Published on May 29th, 2019 |
by Jennifer Sensiba
May 29th, 2019 by Jennifer Sensiba
Yes, I know. The Wankel rotary engine has a reputation for poor gas mileage, bad emissions, and even longevity issues. That’s why I was skeptical, too, when I heard that Mazda is bringing the rotary back not only as a range extender, but to provide direct power in some cases. After doing a lot of reading on recent research and development, I found that the rotary engine may really come back with its problems solved.
If this goes the way I now think it will, the return of the rotary engine may prove to be a good step toward the transition to renewable energy.
In this article, I will cover the following:
- A quick introduction to the engine technology and its history (for those unfamiliar)
- More recent developments with the technology, including outside of automotive use
- Ways the engines will likely be cleaned up and made more efficient
- Why the engine would be a much better fit to pair with electrification than piston engines and,
depending on the use case, may enable a PHEV that is better than some BEVs
While I don’t have access to a time machine, I’ll start by doing my best to take us back to World War I, where this all began.
- 1 The History and Development of the Wankel Rotary Engine
- 2 A Less Complicated Machine
- 3 1960-1980: Big Names Find Problems With The Design (& Solve Some)
- 4 More Recent Progress with Rotary Engines
- 5 Cleaning Up The Rotary Engine
- 6 Mazda’s Next Steps
- 7 Why A Clean Rotary Is Better For The Environment
- 8 Final Thoughts
The History and Development of the Wankel Rotary Engine
Felix Wankel — A Complicated Man
Like many stories from this era, tragedy hangs over everything. Just days after his 12th birthday, Felix Wankel’s father was killed in a brutal and nasty war. Fighting for the German Empire under Kaiser Wilhelm II, Rudolf Wankel died during the fighting in France. Like many others, Felix Wankel and his family were left emotionally broken and financially impoverished after the war. He struggled with school, and fell for the divisive politics of the NSDAP, or the Nazi Party.
Despite his problems, he did have a talent for visualizing complex machines in his head, and proved himself working on machine projects in garages and sheds. By age 17, he was telling friends that he wanted to build a car with “a new type of engine, half turbine, half reciprocating. It is my invention!” He went on to invent an early version of the rotary engine in 1924 and get a patent for the design in 1929.
Yet more war, and Felix’s anger at the loss of his father, kept him from working further on his engine for decades. He was kicked out of the Nazi Party two times, and was even imprisoned for militaristic views that were too much for even the SS. However, he was quite skilled at designing seals and valves for various military equipment, and this earned him the respect of Nazi leadership, who freed him and kept him working through the war on various mechanical engineering projects.
After the war ended, Wankel was imprisoned for a time for his Nazi ties, but was eventually freed. Ironically, his extreme views may have prevented him from getting too close to the leadership, which may have kept him from being involved in war crimes he seems to have been comfortable advocating for. In later life, he told people that the things he said and did during that time in his life were a mistake. Like many other Germans who had fallen for Hitler’s scapegoating and violence, Wankel changed his ways and moved on with his life. It was at this point where he began to see more success.
He did keep up his interest in rotary engines, and eventually was able to secure the funding to build prototypes with Goetze AG and NSU Motorenwerke AG. Wankel’s original design was somewhat complicated (but still simpler than a piston engine), and another designer helped him to simplify it even further.
By the late 1950s, he had running prototypes, and started selling licenses for the engine to a growing number of major automakers and motor companies. In the 60s and 70s, Curtiss-Wright, Daimler-Benz, General Motors, Nissan, Toyota, Mazda, Ford, Suzuki, Yamaha, Kawasaki, OMC, AMC, and many other companies all paid large licensing fees to build their own rotary engines.
Felix Wankel continued to work with his company, Wankel GmbH, and made quite a bit of money during his life improving and licensing the design. Oddly, he never had a driver’s license due to poor vision, but had a chauffeur drive him around in a rotary car. He eventually passed away of old age in the late 80s, but continued to work on rotary engines nearly until his death.
You can learn a lot more about Felix Wankel on Wikipedia.
A Less Complicated Machine
While there are currently no mass-market cars sold with a Wankel rotary engine, its unique characteristics are what generated so much interest in them during the 60s and 70s. The engine’s compact size, simple design, fewer moving parts, and low weight all attracted investment and licensing.
Advantages of the engine include:
- High power to weight ratio compared to a piston engine
- Smaller than piston engines (about ⅓ the size for the same power output)
- (this makes it much easier to put in small spaces and gives auto designers greater freedom to design the rest of the car)
- No abrupt changes in the directions parts move, meaning far less vibration.
- Higher engine speeds because the parts inside are few and move ⅓ the speed of the shaft.
