Piston ring spacing - looking for information

[pmtoo]

I am working on a project that involves a large gap (measured axially) between the top and middle (scraper) ring. The space between the two will most likely be over 14mm. The piston is of similar design to an automotive diesel engine. I am trying to find any resource describing the importance of the space between the rings. Does anyone have some ideas?

-Matt

 

[ivymike]

you'd probably do well to search online for papers discussing variations on (and combinations of) the following themes:
- Second land volume
- Accumulator groove volume
- Ring dynamics
- top ring motion
- top ring flutter
- oil consumption

You mentioned that the piston is similar to an automotive diesel piston, but you didn't mention the intended application. The importance of land volumes will depend to an extent on the intended application (especially operating speeds, cylinder pressures, engine aspiration). There are several software packages on the market which will allow simulation of ring motion, including the effects of land volumes and gap sizes. AVL, Ricardo, and MIT each have at least one such software package.

 

[pmtoo]

Thank you, those terms are coming up with a few items, it will take some time to narrow down what I need.

For all intensive purposes the piston is a modern diesel engine piston (bore of ~80mm). I work in combustion, so the top of the piston is my area, but due to manufacturing constraints I need to extend that described space for a new design. I want to learn what I can about the importance of the region before bringing the new design to a piston manufacturer for review at the end of the week to avoid any major gaffs.

The problem was that most of my resources were on the design of rings, grooves, and cylinder walls, not the spacing lower grooves (though I do have papers on grooves above the rings) . I think your suggestions will help.

 

[ivymike]

if you're just looking to make the piston a bit taller, what stops you moving all three grooves up (nevermind that a taller second land may be beneficial)?

 

[pmtoo]

The piston, like all of our units, is for R&D purposes, not a production ready part. The new design we want to try out requires that a uninterrupted space of ~14mm is left about 12mm below the piston crown. Using "standard" diesel ring placement this will only allow for one ring, however, we may be able to compress both rings into the area. The space is left so that the piston can be manufactured the way we require and to add strength to an otherwise weak design that has a complex internal piston structure. Unfournately I can not increase the height from the pin to crown.

 

[ivymike]

I'm having a bit of trouble following your description. Referring to the attached sketch, are you saying that you must keep the compression height (A) constant, but you want to make the second land (C) height at least 14mm while keeping the top of the second land within 12mm of the top of the piston?

 

[dicer]

It is partialy the constraints you have to work with.
Meaning the areas above and below the wrist pin, and how many rings you want to place in those areas.

 

[pmtoo]

ivymike,

Your description of my restraints is correct. It seems as though the area acts as a gas containment area to keep the high pressure differential on the top ring and as a piston to liner contact area in most designs. I feel as though I am safe in presenting the design to our piston manufacturer (who handles piston body design and machining).

My concern was that the extended area was going to obliterate a working system that I did not fully understand.

 

[ivymike]

I wouldn't say that it's a contact area in MOST designs, but it certainly is in many of them. As long as you can still fit your rings on the piston without pushing the top ring much (any?) higher, you should be okay.

 

[tbuelna]

pmtoo,

The large spacing between the compression and secondary ring on high-pressure diesels is to provide a "buffer" volume behind the compression ring. With high combustion pressures, the gas leakage passing through the ring gap and accumulating behind the compression ring can cause the compression ring to lift from its land and go into flutter. Having that extra volume space between the compression and secondary rings dampens the max pressure rise in that space due to leakage.

The distance from the crown to the (top) compression ring should not be reduced, since that distance provides thermal isolation. The ring interface temperatures (ie. groove land and liner surface) should be kept within the flash temperature capabilities of your lubricant, otherwise oil coking, ring sticking and scuffing will occur.

If you want to move the secondary ring up closer to the compression ring, you can still create a buffer volume by slightly undercutting the land area between the two rings. Just make sure to do some analysis to verify that the amount of ring land deflection is not causing the ring to twist excessively.

You may also gain some space by moving the oil control ring as close to the wrist pin bore as possible. It appears as though you're not using a composite piston and it's a 4-stroke engine, so the oil ring groove lower edge can be located within about 1mm of the wrist pin bore edge. If you do this, make sure to also extend the crown diameter relief cut lower. Unless you have a very short conrod, your piston skirt thrust faces should still be adequate.

Good luck.
Terry