Methodology to separate the two burn stages of natural-gas lean premixed-combustion inside a diesel geometry

•Two-stage combustion process compared to conventional gasoline engine.•Inflection points of the heat release rate separated the fast burn stage.•Flame speed inside the bowl was ∼3 times higher than inside the squish.•In-cylinder conditions at the transition between the two stages were important. Heavy-duty diesel engines can be modified to natural-gas spark-ignition operation by replacing the fuel injector with a spark plug and fumigating the gas inside the intake manifold. As natural gas premixed combustion in a diesel geometry is a two-stage combustion process with each stage influencing engine efficiency and emissions, determining the end of the fast burn inside the bowl can help optimize engine operation. This study proposed a methodology that identifies the burn inside the bowl using the inflection points of the heat release rate. A 3D CFD simulation that changed spark timing, equivalence ratio, and engine speed showed that this methodology can separate the fast combustion stage from the rest of the combustion event compared to the conventional spark-ignition theory that could not. In addition, this methodology showed that operating conditions controlled the flame location and combustion behavior inside the squish. For example, advanced spark timing, higher equivalence ratio, and lower engine speed advanced the transition point between the fast and the slow combustion stages but increased the mass of fuel inside the squish region. Moreover, the flame speed inside the bowl was ∼3 times higher than that inside the squish region. In addition, the lower burning rate after the transition point can decrease engine efficiency. Moreover, a higher engine speed optimized in-cylinder conditions at the transition between the two combustion events (which would suggest a better late combustion stage) but decreased the time available to complete the late combustion, which will probably increase emissions compared to conventional spark-ignition engines.