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Comment by npunt | Hacker News Reader

I've always seen rotary engines as one of the best examples of a particular approach to design that solves something very elegantly but relies too much on one critical component where all the pressure of the design is applied. In the case of a rotary engine, it's the apex seals.

They're a great warning for designers/architects/engineers to not get too enamored with the elegance of a system if parts of it are not yet completely solved. It's so easy when designing to try to shove aside some complex problem and say you'll solve it later, or play some shell game where every time you hit some hard to solve problem you wind up shuffling it around to someplace else [1], but that kind of instinct ultimately leads to unworkable things in practice.

[1]: 'we'll solve the seals problem later.. maybe materials has an answer' or 'just add oil in the mix to protect the seals there, we'll solve emissions later'

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I would just add that sometimes ideas exceed the technology of their time. So revisiting a design that had deficiencies (weak points, high production costs, bad emissions, etc.) with new tools, materials, etc. can lead to breakthroughs. Not that that's what is happening here, just why some ideas that previously didn't work seem to circle back around.

I feel like software is the poster child for this. My hunch is that a lot of techniques have been dropped for performance penalties that may be on the order of 20-30% (totally guessing here) when Moore’s Law often has covered that gap in mere months.

I read a comment on HN a while back that you can look like a genius in software by going back about 10 years and finding something forgotten. The whole web development shift from server-side to client-side and now drifting back to server-side as if it’s something new seem to validate this. Though in this case it just seems like the extension of the decades long back and forth of going from mainframe with super minimal client, to PC only, to networked, to client/server, to server-side web, etc, etc.

>20-30%

Your total guess is orders of magnitude off.

Most software today is in the ballpark of 10,000x slower than it should be.

That's not what they're saying. They're talking about things that couldn't be done due to performance issues but are now possible thanks to improved hardware.

Software developers rarely question the hardware they are developing on/for. Unless you were developing in the pentium and earlier era, it has almost definitely not been true for you that “the hardware is not capable of running this idea” (ignore certain obvious high performance sectors).

Dunno, there are some pretty large exceptions to that.

The AI boom is almost entirely a thing due to advancements in hardware. You simply couldn't build ChatGPT with 2014 hardware.

10 years ago we designed a system to approximate a chunk of maths we did not have time to do every second, too slow.

If you started from scratch you'd never even consider doing anything other than the original obvious "correct" maths. Approximation be damned.

This factoid about web dev misses the fact that the JS developers today are often the same people who were writing CGI, Perl and PHP, some of us also ASP and later ASP.NET, or some Java. We always knew there are good parts about the old ways and merging the two was always the goal - it just takes some time to get the client side right and then to get the merging right.

This especially applies to React core devs. Remember XHP?

It’s hard to tell if you can improve technology until it’s good enough (jet engines) or if it’s a futile dead end.

Kelly Johnson of Skunkworks had a keen mind for differentiating between them.

It is indeed and sometimes it can go either way depending on the time point. A good example is what eventually became the PW GTF, the PW8000, discussed here:

https://cornponepapers.blogspot.com/2006/04/short-life-and-u...

Just a little bit to early but when they came back to the idea they ended up with a technology that they use on all their new (big) commercial engines.

For anyone that doesn't know the name Kelly Johnson I recommend "Kelly: More Than My Share of It All". A rare person who combined technical genius with an ability to get large scale things done.

Adding to the list of deficiencies, Apex seals!!

No, rotary engines are terrible. For one, the equivalent of variable valve timing doesn't exist/cannot exist for rotary engines, unless Mazda creates a miracle. VVT and VVL lets you do some insane things in terms of efficiency and good engine behavior, so the rotary falls behind a lot.

The rotor is pretty much unable to be cooled too.

I'm not saying rotaries are great, but two stroke piston engines can have variable port timing (typically called a "power valve"). I don't see why the same method couldn't be applied to a rotary.

The same reason we don’t use two strokes anymore? They’re environmental disasters and wildly inefficient.

The comment I replied to said a rotary couldn't have variable timing and so I was simply refuting that claim.

I'm not trying to make a case for either engine here.. there's clear reasons why four cycle piston engines won out for the most part (other than a few powersport and power equipment niches).

