Compression ratio Totally Explained

Everything about The Compression Ratio totally explained

The compression ratio is a single number that can be used to predict the performance of any engine, particularly piston engines (but can be used on essentially any internal-combustion engine or external combustion engine as well).
   In a piston engine it's the ratio between the volume of the cylinder, when the piston is at the bottom of its stroke, and the volume when the piston is at the top of its stroke.
   Picture a cylinder with the piston at the bottom of its stroke containing 1000 cc of air. When the piston has moved up to the top of its stroke inside the cylinder, and the remaining volume inside the head or combustion chamber has been reduced to 100 cc, then the compression ratio would be proportionally described as 1000:100, or with fractional reduction, a 10:1 compression ratio.
   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 adiabatic 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 and this can reduce an engine's efficiency or even physically damage it. Diesel engines on the other hand "knock" because they operate on the principle of compression ignition.

Formula

The ratio is calculated by the following formula: ť

mbox PR

Since T₂ is much higher than T₁ (compressing gases puts work into them, for example heats them up), CR is much lower than PR.

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