On the Phenomenology of Hot-Spot Induced Pre-Ignition in a Direct-Injection Hydrogen-Fueled, Heavy-Duty, Optical-Engine 2023-32-0169

Hydrogen-fueled internal combustion engines are highly susceptible to pre-ignition from external sources due to its low minimum ignition energy despite the hydrogen’s good auto-ignition resistance. Pre-ignition leads to uncontrolled abnormal combustion events resulting in knocking and / or backfire (flashback) which may result in mechanical damage, and as such represents tenacious obstacle to the development of hydrogen engines. Current pre-ignition mitigation strategies force sub-optimal operation thereby eroding the efficiency / emissions advantages of hydrogen fuel making the technology less attractive. Hydrogen pre-ignition phenomenon is poorly understood and knowledge gaps about the underlying mechanisms remain.

To this end, a phenomenological study of hot-spot induced pre-ignition is carried out in a direct-injection hydrogen-fueled, heavy-duty, single-cylinder optical engine. Pre-ignition is induced with an electrically heated glow-plug which creates a hot-spot with varying surface temperatures based on the applied excitation voltage. The effect of engine speed, hot-spot temperature and hydrogen injection timing on pre-ignition frequency and phasing is studied using optical diagnostics. First, the hot-spot temperature during engine operation is characterized using infrared (IR) imaging, which relied on pre-calibration using thin wire thermocouple. Thereafter, the mixture field surrounding the glow-plug is characterized by tracer PLIF using hydrogen seeded with anisole. High-speed OH* chemiluminescence imaging is used in conjunction with cylinder pressure measurements to characterize pre-ignition timing. Experimental results are complemented with closed homogeneous reactor chemical kinetic calculations to understand the effect of varying in-cylinder temperature and pressure on hydrogen ignition delay. An interplay between surface temperature, in-cylinder pressure, and injection timing is revealed, which explains the tendency of pre-ignition to occur during the gas-exchange or in early compression stroke.