Oil more likely to cavitate as it gets hotter

[Linctex]

https://jalopnik.com/the-fascinating-reason-why-nascar-engines-run-so-hot-1835071544 The Fascinating Reason Why NASCAR Engines Run So Hot The average car on the road today keeps its engine running at around 200 degrees Fahrenheit, but a NASCAR racing engine runs around 280. Here's the reasoning behind this, why teams run ice-water through their engines, and more fascinating facts about NASCAR engine cooling systems. { SNIP } As far as the current setup, which incentivizes cooling system compromises in favor of drag reduction, Randolph admits that it's a "reliability issue," and that it's expensive to design an engine to handle those elevated temperatures and pressures. In an email, he described how much stress those conditions put on engine components, saying: Components that suffer from high temperatures include rod bearings (oil more likely to cavitate as it gets hotter), pistons (aluminum softens as the average operating temperature increases), valve seats (seat wear/erosion proportional to operating temperature), and cylinder heads (life shortened due to aluminum softening). Lubrication becomes problematic as oil film thickness reduces with temperature and in fact the oil itself begins to dissociate above 350F (engines smell like sulfur when disassembled due to oil dissociation). Bore roundness deteriorates, bolt clamping forces change (some increase, some decrease), chances of knock and/or preignition increase. Lots of bad stuff. To help increase engine durability, Randolph says his teams uses special coatings, alloys, and sealing materials. And the cooling system uses high-temperature o-rings and threaded connections in place of rubber hoses. But of course, this all adds cost. Stephen Papadakis, the owner of Papadakis Racing and the subject of our story last year on the early tuner street racing culture of Los Angeles, also runs an excellent YouTube channel that provides all sorts of insights into race car technology. In his latest video, "7 Things You Didn't Know About NASCAR Cup Technology," he gets into some hot, steamy NASCAR cooling system details:

 

[KGMtech]

Thanks for sharing the video, interesting stuff.

 

[RDY4WAR]

I'm assuming they mean cavitation as a result of insufficient oil film? Those engines run on 0w-16 equivalent oil so it's not surprising. I haven't watched the video yet. Most cavitation issues I've run into has been a result of excessive aeration which decreases with higher temperature / lower viscosity. Most of those cases are from guys turning 9,000 rpm with 20w-50 oil that's only 130*F.

 

[DoubleWasp]

Engine oil beginning to aerate and foam at very high engine temps is nothing new. I remember someone here posted about a circle track car they were testing with VR1 that foamed more and more the hotter they got it.

 

[RDY4WAR]

Originally Posted by DoubleWasp

Engine oil beginning to aerate and foam at very high engine temps is nothing new. I remember someone here posted about a circle track car they were testing with VR1 that foamed more and more the hotter they got it.

That may have been me that posted that. I had that experience in a circle track car. VR1 was highly aerated after 20 laps where as Red Line was not. However, the oil was getting hotter and hotter because of aeration, not the other way around.

 

[oldhp]

They can cool the engines down with ice water because they found out about a special metal to cast the blocks with. I'm pretty sure that technology came from Germany. NASCAR has a limit on how much water pressure they run today because it(money) was getting out of hand what these cooling systems were costing. These guys are geniuses. No wonder the rule book is thick.

 

[RDY4WAR]

Also keep in mind that these engines run on oil that has a KV100 = 5.6-6.0 cSt and HTHS around 2.3 cP.

 

[ZeeOSix]

Oil aeration and oil cavitation are two different things. Cavitation occurrs when the fluid can't flow fast enough towards a low pressure area, and as a result the fluid pressure decreases to the point where it cavitates. Aeration is just the mixing and formation of air bubbles in the oil, but the air is not a result of cavitation. I had thread a while ago about journal bearing oil cavitation - see link in the link. https://www.bobistheoilguy.com/foru...eristics-in-journal-bearings#Post5082094

 

[ZeeOSix]

Oil aeration and oil cavitation are two different things. Cavitation occurs when the fluid can't flow fast enough towards a low pressure area, and as a result the fluid pressure decreases to the point (vacuum) where it cavitates (vapor implosions). Aeration (foaming) is just the mixing and formation of air bubbles in the oil, but the air is not a result of cavitation. I had thread a while ago about journal bearing oil cavitation. Looks like the link to the PDF test report is now broken. I summarised the conclusions in the link below. https://www.bobistheoilguy.com/foru...eristics-in-journal-bearings#Post5082094 More info on oil cavitation. https://www.munciepower.com/company...n_what_is_it_and_how_can_you_prevent_it_

 

[ZeeOSix]

Originally Posted by Linctex

As far as the current setup, which incentivizes cooling system compromises in favor of drag reduction, Randolph admits that it's a "reliability issue," and that it's expensive to design an engine to handle those elevated temperatures and pressures. In an email, he described how much stress those conditions put on engine components, saying: Components that suffer from high temperatures include rod bearings (oil more likely to cavitate as it gets hotter), pistons (aluminum softens as the average operating temperature increases), valve seats (seat wear/erosion proportional to operating temperature), and cylinder heads (life shortened due to aluminum softening). Lubrication becomes problematic as oil film thickness reduces with temperature and in fact the oil itself begins to dissociate above 350F (engines smell like sulfur when disassembled due to oil dissociation).

Sounds more like oil vaporization going on, not cavitation. If oil is very low in viscosity it should flow very easily, which would decrease the likelihood of cavitation. Of course the NASCAR engines are running high RPM which causes lots of heat and shearing in the bearings. That's probably what's vaporizing (disassociating) the oil.

 

[RDY4WAR]

I read that thread when you posted it. There is some good info there. I also feel it's a matter of sloppy clearances. What I was thinking was that highly aerated oil entering a rod bearing where there's negative pressure in the cavitation zone would allow for entrained air to escape into that zone, displacing the oil, and thus compromising the MOFT and load carrying capacity of that oil. I was thinking of it as cavitation in the loaded side of the bearing caused by aeration, but I now believe that to be an incorrect meaning/use of the word cavitation.

 

[RDMgr]

Earlier comments are correct regarding what cavitation is not. It is not aeration, foaming or air entrainment which are typically caused by air leaks, turbulence or churning, contamination, lubricant level, oil formula and similar things. Cavitation is a function of pressure change. Oil flow through line direction changes or exiting from confinement zones can cause a sudden pressure drop and dissolved gas may be released in the form of vapor filled bubbles or voids. This occurs when pressure falls below the pressure necessary to keep the gas in solution. When high pressure is applied to these bubbles they compress, eventually collapse forming a high pressure jet or shock wave that hits the component surface causing surface erosion. In an engine main bearing, cavitation can be caused by the pressure drop as oil flows over the edges of oiling holes or grooves. Possible solutions may be rounding edges of oiling holes and grooves or using more damage resistant bearing materials. Cavitation is seen in a lot of applications including bearings, gears, hydraulics, pump impellers and boat propellers.