SIST EN ISO 17507-2:2026

SLOVENSKI STANDARD
01-maj-2026
Zemeljski plin - Izračun metanskega števila za plinasta goriva za motorje z
notranjim zgorevanjem - 2. del: Metoda PKI (ISO 17507-2:2025)
Natural gas - Calculation of methane number of gaseous fuels for reciprocating internal
combustion engines - Part 2: PKI method (ISO 17507-2:2025)
Erdgas - Berechnung der Methanzahl von gasförmigen Kraftstoffen für
Verbrennungsmotoren - Teil 2: PKI-Verfahren (ISO 17507-2:2025)
Gaz naturel - Calcul de l'indice de méthane des combustibles gazeux pour les moteurs
alternatifs à combustion interne - Partie 2: Méthode PKI (ISO 17507-2:2025)
Ta slovenski standard je istoveten z: EN ISO 17507-2:2025
ICS:
27.020 Motorji z notranjim Internal combustion engines
zgorevanjem
75.060 Zemeljski plin Natural gas
2003-01.Slovenski inštitut za standardizacijo. Razmnoževanje celote ali delov tega standarda ni dovoljeno.

EN ISO 17507-2
EUROPEAN STANDARD
NORME EUROPÉENNE
December 2025
EUROPÄISCHE NORM
ICS 75.060
English Version
Natural gas - Calculation of methane number of gaseous
fuels for reciprocating internal combustion engines - Part
2: PKI method (ISO 17507-2:2025)
Gaz naturel - Calcul de l'indice de méthane des Erdgas - Berechnung der Methanzahl von gasförmigen
combustibles gazeux pour les moteurs alternatifs à Kraftstoffen für Verbrennungsmotoren - Teil 2: PKI-
combustion interne - Partie 2: Méthode PKI (ISO Verfahren (ISO 17507-2:2025)
17507-2:2025)
This European Standard was approved by CEN on 29 October 2025.

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. Up-to-date lists and bibliographical references
concerning such national standards may be obtained on application to the CEN-CENELEC Management Centre or to any CEN
member.
This European Standard exists 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.

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United Kingdom.
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. EN ISO 17507-2:2025 E
worldwide for CEN national Members.

Contents Page
European foreword . 3

European foreword
This document (EN ISO 17507-2:2025) has been prepared by Technical Committee ISO/TC 193
"Natural gas" in collaboration with Technical Committee CEN/TC 408 “Biomethane and other
renewable and low-carbon methane rich gases” the secretariat of which is held by AFNOR.
This European Standard shall be given the status of a national standard, either by publication of an
identical text or by endorsement, at the latest by June 2026, and conflicting national standards shall be
withdrawn at the latest by June 2026.
Attention is drawn to the possibility that some of the elements of this document may be the subject of
patent rights. CEN shall not be held responsible for identifying any or all such patent rights.
Any feedback and questions on this document should be directed to the users’ national standards
body/national committee. A complete listing of these bodies can be found on the CEN website.
According to the CEN-CENELEC Internal Regulations, the national standards organizations of the
following countries are bound to implement this European Standard: 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 the
United Kingdom.
Endorsement notice
The text of ISO 17507-2:2025 has been approved by CEN as EN ISO 17507-2:2025 without any
modification.
International
Standard
ISO 17507-2
First edition
Natural gas — Calculation of
2025-12
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
Reference number
ISO 17507-2:2025(en) © ISO 2025

