compound numbers

[CSM diesel]

What?

You take the exact same scenario of shaft speed and PR that you tested before. Now only change two things, the pressure and temperature of the room in which the test is performed. Double the ambient temperature and double the ambient pressure and re-record your mass flow reading.

Do you suppose that the mass flow will be the exact same when the exact same shaft speed and PR are tested? As you stated that mass was constant and volume depended on pressure and temp?

Of course not. The reality is that mass flow would be around half what you recorded the first time, whereas the volume flow would still match what you had noted on test one.

What?

That is bass-akwards.

Mass flow is unchanging, measured in lbs/min. Mass of working fluid per unit time.

Volume flow is dependent, measured in CFM, m^3/s, etc. The measurement of which takes fluid density, friction, pressure, and flow regimes into account. Compressible flow makes this a little funny to deal with.

So help me out here...


However, as I have stated more than once in this thread, if mass is the correct unit of measure for compressor wheel flow performance, then why does the map become completely useless as soon as you begin dealing with a compressor working under non STP conditions, such as is the case with any second stage compressor?

It is useless at these conditions. The conditions seen by the secondary pump are attributed to by the primary pump, which flows above and beyond what the second stage is capable of. So you are in a sense trying to plot something that is not possible by that particular pump at standard temp and PRESSURE.

Or, another way of reasoning out this issue as I see it:

Having noted the max flow for each respective wheel as shown on their compressor maps, while my first stage compressor is capable of moving ~140lbs/min, my second stage is only capable of flowing ~72lbs/min max.

This should raise concern, I would hope. Because either one of two subsequent issues must be addressed in that case.

1. Is my second stage being operated at a point around two times the maximum value shown on it's compressor map at all times when I am running WOT in my truck? If so, what in God's name is the shaft speed at that time? Since ~72lbs/min occurs at a shaft speed of 110,000rpm, if I was fully utilizing my first stage compressor, and moving roughly twice that mass through the system, am I to assume that the second stage compressor is then approaching 200,000rpm? I think we can all rule that out right now.

2. If I pull in ~140lbs/min on the first stage, and the second stage is only capable of moving ~72lbs/min at max flow (so says the compressor map) then is it possible that close to 70lbs/min of atmosphere is consumed by the system somewhere between the first stage compressor inlet and the second stage compressor inlet? I don't think we need to delve into E = MC^2 to see the magnitude of nuclear reactor that would entail... So no, the mass is not disappearing between stages, so we still cannot explain how the second stage is moving roughly twice the max flow listed on it's map.

The flow rates are not additive. For this to happen, the pumps would have to be hooked up in parallel, both working ends sharing the same discharge.

The big pump is pushing through the little one, and that is why we waste-gate around the little pump when it has done it part.

This is only made more clear in the fact that the entire mess above, that is unexplainable (at least by me) in terms of mass flow maximum for a second stage wheel, is simultaneously Perfectly explained if the wheel is instead viewed in terms of volume flow. All of a sudden compressor flows make perfect sense, and plotting the operational point for the second stage goes from a point 8 inches to the right of the paper/screen to a point coincidentally right where you expected one to be. And coincidentally, right to a point where outlet temps and shaft speeds are right where they should have been all along.

You are CORRECTING the flow of the pump using the pressure ratios supplied by the first stage.

You answered your own question to weather or not Mass flow is constant and volume flow is dependent with the statement above.

:ft:

 

[Charles]

What?

That is bass-akwards.

Mass flow is unchanging, measured in lbs/min. Mass of working fluid per unit time.

Mass flow throughout the system is unchanging, yes, absolutely. However, mass flow through any given compressor for any given shaft speed/PR is VERY MUCH SO changing based on ambient temperature and pressure. If you are an educated engineer you cannot argue this point. Now that we have that out of the way, go ahead and note that while mass flow across the compressor wheel will indeed be highly variable relative to any variances in ambient conditions, volume flow WILL NOT. For any shaft speed/PR the volume flow across that wheel will be consistent.

Volume flow is dependent, measured in CFM, m^3/s, etc. The measurement of which takes fluid density, friction, pressure, and flow regimes into account. Compressible flow makes this a little funny to deal with.

Right with you.

It is useless at these conditions. The conditions seen by the secondary pump are attributed to by the primary pump, which flows above and beyond what the second stage is capable of. So you are in a sense trying to plot something that is not possible by that particular pump at standard temp and PRESSURE.

And....

