ISO/FDIS 17507-2

Natural gas — Calculation of methane number of gaseous fuels for reciprocating internal combustion engines — Part 2: PKI method

Part 2 of ISO 17507 describes the calculation method for the methane number of a gaseous fuel according to the methodology developed by DNV in a consortium with leading engine OEMs and fuel gas suppliers.

Gaz naturel — Calcul de l'indice de méthane des combustibles gazeux pour les moteurs alternatifs à combustion interne — Partie 2: Méthode PKI

General Information

• Status: Not Published
• ICS: 75.060 - Natural gas
• Technical Committee: ISO/TC 193 - Natural gas
• Drafting Committee: ISO/TC 193 - Natural gas
• Current Stage: 5000 - FDIS registered for formal approval
• Start Date: 25-Apr-2025
• Completion Date: 01-May-2025

Standards Content (Sample)

FINAL DRAFT
International
Standard
ISO/TC 193
Natural gas — Calculation of
Secretariat: NEN
methane number of gaseous
Voting begins on:
fuels for reciprocating internal
2025-08-01
combustion engines —
Voting terminates on:
2025-09-26
Part 2:
PKI method
Gaz naturel — Calcul de l'indice de méthane des combustibles
gazeux pour les moteurs alternatifs à combustion interne —
Partie 2: Méthode PKI
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 SUPPOR TING DOCUMENTATION.
IN ADDITION TO THEIR EVALUATION AS
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/CEN PARALLEL PROCESSING LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
Reference number
FINAL DRAFT
International
Standard
ISO/TC 193
Natural gas — Calculation of
Secretariat: NEN
methane number of gaseous
Voting begins on:
fuels for reciprocating internal
combustion engines —
Voting terminates on:
Part 2:
PKI method
Gaz naturel — Calcul de l'indice de méthane des combustibles
gazeux pour les moteurs alternatifs à combustion interne —
Partie 2: Méthode PKI
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 SUPPOR TING DOCUMENTATION.
© ISO 2025
IN ADDITION TO THEIR EVALUATION AS
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this publication may
BEING ACCEPTABLE FOR INDUSTRIAL, TECHNO-
ISO/CEN PARALLEL PROCESSING
LOGICAL, COMMERCIAL AND USER PURPOSES, DRAFT
be reproduced or utilized otherwise in any form or by any means, electronic or mechanical, including photocopying, or posting on
INTERNATIONAL STANDARDS MAY ON OCCASION HAVE
the internet or an intranet, without prior written permission. Permission can be requested from either ISO at the address below
TO BE CONSIDERED IN THE LIGHT OF THEIR POTENTIAL
or ISO’s member body in the country of the requester.
TO BECOME STAN DARDS TO WHICH REFERENCE MAY BE
MADE IN NATIONAL REGULATIONS.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: [phone]
Email: [e-mail]
Website: www.iso.org
Published in Switzerland Reference number
ii
Contents Page
Foreword .iv
Introduction .v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 2
5 PKI method. 2
5.1 Introduction .2
5.2 Applicability .2
5.2.1 Standard gaseous fuel composition range .2
5.2.2 Handling of other gaseous fuel components .3
5.3 Methodology to calculate the MN .4
PKI
5.3.1 General .4
5.3.2 Step 1: Calculation of the PKI .4
5.3.3 Step 2: Calculation of the MN .5
PKI
5.4 Expression of results . .5
5.5 Uncertainty error and bias .5
6 Example calculations . 6
6.1 Example calculation 1 .6
6.2 Example calculation 2 .6
Annex A (normative) Listing of coefficients used in Formula (1) and Formula (4) . 9
Annex B (informative) PKI and MN values for selected gaseous fuel compositions .13
PKI
Annex C (informative) Tools for users of the MN method .15
PKI
Annex D (normative) Uncertainty error and bias .16
Annex E (informative) Natural gas-based fuels for reciprocating internal combustion engines .18
Annex F (informative) Basis of the PKI method . 19
Bibliography .22

