How a rotary works & why it makes the most engineering sense (on paper)

How a rotary works & why it makes the most engineering sense (on paper)

If you read THIS STORY you'd know the Otto cycle rotary engine isn't solely a Mazda engine. So what is the appeal of the tiny triangle donk, and how to they work?

While it was first drawn up in the late 1920s, it wasn't until the mid-1950s that Felix Wankel's rotary engine really came to be A Thing. Wankel was after a smooth-running powerplant that didn't suffer from vibrations found in piston engines of the time, and this has always been a key highlight of the rotary.

While a piston engine has to transfer the up-and-down motion of the pistons and conrods into a circular motion of the crankshaft, the rotary always spins in a cyclical motion - this makes for a far more compact and light engine, reduces vibrations and opposing forces, and promotes high-RPM power.

Using an Otto cycle the Reuleaux (curved-sided) triangle rotor spins on a central crankshaft in an eliptical path, inside a housing (like a cylinder in a piston engine).  The air/fuel mixture enters the housing in the first gap in the chamber, is compressed in the second as the rotor spins, and ejected out the third. 

Rotaries use multiple housings and end plates stacked up to create the whole engine. A two-rotor engine (ie: 10A, 12A, 13B) has two housings with rotors. A triple-rotor (20B) has three, and a four-rotor has 12... just kidding, it has four. Ground-breaking, i know. 

The combined size of the gap between the housing and the rotor is how the engine's capacity is calculated. This is how we get the 10, 12, 13 and 20 sizings in famous Mazda rotary engines. 

Part of the genius is the rotary produces three power pulses per revolution and this, with its ability to spin to truly ridiculous RPM, means the Wankel rotary can produce far greater horsepower than its diminutive capacities suggest. And once you get into the world of ported housings, then the power (and noise) of the rotary really clicks the next gear. 

Rotary tech heads often throw out words like "bridgeport", "monster", and "J", and these are the secrets to how modified rotaries really beat-up piston engines. Enlarging the diameter and shape of the intake and exhaust ports in a rotary engine is all about getting more air in and out, which is key for making power. 

Port-tuning comes down to which direction you open the port up, as this can affect whether the air/fuel mixture enters and exits the chamber early or late. This matters for the engine combo you're putting together (NA, turbo, circuit, drag, street, etc). 


The addition of turbochargers and EFI to the rotary engine really brought them alive in the 1980s, with Mazda's 20B-REW arguably the greatest production iteration of the platform. With three large rotors and a turbo feeding it, it was a shame Mazda only fitted them to the Cosmo luxury coupe instead of a super-hot RX-7. 

The aftermarket picked up these compact, power titans and turned them into monsters which hosed far larger engines on drag strips, circuits and streets all  over the world. Today, the aftermarket love for rotary engines means you can buy complete billet engines which can produce thousands of horsepower from an engine the size of a large sports bag. 


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