The reason for this? I have clearly explained above, that even though the second stage compressor is indeed moving twice the maximum value listed on it's map, rendering that map useless as we agree on, if you had merely converted the map values to volume flow to begin with, you would see that while mass flow is out the window, the volume flow is directly inline with the map (if you have chosen a proper turbocharger in the first place). This is because the wheel does not care about mass flow. It is merely a physical object, with a specific diameter, fin count and pitch, so on and so forth. For any given shaft speed and any given pressure ratio across that wheel it will move ____ Cubic Feet of media per time. Period.

The flow rates are not additive. For this to happen, the pumps would have to be hooked up in parallel, both working ends sharing the same discharge.

The big pump is pushing through the little one, and that is why we waste-gate around the little pump when it has done it part.

We are not even remotely on the same page, and I'll assume fault for that on this go round. I haven't the foggiest idea where you assumed that, being that the paragraph this was in response to was addressing the fact that mass flow from the first stage must be traveling through the second, else we're going nuclear. And a more important point is this, we are talking in terms of compressors here. There is no flow path around the second stage compressor my friend. Every last bit of incoming air from the first stage will pass through the second stage on it's way to the engine. The wastegates serve to bypass the turbine sections of each respective charger. This has no relevance to this discussion.

You are CORRECTING the flow of the pump using the pressure ratios supplied by the first stage.

You answered your own question to weather or not Mass flow is constant and volume flow is dependent with the statement above.

:ft:

Why you would choose to try and correct for everything all the time just to chase some magic mass flow unit instead of just using volume flow in the first place is beyond me.

For example, calculate how many lbs/min a 5.9L engine is consuming at 3000rpm with a VE of 80%. Oh wait, you can't, because I haven't listed ambient conditions. Okay, so then I'll give you that and say, 70 degrees and 14.7psia. Oh wait, you still can't do anything if this is a turbocharged engine because you need to know manifold pressure and compressor efficiency. Two chargers, oh boy, twice the calculation.

Yet....

We knew all along that the damned thing was displacing 5.9 Liters every two revs at 100% VE. So at 3k rpm we've got 2.36 Liters per minute traveling through that engine at 80%VE.

Done, and done. Excuse me if I think that trying to dick around in mass flow is a bit absurd.




It is clear that you and I are not on the same page in our own understandings of compound turbocharging. I'll see how you react to the above and then proceed from there.

 

[CSM diesel]

Why you would choose to try and correct for everything all the time just to chase some magic mass flow unit instead of just using volume flow in the first place is beyond me.


Done, and done. Excuse me if I think that trying to dick around in mass flow is a bit absurd.

It is clear that you and I are not on the same page in our own understandings of compound turbocharging. I'll see how you react to the above and then proceed from there.

I believe we are still on the same page here, just not using the same thought process.

Here is my $.02 on how I understand compound chargers.

I pick the secondary charger based on the corrected flow from the engine and using standard PR's. Then decide when the secondary charger has "done its part" and waste-gate around it (turbine side obviously,) effectively rendering that charger to its "sweet spot" on the map (using mass flow.) I know this does not happen as the primary charger comes up to speed, but this is how I do it.

I then pick the primary charger based on standard PR's using smaller flow numbers as the secondary charger is taking some of the fun away from it. Using mass flow of course.

I believe that once the primary charger is up to speed, that the secondary charger is doing what it can to impart velocity to the medium, it is just doing it very poorly with the PR's it is seeing, and that is why it is "off the map."

Thoughts and questions?:st:

 

[B18B1LS1]

I've been wondering something and this seems like a good place to throw it in. Most likely silly compared to these last few posts that are WAY above me. Would it ever be beneficial to wastegate some of the air around the 2nd stage compressor when you are feeding it too much from the 1st stage? I guess a proper set of turbos would never have this issue, but I'm guessing there are a ton of mismatched combos out there. Probably mine.

 

[Charles]

I believe we are still on the same page here, just not using the same thought process.

Here is my $.02 on how I understand compound chargers.

I pick the secondary charger based on the corrected flow from the engine and using standard PR's. Then decide when the secondary charger has "done its part" and waste-gate around it (turbine side obviously,) effectively rendering that charger to its "sweet spot" on the map (using mass flow.) I know this does not happen as the primary charger comes up to speed, but this is how I do it.

I then pick the primary charger based on standard PR's using smaller flow numbers as the secondary charger is taking some of the fun away from it. Using mass flow of course.

I believe that once the primary charger is up to speed, that the secondary charger is doing what it can to impart velocity to the medium, it is just doing it very poorly with the PR's it is seeing, and that is why it is "off the map."