iii
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out through
ISO technical committees. Each member body interested in a subject for which a technical committee
has been established has the right to be represented on that committee. International organizations,
governmental and non-governmental, in liaison with ISO, also take part in the work. ISO collaborates closely
with the International Electrotechnical Commission (IEC) on all matters of electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are described
in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the different types
of ISO document should be noted. This document was drafted in accordance with the editorial rules of the
ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of (a)
patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed patent
rights in respect thereof. As of the date of publication of this document, ISO had not received notice of (a)
patent(s) which may be required to implement this document. However, implementers are cautioned that
this may not represent the latest information, which may be obtained from the patent database available at
www.iso.org/patents. ISO shall not be held responsible for identifying any or all such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and expressions
related to conformity assessment, as well as information about ISO's adherence to the World Trade
Organization (WTO) principles in the Technical Barriers to Trade (TBT), see www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 193, Natural gas.
A list of all parts in the ISO 17507 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.

iv
Introduction
The globalization of the natural gas market and the drive towards sustainability are increasing the diversity
of the supply of gases to the natural gas infrastructure. For example, the introduction of regasified liquefied
natural gas (LNG) can result in higher fractions of non-methane hydrocarbons in the natural gas grid
than the traditionally distributed pipeline gases for which these hydrocarbons have been removed during
processing. Also, the drive towards sustainable gaseous fuels, such as hydrogen and gases derived from
biomass, results in the introduction of “new” gas compositions that contain components that do not occur
in the traditional natural gas supply. Consequently, the increasing variations in gas composition affect the
knock resistance of the gas when used as a fuel. This can affect the operational integrity of reciprocating
internal combustion engines.
For the efficient and safe operation of gas engines, it is of great importance to characterize the knock
resistance of gaseous fuels accurately. Engine knock is caused by the autoignition of unburned fuel mixture
ahead of this mixture being consumed by the propagating flame. Mild engine knock increases pollutant
emissions accompanied by gradual build-up of component damage and complete engine failure if not
counteracted. Severe knock causes structural damage to critical engine parts, quickly leading to catastrophic
engine failure. To ensure that gas engines are matched with the expected variations in fuel composition, the
knock resistance of the fuel is to be characterized, and subsequently specified, unambiguously.
Traditional methods for characterizing the knock resistance of gaseous fuels, such as the methane number
method developed by Anstalt für Verbrennungskraftmaschinen List (AVL) in the 1960s, relate the knock
propensity of a given fuel with that of an equivalent methane/hydrogen mixture using a standardized test engine
(see References [1], [2] and [3]). Several other methane number methods have since been developed, sometimes
based on either the approach or data, or both from the original experimental work performed by AVL.
In recognition of the need to standardize a method for characterizing the knock resistance of gaseous fuels,
several existing methods for calculating a methane number have been considered, including the propane
knock index (PKI) method outlined in this document. ISO 17507-1 describes the MN method.
C
Methods to calculate a methane number are based on the input of the gas composition under investigation.
While methods can be fundamentally different in their development approach, ideally the methods produce
similar methane numbers for the range of gas compositions they are valid for. Yet, differences in outcome can
be observed. Engine manufacturers typically determine the calculation method to be used when specifying
a methane number value for their engines as part of their application and warranty statements. In all cases,
when specifying a methane number based on either method, or any other method, the method used should
be noted.
The PKI method was developed by Det Norske Veritas (DNV), headquartered in Oslo, Norway in a consortium
of engine Original Equipment Manufacturers (OEMs) and natural gas fuel suppliers. The method is based on
the physics and chemistry of the air-fuel mixture during the compression and combustion phases of the engine
working cycle that determine engine knock, using an experimentally verified engine combustion model.
The PKI method uses two polynomial functions to compute the methane number from the gaseous fuel
composition input. The development and experimental verification of the PKI method is documented in a
series of publications (see References [5] to [18]). A more detailed history of the PKI method can be found in
Annex F.
A version of the PKI method dedicated to LNG fuels is described in ISO 23306:2020, Annex A.

v
FINAL DRAFT International Standard ISO/FDIS 17
...

