Fuel pressure damper & regulator functions?
07-30-2004, 10:14 PM
#2
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I *believe* that the regulator is controlled by engine vacuum working on an internal diaphragm that allows more or less fuel to flow thru it. Engine vacuum is an indication of the load the engine is under... The dampener is used to reduce the pulsations in the fuel lines caused by the injectors opening and closing, resulting in smoother fuel flow in the system. How it works escapes me for the time being... anyone??
07-31-2004, 07:39 PM
#3
Burning Brakes
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Well...
Yes Bill, you've got it basically correct. Couple of caveats on our primitive (but quite effective) L/LH systems.
In both systems, the injectors batch fire once per rev, twice for each 4 stroke cycle. In other words, two squirts per putt. Half the fuel is injected each time.
It is a "batch fire" system - that is, ALL the injectors fire at once. "Sequential" (the other system) precisely timed to attempt to fire most of the fuel charge at the open valve *can* get you another 1-2% HP/TQ, an (why manufacturers do it) is because you get slightly better emissions.
On a 928, you'll notice that the "damper" or "dampener" is located at the "IN" end of the fuel rail. This seems odd at first, because the damper is there to absorb pressure waves in the fuel rail from the injectors, therefore it should be at the end of the rail, no? No. Since the 928 does batch fire, the injectors all fire at once , ergo no big waves.
However, the Bosch roller cell style fuel pump does generate some pressure waves - so the damper is really there to absorb those waves, and to act (sort of) like an accumulator. It builds up a "pressure reserve" before all them durn injectors fire again.
The regulator does just what you think it does - it regulates the pressure the injectors 'see' by allowing a certain amount of fuel to be returned to the tank. The vacuum reference allows more fuel to return to the tank during periods of high vacuum (like ~25 kPa absolute) like you see at idle, or during over-run.
How? The damper is just a spring loaded diaphragm and so is the regulator. Though the 928's damper is vac referenced, most aren't. Think of it as a trampoline in a tube. Or (almost exactly) like one of those anti-hammer jugs you put on your hot water lines. Tube contains a flexible membrane on one side, and a spring on the other. Fuel flows under the membrane, membrane flexes at some predetermined rate to absorb the 'jumping' fuel pressure.
Presure regs - though the more pedantic call them 'relief valves', which is technically correct since they are on the "OUT" end, not the "IN" end... I call 'em regulators, because that's what Bosch calls them.
Regs are kinda like dampers. Fuel flows under a diaphragm. This time the spring is a bit more serious in resistance, and the vac ref is really helpful to allow the regulator to keep the fuel at a consistent pressure thoroughout the load range.
HTH,
Greg
Yes Bill, you've got it basically correct. Couple of caveats on our primitive (but quite effective) L/LH systems.
In both systems, the injectors batch fire once per rev, twice for each 4 stroke cycle. In other words, two squirts per putt. Half the fuel is injected each time.
It is a "batch fire" system - that is, ALL the injectors fire at once. "Sequential" (the other system) precisely timed to attempt to fire most of the fuel charge at the open valve *can* get you another 1-2% HP/TQ, an (why manufacturers do it) is because you get slightly better emissions.
On a 928, you'll notice that the "damper" or "dampener" is located at the "IN" end of the fuel rail. This seems odd at first, because the damper is there to absorb pressure waves in the fuel rail from the injectors, therefore it should be at the end of the rail, no? No. Since the 928 does batch fire, the injectors all fire at once , ergo no big waves.
However, the Bosch roller cell style fuel pump does generate some pressure waves - so the damper is really there to absorb those waves, and to act (sort of) like an accumulator. It builds up a "pressure reserve" before all them durn injectors fire again.
The regulator does just what you think it does - it regulates the pressure the injectors 'see' by allowing a certain amount of fuel to be returned to the tank. The vacuum reference allows more fuel to return to the tank during periods of high vacuum (like ~25 kPa absolute) like you see at idle, or during over-run.
How? The damper is just a spring loaded diaphragm and so is the regulator. Though the 928's damper is vac referenced, most aren't. Think of it as a trampoline in a tube. Or (almost exactly) like one of those anti-hammer jugs you put on your hot water lines. Tube contains a flexible membrane on one side, and a spring on the other. Fuel flows under the membrane, membrane flexes at some predetermined rate to absorb the 'jumping' fuel pressure.
