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

The amount of conversation in the industry regarding diesel fuel related concerns and analysis has been substantial. This three part article is intended to provide guidance to folks in the area of diesel fuel testing. Part I and II will contain an overview of table 1 from ASTM D 975 Standard Specification for Diesel Fuel Oils. Part III will provide further guidance on other common diesel fuel tests performed that are not specified in the Table 1 requirements. While Table 1 encompasses the detailed requirements for several grades of Diesel Fuel, this overview will focus primarily on the No. 2-D diesel fuel grade.

Flash Point – Flash Point measures the temperature at which vapors on the surface of the fuel will ignite when exposed to a flame. Flash point is generally specified for legal and safety concerns in regards to fuel handling and storage. When the flash point does not meet the minimum specification, this provides an indication that the fuel may be contaminated with a more volatile product. If enough contamination of the higher volatile material is present the cetane number will likely be adversely affected which may result in poor engine performance. The minimum specification for No. 2 diesel is 52°C (125°F). When No.1 and No2 diesel are blended for low temperature operability the minimum specification is reduced to 38°C (100°F).

Water and Sediment – Water and Sediment analysis measures the amount of free water and sediment present in the fuel and is determined after subjecting the fuel to centrifugation. Keeping the water content under control can prevent several water related problems. Water contamination can corrode fuel system components and lead to increased wear. Too much water impairs the fuels ability to properly lubricate. Microorganisms require water to grow and, since most microbial growth occurs at the fuel water interface, keeping fuel systems dry will greatly reduce the likelihood of microbial contamination and its related problems. High levels of sediment can lead to increased filter plugging potential as well as accelerated levels of fuel system wear and injector failures. Particulates are of special concern with modern high pressure common rail fuel injection systems, as any hard particles can cause abrasive wear to injectors at the high pressures employed. The maximum limit for water and sediment is 0.05% for all No. 1-D grades and No. 2-D grades.

Distillation – Distillation is a measure of the boiling range and fractional cut of diesel fuel. It is determined by distilling a specimen of the sample under prescribed conditions while observations of temperature readings and volumes of the condensate are made. The distillation profile, which is a fundamental property of fuel, provides a measurement of fuel’s volatility thus providing an indication of quality. Generally engine design dictates the fuel volatility requirements, and acceptable fuel volatility is important to maintain good engine performance. While several temperatures are generally determined and reported with a distillation analysis, the only temperature that is specified in table 1 is the temperature at which 90% recovery occurs. The minimum and maximum temperatures specified for the 90% recovery temperature for No. 2 diesel is 282°C (540°F) and 338°C (640°F) respectively.

Kinematic Viscosity at 40°C – Kinematic Viscosity is determined by measuring the time that a fixed volume of the fuel to flows under gravity, through a calibrated capillary viscometer tube and is commonly reported in centistokes (cSt). Viscosity plays an important role in fuel systems. Viscosity affects the fuel’s ability to lubricate fuel system components, as well as atomization. Improper fuel atomization can result in poor combustion, which may yield a variety of issues including loss of power and fuel economy. Low fuel viscosity may result in fuel pump and injector leakage. Improper viscosity can lead to increased fuel system wear. ASTM D 975 Table 1 specifies the minimum and maximum kinematic viscosity for No. 2 diesel as 1.9 cSt and 4.1 cSt respectively. The table 1 requirement does allow for a minimum of 1.7 cSt when No. 1-D and No-D grades are blended to improve low temperature operability.

Ash – Ash % determination provides a measurement of the ash-forming materials that are present in the fuel. Ash is determined by weighing the ash remaining after burning a weighed amount of the fuel. Ash-forming materials are generally considered to be resultant of contamination or impurities. Ash-forming materials may be found from a variety of sources and are normally present in fuel in the form of soluble metallic soaps and/or abrasive solids. Soluble metallic soaps can result in increased engine deposits. Abrasive solids can also contribute to engine deposits as well as increased pump, injector, and piston wear. ASTM D 975 Table 1 specifies the maximum amount of ash as 0.01% for No.1 and No. 2 diesel.

Sulfur – ASTM D 975 allows for a variety of different analytical techniques such as General High Pressure Decomposition Device Method, Ultraviolet Fluorescence, and X-ray Fluorescence to determine Sulfur content in diesel fuel. Sulfur is commonly reported in ppm. Today, limitations on the amount of sulfur allowed in diesel are driven by emissions standards more than operability concerns as in the past; however use of higher sulfur fuels will increase the acids formed during combustion and can result in a more rapid base number depletion. Also, fuels that do not meet the required sulfur requirement can poison catalysts used in some of the advanced emission control devices resulting in increased maintenance costs. The maximum sulfur content has been uniquely built into the fuel grade nomenclature. In the case of No. 2 diesel there are actually three grades; an S15, an S500, and an S5000. The grade refers to the max limit of sulfur i.e; No. 2 diesel S15 has a maximum limit of 15 ppm sulfur. This same structure applies to the No. 1-D grade as well.

Copper Strip Corrosion – Analysis of diesel for copper strip corrosion provides an indication to issues that may arise with copper components in the fuel system and in general provides a relative degree of the corrosiveness of the fuel. The analysis encompasses immersing a polished copper strip in the fuel for three hours at 50°C. The copper strip is then washed and the tarnish level is determined by comparing the copper strip to the ASTM Copper Strip Corrosion Standard. ASTM D 975 Table 1 specifies the maximum limit for copper strip corrosion rating of No. 3 for all No 1-D and 2-D grades.

We are about half way through Table 1 of D975 at this point and a good place to take a break. I hope this overview has been informative and helpful thus far. Please stay tuned for part II and III in upcoming eSource distributions.