Rotary Engines Reimagined

27 February 2017
The Mazda rotary engine is adapted from the Wankel engine of the 1950s.

The last rotary engine-powered production vehicle rolled off a Mazda auto assembly line in 2012 due to slow sales coupled with stricter fuel efficiency and emissions regulations.

Since then, auto enthusiasts, engineers, fans, media, and Sunday drivers alike have speculated about the fate of rotary engines.

Mazda is the only automaker to mass manufacture and sell a rotary engine vehicle in the last 40 years and has single-handedly prevented the technology from becoming an automotive history footnote. It was the only car company to heavily invest in its development in the 1960s, to the extent that the company almost went extinct during the 1970s' energy crisis due to the engine's inefficiency.

Mazda hopes that its rotary engine technology will once again go "zoom zoom."Mazda hopes that its rotary engine technology will once again go "zoom zoom."Almost all conversation regarding the future of rotary engines for auto applications begins and ends with Mazda. Over the past decade, Mazda has employed anywhere from 30-50 engineers on developing the next generation of rotary engine technology.

(Watch a video to see how rotary technology works.)

Several advantages to rotary engine technology motivate Mazda to continue its development. Rotaries are valued for their simplicity, operating smoothness, compactness, high RPMs, and power-to-weight ratios, which make the power plant a natural choice for fast, agile vehicles—something for which the company with the “zoom zoom” advertising tagline is well known.

But the shortcomings (including poor fuel efficiency, excessive oil usage, and relatively high emissions) have been the technology’s Achilles heel. The immense head start enjoyed by reciprocating engine technology also means that Mazda is virtually alone in its pursuit of rotary engine modernization. For a small automaker with just 2% of the global market share, it is a risky endeavor.

Tech Check

In a reciprocating engine, pistons compress air in a chamber where fuel is ignited. The resulting rapid expansion of the gas drives the piston downward and transmits torque to the crankshaft via linkages that turn linear force into rotary force, while also providing the compression force for an opposing piston.

Rotary engines deliver torque to the crankshaft, known as the eccentric shaft, or e-shaft, quite differently. Instead of pistons compressing air and delivering torque, a Reuleaux triangle-shaped rotor spins and also completes orbital revolutions around the e-shaft within an elliptic-like chamber.

As the rotor moves throughout the chamber, seals on the corners of the rotor create a cavity between the curved surface of the chamber and corners of the rotor. This cavity first creates suction against the air intake quadrant of the housing, and then seals and compresses the air in the second quadrant as the rotor revolves and spins. In the third quadrant, the air is laden with fuel and ignited. The explosiveness transmits torque to the e-shaft while also initiating the compression phase of the subsequent engine cycle. As a result, power transmission is exceptionally smooth.

History and Nomenclature

Félix Millet, a Parisian, patented the internal combustion rotary engine in 1888. A few years later, the rights to the engine were purchased by a London company that began manufacturing motorcycles. From there, the technology slowly evolved; by World War I the rotary engine had become a mainstay in aircraft due to its smoothness, excellent cooling, and favorable power-to-weight ratio as it needed no flywheel.

The Millet motorcycle used rotary engine technology.The Millet motorcycle used rotary engine technology.But over time the term "rotary engine" became misunderstood.

Technically, a rotary engine spins the housing around a stationary central shaft, as with early plane propellers. Thus, Mazda’s power plant is in fact a radial engine, where the crankcase is stationary and the torque is delivered to the crankshaft.

The truth is that there isn’t much consistency between the terms rotary engine and radial engine, primarily because the operational principles remain the same. Both usages, arguably, are correct.

Mazda rotary engines are considered Wankel-type engines after German engineer Felix Wankel developed a modernized rotary engine in 1951. While working for NSU Motorenwerke AG (today’s Audi), Wankel’s contribution was to integrate vane seals on the rotor tips, which greatly increased the engine’s efficiency. However, Wankel’s colleague, Hanns Dieter Paschke, several years later converted the engine from rotary to radial, parlaying the motor for use in automobiles.

By the early 1960s, no less than a dozen auto and equipment manufacturers had licenses to Wankel engines, including Mazda. Some automakers, such as General Motors, believed Wankel engines offered little benefit at an increased price tag. Others, such as Mercedes and Citroën, integrated it into concept cars or certain model lines but never adopted the motor enthusiastically.

