Pilot Injection Timing and Dwell-Time Variation Effect on Hydrogen Split Injection

This study evaluated split hydrogen injection with pilot ignition, employing two separate injectors for hydrogen and pilot fuel in a fixed converging configuration. Fuels were injected into a quiescent charge within an optically accessible chamber. Experimental conditions were maintained at an 870 K ambient gas temperature, 23.3 kg/m3 ambient gas density, and 15 vol % O2 concentration. The pilot-fuel quantity was kept constant, while hydrogen was delivered via a split-injection schedule. High-speed Schlieren imaging and pressure measurements were performed to measure the jet and flame development and the corresponding heat release performance. The investigation comprised two phases: (i) a fixed hydrogen split-injection schedule with a constant dwell period but varied pilot-jet timing and (ii) fixed pilot-jet timing relative to the second hydrogen injection (co-injection), with dwell time between the first and second hydrogen injections varied. The first-phase experiments showed that when pilot-jet ignition occurred after the end of the first split, rather than during it, the mixing and leaning of the first hydrogen split resulted in more limited flame spreading and larger unburned regions. This altered the burned-mixture state encountered by the incoming hydrogen jet and allowed greater hydrogen-air mixture build-up ahead of ignition, while the early injected reacted and unreacted charge continued to mix. These combined effects changed the subsequent combustion-spreading process and heat release profile. The apparent combustion efficiency decreased by 14% when ignition occurred after the first split compared to the reference case, where ignition occurred during it. The second-phase experiments showed that when pilot-jet ignition occurred after the end of the first split, a short dwell time between the first and second splits created a closer coupling that reduced the apparent combustion efficiency to 5% relative to the reference case. This reduction was associated with a comparatively faster combustion-spreading process after ignition.