Chamber pressure vs. rail pressure

[Nick]

I've been getting some inquiries about chamber pressure in relation to rail pressure. While I'm happy to do testing for curious minds, I'd like to get some more input from those who want to know.

Some things to consider: The highest chamber pressure recordings I've been able to muster have been near peak torque (~2500 RPM in my tunes). Most stock CP3's can keep up pretty well in this area (~23kpsi @ 3000uS). The difference with dual CP3's is most noticeable in 4th and 5th gear at high RPM
(3100 RPM+).

If dual CP3's are really responsible for bent rods, then my mickey mouse reasoning leads me to think the damage is being done over 3000RPM, and is the result of inertial stress compounded with healthy cylinder pressure. A combination of factors really. This would mean the high revving guys are not out of the woods.

Of course I have no facts yet, but I have some a neat tools and some clever friends.

What do you want to see?

Nick

[Fingers]

KISS.

Three runs. 23K, 20K and 17K PSI . Same timing for starters.

I think the damage happens on the way down through the revs as the engine loads up with a hot chamber. That would be in the 3-4 and 4-5 shift on the drag strip and when the pan hits on the pulling track. Obviously, the happens when the towing vehicle bogs down as it hit the bigger hill too.

[neversatisfied]

And a 26K please.

This will answer my PM. Thanks Nick.

[blksmok]

I've been getting some inquiries about chamber pressure in relation to rail pressure. While I'm happy to do testing for curious minds, I'd like to get some more input from those who want to know.

Some things to consider: The highest chamber pressure recordings I've been able to muster have been near peak torque (~2500 RPM in my tunes). Most stock CP3's can keep up pretty well in this area (~23kpsi @ 3000uS). The difference with dual CP3's is most noticeable in 4th and 5th gear at high RPM
(3100 RPM+).

If dual CP3's are really responsible for bent rods, then my mickey mouse reasoning leads me to think the damage is being done over 3000RPM, and is the result of inertial stress compounded with healthy cylinder pressure. A combination of factors really. This would mean the high revving guys are not out of the woods.

Of course I have no facts yet, but I have some a neat tools and some clever friends.

What do you want to see?

Nick

Maybe I'm thinking about this wrong, but I can't see the corelation between rail pressure and cylinder pressure.

I'll explain my thoughts and please tell me where I don't get it.:D Rail pressure is what it is so that the injector can deliver the requested fuel amount in the small alloted time it has to do so. Based on RPM (time) and commanded fuel (your foot), a certain amount of time is needed to deliver the requested fuel. In order to deliver the fuel in a smaller amount of time, you need more pressure. I can't see how any of this causes bent rods. Rods are bent due to the pressures being exerted on them (and possibly heat). The pressure is cylider pressure. The high rail pressure is just the necessary device to deliver the amount of fuel to give you the power that you want, which results in cylinder pressure. If you are saying that bent rods are caused by rail pressure, you could say bent rods are caused by throttle pressure.

Okay, where am I missing the point?:o

[neversatisfied]

Maybe I'm thinking about this wrong, but I can't see the corelation between rail pressure and cylinder pressure.

I'll explain my thoughts and please tell me where I don't get it.:D Rail pressure is what it is so that the injector can deliver the requested fuel amount in the small alloted time it has to do so. Based on RPM (time) and commanded fuel (your foot), a certain amount of time is needed to deliver the requested fuel. In order to deliver the fuel in a smaller amount of time, you need more pressure. I can't see how any of this causes bent rods. Rods are bent due to the pressures being exerted on them (and possibly heat). The pressure is cylider pressure. The high rail pressure is just the necessary device to deliver the amount of fuel to give you the power that you want, which results in cylinder pressure. If you are saying that bent rods are caused by rail pressure, you could say bent rods are caused by throttle pressure.

