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.