I completely agree, there's no reason any engine can't have VVT. I believe that famous northern european supercar guy, I think "Keurig" or something like this is is name - he took an old beater car 4 cylinder engine and superpowered it with some pneumatic / AI controlled valves - I think it's called "FreeValve" or something like this.

As for two strokes being "less efficient", efficiency is in the eye of the beholder. We see on the recent key bridge collapse lore that indeed these mammoth cargo ships run two stroke engines, in this case, two stroke turbodiesels which formerly were the mainstay engine of american trucking. The humble weed eater or chainsaw also tends to use two-stroke - why? because they're far more efficient, almost twice as efficient, in power-per-pound of engine i.e. in applications where lightweight is needed. An added benefit is mechanical simplicity. The double-power notion follows from the name, these engines give power every two strokes versus every four strokes. Two strokes are also really interesting in that exhaust manifold geometry interacts with the engine stroke with pressure waves, look at any two-stroke dirtbike and some will have these goofy-appearing exhaust manifolds for this reason. Granted the major challenge of two-strokes is dirty/pollution and being oil-burning by their nature.

The two stroke diesel isn't really comparable to typical two stroke gas engine ala. chainsaws and dirtbikes.

Because they use forced induction, they don't use the crankcase for scavenging and thus have a "normal" four stroke style oil sump.

Since the total loss lubrication system isn't used, they aren't actually any dirtier on principle than a four stroke.

The big ship engines also turn very slow so a lot of the physics behind it is different, and different fuels are used compared to road going diesel (different grades of bunker fuel).

You are correct that GM used to make a two stroke diesel for over the highway and medium-sized equipment use but they were phased out because they weren't competitive on fuel consumption and their crude superchargers were incredibly loud. Growing up my Dad owned a compactor with one and I'd have to wear ear plugs under my ear muffs to not be deafed by "the screaming jimmy" (Detroit 471, IIRC).

That hasn't been true for a long time. Two strokes barely produce more power per CC, and the emissions are not compatible with any modern standards. They're used in yard tools becuase they're CHEAP.

Power per CC is the wrong way of looking at it. A 250cc displacement two stroke is still much lighter and more compact than a 250cc four stroke engine so you want to focus on power per weight.

The cost argument probably is the main driver for yard tools (Honda makes a great 4 stroke string trimmer for instance) but Endurocross bikes, trials bikes, and mountain snowmobiles especially still use two strokes for the power to weight advantage. The BRP snow machines even have direct injection which really helps emissions, though obviously they still have total loss lubrication so they're dirtier than a four stroke.

I think it only turns to charge the battery, so probably it turns at it's most efficient speed at all times. Does variable valve timing help in that case?

That said, the article doesn't describe any benefit to using the rotary engine here either.

I wonder if we're missing something?

Mazda implemented "auxiliary ports" in the fb to Fc transition ~85, the 5th and 6th ports were vacuum actuated rotating sleeves that increased intake volume and duration; this was available before VTEC and Variocam.

Mazda has a super interesting way of doing this. When paired with an electric motor/generator, they can speed up/slow down specific parts of the combustion cycle to essentially get vvt. I think they can only really do this as a range extender due to the fact it makes power delivery a little bit weird.

https://carbuzz.com/news/mazdas-new-rotary-engine-variable-v...

In hub motors might be another automotive example of this. They were used by Ferdinand Porsche and invented in 1896. Ever since engineers keep trying them out as a simple elegant solution. Eventually they will break through I think, but there’s the same allure in the simplicity of it all.

I admire the idea of hubmotors as well, but once mentioned them to a car engineer, who basically said "think about unsprung weight".

Which, now that I think of it again, is a bit silly because most EV mass is battery, not motor.

Shaving unapeung weight down, even by a small fraction of the total vehicle weight, will dramatically affect handling and perceived ride quality. Even though the battery makes an EV heavier than an equivalent ICE car, there is still the need to reduce unsprung weight. Thus, it would be better to have the motor inboard and run a cv axle out to the wheel than have the entire mass of the motor out on the wheel.

Unsprung weight does not have a linear impact.

Increasing the unsprung weight by 10% causes a 100% deterioration in handling and ride quality.

You're suggesting increasing the unsprung weight by about 400%.

> once mentioned them to a car engineer .... now that I think of it again, is a bit silly

No offense, but this right here is peak HN.

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