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BRAKE POWER (BP) and Types of Brakes used in I.C Engines

 Brake Power (BP) :

 Brake Power is the useful power available at crankshaft . It is always lesser than indicated power. The brake power of an I.C Engine is usually measured by means of brake mechanism. Brake Power measurement involves the determination of the torque and the angular speed of the engine output shaft. The torque measuring device is called a dynamometer. the term brake and dynamometer means one and the same. A dynamometer is also a brake except the measuring devices are included to indicate the amount of force required in attempting to stop the engine.

There are following two types of brakes used in I.C Engines for measuring B.P ,

a) Prony Brake : 

It consists of a frame with two brake shoes gripping the flywheel. The pressure of the brake shoes on the flywheel can be varied by spring loaded nuts on the top of the frame. 

A loaded bar extends from the top of the brake and a load carrier is attached to the end of the load bar. Weight kept on this load carrier is balanced by the torque reaction in shoes. During the test, whole power is absorbed and converted into heat. Hence, This type of dynamometer must be cooled. 




Torque is given by T = WR (kNm) 

       Brake Power Bp = 2 * 3.14 * NWR (kW)

       Where ,         W = Net load on brake (kN)

                             R = Effective radius of brake drum (m)

                             N = Speed of the engine in r.p.s 

b) Rope Brakes :

 Brake power can also be measured by rope brake. It is also one type of mechanical dynamometer. The arrangement of the rope brake system. It consists of a number of turns of rope wound around the rotating drum attached to the output shaft. One end of the rope is attached to spring balance. The other end is attached to load. The power is absorbed is due to friction between rope and the drum. To avoid friction between rope and wheel, it is water - cooled. This method is mainly used for slow speed engines. 




Let D1 = Diameter of the brake drum is m,

      d1  = Diameter of the rope or thickness of band in m,

      W1 = Weight applied in kN. 

      W2 = Spring Balance reading in kN. 

       N   = Speed of the engine in rps. 

The effective diameter of the brake drum D = D1 + d1 in m. 

            Effective Brake Radius    , R      = D / 2 = (D1 + d1 ) / 2

            Net Brake load ,                  W     = W1 - W2 in kN.

            Brake Torque        ,              T      = WR kN-m ,

Brake Power                                = 2 (3.14) ( NT)  = 2 (3.14) N W R 

                                                     = (3.14) DNW in kW ( 2R = D ) 

If  the brake mean effective pressure is given , the

 Brake Power , BP         = Pmb  l a n k in kW

Where , Pmb                   = Brake mean effective pressure in kN / m2 . 

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