Mazda’s Rotary Range Extender Might Succeed Where Others Have Failed

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In a recent press release, Mazda Europe finally announced its highly-anticipated MX-30 e-Skyactiv R-EV for the European market. The announcement happened at the Brussels Motor Show, and since then various automotive news outlets have been sharing details like pricing (starting at just under $35,000). But, I want to discuss why this new application for a technology known for being dirty could be an important bridge to a clean, fully electric future.

Talk Nerdy To Me

This is a cleantech site, so I’m going to start with the technological details first, and then explain further down why this isn’t just yet another plugin hybrid.

For those unfamiliar, the MX-30’s rotary engine doesn’t work like most other automotive engines. Instead of reciprocating pistons, connecting rods, and a crank (all fed by a complicated set of valves), a rotary engine is best described as a “spinning Dorito.” Here’s a video that uses a 3D-printed model of a rotary engine to describe how it all works:

The design allows for a small engine to do the work of a much bigger engine (with some important limitations I’ll get to in a minute), and it runs a lot smoother than most combustion engines.

Instead of using this kind of engine like it did with the RX-7 and RX-8, Mazda decided to only use its latest rotary engine to turn a generator. There’s no mechanical link between the rotary engine and the wheels, and the only way power from the engine can get to the road is via the generator, battery pack, and main electric motor that pushes the car. In other words, it’s a series hybrid, and not a parallel hybrid like many hybrids these days.

The MX-30’s battery pack is only good for around 85 km (around 50 miles) of range, which is more than enough for most people’s daily driving needs. If the battery gets low (or sooner if you desire), the rotary engine can burn some gas to recharge the battery pack and give you more driving range. Put more simply, it’s an EV for 50ish miles and then it’s a hybrid with an unusual engine.

The press release says that you can also use the rotary engine to generate electricity for emergencies, camping, or jobsites.

It Took Some Work For The Rotary To Serve In This Role

Various automakers experimented with the Wankel Rotary design during the 2oth century, but most abandoned it because it was too inefficient. Mazda was particularly stubborn, and knew that a low-volume production car with the rotary’s unique driving characteristics (smooth power up to 9,000+ RPM) and sound could be successful while it worked to improve the design further.

Ultimately, though, everybody but Mazda gave up on it when emissions standards became a thing, and Mazda really struggled just to keep one model (the RX-7, followed by the RX-8) on the market. The problem is that in city driving, a rotary engine both has a short lifespan between rebuilds, often lasting less than 100,000 miles. Plus, even the last version of the engine, the Renesis, gave a small car truck-like fuel efficiency.

Some owners got much longer lives out of their engines and much better than average fuel efficiency, but it was almost always people who primarily drove their RX-8 on the highway at a steady speed.

When the last rotaries went off the market in 2012, Mazda kept experimenting with the design behind the scenes. Mazda knew that at a steady speed, the engine did pretty good, but people don’t drive just one speed. Plus, they figured out how to incorporate homogenous charged ignition (Skyactiv-X) to the design, further increasing its efficiency.

What this ultimately gives you is a gasoline engine that has the efficiency of a diesel engine (because of the partial compression ignition), but without the diesel exhaust particulates and other nasty diesel exhaust! Putting a Skyactiv-X rotary engine in a generator role further improves fuel efficiency because it can always run at a stead RPM instead of having to go up and down in RPM as the vehicle’s speed varies (especially in city driving).

Other Advantages Of The Rotary As A Range Extender

There are several big advantages to using a rotary engine over a piston engine for a range extender.

Perhaps most importantly, rotary engines are small. This means that instead of having to design the vehicle around the engine (like you would in ICE and hybrid vehicles), you can optimize the vehicle to be an EV first and then figure out where to put the rotary engine and the generator it turns. This means the MX-30 is natively an EV and not a modified ICE car with batteries put in somewhere as an afterthought. Interior space, handling, and overall usefulness is greatly improved.

Another major benefit is that rotary engines are light. Most plugin hybrid drivers (at least the ones who pay for their own fuel, unlike the subjects of most flawed studies on the topic) plug the car in at night. This means that 90% of the time, they’re lugging around a big, heavy piston engine they’re not using. This is bad for overall efficiency. But, the rotary is small and light, so it doesn’t waste a bunch of electricity most days.

Another big advantage is that rotary engines are inherently very low on harshness and vibrations. They don’t reciprocate, so they won’t vibrate your car and make other common engine noises. They aren’t completely silent (especially if you don’t have a good exhaust), but a rotary range extender won’t disturb the peace of an EV driver nearly as much as a piston engine.

Why This Is Important

Ideally, we’d make all cars EVs. I’m not going to argue that plugin hybrids are inherently better, because that would be foolish in most cases. But, battery supplies are limited right now. We simply don’t have enough battery cells to give every driver an EV with 300 miles of range. Fixing that situation will take a number of years, as we have to get permits, check environmental impacts, and actually dig the mine. Refining facilities must be built. Battery production lines must then be built. Getting enough batteries for everybody to do everything on battery could take a decade.

But, it’s a decade we really don’t have. If we can get more people to switch to a plugin hybrid today and then make their next car a full BEV, that 90% reduction in emissions is a damned good start.

This is particularly important for electric pickup trucks. The American way is probably going to require putting massive 200-500 kWh battery packs to get good range while towing and just lug heavy vehicles like the Hummer EV around. Making those massive battery packs and then getting poor MPGe ratings means that these vehicles may be just as dirty as some of their gas and diesel-powered brethren.

But, we could skip those impacts and save vital battery materials for more vehicles by incorporating rotary range extenders into electric pickups. With enough battery power for local driving (often a commute to an office job), but enough range on gas to go camping a few times a year, the overall environmental picture would be a lot better for the industry.

All of this having been said, it’s important to keep in mind that this is just a stepping stone. We can’t let the industry get hooked on range extenders and not actually come up with the battery supply for the next generation of EVs to have batteries.

Images provided by Mazda.