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Jul 01, 2023

10 Facts About The Rotary Engine: How It Works And Its Advantages

The automotive industry has seen various engine designs, but none took the world by storm more than the unique rotary engine. Throughout history, the automotive industry has spawned various engine

The automotive industry has seen various engine designs, but none took the world by storm more than the unique rotary engine.

Throughout history, the automotive industry has spawned various engine designs and configurations. The most common engine type is the piston engine, where cylinders are arranged in a straight line, horizontally opposed (boxer), or in a "v" configuration. Hidden among these gems is the out-of-the-box and unique rotary engine that took the world by storm during the second half of the 20th century.

Unless you've owned or interacted with one, chances are you only have a basic understanding of how the rotary engine differs from the traditional piston engine. Highly associated with Mazda, many people don't know that the rotary engine powered everything from aircraft to tractors and motorbikes. But thanks to HotCars, here is your chance to learn everything about the revolutionary engine. Spend the next few minutes learning why this oddball was an engineering marvel. That said, here are 10 facts about the rotary engine, how it works, and its advantages.

In a nutshell, a rotary – or Wankel engine – is a type of Otto cycle engine that, instead of using pistons for internal combustion, it uses a triangular-shaped rotor(s) to convert combustion pressure into rotating motion. A central rotor spins to produce power, which then gets transferred to the wheels via a transmission system.

Unlike the traditional piston engine with a fixed cylinder block and a rotating crankshaft, the rotary engine features a stationary crankshaft and an entire cylinder block (triangular rotor) that rotates around it. The rotary engine became an alternative to the piston engine in the car industry.

Although many people believe the rotary engine came along in the 20th century, the truth is that its origin dates back as far as the 16th century. Yeah, that's right. Agostino Ramelli, an Italian engineer, invented the first rotary-piston-type water pump in 1588. About two centuries later, in 1769, James Watt developed the first rotary steam engine.

However, Dr. Felix Wankel (1902-1988) was the genius behind popularizing the rotary engine in the automotive industry. At the young age of 22 years, Wankel – having no formal training – set up a research lab and began pursuing his childhood dream of inventing a half-turbine, half-reciprocating engine. In 1951, he partnered with NSU (a motorcycle and car manufacturer). Six years later, the two completed the first working prototype rotary engine, called the DKM. Although it had a triangular rotor and a rotating cocoon-shaped rotor housing, the 1958 DKM with a static rotor housing became the true forefather of the revolutionary rotary engine.

RELATED: Check Out The Incredible 12-Rotor Rotary Engine That's The Only One Of Its Kind

At the heart of the Wankel or rotary engine is a trochoid/triangular-shaped rotor that spins around an unconventional shaft within a cocoon-shaped rotor housing. This internal design creates three spaces between the rotor housing and the rotor, creating chambers for the combustion process.

The rotor has a fixed inner-toothed gear ring, while the eccentric shaft has a fixed outer-toothed gear. During the rotating process, the turning speed between the rotor and shaft is 1:3. There are seals at the apex of the rotor seal against the rotor housing. On one side of the rotor housing are intake and exhaust ports, while spark plugs are located directly opposite.

The operating principle of the rotary engine is similar to the typical Otto principle in a four-stroke piston engine. That means one side of the rotary engine completes the four-stage cycle (intake, compression, ignition, and exhaust) in each rotor revolution.

But unlike a piston engine, the rotary engine has three sides, meaning all sides operate simultaneously in one complete rotor revolution. This means the rotary engine has three power pulses per rotor revolution. And since the output shaft spins three times faster than the rotor, the Wankel engine produces one power pulse in each shaft revolution. For reference, a piston engine completes the four-stroke cycle in two shaft (crankshaft) revolutions. The rotary engine, therefore, has twice as many power pulses per shaft revolution.

RELATED: Why The Flawed Mazda RX-8 Nearly Killed Off The Rotary Engine

The rotary engine is an engineering marvel, and we can't deny its simple and efficient design. The biggest advantage of this engine is that it has a better power-to-weight ratio over its piston adversary, which makes it a better fit for performance cars.

