Strange 911 questions

[Beethoven]

1) Can any of the engine gurus give me a succinct technical reason why combustion (and steam) engines use cylinders and pistons rather than cubes and whatever the orthogonal equivalent of a pistons is?

2) Is there any functional (i.e. engine or drive train related) part in the 911 that is perfectly spherical or perfectly square?

If it sounds bizarre, this is for a research project on the use of the cylinder in engines and machines. Whom better to ask than a bunch of flat-sixers?

Thanks

[radcon]

The use of "round" pistons and cylinders makes it possible to use circular piston rings, which makes assembly much easier and provide a better seal between the piston and cylinder wall. It would me much more complex if you had to affix some type of sealing apparatus to the four sides of a square piston.

[kach22i]

At one time or another there may have been an elliptical cylinder shape.

Also there is a wedge shaped engine under development, hinged on one pivot point, rectangular in section.

Let's not forget the Wankle/Rotarty.

[RickM]

The uniform pressure on a round piston helps with strength and longevity. I'd imagine the peaks of the corners on a square piston would be succeptable to breakage and excessive wear.

[vash]

easier to drill a round hole or turn a round cylinder than any other shape. the only reason honda is playing around with eliptical shapes is to make a motor narrower, like in motorcycles, which is a good thing. but hecka expensive.

[MotoSook]

Honda made an oval piston engine for their race bikes back in the 90's. see edited note..

Round is good. As Rick said, the circular surface of the piston makes it a good shape for distribution of the pressure of combustion. If the piston were square, the corners would never see the full pressure. Most IC engines have spark plugs that are at the center of the combustion chamber, and in an ideal situation, the flame front would radiate outward from the spark at the center so that when combustion is complete, the flame front has just covered the radius of the piston. Think of the ripples of a single rain drop in a pool. What happens? Does the ripple travel in a square shape? No.

It's obviously easier and most efficient to design a circular ring for the piston. I can't imagine why anyone would prefer a square ring over a circular ring.

As for a perfect sphere or cube in a 911...any of the ball bearings are likely to be as close as you are going to get to a perfect sphere.

And if you are working on some abstract comp lit paper..you could romanticize that the fuel droplets from your injectors are about as close to perfect as you're gonna get...but only when the flow stalls long enough and the droplet is oriented just right such that the effects of gravity and all the forces on that droplet are equal in 3-D.


EDIT: Here is a tidbit about Honda's oval piston engined bike...(note: nowadays a traditional 750cc inline-4 can make well over 130 HP and in normally aspirated form)

"About the Honda NR 750

In 1979, 500cc motorcycle racing was dominated by 2-stroke machines. Honda introduced the NR500 4-stroke racer that year, in an attempt to break this 2-stroke hold on 500cc Grand Prix Championships. Honda's bike had an epoch-making aluminum monocoque frame, 16-inch wheels, and was powered by a 32-valve, V-4 cylinder engine using oval-pistons. Each cylinder had 8-valves and two spark-plugs that enabled it to turn at an astonishing 20,000rpm, and producing horsepower equivalent to a 2-stroke engine. The NR500 failed as a G.P. racing machine, but Honda continued to develop it because of its performance potential. After years of trial and error, the NR reappeared on the racing scene in 1987 as a 750cc endurance racer. This proved that an oval-piston concept was workable. During the 1991 Geneva and Tokyo Motor Shows, Honda introduced a production version of this hi-tech motorcycle which they simply called "NR." The sophisticated powerplant of the 750cc "NR" uses fuel injection and is capable of producing 130 HP at a realistic 14,000rpm. This phenomenal engine is mounted on an exceptionally rigid all-aluminum twin-spar frame which rides on 16 inch front & 17 inch rear cast magnesium wheels, like those seen on racing machines. The cowling is fabricated from carbon-composite for durability and weight savings. The Honda NR is a true "dream-bike" that exceeds performance standards of today's motorcycle technology, taking a big leap into the future."

[xlr8]

pivotal engine

[yelcab1]

If this is a math project, then you can express the area as a function of its perimeter, do an integration to prove that:

The most surface area for a given perimeter is a circle. Hence the most efficient use of space, volume, and material is the circular piston.

Some math PhD will probably prove that I am full of wax, which I already know.

