oSIST prEN 16715:2025

Liquid petroleum products - Determination of ignition delay and derived cetane number (DCN) of middle distillate fuels - Ignition delay and combustion delay determination using a constant volume combustion chamber with direct fuel injection

This document specifies a test method for the quantitative determination of ignition and combustion delays of middle distillate fuels intended for use in compression ignition engines. The method utilizes a constant volume combustion chamber with direct fuel injection into heated, compressed synthetic air. A dynamic pressure wave is produced from the combustion of the product under test. An equation is given to calculate the derived cetane number (DCN) from the ignition and combustion delays determined from the dynamic pressure curve.
This document is applicable to middle distillate fuels, fatty acid methyl esters (FAME) and blends of diesel fuels and FAME. The method is also applicable to middle distillate fuels of non-petroleum origin, oil-sands based fuels, blends of fuel containing biodiesel material, diesel fuel oils containing cetane number improver additives and low-sulphur diesel fuel oils. However, users applying this document especially to unconventional distillate fuels are warned that the relationship between derived cetane number and combustion behaviour in real engines is not yet fully understood.
This document covers the ignition delay range from 2,6 ms to 3,9 ms and combustion delay from 3,78 ms to 6,56 ms (62,78 DCN to 39,44 DCN).
NOTE   The combustion analyser can measure shorter or longer ignition and combustion delays, but precision is not known.
WARNING - The use of this document can involve hazardous materials, operations and equipment. This document does not purport to address all of the safety problems associated with its use. It is the responsibility of users of this document to take appropriate measures to ensure the safety and health of personnel prior to application of the document, and fulfil statutory and regulatory requirements for this purpose.

Flüssige Mineralölerzeugnisse - Bestimmung des Zündverzugs und der abgeleiteten Cetanzahl (ACZ) von Kraftstoffen aus Mitteldestillaten - Bestimmung des Zündverzugs und des Verbrennungsverzugs in einer Verbrennungskammer mit konstantem Volumen und direkter Kraftstoffeinspritzung

Dieses Dokument legt ein Prüfverfahren zur quantitativen Bestimmung des Zündverzugs und des Verbrennungsverzugs von Kraftstoffen aus Mitteldestillaten fest, die in Dieselmotoren eingesetzt werden. Dieses Verfahren nutzt eine Verbrennungskammer mit konstantem Volumen mit direkter Kraftstoffeinspritzung in erwärmte und komprimierte synthetische Luft. Durch die Verbrennung des zu prüfenden Produkts wird eine dynamische Druckwelle erzeugt. Eine Gleichung zur Berechnung der abgeleiteten Cetanzahl (ACZ) mit den anhand der dynamischen Druckkurve ermittelten Werte für den Zünd- und den Verbrennungsverzug ist angegeben.
Dieses Dokument gilt für Mitteldestillatkraftstoffe, Fettsäuremethylester (FAME) sowie Gemische aus Dieselkraftstoffen und FAME. Das Verfahren ist ebenfalls auf Mitteldestillatkraftstoffe nicht mineralölstämmiger Herkunft, auf ölsandbasierte Kraftstoffe, auf Biodieselmaterial enthaltende Kraftstoffmischungen, auf Dieselkraftstoffe, welche Zündverbesserer enthalten und auf schwefelarme Dieselkraftstoffe anwendbar. Anwender, die dieses Dokument im Besonderen auf alternative Destillatkraftstoffe anwenden, werden jedoch darauf hingewiesen, dass die Beziehung zwischen der abgeleiteten Cetanzahl und dem Verbrennungsverhalten in realen Motoren noch nicht völlig geklärt ist.
Dieses Dokument umfasst den Zündverzugsbereich von 2,6 ms bis 3,9 ms und den Verbrennungsverzugsbereich von 3,78 ms bis 6,56 ms (ACZ 62,78 bis ACZ 39,44).
ANMERKUNG   Das Verbrennungsprüfgerät kann einen kürzeren oder längeren Zündverzug und Verbrennungsverzug messen, die Präzision ist jedoch nicht bekannt.
WARNUNG - Die Anwendung dieses Dokuments kann die Anwendung gefährlicher Stoffe, Arbeitsgänge und Geräte mit sich bringen. Dieses Dokument beansprucht nicht, alle damit verbundenen Sicherheitsprobleme zu behandeln. Es liegt in der Verantwortung des Anwenders dieses Dokuments, vor dessen Anwendung geeignete Maßnahmen für die Sicherheit und den Gesundheitsschutz des Personals zu ergreifen, und dafür Sorge zu tragen, dass behördliche und gesetzliche Maßnahmen eingehalten werden.

