Overview
ISO 17507-2:2025 - "Natural gas - Calculation of methane number of gaseous fuels for reciprocating internal combustion engines - Part 2: PKI method" specifies the PKI (Propane Knock Index) method for calculating the methane number (MN) of gaseous fuels using only the gas composition as input. The standard applies to natural gas, biomethane, and their admixtures with hydrogen, and is intended to characterize fuel knock resistance for reciprocating internal combustion engines.
Key topics and technical requirements
- Scope and applicability: The PKI method is applicable to a defined range of gaseous fuel components (methane, C2–C6 hydrocarbons, hydrogen, CO, CO2, N2, H2S) and to the boundary limits listed in the standard.
- Component limits (mole fraction %):
- Methane: 65–100%
- Ethane, Propane: 0–20%
- n-/i-Butane: 0–5%
- n-/i-/neo-Pentane: 0–2%
- Hexanes and higher (treated as n‑hexane): 0–1.5%
- Hydrogen: 0–35%
- CO: 0–10%, CO2: 0–20%, N2: 0–20%, H2S: 0–0.5%
- Methodology: Calculation involves two polynomial functions - first computing a PKI value, then converting PKI to the MN. Gas composition must be expressed in mole fraction; conversions from volume or mass fractions are to follow ISO 14912.
- Handling of special components: Oxygen and water vapour are ignored and the composition normalized; guidance is provided for other non-listed components.
- Quality and traceability: The standard includes annexes with coefficients, example calculations, tools for users, and provisions on uncertainty, error and bias to support implementation and software validation.
Practical applications and target users
- Engine OEMs: to specify permitted methane number for warranty and engine tuning.
- Gas suppliers and biomethane producers: to certify fuel suitability and communicate MN values.
- Engine operators and fleet managers: to assess fuel compatibility, prevent engine knock, and manage maintenance risk.
- Consulting engineers and instrument OEMs: for combustion modelling, gas analysis systems, and control strategies.
- Software developers: validation against provided examples and annex coefficients.
Related standards and references
- ISO 17507-1 (other MN methods) - users should note which method is used when specifying MN.
- ISO 14912 (gas composition conversion) - mandatory for mole-fraction conversions.
- ISO/IEC Guide 98-3 (GUM) - used for expressing measurement uncertainty.
Keywords: ISO 17507-2:2025, methane number, PKI method, natural gas, biomethane, hydrogen admixtures, gas composition, reciprocating internal combustion engines, knock resistance, ISO 14912.
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