Posted 21 June 2011 - 03:15 PM
In essence, it is both. It is the volume moved by the pistons, which could be gas or fluid. The best description I can find is below:
Engine displacement, also referred to by the less common, but more technically accurate term piston displacement, is the measure of the volume moved by the pistons of an internal combustion engine as they move through a single, complete revolution. Piston displacement, which is expressed in volumetric units such as liters, cubic centimeters, or cubic inches, is directly proportional to the overall power generated by an engine. It is calculated according to an equation taking into account various engine characteristics, including the number of cylinders, bore, and stroke.
Bore is the inside diameter of the hollow cylinders that house the pistons. Stroke is the distance a piston moves over the course of a revolution. With this in mind, the mathematical formula for displacement is: the number of cylinders multiplied by bore squared, multiplied by the stroke, multiplied by Pi divided by four.
Generally speaking, the larger the bore and stroke of an engine, the greater its displacement and horsepower. With technological advances, however, this correlation has become less absolute as variable displacement engines, which can turn entire cylinders on and off to provide greater performance or economy, have become more prevalent. Barring variable displacement, piston displacement can generally be used to estimate an engine's power and economy, and is particularly useful in comparative situations.
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