Cetane number testing is the definitive laboratory procedure for evaluating the ignition quality of diesel fuels, measuring how quickly fuel ignites under compression-ignition conditions. Performed in accordance with internationally recognized standards such as ASTM D613, EN ISO 5165, and GB/T 386, the test is essential for fuel compliance, refinery blending, engine development, and emissions control. This article explains the principle of the cetane number test, typical sample types, testing items, instrumentation, and practical applications.
Technical Overview
The cetane number (CN) is the fundamental measure of the combustion quality of diesel fuel during compression ignition. It quantifies the ignition delay, defined as the time interval between the start of fuel injection and the start of combustion. A fuel with a higher cetane number has a shorter ignition delay, ignites more readily, and burns more smoothly, which translates into easier cold starting, reduced combustion noise (engine "knock"), and lower hydrocarbon and carbon monoxide 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. The cetane number of a sample is determined by comparing its ignition delay characteristics with blends of these two reference compounds under standardized engine or constant-volume combustion chamber conditions. Typical automotive diesel fuels fall in the range of 40 to 60 cetane number, with many premium and European-grade diesels requiring minimum values of 51 or higher.
- Higher cetane number: shorter ignition delay, faster cold start, smoother combustion, quieter engine operation
- Lower cetane number: prolonged ignition delay, pressure rise spikes, increased noise, harder starting, elevated white smoke during warm-up
- Cetane number is a measured property and must not be confused with the calculated cetane index, which is estimated from density and distillation data
Testing Samples
Conventional Petroleum Diesel
Automotive diesel fuels such as those conforming to EN 590, ASTM D975 (Grades No. 1-D, No. 2-D, and No. 4-D), and GB 19147 constitute the majority of submitted samples. This category includes ultra-low-sulfur diesel (ULSD), winterized and low-cloud-point grades, and low-temperature Arctic-class fuels.
Biodiesel, Renewable Diesel, and Blends
Neat biodiesel (B100) conforming to ASTM D6751 or EN 14214, commercial blends such as B7 and B20, hydrotreated vegetable oil (HVO), and gas-to-liquid (GTL) synthetic diesels are routinely characterized for ignition quality. Fatty acid methyl esters typically exhibit high cetane numbers, while synthetic paraffinic diesels may reach values well above 70.
Marine, Railway, and Off-Highway Distillate Fuels
Marine distillate grades within ISO 8217 (DMX, DMA, DMZ, DMB categories), railway diesel, and fuels for mining and agricultural machinery are tested to verify category compliance, where minimum cetane number requirements commonly range from 35 to 45 depending on the grade.
Additive-Treated Fuels and Research Samples
Fuel samples treated with ignition improvers such as 2-ethylhexyl nitrate (2-EHN), experimental refinery streams, catalyst evaluation samples, and stability study aliquots are tested to quantify additive response or process effects on ignition quality.
Proper sampling practice is critical: samples should be collected in clean, sealed containers (typically amber glass or lined metal), protected from light, heat, and air, and delivered with sufficient volume. The CFR engine method generally requires approximately 1 L or more of sample, whereas constant-volume combustion chamber instruments typically require 100 to 200 mL.
Testing Items
Depending on the specification and the purpose of the evaluation, the following ignition-quality parameters can be determined:
- Cetane Number (CN): the engine-determined rating obtained by bracketing against primary reference fuel blends
- Derived Cetane Number (DCN): the instrument-determined rating obtained from measured ignition delay in constant-volume combustion chamber devices
- Calculated Cetane Index (CCI): an estimated value derived from density and distillation recovery temperatures, by the two-variable or four-variable procedure
- Ignition delay: expressed in crank angle degrees (engine method) or milliseconds (chamber methods)
- Supporting data: density at 15 °C and distillation curve (required inputs for cetane index calculation and cross-verification)
Testing Methods
CFR Engine Method (ASTM D613 / EN ISO 5165 / GB/T 386)
The reference method uses a specially instrumented single-cylinder Cooperative Fuel Research (CFR) engine with a variable compression ratio. The engine operates at a fixed speed of 900 rpm with injection timing set at 13 degrees before top dead center. The operator adjusts the compression ratio until the ignition delay of the sample equals 13 crank angle degrees, meaning combustion begins exactly at top dead center. The corresponding handwheel reading is bracketed between two primary reference fuel blends that differ by no more than 5 cetane numbers, and the cetane number of the sample is obtained by interpolation. Regular verification with check fuels, such as n-tetradecane, is required to confirm engine condition and operator technique.