Mazda's Cosmo model dates from 1967 and buoyed the company's rotary engine prospects.Mazda's Cosmo model dates from 1967 and buoyed the company's rotary engine prospects.But Mazda was seeking a means to stand out in a crowded Japanese auto market. While NSU’s vehicles suffered poor sealing issues and engine failures, Mazda had produced a reliable Wankel-powered sports car, the Mazda Cosmo, by 1967. With more horsepower at a lighter curb weight than similar reciprocating engine cars, Mazda had found its niche. It began producing pickup trucks, station wagons, and buses with Wankel-type power plants.

The 1970s' energy crisis crippled the rotary engine market and nearly bankrupted Mazda. The engine’s poor fuel economy was exacerbated by primitive sealing and air-fuel ratio technologies. This marked Mazda’s necessary return to reciprocating engines for many vehicles. The company repositioned its rotary engines solely as a power plant for sports cars. In the U.S. market, all Mazda rotary engines sold from 1978 onward were outfitted in the RX family of sports cars.

Tomorrow’s Rotary Engine

Development news of Mazda’s next rotary engine sports car has been nothing short of a whirlwind.

At the 2015 Tokyo Auto Show, the company debuted the RX-Vision, with a look reminiscent of the RX-7 and an engine known as the Skyactiv-R. Vehicles with standard Skyactiv are equipped with some permutation of a high-compression reciprocating engine, two-stage turbocharger, high efficiency or lightweight transmissions, lightweight body designs and electronic steering control. Skyactiv-R would replace the reciprocating engine with a next-generation turbocharged rotary engine. Mazda officially filed for a patent on the technology in early 2016.

Mazda says it can convert its rotary engine design to hydrogen fuel. Mazda says it can convert its rotary engine design to hydrogen fuel. Even at the time of the RX-Vision’s unveiling, Mazda officials were quick to throw cold water on the rotary engine hot stove, stating that the concept was just that--a concept--and that no active development on the next RX vehicle was occurring. Yet by summer 2016, reports from Japanese auto industry sources stated that the RX-9 would be introduced by 2019.

Then, in an interview with Automotive News in December 2016, Mazda CEO Masamichi Kogai was quoted as saying that there was zero development work on any RX or rotary-powered vehicle. He reportedly insisted that any Mazda rotary technologies in the near future were more likely to be used as a range extender (REX) in electric vehicles (EVs).

REXs are combustion engines that recharge the electric bank of an EV and do not provide primary locomotion. Indeed, the company has had to play catch-up in the electric market and a rotary REX could be a major asset. Some analysts even expect a rotary REX in a next-gen EV sports car.

“We forecast Mazda will introduce a rotary engine with gasoline for its REX,” said Toru Hatano, IHS Markit automotive analyst in Tokyo. “The rotary engine has advantages for size and weight, however it has disadvantages for cost and emissions. Normally, OEMs will use reciprocating engines for REXs; for Mazda, the rotary engine is an icon of the brand.”

The company has earned renown with previous rotary successes and is keen to remind automotive fans and reporters of its previous engineering prowess, such as with its sponsorship of the rotary engine-only Mazda Star Championship racing series.

“To use a rotary engine for REX is good to reduce a cost of sports vehicles and distinguish its REX from other OEM’s REXs,” says Hatano.

The company is actively pursuing the technology, but engineers intend to develop rotary engines for EVs and not for traditional fuel vehicles. This could explain the disparate reports about Mazda’s rotary engine development over the past two years. It would also clarify why details surrounding Skyactiv-R, such as rotor configuration, output potential and efficiency improvements, have remained slim despite a reputed decade of research and development. If so, then Skyactiv-R and the RX-Vision were likely red herrings meant to confuse competitors or build hype around rotary engine developments.

It seems that rotary engine fans may have to live with the next Mazda rotary engine solely as a REX, possibly as early as 2019. Those that might consider it an affront to the RX legacy should also bear in mind that any return of the rotary engine confirms Mazda’s commitment to the technology.

Yet usage of a rotary REX doesn’t preclude a future rotary engine-powered sports car, especially in light of a 2015 partnership between Mazda and Toyota that saw them share Skyactiv and hydrogen fuel cell technologies. Hydrogen is an excellent potential rotary fuel source due to its clean and rapid burning nature. Mazda itself has said it can convert its rotary engine design to hydrogen fuel at low cost and has manufactured some concepts.

Ultimately, Mazda is a small automaker in the midst of a culture change to improve recent customer retention rates and sales. What’s clear is that Mazda values its history and is dedicated to keeping rotary engines alive in some form. The return of its prestige product would be nothing less than a triumph.

