Injectors 101

[F51]

How does air in the line between the injector pump and the injector prevent the injector from functioning? I'm guessing it is because fuel is not compressible but the air is, thus preventing the injector from spraying.

[shakey doug]

You have it in one.

[skipmac]

If you have air in the line then the injector is not getting fuel. It's not a matter of compressing air or compressing fuel, you have to have fuel there to inject.

To make it work you have to loosen the fuel line at the injector and pump or run the engine until the air is gone and you get fuel.

[weto]

CUZ air is compressible and liquids are not just as Skip stated.

[delmarrey]

Originally Posted by weto

CUZ air is compressible and liquids are not just as Skip stated.

The injector will not open, allowing the air to escape. That's why one has to bleed at the nut.

That's unless you have the new pressure rail system with electronic injectors. But that requires electricity. Bad for boats IMO.

[F51]

Originally Posted by skipmac

If you have air in the line then the injector is not getting fuel. It's not a matter of compressing air or compressing fuel, you have to have fuel there to inject.

To make it work you have to loosen the fuel line at the injector and pump or run the engine until the air is gone and you get fuel.

The line would not likely be completely empty, would it? I believe the compressibility of the air prevents the force of the pump from being transferred to the injector. Thus the injector does not open or opens incompletely. What I am looking for is confirmation of that belief. The injector has to get the force to open from somewhere--I cannot see any other medium of transfer than the incompressible fuel.

I understand HOW to bleed. I am trying to understand what makes the injector operate.

[perchance]

The column of oil acting on the pintle or needle overcomes the spring pressure holding the injector closed allowing the oil to exit the nozzle. See Youtube for a more detailed explanation

[a64pilot]

Diesel 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

[skipmac]

Originally Posted by F51

The line would not likely be completely empty, would it? I believe the compressibility of the air prevents the force of the pump from being transferred to the injector. Thus the injector does not open or opens incompletely. What I am looking for is confirmation of that belief. The injector has to get the force to open from somewhere--I cannot see any other medium of transfer than the incompressible fuel.

I understand HOW to bleed. I am trying to understand what makes the injector operate.

OK, in practical terms (yes for the nit pickers I know on a micro level this isn't exactly correct) but a gas will compress and a liquid will not.

The injector pump pushes a small amount of fuel into the line to the injector. Since the liquid fuel does not compress then it will push into the injector IF there is liquid only all the way to the injector.

Gas compresses, so little if any gas will push into the injector. If there was a very large piston pushing a lot of gas then it might push into the injector but the quantity is very small so the gas essentially absorbs the pressure from the injector pump, like a shock absorber.