Car lubricants play four major roles – they control friction and wear in the engine, they protect the engine from rusting, they cool the pistons, and they protect the engine oil stored in the sump from combustion gases.

 

Some 75%-95% of a typical engine lubricant is made up of a base oil – a mineral oil that has come directly from a refinery. These base oils can naturally contain straight or branched chains of hydrocarbons, hydrocarbon molecules with aromatic rings attached, or these chains can be produced by further chemical reactions of the base oils.

 

The remainder of the lubricant comprises a variety of additives, which are used to improve performance. Typically these include anti-wear additives, corrosion inhibitors, antioxidants, detergents, dispersants, antifoam additives, and large polymer molecules known as viscosity modifiers, which are added to improve the viscosity variation of the lubricant with temperature.

 

Indeed, the viscosity is the most significant physical property of a lubricant. The way in which it varies with temperature, shear rate and pressure determines to a great extent how the lubricant performs in an engine. But the chemistry of the lubricant is also important: it must be resistant to oxidation and it must be able to “lay down” a protective film to combat wear and tear where metallic contact is inevitable.

 

The behaviour of an oil film trapped between two moving surfaces is quantified by the dynamic viscosity, which is measured in millipascal seconds (mPa s). More accurately, the dynamic viscosity relates the shear stress, the shearing force acting on the oil per unit area, and the shear rate, the difference in velocity between the two surfaces divided by their separation. However, it is often more convenient to measure a quantity known as the kinematic viscosity, which is the dynamic viscosity divided by the fluid density and is measured in mm2 s-1 or centiStokes (cSt).

 

Lubricants fall into two broad categories – monograde and multigrade – depending on whether their viscosity changes significantly with temperature or not. A more detailed classification system has been devised by the Society of Automotive Engineers (SAE) (see table). One common lubricant is described according to this scheme as an SAE-10W/30 multigrade. The first number (10W) refers to the dynamic viscosity measured at low temperatures, while the second (30) describes the kinematic viscosity at 100 oC. Lower numbers describe runnier lubricants – the viscosity of an SAE-5W/30 multigrade, for example, is five times lower than that of SAE-20W/50 at -20 °C. Roughly speaking, the energy lost due to friction varies with the square root of the viscosity: at -20 °C the friction losses of the low-viscosity oil will be approximately half those of the thicker oil, allowing the engine to start more easily.

 

The viscosity grade of a multigrade oil is different at high and low temperatures due to additives known as viscosity modifiers. For example, SAE-10W/30 has a similar viscosity to the monograde lubricant SAE-30 at 100 °C. At lower temperatures, however, SAE-10W/30 is much thinner than the monograde oil. This means that the multigrade oil provides protection at high temperatures and is runny enough at low temperatures to enable engines to start easily in most European countries. In contrast, the thick monograde oil would simply be unsuitable in winter.