Okay, where am I missing the point?:o

Why have we seen more problems once dual cp3's have been added?
You said so yourself that high rail psi will result in increased chamber psi. Too high of chamber psi will result in bent rods. Dual cp3's can maintain rail psi or add psi if you tune for it whereas a single cp3 most would be happy to maintain 20k on a big tune. So the point is what is the difference in chamber psi at 17k, 20k, 23k, 26k or whatever rail psi. Do we need to decrease timing with higher rail psi in order to lower chamber psi? I think so but how much

[blksmok]

The rail pressure just allows you to deliver the amount of fuel you have commanded in the given time you have based on rpm and timing.

Rail pressure isn't going to increase cylinder pressure. Only the amount of fuel you get into the cylinder and when you put it in is going to increase cylinder pressure. The rail pressure is only a tool you use to get the mm3's of fuel into the cylinder within the give time constraint.

If you are seeing more problems once dual cp3's are installed, then it would suggest that prior to the duals, the desired amount of fuel (per the tune) was not being delivered. Once you get the fuel you have commanded in the timing you have specified, if it is bending rods, then one of those two (fuel amount or timing) is the problem, not the rail pressure.

I guess I'm saying, you could take a stock tune that keeps the rail pressure maxed all the time and nothing would bend.

The above is all JMO.

Nick and Jon are way smarter about this stuff than I am, so I'll go back to the spectators bench and watch. :D I'm just trying to make sense of this in my mind...

[blksmok]

I think with a single cp3 you weren't getting the true commanded amount of fuel. With duals, you are and it's too much fuel or too much timing in the given tune. I think you have to adjust one or both of those two while keeping the rail at a known value with the duals.
But the rail pressure isn't what is causing the high cylinder pressure... it's fuel amount or timing or a combination of the two... the rail pressure is just making sure you get what you said you want.

[Nick]

I think you guys have a good idea of what's going on in your heads (pressure wise at least;)).

The main considerations with chamber pressure as it relates to timing is this: How much fuel can we get into the cylinder BTDC.

Arbitrary # warning

20* of timing at 2500 RPM gives us a defined 'Time Window', injector nozzle sized is also fixed, so the only variable is pressure. As pressure on the rail increases, quantity of fuel injected in the cylinder increases with the square root of the pressure increase. So if pressure increases by 20%, flow will increase by the square root of 1.2 (9.5%).

As quantity of fuel in the chamber BTDC increases, so will peak chamber pressure.



Nick

[blksmok]

Right, but it's still the amount of fuel and timing that directly affects the chamber pressure. Rail pressure is just the tool you use to get your requested fuel amount into the cylinder in the given time window right?

[Fingers]

The faster you get fuel into the chamber, the faster the Chamber pressure rise. Agreed?

If we dial in the timing so Peak Pressure happens at the same crank angle for two given Rail Pressures, the Peak Pressure will be higher with a higher Rail Pressure. Why? Because more fuel will be injected and lit prior to the Peak Pressure point with a higher Rail Pressure.

[Nick]

Right, but it's still the amount of fuel and timing that directly affects the chamber pressure. Rail pressure is just the tool you use to get your requested fuel amount into the cylinder in the given time window right?

Amount has 4 factors...

1.Time (timing and pulse duration)
2.Orifice size (nozzle)
3.Fuel viscosity (fuel temp)
4.Pressure differential between the rail and chamber

Increasing the pressure would increase the amount of fuel in similar to increasing injector size, raising timing, or lowering viscosity.

:blahblah:...right... :lol:

[blksmok]

The faster you get fuel into the chamber, the faster the Chamber pressure rise. Agreed?

If we dial in the timing so Peak Pressure happens at the same crank angle for two given Rail Pressures, the Peak Pressure will be higher with a higher Rail Pressure. Why? Because more fuel will be injected and lit prior to the Peak Pressure point with a higher Rail Pressure.

Amount has 4 factors...

1.Time (timing and pulse duration)
2.Orifice size (nozzle)
3.Fuel viscosity (fuel temp)
4.Pressure differential between the rail and chamber

Increasing the pressure would increase the amount of fuel in similar to increasing injector size, raising timing, or lowering viscosity.

