Active and passive prechamber assisted engine combustion: simultaneous 50kHz formaldehyde PLIF and OH* visualization

Prechamber-assisted internal combustion engines allow lean limit extension by replacing the spark plug with multiple, high-temperature, radical-rich jets that entrain and ignite the main chamber charge. This work reports the first investigation of active ultra-lean, active rich, and passive prechamber assisted prechamber engine operation (with a fixed prechamber geometry) through 50 kHz cycle-resolved measurements of formaldehyde planar laser-induced fluorescence (PLIF) and OH* imaging in a heavy-duty optical engine operating at 1200 rpm. The third harmonic of a burst mode laser (355 nm) provided the excitation source for formaldehyde PLIF, and a high-speed CMOS camera collected the fluorescence signal from the side window of the engine. Bottom-view imaging of OH* chemiluminescence was achieved using an intensified high-speed camera. The high-speed image sequence of the prechamber jet formaldehyde layer and OH* captures the main chamber heat release stages and the interaction between the moving piston and the prechamber jet. The prechamber-assisted combustion process in the main chamber could be segmented into three phases: initial jet, jet wall interaction, and Post Jet Combustion. The first two phases differ between active and passive operations owing to the global lambda, while the last phases mostly remain flame propagation driven. Jet wall interaction leading to mixing and vortical transport across the main chamber is pivotal in main chamber heat release rate shaping. The study reports the dynamics and evolution of main chamber combustion in detailed time-resolved setups.