In a Wankel engine, the “piston” or rotor, similar in shape to a Reuleaux triangle, spins eccentrically inside an oval-section (epitrochoidal) chamber.
The rotor is mounted on an output shaft featuring an eccentric lobe (or lobes if more than one rotor is used, as in the Norton). The rotor is also geared to a stationary gear. The shaft’s eccentric defines the rotor’s throw while the gear defines the rotor’s axis. As the rotor spins it pushes against the eccentric, causing the output shaft to spin. The motion of the rotor alternately compresses and expands volume in three zones within the chamber. Intake ports are arranged to allow air/fuel mixture into the chamber as the rotor opens the first zone. A spark plug ignites the mixture as it’s compressed in the second zone, the expansion of combustion applying force to the rotor. The third zone has an exhaust port, allowing the rotor to expel spent exhaust gases.
A Wankel engine embodies all the functions of a 4-stroke engine — suck, squeeze, bang, blow — but produces one power stroke for each revolution of its output shaft. Because each rotor face is always working one part of the cycle, each revolution of the rotor creates three combustion phases. However, as the output shaft spins three times for each rotor revolution there is only one combustion phase for each revolution of the output shaft.
The swept volume of a rotary doesn’t correlate with conventional engines. Most sanctioning bodies adopted an equivalency formula of 1.7:1, so that in performance terms the 588cc Norton rotary was deemed comparable to a piston 4-stroke of 1,000cc. MC