ISO 17507-2:2025(en)
© ISO 2025
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
Email: copyright@iso.org
Website: www.iso.org
Published in Switzerland
ii
ISO 17507-2:2025(en)
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
ISO 17507-2:2025(en)
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, in collaboration with the
European Committee for Standardization (CEN) Technical Committee CEN/TC 408, Biomethane and other
renewable and low-carbon methane rich gases, in accordance with the Agreement on technical cooperation
between ISO and CEN (Vienna Agreement).
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 17507-2:2025(en)
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
International Standard ISO 17507-2:2025(en)
Natural gas — Calculation of methane number of gaseous
fuels for reciprocating internal combustion engines —
Part 2:
PKI method
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
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.
ISO 14532, Natural gas — Vocabulary
ISO 14912, Gas analysis — Conversion of gas mixture composition data
ISO/IEC Guide 98-3, Uncertainty of measurement — Part 3: Guide to the expression of uncertainty in
me a s ur ement (GUM: 1995)
3 Terms and definitions
For the purposes of this document, the terms and definitions given in ISO 14532 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
methane number
MN
numerical rating indicating the knock resistance of a gaseous fuel
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
same knock resistance as the gaseous fuel to be examined.
3.2
PKI methane number
MN
PKI
numerical rating index obtained by calculation, indicating the knock resistance of a gaseous fuel according
to this document
Note 1 to entry: This analytical estimate of a methane number is based on using mole fraction gaseous fuel composition
as input.
ISO 17507-2:2025(en)
4 Abbreviated terms
MN methane number
PKI propane knock index
MN PKI methane number
PKI
5 PKI method
5.1 Introduction
The methane number of a gas quantifies the knock propensity of that gas when used as a fuel in a
reciprocating internal combustion engine. The higher the methane number, the more knock resistant the
gaseous fuel is, and vice versa.
The methane number of a gaseous fuel is calculated from its composition according to several different
methods, all of which can give different results. The PKI method is used by engine OEMs, gaseous fuel
suppliers, engine operators, consulting engineers and engine control and gas analyser equipment OEMs. It
has been adopted in ISO 23306. When referring to a methane number value, the method used should be
noted.
The PKI method described in this document has been developed for a range of gaseous fuel compositions
exceeding the typical composition range of natural gas-based fuels used in reciprocating internal combustion
engines shown in Table E.1.
The PKI method can be used to calculate the methane number of any gaseous engine fuel as long as the gas
composition input ranges, shown in Table 1, and further boundary conditions of this method are adhered to.
The boundary conditions for the PKI method are set out in this document.
The method is based on gaseous fuel compositions in mole fraction, expressed as a percentage. If the gas
composition is available either as volume fraction or as mass fraction, conversion to mole fraction shall be
performed using the methods specified in ISO 14912.
Calculation of the MN from the gas composition involves two polynomial functions, as described in
PKI
5.3. Numerical examples are provided to enable software developers to validate implementations of the
methodology described in this document.
5.2 Applicability
5.2.1 Standard gaseous fuel composition range
The PKI method described in this document has been developed for and is applicable to all reciprocating
internal combustion engines using a gaseous fuel.
In general, the use of any method for calculating the methane number of a gaseous fuel requires either
careful consideration or consultation or both with specialist industry parties, such as engine suppliers, fuel
suppliers and consulting firms.
The PKI method described in this document is applicable to gaseous fuels comprising the following
