A team from Oak Ridge National Laboratory and General Motors, led by ORNL researcher Dr. Jun Qu, has developed a new group of ionic liquids as lubricant additives that could help improve the energy efficiency of cars and trucks.

Prototype low-viscosity ionic liquid-additized engine oil demonstrated a 2% improved fuel economy compared to Mobil 1 5W-30 engine oil (3.93% over the 20W-30 baseline oil) in standard fuel efficiency engine dynamometer tests. The prototype oil also successfully passed a 100-hour high-temperature, high-load engine dynamometer test.

Friction is the cause of the loss of ~10-15% of the energy in an internal combustion engine; for the transportation sector, parasitic friction (primarily induced by elastohydrodynamic drag between the piston rings and cylinder liners, which is proportional to the lubricant viscosity) consumes approximately 400 million barrels of oil annually in the US. Accordingly, the US Department of Energy’s (DOE’s) Vehicle Technologies Office, which sponsored this research, has set a goal of 2% fuel economy improvement via lubricant advances by 2015.

This breakthrough ionic lubricant technology could potentially save the US tens of million barrels of oil annually.

Ionic liquids are “room temperature molten salts”, composed of cations & anions, instead of neutral molecules. In a 2012 paper published in the journal ACS Applied Materials and Interfaces, Qu and his colleagues noted that:

Ionic liquids [ILs] have been explored for lubrication applications since 2001. The majority of the literature has focused on using ILs as neat lubricants or base stocks, which would be suitable for special bearing applications such as for operation at >250 °C where conventional hydrocarbon lubricants start decomposing, but many ILs are stable.

Another approach is to use ILs as lubricant additives, which has significant practical merit because it may get into the lubricant market without requiring major supply and distribution changes. However, most ILs have little or no solubility (<1%) in nonpolar hydrocarbon oils. Most previous studies used unstable oil-IL emulsions or low concentrations of ILs in nonpolar base oils, and others used a polar base stock for better compatibility with ILs. Therefore, there is a significant opportunity for developing ILs with good solubility in nonpolar lubricating base oils.

In this paper, we report the results of research on the IL trihexyltetradecylphosphonium bis(2-ethylhexyl) phosphate, which is not only soluble but fully miscible in common non-polar hydrocarbon oils. Results with this IL reported here have demonstrated high thermal stability, noncorrosiveness, and high effectiveness in reducing friction and wear when blended into lubricating oils. Surface boundary film examination revealed the antiwear mechanism for the IL additive and its synergistic effects with an existing additive package.

In general, they noted, ions and non-polar neutral organic molecules are immiscible, because ions are attracted by polar forces, whereas non-polar molecules are held together by dispersion forces. They hypothesized that the exceptional oil-miscibility of their ionic liquid is due to its three-dimensional quaternary structures with high steric hindrance (long hydrocarbon chains) that dilute the charge of the ions and therefore improve the compatibility with neutral oil molecules.

In contrast, most ILs that have been studied contain either two-dimensional cations or small anions, and thus cannot dissolve in oils. Their experiments also found that, in addition to the quaternary structures, an oil-miscible IL needs to contain at least one alkyl with four carbons or more for both the cation and the anion.

In a newer paper in Tribology International, the team reports the anti-scuffing/anti-wear behavior and mechanism of the oil-miscible ionic liquid in a base oil at 1.0 wt% concentration under both room and elevated temperatures. Results are benchmarked against those for a conventional anti-wear additive, zinc dialkyl-dithiophosphate (ZDDP).

They conducted reciprocating sliding, boundary lubrication tests using a piston ring segment against a cylinder liner piece cut from actual automotive engine components. Although the IL and ZDDP worked equally well to prevent scuffing and reduce wear in the room-temperature tests, the IL significantly outperformed ZDDP in the 100 °C tests.

Furthermore, they found that ionic liquid additives have potentially less adverse impact on TWC (three-way catalysts) compared to ZDDP.