Wankels have tremendous power-to-weight and power-to-size ratios. Their main problem is reliability. The generally accepted solution to improve rotary engine reliability (oil injection) results in poor emissions. The wide, flat-ish combustion chamber doesn't help the emissions problem, either.
The Wankel is at its most efficient and its most reliable when operating at a constant RPM. Conveniently, the EV generator application demands a pretty flat RPM band. As a result, the engine doesn't need to lean as hard into those emissions-increasing compromises.
Thus, EVs allow the Wankel's benefits over a reciprocating-piston engine to be reaped without the same costs as before. In theory, at least. It remains to be seen if the benefits will outweigh the drawbacks. I'm glad they're at least going to give it a try.
Technically, the Wankel rotary engine design allows much higher power output than an I-4 or even a V-6 engine design due to no strains on the rods, and much less strains on the piston(s) as those literally blow up all the time in I4 engine designs in similar power output ranges.
I've seen many very reliable 3 or 4 rotary wankel swaps in RX7 FD3S that output far beyond anything imaginable any V8/V12 could produce. 2000hp is not a joke here, when being run on pure ethanol as fuel input. Apart from fuel injectors, clutch and gearbox, these engines run very stable and reliable.
There was an RX-8 Blue model being sold in Japan which was burning hydrogen directly, effectively producing water as output, which, in the prototype was being converted back to hydrogen via a fuel cell. And this was in 2004.
I wish there were more Wankel engines being used as "pocket generators", because they can reliably run on synthesized alcohol and hydrogen and be a potential generator replacement for all that Diesel based crap that's being used in rural areas.
Imagine a solar roof on your house that produces hydrogen with some fuel cells (which also produce heat for your home). This could be the optimum cycle for use in a decentralized home, as chemical energy storage has no loss compared to li-ion batteries that have a limited lifetime. The multiple use of hydrogen (e.g. a stove just needs to burn the gas) also makes it very low tech, and possibly much more reliable than a circuit based system where transformers might fail over time.
But of course, can't sell decentralized approaches via gas stations, so it will never take off...
While I also share in your love of Wankel engines I think you are grossly skipping over their unreliability (mostly around the seals).
There's an extremely good reason "rural" applications are reliant on diesels. It's not uncommon for a diesel engine to hit 300k+ miles and still operate reliably. And not to mention simply. They are relatively easier to maintain than a standard gasoline engine.
Diesels also greatly benefit from being on a fixed power band. It's why they work so dang well on large ships.
Where as rotary engines(rx-7's at least) need a rebuild every 80-100k miles. So I think without really knowing what you're doing mechanically or having access to a mechanic that can repair those kinds of engines you aren't going to be super reliable with a rotary.
Green hydrogen approaches via rotary engines does seem quite interesting. I think if the reliability of the rotary could be improved that could have some serious merit. Or at least making small enough generators that you can easily ship them out to be replaced/repaired could have serious merit.
My guess is this: Mazda believes its battery-only range is enough for most driving situations. If they're right, the rotary engine's more limited lifespan would be a moot point since it will still last the vehicle's entire lifetime. One can only hope.
The maintenance thing is concerning, though. Nobody knows how to work on these things. As an RX-7 or RX-8 owner, you're either doing most of your engine work by yourself, or you're driving very long distances to go to a specialty shop. We can pray that it's reliable enough for this to not be a problem, but if these engines start failing, it's going to be a huge mess.
Side note - the geometry of a Wankel rotary engine makes a diesel version impossible - you can't get enough compression. Diesel rotaries do exist though. An American company called Liquid Piston is making them for the Army. The intent is to use them in diesel generators, since the resulting generator is much smaller and lighter than more traditional diesel generators.
"The multiple use of hydrogen (e.g. a stove just needs to burn the gas)"
Oh, no. You don't want to be cooking over a flame capasble of melting platinum. You will utterly destroy your pots and pans temper. I do jewelry work, hydrogen gas is one of the hardest gases to properly work with. I'd rather let a newbie play with oxy-acetylene than a hydrogen torch.
Could you elaborate on why this is? Naively, the temperature of a simple hydrogen flame is not too much hotter than a methane flame. And in torch form, it seems like MAPP is several hundred degrees (C) hotter? And oxy-acetylene a few hundred more on top of that. Any torch seems to have way more than enough heat to melt platinum (1700C)?
A hydrogen-oxygen flame (this is what happens naturally since you're burning hydrogen in open atmosphere) burns closer to 2800C, and also has the fun property of causing embrittlement (which is especially worsened at higher temperatures where metal expands and becomes more porous.) Your steel pots and pans are essentially toast under a hydrogen flame.