components:
— methane
— ethane
— propane
— n-butane
ISO 17507-2:2025(en)
— i-butane
— n-pentane
— i-pentane
— neo-pentane
— hexanes
— hydrogen
— carbon monoxide
— carbon dioxide
— nitrogen
— hydrogen sulfide
Upper and lower limits for gaseous fuels applied to this method are shown in Table 1.
Table 1 — Upper and lower limits of gaseous fuel components
for the PKI method
Amount of substance
Component
mole fraction / %
methane 65 to 100
ethane 0 to 20
propane 0 to 20
a
n-butane 0 to 5
a
i-butane 0 to 5
a
n-pentane 0 to 2
a
i-pentane 0 to 2
a
neo-pentane 0 to 2
+ b
hexanes 0 to 1,5
hydrogen 0 to 35
carbon monoxide 0 to 10
carbon dioxide 0 to 20
nitrogen 0 to 20
hydrogen sulfide 0 to 0,5
a
The PKI method differentiates between the isomers of butane and pentane in
recognition of their difference in knock propensity.
b
The PKI method treats the sum of hexanes and higher hydrocarbons including their
+
isomers (listed as hexanes ) as n-hexane.
Gas composition analyses can comprise of (non-)hydrocarbon components not listed as valid gas input
components for the PKI method as per Table 1. To provide guidance towards the correct use of and optimum
results from the PKI method, instructions for selecting non-listed gas components are given in 5.2.2.
5.2.2 Handling of other gaseous fuel components
5.2.2.1 Oxygen and water vapour
Any mole fractions of oxygen and water vapour present in the gaseous fuel under investigation shall be
ignored. The resulting gas composition shall be normalized as a composition free of oxygen and water
vapour.
ISO 17507-2:2025(en)
5.2.2.2 Argon and helium
Any mole fractions of argon or helium present in the gaseous fuel under investigation shall be assigned to
the mole fraction of nitrogen.
5.2.2.3 Other non-listed gaseous fuel components
Any component present in the gaseous fuel under investigation not listed as valid gas input component
for the PKI method as per Table 1 and not listed in 5.2.2.1 or 5.2.2.2 shall be ignored. The resulting gas
composition shall be normalized as a composition without that component.
5.3 Methodology to calculate the MN
PKI
5.3.1 General
The MN of the gaseous fuel under investigation is calculated from its composition in two steps:
PKI
a) calculation of the PKI, based on the gas composition;
b) calculation of the MN , based on the PKI calculated in step a).
PKI
Several calculation tools have been developed to support users of this method. See Annex C.
5.3.2 Step 1: Calculation of the PKI
To calculate the PKI, a polynomial function, Formula (1) is used.
n k m
PKIX  XX (1)
nk m
i  i j
n1 i m=1 k=1 ()ij
where
PKI is the propane knock index of the gaseous fuel;
X is the (normalized) mole fraction of component i or component j in the gaseous fuel;
α is a coefficient relating to component i in the gaseous fuel; α values are given in Table A.1
(α values not listed in Table A.1 shall be assumed zero);
β is a coefficient relating to combinations of components i and j in the gaseous fuel;
β values are given in Table A.1 (β values not listed in Table A.1 shall be assumed zero);
i is an index indicating a gaseous fuel component from the range: CH , C H , C H , n-C H ,
4 2 6 3 8 4 10
i-C H , n-C H , i-C H , neo-C H , CO , CO, H or N ;
4 10 5 12 5 12 5 12 2 2 2
j is an index indicating a gaseous fuel component from the range C H , C H , n-C H , i-C H ,
2 6 3 8 4 10 4 10
n-C H , i-C H , neo-C H , CO , CO, H or N ;
5 12 5 12 5 12 2 2 2
k 1, 2
m 1, 2
n 1 to 4
The result of Formula (1) is only valid if all of the following conditions are met:
a) the composition of the gaseous fuel used as input for Formula (1) conform with the limits/ranges listed
in Table 1;
b) the total mole percentage of the gaseous fuel used as input for Formula (1) is 100 %, or unity in the case
of using mole fractions;
c) the PKI value resulting from Formula (1) is ≤ 20.
Scaling factors are used to account for the presence of hexanes and higher hydrocarbons (denoted as
+
hexanes ) and hydrogen sulfide in the gaseous fuel under investigation. These factors convert the effect
of the components on the knock resistance of the gas to that of methane and n-pentane respectively. The

