Why is the displacement of a car measured in cc/litres?


Or rather, why is it called displacement?

In: Engineering

Lets say you have a one cylinder engine, the volume of air displaced from the cylinder when the piston rises is the displacement. If you have more cylinders, multiply.

The displacement is the area of the space where the fuel/air mixture burns (the inside of the cylinders).

Some American cars measure them in cubic inches, but obviously cc or liters are the more universal unit of measurement.

It’s not very predictive of performance, but it can give you a relative idea of the size of the engine compared to other cars.

The specific thing being measured is the Engine Displacement, or the total volume swept by all of the pistons within the engine’s cylinders. This displacement correlates with the amount of air ingested by the engine in a single engine cycle (2 full revolutions of the crankshaft for a modern 4-stroke engine). Because the engine functions on the combustion of air and fuel, the displacement is the hard limit on how much air you can run through the engine, and thus how much combustion you can get. More displacement means more power, which in turn means a higher top speed.

Because it’s a measure of volume, it uses units of volume, of which cc and liters are two options.

It refers to the total space, ie volume, contained in all the cylinders of the engine, added together. Since volume is a three-dimensional quantity, it is measured in cc/liters/cubic inches just like other volumes.

The volume of an engines cylinder consists of

* The volume that is swept by the piston going up and down

* The unswept volume above where the piston stops, ie the combustion chamber.

The ratio of volume between the total volume and unswept volumes gives the (static) compression ratio. An example would be 1:10 for a petrol engine.

Displacement only refers to the swept volume. ie the volume of air+fuel mixture that is displaced by the piston.