[Beethoven]

Souk, you just had to out me as a comp lit fuzz head, didn't you?

And what ball bearings? The wheel bearings have cylindrical pegs, as we've discovered.

The argument that it's easier to bore and turn a round hole is itself circular, because that depends on the fact that the turning machines are based on cylindrical parts themselves, etc.
Same with the one about the distribution of pressure, I suppose, if we disregard manufacturing convenience?
The one about the radiation of the flame front, however, strikes me as genuine, because it makes the form of the cylinder depend on something external, not on another cylinder bored by another cylinder, etc.

If you keep it coming at this rate, the 'Book of Cylinders' will be coming to a bookstore near you soon!

[MotoSook]

Don't fret H. You're better off as a gear head than a lot of engineers I know. As for ball bearing...your CV joints have huge balls

And now that you are looking for deeper meaning...

F=P*A is good no? As yelcab pointed out, a circle is a very efficient shape for max area for a given perimeter. Maximizing the area for the pressure to act upon while keeping the shape compact is important...but then we go back to maufacturing...or rather packaging for the masses

[Beethoven]

Ok, the geometrical argument about maximizing surfaces is a good one as well.

That pivotal engine sure looks interesting. Has it been built?

[MotoSook]

Book of cylinder..hmm...

I recall a European group several years back that designed a valve-less engine. They installed it in what looked to be an old F-1 car for testing. Guess what they used instead of valves?...a cylinder with cuts in it to allow for intake and exhaust flow.

And you can't forget the radial engine.....

[Superman]

I think it's sexual. That's the reason. We like to think we're objective, empirical creatures but we're not. We do things the way we do because we LIKE them that way. There are engine designs that are superior to these piston things, but we feel pretty comfortable with a piston thrusting in and out of a cylinder. If women had been the principle design engineers, then it might have developed differently. I'm pretty sure about that. Engines would be pretty. In fact, they'd all be motorcycles, probably.

Hope this answers your questions.

[jluetjen]

There's an engine running now that uses barrel valves. I guess they had a heck of a time solving the problem of laterally sealing the valves.

As far a square pistons are concerned, in addition to a circle's ability to encompass the largest area with the smallest outside area, there is also the problem of the corners which are murder to good combustion since mixture will collect there and be the last area to be burned for the smallest gain in recovered energy.

[kach22i]

Originally posted by Beethoven
That pivotal engine sure looks interesting. Has it been built?

The one I saw on cable TV was rocking away in a motorcycle and doing good. They were trying to upscale it, some thermodynamic problems involved, like trying to scale up a Cox airplane model engine.

That link looks like what I saw last year.

Anyone want to talk about two-stoke diesels?

Orbital?
http://www.orbeng.com.au/orbital/orbitalTechnology/techPapers/a_nov97.htm

[yelcab1]

By the way, Mazda engineers obviously are not participating in this thread. They designed a Rotary engine instead.

[kach22i]

Originally posted by kach22i
Let's not forget the Wankle/Rotarty.

Dang, I forgot already.

[Jim Sims]

"The argument that it's easier to bore and turn a round hole is itself circular, because that depends on the fact that the turning machines are based on cylindrical parts themselves, etc.
Same with the one about the distribution of pressure, I suppose, if we disregard manufacturing convenience?"

This is not correct; James Watt would laugh. The argument for a round piston is not based on circular logic or manufacturing convenience. The shapes and forms of machinery can be traced to the process of satisfying the requirements of statics, kinematics and other physical laws of motion in the most efficient ways possible. These topics are usually covered in the sophomore or early junior years of a mechanical engineering curriculum.

If you want a real puzzle and newfound respect for early machine tool builders consider the problem of how lead screws have become better and better despite the fact that one lead screw is made by another and the accuracy should degrade at each step. Cheers, Jim

[beepbeep]

Thermal efficiency. Round cylinder walls have least area exposed to flame-front so it can achieve higher temp and thus higher effective pressure in cylinders. That is what "hemi" is all about...

[Porschekid962]

all the mazda engineers was to improve upon an original design, henry wenkel a german of course. they are neat engines but there are a lot of common misconceptions about them especially their actual engine size, plus lots of heat problems in turbo apps. didnt stop them from winning le mans though.