⛽ Cetane number of diesel fuel — engine method: single-cylinder engine with pre-combustion chamber and variable compression ratio, interpolation between two primary reference fuel blends (hexadecane / heptamethylnonane or pentamethylheptane); PN-EN ISO 5165

Petrochemistry PN-EN ISO 5165

Overview

WHAT THE STANDARD COVERS. ISO 5165:2020 (clause 1) establishes the rating of diesel fuel on an arbitrary scale of cetane numbers (CN) using a standardized single-cylinder, four-stroke engine with variable compression ratio and indirect injection; the cetane number is a measure of the ignition performance of diesel fuel in compression-ignition engines, determined at constant speed in a pre-combustion chamber type test engine. The standard cautions that the relationship of test engine results to the performance of full-scale engines with variable speed and load is not completely understood. The scale covers 0–100 CN, typically the range 30–65 CN is tested. A CEN interlaboratory study of 2013 (10 samples in the range 52.4–73.8 CN) confirmed that paraffinic diesel fuel from synthesis or hydrotreatment, containing up to 7 % (V/V) of fatty acid methyl esters (FAME), can be tested by this method with a precision comparable to conventional fuels. The method can be applied to non-conventional fuels (synthetic, vegetable oils), but their precision has not been established. Samples whose properties hinder the gravity flow of fuel to the pump or delivery through the injector are not suitable. The "Scope" field of the PKN card of PN-EN ISO 5165:2021-02 (Polish version), fragment: "CN is a measure of the ignition properties of diesel fuel in compression-ignition engines. (…) This document applies to the whole scale range from 0 CN to 100 CN, but typical testing is carried out in the range from 30 CN to 65 CN" (our translation). The card adds that the relationship between the performance of the test engine and that of a full-scale engine running at variable speed and load is not completely understood.

PRINCIPLE (ISO 5165:2020, clauses 3–4). The cetane number of diesel fuel is determined by comparing its combustion characteristics in a test engine with blends of reference fuels of known cetane number under standardized operating conditions — by the "bracketing handwheel" procedure: for the sample and for each of two reference fuels bracketing it, the compression ratio (handwheel reading) is varied to obtain a specified ignition delay, which permits interpolation of the cetane number in terms of the handwheel reading. The ignition delay is the time from the start of injection to the start of combustion, expressed in degrees of crank angle; it is displayed by a cetane meter (the standard lists three approved generations: Mark II Ignition Delay Meter of 1974, Dual Digital Cetane Meter of 1990 and XCP Cetane Panel of 2014).

REFERENCE FUELS (points 3.11–3.13 and clause 5). The primary reference fuels (PRF) are hexadecane (n-cetane, 100 CN), 2,2,4,4,6,8,8-heptamethylnonane (HMN, 15 CN), 2,2,4,6,6-pentamethylheptane (PMH, 16.3 CN) and their two-component volumetric blends; the scale is defined by the formulas CN = Pr + 0.15·P(HMN) and CN = Pr + 0.163·P(PMH), where Pr is the percentage of hexadecane. Historically the scale was defined by 1-methylnaphthalene (0) and hexadecane (100); in 1962 1-methylnaphthalene was replaced by HMN, and in 2018 PMH was added as a material of higher purity and availability. Requirements of Table 1: purity of at least 990 g/kg (hexadecane), 980 g/kg (HMN), 995 g/kg (PMH) — gas chromatography; hydroperoxide number not more than 5.0 mg/kg expressed as oxygen (ASTM D3703). Hexadecane melts at 18 °C, which is why PRFs are stored and used at a temperature of at least 20 °C. The secondary reference fuels (SRF) are blends of fuels "T" (high CN, Table 2: 73–76) and "U" (low CN: 19–22), rated by the ASTM Diesel National Exchange Group and not interchangeable between batches; check fuels — 38–42 (low) and 50–55 CN (high). Cylinder jacket coolant: water of grade 3 according to ISO 3696, boiling at 100 °C ± 2 °C; engine oil SAE 30 (9.3–12.5 mm²/s at 100 °C, viscosity index at least 85), recommended change every 50 hours of engine operation.

APPARATUS (clause 6). A single-cylinder engine with a standard crankcase with a fuel pump assembly, a cylinder head with a pre-combustion chamber, a thermosiphon jacket cooling circuit, several fuel tanks with selector valves, an injector with a specified nozzle and an exhaust; the engine is belt-coupled to an electric motor which starts it and absorbs power at constant speed. According to the standard, the engine equipment and instruments are supplied by a single manufacturer. The contents also list a device for dispensing reference fuels (6.3) and an injector nozzle tester (6.4).

