As availability of Avgas decreases, alternative power plants and fuels are becoming more affordable and reasonable to consider. Compression ignition (AKA: diesel or CI) aircraft power plants are a viable alternative and several companies have developed quality engines to fill the need.
Advantages of diesel
Proponents of diesel engines point out the many advantages of a CI power plant over a spark ignition (SI) engine. For example, diesel fuel is safer to handle because of its lower volatility than gasoline. Also, since there is no need for an ignition system in a CI engine, there is less electronic interference with navigation and communication systems, not to mention less weight because of unnecessary components such as magnetos, spark plugs, etc. Some diesel power plants use the two-cycle system, which eliminates even more weight due to fewer mechanical components. With fewer engine components, there are fewer maintenance procedures and simpler operational handling. In some cases there are lower operating costs due to lower fuel consumption, and possibly longer range. In addition, with the liquid cooling systems used by many CI engines, there is a reduced variance in engine temperature, thus avoiding overcooling during descents. As with any new design or system, there is bound to be controversy.
Debate about fuels
A debate has arisen regarding the acceptability of using Jet-A fuel in these CI aircraft power plants. Because Jet-A fuel is readily available, and because of its similarity to diesel fuel, the question has become just how efficient and safe Jet-A fuel is if used in CI power plants instead of standard diesel fuel like that used in CI engines that power ground equipment, trucks, and generators.
The debate was ratcheted up a notch last November when ExxonMobil sent a directive to its distributors saying in part: “ExxonMobil Aviation Global Technical Group has made the technical decision that ExxonMobil does not support or endorse the supply of jet fuel to aircraft powered by diesel engines.” The letter did not prohibit dealers from selling Jet-A fuel to diesel engine customers. Included with the letter was an indemnity agreement that each dealer was requested to sign and return, agreeing to either not sell Jet-A fuel to diesel engine-powered aircraft customers or to explain the situation to said customers and have them sign a waiver of responsibility should anything happen related to fuel delivery during engine operation. The letter has generated many responses from engine and aircraft manufacturers, designers, and owners of CI engine aircraft.
A diesel refresher
Mr. Diesel’s original design many decades ago included the familiar crankshaft, cylinders, and piston assemblies, drawing air into a combustion chamber which, when mixed with fuel and ignited, created a strong downward force by the piston and rotary force on the crankshaft. However, unlike spark ignition (SI) engines, which mix the fuel and air together, then ignite the compressed mixture with a spark plug, the CI design compresses only air in the combustion chamber. Because the compression ratio of a CI engine is so much higher than an SI engine, creating much more heat, the fuel needs no help for ignition. It ignites almost immediately upon entering the combustion chamber. In order to avoid the damaging detonation during ignition that gasoline would cause in a CI engine, less volatile diesel fuel is used, creating a smoother burn. The “anti-knock” ratings of a fuel are the result of extensive testing at various temperatures. Diesel fuel is given a cetane rating that describes its anti-knock characteristics, just as gasoline is given an octane rating.
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.”
Peter Mauer, CEO of Diamond Aircraft (which uses Thielert (TAE) diesel engines), responds that the technical concerns expressed specifically by fuel supplier ExxonMobil are, “valid general concerns for operation of diesel engine powered aircraft. In the case of TAE and Diamond, all listed reasons, specifically ignition quality, freezing point, and lubricity, have been addressed as part of the engine and aircraft certification process and approved by the responsible airworthiness authorities.”
The FAA says: “It is anticipated that CI engines will use jet fuels, such as Jet-A, which are produced in accordance with the ASTM International Specification D1655, or automotive diesel fuels produced in accordance with automotive fuel specifications.”
ExxonMobil’s bottom line, as of November, is: “No fueling of diesel engine aircraft with Jet Fuel may be performed without a valid indemnity agreement signed by the customer in place.”
So the debate continues. As the aviation industry shifts to more efficient and more “green” power plant and fuel alternatives in the coming years, debates like these will become more and more prevalent. They are necessary since the voicing of differing viewpoints is an effective way to ensure that we eventually end up with safer, more cost effective, environmentally friendly power plants and aircraft.
Scott Fisher is an A&P and received his training at Dakota Aero Tech in Fargo, ND.