Things that keep me awake at night Part one:

 

I’ve played with various engines in my time, fitted them, their pistons/rings, crankshafts, become well versed on what happens when you get the plug leads on wrong 💥etc and therefore know this:

 

Piston rings are never a complete circle, they have to spread during installation, to get over the top of the piston before dropping into their allocated piston grooves. They also need to be a non-complete, sprung circle, to expand properly against the piston wall. Most pistons have 2 compression rings and 1 oil control ring.

 

My issue is regarding compression and a thought that has always bugged me:

 

How do (some) combustion and exhaust gases not get past the incomplete circle gaps (where the ends of the rings meet) of those 3 rings, and thereby end up in the crankcase?

 

If they do, doesn’t this, albeit only slightly, degrade engine performance and cleanliness? I know that the gaps are very small on installed piston rings, and are only partially proud of the piston wall but all the same, they must still be a source of compression loss however small, even if the gases do have to track through all 3 ring gaps, which are most likely not in line with one another either, thus making the journey longer before making it to the crankcase.

 

One thought I have is that the adhesion of the engine oil might be enough to plug the gaps but I don’t know for sure and I can’t really envisage this happening 100% ie the oil viscosity withstanding those compression forces, especially during the ignition stroke 🤔

 

So at the moment I am a lost soul, staring at the wall for hour upon hour, and if anyone can put me out of my misery with some words of wisdom then I’ll be happy once more 🙂

 

Edited 18 October, 20241 yr by davembk
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When I built engines, I staggered the ring gaps around the piston as was the recommended practice. 

11 hours ago, davembk said:

 

Things that keep me awake at night Part one:

 

I’ve played with various engines in my time, fitted them, their pistons/rings, crankshafts, become well versed on what happens when you get the plug leads on wrong 💥etc and therefore know this:

 

Piston rings are never a complete circle, they have to spread during installation, to get over the top of the piston before dropping into their allocated piston grooves. They also need to be a non-complete, sprung circle, to expand properly against the piston wall. Most pistons have 2 compression rings and 1 oil control ring.

 

My issue is regarding compression and a thought that has always bugged me:

 

How do (some) combustion and exhaust gases not get past the incomplete circle gaps (where the ends of the rings meet) of those 3 rings, and thereby end up in the crankcase?

 

If they do, doesn’t this, albeit only slightly, degrade engine performance and cleanliness? I know that the gaps are very small on installed piston rings, and are only partially proud of the piston wall but all the same, they must still be a source of compression loss however small, even if the gases do have to track through all 3 ring gaps, which are most likely not in line with one another either, thus making the journey longer before making it to the crankcase.

 

One thought I have is that the adhesion of the engine oil might be enough to plug the gaps but I don’t know for sure and I can’t really envisage this happening 100% ie the oil viscosity withstanding those compression forces, especially during the ignition stroke 🤔

 

So at the moment I am a lost soul, staring at the wall for hour upon hour, and if anyone can put me out of my misery with some words of wisdom then I’ll be happy once more 🙂

 

IFAIK, there are several factors at play that ensure a good gas seal, and not all rings are fitted to do the same job, they can have different purposes and profiles.

 

1) There are several piston rings which are normally staggered when fitted, as @Warrior193 said

2) The gap in the piston ring closes somewhat when the piston is fitted (squeezed into) into the cylinder. (Otherwise you wouldn't be able to fit the piston without risking damage to the cylinder.)

3) Thermal expansion takes up a bit of the gap too.

4) The oil film also helps the gas seal.

5) The pressure of the combustion also contributes to forcing the rings outwards to form a tight seal against the cylinder wall.

 

The sum of those factors results in a good gas seal for effective compression and power stroke.