How does petrol engine works ?

Four stroke petrol engine

                          Four stroke cycle petrol engines work on Otto cycle. the suction, compression, power and exhaust are completed in four strokes of the piston (or) two revolutions of the crank shaft. the main components are (1) cylinder block and head  (2) piston assembly  (3) connecting rod  (4) crank shaft  (5) valves and  (6) crank.

1. Suction stroke

• piston moves downwards from T.D.C creating a partial vacuum inside the cylinder.
• inlet valve is open and the exhaust valve is closed.
• fuel air mixture is sucked into cylinder due to the partial vacuum created in the cylinder.
• the crankshaft moves through one half of a revolution
• 
  

       2. compression stroke

• piston moves upwards from B.D.C
• both intake and exhaust valves are closed.
• drawn in fuel air mixtures compressed into the combustion chamber to a pressure of 5-10 kg/cm and temperature of 600-700 celcius, this temperature being less than the self ignition temperature of fuel.
• the crankshaft moves through another one half of a revolution.  

   3. power stroke

• At the end of compression when the piston reaches the T.D.C, a spark is provided by the spark plug, which ignited the compressed fuel air mixture. the pressure and temperature rise to 35 kg/cm and 2200 celcius.
• rapidly expanding burning gases, thrust the piston downward from T.D.C to B.D.C.
• both intake and exhaust valves are closed.
• The downward movement of piston is transmitted to the crankshaft connecting rod.
• now, the crankshaft has completed one and the half.

    4.Exhaust valve 

• piston moves upwards from B.D.C.
• exhaust valve is open while the intake valve closed.
• burnt gases are escaped from the cylinder through the exhaust valve.
• now the crank shaft has completed in two revolution of crank shaft.

Thus the cycle is completed in two revolution of crank shaft. four stroke petrol engines are used in HERO HONDA, YAMAHA, AMBASSADOR, HYUNDAI SANTRO, TATA INDICA etc.

Two stroke petrol engine

            In two stroke petrol engine the cycle of operations, viz. suction, compression, power and exhaust can be completed in two stroke of the piston (or) in one revolution of the crank shaft.

        

the main engine components are (1) piston with a deflector, (2) cylinder block with port - intake, transfer and exhaust port (3) crank case (4) connecting rod, (5) crankshaft and flywheel (6) a carburetor, (7) intake and exhaust manifolds.

working

1. upward stroke

• piston travels upwards from B.D.C to T.D.C closing exhaust port and uncovers the inlet port
• fresh change of fuel air mixture enters into the crank case from the carburetor through the inlet port due to vacuum produced in it.
• Transferred in fuel air mixture is compressed into the combustion chamber.
• thus, in one stroke of the piston, two operation viz suction and compression are carried out.
• The crankshaft moves through one half of  revolution.

2.Downward stroke

• At the end of compression, ignition takes place by means of a spark supplied by the spark plug.
• Piston receives power impulse from the rapidly expanding gases and move downwards from T.D.C to B.D.C. 
• Fresh charge of fuel air mixture which had entered into the crank case during upward stroke is pump into the cylinder through the transfer port, by the downward movement of piston.
• The piston uncovers the exhaust port and allows burnt gases go out of the engine.
• When the piston reaches the B.D.C, the pre compressed fuel air mixture travels the crank case to the combustion chamber through the transfer port.
• The fresh fuel air mixture is fed the combustion chamber with help of a deflector on the piston head.
• The deflector also allows explosion of exhaust gases by the fresh fuel air mixture. this process is known as scavenging.

We conclude that during the downwards stroke two operations, viz. power and exhaust are completed. The crankshaft moves through the other half of a revolution. 

Thus the four cycles of operations, compression, power and exhaust are completed in one revolution of the crank shaft.

  

                    Two stroke petrol engines are used in mopeds, scooters, motor cycles lawn movers.