Rotary Engines: What Are They and How Do They Work?
In an industry as rich with change and new technologies as automotive, one thing has remained relatively untouched throughout the development and advancement of the modern automobile: the piston engine design.
That’s not to say engines haven’t seen extensive innovation over the years, but the underlying principle that powers vehicles now is the same as it was in the ‘40s. The piston engine has remained the king of automotive engine designs since the beginning and has been relatively unchallenged for its reign.
However, there was one formidable competitor to the piston engine that made its way onto the roads and captured a portion of the market along with a dedicated cult following; the rotary.
In this article, we’ll explore what made the rotary seem so promising for manufacturers to develop, how it was applied in mass-produced vehicles, and the issues that caused the rotary to fall out of favor.
Birth of the Rotary
The rotary engine (sometimes known as the Wankel engine) was developed in 1954 by German engineer Felix Heinrich Wankel as a smoother-running and less vibration-prone competitor to the traditional piston engine design.
After a few more years of development into a practical design for mass production by the German automotive brand NSU (which later became part of Audi), the rotary gained notoriety from other large players in the market who saw its possible advantages as well.
The first vehicle to make use of a rotary engine was NSU’s Spider, a lightweight roadster released in 1963.
Rotary vs. Piston Engines
Modern piston engines rely on the same method applied to the first gasoline internal combustion engines, harnessing the power generated by combustion through a piston that turns a crankshaft. These motors rely on a four-stroke cycle to produce power: intake, compression, combustion, and exhaust.
Rotaries use the same four-stroke operation as piston engines but accomplish them in completely different ways. Rather than using valves to manage the intake and exhaust along with a piston to compress the air/fuel mixture, Wankel designs instead use a central rotor to take care of both.
This rounded-triangle rotor has an inside gear (the crown gear) that spins on a stationary center pinion gear, as well as rotates itself (think of how the Moon rotates around the Earth while also spinning on its axis independently). As the rotor completes this motion, it creates compression in one portion of the stator housing at a time. At different points in its cycle, it will complete all four strokes of a combustion engine.
You can split a rotary engine into four separate chambers (one for each of the combustion strokes). As the rotor spins, it’s consistently changing which chamber is being compressed. For instance, if the rotor is currently compressing the air/fuel mixture to create combustion, the following rotation will take the exhaust gasses and push them out of the exhaust port in the next chamber of the motor. The chambers that have been “expanded” from the rotor’s movement will produce a vacuum, pulling in the air and fuel needed for combustion into the proper chambers of the stator housing.
The beauty of the rotary is that the engine can accomplish these steps without the use of delicate valves and other moving parts, allowing rotaries to rev extremely high, save on weight, and fit in compact areas.
Benefits of the Rotary Design
This radically different design for an engine led to some pretty significant benefits, including:
- Improved power-to-weight ratios
- Compact for smaller vehicle applications
- Able to reach higher RPM than traditional motors
- Decreased vibrations
- Fewer moving parts to fail
- Cheaper to produce
- Provides torque for two-thirds of the combustion cycle as opposed to one-fourth
Mass Market Applications
After the relative success of NSU’s Wankel-powered Spider, other auto manufacturers started to take notice. Citroen, General Motors, and Mercedes-Benz all took cracks at some sort of rotary-powered vehicle, most of them dying as just prototypes.
In modern terms, the rotary engine is almost synonymous with Mazda. This long-lasting relationship started in 1967 when Mazda first released the Cosmo. Not only was the Cosmo Mazda’s (and Japan in general’s) first attempt at a mass-production rotary vehicle, but it was also the first in the world to use a twin-rotor design.
Mazda continued to develop and make use of the Wankel in various vehicles in the coming years. Even a B-Series pickup got the rotary treatment, creating one of the most interesting combinations of engine and vehicle we’ve ever seen.
The RX Series
However, Mazda’s product line most famous for its rotary powerplants is the RX line. The RX cars were the perfect formula for a rotary sports car; lightweight, rear-wheel drive, and with the ability to rev extremely high.
The RX-3 (sold as the Mazda Savanna domestically in Japan) was the first taste of Mazda’s rotary seen in the United States, debuting in 1971 but not hitting American shores until 1973. The Savanna sold very well in Japan, partially because it skirted many of the strict regulations and taxes placed on larger engine sizes by the Japanese government at the time.
The RX-7 was the mass-market rotary success Mazda needed in the United States. The first generation, often known as the FB, debuted in 1978. At this point, the rest of the major rotary projects from other manufacturers had fizzled out, and Mazda was left as the sole producer of rotary vehicles. The second-generation FC RX-7 followed the same formula but had been designed with the North American market in mind and with significant upgrades in suspension technology to make it more capable around corners (something the FB was already pretty good at).