Produits pétroliers liquides — Détermination du délai d'inflammation et de l'indice de cétane dérivé (ICD) des distillats moyens — Détermination du délai d'inflammation et de combustion en utilisant une chambre à volume constant avec injection direct du carburant

Le présent document spécifie une méthode d'essai pour la détermination quantitative des délais d’inflammation et de combustion des distillats moyens utilisés comme carburants dans les moteurs à allumage par compression. Cette méthode utilise une chambre de combustion à volume constant avec injection directe de carburant dans de l'air synthétique comprimé et chauffé. Une onde de pression dynamique est générée par la combustion du produit soumis à l'essai. Une équation est donnée pour calculer l'indice de cétane dérivé (ICD) à partir des délais d’inflammation et de combustion déterminés à partir de la courbe de pression dynamique.
Le présent document s'applique aux distillats moyens, aux esters méthyliques d’acides gras (EMAG) et aux mélanges de carburants diesel et d'EMAG. La méthode est également applicable aux distillats moyens d'origine non pétrolière, aux carburants produits à partir de sables asphaltiques, aux mélanges de carburants contenant du biodiesel, aux carburants diesel contenant des additifs améliorateurs de cétane et aux carburants diesel à faible teneur en soufre. Cependant, l'attention des utilisateurs du présent document, en particulier lorsqu’il est appliqué aux distillats combustibles non conventionnels, est attirée sur le fait que la relation entre l'indice de cétane dérivé et les phénomènes de combustion dans les moteurs n’est pas encore parfaitement comprise.
Le présent document couvre les délais d'inflammation compris entre 2,6 ms et 3,9 ms et les délais de combustion compris entre 3,78 ms et 6,56 ms (ICD compris entre 39,44 et 62,78).
NOTE   L’analyseur de combustion peut mesurer des délais d’inflammation et de combustion plus longs ou plus courts, mais la fidélité n’est pas connue.
ATTENTION - L'utilisation du présent document peut impliquer la mise en œuvre de produits, d'opérations et d'équipements à caractère dangereux. Le présent document n'est pas censé aborder tous les problèmes de sécurité concernés par son usage. Il est de la responsabilité des utilisateurs de ce document de prendre les mesures appropriées pour assurer la sécurité et préserver la santé du personnel avant son application, et pour répondre aux exigences réglementaires et statutaires à cette fin.

Tekoči naftni proizvodi - Ugotavljanje zakasnitve vžiga in izpeljanega cetanskega števila (DCN) v srednje destilatnih gorivih - Določanje zakasnitve vžiga in sežiga z uporabo komore s konstantno prostornino z direktnim injiciranjem goriva

General Information

• Status: Not Published
• Public Enquiry End Date: 04-May-2025
• ICS: 75.160.20 - Liquid fuels
• Technical Committee: NAD - Petroleum products, lubricants and related products
• Current Stage: 4020 - Public enquire (PE) (Adopted Project)
• Start Date: 10-Feb-2025
• Due Date: 30-Jun-2025
• Completion Date: 07-May-2025
• Ref Project: prEN 16715 - Liquid petroleum products - Determination of ignition delay and derived cetane number (DCN) of middle distillate fuels - Ignition delay and combustion delay determination using a constant volume combustion chamber with direct fuel injection

Relations

• Revises: SIST EN 16715:2015 - Liquid petroleum products - Determination of ignition delay and derived cetane number (DCN) of middle distillate fuels - Ignition delay and combustion delay determination using a constant volume combustion chamber with direct fuel injection
• Effective Date: 01-Apr-2025

Overview

prEN 16715 (draft 2025) is a CEN standard method for measuring ignition delay (ID) and combustion delay (CD) of middle distillate fuels and for deriving a derived cetane number (DCN). The test uses a constant volume combustion chamber (CVCC) with direct fuel injection into heated, compressed synthetic air and analyses the dynamic pressure trace to determine ID, CD and a DCN via a correlation equation. The method is applicable to diesel, FAME (fatty acid methyl esters), biodiesel blends, non‑petroleum distillates and fuels containing cetane improvers, with defined measurement ranges and precision considerations.

