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

This brief article is part two of a three part article series intended to provide guidance to folks in the area of diesel fuel testing. In Part I we began our overview of the detailed requirements specified in Table I of ASTM D975 which is the Standard Specification for Diesel Fuel Oils. In this, part II of the series I will continue the overview of the remainder of the table 1 of ASTM D 975 Standard Specification for Diesel Fuel Oils that was not covered in Part I. In the upcoming part III, I will be providing further guidance on other common diesel fuel tests performed that are not specified in the Table 1 requirements. As a reminder, the primary focus of this series is on the No. 2-D diesel fuel grade.

Cetane Number – Cetane number is a measure of the ignition quality of diesel fuel which relates to ease of combustion. Cetane number is essentially a measure of a fuel's ignition delay; the time period between the start of injection and the actual start of fuel combustion in a pre-combustion chamber type compression ignition test engine. Generally, fuels with higher cetane numbers provide a shorter ignition delay period than fuels with lower cetane numbers. Diesel fuel that possesses good ignition quality should provide good cold start performance. The cetane number scale covers the range from 0 to 100, but typical testing results are is in the range of 30 to 70. Calculated cetane index is commonly used to estimate the cetane number of diesel fuel. Cetane index is calculated using a four variable equation which uses the density of the fuel and the 10%, 50%, and 90% recovery temperatures determined by distillation. Please note that the cetane index is not generally affected by cetane improvers. ASTM D 975 Table 1 specifies a minimum limit for cetane number of 40 for all No 1-D and 2-D grades.

Table 1 specifies that one of two conditions must be met as specified in 40 CFR Part 80 controls and prohibitions on diesel fuel quality. The two parameters specified are cetane index and aromaticity %. Either the cetane index is at least 40; or the maximum aromatic content of 35 volume percent must be met. This requirement is aimed to control aromatics content as they may have a negative impact on emissions

  • Calculated Cetane Index - Cetane index which is commonly used to estimate the cetane number of diesel fuel is also specified as part of 40 CFR Part 80 as noted above. In this case, the method specified is ASTM D 976 which varies from the cetane index equation that I discussed above. The D976 equation only requires the mid boiling temperature and the density of the diesel fuel in question. This requirement is specified to limit high amounts of aromatics. ASTM D 975 Table 1 specifies a minimum limit for cetane index of 40 for the S15 and S500 grades of No. 1-D and No. 2-D.
  • Aromaticity - ASTM D1319 is the Standard Test Method for Hydrocarbon Types in Liquid Petroleum Products by Fluorescent Indicator Adsorption. This method covers the determination of the hydrocarbon types aromatics, olefins, and saturates in petroleum fractions. In this case, the aromatics % is of interest as specified as part of 40 CFR Part 80 as it is important in characterizing the quality of petroleum fractions and specifically the potential impacts on emissions. ASTM D 975 Table 1 specifies a maximum limit for aromatics of 35 % volume for the S15 and S500 grades of No. 1-D and No. 2-D.

Table 1 includes an operability requirements specification that includes performing one of three low temperature operability tests. The three tests that are listed are Cloud point, Cold Filter Plugging Point (CFPP), and Low-Temperature Flow Test (LTFT). First I will briefly discuss low temperature operability in general, then we will cover the three tests specified.

