Engine Physics 101: Compression Ratio



Continuing our series about the physics of engines, today we focus on compression ratio.

Compression ratio is directly related to the amount of power an engine produces. In general, the higher the compression ration, the more powerful the engine. This ratio is measured from the top of the stroke to the bottom of the stroke. So, what is compression ratio?

Compression ratio is the ratio of volume within the engine’s combustion chamber from greatest to smallest capacity. For instance, the compression ratio of our VGT 500 is 18:1. That means that the volume of the bottom of the stroke is 18 times larger than the volume at the top of the stroke. It is how much air is compressed.

What does this have to do with power?

Since diesel engines use heat and pressure to combust fuel, the more pressure applied, the greater the amount of energy produced. So what is the limit? The amount of pressure that the cylinder head and piston are designed to withstand. On diesel engines from most manufacturers, the compression ratio ranges from 14:1 to as high as 22:1. On most petrol cars, by way of comparison, compression ratios are nearly always under 14:1, and usually no higher than 10:1, since they use a spark for combustion.

It is for this reason that diesel engines of a similar size to their petrol counterparts normally produce much greater levels of power and torque.

The formula for determining compression ratio is here:

mbox{CR} = frac { tfrac{pi}{4} b^2 s + V_c } {V_c}

whereb; = cylinder bore (diameter)

s; = piston stroke length

V_c; = clearance volume. It is the volume of the combustion chamber including head gasket.