Motoring corner With Carlove

r“Know Your Oil (Cont’d)”We looked at the viscosity of oils, most of the time, viscosity is explained in words that are used that are too technical for the average person to quickly grasp. This leaves them still wondering what the viscosity numbers really mean on a bottle of motor oil.

r“Know Your Oil (Cont’d)”

We looked at the viscosity of oils, most of the time, viscosity is explained in words that are used that are too technical for the average person to quickly grasp. This leaves them still wondering what the viscosity numbers really mean on a bottle of motor oil.

Simply put, viscosity is the oil’s resistance to flow or, for the layman, an oil’s speed of flow as measured through a device known as a viscometer. The thicker (higher viscosity) of an oil, the slower it will flow. You will see oil viscosity measurement in lube articles stated in kinematic (kv) and absolute (cSt) terms. These are translated into the easier to understand SAE viscosity numbers you see on an oil container. What about 0W oils then?  Good question. Given that you can’t have 50ml, 60ml or 200ml of oil flowing through any size hole in zero seconds, what on earth does the 0W rating denote? Well it’s a special case denoting a difference in the ‘pour point’ of the oil. Most 5W oils have a pour point at -40°F (-40°C) The base oil is the same in
0W40, but it’s pour point has been lowered even further - sometimes to as much as -50°F (-46°C).

Pour point is 5°F above the point at which a chilled oil shows no movement at the surface for 5 seconds when inclined. This measurement is especially important for oils used in the winter. A lot of manufacturers tend to quote pumping temperature rather than pour point. Pumping temperature is the temperature at which the oil will pump around the engine and maintain adequate oil pressure. This is typically 20°F above the pour point - ie. 25°F above the point at which the oil is basically a gel.  So 0W oils don’t flow through a viscometer in zero seconds - they rate at 5 seconds like a 5W oil, but they will be pourable at a much lower temperature. The bottom line then is that if you think your car is ever likely to see a cold morning in the -45°F (-43°C) range, you should be considering 0W40 oil. If not, 5W40 will do. Note that at -45°F, you’ll probably have more to worry about than your engine oil - like your radiator fluid, brittle tyres, frozen locks, permafrost on the windscreen etc, in Africa, we will not

be worrying about that!

In addition to measuring the viscosity of oil, motor oil manufacturers typically also use a cold crank simulator (CCS) to measure the apparent viscosity of the oil in cold weather (from -5°C own to -40°C). This in turn translates to the resistance that the oil presents to the moving parts of an engine during a cold startup. The CCS tries to simulate this using a cylinder with a piston-like moving part inside it that is linked to an electrical engine. The piston and cylinder are separated by few millimeters and oil is pumped into this space. The whole lot is then cooled to the temperature required to run the test and given time to reach equilibrium. The motor then tries to rotate the piston and the resistance to that rotation is measured and compared to the resistance presented by a standard oil of known resistance. According to the viscosity range, the test is carried at a defined temperature. For example, for a 25W oil, the test is carried at -10°C, for a 20W, -15°C, for a 15W, -20°C and so on. 

 

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