Engine displacement, or piston displacement, is the volume moved by an engine’s pistons in one revolution. It’s directly proportional to an engine’s power and is calculated based on the number of cylinders, bore, and stroke. The larger the bore and stroke, the greater the displacement and horsepower. However, variable displacement engines have become more prevalent, making the correlation less absolute. Piston displacement is measured in liters, cubic centimeters, or cubic inches and is often used in model nomenclature. Compression ratio, which measures a combustion chamber’s capacity, is closely related to piston displacement and is considered a desirable trait for engines.
Engine displacement, also known as 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. Expressed in volumetric units such as liters, cubic centimeters, or cubic inches, piston displacement is directly proportional to the total power generated by an engine. It is calculated according to an equation that takes into account various engine characteristics, including the number of cylinders, bore, and stroke.
The bore is the internal diameter of the hollow cylinders that house the pistons. Stroke is the distance a piston moves in the course of one revolution. With this in mind, the mathematical formula for displacement is: number of cylinders times bore squared, times stroke, times Pi divided by four.
Generally speaking, the larger an engine’s bore and stroke, 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 increased performance or economy, have become more prevalent. Except for variable displacement, piston displacement can generally be used to estimate an engine’s power and economy, and is particularly useful in comparative situations.
The units in which piston displacement is measured have undergone a historic change, from cubic inches in the muscle car era of the 1960s and 1970s, to metric units (liters and cubic centimeters) today. Automakers have traditionally incorporated piston displacement into model nomenclature, marketing performance models in particular by emphasizing the size of their engines. Famous examples of this technique include Ford’s 427 cubic inch engine, which was an eight-cylinder V8 configuration engine featured in much of Ford’s product lineup throughout the 1960s, and the 5.0 V8. liters that was an outlet on Ford Mustangs in the late 1980s and early 1990s.
In addition to its standalone utility as a measure of capacity, piston displacement is also closely related to another common automotive measurement called the compression ratio. Compression ratio is the comparison of a combustion chamber’s largest capacity to its smallest capacity, from the top of a piston’s stroke to the bottom. Bore and stroke are again integral components in the formula for compression ratio. A higher ratio means that an engine can generate more mechanical power from a given amount of fuel-air mixture. Unlike piston displacement, which can be subjectively assessed based on an engine’s potential usage, a high compression ratio is almost always considered a desirable trait.
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