Fuel Pressure Regulator

[dlaing]

Now, say you cruise on 5th gear at exactly 3000 rpm. Then wind on 75% throttle . Will this produce a totally different manifold pressure? How?

 

Or, to have even more differing scenarios: When hitting that cell during acceleration (low gear) from lower rpm, will the manifold pressure be significantly different from when hitting it during deceleration (too high gear) from higher rpm? Sorry if I'm being a complete moron, I just can't picture this.

I am having trouble picturing what is happening in the manifold, but in the engine the 5th gear, 3000rpm, 75% will be more likely to ping. I think this will somehow effect what is happening in the manifold, but yah, I too doubt it would be significant, and closing off the pressure regulator's line to atomosphere probably won't make a significant difference.

Likewise under any scenario of 75% throttle, I doubt there would be a significant difference.

The significant differences would be at the throttle rpm situations listed with the greatest vaccuum being at about redline with throttle closed.

Thanks Dan for the numbers and analysis.

I am sure the numbers will vary, mostly depending on what you have the idle set too.

[Ryland3210]

Maybe we need a poll to determine how many people still have the canister(s).

I kind of assumed most people considering the mod had no canister(s).

The diagram shows no canisters.

Was your test done with or without canisters?

If you have canisters, that would certainly explain why you noticed no difference!!!

 

In Ratch's earlier post, he indicated his bike came with cannisters, but his test was done without them.

[Ryland3210]

Another thought: Why would the alpha-n system not be enough by itself? Why would this FPR connection, or a MAP sensor, ever be needed? Say you have a map cell at 3000 rpm and three quarter open throttle. Could you be running at that very cell with completely different manifold pressures? If so, how? Why? Other than the effect of dirty filters, of course.

 

Say you cruise on 3rd gear at exactly 3000 rpm. Then wind on 75% throttle. This results in a pressure of x in the intake manifolds before rpm actually increases.

 

Now, say you cruise on 5th gear at exactly 3000 rpm. Then wind on 75% throttle . Will this produce a totally different manifold pressure? How?

 

Or, to have even more differing scenarios: When hitting that cell during acceleration (low gear) from lower rpm, will the manifold pressure be significantly different from when hitting it during deceleration (too high gear) from higher rpm? Sorry if I'm being a complete moron, I just can't picture this.

 

At 3000 RPM, three quarter open throttle, think of it this way: The RPM and throttle position determine the torque the engine puts out, all other things being constant, so the load on the bike determines how long it stays at that RPM. If on an an uphill grade, it will accelerate slower than on a downhill. As long as it stays at that RPM, the intake manifold absolute pressure will remain the same.

 

Whether you are in one gear or another has no effect on manifold pressure. It will only affect the bike's rate of acceleration/deceleration. Of course, that will change RPM.

 

To say it another way: the engine is a fixed displacement vacuum pump as far as its suction of the air/fuel mixture is concerned. Setting aside the effects of valve timing, resonance effects in the intake passage, etc., for the moment, the rate of suction from the engine is proportional to its RPM. The atmosphere is supplying air at 29.92 inches of Mercury through the filter and throttle body to the intake passage . The manifold pressure is 29.92 minus the air filter restriction, minus the throttle body restriction. So for a given RPM, the intake manifold pressure is determined by those restrictions alone, not the load on the engine, as it is affected by the grade or whichever gear one is in.

[Dan M]

Wow! A lot of water under the bridge since the last time I checked in.

 

First, Dan M's reading of 15" Hg at idle, is about - 0.5 Bar, while the fuel pressure regulator target is 3 bar. So if its reference port is connected to the intake manifold, it becomes 2.5 bar. Fuel flow would be reduced by 8.7%.

 

I did not read 15"Hg at idle, I applied 15"Hg to the regulator while the engine was idling. The vacuum from the manifolds bounces too much to keep the regulator actuated.

