Diesel Additives Can Still Harm Engines

Instability Can Create Solids in Fuel 
Much of the world is required to use Ultra Low Sulfur diesel (ULSD).
With the removal of sulfur, fuel properties change radically, requiring the increased use of additive in order to meet specifications. In the 
real world, maintaining these fuel properties can be a challenge. Diesel is tested at many points along the distribution channel, from refinery 
to terminal to distributor to end user. Chemical additives are used to correct or enhance the fuel at each of these locations. 

Chemically speaking most additives are very similar to diesel itself (i.e. long chain hydrocarbons) so that they dissolve into the fuel. Additives are composed of a long hydrocarbon "tail" at the "head" of which there is another element, the one intended to have a beneficial effect on the diesel. This head typically contains atoms of nitrogen, oxygen, and/or sulfur in addition to carbon and hydrogen. Unlike the hydrocarbon tail, the head is polar. The beneficial element is intended to stay dissolved in the fuel so that it can do its jobSince head elements are polar, they have a strong tendency to want to escape from the non-polar hydrocarbons and associate with other polar substances such as water, metal, dirt, degradation particles or asphaltenes. You might think of them as being magnetically attracted to each other. 

Sometimes, usually under adverse circumstances, the polar elements manage to come out of solution and form a bond with other polar elements. When this happens, there are two consequences: 

  • T​he chemical may no longer be doing the job it was intended to do.
  • The chemical, now bonded with another substance, drops out of solution and turns into a soft contaminant in the fuel. 

This phenomenon is most frequently observed in the presence of poor fuel storage conditions and fluctuating temperatures. Water, improper blending practices, and cold temperatures are leading contributors to these problems. 

When unstable additive drops out of solution, it forms soft, sticky, solids which can cause rapid filter plugging and deposit formation. Engine manufacturers have observed injector deposits caused by these soilds, which are chemically identical to the substances found in many rapidly plugged filters, especially the high efficiency filters specified by OEs to protect HPCR systems. 

The image to the right is of filter media at 1000x magnification under a scanning electron microscope (SEM).
The media was cut from a high efficiency filter of the type used in high pressure common rail fuel systems. This filter clogged in under 30 minutes of use. Between the media fibers you will notice a film. This kind of soft contaminant coats media as if it were covered in plastic wrap, effectively disabling the filter.
filterThe filter to the left has been completely disabled by soft contaminant. It looks clean because it did not have the time to become loaded with dirt. Instead, you will notice a sort of sheen, as if the filter media had been coated with wax. This is not due to wax (fuel gelling), but to soft contaminants that have dropped out of solution in the fuel. The pink color is red dye from off-road diesel. Upon further analysis, these soft contaminants were found to have chemical profiles identical to certain components of common diesel additives such as corrosion inhibitors, cold flow improvers, lubricity improvers and others. One thing that is important note, the root causes of this specific chemical instability are not yet completely understood. They are under rigorous investigation by Donaldson Company and other industry leaders.