Natural gas (NG)-diesel dual fuel engines have been highlighted for their fuel flexibility and high thermal efficiency comparable to diesel engines. However, the addition of NG to compression ignition diesel engines was reported to elongate ignition delay and to increase the emissions of carbon monoxide (CO), unburned methane (CH4), and nitrogen dioxide (NO2). Past research on dual fuel engines has focused on the experimental research on the engine performance, combustion process, and exhaust emissions. The research on detailed mechanism dominating the impact of CH4 on formation of CO and NO2 in cylinder, and the mechanism for CH 4 to survive the combustion process and slip through the cylinder is limited. The examinations of these mechanisms require the simulation of dual fuel engine combustion using a CFD model coupled with chemical kinetic mechanism.
This research numerically investigates the combustion process and exhaust emissions from two NG-diesel dual fuel engines using a CFD model coupled with a reduced primary reference fuel (PRF) chemistry. The CFD model used is Converge-SAGE model with a maximum of 300000 grid points. The fuel chemistry used is a reduced PRF mechanism with 45 species and 142 reactions including a reduced NOx mechanism with 4 species and 12 reactions. The CFD model with reduced PRF chemistry has been validated against experimental data measured in a single-cylinder compression-ignition engine over a wide range of CH4 substitution ratio. A post-processing tool has been developed to calculate, analyze, and visualize the instantaneous rate of production (ROP) of key species in each cell with the known temperature, pressure, and species concentration exported by CFD code. The simulation results are further post-processed to numerically investigate the combustion process and the formation mechanism of CO, and NO2 in a dual fuel engine. The mechanism for CH4 to survive the main combustion process and post-combustion oxidation process is numerically examined.
The research on NO2 formation identified NO+HO2→NO 2+OH as the key reaction dominating the increased formation of NO 2 in dual fuel engines. The HO2 necessary for the formation of NO2 emitted by the engine is produced through the post-oxidation of CH4 that survived the main combustion process. The CO emitted from the NG-diesel dual fuel engine is formed through the oxidation of CH 4 during the late combustion process and post-combustion CH4 oxidation. The CH4 that survived the main combustion and post-combustion oxidation process is mainly distributed in region far from the spray plume of the pilot fuel and its combustion products.
This research also examined approaches capable of significantly reducing the emissions of CH4 from a dual fuel engine. The preliminary results concluded that CH4 emissions can be significantly reduced through optimizing injection timing, and the application of two-pulse fuel injection strategy. Adjusting injector fuel spray angle can also significantly reduce CH4 emissions which should be considered in developing dedicated dual fuel engine.
|Commitee:||Akkerman, V'yacheslav, Dumitrescu, Cosmin, Singh, Satbir, Wayne, Scott|
|School:||West Virginia University|
|Department:||Engineering & Mineral Resources|
|School Location:||United States -- West Virginia|
|Source:||DAI-B 80/01(E), Dissertation Abstracts International|
|Subjects:||Engineering, Mechanical engineering|
|Keywords:||CFD, CH4 emission, Dual fuel engine, Instantaneous kinetic study, NO2 emission, Natural gas|
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