EDITION. The fifth edition (2020) replaced ISO 5165:2017 and is harmonized with ASTM D613; changes according to the foreword: requirements for PRF, SRF and check fuels, PMH as an alternative to HMN, new requirements for the test report.

STATUS (PKN card as of 27.10.2026). The card of PN-EN ISO 5165:2021-02 (Polish version) has no withdrawal label: publication 29-06-2022, 28 pages, Chemistry Sector, KT 222/PK 1 Liquid Fuels, "Adopts: EN ISO 5165:2020 [IDT], ISO 5165:2020 [IDT]", "Replaces: PN-EN ISO 5165:2018-03 - Polish version, PN-EN ISO 5165:2018-03 - English version", ICS 75.160.20. PKN search, search term "ISO 5165" (27.10.2026): alongside it there is the English version PN-EN ISO 5165:2021-02; the editions PN-EN ISO 5165:2002, 2003 and 2018-03 are withdrawn. In the iTeh catalogue (read on 27.10.2026) ISO 5165:2020 has the status "Published" and the stage "9092 – International Standard to be revised" (standard to be revised), and EN ISO 5165:2020 — "Published".

REGULATIONS. Regulation on test methods for the quality of liquid fuels (Journal of Laws 2024 item 1058), annex, part II points 1 and 1.1: the cetane number of diesel fuel is determined by one of five methods, and for the engine method comparing self-ignition properties with reference fuel blends the procedure, apparatus, calculations and precision are specified by PN-EN ISO 5165; the other methods are the derived cetane number (DCN) from a constant-volume chamber (standard adopting EN 15195), a high-temperature chamber with a pressure sensor (PN-EN 16715), an engine comparing the mass of intake air (standard adopting EN 16906) and the indicated cetane number ICN (PN-EN 17155); according to point 1.2 DCN is an approximation of the cetane number determined according to PN-EN ISO 5165. Regulation on quality requirements for liquid fuels (Journal of Laws 2024 item 1018), Annex No 3, item 1: cetane number of diesel fuel at least 51.0. For biofuels: regulation on test methods for the quality of liquid biofuels (Journal of Laws 2025 item 1341, "in force"), part I point 5.1 — for the methyl ester as a self-contained fuel the procedure according to PN-EN ISO 5165, and the precision according to Annex A to PN-EN 14214; regulation on quality requirements for liquid biofuels (Journal of Laws 2025 item 426, in the ELI database "act covered by a consolidated text"), Annex No 1, item 5: cetane number of FAME at least 51.0.

HOW MANY LABORATORIES AND IN WHAT FORM (copy of the accreditation scope database refreshed from the production server on 27.10.2026 and current PCA documents downloaded on that day). Number 5165 appears in 8 records at 5 Polish laboratories: AB 009, AB 387, AB 474, AB 484, AB 773. The PCA documents confirm the standard at four of them; AB 474 has no scope document on the PCA website on 27.10.2026. No entry is suspended. Forms: "PN-EN ISO 5165:2021-02" (AB 474, AB 484) and "PN-EN ISO 5165" without the year (AB 009, AB 387, AB 773). The "Laboratories" tab ("PN-EN ISO 5165") shows 7 laboratories: these 5 Polish ones and 2 Czech ones (tab query on labcoda.pl, 27.10.2026).

WHAT THE LABORATORIES TEST (database copy and PCA documents). All entries are the engine method. Ranges: 40.0–56.0 (AB 009: diesel fuel, biohydrocarbons for compression-ignition engines, diesel fuel with FAME, FAME), 30.0–65.0 (AB 474 and the Instrumental Laboratory of AB 484: diesel fuels, light heating oil, marine fuel, petroleum distillates), 50.0–60.0 (AB 484: Fuels and Aromatics Laboratory — diesel fuel and FAME, service point in Ostrów Wielkopolski — diesel fuel), 45.0–60.0 (AB 773: diesel fuel, also with FAME); AB 387 (Nowa Wieś Wielka) gives no range.

ALONGSIDE THIS STANDARD (database copy, 27.10.2026). The cetane number determined by other methods is in the scopes of AB 387 and AB 484 — according to PN-EN 16715 (high-temperature chamber, derived cetane number).

