Cetane number improvers are the most widely used performance additives in modern diesel fuel, and verifying their identity, purity, and actual ignition benefit requires specialized laboratory testing. This technical article explains how diesel additive cetane improvers such as 2-ethylhexyl nitrate are evaluated, covering sample types, key testing items, internationally recognized methods including ASTM D613, ASTM D6890, EN 15195, and GB/T 386, and the industrial applications of the results. Whether you are an additive manufacturer, refinery blender, or fuel distributor, understanding this testing framework is essential for compliance with specifications such as EN 590, ASTM D975, and GB 19147.
Technical Overview
Cetane number (CN) is the fundamental measure of the ignition quality of diesel fuel. It quantifies the ignition delay period between the start of fuel injection and the onset of combustion in a compression-ignition engine. A higher cetane number means a shorter ignition delay, smoother and quieter combustion, faster cold starting, improved fuel economy, and lower hydrocarbon, carbon monoxide, and particulate emissions.
Cetane number improvers are fuel-soluble organic compounds added to diesel at low treat rates to raise the effective cetane number of the fuel. The dominant chemistry in commercial use is alkyl nitrates, principally 2-ethylhexyl nitrate (2-EHN), followed by organic peroxides such as di-tert-butyl peroxide (DTBP). These compounds decompose readily at the high temperatures and pressures inside the engine cylinder, generating free radicals that accelerate pre-flame reactions and shorten the ignition delay. Typical treat rates range from approximately 100 to 2000 mg/kg, delivering cetane improvements of roughly 2 to 8 numbers depending on the base fuel composition and dosage.
Testing of cetane number improvers serves several purposes: quality control of the additive concentrate itself, verification of the cetane lift achieved at a declared treat rate, confirmation that the additive does not degrade other fuel properties, and demonstration of compliance with regulatory fuel specifications. European standard EN 590 requires a minimum cetane number of 51, ASTM D975 in North America specifies a minimum of 40 for Grade No. 2-D, and the Chinese national standard GB 19147 imposes grade-dependent minimums, so cetane improvers are frequently the most economical route for refiners and blenders to meet these limits.
Testing Samples
Neat Cetane Improver Concentrates
Technical-grade 2-ethylhexyl nitrate, di-tert-butyl peroxide formulations, and multifunctional additive packages in which the cetane improver is combined with lubricity improvers, detergents, corrosion inhibitors, demulsifiers, and antifoams. Laboratory evaluation of these samples focuses on identity confirmation, purity, active content, and physical-chemical properties.
Base Diesel Fuel Samples
Unadditized diesel fuel drawn from refinery streams, storage tanks, or certified reference fuel batches. These samples establish the baseline cetane number against which the performance of the additive is measured. Characterization of density, distillation curve, aromatics content, and sulfur level supports interpretation of the additive response.
Additized Fuel Blends
Laboratory-prepared blends of base diesel treated with the cetane improver at defined dosages, commonly 250, 500, 1000, and 2000 mg/kg. These blends are used to construct the dose-response curve and to determine the treat rate needed to achieve a target cetane number increase.
Finished Commercial Diesel
Market fuels from terminals, service stations, or bulk deliveries. Testing verifies conformity with the applicable specification, confirms the presence and effectiveness of any declared cetane additive, and supports dispute resolution between fuel suppliers and end users.
Testing Items
A comprehensive evaluation program for a cetane number improver typically covers the following parameters:
- Cetane number of the base fuel and of the treated fuel (ASTM D613, GB/T 386)
- Derived cetane number and ignition delay (ASTM D6890, ASTM D7170, ASTM D7668, EN 15195)
- Cetane number improvement (delta CN) at specified treat rates and full dose-response behavior
- Identity confirmation of 2-EHN or organic peroxide chemistry by FTIR spectroscopy and GC-MS
- Active content and purity of the additive concentrate by gas chromatography with flame ionization detection
- Total nitrogen content by oxidative combustion with chemiluminescence detection (ASTM D4629)
- Density (ASTM D4052), flash point (ASTM D93), and kinematic viscosity (ASTM D445) of the additive
- Water content by coulometric Karl Fischer titration (ASTM D6304) and appearance and color assessment
- Acid number (ASTM D664) and trace chloride content of the concentrate
- Effect of the additive on fuel lubricity by HFRR wear scar diameter (ASTM D6079, ISO 12156-1)
- Effect on oxidation stability (ASTM D2274, EN ISO 12205, and EN 15751 for fatty acid methyl ester blends)
- Copper strip corrosion (ASTM D130) and storage stability of treated fuel
- Cold flow properties including cloud point, pour point, and CFPP to confirm no adverse interaction
- Sulfur content (ASTM D5453) to confirm the additive does not affect ultra-low sulfur diesel compliance
Testing Methods
Engine-Based Cetane Number Determination
The classical reference method is ASTM D613, equivalent in principle to the Chinese standard GB/T 386. A single-cylinder, variable compression ratio CFR cetane engine is operated under strictly standardized conditions, and the ignition quality of the sample is bracketed against primary reference fuels: n-hexadecane with an assigned cetane number of 100 and heptamethylnonane with an assigned cetane number of 15. The method requires approximately one liter of sample per determination and provides the legally and contractually recognized cetane number, with repeatability and reproducibility limits defined in the standard. Because reproducibility between laboratories is on the order of several cetane numbers, booster claims of a few points are often corroborated with instrumental methods or replicate testing.
