Non-cylindrical pistons?

[Retracnic]

Does anybody know of any experimentation into using non-circular pistons and bores? For over a century engine designers have relied heavily on the standard cylindrical piston/bore combination. I do know some makers have had experimented briefly with oval pistons (notably Honda's NR engine) but there have been few follow ups.

It would seem that modern day manufacturing techniques would open the door to a wide range of designs that were difficult/impossible to build in the past. So what do you guys think? What are the Pros and Cons of deviating from the norm? Also what other shapes show promise?

Regards,

Bryan Carter

 

[EricFahlgren]

Here's a pic of a VW diesel that had the same bore centers as their 1.9 l, but was jacked up to 2.3 with oval pistons:

http://not2fast.wryday.com/vw/stuff/1990_2.3d_oval2.jpg

I have no details on the result, but I assume that they didn't like what they saw, as this is the only evidence I have of the project...

I would guess that they had ring seal problems.

 

[glueontop]

A round cylinder is the strongest shape to not deform under pressure. Other shape of cylinder would require a much ticker wall and would give a heavier block. Look at all the pressure vessel and they are all round: compressor storage tank, diving bottle, ... An other point is that after the explosion, the pressure may not be distributed evenly on the piston, if it is supported only by the center, it will twist and jam.

 

[IRstuff]

Additionally, any cylinder that's not close to round will have a corresponding pistton ring seal that will not equal wear and seal performance.

A rectangular prism would be extremely difficult to design for good sealing and wear.

TTFN

 

[CanadaTech]

I was of the impression that Honda made a bean shaped piston with 2 supporting connection rods. Of the impression it was run in a line of sport bikes a few years back. Also of the impression they had some problems. I looked briefly, but couldn't find anything to verify this. Anyone else?

 

[patprimmer]

I heard there was an oval pistoned Honda some time ago, possibly with ceramic piston and bore, if I remember correctly.

As well as being the strongest shape from an internal pressure point of view, a cylinder also has the lowest surface area to volume ratio of any shape that is straight on the walls in one plane, hence it gives the best thermal efficiency, and concequently potentially better fuel efficiency than other shapes

Regards
pat

 

[kradicke]

More importantly, how many machine shops are going to recondition an oval cylinder block when it requires at least double the labor over a round cylinder block.

 

[wildcatgeorge]

The little reading I did on the Honda, was that to comply
with the racing class rules, the engine would have to be
a 4 cylinder, so the ovaled out an 8 cylinder engine into
a 4 cylinder model, simple as "the mother of invention
is nessesity"

 

[somejeffguy]

yes, the honda nr was essentially a v8 with each adjacent pair of cylinders siamesed together, to form a v4. an 8 valve per cylinder, 2 connecting rod per piston, quad cam v4.

it wasn't particularly successful against its 2-stroke competition. something about reliability....

the grand prix rules were later updated to prohibit the use of non-circular cylinders.

 

[PJGD]

SAE paper 930224 by Kanazawa et al of Honda describes this piston development activity. Four and a half pages out of 12 are spent discussing the problems of oil control!

PJGD

 

[wildcatgeorge]

Well if Honda gave up and couldn't take care of oil control
it just ain't gonna happen. Looks like it's back to the old
boring round pistons....again.
LOL

 

[Pancholin]

Hi Retracnic.
Back in "the olden days" auto parts stores bought pistons in oversize. Then ground them to the desired oversize. At one time I had wone of the piston grinding machines and a set of the different cams. HD motorcycles even had a special cam. Most of what I have read says to measure the piston size just below the rings and perpendicular to the wrist pin. The ring lands are round but progressively smaller than the skirt toward the hotter top, and the skirt area is slightly oval shaped and usually the wrist pin is not in the center either. Find an old piston and measure one paying attention to the power and compression thrust faces, and noticing that unless they are pretty badly worn that the ring lands have never touched the cylinder. A really, really oval cylinder might allow the use of HUGE valves.
Pancholin.

 

[Rob45]

Few if any automotive pistons are truly cylindrical - most are cam ground - but they go into cylindrical bores for all the reasons mentioned above plus one more - how exactly do you drill an oval hole?
Yeah, you can do it with NC machinery, but why bother?

Sometimes the old way IS the best way; not everything needs reinvention.