Thoughts and questions?:st:

I don't believe the second stage is off the map at all. And furthermore, I believe if you were to measure inlet temp/press and compare to outlet temp/press and shaft speed you would very quickly see that you are very much on the map. (If the charger is even remotely in the appropriate range on size anyway). Second stage intake air outlet temps do not agree with compressor maps labeled in terms of mass flow.

This is, again, because IMO, the wheel doesn't care about mass flow, it just moves ___ cubic feet of air per rev at ___ shaft rpm and ___PR.

If you take a stab at some calculations, you will see that while mass flow is obviously constant at all points in the system, volume flow is not. And in fact, volume flow will be less at the second stage by an amount roughly proportionate to the density ratio between atmosphere and the second stage inlet. For example, you might have 1500CFM entering the first stage, and only 800CFM leaving it and subsequently entering the second stage. As such, the much smaller second stage is right at home moving the 800CFM, even though those 800 cubic feet per minute might weigh twice as much as they did when that compressor was originally tested by the manufacturer on their flowbench and corrected to STP, yielding the very compressor flow map that would argue that the second stage was doing the impossible.

I believe this is due to the wheel being improperly labeled in terms of mass flow, when it should have been labeled in terms of volume flow in the first place. As many are I might add.

 

[Charles]

I've been wondering something and this seems like a good place to throw it in. Most likely silly compared to these last few posts that are WAY above me. Would it ever be beneficial to wastegate some of the air around the 2nd stage compressor when you are feeding it too much from the 1st stage? I guess a proper set of turbos would never have this issue, but I'm guessing there are a ton of mismatched combos out there. Probably mine.

I believe one concept will ease your mind.

At all times, remember this. No matter how much air you compress into an engine's intake manifold, that engine will never flow any more CFM than it has displacement and rpm for. The internal displacement is what it is. For a 5.9L engine there is quite literally only 2.95 liters of room in that engine every rev at 100% VE.

To put it in simpler light, the second stage can't get "in the way" any more than the engine itself can. Always remember, that at the end of the day, the engine is only actually moving air in the 300ish CFM range for most of us. And that the second stage doesn't care what the first stage is doing. To the second stage the first stage is just atmospheric conditions. For all it knows you're just driving around a few miles below sea level. Same goes for the engine. For all it cares you're driving around near the earth's core.

 

[B18B1LS1]

Nice! A post I can understand! :Cheer:

 

[Diesel Freak]

the secondary charger is almost never off it's map when run in a sequential application. All you need to do is density compensate to plot out where the wheel is running.

 

[Charles]

the secondary charger is almost never off it's map when run in a sequential application. All you need to do is density compensate to plot out where the wheel is running.

How so? I'm not disputing you, I'm wondering the best way to go about such a thing.



And secondly, why keep fighting to use mass flow when volume flow seems so obvious?



Lastly, hopefully you can see this as a legitimate question (because it IS), but did you mean Sequential, or did you mean compound? Because when people choose to use my least favorite terms of "Primary" and "Secondary" we now have yet another case for confusion.

For instance, while in a compound setup "Primary" (most oftenly...) refers to the first stage compressor. Although obviously even this is still somewhat subject to each individual. Whereas in a sequential setup, primary would actually refer to the smaller charger that the vehicle drove around on primarily, meaning that in that case "Secondary" would actually be the large turbo.

So did you mean that, or were you referring to the second stage compressor in a compound setup when you said "secondary"?

Again, I DON'T bring this up as if I'm picking on semantics like a dick. I bring it up because it becomes a drastically different point depending on how your terms are interpreted.

 

[Charles]

Think Super Troopers...

Thorny:
Are you okay?

College Boy 2:
Yeah, sure.

Thorny:
Yes sir?

College Boy 2:
Yes sir.

Thorny:
Now, did you say, "yes sir."?

Rabbit:
I think he said "yeah, sure."

College Boy 1:
What'd you say man?

College Boy 2:
Well I said, "yeah, sure", but what... literally what I said was "yeah, sure....... sir."

Thorny:
So you are okay then?

College Boy 2:
Yes sir.

 

[Diesel Freak]

How so? I'm not disputing you, I'm wondering the best way to go about such a thing.



And secondly, why keep fighting to use mass flow when volume flow seems so obvious?



Lastly, hopefully you can see this as a legitimate question (because it IS), but did you mean Sequential, or did you mean compound? Because when people choose to use my least favorite terms of "Primary" and "Secondary" we now have yet another case for confusion.

For instance, while in a compound setup "Primary" (most oftenly...) refers to the first stage compressor. Although obviously even this is still somewhat subject to each individual. Whereas in a sequential setup, primary would actually refer to the smaller charger that the vehicle drove around on primarily, meaning that in that case "Secondary" would actually be the large turbo.

