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Comment by hatsunearu | Hacker News Reader

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.

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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.

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.

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).

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...

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