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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.