Jet-A versus Diesel fuel

The GAfuels Blog is written by two private pilots concerned about the future availability of fuels for piston-engine aircraft: Dean Billing, Sisters, Ore., an expert on autogas and ethanol, and Kent Misegades, Cary, N.C., an aerospace engineer, aviation sales rep for U-Fuel, and president of EAA1114.

Recently, your bloggers were discussing the use of diesel fuel at airports with Mike Webb, founder of U-Fuel and former owner of the FBO Oshkosh Aero. Mike had mentioned the common practice at airports of using waste Jet-A fuel from daily tank sumpings (knowing at airports as “thiefing”) in ground equipment powered by diesel engines. But are Jet-A and diesel fuel really the same? GAFuels blogger Dean Billing says no, but its use in older diesel vehicles is probably OK. This lead to a broader discussion of the use of Jet-A in aircraft compression ignition engines.

From Dean Billing: If one does a little sleuthing on the Internet about the differences between Jet-A and diesel fuel, especially if one is considering using Jet-A in an aircraft diesel engine or using Jet-A in that airport service vehicle, one will soon find out that it is not a particularly wise idea. There are some major differences between Jet-A and diesel:

1. Jet-A is a relatively high sulfur fuel, diesel is low sulfur and EPA requirements are getting more stringent about sulfur in diesel every year. After all, we are now in the Ultra-Low Sulfur diesel era. Using Jet-A in your truck will be highly frowned on by your EPA inspector and could lead to fines.

2. Jet-A is “dry.” Diesel is made in such a way, or additives are mixed in, to lubricate the injector system of a diesel engine.

3. Jet-A is closer to kerosene and Diesel #1. Most modern diesel engines specify Diesel #2.

4. The viscosity specifications for the two fuels is different. Jet-A and Diesel #1 tend towards lower viscosities than Diesel #2. Lower lubricity is likely as the viscosity decreases. This may not cause catastrophic instant damage, but it may cause long-term wear of pumps, injectors, etc.

5. Cetane number. Diesel #2 is manufactured with a required cetane number. ASTM D975 specifies a minimum of 40. Most Diesel #2 in the U.S. is 42-45. The ASTM specification for Jet-A, ASTM D1655, has NO minimum cetane rating, because cetane pertains to compression ignition engines, and has no meaning in turbine engines. Using a fuel with too low a cetane number in a diesel engine will just result in a rough-running or not running at all engine. This is why Exxon required pilots of Diamond aircraft with Thielert diesel engines to sign a liability release to fill up with Jet-A at Exxon airport supplier, as described in this article.

One can learn more than they ever wanted to know about Jet-A and diesel at Wikipedia on Jet-A, Diesel fuel and cetane numbers.

This article explains the differences between Jet-A and diesel on a diesel engine forum. Scroll down to the post by Blitzkrieg who references information from Chevron. Probably more than you ever wanted to know about heavy distillates, as they are known in the trade.

Nobody has ever claimed using Jet-A in a diesel engine wouldn’t work, not even Exxon. In fact it worked pretty well as far as I know in the Diamond aircraft with diesel engines. Never heard of a problem. The problem was liability since all motor diesel in the U.S. has a required cetane rating in the ASTM specification and all diesel engines have a minimum certain requirement. But that requirement does not indicate that the engine will not work with a lower rating, only that the manufacturer will not warrant the engine if you use a fuel with a lower cetane rating. Jet-A has no cetane rating because it is not necessary for a turbine engine, thus Exxon was very concerned about liability with the spread of the Thielert engines. Actually Jet-A has a widely varying cetane rating, it is just not controlled nor guaranteed. Might be that the Jet-A you get has a cetane rating in excess of what your engine is specified for and it may vary from batch to batch.

From Mike Webb: Dean, thanks for the great explanation. In the North where it cold, people burn Diesel #1 to keep the engines running. Also, most independent FBOs have some ramp equipment that is not new — in fact, as old as the airplanes they are servicing, 20 to 40 years. I am not saying Jet-A is good for new diesel engines. I am just saying it is “free” and you do not have to “pay for a disposal of a hazardous liquid” if you recycle it through an old tug. And, that is what people do. Also, many FBOs use waste oil taken when they change the oil in aircraft engines to heat their building. I believe you can throw a little Jet A in there too to thin things out. The above comments are a reflection of how things really work in a profitable FBO and not necessarily the opinions of the writer. However, the writer ran a profitable FBO …