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        In the steam or I.C. engines, the fluctuations of turning moment exerted on the crankshaft are reflected in fluctuations of the speed of the crankshaft.

  • The flywheel is used to control the fluctuations in speed during each cycle of an engine.
  • lt a rotating mass which 1s attached to the crankshaft and used as an energy reservoir.
  • A flywheel stores energy when the supply of energy in an engine is more than the requirement, and it releases energy during the periods in which the requirement is more than the supply.
  • Thus, the flywheel makes it possible to get uniform distribution of available energy through the cycle as per the load requirement and hence reduces the fluctuations in the speed of the engine.
  1. Turning Moment Diagram: –
  2. Turning Moment diagrams drawn on cartesian co-ordinates are known as rectangular turning moment diagram
  3. In the turning moment diagram, the turning moment (torque) is plotted along the vertical axis (ordinate) and the crank angle is plotted on the horizontal axis (abscissa)
  4. These diagrams are also known as T-0 diagrams or crank effort diagrams.
  1. Turning Moment Diagram for an Internal Combustion Engine: –

       The indicator diagram for Single cylinder four stroke petrol engine is shown in Fig.1.1

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Single cylinder four stroke petrol engine

The various strokes of the engine are: –  

  • Suction: The piston draws the mixture of fuel and air during its outward s stroke The pressure inside the cylinder is less than the atmosphere as represented by the curve ab.
  • Compression: During the next stroke the piston compresses the mixture up to the clearance volume of the cylinder as shown by the curve bc
  • Expansion: At the end of compression the mixture is ignited and the combustion takes place nearly at constant volume. The pressure and temperature of the gas rise to d and then it its outward stroke up to e. The work is done by the gases on the piston
  • Exhaust: Burnt gases are expelled into the atmosphere during the inward stroke of the piston. The turning moment on the crankshaft for different crank angles can be calculated in the same way as in the case of the steam engine. The only difference is that in a four-stroke IC engine the diagram repeats after two revolutions instead of one revolution.

                   The torque diagram for gas torches 18 is shown by dotted lines in Fig. 1.2 The pressure on the gases during suction stroke is slightly less than atmospheric due to which there is a very small negative loop. During the compression stroke, the negative loop is obtained because the work is done by the piston on the gases. During the expansion stroke, we get a positive large loop since the work is done by the gases on the piston while the work is done on the gases by the On the piston during exhaust stroke due to which we again set a small negative loop. It should be noted that the pressure of the gases inside the cylinder is slightly higher than the atmosphere.

                The torque diagram for inertia forces has been shown by long broken lines. After Superimposing the inertia torque on gas torque, the net turning moment diagram is obtained as shown by firm lines in Fig. 1.2

Fig 1.2
  • Coefficient of Fluctuation of Speed (ks):-

             It may be observed that the change of speed of the flywheel is essential so that the flywheel can store the energy when the energy developed by the engine is more than the demand and give away its stored energy when the demand is more than the energy developed by the engine. However, this change of speed is limited to a small value depending upon the application. Hence, we define the coefficient of fluctuation of speed, ks, as the ratio of the difference between the maximum and minimum angular speeds of the flywheel or crankshaft to its mean angular speed. Mathematically,

                   Ks = w1-w2 /w = N1- N2/N


         W1 = Maximum angular speed of the crankshaft, rad/s

         W2 = Minimum angular speed of the crankshaft,rad/a

          W = Mean angular speed of the crankshaft,  rad/a

          N = Speed in r.p.m

  • The value of k, depends upon the system. For certain applications, the average values of ks are given  below when an engine is used


                        As explained earlier, the function of the flywheel is to absorb energy during the periods of crank rotation when the torque developed by the engine is more than the resisting torque and supply the energy during the periods of crank rotation when the torque developed is less than the resisting torque. Absorption of energy increases the speed of the flywheel and accordingly the coefficient of fluctuation of speed was defined which depends upon the specific applications. To design the size of the flywheel, we can relate the moment of inertia of the flywheel in terms of coefficients of fluctuation of speed and energy as follows:

Let         m = Mass of the flywheel

               K = Radius of gyration of the flywheel


In the preceding section, the function of the flywheel is discussed.

 The flywheel controls the cyclic fluctuations in speed due to fluctuations of energy. The internal combustion engines come across the change in speed due to changes in load which cannot be controlled by the flywheel. When the load on the engine decreases the speed of the engine increases. Similarly, when the load on the engine increases, the speed of the engine decreases.

The variation in speed occurring due to variation in load is controlled by making a variation in fuel supply. This function is achieved by a mechanical device called a governor.

 Thus, the function of the governor is to automatically maintain the speed of an engine within the prescribed limits for varying load conditions.

Types of Governors: – 

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Types of Governors
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Categories: Automobile Engineering, Mechanical Engineering