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Engine oil viscosity

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Engine oil viscosity statement

What does oil viscosity mean?

In addition to defining engine oil viscosity, this article explains why the viscosity of the oil is so important to the engine and clears up misunderstandings when it comes to engine oil viscosity.

The viscosity says how “viscous” the engine oil is at a certain temperature. But initially it says nothing about the quality of the engine oil.

Low viscosity = low resistance to deformation
thin"(Example: water)

High viscosity = high resistance to deformation
thick"(Example: honey)

Are you looking for a special motor oil? Then this way: Motor oil manufacturer approvals and ACEA specifications. If you are looking for simple explanations on the complex topic of viscosity, just read on.

 

While engine oil with a low viscosity flows more easily and is easier to transport through the engine, engine oil with a high viscosity can withstand higher mechanical loads.

And here the dilemma begins:
On the one hand, when starting cold starts in winter, you want a reasonably thin, easy-to-pump engine oil. On the other hand, it should withstand the high shear forces at high temperatures and at high speeds, lubricate all important engine parts well and thus protect against wear.
Particularly in the bearing and valve area, the camshaft and on the cylinder wall, very high speeds and loads occur under load in the upper speed range. The oil must then not be too “thin”. The problem:

The Engine oil viscosity changes with:
- different temperatures
- different pressure
- Different loads (shear gradient) for engine oils with VI improvers

In our latitudes, where almost no one switches from summer to winter oil (single-grade oils), but almost everyone drives with multigrade oils, one would prefer an engine oil that does not change its viscosity, especially at the different temperatures in winter and summer.
Unfortunately, there is no such thing as an engine oil, all change their viscosity - but differently. The viscosity index describes precisely these differences.

Viscosity index VI

If the viscosity of an engine oil decreases only slightly with increasing temperatures, it has a high one Viscosity index (VI). The lower the VI, the more the viscosity changes.

To put it simply: engine oil with a low VI becomes thinner when heated and more viscous when cooled than an oil with a high VI. Neither is good in a car engine. In this respect, the viscosity index (VI), in contrast to the viscosity, can say something about the quality of an engine oil.

The VI cannot be measured directly. It is calculated from two viscosity measurements (at 40 ° C and at 100 ° C).

In general, well refined base oils have a VI of about 90 to 100. Hydrocracked and synthetic (PAO) oils have a VI of about 110 to 150.

Fully synthetic engine oils therefore “naturally” have a higher viscosity index than mineral oils and the hydrocracked oils made from them - even if their VI was artificially improved during the formulation using VI improvers. There are already differences in the base oils of mineral engine oils: Group II and III oils have a better VI than the cheaper Group I oils.

Motor oil also changes its chemical composition and the structure of its molecular chains during operation (especially under heavy loads). And that in turn changes the oil viscosity. Multigrade oils in particular, which should cover a high temperature range with the help of a particularly large number of VI improvers, tend to age more quickly and the resulting change in viscosity.

Another challenge for engine oil are two contradicting requirements: On the one hand, the engine oil should protect the engine from wear and tear. An oil with a higher viscosity would be good for this, because it withstands the (shear) forces in the engine better than a low-viscosity oil. Unfortunately, the highly viscous oil has a higher internal friction, which leads to (slightly) increased fuel consumption.

This is where environmental protection and car manufacturers come into play: They are interested in lowering fuel consumption - which in turn would speak in favor of a motor oil with a lower viscosity at operating temperature, such as that offered by so-called low-viscosity oils with a reduced HTHS value.

HTHS viscosity engine oil (HTHS value)

The HTHS value or the HTHS viscosity (also high temperature viscosity) of an engine oil describes the viscosity (viscosity) of the oil at high temperature and high shear rate.

HTHS is the abbreviation for the English "High Temperature High Shear". The HTHS viscosity is at 150 ° C and at a load (shear gradient) of 106 s-1 measured with a rotational viscometer (e.g. according to ASTM D4683.)

A high HTHS value would 3.5 mPas.
A low HTHS value moves between 2.9 and max 3.5 mPas

High temperatures and high shear speeds prevail in the engine, especially where components move quickly (including cylinder liners, crankshafts, valves) and reach their extreme values ​​at high engine speeds, for example when driving fast on the motorway.