 

Improvement in economy would not only occur at idle or closed throttle decels. Part throttle cruising economy would also be improved, although to a lesser degree, if the vacuum is less than at idle. By the way, that isn't necessarily so. Had Dan M's idle vacuum reading been in the 20-21" Hg range, I would expect it to be far lower during cruise, but at 15", I'm not sure at all. I've seen engine vacuums ranging anywhere from 10 to 16" under cruise conditions. Under deceleration, fuel flow would decreased by a higher percentage, due to the higher vacuums achieved.

 

No, once actuated the pressure does not change. This engine's vacuum characteristics are not what everybody is used to looking at. If there was a single manifold to both cylinders and a single throttle body supplying both this set up may work, but with only 2 cylinders it will still pulse. As it is though there is wildly pulsing vacuum at most RPMs.

Further, This depends on the threshold of the regulator spring and what actual vacuum is at a given RPM/throttle opening. This particular regulator is activated at just about 15"Hg. Once the spring is overcome and the pressure is reduced, that is it. It is not a variable regulator, it has two values, with vacuum applied and without. The spring is overcome in a range of 1 or 2 "Hg. So it is either on or off so to speak. Usually the difference is 5-10 psi.

 

With smart enough software in an ECU, anything which can be done by connecting the FPR to the intake manifold could be duplicated, since it has data on TPS and RPM, BUT only if the air filter pressure loss and barometric pressure is consistent with the software's assumption (there's that dangerous word again!), OR if an ECU's absolute pressure sensor is working and connected to the airbox between the filter and the throttle body, then there's no problem.

 

Don't forget about fast enough ECU. The mechanical regulator is used to provide instant response. It can all be done with pulsewidth if the computer can respond fast enough. It would also need an air flow meter or a map sensor. Our system, especially the pre O2 & Cat years is a pretty basic one.

 

So under those conditions, mapping the ECU can do the same thing. So why bother considering connecting the FPR to the intake manifold? Because it's cheap!

 

Ratch's comment that his water manometer (Mercury's density is 13.6 times higher) does not fluctuate much lends some comfort that the FPR diaphragm might not be unduly fatigued. Moreover, the worst case for engine pulses would be at idle, wherein the frequency would be lowest. The 0.030" orifices act as a low pass filter, so the higher the RPM, the less the fluctuation. Also, the farther open throttles are, the less the fluctuation. Idle is the worst case for both reasons.

 

Connect a vacuum gauge to your intake and reconsider. This is the fact that will change your mind. You have to see it for yourself. If the actuation vacuum is say 15" and the vacuum bounces from 11 to 18 the diaphragm will be fatigued

 

If this regulator is constructed in the conventional fashion, its diaphragm is controlling a needle, with fuel pressure on one side of the diaphragm and ambient pressure and a spring on the other. So it's not as though the diaphragm makes a sudden movement when the ambient pressure (vacuum) reaches a threshhold. Still, it is a concern that I don't wish to discount altogether.

 

See above, it is not variable. It does make a sudden movement

 

Would I do it anyway in the name of science and fuel conservation? You bet! I like the cleaner exhaust as well. Based on Dan M's analysis, it's as if the idle mapping was done with it in mind.

I agree completely. It just doesn't work with the pulsing vacuum of a v-twin. I think they overlooked this fact in the design stage and then decided to disconnect it once they found it useless.

[Guest ratchethack]

It just doesn't work with the pulsing vacuum of a v-twin. I think they overlooked this fact in the design stage and then decided to disconnect it once they found it useless.

Seems reasonable enough to me also.

 

Wot d'you bet the FPR was sourced via low bid contract from a supplier of automotive components and was originally designed for EFI on 4, 5, 6, or 8 cylinder motors not exhibiting anything close to the intake pulses of the V11?

 

Again -- Nicely done, Dan.

 

FWIW, not related to your analysis, and I'm not going to find and post the reference above, but somebody above (who oughtta know better! ) has evidently confused changes in fuel pressure with a direct correlation to changes in fuel flow rate through the injectors. Naughty! Mustn't do that. . .

[BrianG]

......................... has evidently confused changes in fuel pressure with a direct correlation to changes in fuel flow rate through the injectors. Naughty! Mustn't do that. . .

By "direct" do you mean 1:1, or do you simply mean that there is no correlation?