Key topics and technical requirements

• Test principle: Direct injection of a sample into a temperature‑ and pressure‑controlled CVCC; pressure sensor records combustion cycles; ID and CD measured from injector signal rise to features on the pressure curve.
• Measurement sequence: 5 preliminary injections then 15 analysis injections; outlier removal using Peirce’s Criterion and averaging of remaining cycles.
• DCN calculation: An explicit correlation equation (Clause 12) converts measured ID/CD to a derived cetane number (DCN). Clause and correlation are periodically reviewed by CEN/TC 19.
• Applicable range: ID 2.6–3.9 ms and CD 3.78–6.56 ms (approx. DCN 62.78 to 39.44). The analyser may measure outside these ranges but precision is not established.
• Calibration & QC: Use of calibration reference fluid (40:60 hexadecane / 2,2,4,4,6,8,8‑heptamethylnonane with assigned ID 2.96 ms, CD 4.90 ms) and verification fluid (methylcyclohexane, ARV ID 11.0 ms, CD 17.0 ms). Purity checks and peroxide removal (e.g., molecular sieves or column) are specified to avoid contamination effects.
• Safety note: Standard warns of hazardous materials and equipment; users must implement appropriate safety and regulatory measures.

Applications and who uses it

• Engine manufacturers and OEMs: fuel/engine matching and ignition characteristic assessment for compression‑ignition engines.
• Refiners and fuel suppliers: quality control, specification testing and monitoring of cetane performance in diesel and biodiesel blends.
• Research labs and test houses: characterisation of conventional and unconventional middle distillates, evaluation of cetane improvers, and inter‑laboratory comparisons.
• Regulatory and procurement bodies: specification compliance and comparative DCN data for fuel selection.

Related standards

• EN ISO 5165 (conventional engine cetane number)
• ASTM D7668 (technical equivalent basis)
• ISO 4259‑1:2017 (precision statements/correlation review guidance)
• EN ISO 3170 / EN ISO 3171 (sampling), IP 537 (purity test for DCN reference fluids)

Keywords: prEN 16715, ignition delay, combustion delay, derived cetane number, DCN, constant volume combustion chamber, middle distillate fuels, FAME, cetane number, combustion analyser.

Standards Content (Sample)

SLOVENSKI STANDARD
01-april-2025
Tekoči naftni proizvodi - Ugotavljanje zakasnitve vžiga in izpeljanega cetanskega
števila (DCN) v srednje destilatnih gorivih - Določanje zakasnitve vžiga in sežiga z
uporabo komore s konstantno prostornino z direktnim injiciranjem goriva
Liquid petroleum products - Determination of ignition delay and derived cetane number
(DCN) of middle distillate fuels - Ignition delay and combustion delay determination using
a constant volume combustion chamber with direct fuel injection
Flüssige Mineralölerzeugnisse - Bestimmung des Zündverzugs und der abgeleiteten
Cetanzahl (ACZ) von Kraftstoffen aus Mitteldestillaten - Bestimmung des Zündverzugs
und des Verbrennungsverzugs in einer Verbrennungskammer mit konstantem Volumen
und direkter Kraftstoffeinspritzung
Produits pétroliers liquides — Détermination du délai d'inflammation et de l'indice de
cétane dérivé (ICD) des distillats moyens — Détermination du délai d'inflammation et de
combustion en utilisant une chambre à volume constant avec injection direct du
carburant
Ta slovenski standard je istoveten z: prEN 16715
ICS:
75.160.20 Tekoča goriva Liquid fuels
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