So did you mean that, or were you referring to the second stage compressor in a compound setup when you said "secondary"?

Again, I DON'T bring this up as if I'm picking on semantics like a dick. I bring it up because it becomes a drastically different point depending on how your terms are interpreted.

LMAO, this whole thread reads like a lesson in semantics. Twins, Sequential, Compound....all mean the same thing in the CTD world. Actually you are over complicating it (yes, Cliff really did just type that). The whole thing reminds me of one of my favorite quotes.

T'is best to be relatively close and understandable than to be academically correct and unintelligible" -Sir Oliver Lyle

Mass flow, volume flow...it really dont matter....if you know what you are doing...you can go back and forth between the two and still be correct.

The only difference is that with Volume Flow, you have to stipulate the temperature and pressure of the gas. With Mass Flow, you don't have to attach the extra data.

 

[Charles]

LMAO, this whole thread reads like a lesson in semantics. Twins, Sequential, Compound....all mean the same thing in the CTD world. Actually you are over complicating it (yes, Cliff really did just type that). The whole thing reminds me of one of my favorite quotes.




Mass flow, volume flow...it really dont matter....if you know what you are doing...you can go back and forth between the two and still be correct.

The only difference is that with Volume Flow, you have to stipulate the temperature and pressure of the gas. With Mass Flow, you don't have to attach the extra data.

I'll try and be collected while I wipe your spit from my face...

The only difference is that with Volume Flow, you have to stipulate the temperature and pressure of the gas. With Mass Flow, you don't have to attach the extra data.

So.... tell me smartass.... :hehe:

Why is it then.... that the MASS FLOW compressor maps you see everywhere are the ones CORRECTED TO STP!!! And if you run the compressor at the EXACT SAME shaft speed and the EXACT SAME pressure ratio you get a DIFFERENT mass flow if the temp and pressure vary at ALL!!! It's because MASS FLOW requires the G'damn temp and press data due to the fact that air density changes wildly with any changes to temp and press DA.

Yet volume flow maps are NOT CORRECTED, nor do they NEED TO BE. It is quite literally ____CFM per any given shaft speed/pressure ratio combination. Pull head from ass.... then and only then respond por favor.

When my first stage compressor is moving over 120lbs/min how does my second stage handle that when the compressor map only reads to 72lbs/min at choke?

If it doesn't matter which you choose, then why don't you explain why mass flow compressor maps make the second stage look like an enormous roadblock?

Explain how to size two chargers based on mass flow. Any example you wish to give.

I now expect your smart ass to show us all how to size each stage for any given engine you choose, at any given rpm you choose, and any given manifold and interstage pressure you choose based on mass flow. And don't even THINK about using volume flow in your example.

Ready..... Set.... choke.

 

[UNBROKEN]

Charles....at least TRY to make a post when someone disagrees with you where you don't resort to the same childish crap that got you removed elsewhere.
If you seriously cannot have a conversation without it degrading to 2nd grade level insults...then maybe you really do need to stay off these forums.

I will NOT deal with your crap here like I did elsewhere.

Consider this your friendly reminder...ruler across the hand...whatever. Just don't start acting like an ass.....again. You've been doing so well......

 

[Charles]

Charles....at least TRY to make a post when someone disagrees with you where you don't resort to the same childish crap that got you removed elsewhere.
If you seriously cannot have a conversation without it degrading to 2nd grade level insults...then maybe you really do need to stay off these forums.

I will NOT deal with your crap here like I did elsewhere.

Consider this your friendly reminder...ruler across the hand...whatever. Just don't start acting like an ass.....again. You've been doing so well......

The dude takes this entire thread-worth of information, spits on it, laughs at it, explicitly STATES that we are over-complicating something apparently simple "if you know what you're doing" (when the purpose of the thread was to discuss PRECISELY what was being discussed) and finishes it off by labeling me unintelligible.

Let me break it down farther...

Here is my response to him before he came back with that crap:

How so? I'm not disputing you, I'm wondering the best way to go about such a thing.



And secondly, why keep fighting to use mass flow when volume flow seems so obvious?



Lastly, hopefully you can see this as a legitimate question (because it IS), but did you mean Sequential, or did you mean compound? Because when people choose to use my least favorite terms of "Primary" and "Secondary" we now have yet another case for confusion.

For instance, while in a compound setup "Primary" (most oftenly...) refers to the first stage compressor. Although obviously even this is still somewhat subject to each individual. Whereas in a sequential setup, primary would actually refer to the smaller charger that the vehicle drove around on primarily, meaning that in that case "Secondary" would actually be the large turbo.