It also has smooth power delivery since there are no reciprocating parts, eliminating engine vibrations. A two-rotor Wankel engine, for example, is twice as smooth as a four-cylinder piston engine. Fewer moving parts make the rotary engine lighter, more reliable, and cheaper to package and mass-produce. It can also reach higher engine speeds (revolutions per minute) than a piston engine.

French aircraft manufacturer Gnome et Rhône began producing rotary power plants for aircraft in 1910. These aircraft played a significant role in World War I, and the rotary engine became an important engine design in the aviation industry.

Another successful rotary engine application was in the 1989 Norton F1 motorbike, which had an incredible top speed of 140 mph. In the automotive industry, several manufacturers like NSU, American Motors, Daimler-Benz, General Motors, Ford, and Citroën tested the rotary engine in their cars, but it's Mazda who made the final breakthrough.

Without Mazda, we reckon the rotary engine wouldn't be as popular as it is today. The Japanese automaker is responsible for adopting Wankel's design concept and nurturing it into a full-blown car engine. Mazda's tale with the rotary engine goes way back to 1961 when Tsuneji Matsuda (the then Mazda president) negotiated a licensing agreement with NSU.

In 1967, Mazda dropped the world's first two-rotor Wankel engine in the now popular and highly collectible Cosmo Sport. With an output of 110 hp, the 10A rotary engine featured new-developed carbon apex seals that didn't wear out quickly. The engine didn't show the chatter marks affecting the previous rotary engines. The tightening emission regulations and the engine's high appetite for gas forced Mazda to stop using the engine in passenger cars. Instead, they shoehorned it in sports cars like the RX-3, the iconic RX-7, RX-8, and the Le Mans-winning 787B.

You might wonder why, despite its advantages, the rotary engine isn't common in modern cars. Well, it's because of tailpipe emissions and fuel efficiency. The rotary engine tends to burn oil meant for lubricating the apex seals. If it runs out of oil, the engine overheats and damages the apex seals, requiring a complete engine rebuild or an entirely new engine.

Additionally, the rotary engine is arguably the least fuel-efficient engine. The engine's design doesn't compress the air/fuel mixture as efficiently as the traditional piston engine. It maintains poor compression at lower revs, making it inefficient for city driving.

RELATED: How This Tiny New Wankel Rotary Engine Apex Seals Are Designed Not To Fail

Following its success in the iconic Mazda RX-7, the rotary engine garnered popularity among tuners and hot rodders due to its massive tuning potential. Its incredible power-to-weight ratio and high-revving nature make it a common swap in drift cars and other track weapons.

A quad-rotor Wankel engine, for example, is smaller than a flat-six piston engine in size, but make no mistake, it produces the kind of grunt you'd get in a massive V8 engine. A rotary engine can produce an output of about 150 hp per rotor. Throw in a turbocharger, and the engine's output increases to about 300 hp per rotor. The engine revs hard, producing hot exhaust pulses that can easily spin massive turbos. Top Gear recently featured a tire-burning 2,000 hp rotary-powered RX-7 that produces one of the best soundtracks.

Mazda kept the rotary engine alive for decades but dropped it in 2012 when the Japanese automaker retired the RX-8. Although Mazda promised to bring back the rotary engine on various accounts since then, many pronounced it dead. About a decade later, Mazda re-introduced the rotary engine in the 2023 MX-30 R-EV.

The MX-30 R-EV is a plugin hybrid crossover, and the rotary engine functions as a range extender and not the car's primary power source. It is a single-rotor naturally aspirated Wankel engine that produces 74 hp and kicks in when the car's battery depletes, extending the MX-30's range to 372 miles. While it's not the legendary rotary engine of the past, the MX-30 R-EV proves that the rotary engine is very much alive.

Sources: Mazda, Energy Education, How a Car Works, Rotary Revs, and How Stuff Works

Dennis Kariuki is a tech enthusiast who writes for Hotcars.com. Previously, he wrote for the.car .He likes covering the marriage between technology and cars. Besides that, he is big f1 fan. After working tirelessly through the week, you can be sure that on Sundays he is most likely enjoying and following motorsports events.