Molecular dynamics simulations of engine lubricant additives

Abstract

The environmental impacts of human activity are causing increasingly severe repercussions. Travel and shipping accounts for a large percentage of the man-made greenhouse gas pollution. Increasing the efficiency of the conventional combustion engine offers significant energy savings, reducing the consumption rate of fossil fuels and decreasing the footprint of existing technology while new alternatives are developed and implemented on larger scales. Since the majority of energy losses within any engine are due to friction, effective lubrication results in notable improvement of engine functionality. The molecular-level behaviour of various additives can have a significant impact on the behaviour of a lubricant, but the knowledge of the mechanics on the nano-scale is rather incomplete. Interpreting experimental data often relies on assumption and approximation, and this process can be complemented by computational methods, which offer the possibility to scrutinise the molecular behaviour at an atomic level. Exploration of extreme conditions, such as high pressure, are also much more available computationally. Therefore, the development of new lubricant formulations can be significantly aided by computational modelling. With a robust method, new additives can be analysed and screened for functionality, supplementing the experimental formulation studies and expediting the development process. To this end, various lubricant systems are explored in this work, using the LAMMPS software package to run molecular dynamics simulations.

A comprehensive method to study the bulk liquid and solid-liquid behaviour of an additive is adapted and tested on a proprietary polymer of mixed design (for viscosity control and surfactant-like behaviour) in nonpolar solvent. Both the bulk and confined simulations show excellent agreement with experimental data, proving the validity of the method.

The aggregation behaviour of the additive in nonpolar solvent is also explained. The mechanics of friction reduction under high load, in a system confined between iron oxide surfaces, are uncovered through simulation analysis. The confined simulations show the exact surface adhesion tendencies on the molecular level. The findings obtained from the computed models provide novel insights on additive behaviour. The method is further tested on a polar, surfactant-type additive, Aerosol OT (AOT) in cyclohexane. New charge parameters are developed for the polar head group to study its initial bulk self-assembly. Self-assembly from a completely randomised starting distribution is successfully achieved.

Simulations show excellent agreement with small angle neutron scattering (SANS) data, giving proof of principle for both the parametrisation and the overall method. The surface confinement simulations of AOT show good agreement with experimental neutron reflectivity results, but uncover the tendency of the surfactant to vary reverse-micellar shape with solvent. This is explored further by AOT bulk simulations in dodecane. The formation of non-spherical micelles, is further examined by simulations of MgAOT, to complement SANS data of different cationic counterparts for the docusate anion (paired with sodium in AOT). The results as a whole show how computation can be used to aid formulation and explain molecular behaviour of engine lubricant additives. In the future, the method may be applied to other compounds of similar design. There is a potential for building up a library of possible additive molecules and the varying effects they have on properties of interest for lubricant design. In this manner, computational modelling could be used for compound screening and establishing formulation benchmarks.