Working of a Four-stroke Engine: To understand the working of a four-stroke motorcycle engine, the whole operation can be divided into four parts, known as strokes. The strokes are named after their functions such as, Suction stroke, Compression stroke, Power or Expansion stroke and Exhaust stroke. Hereunder, the line diagram is showing all the four strokes of operation of an engine.

Note : The direction of arrows  indicates the direction of movement of piston.

Suction Stroke

The pressure decreases from atmospheric to some lower value inside the cylinder. This happens because of downward piston movement. Because of the pressure difference of outside and inside the cylinder, the charge, which is mixture of air and fuel vapour, is sucked into the cylinder.

Compression Stroke

The charge is compressed during this stroke. The compression enhances the fuel air mixing and makes it closer to perfect mixture, and then a sparking takes place at the end of compression stroke.

Power Stroke

In the beginning of this stroke, the spark causes combustion and the gases expand, exerting pressure on the piston. By the virtue of expansion of gases the pressure increases inside the cylinder on the top of the piston. Because of pressure exerted on the piston, piston moves downward and rotates the crankshaft via connecting rod.

Exhaust Stroke

At the end of the power stroke the exhaust valve opens. The piston moves upward and pushes the burnt gases out.

Even though reciprocating internal combustion engines look quite simple, yet these are highly complex machines. These have hundreds of components, which have to perform their functions satisfactorily to produce power. Before going through the working principle of this complex machine, a brief description of the important engine components and the nomenclature.