Oil question....
Discussion
Okay I don't want to derail this thread but maybe you can help me get this down once and for all.
SAE multi grade oil ratings might tell you the oil is grade 5 when cold and 30 when hot, with 30 being more 'viscous' according to their viscosity test. What am I missing? How is it that the oil is actually less viscous when hot?
SAE multi grade oil ratings might tell you the oil is grade 5 when cold and 30 when hot, with 30 being more 'viscous' according to their viscosity test. What am I missing? How is it that the oil is actually less viscous when hot?
http://www.viscopedia.com/viscosity-tables/substan...
Chart of various grades of engine oil viscosity vs temperature.
Chart of various grades of engine oil viscosity vs temperature.
HustleRussell said:
Okay I don't want to derail this thread but maybe you can help me get this down once and for all.
SAE multi grade oil ratings might tell you the oil is grade 5 when cold and 30 when hot, with 30 being more 'viscous' according to their viscosity test. What am I missing? How is it that the oil is actually less viscous when hot?
The 5 and the 30 in a 5W-30 multigrade oil are NOT absolute measures of viscosity. They just tell you which two grades of single grade oil will have the same viscosities when the oil is cold (the 5W) and when hot (the 30) as the multigrade does at both temperatures. Look at the chart I previously listed.SAE multi grade oil ratings might tell you the oil is grade 5 when cold and 30 when hot, with 30 being more 'viscous' according to their viscosity test. What am I missing? How is it that the oil is actually less viscous when hot?
The 0W-30 and the straight 30 grade have almost the same viscosity at 100c. However the 0W-30 is much thinner than the straight 30 grade when cold. In fact it has the same viscosity when cold as a straight 0 grade oil would have. That's what the 0W means.
So the multigrade loses less viscosity as its temperature rises but every type of oil, as with every other liquid, gets thinner as it heats up.
An ideally perfect oil would have the same viscosity at all temperatures. This target viscosity is a function of the engine design and bearing clearances. A straight grade oil with the correct viscosity when it's hot is actually far too thick for the engine's needs when it's cold. Multigrades preserve the hot viscosity required while being thinner than the straight grade when cold although still far too thick for the engine's ideal needs.
Edited by Pumaracing on Tuesday 1st September 00:31
Here is a link to a good pdf from Opie Oils, they are a very good source of oils and lubricants and even offer sponsorship for race cars displaying Opie Stickers.
http://www.opieoils.co.uk/pdfs/tech-articles/Engin...
Peter
http://www.opieoils.co.uk/pdfs/tech-articles/Engin...
Peter
There are two opposing mechanisms at work here. Over time the long chain polymer molecules that help give a multigrade oil its high temperature viscosity get chopped up by the oil pump gears, piston rings etc and the oil loses viscosity.
However contamination from soot, other by-products of combustion and wear particles increase viscosity and this is by far the biggest factor, especially in a diesel engine. So generally viscosity increases with mileage.
However contamination from soot, other by-products of combustion and wear particles increase viscosity and this is by far the biggest factor, especially in a diesel engine. So generally viscosity increases with mileage.
Pumaracing said:
There are two opposing mechanisms at work here. Over time the long chain polymer molecules that help give a multigrade oil its high temperature viscosity get chopped up by the oil pump gears, piston rings etc and the oil loses viscosity.
However contamination from soot, other by-products of combustion and wear particles increase viscosity and this is by far the biggest factor, especially in a diesel engine. So generally viscosity increases with mileage.
so it gets thinner?However contamination from soot, other by-products of combustion and wear particles increase viscosity and this is by far the biggest factor, especially in a diesel engine. So generally viscosity increases with mileage.
Alex said:
It is important to note that the winter rating (e.g. "5W") and the SAE viscosity (e.g. "30") are not related and do not use the same measurement or scale.
That's correct. I'll conclude this with a brief look at how multigrade and single grade engine oils compare and exactly how multigrades are rated.What the engine requires is a certain viscosity from its oil at normal operating temperature. That's where the engine obviously spends most of its time. Single grade oils, which was all that was available until multigrades were invented, were rated for viscosity (and still are) at 100c. This is done by pouring the hot oil through a cup with a small hole in it and timing it. The more viscous obviously the longer the cup takes to drain. The oil is then graded according to its hot viscosity into a number of SAE grading intervals ranging from about 60 down to 0, the thinnest.
Over the years the average engine has seen a fall in the hot viscosity required from about 50 grade oil back in'th day to around 30 nowadays. This has partly been driven by fuel efficiency requirements as thinner oil saps less power from the oil pump and other mating components in the engine and also partly because modern additives and friction modifiers have enabled thinner oil to lubricate as well as older less sophisticated thick grades.
The bigger number in a multigrade oil rating is also the viscosity at 100c measured in the same way. So any grade of oil with xW-30 i.e. 0W-30, 5W-30 will have the same hot viscosity as straight 30 grade oil. This is in fact the only number that's really applicable to your engine's needs when it's at normal temperature.
Enter the multigrade oil. Scientists discovered that if you added long chain polymer molecules that were soluble in engine oil they had the property of increasing the viscosity of the oil when it was hot but had little effect when it was cold. They didn't really understand how this worked and assumed that it was because the polymer molecules expanded when they were hot and became harder to pump and contracted again when they were cold. In fact it's more to do with the polymer solubility as the oil heats up but that's another story.
So if you take a straight 20 grade oil and add enough polymer it acts like a 20 grade oil when cold and a 50 grade oil when hot hence the 20W-50 that most cars used back in the 60s and 70s.
But all oils are too thick for the engine when cold so why not take a 0 grade oil and add enough polymer to make it act like a 50 grade when hot? The problem is twofold. Firstly the polymer molecules don't actually lubricate, they just thicken the hot oil like cornflour in a stew. Too much of them and the oil doesn't lubricate very well. Secondly the polymers get chopped up over time by the engine components and lose their effect. Then you end up with only the thin base oil left which is far too thin to lubricate properly when it's hot and the engine goes bang.
As science discovers better polymers oil will be able to have a wider viscosity spread. Thinner when cold while still retaining the desired hot viscosity.
Finally what is the xW measure in a multigrade oil? The big number (the 30 in 5W-30) is still viscosity at 100c. The small number (the 5W), W means Winter btw, is a measure of the oils pumpability at very cold temperatures, like below zero degrees c. In fact it's measured at different temperatures depending on the grade, all the way from about -5c down to as low as -40c. It's really more applicable to whether your engine will even start in places like Siberia or Northern Canada than to anything we see in the UK. But it's still a good indication of how thin the oil is when cold and the thinner the better as all oil is too thick for the bearings when the engine is cold.
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