It gets stuck in the groove, and can’t move to conform to the cylinder wall. Oil then can move past it, and combustion gasses can move into the crankcase. If you take rebuild an old engine, there will be carbon in the piston groove that has to be removed, I’ve used a broken ring as a scraper, you can also use something like this:
Still confused. When installing a piston into the cylinder, isn’t there a tool that sort’ve compresses the ring around the piston so that the whole unit (piston with rings) can slide into the cylinder? As the piston moves up and down in the cylinder, from brand new assembly, the ring is constantly putting slight outward force against the cylinder wall as to create a seal. Saying that it can’t move to conform to the cylinder wall makes it sound like the cylinder would have varying widths or something. As if it were tapered. Doesn’t make sense to me.
After rereading the comment “it gets stuck in the groove” I guess you mean an area of the ring gets compressed inward into the groove so it cannot provide the outward force towards the cylinder wall to create a good seal.
I’m confused as to how that could even happen mechanically. I mean I understand how the carbon buildup could occur but what would cause the ring to get pushed or compressed inward toward the piston? Sorry for all the questions, just trying to understand it better.
If enough carbon builds up the ring jams in the groove.
And it’s not the spring tension that holds the compression rings against the cylinder walls, it the pressure of the combustion gasses that get behind the ring that does that. If that’s blocked by carbon buildup the ring stops sealing as wells.
Another cause of blowby on the Geo Metro engines is the fact that the PCV system is notorious for not handling anything more than just a little excess blowby past the compression rings. So, pressure builds in the crankcase and the 4 oil drain holes in the head up to the valve cover area don’t allow oil to drain back very well. The oil begins pooling in the valve cover/camshaft area and whipped into a mist that is sucked through the breather. In really bad cases, the oil level can reach the top of the valve cover where it is sucked into the breather in liquid form and directed straight into the throttle body, generating massive smoke clouds.
There was obviously a little of this blowby going on as I saw some oil residue under the air cleaner when I first got the car. I cleaned it up with carb cleaned and noticed some again after I realized the car was using oil. I cleaned it again after the high-load runs and haven’t seen any oil return. This problem may have been as much about sticky compression rings as it was about oil rings. These engines are known for sticking rings if neglected. The problem was they were a cheap car to start with and many people neglected them because of that fact. I replaced every seal and gasket on this one besides the head gasket and could tell it hadn’t gotten TLC in regards to oil changes. There was some sludge in the dark recesses of the engine and any of the bare metal had a nasty brown film/coating. Besides this, the engine seems to run quite well so I decided to gave it a chance.
Look up “Engine break in” on Google and select “Break In Secrets - MototuneUSA” I would post the link but these never seem to appear. This looks very similar to the method I describe. I guess the engine vacuum pulls out bits of crud so they are exhausted instead of staying in the ring area. They make sure to stress closed throttle decel/engine braking in this article.
I have also noticed that the new oil isn’t getting dirty as fast. I can still see that things are super clean inside by pulling the oil cap but suspect that blowby is greatly reduced, resulting in less contamination of the oil. I am running a European spec synthetic in this to try and clean it out. All the seals/gaskets are new so I am not worried about synthetic oil causing leaks.
@Fender, The piston does not have perfectly vertical forces acting on it. It gets pulled/pushed side to side because the connecting rod pushes/pulls at an angle on the piston except at TDC/BDC. That sideways force tends to compress the piston rings into the groove. When the oil ring gets gummed up and stuck in the groove, it can’t spring back against the cylinder wall as the sideways force is periodically removed/reversed as the crankshaft rotates.
Yeah, I agree that this is the lazy way of solving the problem but why not try it? If an engine is worn out, what have you got to lose? It needs to be replaced or rebuilt anyway. It is also basically free except for the extra gas you burn during the high load runs. Now I guess if an engine is knocking, then you might cause it to fly apart doing this.
Here’s a diagram showing how the compression ring works. It’s for a compression ring with an added “rail” at the bottom to reduce gas blowby, most compression rings don’t have that rail.
They move within their grooves, constantly expanding and contracting to conform to the cylinder walls. It’s that ability to move within the grooves that allows them to press against the walls upon compression and to compensate for wear. Cylinders wear the greatest at the top, because that’s where the forces are felt the most. They become tapered over time. A piston ring will have a gap in it that allows it to become larger in diameter and smaller as the piston goes up and down.
The gap also allows for heat expansion, but that’s another question.
As Texases stated, if it gets stuck in the groove it can no longer do this.
Wow ok, now we’re getting somewhere. So all cylinders have a slight taper to them? Even from new? If this was the case, then the gap in the ring makes sense to me. That gives it room to compress where the cylinder walls are smaller in circumference.
No, not from new, it develops over time. The gap is required, even in a perfect wellbore, to account for manufacturing variation and, most importantly, thermal expansion.
Very interesting. I think I’m starting to grasp it. I appreciate it guys.
It begs the question, why can’t we have a block and piston that are made from the same material so as to avoid mismatched thermal expansion/contraction times, and also, why can’t it be precision machined so that there’s just a piston with no ring needed? Are we limited by the science and material or are we limited because of a cost ratio for mass producing vehicles for the public?
Limited by manufacturing precision, and even if you had a ‘perfect’ fit, the friction could be a real problem. This way the friction’s only at the ring contact, and mostly limited to the power stroke. Much less friction on the other 3 strokes.
A cylinder is hotter at the top where the combustion process happens than at the bottom where much of the heat energy has been expended. Since material expansion is a function of its coefficient times the temperature change, it could be argued that when at full operating temperature even a new engine’s cylinders will be tapered due to the variation in temperature along the cylinder walls. Computer modeling with modern design software can actually calculate and illustrate these variations.
But a cylinder is not designed or manufactured with a taper. The engine is simply designed to accommodate a certain amount of cylinder taper. That design element that allows I to work with a taper is the rings.
In addition, the pistons don’t expand exactly the same amount as the cylinders. Their materials are different and their manufacturing processes are different, and both affect the expansion coefficient. The rings compensate for that too.
And there truly is the manufacturing variation that Texases describes. Cylinders are not perfect, and if there’s more than one no two will be exactly the same. The rings compensate for that.
Very good. Thank you guys. Sorry to hijack the thread there a bit.
No problem. It’s actually relevant, or at least valuable to, the original question anyway.
If there was no piston ring, there would be a lot more friction, and I’m not sure oil could get down next to the piston. Either the oil couldn’t get around the piston or it would all leak down into the combustion chamber and burn.
even if they were the same material they would not expand the same. the mass and shape are different, which makes them expand differently
Good point wes. And very true.
Quoting @wesw "after I got it, whenever I stepped on the gas , clouds of black smoke would come out of the tail pipe.
this was handy when I was being tailgated."
Love it wes.
Reminds me of a car I had once that had one very poorly aimed windshield squirter. It would spray completely past the top and hit the “gater”. I knew it was working when they backed off and turned their wipers on.