ASTM Diesel Fuel Specification Overview | Part III
by Ambrose Hughey, OMA I
Business Manager Northwest, Tribology

This brief article is part III and the conclusion of a three part article series intended to provide guidance to folks in the area of diesel fuel testing. In Part I and part II, I covered the detailed requirements specified in Table I of ASTM D975 which is the Standard Specification for Diesel Fuel Oils. In this final part of the series, I will provide an overview of common tests performed on diesel fuel that are not covered in in the Table 1 requirements. As a reminder, in this series the primary focus will be on the No. 2-D diesel fuel grade.

Acid Number – Acid Number is a measure of the acidic constituents in fuel. Traditionally, acid number was primarily recommended for storage applications to monitor any acids being formed in the tank. With the introduction of biodiesel blends, acid number is now also used as a way to monitor the level of free fatty acids or processing acids that may be present as a result of the biodiesel portion of the blend.
Appearance – Most distillate fuel specifications include a workmanship statement that states that the fuel should be bright and clear with no visible sediment or free water.

Particulate contamination - Particulate contamination analysis measures the cleanliness of fuel by determining the total particulate amount by filtering a specified amount of the fuel through membrane filter. High levels of particulate contamination can lead to increased filter plugging potential as well as accelerated levels of fuel system wear and injector failures.

Water content - There will always be some level of water in middle distillate fuel, but most quality diesel fuel possesses water content less than 100 ppm. Water content at this low level is not of any particular concern. Generally speaking, once the water content approaches 200 ppm the first alarm level is triggered. Keeping the water content under control can prevent several water related problems such as corrosion of fuel system components and increased wear. Excessive water also impairs the fuel’s ability to properly lubricate. Keeping fuel systems dry will greatly reduce the likelihood of microbial contamination outbreaks and its related problems.

Microbial Contamination – Microorganisms require water to grow and most microbial growth occurs at the fuel water interface. Some of the common problems with microbial contamination is the increased potential for filter plugging, corrosion of fuel system components, and decreased stability. The wastes created by microbial growth activity create a potential cause of premature filter plugging. Microbes may produce organic acids which accelerate the corrosion process by chemically etching metal surfaces. Good housekeeping is essential to controlling microbial growth.

Biodiesel Content – This test provides a percentage of the FAME (biodiesel) content that is present in the diesel fuel. Most OEMs have a statement and/or guidance regarding the use of biodiesel blends and the allowable amount of the biodiesel portion in the fuel. ASTM D 975 currently allows for up to 5% biodiesel (B5 blend) and ASTM D7467 is the standard specification covering blends from B6 to B20.

API Gravity – API Gravity is a measure of the fuel’s density. The energy content of fuel, which in general correlates with its density, has an impact on power. API gravity is also used in conjunction with distillation to provide the calculated cetane index.

Storage and Thermal Stability – Diesel fuel will form insoluble particulates that can contribute to issues with fuel systems such as premature filer plugging and injector deposits to name a few. These particulates may form slowly over time as the fuel naturally degrades or may form more rapidly when subjected to heat, as in the case of fuel system recirculation. Users may experience increased issues when fuels do not possess good thermal stability. In general, diesel fuel has adequate storage stability for normal use; however, a fuel monitoring program is important to successful long-term storage of fuel (more than 12 months).

This completes our journey through the detailed Requirements of Table 1 of the standard specification for diesel fuel oils and an overview of some common diesel tests not covered in Table 1. I hope this overview has been informative and helpful.