Diesel cetane number analysis is the definitive laboratory procedure for evaluating the ignition quality of diesel fuel, a parameter that directly governs combustion smoothness, cold-start performance, engine noise, and exhaust emissions in compression-ignition engines. This technical article explains the fundamental definition of the cetane number, the sample types suitable for testing, standardized test methods such as ASTM D613 and ASTM D6890, and practical answers to frequently asked questions for refiners, fuel suppliers, and engine manufacturers.
Technical Overview
The cetane number (CN) is a dimensionless measure of the ignition quality of diesel fuel, quantifying the time delay between fuel injection into the combustion chamber and the onset of combustion in a compression-ignition engine. A higher cetane number indicates a shorter ignition delay, which produces smoother and more complete combustion, faster cold starting, reduced engine knock and noise, lower white smoke during start-up, and decreased hydrocarbon, carbon monoxide, and particulate emissions.
The cetane scale is defined by two primary reference compounds: n-hexadecane (n-cetane), assigned a cetane number of 100, and heptamethylnonane, assigned a cetane number of 15. The cetane number of a test fuel is determined by bracketing its ignition delay between calibrated reference fuel blends. Most commercial diesel fuels exhibit cetane numbers in the range of 40 to 60, and regional specifications differ accordingly: ASTM D975 requires a minimum cetane number of 40 for standard grade diesel in North America, EN 590 mandates a minimum of 51 in Europe, and the Chinese national standard GB 19147 specifies a minimum cetane number of 49 (or cetane index of 46) for conventional diesel grades, with relaxed limits for certain low-temperature grades.
Fuel composition strongly influences ignition quality. Normal paraffins possess high cetane numbers, while aromatics and heavily branched iso-paraffins exhibit low cetane numbers. Consequently, cetane number analysis serves as a critical control point in refinery blending, fuel certification, and regulatory compliance testing performed by accredited third-party laboratories.
Testing Samples
Refinery and Terminal Diesel Products
Finished commercial diesel (for example, No. 5 and No. 0 diesel in China, EN 590 Class A/B diesel in Europe, and ASTM Grade No. 2-D in North America), straight-run gas oil, hydrotreated diesel, hydrocracked distillate streams, and blended intermediate products from refinery blending units are routinely submitted for cetane number verification.
Blended and Alternative Diesel Fuels
Biodiesel blends such as B5, B7, B10, B20, and B30, fatty acid methyl ester (FAME) components, renewable diesel, gas-to-liquid (GTL) synthetic diesel, and dimethyl ether based fuel formulations are tested to confirm that blending has not compromised ignition quality. Biodiesel components generally raise the cetane number of the finished blend, but verification against specification remains mandatory.
Research, Storage, and Field Investigation Samples
Fuel development samples from pilot plants, fuels subjected to long-term storage stability studies, fuels recovered from engine durability trials, and complaint or dispute samples drawn from retail stations, pipelines, vessels, or customer tanks are analyzed to diagnose poor cold starting, excessive combustion noise, or non-conforming fuel quality.
Testing Items
Typical cetane related test items reported by accredited third-party laboratories include:
- Cetane number (CN) determined by engine-based ignition delay measurement
- Derived cetane number (DCN) determined by constant volume combustion chamber methods
- Calculated cetane index (CI) derived from density and distillation recovery temperatures
- Compliance assessment against specification limits such as ASTM D975, EN 590, and GB 19147
- Comparative evaluation of cetane improver additive effectiveness
- Ignition quality trend monitoring for storage stability and quality assurance programs
Testing Methods
ASTM D613 / EN ISO 5165 / GB/T 386 - CFR Engine Method
The Cooperative Fuel Research (CFR) engine method is the internationally recognized referee method for cetane number determination. A single-cylinder, variable compression ratio engine operated under standardized speed, injection timing, and intake air conditions is run alternately on the test fuel and on primary reference fuel blends of n-cetane and heptamethylnonane. The compression ratio required to achieve a fixed ignition delay is bracketed, and the cetane number is calculated by interpolation between the reference blends. This method offers the highest authority for specification disputes but requires a large sample volume, a skilled operator, and a long analysis time.
