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Old 20th April 2017, 09:55 PM   #1
LBCportagee
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Fuel Pressure explained

Nothing ground breaking here but this short article put a couple concepts together for me especially concerning Fuel pressure and injectors on forced induction systems and return vs returnless so I thought I'd share it...

Fuel Pressure Explained

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Often times the term fuel pressure is used with little understanding of what it really means. This leads to confusion with respect to injector flow rate, and people lose sight of how their injectors really work. Understanding how fuel pressure works and is applied in both returnless and return style fuel systems is important if a user wants to properly set up their injector characterization and get predictable fueling. Knowing what to expect also allows a user to diagnose problems with their fuel system and ultimately make the vehicle function as intended.

There are two pressures that people need to consider: rail pressure and effective (or differential) pressure. For the purposes of the rest of this article, it will just be called effective pressure. Rail pressure is self-explanatory; it is the pressure inside the rail. When you stick a fuel pressure sensor on the end of a rail, it is reading the pressure inside of the rail. While this number is important, it is only half of the story.

Effective pressure is the actual applied pressure for the injector, and is the pressure differential ACROSS the injector. Effective pressure is what injector flow rate is ultimately based off of. When an engine is idling, there is a vacuum in the intake manifold. This vacuum pulls fuel out of the injectors, and increases the effective pressure across the injector to a pressure higher than the rail pressure itself. When a supercharged or turbocharged vehicle is in boost, the pressure inside the manifold is trying to push fuel back into injector, resisting the flow and decreases the effective fuel pressure below that of the rail.

This concept is important because it changes how the fuel system needs to be set up in the PCM. There are two generic types of fuel system setups: returnless and return style. A returnless system does just as the name implies and doesn’t return fuel to the tank. Return style systems will bleed excess fuel back to the tank through the regulator. Return style systems hold a big advantage in that with a vacuum/boost referenced fuel pressure regulator, the system can maintain a CONSTANT effective fuel pressure, which can extend the range of fuel injectors and help them function at lower fuel demands as well.

With a return system, the base pressure is set with the engine off, but the pump running. For a GM, this pressure is usually set to 58psi (factory fuel pressure in the rail). The vacuum/boost referenced regulator will help to change the pressure in the rail based on the pressure in the manifold. When an engine is idling, it may be pulling 20 inHg of vacuum, which translates to roughly 10psi. The reference to the regulator will allow it to adjust and lower the pressure in the rail to 48psi, resulting in 58psi effective pressure, which is the same as the base pressure. When the engine is making 10psi boost, the regulator will adjust and increase rail pressure to 68psi, again resulting in 58psi of effective pressure. The regulator will constantly bleed off pressure inside of the rail to maintain the same effective pressure at all operating conditions. This helps to prevent a loss of effective pressure during wide open throttle, and also helps to prevent injectors from having to run extremely low pulse widths to fuel at idle. A downfall of return systems is the fact that they circulate fuel through a very hot engine bay, ultimately carrying that heat back into your fuel tank.

A return style system that isn’t variable will maintain a certain pressure inside the rail, regardless of what is happening in the manifold. For instance, take a GM system with the standard 58psi in the rail (usually there is a mechanical regulator near the pump to bleed pressure back into the tank and keep the rail itself at 58psi). No matter what operating condition (short of demanding more fuel than the pump can supply), pressure in the rail will always be 58psi (or pretty close). When idling at 20 inHg, this means effective pressure will rise to 68psi because the vacuum in the manifold is adding 10psi to the rails 58psi. This requires injectors to pulse shorter so as to not overfuel the engine and cause a rich condition. By contrast, when a naturally aspirated engine is wide open throttle, the manifold pressure is not in vacuum or in boost, so the effective pressure is the 58psi of rail pressure and nothing more. However, a boosted engine at 10psi will be resisting the fuel, causing effective pressure to drop to 48psi from the 58psi in the rail. This lowers the ultimate output of the injectors.

Some returnless systems will actually vary the pump output to emulate a referenced system, or to offer more fuel pressure at higher demands and less fuel pressure at lower demands. Ford fuel systems modulate the pump in an effort to maintain effective fuel pressure at 3 bar. The Corvette ZR1 runs fuel pressure in the 30s until an increased demand is on the system, at which point it will ramp the fuel pressure up to 88psi in the rail. Systems like these use sensors that record the fuel pressure, and when combining that pressure with the manifold pressure, the PCM knows what the effective pressure is and will determine a pulse width for the injector accordingly. Systems like these offer the best of both worlds.

Ultimately, what we need to know is the effective fuel pressure in any given situation though. GM uses manifold pressure to subtract away from the rail pressure (which it always assumes is 58psi) to calculate pulsewidth. By referencing the flow rate table, in which the flow rate at various effective pressures is programmed in, the PCM knows what flow the injector is capable of at any given operating system. To convert a GM vehicle to work with a boost referenced return system, one must simply populate all of the various pressures with the same flow value, since the effective pressure (and consequently injector flow rate) will remain constant, regardless of manifold pressure. Word to the wise, when you see injectors advertised to flow X amount of fuel at a certain pressure, if you have a boosted vehicle, they will actually flow less while in boost unless you have a boost referenced system!

