Visualization of Pre-Chamber Combustion and Main Chamber Jets with a Narrow Throat Pre-Chamber

Pre-chamber combustion (PCC) has re-emerged in recent last years as a potential solution to help to decarbonize the transport sector with its improved engine efficiency as well as providing lower emissions. Research into the combustion process inside the pre-chamber is still a challenge due to the high pressure and temperatures, the geometrical restrictions, and the short combustion durations. Some fundamental studies in constant volume combustion chambers (CVCC) at low and medium working pressures have shown the complexity of the process and the influence of high pressures on the turbulence levels. In this study, the pre-chamber combustion process was investigated by combustion visualization in an optically-accessible pre-chamber under engine relevant conditions and linked with the jet emergence inside the main chamber. The pre-chamber geometry has a narrow-throat. The total nozzle area is distributed in two six-hole rows of nozzle holes. A novel optical pre-chamber assembly was designed and tested on an optical engine, with combustion only in the pre-chamber volume, using methane (99.5% purity CH₄) as fuel. A separate measurement was carried out using a metal pre-chamber assembly to study the emergence of the pre-chamber jets into the main chamber to assess the difference between the pre-chambers. The combustion process was captured using a broadband camera and pressure transducers in both chambers (pre- and main chamber). The combustion inside the pre-chamber starts at the spark location; the flame propagates along the cone and subsequently propagates fast inside the throat. The broadband combustion luminosity exhibited a non-proportional relation with the heat release rate (HRR) due to the CO₂* chemiluminescence. An HSV-model (Hue Saturation Value) analysis revealed the predominance of premixed combustion during the first stage of the process, followed by infrared flame radiation. Finally, although the optical pre-chamber assembly showed a faster pressure increase, the combustion process is similar to the metal pre-chamber assembly on a mass burnt scale.