:blahblah:...right... :lol:

Cool. I'm with you. Thanks for helping me reach the handle on this one.

[Nick]

http://i105.photobucket.com/albums/m201/vortecfcar/16K_RailPressureTest.jpg

http://i105.photobucket.com/albums/m201/vortecfcar/CPT_19KRailPressure.jpg

http://i105.photobucket.com/albums/m201/vortecfcar/CPT_22KRailPressure.jpg

http://i105.photobucket.com/albums/m201/vortecfcar/CPT_245KRailPressure.jpg

[blksmok]

The graphs are a little tough to read but it looks like cylinder pressure came up a couple hundred psi from 18K rail pressure to 24K rail pressure?

[Fingers]

Looks that way. Wish I could read the peak pressure timing.

[swatkins]

What I would like to know is how the pressure in the rail gets so high. Fluids don't compress to good :)

[Fingers]

But the things holding them in expand elastically.

[swatkins]

I don't know a thing about the fuel system except it has a pump and injectors . Can you tell? :)

[Fingers]

Then you have been lucky enough not to pop the FPRV.

[Nick]

Sorry about the bad resolution, I was trying to give you guys something to talk about before I had to leave for dinner.

Didn't work out so well.

Give me a half hour,

Nick

[Nick]

The bold details correspond to the graphs beneath them. See the actual rail pressures in the scan tool screen shots (third number down on the left of the scan tool screen shot). Sorry for the poor resolution, I don't think I can get it any better without separating the scan shots and chamber plots.

2392PSI/2533 RPM/8.1*ATDC/59.1

http://i105.photobucket.com/albums/m201/vortecfcar/16k_good.jpg

2450PSI/2459PSI/8.8*ATDC Peak/62.7

http://i105.photobucket.com/albums/m201/vortecfcar/19k_good.jpg

2613PSI/2597RPM/7.5*ATDC Peak/59.9

http://i105.photobucket.com/albums/m201/vortecfcar/22K_good.jpg



2680psi/2531RPM/7.5*ATDC PEAK/63.3 Peak

http://i105.photobucket.com/albums/m201/vortecfcar/24K_Good.jpg


Nothing too conclusive, need a lot more data to get some decent statistical info. I think there's a trend though.

Hmmm...

Nick

[Fingers]

The only thing bothering me is the peak at ~8* ATDC. :) I have to get that torque factor thing figured out better I guess.

Interesting first cut on the affects of Rail Pressure on Peak Pressure. Not conclusive, but I detect a trend too. Now, which make more power?

[swatkins]

Is there anything I could do to help you with your pictures Nick? Host them on the site at full scale maybe?

My eyes are getting worn and I am having to hold my monitor at arms length now :)

[gmduramax]

:idea:
So if the 2nd CP3 is letting it hold rail pressure. Its increasing the cylinder pressure. I think I got that. So if you take out some rail pressure and some timing the d max can make more than 1200ftlbs on the stock rods
Is that what your getting at?

[Fingers]

It is a balancing act.

Graphically:

Torque is roughly the area under the gray curve in the plots.

The gray line is the torque applied to the crank at each Crank Degree from the Chamber Pressure represented by the black line.

By shifting the peak with timing and/or Rail Pressure, you might be able to get as much if not more torque with less Peak Pressure.

That is, in fact, what all this playing around with Chamber Pressures is about IMO. Optimizing the timing and other factors to get the most torque with the least Cylinder Pressure.

[Nick]

Is there anything I could do to help you with your pictures Nick? Host them on the site at full scale maybe?

My eyes are getting worn and I am having to hold my monitor at arms length now :)

I think that may be helpful :), I tried a few different ways wasn't able to get them to show any larger.

The only thing bothering me is the peak at ~8* ATDC. :) I have to get that torque factor thing figured out better I guess.

Interesting first cut on the affects of Rail Pressure on Peak Pressure. Not conclusive, but I detect a trend too. Now, which make more power?

