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Author: Subject: Alternative piston shapes in engines?
OneEyedPyro
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[*] posted on 28-2-2024 at 06:05
Alternative piston shapes in engines?


I know this isn't exactly a mechanical engineering forum, but someone here surely has a better grasp on this than I do.

Why are pistons traditionally cylindrical? A square or oval piston for example would allow for a greater displacement in a given sized engine and allow for larger intake and exhaust ports over a cylindrical design at a given displacement.

Maybe I'm missing something obvious here which is why I put this in the beginnings section, but I can't think of any glaring disadvantage aside from perhaps a machining/manufacturing aspect.
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unionised
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[*] posted on 28-2-2024 at 06:08


The friction increases broadly in line with the length of the perimeter.
Also a round piston won't jam if it rotates slightly about the axis.
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[*] posted on 28-2-2024 at 06:33


The square piston would suffer too much wear in the corners during normal use, and the corners would also make it harder to seal. The cylindrical piston can support the sudden increase in pressure better than the oval or the square.

In short, the cylindrical piston is better to seal and lasts longer than the others.




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OneEyedPyro
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[*] posted on 28-2-2024 at 06:47


Quote: Originally posted by unionised  
The friction increases broadly in line with the length of the perimeter.
Also a round piston won't jam if it rotates slightly about the axis.


I'd like to think a piston moves rather freely up and down in the cylinder but I suppose there's a certain amount of non linear force as the connecting rod changes angle. I'd also think the rings would allow for some degree of play for the piston to wobble without binding.
Maybe an oval/rectangular piston with two rounded sides would behave very much like a cylindrical one while still enjoying the benefits of having less useless space between cylinders?
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fx-991ex
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[*] posted on 28-2-2024 at 06:56


Its because of the squish band and ease of assembly/machining.

Gaping the rings on a square piston will be very impractical, the piston/cylinder clearance will be much more complicated too.
Same thing when servicing the cylinder, boring/honing. much easier when round.

Also, theres the combustion chamber squish band, creating turbulance in the air/fuel mixture, promoting better mixture of the fuel/air mixture and it also favor faster/more even combustion because of the flame front it's also reducing hotspot/better cooling.

https://en.wikipedia.org/wiki/Squish_(piston_engine)



[Edited on 28-2-2024 by fx-991ex]
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[*] posted on 28-2-2024 at 11:51


Keep it round and just increase the stroke length...or turbo charge it.
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[*] posted on 28-2-2024 at 13:02


Quote: Originally posted by fx-991ex  
ease of assembly/machining

This would be my guess, square holes are difficult to cut
Force distribution would also be a big problem, equal pressure will be applied during combustion but the substrate support at the corners would be a lot less, like <10% vs non corner areas

[Edited on 28-2-2024 by Rainwater]




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[*] posted on 28-2-2024 at 14:01


Don't forget ignition speed. For the same piston area its going to take longer for the flame front to reach the the corners of a square than to reach the edge of a circle. We're now in a world where every bit of performance matters, and you can be certain that engineers have been turning over every stone looking for that little bit extra. If there were any advantage to square pistons then we'd be using them.

Closest we have to a square piston is the distorted triangular piston in a Wankel rotary. An engine with high power to weight, but poor fuel efficiency. And a serious weakness with the apex seals.




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OneEyedPyro
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[*] posted on 28-2-2024 at 14:52


Quote: Originally posted by Twospoons  
Don't forget ignition speed. For the same piston area its going to take longer for the flame front to reach the the corners of a square than to reach the edge of a circle. We're now in a world where every bit of performance matters, and you can be certain that engineers have been turning over every stone looking for that little bit extra. If there were any advantage to square pistons then we'd be using them.

Closest we have to a square piston is the distorted triangular piston in a Wankel rotary. An engine with high power to weight, but poor fuel efficiency. And a serious weakness with the apex seals.


How truly important is flame speed? I suppose it's best to achieve the most combustion possible while compression is highest?
I've seen engines with offset plugs, why not in say a stadium/pill shaped piston engine use two plugs placed 1/3rd from either end?

The only valid advantage would be compactness. Specifically reducing the length of the block on an inline engine which would be especially useful in ATVs, motorcycles, PWCs etc.
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[*] posted on 28-2-2024 at 17:03


Plugs are offset from center not to offset from center, but to put the spark in the best place. A perfect fuel air ratio burns best but ignition perfers a different ratio, so the plugs are positioned where the injector distribution pattern creates this "preferred" ratio. Serious study and dollars go into investigating how the flame front moved along with the compression wave. In simple terms I think of the fuel air mix as a heterogeneous mixture, in reality it is not, but some genuine genius figured out thats a real advantage. By altering the fuel injection geometry, they can create very fined tuned flow patterns and alter the timing of the combustion event. Directing it towards the center of the cylinder volume, tho intuitive, is not the goal. One of my prof. Says a donut shaped ring is the current standard, providing the most flame front surface area to increase the deflagration rate
The theoretical maxium power achieveable would be from a complete and instant combustion event.




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[*] posted on 1-3-2024 at 06:40


my guess, maybe resonance has an important role. Its better for resonace tweaking to use a cylinder than a square (thats why magnetrons inside are round also high frequency filters are mainly round)

also found this (so it seems resonance is studied in motor design)

Determination of the resonance response in an engine cylinder with a bowl-in-piston geometry by the finite element method for inferring the trapped mass

"Abstract
Cylinder resonance phenomenon in reciprocating engines consists of high-frequency pressure oscillations excited by the combustion. The frequency of these oscillations is proportional to the speed of sound on pent-roof combustion chambers and henceforth the resonance frequency can be used to estimate the trapped mass, but in bowl-in-piston chambers a geometrical factor must be added in order to deal with the bowl disturbance"

https://journals.sagepub.com/doi/10.1177/1468087415589701

here is other information:

https://www.cycleworld.com/blogs/ask-kevin/motorcycle-piston...




[Edited on 1-3-2024 by RU_KLO]




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[*] posted on 23-3-2024 at 04:03


Oval pistons have been tried in motorcycle racing:

https://en.wikipedia.org/wiki/Honda_NR
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[*] posted on 24-3-2024 at 09:06


Quote: Originally posted by Rainwater  
The theoretical maxium power achieveable would be from a complete and instant combustion event.


But that would be knocking, where there is a complete combustion in a short time, also known as an explosion, rather than a deflagration. Internal combustion engines work best with a slower, slowly propagating cumbustion that goes over much of the time that the piston is moving up. Otherwise the piston gets one push and then none, causing the engine to get great stress.
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[*] posted on 16-4-2024 at 07:39


Well then. The answer to the question asked by the author lies in the question itself. You are absolutely right and answered accurately - the reason is that the pistons are cylindrical, easy to manufacture and, as a consequence, price. Theoretically, any shape can be made - almost all the advantages obtained from a complex shape can be completely offset by the price and complexity of manufacturing.
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[*] posted on 27-4-2024 at 07:20


Some uncommon cylindrical pistons ...

We make copper/brass/bronze pistons – what will happen?
https://www.youtube.com/watch?v=RLWSljLIBaw

[Edited on 27-4-2024 by Morgan]
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