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What is friction power and how it is determined ?

Friction power of  an engine may be defined as the difference between the indicated power and brake power. Two kinds of losses occur in the engine. They are pumping losses and friction losses. The pumping losses occur during suction and exhaust stroke. The friction loss is due to the friction between the piston and cylinder walls , piston rings and cylinder walls, and between the crankshaft and cam shaft and their bearings etc. The friction power can be determined by using, 

Friction power = Indicated power - Brake Power 

                F.P    = I.P    -   B.P 

The friction power of an I.C Engine can be determined by the following methods : 

i) Willan's line method

      In Willan's line method , the gross fuel consumption is plotted against B.P and the line so obtained is extended backward to zero fuel consumption. The negative intercept on the B.P. axis gives the value of the friction power (F.P.). This negative work represents the combined loss due to mechanical friction, pumping etc,



 

ii) Morse Test 

Morse test is a method of obtaining approximate indicated power (I.P) of a multi-cylinder engine. This method is used for both S.I (petrol) and C.I (diesel) engine.

In this method each cylinder is made inoperative one by one.

Cylinder is made inoperative -

  • In diesel- by cutting off the supply of fuel to each cylinder.
  • In petrol- by shorting the spark plug.

When one cylinder is made inoperative brake power obtained is with one cylinder less ,while friction power is constant.

iii) Motoring Test 

The engine is started with help of an external motor called motoring method. If the power consumed by this electric motor is corrected for mechanical and other losses, it will give the FHP of the engine. The main criticism is no actual firing. It differs from the firing condition and also the FHP value obtained different.

Advantages : 

1. Successive "stripping off" the engine occurs

2. The overall engine friction can be accurately obtained. 

iv) Deceleration Method 

In this method, a running engine is left free after cutting off fuel supply. By this, deceleration takes place due to effect of friction. This deceleration can be measured in the form of polar moment of inertia of the engine because FHP is the product of polar moment of inertia and initial deceleration.

v) Difference between H.P and B.P. 

Usually, F.H.P is finding by differentiating IHP obtained from indicator diagram and BHP obtained from a dynamometer. 

Limitations :

Due to some difficulties in obtaining accurate indicator diagrams especially at high - speed engines , this method is mainly used in research laboratories.

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