The final generation of the RX-7 was the FD, which you might have seen in some popular TV shows and movies. The FD was a stark departure design-wise from the previous RX-7s, with its curvy and bulbous design reminiscent of Miatas from the same era.
This is the vehicle that cemented the rotary as a beloved cult classic, coming stock with a sequentially twin-turbocharged 1.3L 13B motor. Much of the current aura around the FD comes from the fact that it’s widely regarded as “the last great rotary”, but the car itself was ahead of its time and provided a driving experience unlike anything else on the market. For those wondering, you unfortunately won’t be able to pick one up for cheap anytime soon. Well-maintained FDs with relatively low mileage can fetch over $70K on the used market (as of this article being published).
Mazda followed this up in 2002 with the RX-8, a controversially styled sports car with a pair of oddly shaped suicide doors and a knack for killing catalytic converters. There wasn’t one specific factor that led to its commercial failure, but rather a perfect storm of unlucky scenarios, engineering oversights, and an international economy not in any shape to support such an unusual vehicle at the time.
The RX-8 was plagued with mechanical issues immediately after hitting the production line. Rapid oil consumption and failing ignition coils were just a few of the fundamental issues with the 1.3L RENESIS twin-rotor engine.
Past the 30,000-mile mark, it was common for the ignition coils to completely fail. At the time this exact cause of engine failures was unknown, and led to many RX-8s receiving expensive engine replacements rather than just having the coils rectified.
One of the most frightening issues the RX-8 faced, the power steering system has been known to cut in and out abruptly, despite all of the fluid and mechanical connections being tight and secure.
Chief among these issues though was the emissions equipment disaster that Mazda faced. The RX-8 had a habit of blowing through catalytic converters, which were not cheap to replace. In rare cases, the failed exhaust components would build up so much heat they could melt components around the fuel tank and cause dangerous fuel leaks. In 2003 and 2004 alone, Mazda had to recall over 15,000 RX-8 vehicles due to these exhaust and emissions concerns.
In 2012, Mazda pulled the RX-8 from dealerships to address the car’s problems passing emissions testing. From there, emissions regulations only got stricter and made the gas-guzzling (for its size) rotary engine seem less and less practical. The RX-8 never returned to production, and Mazda pulled the plug on it that same year.
What Killed the Rotary?
So, was it just a single failed model that killed the rotary dream entirely? Not quite.
Despite all the benefits that make them seem like an engineer’s dream, the Wankel engine had various drawbacks compared to a traditional piston design.
As a result of the shape of the chambers in a rotary motor, combustion occurs slowly due to the extra surface area causing low thermal efficiency. This leaves a significant amount of unburned fuel to be expelled out of the exhaust (this is also why rotaries backfire so often). Fuel efficiency wasn’t the concern of most rotary vehicle owners back in the day, but in a more environmentally and cash-conscious America, it became hard for most to justify.
One of the most infamous downfalls of the Wankel was poor rotor sealing. Rotaries rely on carbon and ceramic “apex seals” on the corners of the rotor to create air-tight seals between the chambers, but over time these seals can wear down due to carbon build-up from burnt oil. This abrasive carbon residue is swept around the walls of the chambers by the spring-loaded apex seals, leading to metal marring and improper seals for compression.
These fundamental issues with the rotary concept combined with shifting consumer priorities made Mazda, the company that had kept it alive for so long, inevitably kill off its most legendary engineering achievement.
Maybe if the Wankel had continued to be developed at the same pace as the piston engine by other manufacturers we’d be living in a world full of fuel-efficient and reliable rotary vehicles, but for now, it seems to be a relic of an innovative period in the automotive industry that never fully materialized.
So don’t be sad the rotary is gone, just be glad it existed in the first place.
The Return of the Wankel?
Okay okay, the rotary isn’t gone just yet.
However, it’s also not being used exactly how you’d expect either. The new Mazda MX-30 e-SKYACTIV R-EV crossover is equipped with a 0.8L rotary motor to be used as a range extender for the electric motors that power the vehicle. The engine kicks in when the battery is low and provides a great deal of extra range.
While word of a Mazda “RX-9” made its rounds on the internet, furthered by an official Mazda RX-VISION rotary sportscar concept in 2016, it seems we won’t be getting a true rotary-powered car anytime soon.
It may not be the smooth, high-revving, potent Wankel motor of years past, but it’s reassuring to see that the rotary is not yet extinct and still under the development of a major automaker.
Mazda Services at Micro Import Service
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