Constant Volume Combustion Chamber Methods
Instrumental alternatives measure ignition delay in a constant volume combustion chamber and calculate a derived cetane number (DCN). ASTM D6890 and its European counterpart EN 15195 use the ignition quality tester principle, requiring well under 200 mL of sample and delivering results within a fraction of an hour. ASTM D7170 employs a fuel ignition tester with fixed injection volume, while ASTM D7668 determines both ignition delay and combustion delay. These methods are formally correlated with ASTM D613 and are widely used for additive screening, dose-response studies, and high-throughput quality control.
Chemical Composition Analysis
Gas chromatography with mass spectrometric detection confirms the identity of 2-ethylhexyl nitrate and reveals synthesis by-products, solvents, or degradation products in the concentrate. Gas chromatography with flame ionization detection quantifies the active content, which for commercial 2-EHN is typically specified at 97 percent or higher. FTIR spectroscopy verifies the characteristic nitrate ester absorption bands, and total nitrogen determination by ASTM D4629 provides an independent check, since pure 2-EHN contains approximately 8 percent nitrogen by mass. These techniques can also detect residual nitrate additive in finished fuel at low concentrations.
Physical and Performance Testing
The concentrate and treated blends are characterized for density, flash point, viscosity, water content, and corrosion behavior. Because cetane improvers are reactive chemicals, flash point testing is also relevant to safe storage, transport, and handling classification. Performance testing then evaluates whether the additive degrades any regulated fuel property, including lubricity, oxidation stability, cold flow behavior, corrosion, and foaming tendency, with the final treated fuel certified against the relevant limits in EN 590, ASTM D975, or GB 19147.
Treat-Rate Evaluation Protocol
A typical evaluation proceeds by establishing the baseline cetane number of the base fuel, preparing blends at a series of dosages, determining the cetane number or DCN of each blend, plotting the delta CN response, and confirming that secondary properties remain within specification at the selected commercial treat rate. This protocol allows the supplier to define minimum recommended dosage and allows end users to verify supplier performance claims objectively.
Applications
Testing of cetane number improvers supports a wide range of commercial and technical activities across the fuel supply chain:
- Quality control and batch release at additive manufacturing and blending plants
- Refinery cetane shortfall management, allowing refiners to meet EN 590, ASTM D975, or GB 19147 cetane limits without re-blending crude-derived components
- Formulation development of multifunctional diesel additive packages and comparative benchmarking of competing chemistries
- Verification of performance claims by fuel marketers, fleet operators, and premium diesel brand owners
- Regulatory compliance verification, import and export inspection, and customs classification support
- Dispute resolution, warranty investigation, and root-cause analysis of field complaints such as hard starting or excessive combustion noise
- Research into alternative ignition improvers, renewable diesel, and synthetic paraffinic fuels covered by EN 15940
FAQ
What is a cetane number improver?
A cetane number improver is a fuel-soluble additive, most commonly 2-ethylhexyl nitrate or an organic peroxide, that decomposes under engine cylinder conditions to accelerate pre-flame chemistry and shorten ignition delay, thereby raising the effective cetane number of the treated diesel fuel.
How much cetane improvement can a typical additive deliver?
Response depends on base fuel composition and dosage. As a general guideline, treat rates of 500 to 1000 mg/kg of 2-EHN typically raise the cetane number by roughly 2 to 6 points, with diminishing returns at higher dosages. A laboratory dose-response study provides the reliable figure for any specific fuel.
What is the difference between cetane number and derived cetane number?
Cetane number is measured directly in a CFR cetane engine by ASTM D613 or GB/T 386, while derived cetane number is calculated from ignition delay measured in a constant volume combustion chamber by ASTM D6890, ASTM D7170, ASTM D7668, or EN 15195. The two values are statistically correlated, and the instrumental DCN methods are faster, cheaper, and require far less sample.
Can the presence of a nitrate cetane improver be detected in finished fuel?
Yes. GC-MS can identify 2-EHN in extracted or direct-injected fuel samples, and elevated nitrogen content measured by ASTM D4629 can corroborate the presence of a nitrate-type improver, since untreated diesel is normally very low in nitrogen.
Do cetane improvers affect other fuel properties?
At normal treat rates, well-formulated cetane improvers have minimal effect on density, distillation, and cold flow, but nitrate additives can influence oxidation stability, storage stability, and elastomer compatibility over time. A complete qualification program therefore checks lubricity, stability, corrosion, and full specification compliance of the treated fuel.
How much sample is required and how long does testing take?
Engine cetane number testing generally requires about one liter of fuel, while instrumental DCN methods need less than 200 mL, and chemical characterization of an additive concentrate requires only tens of milliliters. Typical turnaround ranges from a few working days for DCN and composition testing to one to two weeks for full engine-based cetane certification with a complete property panel.