ISO 17507-2:2025(en)
method to adjust the methane and n-pentane mole fractions used as input for Formula (1) is given in
Formulae (2) and (3).
XX03, X (2)

CH44 CH
CH614
XXXX13, (3)

nC51HC25 n HH12 2S
CH614
where
X is the adjusted mole fraction of methane in the gaseous fuel;
CH4’
X is the initial (normalized) mole fraction of methane in the gaseous fuel;
CH4
+ +
X is the (normalized) mole fraction of hexanes in the gaseous fuel;
C6H14
X is the adjusted mole fraction of n-pentane in the gaseous fuel;
n-C5H12’
X is the initial (normalized) mole fraction of n-pentane in the gaseous fuel;
n-C5H12
X is the (normalized) mole fraction of hydrogen sulfide in the gaseous fuel.
H2S
5.3.3 Step 2: Calculation of the MN
PKI
To calculate the MN , a polynomial function, Formula (4) is used:
PKI
2 3 4 5 6
MN = a · PKI + a · PKI + a · PKI + a · PKI + a · PKI + a · PKI + b (4)
PKI 1 2 3 4 5 6
where
MN is the PKI methane number of the gaseous fuel;
PKI
PKI is the propane knock index of the gaseous fuel as calculated with Formula (1);
a to a are coefficients; values are given in Table A.2;
1 6
b is a coefficient; value is given in Table A.2.
The result of Formula (4) is only valid if all of the following conditions are met:
a) the PKI value used as input for Formula (4) is ≤ 20;
b) the MN value resulting from Formula (4) is ≥ 53.
PKI
5.4 Expression of results
To express the final result, the calculated methane number is expressed as an integer. The method used
should be noted, e.g. 74 MN as per ISO 17507-2. Rounding to an integer value according to ISO 80000-1 is
PKI
recommended as a higher numerical resolution of the MN value is not relevant in practice.
PKI
5.5 Uncertainty error and bias
The MN is calculated from the mole fraction composition of the gaseous fuel under review as the sole
PKI
input, using two polynomial functions. The coefficients used in both polynomial functions have a fixed value
based on the particular composition of the gaseous fuel under review. This means that there can only be one
MN value for a given gaseous fuel composition. For the purposes of this document, the MN values thus
PKI PKI
calculated are deemed to be exact according to the PKI method. Hence, any error or bias in an MN value
PKI
arises solely from errors in the gaseous fuel compositions used as input.
The resulting uncertainty shall be estimated according to Annex D.

ISO 17507-2:2025(en)
6 Example calculations
6.1 Example calculation 1
The determination of the MN (and PKI) of a gaseous fuel is illustrated here by an example calculation for a
PKI
gas with mole fractions expressed as percentages of 90 % methane and 10 % ethane.
This gaseous fuel meets the gas composition limits/ranges of the PKI method as given in Table 1. The
composition of the gaseous fuel adds up to 100 %, meaning that the PKI method can be applied.
For the example gas composition noted, Formula (1) for the calculation of the PKI of this gaseous fuel can be
simplified to Formula (5):
2 2 3 3 4 4 2 2
PKI = X · α + (X ) · α + (X ) · α + (X ) · α + X · α + (X ) · α +
CH4 CH4 CH4 (CH4) CH4 (CH4) CH4 (CH4) C2H6 C2H6 C2H6 (C2H6)
3 3 4 4
(X ) · α + (X ) · α + X · X · β (5)
C2H6 (C2H6) C2H6 (C2H6) CH4 C2H6 CH4 · C2H6
Based on the gaseous fuel composition, the methane mole fraction X and ethane mole fraction X
CH4 C2H6
amount to 0,9 and 0,1 respectively. Using these fractions and the relevant α- and β coefficients from Table A.1
in Formula (5), the calculation of the PKI for this gaseous fuel amounts to:
2 3
PKI = 0,9 · 569,285 536 016 002 0 + (0,9) · −650,854 339 490 7 + (0,9) · 64,359 575 257 386 2 +
4 2
(0,9) · 17,214 959 222 053 6 + 0,1 · −645,099 966 662 855 0 + (0,1) · 694,229 376 857 102 0 +
3 4
(0,1) · −675,381 075 231 165 0 + (0,1) · 1 474,790 791 373 33 + 0,1 · 0,9 · 201,788 909 592 169
The calculation delivers a PKI value of 3,443 after rounding to three decimals.
This PKI value meets the criterion of PKI ≤ 20 and thus can be used as input for Formula (4). Using the PKI
value of 3,443 and the a and b coefficients from Table A.2 in Formula (4), the calculation of the MN of this
PKI
gaseous fuel amounts to:
2 3 4
MN = −9,757 977 · 3,443 + 1,484 961 · 3,443 - 0,139 533 · 3,443 + 0,007 031 306 · 3,443 −
PKI
5 6
0,000 177 002 9 · 3,443 + 0,000 001 751 212 · 3,443 + 100
The calculation delivers a MN value of 79 after rounding to an integer value.
PKI
This MN value meets the criterion of MN ≥ 53 and thus is a valid result of the PKI method for this
PKI PKI
example gaseous fuel.
6.2 Example calculation 2
The determination of the MN (and PKI) of a gaseous fuel is illustrated here by an example calculation for
PKI
a gas with mole fractions expressed as percentages of 84,5 % methane, 6,0 % ethane, 4,0 % propane, 1,5 %
+
i-butane, 0,5 % n-pentane, 0,4 % hexanes , 3,0 % nitrogen and 0,1 % hydrogen sulfide.
This gaseous fuel meets the gas composition limits/ranges of the PKI method as given in Table 1. The
composition of the gaseous fuel adds up to 100 %, meaning that the PKI method can be applied.
Before calculating the PKI of this gaseous fuel, the knock resistance of the hexanes and higher hydrocarbons
+
(denoted as hexanes ) and hydrogen sulfide fractions in the gas shall be accounted for. This is done by
adjusting the methane and n-pentane mole fractions of the gaseous fuel according to Formulae (2) and (3).

