4 Stroke Engine (Spark Ignition)
The modern petrol 4 stroke engine operates on Otto (constant volume) cycle. 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 an Otto cycle, spark ignition 4 stroke engine, the 4 strokes are as follows:
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 motor. The inlet valve remains open and the exhaust valve is.closed during this stroke. The downward movement of the piston sucks the air-fuel mixture in the cylinder from the carburetor 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 the 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 a more homogeneous mixture of air and petrol inside the cylinder. The heat makes the petrol easier to burn while the compression forces it into a 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 the top dead center. Both the inlet and exhaust valves remain closed during the compression stroke.
Working, Power or Expansion Stroke
The expansion of the gases due to the heat of combustion exerts a pressure on the cylinder and piston. By this impulse, the piston moves downward thus doing reciprocating work. Both the valves remain closed during this stroke.
During this stroke, the inlet valve remains closed and the exhaust valve opens. The major part of the burnt gases escapes due to their own expansion. The piston then moves upward and pushes the remaining gases out of the exhaust valve. Only a small quantity of exhaust gases remains in the clearance space which will dilute the fresh incoming charge.
Thus, in this type of engine, four strokes of the piston are required to complete the cycle, and the four strokes make two revolutions of the crankshaft. The operations are repeated over and over again in running the engine. Each revolution of the crankshaft has one power stroke.