- Doesn’t “knock” easily, so a wider range of octane fuels can be used.
- Cheaper to mass-produce, because the engine contains fewer parts
- Easily adapted to a wide range of alternative fuels, including diesel, kerosene, various gases, and hydrogen.
I could give a detailed explanation of how the engine works, but that has been done well many times on the internet, so I’ll direct you to the best resources. If you’re more of a reading learner, MechStuff has a very simple explanation with illustrations, and HowStuffWorks has a longer explanation with pictures of the parts. If you’re a visual learner, I’d recommend the below video that explains the differences between piston and rotary engines, and EngineeringExplained’s use of a 3D printed model of the engine to explain it. I’d also recommend another EE video that shows the real parts in action.
Finally, don’t miss out on this video showing slow motion of a see-through miniature rotary engine:
If you look closely, you can see the intake, the combustion (burning), and the little puffs of smoke exiting on the top-left of the little engine.
1960-1980: Big Names Find Problems With The Design (& Solve Some)
Now that you have a basic idea of how the engine works, let’s talk about why we don’t see rotary engines in new cars today. After licensing the Wankel/NSU design, automakers like Mazda, GM, and AMC tried to make more refined designs to go in real cars. That’s where the design’s problems really started to become apparent.
Mazda was the first to really jump in. They were among the first to license the design in the early 1960s, and immediately started finding problems with their first prototype, which they called the 40A. Apex seals would vibrate and resonate as the rotor turned, and the engine was burning a LOT of oil. With some design changes, Mazda’s engineers were able to solve these problems and, by 1965, were selling cars powered by the engine.
Mazda continued to make gradual improvements and experiment with different designs. They eventually were producing fuel injected versions and some turbocharged models. They were putting rotaries in all sorts of vehicles, but when buyers started wanting better fuel efficiency for mainstream cars in the late 1970s, Mazda started only putting the engine in sports cars like the RX-7.
General Motors also had a fairly serious rotary R&D program that almost made it to production. After licensing the design, initial testing showed that the rotary’s simple design could last to 500,000 miles and make better power than 1970s V6 and V8 engines, while being smaller and lighter than their first aluminum engines (which ultimately proved to be a disaster). They decided to work toward producing a rotary for the new Chevrolet Vega.
After years of delays as problem after problem were identified, they decided to use a new car to showcase the rotary engine, the 1975 Chevrolet Monza. The Monza was based on the Vega, but changed to shed the poor reputation the Vega had earned. Monza bodies were even made with a larger transmission tunnel to better accommodate the rotary, but the project was canceled at the last minute and the Monza got piston engines.
GM’s engineers claim to have solved the last longevity and consumption problems right before cancellation, but no information is publicly available as to what they did to accomplish the longevity goal of 500,000 miles.
The demise of GM’s rotary program affected AMC, the company that owned Jeep at the time. They planned on releasing a new car, the Pacer, built around a GM-licensed rotary engine. The car’s unusual styling was made to maximize safety based on the advantages of the compact rotary. AMC also had plans to put rotary engines in Jeeps and other vehicles. With the cancellation of the GM rotary, AMC had to redesign the Pacer to have a less powerful inline 6 piston engine, which ended up taking away most of what would have made the Pacer unique and desirable. Of course, Jeeps and other AMC vehicles never saw a rotary engine under their hoods, at least from the factory.
Despite the problems other manufacturers had bringing the rotary to market, Mazda pressed on for decades. Gradual improvements continued over three generations of the RX-7, but rotary engines continued to need rebuilds far before comparable piston engines. Emissions issues eventually resulted in the RX-7 not being sold in the United States after 1995, but Japanese production and sales continued on a limited basis.
The last production vehicle to get a rotary was the RX-8. Sold from 2003-2012, it had a much improved design that made more power without turbocharging and produced less harmful emissions. All but the last few years of the vehicle were plagued with reliability problems, but many of these kinks were worked out for the 2009 and later vehicles, while earlier cars often got replacement motors under warranty. Once again, stricter fuel economy and emissions standards made it uneconomical for Mazda to continue selling the RX-8. European sales ended in 2010, and production ended in June 2012.
More Recent Progress with Rotary Engines
While automobiles stopped coming with rotary engines, research into improving them did not stop.
The weight advantages of the design found fertile ground in the growing military UAV market. While larger drones like the Predator or Reaper continue to have piston or turbine engines, smaller models like the RQ-7 shadow made good use of the design. Not only were weight savings and low vibration an advantage, but the ability to use different fuels in combat areas with iffy fuel availability was a big plus. This success, while not perfect, led to a variety of other research grants.