Presure regs - though the more pedantic call them 'relief valves', which is technically correct since they are on the "OUT" end, not the "IN" end... I call 'em regulators, because that's what Bosch calls them.
Regs are kinda like dampers. Fuel flows under a diaphragm. This time the spring is a bit more serious in resistance, and the vac ref is really helpful to allow the regulator to keep the fuel at a consistent pressure thoroughout the load range.
HTH,
Greg
08-03-2004, 05:25 PM
#5
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2V4V,
Thanks for a good explanation. What does it mean in practice that the dampers are vacuum referenced? I.e. what is the vacuum signal needed for and how would the dampers behave if the vacuum line was disconnected? How does the amount of vacuum affect the operation of the damper?
Best regards
Niklas Kampe
928 S4 1987
Finland, Europe
Thanks for a good explanation. What does it mean in practice that the dampers are vacuum referenced? I.e. what is the vacuum signal needed for and how would the dampers behave if the vacuum line was disconnected? How does the amount of vacuum affect the operation of the damper?
Best regards
Niklas Kampe
928 S4 1987
Finland, Europe
08-03-2004, 05:54 PM
#6
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As far as I know, the vacuum from the intake "tells" the regulator how much fuel to let through. The amount of vacuum dictates how much fuel is returned to the tank. If the vacuum line were disconnected, the regulator would allow all the fuel by at all times, and it wouldn't be regulating the pressure. I.e. max fuel pressure at all times. Someone correct me if I'm wrong.
08-03-2004, 06:01 PM
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FR, you are on tract. I'll only add that the fuel damper is on the supply to the fuel rail and functions to "dampen" the pulsations caused by the fuel injectors. No vacuum on the damper, no dampening of the pulsations. When I got my 80' it had a golf T shoved in the line, thus giving no vacuum.
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08-03-2004, 06:59 PM
#8
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... to add another way of looking at it, for 2v4v has it covered, is that the fuel pressure regulator (FPR) is in effect, a control valve for the pump discharge: It has two chambers, either side of a diaphragm. The spring on the fuel side of the diaphragm pressures a check valve, and this valve/spring is calibrated to "resist" the pump output pressure until its set limit is reached - say 3.6 bar or ~ 55psig with atmospheric pressure on the other side of the diaphram.
The 'vacuum side' of the diaphragm, if open to atmospheric pressure, would stay constant at 3..6 bar; however, the diaphragm is sealed off and opened to the engine vacuum system. Consequently, when vacuum is applied, the diaphragm moves to release pressure on the check valve spring - in doing so, the pressure maintained at the check valve drops to, say 45psig.
As the engine normally operates under reasonable vacuum ( ( high volumetric pumping factor, high efficiency, etc.), the normal pressure seen by the injectors would be the lower value of ~45psig.
When you mash the throttle to the floor with your size 12, it is assumed that you want maximum power: the engine vacuum drops close to zero, and the FPR diaphragm collapses on its spring thereby raising the pressure. Since the injector flows proportionally to applied pressure, you get more fuel per squirt when it is most needed! At least, this is one guesstimate as to how it operates ....
This is only one of the several ways used to enrich the mixture, or at least keep up with the demand.
The 'vacuum side' of the diaphragm, if open to atmospheric pressure, would stay constant at 3..6 bar; however, the diaphragm is sealed off and opened to the engine vacuum system. Consequently, when vacuum is applied, the diaphragm moves to release pressure on the check valve spring - in doing so, the pressure maintained at the check valve drops to, say 45psig.
As the engine normally operates under reasonable vacuum ( ( high volumetric pumping factor, high efficiency, etc.), the normal pressure seen by the injectors would be the lower value of ~45psig.
When you mash the throttle to the floor with your size 12, it is assumed that you want maximum power: the engine vacuum drops close to zero, and the FPR diaphragm collapses on its spring thereby raising the pressure. Since the injector flows proportionally to applied pressure, you get more fuel per squirt when it is most needed! At least, this is one guesstimate as to how it operates ....
This is only one of the several ways used to enrich the mixture, or at least keep up with the demand.
08-04-2004, 02:46 AM
#9
Burning Brakes
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Garth, et.al.,
I think you've got most of it, but just a coupla finer points...
