Car lubricants: fact and friction

Lubricants are essential in modern life. Car engines and gearboxes run smoothly thanks to sophisticated oils and greases, while computer hard disks rely on thin organic films to ensure that the "read/write" head can move reliably at high speeds across the recording medium. According to some analysts, however, the direct costs of friction and wear can account for nearly 10% of the gross national product (GNP) in many industrial nations. Moreover, they estimate that cost savings of up to 1% of the GNP could be achieved simply by using the right lubricant for the job.

Lubricants are remarkable fluids. During winter in Detroit, for example, the same car-engine oil has to operate reliably over temperatures ranging from -40 °C to above 250 °C - the temperature near the top piston ring. It also has to cope with pressures between 105 and 109 pascals, as well as contaminants including metal particles and soot. The final straw is that this fluid must deal reliably with these conditions every day for up to two years - the recommended time between oil changes, according to some vehicle manufacturers.

Surprisingly, one of the major driving forces behind the development of lubricants is the environment. Modern vehicles are required to emit far fewer pollutants than older cars and lorries. Indeed, the emissions from a typical modern vehicle are some 50 times lower than those manufactured in the 1960s.

Carbon dioxide is a natural by-product from the combustion of fuel and is among the most significant pollutants being targeted for reduction. Indeed, vehicles that have a high fuel consumption emit large amounts of carbon dioxide. However, the European Union is taking a strong lead in tackling this problem, and has indicated that the average amount of carbon dioxide emitted by every vehicle should be reduced from today's average of 200 grammes per kilometre to less than 140 grammes per kilometre from 2008. This is roughly equivalent to improving the average fuel consumption from 33 to 47 miles per gallon. Such an increase would lead to big cuts in terms of carbon dioxide. In the UK alone - where there are roughly 20 million cars, each covering an average of 16 000 km every year - the total annual drop in CO2 would be about 19 million tonnes.

Clearly manufacturers are making a number of engineering changes to their vehicles to try to improve fuel economy. Less well known, however, is the fact that the fuel consumption can be significantly improved just by changing the lubricants. For example, it is possible to decrease the amount of fuel consumed by modern cars by up to 5% simply by switching from a typical multigrade oil to a "friction-modified" lubricant with a lower viscosity. This would lead to an annual CO2 drop of roughly 3 million tonnes in the UK. Remember that this figure is just for the UK and just for cars. Greater CO2 savings are clearly possible if optimized lubricants were also used in trucks and in other machinery.

What is a lubricant?
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) .

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.