Use of a convenient thermodynamic model to study the effects of operating parameters on nitrogen oxides emissions for a liquefied methane fueled spark-ignition engine

With the increasingly restrictive emission regulations, the emission requirements for internal combustion (IC) engines become more stringent, especially the amount of nitrogen oxides (NOx) emissions. In this investigation, a convenient thermodynamic model was employed to study the effects of operating parameters on NOx emissions for a liquefied methane fueled spark-ignition engine. The selected GRI-MECH 3.0 mechanism was verified by the experimental data and the thermodynamic model was calibrated by the in-cylinder pressure and heat release rate (HRR). The validated model with detailed combustion chemistry mechanism was utilized to perform the numerical investigation. The results showed that equivalence ratio and engine load have the greatest impact on NOx emissions, followed by the intake gas temperature, and finally the engine speed and intake gas pressure have the least impact on NOx emissions. The intake gas temperature, engine speed and intake gas pressure have a limited influence on NOx emissions in ultra-lean mixtures (phi = 0.5 and 0.6). However, under stoichiometric conditions, a reduction appears in NOx emissions compared to the lean fuels of corresponding operating conditions. In addition, > 90% NOx emissions are NO pollutants, with an occupation of 85% from thermal mechanism and a proportion of 12% from N2O-intermediate mechanism. More importantly, liquefied methane shows an advantage in terms of NOx emissions compared to multi-component natural gas. All these have provided research direction for the following 3-D CFD simulation work, and offered theoretical basis for selecting technology route to meet emission regulations of liquefied methane engine (LME).