A statistical analysis of emission features in non-road small SI engines with the same displacement

Emission share rates at various operating modes for non-road small spark-ignition engines are comprehensively analyzed, based on 25 sets of emissions data from one particular type of engine with the same displacement manufactured by different factories. The results of the statistical analysis show that the greatest contribution for CO and HC emissions occurs at mode 3, while that for NO_x emissions occurs at mode 2. At the rated power mode 1, the average share rates for each pollutant, especially NO_x, are beyond their weighting factors. The sum of contributions at modes 2–4 to CO and HC final emissions is close to their summary weighting factors, while the NO_x emission characteristic differs greatly. As for each pollutant, the share rates of modes 5–6 are very low and negligible. The maximum standard deviation of share rate for NO_x emission appears at mode 1, while the coefficient of variation differs greatly, especially at small-load modes. At the representative mode 1, the correlation coefficients between specific CO, HC and NO_x emissions with the excess air ratio are −0.874, −0.762 and 0.946, respectively. Even more, the specific HC + NO_x emission shows a positive correlation as high as 0.933, despite the negative correlation of HC. Therefore, a proper excess air ratio of [0.8, 0.9] is recommended for reducing more NO_x at mode 1. While at middle and small loads, the excess air ratio should be moderately increased to reduce HC and CO emissions, as well as aiding better fuel consumption. From these results, this study can do a great service in the low-emission management, for non-road gasoline engines without after-treatment devices, to meet current and future emission directives.