Displacement is the cylindrical volume that a piston displaces as it moves through one stroke. It is equal to the area of the piston multiplied by the length of the stroke. Displacement is one of the factors that determine the amount of horsepower an engine will produce, the greater the displacement, the greater the volume of air and fuel that is burned during combustion, which means more power. Expressed mathematically, piston displacement is:
where PD = Piston displacement in cubic inches (in3); B = Bore of cylinder (diameter), in inches (in); S = Stroke of piston in inches (in).
The bore and the stroke of an engine has been traditionally expressed as B x S with the dimensions in inches. Thus, if B x S = 3.50 x 4.00, the piston displacement is:
Larger engines are constructed with more than one cylinder. For multicylinder engines, the term engine displacement (ED) is used. Engine displacement is the product of the cylinder displacement times the number of cylinders. Expressed in equation form:
ED = PD x n where ED = Engine displacement (in3); PD = Piston displacement (in3); n = Number of cylinders.
If the engine in the previous example is a four-cylinder engine, then the engine displacement is:
It is a common practice to combine these two equations. For a single cylinder engine the value for n = 1.
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