DRAFT
EUROPEAN STANDARD
NORME EUROPÉENNE
EUROPÄISCHE NORM
March 2025
ICS 75.160.20 Will supersede EN 16715:2015
English Version
Liquid petroleum products - Determination of ignition
delay and derived cetane number (DCN) of middle
distillate fuels - Ignition delay and combustion delay
determination using a constant volume combustion
chamber with direct fuel injection
Produits pétroliers liquides - Détermination du délai Flüssige Mineralölerzeugnisse - Bestimmung des
d'inflammation et de l'indice de cétane dérivé (ICD) Zündverzugs und der abgeleiteten Cetanzahl (ACZ) von
des distillats moyens - Détermination du délai Kraftstoffen aus Mitteldestillaten - Bestimmung des
d'inflammation et de combustion en utilisant une Zündverzugs und des Verbrennungsverzugs in einer
chambre à volume constant avec injection direct du Verbrennungskammer mit konstantem Volumen und
carburant direkter Kraftstoffeinspritzung
This draft European Standard is submitted to CEN members for enquiry. It has been drawn up by the Technical Committee
CEN/TC 19.
If this draft becomes a European Standard, CEN members are bound to comply with the CEN/CENELEC Internal Regulations
which stipulate the conditions for giving this European Standard the status of a national standard without any alteration.

This draft European Standard was established by CEN in three official versions (English, French, German). A version in any other
language made by translation under the responsibility of a CEN member into its own language and notified to the CEN-CENELEC
Management Centre has the same status as the official versions.

CEN members are the national standards bodies of Austria, Belgium, Bulgaria, Croatia, Cyprus, Czech Republic, Denmark, Estonia,
Finland, France, Germany, Greece, Hungary, Iceland, Ireland, Italy, Latvia, Lithuania, Luxembourg, Malta, Netherlands, Norway,
Poland, Portugal, Republic of North Macedonia, Romania, Serbia, Slovakia, Slovenia, Spain, Sweden, Switzerland, Türkiye and
United Kingdom.
Recipients of this draft are invited to submit, with their comments, notification of any relevant patent rights of which they are
aware and to provide supporting documentation.

Warning : This document is not a European Standard. It is distributed for review and comments. It is subject to change without
notice and shall not be referred to as a European Standard.

EUROPEAN COMMITTEE FOR STANDARDIZATION
COMITÉ EUROPÉEN DE NORMALISATION

EUROPÄISCHES KOMITEE FÜR NORMUNG

CEN-CENELEC Management Centre: Rue de la Science 23, B-1040 Brussels
© 2025 CEN All rights of exploitation in any form and by any means reserved Ref. No. prEN 16715:2025 E
worldwide for CEN national Members.

Contents Page
European foreword . 3
Introduction . 4
1 Scope . 5
2 Normative references . 5
3 Terms and definitions . 5
4 Principle . 7
5 Reagents and materials . 7
6 Apparatus . 8
7 Sampling . 8
8 Apparatus assembly and installation . 10
9 Preparation of apparatus . 10
9.1 System start-up and warm-up . 10
9.2 Standard operating and test conditions . 10
10 Calibration . 11
10.1 General. 11
10.2 Calibration, verification and quality control . 11
10.3 Apparatus verification . 12
10.4 Quality control (QC) . 13
11 Test procedure . 13
12 Calculation . 13
13 Expression of results . 14
14 Precision . 14
14.1 General. 14
14.2 Repeatability, r . 14
14.3 Reproducibility, R . 14
15 Test results . 15
Annex A (normative) Combustion analyser description . 16
Annex B (normative) Operational details in support to the standard test procedure . 19
Bibliography . 23