ISO/TC 193
Secretariat: NEN
Date: 2025-07-17
Natural gas — Calculation of methane number of gaseous fuels for
reciprocating internal combustion engines —
Part 2:
PKI method
Gaz naturel — Calcul de l'indice de méthane des combustibles gazeux pour les moteurs alternatifs à combustion
interne —
Partie 2: Méthode PKI
FDIS stage
ISO 17507-2:2024(E)
All rights reserved. Unless otherwise specified, or required in the context of its implementation, no part of this
publication may be reproduced or utilized otherwise in any form or by any means, electronic or mechanical,
including photocopying, or posting on the internet or an intranet, without prior written permission. Permission can
be requested from either ISO at the address below or ISO’s member body in the country of the requester.
ISO copyright office
CP 401 • Ch. de Blandonnet 8
CH-1214 Vernier, Geneva
Phone: + 41 22 749 01 11
E-mail: [e-mail]
Website: www.iso.org
Published in Switzerland
ii © ISO 2024 – All rights reserved

Contents
Foreword . iv
Introduction . v
1 Scope . 1
2 Normative references . 1
3 Terms and definitions . 1
4 Abbreviated terms . 2
5 PKI method . 2
5.1 Introduction . 2
5.2 Applicability . 2
5.3 Methodology to calculate the MNPKI . 4
5.4 Expression of results . 6
5.5 Uncertainty error and bias . 6
6 Example calculations . 7
6.1 Example calculation 1 . 7
6.2 Example calculation 2 . 7
Annex A (normative) Listing of coefficients used in Formula (1) and Formula (4) . 10
Annex B (informative) PKI and MN values for selected gaseous fuel compositions . 15
PKI
Annex C (informative) Tools for users of the MNPKI method . 17
Annex D (normative) Uncertainty error and bias . 18
Annex E (informative) Natural gas-based fuels for reciprocating internal combustion
engines . 21
Annex F (informative) Basis of the PKI method . 22
Bibliography . 26

iii
ISO 17507-2:2024(E)
Foreword
ISO (the International Organization for Standardization) is a worldwide federation of national standards
bodies (ISO member bodies). The work of preparing International Standards is normally carried out
through ISO technical committees. Each member body interested in a subject for which a technical
committee has been established has the right to be represented on that committee. International
organizations, governmental and non-governmental, in liaison with ISO, also take part in the work. ISO
collaborates closely with the International Electrotechnical Commission (IEC) on all matters of
electrotechnical standardization.
The procedures used to develop this document and those intended for its further maintenance are
described in the ISO/IEC Directives, Part 1. In particular, the different approval criteria needed for the
different types of ISO document should be noted. This document was drafted in accordance with the
editorial rules of the ISO/IEC Directives, Part 2 (see www.iso.org/directives).
ISO draws attention to the possibility that the implementation of this document may involve the use of
(a) patent(s). ISO takes no position concerning the evidence, validity or applicability of any claimed
patent rights in respect thereof. As of the date of publication of this document, ISO had not received notice
of (a) patent(s) which may be required to implement this document. However, implementers are
cautioned that this may not represent the latest information, which may be obtained from the patent
database available at www.iso.org/patents. ISO shall not be held responsible for identifying any or all
such patent rights.
Any trade name used in this document is information given for the convenience of users and does not
constitute an endorsement.
For an explanation of the voluntary nature of standards, the meaning of ISO specific terms and
expressions related to conformity assessment, as well as information about ISO's adherence to the World
Trade Organization (WTO) principles in the Technical Barriers to Trade (TBT), see
www.iso.org/iso/foreword.html.
This document was prepared by Technical Committee ISO/TC 193, Natural gas.
A list of all parts in the ISO 17507 series can be found on the ISO website.
Any feedback or questions on this document should be directed to the user’s national standards body. A
complete listing of these bodies can be found at www.iso.org/members.html.
iv © ISO 2024 – All rights reserved

