Ignition and Emission Characteristics of Surrogate and Practical Jet Fuels

Aircrafts have been identified as a significant source of particulate matter (PM) emissions, which have been associated with adverse health effects and decreased ambient air quality. Recent published studies have shown that synthetic fuels, such as iso-paraffinic Fischer-Tropsch (F-T) fuels, can significantly reduce PM emissions compared to operation with petroleum-derived fuels. It is believed that the absence of aromatics in synthetic fuels slows the molecular growth to higher ringed polycyclic aromatic hydrocarbons (PAHs), which ultimately leads to lower soot emissions. However, it is not known if differences in chemical composition between synthetic and conventional (JP-8) fuel also change the ignition and combustion characteristics, which likely impact high altitude relight and combustor lean blowout (LBO) limits. This study was conducted to investigate how differences in fuel composition can impact the chemical ignition and emissions characteristics of a fuel and to help deconvolute the effect of physical and chemical processes on combustion efficiency. Only the chemical ignition delay was considered as the fuels were prevaporized and premixed prior to ignition. The fuels investigated in this study were a F-T processed synthetic jet fuel (synjet), a conventional JP-8, 2-methylheptane (surrogate synjet fuel), and a mixture of 80% heptane and 20% toluene (surrogate JP-8). The experiments were conducted using a modified single-pulse reflected shock tube instrumented to measure fuel ignition delay times and PAH and soot yields. The shock tube results show that both the actual and surrogate synjet fuels produced lower soot and PAH emissions than the actual and surrogate JP-8 fuels at temperatures of interest. These results are consistent with previous turbine engine test results with the actual fuels. It was observed, however, that, for the conditions tested, both surrogate and actual synjet and JP-8 fuels evaluated had similar ignition delays, which may imply that there is no correlation between soot emissions and chemical ignition delay. However, these results may be controlled by the test conditions, and accordingly, future tests at different conditions (e.g., higher equivalence ratios, pressures, lower pre-ignition temperatures) are warranted.