market dynamics

The definition of displacement and compression ratio

Release date：2019.08.15

The volume of exhaust is actually closer to the volume of air inhaled, but we cannot simply assume that the volume of exhaust is the same as the volume of air inhaled by the engine, because after a series of compression, ignition and expansion reactions within the engine, the volume of exhaust contains the part of combustion expansion.

Displacement is the amount of volume a piston can drain from one stroke to another. For a quick explanation, look at the image below.

We can see that the piston within the cylinder at the movement, the movement to the top position is called top dead center, the movement to the next position is called bottom dead center, the distance from the top and bottom dead center movement called piston stroke, using the inner diameter of the cylinder and the travel distance to calculate the volume of this cylinder, is the displacement of a cylinder. And then you take this displacement times the number of cylinders, and you get the displacement of the entire engine. Don't understand the picture below.

Simply expressed in mathematical formula, displacement = stroke x internal diameter x cylinder number

So what's the compression ratio?

It is the ratio between the volume before gas compression and the volume after gas compression, that is, the ratio between the total volume of the cylinder and the volume of the combustion chamber is called the compression ratio.

To put it more simply, it's the ratio of the product of the contents of the cylinder when the piston is at the bottom dead center to the product of the contents of the cylinder when the piston is at the top dead center.

Generally speaking, the larger the compression ratio, the greater the power, but not the compression ratio is better, because the compression ratio of gasoline engine is too large to appear deflagration and other abnormal combustion phenomenon. Therefore, the gasoline engine compression package is usually controlled at about 10 is better.

Compression ratio and gasoline label:

The compression ratio is 7. 5 ~ 8. 0 should choose no. 90 motor gasoline;

The compression ratio is 8. 0 to 8. 5 should choose no. 90 ~ 93 automotive gasoline;

The compression ratio is 8. 5 ~ 9. Should choose no. 93 ~ 95 automotive gasoline;

The compression ratio is 9. 5 ~ 10 should choose no. 95 ~ no. 97 automotive gasoline.