With cheap sensors and machine learning, researchers are working to make pre-ignition in hydrogen engines instantly detectable. Image Credit: Juan Roballo/Shutterstock.comWith its high energy density and clean H 2 O emissions, hydrogen fuel holds strong promise as an environmentally neutral energy source. However, its use in internal combustion engines muddies the fuel's plausibility. Pre-ignition, the uncontrolled combustion event that occurs before the spark plug fires, is particularly troublesome in hydrogen fuel development. Because the factors behind it are varied and hard to track, traditional methods have struggled to identify pre-ignition reliably, let alone in real time.