This type of injector requires hydraulic fluid (usually diesel) under high pressure.

The injection pressure is limited to 200 bar. During the actual injection the solenoid is activated which then releases the pressure in the diesel which is in the upper part of the injector, the hydrogen which is under high pressure can then lift the injection needle from its seat and the injection takes place. In this type of injector, there must be such a high pressure on the diesel that the needle seals against its seat in the closed position. The diesel also ensures that certain moving parts in the injector are lubricated.[1]


[1] (Yip et al., 2019,  s. 18)