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Black cloudy Diesel – No Diesel Bug? What’s going on?
You may have asphaltenes agglomerating in the fuel, yet another problem associated with Ultra Low Sulphur Diesel (ULSD).
We are answering increasing numbers of enquiries from owners who have dark fuel or black sludge in the filters or bottom of the tank. Initially this can look like Diesel Bug, and often is, but on investigation it is becoming apparent that asphaltenes are also to blame. “Sweet Crude” was the crude of choice in the past, already with low sulphur levels it required relatively simple refining to produce distillate fuels. But that resource is becoming scarcer and being replaced with higher sulphur “sour crude”. Being more toxic and corrosive and with higher sulphur levels it comes at a time when global legislation is limiting the sulphur content of fuels so requires greater processing. Additionally this crude can be mixed from various sources which can promote flocculation of asphaltenes. The additional processing results in ULSD fuels being unstable and then we add a percentage of FAME (Fatty acid methyl ester) making it more unstable again.
Asphaltenes occur naturally in fuel, passing easily through the filter they are too small to be seen, indeed the presence of asphaltenes contribute to the heat energy of the fuel. The asphaltenes normally stay in suspension, but as fuel degrades and oxidises the asphaltenes begin to agglomerate. This process is further accelerated in the present of heat, pressure and turbulence through the fuel pump further darkening and degrading the fuel.
If you imagine the life of a small molecule of fuel; it can be subject to heat and extreme pressure many times as it is pumped through the engine, cooling the fuel nozzles then returning to the tank before eventually being injected to burn in the cylinder. This process also heats the tank giving rise to condensation that collects as free water on the bottom which further contributes to the problem as it reacts with the fuel and harbours Diesel Bugs, both contributing to the formation of acids and further fuel degradation.
Once the sludge has formed it is difficult to remove, there are solvents but they have limited success so the goal is to prevent it in the first place. Mixing fuel from different sources should be avoided and additives can help to maintain stability and keep asphaltenes in suspension, we have our own DieselAid® range of additives.
Most importantly we recommend, as always, remove the water, if you can get to the bottom of the tank, use the drain valve daily. If you cannot we can supply an automatic de-watering system (Diesel Drake®) specifically to drain free water from the tank. Timed to operate twice a day it samples the bottom of the tank and automatically discharges water.
Today’s high pressure common rail diesel engines are highly sensitive to fuel quality and require exceptionally clean dry diesel. When you consider an ECU driven modern fuel injector can operate 100 times a second at 2000 rpm whilst the humble diesel in some cases is not fit for purpose unless
maintenance of the fuel is deemed as important as maintenance of the engine.
24th May 2017
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