[Guest ratchethack]

By "direct" do you mean 1:1, or do you simply mean that there is no correlation?

Yes. Direct = 1:1, but also 1:2 or 1:x. I mean simply that flow rate is a function of volume over time, and there is NO KNOWN correlation between flow rate and pressure without knowing the RESISTANCE to flow (in this case, for practical purposes, the resistance to flow is provided by the injector nozzles). So a given percentage change in fuel pressure DOES NOT necessarily -- make that WILL NOT EVER -- translate to the same percentage change in fuel flow rate (delivery at the injectors), as has been suggested.

 

SOURCE: U. of Florida UF/IFAS, Orifice Equations and Tables linked here:

 

http://edis.ifas.ufl.edu/AE107

[Ryland3210]

Thanks, Dan, for the info. on how this regulator actually works. It surprises me.

 

Did you make note of the actual manifold vacuum at idle? If so, what was it?

 

Can you clarify that you meant to say the threshhold is really only 1-2" Hg or is it 15"?

If it is the lower value, it would be affecting the regulator virtually all the time. It would require a pretty free flowing air filter and WOT at low RPM for manifold vacuum to drop below 1-2"

 

Some comments:

 

I thought the need for a fast enough ECU went without saying, but it is important to note.

If you believe it would work with a single carb setup, it seems to me the equivalent is connecting the regulator to a tee which connects to both intakes, as is actually shown in the diagram.

 

An ECU could do pretty well without an air flow/map sensor if it was "smart" enough to contain a model of the engine's flow characteristics and provided it at least has an absolute atmospheric pressure and temperature sensor. That leaves out relative humidity which a map sensor would take into account.

 

On the question of fatigue, the degree of dampening out the pulses depends on the size of the orifice and the volume contained in the hoses and regulator. The tradeoff of having the orifice large enough relative to the volume to provide a fast enough response time is the tricky one.

 

Are you aware of any actual data on why the connection was discontinued? Was it actually wear and tear on the regulator, or simply that later models have refined mapping that accomplishes the same thing? Maybe the relocation of the regulator to inside the tank made connection to the intake manifold too inconvenient.

[Troy]

Yes. Direct = 1:1, but also 1:2 or 1:x. I mean simply that flow rate is a function of volume over time, and there is NO KNOWN correlation between flow rate and pressure without knowing the RESISTANCE to flow (in this case, for practical purposes, the resistance to flow is provided by the injector nozzles). So a given percentage change in fuel pressure DOES NOT necessarily -- make that WILL NOT EVER -- translate to the same percentage change in fuel flow rate (delivery at the injectors), as has been suggested.

 

SOURCE: U. of Florida UF/IFAS, Orifice Equations and Tables linked here:

 

http://edis.ifas.ufl.edu/AE107

 

 

 

Ratchet, your orifice equation exhibits the same characteristic as a fuel injector. The last term in the flow equation you referenced is the square root of the water column height. Water column height is proportional to pressure. So the flow rate is proportional to the square root of pressure.

 

In an earlier post which I can't locate, John quoted this same relationship for our fuel injectors.

 

(Flow rate 2 / Flow rate 1) = Root(2) of (P2 / P1)

 

 

Go here and plug in values to prove it to yourself. If you double the pressure, the flow increases by 41%.

 

http://www.csgnetwork.com/fiflowcalc.html

[dlaing]

In Ratch's earlier post, he indicated his bike came with cannisters, but his test was done without them.

Thanks!

One of these days I will read everything the man babbles on about, or maybe not.

 

snip

So for a given RPM, the intake manifold pressure is determined by those restrictions alone, not the load on the engine, as it is affected by the grade or whichever gear one is in.

I disagree.

What you say would only be true in a theoretical static condition where RPM did not begin to change. With less load the RPM will change faster.

If the engine has a high load it will effect the manifold pressure.

A higher load facing the same Throttle position and RPM will be more likely to ping.

I can only conclude that if it is more likely to ping, then something different is happening to the flow and manifold pressures.

Is that difference significant? Is more load going to result in more vacuum, or less vacuum? I don't know, but I know it will be different.