Under these conditions, the engine oil is exposed to particularly heavy loads and the oil film must not crack under any circumstances. From the point of view of the engine manufacturer - and certainly also the car owner - one would say: the more stable and resilient the engine oil is under these conditions (temperature, mechanical load), the better.

An engine oil with a relatively high HTHS viscosity would therefore be desirable with a view to protecting the engine against wear.

On the other hand, the use of engine oils with a reduced HTHS value can save fuel due to their lower internal friction. On the one hand, this enables better exhaust gas values ​​(environmental aspect) and the car manufacturers also like to advertise with lower consumption values.

Conclusion: When choosing the engine oil, it is necessary to find a good compromise between the best possible engine protection and low fuel consumption.

Viscosity of the SAE classes - multigrade oils

The viscosity of the engine oil changes during operation due to temperature, pressure and load (see viscosity index). For the (multigrade) engine oils commonly used today, the SAE the division into viscosity classes made while the ACEA specifications take care of for which type of engine the oil is suitable and how powerful it is. Based on the SAE class, it can be said for which temperature range an engine oil is suitable.

The information on the SAE classification on an engine oil packaging gives two values, thus describes a "from ... to ..." range and typically looks like this:

 

SAE 5W-30

 

  • SAE means nothing else,
    rather than the classification according to the SAE standard.
  • 5W is the value of Low temperature viscosity
    (the W stands for winter suitability).
  • 30 is the value of High temperature viscosity.

Tip: Motor oil 5W-30 test. The Low temperature viscosity or Limit pump temperature provides information about the minus degrees (cold oil = more liquid) up to which the engine oil can be pumped through the engine by the oil pump and can flow back into the oil pan.

According to the SAE requirements, pumpable motor oil for passenger cars has a dynamic viscosity of no more than 60,000 mPa · s. In order to classify an engine oil, one checks the temperature up to which (getting colder) the oil can still achieve this degree of "thin liquid". The values ​​for the assignment are:

SAE class
Winter suitability
Limit pump temperature
(for low temperature viscosity)
SAE 0W-40 ° C
SAE 5W-35 ° C
SAE 10W-30 ° C
SAE 15W-25 ° C
SAE 20W-20 ° C
SAE 25W-15 ° C

In order to be on the safe side with the engine oil at certain temperatures, one should calculate a "safety margin" due to the different engine constructions. The values ​​for this can be found in the viscosity temperature table.

The High temperature viscosity says how thick or thin the engine oil is at 100 ° C. This allows you to find out something about the resilience of the oil during operation. However, the HTHS viscosity, which is also used for classification in ACEA classes and for manufacturer approvals, is more realistic.

In order to always choose the correct viscosity range, the SAE information as it is on the label is sufficient. The corresponding values ​​according to SAE:

SAE class(High temperature) viscosity
minimum value at 100 ° C
SAE 16
This SAE class for even more fuel-efficient engine oils is relatively new (2013) and should never be used in older engines or vehicles without an explicit approval for such an oil.
5.6 mm² / s
SAE 206.9 mm² / s
SAE 309.3 mm2 / s
SAE 4012.5 mm² / s
SAE 5016.3 mm² / s
SAE 6021.9 mm² / s

For which temperature ranges the multigrade engine oils according to the SAE specifications then are suitable in practice, see the temperature table below.

Engine oil viscosity temperature table (SAE)

SAE classTemperature range
For these ambient temperatures
is the respective oil suitable *
SAE 0W-20-35 ° C to 20 ° C
SAE 0W-30-35 ° C to 30 ° C
SAE 0W-40-35 ° C to 40 ° C
SAE 5W-20-30 ° C to 20 ° C
SAE 5W-30-30 ° C to 30 ° C
SAE 5W-40-30 ° C to 40 ° C
SAE 10W-30-20 ° C to 30 ° C
SAE 10W-40-20 ° C to 40 ° C
SAE 10W-50-20 ° C to 50 ° C
SAE 10W-60-20 ° C to 55 ° C
SAE 15W-40-15 ° C to 40 ° C
SAE 15W-50-15 ° C to 50 ° C
SAE 20W400 ° C to 40 ° C
SAE 20W500 ° C to 50 ° C
* The ambient temperatures (i.e. the outside temperatures) in which the respective oil can be used Viscosity table read are approximate values. Why? Because the SAE J300 specification describes the “viscosity” the oil must not exceed at certain temperatures, but car engines are designed differently. That's why car manufacturers give the exact engine oil specifications for every car.

