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
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.