So did you mean that, or were you referring to the second stage compressor in a compound setup when you said "secondary"?

Again, I DON'T bring this up as if I'm picking on semantics like a dick. I bring it up because it becomes a drastically different point depending on how your terms are interpreted.

HOW do you make it any more G'damn clear that you are NOT interested in stepping on toes nor having a debate on semantics for the sake of semantics? Some people need to under****ingstand that you can't have a discussion until you're talking the same language. And that it IS a big deal. Else STFU and stay out of the debate if you don't think the debate is important enough for everyone to at least be talking about the same things when they write the same words.



Secondly...

There were opposing viewpoints FAR earlier in the thread where you can see that none of this occurred. Why you might ask? Because the opposing ideas were presented by INTELLIGENT people, willing to have a real discussion on a topic. Not by someone who has contributed zero worthwhile information, or explanation, yet presented themselves as superior to the entire discussion, downplaying it to the hilt. And just so you know, I was actually LOOKING FORWARD to a response from "CSM Diesel" in hopes that he COULD help me understand why mass flow is used so often.


If you think I got pissed because he disagreed you would be dead wrong. First of all, he didn't even disagree. That should sufficiently crush your assumption that I was frustrated because he disagreed. G'damn that is stupid.

I was frustrated because someone, who based on the miniscule amount of text provided, seems to have only the most generalized conception of what is being discussed, but that went and downplayed the entire thing.

Statements like this with zero backing are unacceptable.

Mass flow, volume flow...it really dont matter....if you know what you are doing...you can go back and forth between the two and still be correct.

Wow....

Now I want an example. And yes.... the SECOND time, after being called unintelligible, I asked for his wonderfully concise, YET CORRECT explanation with a bit less restraint. You got me.

Having someone who hasn't displayed the ability to back up their mouth make light of a discussion, and the time spent explaining as best I can twice in a row doesn't set well with me. Especially when I get directly insulted for my effort part-way through.

 

[UNBROKEN]

I feel ya man...I do.
Just try to not let your frustration derail a damned good thread into a pissing contest. Make sense?

 

[CSM diesel]

What just happened?

 

[Charles]

I feel ya man...I do.
Just try to not let your frustration derail a damned good thread into a pissing contest. Make sense?

Only one way to make that happen.

Abandon the thread.

 

[CSM diesel]

the secondary charger is almost never off it's map when run in a sequential application. All you need to do is density compensate to plot out where the wheel is running.

:doh:

Seems like a forgot a pieceLOL It all comes together. I better get the fluids book out... Oh the things you forget.

 

[Diesel Freak]

What just happened?

no idea...looks like a complete mental break to me...

all I know...air is just a gas...and a relatively simple one at that. When the dust settles, it's just a ballance between fuel and air.

Sequential Turbos are basicly using a Biger turbo to Turbocharge an already turbocharged engine...or...just think of it as if the primary turbo is on a naturally aspirated engine. The primary turbo dictates what the whole thing is capable of.

hmmm so lets assume a S400 or GT42-02 is the primary...in theory, a 75mm primary would work best with a 53mm seccondary...here is the point where theory dies and practice takes over. A 53mm turbo is smaller than the OEM unit. The turbine side of a stock HX35 is too small to be of much use on a truck making much past 500 flywheel HP when used as a seccondary on a twinset. The drive pressures go WAY out of whack when trying to compound...however, it does spool quite well.

At this point it is very beneficial to go to at least a 60mm HX 40 (some Holset Maps include both CFM and kg/s on opposing axes) or a S300 with a larger turbine section and a 62mm compressor. This is a compromise...allowing more compounding without drive pressures getting too funky.

or,

Assume plenty of air (A/F ratio above 20:1...29:1 would be great, but there is no way we can maintain 200% excess air throughout the powerband), BSFC (brake specific fuel consumption in #/BHPxHR) somewhere between 0.32 and 0.36 (this is a street truck and we want EGT under 1200F), choose an "expected power level" then compute the required air mass flow...this is the approximate air flow the primary needs to move at 30 PSIG....or a PR of 3:1 at choke. This gets you in the ball park.

For the Seccondary, it's even easier....cut the air flow by about 40%...and find a turbo that runs well at a PR of 3:1 at that flow and at choke.

it really is not that hard....

Charles, quit "Nuking" it out...

 

[Tate]

Mass flow vs. volume flow, can't we all just get along? If you want to use CFM instead of lbs/min, just multiply lbs/min by 12.5 and you get CFM at STP. Then you can easily bounce back and forth.