Note, to read and understand much of the information in a "Compressor Performance Map, CPM" it helps to read and understand this, because these maps are calibrated in terms of air flow!

I was preparing to start a thread with a title along the lines of "Turbos, Compressor Maps, Surge, Noises, Sounds, Chirp, Chuffa, Huffa, Whoosh, Rattle, Buffet, Flapping..." where the title was intended to match search words for users worried about weird noises coming from their turbos, and the thread was to provide a place for posting information about these sounds, under what circumstances they're heard, what causes them, what harm might they do, how understanding the surge and choke lines and other features on a CPM can help for avoiding them and seeing how air inlet and turbo upgrades mitigate them, and any other information concerning them that anyone might want to post. It seems I see a lot of new questions along these lines, and I thought it might be convenient to have a bunch of info on the general subject in one place that could be easily referenced.

Well that was the plan until I started reading the "stuff" I was going to post as the lead in for the thread, and some of my verbiage and equations were even confusing me! I tried to see if I could explain it to my wife, and then incorporate those words into my text, but she just put her headset back on and turned the music up a little louder. So I've decided to post one piece of my "stuff" at a time, and hopefully get a little feedback on each, and then post a summary of everything with links to the details.

I've been doing some internet research, and in the process discovered that I'm not the only one who find this subject confusing. I've read on several forums and web sites statements like "adding a turbocharger to your engine increases the air flow velocity and the cfm volume air flow through the engine, which allows it to produce more HP." If you just look at the "bottom line" equation that combines several factors that "appear" to effect air flow, this statement looks reasonable, however, if you derive the equation step by step, by the time you get to the same "bottom line" result, you see that not only is this statement untrue, but buying into it leads to no end of confusion when it comes to reading a CPM.

Just to make sure there's no confusion on what I'm claiming to be an "untrue statement", consider this thought experiment. Disconnect the CAC boot from your turbo outlet, and go for a drive holding the RPM constant under load like pulling a long grade. Assume you've hooked up a way to measure either the Air Flow Velocity, AFV in ft/min=fpm or the Volume Air Flow, VAF in ft3/min=cfm in the CAC tube, as these two parameters are related by the CAC tube Cross Section Area, CSA in ft2=sf, as VAF=(AFV)(CSA). Now stop and hook the boot back up and make the same run with the turbo producing a Boost Pressure, BP=15 psi. As long as you maintain the same RPM as before, I claim you'll see exactly the same values for AFV and VAF in the CAC tube as you saw before with the turbo disconnected! The "untrue statement" claims that with BP=15 psi the AFV in the CAC tube has doubled because the driving pressure that's forcing the air into the cylinders has doubled, and this doubles the VAF through the tube.

In fact, by searching FTE I came across the following post from someone who had obviously read and believed the "untrue statement" I'm trying to refute, "Boost provides an additional pressure difference (delta P) from the turbo outlet to the inlet to the cylinders. If the air flow through this path is Laminar, the additional flow rate (cfm) is directly proportional to the additional driving pressure (delta P). In this case, a boost of 14 psi would increase the MAP by a X2, and double the flow rate into the cylinders." Yes, (BLUSH) that's one of mine (and would be true if the engine wasn't hooked to the other end of the tube), but if you search to find the rest of it, you'll see it was from a time long and now (fortunately) in a place far away, and soon to be replaced by the "Gatling gun" explanation for air flow through an engine.

The PSD Cylinder Voulme, CV=(444)/(8)=55.5 in3. Each CV fills almost exactly an 8" long (7.85" to be exact) segment of the 3" diameter CAC tube. Imagine 4 of these 8" long segments of air laid end-to-end along the CAC tube. Each time the crankshaft turns one revolution, 4 of these 8" segments of air advance along the CAC tube and through the engine, kind of like rounds in a Gatling gun ammo belt advancing as it fires 4 rounds out its barrels. This is true whether or not the turbo is hooked up, every 1/4 revolution of the crankshaft advances 1 of these 8" rounds of air through the engine, and since the RPM (firing rate of the gun) is the same, the rounds move along the CAC tube with the same velocity as before, and therefore the VAF is the same as before. Now if you're running at BP=15 psi, each of these 8" long rounds contains twice as many air molecules as they did with BP=0 psi, but the 8" long segments of now pressurized air, move at the same speed along the tube as before, and 4 of them advance through the engine for each revolution, giving the same volume flow rate as before. Its like packing twice as much gun powder into a bullet and getting twice the bang even though the firing rate is unchanged.

