Study of NO_x Formation at Lean Conditions in a Prechamber-Ignited Two-Stroke Natural Gas Engine

A computational fluid dynamics study of a large-bore, two-stroke, integral compressor engine is conducted. The engine under study operates on natural gas and uses a prechamber for ignition. The purpose of this work is to ascertain how property variations in the prechamber jet affect NO_x formation in the main combustion chamber. Several testing suites were performed using a CFD model of a Cooper-Bessemer GMV four-cylinder engine. The chemical composition and temperature of the prechamber jet were manually modified for each simulation. NO emissions were found to be more sensitive to increases in the jet temperature than decreases in the jet temperature. NO₂ emissions were largely insensitive to any jet temperature modification. Jet composition was modified to represent discrete prechamber fuel-air ratios. A non-monotonic relationship exists between jet composition and NO_x formation. In general, the optimal balance among NO_x, residual CH₄, and CO was found for simulations using a prechamber equivalence ratio slightly rich of stoichiometric. NO tended to form relatively early in the cycle in areas surrounding the PCC jet. Later in the cycle, NO collected around the perimeter of the cylinder. The extended Zeldovich and N₂O mechanisms were the highest contributors to NO formation. NO₂ formed relatively late in the cycle in areas of high NO concentration.