Improved NO_x Submodel for In-Cylinder CFD Simulation of Low- and Medium-Speed Compression Ignition Engines

Abstract

The NO x submodels currently used in the CFD-based simulation of engines have been developed for conditions where homogeneous and stoichiometric mixtures of fuel and air are burned. Such conditions apply well for spark ignition engines (SIE) but not for compression ignition engines (CIE), where combustion takes place in a highly heterogeneous environment. As a consequence, current NO x submodels do not satisfactorily describe the fate of nitrogen oxides in CIE. The aim of this work was to determine, by detailed chemical kinetic investigation, the mechanisms leading to in-cylinder NO x in low-and medium-speed CIE and to develop a more accurate submodel for the prediction of NO x in CIE by means of CFD. Calculations at constant pressure (1-150 bar) and constant temperature (1500-2200 °C) under ideal plug flow conditions show that, relative to the proportion of NO, the proportion of NO 2 in NO x is negligible and that the formation and destruction of NO occurs mainly via 10 reactions. These reactions can be organized into three NO mechanisms: thermal, N 2 O intermediate, and N 2 O extension, the latter being a set of five reactions that oxidize N 2 O to NO via NH and HNO intermediates. To our knowledge, the importance of the N 2 O-extension mechanism is reported here for the first time. An improved NO x submodel was developed taking into account all three mechanisms, after introducing approximations relevant to CIE. According to our kinetic investigations, partial equilibrium approximation can be applied to O, OH, and H and a quasi-steady-state approximation to N 2 O, N, NH, and HNO. The performance of the improved submodel is illustrated by comparing its NO x predictions with those of a detailed kinetic scheme and a NO submodel currently used in CFD. The comparison shows that the improved submodel always produces more accurate predictions of NO x than does the current submodel.

Reduced Kinetics Schemes for Oxides of Nitrogen Emissions from a Slow-Speed Marine Diesel Engine

“…Essential features of NO x formation in slow-speed engines include formation primarily on the lean side of the flame and mixing of burnt gas with oxygen rich unburnt gases. − The NO x model attempts to incorporate these features into a real time model in a meaningful way.…”

Reducing NOx Emissions Using Fuel Staging, Air Staging, and Selective Noncatalytic Reduction in Synergy

“…As indicated above, reaction kinetics were deduced from the gas-phase detailed mechanism in ref . This very mechanism has been used successfully for the past years to simulate high-temperature reburning, SNCR, and air staging, ,,,,, which encourages us to consider it a valid choice here.…”

“…Nitrogen oxides (NOx = NO + NO 2 ) are pollutants that form during most combustion processes. Nitrogen oxides form from the nitrogen contained in fuel and air via a number of routes that depend on fuel and process. − Fuel staging (FS), air staging (AS), and selective noncatalytic reduction (SNCR) techniques were introduced decades ago for reducing NOx from combustion devices such as furnaces and engines. Alone, each technique reduces NOx by some 50 to 70%, but only within well-defined applicability limits.…”

Development and evaluation of a semi-empirical multi-zone thermodynamic model for the estimation of nitric oxide emissions and formation history in diesel engines using the measured cylinder pressure trace

“…For the calculation of NO, the reduced kinetics scheme developed by Zabetta and Kilpinen [185,186] was used because it was derived in the context of large marine propulsion Diesel engines. It involves the extended Zeldovich mechanism, a two-step N2O-intermediate path and five additional reactions, called the "N2O extension" path, which involves oxidation of N2O to NO via NH and HNO intermediates.…”

“…Multi-zone, 3D, phenomenological models [91,92,93,94,95] simulate the fuel spray in the three dimensions by dividing the spray elements into zones of which their spatial location and evolution inside the combustion chamber are determined according to the injection and spray evolution and the considered spray pattern. On the other hand, CFD models [96,97,98,99,100] are characterized from their robust physical background and detailed space discretization (mesh). These models attempt to simulate the actual procedures taking place inside the cylinder providing detailed information for the in-cylinder conditions spatially and temporally.…”