Don't get so hung up on positive and negative voltage.
Voltage=Stress
An example:
You have one end of a rubberband secured at a level of 2ft.
Pull up the other end 1ft. How much tension did it take to get it to stretch up 1ft? This equals +voltage.
Now stretch it down 1ft. How much tension did it take to pull it down 1ft. This equals -voltage.
What's the difference?
None.
Positive and negative are our imaginary references to either earth ground or other parts of a circuit, ie...primary of a transformer(maybe has one end of the winding tied to ground, or negative terminal of battery) and then the secondary has NOTHING connected to ground(or batt-) in the circuitry common with the primary side, so there is no exact reference level, just what we measure as a stress field between the ends of the secondary winding.
That's what an isolated ground is. No particular reference to anything, just a stress field between two points.
On the injector, the outside IS the positive IF the diode(s) on the VIC is reversed.
The engine block is 0V, the higher point of the scale.
The applied voltage is -40KV(40KV as an example).
You have a stress field across the water of 40KV.
Voltage is going to do the work, right?
What does it matter if "our" point of reference is positive or negative?
The water doesn't care. It has 40 KV of stress across it.
I hope this makes sense. I'm starting to get stoned. Oh, not the smoke...Keystone (beer)
Mike