Part load operating point for a controlled auto-ignition four-stroke internal
combustion engine is reduced without compromising combustion stability through
load dependent valve controls and fueling strategies. Optimal fuel economy is achieved
by employing negative valve overlap to trap and re-compress combusted gases below
a predetermined engine load and employing exhaust gas re-breathing above the predetermined
engine load. Split-injection fuel controls are implemented during low and intermediate
part load operation whereas a single-injection fuel control is implemented during
high part load operation. Split-injections are characterized by lean fuel/air ratios
and single-injections are characterized by either lean or stoichiometric fuel/air
ratios. Controlled autoignition is thereby enabled through an extended range of
engine loads while maintaining acceptable combustion stability and emissions at
optimal fuel economy.