[dlaing]

I wrote

I suppose you could put a vacuum guage on it, divide the fuel pressure by the vacuum pressure in atmospheres and subtract the result from the fuel pressure at 1 atmosphere to give you a theoretical estimate of the change in fuel flow. Actual flow variation from a change in pressure will offset the answer. But I don't know how to calculate that. I imagine efficiency of flow decreases geometrically as pressure builds, but to what extent, we would need to chart the fuel injectors.

To which Ratchet replied,

Err, WOT?!?! That last statement was a Lu-lu.

 

Long as y'er at it, why not throw in a multipilcation by pi, an addition of Avogadro's number, and a division by Alfred E. Neuman's hat size?!

Lu-Lu only if you don't try to use your noggin.

But no doubt I was wrong and the formula is more simple, as Troy will later describe the math that Ryland used.

Ryland laid down some numbers without showing the math...

First, Dan M's reading of 15" Hg at idle, is about - 0.5 Bar, while the fuel pressure regulator target is 3 bar. So if its reference port is connected to the intake manifold, it becomes 2.5 bar. Fuel flow would be reduced by 8.7%.

Since 2.5bar is about 83% of 3 bar, we have a reduction of about 17% in pressure, which I presume Ryland did further math to arive at a reduction in flow of 8.7%, but then Ratchet did not see the invisible math, so he said,

FWIW, not related to your analysis, and I'm not going to find and post the reference above, but somebody above (who oughtta know better! ) has evidently confused changes in fuel pressure with a direct correlation to changes in fuel flow rate through the injectors. Naughty! Mustn't do that. . .

And then Troy nailed it down.

Ratchet, your orifice equation exhibits the same characteristic as a fuel injector. The last term in the flow equation you referenced is the square root of the water column height. Water column height is proportional to pressure. So the flow rate is proportional to the square root of pressure.

 

In an earlier post which I can't locate, John quoted this same relationship for our fuel injectors.

 

(Flow rate 2 / Flow rate 1) = Root(2) of (P2 / P1)

 

 

Go here and plug in values to prove it to yourself. If you double the pressure, the flow increases by 41%.

 

http://www.csgnetwork.com/fiflowcalc.html

41% of 17% is about 7% doing my sloppy math, which is not far from Ryland's probably more accurate 8.7%

 

I think if someone wanted to give this a try they should consider no canisters and they should select a smaller orifice at the T to reduce pulsing just to the point where no visible pulsing occurs with a vacuum meter.

I have dial cutoffs that I added to my Twin Max and they work great. If I dial them too loose, I get too much pulsing, but if I dial them too tight, I get a lack of instantaneous response. Luckily it is easy to find the happy medium right at the point where pulsing disappears. I think it would be trivial to find remedy to this through proper orifice selection

Of course the other points do seem to indicate that it may not be a good idea to loop the FPR with the intake manifolds.

[Guest ratchethack]

Ratchet, your orifice equation exhibits the same characteristic as a fuel injector. The last term in the flow equation you referenced is the square root of the water column height. Water column height is proportional to pressure. So the flow rate is proportional to the square root of pressure.

 

In an earlier post which I can't locate, John quoted this same relationship for our fuel injectors.

 

(Flow rate 2 / Flow rate 1) = Root(2) of (P2 / P1)

 

 

Go here and plug in values to prove it to yourself. If you double the pressure, the flow increases by 41%.

http://www.csgnetwork.com/fiflowcalc.html

Troy, you've illustrated my point for me. Actually, using the calculator at your link above, I get ~39% increase in flow rate with a doubling in pressure, but I'm rounding off my numbers (which accounts for this), and who's quibbling about a few percentage points well within the margin of error in actual application?

 

To take a few more examples to extend the illustration, again using the calculator at your link above, an increase in pressure of 50% translates to an increase in flow rate of ~22%. An increase in pressure of 200% translates to an increase in flow rate of ~72%. Again, my point was this:

So a given percentage change in fuel pressure DOES NOT necessarily -- make that WILL NOT EVER -- translate to the same percentage change in fuel flow rate (delivery at the injectors), as has been suggested.

