A measurement of how tightly the air-fuel mixture is compressed or squeezed within the cylinder by the piston. This can be determined by taking the total volume of the cylinder when the piston is at its lowest point (Bottom Dead Center, or BDC) and dividing it by the volume when the piston is at its highest point (Top Dead Center, or TDC).
A high compression ratio is desirable because it allows an engine to extract more mechanical energy from a given mass of air-fuel mixture due to its higher thermal efficiency. High ratios place the available oxygen and fuel molecules into a reduced space along with the heat of compression - causing better mixing and evaporation of the fuel droplets. Thus they allow increased power at the moment of ignition and the extraction of more useful work from that power by expanding the hot gas to a greater degree.
Higher compression ratios will however make gasoline engines subject to engine knocking, also known as detonation or pre-ignition, and this can reduce an engine's efficiency or even physically damage it.
May we suggest:
| Air/Fuel Ratio | Combustion Chamber | Cylinder | Knocking | Piston | |
