Spark ignition and early flame development of lean mixtures under high-velocity flow conditions: An experimental study

This study set out to experimentally investigate spark ignition and the subsequent early flame development of lean air-fuel mixtures of A/F = 20-30 under high-velocity flow conditions using a uniquely designed swirl chamber. The swirl chamber realizes a high-velocity flow of 65 m/s at the spark plug gap, as well as internal temperature and pressure histories that are equivalent to those of spark-ignition engines, being equipped with an optically accessible engine. The designed swirl chamber clearly captures the characteristic behavior of the spark channels and flames in the vicinity of the spark plug. The results show that the spark channels stretch downstream following the flow and are subject to short circuits or restrikes. In the case of a high ignition energy of 200 mJ, short circuits of the spark channels occur in the early part of the discharge, while restrikes occur in the later parts. With a decrease in the ignition energy, restrikes occur in the earlier parts of the discharge. With a low ignition energy of 65 mJ, restrikes can occur immediately after the electrical breakdown without any significant spark stretch. At a sufficiently low dilution degree of A/F = 20, flames can hold behind the ground electrode of the spark plug, which significantly suppresses the cycle dispersion while also enhancing the combustion in the early stage. With further air dilution, that is, A/F > 20, flames develop, flowing downstream without flame holding. However, temporal flame attachment to the ground electrode is observed during the discharge duration even at A/F = 30, while the attached flames eventually blow off downstream at the end of the spark discharge.