Numerical study of a boosted HCCI engine fueled with n-butanol and isobutanol

Butanol, as a fuel in internal combustion engines, has many advantages over ethanol as biologically derived component of gasoline. The benefits include higher energy density, less water solubility, and better engine compatibility. This study assesses four different chemical kinetic mechanisms for two butanol isomers (n-butanol and isobutanol) in a single-zone engine model and compares the results to experiments conducted on a Homogeneous Charge Compression Ignition (HCCI) engine. The simulations, and experiments, spanned a range of intake pressures (1.0-1.8 bar) and equivalence ratios (0.3-0.4). As was seen in the experiments, the numerical model was able to reflect delayed combustion timings as the intake temperature was decreased. Additionally, the mechanisms were able to qualitatively differentiate the fuel sensitivity of the two isomers, with n-butanol being more reactive than isobutanol across all cases. However, for each mechanism, the computational results for required intake temperature at a given combustion timing were different than that observed in experiments by 20 to 40 °C. Furthermore, the rate of change of intake temperature versus combustion timing varied across the mechanisms, which were all different than that of the experiments. Heat release rates, generation of intermediate species (OH, HO2, H2O2, CH2O, and CO), and temperature/pressure histories corresponded well with combustion timing, though significant differences existed between the mechanisms. The results indicated that kinetic mechanisms for butanol isomers do not reliably capture the ignition behavior of HCCI engines, which points to an acute development need as low temperature combustion strategies become an increasingly important part of the pathway towards engines with higher efficiencies and lower emissions.