piston ring position
piston ring position
(OP)
In an engine, what is the purpose of offsetting rings 120 degrees when installing the pistons in the bores? Out of several engines I’ve pulled apart and looked specifically at the ring positions, not a single piston has had the rings at 120 degrees.
No bores, pistons, rings, blocks etc are perfect and considering expansion/contraction & that the rings have been up and down millions of times during the engine’s working life, it seems hardly surprising that the rings want to move around. Don’t they just wriggle around after first start up and assume a position which most accurately aligns the new (and not perfectly round) ring with the not perfectly round bore and then bed in at that point, staying put until strip down?
Wouldn't manufacturers install some locating method in the ring groove if they wanted the ring to stay in a particular position?
No bores, pistons, rings, blocks etc are perfect and considering expansion/contraction & that the rings have been up and down millions of times during the engine’s working life, it seems hardly surprising that the rings want to move around. Don’t they just wriggle around after first start up and assume a position which most accurately aligns the new (and not perfectly round) ring with the not perfectly round bore and then bed in at that point, staying put until strip down?
Wouldn't manufacturers install some locating method in the ring groove if they wanted the ring to stay in a particular position?





RE: piston ring position
On 2-cycle engines where the ports are in the cylinder bores, some manufacturers have a pin in the ring groove that meets with a notch in the ring end to prevent it from moving to the cylinder port. All in all, I guess it would be good practice to stagger the ring ends, if nothing but to prevent the cylinder bore from developing a vertical ridge, but certainly to minimize blowby.
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RE: piston ring position
There have been a number of studies done with running engines & various methods of seeing inside, either by sensors or windows, and all that I'm aware of have shown all the rings rotating in fits & starts or even quite rapidly.
One thing which would tend to drive them around is the rocking of the piston back and forth in the bore (in the thrust plane) combined with any transient offset of the piston to one side or the other in the pin plane. You'll also have uneven distribution of friction at the ring-groove interface (which works w/rocking to move the ring), and you may have some amount of (aerodynamic) thrust from the blow-by gas depending on the details of the ring groove geometry.
Some people will tell you that they always find the rings in certain orientations when they tear down engines - I have never seen a scientific study of whether this occurs - I suspect it's bias on the part of the observer which skews their recollections. I have definitely seen more than one warranty claim where the servicing mechanic reported that the rings were not "clocked" correctly when the engine was built (ahem, BS).
RE: piston ring position
Ed Danzer
www.danzcoinc.com
www.dehyds.com
RE: piston ring position
Many pistons now contain a phosphate coating to prevent micro welding of the ring to the ring land to promote longer ring land life.
RE: piston ring position
After the engine starts, and you have at least a bit of ring seal, it doesn't matter any more. The gaps will be in random positions - but generally not aligned.
RE: piston ring position
RE: piston ring position
Although >>some<< swear by gapless rings in various configurations.
(Last time I looked a few weeks back total seal said go gapless in either 1 or 2, my choice)
I question if there would be any functional superiority over a well done conventional ring installation. (well done = about 30 well controlled parameters. some examples - cylinder roundness and taper with head installed, cylinder finish, and pre-assembly cleanlinesss, piston ring groove finish, flatness, back clearance, break-in procedure, crankcase ventilation and pressure management)
One of the mechanisms behind shaft pressure sealing devices like o-rings, lip seals and piston rings is a pressure differential forcing the seal radially outward. The greater the differential the better the seal. No differential and the pressure can puff right past the sealing faces. Piston rings have inertia, so when the piston is accelerating fast down the bore from TDC the ring would prefer to migrate to the top of the groove. If the pressure (differential) is insufficient to keep the ring against the bottom of the groove and the ring lifts off, the pressure in the back clearance will bleed off and the main seal against the cylinder wall will be lost. that one reason why high rpm requires narrow (light) rings.
Speed Pro recently has been speaking publicly about adding a little extra gap clearance to the second ring gap, to intentionally release pressure that build up between the 1st and 2nd rings during the power stroke to bleed down, ensuring the greatest possible differential across the first ring and best sealing under extreme rpm.
Ring gap for the top ring remains the smallest that will still not butt during operation.
A total seal ring in the 2nd groove would seem to invite capturing pressure between 1 an 2, ruining ring 1's stability at higher rpms.
RE: piston ring position
Rod
RE: piston ring position
FYI, on large slow speed engines you can have fitted ring wear measuring equipment. using this you can actually see the rings wandering around.
On one medium speed engine design not only to the rings rotate but the piston does as well (Sulzer ZA)
the angle isn't critical so long as you make the 'torturous path' commented above.
On Slow Speed engines its nearly alwys the 2nd ring that breaks initially
RE: piston ring position
I'm very interested in your comment about ring wear measuring equipment on large, slow speed engines (I'll assume you mean very large ship engines). I'm about to start working on the design of my first engine of this type (all my previous experience is with automotive and off-highway diesels), this engine will be for testing lubrication systems so ring wear is one of the key parameters.
What sort of system can be used to measure the ring wear as the engine operates? We're going to have to investigate what's possible in this area but any pointers you could give that would help me short-cut to real world systems would be appreciated.
Thanks.
RE: piston ring position
When I was involved with this technology, there was only one commercial proponent, TMT (Tribologiemesstechnik) in Karlsruhe, Germany. They may have competitors by now, but I don't know.
Here is a publication of a project I worked on using this technology.
Correlation of Radiotracer Wear Measurements with conventional Methods in the Navy 1000-hour High-speed Diesel Durability Test.
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz
RE: piston ring position
RE: piston ring position
"Schiefgehen will, was schiefgehen kann" - das Murphygesetz