Diesel fuel vs. Jet-A
While the processes for developing the two fuels at the refinery are very similar, there are some differences between the two fuels. Diesel fuel is distilled in the cracking tower at between 200 C and 350 C. There are two main types: Diesel #1, which is closer in characteristics to Jet-A, and Diesel #2 (more common, and also called “Road Diesel”), the familiar fuel for trucks, tractors, and stationary generators. Jet-A is created at between 150 C and 275 C (somewhere between diesel fuel and gasoline). This translates into Jet-A having about 5 percent less energy per measurable unit (such as gallons) than diesel fuel, though that number is insignificant when it comes to running either fuel in a diesel engine. While diesel fuel certainly has the potential of being more efficient and environmentally friendly than Avgas, using Jet-A fuel in its place presents some questions regarding performance characteristics. Proponents of using Jet-A point out the more rigorous testing, delivery, and storage methods involved with Jet-A, thus creating a safer fuel to use than standard diesel. They also point out that diesel fuel’s propensity for attracting moisture would not be as much of a factor with Jet-A. But there are three main points of contention involved in the debate that aren’t so easily resolved: ignition quality, freezing point, and lubricity.
The testing temperatures of Jet-A and diesel fuel are very different from each other. Kerosene-based fuels such as Jet-A are not tested for or given a cetane rating, making it difficult to compare efficiency in combustion when ignition occurs in the engine. The FAA says, “using the appropriate cetane rated fuel in a diesel engine is critical to developing the appropriate power. It is not anticipated that commercial turbine fuels will be given a cetane rating; therefore, aircraft diesel engines will be approved and certificated to operate on a specific turbine fuel(s). An appropriate ELOS (equivalent level of safety) or special condition for each engine model must be determined on a case-by-case basis in accordance with Part 21 and Part 11.”
ExxonMobil’s position is that, “the minimum cetane for airworthiness has not been determined, in combination with the fact that cetane is not measured as part of the jet fuel specification, means that ExxonMobil cannot guarantee the ignition performance of the jet fuel it supplies and cannot know if the aircraft will be airworthy after fueling.”
Diane Doers, CEO of DeltaHawk, manufacturer of aircraft using CI engines, says: “Difficult cold weather starting is the primary effect of extraordinary low cetane on diesel engines, but once running, the engine will perform within normal engine limits. DeltaHawk engines were specifically designed to handle low-cetane levels.”
Diesel fuel has a freezing point of 0 C while Jet-A freezes at minus 47 C, which would seem to be an advantage. However, ExxonMobil points out that, “Piston-powered aircraft do not reach speeds that cause the heating of the fuel in the wing due to friction caused by airflow. It is possible that an aircraft powered by a diesel engine could reach altitudes where the fuel would begin to freeze in flight, particularly in cold climates where the ground temperature in the winter can be close to the jet fuel freezing point. Whilst the fuel may not freeze solid, other physical properties such as viscosity can change. This may have adverse effects on engine components such as fuel pumps and fuel injectors.”
DeltaHawk’s position: “Many jet-fueled aircraft which do not fly at ‘speeds that cause heating of the fuel in the wing due to friction caused by airflow’ have been operating safely for years (i.e. Beech King Air and turboprop conversions of piston aircraft).” Doers also points out the possibility of using additives and/or “built-in heated fuel recirculation and engine liquid cooling systems” to avoid the freeze point adversities.
Because kerosene-type fuels tend to absorb water more readily than gasoline, the U.S. military often uses anti-icing (or “thermal stability”) additives as a precaution in its jet aircraft. However, some of these additives have questionable “environmentally friendly” attributes and are (or soon will be) banned in some overseas facilities.
The lubricating properties of diesel fuel, with its higher viscosity, are much better than those of the kerosene-based Jet-A. Thus, any internal engine or fuel delivery parts (such as high pressure pumps) that rely on the lubricating properties of the fuel receive less lubrication with Jet-A than diesel fuel. ExxonMobil’s view is: “The fuel pump and injectors of current certified diesel aircraft engines are the components most susceptible to fuel lubricity and, to ensure reliability and safety in flight, should be tested in the same fashion as jet engine components.
Consequently, ExxonMobil Aviation cannot guarantee that the lubricity performance of the jet fuel that it supplies will meet the requirements of aviation diesel engines.”
Doers points out that lubricity is “only an issue if you use the fuel as a lubricant for your fuel pumps. DeltaHawk diesel engine fuel pumps and injectors do not. No. 1 Diesel will actually produce the worst case of lubricity due to its tighter sulfur specifications.”
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