Experimentation and modeling of NOx formation from a micro-pilot ignited natural gas engine.

摘要

A diesel micro-pilot ignited natural gas engine under study in the Internal Combustion Engines Laboratory in the Department of Mechanical Engineering at the University of Alabama shows good promise for future alternative fuel use. The ultra-low NOx emissions and diesel-like fuel conversion efficiency make it a good substitute for diesel engines. In this dissertation, experimental and modeling results for NOx formation in the research engine are described. NOx was measured for varying diesel pilot injection timing, intake temperature, and diesel pilot quantity. It is shown that for the "ALPING" (Advanced Low Pilot Ignited Natural Gas) region, consisting of injection timings advanced beyond 50° before top dead center, NOx emissions are lower than 0.2 g/kWh with diesel-like fuel conversion efficiency of around 40 percent.; NOx modeling coupled with sensitivity analysis of NOx formation reactions in an ALPING engine is performed based on the Krishnan's phenomenological combustion model (2005). The NOx formation model incorporates a super-extended Zel'dovich mechanism (SEZM) with up to 43 reactions. NOx predictions are based on both the extended Zel'dovich mechanism (EZM) and the SEZM. It is shown that for injection timings between 20° and 35° BTDC, the SEZM improves the NOx prediction over the standard EZM by as much as 50 percent. For injection timings between 40° and 60° BTDC, the combustion model using either the EZM or SEZM overpredicts NOx emissions compared to experimental results. The sensitivity analysis compares the normalized sensitivity coefficients for each major NOx formation reaction, and identifies the rate controlling NOx formation reactions. Results show that the following reactions are important only in the packet zone: O+N2=NO+N R1 N+O2=NO+O R2 while the following reactions are important mainly in the burned zone: N2O+M=N2+O+M R7 N2O+O2=NO+NO2 R10 Reactions 7 and 10 are also important in packets leaned out by the effect of advanced injection timing.; Reaction 21 below is important in both high temperature and low temperature regions, thus it is important for both the fuel packets and burned zone: N2+HO2=NO+HNO R21 Rate parameters for these reactions affect NOx formation the most, therefore, they should be known most accurately. Sensitivity analysis can improve the understanding of NOx formation and help provide better insight into modeling of NOx formation from engines. This NOx model can be used for parametric studies to reduce NOx emissions computationally prior to experimentation for validation purposes. Finally, because the quasi-steady-state approximation is commonly used for certain species in NOx modeling, the relative error is estimated to evaluate its use. The transient relative error in NOx prediction for this assumption is of the order of 2 percent.