Brain fade re oil viscosity

[TwinKam]

It was explained to me once to imagine synthetic multi-grade oil molecules as coiled clock-springs and, as they get hotter, they uncoil, hence increasing their 'size'.

[GreenV8S]

Caddyshack said:

It is just that the bigger the number on the end means thicker and is the hot reading.

No, that is not what the numbers mean.

All oils get thinner with temperature. Over the range from 0C to 100C most engine oils change from as thick as treacle to as thin as water.

You can measure the viscosity of oil by measuring how easily it flows. If you plot viscosity against temperature for a single grade oil you will get a curve showing it getting much thinner as it gets hotter. Plot the same for diferent grades of oil and you'll get a set of curves that are broadly parallel. Each grade of oil will have its own characteristic curve. The higher grade oils are thicker at a given temperature than the lower grade oils, but the difference between grades is small compared to how much the viscosity changes with temperature.

If you plot viscosity against temperature for a multigrade oil you will find it crosses the curves for single grade oils ie it can behave like a 10 grade oil when it is cold and a 40 grade oil when it is hot. You'd see this labelled as 10W40. The grade numbers are not a direct measure of viscosity, they just identify the curve of viscosity against temperature.

The benefit of a multigrade oil is that you can have the right viscosity when the engine is up to temperature, without the oil thickening up so much when it gets cold. 40 grade oil might be a pig to start in cold weather because it's too thick when it's cold. 10W40 would behave the same when it's hot, but much easier to start when the engine is freezing.

[Caddyshack]

GreenV8S said:

Caddyshack said:

It is just that the bigger the number on the end means thicker and is the hot reading.

No, that is not what the numbers mean.

All oils get thinner with temperature. Over the range from 0C to 100C most engine oils change from as thick as treacle to as thin as water.

You can measure the viscosity of oil by measuring how easily it flows. If you plot viscosity against temperature for a single grade oil you will get a curve showing it getting much thinner as it gets hotter. Plot the same for diferent grades of oil and you'll get a set of curves that are broadly parallel. Each grade of oil will have its own characteristic curve. The higher grade oils are thicker at a given temperature than the lower grade oils, but the difference between grades is small compared to how much the viscosity changes with temperature.

If you plot viscosity against temperature for a multigrade oil you will find it crosses the curves for single grade oils ie it can behave like a 10 grade oil when it is cold and a 40 grade oil when it is hot. You'd see this labelled as 10W40. The grade numbers are not a direct measure of viscosity, they just identify the curve of viscosity against temperature.

The benefit of a multigrade oil is that you can have the right viscosity when the engine is up to temperature, without the oil thickening up so much when it gets cold. 40 grade oil might be a pig to start in cold weather because it's too thick when it's cold. 10W40 would behave the same when it's hot, but much easier to start when the engine is freezing.

Thanks, interesting read.

So, how would you describe a 50 at the end compared to a 30 at the end assuming the small w number was the same? Would it just mean a wider spread on the curve?

[GreenV8S]

Caddyshack said:

So, how would you describe a 50 at the end compared to a 30 at the end assuming the small w number was the same? Would it just mean a wider spread on the curve?

For example 10W30 and 10W40 would have similar viscosity when cold. When hot, the 10W40 would be thicker than the 10W30.

The 'hot' grade is usually the most important one because it determines the oil viscosity when the engine is up to temperature.

The 'winter' (W) grade tells you how much thicker the oil gets when it is cold.

[Caddyshack]

GreenV8S said:

Caddyshack said:

So, how would you describe a 50 at the end compared to a 30 at the end assuming the small w number was the same? Would it just mean a wider spread on the curve?

For example 10W30 and 10W40 would have similar viscosity when cold. When hot, the 10W40 would be thicker than the 10W30.

The 'hot' grade is usually the most important one because it determines the oil viscosity when the engine is up to temperature.

The 'winter' (W) grade tells you how much thicker the oil gets when it is cold.

Thanks

So, when I said that the bigger number is the hot number and the higher the number the thicker it is you said that wasn’t the case but what you have said above supports that? (The bit you quoted me in above). Not having a go or argument but I cannot understand the difference, I think I just didn’t use the normal terminology?

[Scarletpimpofnel]

Thanks all. At times I have become more baffled by this evolving thread but some of you put your finger on it. But I had to look at the graphs available here to understand it myself - https://www.widman.biz/English/Tables/gr-motores.h...

Basically a 5w/30 oil is as thin as a "5" at cold temps and as thick as a "30" at hotter temperatures. BUT overall the multigrade 5W/30 is still thinner at the hotter temp than it is at the lower temp (this is what I wasn't understanding).

A multigrade oil is a clever thing then if can change its inherent viscosity as temperature increases (resulting from the coiled spring analogy someone gave previously).

I always thought oil was oil and didn't care if i was topping up with the wrong grade or not but now I can see that the wrong grade could make the engine work too hard when cold and the oil be too thin when hot thus potentially causing premature engine wear. I never thought the grade of oil was that important. Perhaps it's a lot more important than I had thought.