When I was doing a bit of research, I was coming up with 2050C or so for hydrogen open flame (in air; with pure oxygen it was 2600C), and about 1950C for methane in air. Natural gas is mostly methane and has a pretty similar flame temperature, but it does not melt our pots and pans. Why would hydrogen? It is hotter, sure, but not dramatically so.
Hydrogen really likes to strip out all the carbon in your steel, leaving you with pure iron (given enough time). It's a well known phenomenon called hydrogen embrittlement.
Where are you seeing all these magically rotaries?
Yeah the guys in NZ and maybe some Americans(Rob Dahm) have some high horsepower rotaries but let's not pretend it's the norm and that they are more reliable than an equivalent piston engine.
Built Nissan VR38DETT V6 produces 2000hp. A full billet block will produce 3000hp+. Nissan GT-R guys(T1, ETS,AMS)
Let's not even get into the big block V8's because these things will run all day at that hp and be a whole lot more reliable than anything mentioned here.
Reliability is something easily discounted because the data to characterize it is much more difficult to capture than performance data. In most applications you can work around this with redundancies and diverse technologies, but no one makes a fault tolerant powertrain due to cost.
I don't think there's a good reason to keep pushing down deadend reliability paths. We should be responding to our hard earned decades of learning and be pressing advantages. Not every novel and viable solution ends up being an enhancement.
Except in Mazdas case the data was there to characterize it. They were notorious for failing very early and suffered bad seals around the rotary shaft. This has been well known for a long time. Many Rx owners knew ride it for 100k miles then sell it before it is too late.
Right but then why not use a more efficient and reliable engine? The motors will last hundreds of thousands of miles. The power circuitry should have a similar run if kept cool. Why have an ICE that's more likely to fail with lower thermal efficiency?
The engine used in Toyota's most popular cars runs in Atkinson most of the time with a 40% thermal efficiency.
Because the engine isn’t the only thing being optimized. Suppose your options are 50 miles of effective plug in range and a 40% efficient engine or a 35% efficient engine and a 100 miles of plug in range. The second option might use a lot less gas.
It’s not just the cost of batteries that’s a concern resulting in short plug in hybrid ranges. Weight is a real limiting factor when you also need a gas engine and large fuel tank.
Atkinson engines have poor power to weight ratio at maximum efficiency, if you’re going to maintain that 40% then the engine needs to be oversized and under utilized. This is why many hybrids have seemingly much more power than they need. Thus the 194HP Prius vs 75 HP on a CX-90 plug in SUV.
Also it’s not just that you can have extra Li+ it’s also pushing around less dead weight when the engine is off. So you can get fairly close to a 50 mile range difference depending on exact setup.
Alternatively you can have a smaller Atkinson engine and a larger reserve on the battery, but that also costs range while still being heavier.
It could make more sense to use a conventional engine, but get extra expansion (the kind an Atkinson/Miller cycle engine gets from the longer expansion stroke) by turbo-compounding. That is, exhaust gases (still at greater than ambient pressure) go through a turbine that is coupled to the output shaft, or to another generator, rather than being used to drive a compressor as in a conventional turbo. Or, attach a motor/generator to a conventional turbo and be able to have it operate in both modes and use the battery to help overcome turbo lag.
It might make sense to look into a turbine engine on a hybrid. Though that’s probably been investigated I doubt anyone has invested the kind of time and money needed to make a real shot of it.
> The engine uses port and side direct fuel injection systems (PFI/GDI, referred to by Toyota as D-4S); a cooled, external exhaust gas recirculation system (cEGR); and a wide range of authority variable valve timing with electric phasing on the intake camshaft and hydraulic phasing on the exhaust camshaft. Atkinson Cycle is implemented using late intake valve closing (LIVC). Effective compression ratio is varied by varying intake camshaft phasing.
I had an RX-8 for years. It was made in 2007. Someone defaulted on the loan or lost it in a divorce and drove it into a lake, marking it salvage.
I bought it for 25k and was the owner from 2009 to 2019, from 60k to over 150k mi. The maintenance was a bit pricey, due to the fuel injection problems and the electronics would fail in extreme heat of sustained 110+ outside temp (southern california heat) or cold (seattle mornings). I eventually sold it for 9k to a collector. Not a bad deal at all.
$25k USD? That does sound really high for a salvage RX-8 in 2009. My 2005 RX-8 was about $12k CAD in 2011 and was not salvage status. Though I guess yours was just a couple years old, mine was older, but still.
In regards to reliability, not long into owning it I had to get the engine rebuilt due to low compression (luckily JUST within the extended warranty period which Mazda offered due to the very issue everyone in here is mentioning, loss of compression due to failing seals).
