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WORKING OF FOUR STROKE CYCLE ENGINE (S. I ) Engine :

 In four- stroke cycle Engine, one working cycle is completed in four stroke of the piston or two revolution of  the crankshaft. hence, it is called as four stroke engine. In 4 - stroke engine 2 valves are placed instead of  port as that of  two stroke engines. These are termed as inlet and exhaust valves.

It consists of four strokes 

1. Suction Stroke 

2. Compression Stroke 

3. Power or expansion Stroke 

4. Exhaust Stroke



I) SUCTION STROKE : 

    At the beginning of  the stroke , the piston is at the top most position (TDC) and is ready to move  downward . As the piston moves downwards , vacuum will create inside the cylinder. Due to this  vacuum , air fuel mixture from the carburetor is sucked into the cylinder through inlet valves till the piston reaches Bottom most position (BDC). During the suction stroke, exhaust valve remains in closed  condition  and inlet valve remains open. At the end of the suction stroke , the inlet valve will be closed. 


II) COMPRESSION STROKE :

    During the compression stroke, both the inlet and exhaust valves are in closed condition and the piston moves upward from BDC to compress the air fuel mixture. This process will continue till the piston   reaches TDC. The compression ratio of engine varies from 6 to 8. The pressure at the end of  compression is about 600 to 1200 kN/m2 .

    The temperature at the end of  the compression is 250 to 300*C. At the end of this stroke, the mixture is ignited by spark plug. 

    It leads to increase in pressure and temperature of the mixture instantaneously.  


III) POWER OR EXPANSION STROKE : 

    Both the pressure and temperature range of  the ignited mixture are 1800 to 2000 * C and 3000 to 4000kN / m2 respectively. During the expansion stroke, both the valves are remains closed. The rise in  pressure of  the mixture exerts an impulse on the piston and pushes it downward. There fore the piston   moves from TDC  to  BDC. This Stroke is known as Power Stroke. 


IV) EXHAUST STROKE : 

    During the exhaust stroke, the piston moves from BDC to TDC, the exhaust valve is opened and inlet valve is closed. The burnt gases are released through the exhaust valve when the piston moves upward. As the piston reaches the TDC, again the inlet valves will open and the fresh air fuel mixture enters into the cylinder for the next cycle of operation. 

   It is obvious from the above operations; only one power stroke is produced in each and every four  stroke of the piston or two revolution of  the crankshaft. Hence, it is termed as four - stroke engine. 

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