: Why are pistons round?



weister42
03-20-07, 04:42 PM
Why not any other shape like oval, triangle or square?

illumina
03-20-07, 04:50 PM
My best guess would be that because of the cylindrical shape, said shape promotes balance, strength, proper combustion, less friction, shit like that. Think about it, would the integrity of a square be superior than that of a cylinder? How about square piston rings?

Some "physics" food for thought...

RunningOnEMT
03-20-07, 04:55 PM
the circle is the maximization of area vs. circumference

so there is the least amount of "edge" in contact with the cylinder bore on a circular piston

there are some ovoid pistons, but not on common engines, i know westinghouse uses them on some of their 2 stage compressors so they can fit big enough valve assemblies on the head

and as for shapes with corners, sharp edges are concentration points for heat, too much heat concentration and you'll get autoignition of fuel/air mixture

weister42
03-20-07, 05:06 PM
There's a motorcycle engine that has oval pistons to fit more valves since it's longer, forgot which one but it's from Japan I think.

I was just thinking if pistons were say a triangle with smooth edges, they'll be able to fit more pistons in the same block as round pistons. If it's square with the edges rounded off then it'll have more displacement per cylinder than a round one. I'm sure if you design something like that you'll have to redesign everything else too.

RunningOnEMT
03-20-07, 05:14 PM
There's a motorcycle engine that has oval pistons to fit more valves since it's longer, forgot which one but it's from Japan I think.

I was just thinking if pistons were say a triangle with smooth edges, they'll be able to fit more pistons in the same block as round pistons. If it's square with the edges rounded off then it'll have more displacement per cylinder than a round one. I'm sure if you design something like that you'll have to redesign everything else too.

how would you hone the clinders? anything with a radius like that would require high precision deep bore milling

with a circle you can use a cheap 3 stone hone one a drillpress

if there is a special need to have something other than round i'm sure they'll make it ... like that motorcycle

but everything in the common market is geared towards round pistons

and sure with a square piston you can get more area, but then where do you put the water jacket, and you greatly increase the perimeter, thus the friction, then you also increase heat/surface contact, and you've also compromised strength/rigidity on your cylider rings by introducing straight edges

Patrick7997
03-20-07, 06:14 PM
It was 1991 or 1992, if I remember right....

Honda made a Sportbike, there was something along the lines of 750NR in the model number....

Handbuilt oval piston race bike.

They made a few hundred and sold them for around $55,000, I think...

Manufacturing nightmare, obviously... as was pointed out, the idea is to get more valve area...

Night Wolf
03-20-07, 06:23 PM
corners are also concentrations for stress

cooling of a triangle piston would be hard, the distance form the center to the side is alot less then from the center to a corner.

I've heard of the oval pistons a few times.... but thats it, round is the only thing that makes sense.

c5 rv
03-20-07, 06:50 PM
Why not any other shape like oval, triangle or square?

They tried those other shapes, but they don't fit into the round holes as well.

EDBSO
03-20-07, 07:58 PM
http://static.howstuffworks.com/gif/rotary-engine-intro.jpg

Nice triangular piston here.

ShadowLvr400
03-20-07, 08:08 PM
Another issue I see would be tring to get an even burn. Fuel would pool in the corners, and might not burn fully. Plus you'd also see concentrations of sediment as time went on.

EcSTSatic
03-20-07, 08:59 PM
Nice triangular piston here.

I knew someone would bring up the Wankel. I almost bought a new RX7 in '79 but the salesman pissed me off!

http://www.keveney.com/img/wankel.gif

RunningOnEMT
03-20-07, 09:22 PM
i didnt like the wankel in my rx7 ... i had it pulled out and i dropped in a 350

RunningOnEMT
03-20-07, 09:22 PM
http://static.howstuffworks.com/gif/rotary-engine-intro.jpg

Nice triangular piston here.

nice 2.6 litre POS

Florian
03-20-07, 09:32 PM
you said Wank...


F

EcSTSatic
03-20-07, 09:35 PM
you said Wank...


F

he said "pulled it out"

I~LUV~Caddys8792
03-20-07, 10:09 PM
I gotta wonder why the Wankel Rotary never hit it off bigger?

RunningOnEMT
03-20-07, 10:25 PM
I gotta wonder why the Wankel Rotary never hit it off bigger?

GOD I LOVE THIS DEBATE... i really do ...

here's my take on it... its an amazing piece of technology ... theoretically no redline, tons of power/cubic inch (another debate on functional CID), small, balanced

my theory on why it never made it, is the fanboi crowd got to it ....

its a racing motor, meant to be operated at high RPM and pretty steady state...works great for something like le mans or cart style racing.

but the highly transient city driving and even most urban/suburban freeway/highway driving is murder on these engines.

THEN THE fanboi's ... the typical ricercrowd goes and slaps a fartcan on anything that will make noise, well on these engines, especially the turbo 13brew engines exhaust heat is your enemy, apex seals are already tenuous. well the old rotary addage of boost in, apex seals out...its true you put a free flowing exhaust on a 13brew and it spins up the stock turbos really quickly and the it hits overboost and BOOM!!!!

if there was a society of competant tuners that didnt fight all the time the aftermarket would be more solid and cohesive, and the fanbois might not make such a bad name for the wankel by blowing every single one of them shy ****ing high!

