​Inside an engine, acidic species form that can damage the engine. To prevent this damage, the oil used for lubrication contains detergents that are 'overbased' to neutralise these acids.

In the past, the dominant acidic species were inorganic mineral acids formed as a by-product of the combustion of sulfur-containing fuel. However, legislation has reduced the sulfur content in fuel and the rise in the use of biofuels has shifted the focus from inorganic acids to the short chain organic acids formed by fuel oxidation, incomplete combustion, or degradation.  

Understanding the effectiveness of conventional overbased detergents in neutralising these organic acids is increasingly important, especially with the drive towards greener fuels. Researchers from Lubrizol have used ISIS to learn more about the interactions between organic acids and lubricant additives. 

Traditionally, the effectiveness of an overbased detergent in neutralising acidic species is described by its total base number (TBN). However, using small angle neutron scattering (SANS) on Sans2D and infrared spectroscopy, the researchers found that the TBN alone was actually not an accurate predictor of how well an overbased detergent would neutralise an organic acid.

A stopped-flow method was used for both techniques, enabling the neutralisation to be monitored at specific time periods after the overbased detergent and acid species were mixed. Data showed that three overbased detergents diluted to the same TBN value exhibited very different neutralisation rates.

The group went on to study other factors that could be influencing the neutralisation. They found that the surface area to volume ratio of the overbased detergent particles was important, where those with higher surface area to volume ratios neutralised more acid in the same time period. This was supported by their comparison of overbased detergents with different surfactant shells, and with the type (and therefore base strength) of metal ion used in the overbased detergent core.

Because of the different interactions between neutrons and hydrogen versus deuterium, the researchers were able to design an experiment that used deuterated acetic acid to understand the mechanism of the acid molecule's interaction with an overbased detergent. This showed that the acid molecules reacted on the surface of the overbased detergent particle, confirming the importance of surface area in these reactions.

“Our study found that total base number was not always an accurate predictor of acid neutralisation rate," explains David Growney, Lubrizol; “and its use could actually be misleading. Considerations must be made based on the learnings in this study when formulating engine lubricants to ensure organic acids are effectively neutralised, therefore preventing premature engine degradation."

He adds; “using SANS at ISIS has enabled brand new insights relating to the mechanisms involved in acid neutralisation, and therefore engine protection at a molecular level, and has advanced the industry understanding in this space."

Further information

The study has been published online at DOI: 0.4271/04-14-03-0013

Lubrizol accessed ISIS as part of the ISIS Collaborative R&D programme.