A Knock-Limited Otto Reference for Fuel–Engine Realization Diagnostics in Gaseous-Fueled Spark-Ignition Engines

Gaseous-fueled spark-ignition engines need diagnostics that separate fuel knock resistance, the ideal thermodynamic opportunity it enables and the fraction an engine realizes. A previously proposed methane-normalized fuel factor, XfMN, forced methane to unity, whereas its measured realization ratio is 0.544. This work reformulates XfMN as the ratio between the measured electrical efficiency at the knock threshold and a knock-limited Otto reference built from the methane number, the critical compression ratio of a Cooperative Fuel Research (CFR) engine and the specific-heat ratio of the fuel gas. Nine fuels from a biogas spark-ignition (BSI) campaign were reanalyzed on a corrected thermochemical basis; a reconstructed 84-record matrix served only to delimit the applicability domain. A two-descriptor model based on the hydrogen-to-carbon ratio and the specific-heat ratio is identifiable, whereas the full five-descriptor model is not. Under the nominal methane-number assignment, the reduced model lowers the leave-one-out error of electrical efficiency prediction by a factor of 2.10 relative to a null model; this advantage disappears if composition-consistent methane numbers are assigned to five engine blends. Only 23 of 84 records lie inside the calibration domain, and 58 require extrapolation. XfMN is therefore a local domain-aware realization diagnostic, not a universal fuel-quality ranking, and it can support fuel matching in adaptive multifuel engines.