Fuel Cell This is a process of splitting water with voltage doing the work and not the amps, thus making no heat. This use a water cell that is ran by a circuit which produce a surplus of two parts Hydrogen and one part Oxygen directly into a standard combustion engine no mater the regular fuel.
Injector This process is similar to the first but is more easily adapted to a standard engine by replacing the spark or glow plug. Though easier to install, this is much more difficult to make. It uses a complex process to run water straight into the engine and simultaneously atomize the water, split it, and ignite it.
In a Other Engines and Turbines Not only can you modify a standard engine to run a lot better on hydrogen, but there are other more efficient engines and options. One would be the Massive Yet Tiny (MYT) engine, which can replace your 800lbs engine in your car with a 25lbs engine.