It is most important to perform the engine as per our requirement.The performance of engine depends on mean effective pressure and rotational speed.Since mean effective pressure is limited by fuel and strength of materials,the only way to have higher performance is to have higher rotational speed.
In high performance engines it is usual to open the valves earlier and close them later so that the opening time is more thus ensuring the maximum amount of fresh mixture at higher rotational engine speed and greatest possible clearance of exhaust gases.
In some unsupercharged racing cars inlet valve opens as early as 25 degree before top dead centre and closes 60 degree after bottom dead centre.Exhaust valve opens 55 before bottom dead centre and closes 25 degree after top dead centre.
Thus we see that the inlet and exhaust valves open and close in four stroke Otto cycle engine as follows,
01) Inlet valve opens 10 to 30 degree in advance of top dead centre position.Closes 30 to 60 degree after bottom dead centre.
02) Exhaust valve opens 30 to 60 degree in advance of the bottom dead centre position.Closes 8 to 25 degree after the top dead centre position.
It has been found practically that the actual maximum pressure in such engine is only about half of the theoretical value,because the inlet and outlet valves do not open and close suddenly but take some time to operate.
Because of the resistance of the inlet valve of the entering charge,the actual pressure inside the cylinder during the suction stroke is slightly less than atmospheric pressure.Similarly because of the resistance of exhaust valve to exhaust gases leaving the cylinder,the actual pressure inside the cylinder during exhaust stroke is slightly higher than atmospheric pressure.
Due to this there are some losses occur which is called as "Pumping Loss".This loss is treated as negative and therefore subtracted from the positive work done,thus we get net work done during the cycle.
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