Fuel Rail Pressure's relationship to fuel economy.

[duramaximizer]

Is there a relationship between the Actual Fuel Rail pressure and the amount of fuel required to propel the truck down the road? I am debating this with myself as to whether I can prove that fuel rail pressure can be tuned up to run at a higher level and actually increase the fuel economy. I have sucessfully raised fuel rail pressure and decreased the required pulse width to idle esp. This in theory (kinda) would prove the be a sort of control for the experiment.

My problem is proof. I have to have a computation on what happens at the injector level when FRP is increased 10% or even 20% at idle. 30-35 etc etc. If my injector pulse width is decreased the same 10% or 20% did I even gain anything in terms of a decrease in the amount of fuel used. I know that as you increase pressure, flow is also increased. If anyone can help me out on the injector fuel flow data at different pressures I would appreciate it.

I also have to figure out how timing plays a part in the fuel rail pressure equation. I know it's the same as adding timing.. or so I was told. I have an older FRP-PW-Timing calc.

Part 2 is if frp even matters, or if it can be tuned better with timing to control cylinder pressures for the same economy. At a point you can't get blood from a turnup.

[bballer182]

In my tests so far i have found on my LBZ that the factory FRP is just about right on the money for MPG (in the cruising range) i did find also that raising FRP and advancing timing at idle did decrease fuel consumption quite considerably.

[duramaximizer]

Thanks for the reply. As pressure increases and as pulse width decreases, how do we know that the final fuel quantity also decreases?

I also found that decreasing the idle speed to 600 also decreased fuel consumtion a lot more and also idles smoother and with a nicer deeper rumble than before. ;)

[bballer182]

Thanks for the reply. As pressure increases and as pulse width decreases, how do we know that the final fuel quantity also decreases?

I also found that decreasing the idle speed to 600 also decreased fuel consumtion a lot more and also idles smoother and with a nicer deeper rumble than before. ;)

Yes i concur. I have lowered mine to 630-640. 600 i thought was just a little too low. (oil pressure)

and the answer to your question lies withing the equation

fuel flow (v) is equal to injection pulse width (t) times the square root of pressure (p).


V=t*sqrt(p)


a simple calc_pid will yield a plethora of valuable information.

[killerbee]

consider that there is a mechanical latency in the pulse width measurement. 800us is not double the fuel of 400us. The latency measurement appears to shorten with increasing pressure, as if this problem wasn't complicated enough. :)

FWIW, I typically have not increased pressure, in fact, on some platforms I lower pressure at lower loads. So I use a wider range of pressure, spread across a range of loads. I do this with a timing adjustment that allows for greater economy, and lower noise.

Your post is a good discussion starter

[LBZoom]

Yes i concur. I have lowered mine to 630-640. 600 i thought was just a little too low. (oil pressure)

and the answer to your question lies withing the equation

fuel flow (v) is equal to injection pulse width (t) times the square root of pressure (p).


V=t*sqrt(p)


a simple calc_pid will yield a plethora of valuable information.

Is there a way to make that calc_pid? I would like to be able to monitor that. I have been curious on this very topic as well.

In Michael's post.... would lowering pressure not decrease power and economy? Would you have to couple that change with an adjustment in pulse for proper atomization?

Also, when increasing pressure but not decreasing pulse, I have found that the truck appears to override my pressure adjustments for the most part and utilize values close to stock, am I seeing that right? So in order to actually get a noticeable change in pressure when cruising I have no choice but to shorten pulse time? I'm still experimenting but hopefully this will speed up my learning curve. Thanks :cheers:

[killerbee]

Would you have to couple that change with an adjustment in pulse for proper atomization?



You are asking all the right questions and the answers are not so intuitive. :)

Remember firstly and lastly, for a given set of conditions, and total drag, the dmax is going to find the power to exactly offset the drag for steady state conditions. If you lower pressure, and do nothing else, then the truck will have to have more pulse. Or you will have to command more pressure with pedal. Keep digging into these concepts. It is a great (albeit painful) learning curve.

[duramaximizer]

It might be such that you have to have a pilot and a post for idling and no main. Get the smallest shot possible to light a charge and then a post to give it enough downward momentium to keep to running smooth.

[LBZoom]

You are asking all the right questions and the answers are not so intuitive. :)

Remember firstly and lastly, for a given set of conditions, and total drag, the dmax is going to find the power to exactly offset the drag for steady state conditions. If you lower pressure, and do nothing else, then the truck will have to have more pulse. Or you will have to command more pressure with pedal. Keep digging into these concepts. It is a great (albeit painful) learning curve.

Thanks, I'm never finished, only finished for a few days at a time haha. :cheers:

[DmaxHawk]

consider that there is a mechanical latency in the pulse width measurement. 800us is not double the fuel of 400us. The latency measurement appears to shorten with increasing pressure, as if this problem wasn't complicated enough. :)

FWIW, I typically have not increased pressure, in fact, on some platforms I lower pressure at lower loads. So I use a wider range of pressure, spread across a range of loads. I do this with a timing adjustment that allows for greater economy, and lower noise.

Your post is a good discussion starter

Micheal, I was under the impression that increasing pressure would better atomize fuel and therefore create a better burn. Or is it not much of a difference?

