Oil Viscosity and Turbocharger Failure

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There is a thread on another board where we are discussing the possibility that the use of XW30 oil could be causing Borg-Warner KKK04-1 and KKK04-15 turbos to fail, due to rotordynamic instability caused by thin oil. The shafts on these turbos are supported by a pressurized oil film inside a sleeve, and the theory is that XW30 is too thin to keep the compressor from contacting the housing. I have run 5W30 in a KKK03 (same housing and shaft as the K04 in question, slightly smaller turbine and compressor) without incident. Comments? Experiences? [ September 04, 2002, 08:05 PM: Message edited by: BOBISTHEOILGUY ]
 
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If the XW-30 were mineral based, a possibility indeed. An XW-30 synthetic, unlikely. As mentioned in previous posts, the synthetic oil used in turbine engines turning at 30,000 rpm is generally in the 5W viscosity range and obviously works very well with no instability problems. However, it is very much depending on the engine's requirements; i.e. diesel, etc. George Morrison
 

MolaKule

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Coking of pure dino's is a possibility with turbos. Most turbo makers specifiy at least a 10W30 full synth because of its high temperature stability and thicker film. Also, some turbo makers specify oil changes within 3k to avoid overtaxing the oil's high temp anti-oxidants.
 

dbrowne1

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George- These turbos spin at close to 200,000 rpm (no, that's not a typo). The only thing keeping the compressor blades from hitting the housing is the oil film. Molakule- It's not a coking issue, it's an issue of the oil being viscous enough to keep the shaft play to a low enough level.
 
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dbrowne1:" The only thing keeping the compressor blades from hitting the housing is the oil film." Yes, in that the proper oil will prevent the turbo shaft bearings from failing. The shaft itself must be perfectly balanced or no simple oil film will prevent it from tearing itself up in about a second at 200,000rpm. Also, there must be minimal play in the bearings. The two together = no lateral movement in the shaft. A top quality oil is a must to protect in this harsh application of high temp, high rpm, low clearance. My own conclusion....synthetic only. I'm currently running Amsoil XL-7500 10W-30 in my turbo 2002 Volvo S40. With 3000 miles on the oil with a good deal of hard driving, it's still pretty clear and hasn't darkened much. I hope this means no coking....the most common cause of turbo failure. Looking forward to the analysis on this!
 

dbrowne1

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quote:
Originally posted by mormit: dbrowne1:" The only thing keeping the compressor blades from hitting the housing is the oil film." Yes, in that the proper oil will prevent the turbo shaft bearings from failing. The shaft itself must be perfectly balanced or no simple oil film will prevent it from tearing itself up in about a second at 200,000rpm. Also, there must be minimal play in the bearings. The two together = no lateral movement in the shaft.
Here is what one person at vwvortex found: Excerpt from Automotive Engineering (February 97) "One of the more prevalent turbocharger failure modes observed in motorsports is extreme wear of the trust-bearing face due to angular shaft displacement associated with excessive dynamic response and rotordynamic instability. Severe operational surge loading further aggravates the limited trust-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. [ September 04, 2002, 10:53 PM: Message edited by: dbrowne1 ]
 
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[Eek!] Wow! Amazing to think that an oil viscosity could prevent such movement. I still suspect that the rotor balance and tight tolerance in the bearings are the limiting factors in wobble in the shaft. Oil is the critical factor that keeps the above mentioned items from wearing to out of tolerance. This is assuming that the oil used is within the auto manufacturer's specification. Wrong weight of oil?? Then sure I would agree that the viscosity and film strength would not match the requirements for the application.
 
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mormit That identical Volvo S40 turbo here in Oz has minimum rec oil viscosity of a full syn 5W40 with 10W or 15W being the norm. It's unreal how CAFE screws up viscosity thinking in the US. I wouldn't run a turbo hard with 30W anything in it, I want it to last!
 
