There are some similarities and some differences between biodiesel and ULSD. One of the main differences is that biodiesel actually has very good lubricity properties. ULSD on the other hand does not. ASTM standards in the US for diesel fuel are greatly lacking. The current standard for measuring a fuels lubricity is ASTM 6079 with a maximum wear scar diameter of 520 microns. OEM’s are generally not satisfied with 520 micron limit and prefer 400 to 460 micron protection. The European standard is also better than the US. Their standard is 460 microns.
Removing sulfur alone is not really the problem with the lowered lubricity. Sulfur itself does not impart the lubricity quality’s to diesel fuel. However the process that strips sulfur from fuel so it can meet current S15 standards also removes other properties from the fuel that do contribute to the fuels lubricity.
Let’s go over ULSD and some of it’s problems (other than it’s lowered lubricity). ULSD does have increased solvency and some people may have problems with plugged fuel filters after the first couple of tanks full of fuel. The ULSD tends to clean out gunk deposits that have formed in fuel tanks, fuel lines and the engine over the years. Overall not a problem, it’s actually cleaning out the fuel system. However one downfall is a new phenomenon with deposits that are occurring on the injector pintle/needle tip. In older engines these slight deposits are not to much of a problem but with modern injector designs it can lead to poor performance. Modern injector design has come a long way in the past few years in their design and manufacturing processes. Injectors now have much tighter tolerances which are more sensitive to deposits and lacquering. The actual holes in injector nozzles have also changed in recent years. Instead of just being machined/drilled holes they are actually micro honed into precise fuel metering orifices. Modern engines may not be more susceptible to deposits but are much more sensitive to deposits - a little makes a large impact.
The next problem with ULSD was encountered by some here in the US this last winter. Diesel fuel was gelling at temperatures much higher than ever before. To make matters worse some fuel anti-gel additives worked while others made problems worse. The problem seems to be regional and greatly depends on where the fuel was manufactured. Get fuel from one station and you didn’t have a problem, get it from another and you had gelling problems. The main problem with the very low standards on diesel fuel in the US is that it allows for a great difference in fuel quality and composition from one region to another. Examination of the chemical composition of ULSD has shown that the actual different carbon molecules greatly varies in their amount. One sample of fuel may have high levels (2+% by weight of each molecule) of carbon molecules in the C13 to C17 carbon number range while a sample from elsewhere is high at only C15 to C16 and yet another has a fairly level 1% by weight across the board. Different molecules gel at different temperatures. To make matters even worse a new problem was found this last winter. Some fuel anti-gel additives not only didn’t work but actually made problems worse. Remember how some schools were shut down in NY for 1-2 weeks. Analysis of wax formations from the gelled fuel filters showed that the additives in some anti-gels were actually being trapped within the molecular structure. Where you could normally remove a gelled fuel filter, take it inside and it would thaw out the wax was not melting. Samples of the wax had to be heated to around 150 deg F. before they melted. Needless to say there were many people in the industry running around pulling their hair out this winter. Hopefully next winter will not be as bad.
Problems with biodiesel. Where to start? Much higher gel point (poor cold flow properties), fuel foaming, water separation, oxidative stability, injector deposits, lacquer formations, fuel system corrosion and a greatly lowered shelf life (it is recommended to use biodiesel within 6 months).
Once again the US has fallen behind the power curve in oxidation standards. In Europe their standard is EN 14214 for the Rancimat test with a 6 hour minimum induction time limit. In the US our standard is ASTM D6751 3 hour minimum Rancimat induction time. ½ that of the European standard.
OEM’s report damage to fuel injection equipment through use of low stability biodiesel fuels. Relatively small proportion of low stability FAME (fatty acid methal ester)is probably responsible.
What causes low stability in biodiesel. The lack of standardization in biodiesel feed stocks. Biodiesel can be made from almost anything living. Currently the most common feed stocks are Rapeseed, Safflower, Sunflower, Corn, Olive, Soybean, Peanut, Cottonseed, Lard, Beef Tallow, Palm, Butterfat and Coconut. The problem is each of these feed stocks has different levels of saturated fat, monounsaturated fat and polyunsaturated fat. Each being different means a biodiesel made from each is completely different with different properties. Polyunsaturated fats excell in cold flow performance while saturated fats perform good in stability, N0x and Cetane. Monounsaturated shows the best performance across the board. The only problem is that the only real good feed stock for monounsaturated fats is Rapeseed, of course it’s not being used much in the US.
Biodiesel derived from animal fats do not contain any natural antioxidants which strongly inhibit degradation.
Oxidation impacts fuel injection equipment through formation of polymeric materials giving rise to sludge and varnish along with low molecular weight corrosive acids. Solubility of polymerized fatty derivatives decrease in mineral fuel blends.
Just like the problem with injector pintle/needle seat deposits found using ULSD, biodiesel is showing similar problems in this area of injectors. The problems are made worse in modern engines because of the increased engine temperatures and greatly increased fuel injection pressures.
The role of fuel additives. Most of the problems found in biodiesel and ULSD can be counteracted by properly formulated diesel fuel additives. They can help address the stability problems (antioxidants, dispersants, anti-corrosion additives). Other additives can help to improve other performance areas with demulsifiers and antifoams.
So, what’s a person to do. Use a additive or not. It’s your choice. Personally the cost of additives is minimal. At the 1-1000 treat ratio it cost me about 3 cents and at a 1-3000 treat ratio it cost about 1 cent to treat one gallon of fuel. I look at it as cheap insurance.
