A Diesel Conundrum

Is Jet-A OK?


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.

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