Compressed natural gas in aged internal combustion engines: performance, emissions, and challenges – a systematic review
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Date
2026-03-06
Journal Title
Journal ISSN
Volume Title
Publisher
Elsevier Ltd.
Abstract
Compressed natural gas (CNG) offers significant emissions advantages over gasoline and diesel, yet most literature focuses on new or laboratory-optimized engines rather than the aged, retrofitted vehicles common in developing countries. With addition of other studies, the review followed PRISMA 2020 guidelines and a prospectively registered protocol (OSF) − https://osf.io/c8u7f/. Searches across Scopus, IEEE Xplore, and Google Scholar identified 816 records, of which 26 studies met inclusion criteria. CNG consistently lowered CO, HC, PM, and CO2 emissions, but retrofitted SI engines experienced 10–20% losses in power and torque due to methane’s low volumetric energy density and age-related declines in efficiency. High-mileage fleets showed methane-slip increases, catalyst deterioration, and lubricant oxidation, whereas optimized or dedicated CNG engines demonstrated improved thermal efficiency and fuel economy. Retrofit quality and calibration accuracy proved decisive in determining real-world outcomes. The findings highlight that CNG’s environmental and efficiency benefits are achievable but depend on proper engine design, maintenance, and regulatory support, especially in regions dominated by older vehicle fleets. This review provides the first systematic synthesis focused on aged, high-mileage, and retrofitted spark ignition (SI) and compression ignition (CI) engines operating on CNG, integrating evidence on performance, emissions, combustion behavior, methane slip, lubricant degradation, and catalyst aging. By comparing retrofitted and dedicated CNG engines against real-world aged engine across diverse regions, it reveals how engine architecture, retrofit quality, and accumulated mileage shape CNG outcomes and identifies the operational challenges and research priorities needed for durable, efficient, and low-emission operation.
Sustainable Development Goals
SDG 7: Affordable and Clean Energy
Keywords
Compressed natural gas, Aged engines, Retrofitted engines, Engine performance, Emissions performance, Energy transition