Hydrogen gas engine built from a petrol engine, showing how any petrol engine can be converted to run on hydrogen gas. Think of water or water-methanol injection as a way to calm things down inside the engine. Hydrogen burns fast and hot, which can create hot spots that cause knock or pre-ignition—basically the fuel lighting off too early or in the wrong place. Spraying in a little water cools those hot areas, slows things down, and keeps combustion happening when and where it’s supposed to. The