Compression Ratio
The compression ratio compares the cylinder volume with the piston at the bottom to the volume with the piston at the top.
What is Compression Ratio?
Petrol engines typically have ratios around 9:1 to 13:1; diesels around 15:1 to 20:1. Higher ratios improve efficiency but increase the risk of knock in petrol engines. Some engines can change their effective compression ratio by adjusting valve timing. A few production engines vary the ratio mechanically.
How it works
The higher the ratio, the more the air or mixture is squeezed before combustion, which raises temperature and pressure.
Why it matters for your car
Using fuel with the octane rating the manufacturer specifies matters more on high-compression engines. Carbon deposits slightly raise the effective compression ratio over time and can increase knock tendency.
Other terms
- Carburettor
- Certificate Of Conformity (CoC)
- Connector Corrosion
- Defensive Driving
- Diesel Intake Throttle Valve
- Differential
- Emergency Brake Signal (Flashing Brake Lights)
- Firing Order
- Ground Clearance
- Intelligent Battery Sensor (IBS)
- Microfibre Towel
- Mineral Engine Oil
- NOx Storage Catalyst (Lean NOx Trap)
- OEM Parts (Original Equipment)
- Power Steering Warning Light
- Reversing Light Switch
- Roof Rails
- Starting Handle (Crank Handle)
- Toll Vignette
- Vehicle Overloading