How Stuff Works

The rotary engine

’An engine combining high power to weight with extremely smooth operation’

Back in the 1920s, Felix Wankel developed an idea for an engine that eliminated pistons, but it wasn’t until the late 1950s that a working prototype was built.

Although Wankel’s first patents on his invention dated back to 1929, it was Hanns Dieter Pashcke’s design, which used fewer moving parts, that became the basis of the rotary engine more commonly seen today.

It comprises a triangular rotor with outward curved sides and apex seals geared to an eccentric central shaft and rotating in an epitrochoid chambered stationary housing. An epitrochoid is like a radius-cornered rectangle with pinched inwards and curved long sides.

Each rotor tip’s transit describes the epitrochoid perfectly, providing a seal at each corner and constantly changing chamber volume.

Rotor tip 1 passes the inlet port, situated in the side wall of the stationary housing and as it rotates, begins to increase the chamber volume between it and the following tip 2, causing the air/fuel mixture to be drawn in. Tip 2 passes the inlet port and closes the chamber. As the rotor proceeds around the epitrochoid housing, the chamber becomes smaller and the air/fuel mixture is compressed. On full compression, tip 1 passes the spark plug (or plugs), which fires and ignites the compressed mixture, propelling the rotor around towards the exhaust port, as tip 2 also then passes the plugs. The chamber is now increasing in size, along with the expanding gases. Tip 1 passes the exhaust port and as tip 2 follows the shape of the epitrochoid, the chamber between them decreases in size, squeezing the gases out.

The process then repeats, with a complete four-stroke (induction, compression, power, exhaust) cycle achieved in a single rotation of the chamber formed between rotor tips 1 and 2. Two further similar chambers are formed between tips 2 and 3 and again between tips 3 and 1. This means that there are three complete cycles in one rotation of the rotor.

Most car rotary engines have two rotors and housings, equating in effect to six cylinders firing in one revolution. Each rotor is effectively geared to the central eccentric shaft which converts the rotor’s orbital motion into a single rotational axis for output to the gearbox.