Although there are different types of rotary engines, the most common one is the Wankel, which uses three apex seals. The three apex seals are attached at places where the rotor is nearest the housing.

There is a central rotor enclosed in a housing, which has three points. Each of the points is known as an apex; hence the seals are known as apex seals. The fuel is ignited using two spark plugs and motion of the rotor allows both the intake of fuel and release of exhaust. Each time the rotor goes round, the spark plugs are fired once.

The apex seals function by closing the gap between the housing and the seal. They carry out several duties in the process, which include pushing fuel into proper place, creating an ignition chamber within the split seconds when the rotor goes past the spark plugs, and pushing out the exhaust. The whole process takes place within a fraction of a second.

The apex seals need to be very precise in order to function well under such high speeds and temperatures. Broken apex seals are the common reason for failure in a rotary engine. This means that when the apex seals malfunction, the engine will not operate well. Although the rotors provide the necessary power, it is actually the seals that are responsible for efficient functioning of the system.

The apex seals are usually made of either ceramics or steel.