The Combustion and Ignition Characteristics of Varying Blend Ratios of JP-8 and a Coal to Liquid Fischer-Tropsch Jet Fuel in a Military Relevant Single Cylinder Diesel Engine 2015-01-9073

The U.S. Army currently uses JP-8 for global operations according to the ‘one fuel forward policy’ in order to reduce the logistics burden of supplying a variety of fuels for given Department of Defense ground vehicle applications. One particular challenge with using global JP-8 is the lack of or too broad a range of specified combustion affecting properties including ignition quality, high temperature viscosity, and density. In particular, the ignition quality of JP-8 has dramatically varied throughout the past decade on a global basis covering a range of 29 to 70 cetane index. This key combustion affecting parameter was explored in this study by evaluating a synthesized low ignition quality jet fuel blended in 25% volumetric proportions with JP-8 to effectively cover a cetane number range of 25 to 45 in a single cylinder diesel engine operated at various light, medium, and high load operating conditions. The low ignition quality fuel was a Fischer-Tropsch Synthesized, coal-to-liquid (CTL) paraffinic kerosene (FT-SPK) which exhibited low temperature chemistry behavior at light load operating conditions including unstable combustion at lower engine speeds. To better understand this latter combustion behavior, supplemental single cylinder experiments were conducted to explore the impact of cylinder charge density on the ignition delay of the CTL FT-SPK covering mean charge densities of 19, 25, and 30 kg/m³ and a temperature range of 750 K to 950 K. Such experiments revealed a peninsula of excessive ignition delay at mean ignition pressures of less than 55 bar and mean ignition temperatures less than 800 K that were representative of the unstable lower load operating points. Blending the CTL FT-SPK with an ‘average’ JP-8 increased the low temperature activity and yielded stable combustion at lower, light load engine speeds. In particular, the 50-50 blend ratio alleviated any ignition concerns with the CTL FT-SPK at the test conditions included in this study while the 25-75 blend ratio (JP-8/CTL FT-SPK) addressed a major portion of such ignition concerns at full, medium, and certain light load operating conditions. These results are highly sensitive to the initial injection rate. The main purpose of this submission is to explore the effect of blending JP-8 with a known poor ignition quality SPK to assess any potential performance impacts on military relevant diesel engines.