Motor oil viscosity definition

Viscosity is the oil's resistance to deformation. (Colloquially this is the "viscosity".)

Low viscosity liquids have low viscosity internal friction and can therefore flow more easily, i.e. more easily deformed. (Example: water.)

The internal friction of a liquid and thus its viscosity depends above all on the size and similarity of its molecules and the molecular chains:
The larger the molecules, the higher the internal friction. Imagine a bucket with freshly mixed concrete: the more sand and pebbles are stirred in, the more viscous the mixture becomes. In the case of molecules of different sizes, the average molecule size determines the viscosity.

When it comes to motor oil, fully synthetic oils have a clear advantage here: Their molecules are of the same type, so they hardly differ in size compared to mineral oil and there are no foreign molecules that contaminate the oil - one reason why only synthetic oils in 0W-class motor oils Find use.

The best way to illustrate viscosity and the underlying internal friction is using honey as an example. If you try to take honey that has just come out of the refrigerator onto a spoon, the following happens: You need some strength to move the honey in the jar and the honey remains reasonably tight even when the spoon is turned upside down, it hardly flows. When it is cold, honey defends itself against deformation; when cold it has a high viscosity.

If you warm the honey, you can stir it better in the glass and it flows / drips more easily from the spoon: now it has a lower viscosity.

Measure viscosity

There are two measures of viscosity: the kinematic and the dynamic.
Put simply, the kinematic viscosity says how light or heavy the oil flows down due to gravity at a certain temperature. Dynamic viscosity, on the other hand, describes how well or poorly the oil can be pumped or “stirred” at a certain temperature.

The viscosity (of engine oil) is measured with a so-called Viscometer measured, which is available in different versions - depending on whether you want to measure the kinematic or dynamic viscosity:

  • Capillary viscometer
    A certain amount of oil flows through a certain stretch of a capillary (a long, very narrow tube) at the test temperature. The duration determines the viscosity (lower viscosity = thinner = faster).
    There are two options for measuring: Measuring the kinematic or - with additional pressure exerted on the oil and thus a higher shear gradient - the dynamic viscosity.
  • Falling ball viscometer
    To measure the dynamic viscosity. A glass tube is filled with the oil, which is tempered (to the test temperature), in which there is also a metal ball. If you tilt the pipe at an angle, the ball “falls” through the oil in a certain time - faster or slower depending on the oil viscosity.
  • Rotational viscometer
    To measure the dynamic viscosity. To put it simply: You put a small bowl in a larger bowl. The tempered oil is poured into the space in between. Now you measure the force that is necessary to turn the small bowl in the oil bath. You can find a technically correct structure here.

Engine oil - Dynamic viscosity

The dynamic viscosity says how easily or difficult it is to deform oil, e.g. how it can be pumped. She will be in mPa · s specified. (mPa s = milli Pascal second, formerly cP = centi poise).

Engine oil - Kinematic viscosity

Kinematic viscosity says how easily oil deforms due to gravity alone - that is, how well or poorly it flows. She will be in mm² / s indicated (formerly cSt = centi Stokes).

The dynamic and the kinematic viscosity can each be interrelated converted if you know the density of the oil.
Conversion formula dynamic kinematic viscosity.

Mix engine oil with the same / different viscosity

Basically: You can mix engine oil if you pay attention to a few points.
If two (multigrade) engine oils of different viscosity classes are mixed, e.g. an SAE 0W-30 and an SAE 5W-30, the viscosity behavior of the mixture can only be as good as the "worse" of the two oils - in this case 5W-30 .

As long as you meet your automaker's engine oil requirements or the oil you're trying to add exceeds them, mixing isn't a problem. (This is why you should observe the manufacturer's specifications for engine oil, especially for new vehicles.)

Categories car advice, car knowledgeTags engine oil, synthetic oil