An experimental investigation of the ignition properties of low temperature combustion in an optical engine

Homogeneous charge compression ignition (HCCI) engine operation offers the potential to provide fuel economy approaching that of traditional diesel engines, with increased compression ratios and low pumping losses, while simultaneously emitting low NOx and soot due to the homogeneous, low temperature nature of the combustion. HCCI, however, comes with unique challenges as fuel mixture chemical kinetics supplant direct ignition timing control via spark ignition or injection timing. Improved understanding of the ignition phenomena that control in-cylinder combustion phasing and duration in HCCI engines can help overcome the challenges of HCCI. In particular, spark-assisted HCCI combustion has been proposed as a means to extend HCCI operating limits and to facilitate transition between spark-ignition and HCCI operating modes.;The current work presents the results of an experimental study characterizing the ignition phenomena observed during HCCI lean operating conditions using highspeed digital imaging and the optical access provided by a single-cylinder optical research engine. Three fuels (indolene, iso-octane, and pump gasoline) and a range of operating conditions, including spark-assisted HCCI operation, were examined. HCCI combustion was initiated and maintained over a range of lean conditions, from equivalence ratios of &phis; = 0.69 to 0.27. Time-resolved imaging and pressure data showed high rates of heat release in HCCI combustion correlated temporally to rapid volumetric ignition occurring throughout the combustion chamber. Lower rates of heat release were characteristic of spatially-resolved ignition and subsequent propagation of reaction fronts. Gasoline and indolene exhibited similar HCCI imaging characteristics and in-cylinder pressure time-histories, while iso-octane showed a dramatic transition into misfire. Preferential ignition sites within the combustion chamber were identified based on the imaging and were considered clear markers of thermal stratification. The results for iso-octane indicate misfire at low load has distinctly different ignition characteristics (with well resolved, localized ignition and propagation) compared to stable HCCI (with volumetric ignition).;The results of the spark-assisted HCCI study demonstrated that spark assist stabilized HCCI combustion and extended lean operating limits for a window of engine operating conditions. The imaging data showed two ignition regimes exist. One regime is typified by an initial local reaction zone formed around the spark plug, which accelerates subsequent compression ignition sites that consume the fuel charge. The second regime is typified by consumption of the fuel charge by radial reaction front propagation from the spark plug. Regions of high sensitivity of HCCI operation to spark assist were bounded by low and high air preheat conditions. Within the temperature bounds, the effectiveness of spark-assisted HCCI was a strong function of the equivalence ratio and spark timing. Spark assist clearly extended low load HCCI operation within acceptable engine operating metrics, such as rate of pressure rise and indicated mean effective pressure.