§ How it works

The static compression ratio compares the volume above the piston at bottom dead centre with the volume left at top dead centre. The space that remains at TDC is the clearance volume $V_c$ ; the volume the piston displaces is the swept volume $V_s$ .

CR = \frac{V_s + V_c}{V_c} \qquad V_s = \frac{\pi}{4} B^{2} S

The clearance volume is everything the piston cannot reach, added up:

V_c = V_{\text{chamber}} + V_{\text{piston}} + \frac{\pi}{4} B_g^{2}\, t_g + \frac{\pi}{4} B^{2}\, d

$V_{\text{chamber}}$ : the combustion chamber in the head, measured by pouring fluid from a burette ("cc-ing" the head).
$V_{\text{piston}}$ : positive for a dish or valve reliefs, negative for a dome that pokes into the chamber.
$B_g,\ t_g$ : head gasket bore and compressed thickness.
$d$ : deck clearance, the gap between piston crown and block deck at TDC. Negative if the piston stands proud.

Why it matters

For an ideal Otto cycle, thermal efficiency depends only on the compression ratio and the gas property $\gamma \approx 1.4$ :

\eta = 1 - \frac{1}{CR^{\,\gamma - 1}}

Real engines get roughly half of that figure, but the trend holds: more compression extracts more work from the same fuel, until knock sets the limit for the fuel's octane rating.

References

Ideal Otto Cycle — NASA Glenn Research Centerthe thermodynamic cycle behind η = 1 − 1/CR^(γ−1)

§ How it works

CR = \frac{V_s + V_c}{V_c} \qquad V_s = \frac{\pi}{4} B^{2} S

The clearance volume is everything the piston cannot reach, added up:

V_c = V_{\text{chamber}} + V_{\text{piston}} + \frac{\pi}{4} B_g^{2}\, t_g + \frac{\pi}{4} B^{2}\, d

$V_{\text{chamber}}$ : the combustion chamber in the head, measured by pouring fluid from a burette ("cc-ing" the head).
$V_{\text{piston}}$ : positive for a dish or valve reliefs, negative for a dome that pokes into the chamber.
$B_g,\ t_g$ : head gasket bore and compressed thickness.
$d$ : deck clearance, the gap between piston crown and block deck at TDC. Negative if the piston stands proud.

Why it matters

For an ideal Otto cycle, thermal efficiency depends only on the compression ratio and the gas property $\gamma \approx 1.4$ :

\eta = 1 - \frac{1}{CR^{\,\gamma - 1}}

Real engines get roughly half of that figure, but the trend holds: more compression extracts more work from the same fuel, until knock sets the limit for the fuel's octane rating.

References

Ideal Otto Cycle — NASA Glenn Research Centerthe thermodynamic cycle behind η = 1 − 1/CR^(γ−1)

Compression Ratio

Inputsdrag · tap · type

Fig. 03Volume at BDC vs TDC

§ How it works

Why it matters

References

Compression Ratio

Inputsdrag · tap · type

Fig. 03Volume at BDC vs TDC

§ How it works

Why it matters

References