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Auto: Square Pistons

(Disclaimer: I am mostly ignorant of auto tech.)

Why must pistons be round? I'm guessing that it's because it's easy to machine something really accurately if its round, and there's probably also something to be said for even pressure distribution. However, I'm thinking that if you used a square piston with rounded corners, you could get a larger "cylinder" to fit in the same block without compromising the thickness of the walls.

Also, why must ports be round? I can imagine ports that are triangles with rounded corners. This could be used to tune how much air is allowed in or out as the piston is going up and down. This would be a tunable, just like a camshaft.


Square pistons are not the problem. The issues is how do you bore, polish and lap a square hole? The fact is you can't to the same tolerance on all 4 sides of the bore.
Mark Melvin said…
Why must pistons be round?

Because you would never make a seal otherwise. Fabricating a set of rounded-corner piston rings that sealed against the cylinder walls would be...well a challenge, I imagine.

Also, why must ports be round? I can imagine ports that are triangles with rounded corners. This could be used to tune how much air is allowed in or out as the piston is going up and down.

I'm not sure what you mean by this. How is it tunable? 4-cycle engines don't have "ports" in same the sense that 2-cycle engines do (well they have ports, but they are covered with valves that open and close - which are round). So do you mean triangular-shaped valves? If so, then again you'll have major sealing issues, and I still don't see how it could be tunable.
Perhaps you are referring to 2-cycle engine ports (which are in the cylinder walls) - in which case that is an interesting idea, but I think the general goal is to get as much stuff in as quick as possible, and to get it out just as fast, so the end result of your tuning would result in pretty much a big ole' rectangle. ;)
junklight said…
as JohnMc says way easier to make.

Also corners are weak points - that is where cracks and flaws will propagate from. An engine has constant explosions going on it - not a place for weaknesses.
Mike Lowe said…
There are a couple of reasons, and they all center around efficiency. There are 4 things that take away from the kinetic energy produced by the engine, by using a cylinder we can minimize some of them. The first is the friction from the piston rubbing against the cylinder wall. You can minimize this by reducing the area, but we know that reducing the area will mean reducing the perimeter of the cross section which also reduces the power output proportionately. So we use the geometric figure that has the greatest area with smallest perimeter - a circle. Another draw on KE is moving gasses that aren't involved in the combustion. If you take an atomizer and look at the cloud produced it is always conical. So the corners of the rectangle won't get fuel air mix and won't be involved in the combustion and wind up just being in the way. The hemi (as in hemispherical) engines are a good example, the hemisphere that is formed for the ignition chamber closely approximates the fuel air aerosol and produces the greatest efficiency and therefor the most power from equal displacements.

I don't know much about the efficiencies of manufacture, but I would imagine it would be very difficult to produce square corners with the rotary machine tools.

So the short answer is that it would produce a less efficient/less powerful engine at a higher cost.
jjinux said…
Thanks for humoring me with intelligent answers, guys. It's been something on the back of my mind for a couple years.
Varikin said…
Check out the rotary engine. It is different. The mazda rx line uses them.
jjinux said…
Anonymous said…
Same reason we don't have square windows on aircraft nowadays. The first passenger jetliner, the Comet had some "issues" with it's original windows:

Stress around the window corners was found to be much higher than expected, "probably over 40,000 psi," and stresses on the skin were generally more than previously expected or tested. This was due to stress concentration, a consequence of the window's square shape. ...The square windows of the Comet 1 were redesigned as oval for the Comet 2, which first flew in 1953.
jjinux said…
Wow! Such great comments!
Anonymous said…
This is what windows looked like on the original Comet. Interestingly, because of aggressive weight-reduction goals, windows on the latest passenger jets are becoming rounder than what they used to be:

The 777 also features larger, more rounded, windows than most other aircraft. The 777-style windows were later adopted on the 767-400ER and Boeing 747-8.

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