Optical Diagnostics of Pre-Chamber Combustion with Flat and Bowl-In Piston Combustion Chamber

Pre-chamber Combustion (PCC) extends the lean operation limit operation of spark ignition (SI) engines, thus it has been of interest for researchers as a pathway for increased efficiency and reduced emissions. Optical diagnostic techniques are essential to understand the combustion process, but the engine components such as the piston geometry, are often different from real engines to maximize the optical access. In this study, ignition and subsequent main chamber combustion are compared in an optically accessible PCC engine equipped with a “flat” and a real engine-like “bowl” piston geometry. An active fueled narrow throat pre-chamber was used as the ignition source of the charge in the main-chamber, and both chambers were fueled with methane. Three pre-chamber fuel effective mean pressure (FuelMEP) ratios (PCFR) namely 6%, 9% and 11% of the total amount of fuel were tested at two global excess air ratios (λ) at values of 1.8 and 2.0. The optical engine configuration included a metal liner and a quartz piston, and it was operated for 100 consecutive firing cycles, allowing stable emissions measurements. A high-speed intensified camera was used to record the OH* chemiluminescence of the main-chamber combustion. The camera was operated at 50 kHz, resulting in a resolution smaller than 0.2 Crank Angle Degree (CAD), at the engine speed of 1200 revolutions per minute (RPM). Both pistons exhibited similar pre-chamber pressures and an equal pressure difference between pre- and main-chamber (ΔP) at the peak pressure inside the pre-chamber. The free reacting jet behavior was also comparable between the two piston geometries. However, the flat combustion chamber resulted in a shorter free jet time because of the early reacting jet-piston interaction. The interaction also slowed down the combustion propagation and increased the combustion duration for the flat piston.