ISO 17507-2:2025(en)
+
Using the initial mole fractions of methane and hexanes in this gaseous fuel of 0,845 and 0,004 respectively
in Formula (2), the calculation of the adjusted mole fraction of methane amounts to:
X 0,,845 030,,004 0 8438

CH4
+
Using the initial mole fractions of n-pentane, hydrogen sulfide and hexanes in this gaseous fuel of 0,005,
0,001 and 0,004 respectively in Formula (3), the calculation of the adjusted mole fraction of n-pentane
amounts to:
X 0,,,005 0 00113 0,,0040 0112

nC 51H 2
The calculation results deliver an adjusted gas composition comprising of 84,38 % methane, 6,0 % ethane,
4,0 % propane, 1,5 % i-butane, 1,12 % n-pentane and 3,0 % nitrogen.
For the adjusted gas composition noted, Formula (1) for the calculation of the PKI of this gaseous fuel can be
simplified to Formula (6):
2 2 3 3 4 4 2 2
PKI = X · α + (X ) · α + (X ) · α + (X ) · α + X · α + (X ) · α +
CH4’ CH4 CH4’ (CH4) CH4’ (CH4) CH4’ (CH4) C2H6 C2H6 C2H6 (C2H6)
3 3 4 4 2 2
(X ) · α + (X ) · α + X · X · β + X · α + (X ) · α +
C2H6 (C2H6) C2H6 (C2H6) CH4’ C2H6 CH4 · C2H6 C3H8 C3H8 C3H8 (C3H8)
3 3 4 4
(X ) · α + (X ) · α + X · X · β + X · α +
C3H8 (C3H8) C3H8 (C3H8) CH4’ C3H8 CH4 · C3H8 i-C4H10 i-C4H10
2 2 3 3 4 4
(X ) · α + (X ) · α + (X ) · α +
i-C4H10 (i-C4H10) i-C4H10 (i-C4H10) i-C4H10 (i-C4H10)
2 2
X · X · β + (X · X ) · β + X · X · β +
CH4’ i-C4H10 CH4 · i-C4H10 CH4’ i-C4H10 (CH4 · i-C4H10) C2H6 i-C4H10 C2H6 · i-C4H10
2 2 3 3 4 4
X · α + (X ) · α + (X ) · α + (X ) · α +
n-C5H12’ n-C5H12 n-C5H12’ (n-C5H12) n-C5H12’ (n-C5H12) n-C5H12’ (n-C5H12)
X · X · β + X · X · β + X · X · β +
CH4’ n-C5H12’ CH4 · n-C5H12 C2H6 n-C5H12’ C2H6 · n-C5H12 C3H8 n-C5H12’ C3H8 · n-C5H12
2 2 2 2
X · (X ) · β + (X ) · X · β +
C3H8 n-C5H12’ C3H8 · (n-C5H12) C3H8 n-C5H12’ (C3H8) · n-C5H12
2 2 3 3 4 4
X · X · β + X · α + (X ) · α + (X ) · α + (X ) · α +
i-C4H10- n-C5H12’ i-C4H10 · n-C5H12 N2 N2 N2 (N2) N2 (N2) N2 (N2)
2 2 2 2
X · X · β + X · X · β + X · (X ) · β + (X ) · X · β +
CH4’ N2 CH4 · N2 C2H6 N2 C2H6 · N2 C2H6 N2 C2H6 · (N2) C2H6 N2 (C2H6) · N2
X · X · β + X · X · β + X · X · β (6)
C3H8 N2 C3H8 · N2 i-C4H10 N2 i-C4H10 · N2 n-C5H12’ N2 n-C5H12 · N2
Using the mole fractions of the adjusted gas composition and the relevant α- and β coefficients from Table A.1
in Formula (6), the calculation of the PKI of this gaseous fuel amounts to:
PKI = 0,843 8 · 569,285 536 016 002 0 + (0,843 8) · −650,854 339 490 7 +
3 4
(0,843 8) · 64,359 575 257 386 2 + (0,843 8) · 17,214 959 222 053 6 +
0,06 · −645,099 966 662 855 0 + (0,06) · 694,229 376 857 102 0 +
3 4
(0,06) · −675,381 075 231 165 0 + (0,06) · 1 474,790 791 373 33 +
0,843 8 · 0,06 · 201,788 909 592 169 + 0,04 · 499,398 492 651 52 +
2 3
(0,04) · −576,665 945 472 394 0 + (0,04) · 252,193 674 060 28 +
(0,04) · 593,958 975 466 507 0 + 0,843 8 · 0,04 · −865,856 657 223 225 +
0,015 · 735,223 884 113 728 0 + (0,015) · −3 182,614 393 379 67 +
3 4
(0,015) · 20 945,186 725 021 9 + (0,015) · 159 067,868 032 595 0 +
2 2
0,843 8 · 0,015 · −1 023,278 147 470 3 + (0,843 8) · (0,015) · 1 550,095 184 612 58 +
0,06 · 0,015 · −109,983 789 902 769 + 0,011 2 · 2 571,930 793 605 35 +
2 3
(0,011 2) · 10 516,494 109 227 50 + (0,011 2) · −770 539,377 197 693 +
(0,011 2) · 28 633 475,586 565 4 + 0,843 8 · 0,011 2 · −2 811,677 404 325 23 +
0,06 · 0,011 2 · −1 870,347 465 005 63 + 0,04 · 0,011 2 · −1 734,805 682 394 27 +
2 2
0,04 · (0,011 2) · 127 551,642 193 201 + (0,04) · 0,011 2 · 11 318,418 395 072 2 +
0,015 · 0,011 2 · 5 056,603 091 637 61 + 0,03 · −469,428 097 827 742 +
2 3
(0,03) · 352,688 107 288 763 + (0,03) · −220,491 687 402 358 +
(0,03) · 1 419,680 053 962 420 + 0,843 8 · 0,03 · −1,053 973 329 306 09 +
0,06 · 0,03 · 968,887 620 927 515 + 0,06 · (0,03) · 337,464 863 958 288 +
(0,06) · 0,03 · 267,472 766 191 96 + 0,04 · 0,03 · 13,345 337 812 469 +
0,015 · 0,03 · 14,803 895 799 972 4 + 0,011 2 · 0,03 · −1 573,688 937 706 25
The calculation delivers a PKI value of 15,734 after rounding to three decimals.
This PKI value meets the criterion of PKI ≤ 20 and thus can be used as input to Formula (4).