prEN 16725:2025 (E)
European foreword
This document (prEN 16715:2025) has been prepared by Technical Committee CEN/TC 19 “Gaseous and
liquid fuels, lubricants and related products of petroleum, synthetic and biological origin”, the secretariat
of which is held by NEN.
This document is currently submitted to the CEN Enquiry.
This document will supersede EN 16715:2015.
— introduction of the possibility of using a charge air blending system (6.2) and necessary instructions
on the quality control if this is used (under 10.4);
— revision of the correlation equation under Clause 12;
— update of precision statements under Clause 14;
— further clarification of the measurement range towards (39,44 to 62,78) derived cetane number
under Clause 1 in line with the revised ISO 4259-1:2017.
Introduction
This document is derived from joint standardization work within the Energy Institute and ASTM
International. It is based on and technically equivalent with ASTM D7668 [1].
The described method is an alternative quantitative determination of the cetane number of middle
distillate fuels intended for use in compression ignition engines. A correlation study between this method
and EN ISO 5165 [2] has been done and the results of this are incorporated in this document.
The basis of this method is the derived cetane number (DCN) correlation equation as given in Clause 12.
The on-going validation of the equation is monitored and evaluated through the existing American and
European fuel exchange programs. The validation data will be reviewed by CEN/TC 19 with a frequency
of at least every two years. As a result of the review, CEN/TC 19 may make the decision to, if necessary,
modify the existing equation/correlation or develop a new one. As part of this review, the sample types
will be examined, and if certain types are underrepresented, further steps may be taken to evaluate how
they perform.
The ignition delay (ID) and combustion delay (CD) values and the DCN value determined by this test
method provide a measure of the ignition characteristics of diesel fuel oil used in compression ignition
engines. This test is for use by engine manufacturers, petroleum refiners and marketers, and in commerce
as a specification aid to relate or match fuels and engines. This test is also applicable to non-conventional
diesel fuels, such as paraffinic diesel fuel.

