Stabilizing ignition and enhancing combustion within pre-chamber jet by integrating microwave-assisted ignition

Carbon-free combustion in internal combustion engines can be advanced by addressing the low flame velocity and ignition difficulties in lean combustion or ammonia-fueled combustion. Our previous work introduced Turbulent Jet Pre-Chamber Spark Ignition (TJ-PSI) to achieve this goal. While TJ-PSI improves flame speed, ignition failure occurs under strong turbulence. In this study, microwave-assisted ignition is integrated with TJ-PSI to stabilize ignition and accelerate combustion, named Turbulent Jet Microwave-Assisted Ignition (TJ-MAI). The combustion pressure and heat release rate of TJ-MAI are monitored and compared with TJ-PSI and conventional spark ignition. Moreover, a power diagnostic for TJ-MAI is conducted to measure the microwave energy and spark energy, separately. Results show that TJ-MAI leads to more intensive combustion than conventional spark ignition, with a maximum heat release rate increased by over two times. Moreover, TJ-MAI broadens the ignitable time range and improves the ignition success rate compared to TJ-PSI. Then, the principle of microwave stabilizing ignition in pre-chamber jet is proposed from an energy perspective. Microwave radiation inhibits the restrike of the spark channel. Therefore, for TJ-MAI, more spark energy can heat the same bulk of gas mixture to form a self-sustained flame. Meanwhile, microwave energy, absorbed during spark ignition, decreases the minimum ignition energy required for successful ignition in TJ-MAI. According to the chemiluminescence-Schlieren image, the reduction in the minimum ignition energy is due to the kinetic effect of microwave plasma which spurs the population of initial radicals. Additionally, the intensity of initial radicals shows a monotonic increase with microwave energy.