ISO 17507-2:2025(en)
Using the PKI value of 15,734 and the a and b coefficients from Table A.2 in Formula (4), the calculation of the
MN of this gaseous fuel amounts to:
PKI
2 3 4
MN = −9,757 977 · 15,734 + 1,484 961 · 15,734 − 0,139 533 · 15,734 + 0,007 031 306 · 15,734 −
PKI
5 6
0,000 177 002 9 · 15,734 + 0,000 001 751 212 · 15,734 + 100
The calculation delivers a MN value of 57 after rounding to an integer value.
PKI
This MN value meets the criterion of MN ≥ 53 and thus is a valid result of the PKI method for this
PKI PKI
example gaseous fuel.
For further guidance on the calculation of the PKI and MN Annex B lists a selection of gaseous fuels and
PKI,
corresponding PKI and MN values.
PKI
ISO 17507-2:2025(en)
Annex A
(normative)
Listing of coefficients used in Formula (1) and Formula (4)
A.1 Coefficients used in Formula (1)
a
Table A.1 — α and β coefficients used in Formula (1)
Coefficient Value
α 569,285 536 016 002 0
CH4
α −650,854 339 490 7
(CH4)
α 64,359 575 257 386 2
(CH4)
α 17,214 959 222 053 6
(CH4)
α −645,099 966 662 855 0
C2H6
α 694,229 376 857 102 0
(C2H6)
α −675,381 075 231 165 0
(C2H6)
α 1 474,790 791 373 33
(C2H6)
α 499,398 492 651 52
C3H8
α −576,665 945 472 394 0
(C3H8)
α 252,193 674 060 28
(C3H8)
α 593,958 975 466 507 0
(C3H8)
α 934,466 273 223 240 0
n-C4H10
α −86,872 357 077 023 8
(n-C4H10)
α −20 418,906 767 397 9
(n-C4H10)
α 633 286,561 358 521 0
(n-C4H10)
α 735,223 884 113 728 0
i-C4H10
α −3 182,614 393 379 67
(i-C4H10)
α 20 945,186 725 021 9
(i-C4H10)
α 159 067,868 032 595 0
(i-C4H10)
α 2 571,930 793 605 35
n-C5H12
α 10 516,494 109 227 50
(n-C5H12)
α −770 539,377 197 693
(n-C5H12)
α 28 633 475,586 565 4
(n-C5H12)
α −3 582,967 844 353 79
i-C5H12
α 403 155,950 864 334
(i-C5H12)
α −11 917 333,837 932 9
(i-C5H12)
α 1 123,396 367 098 65
neo-C5H12
α 1 679,728 075 248 10
(neo-C5H12)
α −172 182,649 067 176
(neo-C5H12)
α 3 467 918,607 466 990
(neo-C5H12)
α −469,428 097 827 742
N2
α 352,688 107 288 763
(N2)
a
The value of α and β coefficients not listed shall be assumed zero.

