Scientific and Production Company VMPAUTO

 

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Oil viscosity losses in the engine

Viscosity of engine oils and oil thickeners

 

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First of all we should understand what viscosity means. Viscosity is a measure of a fluid's resistance to flow. It describes the internal friction of a moving fluid. More fluid resists to deformation, more viscosity is. Try pouring water into a pail and mixing it with a metal stick, for example.

 You can do it easily. It means that water viscosity is less than oil viscosity. Than take the same pail pouring a cream and try to mix this material. It is harder to do it, which means that cream’s viscosity is more. 

graphic

Stribeck Curve. Different friction modes. More oil viscosity is, more probable is that the friction pair is working in hydrodynamic mode, where there is no wear, but the friction forth can be high. In boundary and mixed modes there is a wear.

How oil viscosity is related to friction? In 1902 year a German scientist Richard Stribeck presented his first results of researches. According to the dependency known as Stribeck Curve, the thickness of an oil film in a friction pair increases when the oil viscosity increases. We can see it easily.

If you really have been mixing water and cream, you had noticed different films’ thickness on the metal stick taking out of the pail. If thickness of this friction pair is enough, the surfaces are securely separated, and there is no units wear (hydrodynamic friction mode). If the film’s thickness is not enough and the surfaces can touch each other, the wear is starting (boundary and mixed friction modes).

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Реометр MCR 102 – прибор для определения деформационных свойств жидких и полутвердых тел в широком интервале температур.

If all engine parts worked in the same conditions and at the same temperature, everything would be very easy. But it is wrong. It may happen that at low temperature high oil viscosity which is good for work in upper piston ring will damage crankshaft work or oil flow in the engine system.

Or on the contrary, low oil viscosity which is preferable for low temperature will transform crankshaft bearings’ and piston rings and skirt friction from hydrodynamic friction mode (non-wear mode) in mixed or even boundary friction mode (high wear mode).

Cause of difficulties and contradiction in oil viscosity standards many years SAE publishes requirements to oil viscosity basing on its rich experience. The SAE requirements are specified in J300 standard. As for today the actual version was published in April 2013. In this standard oil viscosity measuring can be made in the temperature range from +150°C to -35°C according to four specific methods.

Using modern fully automated rheometer Anton Paar MCR 102 in the laboratory of VMPAUTO LLC during 2 hours test we can get an oil viscosity curve in the range from +100°C to -35°C. The datas will not fully correspondent to SAE J300, however they will be linked with those results. Dates received from the rheometer can be used for comparative tests. 

Oil viscosity curves in the range from -100°C to -35°

Blue curve – oil 5W-30. Red curve – the same oil with oil thickener. 

There are too many such additives on the market. They look almost the same: metal or plastic cans 300-400ml with thick yellow liquid. Usually they have similar names like “Motor oil treatment”, “Stop smoke”, “Motor Honey”, etc…

How these additives influence to the motor oil viscosity? The datas we’ve got let us affirm the following: these products increase motor oil viscosity in all temperature range in almost 2 times! It means that when you use such oil thickener in synthetic oil you get semi-synthetic oil. So, according to SAE: from the oil 5W-30 you get oil 10W-40. 

If we calculate in USD you will get: 35$ for 4l can of 5W-30 + 10$ oil thickener = 30$ for 4l can of 10W-40!

In the picture there are oil viscosity curves for the motor oil SAE 5W-30 (blue curve) and SAE 5W-30 with oil thickener (red curve).

Using oil thickener the motor oil viscosity will increase and it may cause problems with cold start. From the other side considerable increasing of oil viscosity at high temperature in case of new engine, for example, will provoke unreasonable losses on friction and power decreasing. Most of all amounts of contaminations will increase.

Short-time positive result can be reached if oil thickener was used in a worn engine with high level of oil burning. In this case the increase of oil viscosity will double oil film on the cylinder surfaces that will lead to all system’s sealing. But this is only a temporary effect, because the oil thickener is a solution of polymers and it will lose its viscosity during engine running.

Use of oil thickeners is preferred by some sellers of used cars: oil burning problem can be easily hidden after use of such oil thickeners. If you buy a car and you suspect that an oil thickener was added you can take an oil sample from carter and make an easy test.

You place the taken sample in one can and pour a sample of the same SAE new motor oil in the second can. At indoor temperature you will not recognize the difference. So place two samples in a freezer from -10°C to -20°C. After 2-3 hours try to mix two samples, than you will understand clearly if an oil thickener was used.

Practically every modern motor oils contain polymers which maintain oil viscosity in a wide temperature range. During the time these polymers deteriorate and the motor oil can become more or less viscous. So may be use of oil additive which keep SAE viscosity has a sense