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, and the output is calibrated to the 700 PS at 9,000 rpm and 608 N·m Mazda published for it. 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 both peripheral ports, holes in the housing's running surface that the passing apex uncovers: intake from 994 degrees of eccentric shaft angle round to 345, exhaust from 737 to 65. That is Mazda's factory peripheral timing, since the R26B's was never published, and it holds both ports open together for 151 degrees, which is why this engine idles rough at 2,400 rpm. Every edge is 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 737 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