Jet A vs. #2 diesel: Which is better?

I received an interesting note from Patrick Puckett who is considering replacing the 100LL engine on his aircraft with a new aviation diesel engine. He had several questions concerning the performance of Jet A vs. #2 diesel fuel.

I received an interesting note from Patrick Puckett who is considering replacing the 100LL engine on his aircraft with a new aviation diesel engine. He had several questions concerning the performance of Jet A vs. #2 diesel fuel.

My answer is almost exactly the same as I have given to those who wish to use auto gas in aircraft engines: It is technically possible to use #2 diesel in an aviation diesel engine, but there are some legal and technical limitations that need to be followed very closely.

Jet A is certified for most of the aviation diesel engines because it is available at most airports, it has a very definite specification, and it will meet all of the low-temperature requirements for almost every location and altitude.

The real concern about using #2 diesel in an aircraft is low-temperature flow and filter plugging. Patrick suggested that he could just use an “”off the shelf additive to prevent gelling.”” There are several problems here. The biggest? These additives seem to help reduce the pouring temperature of diesel, but do not affect the cloud point significantly. There is also the question of how much additive is needed. These additives vary in effectiveness depending on the composition of the original fuel. This means that even with an additive, #2 diesel could cause filter plugging and possible fuel starvation under high-altitude, low-temperature operation.

Being normally clear (vs. dark to opaque for #2 diesel), Jet A is much easier to inspect and check at refueling facilities and at the wing. And water separation filters work well with the product.

Patrick also inquired if injection pumps will work on Jet A and if they are different from ground based units. Injection pumps in aircraft are designed to operate on the low sulfur levels of current Jet A fuels. The lower the level of sulfur in a fuel, the poorer the lubricating properties. Most modern ground-based units are also designed for the lower sulfur fuels; however, some of the older engines use the fuel to lubricate the pump and these units have poor durability on low sulfur fuels.

It is true that there are more BTUs in #2 diesel than in Jet A. This does not have a large effect on power, but it will have a small effect on fuel economy. Engines normally meter fuel based on weight, not on volume. I would expect a small ? maybe 5% ? improvement in fuel economy if one switched from Jet A to #2 diesel and no measurable change in power output.

There is also the effect of the fuels on long term engine cleanliness, soot formation and other differences.

The bottom line on Jet A is that it is available at most airports, it has a documented cleanliness track record, the pour point is always below -40?F, and the engine is designed to operate on it. By comparison, #2 diesel fuel will need to be handled by the pilot, the cleanliness requirements are different, the pour point could be as high as 20?F and the engine is not designed for it.

And then there are the liability and legal issues. Until someone comes up with an STC for using #2 diesel fuel in diesel aircraft engines, I think you will need to stay with Jet A. If and when an STC is issued for the use of #2 diesel in an aircraft, there will be guidelines and limitations that will need to be followed very closely.

Ben Visser is an aviation fuels and lubricants expert who spent 33 years with Shell Oil. He has been a private pilot since 1985. You can contact him at [email protected].