Oils that are too thick for the application

[Patman]

I've been trying to tell my LS1 friends not to run straight 15w50 Mobil 1 in their engines, due to the fact that the thicker oil doesn't transfer heat as well and doesn't flow as good. Someone then responded with this:

quote:

---

I don't know about the rest, but this excerpt is absolutely wrong. This would mirror the thermodynamics of the engine cooling system. If the coolant flows too quickly it doesn't absorb enough heat from the surfaces it is passing over and overheat the parts it is supposed to cool. It seems to me that if the oil runs slower over a part, physics would demand that it draws more heat from the surfaces. So while I agree that the oil flows slower, therefore making cold start wear worse than thinner oil... I disagree with your assertion that the oil wouldn't take away heat.

---

Comments?

 

[Neil Womack]

It seems like a fluid flowing more slowly over a warmer surface will absorb more heat per volume of fluid. However, a faster flowing fluid should be more efficient at heat transfer. Heat transfers more quickly (efficiently) when the temperature difference between the two mediums is greatest. So the slower flowing fluid would remove the heat less efficiently because the temperature differential between the two mediums over the time of contact is smaller. Does this make any sense at all? I think I'm saying for heat transfer all other things being equal faster is better. Additionally would not a thicker fluid squeezing through a given clearance generate some of its own heat through friction.

 

[novadude]

I would expect higher oil temps, since more oil will be pushed through the pump pressure relief with the higher viscosity oil, resulting in heating from shearing. That is a separate subject though.

 

[Rick in PA]

What Neil said. Take it to the extreme. Slow the oil to an absolute crawl... almost no flow. What happens? The oil heats to the temperature of it's surroundings then you get zero heat transfer.

 

[mdv]

If the oil was flowing slower, even if the thicker oil had a greater heat capacity, wouldn't it leave more heat in the higher temperature areas longer than the thinner oil would? So a thicker oil could absorb more heat, but it would take longer to remove it to a cooler part of the system and your oil temp would end up being slightly higher. Not to mention the frictional and pumping effects it would have as well. Am I misunderstanding this?

 

[7TFord]

Do we know for sure as per statictics that a 15/50 flows slower than a 10/30 at operating temps? If so, how much slower can it be to make a significant difference in heat transfer? I'm sure that there are rules of nature that apply to the fluidity of a liquid that don't basically change. As soon as your oil pump pressurizes the system is that not a constant no matter what weight the fluid is? If heat transfer in oil was significantly variable as to viscosity, wouldn't it have to be an SAE standard as to the classification of an oil? Luckily we have the radiator to keep tabs on these temps. I guess to scientifically experiment on this you'd have to do your testing on a motor with no cooling system. A "funny car" I guess!

 

[Steve S]

I was wondering wouldn't the flow of oil be the same because the oil is being pumped at so many gallons per minute? I have allways wondered about what the difference would be . Help me on this.

 

[TooSlick]

Patman, The engine will run significantly hotter with the 15w-50 or 20w-50 ...you can tell your friends that they aren't running Winston Cup cars @ 200+ mph and that this is simply stupid. A 5w-30 or 10w-30 synthetic is all you need here .... TooSlick

 

[Eiron]

OK, let's take the opposite extreme example. The oil flows like water. Now it's so hot-n-thin that it's running off the parts too quickly to absorb any additional heat. Of course, neither extreme example is the actual case, right? One thing I always think about is those d*mn NY taxis. They're running all the time, but they're never going anywhere. The engines are always idling: waiting for fares; waiting in traffic; waiting to get paid; they're rarely under load relative to the number of hours they're running. And what do they use for oil? OK, I've never operated a taxi, so I don't have personal knowledge. But I've read numerous places that they run xW-50. If it's so bad at removing heat, why put it in hundreds of thousands of long-term idlers? And if M1 15W-50 cold-flows as well as some dino 5Ws, why worry about it at the low end? I think all this concern over current synthetic oils being too thick is unfounded. I might agree if all the LS1 guys were running dino 20W-50 year-round in Winnipeg, but they're not, right?

 

[eldawg4100]

i was ****** when i used the M1 15w50... it made something in the bottom end knock like a SOB. and when i drained it (while hot) it was water thin, i didnt understand..

 

[mikemc]

When attending firefighter pump operation classes, one thing that is taught is that pressure and volume are oppositely related when it comes to liquids. Many fire pumps are 2 stage, meaning you can select whether you want more volume or more pressure. When it comes to oil in my motor, I want volume, especially on cold mornings. Thinner oils should flow with more volume and as long as they have a good shear strength and maintain a film of protection on motor surfaces, that's what I prefer.

 

[Ken2]

I've been working with heat exchangers almost daily since the '60s...oil coolers, oil heaters, water coolers, water heaters, steam heaters, steam condensers, air heaters, shell & tube type, plate type, heat transfer from machinery, etc., etc. The velocity of the fluid does not matter unless there's turbulence. Sure, if it's moving faster, it might pick up fewer degrees, but it might be getting more BTUs, and that's what the quantity of heat is measured in. Has anyone made comparisons with an accurate oil temperature thermometer under similar conditions with a 30 wt. oil and a 50 wt? The 50 might run hotter from increased fluid friction in the oil. The most important thing is the size of the oil passages and bearing gaps. If the sizes are small and the oil too thick, it won't lubricate adequately. If the bearing clearances are big enough, you need the thick oil. So...do accurate temperature checks and oil analyses. Ken

 

[7TFord]

quote:

---

Originally posted by mikemc: When attending firefighter pump operation classes, one thing that is taught is that pressure and volume are oppositely related when it comes to liquids. Many fire pumps are 2 stage, meaning you can select whether you want more volume or more pressure. When it comes to oil in my motor, I want volume, especially on cold mornings. Thinner oils should flow with more volume and as long as they have a good shear strength and maintain a film of protection on motor surfaces, that's what I prefer.

