The cetane number is the key indicator of ignition quality for diesel distillates, directly influencing combustion smoothness, cold-start performance, and exhaust emissions in compression-ignition engines. This article provides a comprehensive technical overview of cetane number determination, covering reference standards such as ASTM D613, ISO 5165, and GB/T 386, instrumental derived cetane number methods, calculated cetane index procedures, and typical laboratory testing applications.
Technical Overview
The cetane number (CN) quantifies the ignition quality of diesel fuels by measuring the ignition delay period between the start of fuel injection and the start of combustion in a compression-ignition engine. A higher cetane number corresponds to a shorter ignition delay, more complete and smoother combustion, reduced combustion noise, improved cold-start behavior, and generally lower hydrocarbon, carbon monoxide, and particulate emissions.
The cetane scale is defined by two primary reference fuels: n-hexadecane (n-cetane), assigned a cetane number of 100, and heptamethylnonane (HMN), assigned a cetane number of 15. Binary blends of these reference fuels establish intermediate values, where the cetane number of a blend equals the volume percent of n-cetane plus 0.15 times the volume percent of heptamethylnonane. A diesel fraction with a cetane number of 55, for example, ignites with the same delay as a blend of 57.4 percent n-cetane and 42.6 percent HMN.
Diesel distillates from different sources vary widely in ignition quality. Straight-run and hydrocracked fractions typically exhibit high cetane numbers, while deeply cracked stocks such as FCC light cycle oil contain substantial aromatics and show markedly lower values. Major specifications therefore enforce minimum limits, for example a minimum cetane number of 51 in EN 590 automotive diesel and in the Chinese national standard GB 19147, and a minimum of 40 for most grades in ASTM D975.
Testing Samples
Straight-Run and Secondary-Processing Diesel Fractions
Laboratories routinely evaluate intermediate streams from refinery units, including atmospheric and vacuum gas oil fractions, hydrocracking distillates, fluid catalytic cracking light cycle oil, delayed coker and visbreaker distillates, and coal- or gas-derived synthetic paraffinic fractions. Cetane number data guide hydrotreating severity, stream allocation, and blend recipe optimization.
Finished Commercial Diesel Fuels
Certification testing covers automotive diesel, off-road diesel, heating oil distillates, and marine distillate fuels. Samples are assessed for compliance with EN 590, ASTM D975, GB 19147, and ISO 8217 marine specifications before release to market.
Blend Components and Alternative Diesel Fuels
Fatty acid methyl ester (FAME) biodiesel components and commercial blends such as B5 through B30, hydrotreated vegetable oil (HVO) and Fischer-Tropsch synthetic diesels, and fuels treated with cetane-improving additives such as 2-ethylhexyl nitrate are tested to characterize ignition behavior and verify additive response.
Proper sample handling is essential. Samples should be homogeneous, free from water and particulate contamination, and stored in sealed, light-protective containers. Typical requirements are approximately 1 to 2 liters for the full engine method and roughly 100 to 200 milliliters for instrumental derived cetane number techniques.
Testing Items
- Cetane number (CN) determined by the standard CFR engine method
- Derived cetane number (DCN) determined by constant-volume combustion chamber instruments
- Calculated cetane index (CI) from density and distillation recovery data
- Ignition delay characteristics of diesel fractions and blend components
- Cetane improver additive response evaluation
- Comparative ignition quality assessment for research and blending studies
Testing Methods
CFR Engine Method (ASTM D613, ISO 5165, GB/T 386)
The reference method uses a single-cylinder, variable-compression-ratio CFR cetane engine operated at a fixed speed of 900 rpm with a fixed injection timing of 13 degrees before top dead center. The compression ratio of the engine is adjusted until the test fuel exhibits the standard ignition delay. The fuel is then bracketed between two reference fuel blends of known cetane number that bracket the sample within specified limits, and the cetane number is obtained by interpolation of the compression ratio handwheel readings. ASTM D613, EN ISO 5165, and the Chinese national method GB/T 386 are technically equivalent procedures. The engine method offers repeatability of roughly 2 cetane numbers and reproducibility of approximately 4 to 5 cetane numbers between laboratories.