Another odd place where Wankel engines found use was in the seat belt mechanisms for some vehicles. Tiny versions of the engine were built to run off the gases from a small explosive charge, which in turn tightened up seat belts during accidents.
Though Mazda stopped selling the RX-8, it did not abandon the design. Not much detail is available publicly, but the company does tell a reporter things from time to time and it has released a variety of prototypes and concepts. Some burned hydrogen gas, which would be environmentally friendly, but even more wasteful than hydrogen fuel cells. Other rumors and concepts centered around using a small rotary engine to act as a range extender or serial hybrid.
Cleaning Up The Rotary Engine
Mazda and the other automakers have known for decades how to make an extremely efficient ICE highway car, but until recently have not had a way to make such a car palatable for customers. Cars like the 3-cylinder Geo Metro could get amazing fuel economy on the highway by pushing a small engine to high throttle at low RPMs, achieving much better specific fuel efficiency than a larger engine running at lower throttle. Such cars’ inability to give good acceleration in the city or even when getting on the highway have made them undesirable for most buyers.
Something very similar could be done with the rotary engine, and it would be even better than piston engines due to the light weight and low vibration, but would suffer the same problems unless there were basically two engines that the car had to choose from. A larger engine could power it in town and while going down onramps, and the smaller engine could power it while cruising on the highway. The expense and complexity of this is probably why nobody really tried it.
Electrification makes this a viable option now, though. By using a battery pack and electric motor to accelerate, regeneratively brake, and get up to highway speed, the only thing a small rotary engine would be needed for is to provide highway cruising power and charge the car. The rotary could be run at ideal RPM and throttle full-time, thus achieving maximum efficiency and durability. Whether installed in a PHEV or non-plugin hybrid, this could be a great development (more on this in the next section).
Mazda doesn’t want to stop there, though. It is looking at making the rotary even more efficient than the comparatively heavy and complicated Atkinson-cycle four cylinder engines other manufacturers are putting in hybrids and PHEVs.
Mazda’s executives and engineers tend to be tight-lipped about how they’re doing this, but one interview with Jalopnik did give us a clue. To make it even more efficient, Mazda plans on applying its Skyactiv-X technology to the design. To put it simply, this would mean using precisely timed injections of fuel and controlled sparks to make gasoline compression ignite the same way that diesel fuel does. In Mazda’s piston versions of this technology, the company has managed to get gasoline engines to rival diesel’s efficiency and do better than Atkinson Cycle engines without the nasty particulate matter and other environmental ills.
Here’s a great video by EngineeringExplained that goes more in-depth on the potential for a rotary Skyactiv-X (aka Skyactiv-R):
Mazda’s Next Steps
While Mazda has been tight-lipped about the future of the rotary, it is actually very transparent about its plans for electrification. Most automakers telling the press about their plans don’t get into the specifics of whether electrification means mild hybrids, parallel hybrids, series hybrids, or battery EVs. Mazda is quite clear: By 2030 it wants some sort of hybrid electrification in 95% of vehicles and battery EVs to be 5% of all vehicles.
The company said last year that it plans to start with two electrified vehicles. One will be a pure battery EV (BEV), while the other will have a much smaller battery pack and a rotary range extender. The architecture for these cars will then be used to create other electrified vehicles so all share the same architecture. Additionally, range extended vehicles will be capable of burning propane to be used for a portable power supply. It’s unclear whether this will be allowed when driving, but that could be a huge plus.
One thing I’ve seen around the internet after these announcements is that rotary enthusiasts are disappointed that the engine would only come back as a range extender. One of the unique things about driving a car with a rotary is the way the power is delivered and the way the power feels. In some ways, it’s the opposite of an electric motor. It has very high horsepower, and lower torque. The driver has to really wind it up to get the power out of it, but it gives very high horsepower, especially when trying to drive a highway speeds (and faster).
Mazda has been listening. In a more recent interview, one of Mazda’s executives said that there will be a versatile “XEV” architecture that will use the rotary for more than a range extender in some vehicles, and that even when the rotary engine is running, it will pass even the strictest emissions laws. This also confirmed the plan to directly drive the wheels in some vehicles, giving the rotary enthusiasts the experience they thought they might miss out on.
Why A Clean Rotary Is Better For The Environment
There are a number of reasons that a rotary engine can be better than piston engines in PHEVs, and even better than some battery EVs, depending on the use case.
First, it’s quiet and smooth, and this fits the feel of an EV better. I remember that in a 2013 Chevy Volt I used to own, the car was very quiet and smooth when driving on battery power. When the battery got low and the car decided to kick on the piston engine, it would sound like the car was groaning and complaining at having to start using gas. The noise was far less pleasant than most ICE engines, and gave a lot of vibration.
