Why round pistons not oval????

[Ruralman]

Hopefully someone can give me a serious logical answer to the question - why are all pistons/cylinders round. It would seem, for someone without huge knowledge about the design and engineering of the inside of a motor, that if pistons could be made to work in an oval shape you would be able to make an engine much narrower (but a bit deeper/longer) than the same cc motor with round pistons/cylinders?????
I'm trying to imagine what effect this would have on the Cof G etc.
Given th einternal combustion engine has been around a long time someone will have tried it and presumably because they don't make engines like that there must be some good reason - what is it?
Drilling or shaping the bore would certainly be easier with a circle than an oval but surely there will be technical ways around that?
I look forward to the replies

[Swoop]

Presumably in years gone by, production tools involved the lathe, so things were round. Nowadays with the advent of CNC technology it would be easier to manufacture elliptical pistons?
Rings might be harder to produce though.

Sounds like you have a potential research project on your hands!

[onearmedbandit]

Do some research on the NR500 race bike and NR750 road bike, both were oval pistoned. I believe increased friction was one of the downfalls of an efficient oval-piston engine.

[thehollowmen]

Do some research on the NR500 race bike and NR750 road bike, both were oval pistoned. I believe increased friction was one of the downfalls of an efficient oval-piston engine.

don't forget interesting wear patterns.

[MikeyG]

Do some research on the NR500 race bike and NR750 road bike, both were oval pistoned. I believe increased friction was one of the downfalls of an efficient oval-piston engine.

The designer/engineer of those bikes had an interview in a mag lately. Said they couldn't make them work well with the machining tools at the time.

[Ruralman]

Do some research on the NR500 race bike and NR750 road bike, both were oval pistoned. I believe increased friction was one of the downfalls of an efficient oval-piston engine.

If you take a circle shape andturn it into an oval without changing the outside circumference, does the internal area change? Why would increased friction have been an issue? - I presume something to do with the straight sections of the sides?

[Ruralman]

The designer/engineer of those bikes had an interview in a mag lately. Said they couldn't make them work well with the machining tools at the time.

Does that also mean he said they would be able to do it now?

[Motu]

Well pistons are oval,at least alloy ones are....cast iron are round.As mentioned Honda tried - do a Google and you'll have a month reading to do on the NR500,it's pretty famous.But round is pretty good,all the forces are working in harmony,you don't need tricks to disguise the fact that an oval piston will have different forces acting upon different parts.Maybe you could use valves shaped in a figure 8,and 7 stage carburettors to keep air speed constant,3 sparkplugs per cyl to optimise combustion.....

Once engineers are over the ego trip of their degree,a blinding light appears before their eyes - KISS,it's always been proven to be the road to success.

[classic zed]

check out Paul McLachlan's engine, square flap type pistons.:gob: Prototype built on a TZ bottom end and the test bike is gorgeous.

http://www.pivotalengine.com/

and made in Christchurch, I have seen it close up and despite the fact that he designed it a few years ago its still brilliant:2thumbsup

[degrom]

check out Paul McLachlan's engine, square flap type pistons.:gob: Prototype built on a TZ bottom end and the test bike is gorgeous.

http://www.pivotalengine.com/

and made in Christchurch, I have seen it close up and despite the fact that he designed it a few years ago its still brilliant:2thumbsup

Very interesting concept.. Wish there were more KIWI's like McLaughlin and Britten...

[Edbear]

Did you hear why it doesn't seem to have progressed, Tim?

Perhaps maufacturing costs?

[Ruralman]

check out Paul McLachlan's engine, square flap type pistons.:gob: Prototype built on a TZ bottom end and the test bike is gorgeous.

http://www.pivotalengine.com/

and made in Christchurch, I have seen it close up and despite the fact that he designed it a few years ago its still brilliant:2thumbsup

That thing is fascinating - I suppose its sitting gathering dust now? The only thing I wonder about looking at the demo is that the exhaust gases aren't pushed out - what difference does that make if any.
Was this designed and built in ChCh?

[classic zed]

Did you hear why it doesn't seem to have progressed, Tim?

Perhaps maufacturing costs?

I havent spoken to Paul for a couple of years, I should get in touch.

[Ruralman]

I havent spoken to Paul for a couple of years, I should get in touch.

Better still get in touch and get him to post a reply here

[classic zed]

Better still get in touch and get him to post a reply here

Over the years I have thought about it a lot and still cant work out how you can get a square piston to seal in the corners, its just doesnt seem possible but he has done it. The mans a genius:yes:

[BNZ]

Something to do with seals/gaskets stretching un-evenly?

[dawnrazor]

Do some research on the NR500 race bike and NR750 road bike, both were oval pistoned. I believe increased friction was one of the downfalls of an efficient oval-piston engine.

