Piston Question

[CL8]

I answered a question about pistons on another site then the guy asked me this question
about if it is possible to use rectangular pistons. Any good answers?
Thanks, CL8

Can a rectangular piston be used where in the sides are chamfered and hence reduce friction and also function with same efficiency as cylindrical piston?

[curtis73]

There are a few reasons why they cannot be used. Piston rings get their sealing properties from the pressure in the cylinder. The pressure is able to exert equal sealing all around. Square pistons would not have that sealing ability. Round pistons also are stronger since the flame front (and pressures they see) are more equal. In a square piston you would have widely variable pressures that reach the center first and the corners much later.

The other issue is reciprocating mass. The engine would have to run at typically lower RPMs... you could make the pistons light enough to rev fast, but the corners would likely break off.... or you could make the pistons strong enough to stay intact at the expense of weight which would negate any gains.

Basically (with current technology) a square-piston engine just wouldn't work. Even if you found a way to get the rings to seal, the additional displacement would not be a good trade off for the other shortcomings.

[CL8]

Great, Thanks for the quick reply Curtis!

[MagicRat]

To add to Curtis' reply........Square piston engines would have more cylinder wall surface area for the given displacement, than a conventional cylinder, and (IMO) would be slightly less thermodynamically efficient.

However, square piston engines have been made and did run...... but were made for research purposes only, back in the 1980's. The idea is that one side of the square cylinder was was transparent so combustion dynamics could be observed, photographed in slow-motion and studied to see where improvements could be made in combustion efficiency.

Now, consider oval cylinders. Honda made quite a few high-revving racing motorcycle engines years ago with oval cylinders instead of round ones. One can squeeze more valves into an oval cylinder for better high-rev breathing and, like square cylinders, can allow for a slightly more compact engine. However, manufacturing expense precludes oval cylinders for production purposes.

You can see from this picture, the oval pistons have flat sides..... they are half-way to being square!
From: http://en.wikipedia.org/wiki/Honda_NR

[CL8]

Thanks MR.

I thought by definition a cylinder is something round(ed) with no corners, if so, how could there be a square cylinder?

Also this guy was referring to rectangle pistons which would be a bit longer than a square.

He responded when I told him what Curtis said, here was his response:

All cool with the answer that you provided.thx
But i did mention that there would be no edges but they will be chamfered.

Would that be something like the oval pistons you showed MR, or would the same problem with pressure displacement occur from the center to the edges rounded or not?

[MagicRat]

In structural engineering, a perfect right-angle or corner creates a weak point by focusing structural stresses directly at the angle, or corner.

A chamfer is where a 45-degree angle is made at the corner ( a fillet is a rounded-off corner) which serves to spread-out the stresses thus reducing the chance of cracking or breaking at the corner.

Here is a nice explanation:
http://en.wikipedia.org/wiki/Chamfer