For the original 2013 interlaboratory cooperative test programme results, see [3]. For the exchange programs
correlation data analysis see [4].
prEN 16725:2025 (E)
1 Scope
This document specifies a test method for the quantitative determination of ignition and combustion
delays of middle distillate fuels intended for use in compression ignition engines. The method utilizes a
constant volume combustion chamber with direct fuel injection into heated, compressed synthetic air. A
dynamic pressure wave is produced from the combustion of the product under test. An equation is given
to calculate the derived cetane number (DCN) from the ignition and combustion delays determined from
the dynamic pressure curve.
This document is applicable to middle distillate fuels, fatty acid methyl esters (FAME) and blends of diesel
fuels and FAME. The method is also applicable to middle distillate fuels of non-petroleum origin, oil-sands
based fuels, blends of fuel containing biodiesel material, diesel fuel oils containing cetane number
improver additives and low-sulphur diesel fuel oils. However, users applying this document especially to
unconventional distillate fuels are warned that the relationship between derived cetane number and
combustion behaviour in real engines is not yet fully understood.
This document covers the ignition delay range from 2,6 ms to 3,9 ms and combustion delay from 3,78 ms
to 6,56 ms (62,78 DCN to 39,44 DCN).
NOTE The combustion analyser can measure shorter or longer ignition and combustion delays, but precision
is not known.
WARNING — The use of this document can involve hazardous materials, operations and equipment. This
document does not purport to address all of the safety problems associated with its use. It is the
responsibility of users of this document to take appropriate measures to ensure the safety and health of
personnel prior to application of the document, and fulfil statutory and regulatory requirements for this
purpose.
2 Normative references
The following documents are referred to in the text in such a way that some or all of their content
constitutes requirements of this document. For dated references, only the edition cited applies. For
undated references, the latest edition of the referenced document (including any amendments) applies.
EN ISO 3170, Petroleum liquids — Manual sampling (ISO 3170)
EN ISO 3171, Petroleum liquids — Automatic pipeline sampling (ISO 3171)
EN ISO 3696, Water for analytical laboratory use — Specification and test methods (ISO 3696)
ISO 1998-2, Petroleum industry — Terminology — Part 2: Properties and tests
IP 537, Determination of the purity of Derived Cetane Number reference materials — Gas chromatography
method
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 1998-2 and the following apply.
ISO and IEC maintain terminology databases for use in standardization at the following addresses:
— ISO Online browsing platform: available at https://www.iso.org/obp/
— IEC Electropedia: available at https://www.electropedia.org/
3.1
cetane number
CN
measure of the ignition performance of a diesel fuel in a standardized engine test on a scale defined by
reference fuels
Note 1 to entry: It is expressed as the percentage by volume of hexadecane (cetane) in a reference blend having the
same ignition delay as the fuel for analysis. The higher the cetane number, the shorter the ignition delay.
3.2
ignition delay
ID
period of time, in milliseconds (ms), between the start of fuel injection and the start of combustion
Note 1 to entry: In the context of this test method, the start of fuel injection is interpreted as the rise in the
electronic signal that opens the injector, and the combustion start is interpreted as the first significant increase of
the chamber pressure during the combustion cycle, as measured by a pressure sensor in the combustion chamber.
3.3
combustion delay
CD
period of time, in milliseconds (ms), between the start of fuel injection and mid-point of the combustion
pressure curve
3.4
Derived cetane number
DCN
number calculated by using an equation that correlates a combustion analyser’s ignition delay [3.2] and
combustion delay [3.3] to a cetane number
3.5
accepted reference value
ARV
value agreed upon as a reference for comparison
Note 1 to entry: The value is derived as (1) a theoretical or established value, based in scientific principles, (2) an
assigned value, based on experimental work of some national or international organization, or (3) a consensus
value, based on collaborative experimental work under the auspices of a scientific or engineering group.
3.6
quality control sample
QC sample
stable and homogenous material(s) similar in nature to the materials under test, properly stored to
ensure integrity, and available in sufficient quantity for repeated long-term testing
3.7
calibration reference fluid
stable and homogenous fluid used to calibrate the performance of the combustion analyser
3.8
verification reference fluid
stable and homogenous fluid used to verify the performance of the combustion analyser
prEN 16725:2025 (E)
4 Principle
A test portion of the material under test is injected into a temperature and pressure controlled, constant
volume combustion chamber, which has previously been charged with synthetic air of a specified quality.
Each injection produces a compression ignition combustion cycle detected using a pressure sensor. The
ignition delay and combustion delay are measured from the rise of the electronic signal that activates the
injector solenoid to two specific points along the combustion pressure wave produced by the combustion
cycle.
A complete sequence comprises 5 preliminary injection cycles and 15 subsequent injection cycles used
for the sample analysis. The ID and CD measurements for the last 15 injection cycles are statistically
reviewed and the outlying ID’s and CD’s are eliminated using Peirce’s Criterion [6]. The remaining ID’s
and CD’s are averaged to produce the ID and CD results. An equation is given to calculate the derived
