What is Compression Ratio?
The compression ratio (CR) is defined as the ratio of the volume of the cylinder and combustion chamber when the piston is at the bottom of its stroke (Bottom Dead Center - BDC) to the volume when the piston is at the top of its stroke (Top Dead Center - TDC). Higher compression ratios extract more mechanical energy from the air-fuel mixture, improving engine efficiency and power.
Static vs. Dynamic Compression
Static Compression Ratio (SCR) is calculated based solely on cylinder geometry. It assumes the intake valve closes exactly at BDC, sealing the cylinder for the full upward stroke.
Dynamic Compression Ratio (DCR) takes into account the timing of the intake valve closing (IVC). Since the intake valve usually remains open for some time after BDC to improve filling at high speeds, compression doesn't truly begin until the valve closes. DCR is always lower than SCR and is a better indicator of an engine's octane requirement.
Formula
The basic formula for Static Compression Ratio is:
CR = (Vd + Vc) / Vc
Where:
- Vd = Displacement Volume (Swept Volume)
- Vc = Clearance Volume (Total volume above piston at TDC)