fuel is a high viscosity liquid, to get it to properly atomize, and to get the necessary amount of fuel in the cylinder in the short time available, it has to be delivered at quite high pressure, but since you injector pump
is essentially a miniature engine in itself, the fuel pressure ramps up and down. To prevent injection of fuel at pressures that wouldn't properly atomize it, injectors have quite strong spring that hold them shut, until the fuel hit the injectors "cracking" pressure. When you test injectors you have a hand pump that builds pressure until the injector opens, the pressure is so high that the injector will make a pop sound not unlike an air rifle or something similar.
Now common rail systems are completely different as the fuel pressure is continuously high and the injectors are "fired" electronically, which opens a whole world of possibilities like splitting injection events
, variable timing, and other things not possible with the old mechanical injection pump, but they are complicated and expensive. The parts
cost for eight injectors for my Duramax pickup was around $3,500 if I remember correctly, plus computers
are high energy current
is required. Maybe not the best thing on a boat.
FWIW I think the fuel pressure in my Duramax is up to 30 to 50 thousand PSI.
Oh air in the injection system acts like a spring, not allowing pressures to build high enough to get to the cracking pressure of an injector, so nothing happens until you release the air