Do you think thick oil always protects rings better? Think cavitation!

[Gokhan]

Unlike bearing lubrication, cylinder - ring lubrication is very complicated that involves all regimes of lubrication, including boundary, mixed/elastohydrodynamic, and hydrodynamic.

While thicker oil tends to provide a thicker oil film in general, especially in the hydrodynamic region, it turns out that the oil flow by the rings is very complicated and cavitation -- a complete break in the oil film -- is always present and inevitable regardless of the viscosity and operating conditions.

It turns out that the length of the cavitation (oil-starved) region is proportional to (see the second article):

Length of the cavitation ring section ~ (RPM x viscosity)^0.3

^0.3 represents the 0.3rd power, which reduces the effect of the RPM and viscosity. For example, if you increase the viscosity by 2, cavitation length increases by 23% and not by a factor of 2.

Typical cavitation lengths (oil-film-starved sections) are around a millimeter or less.

This means you need to strike the right balance between cavitation and oil-film thickness. Going thicker won't necessarily reduce ring wear. Cold engines could be even more problematic for thicker oil.

The first link is open access:

https://www.sciencedirect.com/science/article/pii/S0301679X15001760

Not open access:

http://tribology.asmedigitalcollection.asme.org/article.aspx?articleid=1468067

So, it's something to think about before you go arbitrarily thick to reduce ring wear.

 

[double vanos]

Rings? That's the last of my worries! My concerns are the timing chains; thicker oils protect them better ( I hope).

 

[StevieC]

Oh this is going to be good...

 

[Silk]

So, how long is my $500 car going to last on 15W-60 ?

 

[Shannow]

I've driven probably 1M kilometres in a country that specified 20W50 in nearly everything for 40 years.

The roads were never blocked with cars from shoulder to shoulder that had failed in the middle of the road, and neither were the junkyards full of pristine vehicles whose engines had let them down early.

Lawsuits ????

The car companies and lubricant suppliers are STILL IN BUSINESS, indicating that there was no problem ever caused by any of the cars manufactured in an era of 20W50 in everything.

The way you people talk, I would have had to weave my way through 75km of parked relics to get to work in my first year out of uni (instead of 75km in 32 minutes one day I might add)

And please don't post those inflammatory papers in the automotive oils section. People aren't here to be scared by your flim flam use of scientific papers, when we KNOW that millions of cars went hundreds of thousands of miles without ANY ISSUE AT ALL...

THANKYOU

 

[Y_K]

It looks like you have a peculiar view of cavitation, at least from your 'complete break in the oil film'. True, Bernulli Principle applies to all Newtonian fluids, but the oil film thickness and its viscosity are not linearly correlated. Not a big deal, Mr Physicist.

 

[SonofJoe]

One might add that ring damage could also be caused by a giant asteroid hitting your engine!

 

[Shannow]

Originally Posted by SonofJoe
One might add that ring damage could also be caused by a giant asteroid hitting your engine!


I think that was one of the ways that Porsche A40 oil put non Porsche engines at risk in another thread.

 

[OilUzer]

Originally Posted by StevieC

Oh this is going to be good...

 

[Y_K]

I will add an example of an intelligent discussion many can understand

 

[Silk]

I think some people are pushing a thick/thin argument, because they want a thick/thin argument....

Funny, but I'm on an Airhead forum right now pushing a thin oil arument.

 

[Shannow]

Originally Posted by Silk
I think some people are pushing a thick/thin argument, because they want a thick/thin argument....

Funny, but I'm on an Airhead forum right now pushing a thin oil arument.


If I was a betting man, I'd suggest that you are in the straight 30 Newtonian through 15W40 range (HTHS 3.5 to 4) rather than the 4s through 6s that 20W50s, and especially synthetic 20W50s end up at.

Everyone thought I had rocks in my head when I started running A3/B4 (or C3) 5W30s down here...LOL

 

[Shannow]

Originally Posted by Y_K
I will add an example of an intelligent discussion many can understand


There are multiple cavitation issues in fluid dynamics...there's bearing cavitation on the suction side of the bearing, there's the piston ring cavitation per this thread, and there's oil pump cavitation which your thread refers to oil pump cavitation, where the NPSH is insufficient to keep the oil column on the suction side of the oil pump as a "solid" liquid. It pulls the oil apart, and creates bubbles of the gasses that were in the oil, collapsing and leading to failure to supply oil

 

[Gokhan]

To give an intuition of what's going on, this figure from the first paper I linked depicts the cavitation between the ring and liner:

According to the second paper:

length of the cavitation region ~ (speed of the piston x viscosity)^0.3

Since you get the 0.3rd power, the effect of the viscosity is reduced -- about 23% longer cavitation section when the viscosity doubles.

