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Comment by phineastcat | Hacker News Reader

The reason Mazda struggled with the rotary engine is due to what their application was. Rotaries work most efficiently and reliably when run for long periods of time (hours) at a consistent RPM. Vehicles with traditional manual or automatic transmissions (what most Mazda rotary cars had afaik) do anything but keep consistent RPM. Mazda subsequently struggled with keeping emissions down, and fuel economy up, while prolonging the life of the engine.

If you tie a rotary to a generator, or true CVT transmission, they're much better suited to the task.

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That sounds like a good choice for a hybrid that uses the rotary ICE to periodically charge the battery.

Or a range extender generator like in the BMW i3

Mazda actually had a Mazda2 EV prototype with a rotary range extender that they unveiled in 2013. Not sure what ever happened with it, though.

I first saw this on the Audi A1 e-tron concept; it was a 250cc wankel for the range extender. I thought that was the future when I saw it, and here I am now with a Vauxhall Ampera (Chevy Volt)!

> or true CVT transmission

What I never understood: why don't do it like on a ship, attach a generator to the fossil-fuel engine, add a battery and supercap pack, and four small-ish wheel electric motors?

That should save the weight of a gear-shift box, complex mechanics on FWD cars (because the axle transmitting the movement energy must be flexible), and it would allow the motor to run at its most efficient RPM count, as well as providing full torque from 0 km/h.

In essence, a Tesla with a fossil-fuel engine in the frunk...

I think a more apt comparison would be with a Diesel-Electric locomotive. They're already the kings of transportation efficiency (or possibly a close second to container ships).

I don't know why tractor trailers haven't followed this same diesel-electric pattern. Max torque from 0 RPM plus the elimination of 18 speed manual gear boxes?

> I don't know why tractor trailers haven't followed this same diesel-electric pattern.

A high-load tractor trailer can deliver around 500 kW of power. I believe the main problem is that there is no wheel-mountable motor available to transfer that kind of power. The Tesla, for example, is rated 345 kW peak in the P85D, but a tractor trailer needs this amount of power continuously. When we do the math and assume 500V (which is a ridiculous high voltage anyway!), you'll need 1000A-rated cables, switches and motors/generators.

That's not your home 2.5mm2 cable, and discarding the waste heat (assuming 95% end-to-end efficiency) at this scale is a hard task as you have, essentially, a rolling 25 kW heater.

With ships there is MASSIVELY more space available, and the heat can be (and often is) used for heat distribution, and a diesel loco also has lots of space available to put components in.

A loco can take up to eight times the same space as a single tractor-trailer...

In ships a portion of the waste heat from the engine is also used to pre-heat the fuel tank and lines to make it easier to pump, as heavy bunker oil is extremely thick at 'room' temperature.

No one uses wheel motors on cars; they sound good at first, but the problem is that motors are heavy, and the last thing you want on car wheels is more unsprung weight: it's very bad for ride and handling. So you still need flexible axles to transmit power from the motor to the wheels. Go look at a Tesla at the showroom and you'll see this in the bare-chassis floor models they exhibit.

Otherwise, you're basically describing the Chevy Volt. There's a reason these cars aren't more popular: they're expensive, and have a lot of drawbacks. Instead of a single drive system like the Tesla (electric motor, axles, single-speed gearbox, and motor controller, plus a big battery pack for energy storage), now you need two drive systems (the former, plus a traditional fossil-fuel engine, and a generator, and a fuel tank). All that hardware takes up space, and costs money, so you end up with a car that's a lot more expensive than a comparably-sized and appointed gasoline car. You could downsize the battery pack greatly to save cost and space and eliminate the 50+ mile electric-only range, but now you're stuck with unimpressive fuel economy: take a look at the Volt's fuel economy when in gas-only mode, it's not anything special. The Prius gets much better fuel economy than the Volt when comparing gas-only operation, and for good reason: serial hybrids are inherently inefficient because of all the power conversions. The Prius does better because it's a parallel hybrid.

Now moving away from the Volt's gas-powered piston engine to a far more compact diesel-powered rotary optimized for single-speed operation might change this equation, I don't know. Of course, as we've seen with VW, diesel has problems with emissions.

I still want a serial hybrid with 4 motors.

The motors wouldn't have to be unsprung weight. They could be inboard, as on the Tesla cars but in pairs. They could be inboard, tipping as needed to eliminate 1 joint per wheel. They could be out at the wheel, affixed to the body, missing the top and bottom to allow the wheel to move around them -- the car effectively being a mag-lev inside centerless wheels.

I want to not have sloppy differentials. Power to each side of the vehicle should be adjusted according to the steering input.

I don't need a battery. Give me a supercapacitor that is just big enough to handle regenerative breaking from 80 MPH to a stop, followed by running the engine while waiting for a drawbridge or freight train.

