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For next-generation engines that operate using low-temperature gasoline combustion (LTGC) modes, a major issue remains poor combustion stability at low-loads. Negative valve overlap (NVO) enables enhanced main combustion control through modified valve timings to retain combustion residuals along with a small fuel injection that partially reacts during the recompression. While the thermal effects of NVO fueling on main combustion are well understood, the chemical effects of NVO reactions are less certain, especially oxygen-deficient reactions where fuel pyrolysis dominates. To better understand NVO period chemistry details, comprehensive speciation of engine samples collected at the end of the NVO cycle was performed by photoionization mass spectroscopy (PIMS) using synchrotron generated vacuum-ultraviolet light.

Negative valve overlap (NVO) is a valve strategy employed to retain and recompress residual burned gases to assist HCCI combustion, particularly in the difficult regime of low-load operation. NVO allows the retention of large quantities of hot residual burned gases as well as the possibility of fuel addition for combustion control purposes. Reaction of fuel injected during NVO increases charge temperature, but in addition could produce reformed fuel species that may affect main combustion phasing. The strategy holds potential for controlling and extending low-load HCCI combustion. The goal of this work is to demonstrate the feasibility of applying two-wavelength PLIF of 3-pentanone to obtain simultaneous, in-cylinder temperature and composition images during different parts of the HCCI/NVO cycle. Measurements are recorded during the intake and main compression strokes, as well as during the more challenging periods of NVO recompression and re-expansion.

A transparent direct-injection spark-ignition engine incorporating a rapid-acting, drop-down cylinder has been built. The design enables access in less than a minute for cleaning windows. Combustion performance of the optical engine is characterized in terms of indicated pressure and coefficient of variation of indicated pressure as a function of injection timing. Piston temperatures are measured and a skip-fire routine is developed so that quartz piston top temperatures agree with a matching non-optical engine. Laser-induced fluorescence imaging of in-cylinder fuel injections highlights the effects of ambient pressure and fuel temperature on spray morphology. Measurements of gasoline vapor distribution provide statistics on heterogeneity of fuel distribution as a function of injection timing. Flame imaging records details of flame development which depend on the degree of fuel mixing.