In part 2 we discussed what “jobs” or functions a motor must do in today’s engines. Today we will discuss Viscosity.
Viscosity refers to the oil’s resistance to flow and is the most important property of an oil. The viscosity of oil varies with changes in temperature – thinner when hot, thicker when cold. Although oil must flow at cold temperatures to lubricate the engine at startup, it must also remain thick enough to protect the engine at high operating temperatures. When an oil is used at a variety of temperatures, as with most engines, the change in viscosity should be as minimal as possible.
Wouldn’t it be handy to have a number that indicated the oil’s viscosity change? We do, and it’s called the Viscosity Index (VI). It is measured by comparing the viscosity of the oil at 40°C (104°F) with its viscosity at 100°C (212°F). The higher the VI, the less the viscosity changes with temperatures changes and the better the oil protects the engine. Synthetics typically have a higher VI than conventional oils.
SAE viscosity grades
For engine and gear oils according to the SAE standards J 300 and J 306
SAE stands for Society of Automotive Engineers. The SAE developed a classification system to define the viscosity or thickness of oil. This system has been progressively modified over the years. It defines “operating temperature” engine oil viscosities for different grades and contains specifications for “cranking” viscosity and pumpability at start up, the “W” grades or winter. A multigrade oil is one that meets both a “W” low temperature viscosity requirement and a 100°C “operating temperature” requirement. The classifications increase numerically; the lower the number, the lower the temperature at which the oil can be used for safe and effective engine protection. Higher numbers reflect better protection for high-heat and high-load situations
For engine oils there is a specification that must be met at 150°C, known as a High Temperature/High Shear (HT/HS) viscosity. HT/HS simulates what happens in high stress areas of the engine e.g. bearings, cams, etc. It measures the viscosity and indicates the oil film thickness under severe high-speed conditions. An oil that is too thin under these conditions may not provide the required protection to prevent significant wear in these critical engine parts.
Centipoise (cP) and Centistokes (cSt) are the units viscosity is measured in. The following chart shows a comparison of different viscosity grades versus temperature rang.
Understanding Viscosities
The number in front of the “W” signifies the viscosity of the oil at cold/start up temperature (tested at -10 to -35°C depending on the grade. Please refer to the SAE J300 – Engine Oils Chart opposite). The lower the first number, the faster the oil flows (pumpability) when the engine is cold. The second number represents the oil’s thickness at operating temperature (100°C). All oils thin out as they get hotter. So, the higher the second number, the less the oil will thin out as it heats up, compared to an oil with a lower second number. Normally, multigrade viscosities are measured on a curve.
In the example above, all three oils have the same operating temperature viscosity but different cold temperature viscosities. All three will be similar viscosities when at operating temperature, measured at 100°C In the example below, all 4 oils gave the same cold temperature viscosity, but different operating temperature viscosities.