The fuel pump runs constantly. As such, it is always trying to "push" the most fuel it can. The job of the regulator(s) is/are to return the unused/unwanted fuel to the tank.
Depending on MY and condition, I have datalogged manifold pressure as low as ~24 kPa (atmo is about 100 kPa depending on altitude/weather). (sorry folks, I just don't do inches of mercury) There are 2 scenarios where you get this high vacuum in the manifold - one is idle, the other is overrun. Overrun is letting off the gas from mid/high RPM. Vacuum is created because the moving cylinders are trying to 'suck' in more air than the throttle is letting past at the moment.
When you slam the throttle open, the first thing that happens is air rushes in past the throttle. This means that the cylinders have more air to 'suck' in. So actually, as the revs go up, the manifiold pressure does too. Vacuum decreases when you open the throttle.
The vac referencing is one more EFI 'fine tuning' point. It helps the springs open the regs on idle/overrun conditons, letting more fuel flow back to the tank because, hey at idle, you don't need much gas to run the car. Also, there is less need to keep the fuel pressure high - in fact, it helps to lower the pressure a bit.
The damper(s) are once again, just 'shock absorbers'. They take the pulses created when the Bosch roller cell pump pushes the gas to the injectors and smoothes them out. It also does serve as an accumulator of sorts to have a bit of 'reserve' for the next time all the injectors fire.
Once again, in L/LH, they ALL fire at the same time. What pressure wave research I have had time to do at this point indicates that there is little in the way of 'wave action' that flows back to the entry mounted damper in L/LH. Since much of the wave action *seems* to follow fuel flow, the regs also serve as 'dampers' since they are at the end of the line. Since L/LH are non-sequential in all iterations, the pressure wave management from the injector firing is not a really huge deal like it would be on a sequential (timed injection setup).
BTW- there's a whole world full of non-vac reference damper and regs out there. Lots are made by Robert Bosch. They work fine. The most impotant part of either assembly is the spring and/or flexible membrane. The vacuum reference makes the system more able to accurately maintain fuel pressure across a wide range of manifold vacuums. The vac ref make it easier to make more power and hit emissions targets.
Greg
I think you've got most of it, but just a coupla finer points...
The fuel pump runs constantly. As such, it is always trying to "push" the most fuel it can. The job of the regulator(s) is/are to return the unused/unwanted fuel to the tank.
Depending on MY and condition, I have datalogged manifold pressure as low as ~24 kPa (atmo is about 100 kPa depending on altitude/weather). (sorry folks, I just don't do inches of mercury) There are 2 scenarios where you get this high vacuum in the manifold - one is idle, the other is overrun. Overrun is letting off the gas from mid/high RPM. Vacuum is created because the moving cylinders are trying to 'suck' in more air than the throttle is letting past at the moment.
When you slam the throttle open, the first thing that happens is air rushes in past the throttle. This means that the cylinders have more air to 'suck' in. So actually, as the revs go up, the manifiold pressure does too. Vacuum decreases when you open the throttle.
The vac referencing is one more EFI 'fine tuning' point. It helps the springs open the regs on idle/overrun conditons, letting more fuel flow back to the tank because, hey at idle, you don't need much gas to run the car. Also, there is less need to keep the fuel pressure high - in fact, it helps to lower the pressure a bit.
The damper(s) are once again, just 'shock absorbers'. They take the pulses created when the Bosch roller cell pump pushes the gas to the injectors and smoothes them out. It also does serve as an accumulator of sorts to have a bit of 'reserve' for the next time all the injectors fire.
Once again, in L/LH, they ALL fire at the same time. What pressure wave research I have had time to do at this point indicates that there is little in the way of 'wave action' that flows back to the entry mounted damper in L/LH. Since much of the wave action *seems* to follow fuel flow, the regs also serve as 'dampers' since they are at the end of the line. Since L/LH are non-sequential in all iterations, the pressure wave management from the injector firing is not a really huge deal like it would be on a sequential (timed injection setup).
BTW- there's a whole world full of non-vac reference damper and regs out there. Lots are made by Robert Bosch. They work fine. The most impotant part of either assembly is the spring and/or flexible membrane. The vacuum reference makes the system more able to accurately maintain fuel pressure across a wide range of manifold vacuums. The vac ref make it easier to make more power and hit emissions targets.
Greg
