A portion of this waste heat may be channeled tothe passenger compartment to heat it.The lubricating oil in the engine removes aportion of the waste heat.In addition to energy lost through waste heat, thereare the following inherent losses in the piston engine.Much energy is consumed when the piston mustcompress the mixture on the compression stroke.Energy from the fuel is consumed to pull theintake mixture into the cylinder.Energy from the fuel is consumed to push theexhaust gases out of the cylinder.The combination of all these factors in a pistonengine that uses and wastes energy leaves the averageengine approximately 20 to 25 percent thermallyefficient.LINEAR MEASUREMENTSThe size of an engine cylinder is indicated in termsof bore and stroke (fig. 2-24). BORE is the insidediameter of the cylinder. STROKE is the distancebetween top dead center (TDC) and bottom dead center(BDC). The bore is always mentioned first. Forexample, a 3 1/2 by 4 cylinder means that the cylinderbore, or diameter, is 3 1/2 inches and the length of thestroke is 4 inches. These measurements are used tofigure displacement.PISTON DISPLACEMENT is the volume ofspace that the pistondisplaces, as it moves from one endof the stroke to the other. Thus the piston displacementin a 3 1/2-inch by 4-inch cylinder would be the area of a3 1/2-inch circle multiplied by 4 (the length of thestroke.) The area of a circle isR^{2}, where R is the radius(one half of the diameter) of the circle. With S being thelength of the stroke, the formula for volume (V) is thefollowing:V =R^{2} x S= 3.14V =(1 .75)^{2} x 4V = 3.14 x 3.06 x 4V = 38.43 cu in.If the formula is applied to figure 2-22, the pistondisplacement is computed as follows:R = 1/2 the diameter = 1/2 x 3.5 = 1.75 in.Figure 2-24.—Bore and stroke of an engine cylinder.The total displacement of an engine is found bymultiplying the volume of one cylinder by the totalnumber of cylinders.38.43 cu in. x 8 cylinders = 307.44 cu in.The displacement of the engine is expressed as 307cubic inches in the English system. To express thedisplacement of the engine in the metric system, convertcubic inches to cubic centimeters. This is done bymultiplying cubic inches by 16.39. It must be noted that16.39 is constant.307.44 cu in. x 16.39 = 5,038.9416 ccTo convert cubic centimeters into liters, divide thecubic centimeters by 1,000. This is because 1 liter =1,000 cc.5,038.9416 = 5.03894161,000The displacement of the engine is expressed as 5.0 litersin the metric system.2-18

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