Other owners of PHEVs have complained about the unpleasant transition from battery power to hybrid power. It makes people not want to even drive the car long distances. Even worse, some PHEVs will kick in the piston engine at full throttle, even on a full battery, and it feels almost like a punishment for the sin of pressing the skinny pedal down too fast or too hard.
Having a smooth transition back to fossil fuels can make the overall experience much more positive and pleasant for people who might not otherwise consider an electrified vehicle.
Another great benefit is that the engine is lighter and allows smaller/lighter battery packs. While not burning fossil fuels at all is probably better, we have to keep in mind that about 90% of EV charging already happens at home. Public chargers, superchargers, and even level 2 charging at stores and malls aren’t used that much by any given EV owner. We do want to have the range to be able to drive that other 10% of the time, so BEVs need to have large, heavy, and expensive battery packs, and we need access to level 3 charging, which can be expensive and wastes a lot more electricity than level 2.
The environmental cost we don’t consider is that we are carrying around all that extra battery we don’t typically use, and for PHEVs, we’ve had to carry around a big and heavy piston engine that typically doesn’t get used. Neither of those options are particularly good for the environment, and with the limited availability of battery cells, it drives the cost of them up, further slowing down EV and/or PHEV adoption.
By having a battery pack that fits nearly all of our driving and having a small, lightweight range extender, the vehicle will get better range with less electricity used per mile.
Another big plus is the size of the engine. When building a PHEV with a piston engine, automakers have to design the platform around the engine. With a rotary engine’s small size, it can be placed in a variety of locations that best suit the overall goals of the vehicle.
Tesla has proven that flexibility in design can reap many benefits. Because it can place the relatively small drive units and flexibly-shaped battery pack where it wants, the company has been able to achieve:
- Much higher crash safety ratings
- Much better interior room and storage space
- Much lower drag coefficients
- More flexibility with the aesthetics of the car while achieving all of the above
With a small rotary range extender, PHEVs can get many or all of the packaging advantages Tesla achieved. When you don’t have to design the car around a piston engine, good things happen to the rest of the car.
Even for non-plugin hybrids with a rotary engine, the flexibility afforded by the small size can still be leveraged for most of the same benefits.
There’s also the full lifecycle from manufacture to recycling to consider. Because rotaries are smaller, they use less metal and thus take less energy to build. Because they are less complex, less energy is needed to shape many fine components. Even the assembly process of a rotary takes less energy. All of these factors come together to make recycling easier, too.
The other thing is that smaller engines use less oil and coolant. This means less oil will be needed to keep it maintained over time, and the risk of harm to wildlife and loose pets will be far less should a coolant leak occur.
Rotaries are also generally cheaper and easier to rebuild than piston engines. The lower complexity means that unless there has been a catastrophic failure, the rebuilder generally only has to replace worn seals and reassemble the unit. This can make rebuilds more likely than just junking the whole engine or the whole car when an expensive piston engine gives out. Many novice mechanics who wouldn’t consider rebuilding a piston engine have successfully rebuilt rotaries at home after watching instructional videos.
The rotary could prove to be a much better engine for the environment from cradle to grave than piston engines, and help reduce the number of cars that are needlessly scrapped altogether.
Finally, the rotary engine could do a great job of filling in where electric motors have often proven weak. As I pointed out earlier, rotaries are in many ways the opposite of an electric motor. EVs make torque at low RPM, but don’t make as much power at higher RPMs you’d encounter on the highway. This is a perfect time for a rotary engine to kick in as needed and supply some extra highway horsepower for passing and other needs.
The synergy of the electric motor + rotary can equal a vehicle that outperforms most gasoline and most electric vehicles at a much lower price than the competition, while providing the environmental benefits we all need.
It’s important to keep in mind that both piston engines and electric vehicles took decades to improve before they were as good as they are now. The piston engine and EVs both have roots in the 19th century, and competed on roads in the early 20th. Lack of good battery technology held early EVs back, and the introduction of electric starters for piston engines solved many of the problems, allowing for mass adoption of piston engines through most of the 20th century.
After decades, EVs are starting to gain ground again, with better battery chemistries allowing greater range and power. This is an ongoing process that will continue to improve in the coming decades.
The rotary engine is a relative newcomer, and has been improved in thousands of small ways since originally conceived in the mind of a teenager in the late 1910s. Now that it has had some more time to mature, it looks like it just might be able to hold its own and rejoin the automotive world while improving the experience in ways other technology can’t.
We should give Mazda a chance to develop this technology further, and hope for the best. Any effort that helps push our society toward cleaner, more sustainable energy is worthy of our support.