cetane number (DCN) from the ignition and combustion delays determined from the dynamic pressure
curve. The DCN obtained by this procedure is an estimate of the cetane number (CN) obtained from the
[2]
conventional large-scale engine test EN ISO 5165 .
5 Reagents and materials
5.1 Calibration reference fluid, 40:60 mixture by weight of hexadecane and 2,2,4,4,6,8,8-
heptamethylnonane, respectively, measured with an accuracy of 0,01 percent. For peroxide-free material
the assigned ID is 2,96 ms and the assigned CD is 4,90 ms
ARV ARV
5.1.1 Hexadecane, minimum purity of 99,0 mass fraction percentage
5.1.2 2,2,4,4,6,8,8-Heptamethylnonane, minimum purity of 98,0 mass fraction percentage
IMPORTANT — Hydrocarbons can form peroxides and other free radicals forming contaminants that
can influence the ID and CD. Experience has found some 40:60 blends of hexadecane and 2,2,4,4,6,8,8-
heptamethylnonane meeting the purity specification can contain peroxides and other free radically form
contaminants. Typically, the peroxides and other free radically formed contaminants can be removed
from the 40:60 mixture of hexadecane and 2,2,4,4,6,8,8-heptamethylnonane by subjecting the blend to
activated 4 Å molecular sieves.
5.2 Verification reference fluid, methylcyclohexane (MCH) of a purity of minimum 99,0 mass fraction
percentage to be used as the designated 11,0 ms ignition delay (ID ) and the designated 17,0 ms
ARV
combustion delay (CD ) assigned accepted reference value material
ARV
If the initial purity is not known the purity shall be checked in accordance with IP 537.
Even if the verification reference fluid meets the purity specification, it might not meet the Ignition and
Combustion delay requirements (see Table 2). It is recommended to either pass the suspect MCH through
a filter column to remove peroxide-based impurities or to test a bottle of MCH that has been shown to
meet the ID and CD requirements. It is recommended that each bottle of MCH is tested prior to its use as
a verification reference fluid to confirm it is of acceptable quality.
5.3 Quality control sample, stable and homogenous distillate fuel, similar in nature to the materials
under test (see 3.6).
5.4 Charge air, a compressed synthetic air mixture containing (20,0 ± 0,5) volume percent oxygen
with the balance nitrogen, less than 0,003 volume fraction percentage hydrocarbons, and less than
0,025 volume fraction percentage water.
NOTE Oxygen content of bottled charge air can vary between batches (cylinders). Significant variation leads to
changes in ignition delay and combustion delay (higher oxygen content leads to shorter ignition and combustion
delays).
5.5 Heptane, (n-Heptane) with a minimum purity of 99,5 mass fraction percentage.
5.6 Water, unless otherwise specified, shall meet the requirements of grade 3 of EN ISO 3696.
5.7 Coolant system fluid, 50:50 volumetric mixture of commercial grade ethylene glycol-type radiator
antifreeze with water (5.6).
5.8 Compressed nitrogen, of minimum purity 99,9 volume fraction percentage
5.9 Instrument air, a compressed air mixture containing (20,0 to 22,0) volume fraction of oxygen with
the balance nitrogen, for use with a charge air blending system (6.2).
6 Apparatus
6.1 Combustion analyser
The apparatus is described in more detail in Annex A. For the installation and set-up procedures, and for
detailed system description, refer to the manufacturer’s manual.
The system described in this document comprises a temperature and pressure-controlled combustion
chamber (6.1.1) with fluid cooling of designated areas, chamber inlet and exhaust valves and associated
piping, an electronically controlled fuel injection system, a fuel delivery system, a recirculating coolant
system, solenoids, sensors, controls and connection fittings for the compressed gas utilities. Figure 1
gives a schematic outline of the analyser.
6.1.1 Combustion chamber, a steel combustion chamber of capacity 0,473 l ± 0,005 l. Annex A gives
further details.
6.1.2 Filter medium, a removable polytetrafluoroethylene filter with a 5 µm pore size placed
downstream from the sample vessel to filter particulate matter from the test portion.
6.2 Charge air blending system, an instrument air (5.9) and nitrogen (5.8) gas mixer that provides a
constant oxygen concentration as prescribed by this document.
NOTE The oxygen content of a new charge air cylinder can differ from that of the previous source, which has a
significant effect on the delay measurements, while a charge air blending system provides a constant oxygen
content.
7 Sampling
7.1 Unless otherwise specified, obtain samples in accordance with the procedures given in EN ISO 3170
or EN ISO 3171.
7.2 To minimize exposure to UV emissions that can induce chemical reactions, which may affect
ignition and combustion delays measurement, collect and store samples in sample containers that are
either constructed of materials that minimize light reaching the sample such as a dark brown bottle, metal
can or containers that shall be wrapped or boxed in light-proof containers immediately after filling. If the
fuel is not to be analysed within 24 h, retain in a dark, cool environment, and preferably under an inert
gas.
prEN 16725:2025 (E)
NOTE 1 Exposure of petroleum fuels to UV wavelengths of less than 550 nm for even a short period of time has
been shown to affect ignition delay [5].
NOTE 2 The formation of peroxides and radicals, which affect the ignition delay and the combustion delay, is
minimized when the sample is stored in the dark, under a nitrogen blanket in a cool environment.
7.3 Condition the diesel fuel oil sample before opening the storage container, so that it is at room
temperature, typically 18 °C to 32 °C.
7.4 Inspect the sample for wax precipitation. If precipitants are present, bring the test sample to a
temperature of at least 14 °C above the expected cloud point of the material being tested, taking care not
to lose any lower boiling range components. Agitate the sample to return precipitants back into the
solution, ensuring the sample is homogeneous before proceeding.

