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The posted number for LSD at 500ppm is the maximum allowed. The posted number for ULSD at 15ppm is also the maximum allowed.
As far the definition of cetane....it is a rating for diesel oil that indicates how easily the fuel ignites and how fast it will burn. The rating is obtained by measuring the time lapse between fuel injection and ignition. This is easily obtained in the refinery during the distillation process by controlling the different parameters of heat, cooling and pressure.
Kerosene (also known as jet fuel) is the next lightest component in the crude chain above diesel. Depending upon the market's demand for kerosene allows the refinery to set control points for diesel. They will pull as much diesel into the kerosene as possible in order to keep the supply up for the airlines and for processing economics. Rich's comment about low cetane and the specifications are mostly correct except it does not depend upon the type of crude being refined. However, by rereading the above definition of cetane it is very simple to increase cetane.....add a gallon of gasoline. The power curve for increased performance will not occur after cetane has reached a 50. It not recommended that diesel contain more than 20% gasoline.
I have looked at the labels on diesel additives and it does not give composition. I suspect since it requires only adding a few ounces to a tank of diesel that it might be something like toluene or xylene, both of which are very high in octane.
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