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I think this may be the time to come out and explain something here that is being over looked in this issue and GeorgeCLS or Mola can correct me if I miss the point. So here it is, Newtonian vs Non newtonian fluids. In the case of a mineral oil, you start out with a base oil of 15wt(15w40 for example). You then add some VII to enhance it to the 40wt as it heats up. This is a non newtonian fluid. 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. This is a netownian fluid or actually a straight wt oil with the flow properties of some non netonian fluids. Now there is some so called full synth's that need some assistance to get a further spread on the numbers and there is some help added there but in the case of full synths this is the basic premise. Mobil stands out as being one of those from my understanding. Castrol on the other hand would not be. Now given that, where these turbine bearings come into play, first lets point out that in bearings, you have a wedge effect. How that works is where you have a bearing, inside is little rollers. Oil is sitting in front of the roller and as the roller starts to move around, it pushes the oil in front of it. The faster the bearing spins, the more hydrodynamic fluid is going to be present inbetween the roller and the race as it is forcing more oil under it. The slower it moves the less hydrodynamic film in going to be present. So, If too thick of an oil is present, this wedge will not shear down as easily, thus failing to supply enough hydrodynamic film under the roller therefore causing less film, and more wear. This is why too thin is not good, but too thick also not good. Another point is when you get too thick, you will create a drag effect on the bearings as it will create a bigger wedge in front of the roller the faster it spins. This priciple is the same in both mineral oil and full synths. The main reason bearings in a turbo/turbine will fail sooner is not because the synth has a better film strength over a mineral oil but because the mineral oil will start to oxidize faster due to the high levels of heat it is exposed to therefore it cannot maintain it's viscosity like a full synth and ulitmatly coking will start to appear do to all the VII's used, excessive heat and the VI curve comes into play. So, because the full synth has a better bonding effect on the molecular level, it takes more punishment as the oxygen molecules cannot break the bond as easily unlike a mineral oils molecular chain cannot withstand these forces therefore oxygen atoms will take over the hydrogen atoms and will break down the molecular chain sooner. So, synth's molecular film strength is much better than a mineral oils, but not hydrodynamic film strength when both are new. So, to recap, Synths' advantage in use for turbines and such is not because if produces a better film strength but maintains a more consistant file strength due to the VI index. In cases of mineral oils it heats up, oxidizes, thickens up creating more of a wedge effect, putting more drag on the bearing, less hydrodynamic film and cause more bearing failure. Sorry George, but the idea of synth's hydrodynamic film strength being stronger than a counter part mineral is not why turbines last longer but because full synth's can supply a consistant viscosity over a mineral when an extreme amount of heat is applied. This is One Application(jet engine turbines) that is very important to use a full synth because of the high heats vers the viscosity index. There is no way a mineral will withstand those higher temps for extended periods as it will cause coking. As for the viscosity helping maintian the tightness of the shaft wobble is not going to fly either as the hydrodyamic film will change when the speed changes. The shaft play is a mechanical function that cannot be reliant on the film of any oil to maintain wobble free operation. The surge load reffered to is what causes momentary shearing of the oil's film, and this is why it is necessary to have a really good barrier additive in place as shock loads do happen in every application at one time or another. As for oil whip, this is where the oil falls below the lubrication zone of a bearing and this creates an agitation or air bubbles, and as many of you know, air trapped in oil is going to generate heat and in turn more heat more stain on the oil.
 

dbrowne1

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Manufacturers recommendation (for the car) is an oil that meets one of the following: ACEA A2 or A3, API SJ, or the VW specs. Manual says 5W40 is factory fill, 5W30 is acceptable as well. The problem is that the failing turbos are NOT stock...they are a slight upgrade from the stock one. Basically the same shaft and housing with a slightly larger/redesigned turbine and compressor. Also, the turbos are run right at the edge of their efficiency (unlike the stock turbo and programming). So the VW recommendations do not necessarily apply. I do not know what Borg-Warner recommends. The stock turbo (which I have and run right at the limit) seems to do fine on XW-30 oils.
 

dbrowne1

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quote:
Originally posted by BOBISTHEOILGUY: Now given that, where these turbine bearings come into play, first lets point out that in bearings, you have a wedge effect. How that works is where you have a bearing, inside is little rollers. .
I'm not so sure that this is how the bearing in this turbo works. I don't recall any rollers being involved...it's not a ballbearing turbo or anything similar.
 
