A Review of the Performance of a Spark Ignition (SI) Engine Retrofitted to Operate on Biogas

Biogas has significant potential as a renewable fuel for spark ignition (SI) engines, particularly in waste-to-energy applications. However, its relatively low heating value, high carbon dioxide content, variable methane concentration, and combustion characteristics can affect engine performance and emissions. This review examines recent developments in biogas-fuelled SI engines, focusing on retrofitting, combustion, performance, emissions, optimisation, and predictive modelling. A structured literature search was conducted using Scopus, ScienceDirect, Wiley Online Library, Sage Journals, Taylor & Francis Online, and Google Scholar. Publications from 2020 to August 2026 were screened using predefined criteria, resulting in 80 publications included in the review. The literature was categorised into pure biogas SI, biomethane, and blended or dual-fuel systems, including biogas–gasoline, biogas–hydrogen, biogas–ethanol, and biogas–diesel configurations. ML studies involving unrelated fuel or engine systems were considered only for methodological insights. The findings indicate that methane concentration, compression ratio, ignition timing, air–fuel ratio, and fuel-delivery characteristics strongly influence engine performance and emissions. Carburettor modification, biogas upgrading, and operating-parameter optimisation can improve engine operation. Key challenges include fuel composition variability, adaptive control, durability, and the limited validation of predictive models specifically for biogas-fuelled SI engines.