ASTM D613-08 Standard Test Method for Cetane Number of Diesel Fuel Oil

The standard "ASTM D613-08 Standard Test Method for Cetane Number of Diesel Fuel Oil" published by the American Society for Testing and Materials (ASTM) describes a test method for evaluating diesel fuel oil on an arbitrary cetane number scale using a standard single-cylinder, four-stroke cycle, variable compression ratio, indirect injection diesel engine. The cetane number scale covers the range from zero (0) to 100, but typical testing is in the cetane number range of 30 to 65.

For the purposes of this standard, the following terms and definitions apply:

• An accepted reference value refers to a value that serves as an agreed reference for comparison and is derived as follows:
  • A theoretical or established value based on scientific principles
  • An assigned or approved value based on empirical studies by some national or international organization.
  • A consensus or approved value based on collaborative experimental work under the auspices of a scientific or engineering group.

In the context of this test method, the accepted reference value is understood to apply to the cetane number of specific reference materials determined experimentally under repeatability conditions by the National Interchange Group or another recognized interchange testing organization.

• Cetane number is a measure of the ignition performance of a diesel fuel oil as compared to reference fuels in a standardized engine test. In the context of this test method, ignition performance means the ignition delay of the fuel as determined in a standard test engine under controlled conditions of fuel flow rate, injection timing and compression ratio.
• Compression ratio refers to the ratio of the volume of the combustion chamber, including the precombustion chamber, when the piston is at bottom dead center to the comparable volume when the piston is at top dead center.
• Ignition delay is the time period, expressed in degrees of crank angle rotation, between the start of fuel injection and the start of combustion.
• Injection timing (injection advance) is the time in the combustion cycle at which fuel injection into the combustion chamber is initiated, measured in degrees of crank angle.
• Repeatability conditions are those conditions under which independent test results are obtained by the same method on the same test items in the same laboratory by the same operator using the same equipment at short intervals. In the context of this test method, a short time interval between two ratings on a sample fuel is understood to be not less than the time required to obtain at least one rating on another sample fuel between them, but not so long as to permit any significant change in the sample fuel, the test equipment or the environment.

In performing the test, the cetane number of a diesel fuel oil is determined by comparing the combustion characteristics for reference fuel blends of known cetane numbers under standard operating conditions with the combustion characteristics in a test engine. This is accomplished by using the bracketing handwheel procedure, which obtains a specified ignition delay by varying the compression ratio for the sample and each of the two bracketing reference fuels, allowing the cetane number to be interpolated with respect to the handwheel reading.

The cetane number provides a measure of the ignition properties of diesel fuel oil in compression ignition engines. This test method is used by engine manufacturers, petroleum refiners and marketers, and in the trade as a primary specification measure for matching fuels to engines.

The cetane number is determined at constant speed in a precombustion chamber type compression ignition test engine. The relationship of test engine performance to full-scale, variable speed, variable load engines is not fully understood.

This test method can be used for unconventional fuels such as synthetics, vegetable oils, and the like. However, the relationship of such materials to performance in full-scale engines is not fully understood.

It is necessary to avoid exposing sample fuels and reference fuels to sunlight or fluorescent lamp UV emissions to minimize induced chemical reactions that could affect cetane number ratings. Brief exposure of these fuels to UV wavelengths shorter than 550 nm can significantly affect cetane number ratings. Certain gases and fumes present in the area of ​​the cetane test engine may have a measurable effect on the cetane number test result. This test method is not suitable for rating diesel fuel oils that have fluid properties that interfere with unimpeded gravity flow to the fuel pump or delivery through the injector nozzle.

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