I.E what does the volume actually reflect? What’s the difference between a 2.4L engine and a 7.3L?
In: Engineering
It’s the overall volume of empty space inside the cylinders when the pistons are at bottom dead centre.
The space is filled with a fuel air mix that goes bang forcing the pistons round/down.
They are usually measured in cubic centimetres or “CC’s” which has a direct translation into litres because they are metric. eg a 2500cc engine is 2.5 liters.
Generally speaking the bigger the volume of fuel/air you can make go bang, the fast or more powerful an engine will be.
As the pistons go up or down in the cylinders they sweep out a certain volume between the top and bottom of their motion. The total volume for the whole engine is given in litres or cubic inches.
The volume they reflect is the total volume of the cylinders. Since each of the cylinders will fire once per revolution, the sum of the cylinder volumes is the volume of gas/air you combust for each revolution.
The reason sizes are listed is because it sort of reflects performance: more cylinder volume means you can burn more fuel at the same time, which means you’re putting out more power per revolution.
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The “size” you’re referring to is called *displacement*. It is also known as the *swept volume*. It is the volume displaced by one complete movement of the pistons, from one extreme of the pistons’ travel to the other extreme.
A given piston will move up and down inside its cylinder as the crankshaft rotates. In a four-stroke engine the piston will move from top to bottom to draw in air (the “intake stroke”), move from bottom to top to compress it (the “compression stroke”), move top to bottom after the charge of air and fuel is ignited (the “power stroke”), and move from bottom to top to expel the products of combustion (the “exhaust stroke”).
All things being equal (and they’re not really, at all, but pretending as though they were) the volume of air that gets pushed or pulled through one of these strokes of the piston is roughly analogous to the amount power that piston’s movement can produce. In very rough terms more air will allow for more fuel to be burned and it will resultantly make more power.
To calculate displacement it is simply:
displacement = stroke * pi * [(1/2)*bore]^2 * number of cylinders
Where piston *stroke* is the distance travelled by the piston from top to bottom or vice versa, and *bore* is the cylinder diameter. Do you remember basic geometry you would have learned in school? This is quite simply just the calculation of a cylinder volume, pi * radius^2 * height.
Because displacement is roughly analogous to power and fuel economy many governments around the world have instituted taxes on cars based on their engine displacement. This is meant to encourage people to buy smaller cars, which is a desirable political outcome in many densely-populated and/or resource-poor countries. The difference between, say, a 1.4 L engine and a 2.4 L engine may be hundreds or even thousands of dollars worth of taxes. The populace of countries with displacement taxes will find it to be an important measure, whereas in countries where there are no tax implications—e.g. the US, Canada, Australia—it’s mostly irrelevant and is just used in advertising.
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