One of the ways to supply the engine with hydrogen is via injection valves mounted in the intake manifold on the outside of the intake valve and thus allow the hydrogen to start mixing with the air/oxygen already outside the combustion chamber, and then with the help of the engine's negative pressure, it is sucked into the cylinder when the intake valve opens. This model has a certain risk of self-ignition because the fuel enters as soon as the inlet valve opens and there may then be a risk of the hydrogen coming into contact with parts that have such a high temperature that it ignites. It could be the spark plug that has too low a heat number and still glows, it could be residual carbon and soot deposits from the engine previously running on fossil fuel before the conversion. Or oil residues that are forced up between the cylinder wall and the piston rings due to somewhat substandard crankcase ventilation, which can glow in the cylinder. Detonation of the fuel/air mixture can also occur due to cam timing with too high an overlap and the fuel then comes into contact with the exhaust port. 

 

The other way in which hydrogen can be supplied to the engine is to use a so-called direct injection model of engine. This means that the injection valve must sit directly in the combustion chamber. The advantage of this method is that you can control more precisely when you want the injection to take place.