A new RX8 Gran Touring in 2009 was ~38k out the door, iirc.
I completed the purchase of one red new gran touring from Mazda, for delivery. The inventory was already taken, so they tried to swap a yellow and I cancelled. The next shipment was in another 8 months. Obviously, the internet RX8 community was active and I knew what was a reasonable pricing, at the time. I looked locally and I had a salvage checked out at Mazda. It was clean. As I mentioned, I sold it to a collector for 9k ~10 years later, as it was still in near-mint condition, regardless of the title status.
> I don't think there's a good reason to keep pushing down deadend reliability paths. [..] Not every novel and viable solution ends up being an enhancement.
That's a strange take.. It took decades to get reciprocating engines to be reliable, even deployed at scale. It also took decades to get jet engines reliable enough to comply with ETOPS requirements, to the point that it was long considered a pipe dream.
If it hadn't been for continuous investments over long periods of time and incremental adoptions of improved technologies, we wouldn't be enjoying any of their benefits today.
>In most applications you can work around this with redundancies and diverse technologies
Are you proposing that Mazda should put two engines in their cars? Or am I misreading this?
>hard earned decades of learning and be pressing advantages. Not every novel and viable solution ends up being an enhancement.
That's precisely what Mazda is doing. They poured decades of R&D into the rotary engine, and are leveraging that research to their advantage. Their engine's diminutive size means they can provide more interior space in a compact vehicle, which is one of the key benefits of an EV that you can't get with a conventional hybrid. There are innate material advantages to the rotary in this application, it's not just hand-waving.
There's nothing overly novel about series hybrids, either. Chevy proved they can deliver EV driving dynamics and efficiency with ICE convenience, even at a larger scale than Mazda appears to be targeting.
This entire comment perplexes me. I can't tell if you like what Mazda's doing, or if you dislike it.
The F1 MGU-K is geared to the crankshaft via the timing gears. The wheels are absolutely NOT electrically driven, it’s working as a torque fill for the conventional powertrain.
What happened to the inside out rotary that was supposed to be extremely compact and would enable cheaper PHEVs? IIRC the patents were about 5 years ago.
If you're referring to Liquid Piston's "X-engine," as they call it, it's doing well. It landed a substantial military contract and will likely be used to make super compact, lightweight diesel generators.
> The Wankel is at its most efficient and its most reliable when operating at a constant RPM. Conveniently, the EV generator application demands a pretty flat RPM band.
If I understand the article correctly, the "series" hybrid configuration means precisely that it cannot operate at the ideal rpm when charging the battery, because it is always driving the wheels directly as well.
Exactly the opposite. In a series hybrid, there is no mechanical connection between the engine and the wheels - only an electrical one, with the battery still providing considerable power in most cases, even when it's depleted.
No hybrid ever runs its battery all the way to zero, and series hybrids are no exception. The Chevy Volt (the other notable series hybrid in Western markets) keeps its engine at a near-constant RPM by using the battery to supply torque, even when the battery is "dead." I would expect the MX-30 to do the same.
IME in several thousand miles driving/riding in a Volt, it hums away within a very narrow RPM band unless you hoon it on a dead battery in cold weather. Even then, it's not nearly as wide of a rev band as a conventional hybrid.
The part that's got me confused is the illustration near the end, with the caption: "In the MX-30 e-Skyactiv R-EV’s electric propulsion unit, the already compact rotary engine shares a common axle with the generator and motor."
I don't get how the engine can be decoupled from the wheels if it's mounted to the same axle as the motor?
The Volt is a series/parallel hybrid. It has situations where the motor is directly connected to the wheels. The BMW i3 does has a series only set up, but that’s probably because it also has a BEV only version.
I think you have series and parallel hybrids backwards. Series hybrids have no mechanical connection between engine and wheels. It's just an EV with a generator wired in.
Wankels have tremendous power-to-weight and power-to-size ratios. Their main problem is reliability. The generally accepted solution to improve rotary engine reliability (oil injection) results in poor emissions. The wide, flat-ish combustion chamber doesn't help the emissions problem, either.
The Wankel is at its most efficient and its most reliable when operating at a constant RPM. Conveniently, the EV generator application demands a pretty flat RPM band. As a result, the engine doesn't need to lean as hard into those emissions-increasing compromises.
Thus, EVs allow the Wankel's benefits over a reciprocating-piston engine to be reaped without the same costs as before. In theory, at least. It remains to be seen if the benefits will outweigh the drawbacks. I'm glad they're at least going to give it a try.