Fuel Flow Vs. Line Size
07-15-2007, 09:45 AM
#1
Supporting Vendor
Thread Starter
Fuel Flow Vs. Line Size
I've made this post before, but it was a long time ago and I guess we need to examine this topic again - specifically to address Tony Dee's thread. I thought it better to make it a new separate post so new members a year from now could find it more easily. This post will address the required fuel line size from your fuel tank to the injector rails as well as the injector rails themselves.
Let's start with a random off the shelf injector - say a 60 lb/hr. In order for the flow rating of any injector to be meaningful we must also have a pressure rating. Most injectors' flows are rated at around 42 to 45 psig (pounds per square inch gage) and typically 80% duty cycle.
So we take this injector and hook it up to a test rail. The test rail has a regulator on it which we set to say 42 psig. We also attach an accurate fuel pressure gage right at the rear of the injector to verify the pressure setting. Now we run the injector at 80% duty cycle with its business end pointed into a calibrated cylinder so we can measure its' flow over time. Some labs have very accurate in-line flow meters that make the measuring task much easier/quicker. With everything in place and functioning we measure a flow rate of 60 lbs/hr.
Now I ask you did it matter whether I fed the fuel rail with a 1/8" id fuel line or a 3/8" id fuel line or a 1/2" id fuel line or a 1" id fuel rail as long as I was able to maintain the 42 psig? No - it does not matter what the supply line size is as long as I can maintain the desired pressure at the injector head !!!
I took this round about way of explaining flow because some people have it stuck in their heads that they must increase their line size when making big HP numbers like a thousand or more. A #6 AN line can easily support 2000 HP as long as you have enough pump head !!!
That is to say as the flow thru any given line size increases so do the friction losses. These losses are published in engineering manuals such as the Cameron Hydraulic Data Handbook published by Ingersoll-Rand. Any engineer who works with designing pump installations has one of these manuals.
Let's take a typical Corvette. I'll say there is 20 feet of steel 3/8" id fuel line from tank to rail (there is actually less but we'll be conservative because bends in the line account for some pressure losses also - each bend adds 'x' amount of equivalent feet).
Now you go into the table and look up the pressure drop for say 110 gph (gallons per hour) for 20 feet of 3/8" id steel line (12 HP/ gallon of gasoline is a conservative number): it's going to be around 2 to 3 psig.
This is the same way an electrical engineer determines what size wire to run for a given load. If the load is say 40 amps he could use a #14 AWG or he could use a #8AWG. Both size wires can handle the load, i.e., 50 amps (or if gasoline amps = volume in gallons). However the smaller wire has a much higher voltage drop (in gasoline thats pressure). If our voltage is fixed at 120 VAC the engineer will go with the bigger wire to minimize the voltage drop. Your Corvette pump head (i.e., voltage) is not fixed until after you pick your pumps etc.
So what this means is that if you need to maintain 42 psig at the fuel injector you must have enough pump head to not only make the 42 psig but an extra 2 or 3 psig for the line losses. In other words if your pump(s) can deliver 110 gph at say 60 psig, you have plenty of pump with some to spare.
Where all these line size "wifes' tails" started is back in the days of carbureautors and low pressure electric pumps or low pressure mechanical pumps. These pumps had plenty of flow but at low pressures. These pumps could not afford a 3 psig drop from front to back. So they increased line size - without looking it up perhaps a #8 AN line has only a 1 psig drop at 110 gph for the given 20 feet.
The whole point of this discussion is that if you have an accurate fuel pressure gage placed at the end of your fuel rail and you are able to maintain the head i.e., desired fuel pressure PSIG, your fuel line size is fine!!! If the fuel pressure starts to fall as you go up in HP you need more pump - if for some reason you cannot get more pump then line size can be considered - but if you look at the numbers for #6 vs. #8 there is very little difference.
Tony Dee's thread also addressed hydraulic shock from the opening and closing of the injectors. I've used for years a small Bosch unit #0 280 161 212. It is 2" in diameter and about 1.5" high. It is suitable for pressures well in excess of 100 psig. It's a little difficult to use because it attaches with a M10 banjo bolt - hence you need a banjo adaptor. A pair of these (one on each rail) virtually eliminates the fuel pressure spikes which can be disastrous. The biggest problem with these pulsation dampers are $. They may cost you in excess of $125 each and then you still have to adapt them to your rail.
