A fuel is considered unstable when it undergoes chemical changes that produce undesirable consequences such as deposits, acidity, or a bad smell. There are three different types of stability commonly described in the technical literature: thermal stability, oxidative stability, and storage stability.

Thermal stability addresses fuel changes that occur due to elevated temperature. These changes may occur at conditions commonly found in diesel fuel injection systems (60 - 100 degrees C) and particularly at conditions found at the fuel injector tip (300 degrees C).

Oxidative stability refers to the tendency of fuels to react with oxygen at temperatures near ambient. These reactions are much slower than those that would occur at combustion temperatures, and they produce varnish deposits and sediments. 

Storage stability is also a frequently used term and refers to the stability of the fuel while it is in long-term storage. These terms are not necessarily exclusive terms. For example, oxidative attack is probably one of the primary concerns of storage stability but storage stability might also involve issues of water contamination and microbial growth. For this reason, we won't focus on the use of these terms but will describe the actual processes that cause the chemical changes in the fuel.

Vegetable oils are generally more susceptible to oxidative attack because they are less saturated, that is, they contain more carbon-carbon double bonds.  When unsaturated oils, and the biodiesel made from them, are exposed to oxygen, the oxygen attaches to a carbon that is immediately adjacent to those involved in the double bond (a beta carbon).  This forms a hydroperoxide molecule.  The presence of these compounds is measured with the Peroxide Value, which is an indicator of early steps in the oxidation process.

Depending on the physical conditions for the oil, the hydroperoxides can either break apart to form short chain aldehydes and acids or they can attach together to form dimers and polymers.  The short chain acids can be volatile and cause a foul smell, and a lowering of the flashpoint.  Polymerization can cause an increase in viscosity and the formation of insoluble sediments and varnish deposits.

As mentioned earlier, unsaturated molecules are more susceptible to oxidation than saturated molecules.  A commonly used measure of saturation is the Iodine Value.  This test uses iodine to measure the number of double bonds in an oil or fuel.  Oils with high Iodine Values, such as soybean oil (IV = 130-135) are very susceptible to oxidation while animal fats with low Iodine Values, such as tallow (IV = 30-48) are much less susceptible.  The primary drawback of the Iodine Value is that it does not recognize that some double bonds oxidize more readily than others.  Methyl linoleate, with two double bonds, will oxidize approximately 50 times faster than methyl oleate, with only one double bond.  Methyl linolenate, with three double bonds, will oxidize even faster, although not by the same level of increase.  Simply counting double bonds to indicate the susceptibility to oxidation is only a crude measure and can easily provide an incorrect result.  

There are numerous test procedures for characterizing fuel stability. ASTM D2274 is a commonly used method for diesel fuels. The method consists of accelerated oxidation of the fuel by bubbling oxygen through it at elevated temperatures and then filtering the fuel to measure the amount of insoluble sediment that was formed. Unfortunately, the method described in the ASTM standard is not suitablefor use with biodiesel because the filters absorb biodiesel and this is falsely indicated as excessive sediment. Alternative filter materials have been proposed but no new ASTM procedure is suggested. Concerns have also been expressed about whether the solvent used in D2274 can transport all of the adherent sediments without dissolving them.

An example of the tradeoffs that commonly occur in fuel properties is the relation between the degree of saturation, the susceptibility to oxidation, the cold flow properties, and the cetane number.  Cold flow properties and cetane number are described elsewhere in this website.  Highly saturated fuels, such as those made from tallow, are very resistant to oxidation and have high cetane numbers.  However, they tend to have poor cold flow properties, often starting to crystallize at temperatures as high as 50 - 60 degrees F.  Unsaturated fuels, such as those made from soybean oil, oxidize readily and have lower cetane numbers.  However, they will generally stay liquid at temperatures down to 32 degrees F.  In the case of soybean oil, naturally-occurring antioxidants known as tocopherols, can inhibit oxidation.  Other artificial additives, such as TBHQ, can also inhibit oxidation.

Biological attack

Certain types of bacteria and fungi can grow in diesel fuel storage tanks.  These microorganisms can be either aerobic or anaerobic but typically require some water to be present.  The organisms generally grow at the interface between the fuel and water.  They can plug fuel filters and increase the acidity of the fuel, causing corrosion.  Although very limited test data are available, biodiesel is also expected to be prone to the growth of microorganisms.  The preferred method to control growth of microbes in fuel is to eliminate the conditions that allow their growth.  Usually this means removing water from the fuel.  Treatment of the fuel with a chemical biocide can eliminate microorganism growth, but it will also affect the toxicity and biodegradability of the fuel.