But before it can claim victory, Mazda needs to tune up its technology and prove it can finish the race by delivering the next-gen rotary, even if it is a REX.

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Discussion – 10 comments

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Re: Rotary Engines Reimagined
2017-Mar-09 3:24 PM

As I remember the GM part of the story they spent $1B on the Wankel to put in redesigned small cars for 1975 (Skyhawk, Starfire, Monza, Sunbird). Only after spending all that time and money did they decide it could not meet fuel mileage requirements and in total panic bought back Buick V6 tooling from Jeep.

Re: Rotary Engines Reimagined
2017-Mar-09 3:30 PM


Good article, thanks. I worked on the rotary many years ago.


Re: Rotary Engines Reimagined
2017-Mar-14 11:40 AM

As a previous owner of a couple of RX-7 rotary engine powered cars, and an experimental homebuilt aircraft builder who's currently installing a Mazda Renesis (RX-8) rotary engine in my RV-7 aircraft, I've got to say that this article has to be disorienting to anyone who's not already intimately familiar with rotaries. The Millet may well be the 1st rotary *piston* engine, and yes, they were common in WW1 a/c. But the Wankel did not 'improve on the rotary by adding tip seals'. While the terms are similar, the Wankel is a *rotary combustion* engine (shorthand: 'rotary') because the combustion space is actually rotating within the block. The only similarities between early rotary *piston* engines and Wankel style engines is that they both burn hydrocarbons and they both impart net rotational force to do work.

For a double disappointment, the article went on to say a net nothing (that hasn't been said for years) about Mazda's future plans for the rotary.


Re: Rotary Engines Reimagined
2017-Mar-14 12:38 PM

Hi, I agree with Charlie that your article is misleading folks by mentioning the old school rotating radial piston engines that early aircraft and some cars made use of. Plus I didn't see anything worthwhile on new concepts to improve the Wankel. I'm seeking a small turbine engine to run a generator for my electric car and hoped there might be a small newly improved Wankel that could be used instead.

Re: Rotary Engines Reimagined
In reply to #4
2017-Mar-14 1:38 PM

Stan, If you do a google search, there are some good articles about Mazda's proposed 16X rotary (increased displacement; direct injection), and the Skyactive-R sub-100 HP 'sustain-er' rotary that got only a tangential mention in this article.

FWIW, don't waste your time looking for a small turbine. Physics dictate that it's almost impossible to create a small turbine that will have anything better than horrible fuel efficiency. Aircraft guys have been chasing that unicorn for decades. The killer is the turbine's very poor 'compression ratio' compared to modern piston engines; small ones do well to hit 3-1, which is worse than a Model T Ford. Even larger turbines have a difficult time matching piston fuel efficiency; their forte is high altitude operation and cannon-proof reliability. Neither is much of a factor in automotive use.


Re: Rotary Engines Reimagined
2017-Mar-15 6:24 AM

I agree with charlie, though the title of the post suggested something about future of RX series but not much is told about. Moreover there is confusion created between Rotary and Redial Engines.

Another point is that for smaller size turbines are no match with reciprocating IC engines in terms of fuel efficiency only turbines will be non pulsating and vibration free.

Re: Rotary Engines Reimagined
2017-Mar-18 8:00 AM

Use a 50cc two stroke engine

Re: Rotary Engines Reimagined
2017-Mar-28 3:03 PM

Gentlemen, thanks for the note on how inefficient small turbines can be. I should have looked at my notes from Penn State's turbine class I took in 1960. What about using a mini Wankle engine that's used for RC aircraft.

Re: Rotary Engines Reimagined
In reply to #8
2017-Mar-28 6:06 PM

If you're not in too big a hurry, Mazda will likely make one available from a low mileage wreck sometime in the next 3-4 years. They've shown one in the sub-100HP category that's specifically built for that purpose.

There's a high probability that any RC category rotary will be 'charge cooled', and the fuel efficiency will closely approximate a small turbine. :-(

The Europeans have a couple in that power range that are used for drones, self-launching sailplanes, etc. (It's a shame we're the world leader in everything; it makes it really hard to get neat stuff, sometimes....)


Re: Rotary Engines Reimagined
2017-Mar-29 7:40 AM

At least re-design a Twin-Rotary with Twin-Turbo as a starting point. Mazda needs to make it worth while and, I know they can...!!!

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