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Timing diagram and Power card of 4 stroke cycle

  • The timing diagram shows the closing and opening of the valves. The working cycle is illustrated as a ‘P - V’ diagram (pressure-volume). The line ‘l – l’ represents atmospheric line. The piston is considered to have just moved over the ‘top dead centre’ and is on its way down. The air inlet is already open and because of the partial vacuum created when the piston moves towards its bottom position, fresh air is sucked into the cylinder. This process is represented in the 'p-v' diagram by the line ‘1-2’ which is termed suction line. This movement of the piston is called 'Suction Stroke".
  • After the piston has moved over bottom dead centre, the suction valve closes and the volume of air in the cylinder is compressed during the course of the up stroke of the piston. This is represented by the line ‘2-3’ in the above diagram and termed as compression line. This movement of piston is compression stroke.
  • The ignition takes place at point 3 and combustion continues for the duration of fuel injection, ending at point 4. After this combustion products expand to point 5 when the exhaust valve opens. Power is produced between point ‘4 – 5’.
  • The pressure drops in the cylinder to the exhaust line from 5 to 6. The exhaust valve remains open till after piston passes over the top dead center. The combustible gases are expelled. The line 6 to 1 represents this. The pressure is slightly above atmosphere, because of the resistance in the exhaust pipe. This stroke is 'exhaust stroke'.
  • A 4-stroke engine requires two complete revolutions of the crankshaft to finish working cycle.
  • This means inlet, exhaust & fuel valve must only function once for every two revolutions of the crankshaft.
  • In order to activate those valves in the correct sequence, it is necessary to operate them from a shaft, which rotates at half the speed of the crankshaft. This is called camshaft.


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