Various military organizations, namely the U.S. Armed Forces and NATO, fuel diesel engine equipment with JP-8 in order to simplify battlefield logistics. Ignition delay, closely related to cetane number, is an important characteristic of diesel fuel combustion. The diesel fuel specification has a requirement for minimum cetane number, but the specification for JP-8 has no such requirement. Because of this, the cetane number and ignition quality of JP-8 can vary widely; extended ignition delay can cause premature engine failure.
The objective of this work was to measure the ignition delay of JP-8 at ambient densities and temperatures commonly seen in diesel engines at fuel injection. A constant-volume combustion chamber was designed and built to perform the measurements. The chamber was a right cylinder made of high strength steel, with threaded, removable end caps that contained inlet and outlet valves, a mixing fan, and a centrally mounted common rail fuel injector. The chamber was filled with a lean acetylene and oxidizer mixture designed to leave 21% residual oxygen after combustion. The mixture was spark ignited, raising the cylinder temperature and pressure. The combustion products cooled via heat transfer to the cylinder walls, and at the desired chamber pressure and temperature, JP-8 was injected. Ignition delay was measured as the time between injection and a prescribed pressure rise. Premixed charge ignition reliably achieved peak pressures that were about 90% of the peak adiabatic flame pressure. The ignition delay measurement technique used was shown to be robust and repeatable.
JP-8 ignition delays were measured at ambient densities ranging from 7.27 kg/m3to 30 kg/m3, with ambient temperature at injection ranging from approximately 700 to 1200 K. The results show that temperature has a dominant effect on ignition delay, with ambient density playing a lesser role. For an ambient density of 7.27 kg/m3, ignition delay varied from 3100 μs at 820 K to 740 μs at 1030 K. For a density of 18 kg/m3, ignition delay varied from 1230 μs at 820 K to 570 μs at 1060 K. For a density of 30 kg/m3, ignition delay varied from 660 μs at 850 K to 310 μs at 1050 K.
The mixing fan in the constant-volume combustion chamber interior greatly aided premixed combustion, but did not appear to significantly affect ignition delay results. Decreasing the injection pressure to 800 bar slightly increased ignition delays, but rail pressures from 1000 to 1400 bar had no effect on ignition delay. Also, varying ambient oxygen from 17.1% to 20.4% did not have a strong effect on ignition delay. JP-8 with an estimated cetane number of 25 had longer ignition delay than DF-2 with a cetane number of 42.