ISO 17507-2:2025(en)
TTabablele A A.11 ((ccoonnttiinnueuedd))
Coefficient Value
α −220,491 687 402 358
(N2)
α 1 419,680 053 962 420
(N2)
α −953,460 328 339 263
CO2
α 1 148,487 258 682 280
(CO2)
α −601,339 855 375 907
(CO2)
α 448,125 565 457 084
(CO2)
α −5 813,759 963 900 21
CO
α 5 511,721 025 828 67
(CO)
α 1 647,043 065 843 26
(CO)
α −3 471,241 525 554 25
(CO)
α −2 012,525 21 90 63
H2
α 2 059,631 57 03 14 47
(H2)
α −313,277 15 07 26 788
(H2)
α 957,327 60 80 16 344
(H2)
β 201,788 909 592 169
CH4 · C2H6
β −865,856 657 223 225
CH4 · C3H8
β −1 210,227 541 932 4
CH4 · n-C4H10
β 1 331,555 523 696 450
(CH4 · n-C4H10)
β −1 023,278 147 470 3
CH4 · i-C4H10
β 1 550,095 184 612 58
(CH4 · i-C4H10)
β −2 811,677 404 325 23
CH4 · n-C5H12
β 3 363,981 505 063 56
CH4 · i-C5H12
β −1 534,525 674 887 23
CH4 · neo-C5H12
β −1,053 973 329 306 09
CH4 · N2
β 473,574 764 109 71
CH4 · CO2
β −308,259 010 229 21
(CH4 · CO2)
β 5 356,433 570 549 5
CH4 · CO
β 1 227,107 72 94 97 01
CH4 · H2
β 253,206 75 96 21 511
CH4 · (H2)
β 326,009 79 53 02 013
(CH4) ·H2
β −437,695 363 730 406
C2H6 · n-C4H10
β −109,983 789 902 769
C2H6 · i-C4H10
β −1 870,347 465 005 63
C2H6 · n-C5H12
β 3 909,509 060 762 45
C2H6 · i-C5H12
β −886,578 525 827 322
C2H6 · neo-C5H12
β 968,887 620 927 515
C2H6 · N2
β 267,472 766 191 96
(C2H6) ·N2
β 337,464 863 958 288
C2H6 · (N2)
β 1 431,950 116 993 15
C2H6 · CO2
β 6 463,144 442 956 27
C2H6 · CO
β 2 974,729 29 65 84 95
C2H6 · H2
β −118,490 180 710 956
C3H8 · n-C4H10
β −1 734,805 682 394 27
C3H8 · n-C5H12
a
The value of α and β coefficients not listed shall be assumed zero.

