Phenomenological Modeling of Low-Temperature Advanced Low Pilot-Ignited Natural Gas Combustion 2007-01-0942

Recently [1, 2, 3 and 4], the novel Advanced (injection) Low Pilot-Ignited Natural Gas (ALPING) low-temperature combustion (LTC) concept was demonstrated to yield very low NO_x emissions (<0.2 g/kWh) with high fuel conversion efficiencies (>40%). In the ALPING-LTC concept, very small diesel pilot sprays (contributing ∼2-3 percent of total fuel energy) are injected early in the compression stroke (60°BTDC) to ignite lean, homogeneous natural gas-air mixtures. To simulate ALPING-LTC, a phenomenological thermodynamic model was developed. The cylinder contents were divided into an unburned zone containing fresh natural gas-air mixture, several packets containing diesel and entrained natural gas-air mixture, a flame zone, and a burned zone. The simulation explicitly accounted for pilot injection, spray entrainment, diesel ignition (with the Shell autoignition model), spray combustion of diesel and entrained natural gas, and premixed turbulent combustion of the natural gas-air mixture. After a brief validation of the model, parametric studies were performed at different intake manifold temperatures, natural gas equivalence ratios, pilot injection quantities, and injection pressures. From these studies, the profound influence of packet (spray) combustion on the overall premixed turbulent (flame) combustion was identified. The method of flame area computation based on packet volumes provided the direct link between these two combustion processes.