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PRESSURE-VOLUME DIAGRAM OF FOUR-STROKE, OTTO CYCLE ENGINE

Hello friend's in this post we will talk about pressure - volume diagram of four - stroke, otto cycle engine

Fig. 3.8 shows theoretical p-v diagram of a four-stroke Otto cycle engine. During the suction stroke, the charge is admitted in the cylinder increasing the volume from 1 to 2, at the atmospheric pressure. Cylinder volume is the sum of clearance volume and stroke volume. During the compression stroke, the charge is compressed from 2 to 3. Volume decreases to clearance volume and pressure rises. The compressed charge is ignited at constant volume, which increases the pressure as represented by 3- 4 line. During the working stroke the volume increases and pressure decreases as represented by 4-5line. At 5, the exhaust valve opens and exhaust takes place at constant decreases the pressure suddenly as represented by 5-2 line. Finally the piston moves onward rushing out the exhaust gases at atmospheric pressure as represented by 2-1 line. The volume again decreases to clearance volume 1 as at the beginning of the olume, which suction stroke. 
It is to be noted that in this theoretical p-v diagram, every corner is sharp which represents opening and closing of the valve instantaneously. Also, the suction and exhaust take place at atmospheric pressure.
Actually, the opening and closing of valves cannot take place instantaneously but take sometime, by which every corner in the p-v diagram will be round, as shown in actual p-v diagram. (Fig. 3.9). Also, the suction takes place at a pressure slightly lower than the atmospheric pressure due to the resistance of the inlet valve to the entering charge. The exhaust takes place at a pressure slightly higher than the atmospheric pressure due to the resistance to the exhaust valve to the exhaust gases. This gives an area, in the form of a small loop, and indicates what is called the pumping loss of the engine. This area is treated as negative and hence substracted from the area of the larger loop which is treated as positive, giving us the network done during the cycle.

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