The Analysis of Natural Gas Engine Combustion Specificities in Comparison with Isooctane Through CFD Computation

In this paper, the specificities of Natural Gas combustion in IC spark-ignition engines are investigated through CFD calculations. Three reference natural gases are chosen, representative of real compositions distributed in Europe. They consist in different ratios of propane, methane and inert gases. Their combustion behavior is subsequently compared with the combustion characteristics of isooctane. In the first part of the study, a database for the laminar burning velocities for the three natural gases mixtures is built, using the 1-D laminar flame software PREMIX, and based on a complex chemistry scheme for propane and methane. An analysis of the laminar flame behavior for such mixtures is proposed, and a correlation for the laminar burning velocities is built. In the second part of the study, 2D axisymetrical calculations, representative of engine combustion in a simplified geometry, are performed using KIVA II-GSM, in which the flame speed correlation is introduced. A complete database including compression ratio, turbulence intensity, load and engine speed is built for the different mixtures. A comprehensive analysis gives more insight into the specificities of each mixture in the combustion process, and into their response to engine parameters variations. In particular, turbulent flame propagation, heat release rates and pollutant emissions are compared for isooctane and natural gas combustions.