WHERE A SEEMINGLY CORRECT RESULT IS EASY TO GET. First — two reference values for the low-cetane material: HMN is 15, and PMH 16.3 CN; the blend is calculated with the formula appropriate to the fuel used (factor 0.15 or 0.163) — the same volumetric composition gives a different cetane number. Second — hexadecane melts at 18 °C: PRFs are stored and used at at least 20 °C, and fuels "T" and "U" after cold storage are warmed to at least 14 °C above the cloud point, held for 30 minutes and mixed (points 5.3, 5.6). Third — scope of application: the precision is established for conventional fuels and for paraffinic diesel fuel with FAME up to 7 % (V/V), but not for synthetic fuels and vegetable oils; for FAME, which AB 009 and AB 484 test, the regulation on test methods for biofuels gives the precision from Annex A to PN-EN 14214. Fourth — the cetane number (engine) and the cetane index (calculated from density and distillation according to PN-EN ISO 4264) are two different quantities with separate requirements (for diesel fuel 51.0 and 46.0). Fifth — DCN from chamber methods is, according to the regulation, an approximation of the cetane number from PN-EN ISO 5165, not the same result.

WHAT WE DO NOT GIVE. The fragment of ISO 5165:2020 we read ends at point 6.2. We do not give the engine settings (clause 8: speed, timing, injector opening pressure, temperatures, basic ignition delay), engine qualification (clause 9), the course of the determination (clause 10), calculations and precision (clauses 11–13).

Method principle

For the sample and two reference fuel blends bracketing it (hexadecane = 100 CN; heptamethylnonane = 15 CN or pentamethylheptane = 16.3 CN), the compression ratio (handwheel reading) of a pre-combustion chamber test engine is varied to obtain the same specified ignition delay; the cetane number of the sample is interpolated from the handwheel readings ("bracketing handwheel", ISO 5165:2020, clause 4). Cetane number of a reference blend: CN = Pr + 0.15·P(HMN) or CN = Pr + 0.163·P(PMH). WE DO NOT GIVE the engine settings and the course of the test.

Applications

• Diesel fuels for compression-ignition engines — scale 0–100 CN, typically 30–65 CN (ISO 5165:2020, clause 1)
• Paraffinic diesel fuel from synthesis or hydrotreatment with FAME up to 7 % (V/V) — precision comparable to conventional fuels (CEN study 2013)
• Quality control of diesel fuel (requirement CN ≥ 51.0, Journal of Laws 2024 item 1018) and of FAME as a self-contained fuel (CN ≥ 51.0, Journal of Laws 2025 item 426)
• In accreditation scopes: 5 Polish laboratories, engine method; ranges from 30.0–65.0 to 50.0–60.0 CN

Key parameters

Standard

Standard number

PN-EN ISO 5165:2021-02 (wersja polska i wersja angielska; wprowadza EN ISO 5165:2020 i ISO 5165:2020 [IDT])

Title (PL)

Przetwory naftowe — Oznaczanie właściwości zapłonowych olejów napędowych — Metoda silnikowa oznaczania liczby cetanowej

Title (EN)

Petroleum products — Determination of the ignition quality of diesel fuels — Cetane engine method

Step-by-step procedure

1. Sample and reference fuels

   Sampling according to ISO 3170 or ISO 3171 (normative references); PRFs at a temperature ≥ 20 °C, fuels "T" and "U" after cold storage warmed to ≥ 14 °C above the cloud point and mixed.

2. Engine preparation and qualification

   Basic settings according to clause 8, check on check fuels according to clause 9. WE DO NOT GIVE the values.

3. Determination

   For the sample and two reference blends bracketing it, the compression ratio giving the same ignition delay is set; the cetane number is interpolated from the handwheel readings (clause 4). WE DO NOT GIVE the course from clause 10.

4. Result

   Cetane number according to clauses 11–12; assessment against the requirement of 51.0 (diesel fuel, FAME). PN-EN ISO 5165:2021-02 — current (PKN card as of 27.10.2026).

Required equipment and apparatus

Reagents, media and consumables

Health and safety (OHS)

• The PRF and SRF reference fuels and check fuels are flammable, their vapours harmful (warnings in points 5.3–5.7)
• The standard warns that its use may involve hazardous materials, operations and equipment, and that the user establishes safety and health practices (warning before clause 1)
• TECHNICAL NOTE: the derived cetane number (DCN) from chamber methods is an approximation of the engine cetane number, not the same result

Frequently asked questions