions and your air cleaner has zero pressure loss and the fuel is stop cocked, pcomp(ideal) = 14.7 x (16)^1.4 = 14.7 X 48.5 = 712.95 psia.

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BBx

09-06-2006 04:29:07

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Re: Compression ratio converted to PSI in reply to Brad in WI, 09-06-2006 02:14:17

Atmospheric pressure at sea level is about 14.7 PSI. So a 10 to 1 compression ration would give 14.7 x 10 = 147 PSI.

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Josh in Pa

09-06-2006 07:32:55

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Re: Compression ratio converted to PSI in reply to BBx, 09-06-2006 04:29:07

This is only correct if the temperature of the air does not change, which it does when compressed rapidly. I don't believe that there is any chart to convert psi to compression ratio, i think compression ratio must be measured. Josh

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Aaron Ford

09-06-2006 10:23:27

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Re: Compression ratio converted to PSI in reply to Josh in Pa, 09-06-2006 07:32:55

Compression Ratio must be calculated using the volume of the cylinder/gasket/combustion chamber versus the volume of the combustion chamber and the gasket.

Now the cylinder pressure is based on the filling and retention of pressure of the cylinder. This is affected by the cam profile (valve events and overlap) and the sealing of the rings/valves.

The two are related, but there is no direct comparison.

Aaron

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