Naturally Aspirated 4-Rotor 2.6L (26B)

What it is

Eccentricity is 15 mm, the generating radius 105 mm and each rotor 80 mm wide. Run those through the Wankel swept-volume identity and one rotor takes 654.7 cc, so four of them come to 2.619 litres. The rotor count and the chamber displacement match the Mazda R26B. Nothing else here was copied from one. Torque comes out of a Wiebe heat release solved against the epitrochoid chamber volume, twelve faces at once, each stepping through its own 1080 degree cycle.

Every rotor fires once per turn of the eccentric shaft, and the four sit 90 degrees apart on it. That is four power events a revolution, the count a V8 four-stroke needs eight cylinders and two revolutions to reach. At the 9,000 rpm redline the exhaust takes 600 a second. The audio path reads that rate off the rotor count, which is why a four-rotor and a two-rotor sound like two instruments.

A Wankel has no valves. Intake and exhaust are holes in the housing wall that the passing rotor face uncovers, open from 0 to 270 degrees of eccentric shaft angle and from 810 to 1080. Both edges are editable, which is as close as this engine gets to a camshaft swap. The loss that defines it sits in the friction block: 0.1 mm² of equivalent apex and side seal gap, on a log slider reaching 20 mm². Compression is 10:1 on 95 octane. The rotary model reports no knock, so raising it buys efficiency without the detonation bill a piston engine pays.

Things to try

• Walk Seal leakage up from 0.1 mm² toward the 20 mm² top of its log slider. The gas that escapes past the apex seals is compression the face never gets to burn.
• Take Rotors from 4 down to 2 and back up. Displacement, the 90 degree phase spacing and the firing rate all re-derive from that one number, and the exhaust note gains or loses an event per revolution with each housing.
• Move Exhaust opens earlier than 810 degrees. Blowdown starts sooner and the housing empties more freely, at the price of expansion work the face had not finished doing.

Seen in

Mazda 787B

Runs this engine

1990–1991. The R26B's only home, and the one rotary car ever to win Le Mans outright, which it did in 1991 in these colours and this number.

Photo: Pieter Olievier · Unsplash