*Quality and Trends of Automotive Fuels DOI: http://dx.doi.org/10.5772/intechopen.94167*

specifications even though screening tests may not be fully representative of the aging process. ASTM D4625 is a much slower technique which takes up to 24 weeks at a temperature of 43°C. The degradation observed is much closer to real life conditions but is not adequate to be used in specifications due to the length of time required.

A myriad of chemical reaction pathways could be derived from various functional groups in the following mechanisms (**Figure 16**):


**Figure 16.** *Path mechanisms for insoluble fuel degradation products.*

• A well-known oxidation mechanism in gas oil fuels is the acid-catalyzed conversion of phenalenones and indoles to complex indolylphenalene salts. Phenalenones are formed by oxidation of certain reactive olefins; indoles occur naturally in certain blend components of diesel fuel. The required organic acid is either present in a blend component or is generated by the oxidation of mercaptans to sulfonic acids as shown below.

ASTM D 6748 test method can be used for gas oil fuels to determine potential instability caused by phenalenes and phenalenones to determine the potential for storage instability.
