Cylinder heads are designed to help improve the swirl or turbulence of the air-fuel mixture, and prevent fuel droplets settling on the surfaces of the combustion chamber or cylinder walls.

When air-fuel mixture is compressed between the piston and the flat part of the cylinder head, it produces what’s called “squish”. That means squeezing of the gases to increase their velocity and turbulence.

In gasoline engines, the three most popular combustion chamber designs are called hemispherical pent roof, bath-tub and wedge.

A hemispherical, or pent-roof combustion chamber has the intake valve on one side of the chamber and the exhaust valve on the other. This provides crossflow. Air-fuel mixture enters on one side, and exhaust gases exit on the other. Positioning the valves in this way leaves room for relatively large valves and ports, and that helps the engine breathe. Breathing refers to the engine taking in the air or air-fuel mixture. Fuel starts to burn at the plug, then burning travels outward in all directions. This is called flame propagation. With the plug in the middle of the hemisphere, the flame front has less distance to travel than in some other designs, which gives rapid and effective combustion. This design is common in a lot of passenger vehicles.

The bath-tub combustion chamber is oval-shaped, like an inverted bathtub. Valves are mounted vertically and side by side, making them simple to operate. The plug is to one side, and that creates a short flame path. It all helps increase turbulence.

The wedge-shaped combustion chamber tapers away from the plug which is at the thick end of the wedge. The valves are in line and inclined from the vertical. This design usually has a smaller surface area than the others, with less area where fuel droplets can condense. Less fuel is left unburned after combustion, which reduces hydrocarbon exhaust emissions. And since the flame is directed toward the small end of the wedge, damage caused by detonation is reduced.