Dizzying descriptions of rotary and radial engines

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Dizzying descriptions of rotary and radial engines

Post by Careless Whisperer @ Mon Mar 20, 2017 10:53 am

I suspect a definitive answer will be impossible to find here.

Mazda engines, you know, the one's they're well known for. Often described as rotary engines, more accurately as Wankel engines after the inventor of the concept. However, look up the term rotary engine anywhere and you quickly get info about early aircraft engines with conventional cylinders arranged in a circle, then you'll also find the term 'Radial'.

Do some more searching and you'll easily find opposing views on which is a radial and which is a rotary engine. Common views describe the Wankel as a rotary because the cylinder/piston spins, rather than pushes and pulls. The same views describe the aircraft engines as radial because the cylinders are arranged radially, rather than in-line or a vee configuration. The opposing view is that a Wankel is a radial engine as are all conventional car engines because the cylinder block is stationary and the crankshaft spins, whereas a rotary engine has a fixed crankshaft and a spinning engine block - as per most of the early aircraft engines. This gets confusing when you consider the typical aesthetic of the aircraft engines in question originally had a fixed crankshaft and spinning block, then later changed to a fixed block and spinning crankshaft.

So in the absence of knowing any authoritative authors on the subject, where could one look to find a definitive answer?
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Re: Dizzying descriptions of rotary and radial engines

Post by Dirk @ Mon Mar 20, 2017 10:56 am

WHat exactly is the question? You aren't clear.

Not that I'll know the answer
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Re: Dizzying descriptions of rotary and radial engines

Post by FurDonga @ Mon Mar 20, 2017 11:07 am

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Last edited by FurDonga on Mon Mar 20, 2017 11:08 am, edited 2 times in total.
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Re: Dizzying descriptions of rotary and radial engines

Post by Careless Whisperer @ Mon Mar 20, 2017 11:08 am

Which is the correct technical definition (not necessarily the commonly accepted definition) of a Wankel engine - is it rotary or radial?
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Re: Dizzying descriptions of rotary and radial engines

Post by spast1kunt @ Mon Mar 20, 2017 11:09 am

Radial means the cylinders rotate around the crankcase. Rotary means they don't. Wankel engines are rotaries, not radials.

More of us own Wankel engines than we realise, because some manufacturers including VW and Mercedes use gas-powered Wankels as seatbelt pretensioners.

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Re: Dizzying descriptions of rotary and radial engines

Post by Careless Whisperer @ Mon Mar 20, 2017 11:14 am

spast1kunt wrote:Radial means the cylinders rotate around the crankcase. Rotary means they don't. Wankel engines are rotaries, not radials.

Plenty of web info out there suggesting the opposite, as well as plenty agreeing. Hence the question - where is the definitive proof?

spast1kunt wrote:More of us own Wankel engines than we realise, because some manufacturers including VW and Mercedes use gas-powered Wankels as seatbelt pretensioners.

Image

Interesting. That's my new thing for today. :)
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Re: Dizzying descriptions of rotary and radial engines

Post by Careless Whisperer @ Mon Mar 20, 2017 11:15 am

FurDonga wrote:.

Very informative - spot on in fact!
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Re: Dizzying descriptions of rotary and radial engines

Post by minimoog @ Mon Mar 20, 2017 11:19 am

spast1kunt wrote:Radial means the cylinders rotate around the crankcase. Rotary means they don't.


In terms of aero engines, I thought it was the other way round. Radial engines have a fixed bank of cylinders arranged in a circle, and the crank is attached the propellor and rotates in the middle. In a rotary engine the crankshaft is fixed and the whole cylinder bank, which is attached to the prop, rotates around it. e.g. the Gnome engine.

Radial:

Image

Rotary:

Image
Last edited by minimoog on Mon Mar 20, 2017 11:22 am, edited 2 times in total.
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Re: Dizzying descriptions of rotary and radial engines

Post by spast1kunt @ Mon Mar 20, 2017 11:26 am

minimoog wrote:
spast1kunt wrote:Radial means the cylinders rotate around the crankcase. Rotary means they don't.


In terms of aero engines, I thought it was the other way round. Radial engines have a fixed bank of cylinders arranged in a circle, and the crank is attached the propellor and rotates in the middle. In a rotary engine the crankshaft is fixed and the whole cylinder bank, which is attached to the prop, rotates around it. e.g. the Gnome engine.

Yeah. That's what I said, or meant, or something.

Rotary -

Image

Radial -

Image
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Re: Dizzying descriptions of rotary and radial engines

Post by Pigeon @ Mon Mar 20, 2017 11:26 am

The important thing is that the referent of the term describing the unusual feature of the engine defaults to "cylinders" (or equivalent). So for example an engine with 2 camshafts is a "twin cam", while an engine with 2 cylinders is just a "twin". (Of course you can call it a "twin cylinder" if you want to, and people will still know what you mean, but if you do it the other way round and call a "twin cam" engine just a "twin", they won't.)

So, with a shuriken-shaped engine with a fixed block, the unusual feature is that the cylinders are arranged radially to the crank axis, so it is a "radial engine". (You can also have a cylindrical engine with cylinders parallel to the crank axis, which is an "axial engine".)

With a shuriken-shaped engine where the crank is fixed and the block goes round, the unusual feature is that the cylinders rotate, so it is a "rotary engine". (The same default-referent thing applies: the radial cylinder arrangement is the default for this sort of engine, so you don't have to say "rotary radial engine". But you might have to say "rotary axial engine" if you were talking about one of them.)

With a Wankel or any of the zillion variants on the theme, the unusual feature is that the piston goes round, so it is a "rotary-piston engine". The piston isn't the default referent (ie. cylinder or equivalent), so you have to specify it. People often don't, because they are lazy and don't care about being confusing, but that is a silly way to talk.

You could, if you really wanted to, make an engine where the piston goes round and the "cylinder" is parallel to the crank axis, and have a "rotary-piston axial engine". It would probably be shit though. You could also make it with the crank fixed and the block rotating, and have a "rotary-piston rotary axial engine", but that would not make it any less shit.
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Re: Dizzying descriptions of rotary and radial engines

Post by CJ+ @ Mon Mar 20, 2017 12:34 pm

Despite not understanding half of it, I well :heart: this thread.
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