Combustion behaviours and emission characteristics of a retrofitted NG/gasoline duel-fuel SI engine with various proportions of NG-gasoline blends

A baseline gasoline engine was retrofitted into a natural gas (NG)/gasoline duel-fuel engine through increasing a NG injection system at its intake manifold. The combustion behaviours and emission characteristics of the engine with different proportions of NG-gasoline blends were investigated. Numerical simulation and experiments verification were performed on a 4-cylinder SI engine. The results show that the combustion behaviours and emission characteristics of natural gas/gasoline dual fuel engine are between gasoline and natural gas. The maximum in-cylinder pressure (MCP), the maximum heat release rate (MHRR) and maximum pressure rise rate (MPRR) decrease with the increase of NG ratio and combustion phases (CA10 and CA50) are retarded, this implies that the adding natural gas to the gasoline can effectively improve the knocking resistance of the engine, which allows to improve the thermal efficiency by increasing compression ratio. The maximum combustion temperature (MCT) and the total overall equivalence ratio increases with the increase of NG ratio, leading to an increase in NO and soot emissions but their increasing proportion of NO and soot emissions at 50% of NG ratio (by energy) is respectively less than 8.7% and 17.3 with respect to the pure gasoline mode, but at the same time, there is an increase in indicated thermal efficiency (ITE) and a decrease in CO, CO2 and HC, this implies that trading off thermal efficiency and NOx emissions by operating the engine with 50% of NG and 50% of gasoline (by energy) without causing further cost and complexity.