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Piston Engines - Compression Ratios

Cylinder volume and compression ratios in piston engines.

The static compression ratio in a piston engine is the ratio between the volume of the combustion chamber when the piston in the cylinder is at the bottom of its stroke and the volume of the combustion chamber when the piston is at the top of its stroke. It can be expressed as

CR = (Vd + Vc) / Vc  

    = (π d2 s / 4 + Vc) / Vc            (1)


CR = compression ratio

Vd = piston displacement volume - total volume swept by the piston (cm3, in3)

Vc = clearence volume - volume left in the cylinder when the pistion is at the top dead center (cm3, in3)

d = piston diameter (cm, in)

s = piston stroke (cm, in)

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