Foot Notes: Gasoline weighs 5.994 lbs/gal @ 60 degrees F (this number may vary slightly depending on its' octane rating). Divide CC/Min by 10.5 to obtain Lb/Hr. With a little math you'll see that ML/Sec x 5.69722 = Lbs/Hr. At W.O.T. you need 1/2 pound of fuel per hour per HP (that's at a conservative .5 BSFC). Well tuned engines will run at .4 to .45 BSFC at WOT.
Hope the above saves some members the cost of replacing their fuel lines without really needing to. Best regards, Greg
Let's start with a random off the shelf injector - say a 60 lb/hr. In order for the flow rating of any injector to be meaningful we must also have a pressure rating. Most injectors' flows are rated at around 42 to 45 psig (pounds per square inch gage) and typically 80% duty cycle.
So we take this injector and hook it up to a test rail. The test rail has a regulator on it which we set to say 42 psig. We also attach an accurate fuel pressure gage right at the rear of the injector to verify the pressure setting. Now we run the injector at 80% duty cycle with its business end pointed into a calibrated cylinder so we can measure its' flow over time. Some labs have very accurate in-line flow meters that make the measuring task much easier/quicker. With everything in place and functioning we measure a flow rate of 60 lbs/hr.
Now I ask you did it matter whether I fed the fuel rail with a 1/8" id fuel line or a 3/8" id fuel line or a 1/2" id fuel line or a 1" id fuel rail as long as I was able to maintain the 42 psig? No - it does not matter what the supply line size is as long as I can maintain the desired pressure at the injector head !!!
I took this round about way of explaining flow because some people have it stuck in their heads that they must increase their line size when making big HP numbers like a thousand or more. A #6 AN line can easily support 2000 HP as long as you have enough pump head !!!
That is to say as the flow thru any given line size increases so do the friction losses. These losses are published in engineering manuals such as the Cameron Hydraulic Data Handbook published by Ingersoll-Rand. Any engineer who works with designing pump installations has one of these manuals.
Let's take a typical Corvette. I'll say there is 20 feet of steel 3/8" id fuel line from tank to rail (there is actually less but we'll be conservative because bends in the line account for some pressure losses also - each bend adds 'x' amount of equivalent feet).
Now you go into the table and look up the pressure drop for say 110 gph (gallons per hour) for 20 feet of 3/8" id steel line (12 HP/ gallon of gasoline is a conservative number): it's going to be around 2 to 3 psig.
This is the same way an electrical engineer determines what size wire to run for a given load. If the load is say 40 amps he could use a #14 AWG or he could use a #8AWG. Both size wires can handle the load, i.e., 50 amps (or if gasoline amps = volume in gallons). However the smaller wire has a much higher voltage drop (in gasoline thats pressure). If our voltage is fixed at 120 VAC the engineer will go with the bigger wire to minimize the voltage drop. Your Corvette pump head (i.e., voltage) is not fixed until after you pick your pumps etc.
So what this means is that if you need to maintain 42 psig at the fuel injector you must have enough pump head to not only make the 42 psig but an extra 2 or 3 psig for the line losses. In other words if your pump(s) can deliver 110 gph at say 60 psig, you have plenty of pump with some to spare.
Where all these line size "wifes' tails" started is back in the days of carbureautors and low pressure electric pumps or low pressure mechanical pumps. These pumps had plenty of flow but at low pressures. These pumps could not afford a 3 psig drop from front to back. So they increased line size - without looking it up perhaps a #8 AN line has only a 1 psig drop at 110 gph for the given 20 feet.
The whole point of this discussion is that if you have an accurate fuel pressure gage placed at the end of your fuel rail and you are able to maintain the head i.e., desired fuel pressure PSIG, your fuel line size is fine!!! If the fuel pressure starts to fall as you go up in HP you need more pump - if for some reason you cannot get more pump then line size can be considered - but if you look at the numbers for #6 vs. #8 there is very little difference.
