Di-n-buthylether, n-octanol, and n-octane as fuel candidates for diesel engine combustion

In this study, three different C-8 fuels, namely n-octanol, di-n-buthylether (DnBE), and n-octane, are investigated with regard to diesel engine combustion to assess and analyze the effects of fuel structure. DnBE and n-octanol are isomers, i.e. they have the same elementary composition (C8H18O), but a different structure and very different chemical and physical properties. The ignition behavior of these fuels in engine-like configurations is studied in terms of chemical modeling using recently published detailed kinetic mechanisms. The reduced and enhanced homogeneous ignition propensity of the alcohol and ether functional groups, respectively, are explained by reaction pathway analysis. However, the difference in low temperature reactivity of n-octanol and n-octane is relatively small compared to the difference in their cetane ratings. To study the engine behavior of these fuels, experiments and simulations were performed. A single cylinder diesel engine was operated with DnBE, n-octanol, and n-octane as single component fuels. For all three fuels, very low soot emissions at nitrogen oxide emissions within the Euro 6 regulation limits can be reached. Combustion and pollutant formation in the diesel engine are computed using the representative interactive flamelet approach. Results from the numerical simulations show good agreement with the experimental diesel engine tests. A subsequent analysis of the diesel engine simulation results shows that the differences in engine operation and carbon monoxide emissions can be attributed to the different ignitability, reflected by the different cetane ratings of the fuels. The substantially lower cetane rating of n-octanol compared to n-octane is explained by analysis of engine simulations and homogeneous reactor calculations. The effect of the different physical fuel properties does not play a significant role in the operation range studied. However, it is found that one of the main reasons for the large differences in cetane numbers between n-octane and n-octanol is the different stoichiometric mixture fraction, which leads in diesel engines to lower temperatures at the ignition location and hence to longer ignition delay times. (C) 2015 Published by Elsevier Inc. on behalf of The Combustion Institute.