Well as Satoru Horiike (the engineer behind the NR500 and NR750) says, "the oval piston design has a very high potential, but has had a troubled life. I think the idea is very good but ahead of its time, at the time of its introduction (1979) machining technology was not so percise, but now we can make anything, any shape. Technically we could do it now, but it would still be very difficult."

From the horses mouth so to speak, Quotes taken from an interview in BIKE

[avgas]

Be careful now - we're heading into Bimota territory....look where that got them

[Pwalo]

If I recall correctly round pistons also tend to dissipate heat evenly, which is a rather critical factor in engine design.

But as everyone else has said Honda tried, and failed. I'm pretty sure that we'd have seen an oval pistoned RCV by now if the technology was viable.

[vifferman]

The NR had oval pistons not because it was a good idea, but so they could make what was effectively a V8, with only 4 pistons, so it kept to the letter of the law, if not the spirit of the law. Unfortunately, for all the engineering expense and complexity, it didn't produce markedly more power than a V4 could, and in fact a lot less than subsequent V4s, like the RC45, and current MotoGP bikes.

But never mind; it was kewl all the same, and many of the design features of the road-going version (indicators in mirrors, underseat zorsts, single-sided swingarm, etc.) filtered down to subsequent models.

[Lou Girardin]

The Honda had serious ring sealing problems. It was another Honda answer to a question that wasn't asked.

[Motu]

It was just because Honda didn't want to build a dirty stinking 2 stroke.After the failure of the V4/8 they were frog marched down to hall to the 2 stroke room where they finally settled down to play nicely with the rest of the boys.

[steved]

The Honda had serious ring sealing problems. It was another Honda answer to a question that wasn't asked.
Gotta love the 'can-do' attitude. Even though they didn't do it.

[Coyote]

Very interesting concept.. Wish there were more KIWI's like McLaughlin and Britten...
Just give me a few more years

[Flyingpony]

Over the years I have thought about it a lot and still cant work out how you can get a square piston to seal in the corners, its just doesnt seem possible but he has done it. The mans a genius:yes:
They may have gotten an idea from the Mazda Wankel (Rotary) engine.

[Forest]

If you take a circle shape andturn it into an oval without changing the outside circumference, does the internal area change? Why would increased friction have been an issue? - I presume something to do with the straight sections of the sides?

Absolutely - if the perimeter is fixed, then an oval/ellipse will always enclose less area than a circle.

To put it another way. For a given perimeter, a circle always encloses the greatest possible area.

[imdying]

Absolutely - if the perimeter is fixed, then an oval/ellipse will always enclose less area than a circle.

To put it another way. For a given perimeter, a circle always encloses the greatest possible area.

The NR750 is neither oval or ellipse iirc... it's two circles, joined by a rectangle (parallel sides).. or a rectangle with two round ends if you will. Best of both worlds?

[McJim]

Very interesting concept.. Wish there were more KIWI's like McLaughlin and Britten...

It's very difficult for them here - every time someone comes up with a new idea all the other kiwis jump on them and tell them it can't be done - tall poppy syndrome I think it's called - never seen it demonstrated more visibly than here in NZ. Shame coz you're really quite a creative and ingenious bunch.

[slowpoke]

Using the KISS principle it's called Keep It Simple Stupid........hence the round pegs in round holes

[degrom]

It's very difficult for them here - every time someone comes up with a new idea all the other kiwis jump on them and tell them it can't be done - tall poppy syndrome I think it's called - never seen it demonstrated more visibly than here in NZ. Shame coz you're really quite a creative and ingenious bunch.

Is that way, when I built my own pressure pot sand blaster and was looking for nozzles nobody wanted to help me find them.

The one place I asked didn't even want to talk to me about it. They thought I wanted to take over.

Luckily I know places like that won't last... The don't they know the first Zen business rule is that when you share with other they will share with you.

[ZeroIndex]

Why not cube pistons (square ones.. like a box).. everything can be measured in 'cubic' inches, so why aren't pistons done like that instead? :D

[McJim]

Why not oval pistons? Using the KISS principle it's called Keep It Simple Stupid........

Because any shape other than a circle is not Simple any more.

There is friction caused by the piston ring on the cylinder - if we change from a circular piston the contact area and therefore the friction increases which reduces the efficiency of the engine which in turn will bleed power from the system - I imagine that this would become more pronounced the further from a circle shape the piston becomes.

This could all be speculative bullshit of course - I'm sure a real engineer will be along shortly.

[slowpoke]

Because any shape other than a circle is not Simple any more.


Exaccary wot I ment, late on nightshift my finga's wos finkin' for 'emselfs....brayne slowing down....muuuuuust sleeeeeep........ZZZZZZZZZZZZ

[Pixie]

Very interesting concept.. Wish there were more KIWI's like McLaughlin and Britten...

