The chemical origin of octane sensitivity in gasoline fuels containing nitroalkanes

Experimental octane measurements are presented for a standard gasoline to which has been added various quantities of nitromethane, nitroethane and 1-nitropropane. The addition of nitroalkanes was found to suppress the Motor Octane Number to a much greater extent than the Research Octane Number. In other words addition of nitroalkanes increases the octane sensitivity of gasoline. Density Functional Theory was used to model the equilibrium thermodynamics and the barrier heights for reactions leading to the break-up of nitroethane. These results were used to develop a chemical kinetic scheme for nitroalkanes combined with a surrogate gasoline (for which a mechanism has been developed previously). Finally the chemical kinetic simulations were combined with a quasi-dimensional engine model in order to predict autoignition in octane rating tests. Our results suggest that the chemical origin of octane sensitivity in gasoline/nitroalkane blends cannot be fully explained on the conventional basis of the extent to which NTC behaviour is absent. Instead we have shown that the contribution of the two pathways leading to autoignition in gasoline containing nitroalkanes becomes much more significant under the more severe conditions of the Motor Octane method than the Research Octane method.