piston question...

[Raymond]

i was reading some posts about what compression and diameter and such... i found myself wondering why pistons are still round in circumference.

why haven't pistons become a rounded square, rectangular, or oval? i know in the 80's/'90's, honda was experimenting with a peanut or 8 shaped piston, but apparently it held no advantage over the circular one.

with the layout of the flat 6 and the removable cylinders, it seems like rounded square pistons would work...?...

just wondering if anyone knows why this part of an automobile is one of the few things that hasn't changed shape in 100 years.

open forum!

[Emission]

I'd guess it has to do with the aerodynamic properties of a circle vs. any other shape. Airflow in (and out) of the cylinder is critical to good combustion - and a round cylinder just may be about perfect.

[Raymond]

that honda peanut shape was interesting. it was for the superbike at the time.

the 8 shaped piston looked like two single pistons fused together... it had 2 connecting rods and was something of a V2/4.

[Tim Walsh]

I belive emission's right. For the same reason that the most ideal ports are perfectly round so is the cylinder. Both are pieces of an air pump and to get the most efficency out of an air pump round is the way to go.

On a speculation level I belive it's a drag thing. The air closer to the walls of the port/piston are moving slower and so to maximize area and minimize circumfrence (sp?) a circle is the best shape.

[Jim Sims]

Round bores are far less expensive to make and much easier to seal - making piston rings work well in non-circular bores is not trivial. Jim

[David]

$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$$

[ValveFloat]

Just came across this. Some pictures of the NR, the Honda MC with oval pistons:
http://www.sportbikez.net/pictures/mod/nr750

[Emission]

Is that eight valves per cylinder?

[Emission]

Um... Yes.

[jim72911t]

Jim and David are right; the manufacturing costs for pistons and cylinders would go up exponentially if the design specified anything but circular bores. I do admit, though, that if somebody else was paying the development costs, it'd be fun to build an oval piston engine.

Jim

[arerrac]

This interested me so I did a search on the Internet. Found a gentleman at the University of Maryland in the mechanical engineering department. I asked about the reasoning behind round vs square postons & cylinders , and this is what he had to say,

"There are several reasons that come to mind as to why a round cylinder is
optimal.

1) A round cylinder minimizes surface area to volume ratio and thus heat losses from the hot combustion products to the walls. Heat losses reduce power and increase cooling loads.

2) A round chamber allows for minimum distance for flames to travel from a centrally located spark and shorter combustion times (without detonations) provides better ability to optimize power out per stroke.

3) Round chambers are far more easy on design of seals and manufacturing of liners etc...

I hope this helps and enjoy your Porsche. Be thankful you can enjoy such a
fine vehicle.

Greg Jackson
University of Maryland"