Fuel Pressure

[13BeastREW]

In a perfect world a certain in-tank fuel pump will flow 200 liter/hour and read 40 PSI on pressure gauge connected to the outlet of the pump.

Now for 2 scenarios...

The fuel tank is nearly empty and not moving, there's only enough fuel in the tank to cover the inlet of the fuel pump. Assuming the change in fuel pressure from the inlet to the outlet was instant, at that moment would the fuel pressure still be 40 PSI or would it start to drop off.

Second scenario... A fuel pump running at full speed in a fuel tank with no fuel in it, only air at atmospheric 15 PSI (which the pressure guage ignores), will the pressure gauge reading be zero or a number greater than zero.

 

[schmidtj86]

At the outlet of the pump going into the fuel line towards the pressure regulator,(if the gauge were at 40 at the pressure regulator) there would be above 40 psi, by whatever psi it takes to push the fuel through the length of line to reach the regulator at the flowrate achieved (pressure drop along the line to achieve 40 psi at the regulator if the gauge is at the regulator). If the gauge is at the pump, and the gauge read 40, the regulator would be lower than 40. IF there were no regulator, and a free flowing tube, there would be NO pressure, other than the pressure it takes to flow whatever flow is flowing through the length of tube. A pump will not make pressure, the pump makes flow (dependant upon pressure), and the regulator makes pressure. If there is no regulator, it almost all goes to flow. The fuel pump should have a flow vs pressure curve that's fairly linear that a manufacturer can provide if poked in the right places, or you can do it yourself with a free flowing flow measurement, a flow measurement at a pressure, and a deadheaded pressure reading.

Air will stop a pump in its tracks, and reduce pressure - if there are injectors releasing pressure - it will drop it to zero. If the regulator is still there and the injectors are off, it may bounce a bunch if there is fuel in the line and drop pressure. If it's all air, the pump will not create pressure - at least with most common fuel pumps - as they generally seal better with a liquid. This can be seen with the difficulty of getting an inline fuel pump to "prime" on initial startup if there is no fuel in the line.

Kinda rambly, but I hope I answered your questions.

 

[13BeastREW]

For as mush as I can tell, you've completely answered my questions. Thanks. I actually may have a few more but with the information you've given I may be able to figure the rest out myself.

 

[13BeastREW]

Actually I've managed to come up with another theory that I'm having trouble finding the information need to confirm it with.

The fuel pump creates no real pressure itself... The only pressure actually measurable imediately following the pump is the pressure created by the liquid itself while trying to expand within the tubing (level out) and the pressure created by the liquid pushing through the tubing. A fuel pressure of 40 PSI read at the fuel pressure regulator is created by the pressure required to open the bypass valve. So... At this point I'm assuming that pressure reading is directly related to the volume/density of the fuel with in the system.

Say for example that an unpressurized fuel system's volume is 1 liter. When only 1 liter of of fuel is in the system, a pressure gauge should read 0 PSI. For the gauage to read 1 PSI, does this mean the volume of fuel in the system needs to be doubled to 2 liters? up to 41 liters to establish a pressure reading of 40 PSI?

 

[notnats]

13beastREW
To answer your last question, ... no.

For practical purposes, liquids are incompressible. The only extra fuel required in the line to raise the pressure from 0 to 40 PSI is the amount needed to allow for elastic expansion of the fuel lines, ie very little.

The fuel pump does produce pressure.

The pump will have performance curves which relate flow and pressure.

If the bypass valve fails to open pressure will rise to the pressure predicted by the curve for the flow rate at that moment.

Diaphragm type pumps can usually pump air. So as the tank empties and the pump begins to suck air, the line will fill with air and fuel will still be supplied to the engine for a brief time. The pressure will probably drop unless the fuel flow is small because air is a compressible fluid and will need to be supplied in greater volumes as you describe in your last paragraph.

Jeff

 

[13BeastREW]

Ok, well I plan to use this information in a quick and dirty computer simulation before I put it to practical use and I need to figure out how to get some numbers out of this problem. I am aware that there are several types of pumps such as vane, centrifugal, flex impeller, diaphragm, etc. My problem is modeling these.