Dave Steck
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Old 21st April 2017, 01:23 PM   #2
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Re: Fuel Pressure explained

Thanks Scott. It never occurred to me to go ahead and run a MAP hose to the regulator feeding my rails. Makes perfect sense to do that, but I simply worked off a flat 58 PSI ever since I've completed the car. Will look into that.
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Old 1st May 2017, 02:09 PM   #3
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Re: Fuel Pressure explained

Thanks Scott, very informative, and relevant for me as I am in the middle of converting my 302 to efi.
The only part that wasn't clear was, 'A downfall of return systems is the fact that they circulate fuel through a very hot engine bay, ultimately carrying that heat back into your fuel tank'.
I can see that is true, but is it also true that the downfall of a returnless system is that the heat build can boil the fuel in the headers, especially for hot starting? Maybe the 58psi is enough to prevent that, as when I looked up vapour pressure tables, I saw that the most volatile gasoline vapour pressure was 16psi at 100F, I tried to find data, and while not finding anything definitive, my best data suggests 58psi at 250F which seems adequate.
When a hot engine is stopped there is definitely a possibility of boiling, does anyone advocate running the pumps for a period after shutdown, any knowledge on this here?
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Old 1st May 2017, 03:33 PM   #4
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Re: Fuel Pressure explained

Quote:
Originally Posted by wolodymyr View Post
Thanks Scott, very informative, and relevant for me as I am in the middle of converting my 302 to efi.
The only part that wasn't clear was, 'A downfall of return systems is the fact that they circulate fuel through a very hot engine bay, ultimately carrying that heat back into your fuel tank'.
I can see that is true, but is it also true that the downfall of a returnless system is that the heat build can boil the fuel in the headers, especially for hot starting? Maybe the 58psi is enough to prevent that, as when I looked up vapour pressure tables, I saw that the most volatile gasoline vapour pressure was 16psi at 100F, I tried to find data, and while not finding anything definitive, my best data suggests 58psi at 250F which seems adequate.
When a hot engine is stopped there is definitely a possibility of boiling, does anyone advocate running the pumps for a period after shutdown, any knowledge on this here?
Dave
Hi Dave
I would go with a return system, if you ever get a bubble of air with hard braking etc it will probably get returned to the tank. Return less it gas to go through an injector.

Hot fuel, fit an air to fluid cooler in the return. Look at small diesel vans, especially Citroen as they have them.

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Old 1st May 2017, 04:16 PM   #5
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Re: Fuel Pressure explained

Thanks Ian, already going with a return system and it seems natural since I would rather return heat to the tank than have it build up at the engine, hence my comment on the negative statement on a return system.
There is plenty of heat loss in an aluminium tank anyway so it doesn't seem like an issue to my mind.
The cooler is an interesting point, has anyone found that necessary tho? Ultimately, the unused fuel flows back to the tank and loses heat there, maybe less so for bladder tanks tho.
With carbs, some have had vapour lock issues, especially when hot starting. I was just wondering if anyone has had issues with an efi system.

Just learning as I go along, so on the subject of maintaining a constant effective pressure across the injectors, I checked the pressure regulator I was supplied with, it's a Malpassi Fuel Pressure Regulator 30069INJE409 and has a vacuum/boost port (on the top by the adjuster) which will do exactly that, so spot on from Emerald.
Dave
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Old 1st May 2017, 04:28 PM   #6
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Re: Fuel Pressure explained

Quote:
Originally Posted by wolodymyr View Post
Thanks Scott, very informative, and relevant for me as I am in the middle of converting my 302 to efi.
The only part that wasn't clear was, 'A downfall of return systems is the fact that they circulate fuel through a very hot engine bay, ultimately carrying that heat back into your fuel tank'.
I can see that is true, but is it also true that the downfall of a returnless system is that the heat build can boil the fuel in the headers, especially for hot starting? Maybe the 58psi is enough to prevent that, as when I looked up vapour pressure tables, I saw that the most volatile gasoline vapour pressure was 16psi at 100F, I tried to find data, and while not finding anything definitive, my best data suggests 58psi at 250F which seems adequate.
When a hot engine is stopped there is definitely a possibility of boiling, does anyone advocate running the pumps for a period after shutdown, any knowledge on this here?
Dave
I've struggled with this ever since completion of the car. It all started when vapor lock dropped my high-pressure from 58 PSI down to around 20-30. Stopped and shut down the car, and listened to the surge tank boiling fuel (verified that was what was happening by feeling the vibrations). Fixing this has been a struggle, because even with an open rear panel, and two side scoops to force air into the rear panel, I still get high temperatures for the gas. Recirculation of the gas (main tanks to surge tank/surge tank to fuel rail) eventually over a period of about 30 minutes will raise the temperature of the main tank (depending on which one is being used) to > 170 F. One of the design flaws of my project was placement of the high-pressure pump in the same enclosure as the electric water pump).

