Investigation of light load HCCI combustion using formaldehyde planar laser-induced fluorescence

Planar laser-induced fluorescence of formaldehyde was used to investigate quenching of the homogeneous-charge compression-ignition (HCCI) combustion process at light load in an n-heptane-fueled internal combustion engine. Detailed chemical kinetic calculations indicated that the persistence of formaldehyde after the end of heat release coincided with the existence of a significant mole fraction of carbon monoxide at the end of the cycle. At fuel–air equivalence ratios greater than 0.16, the data indicate that formaldehyde is uniformly formed within the combustion chamber during the first-stage ignition and then completely consumed during the second-stage ignition event. At leaner equivalence ratios, the formaldehyde fluorescence was found to persist after the second-stage ignition process; formaldehyde fluorescence was observed as late at 60 crankangle degrees after top dead center at an equivalence ratio of 0.09. The laser-induced fluorescence images were acquired in the central portion of the combustion chamber away from the cool combustion chamber surfaces. Therefore, the mechanism by which formaldehyde persists is considered to be bulk gas quenching as opposed to quenching due to thermal stratification within the combustion chamber. Data acquired from a uniform formaldehyde mixture over a range of crankangle timings in a motored engine were used to investigate the fluorescence signal as a function of temperature and pressure, and to provide a first order estimation of the absolute concentration. The fluorescence per formaldehyde molecule was found to be constant for a window about top dead center, but was found to increase for advanced or retarded timings where the pressure and temperature were lower.