Why round pistons not oval????

**Ruralman** · 7th September 2006, 17:22

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** · 7th September 2006, 17:27

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** · 7th September 2006, 17:29

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** · 7th September 2006, 17:31

Originally Posted by 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.

don't forget interesting wear patterns.

**MikeyG** · 7th September 2006, 17:36

Originally Posted by 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.

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** · 7th September 2006, 17:43

Originally Posted by 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.

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** · 7th September 2006, 17:44

Originally Posted by MikeyG

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** · 7th September 2006, 17:45

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** · 7th September 2006, 17:47

check out Paul McLachlan's engine, square flap type pistons.

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

**degrom** · 7th September 2006, 17:53

Originally Posted by classic zed

check out Paul McLachlan's engine, square flap type pistons.

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

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

**Edbear** · 7th September 2006, 17:54

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

Perhaps maufacturing costs?

**Ruralman** · 7th September 2006, 17:55

Originally Posted by classic zed

check out Paul McLachlan's engine, square flap type pistons.

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

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** · 7th September 2006, 18:00

Originally Posted by Edbear

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** · 7th September 2006, 18:05

Originally Posted by classic zed

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** · 7th September 2006, 18:11

Originally Posted by Ruralman

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