Introduction
The globalization of the natural gas market and the drive towards sustainability are increasing the
diversity of the supply of gases to the natural gas infrastructure. For example, the introduction of
regasified liquefied natural gas (LNG) can result in higher fractions of non-methane hydrocarbons in the
natural gas grid than the traditionally distributed pipeline gases for which these hydrocarbons have been
removed during processing. Also, the drive towards sustainable gaseous fuels, such as hydrogen and
gases derived from biomass, results in the introduction of “new” gas compositions that contain
components that do not occur in the traditional natural gas supply. Consequently, the increasing
variations in gas composition affect the knock resistance of the gas when used as a fuel. This can affect
the operational integrity of reciprocating internal combustion engines.
For the efficient and safe operation of gas engines, it is of great importance to characterize the knock
resistance of gaseous fuels accurately. Engine knock is caused by the autoignition of unburned fuel
mixture ahead of this mixture being consumed by the propagating flame. Mild engine knock increases
pollutant emissions accompanied by gradual build-up of component damage and complete engine failure
if not counteracted. Severe knock causes structural damage to critical engine parts, quickly leading to
catastrophic engine failure. To ensure that gas engines are matched with the expected variations in fuel
composition, the knock resistance of the fuel is to be characterized, and subsequently specified,
unambiguously.
Traditional methods for characterizing the knock resistance of gaseous fuels, such as the methane
number method developed by Anstalt für Verbrennungskraftmaschinen List (AVL) in the 1960s, relate
the knock propensity of a given fuel with that of an equivalent methane/hydrogen mixture using a
standardized test engine (see References [1], [2] and [3]). Several other methane number methods have
since been developed, sometimes based on either the approach or data, or both from the original
experimental work performed by AVL.
In recognition of the need to standardize a method for characterizing the knock resistance of gaseous
fuels, several existing methods for calculating a methane number have been considered, including the
propane knock index (PKI) method outlined in this document. ISO 17507-1 describes the MN method.
C
Methods to calculate a methane number are based on the input of the gas composition under
investigation. While methods can be fundamentally different in their development approach, ideally the
methods produce similar methane numbers for the range of gas compositions they are valid for. Yet,
differences in outcome can be observed. Engine manufacturers typically determine the calculation
method to be used when specifying a methane number value for their engines as part of their application
and warranty statements. In all cases, when specifying a methane number based on either method, or any
other method, the method used should be noted.
The PKI method was developed by Det Norske Veritas (DNV), headquartered in Oslo, Norway in a
consortium of engine Original Equipment Manufacturers (OEMs) and natural gas fuel suppliers. The
method is based on the physics and chemistry of the air-fuel mixture during the compression and
combustion phases of the engine working cycle that determine engine knock, using an experimentally
verified engine combustion model.
The PKI method uses two polynomial functions to compute the methane number from the gaseous fuel
composition input. The development and experimental verification of the PKI method is documented in
a series of publications (see References [5] to [18]). A more detailed history of the PKI method can be
found in Annex F.
Formatted: Default Paragraph Font
A version of the PKI method dedicated to LNG fuels is described in ISO 23306:2020, Annex A (see
Formatted: Default Paragraph Font
Reference [19]).
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v
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vi © ISO 2024 2025 – All rights reserved
vi
DRAFT International Final Standard

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Natural gas — Calculation of methane number of gaseous fuels for 0 pt, Section start: New page, Header distance from
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reciprocating internal combustion engines — —
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Part 2:
and Asian text, Adjust space between Asian text and
PKI method
numbers
1 Scope
This document specifies the PKI method for the calculation of the methane number of a gaseous fuel, using the
composition of the gas as sole input for the calculation.
This document applies to natural gas (and biomethane) and their admixtures with hydrogen.
2 Normative references
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The following documents are referred to in the text in such a way that some or all of their content constitutes
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requirements of this document. For dated references, only the edition cited applies. For undated references,
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the latest edition of the referenced document (including any amendments) applies. stops: Not at 19.85 pt + 39.7 pt + 59.55 pt + 79.4 pt
+ 99.25 pt + 119.05 pt + 138.9 pt + 158.75 pt +
ISO 14532, Natural gas — Vocabulary 178.6 pt + 198.45 pt
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ISO 14912, Gas analysis — Conversion of gas mixture composition data
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ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
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measurement (GUM:1995)
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3 Terms and definitions
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For the purposes of this document, the terms and definitions given in ISO 14532 and the following apply.
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ISO and IEC maintain terminological databases for use in standardization at the following addresses:
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— — ISO Online browsing platform: available at https://www.iso.org/obphttps://www.iso.org/obp
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— — IEC Electropedia: available at https://www.electropedia.org/https://www.electropedia.org/
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3.1 3.1
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methane number
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MN
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numerical rating indicating the knock resistance of a gaseous fuel
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numbers
Note 1 to entry: It is analogous to the octane number for petrol. The methane number is the volume fraction expressed
as the percentage of methane in a methane-hydrogen mixture, that in a test engine under standard conditions has the
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same knock resistance as the gaseous fuel to be examined.
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...