Oil Weight vs Warm Up Time

[Jaymus]

Ok, I was just wondering how this works.

I'm going to use a wide example.

Let's say you have a 5W-20 and a 5W-40. From what people say on here, a thinner oil supposedly removes heat faster than a thicker oil. If this isn't true, and is just hearsay, please correct me.

With this information, is a 5W-40 going to 'heat up' quicker than a 5W-20 under the same conditions? I need to add more detail to my question, but don't have much time, but generally I am wondering if the 40 weight is going to get to the 40 weight stage somewhat faster than a 20 weight oil.

Sorry I can't word this any better. Hope you guys can answer my curiosity if my post even makes any sense. It just seems that a 40 weight would get within 40 weight spec pretty quick if the first paragraph is true.

 

[Familyguy]

I don't follow. How does "thinner oil supposedly remove heat faster than thicker oil?"

Is there some magic correlation between the heat transfer coefficient (the ability of a fluid to absorb heat from a solid) of oil and its viscosity? That would be news to me, but I'm not a "tribologist" or a chemical engineer.

 

[c3po]

I think there is a guy on here who goes by AeHass that has all of these answers. He can give you some good information.

 

[Liquid_Turbo]

Originally Posted By: Familyguy
I don't follow. How does "thinner oil supposedly remove heat faster than thicker oil?"

Is there some magic correlation between the heat transfer coefficient (the ability of a fluid to absorb heat from a solid) of oil and its viscosity? That would be news to me, but I'm not a "tribologist" or a chemical engineer.


No there isn't. At least this is what I learned in Heat Transfer mechanics in University.

While there would be differences in the molecular interaction between different viscosities, the difference is so minute it would be lead me to believe heat transfer characteristics of different oils are virtually the same. (In terms of warming up the same quantity of oil).

 

[Jaymus]

Originally Posted By: Familyguy
I don't follow. How does "thinner oil supposedly remove heat faster than thicker oil?"


Like I said, people say this on here. I'm not stating it's true or false. Just seems to be common theory/knowledge on this site. And if it's true, it seems like a thicker weight oil would warm up faster. At least faster to being within a percentage of it's stated viscosity at 100C.

 

[labman]

My guess is the thinner. Depending how thick and how cold, part of the oil will go out the pump bypass and back to the pan picking up very little heat. Until the pump bypass closes, the thinner oil will have a greater volume flowing through the engine.

Next, laminar flow. You will have a thin layer of non flowing oil on the outside of any passage. As you work into the center, the oil will be flowing faster and faster. The thicker the oil, the slower the flow next to the wall. Of course as the engine warms, you will get a temperature gradient with the closer to the wall, the warmer. Of course, the faster moving oil in the center will be heated by shear. I don't think oil ever reaches turbulent flow except maybe in the tight clearances in the journals.

Of course, if both are 5W-X, the viscosity, flow, and gradients should be the same initially. The 20 weigh should thin out faster as it heats up.

 

[Familyguy]

Originally Posted By: Jaymus
Originally Posted By: Familyguy
I don't follow. How does "thinner oil supposedly remove heat faster than thicker oil?"


Like I said, people say this on here. I'm not stating it's true or false. Just seems to be common theory/knowledge on this site. And if it's true, it seems like a thicker weight oil would warm up faster. At least faster to being within a percentage of it's stated viscosity at 100C.


"People say?" What people? It just doesn't make sense from a physics standpoint.

 

[aquariuscsm]

Imo,thicker oil would`t "heat up" faster,but it seems to me it would keep moving parts cooler better,since there`s a greater "cushion" per se between the moving parts,keeping the oil film from being breached.

 

[JohnBrowning]

No real difference.....The real issue is the drag inpact on milage and oil pump and such until it is warm. You are loseing a lot to run a thick oil in cold weather or any weather for that matter over a thiner one. The first difference is the main factor not the second.

If their is a difference it would be small like less then a 1-2 minute difference in the real world that is nothing on a lab test for milage for a corerate fleet average it is huge.

 

[elwaylite]

Start the car, wait 15 seconds, put into gear, and enjoy.

 

[Gary Allan]

It's a complicated topic dealing with dynamic conditions.

 

[Familyguy]

Originally Posted By: Gary Allan
It's a complicated topic dealing with dynamic conditions.


Oh my, the gearhead retake on "The View."

 

[Gary Allan]

The bottom line is that it will still take the same "time to temp" regardless of visc (in most cases). The heavier (at the moment - there's that dynamic view there) oil will take more energy to pump and shear ..which will generate more exothermic input and endothermic energy (did I say that right?). The lighter oil (at the moment- it looks like the heavier oil's future ..presently) will pump and shear easier and require and generate less heat.

The main rejection surface of most engines is the pan. There YMMV all over the map (air flow, etc.) and I can "imagine" that the turbulent nature of that would give heat rejection advantages to the thinner oil (at the moment).

 

[labman]

I suspect the chief heating comes from contact with the engine surfaces and there the thin oil should heat up faster. In the oil pan, the surface to mass ratio will limit the cooling of the thinner oil.

 

[Gary Allan]

Help me out with the difference in mass. I don't see a quart of light hot oil having any more or less mass than a frozen solid quart of oil. At least to any amount I can (thinking) measure.

 

[SpitfireS]

Lets hope I've got this right

I think labman means that a quart of oil (a mass of oil) in a sump has a certain contact area with that sump, so that mass of oil will cool by a certain rate no matter how thick or thin it is.
Allthough delta temp has an influence on heat transfer.

 

[Gary Allan]

Well, lets turn it around and use a pan heater. Which fluid convects heat easier? Would that be altered in a forced turbulent environment?

 

[wildLoon]

what heats up quiker in the deep frier, twenty weight or sixty weight?

 

[labman]

An interesting question. Don't let your wife catch you checking it. Oil or any else standing in a container would gain or lose heat at the surface of the container. The more motion within the material, the faster. There is constant motion in an oil pan with the engine running, but not near as much in the small passages and journals. Why do oil coolers divide the oil up into small tubes? Because it cools better than in a big pan.

Oil heated in a deep fat fryer won't get the heat from mechanical shearing, but I doubt that is a big factor in an engine.

 

[saberz]

One would think that the thinner weight oil(20wt) would in theory heat up quicker than the 40 weight as it shouldn't take as long to reach operating temperature.

This is all theoretical, ie from the school of I pulled it out of my butt, but it makes some sense...