Density means the volume weight of a substance. In oils, it is usually indicated in the temperature of +15°C or +20°C, in units kg/m3. Lubricant densities range between about 700 and 950 kg/m3, depending on the quality, viscosity and additive content of the lubricant.
The thicker a fluid is, the greater its viscosity. Nowadays, lubricant viscosity is usually expressed with the units centistoke (mm2/s) and centipoise (mPas).
Centistoke is a unit of kinematic viscosity, based on the amount of force required to beat the internal friction of fluid.
Centipoise is a unit of dynamic viscosity, often used for expressing the internal friction of oil in low temperatures. The connection of cSt and cP is cP = cSt x fluid density.
The temperature must always be given when expressing viscosity with any unit. All oils become much thinner as the temperature rises. A typical viscosity of motor oil SAE 10W at a temperature of -20 °C may be 2,000 cP, but if it heats up to a temperature of +100 °C the viscosity is only 5.2 cSt.
Kinematic viscosity is measured by the pictured Ubbelohde viscometer. It measures the time the oil requires to flow from point m1 to point m2.
Viscosity index (V.I.) describes the fluid’s tendency to thin as the temperature rises. The stronger the fluid thinning the smaller the viscosity index.
The V.I. of single-grade motor oils is about 95-110, and that of multi-grade motor oils even higher than 200.
The flash point expresses the flammability of a fluid. Flash point is the temperature at which, measured from the fluid with a certain method, flammable gases are vaporized so much that they flame up when ignited with a naked flame, but the fluid does not carry on burning.
Ignition point is the temperature at which the gases vaporized from the fluid when heated in an open cup burn for at least five seconds when ignited with a naked flame. The ignition point is usually 10-50 °C higher than the flash point.
Oil thickens as the temperature falls. At a certain temperature it no longer flows by its own weight. This temperature is called the pour point. The pour point depends on, e.g., the viscosity and chemical structure of the oil. In paraffinic oils, stiffening is caused by the wax in the oil, which is distinguishable as crystals.
The more the oil cools down the bigger the crystals grow, eventually forming a flow-preventing network within the oil.
Naphthenic oils have less or no wax, and they remain fluid in lower temperatures than paraffinic oils. The oil eventually becomes so stiff that it no longer flows with its own weight. Fully synthetic oils do not contain wax and their cold properties are excellent.
The pour point can be improved by using an additive that prevents the growth and interconnection of wax crystals. With the pour point, it is possible to describe approximately the cold start properties of oil, but in many cases it is not enough; it is more important to know the true oil viscosity at the starting temperature.
When the motor is running, acid compounds from the burned fuel are mixed in the oil, and they must be neutralised in order to prevent corrosion of the metal parts. Therefore, motor oil contains additives for creating base reserve, the amount of which is expressed as the total base number (TBN).
By monitoring the change in the total base number it is possible to evaluate the condition of the motor oil with certain other tests.
The change in base reserve when using oil.