Oil viscosity and temperature

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131
Location
Sydney
I am still unsure about the meaning of viscosity values given to multi-viscosity oils. Take a 10W - 30 oil. I understand this to mean that the oil has a cold viscosity of 10 and a high temp viscosity of 30. That is, the oil is "thinner" at startup and "thickens" as the engine heats up. From this forum I have learnt that this is due to polymers in the oil which expand as engine temp rises. This makes sense. However, when you pour cold oil out of a container, it flows more slowly than hot oil. I often leave my new oil container in the sun to heat it up prior to pouring into the engine so I won't be waitng all day (for the oil to flow through the strainer in the funnel). The same rationale applies to running the engine a bit before changing the oil so that it flows out of the sump more rapidly. Can someone explain this to me ? How can an oil be "thicker" at high temp with increased visosity and yet flow faster ?? [ January 01, 2003, 10:34 PM: Message edited by: Andrew ]
 

mdv

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531
Location
MA
quote:
Originally posted by Andrew: I am still unsure about the meaning of viscosity values given to multi-viscosity oils. Take a 10W - 30 oil. I understand this to mean that the oil has a cold viscosity of 10 and a high temp viscosity of 30. That is, the oil is "thinner" at startup and "thickens" as the engine heats up. From this forum I have learnt that this is due to polymers in the oil which expand as engine temp rises. This makes sense. However, when you pour cold oil out of a container, it flows more slowly than hot oil. I often leave my new oil container in the sun to heat it up prior to pouring into the engine so I won't be waitng all day (for the oil to flow through the strainer in the funnel). The same rationale applies to running the engine a bit before changing the oil so that it flows out of the sump more rapidly. Can someone explain this to me ? How can an oil be "thicker" at high temp with increased visosity and yet flow faster ??
Everything is relative. The oil does not thicken as it heats up, it actually thins out. The different SAE classes were created to inidcate how well an oil performs at certain temperatures. A simple way to look at it is numbers like 0WXX, 5WXX, 10WXX, 15WXX and 20WXX indicate the relative viscosities of the oil under cold conditions (different temps and limits for the different weights). The 0W class will flow better at colder temps than the 20W oils. SO if you live in a really cold area, you don't want an oil that starts with a high number in winter. The second number like XW20, XW30, XW40, XW50 indicate the relative thickiness of the oil at 100 degrees celsius. 20 weight oils are thinner than 50 weight oils. You need to look for an oil that has the correct range for your car. Most cars would use a 30 wt. or 40 wt. oil. Does that make any sense or did make things worse? Also, the class system is a whole lot more complicated than the way I described, but that should be a start. [ January 01, 2003, 10:58 PM: Message edited by: mdv ]
 
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6,388
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Washington St.
Andrew, Any viscosity number preceeding a W has met a cold viscosity test. Any viscosity number without the W has meet a viscosity test at 100°C. Look at this table. The units for the cold test are centiPoise, and the units for the hot test are centiStoke, ...just a couple of systems of measurement of the oil's resistance to flow. Just look at the Low Temperature Cranking viscosities and temperatures, and look at the Low Shear Rate Kinematic viscosity minimum and maximum @ 100°. http://www.chevron.com/prodserv/NewOronite/library/li_viscosity_motoroil.htm That 10W-30 oil you mentioned has a cold viscosity no greater than 7000 cP (whatever that means) @ -25°C. It has a hot viscosity between 9.3 and 12.4 cSt @ 100°C. As mdv said, look at the viscosity numbers on a relative basis...15W is thicker when cold than 10W, and 40 is thicker when hot than 30. Hope that helps, Ken [ January 01, 2003, 11:29 PM: Message edited by: Ken2 ]
 
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47,644
Location
Duvall WA - Pacific NW USA
quote:
I often leave my new oil container in the sun to heat it up prior to pouring into the engine so I won't be waitng all day...
YIKES!! What kind/viscosity oil are you using??? You live in Sydney, no? Anyhow the other guys did a pretty darn good job of decribing it. Forget about the polymers for a second... Here's another way to think about it: There are some low spread synthetic oils that don't need a lot of polymers to "stay thick" enough to meet the upper number, yet these same oils have low enough cold temp. viscosities to meet the "W" number. The numbers are different test ratings NOT actual comparative viscosities. Food for thought. [Patriot]
 

Andrew

Thread starter
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131
Location
Sydney
Thanks guys, that makes sense now [Cheers!] I have a 1987 V8 Holden Commodore and use 20W - 50 dino oil and frequent changes. Admin, sorry about posting this thread in the wrong forum [Duh!]
 
