🔢 Cetane Index of Diesel Fuel

Petrochemistry PN-EN ISO 4264

Overview

STATUS (as of 30.10.2026). PN-EN ISO 4264:2018-08 — Polish and English versions, implements EN ISO 4264:2018 and ISO 4264:2018 [IDT]; the Polish version was published on 30.10.2020. It replaced PN-EN ISO 4264:2010 and its amendment A1:2013-07. ISO 4264:2018 is the third edition of the standard (published on 22.06.2018, it replaced ISO 4264:2007 and Amendment 1:2013), last confirmed by ISO on 18.10.2023.

SCOPE (clause 1). Calculation of the cetane index of middle-distillate fuels from petroleum-derived sources; the value is called the cetane index by four-variable equation. The standard covers fuels containing non-petroleum derivatives from tar sands and oil shale; it does not apply to pure hydrocarbons or to distillate fuels derived from coal. The calculation does not take into account the effect of additives used to enhance the cetane number. The cetane index is not an alternative way to express the cetane number but a supplementary tool, to be used with due regard for its limitations — it is used to estimate the cetane number when a test engine is not available or when there is insufficient sample for an engine rating.

PROPERTY RANGE AND PREDICTION ERROR (clause 1). Most suitable range of fuel properties: cetane number from 32.5 to 56.5; density at 15 °C from 805.0 to 895.0 kg/m³; 10 % (V/V) recovery temperature from 171 °C to 259 °C, 50 % (V/V) from 212 °C to 308 °C, 90 % (V/V) from 251 °C to 363 °C. Within the cetane number range from 32.5 to 56.5 the expected error of the prediction is less than ±2 cetane numbers for 65 % of the fuels examined; outside this range errors can be greater, and because of sample-specific biases also within it, so users assess the degree of prediction agreement they need. Sample-specific biases have been observed for distillate fuels containing FAME.

EDITION (foreword). The third edition added a statement on sample-specific bias, moved the second procedure — by nomography — into informative Annex A and admitted ISO 3924 (boiling range distribution by gas chromatography) as an alternative to ISO 3405.

INPUT DATA AND CALCULATION (clauses 5–7). The density at 15 °C is determined to the nearest 0.1 kg/m³ according to ISO 3675 or ISO 12185, with corrections according to ISO 91 if appropriate; the 10 %, 50 % and 90 % (V/V) recovery temperatures — to the nearest 1 °C, corrected to standard barometric pressure, according to ISO 3405 or according to ISO 3924 after converting its data to estimated ISO 3405 data. Index: CI = 45.2 + 0.089 2 · T₁₀N + (0.131 + 0.901 · B) · T₅₀N + (0.052 3 − 0.42 · B) · T₉₀N + 0.000 49 · (T₁₀N² − T₉₀N²) + 107 · B + 60 · B², where T₁₀N = T₁₀ − 215, T₅₀N = T₅₀ − 260, T₉₀N = T₉₀ − 310 (temperatures in °C), B = [exp(−0.003 5 · DN)] − 1, DN = D − 850, and D is the density at 15 °C in kg/m³. The index can also be read from nomographs (Annex A), but in case of dispute the calculation by the equation applies. The result is reported to the nearest 0.1.

PRECISION (clause 8). The calculation of the index from the measured density and recovery temperatures is exact; the precision of the index depends on the precision of the input determinations, given in ISO 3675, ISO 12185 and ISO 3405.

WHAT WE DO NOT GIVE HERE. The nomographs of informative Annex A (Figures A.1–A.3) — we only know the description of their use.

Method principle

The density at 15 °C and the temperatures at which 10 %, 50 % and 90 % (V/V) of the distillate is recovered are determined by standard test methods, and the cetane index is calculated from these data using known correlations — the four-variable equation.

The calculation from measured data is exact; the uncertainty of the result comes from the precision of the input determinations, and the agreement with the cetane number from the limitations of the correlation (fuel property range, sample-specific biases).

Applications

• Estimating the cetane number of middle-distillate fuels when a test engine is not available or there is insufficient sample for an engine rating (clause 1, note 3)
• Diesel fuel — requirement of the Polish regulation on quality requirements for liquid fuels: cetane index at least 46.0, alongside a cetane number of at least 51.0 (Dz.U. 2024 item 1018, Annex 3 No 1–2)
• Fuels containing non-petroleum derivatives from tar sands and oil shale (clause 1)
• Outside the scope: pure hydrocarbons and distillates derived from coal; the calculation does not take into account cetane-improving additives (clause 1)

Key parameters

Standard

Standard number

PN-EN ISO 4264:2018-08

Title (PL)

Przetwory naftowe — Obliczanie indeksu cetanowego paliw ze średnich destylatów metodą równania czterech zmiennych

Title (EN)

Petroleum products — Calculation of cetane index of middle-distillate fuels by the four variable equation

Step-by-step procedure

1. Density at 15 °C

   According to ISO 3675 or ISO 12185, to the nearest 0.1 kg/m³, with corrections according to ISO 91 if appropriate (5.1).

2. Recovery temperatures

   T₁₀, T₅₀ and T₉₀ to the nearest 1 °C, corrected to standard barometric pressure, according to ISO 3405 (5.2); alternatively ISO 3924 with its data converted to estimated ISO 3405 data (5.3–5.4).

3. Calculation

   DN = D − 850; B = [exp(−0.003 5 · DN)] − 1; T₁₀N = T₁₀ − 215, T₅₀N = T₅₀ − 260, T₉₀N = T₉₀ − 310; CI according to Formula (1) of 6.1.

4. Assessing the fitness of the prediction

   Comparison of the fuel data with the most suitable property range; outside it and with sample-specific biases (for example fuels containing FAME) the error can be greater (clause 1).

5. Result

   Cetane index by four-variable equation to the nearest 0.1; in case of dispute the calculation of 6.1 applies, not the nomograph (6.2, clause 7).

Required equipment and apparatus

Health and safety (OHS)

• Use of the standard can involve hazardous materials, operations and equipment — safety measures are established by the user (warning of the standard)
• The hazards come from the input determinations — density and distillation; precautions according to ISO 3675, ISO 12185, ISO 3405 or ISO 3924

Frequently asked questions