Compressed natural gas in aged internal combustion engines: performance, emissions, and challenges – a systematic review

Illustrative overview of CNG engine performance, emissions behavior, and operation challenges. The graphic highlights key trends identified in the systematic review, including typical 10–20% power and torque losses; reductions in CO, HC, PM, and CO 2 emissions; methane-slip escalation with mileage; and the dual effect of cleaner combustion but accelerated lubricant oxidation. The figure synthesizes evidence from 22 studies to show how engine architecture, retrofit quality, and accumulated mileage shape real-world CNG outcomes • Provides the first systematic, strata-based synthesis of CNG performance, emissions, and durability in aged and retrofitted SI and CI engines. • Demonstrates that performance and emissions outcomes under CNG are governed by engine design, retrofit quality, calibration strategy, and accumulated degradation rather than fuel properties alone. • Shows that retrofitted SI fleet engines commonly experience power loss and methane slip, while optimized dedicated SI engines achieve higher efficiency through compression ratio and combustion phasing control. • Identifies dual-fuel CI engines as offering strong particulate reduction but requiring careful pilot-injection and EGR management to avoid CO and HC drawbacks at low load. • Highlights durability and lubrication trade-offs under CNG, with reduced soot contamination but increased thermo-oxidative oil stress, emphasizing the need for robust maintenance and calibration practices. 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 CO 2 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.