Part load operating point for a controlled auto-ignition four-stroke internal combustion engine is reduced without compromising combustion stability through negative valve overlap control operative to retain and compress combusted gases within the combustion chamber into which fuel is introduced. Combustion chamber pressures and temperatures are increased as engine load decreases. Various 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.