Closed Loop Fuel Pressure Control - Bleed Required?

Hi everyone.

I am currently setting up a returnless fuel system for my project. I am using a Link Storm ECU and Razor PDM to control the fuel supply via PWM.

In the Link manual under Closed Loop Pressure Control it says “a small bleed is needed to keep some fuel circulating in the system”.

I have done a quick Google search and cannot find any detail regarding a bleed in a closed loop fuel system.

Is a bleed required? If it is, what is the best way to achieve this?

Your help will be greatly appreciated.

Michael,

Returnless systems often have a return within the fuel pump basket/assembly, achieved with a fuel pressure regulator.

Thanks for that Mike.

My understanding is that there are two types of returnless systems.

Mechanical returnless - Uses a mechanical regulator mounted in the fuel tank as part of the fuel pump assembly (as you mention above). Is this regulator the bleed that is mentioned in the Link manual?

Electronic returnless - Uses a pressure sensor to monitor fuel pressure. The ECU then varies the speed of the fuel pump via PWM to increase or decrease fuel flow. A regulator is not required.

I am wanting to set up an electronic returnless system without a regulator. My understanding is that this system does not require a bleed. But I could be wrong.

Can anyone confirm? Thanks.

I think Link is telling you that you should still have some bleed in the fuel tank. It would be practically impossible to manage the fuel pressure if there were no flow. Think of the spikes in fuel pressure as the pump turned on/off completely. If there is always some flow, then you can vary the speed of the pump to keep the pressure where you want it, the bleed would need to be sized correctly to allow the proper pressure range of control. -- perhaps Link can provide some guidance based on their experience.

I would think this looks like a Tee in the fuel line inside the tank, with a cap/plug drilled to the necessary orifice size that just returns fuel to the tank.

As you said, Michael, but can you get any more information on what they mean?

The problem with a "dead-head" is that if there is air in the line post pump/regulator return (if any), the only way it can escape is through the injectors. They will usually have a "Shrader", or similar, valve that will allow manual bleeding. It's unclear if they mean there should be a small return/bleed to remove any air that may get into the line/rail(s)?

With a GOOD swirl pot, or similar setup, that ensures only fuel gets through, it shouldn't be an issue after initial bleeding - about the only time it may be a problem is if the tank fuel level drops to a point where it draws more air than the swirl pot can remove.

I've seen mechanical and I've seen mechanical + pump control. Maybe a pure pump control setup exists in the OEM world, but if you think about it speeding up and slowing a pump down doesn't reduce pressure unless you're injecting more than you're pumping.

For example every time you lift throttle you'd end up over pressure with no means of dropping it until injection resumes.

Any time you're driving and pump speed can't be slowed enough while maintaining stable pump operation to provide the same or less fuel than what's being injected, you'd end up over pressure without a regulator.

Thanks for your comments guys.

What you are saying about the pump flow/pressure and deadheading makes sense.

I am thinking that at a minimum the bleed would need to flow enough to allow the pump to operate above its minimum/stall speed when the throttle is close.

Looks like I will need to do some testing.

Thanks for your help everyone.

The main reason you need a bypass is because you are trying to control pressure by adjusting the motor speed (flow), you need some minimal flow at all times for the pressure to be relative to speed. If there was no flow and you reduced motor speed then the pressure wouldnt reduce. There are several other reasons that may apply also - motor cooling, minimum duty cycle, stall of positive displacement type etc. In my flow bench which has a walbro 255 pump, I just have a -4AN tee with a random holley jet tapped into one leg of it for the bypass, it is 0.075" from memory which is possibly larger than it needs to be, but it works. I suspect something more like 1mm would work for most, it is surprising how much fuel comes out of that hole.

I have seen 1mm or 0.040" seems to be a common choice for the few commercially available kits. Example: https://www.vaporworx.com/shop/product/an6-male-male-restrictor-fitting-w-1-8-npt-on-side1/

Thanks for your input on this Adam. Thats very helpful.

Your suggestion of using a carb jet is a good idea. You can change the bypass flow very easily by changing the jet.

Thanks again.