Working of Four Stroke Petrol Engine.

 Construction And Working of Four Stroke Petrol Engine.

        In four stroke cycle engine, cycle is completed in two revolutions of crankshaft or four stroke (operation) of piston.

            Each stroke consists of 1800 of crankshaft rotation. Therefore, the cycle consists of 7200 of crankshaft rotation.

Working of four stroke petrol engine.

    Cycle consists of following four strokes:

1. Suction/Intake Stroke
2. Compression Stroke
3. Expansion/Power/combustion Stroke
4. Exhaust Stroke.

    We explain them in detailed below.      

This GIF shows how strokes can be worked.

   

• Suction/Intake Stroke:-

            Suction stroke starts when piston is top dead center and about to move downwards. During suction stroke inlet valve is open and exhaust valve is close.

             Due to low pressure created by the motion of piston towards bottom dead centre, the charge consisting of fresh air mixed with the fuel is drawn into cylinder. At the end of suction stroke the inlet valve closes.

• Compression Stroke:-

              During compression stroke, the compression of charge takes place by return stroke of piston, i.e. when piston moves from bottom dead centre to top dead cenre.

              During this stroke, both inlet and exhaust valve remain closed. Charge which is occupied by the whole cylinder volume is compressed upto the clearance volume. Just before completion of compression stroke, a spark is produced by the spark plug and fuel is ignited. Combustion takes place when the piston is almost at top dead centre.

                Thus, combustion of fuel takes place due to production of spark, therefore engine is called as 'spark ignition engine'.

• Expansion/Power/combustion Stroke:-

             Piston gets downward thrust by explosion of charge. Due to high pressure of burnt gases, piston moves downward to the bottom dead centre.

           During expansion stroke, both inlet and exhaust valve remain closed. Thus, power is obtained by expansion of products of combustion. therefore it is called as 'power stroke'.

             Both pressure as well as temperature decreases during expansion stroke.

• Exhaust Stroke:-

               At the end of expansion stroke, the exhaust valve open, the inlet valve remains closed and the piston moves from bottom dead centre to top dead centre.

            During exhaust stroke the burnt gases inside the cylinder are expelled out. The exhaust valve closes at the end of the exhaust stroke but still some residual gases remains in cylinder.

            Each cycle of four stroke engine completes the above four strokes or operation in two crank revolution.

            One revolution of crank shaft occurs during the suction and compression strokes and second revolution during the power and exhaust stroke.

           Thus, for one complete cycle i.e. for two revolutions of crankshaft, there is only one power stroke.

                Example of four stroke engine is passenger car.