Model for Nitric-Oxide Formation in a Large-Bore Spark Gas Engine

Several characteristics distinguish the combustion in large-bore engines from ordinary automotive engines; namely low rpm and associated long combustion duration, natural gas fuel, multiple spark plugs, lean (Φ ≈ 0.8) operation, substantial unmixedness due to direct gas injection during compression, and low surface to volume ratio. In order to better appreciate the effects of these factors on NO_x formation, a phenomenological model of combustion in a 20-inch bore two-cycle natural gas engine is described. The model accounts for random “unmixedness” of the initial charge, stratification, and temperature gradients arising from successively burned zones. Predictions are compared with experimental data for the effects of variations in timing, fuel-air ratio, stratified charge, EGR, and turbulence.