The gasoline HCCI engine holds a promise of achieving very high part-load efficiency combined with extremely low NOx and soot emissions. However, the load range of HCCI operation is limited by the misfire limit at the low end, and knock limit at the high end. Therefore, the future practical implementation will likely be a dual-mode engine, operating in the HCCI mode at part load and switching to SI at higher loads. Expanding the limits will be critical for maximizing the fuel economy benefits in the vehicle. The mixture stratification, both thermal and compositional, can have very tangible impact on HCCI combustion; and gaining a deeper insight into these effects is critical for expanding the HCCI range of operation. This paper presents results of the comprehensive experimental investigation of the mixture preparation effects on a single-cylinder gasoline engine with exhaust re-induction. The effects include type of mixture preparation (external mixing vs. direct injection), charge motion, and injection timing. A combination of pressure-based combustion diagnostics, emissions analysis, and heat flux measurements on the combustion chamber wall quantifies the effects on combustion and provides insight into reasons for observed engine behavior. As an example, the instantaneous temperature and heat flux measurements show the fuel impingement locations and allow assessing the fuel film dynamics and their effect on mixture stratification. The effects of direct injection and partial closing of the swirl control valve were relatively small compared to extending the injection timing late into the intake process or completely closing the swirl control valve and allowing charge storage in the port.

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