How Does a Wankel Rotary Engine Work?
A Wankel rotary engine has a triangular rotor in an oval-like housing, and its three sealed pockets each complete intake, compression, power and exhaust in turn.
A Wankel engine is an internal combustion engine in which a triangular rotor turns inside a housing. Three sealed pockets form between the rotor faces and the housing, and as the rotor turns, each pocket takes in air and fuel, squeezes it, fires it and empties the exhaust. The engine has no pistons, connecting rods or valves, and each pocket completes one cycle for every turn of the rotor.
What are the main parts?
The engine has three main parts. The housing is a fixed casing whose inner wall is an epitrochoid, a curve that looks like an oval with pinched sides. The rotor is a thick triangle whose three faces are slightly convex, and it carries an internal gear at its centre. The output shaft has an offset lobe, called the eccentric, on which the rotor rides.
Seals run along the three corners of the rotor, called apex seals, and along its faces. A fixed gear on the side housing meshes with the rotor's internal gear, so the rotor stays in step with the housing. For every turn of the rotor, the output shaft turns three times.
How does one pocket complete a cycle?
Follow one pocket as the rotor turns. The space behind a rotor face grows as that face sweeps past the intake port, drawing in a fuel and air mixture. As the rotor continues, the pocket shrinks and compresses the mixture. A spark plug fires near the point of greatest compression, and the expanding gas pushes the face hard, which turns the rotor and the shaft. Finally the face uncovers the exhaust port and sweeps out the burnt gas as the pocket shrinks again.
The intake and exhaust ports are simply openings in the housing wall. Nothing opens or closes them. The passing rotor face uncovers and covers each port in turn, which is why the engine needs no camshaft or valve springs.
How does it compare with a piston engine?
In a piston engine these four events happen in sequence in one cylinder. In a Wankel they happen at the same time in three pockets, each at a different stage. The rotor turns about its own centre while that centre orbits the shaft, so the pocket volume changes smoothly with no piston reversing at the top or bottom of a stroke.
The table compares the two designs.
| Feature | Wankel rotary | Four-stroke piston |
|---|---|---|
| Main moving part | Triangular rotor | Piston and connecting rod |
| Valves | None, since the rotor uncovers the ports | Poppet valves driven by a camshaft |
| Power strokes per crankshaft turn | One per rotor | One per cylinder every two turns |
| Crankshaft turns per rotor turn | Three | Not applicable |
Gas turbines, described in how jet engines work, also avoid reciprocating pistons, but they burn fuel in a continuous flow rather than in sealed pockets.
What are the strengths and weaknesses?
The rotary design is smooth and compact. It has few moving parts and little reciprocating mass, so it runs with low vibration and suits small, high-speed engines. Mazda used it in sports cars such as the RX-7, and its four-rotor racing engine won the 24 Hours of Le Mans in 1991.
The weaknesses come from the shape. The combustion chamber is long and thin, with a large surface area that loses heat and leaves more unburnt fuel in the exhaust. Apex seals rub against the housing at high speed and wear over time. The engine needs oil for lubrication, some of which burns, so oil consumption is higher than in a piston engine. Fuel economy has generally been lower as well. Seal wear and friction are a central problem in machines like this, covered in tribology.
Felix Wankel developed the design at NSU in Germany during the 1950s, and it is named after him. Its lasting appeal is the simplicity of the idea: one rotor on an eccentric shaft does the work that pistons, connecting rods and valve trains do in a piston engine. For the broader thermodynamic principles, see the four-stroke engine and the second law of thermodynamics.
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engines internal combustion mechanical engineering rotary engine thermodynamics