High Turbulence Combustion Chambers

Small cylinders with hemispherical and pentroof type combustion chambers are now more commonly used in SI engines.

• Small cylinder engines can be operated at higher speeds whicht increases turbulence and tends to reduce HC emissions..
• Smaller cylinders have smaller amount of burned gases that form the high temperature adiabatic core. More heat transfer takes place from the burned gases as the walls are nearer to the bulk gases. It results in lower NO_x.
• The compact hemispherical combustion chambers shape (Fig 5.2a) provides the lowest surface to volume ratio and minimum tendency to engine knock.

The hemispherical combustion chamber although may employ multiple valves,  the two valve configuration is more common as it is difficult to accommodate 4-valves at the necessary valve positioning angles. The valve heads along with the surface of combustion chamber form a profile that looks like a hemisphere. Both the intake and exhaust valves are inclined increasing valve port area that results in higher volumetric efficiency. The chamber has a low surface to volume ratio.  The intake ports are provided with a suitably curved geometry to generate high rate of air swirl.