  Mixing, ignition and combustion behavior in a rapid compression and expansion
machine operated under Premixed Charge Compression Ignition (PCCI) relevant
conditions are investigated by combined passive optical and laser-optical
high-speed diagnostics. The PCCI concept is realized using a split injection
schedule consisting of a long base load injection and two closely separated
short injections near top dead center. Previous studies of close-coupled double
injections under constant ambient conditions showed an increased penetration
rate of the subsequent fuel spray. However, the aerodynamic gain from the
preceding injection is counteracted by the density rise during the compression
stroke under transient engine conditions. The study confirms that the rate of
mixing of the subsequent fuel spray is significantly increased. Regarding
combustion behavior, the thermodynamic analysis exhibits contributions of low
temperature oxidation reactions of more than 20 % to the total heat release,
with a notable amount of unburnt fuel mass varying from 25 to 61 %. The
analysis of the optical data reveals the multi-dimensional impact of changes in
operating parameters on the local mixture field and ignition dynamics. The
onset of low temperature reactivity of the first short injection is found to be
dominated by the operating strategy, while the location is strongly related to
the local mixing state. Low temperature ignition of the consecutive fuel spray
is significantly promoted, when upstream low temperature reactivity of the
preceding injection is sustained. Likewise, it is shown that high temperature
ignition is accelerated by the entrainment of persistent upstream low
temperature reactivity.