Mitigation of heat sinking (in order of application) included:
1) Insulation of fuel lines throughout the engine bay (undetermined impact)
2) Heat barriers and shielding from radiative heat of the headers (moderate impact).
3) Scoop under side pods to force air around main tanks and out the rear of the car (small impact)
4) Scoop from front oncoming air piped to the HP fuel pump/water pump enclosure, to exit out the back (moderate impact).
5) Placement of a 5" X 5" Setrab cooler sealed against one of the side scoops to cool fuel in the HP circuit (greatest impact).



In an emergency, I can close off the recirculation, which then pressurizes the surge tank to about 5 PSI, which may buy me time enough to get the car to a safe location and allow it to cool down. To date, I no longer have fuel pressure issues on 95 F or less days . I'll find out this summer what the vapor lock threshold will be.
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Last edited by blueovalz; 1st May 2017 at 04:41 PM.
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Old 2nd May 2017, 08:09 AM   #7
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Re: Fuel Pressure explained

Terry, you must have had a hard with that one. Looks like you have some interesting data there. So your experience modifies my possible 58psi at 250F down to maybe 200F?
Do you have a system diagram, and do you know what capacities and flows you have?
Here in Scotland, we don't have 95deg days so that looks like a benefit from that point of view, but heat barriers sound like a good investment, I think that a cooling air flow through the sponsons would also take advantage of the large area of tank available for heat loss.
Dave
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Old 2nd May 2017, 10:14 AM   #8
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Re: Fuel Pressure explained

LP (Holley Blue, not Red) pumps flow 110 GPH free-flow, and the HP A1000 pump flows about the same. Not posted earlier was the thermal barrier applied to the headers, which helps a lot for underhood temps.

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Old 3rd May 2017, 08:20 AM   #9
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Re: Fuel Pressure explained

Terry, good diagram.

my first observation is that the surge tank will be at virtually atmospheric pressure as the LP pump pumps through the surge tank and straight into the overflow. In this case there is no pressure to raise the boiling temperature unless there is an orifice or resistance in the line.

second, the hp loop will tend to recirculate hot fuel so some heat build up from the engine heat transfer.

If the HP circuit returned to the tank, and the tank took heat out of the fuel, there could be a double benefit of higher pressure in the surge tank to raise the boiling temperature, and the HP circuit would receive cooler fuel.

Oh, the other thing, a typo, is that the one way valves are drawn reversed.

Need to think about that further to establish feasibility, and would require knowledge of flow characteristics of the pumps etc, but my view is that raising the pressure in the surge tank would be key to preventing boiling.

Dave
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Old 3rd May 2017, 03:41 PM   #10
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Re: Fuel Pressure explained

Dave

HP return cannot return to tank as if you are on brakes or downhill on low fuel do LP is drawing air the hp pump will empty the swirl in very quick time.

The LP pump should have higher flow rate than the HP so it can catch up when the swirl is not totally full.

On my set up I have 8mm to the swirl and 6mm return to tank so yes swirl is slightly pressurised.

Oh and my swirl is mounted low on the top of bottom chassis rail so can get good air past, even so it will run warm to hot especially in slow traffic. I also find once run in traffic for a while swapping tanks causes better running so presume the tank fuel is likewise getting hot.

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Old 3rd May 2017, 05:34 PM   #11
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Re: Fuel Pressure explained

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Originally Posted by IanAnderson View Post
Dave

HP return cannot return to tank as if you are on brakes or downhill on low fuel do LP is drawing air the hp pump will empty the swirl in very quick time.

The LP pump should have higher flow rate than the HP so it can catch up when the swirl is not totally full.

On my set up I have 8mm to the swirl and 6mm return to tank so yes swirl is slightly pressurised.

Oh and my swirl is mounted low on the top of bottom chassis rail so can get good air past, even so it will run warm to hot especially in slow traffic. I also find once run in traffic for a while swapping tanks causes better running so presume the tank fuel is likewise getting hot.

Ian
Ian,

Exactly as I've seen and practiced this myself. As you both have noted, I can dead-end the swirl pot to pressurize it at the LP pump's pressure (to decrease risk of boiling), but I would only do this to temporarily get me out of a bind to find a safe place for cool-down.
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Old 4th June 2017, 09:03 AM   #12
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Re: Fuel Pressure explained

Ian, interesting points, agreed fuel return needs to come from the surge tank to take away any air (two returns would be ott I think), and you made me realise that with low tanks under braking, air can easily be drawn in. What I am thinking is that the LP pump delivers a rated flow at a pressure. With no orifice and just the pipework back pressure, the pressure in the surge tank will be well below that rated pressure. If the system has a correctly sized orifice in the return, the pump will run at its rated pressure and that increased pressure will help to prevent boiling in the surge tank. It might also help prevent cavitation when at full throttle and HP Pump demand is high. To size the orifice correctly, one would need to understand the LP and HP pump characteristics (pressure v flow), the PRV characteristic and the engine fuel demand at WOT.
Of course when the engine is switched off, the pressure isn't there any more, so if it's hot enough it will boil anyway.
At first I wasn't too worried about what happens when the car has been running for a while, but now look what you've done
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