How to work out compression ratios

[obi_waynne]

I'm stuck, what is the formula to calculate the compression ratio of an engine? How do you work out a compression ratio?

Is it:-
Final Compression Ratio or FR= [(Boost / 14.7) + 1 ] x Compression Ratio
NB: We have taken the Volumetric Efficiency to be 14.7 in this example, but it will vary from engine to engine.

Or someone suggested
CompressionRatio = (swept volume+combustion chamber volume at TDC)/combustion chamber volume at TDC

(We'd also need how to get the swept volume for the latter formula to work)

 

[old-git]

The compression ratio is simply the difference between the volume above the top of the piston at BDC and TDC. Boost pressure has absolutely nothing to do with it as we are talking ratios not pressure.

 

[obi_waynne]

So the latter one is correct then.

How do you work out swept volume then?

 

[old-git]

Yes

Stroke x bore cross sectional area.