Experimental study of microwave assisted spark ignition on expanding C2H2-Air spherical flames

Abstract Microwave Assisted Spark Ignition (MAI) is a potential technology to improve lean combustion characteristics in spark ignition engines. This paper investigates the MAI performance with C2H2-Air spherical expanding flames in a Constant Volume Combustion Chamber (CVCC) system under ambient pressures from 0.1 to 0.6 MPa and equivalence ratios (φ) from 0.5 to 1.9. Results show that MAI greatly increases early flame kernel radius compared with SI mode particularly in the cases close to lean or rich limit under 0.1 MPa. The enhancement decreases obviously as equivalence ratio approaches stoichiometric ratio. With ambient pressure increasing, the MAI enhancement gets weakened. Normalized Reduced Electric Field (REF) is found to correlate well with the decreasing trend of normalized flame radius enhancement with increasing ambient pressure, indicating that REF is probably one of the key factors affecting MAI performance. The influence of microwave Pulse Repetition Frequency (PRF) on MAI under PRF from 1 to 80 kHz are studied and discussed for both non-combustion discharge and ignition tests. Increasing PRF, the enhancement of MAI is strengthened as more microwave energy is injected in high-absorption-efficiency period which is dominated by electron density. At last, the formation mechanism for “fluorescence spot” and its dependence on pressure are offered to explain the observed increasing light intensity in the flame kernel with ambient pressure.