| Energy crisis and environmental pollution problems have become increasingly serious. Thus, natural gas, which is an alternative fuel with low emissions, has been paid more attention to. Composition and content of natural gas are different in different regions, which could result in differences in the combustion and emission characteristics of natural gas engines. Studying the effects of composition and the corresponding fraction on the combustion process and emissions by adding other gases to methane is crucial in guiding the calibration of natural gas engine and the development of the emission control strategy. Also, it has great significance for understanding the differences in natural gas combustion characteristics of different sources.In this paper focused on WP7 natural gas engine. Ignition, flame propagation and production process of exhaust pollutants were calculated based on a detailed chemical kinetic model of multicomponent gas combustion. The simulated cylinder pressure curves were compared with the experimental ones to verify the model accurately. On the basis of the validated simulation model, the effects of gas composition to combustion heat release rate, temperature and pressure in cylinder, NO and CO generation rules were analyzed by adding different proportions of hydrogen gas, propane, and exhaust gas to methane.Analysis and calculation results of different hydrogen volumetric fractions show that with the increase of the proportion of hydrogen in gaseous fuel, the heat transfer in cylinder becomes faster and faster, which increases the combustion rate, shortens the combustion duration, and subsequently, raises the combustion temperature and pressure. When hydrogen mixing ratio raises from 0 to 25%, the in-cylinder pressure increases by about 14.78% at engine speed of 1600r/min and increases by about 17.23% at engine speed of 2100r/min; moreover, the phase angle of the maximum cylinder pressure move towards the TDC gradually. However, the hydrogen increases the combustion temperature in the cylinder, which provides favorable conditions for the formation of nitrogen oxides and lead to the increase of NO emissions. At 50°TDC, as hydrogen mixing ratio raises from 0 to 25%, the NO concentration is doubled at the engine speed of 1600r/min while at engine speed of 2100r/min, the concentration of NO with 25% hydrogen added is 2.3 times as much as that without hydrogen.The effects of propane volumetric fraction on combustion process and emissions of the engine are similar to that of hydrogen. The pressure and temperature in the combustion process increased with the increase of propane ratio. When propane mixing ratio raises from 0 to 25%, the in-cylinder pressure increases by about 11.1% and the concentration of NO is 1.84 times as much as that without propane at engine speed of 1600r/min; when propane mixing ratio raises from 0 to 25%, the in-cylinder pressure increases by about 12.8%, and the content of NO is 1.92 times as much as that without propane at engine speed of 2100r/min.Different EGR models were established to explore the influence of EGR on engine combustion and emission by gradually increasing the exhaust gas ratio of the in-cylinder charge. The results show that the increases in engine EGR rate could cause reductions in heat release rate due to the dilution effect of exhaust gas and its absorption of heat release. Due to the reduction of the heat release rate, the maximum average pressure in the cylinder and the maximum average temperature are also decreased. The effect of EGR on the emission is also very significant. The emission of NO is obviously decreased with the increase of EGR rate. At the engine speed of 1600r/min, the NO emission decreases by about 94.8% as EGR rate increases from 0 to 20%; At engine speed of 2100r/min, the NO emission decreases by about 87.9% as EGR rate increases from 0 to 16%. |