Now the reason I say the pumps dont actually create the pressure is that a pump can pump as much as it wants but without some sort of restriction there will be no pressure. For example a fire hose as a cone head on it that forces the creates the restriction. Flowing water over the limits of the outlet of the cone head will produce pressure. Pressure is obviously increased by raising the speed of the water pump motor.

In an automotive application however, the fuel pump usually runs at a static voltage and therefore can only push the fuel with so much force. When the force pressing back on the fuel pump grows beyond the messure of that force, the pump's flow rate is reduced to 0. Thus the 0 portion of the flow chart. With no force at all going back to the pump the pump will reach its max flow rate and you get the high part of the flow chart.

So... somewhere in there, some extra fuel is squeezed passed the fuel pump outlet, cramming more fuel into the fuel system than its ability at a given messure of atmospheric pressure. While it may be true that liquids are far less compressible than a gas such as air, there must be some messure of compression happening for a fuel system to pressurize. So I need to know how much extra fuel that is...

 

[notnats]

Yes, when there is no restriction there will be no pressure.

If the pump is supplying more fuel than the engine needs, a surplus valve will maintain the line pressure and dump the surplus back to the tank.

When carburettor engines had a fuel pump driven off the camshaft, if there was no fuel flow a spring under the pump diaphragm maintained the line pressure while the cam lever "freewheeled".

Yes fuel is slightly compressible, so is concrete.

You won't measure the extra fuel in the line as the pressure increases to 40PSI, unless you have air locks in the line or weak hoses that are about to burst.

Jeff

 

[sailoday28]

"Now the reason I say the pumps dont actually create the pressure is that a pump can pump as much as it wants but without some sort of restriction there will be no pressure."

Not true! For example, a centrifual pump has a characterist Head vs Flow at a given RPM. Generally, the pressure (and flow) is then determined based on the system resistance (head vs flow). ie 2 equations, 2 unknowns.
A positive displacement pump attempts to yield flow rate vs RPM. The pressue is then determined from system resistance.

 

[13BeastREW]

OK, please correct me if I'm wrong but I was under the impression that most automotive fuel pumps were vane type. Are there also centrifugal type automotive fuel pumps?

At this point I'm also thinking that I should be targeting the forces created by the pump versus the forces exerted by the bypass valve of the fuel pressure regulator then using the sum of those forces to determine the fuel pressure. Based on that I'm also guessing that I'll need to know the efficiency of the vane/impeller at various RPMs. Or is there another way to calculate the forces created by the pump given its type and maybe even some known information about its flow vs pressure characteristics?

 

[sailoday28]

I did not realize that your discussion was limited to vane pumps, which are positive displacement.

 

[Tmoose]

http://www.magnumforceracing.com/images/product_images/magnaflow/Fuelpumpgraph.jpg

http://www.vectorbd.com/peugeot/images/mot.jpg

A positive displacement pump WILL try hard to pump a certain volume. Pressure regulation is normally accomplished by simply by-passing the amount of fuel not "needed." At idle a lot of the fuel bypasses. At max power much less fuel bypasses. The pressure gage downstream of the regulator (item 5) does not know the difference.

 

[13BeastREW]

Tmoose, thanks for the information. I am however, well aware of the mechanics behind automotive fuel system (automotive systems for that matter), just not the physics behind them.

Due to difficulty in finding any information on the actuall design specs of fuel pumps in use these days I'm now just trying to come up with an educated guess on their operation.

I'm specifically working around the basis of a fuel pump that flows about 240 liters per hour at 35 PSI. To estimate the pump operating rpm range I first determined that the pump will flow 4 Lpm @ 35 PSI then due to the fact that it is a 12V electric motor used for pumping, I'm guessing it has an operational load RPM range of 1500 - 2000 RPM at its normal voltage. This would produce a flow efficiancy of .002 liters per revolution.

Does that seem reasonable to anyone else?