Cetane ain’t octane

Dr. Jason Gislason from ConocoPhillips points out that one of the problems with aircraft diesel engines using Jet A is that there is not a minimum cetane rating in the ASTM D-1655 specification (the spec for all Jet A sold in the U.S.).

Dr. Jason Gislason from ConocoPhillips points out that one of the problems with aircraft diesel engines using Jet A is that there is not a minimum cetane rating in the ASTM D-1655 specification (the spec for all Jet A sold in the U.S.).

The cetane rating “”is a measurement of the ignition quality of the fuel and influences combustion roughness,”" according to the ASTM D-975 spec for diesel fuel oils. To measure the cetane rating, fuel is run in a single cylinder test engine with a variable volume pre-combustion chamber. The engine is a diesel version of a CFR test engine, which is used for octane testing of gasoline. In the test, the engine is run at a constant rpm and load. Then the pre-combustion chamber volume is decreased while the number of crankshaft degrees between injection and auto-ignition is measured. When the angle is at the specified number of degrees, the volume of the pre-combustion chamber is then correlated to a series of reference fuels to determine the cetane number of the fuel.

The cetane number is almost the opposite of the octane number. This is very logical as in a diesel one depends on the fuel to auto ignite (the proper name for diesel engines is compression ignition or CI engines). By comparison, in a gasoline or spark ignition (SI) engine, one depends on the spark plug to start the fire. The octane number is the measure of a fuel’s ability to resist auto ignition in the far reaches of the combustion chamber.

In an SI engine, if you use a fuel with an octane rating less than that needed by the engine, you will experience knock. This knock can increase the temperature in the combustion chamber enough to lead to pre-ignition, which can destroy an engine in a very short time. In a CI engine, operation of the engine on a fuel with too low a cetane rating usually will not harm the engine.

The main problem associated with a low cetane number is hard starting. This is especially true for small bore CI engines at very low ambient temperatures. (The Centurion diesel engines would be considered small bore.) The other concern is that inadequate cetane in a CI engine will effectively retard the injection timing, which could result in a small reduction in power and an increase in the noise and roughness level of the engine.

The cetane rating test was developed and the specification first applied to diesel fuels for submarines. It was determined that fuels with higher cetane ratings ran quieter, which was a very critical parameter in a sub.

Currently the cetane spec on diesel fuels is a minimum of 40. There is no minimum spec for Jet A.

However, most samples of straight run distillate Jet A I have seen had a cetane rating over 40. The concern is that there is a significant amount of non-straight run distillate Jet A fuel being sold in the U.S. These fuels are perfectly legal and meet all of the requirements of the ASTM D 1655 specification. On the limited amount of testing that I have run, many of these fuels had cetane ratings well below 40.

One of the concerns pointed out by Dr. Gislason is that the two diesel engines currently available were designed and tested in Europe. This is not a problem, except that almost all of the Jet A fuels available in Europe are straight run distillates, which should have cetane ratings well above 40.

You probably think that the solution here is simple ? put a minimum cetane limit in the Jet A spec. It sounds simple, but may be almost impossible to do. Part of the problem is that refiners are against any unnecessary tests. But the biggest problem would be for the refiner who has several million galllons of Jet A that doesn’t meet the cetane spec, but would work fine in a jet engine. Can the product still be sold to the airlines? Is it off-spec? And how will the products that do not meet the cetane spec be differentiated from the products that do meet the spec?

So what should be done? If the aircraft diesel engine manufacturers have not run a series of tests on some very low cetane rating Jet A fuels, they should consider doing so. They should test for low temperature starting, noise level, engine roughness and max power at a range of temperatures and density/altitudes. They also may want to consider running some long-term durability tests on non-conventional Jet A fuels and thoroughly check out the engine and prop performance and dependability following extended real world service.

In addition, once the number of aero diesel engines becomes significant, it may become feasible to provide a calculated cetane index for each batch purchased by an airport.

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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