The papers are trying to optimize the ring geometry to decrease the length of the cavitation region.

 

[Shannow]

Originally Posted by Gokhan
To give an intuition of what's going on, this figure from the first paper I linked depicts the cavitation between the ring and liner:

According to the second paper:

length of the cavitation region ~ (speed of the piston x viscosity)^0.3

Since you get the 0.3rd power, the effect of the viscosity is reduced -- about 23% longer cavitation section when the viscosity doubles.

The papers are trying to optimize the ring geometry to decrease the length of the cavitation region.


Thankyou Gokhan for cutting to the chase so to speak.

For those of you unfamiliar with cavitation, the phenomenon occurs when the local vapour pressure within a column of fluid exceeds the local mechanical pressure applied by the equipment.

For an example, take your insulin syringe, and put it in a tub of water...pull the plunger, and the plunger will be really hard to pull, until at some point, it suddenly "pulls a vacuum" in excess of the local vapour pressure of the fluid, and suddenly, the syringe "releases", and the plunger moves rapidly as the syringe fills with nothing...well it's nothing, in that it's the vapour pulled from the fluid.

Let go of the plunger, and the nothing collapses creating a huge pressure spike...that will chew the propellers off a submarine if they are poorly designed.

Note in Dr Gohkan's above picture, that the moving piston ring has a converging diverging face. As the ring moves, the oil is jammed into a progressively smaller space, which applies a pressure to "keep em separated", pushing the faces apart...that's hydrodynamic lubrication.

Now, on the diverging face, the oil has to slow down enough to increase it's pressure (Bernoulli), to keep the faces flooded with oil.

If the dynamic forces due to the rate of change of the opening overcome the vapour pressure of the oil, then the oil drops into vapour phase, and creates a region of cavitation...and yes, move viscosity, the greater that...and note that the equation quoted doesn not provide either a zero point, nor an asymptote.

The area of cavitation is at pretty close to a perfect vacuum, which further pulls the ring towards the cylinder, increasing the effective load, and reducing the oil film thickness, all things being equal.

NOTE : h min is the minimum oil film thickness...and the region of cavitation is not in and of itself dry. The region of cavitation is clear of the cylinder in Gokhan's picture, the surfaces are NOT dry, as the entiretly of the oil present has not evaporated, and one wonders how the title of the thread relates to the imagery shown.

 

[Silk]

Originally Posted by Shannow


If I was a betting man, I'd suggest that you are in the straight 30 Newtonian through 15W40 range (HTHS 3.5 to 4) rather than the 4s through 6s that 20W50s, and especially synthetic 20W50s end up at.

Everyone thought I had rocks in my head when I started running A3/B4 (or C3) 5W30s down here...LOL


Drivetrain too, I'm using 75W-90 syn, everyone else won't budge from dyno 90.

 

[demarpaint]

Here we go again, I'll play. I seriously doubt 5W30 in a vehicle calling for 5W20 is going to cause any phenomenon that will cause damage to my engines. In my case I believe it will benefit the engine over time. In fact I doubt a 40 grade oil will be a problem either.

 

[CarbonSteel]

Originally Posted by Shannow
People aren't here to be scared by your flim flam use of scientific papers, when we KNOW that millions of cars went hundreds of thousands of miles without ANY ISSUE AT ALL...
If he cannot dazzle them with brilliance (which is usually lack-luster at best), he can always baffle them with...

 

[OilUzer]

You can blindly & safely pick 0Wx30 (or even 0Wx40) for all passenger cars/trucks on this planet with no issues. Racing excluded ...

You can't say the same about 0Wx16 or 0Wx20

 

[demarpaint]

Originally Posted by OilUzer
You can blindly & safely pick 0Wx30 (or even 0Wx40) for all passenger cars/trucks on this planet with no issues. Racing excluded ...

You can't say the same about 0Wx16 or 0Wx20