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Running Amsoil 10W30 in my twin turbo. Manual calls for the 10W30. I am third owner but previous owners ran Mobil 1 10W. Only 20,000 miles so may be too early to tell anything. I do let the turbos spin down prior to shut down though which no one seems to have mentioned previously [ September 05, 2002, 01:15 PM: Message edited by: Spector ]
 

dbrowne1

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quote:
Originally posted by Spector: I do let the turbos spin down prior to shut down though which no one seems to have mentioned previously
This is not as big an issue with the Borg-Warner K03/K04 turbos that I'm talking about, because they are water cooled. Even if the exhaust housing is glowing red (not hard to do if you drive hard for a little while, or sustain 100mph for 5 minutes or so) the water and oil combined tend to keep the bearing temps low enough to prevent coking. A Group IV synthetic will be able to handle it even without a cool down period. It is still a good idea to let it idle though after heavy load driving, either by staying there or using a turbo timer.
 

MolaKule

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"As for the viscosity helping maintian the tightness of the shaft wobble is not going to fly either as the hydrodyamic film will change when the speed changes. The shaft play is a mechanical function that cannot be reliant on the film of any oil to maintain wobble free operation." The shaft is going to wobble (gyrate in an eccentric motion) regardless of the film thickness. How much it wobbles (amplitude of oscillation) depends on the clearance(s) created by wear and the viscosity of the oil. The main point here is the dampening of the oil on the shaft. Just as in shock absorbers, the thicker the oil in the shock, the less time it will take to dampen (kill) the oscillations. In other words, the "amplitude" (displacement) of oscillation is reduced faster with thicker oil over a thinner oil. Think of this system as a weight hung on a spring, with the weight in air. Thump the weight into oscillation and it will take a long time for the oscillations to die out (boing, boing, boing!). Place the weight in oil, and then give it a thump; the oscillations die out much quicker. The turbo shaft has a weight (mass) on it (compressor blades). The shaft wobbles (oscillates, gyrates); the thicker the oil, the smaller the wobble (displacement) of the shaft from the centerline of the bore. [ September 06, 2002, 12:20 AM: Message edited by: MolaKule ]
 

MolaKule

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"So, to recap, Synths' advantage in use for turbines and such is not because if produces a better film strength but maintains a more consistant file strength due to the VI index." If the film stength is more constant over wider temperature range, doesn't that mean the film strength is greater overall? In analogy - Take a container of 5 very thin rubber bands and tie them together. Then take a set of 5 wide and strong rubberbands and tie them together. Which ones will be harder to stretch? I liken the harder and stronger rubber bands to the molecular bonding and arrangement of synthetic oil molecules. [Cheers!]
 
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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. I have not mentioned this in previous posts but I started out life teaching at Embry Riddle University and in the turbine engine lab we worked to quantify exactly what I have been sharing. It is not the high temps nor the temperature variations during operation but the extreme loading of the bearings. In the jet engine test cell we circulated the oils at 100 C, with temperatures never higher, even on shut downs as we could do very high flows to keep temps cool. We did this under the guidance of our instructor who worked in the initial jet engine development program prior to/during WW II. He was sent to Germany the day the war ended to gather up all the turbine lab engines, research, oils, etc. The reason he was sent was that the allied turbine engines were not living! It was not until the German "secret" was discovered that our turbine engines began to "live".. The secret was the synthetic oil, which the German's thought was inferior to "real" oil; they had to use synthesized oil due to the oil embargo... The U.S. designers mistakenly thought it was superior German bearing design, etc. The same situation occurred with the Soviet Union as they had to run mineral based oil until recently. They designed their turbine engine oil cooling systems so the very high quality mineral based oils never saw temps much above 100C, even on shutdown as the continued oil circulation at high rates; engine bearing life was many times measured in hundreds of hours vs. our turbine bearing life in the tens of thousands of hours. Sorry for the long diatride on this but needless to say I have spent many years with this subject.
 
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[Off Topic!] I am a graduate of ERAU myself George. Great School. There are a few of us lurking on this site.
 
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