Ignition Quality Tester (ASTM D6890, SH/T 0765)
This instrumental method injects a small, metered quantity of fuel into a heated, pressurized constant-volume combustion chamber charged with air at precisely controlled temperature and pressure. The measured ignition delay is converted through a defined correlation into a derived cetane number. The method requires only a small sample volume, delivers results in a fraction of the time needed for engine testing, and provides improved precision, making it attractive for high-throughput refinery quality control and research on limited-volume samples.
Fuel Ignition Tester (ASTM D7170)
The fuel ignition tester similarly measures ignition delay in a constant-volume chamber under automated conditions and reports a derived cetane number. It is suitable for rapid screening of diesel fractions and blended components within its stated applicability range.
Cetane Ignition Delay Instrument (ASTM D7668)
This method employs a constant-volume combustion chamber instrument in which the fuel is injected into heated compressed air and the ignition delay is recorded to calculate the derived cetane number. It offers rapid analysis with low sample consumption and close correlation to the engine method for conventional diesel fuels.
Calculated Cetane Index (ASTM D4737, ASTM D976, GB/T 11139)
The four-variable equation of ASTM D4737, also adopted in Chinese standard GB/T 11139, estimates ignition quality from the density at 15 degrees Celsius and the 10 percent, 50 percent, and 90 percent distillation recovery temperatures. ASTM D976 provides an earlier two-variable correlation still referenced in some specifications. Calculated cetane index is not valid for fuels containing cetane improvers or oxygenates such as FAME, and certain specifications restrict its use as an alternative to the measured cetane number.
Quality Assurance of Measurements
Reliable results depend on calibration with certified primary and secondary reference fuels, scheduled instrument verification with check standards, strict control of chamber or engine operating conditions, and participation in interlaboratory proficiency programs. Accredited third-party laboratories maintain these controls to ensure traceable, defensible cetane number data.
Applications
- Refinery stream characterization and diesel pool blend optimization
- Certification of finished diesel against EN 590, ASTM D975, GB 19147, and customer specifications
- Hydrotreating and hydrocracking process severity monitoring and catalyst performance studies
- Cetane improver additive screening and treat-rate optimization
- Qualification of alternative fuels including FAME, HVO, GTL, and CTL distillates
- Engine calibration, combustion modeling, and emission correlation studies
- Custody transfer verification, supplier qualification, and commercial dispute resolution
- Regulatory compliance demonstration for fuel quality surveillance programs
FAQ
What is the difference between cetane number and cetane index?
The cetane number is a direct measurement of ignition quality obtained on an engine or combustion instrument, while the cetane index is an estimate calculated from density and distillation data. The cetane index cannot account for the effect of cetane-improving additives and is not applicable to fuels containing oxygenates such as biodiesel.
What is the difference between cetane number and derived cetane number?
The cetane number is determined directly by the CFR engine reference method, whereas the derived cetane number is calculated from the ignition delay measured in a constant-volume combustion chamber instrument. The two values are correlated and, for conventional diesel fuels, agree closely within stated precision limits.
Can the calculated cetane index replace the engine cetane number for specification compliance?
Some specifications, including EN 590, permit a minimum cetane index as an alternative criterion for conventional hydrocarbon diesel, but not for fuels treated with cetane improvers or containing FAME. Always verify the acceptance criteria of the applicable specification before substituting a calculated value for a measured one.
How much sample is required for cetane number testing?
The CFR engine method typically requires 1 to 2 liters of sample, while derived cetane number instruments generally need only about 100 to 200 milliliters, which is advantageous for research samples and pilot-plant distillates.
Do biodiesel blends require special handling in cetane testing?
Biodiesel components such as FAME usually exhibit high cetane numbers and are testable by derived cetane number methods as well as the engine method. However, calculated cetane index correlations are not valid for oxygenated blends, so a measured method must be used.
How precise are cetane number determinations?
Under the CFR engine method, within-laboratory repeatability is on the order of 2 cetane numbers and between-laboratory reproducibility is approximately 4 to 5 cetane numbers, depending on the fuel. Modern constant-volume chamber instruments typically demonstrate tighter precision, particularly on repeatability.