Triangular rotor instead of piston: How does a Wankel engine actually work?

After World War II, the Wankel engine was considered the automotive industry's brightest hope—but instead of a breakthrough, it was a flop. To this day, it still has loyal fans.

The Wankel engine is one of the most fascinating, yet controversial, developments in automotive history. Unlike the highly successful reciprocating piston engine, in which pistons work up and down in cylinders, the Wankel engine relies on a rotating motion. At its center is a triangular rotor that rotates eccentrically within an oval housing. This special geometry allows the four strokes of the combustion process—intake, compression, power, and exhaust—of a traditional gasoline engine to run continuously in separate chambers. It is thus still a four-stroke engine, but one that does not require traditional pistons.

Although the Wankel engine is technically called a rotary piston engine, it does not have pistons in the traditional sense. Instead of the typical linear up-and-down motion, triangular rotors compress the air-fuel mixture, initiate combustion, and transfer the energy as rotary motion directly to the drive shaft. The rotors perform a combined rotational and wobbling motion, which fundamentally differs from that of a reciprocating piston engine.

This operating principle promises smoother running and less vibration. Due to its simple design, which eliminates the crankshaft, connecting rod, camshaft, and valve train and control, a significantly more compact construction is possible. Furthermore, a Wankel engine is potentially lighter. Its specific power, i.e., power relative to volume, was impressive even early on.

This engine type was invented by German engineer Felix Wankel. He had the idea of ​​a rotary combustion unit as early as the 1920s, but it wasn't until after the Second World War that he began concrete work. The first functional prototype, the DKM 54 model, was presented at NSU in Neckarsulm in 1957. The Wankel engine went into series production at NSU in the 1960s. The first model was the NSU Spider in 1964, followed by the Ro 80 in 1967, which was celebrated for its technical pioneering spirit, but soon revealed teething problems and conceptual drawbacks that prevented its breakthrough.

A major problem lies in the seal between the rotor and the housing. The so-called edge or apex seals are particularly stressed, as they are in direct contact with the combustion chamber wall with every rotation of the rotor. Wear is high, and oil and fuel consumption, as well as emissions, are comparatively high. Emissions regulations were first introduced in Germany in 1971, followed by the oil crisis in 1973. NSU, which had since merged with Audi, subsequently discontinued Wankel production.

Mazda also gave up at some point

Despite these challenges, the Japanese manufacturer Mazda licensed the NSU technology and further developed it independently. The Mazda RX-7, built in several generations between 1978 and 2002, became legendary, achieving success primarily in motorsports. In 1991, the Mazda 787B became the first car with a Wankel engine to win the 24 Hours of Le Mans, a feat that remains unique to this day. The last production model with a rotary engine was the Mazda RX-8, whose production ceased in 2012.

By this time, European and US automakers had long since recognized the Wankel engine as a dead end. Audi, for example, abandoned the technical development of the NSU legacy. Other manufacturers that had experimented with the technology, such as Mercedes-Benz and Citroën, also abandoned their Wankel ambitions. In 2010, Audi presented the A1 E-Tron, which used a Wankel engine as a range extender.

Even at Mazda, the Wankel engine had been quiet for many years. However, in 2023, it experienced a small renaissance in series production: Since then, the Japanese company has been using the rotary engine as a range extender in the primarily electric MX-30 e-Skyactiv R-EV. The compact design and smooth running of the rotary engine are ideal for this purpose. However, even here, the Wankel engine results in comparatively high fuel consumption. Even in this niche, the Wankel engine seems to be losing ground.

Given the increasing electrification of drive systems, it's questionable whether it will ever experience a comeback. Its specific weaknesses, particularly in fuel consumption and emissions, are difficult to address. Its history is therefore likely to be one thing above all: a lesson in technical innovation and the pitfalls that even the most elegant ideas can bring.