Initiation characteristics of spark-ignited premixed hydrogen-oxygen flames with diluents of argon/nitrogen/carbon dioxide

The spark-ignition features of premixed H2 under Ar–O2 atmospheres are essential for the development of high-efficiency and zero-emission Argon Power Cycle engines. Therefore, constant-volume combustion vessel experiments are conducted at an initial temperature of 287 K, initial pressures of 0.1, 0.3, and 0.5 MPa, with diluents of Ar/N2/CO2. Excess oxygen ratio, defined as the ratio of O2 supplied to the O2 needed for stoichiometric combustion, ranges from 1.0 to 4.7. Dilution ratios, defined as the molar fractions of diluents in the diluent-O2 mixture, were separately 70%, 79%, and 88%. Results show that both high dilution ratios and high excess oxygen ratios lengthen the flame-initiation periods. With excess oxygen ratio of 3.3, when Ar dilution ratio increases from 79% to 88%, the flame-initiation period rises by a factor of five to ten. With Ar diluent ratio of 79%, when excess oxygen ratio increases from 2.5 to 4.7, the flame-initiation period rises from under 10 ms to over 60 ms. Both the application of Ar as the diluent and high initial pressures extend the flammability limits. With Ar dilution ratio of 79% and pressure no less than 0.3 MPa, the flammability limit on excess oxygen ratio is over 4.7. Furthermore, Ar can additionally shorten the flame-initiation periods. At 0.1 MPa, excess oxygen ratio of 2.5, when diluents are respectively Ar, N2, and CO2, the flame-initiation periods are 10, 19, and 41 ms. The research reveals that a spark-ignition Argon Power Cycle engine running with ultra-lean and highly diluted mixtures may be feasible.