  • Low Temperature Operability – Diesel fuel contains paraffin which will form wax when subjected to low temperatures. Low temperature operability considerations are very important as this wax formation can clog fuel filters on equipment, which can lead to fuel starvation and shutdown of the equipment. This wax formation will cause diesel to gel and eventually solidify if subjected to cold enough temperatures, not that dissimilar to water freezing. However, unlike water which has a known freeze point with little variance depending on the source, middle distillate fuels from different sources can potentially have a wide variance in cold temperature performance. This variance is attributed to the differences in crude oil, refining processes, and additives. The low temperature operability of diesel can be measured by methods such as cloud point, cold filter plugging point (CFPP), and low-temperature flow test (LTFT).
  • Cloud point – The cloud point is the first temperature at which the fuel begins to display a haze or cloud caused by wax formation. The analysis entails cooling a sample at a specified rate and examining periodically for the initial precipitation of the wax. The temperature at which the cloud is first observed is recorded as the cloud point. Use of diesel at or below its cloud point may cause operability issues. Fuel starvation may occur as wax crystals trigger fuel system plugging and/or inadequate flow. Low temperature operability additives may be blended into the fuel to improve performance well below the cloud point.
  • Cold Filter Plugging Point (CFPP) and Low-Temperature Flow Test (LTFT) – The CFPP is the temperature at which wax crystals are sufficient enough to stop or slow down the flow through a 45 micron screen. At each decrease in temperature, a vacuum is applied to draw the sample from a test jar through the screen to fill a standard pipet and timed. The test is completed when; the time taken to fill the standard pipet exceeds 60 seconds or the fuel fails to return completely to the test jar before cooling to the next temperature reading. The LTFT pass temperature is the lowest temperature, at which a test specimen can be filtered in 60 s or less through a 17 micron screen. Both CFPP and LTFT are commonly used to investigate fuel additive performance for low temperature operability.

While the above low temperature operability tests can provide estimates of the fuel’s performance in cold weather, ASTM Table 1 does not specify limits for these properties as the needs of low temp operability greatly vary depending on time of year and location. ASTM D975 appendix X5 provides some guidance on low temperature operability.

  • Carbon Residue – Carbon residue analysis entails quickly heating a sample to the point at which all volatile matter is evaporated out of a bulb with or without decomposition while the heavier residue remaining in the bulb undergoes cracking and coking reactions. The residue remaining at the test conclusion is calculated as a percentage of the original sample. The amount of carbon residue provides a measure of the coking tendency of the fuel and can be used to estimate the carbon depositing potential of the fuel. Carbon deposits can lead to performance issues and maintenance challenges. ASTM D 975 Table 1 specifies the maximum carbon residue as 0.15% and 0.35% for grades No. 1-D and No. 2-D respectively.
  • Lubricity – Diesel fuel provides lubrication to most components of the fuel injection system. Fuel with poor lubricity can lead to a service life reduction and maintenance concerns of fuel injection equipment. Table 1 specifies a requirement to meet maximum wear scar as determined by the HFRR test Method for Evaluating Lubricity of Diesel Fuels by the High-Frequency Reciprocating Rig (HFRR). The analysis entails a 2-mL sample of diesel being placed in the test reservoir of an HFRR holding a steel ball loaded with a 200-g mass which is lowered until it contacts a test disk completely submerged in the fuel. The ball is caused to rub against the disk with a 1-mm stroke at a specified frequency for 75 min. At the test conclusion, the image of the wear scar is captured and recorded using a microscope digital camera. ASTM D 975 Table 1 specifies a maximum limit for lubricity of 520 microns for all No 1-D and 2-D grades.
  • Conductivity – The conductivity requirement is primarily in place for safe handling concerns. Risks may be associated with static electrical discharge and many factors can contribute to this risk. The conductivity requirement will help decrease the risk. As stated in the specification “The intent of this requirement is to reduce the risk of electrostatic ignitions while filling tank trucks, barges, ship compartments, and rail cars, where flammable vapors from past cargo can be present. Generally, it does not apply at the retail level where flammable vapors are usually absent.” ASTM D 975 Table 1 specifies a minimum limit for conductivity of 25 pS/m for all No 1-D and 2-D grades. This requirement applies to all instance of high velocity transfer such as mentioned above.

This completes our journey through the detailed requirements of Table 1 of the standard specification for diesel fuel oils. I hope this overview of Table 1 has been informative and helpful to you. In the last part of this three part series I will be covering some common tests performed on diesel fuel that are not covered in in the Table 1 requirements. Please stay tuned for part III in an upcoming eSource distribution.