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What Is Engine Displacement?

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  • Written By: James Doehring
  • Edited By: Lauren Fritsky
  • Last Modified Date: 30 October 2014
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Engine displacement is a measure of the volume in an internal combustion engine. Though not directly proportional to total power produced, it typically correlates strongly with output power. As cylinders move within an engine, they do work by displacing a certain volume of air. All internal combustion engines have a certain volume displaced by cylinders. High engine displacement will generally result in low-fuel economy, and various governments have used the figure as a basis for taxation.

An internal combustion engine burns fuel in cylinders to produce power. A piston compresses air within a cylinder, while fuel, typically gasoline, is injected into the air. A spark plug ignites the fuel in the compressed air, which further increases the air pressure within the cylinder. This high-temperature, high-pressure gas pushes the piston down a distance called the stroke. The piston is directly connected to the vehicle’s transmission, which in turn can spin the wheels.

Engine displacement refers to the volume swept by all the pistons in an engine. It does not include the volume of air above the piston where the initial spark fires. It can be calculated with the following formula:

Engine displacement = π/4 * (cylinder diameter)2 * stroke * number of cylinders

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In essence, this is the total volume all cylinders can change by. The figure only refers to the volume displaced by pistons, so it is the internal cylinder diameter that must be used in this calculation. Cylinders in an internal combustion engine are never at their maximum volumes at the same time, however. Their positions are staggered so that a more continuous power can be produced, rather than bursts of power.

Engine displacement is typically given in liters, but it can also be expressed in cubic centimeters (cc) or cubic inches. Many engines in motorcycles and lawn equipment have a total displacement of less than one liter, so the cc figure is preferred. Economy-class automobiles generally have four cylinders with total displacements between one and two liters. Eight-cylinder sport utility vehicles (SUVs), on the other hand, can have displacements upwards of six liters. The total engine displacement is often incorporated into a vehicle’s name.

In some countries, engine displacement has a legal significance. Engine sales are frequently taxed based on displacement, rather than total power output. For this reason, it is common to see displacements slightly under some threshold value. Many countries, such a Sweden, are beginning to tax vehicles based on total carbon dioxide emissions instead of engine displacement, however.

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anon346987
Post 4

Engines do include the volume of air above the piston if the engine scavenges. Scavenging displaces or moves the air in the combustion chamber.

Mechanical displacement includes the pistons movement, and if supercharged, the rated output times the speed it moves in comparison to the engine.

Actual engine displacement can be 125 percent of the engine's mechanical displacement thank to scavenging and momentum from properly tuned intake runners pushing gas into the cylinder.

You have listed mechanical engine displacement, not actual effective engine displacement.

Amphibious54
Post 3

Should I consider an engine replacement or should I rebuild the engine on my vehicle? I blew a head gasket, possibly from a detonation/knocking problem. The mechanic said it would be an expensive fix either way. I think the motor in my vehicle would still be good once fixed, but the replacement would be a little cheaper. I just worry that the replacement would be an engine that was also rebuilt with hidden problems.

Can anyone give me any advice on what I should do? I need to keep the car because I owe more than it would cost to repair.

cougars
Post 2

@glassaxe- I think that there should be stricter regulations on fuel efficiency rather than a tax on the engine size, performance, or emissions. If car companies had higher fuel efficiency requirements, they could design better high displacement motors for larger vehicles. From what I understand about the auto industry, car companies push their large vehicles because they generate so much profit compared to smaller cars.

Stricter fuel regulations would be a better way to create a disincentive for pushing gas-guzzlers on the public. Rather than taking away the choices of the consumer, the government should focus on improving the quality of the choices for consumers.

GlassAxe
Post 1

I believe vehicles in the United States should be taxed based on their engine displacement or their emissions as well. A truck or sports car with an engine size over 4.0 liter is likely to use more fuel than a vehicle with a lower displacement value. This will help deter people from buying so many polluting light trucks and SUVs that skirt the fuel economy standards outlined for cars in the CAFE legislation. It would be more of a disincentive for people who really do not need these large vehicles for anything other than a status symbol.

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