Overview
EN ISO 17507-2:2025 - Natural gas: Calculation of methane number (PKI method) specifies the PKI (Propane Knock Index) method for calculating the methane number (MN) of gaseous fuels for reciprocating internal combustion engines. The standard uses the gas composition as the sole input and applies to natural gas, biomethane and their admixtures with hydrogen. The PKI method is physics- and chemistry-based and was developed and experimentally verified with engine combustion models.
Key topics and requirements
- Scope and applicability
- Applies to natural gas, biomethane and hydrogen blends used in reciprocating engines.
- Requires gas composition expressed as mole fraction (percentage); conversions from volume or mass fractions must follow ISO 14912.
- Calculation method
- The PKI method computes an MN value (denoted MN_PKI) using two polynomial functions that take the mole-fraction composition as input.
- Boundary conditions, composition ranges and handling of atypical components are defined to ensure valid results.
- Data and verification
- Normative references include ISO 14532 (vocabulary) and ISO 14912 (composition conversion).
- The document includes example calculations and coefficient tables to support correct implementation and software validation.
- Quality and uncertainty
- The standard addresses uncertainty, error and bias for MN results and includes annexes with methods and normative information to assess accuracy.
- Supporting annexes
- Includes annexes with coefficient listings, PKI/MN values for selected compositions, user tools, uncertainty treatment, and the technical basis of the PKI method.
Practical applications and who uses it
- Engine OEMs: specify acceptable fuel MN for engine performance, warranty and calibration.
- Gas suppliers and producers: certify fuel quality (natural gas, biomethane, H2 blends) for engine markets.
- Power plant and vehicle operators: evaluate fuel suitability and predict knock risk to avoid engine damage.
- Fuel analysts and laboratories: implement the PKI calculation from compositional analysis.
- Control-system and gas-analyzer OEMs: integrate MN_PKI computation into instrumentation and engine management systems.
- Software developers: validate implementations using provided numerical examples and coefficient tables.
Related standards
- ISO 17507-1 - alternative methane number calculation methods (MNc).
- ISO 23306 - LNG fuel specification (PKI variant included in annex for marine LNG).
- ISO 14532 and ISO 14912 - vocabulary and composition conversion referenced for correct input data handling.
This standard enables consistent, repeatable calculation of methane number using the PKI method, supporting safe and efficient use of evolving gaseous fuels in reciprocating internal combustion engines. Keywords: methane number, PKI method, EN ISO 17507-2:2025, natural gas, biomethane, hydrogen blends, gas composition, engine knock resistance.
