Oil additives, friction modifiers and DLC coatings

Monday, December 02, 2013

Tags :  oil-system

In a previous RET-Monitor we looked at the basics that go into producing modern racing oils, particularly the base oil stocks from which they are formulated. Although the base oils, particularly the highly refined PAO and ester-based oils, used in most motorsport applications have excellent inherent lubrication characteristics, oil manufacturers will invariably add other chemicals to tailor the oil for specific purposes and improve lubrication performance.

These additives allow the lubricant to work at a wider temperature range and at higher loads as well as prolonging the life of the lubricant and the mechanical parts involved. Oil producers will keep their exact additive packages a closely guarded secret, but in general terms these additives can be put in to the following categories. 

Friction modifiers (FM)

These additives are introduced to lubricants to reduce friction. Typically they consist of chemical compounds that have a high affinity for metal surfaces and possess long alkyl chains that are adsorbed on the contact surfaces in either single or multiple layers, resulting in a thin film forming. The nature of this attachment means FM molecules only function in the boundary or mixed lubrication regimes, for example at the camshaft-lifter interface. 

Anti-wear (AW) additives

Compared to friction modifiers, anti-wear additives are adsorbed more strongly onto metal surfaces, and are able to reduce wear of the surfaces involved thanks to a thicker film that serves to separate the surfaces even under high loads. The most common AW additive is a compound of zinc, sulphur and phosphorus, called ZDDP (zincdialkyldithiophosphate), the benefits of which were covered extensively in a previous RET-Monitor article.

As noted in that article, ZDDP has fallen victim to more stringent environmental regulation, so oil manufacturers have undertaken extensive research into similarly effective alternatives, including non-metal based compounds featuring a high phosphorous content. Alternatively, particularly when dealing with engines that use hydraulic tappets, the use of low-friction, wear-resistant coatings such as DLC has removed the need for additives such as ZDDP. However, in engines such as the flat-tappet units used in NASCAR’s Cup series, ZDDP is still the only viable solution to ensuring component longevity.  

Extreme pressure (EP) additives

EP additives are used primarily in applications that see exceptionally high contact pressures, and are more often found in transmission rather than engine oils. Both AW and EP additives are activated by pressure and temperature as well as shear forces, although EP additives tend to activate at higher temperatures than AWs.

At elevated temperatures, they ‘chemisorb’ – chemically bind – at surfaces, the compound being adsorbed onto the metal surface by a chemical reaction. However, at lower temperatures they are more likely to physically adsorb. Therefore, under moderate loads, AW additives will provide the protection needed, with the EP additives coming into play in more extreme conditions. 

Viscosity modifiers

The purpose of viscosity modifiers is to allow the lubricant to work effectively over a wide temperature range. They are particularly important in race engines that tend to run at higher temperatures than general automotive units. For example, one oil supplier to NASCAR teams had to develop a new viscosity-modifying additive package to ensure oil film stability at the elevated temperatures encountered when cars were drafting, as a result of the practice’s impact on airflow through the car’s oil coolers. 

Viscosity modifiers tend to consist of polymers of high molecular weight that unfold at high temperatures, leading to an increase in viscosity. At low temperatures they curl up, leading to a less pronounced increase in viscosity. By carefully selecting the viscosity modifiers, an oil can be produced that has a low viscosity at normal operating temperatures, thus reducing frictional losses, yet still retains its lubrication characteristics, preventing excessive wear, if temperatures rise.

Written by Lawrence Butcher

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