90Brougham350
03-20-07, 10:33 PM
Any otherwe shape thanw a cirlce woudln't have good squish an sd qujench properies. An ovol perjhaps, but triangleas and squares would be diffcult to machine and fiffdicultto to pro duce good saeling ring for

RunningOnEMT
03-20-07, 10:34 PM
durnk pzotsting?

danbuc
03-20-07, 11:53 PM
They make heavy duty apex seals for higher boost applications but they are expensive and require a complete tear down....basically making them out of reach for the average ricer. I would like the rotary engines a little more if they weren't so damn inefficient. They burn massive quantities of fuel relative to they're actual output, and have near zero torque. Unless your drag racing it, it sucks to drive on a daily basis. I've seen some later gen RX-7's hook like crazy because they don't have the low end torque to spin the slicks off the line. Totally impractical for street use. Plus they just sound like ass in general. Anything that sounds like driving around in a pulse jet is just stupid. Oh....and they burn a butt load of oil once those apex seals go too.

xxpinballxx
03-21-07, 09:28 AM
the NR500 was the first use of the oval pistons in the hondas and came into play in the Production NR750 like said before. 1979 was the NR500...so the ideas always made engineers wonder and experiment.

dkozloski
03-21-07, 10:40 AM
Nearly all automotive pistons are oval when cold by a very small amount but become round at operating temperatures. This is called "cam grinding" pistons and reduces piston slap during warmup. The cylinders bores are always round. The reason cylinder bores and pistons are round is because it would be very difficult and expensive to do the machining required to get the system to seal and not have blowby. Besides that the rings rotate on the piston while the engine is running to keep them free in the grooves. Pistons and cylinders other then round would only be a curiosity of no discernible benefit. Besides as any country boy knows pie are round, cornbread are square.

hardrockcamaro@mac.c
03-21-07, 03:43 PM
The rotary suffers badly from wear on the rotor tips. 50,000 or so miles and you're rebuilding it.
It also uses a lot of gas.

dkozloski
03-21-07, 05:31 PM
The rotary suffers badly from wear on the rotor tips. 50,000 or so miles and you're rebuilding it.
It also uses a lot of gas.
The rotor seal problem was cured on the second generation RX-7 engines. In fact it is now the most reliable automotive engine ever made. Only 2 out of every 1000 owners reports engine problems of any kind and the average milage of the problems is 168,000 miles. The rotor tip seals are so durable there is a special box to store them in at overhaul so that they can be reused.

c5 rv
03-21-07, 07:09 PM
In the 70s, the AMC pacer was designed to use a Wankel engine being developed by GM. (AMC used to borrow liberally from the big 3.) GM shelved the Wankel project due to the emissions problem and AMC was forced to shoehorn an existing I-6 under the hood.

weister42
03-21-07, 07:35 PM
Here's a pic of the oval piston in that Honda sportsbike...eight valves per cylinder:eek:

http://world.honda.com/history/challenge/1979pistonengine/img/pho_02.jpg

Since it has two cranks maybe this has the same area as two separate round pistons. A V6 that outputs like a V12?

dkozloski
03-21-07, 07:51 PM
The power output of an engine, if you're holding displacement constant, goes up as you increase piston surface area. Following that to it's ultimate conclusion you would have one very large piston with the stroke little more than a vibration or a whole host of tiny cylinders like a BRM H-24. The Honda example is a good engineering exercise headed in another direction. The success of the idea can be judged by the number of these things you see running on the streets. I'll bet that the guy that thought that thing up is on a different career path now. 35 years ago Honda built a 250cc 6cyl bike motor that had a sixteen speed transmission and turned 22,500RPM. You don't see many of those either.

70eldo
03-27-07, 06:08 AM
Here's the intro to the full story on the Honda oval pistons:
http://world.honda.com/history/challenge/1979pistonengine/

"When I look back at it, I'm not sure if we were experimenting with cutting-edge technologies or obsessed with foolish ideas," recalled Toshimitsu Yoshimura, an engineer involved in the development of the NR500's oval piston engine.

If you see some japanese TV games and inventor challenges, you understand the above phrase.
It also seems that the motorcycle racing world has a 10kg penalty on the minimum bike weight when you use oval pistons.

Google gave me this very interesting result of a patent to make oval cilinders:
http://www.hotelherradura.com/data/manufacture_pistons.html
http://www.freepatentsonline.com/4638694.html

Here are some interesting Wankel examples:
http://rkm.schapiro.org/demos.php

Space saving oval pneumatic pistons:
http://www.taiyo-ltd.co.jp/eg/login/products/products2/2_A10010.html

Many sources claim and explain why a non-circular piston would be the basis of a more efficient combustion engine, but with the current state of technology it would be a very costly engine. Maybe this/these patent(s) will make some change in them and we will see non-circular/oval piston engines more in the future.

Thanks for bringing up an interesting topic!

