Ask most people what a rotary engine is, and they’ll describe the Mazda RX-7 which has a triangular rotor rotating inside a housing, with combustion occurring in the sequence of the rotor rather than a row of pistons. That engine is a Wankel. But “rotary engine” is a broad category occupied only by Wankel, and there are other ways to build it.
The Wankel is the most commercially successful pistonless rotary engine ever built. Its roughly triangular rotor rotates eccentrically inside an epitrochoid housing – shaped like a two-lobed oval – creating three separate combustion chambers as it moves. Each face of the rotor cycles through intake, compression, combustion, and exhaust, producing three power strokes per rotor orbital revolution and one power stroke per eccentric-shaft revolution. That frequency, combined with fewer moving parts than a piston engine, is the Wankel’s main appeal.
The defining feature of the rotary engine is the absence of a reciprocating piston. In a conventional engine, pistons travel back and forth, converting linear motion into rotation via a crankshaft. A rotary eliminates that conversion – a continuously rotating element keeps moving inside a housing – and intake, compression, ignition and exhaust all occur through that rotation alone. This is why the more accurate term is “piston-less rotary” – this distinguishes these engines from the radial-piston rotaries common in early aviation, a design that became obsolete by the early 1920s.
Rotary engine problems are equally well documented: apex seal wear, poor fuel economy at low loads, and higher emissions than comparable piston engines. This eventually led every automaker to do away with the design, with Mazda being the last to discontinue the RX-8 in 2012. Rotary technology briefly returned to Mazda’s lineup in the form of a single-rotor range extender in the 2023 MX-30 R-EV.
Rotary identity goes beyond Wankel
LiquidPiston’s X Engine is a rotary which is clearly not a Wankel. The company’s own website describes it as As “non-Wankel rotary engine” “With fundamentally different thermodynamic cycles, architecture and operation”. Where the Wankel rotated a triangular rotor inside a fixed epitrochoid housing, the X engine reverses that geometry: a two-lobed, figure-eight-shaped rotor rotates inside a roughly triangular housing. The result is still three combustion events per rotor revolution, but through a patented high-efficiency hybrid cycle LiquidPiston claims it delivers 30 percent better efficiency than conventional diesel engines.
An At full growth the grapefruit-sized package is expected to reach five horsepower at 15,000 rpm.
Wankel proved that a pistonless rotary could provide enough power for a production car to win at Le Mans. LiquidPiston is a reminder that it was never the only way to solve the problem.
