Elucidating the Properties of Surrogate Fuel Mixtures Using Molecular Dynamics

Abstract

The wide compositional differences between conventional and alternative fuels have resulted in much research aimed at determining which alternative fuels can be used, and in what proportions, in conventional engines. Atomic-scale modeling is uniquely positioned to lend insight into this question without extensive large-scale tests. The predictive power such modeling affords could narrow the phase space that must be examined experimentally. This study utilizes molecular dynamics (MD) simulations to predict the properties of a set of pure hydrocarbons, as well as binary and multicomponent surrogate fuel mixtures for alternative fuels created from these pure components. The accuracy and transferability of the modified Lennard-Jones adaptive intermolecular reactive empirical bond-order potential (mod-LJ AIREBO) [J. Comput. Chem.200829601611] was assessed by calculating densities, heats of vaporization, and bulk moduli of pure hydrocarbons and the mixtures of these hydrocarbons, i.e., surrogate fuels. Calculated results were compared to experimentally determined values and to values obtained with the nonreactive, all-atom version of the optimized potential for liquid simulations (OPLS-AA) [J. Am. Chem. Soc.19961181122511236]. The mod-LJ AIREBO potential quantitatively predicts the densities of the pure hydrocarbons and binary mixtures of n-dodecane and 2,2,4,4,6,8,8-heptamethylnonane (isocetane). It is interesting to note, that despite doing an excellent job predicting the densities of the pure hydrocarbons, the performance of the mod-LJ AIREBO potential degrades when predicting the densities of the multicomponent surrogates and mixtures of n-hexadecane and isocetane, implying that it is not straightforward to extend potentials fit with pure compounds to mixtures. The OPLS-AA potential also has difficulty quantitatively predicting the densities of mixtures, although a new parameter set for long-chain hydrocarbons (L-OPLS) [J. Chem. Theory Comput.2012814591470] yields some improvement for binary mixtures. Heat of vaporization predictions using both potentials also agree reasonably well with experiment. Bulk moduli predictions using the mod-LJ AIREBO potential are consistently higher than, and do not quantitatively agree with, the experimental values. In contrast, bulk moduli predictions using the OPLS-AA potential are generally in good agreement with experimental values. Despite the success of the OPLS-AA potential predicting the bulk moduli of individual hydrocarbons, it is unable to quantitatively predict the bulk moduli of the multicomponent surrogates. Interestingly, the use of the L-OPLS parameter set improves density predictions but not predicted bulk moduli values.

Densities, Speeds of Sound, and Viscosities of Binary Mixtures of ann-Alkylcyclohexane (n-Propyl-,n-Pentyl-,n-Hexyl-,n-Heptyl,n-Octyl-,n-Nonyl-,n-Decyl-, andn-Dodecyl-) withn-Hexadecane

“…Previous studies calculated the densities, bulk moduli, and heats of vaporization of 12 pure hydrocarbons, as well as densities and bulk moduli of multicomponent fuel surrogates containing up to 12 components, and showed how molecular structure and composition affected the properties of binary mixtures of n-hexadecane and n-alkylbenzenes. 40,41 The properties tested in the current study are those that affect the transport and injection of a fuel into a combustion chamber (bulk modulus, density, viscosity). Both the American Society for Testing and Materials and the military specify ranges of values for density and viscosity.…”

“…Molecular dynamics (MD) simulations can be used to predict properties of hydrocarbon mixtures; in addition, they can model high temperatures and pressures and can give an atomic-level picture of fuel behavior that is difficult or impossible to obtain experimentally. Previous studies calculated the densities, bulk moduli, and heats of vaporization of 12 pure hydrocarbons, as well as densities and bulk moduli of multicomponent fuel surrogates containing up to 12 components, and showed how molecular structure and composition affected the properties of binary mixtures of n -hexadecane and n -alkylbenzenes. , …”

Viscoelasticity of Short Polymer Liquids from Atomistic Simulations

“…The search for alternative fluid mixtures for internal combustion engines is another field where molecular simulations could play a role, if both fluid viscosity and chemical reaction can be properly accounted for. 51 We do not expect chemical reactions such as bond scission to occur around room temperature and atmospheric pressure, as in our simulations. Nonetheless, as a starting point, it seems useful to test the viscosities of these reactive models 43,44 even under ordinary conditions.…”

Probing the high-pressure viscosity of hydrocarbon mixtures using molecular dynamics simulations

“…A common theme in the successful entries was the use of an allatom force field, which has been shown previously to be important for accurate viscosity predictions of long-chain hydrocarbons. 28 Several other studies have used MD simulations to investigate the viscosity of hydrocarbon mixtures; [37][38][39][40][41][42][43][44][45][46] however, only a few of these have calculated viscosities at elevated pressure. In a recent study, Verma et al 42 studied the viscosity of n-octane + n-dodecane and n-hexadecane + ndecylbenzene binary mixtures up to 200 MPa using the Green-Kubo method.…”