Key
Digital signals Analogue signals
L1 upper-level sensor T1 coolant temperature
L2 lower-level sensor T2 inner wall temperature
TF thermal fuse P1 fuel pressure
M1 hydraulic pump P2 chamber dynamic pressure
N1 injector P3 chamber static pressure
V1 flush valve P4 nitrogen pressure
V2 air inlet valve FR1 coolant flow rate
V3 exhaust valve
Vx nitrogen circuit valves
Analyser lines Analyser parts (no signal registration)
AI air inlet CC combustion chamber
CI coolant input F1, F2, F3 filter
CO coolant output M2 multiplier
EX air exhaust RD rupture disk
NI nitrogen input S1 safety valve
SI sample inlet SV fuel reservoir with cap on top
WB sample waste drain
H1 clam shell heater
Figure 1 — Schematic overview of combustion analyser
8 Apparatus assembly and installation
Annex A gives the details on the apparatus assembly and installation. The apparatus requires placement
on a bench with facilities for the hook-up of all utilities and gases. The user shall ensure compliance with
all local and national codes. The apparatus requires an environment with a temperature of 10 °C to 35 °C.
The exhaust gases shall be directed into a low suction pressure fume extraction system.
9 Preparation of apparatus
9.1 System start-up and warm-up
9.1.1 Open the valve at the source of the combustion charge air supply and adjust the pressure
regulator as needed to provide the specification pressure (5.4).
9.1.2 Open the valve at the source of the nitrogen (5.8) supply and adjust the pressure regulator as
needed to provide a pressure of (0,6 to 1,0) MPa to the instrument (6.1).
9.1.3 Switch on power to the combustion analyser and the coolant pump.
9.1.4 After the chamber wall temperature has stabilized a chamber leakage test will be performed to
determine the chamber leakage rate. Combustion chamber leakage rate shall be less than 0,75 kPa/s, as
measured during the automated check of the sealing integrity of the combustion chamber. If the leakage
test fails, a warning is issued.
9.1.5 For more details and in case of error messages refer to the manufacturer’s manual.
9.2 Standard operating and test conditions
9.2.1 Operation of the combustion analyser requires setting a series of testing variables to prescribed
specifications. Some of these settings are established by the operator, others are operating conditions
that are monitored or controlled by the computer software.
9.2.2 The set points for chamber wall temperature and the injection time are determined during the
calibration procedure.
9.2.3 Check that the parameters are according to Table 1. Refer to manufacturer’s manual in case the
parameters are outside the limits.
prEN 16725:2025 (E)
Table 1 — Standard operating test conditions
Parameter Limits
Chamber static pressure (2,00 ± 0,02) MPa
Chamber wall temperature 560 °C to 640 °C
Stability during the 15 injections ±0,2 °C
Injector nozzle coolant temperature (50 ± 2) °C
Injection Pressure (100 ± 1,5) MPa
Injection time 2 000 µs to 2 700 µs
10 Calibration
10.1 General
Calibrate and verify the apparatus at each of the following occasions:
— after it is installed and commissioned,
— after replacement of critical parts or components of combustion chamber assembly, fuel injection
system, fuel injection system instrument sensors, chamber static pressure or chamber dynamic
pressure sensors,
— after calibration of the chamber static pressure, or chamber dynamic pressure sensors, or
— whenever QC sample determinations are not in statistical control, and the assignable causes for QC
non-compliance have been suitably addressed.
10.2 Calibration, verification and quality control
10.2.1 Clean the fuel system in accordance with B.2.
10.2.2 Bring the calibration reference fluid (5.1) to a temperature greater than 20 °C and agitate before
use.
10.2.3 Remove the fuel reservoir cap from the fuel reservoir (see Figure 1) and wash the stem and
threads and the fuel reservoir with approximately 50 ml of the calibration reference fluid. Reinstall the
fuel reservoir cap.
10.2.4 Flush the entire aliquot of the calibration reference fluid through the fuel system.
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