Oil Viscosity and Turbocharger Failure

[MolaKule]

"I must disagree with respect to the turbine oil must perform under high temperatures. Turbine bearings operate much cooler than recip engines as jet engines operate with an abunance of good cool air flow."

In automotive applications, the exhaust heat is conducted to the bearings and the bearing temps are very close to exhaust temps, hence the need for a high-temp, low oxidation oil. With water cooled turbos, the temps are not so high.

In most jet engines, you have two coolers to keep the oil cooled down; the fuel-to-oil cooler and the air-to-oil cooler. When the oil is no longer cooled by fuel and/or air (as in shutdown), the bearing cell temps may spike, as the heat is conducted from the combustion chamber to the turbine rotor bearings.

ERAU is a great school; a graduate of 1997 I am!

 

[Patman]

quote:

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Originally posted by BOBISTHEOILGUY:


In the case of a real full synth, You take a 40wt base oil synth. In most cases it has the ability to flow in lower temps to what ever it is tested to.So for example, a company like M1 can make a 40wt synth, test it to the flow propertis of a 15wt and label it, take the same oil, retest it to a 10wt property and label it,again use the same oil and test it to a 5wt and 0wt and relabel those, all being the same oil. This is not a bad thing, but in fact is a win win for everyone as it lowers the cost of production to one oil for many viscosities and also, you could have a 15w oil that actually perform down to a 5wt oil in really sub cold temps if needed. But in all, it's still a straight 40wt with no VII added, therefore it has a natural ability to resist the cold flowing properties.

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Do any oil companies really do this? If so, which ones?

 

[GeorgeCLS]

Taught at ERAU 1966-68.... Dating myself.. Just visited in May, wow, what a difference 30 years makes! Beautiful campus...

 

[The_Oz]

quote:

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Originally posted by Patman:
[QBDo any oil companies really do this? If so, which ones?[/QB]

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Patman,

You asked before I did, you silly!!

http://theoildrop.server101.com/ubb/ultimatebb.php?ubb=get_topic;f=1;t=000438

Oz

 

[VeeDubb]

My wife's Volvo 850 Turbo has 94k on it and it is fine (cross my fingers). Before she married me, she used to take it to Walmart for oil changes and they put in dino. She doesn't cool it down after hard driving and she doesn't know what type of oil they used (weight or brand). She loans it to friends who don't warm it up (drive it hard right after startup) and often take 2 minute trips. Was she just lucky or are Volvo turbos well built? I recently switched her over to Mobil 1 0w-40 and wouldn't it be ironic if we had a turbo failure?

 

[MolaKule]

"One of the more prevalent turbocharger failure modes observed in motorsports is extreme wear of the [thrust-bearing] face due to angular shaft displacement associated with excessive dynamic response and rotordynamic instability. Severe operational surge loading further aggravates the limited [thrust-support capacity of the conventional [trust-bearing] face design. The resulting axial play of the rotating group can then become sufficient to permit wheel-to-housing contact and complete turbocharger destruction"

One design problem is that the first bending mode may be near the operating range of the turbocharger. The article states "This instability will manifest itself as rotor whirl and destructive oil whip, potentially leading to rubbing and subsequent failure." Excessive amplitude as you near the resonance point is restrained by viscous damping of the oil in the bearing journal. Lack of restraint could be caused by insufficient film thickness or viscosity. One cheap test
would be to drain the oil and replace it with something higher in viscosity and then test for a difference in feel or sound."

It sounds like a snowball effect. If the film thickness is insufficient to resist wear, then the bearing wears (since it is softer than the shaft), the clearances become larger, and the rotor shaft then wobbles or oscillates in the bore, leading to more whipping of the oil and aeration, which reduces the film thickness and increases eccenttric
motion of the shaft, which then means the blades rub the walls and destruction results.
At the resonance frequency of the rotor, the rotor shaft would receive massive amount of mechanical energy which causes pounding of the bearing faces and bore.
The greater the oil viscosity, the more damping of the rotor oscillations.

Bottom line, you need to start out with thick oil film (in my view at least SAE 30) and a lube with a good AW addtive to prevent boundary friction and subsequent wear. Sounds like a good application for #132.

[ September 05, 2002, 12:08 PM: Message edited by: MolaKule ]

 

[MolaKule]

The 850 turbo has large clearances with finely balanced components. Its a tough one indeed.

 

[sprintman]

VeeDubb
Mobil 1 0W40 is a good oil as used in Porsche turbo. if your turbo fails that won't be the cause IMHO.

 

[widman]

Right, Molakule, vehicle turbos are hot. I once tried to check with my laser pyrometer and it was out of range. (over 500 F).
I run Group I 20w50 in my 7 ton truck. One turbo failure in 135,000 miles caused by a stupid driver who stopped on the highway to relieve himself and turned off the engine without cooling. I have Delo 400 15w40 in my Turbo Diesel 4 Runner, but only about 19000 miles so far, most with turbo boost as it has no power until it kicks in at 1600 rpm.

 

[GeorgeCLS]

As an over-all statement: fleet customers I work with that run mineral based oils all seem to have a skid over in a corner of their garages with turbo-cores on them. However, the fleets that run full synthetic do not... It is one of the clear advantages of synthetic vs. mineral in terms of turbo life with the advantages of no coking and better heat dissapation, much more forgiving operationally.
George

 

[TooSlick]

Mormit,

For fifty cents more a quart, the regular Amsoil 10w30 (ATM) is a significantly better oil than the XL-7500 stuff. It is formulated towards the high end of the 30wt range (12 centistokes vs 10 for the XL-7500) and has a more robust additive package, with higher levels of antiwear and detergent/dispersant additives.

TooSlick

 

[Dick in Falls Church]

Patman-
It is my understanding that an oil must be labelled at the lowest xW rating it will pass.
Thus, a 5W- must therefore exceed the cold temperature requirements for 20W-, 15W-, and 10W-.

Perhaps GeorgeCLS can comment on this

 

[mormit]

quote:

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Originally posted by sprintman:
mormit
Just talked to Shell and they say 15W50 Full Synthetic for S40 turbo. Are you sure you really want 10W30 in your turbo bearings?

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The owners manual and dealer both say 10w30 up to 104degF ambiant temp. Hottest day I encountered this summer was 101degF. I did 6 hrs of 90mph highway driving that day. The oil has 3000mi on it now and looks very clear still with little darkening. I'm gonna pull a sample this week for analysis just for good measure.