Tony Dee's thread also addressed hydraulic shock from the opening and closing of the injectors. I've used for years a small Bosch unit #0 280 161 212. It is 2" in diameter and about 1.5" high. It is suitable for pressures well in excess of 100 psig. It's a little difficult to use because it attaches with a M10 banjo bolt - hence you need a banjo adaptor. A pair of these (one on each rail) virtually eliminates the fuel pressure spikes which can be disastrous. The biggest problem with these pulsation dampers are $. They may cost you in excess of $125 each and then you still have to adapt them to your rail.
Foot Notes: Gasoline weighs 5.994 lbs/gal @ 60 degrees F (this number may vary slightly depending on its' octane rating). Divide CC/Min by 10.5 to obtain Lb/Hr. With a little math you'll see that ML/Sec x 5.69722 = Lbs/Hr. At W.O.T. you need 1/2 pound of fuel per hour per HP (that's at a conservative .5 BSFC). Well tuned engines will run at .4 to .45 BSFC at WOT.
Hope the above saves some members the cost of replacing their fuel lines without really needing to. Best regards, Greg
07-15-2007, 10:22 AM
#2
Safety Car
I always enjoy Greg's technical view on these subjects. As a ChE by formal education, I have a blue backed Cameron Handbook on the shelf; I used to use it regularly when sizing lines for the plant.
I struggled with fuel supply issues for years when I was running twin Walbros and a -6AN feed with 42 lb/hr FMS, an FMU, and the old D1. When I upgraded to aftermarket ECM, new engine and new blower, I wanted no fuel supply concerns and a gent by the name of Travis Quillian (well known in the Pro 5.0 crowd) led me in the direction of the fuel system I have now.
Since fixing an early leaky injector issue, I have had no fuel supply related issues since upgrade. Though I seem to be getting close to the limits of my injectors.
Again Greg, thanks for taking the time to write the detailed technical above. Your knowledge is always appreciated.
Aaron
I struggled with fuel supply issues for years when I was running twin Walbros and a -6AN feed with 42 lb/hr FMS, an FMU, and the old D1. When I upgraded to aftermarket ECM, new engine and new blower, I wanted no fuel supply concerns and a gent by the name of Travis Quillian (well known in the Pro 5.0 crowd) led me in the direction of the fuel system I have now.
Since fixing an early leaky injector issue, I have had no fuel supply related issues since upgrade. Though I seem to be getting close to the limits of my injectors.
Again Greg, thanks for taking the time to write the detailed technical above. Your knowledge is always appreciated.
Aaron
07-15-2007, 03:13 PM
#4
Supporting Vendor
Thread Starter
Thanks Aaron and Tony for the kind comments: much appreciated! I will add the following; my mini-discertation only applies to pushing the fuel thru the lines, i.e., rear mounted pumps with LARGE suction line. That's also why in the days of carbs people ran large lines to the mechanical pump up front that had to suck the fuel thru the lines. Warm fuel with negative pressure can flash. Also I don't like rear mounted parallel pumps because they can fight each other; that's why I've always been an advocate of the 340 in series with the 392. Both these pumps by themselves can support in excess of 700 HP with no voltage booster if fuel pressure needs are well below 60 psig. In series these two pumps can easily support 700 HP at 100+ psig. If I had an application that required the volume of two 340's (1400+ HP) then I'd run the dual feed lines and feed each rail separately with a common return, as I guess Aaron and many others have done. Best regards, Greg
07-15-2007, 05:03 PM
#5
Drifting
I can't remember where I saw it, but I saw a pulse damper that was just a small tank with a fitting at the bottom. The air trapped in the tank got pressurized so no spring or membrane was needed.
Cheap and effective!
The only problem I see with the design is that the air might and probably will dissolve into the fuel with time and then the pulse damper will stop working. You could put a schrader valve at the top of the tank and racharge it with air every now and then. On a race car this is not an issue.
Cheap and effective!
The only problem I see with the design is that the air might and probably will dissolve into the fuel with time and then the pulse damper will stop working. You could put a schrader valve at the top of the tank and racharge it with air every now and then. On a race car this is not an issue.