[Scarletpimpofnel]

TwinKam said:

It was explained to me once to imagine synthetic multi-grade oil molecules as coiled clock-springs and, as they get hotter, they uncoil, hence increasing their 'size'.

I had no idea what you meant when you said this but now I get it. Basically the "uncoiling springs" stop the oil getting as thin as it otherwise would at higher temperatures. TY

[Caddyshack]

Scarletpimpofnel said:

Thanks all. At times I have become more baffled by this evolving thread but some of you put your finger on it. But I had to look at the graphs available here to understand it myself - https://www.widman.biz/English/Tables/gr-motores.h...

Basically a 5w/30 oil is as thin as a "5" at cold temps and as thick as a "30" at hotter temperatures. BUT overall the multigrade 5W/30 is still thinner at the hotter temp than it is at the lower temp (this is what I wasn't understanding).

A multigrade oil is a clever thing then if can change its inherent viscosity as temperature increases (resulting from the coiled spring analogy someone gave previously).

I always thought oil was oil and didn't care if i was topping up with the wrong grade or not but now I can see that the wrong grade could make the engine work too hard when cold and the oil be too thin when hot thus potentially causing premature engine wear. I never thought the grade of oil was that important. Perhaps it's a lot more important than I had thought.

I am with you on that one. Last time I gave it thought was doing A levels and that was a while ago.

I wonder what actually happens inside an engine when you mix two viscosity’s?

[Super Sonic]

said:

If you look at Green V8s post above, his explanation is a lot clearer than my attempts.

[JohnMcL]

GreenV8S said:

If you plot viscosity against temperature for a multigrade oil you will find it crosses the curves for single grade oils ie it can behave like a 10 grade oil when it is cold and a 40 grade oil when it is hot. You'd see this labelled as 10W40. The grade numbers are not a direct measure of viscosity, they just identify the curve of viscosity against temperature.

The benefit of a multigrade oil is that you can have the right viscosity when the engine is up to temperature, without the oil thickening up so much when it gets cold. 40 grade oil might be a pig to start in cold weather because it's too thick when it's cold. 10W40 would behave the same when it's hot, but much easier to start when the engine is freezing.

Explained to me years ago as:
a 10w40 oil never gets thicker than a 10 grade in winter and never gets thinner than a 40 grade in summer.

[ChocolateFrog]

GreenV8S said:

No, that is not what the numbers mean.

All oils get thinner with temperature. Over the range from 0C to 100C most engine oils change from as thick as treacle to as thin as water.

You can measure the viscosity of oil by measuring how easily it flows. If you plot viscosity against temperature for a single grade oil you will get a curve showing it getting much thinner as it gets hotter. Plot the same for diferent grades of oil and you'll get a set of curves that are broadly parallel. Each grade of oil will have its own characteristic curve. The higher grade oils are thicker at a given temperature than the lower grade oils, but the difference between grades is small compared to how much the viscosity changes with temperature.

If you plot viscosity against temperature for a multigrade oil you will find it crosses the curves for single grade oils ie it can behave like a 10 grade oil when it is cold and a 40 grade oil when it is hot. You'd see this labelled as 10W40. The grade numbers are not a direct measure of viscosity, they just identify the curve of viscosity against temperature.

The benefit of a multigrade oil is that you can have the right viscosity when the engine is up to temperature, without the oil thickening up so much when it gets cold. 40 grade oil might be a pig to start in cold weather because it's too thick when it's cold. 10W40 would behave the same when it's hot, but much easier to start when the engine is freezing.

I wrote a post the other day and immediately deleted it because I couldn't get across what was in my head coherently.

Your explanation is better anyway.

I find Tribology quite interesting.

[ChocolateFrog]

JohnMcL said:

GreenV8S said:

If you plot viscosity against temperature for a multigrade oil you will find it crosses the curves for single grade oils ie it can behave like a 10 grade oil when it is cold and a 40 grade oil when it is hot. You'd see this labelled as 10W40. The grade numbers are not a direct measure of viscosity, they just identify the curve of viscosity against temperature.

The benefit of a multigrade oil is that you can have the right viscosity when the engine is up to temperature, without the oil thickening up so much when it gets cold. 40 grade oil might be a pig to start in cold weather because it's too thick when it's cold. 10W40 would behave the same when it's hot, but much easier to start when the engine is freezing.

Explained to me years ago as:
a 10w40 oil never gets thicker than a 10 grade in winter and never gets thinner than a 40 grade in summer.

Although in reality it will because most oils will shear out of grade at the slightest provocation. That 40 weight oil could quickly thin to a 20 weight equivalent or likely worse on a hot track day.

Good quality oil is important.

[GreenV8S]

ChocolateFrog said:

Although in reality it will because most oils will shear out of grade at the slightest provocation.

The greater the multigrade range the more compromises are necessary to achieve it.