The modern petrol (gasoline) engines operate on Otto (constant volume) cycle as shown in Figure.

This cycle-was introduced in practical form by a German scientist Otto in 1876, although it was described by a French scientist Beande Roches in 1862.

In a four stroke otto cycle spark-ignition engine the four strokes are as follows:

(1) Suction Stroke : 

During suction stroke, the piston is moved downward by the crankshaft which is revolved either by the momentum of the flywheel or by the power generated by the electric starting motors The inlet valve remains open and the exhaust valve is closed during this stroke.


The downward movement of the piston sucks air fuel mixture in the cylinder from the carburettor through the open inlet valve. Here the fuel is petrol mixed with air broken up into a mist and partially vaporized in the carburetor.


During compression stroke the piston moves upward, thus compressing the charge. Ignition and much of the compression also take place during this stroke. The heat produced by the compression makes more homogeneous mixture of air and petrol inside the cylinder.

The heat makes the petrol easier to burn while the compression forces it into closer combination with the air The mixture under compression is ignited by the spark produced by a spark plug and the combustion is about half completed when the piston is at top dead centre. Both the inlet and exhaust valves remain closed during the compression stroke.



The expansion of the gases due to the heat of combustion exerts a pressure on the cylinder and piston. Under this impulse, the piston moves downward thus doing useful work. Both the valves remain closed during this stroke.


During this stroke, the inlet valve remains closed and the exhaust valve opens The greater part of the burnt gases escapes because of their own expansion. The piston moves upward and pushes the remaining gases out.


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