I've made an error, however. When I posted:

Yes. Direct = 1:1, but also 1:2 or 1:x.

I was thinking in terms of x = a constant, and should have so specified.

 

As far as any calculations that Dave has done -- errr. . . attempted, I have no comment, as I'm strictly limited to Blue Planet Physics, and his calculations are typically far far beyond me limited earthbound comprehension.

[Dan M]

Thanks, Dan, for the info. on how this regulator actually works. It surprises me.

 

Did you make note of the actual manifold vacuum at idle? If so, what was it?

 

It was at idle that there was the most fluctuation, but it happens at all ranges. With the orifice in place it was literally bouncing the needle on my gauge from about 5" to 12"Hg. With the orifice removed and testing directly at the manifold, the true reading was fluctuating from slight positive pressure to over 15" The little "burp" from valve overlap causes the wildly varying reading. On a multi cylinder engine the vacuum pulses smooth out.

 

Can you clarify that you meant to say the threshhold is really only 1-2" Hg or is it 15"?

If it is the lower value, it would be affecting the regulator virtually all the time. It would require a pretty free flowing air filter and WOT at low RPM for manifold vacuum to drop below 1-2"

 

When I was applying vacuum (with a vacuum pump) to the regulator I was doing it in about 5" increments. First 5 then 10 and so on. By 15" the regulator was actuated and the actuation happened over about a 2" range. I'd have to spend some more time to get exact numbers but I can confidently say that at about 12" the regulator was not applied but by 14" is was. So somewhere in that 2" range the application took place.

 

Some comments:

 

I thought the need for a fast enough ECU went without saying, but it is important to note.

If you believe it would work with a single carb setup, it seems to me the equivalent is connecting the regulator to a tee which connects to both intakes, as is actually shown in the diagram.

 

No, the volume of a manifold is many times greater than the volume of a hose. The pulses from valve overlap would be absorbed in the manifold more than they can be in a small hose. With a manifold connecting the cylinders the small pressure pulses would be counteracted by the vacuum from the other cylinder. Like I said, being a twin the vacuum will always fluctuate. I'm sure you've seen how smooth the needle can be on an 8 cyl. the pulses are too big and too far spaced on a twin.

 

An ECU could do pretty well without an air flow/map sensor if it was "smart" enough to contain a model of the engine's flow characteristics and provided it at least has an absolute atmospheric pressure and temperature sensor. That leaves out relative humidity which a map sensor would take into account.

 

The trouble with this is the computer can not know load. It needs the air or map values to calculate load. Just for example, half throttle at 2000 rpm in first gear is a far lower load than half throttle at 2000 rpm in sixth.

 

On the question of fatigue, the degree of dampening out the pulses depends on the size of the orifice and the volume contained in the hoses and regulator. The tradeoff of having the orifice large enough relative to the volume to provide a fast enough response time is the tricky one.

 

True enough, but for the system to work it has to go from high vacuum (actuated regulator at closed throttle) to low vacuum (closed regulator at open throttle) The point is the vacuum constantly bounces from high to low with every other stroke of the engine.

 

Are you aware of any actual data on why the connection was discontinued? Was it actually wear and tear on the regulator, or simply that later models have refined mapping that accomplishes the same thing? Maybe the relocation of the regulator to inside the tank made connection to the intake manifold too inconvenient.

 

This is the question that nobody has been able to answer. I speculated yesterday that perhaps once they discovered it wouldn't operate they discontinued the connection. I have seen quite a large number of vacuum regulators fail on cars over the years and those only change position with throttle opening. I'd have to conclude that if you applied constant pulsing vacuum to one, the service life would be very short. I don't know when they made the changes but I'd guess the internal (constant pressure) regulator came with the addition of a closed loop system (oxygen sensors and catalysts) Mapping has to be much more precise or the catalysts will be destroyed

 

 

One more thing. There seems to be some confusion about the relationship between vacuum and fuel pressure. There is none. Vacuum is just used to trigger a regulator at a specific value. Fuel pressure is governed by the regulator at two preset values. Let's say for the sake of agrument that the regulator applies at 15"Hg. and the fuel pressure spec is 35psi with the regulator applied and 40psi with it not applied. Vacuum is just the switching device. If the engine is producing less than 15" vacuum the fuel pressure in this model will be 40psi. If the engine is producing more than 15" vacuum the fuel pressure will be 35psi. The idea is high load = low vacuum ~ low load = high vacuum. This is why vacuum is a handy means of actuation. Make sense?

