OneEyedPyro
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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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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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bnull
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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.
Quod scripsi, scripsi.
B. N. Ull
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OneEyedPyro
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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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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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Chemgineer
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Keep it round and just increase the stroke length...or turbo charge it.
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Rainwater
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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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Twospoons
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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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| 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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Rainwater
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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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RU_KLO
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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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Bronstein
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Oval pistons have been tried in motorcycle racing:
https://en.wikipedia.org/wiki/Honda_NR
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Dr.Bob
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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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Grizli7
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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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Morgan
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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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Morgan
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Moving on to the valves ...maybe of some vague interest.
Rotary Valves Make Normal Valves Look Silly - Why Aren't We Using Them?
https://www.youtube.com/watch?v=DZU8tsNNxGA
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