For proper mixing of fuel and air in the combustion chamber the various methods of air movement are employed called air swirl. Various types of air swirl being discussed below.

Induction Swirl

In this method the flow of air is directed in desired direction through a port tangential to the piston or by masking or shrouding the intake valve head or by squish during the compression stroke. This method is generally employed in case of open or direct type combustion chamber.

Fig. 1(a) illustrates a tangential port method of induction swirl.

Fig. 1(b) illustrates the method of producing air swirl by masking or shrouding one side of the inlet valve, permitting the air flow only around the part of periphery of the valve.

The angle of mask used usually varies 90o to 140o. The best tangential direction of air movement can be obtained by turning the valve around its axis.

Since such a method imposes a restriction on area of flow, it reduces the volumetric efficiency.

Fig. 1(c) illustrates the method of producing air swirl by casting a lip on one side of the inlet valve.

Though it has been established that the air motion induced by the above method during suction stroke persist even during the compression stroke but these air movements are not enough for proper mixing of fuel and air.

Therefore, it is further augmented by squish method illustrated in Fig. 1(d).Squish is the flow of air from periphery to the centre of the cylinder into the combustion recess by squeezing it out from between the piston and cylinder head towards the end of compression stroke.

Fig.1. Methods of induction swirl

Compression Swirl.

In this method the air is forced through a tangential passage into a separate swirl chamber during compression stroke. The method is used in swirl combustion chambers.

The compression swirl method is illustrated in Fig. 2. During the compression stroke the air is forced into the chamber and during the expansion or power.

Stroke the burnt gases pass out to the main combustion chamber through the throat of swirl chamber resulting into excessive turbulence and better combustion.

Fig.2. Compression swirl

Combustion Induced Swirl.

In this method a small pressure rise is obtained by combustion of fuel and air in a separate cell and then it is forced through a small hole at high velocity to create swirl turbulence. This method is employed in precombustion and air cell combustion chamber designs.