I don't know why peak pressure is still so early on the graphs too. My tests don't seem to jive well with yours, or at least my truck will not tolerate anywhere near as much timing per the plots. I need to get on that superflow to back up my SOTP.

[Fingers]

Nearly all my plots are with 50% over injectors. That, at the minimum, will change the shape of the plot. My Plots also span a few generations of Sensor setups. Still, I don't know why peak so close to TDC is giving you the best power. Then again, maybe you're right and I am wrong. ;)

So many questions.

[neversatisfied]

I'll have to get on another computer as this stupid one wont let me view the pictures.:mad: Unless you guys decide to host them here.
Thanks for testing this, Nick.

[swatkins]

Nick send me anything you would like posted and I will take care of it for you :)

[TheBac]

Hmm. Let me see if I understand you guys. I'm probably way out in leftfield.

Figure "max safe cyl pressure" as a number we want to stop at, no matter the RPM.

MSCP = fuel amount (regulated by CP3, not injector size) x timing. Correct?

As timing increases and fuel increases (like with dual CP3s) you eventually unbalance the equation where MSCP becomes higher than the predetermined number, which then bends the rods, right?

So as timing increases, the CP3 losing pressure has actually been keeping MSCP at a reasonable level, thereby keeping the equation in balance.

Is there a way to lower timing as RPM increases when using dual CP3s and still get the power numbers you want?

[Ridge_Runner_Diesel]

PW, Fuel Pressure, Timing, and Boost effect the CP.

;)

[TheBac]

Just remember, I dont know jack about EFI tables.....:lol:

Wouldnt the amount of fuel being injected at whatever PW be contingent on fuel pressure availablilty and injection timing?


OK, I forgot about boost. I was trying to make it simple and deal with fuel. I dont think I've heard of anyone breaking a rod from too much boost. The headgaskets would go first, wouldnt they. I dont think we've hit a point of boost pressure where the air is turning into a "liquid" mass yet, have we?

Just seems that when dual CP3s came out is when rods started to bend. That leads me to think the amount of fuel related to timing is the culprit.

[Nick]

Is there a way to lower timing as RPM increases when using dual CP3s and still get the power numbers you want?

Yes, use bigger injectors :D


But seriously, that's what we're trying to nail down....with data.


Nick

[Fingers]

As I see it, once we understand how the pressures come up for a given set of conditions, we can tune to the other side with confidence. Additionally, we can also use what we know to make lean and mean tunes.

So much more data to gather....

[neversatisfied]

Yes, use bigger injectors :D


But seriously, that's what we're trying to nail down....with data.


Nick

I've got the injectors, now I just need the perfect tune.
I wish I had a chamber monitor

[Fingers]

That can be arranged. :)

[neversatisfied]

Is that so?:cool:

[TheBac]

Where'd everyone go? Did I kill the thread?

[Fingers]

Where'd everyone go? Did I kill the thread?

No, you didn't. I personally don't have a good answer for you. I have some theories, but that is about it right now.

[TheBac]

I was thinking about this yesterday...

At what pressure does air become uncompressable?

I'm thinking that if the engine's compression ratio is too high, the fuel/air mix becomes incompressible and "hydro locks".

Is this possible?

[minisub]

I was thinking about this yesterday...

At what pressure does air become uncompressable?

I'm thinking that if the engine's compression ratio is too high, the fuel/air mix becomes incompressible and "hydro locks".

Is this possible?

Tom,

PV=NRT......):h

The point air becomes uncompressable is when it becomes a liquid.

Basically can't happen in an internal comustion engine as "T" is quite high. ;) IIRC the point of liquification for O2 is something like -300*F at atmospheric pressure....Not sure where it is for air as that is mostly nitrogen, but it gives you an idea of what it takes to liquefy a gas...

[TheBac]

Um....what is PV=NRT?

I figured air becomes a liquid at very low temps....I used to watch the Apollo missions! :lol:

I just wondered if compressing air to a certain point does the same thing.

[blksmok]

Anything new with the Chamber pressure monitors?

