Fuel Pressure Regulator Properties

[Andy330hp]

We are putting together a fuel system, and have a question about fuel pressure regulators. We have found that you can achieve the desired average fuel pressure with a number of different spring/installed height combos, which makes perfect sense. However, the fuel pressure stability is affected, some are worse and some are better. It seems that orifice sizes are also a player, but since they are not as easily modified I am considering it secondary for now.
Is there an on-paper method to predict the best combo? We have fuel pump curves. Our current idea is to shoot for best stability around idle, as small changes have the biggest effect there. Thus, we know consumed fuel rate. Since it is a full return system, flow rate through the regulator can be found from the pump curve minus consumed fuel. However, the dynamics I'm afraid might get a little tricky....
I have heard that there are such things as regulator curves, that plot "regulator efficiency" vs something else. As you can tell, my info on that is very weak. But, it may be a good way for us to empirically find the best regulator for our system if a paper solution is difficult to come by. Does anybody have experience with these curves, and if so can you tell me (1) what information is shown (2) how to use them (3) how to generate them. A sample curve would be great. Thanks a lot

Andy

 

[techguru]

whoa. when you figure this out let me know. I have several setups that aftermarket regulators are not available for and id like to change or modify them.

 

[WGJ]

Andy330hp - send your email address to me (bjones6@hotmail.com) and I'll send you the sketched graph that goes with the following.

Ps Pressure 'set point'
Pu Pressure at the upper limit of regulation / proportionality
Pl Pressure at the lower limit of regulation / proportionality

Qs Flow at the pressure set-point
Qu Flow at the upper limit of regulation / proportionality
Ql Flow at the lower limit of regulation / proportionality

No conventional mechanical (spring & diaphragm) regulator will guarantee a fixed pressure with a wide range of fuel flow, there will be some variation in pressure with the variation in fuel flow that is experienced due to engine demands and system voltage variation (electric pump speed changes).
However, good regulators will ensure that a significant portion of their flow range is relatively pressure linear with a small change in pressure due to flow.
This 'linear' portion of the regulator's performance might have a gradient of about 0.05kPa per litre/hr flow change, so a 3 bar fuel system with an engine demand ranging from 10 to 100 litre/hour would see a pressure change of 0.05 x 100 = 5kPa over that flow range, assuming a correct choice of regulator was made.
In choosing the regulator yourself, you have to know what the engine requirement will be for wide-open throttle fuel flow and then add the regulator's 'lower limit of proportionality' or 'knee point' figure to this to get a reliable minimum regulator flow number (Ql on graph). Then you need to check that the regulator you pick has a suitable 'upper limit of proportionality' (max engine flow + pump wear out allowance + a hot fuel handling allowance, altogether say 25% more than your engine's max requirement). If you don't pick a Qu high enough, the fuel pressure at low engine fuel usage may exceed your requirements.

Bosch 3 bar regulators have had 'knee points' in the range 15 to 40 litre/hour over the years, with the lower values tending to be the later designs. At one point, there were adjustable regulators for 3 bar fuel systems - they had a screw and locknut behind the regulator spring, but whether a commercial Bosch parts outlet would admit to this, I don't know.

 

[Andy330hp]

I sent an email, but just in case my address is aallen@injectionlogic.com or ajallen@hotmail.com
Thank you very much!