[dlaing]

One more thing. There seems to be some confusion about the relationship between vacuum and fuel pressure. There is none. Vacuum is just used to trigger a regulator at a specific value. Fuel pressure is governed by the regulator at two preset values. Let's say for the sake of agrument that the regulator applies at 15"Hg. and the fuel pressure spec is 35psi with the regulator applied and 40psi with it not applied. Vacuum is just the switching device. If the engine is producing less than 15" vacuum the fuel pressure in this model will be 40psi. If the engine is producing more than 15" vacuum the fuel pressure will be 35psi. The idea is high load = low vacuum ~ low load = high vacuum. This is why vacuum is a handy means of actuation. Make sense?

No, it does not make sense.

First you say there is no relationship between vacuum and fuel pressure and then you show that there is a relationship.

But your explanation of the relationship is a good explanation of why vacuum is a handy means of changing the fuel pressure.

You said the it regulates at two pressures. That makes sense, since otherwise the regulator would go crazy, constantly opening and shutting, trying to keep it at 3.0 bar.

For sake of argument lets say that the regulator closes going from high pressure to 2.9 bar and opens going from low pressure to 3.0 bar.

If we apply 0.0 bar vacuum we will get between 2.9 and 3.0 bar fuel pressure relative to atmosphere.

If we apply 0.5 bar vacuum we will get between 2.4 and 2.5 bar fuel pressure relative to atmosphere.

If we apply 1.0 bar vacuum we will get between 1.9 and 2.0 bar fuel pressure relative to atmosphere.

Right? or am I missing something.

[Dan M]

No, it does not make sense.

First you say there is no relationship between vacuum and fuel pressure and then you show that there is a relationship.

But your explanation of the relationship is a good explanation of why vacuum is a handy means of changing the fuel pressure.

You said the it regulates at two pressures. That makes sense, since otherwise the regulator would go crazy, constantly opening and shutting, trying to keep it at 3.0 bar.

For sake of argument lets say that the regulator closes going from high pressure to 2.9 bar and opens going from low pressure to 3.0 bar.

If we apply 0.0 bar vacuum we will get between 2.9 and 3.0 bar fuel pressure relative to atmosphere.

If we apply 0.5 bar vacuum we will get between 2.4 and 2.5 bar fuel pressure relative to atmosphere.

If we apply 1.0 bar vacuum we will get between 1.9 and 2.0 bar fuel pressure relative to atmosphere.

Right? or am I missing something.

 

You are missing something. Other than the threshold for overcoming the regulator spring, fluctuations in manifold vacuum have no effect on the pressure in the fuel lines. The regulator is just using vacuum to mechanically switch the regulator. Think of it as a regulator with a manual lever to throw to switch from low pressure to high. the "lever" that is getting thrown is the vacuum diaphragm. The regulator can be considered as on or off. "On" with vacuum applied allowing fuel to pass back to the tank at a lower pressure than "off". The fuel pressure is in the fuel circuit (pump, lines, injectors, and finally regulator) atmospheric pressure and manifold vacuum do not alter it.

The regulator is the last component in the circuit, it blocks the flow back to the tank causing pressure in the system. When vacuum is applied to the port, it overcomes one of the springs via the diaphragm and allows fuel to pass more easily back to the tank reducing pressure. It only has two values, with or without vacuum applied, high or low, if you will. If there was a mechanical switch instead of a vacuum diaphragm there still would only be two values, how hard you pushed the switch would have no effect.

When you apply vacuum, nothing changes until you get to the threshold when the vacuum overcomes the spring. Then the change is made and it remains that way until sufficient vacuum is released for the spring to relax.

 

Whew. Pretty redundant here. Sorry if I beat the shit out of this topic.