ISO 17507-2:2025(en)
TTabablele A A.11 ((ccoonnttiinnueuedd))
Coefficient Value
β 127 551,642 193 201
C3H8 · (n-C5H12)
β 11 318,418 395 072 2
(C3H8) ·n-C5H12
β 3 318,968 208 193 38
C3H8 · i-C5H12
β 13,345 337 812 469
C3H8 · N2
β 292,275 289 330 565
C3H8 · CO2
β 5 403,502 607 948 29
C3H8 · CO
β 2 333,823 463 429 21
(C3H8) ·CO
β 2 067,292 42 46 09 78
C3H8 · H2
β 3 500,702 828 522 74
n-C4H10 · i-C4H10
β −4 737,328 494 949 99
n-C4H10 · n-C5H12
β 525 591,310 711 326
n-C4H10 · (n-C5H12)
β 297 556,039 242 685
(n-C4H10) ·n-C5H12
β 6 095,059 988 750 87
n-C4H10 · i-C5H12
β −953,002 183 779 388
n-C4H10 · neo-C5H12
β −103,571 484 346 062
n-C4H10 · CO2
β 5 869,190 506 527 74
n-C4H10 · CO
β 2 377,694 85 62 41 19
n-C4H10 · H2
β 5 056,603 091 637 61
i-C4H10 · n-C5H12
β 6 619,278 776 370 44
i-C4H10 · iso-C5H12
β −1 363,961 016 448 41
i-C4H10 · neo-C5H12
β 14,803 895 799 972 4
i-C4H10 · N2
β 211,752 602 673 394
i-C4H10 · CO2
β 5 786,325 257 174 88
i-C4H10 · CO
β 2 567,653 63 14 925
i-C4H10 · H2
β 12 268,283 772 748
n-C5H12 · i-C5H12
β −1 573,688 937 706 25
n-C5H12 · N2
β −898,466 856 535 774
n-C5H12 · CO2
β −42 401,411 139 182 4
(n-C5H12) ·CO2
β 3 985,110 420 511 03
n-C5H12 · CO
β 48 265,319 103 373 7
(n-C5H12) ·CO
β 9 9313,950 84 34 517
(n-C5H12) ·H2
β 3 773,449 267 853 97
i-C5H12 · neo-C5H12
β 4 490,678 300 326 75
i-C5H12 · N2
β 5 122,009 935 455 09
i-C5H12 · CO2
β −28 087,848 186 432 6
(i-C5H12) ·CO2
β 10 248,340 825 423 2
i-C5H12 · CO
β 6 575,397 11 80 68 26
i-C5H12 · H2
β −642,170 828 416 611
neo-C5H12 · N2
β −11 320,112 689 948 1
(neo-C5H12) ·CO2
β 4 772,677 301 186 82
neo-C5H12 · CO
β 1 108,926 38 47 52 54
neo-C5H12 · H2
β 1 156,200 327 160 21
N2 · CO2
β 359,342 203 118 816
(N2) ·CO2
a
The value of α and β coefficients not listed shall be assumed zero.

ISO 17507-2:2025(en)
TTabablele A A.11 ((ccoonnttiinnueuedd))
Coefficient Value
β 6 076,818 092 916 31
N2 · CO
β 389,853 153 629 781
(N2) ·CO
β 367,319 351 280 689
N2 · (CO)
β 2 616,219 56 43 13 42
N2 · H2
β 6 557,376 349 418 7
CO2 · CO
β 1 824,585 879 374 03
(CO2 · CO)
β 3 034,741 34 60 668
CO2 · H2
β −1 664,28 09 40 74 521
(CO2 · H2)
β 8 006,508 20 72 31 09
CO · H2
β 884,142 62 53 84 53
(CO · H2)
a
The value of α and β coefficients not listed shall be assumed zero.
A.2 Coefficients used in Formula (4)
Table A.2 — a and b coefficients used in Formula (4)
Coefficient Value
a −9,757 977
a 1,484 961
a −0,139 533
a 0,007 031 306
a −0,000 177 002 9
a 0,000 001 751 212
b 100
SIST
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