The measurement of an engine’s bhp involves the  measurement of a quantity known as torque or twisting  moment. Torque is the product of a force and the distance  of the force from the axis about which it acts, or stroke in feet gives the work performed in one power stroke,  which, multiplied by the number of power strokes per minute,  gives the number of ft-lb per minute of work produced by one  cylinder. Multiplying this result by the number of cylinders  in the engine gives the amount of work performed, in ft-lb,  by the engine. Since hp is defined as work done at the rate  of 33,000 ft-lb per minute, the total number of ft-lb of work  performed by the engine is divided by 33,000 to find the  indicated horsepower. Torque is a measure of load and is properly expressed in  pound-inches (lb-in) or pound-feet (lb-ft). Torque should not  be confused with work, which is expressed in inch-pounds  (in-lb) or foot-pounds (ft-lb). Given: There are numerous devices for measuring torque, such as  a dynamometer or a torque meter. One very simple type of  device that can be used to demonstrate torque calculations is  the Prony brake.  [Figure 1-40]  All of these torque-measuring  devices are usable to calculate power output of an engine on  a test stand. It consists essentially of a hinged collar, or brake,  which can be clamped to a drum splined to the propeller  shaft. The collar and drum form a friction brake, which can  be adjusted by a wheel. An arm of a known length is rigidly  attached to or is a part of the hinged collar and terminates  at a point that rests on a set of scales. As the propeller shaft  rotates, it tends to carry the hinged collar of the brake with it  and is prevented from doing so only by the arm that rests on  the scale. The scale indicates the force necessary to arrest the  motion of the arm. If the resulting force registered on the scale  is multiplied by the length of the arm, the resulting product  is the torque exerted by the rotating shaft. For example, if  the scale registers 200 pounds and the length of the arm is  3.18 feet, the torque exerted by the shaft is: Once the torque is known, the work done per revolution of  the propeller shaft can be computed without difficulty by  the equation: The indicated horsepower calculation discussed in the  preceding paragraph is the theoretical power of a frictionless  engine. The total horsepower lost in overcoming friction must  be subtracted from the indicated horsepower to arrive at the  actual horsepower delivered to the propeller. The power  delivered to the propeller for useful work is known as brake  horsepower (bhp). The difference between indicated and  brake horsepower is known as friction horsepower, which  is the horsepower required to overcome mechanical losses,  such as the pumping action of the pistons, the friction of the  pistons, and the friction of all other moving parts. If work per revolution is multiplied by the rpm, the result is  work per minute, or power. If the work is expressed in ft-lb  per minute, this quantity is divided by 33,000. The result is  the brake horsepower of the shaft.  1-32 2πr × force on the scales (lb) × length of arm (ft) × rpm 33,000 Power = Work per revolution × rpm and bhp =  Work per revolution × rpm  33,000 Work per revolution = 2π × torque 200 lb × 3.18 ft = 636 lb-ft Torque = force × distance (at right angles to the force) Example Indicated mean effective pressure (P)  = 1.65 lb/in 2 Stroke (L)  = 6 in or .5 ft Bore = .5 in rpm  = 3,000 No. of cylinders (K)  = 12 Indicated hp =  PLANK 33,000 ft-lb/min Find indicated hp. Brake Horsepower A = ¼πD 2 A = ¼ × 3.1416 × 5.5 in × 5.5 in = 23.76 in 2 A is found by using the equation N is found by multiplying the rpm by ½: N = ½ × 3,000 = 1,500 rpm Indicated hp = 1.65 lb/in$^{2}$ × .5 in × 23.76 in$^{2}$ × 1,500 rpm × 12  33,000 ft-lb/min Indicated hp = 1069.20 Now, substituting in the formula: