A comprehensive review of emission characteristics and mitigation strategies for non-road small spark-ignition engines

Despite the global shift toward electrification, non-road small spark-ignition (SI) engines continue to dominate as primary power sources for garden equipment, agricultural machinery and generator sets. However, their operation with the rich air-fuel mixture leads to significant emission challenges. This comprehensive review systematically covers the emission characteristics, regulations and available control technologies for small SI engines. Current analyses indicate that these engines exhibit distinct emission profiles, characterized by elevated hydrocarbon (HC) and carbon oxide (CO) emissions, along with moderate nitrogen oxide (NOx) level. Among various operating and structural parameters, the excess air coefficient is identified as the most critical factor governing emission performance. The review identifies several promising mitigation strategies. Alcohol-gasoline blends consistently reduce HC and CO emissions by 30–40 %, although their impact on NOx emissions remains variable. In contrast, electronic fuel injection (EFI) systems outperform carburetors, improving mixture quality and reducing HC and CO emissions by 50–70 %. Furthermore, the integration of secondary air injection (SAI) with oxidation catalysts reduces HC and CO emissions by 66–88 %, whereas three-way catalytic converters (TWCs) achieve up to 90 % reductions in HC and CO and 75 % in HC + NOx. While many emission control technologies from automotive applications show potential for adaptation, their implementation in small SI engines must consider critical constraints related to cost-effectiveness and spatial limitations. This review establishes a systematic framework for understanding and addressing emission reduction challenges in small SI engines, providing valuable insights for researchers, manufacturers and policymakers in the field.