---

I understand the comparison but is it the same since with the fire pump you have water in and water out and you are after an increased flow/pressure at the one end. The auto engine oiling system is a closed loop with no interruption (you hope) in the feed of fluid, thus a constant flow/pressure. I know a lot of importance is placed on oil flow at start up which is a critical time, but the engine spends more time running at operating temps and the oil isn't cold for long so high temp characteristics are probably more significant.

 

[Jay]

Thicker oils runs hotter for 3 main reasons: 1. Thicker oil will generate more heat from it's own internal friction. 2. Thicker oil will not absorb heat as readily from hot metal parts. 3. Thicker oil flows more slowly. On item two, put equal amounts of water and tomato paste in equal sized pans and apply equal amounts of heat. Which one boils first? The tomato paste. Why? There's little or no turbulent circulation in the tomato paste allowing localized hot spots. How do products like "Water Wetter" work? They work because a mixture of "Water Wetter" and water is less viscous than ethylene glycol and water. On item 3, why does the CPU in your computer need a fan? To speed more air over the surface of the heat sink. A larger volume of air can remove more heat than a smaller volume. The same thing is true with oil (and coolant) circulating in the engine.

 

[G-MAN]

quote:

---

Originally posted by Jay: On item two, put equal amounts of water and tomato paste in equal sized pans and apply equal amounts of heat. Which one boils first? The tomato paste. Why? There's little or no turbulent circulation in the tomato paste allowing localized hot spots.

---

If the thicker tomato paste boils first, isn't it absorbing more heat, or at least absorbing heat at a faster rate, than the water?

 

[Ken2]

No, hot spots--poor internal circulation. But motor oil is always circulating. Hot 50 wt. might circulate more slowly than hot 30 wt. if the increased resistance to flow causes the oil pump's pressure control valve to dump more of the output directly to the sump. But, I don't buy the tomato paste analogy. The oil is constantly circulating through the oil passages and bearings and returning to the sump. Maybe 50 wt. does transfer less heat from the metal to the oil, but it takes careful measuring and research to document it. WaterWetter works by decreasing the surface tension of the coolant. You can't compare WaterWetter and water to glycol and water...different mixtures. You can compare glycol and water to glycol, water, and WaterWetter mixed; Redline says it cools better, and guys I know says that it makes no difference. Ken

 

[oilboy28]

it is not really absorbing it quicker, it is suffering faster because it cannot dissipate as well.

 

[satterfi]

Thicker oils should run cooler. It's the engine that runs hotter. Thicker oils absorb more heat at the metal surface because the hydrodynamic boundary layer is thicker. This transfers more heat into the bulk flow. However, thinner fluids are more turbulent and macroscopic elements of fluid will transport more energy and momentum than individual molecules. Here’s an equation for flow over a flat plate that describes these two offsetting characteristics. Nu=.332Re^.5*Pr^.33 Nu is basically the heat transfer coefficient. Re is how easily the fluid goes turbulent. Pr is how thick the boundary layer is.

 

[Shannow]

Nusselt, Reynolds and Prandtle. I hated those guys in thrid year engineering. The heat transfer oils are ISO46, probably to optimise heat transfer, and minimise pumping losses I'd guess.

 

[MolaKule]

There three major variables in determining oil viscosity verses heat rejected in a bearing: 1. Bearing clearances in units of inches or um 2. viscosity of the oil in KV 3. volumetric oil flow, in units of in^3/sec. The heat generated in a bearing is a major function of two variables (although there are others), bearing clearances and oil viscosity, and they are directly related. The greater the oil viscosity or the smaller the diametral clearance, the greater the heat generated within a bearing. Since the bearing is generating heat due to friction (a function of speed, load, oil viscosity and clearances), this heat has to be removed. The only way to remove the heat in an engine bearing is through oil flow. Too fast an oil flow and amount of heat transferred to the oil will be low, too slow a flow of oil and the oil temp will rise too quickly and oxidize. So there has to be a certain amount of volumetric flow of oil to efficiently remove the heat. A temperature difference of approx. 20 C between the oil and bearing has to maintained for efficient heat transfer. This can vary between engine designs. The volumetric oil flow has to be such that the heat rise in the bearing is kept low so as to not soften the bearing metal, as in journal bearings, and to keep the oil temp from rising to too high a level. Too high a viscosity will result in increased "viscous friction" in the bearing clearances (and a subsequent rise in bearing temperature), and pumping losses in the oil pressurization system. So there is a trade-off between oil viscosity and the volume of oil flow allowed. A good XW30 should be the optimum starting oil viscosity. A higher viscosity is needed only when higher temperatures might reduce the oil viscosity and reduce the thickness of the hydrodynamic lubricating film, or when fuel dilation might reduce viscosity, as in alcohol fuel engines. A 15w50 is not necessary for tight clearance engines run in street mode. [ February 22, 2003, 06:58 PM: Message edited by: MolaKule ]