It's not about fuel economy. It's about acceleration, including 4-wheel fully independent anti-lock acceleration. It's about never feeling the jolt of changing gears, and doing so without the wimpyness of a CVT. It's about getting all this when there is no place to charge an electric car.

If your 4-motor idea really made sense, I think Tesla would have done that. They didn't: they use a single large motor (or two, for the P85D, front and rear), driving a single-speed gearbox, through a differential, to two standard CV axles.

The Chevy Volt, too, uses a single motor. I'm not sure about the Bolt or the LEAF but I suspect they're the same.

>I don't need a battery. Give me a supercapacitor that is just big enough to handle regenerative breaking from 80 MPH to a stop, followed by running the engine while waiting for a drawbridge or freight train.

Then you're going to have lousy fuel economy. If this idea made sense, the Chevy Volt would have used it. They didn't. No one wants to buy an serial hybrid car that has crappy fuel economy. They can just buy a gasoline car for a lot less money.

>It's not about fuel economy. It's about acceleration

The Tesla P85D has this already, and without all the complexity you're proposing.

>It's about getting all this when there is no place to charge an electric car.

If you have a house with electricity, you have a place to charge your car. If you have an apartment with a parking space and no way to get power there for a charging station, then you're too poor for anyone to bother making a car like this for you.

>They could be out at the wheel, affixed to the body, missing the top and bottom to allow the wheel to move around them -- the car effectively being a mag-lev inside centerless wheels.

As an engineer, I have no idea what you're talking about here.

Prior to the P85D, one could have said "if a 2-motor idea really made sense, somebody would have done that". One would have been wrong to say it doesn't make sense.

Prior to the Prius, one could have said the same about hybrids. Prior to Tesla, once could have said the same about electric cars -- and at the time it was true, with nobody wanting what little was offered.

There are numerous city dwellers with plenty of money and no place to charge. Often they live in historical neighborhoods that are all the rage these days. There are also homeowners without garages, and with garages full of stuff like boats and model railroads and other cars.

Also, range anxiety is real. No, I don't want to drive along a route defined by Tesla's superchargers.

That third option for motors probably requires an animation to explain. Never mind; there are at least two other ways that work fine without the unsprung weight problem.

I'm assuming all that weight still gets better mileage than what would basically be a petrol->kinetic->electric->kinetic convertor.

Modern engines are pretty efficient, adding an electric stage would likely reduce that efficiency.

It's not only about the weight. An electric intermediate circuit can:

a) absorb braking energy - that's why I mentioned super-caps, because batteries can't ingest the vast amounts of energy that braking could theoretically produce (usually the brake power is 4x engine power, which means for a 250 kW motor a minimum brake power of 1 MW, and for sportscars likely muuuch more)

b) keep the engine, if not in idle (i.e. you're stuck in traffic), at its most fuel-efficient RPM range - this is something any cheap-ass scooter does, and every diesel-electro locomotive and ship, but no car! Not to mention it isn't just about fuel-efficiency, but also many exhaust gas treatments only work at peak performance under very specific driving conditions (esp. temperature).

The efficiency loss by conversion (97% and better in efficiency class IE4, see https://de.wikipedia.org/wiki/Elektromotor#Wirkungsgrad_und_...) is, taking especially the recovered braking energy into account, smaller than the efficiency delta gained by loss of hundreds of kg of weight (batteries are heavier, yes, but they can also replace "dumb weights" needed before to keep the car on the ground in the "elk test").

Also, less parts that are prone to (dirt) failure, e.g. the power-loaded joints on FWD cars, clutches, bearings, the entire gearbox system, differentials (in 4WD/multi-WD systems!). And you save on tires because in curves the individual motors can adjust speed. If done well, it should be pretty cost-efficient (less stops in the shop), too.

That's a different take on hybrids then? I dunno, I guess manufacturers went for straight electric and skipped that idea, since you can have ICE+fuel OR the same energy in batteries?

Straight electric is usually done without supercaps, thus losing the ability to fully recover braking energy, and batteries still don't have the same range as a tank of fuel, and can't be recharged as fast.

There is one thing that has the potential to kickstart the EV industry: sell the cars, but rent the battery packs, with an industrywide standard on how battery packs look like and how they are swappable by machines.

It's no good if a gas station has to carry swap stations and swap batteries for dozens of different models.

That is what the Nikola Motors One is doing, except it is using a turbine engine.

https://nikolamotor.com/one

How is their maintenance/durability? My understanding on rotary engines is that in order to maintain compression, they have to have a solid seal on very fast moving surface contact points, which is incredibly hard to do. Naturally, the material that produces the seal is going to wear down over time, and to replace it the entire engine will have to be taken apart.

