Molecular dynamics simulation of complex molecules using quantum-chemical potentials: application to modelling fuel droplets

This project was concerned with the development of a new hybrid quantum mechanics/ molecular dynamics (QM/MD) model for the simulation of complex hydrocarbon molecules and the application of this model to the simulation of n-dodecane and a mixture of n-dodecane and dipropylbenzene molecules in Diesel engine-like conditions.

The solution of the time independent Schrodinger equation allowed researchers to obtain the equilibrium geometry of a molecule or an ensemble of molecules, and to calculate the potential energy for any position of atoms and electrons in the system. This approach gave the potential energy of interacting molecules as a function of their geometry. Comparison of this energy for interacting individual C and H atoms and molecules with the interaction energy calculated by the conventional MD approach (taking into account the internal degrees of freedom of molecules, used in our EPSRC project EP/H001603/1), Development of a new quantitative kinetic model for the analysis of heating and evaporation processes in complex hydrocarbon fuel droplets) for the same inter-atomic distances allowed the investigators to analyse the differences in the QM and classical potentials.

Results were used to calculate the corrections for the potentials used in the classical MD calculations. The new hybrid model was then used for the analysis of the dynamics of n-dodecane molecules in liquid and gas phases and at the liquid/gas interface, using techniques developed during the work on EPSRC project EP/H001603/1. At this stage investigators were able to establish the range of applicability of the conventional MD approach. A new approximate method of taking into account the QM corrections to the classical results was developed. Also, the previously developed kinetic model, taking into account the presence of two components (fuel vapour and air) in the kinetic region was generalised to take into account the presence of the three components (two species of fuel and one of air) there.

These new models were applied to the analysis of Diesel fuel droplet heating and evaporation in realistic engine conditions. In contrast to the previously developed models, the kinetic effects were taken into account alongside the effects of temperature gradient and recirculation inside droplets and the effects of the moving boundary during the evaporation process. There was no awareness of any previous research in this area.

This was a collaborative project involving visiting researchers Professor Vladimir M. Gun'ko (Chuiko Institute of Surface Chemistry of the National Academy of Sciences of Ukraine, Kiev, Ukraine) who is an internationally recognised expert in interfacial phenomena, Dr Bing-Yang Cao (Tsinghua University, Beijing, P.R. China), whose expertise includes the development of numerical algorithms for molecular dynamics simulation and Dr Irina Shishkova (Moscow Power Engineering Institute, Russia), whose expertise is focused on the development of numerical codes for the solution of the Boltzmann equation.

It was led by Professor Sergei Sazhin, whose expertise includes the development of new physical models of fuel droplet heating and evaporation with a view of applications to modelling the processes in internal combustion engines. The Co-investigator Professor Morgan Heikal advised the project members on the relevance of the models to automotive applications. A Research Fellow was also included in the project. This project built upon the EPSRC project EP/H001603/1, supporting the collaboration between the PI, Dr B-Y. Cao and Dr I. Shishkova, and previously funded EPSRC projects EP/C527089/1 and EP/E02243X/1 (A kinetic algorithm for modelling the droplet evaporation process in the presence of a heat flux and background gas), and a Royal Society Joint project with Russia, supporting the collaboration between the Principal Investigator and Dr I. Shishkova.