[killerbee]

In my personal experience, contrasting conventional wisdom, I found lower pressure for lower load to be the way to go, with do real downside in efficiency.

It realistically comes down to keeping pulsewidth to a reasonable CADeg. At higher loads, you would really need the pressure to lower CADeg, which can get as high as 30-40 degrees is some of the wilder power requirements.

[LBZoom]

I'm going to experiment with my fuel pressure some more in the next couple weeks, started a new job with about 30 miles of interstate driving every day so I should get some good testing in. I'm going to attempt to lower my pressure slightly, and I'll have to open my pulse some to still allow the fuel in I suppose... correct?

Or should I maybe lower my crusing pulse time and map the actual FRP vs. my current base FRP table and go that route? I guess only experimentation will tell.

[killerbee]

This is by no means the definitive technique, but I will say what worked for me on a long road, where I made plenty of learning mistakes before I settled on this. Find a pressure table that you like the feel of (easier said than done). Then create a pulse table that keeps around 800-900us as you climb the power ladder. As you approach your pressure cap, pulse should begin to grow past 1000us into the real power range. This was very important to my end product refinement, and this one process can take weeks or months if you are a perfectionist.

No load highway cruise uses 25-35HP. For me, I find no use for more than 60Mpa. That should give a good place to start from. Eventually you will draw your own conclusions, but trial and error is definately involved. Develop the buttmeter to tell you what you need, trust the force. :) Eventually, you will develop keen ideas of what changes the feel of things, and make correlations that the Dash will provide. I use dashB almost exclusively.

[DmaxHawk]

In my personal experience, contrasting conventional wisdom, I found lower pressure for lower load to be the way to go, with do real downside in efficiency.

It realistically comes down to keeping pulsewidth to a reasonable CADeg. At higher loads, you would really need the pressure to lower CADeg, which can get as high as 30-40 degrees is some of the wilder power requirements.

So lower pressure will lower CADegs? I'm a little confused

[killerbee]

No. As you probably know, :) lowering pressure, changing nothing else will increase CADeg.

I know some are wondering what CADeg is. It is the pulswidth measured as degrees of crank rotation.

[DmaxHawk]

No. As you probably know, :) lowering pressure, changing nothing else will increase CADeg.

I know some are wondering what CADeg is. It is the pulswidth measured as degrees of crank rotation.

Ok so lowering pressure will increase the pulse because it is still trying to get the same mm3s into the cylinder? The increase in pulse is compensation for lower pressure?
Is this right?

[killerbee]

Yes. More or less, a given HP requirement (or drag total) requires the same fuel volume.

The actual fuel volume is best defined by fuel pulse multiplied by the pressure square root. Don't get too hung up on "mm3" as illustrated in the tables. It would be best considered a unitless index.

[bballer182]

I'm going to experiment with my fuel pressure some more in the next couple weeks, started a new job with about 30 miles of interstate driving every day so I should get some good testing in. I'm going to attempt to lower my pressure slightly, and I'll have to open my pulse some to still allow the fuel in I suppose... correct?

Or should I maybe lower my crusing pulse time and map the actual FRP vs. my current base FRP table and go that route? I guess only experimentation will tell.

As posted by Michael earlier there is no need to alter the pulse width table when lowering or raising the pressure you are automatically increasing or decreasing the PW. you cant expect to run @ 65 mph and have a pressure of 60Mpa and a PW or 800 and then change the pressure to 70Mpa and have the same PW. you would be creating more power than is required to move the truck down the road @65. so just leave the PW alone.

[LBZoom]

I've already noticed a slight difference in my economy with the lower pressure.... Why exactly does lower pressure improve MPG's if the fuel volume essentially remains static? Just curious...
:cheers:

[bballer182]

you are playing with how the fuel burns. either short and quick with a fast pressure spike or a slow burn with a slow rolling pressure curve.

[LBZoom]

Okay I got it now. Thanks for the help guys! :cheers:

[killerbee]

The CP3 is a parasitic accessory.

[DmaxHawk]

The CP3 is a parasitic accessory.

What would be better?

[killerbee]

Nothing. Just pointing out that higher fuel pressure equals more parasite. When commanding more pressure without a commensurate power increase requirement, the impact (IMO) is more drag.

As Bballer said, there may be some burn effect differences that are not as easy to predict.

[duramaximizer]

I figured the CP3 is always making 30k internally, but the fuel pressure regulates output of that pressure to the rail. What side of the pumps is the FRP on low side or high side. My guess is low side, but I always thought it was on high side. :doh2:

[killerbee]

regardless of output, it doesn't work for free. :)

[bballer182]

Nothing. Just pointing out that higher fuel pressure equals more parasite. When commanding more pressure without a commensurate power increase requirement, the impact (IMO) is more drag.

As Bballer said, there may be some burn effect differences that are not as easy to predict.


Good point, i forgot to mention that.

[DmaxHawk]

Okay I got it now. Thanks for the help guys! :cheers:


How much did you lower it by?

[duramaximizer]

regardless of output, it doesn't work for free. :)

I just figured its hp scavenged was a given amount at a given RPM range not a variable hp amount at a variable pressure level. Can anyone confirm this on bench tester based on resistance?