McLaughlin has some original ideas.
Britten merely stole and cobbled together other peoples ideas

[Shaun]

McLaughlin has some original ideas.
Britten merely stole and cobbled together other peoples ideas


You are a Nice guy, did you know john? Did he ignore you some time

[slowpoke]

Britten merely stole and cobbled together other peoples ideas

OUCH!!!!!!

[Motu]

There is no inovation in the motor game - it has all been done before....it's just reapplied with new technology.

[craigs288]

If you can take several good ideas from other people and fit it all together in a way that's better than what you originally had, that's still pretty ingenious.

[The Pastor]

That square piston engine looks very good. I really hope that engine gets into some bike! Even if it IS a hondurrr

[Biff]

According to Honda chief design cheese, in an interview in Bike magazine a couple of months ago, oval pistons will become the norm within the next 10 years for virtually all motorbikes.

Earlier manufacturing costs and design problems made them overly expensive for mass production machines.

I think the guys name was Tommy Tookamoto. Japanese chap.

[Motu]

Bugger eh - so will one of those oval piston motors be powering my sucky blowy hovercar? We were told we were going to be driving those in 10 years too - 50 years ago!

[sAsLEX]

Bugger eh - so will one of those oval piston motors be powering my sucky blowy hovercar? We were told we were going to be driving those in 10 years too - 50 years ago!

What about this concept
http://www.craftsmanshipmuseum.com/Tomlinson.htm

and lots more engine porn here
http://www.advrider.com/forums/showthread.php?t=80568&page=1&pp=15

[Speedracer]

"This was followed by a 400cc twin chamber motorcycle engine. This engine ran up to 11,500rpm and drove the motorcycle to speeds exceeding 160kph. "

Then something about a 90kw 1.1L engine.

I don't think its amazingly powerful, but that pivotal engine design does look pretty darn neat!

[cowpoos]

"This was followed by a 400cc twin chamber motorcycle engine. This engine ran up to 11,500rpm and drove the motorcycle to speeds exceeding 160kph. "

Then something about a 90kw 1.1L engine.

I don't think its amazingly powerful, but that pivotal engine design does look pretty darn neat!
can you find a web link to that engine??

[Lou Girardin]

You are a Nice guy, did you know john? Did he ignore you some time

Is Pixie wrong then?

[vtec]

Oval pistons mean that you need to have near perfect design and macining to get them to fit. With oval, you need to line up the conrod connection perfectly with the cylinder, so that you aren't putting twisting force on the oval piston causing strange wear on the piston and cylinder walls, whereas with a circular piston, you just have to make sure that the conrod is in the centre of the cylinder.

Also, with oval piston, there is much more cylinder wall area for less piston surface area, thus meaning that more heat energy will be absorbed into the cylinder walls, effectively absorbing more of the explosive energy of the combustion than a round piston engine. This effect may be minimal though.

Another point was that if it's not circular, then there is more piston circumference for the same piston surface area meaning more piston to cylinder wall friction, again, I think this is minimal. But to be quite honest, I don't see the need for any of this fancy technology. All it seems to do is make it harder.

I haven't even put much thought into the piston ring difficulties that would come with oval pistons.

So, what are the advantages? You can make a larger bore engine skinnier... We don't really need to, the current flock of inline 4 engines are seriously bitchin'. If you want to make it skinnier while being large bore, then just use a V configuration... V4 or V5 really kicks the Llamas ass.

Anybody think of any other advantages? Apart from bending racing rules of course.

[Fooman]

Oval pistons mean that you need to have near perfect design and macining to get them to fit. With oval, you need to line up the conrod connection perfectly with the cylinder, so that you aren't putting twisting force on the oval piston causing strange wear on the piston and cylinder walls, whereas with a circular piston, you just have to make sure that the conrod is in the centre of the cylinder.

The Honda engine had 2 conrods per piston - Also any small misalignment in the cylinder is normally accounted for in the rings, they are providing the actual sealing surface



I haven't even put much thought into the piston ring difficulties that would come with oval pistons.


Honda have a few patents relating to the oval rings. That may be why others haven't come up with oval pistons - Honda may control the technology to make them work.



Anybody think of any other advantages? Apart from bending racing rules of course.

As previously mentioned, the reason Honda used oval pistons was to effectively have V8 valve area in a (limited by the rules) V4 motor. For the same displacement, the higher the number of pistons, the larger the effective valve area. The more valve area, the more efficient the combustion (more energy out). And pistons are smaller, so they generally rev higher.

More valves = more efficient * higher revs = more power.

As many have also pointed out, the most likely reason why we use circular pistons is that it is trivial to machine something with a circular cross section, both internal and external. Oval pistons are a lot harder = a lot more expensive.

FM