Hydrogen effect on lean flammability limits and burning characteristics of an isooctane–air mixture

The flammability limit is a hurdle to realizing ultra-lean combustion and achieving high thermal efficiency in spark ignition engines. Hydrogen, known for its wide explosion limits, has been utilized in gasoline to improve the lean limit. The influence of hydrogen on lean flammability limits of isooctane was studied at initial temperatures of 393–513 K and an ignition energy (IE) of 3000 mJ, while the burning characteristics were studied at an initial temperature of 393 K and IE = 37 mJ. It is found that a spherical flame appeared when the isooctane–air–H₂ mixture was ignited from λ = 0.8 to λ = 1.2 at IE = 37 mJ, whereas a leaner mixture was burnt at IE = 3000 mJ with unstable flame growth during the early stage. Flame instability eventually evanesced and buoyancy was observed near the lean limits. The lean limits of isooctane–air increased to 2.1λ with 0–90% hydrogen. Raising the initial temperature from 393 to 513 K increased the lean limits of isooctane–air–H₂ to 0.2λ. Hydrogen obtained from CH₂O (C, H, and O species) makes a considerable contribution to improving the burning near the lean limits. The mole fraction of the short-lived radicals H, OH, and O decreased when the excess air coefficient moved toward the lean limits and increased by introducing hydrogen, which improved the lean flammability limits and burning velocity.