Reduction in the cycle-to-cycle variation (CCV) of combustion in internal combustion engines is required to reduce fuel consumption, exhaust emissions, and improve drivability. CCV increases at low load operations and lean/dilute burn conditions. Specifically, the factors that cause CCV of combustion are the cyclic variations of in-cylinder flow, in-cylinder distributions of fuel concentration, temperature and residual gas, and ignition energy. However, it is difficult to measure and analyze these factors in a production engine. This study used an optically accessible single-cylinder engine in which combustion and optical measurements were performed for 45 consecutive cycles. CCVs of the combustion and in-cylinder phenomena were investigated for the same cycle. Using this optically accessible engine, the volume inside the combustion chamber, including the pent-roof region can be observed through a quartz cylinder. CCV of in-cylinder flow for 45 continuous firing cycles were measured by Time-Resolved Particle Image Velocimetry (TR-PIV) technique. The in-cylinder flow was measured at intervals of 2 crank angle degrees from the intake to compression strokes using a dual-cavity, high-frequency Nd:YLF laser. In order to analyze the CCV, the measured instantaneous flow was converted to a time-averaged flow by low-pass filtering to remove the high-frequency component. Moreover, CCVs of fuel distribution at intake valve closing (IVC) and just before ignition timing were obtained by Planar Laser Induced Fluorescence (PLIF) technique. The fourth harmonic generation of a dual-cavity Nd:YAG laser was used as the excitation light source. 3-Pentanone, which was mixed with iso-octane and injected to the intake port, was used as a PLIF tracer. These two visualization techniques were applied simultaneously during the continuous firing cycles.
As a result, it was confirmed that the CCVs of in-cylinder flow and fuel distribution significantly affect the CCV of combustion at low-load conditions. In particular, flow in a direction opposite to the tumble flow was observed in the lowest load cycle.