ASTM D6890 / EN 15195 / SH/T 0764 - Ignition Quality Tester (IQT)
The Ignition Quality Tester method determines the derived cetane number (DCN) by injecting a small quantity of fuel into a heated, constant volume combustion chamber charged with compressed air. The measured ignition delay is converted to a DCN through a correlation with the ASTM D613 engine method. The IQT method requires far less sample, delivers results in a fraction of the time, and maintains good precision, which makes it well suited to refinery process control and high-throughput laboratory screening.
ASTM D7170 - Constant Volume Combustion Chamber Method
ASTM D7170 employs an automated constant volume combustion chamber instrument to measure ignition delay and combustion behavior, reporting a derived cetane number. It is a rapid, low-sample-consumption alternative used for research screening, blend optimization, and quality verification where the full engine method is impractical.
ASTM D976 and ASTM D4737 / EN ISO 4264 - Calculated Cetane Index
The cetane index is an estimate of the cetane number calculated from easily measured bulk properties, specifically the density of the fuel and the distillation recovery temperatures at 10 percent, 50 percent, and 90 percent volume evaporated. ASTM D976 uses a two-variable equation, while ASTM D4737 uses a four-variable equation and is preferred for modern fuels. The cetane index is not valid for fuels containing cetane improver additives or significant quantities of oxygenated components such as biodiesel, and it is generally accepted as a specification alternative only for straight-run or conventionally refined diesel.
Applications
Cetane number analysis supports a wide range of technical and commercial applications across the fuel supply chain:
- Refinery blending optimization for straight-run, cracked, and hydrotreated diesel components
- Certification and regulatory compliance testing against ASTM D975, EN 590, and GB 19147 specifications
- Verification of biodiesel blend ignition quality and blend ratio effects
- Engine calibration, combustion development, and fuel effects research by engine manufacturers
- Emission control programs, since higher cetane number typically reduces unburned hydrocarbons and carbon monoxide
- Custody transfer, import and export inspection, and dispute resolution by accredited third-party laboratories
- Investigation of field complaints involving hard starting, rough idling, diesel knock, and excessive smoke
- Quality monitoring of cetane improver additive dosing during fuel terminal operations
FAQ
What is the difference between cetane number and cetane index? The cetane number is a directly measured physical property obtained from engine or combustion chamber testing, whereas the cetane index is a calculated estimate derived from density and distillation data. The cetane index cannot account for cetane improver additives and is therefore not applicable to additized or oxygenated fuels.
What is the difference between cetane number and derived cetane number? The derived cetane number (DCN) is obtained from rapid constant volume combustion chamber methods such as ASTM D6890 or ASTM D7170 and is correlated to, but not identical with, the engine-measured cetane number of ASTM D613. Precision statements for each method govern which result is contractually or regulatorily authoritative.
How does the cetane number differ from the octane number? The two parameters describe opposite ignition behaviors. Diesel engines rely on autoignition and therefore require a high cetane number for short ignition delay, while gasoline engines resist autoignition and therefore require a high octane number. A fuel with excellent diesel ignition quality would exhibit poor gasoline knock resistance, and vice versa.
What is considered a good cetane number? Most modern diesel specifications require minimum values between 40 and 51. Values between 45 and 55 generally provide excellent cold starting, low combustion noise, and favorable emissions, while benefits diminish noticeably above approximately 55.
What factors influence the cetane number of diesel? Normal paraffins and saturated straight-chain hydrocarbons raise the cetane number, whereas aromatics, naphthenes, and heavily branched iso-paraffins lower it. Deep hydrotreating and hydrocracking increase cetane number by saturating aromatics, while thermal cracking and coker distillates reduce it.
Can the cetane number be improved? Yes. Alkyl nitrate cetane improver additives, such as 2-ethylhexyl nitrate, effectively raise the cetane number by accelerating pre-flame oxidation chemistry. Blending with high-cetane components such as biodiesel, GTL diesel, or straight-run paraffinic stocks also increases ignition quality.