*5.4.5 Last part of pre-mixed combustion (7.0–9.0° ASI)*

As fuel-air mixing rate increases, PAH and soot formation are enhanced towards the end of pre-mixed combustion, and this extends length of the jet downstream. At about 8.0° ASI, the soot concentration is high and almost fills the jet cross-section bulging more towards the leading edge to form a head vortex. At the periphery of the jet, the thin sheet of turbulent diffusion flame produces larger soot particles that move inwards to mix with the inner smaller soot particles in the bulk volume of the jet, this is more active at the leading edge where there is more soot concentration. As the last portion of the premixed charge burns, smaller size soot particles are formed over the entire jet volume extending up to 27 mm from the tip of the injector but soot is entirely lifted off the nozzle tip for the reference low sooting fuel.