Now for the equation, 4 rounds per rev gives a Cylinder VAF, CVAF=4(CV)(RPM)=4(55.5)(RPM)=222(RPM) in3/min as the volume flow rate through the cylinders. Converting to cfm gives, CVAF=(222)(RPM)/(1728)=(0.1285)(RPM) cfm. Therefore, the maximum CVAF for a PSD is (0.1285)(3400)=436.9 cfm, and even installing multiple turbos can't increase this CVAF, but they would increase the BP at which this CVAF occurs.

Now what about the so called Volumetric Efficiency, VE factor that's supposed to be multiplied by the above equation to reduce the CVAF because of the limitations of the valves at higher RPM? Everywhere you look, you'll see it written as CVAF=(VE)(Engine Displacement)(RPM/2), implying that the VE factor is somehow reducing the CVAF through the engine, but this is also incorrect and confusing compared to what is actually happening.

Pull the heads and spin the engine using the starter. At BDC of each intake stroke, each cylinder has its entire volume, CV filled with air at Atmospheric Air Pressure, AAP. Now replace the heads and spin the engine at the same RPM. Due to the vulgarities of the air flow through the valves, now at BDC on the intake strokes there's slightly less than AAP in the cylinders and this is like a "negative turbo effect" that reduces HP, and this effect increases at higher RPM due to the air having inertia, turbulence, and other effects.

By including the VE factor as part of the equation for CVAF in a normally aspirated, or for that matter a turbocharged engine, you're calculating a lower than "actual" CVAF which is a "fictitious" way to account for the reduced HP due to a lower Air Density, AD in lb/ft3 in the cylinders, and likewise trying to claim that the CVAF is doubled with a turbo at BP=15 psi is an equally "fictitious" way to account for the HP increase due to doubling the AD in the cylinders. In both cases, the CV fills and empties the same volume of air at a rate that only depends on RPM, it's just that when it fills, there are either fewer or more air molecules than there would be if the cylinder pressure was at AAP.

Well there's a lot more to this story, but I'd better wrap up this installment because I doubt many have read this far down anyway. If you did, a little feedback (like the max # of equations/words you can digest all at once) might help me to better present the next installment which is where the story really starts to become interesting, but requires more equations to understand where the VE factor really belongs, mass air and fuel flow and how much HP your engine can produce, how to read a CPM, and a bunch of other neat stuff.

 

Makes sense to me as long as you're saying that turbocharging an engine will increase the total air flow through the engine. I know we are still dealing with the 8" "bullets" but they contain more air by volume. At least that is what I got from all of this. Since that is the case, the engine will flow more CFM under boost and have increased potential for power. I'm sure the actual flow is not directly proportional to boost due to cylinder fill facors.

I am curious about the next installment.

 

I too am curious about what is up next. I am not entirely sure what your main point you are trying to get at with the saga though. Are you contesting that more boost pressure doesn't increase airflow through an engine?

 

Ernest, i have noticed , that the whole flow and CFM thing it has been bogging you, keep it comming, we all learn somthing form you reasearch
i wonder whats next?

 

Ohhhh no! I just wrote what I thought was a very convincing explanation as to why a turbocharger can't and doesn't increase the "Volume Air Flow, VAF in ft3/min=cfm in the CAC tube", it doesn't do it there, and doesn't do it downstream of there either where the engine is located! A turbo does increase MAF= Mass Air Flow in lb/min, but that's covered in the next installment. CPM's are sometimes calibrated in VAF (ft3/min) which isn't effected by BP or even having a turbo, and sometimes in MAF (lb/min) which does depend on BP.This time it's a double NO! A bullet that's 8" long and 3" in diameter HAS A FIXED VOLUME PERIOD, no matter how much air you cram into it!No this is not true, even though (as I've readily admitted) I posted just such a notion several months ago. I tried to explain how confusing this subject is in the lead in! The cfm flow through an engine IS NOT effected by BP or by even having a turbo installed!

Try this thought experiment. Unhook both ends of the CAC tube, the one going to the turbo and the one going to the engine. Put a gate=slide valve at the engine end and open the valve. Place a really big leaf blower at the other end and turn it up to max, say 1000 cfm. Now close the slide valve and say the blower can make a BP=50 psi under stall conditions with cfm=0. Now get a lot of jars, each with a cylinder volume=CV. Take a jar and place it over the end of the tube, open the valve, let the jar fill with air (if it doesn't get quite full, this is where the VE factor applies) and then close the valve. Each time you do this, you've removed a VOLUME of air = CV. If you repeat the jar filling once every min, you have a VOLUME AIR FLOW out the end of the tube = jar volume=1 CV per min no matter how much pressure is built up in the tube. Having the engine at the end of the tube and running at some RPM, is just like filling a bunch of jars in rapid succession and the resulting VOLUME AIR FLOW is many "jar volumes" per min, and the value of "many" only depends on RPM, which determines the # of jars per min being filled, but not on the value of BP.