Oh, I was interested in the RX8 very shortly, until I saw that it uses twice the fuel for its power output. No thanks...

lawfive
03-27-07, 10:46 AM
They tried those other shapes, but they don't fit into the round holes as well.

lmao! Points!

lawfive
03-27-07, 10:47 AM
the circle is the maximization of area vs. circumference

so there is the least amount of "edge" in contact with the cylinder bore on a circular piston

there are some ovoid pistons, but not on common engines, i know westinghouse uses them on some of their 2 stage compressors so they can fit big enough valve assemblies on the head

and as for shapes with corners, sharp edges are concentration points for heat, too much heat concentration and you'll get autoignition of fuel/air mixture
:worship:

Stoneage_Caddy
03-27-07, 02:29 PM
i would aslo say that a big issue with any other shape then round would be the piston rings themselves ....the stress points at the corners on a set of rings that were anything but round would be so bad theyd break ...

ive never had any issues with how pistons are desinged , ive always hated cylander heads tho , which is why im still a big offenhouser fan , which had no cylander heads ...

i also hate valves and cams, and i think its a damn shame scroll valve and camless technology have not been pushed forward ...

id much rather see the internal combustion engine just go the way of the doto bird ...its a massive chunk of metal that has to be put somewhere in the car ....id rather see a car with one eletric motor per wheel mounted inboard of the suspension line with small cv shafts connecting to each wheel , all four powered by a power source and controlled by a computer ...it would allow desginers to completly concentrate on the form of the body and the safety of the car ....as perfromance could just be "dailed" in

RunningOnEMT
03-27-07, 02:39 PM
rotary valve technology is being worked on at UMCAR, university of maryland center for automotive research, they have a 2 cylinder generator running on rotary valves right now, its long term synchronization that they seem to be having an issue with right now

dkozloski
03-27-07, 08:42 PM
i would aslo say that a big issue with any other shape then round would be the piston rings themselves ....the stress points at the corners on a set of rings that were anything but round would be so bad theyd break ...

ive never had any issues with how pistons are desinged , ive always hated cylander heads tho , which is why im still a big offenhouser fan , which had no cylander heads ...

i also hate valves and cams, and i think its a damn shame scroll valve and camless technology have not been pushed forward ...

id much rather see the internal combustion engine just go the way of the doto bird ...its a massive chunk of metal that has to be put somewhere in the car ....id rather see a car with one eletric motor per wheel mounted inboard of the suspension line with small cv shafts connecting to each wheel , all four powered by a power source and controlled by a computer ...it would allow desginers to completly concentrate on the form of the body and the safety of the car ....as perfromance could just be "dailed" in
The old LeTourneau Land Train had an electric motor in each wheel and all were driven with a central diesel generator. There's pieces of one sitting across the highway from the Trans Alaska Pipeline viewpoint on the Steese Highway north of Fairbanks. John Reeves might sell it to you. There's nothing new under the sun.

codewize
03-27-07, 10:39 PM
What is the strongest geometric shape?

lawfive
03-27-07, 10:46 PM
The rock God created when somebody asked, "Is God so powerful He could create a rock that even He could not lift?"

Then He bought a set of aftermarket lifters.

70eldo
03-28-07, 12:08 PM
Maybe ceramic sleeved cilinders will improve the oval piston technology?

Stoneage_Caddy
03-28-07, 04:24 PM
The old LeTourneau Land Train had an electric motor in each wheel and all were driven with a central diesel generator. There's pieces of one sitting across the highway from the Trans Alaska Pipeline viewpoint on the Steese Highway north of Fairbanks. John Reeves might sell it to you. There's nothing new under the sun.

i acutally talked to him about selling it , it all started with a email to the owner of "big foot" (bob chandler) i had seen a program on bigfoot and one that used the rims from the land train ...i realized there was a land train that i had seen over by wainwright and talked to bob about it as in the program he expressed intrest in bulding another super big foot...

i was sent to the desert and left poor ol bob hanging ....never did get back to him to get him the info so he and the owner of the land train could negotiate a deal

codewize
03-28-07, 04:40 PM
:helpless: Ok so maybe some of you didn't pass 7th grade geometry. None the less the strongest geometric shape is the sphere. Why? because the entire surface has the same pressure applied all the time. In other words there are no corners to fail. As soon as something becomes NOT round it becomes weaker.

If you had a piston of any other shape you would probably also have hot spots causing failure or the structure. IN addition it would be very difficult to design rings to apply pressure in odd directions. How would you design a ring for a square piston? Not going to happen.

Is that enough reasons?

dkozloski
03-28-07, 04:57 PM
rotary valve technology is being worked on at UMCAR, university of maryland center for automotive research, they have a 2 cylinder generator running on rotary valves right now, its long term synchronization that they seem to be having an issue with right now
Johnson outboard motors had rotary intake valves back in the 1920's and 30's. Kawasaki has used rotary valves for at least forty years that I know of. The Napier Sabre engine had rotary sleeve valves and delivered phenominal performance; 2200cu. in., 5500 RPM, 4500HP for 175 hours straight without shutting it off. http://tempest.nerdnet.nl/takeoff.wav