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2,910
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Georgia/Retired
I was under the impression that the cold temperature designation - the number preceding the W - was taken at 0º Celsius (32ºFahrenheit) and the hot temperature viscosity was taken at 100º Celsius (212ºF). Is this correct?
 
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874
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FowVay, for high temp that's correct, but low temp isn't as simple. Each weight has its own cranking cP and temp limits: Lubrizol Doc Note the recent change in enforcement of '99 standards. Another thread here talks about it more. David
 
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2,480
Correct. The oil DOES NOT change with temperature dictated by the number eg. 5-30...it's just a number assigned to an oil which will give it's relative "thickness" at a certain temperature. Eg. a 5 (as in a 5-30) will be thinner than a 10 (as in a 10-30) at say, 0F as a 40 (as in a 5-40) will be thicker at 100C than a 30 (as in a 5-30)...that's all the number's mean. So in real life...check owner's manual as this will spec. out which lower number to go with eg. 5 (as in 5-30) depending on the car's cold cranking abilities and the high number eg. 30 (as in 5-30) for warm temp. protection... Both, numbers are a high debating issue here as there are discrepancies between the manuf. recommendations depending on which continent you live even with the same car/same engine...
 
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chicago
A bit off subject and Molakule may be the expert here. Also, I do not know if this constitutes "thickening", however the universal solvent, water, has a higher specific gravity (density) as it warms. Or at least as it warms from a solid to a liquid (i.e. ice floats) I assume that gravy and pudding don't count. I'll make no further cracks for shucks and grins-but I just had to get that out.
 

Jay

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1,607
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Idaho Falls, ID
Good try. I admit I didn't think of gravy or pudding. They thicken as they cook but then thicken quite a bit more as they cool. This is even further off subject, but water has to be the ultimate non-neutonian fluid. That is, it changes viscosity dramatically under pressure. Ice skaters make good use of that property.
 
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43,651
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'Stralia
I think that the thixotropic fluids thicken with shear rate rather than temp. (corn flour and water being one of the more fun examples). Ice is less dense than water due to it forming a crystaline structure as it freezes. Put a beer can in the freezer to demonstrate.
 
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874
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Pacific NW
I've heard the viscous fluid described as "liquid spider legs". The legs start engaging as they warm from early slippage, increasing friction between the otherwise sliding plates. The effect is similar to an LSD clutch pack without the wearing clutch material. Weakness is if they're overtaxed. With too much force the "legs" shear which leaves you with "unlimited slip". Not sure how the spider analogy holds up to real science. Some of our Jeeps use these in a certain unnamed transfer case. NVG made a rash of the couplers with bad welds allowing ATF (the spec'd tcase lube) to leak in and the viscous fluid out, messing with friction properties and eventually locking them up. Not serious offroad but unpleasant for grocery trips. Darn component is a cylinder 3" across and about the same deep. $400-700 by itself. (spider-legs command a premium) Goes without saying this is not a popular transfer case among the DIY crowd. David
 
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43,651
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'Stralia
Dunno enough about the actions of the fluids to know whether it's extremely localised heating causing the things to thinken up. But do know the cornflour and water "fluid". We used to break wooden spoons in cooking class while making custard. Take about 1/4 cup water, and add corn flour, a bit at a time, until it's about the consitency of thick cream when left to it's own devices (read low shear). Try to rapidly stir the stuff, or drop it from a height. Pour it onto your hands, then rub briskly together. It's weird stuff. Honey behaves similarly. Not a lot to do with the topic, except that there doesn't feel to be a lot of heat in the equation.
 
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874
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Pacific NW
Don't ask me but I'd assume the two (temp & shear) would be hard to separate at the sizes involved. I suppose shear would be the instigator, whatever the sequence of changes. [I dont know]
 
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