Piston Rings and Grooves

[MolaKule]

A small clearance is designed-in between the piston ring and its groove.

In terms of combustion pressure, give a short explanation of the action of the top ring in its relation to the oil film and cylinder wall.

 

[Passport1]

Your question doesn't state compression rings or oil control ring. The bottom ring rides over the small orifices in the piston which allow oil to move from the groove back to the internals of the engine. Its function is not primarily to prevent blow-by gasses from entering the crankcase. The compression rings however don't function to contact a heavy oil film like the bottom ring. Their function is to control the compression from entering the crankcase. The rings are related on the piston only in that the open ends are staggered in position at about 120 degrees and they rotate somewhat as the engine runs.

That's how I understand it.

 

[Kira]

The cylinder wall film and the ring improves the compression efficiency.
Squirting oil into a cylinder during a compression check demonstrates the effect.
Think of the oil film as a cloth wad used when firing a flintlock firearm.

The small clearance you mention in the first sentence has likely been refined over the decades and includes enough looseness to maintain free movement and not so much as to propagate rattling. The correct amount of oil to keep the groove clean as well as lubed has been developed.

 

[MolaKule]

Your question doesn't state compression rings or oil control ring. The bottom ring rides over the small orifices in the piston which allow oil to move from the groove back to the internals of the engine. Its function is not primarily to prevent blow-by gasses from entering the crankcase. The compression rings however don't function to contact a heavy oil film like the bottom ring. Their function is to control the compression from entering the crankcase. The rings are related on the piston only in that the open ends are staggered in position at about 120 degrees and they rotate somewhat as the engine runs.

That's how I understand it.

Let's go with the top compression ring:

A small clearance is designed-in between the piston ring and its groove.

In terms of combustion pressure, give a short explanation of the action of the top ring in its relation to the oil film and cylinder wall.

 

[MolaKule]

The cylinder wall film and the ring improves the compression efficiency.
Squirting oil into a cylinder during a compression check demonstrates the effect.
Think of the oil film as a cloth wad used when firing a flintlock firearm.

The small clearance you mention in the first sentence has likely been refined over the decades and includes enough looseness to maintain free movement and not so much as to propagate rattling. The correct amount of oil to keep the groove clean as well as lubed has been developed.

The question is focused on this:

In terms of combustion pressure, give a short explanation of the action of the top ring in its relation to the oil film and cylinder wall.

 

[Passport1]

Seems to me Kira's explanation answered that.

 

[MolaKule]

Seems to me Kira's explanation answered that.

No, some good points were made but it didn't answer the question.

 

[SubieRubyRoo]

In terms of combustion pressure, there is downward and outward pressure during the “bang” phase, exerted on the top ring; combustion spike forces the ring down, and the small volume of gas that passes between the ring and land can exert some outward pressure on its way to escaping at the end gap.

Is that what you were asking for?

 

[Langanobob]

In terms of combustion pressure, there is downward and outward pressure during the “bang” phase, exerted on the top ring; combustion spike forces the ring down, and the small volume of gas that passes between the ring and land can exert some outward pressure on its way to escaping at the end gap.

Is that what you were asking for?

I’d say you got it.

 

[SubieRubyRoo]

I’d say you got it.

We’ll see… Mola is known to ask seemingly simple questions that end up requiring complex thinking and careful answers…

 

[MolaKule]

In terms of combustion pressure, there is downward and outward pressure during the “bang” phase, exerted on the top ring; combustion spike forces the ring down, and the small volume of gas that passes between the ring and land can exert some outward pressure on its way to escaping at the end gap.

Is that what you were asking for?

Bingo. Thanks to all for responding.

A small vertical clearance around the piston ring in the piston's ring groove allows combustion pressure to get behind the ring and push the piston ring against the cylinder wall to seal it dynamically.

 

[Kira]

There were (still are?) compression rings which have a notch cut out the top of the ring's inner surface extending only half way to the outer. A much thinner ring is set here and it's gap is revolved away from the compression ring's gap to completely span the original, familiar gap. I believe they were called "Perfect Circle". A ring set into and atop the original compression ring, if you will.

Are those still around? I always thought the much thinner additional ring would get dirty and gum up. Maybe they were just for race engines.

 

[MolaKule]

And some pistons have a triangular notch above the top ring and below the oil ring to allow the gas to get behind the ring for that 'dynamic sealing.'

 

[SubieRubyRoo]

There were (still are?) compression rings which have a notch cut out the top of the ring's inner surface extending only half way to the outer. A much thinner ring is set here and it's gap is revolved away from the compression ring's gap to completely span the original, familiar gap. I believe they were called "Perfect Circle". A ring set into and atop the original compression ring, if you will.

Are those still around? I always thought the much thinner additional ring would get dirty and gum up. Maybe they were just for race engines.

Engine Labs just had a test from Lake Speed Jr not too long ago, they went from a thicker "normal" ring and went to the thinner rings more typical of today, and saw an oil temp drop of 20* from the reduced friction.

Video: Explaining How Piston Rings Can Affect Engine Temperature

We all know that thinner rings make more power, but do they affect your engine's temperature in a meaningful way? Total Seal explains.

www.enginelabs.com

 

[SubieRubyRoo]

Also, here's the effect MolaKule was talking about with dynamic sealing...

Dyno Testing Total Seal’s Gas-Ported Rings Against Ported Pistons

Ever since they've hit the mainstream, the question has been, how do Total Seal's gas-ported piston rings compare to actual gas-ported pistons.

www.enginelabs.com

 

[SubieRubyRoo]

Here's the easiest way to improve your cylinder-to-piston seal without buying new pistons:

PRI 2019: Total Seal Piston Rings Debuts New Gas Ported Top Rings

Total Seal Piston Rings is stepping up the performance of the top piston ring, without having to drill any holes in your piston.

www.enginelabs.com

 

[SubieRubyRoo]

Some really detailed info on friction and where the power is actually wasted in ICEs...

Video: Talking Piston Ring Friction With Lake Speed, Jr.

If you locate the number one source of friction inside of your engine, you can then reduce it and improve the system as a whole.

www.enginelabs.com

 

[MolaKule]

Here is the seminal Textbook on ICE written by JB Heywood:

https://www.accessengineeringlibrary.com/content/book/9781260116106

 

[wpod]

Any one remember Dykes style rings?

 

[SubieRubyRoo]

Any one remember Dykes style rings?

Remember the name, never saw them in person. I didn’t have enough money at the time to buy fancy rings, so I had Mahle PowerPaks with standard rings.

The buddies I had who could afford the fancy stuff blew their motors up often enough they didn’t use the fancy stuff, because of cost savings