Automotive Systems

Formerly Automotive Systems I

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Linear Measurements

LINEAR MEASUREMENTS

The size of an engine cylinder is indicated in terms of bore and stroke (fig. 2-24). Bore is the inside diameter of the cylinder. Stroke is the distance between top dead center (TDC) and bottom dead center (BDC). The bore is always mentioned first. For example, a 3 1/2 by 4 cylinder means that the cylinder bore, or diameter, is 3 1/2 inches and the length of the stroke is 4 inches. These measurements are used to figure displacement.

PISTON DISPLACEMENT is the volume of space that the pistondisplaces, as it moves from one end of the stroke to the other. Thus the piston displacement in a 3 1/2-inch by 4-inch cylinder would be the area of a 3 1/2-inch circle multiplied by 4 (the length of the stroke.) The area of a circle is pR2 , where R is the radius (one half of the diameter) of the circle. With S being the length of the stroke, the formula for volume (V) is the following:

V = pR2 x S

 If the formula is applied to figure 2-22, the piston displacement is computed as follows:

R = 1/2 the diameter = 1/2 x 3.5 = 1.75 in.
p = 3.14
V = p (1 .75) 2  x 4
V = 3.14 x 3.06 x 4
V = 38.43 cu in.

 The total displacement of an engine is found by multiplying the volume of one cylinder by the total number of cylinders.

38.43 cu in. x 8 cylinders = 307.44 cu in.

The displacement of the engine is expressed as 307 cubic inches in the English system. To express the displacement of the engine in the metric system, convert cubic inches to cubic centimeters. This is done by multiplying cubic inches by 16.39. It must be noted that 16.39 is constant.

307.44 cu in. x 16.39 = 5,038.9416 cc

To convert cubic centimeters into liters, divide the cubic centimeters by 1,000. This is because 1 liter = 1,000 cc.

5,038.9416/ 1,000  = 5.0389416

The displacement of the engine is expressed as 5.0 liters in the metric system.

Figure 2-24.—Bore and stroke of an engine cylinder.
Published by SweetHaven Publishing Services
Based upon a text provided by the U.S. Navy

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