- 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.
Any opening in a pressure vessel is kept to a minimum and for a man entry an elliptical hole is lesser in size than the corresponding circular hole. More over it is prime concern to have a smoothed generous radius at the corners to eliminate stress concentration. Hence other geometrical shapes like rectangle and square are ruled out. To compensate for the loss of material in the shell due to opening, a doubler ring has to be provided around the opening. The thickness of the ring depends on the axis length along the dirrection in which the stresses are maximum and the thickness of the shell. It is important to align the minor axis along the length of the vessel, as the stress in this direction is maximum. Longitudinal stress: Pd/2t where P= pressure inside the vessel, d= diameter of the arc, t= thickness of the shell plating Circumferential stress: Pd/4t More over a considerable material and weight saving is achieved as minor is along the direction of maximum stress.