I love the concept of a Wankel engine, and it would be awesome if they've overcome that particular aspect.

Mazda eventually got their engines to consistently last over 100,000 miles with the Renesis in the RX-8. The biggest thing is lubricating the seals; as a result all Mazda rotaries burn a little bit of engine oil to lubricate things, via a metering oil pump. That's the biggest reason why they struggled to keep emissions down.

Perhaps they could come up with some system like what we see in traditional piston engines piston seals - a replaceable seal that goes around the rotary triangular err piston for lack of a better word.

If I remember correctly all the other wankel / rotary engines have just had a lump.

Mazda's automotive Wankels have spring-loaded seals on the apex of the rotor and on the sides.

See this image: https://hackadaycom.files.wordpress.com/2016/02/wankel-insid...

Didn't know that - thanks for the information. Was it wear against the side of the casing that led to the issues then?

I don't know, but just looking at that photo, and comparing to a traditional piston engine with its piston rings, I can see a bunch of problems:

1) there's a bunch of irregular shapes here to seal: each "chamber" has shorter seals in two places (the vertices), and two longer seals on the sides. So 4 separate seals per chamber (two of which are re-used for adjacent chambers). In a piston engine, each chamber has usually 2 or 3 seals, with one being the main seal (top of the piston) and the others being either a backup or an oil ring (for scraping off most of the oil). A circle is a much simpler design.

2) 4 seals means 4 places where there's going to be gaps where gases can escape between chambers, causing extra emissions. In a piston engine, there's only 1 gap in each piston ring, and you can rotate the rings so the gaps are not aligned. The sealing on piston engines is extremely good.

3) The piston engine is pretty simple really, as far as sealing and lubrication: oil is either squirted (some engines) or splashed up on the backsides of the pistons and drawn up on the piston walls to lubricate the rings as the piston rises, and then the oil is almost 100% cleaned off by the rings as it travels on the downward stroke. This mechanism works so well that modern piston engines burn a truly negligible amount of oil. Oil burning has always been a problem on rotaries, and looking at that photo it's fairly obvious why.

Mazda (and others) has been working on rotaries with very smart engineers for literally decades, and they still haven't fixed these problems. I don't think they ever will; these problems seem pretty fundamental to the layout of a rotary engine. They had some real promise and advantages, but like many technologies, the small details made it impossible to beat incumbent technologies. Piston engines were bad too, ages ago, before they figured out a lot of small details like how to seal them well, how to optimize intake and exhaust valve design and timing, how to best design combustion chambers to maximize compression ratio and eliminate hot spots causing detonation, etc. But some problems just can't be overcome.

The other challenge vs reciprocating engines is that a piston engine cylinder has two volumes; the area above the piston where combustion occurs, and the area below, where there isn't any combustion. So during the stroke, you can spray the cylinder walls with lubricant, the bottom of piston, etc. So the seals can run on a layer of lubricant, and the piston can be cooled by the lubricant. In a rotary, you don't have that. Both sides of each seal are filled with fuel-air mixture, so to lubricate, you have to spray oil into the mixture. It's the same issue as with two strokes. They have to burn oil, and that is really tough on emissions, and it's really difficult to get heat away from the seal (the only place it can go is to the rotor).

That's an excellent point; I mentioned squirting oil before, but I overlooked that it serves another purpose in cooling the piston and the seals. Lots of modern engines now have oil squirters underneath the pistons to do this.

However, since, as you say, they have to burn oil to cool and lubricate the seals, that does make me wonder if this kind of engine wouldn't work much better with a lubricating fuel, such as diesel or kerosene (jet fuel), rather than gasoline. Kerosene is used in jet turbine engines largely because it is a lubricating fuel, so it serves the purpose of both fueling and oiling. The engine in TFA appears to be a rotary derivative, but they do say that it's designed to run on JP-8 jet fuel, not gasoline.

The sibling post to this covered much of it. As a previous RX-7 owner and racer, I can say the apex seals were prone to damage, especially if the engine was ever overheated. Much more so that overheating piston designs. The side seals didn't tend to cause as much problem.

And of course, with the Wankel, if any single seal fails, the entire engine needs to be torn down. With a piston engine, you can often pull the head to perform a valve job or re-ring a cylinder, without pull the main engine block. Not such a big deal on newer vehicles, but as cars aged, it tended to limit their value in the used car market.

"Where the rotor holds the seals in a normal Wankel, the housing does that job in the X1 engine."

I'm not sure how that will affect durability though.

While that's certainly true, others have developed similar small Wankel derivatives and had little luck moving beyond a prototype phase.

LiquidPiston themselves have developed other engines with no market success that I've seen.

There may be a market for these in small UAVs, but I don't see anything here to indicate LiquidPiston is any more likely to "win" than anybody else.

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