The radius of a circle is equal to ½ the diameter: is approximately horizontal. The amount of this force would  depend upon the roughness of the ground. If the weight were attached to a spring scale graduated in  pounds, then dragged by pulling on the scale handle, the  amount of force required could be measured. Assume that  the force required is 90 lb, and the 100-lb weight is dragged  660 feet in 2 minutes. The amount of work performed in the  2 minutes is 59,400 ft-lb or 29,700 ft-lb per minute. Since 1  hp is 33,000 ft-lb per minute, the hp expended in this case is  29,700 divided by 33,000, or 0.9 hp. Compute the piston displacement of the PWA 14 cylinder  engine having a cylinder with a 5.5 inch diameter and a 5.5  inch stroke. Formulas required are: When other factors remain equal, the greater the piston  displacement, the greater the maximum horsepower an engine  is capable of developing. When a piston moves from BDC to  TDC, it displaces a specific volume. The volume displaced by  the piston is known as piston displacement and is expressed  in cubic inches for most American-made engines and cubic  centimeters for others. The piston displacement of one cylinder may be obtained  by multiplying the area of the cross-section of the cylinder  by the total distance the piston moves in the cylinder in one  stroke. For multicylinder engines, this product is multiplied  by the number of cylinders to get the total piston displacement  of the engine. Since the volume (V) of a geometric cylinder equals the area  (A) of the base multiplied by the height (h), it is expressed  mathematically as: The area of the base is the area of the cross-section of the cylinder. To find the area of a circle, it is necessary to use a number  called pi (π). This number represents the ratio of the  circumference to the diameter of any circle. Pi cannot be  stated exactly because it is a never-ending decimal. It is  3.1416 expressed to four decimal places, which is accurate  enough for most computations. The area of a circle, as in a rectangle or triangle, must be  expressed in square units. The distance that is one-half the  diameter of a circle is known as the radius. The area of any  circle is found by squaring the radius (r) and multiplying by  π. The formula is: From this point on, the calculations are identical to the  preceding example. All internal combustion engines must compress the fuel/air  mixture to receive a reasonable amount of work from each  Another method of calculating the piston displacement uses  the diameter of the piston instead of the radius in the formula  for the area of the base.  Rounded off to the next whole number, total piston  displacement equals 1,829 cubic inches. Substitute values into these formulas and complete the  calculation. Example Piston Displacement Compression Ratio Area of a Circle 1-29 A = πr 2 A =  1 ⁄ 4 (π) (d $^{2}$) Substituting A =  1 ⁄ 4 × 3.1416 × 5.5 in × 5.5 in A = 0.7854 × 30.25 in 2 A = 23.758 in 2 V = A × h r  = d 2 A = πr 2 V = A × h Total V = V × n (number of cylinders) r  = d 2 r  = d = 5.5 inches (in) ÷ 2 = 2.75 in 2 A = πr $^{2}$ = 3.1416 (2.75 in × 2.75 in) A = 3.1416 × 7.5625 square inches (in$^{2}$) = 23.7584 in 2 V = A × h = 23.7584 in$^{2}$ × 5.5 in = 130.6712 cubic inches (in$^{3}$) Total V = V × n = 130.6712 in$^{3}$ × 14  Total V = 1829.3968 in 3