An Optical Study on the Ignition and Flame Development Initiated by a Combination of Passive Pre-Chamber and Spark Ignition

Pre-chamber jet ignition combined with spark ignition in main chamber has advantages in adjusting the combustion phase and improving the burning rate for spark ignition engines. In this study, optical experiments based on a constant volume combustion chamber and high-speed schlieren imaging were conducted to investigate the ignition and flame development under the combined ignition mode with a passive pre-chamber and spark plug using methane fuel. Two modes of flame development were observed with different ignition time interval of pre-chamber ignition and spark ignition (Delta t(ign)). At relatively short Delta t(ign), the "jet-disturbed flame" mode occurs with a three-stage combustion process: laminar flame; flame acceleration by jet-flame interaction and re-ignition at the jet root; flame deceleration. The maximum global flame speed can be increased from 2 m/s to 9 m/s under lambda = 1 and from 1 m/s to 5.5 m/s under lambda = 1.3. The ignition delay is relatively short and the heat release shows a slow and multi-stage feature. Moreover, the ignition delay and combustion duration in main chamber are relatively not sensitive to the variation of nozzle diameter. As the spark ignition becomes later (longer Delta t(ign)), the "spark-initiated turbulent flame" mode occurs. In this mode, a rapidly developing turbulent flame is initiated by the spark plug, with a global flame speed up to 16 similar to 18 m/s under lambda = 1 and 6 similar to 11 m/s under lambda = 1.3. Compared with pre-chamber ignition, a shorter combustion duration is obtained at certain Delta t(ign) and misfire can be avoided because of the synergy of the combined ignition mode.