Fuel Pressure to Fuel Flow (tech question)

[grimreaper]

With my walbro, fuel flow decreases as pressure increases. How is it that i run richer when i increase base pressure considering flow is being decreased?

Looking at charts that have been posted it would appear that you'd run richer with lower fuel pressure to a certain point (keeping injector atomization in mind). In my search it seems the standard on many forums is more fuel needed, turn up the pressure.

I ran across a few hits that stated more pressure on the injector allows more fuel to get through at a given duty cycle. Is that my missing link?

 

[figgie]

ok now that I did that

it is really simple. You are just over thinking the problem

so lets take some arbitrary numbers for examples (these are EXAMPLES ONLY!!)

You walbro 255 lph flows

255 lph @ 0 psi
240 lph @ 10 psi
230 lph @ 20 psi
220 lph @ 30 psi
210 lph @ 40 psi
200 lph @ 50 psi.

So for this example, You base pressure with the hose connected is 40 psi (yes it is lower, I know, This is just an example ).

So the Fuel pump (Fp) is flowing 210 lph through the stock system. We have to make some assumptions.

1. No internal pressure in the system past the Fuel pressure regulator. ie, pressure reading it 40 psi everywhere in the system except past the fuel pressure regulator.
2. fluid has not reached Critical Velocity.

For simplicity sake, the injector in question is flowing 100% at its rated value. We will use the stock 440cc/min @ 40 psi @ 80% DC.

Once we increase the base pressure to 50 psi. The FUEL pump flow reduces by 10 lph to 200 lph. The issue is that the pressure differential is GREATER. before we were going from 54.7 psia to 14.7 psia (40 psig to 0 psig), the new numbers is 64.7 psiga to 14.7 psia (50 psig to 0 psig).

Even though the flow is less. The pressure is more, as long as critical flow is not reached, the injectors have NO CHOICE but to injector more fluid for the given pulse X.

So doing some math (located at the RC injector site).

The new flow reading of the injector for the given PW is

(Sqrt (50psi/40psi)) X 440cc/min = (SQRT 1.25) x 440 cc/min = 1.118 x 440cc/min = 491.93 cc/min.

Why?

well the pressure increases but the internal diameter of the injectors stays the same. That means that we have MORE fluid (higher pressure since the resistance is LESS) passing by the opening until it reaches critical V.

 

[grimreaper]

superb! Glad to know why, and not just what the outcome is. Thank you figgie!:bigthumb:
Is the critical velocity the point in which the pump runs out of steam, or is it in which the fuel system (lines, rails, fittings) cant keep up?


edit: seems to be a combo of both in which things dead head or turn turbulent reducing or preventing flow. Pump and lines have to work together.

 

[figgie]

superb! Glad to know why, and not just what the outcome is. Thank you figgie!:bigthumb:
Is the critical velocity the point in which the pump runs out of steam, or is it in which the fuel system (lines, rails, fittings) cant keep up?


edit: seems to be a combo of both in which things dead head or turn turbulent reducing or preventing flow. Pump and lines have to work together.

Niether actually. Critical Velocity is when the flow of the fluid/air (applies to both) reaches sonic. Once that happens, no amount of pressure vacuum will make it pass that speed.

That is why restrictors work in WRC, Nascar etc.

 

[grimreaper]

Thank you for the clarity!

 

[3p141592654]

As long as the fuel flowing out the injectors is only a fraction of the pump flow rate, the rest will be dumped into the return line to the tank and is effectively wasted, i.e. not used by the engine.

Using Figgie's example at 40psi, six 440cc/min injectors at 80% will use 127 lph, so you have an additional reserve of 83 lph that are dumped back into the tank.

Therefore, as the pressure is raised, more fuel will go out the injectors and less will be returned to the tank. Clearly, at some pressure, the point where no fuel is returned to the tank is reached and you need a bigger pump (actually well before that point, but you get the idea)!