[Fingers]

Nick has the latest Rig now with the latest software. Always something new to add to the software, though the Rig he has is near release configuration. I have one more small set of electronic adjustments to make to the hardware in an attempt to have a more faithful graphing of the falling side of the pressure spike. IFF I get that worked out, then we can do meaningful estimations of torque production from the data.

If anyone is interested, I have enough parts to make a dozen or so Rigs on hand.

[blksmok]

If anyone is interested, I have enough parts to make a dozen or so Rigs on hand.

Are you looking for test monkeys, and if so, what would the test monkey need to be able to do?

[Fingers]

Looking for buyers. :)

[blksmok]

Ballpark on cost?

[minisub]

Nick has the latest Rig now with the latest software. Always something new to add to the software, though the Rig he has is near release configuration. I have one more small set of electronic adjustments to make to the hardware in an attempt to have a more faithful graphing of the falling side of the pressure spike. IFF I get that worked out, then we can do meaningful estimations of torque production from the data.

If anyone is interested, I have enough parts to make a dozen or so Rigs on hand.

Did you get the LBZ voltage issue worked out?

[Fingers]

Did you get the LBZ voltage issue worked out?

Yes, but I need to test it. I am going to use a different probe configuration instead of changing the module electronics. 5x or 10x probe depending on what the voltage actually is. Probably 10x.

[minisub]

Sounds like a testing road trip is in order....;)

Deep fried pork roast for Labor Day?....):h

[Fingers]

Ballpark on cost?

$1,300 is my material cost. I'm thinking I can do $1,500

That includes:

Monitor
One Pressure sensor and adapter (LLY-LB7 or LBZ)
Cables (sensor cable, two insulation piercing probes)
One LBZ injection signal adapter.
Software

Software updates free as long as they are available via download. Additional sensors available.

[keith2500hd]

the dispersion cloud, from injector nozzles will acquire a larger area and ignite over larger surface area of piston. visual inspection should show more even coverage on piston crown, fanned out, maybe cloverleaf pattern in center of bowl. that should show high peak cylinder pressure. wonder what single short pilot inj. would show, saw oldbosch style pump cams that had lead in on ramp to distribute injection pulse. just a thought.

[HotRodSS]

I was thinking about this yesterday...

At what pressure does air become uncompressable?

I'm thinking that if the engine's compression ratio is too high, the fuel/air mix becomes incompressible and "hydro locks".

Is this possible?

Keep in mind that diesel pulling tractors run around 200 lbs of boost, inject copius amounts of water, and large amounts of fuel in their engines. Wonder what their cylinder pressures peak at? :)

[Fingers]

Less than you might think.

No small part of the fuel is used to feed the turbo, and then the meat of the burn is ATDC. If you were to plot it, you would see a nice chubby spike instead of the pointy ones we have been showing. You can burn huge amounts of fuel and not have huge pressure spikes.

[HotRodSS]

If they are injecting most of their fuel ATDC how are they getting the horsepower numbers that they do? Wouldn't that be severely retarding their timing?

[Fingers]

Perfect burn has all of it ATDC. Timing advance is a compromise to handle delays in ignition and to have as much pressure available through the stroke since the fuel burns at a finite rate.

Look at what Nick just did. There is more fuel ATDC than before. At that RPM, the ignition delay is on the line of 10 crank angle degrees.

[HotRodSS]

Sorry, I misread the other post. I was thinking you said that they inject most of their fuel ATDC, but after looking at it again I can see you plainly said most of the fuel burn is ATDC which makes perfect sense. My mistake. :)

[Diesel power]

There is some very good info here, any updates?

[Nick]

What would you like to know?:)

[Diesel power]

What would you like to know?:)

on avg how much cylinder psi increase are you seeing with the addition of nitrous?

Performance engines any different?

[CDR]

WOW absolutely mind blowing post.... Here is my .2 when you command the CP3 to go to max pressure the engine will idle differently and sounds different. I strongly believe its due to the fact that since the pressure is much higher there is way more fuel going in then should be.... So in conclusion rail pressure make a huge difference...