Analysis of two-stage natural-gas lean combustion inside a diesel geometry

Lean-burn natural-gas spark-ignition combustion inside a diesel geometry is characterized by two distinct and successive events: a rapid burning process inside the piston bowl, followed by a much slower burn inside the squish region. This paper used experimental data (including images of flame natural luminosity) to investigate the magnitude and phasing of these two combustion stages. The results showed a substantial impact on the parameters generally used to investigate the combustion phenomena in spark-ignition engines (e.g., the 10-to-90-percent energy release). A better solution to identify the start and end of the rapid- and the slow-burning stages in such an engine is to use the inflection points in the apparent rate of heat release. Moreover, as the higher turbulence in a diesel-like environment accelerates flame inception, the 5% or 10% energy release used traditionally to define the flame inception period should not be used for these engines because the enflamed zone already reached an appreciable size at the two corresponding crank angle degrees. Further, the 2nd inflection point in the apparent rate of heat release (i.e., the point where the slow-burning stage started to dominate the heat release) was much advanced compared to 90% energy release generally used to define the end of the rapid-burning stage in conventional stoichiometric spark-ignition engines. Furthermore, as the slow-burning stage was characterized by a lower flame propagation speed, a lower fuel fraction burning during this stage would increase the engine combustion efficiency. Furthermore, a more advanced phasing of the late-burning stage relative to the fast-burn stage would result in a more optimal combustion environment (i.e., higher in-cylinder pressure and temperature), which would further improve the engine efficiency and probably reduce carbon monoxide and unburned hydrocarbon emissions. As a result, this study suggests that the combustion strategy of such converted diesel engines should advance and optimize the mass of fuel that burns inside the squish region.