Ignition Quality Tester (ASTM D6890)
This constant-volume combustion chamber (CVCC) method injects a metered charge of fuel into heated, compressed air within a sealed chamber. The ignition delay is measured electronically, and an established correlation converts the measured delay into a Derived Cetane Number. The method requires a small sample volume, offers significantly reduced turnaround time compared with the engine method, and delivers improved precision, making it well suited to high-throughput quality control and research involving biodiesel and synthetic fuels.
Alternative CVCC Instruments (ASTM D7170 and ASTM D7668)
ASTM D7170 employs a fuel ignition tester (FIT) while ASTM D7668 uses a diesel fuel ignition tester (DFI). Both introduce fuel into a controlled-volume chamber at elevated temperature and pressure, record ignition delay, and report a Derived Cetane Number through standardized correlations. These methods are accepted in many specifications as equivalents or alternatives to the engine method.
Calculated Cetane Index (ASTM D976 / ASTM D4737 / EN ISO 4264)
The cetane index is computed from fuel density and mid-boiling or distillation recovery temperatures. The two-variable equation uses density and the 50 percent recovery temperature, whereas the four-variable equation incorporates the 10, 50, and 90 percent recovery temperatures plus density. The cetane index is an estimation tool useful for screening and inventory management, but it is not valid for fuels containing ignition-improving additives, biodiesel components, or non-conventional hydrocarbons, because such constituents alter the distillation-density relationship without a corresponding ignition-quality change.
Applications
Cetane number testing supports a wide range of commercial, regulatory, and technical activities:
- Refinery quality control: cetane monitoring of diesel hydrotreaters, blending optimization, and dosing control of cetane improver additives
- Regulatory compliance: verification against EN 590 (minimum cetane number 51), ASTM D975 (minimum cetane number 40 for No. 1-D and No. 2-D grades), GB 19147, and ISO 8217 marine distillate requirements
- Fuel trading and custody transfer: certificate-of-analysis verification, tender specification conformity, and dispute resolution between counterparties
- Engine development: calibration of injection timing and combustion strategies against fuel ignition quality, and fuels research for advanced compression-ignition concepts
- Renewable fuel qualification: ignition characterization of HVO, biodiesel, co-processed diesels, and synthetic e-fuels
- Storage and stability studies: evaluation of ignition-quality degradation in aged, thermally stressed, or contaminated fuel samples
- Accredited third-party laboratory testing: independent certification for importers, distributors, fleet operators, and government agencies
FAQ
What is the difference between cetane number and cetane index?
The cetane number is a directly measured ignition-quality rating obtained on a CFR engine or CVCC instrument. The cetane index is a calculated estimate based on density and distillation temperatures. The index cannot account for ignition-improving additives or biodiesel content, so the two values can diverge substantially for treated or renewable fuels.
How does cetane number differ from octane number?
The two scales are essentially opposite in meaning. The octane number measures a fuel's resistance to autoignition, which is desirable for spark-ignition gasoline engines. The cetane number measures the ease of autoignition, which is desirable for compression-ignition diesel engines. A fuel with excellent octane performance would generally make a poor diesel fuel, and vice versa.
Is Derived Cetane Number equivalent to Cetane Number?
The DCN is statistically correlated with the engine-determined CN, and many modern fuel specifications accept either result. Small systematic differences between methods can exist for certain fuel types, so contract and specification language should always state which method is governing.
How much sample is required and how long does a test take?
The CFR engine method typically requires at least 1 L of sample and several hours per determination, including bracketing runs with reference fuels. CVCC methods such as ASTM D6890 require roughly 100 to 200 mL and can produce a result within an hour, which is why they are preferred for routine quality control.
What cetane number should good diesel fuel have?
Most regulatory specifications require a minimum of 40 to 51. Premium diesel products commonly deliver 52 to 60. Very high cetane numbers above approximately 65 offer diminishing practical benefit in conventional engines, although synthetic paraffinic fuels naturally exceed this level.
Can fuels containing biodiesel be tested for cetane number?
Yes. Both the engine method and the CVCC methods are applicable to biodiesel blends up to B100 within their stated scope. However, the calculated cetane index is not applicable to these fuels, because their non-petroleum composition invalidates the density-distillation correlation.