In my January column, “All 100 octane fuels are not equal,” I explained some of the problems with assuming that an octane rating is an absolute number. One of the comments from that column concerned cetane rating for compression ignition — diesel cycle — engines.
In general, octane ratings and cetane ratings are more or less opposite. In other words, the higher the octane rating the lower the cetane rating and vice versa.
Although it is not a straight line relationship, it does follow a general trend. In octane rating we are measuring a fuel’s ability to limit auto-ignition, while the cetane rating is a measurement of a fuel’s quickness or ease to auto-ignite.
In the octane rating we vary the compression ratio by raising or lowering the cylinder and head assembly relative to the crankshaft.
The cetane engine test is run in a single cylinder diesel test engine. The cetane rating engine has a variable volume pre-combustion chamber to raise or lower the compression ratio. The main measurement in the test is the crank angle degrees between the activation of the injector and actual pressure rise from combustion.
The first step is to chart the volume of the pre-cup versus that for a series of reference fuels for the specified crank angle degrees for the test. The test fuel is then run and the cetane rating is taken from the reference graph.
The bottom line is that a higher cetane rating fuel advances the ignition timing a few degrees.
So why is it important?
The rating system was developed because of diesel-powered submarines. It was found that higher cetane rating fuels ran quieter in subs and that was important. The quieter engines were easier on the men in the sub and it reduced the detectability of the sub.
So what difference does it mean in today’s world? Well, not that much.
In the United States, most diesel fuel has a minimum cetane rating of 40. That is not a hard spec, it is just the norm.
Therefore, most of the engines on the market have injection timing optimized for about 40 cetane, so a higher rating does not make that much difference in performance.
Critical Parameters for Diesel Fuels
So what is the most critical parameter for fuel in aircraft diesel engines?
The three primary fuels available are Jet A, #2 diesel, and kerosene/#1 diesel fuel. These fuels are all straight run distillates and are chemically similar. They come off the distillation column and are then “cleaned” up with hydro treating to reduce sulfur content and may go through other processes.
Jet A has more specifications on it like the Jet Fuel Thermal Oxidation test (JFTOT), so please do not think you can run diesel fuel in a jet aircraft.
I know that most diesel cycle aircraft engines are qualified only on Jet A, but I also know there are people out there who will run diesel fuel based on availability.
What are the most important criteria for fuel if it is not cetane?
The main concern is low temperature flow characteristics. There are two important low temperature characteristics for these fuels, the pour point and the cloud point. These are usually at different temperatures.
When a distillate is cooled down, the first thing that happens is that the long chain wax molecules solidify. This is the cloud point.
If you continue to cool the fuel, the wax molecules will coagulate to form a solid and block flow. This is the pour point.
Both temperatures are important because once you get to the cloud point, the engine can still run, except for the fuel filter, which will plug if the filter is not heated.
Jet A has a pour point of -40°F, but the cloud point can be higher. #1 diesel and kerosene usually have a pour point of -20°F more or less, depending on location. The pour point of #2 diesel depends on where you buy it, with higher values in the summer and in southern climates.
The cloud point of these fuels is anyone’s guess.
A final point is that over-the-counter diesel fuel additives may lower the pour point of a fuel, but usually have little or no effect on the cloud point.
Another concern is that with biodiesel the wax acts very different. With conventional fuels if a filter plugs with wax, once it gets to a warmer temperature, the wax goes back into suspension. With biodiesels, the wax usually does not go back into solution. You would need to replace the filter to get the engine running.
Why is all of this important to aviation?
First, it is important where you mount the filter in a diesel cycle engine. It needs to be in a heated area.
Second, even if you use Jet A, which has no cloud point spec, you could clog a filter at altitude without filter heat.
Third, if a pilot uses regular diesel fuel or even #1, he can get into problems at altitude.
And, finally, the use of biodiesel could cause a problem since a plugged filter will choke off the engine quickly.