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A molecular simulations analysis of cementite in presence of combustion gases using ReaxFF

Banerjee, Sayantan

[Thesis]. Manchester, UK: The University of Manchester; 2016.

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Abstract

Carbonaceous deposits in vehicle engines are high molecular weight compounds, which adhere to different engine components resulting in loss of fuel economy, engine performance and increased emissions of pollutants. The exact mechanisms of their formation are unknown, even though it has been suggested that deposits are formed when fuels and combustion gases interact with the hot engine surface.The work presents an analysis of the molecular level interactions between combustion gases (such as oxygen, hydrogen, carbon monoxide, water vapor, formaldehyde, methane) and the engine surface (which was modeled as cementite-Fe3C), using the ReaxFF simulation approach. A ReaxFF force field was selected for modeling cementite (Fe3C), by validating against energy-volume data from DFT and experimental data. This was followed by analyzing the relative stability of eight cementite surfaces with different Miller’s indices. It was observed that the (111) and (010)-non-stoichiometric cementite surfaces are stable, whereas (100) surface is the most reactive. The ReaxFF force field was then used to study interactions between combustion gases on the (010)-non-stoichiometric cementite surface. Adsorption energies were calculated for six gas molecules (oxygen, hydrogen, carbon monoxide, water vapor, formaldehyde, methane). Our results indicate that most of the selected molecules interact strongly with the cementite surface (chemisorption), except for methane (physisorption). This work shows that it is suitable to use the ReaxFF approach to study fouling in engines.