Fuel Sensitivity Effects on Ignition, Combustion and PAH Emmissions in Diesel Engine Conditions

The amount of non-paraffinic components is directly associated with fuel sensitivity (S), which is an important consideration for using low-octane and low-cetane fuels in direct injection engines. In this research we examine the effects of S and RON (research octane number) on the ignition and combustion behavior of naphtha fuel surrogates in homogeneous mixtures and diesel sprays. Two binary blends (PRF70 and PRF80) and four ternary blends (TPRF70-a, TPRF70-b, TPRF80-a, TPRF80-b), with varying amounts of iso-octane n-heptane, and toluene, are considered. Simulations are performed using a reaction mechanism with 109 species and 543 reactions. The mechanism is validated against the shock tube and rapid compression machine ignition data, and non-reacting spray data. Ignition simulations in homogeneous mixtures are performed using CHEMKIN-Pro for a temperature range of 625-1250K, equivalence range of ɸ=0.5-2.0, and pressure of 55 bar. The study is then extended to examine the transient ignition and flame structure in liquid fuel sprays in a constant-volume combustion vessel using the CFD software CONVERGE. Results indicate that the temperature dependence of ignition characteristics in both homogeneous mixture and spray is strongly influenced by fuel sensitivity. In particular, it affects the NTC behavior and temperature dependence of the 1st and 2nd stage ignition processes. The ignition kernel structure in sprays is also strongly modified by fuel sensitivity, as the ignition kernel in ternary sprays involves rich mixtures, while that in binary sprays contains near stoichiometric mixtures, Consequently, the spray flame structure is modified by sensitivity. While the spray flame is characterized by partially premixed combustion involving a lean premixed zone (LPZ), a rich premixed zone (RPZ), and a nonpremixed zone (NPZ), the effect of sensitivity is to enhance the relative contribution of RPZ compared to those of NPZ and RPZ. In addition, due to enhanced ignitability, the flame in ternary sprays is located closer to the injector compared to that in binary sprays. The effect of higher RON is to increase the ignition delay time. Consequently, the ignition kernel involves relatively leaner mixtures, and the flame structure is characterized by increased contributions from LPZ compared to RPZ, and flame liftoff length is increased. Increasing the initial reactor temperature has the opposite effect. A sensitivity analysis is performed to identify reactions that characterize the reduced and enhanced ignitability of ternary blends at low and high temperatures, respectively. In this work, we also examine the effects of fuel sensitivity on PAHs emissions in partially premixed counterflow flames burning one binary blend (PRF70) and two ternary blends fuel (TPRF70-a TPRF70-b). Four different mechanisms are evaluated for their predictive capabilities for PAHs emission in these flames. The mechanisms include the Park et al. reduced mechanism, CRECK mechanism, Wang et al. mechanism, and Cai and Pitsch mechanism. Counterflow flames are simulated using CHEMKIN-Pro, and these four mechanisms are evaluated by comparing their predictions of PAH species against the PAH LIF signals for fuel blends containing 10% and 20% toluene. The PAH species considered in this evaluation include A1 (benzene, C6H6), A2 (naphthalene, C10H8), A3 (phenanthrene, C14H10), A4 (pyrene, C16H10), and A5 (Benzo[e]pyrene, C20H12) depending upon the mechanism. Park et al. mechanism was found to mimic the trend of increase in PAH with increase of iso-octane in the fuel. For all three fuels, the global flame structures are found to be similar, characterized by a rich premixed zone (RPZ) on the fuel side and a non-premixed zone (NPZ) on the oxidizer side. Most of PAHs are formed near RPZ. The fuel with high sensitivity shows higher PAHs emission, which is due to the presence of toluene (C6H5CH3) that increases the production of benzene (A1), an important precursor for the formation of larger PAHs. The detailed path and sensitivity analysis were carried out to identify the dominant reactions for the formation of A1 and A4 (pyrene). Toluene reacting with hydrogen (H) is observed to be the dominant reaction for the formation A1, while benzyl radical (C6H5CH2) reacting with indenyl radical (C9H7) is the dominant reaction for the formation of A4 in ternary blends. In addition, the effects of equivalence ratio (level of fuel-air premixing) and low temperature combustion (achieved by using N2 dilution of the fuel stream) on PAH emissions are discussed.