The Wankel Engine: A Three-Cornered Rotor in an Epitrochoid

US 2,988,065, 'Rotary Internal Combustion Engine', Felix Wankel and Ernst Hoeppner of Lindau, assignors to NSU Motorenwerke and Wankel GmbH; 38 claims. The specification describes an outer body whose cavity is *"a two-lobed epitrochoid"* and an inner body with *"three apex portions"*, the inner body's outline lying inside the *"inner envelope"* of the epitrochoid — the largest rotor that can turn without hitting the wall. It covers both arrangements: both bodies rotating, or *"the kinematic inversion whereby the outer body is stationary and the inner body performs a planetary rotary movement"* — the form that went into production. The drawing sheet shown with this blueprint is the first form: both bodies turn. Its figures step through the cycle with paired angles — 22.5° of the inner rotor against 33.75° of the outer, 45° against 67.5°, 90° against 135° — the inner turning two-thirds as fast as the outer, while the chambers V1, V2, V3 grow and shrink between them.

Use the notebook's R = 100 mm and e = 15 mm, scaled down by half. Export the epitrochoid points to CAD and make a housing plate with that cavity, 20 mm deep, and a clear acrylic cover. The rotor is a triangle with its sides bowed outward, apexes at the generating radius; give it clearance so it does not bind on the curved flanks. On its face print an **internal gear of 30 teeth**; on the housing, round the shaft, a fixed **external gear of 20 teeth**. That 3:2 ratio makes the rotor turn once for every three turns of the shaft. The rotor rides on an eccentric of 7.5 mm throw on the shaft. Turn the shaft by hand and watch each chamber grow and shrink. Mark one chamber and count: three shaft turns for its full cycle.

**US 2,988,065, 'Rotary Internal Combustion Engine', Felix Wankel and Ernst Hoeppner; filed 17 November 1958 (Serial 774,517), claiming Austrian priority of 11 March 1958; granted 13 June 1961.** NSU's first running engine dates from 1957; NSU and then Mazda built Wankel-engined cars, Mazda for decades. **Honest limits.** The apex seals must slide against the housing at high speed and were the engine's weak point for years. The long, thin combustion chamber has a lot of cold wall per unit of volume, which hurts fuel economy and emissions. And one power stroke per shaft turn per rotor makes it smooth, but it needs high shaft speeds.