The Ignition Behavior of a Coal to Liquid Fischer-Tropsch Jet Fuel in a Military Relevant Single Cylinder Diesel Engine

The U.S. Army currently uses JP-8 for global operations according to the "one fuel forward policy" that was enacted almost twenty years ago in order to help reduce the logistics burden of supplying a variety of fuels for given Department of Defense vehicle and base applications. One particular challenge with using global JP-8 is the lack of or too broad a range of specified combustion and fuel system affecting properties including ignition quality, high temperature viscosity, and lubricity. In addition to these challenges, the JP-8 fuel specification currently allows the use of blending with certain types of synthetic jet fuels up to 50% by volume. This blended fuel also doesn't include an ignition quality or high temperature viscosity specification, but does include a lubricity specification that is much less restrictive than DF-2. One particular currently available synthetic fuel that could be used as a blending agent with JP-8 is produced by Sasol for use in South Africa and is a Fischer-Tropsch's (FT) coal-to-liquid (CTL) aviation fuel that has poor ignition quality (25 cetane number and 52 cetane index) and is more volatile than JP-8. A series of single-cylinder experiments were conducted using the Sasol jet fuel in order to study its ignition behavior over a range of relevant military diesel engine operating conditions. In particular, mean ignition combustion chamber densities of 18, 24, and 30 kg/m₃ were explored over a mean ignition temperature range of 780 to 1000 K. Additional evaluation of this fuel and a 50-50 blend of this fuel with JP-8 was conducted by performing modal testing at high, medium, and low load conditions representative of a military medium-duty truck engine. A comparison of the ignition behavior of this fuel to higher ignition quality fuels showed that the Sasol jet fuel behaved similar (within 20%) to a higher cetane number fuel over a considerable portion of the mean ignition density and temperature range, but also exhibited poor ignition quality behavior in various lower mean ignition temperature and density regimes. Additionally, blending the Sasol jet fuel in a 50-50 volumetric proportion with a typical JP-8 significantly impacted the ignition and heat release profile behavior at medium and light load operating conditions alleviating part of the poor ignition quality concern of using unblended Sasol jet fuel. These results are highly sensitive to the initial injection rate and thus are a strong function of the fuel system which was a hydraulically actuated electronically controlled unit system in this study that inherently has a more gradual initial injection rate in comparison to high pressure common rail systems.