Jet A and diesel engines

In a previous column I talked about going to a recent ASTM meeting and the progress being made on unleaded avgas. At this same meeting, there also was a lot of discussions on the qualification of diesel cycle aircraft engines on Jet A.

There were two main areas of discussion: The first was to establish a recommendation on how to certify these engines; while the second was a report that several oil companies are telling their dealers not to sell Jet A to aircraft owners with diesel engines because of liability concerns.

Those concerns were three-fold: Cetane rating, lubricity and cold temperature flow.

Lubricity is a moot point since the ultra-low sulfur diesel fuels being sold today have significantly lower lubricity properties than almost all Jet A fuels.

The cold flow properties are a concern since most piston planes are designed to operate on a -75°F fuel and Jet A has a -40°F spec. A review of the data showed that there were occasions of aircraft experiencing outside air temperatures of less than -50°F. However, since most diesel engines recirculate fuel to remove heat from the injectors, all agreed that this concern could be addressed with hardware modifications to the aircraft.

The main sticking point is cetane rating. The concern is that the oil companies are against allowing a cetane spec to be added to the D-1655 spec for Jet A.

I can see their point of view, not only because of the cost of running the test, but also the problem of what to do if a batch does not pass. If a refiner had a 1 million gallon batch of Jet A that passed every test except cetane, it would have a big problem. It could not add cetane improver because it is not approved for aircraft use, so it would have a fuel that would work well for 99.9999% of customers, but the company could not sell any of it. So, a cetane spec is not going to happen.

The solution was to find the lowest possible cetane fuel available and test the engines with this fuel. The lowest known cetane fuel to meet the Jet A spec is a synthesized fuel from South Africa that has a rating of 22. The final agreement was that all of the aviation diesel engine manufacturers will need to demonstrate acceptable engine in-flight restartablity and adequate power output on this fuel as part of the certification process.

A typical diesel fuel has a cetane rating of about 40-45. This means that an aircraft diesel engine must run on a fuel with half of the normal cetane rating. Fortunately, the relationship of cetane to engine performance of a diesel engine is not the same as octane is to a spark ignition engine.

If the octane is not adequate in a spark ignition engine, the engine will knock, which can destroy a piston in short order. If the cetane is not adequate in a diesel engine, the engine will continue to operate with little or no problems. In fact, the only problems associated with low cetane are cold starting and slightly retarded timing. If you have a diesel powered aircraft and the engine will not start cold, it is not a safety concern. The only concern is in in-flight restarts. This should not be a problem, especially with a liquid-cooled engine that will retain its temperature even if the engine is off. As far as the loss of power concerns, that will have to be demonstrated during testing.

The bottom line is that we were able to reach an agreement that answered all of the concerns and is workable. It was a good demonstration of the ASTM process, which can be a little slow at times — but it works well because it allows all concerned parties a voice.

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 Visser@GeneralAviationNews.com.

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