 

Let me try to see if I understand what you are saying Gene........

In a non turbo setup the 8X3 cylinder volume is only filled as fast as the pressure differential (piston moving down) allows effectively causing the cylinder not to fill completely full at higher rpm's

In a turbo set up with pressurized air waiting to fill the 8X3 cylinder as soon as the valve opens charge air is pushed into the cylinder effectively giving you more volume as the cylinder is filled quiker and at the higher rpms can be completely filled.

although with air cylinders (fire Fighting Scott Packs) an air bottle filled to 4500psi will contain ~ 87 cu/ft of air even though the volume of the cylinder is only 1.5cu/ft

 

The only problem I see, which is probably yet to be explained, is how you are going to cover air expansion when expelled out the cylinders (jars). I follow the logic to increase of BP doesnt increase doesn't the flow (Velocity= ft/min). But how are you going to explain air density (BP) in relation to volume? All the analogies make sense for water, but gases are compressed. Will be interesting to see where this goes.

 

This is correct. The volume of the cylinder is only 1.5cu/ft (at ambient pressure). And you can prove that with water, as it cannot be condensed, but air CAN be condensed. So at 4500psi, its equivilent to 87 cu/ft of air at ambient pressure. But at 4500 psi, it only takes up 1.5 cu/ft. So in a turbocharged application, such as out trucks, your engine displaces 444 cu/in, but thats at ambient pressure, when you pressurize it (turbo) you put more air into the same amount of volume cylinder. If you released the air out of the cylinder at 25psi, you have more air in there (at ambient pressure) then at 5 psi, which is why you put can more fuel to it at 25psi then 5 psi, because there is more air in the cylinder. Yes its still the same volume, but its pressurized, so theres more air in there (at ambient pressure).

 

ok , now im confused,
let me take it my way: if i might,
you said a turbo has no afect, on the Volume of air pushed through the engine,
i would have to disagree here it goes
to create perfect combustion in the diesel , you need a certain amount of fuel, compressed air and temperature to ignite fuel, so by altering a fuel amount we alter a combustion, you change just fuel without adding more compressed air , your truck will smoke like train, once you upgrade the turbo, you are able to clear it up
so i might agree that every engine has its maximum CFM that its able to get through, but under certain conditions with a turbo, a 1 jar might contain 2cu/ft of air where it originally has .05 cu/ft
its harder and harder to get more flow at some piont and it might not be cost effective, but take for instance an air compressor, lets say it was designed to put out a 135psi, but you can make it to produce 150 psi by allowing to override the pressure switch, and it will do it within 35 min, now more expensive compressor will do the same thing in 10 min, and and produce 135 psi, so the turbo has a crucial role on our trucks, the beter it is (more expensive ) the faster we fill up those jars and by adding more fuel create better combustion conditions, more violant than the motor was designed to do, and create more HP or torque, since bigger BOOM just happen,
why people make the incoming air colder on a lot of racing aplications? to get more volume in to the combustion chamber,


i dont know if this makes any sense . sorry if it does not , TELL me to shut up,

 

Ernest everything you are saying is entirely true. Volumetric flow rate has absolutly nothing to do with the density of the material being moved. This is the same for air at 1 atm or 10 atm or for water, for those who haven't seen atm used as a unit of pressure it is atmospheric pressure at see level (somewhere around 14.7 psi). Therefore boost does NOT increase volumetric flow rate it does however increase mass flow rate.

Also are you getting all of this information from the internet or do you have books too. If you need any equations you can't find I have a internal combustion engines text, with access to another, and a fluid dynamics text.

 

more volume is not being put into the cylinder. the same wolume is more dense therefore it has more mass per unit of volume. This is why you can add more fuel.

 

Does anyone know how I can post these two columns side by side to save space? I'll keep this list updated and alphabetized. You can imagine the # of words I'd have to add to this list for it to make much sense (and this is less than a third of the complete story!), but I wanted to list at least these terms since I've already used most of them so far in this first installment.

For now, I'm just giving enough additional input to try and clarify what I've given so far. I'll use a numerical example to try and illustrate a few key points. For RPM=2000, we get CVAF=Cylinder Volume Air Flow=(0.1285)(RPM)=257 cfm. I'd previously posted that if you run with BP=15 psi, that CVAF doubles to 514 cfm, BUT THIS IS INCORRECT,WRONG, VERY WRONG! The CVAF does not depend on anything except RPM period. If you look around the internet, you'll see this "mistake" a lot, as it seems an intuitive way to explain how the # of air molecules in the cylinder doubles, but what is really happening is that the AD=Air Density, lb/ft3 in the CAC=Charge Air Cooler Tube doubles, and the air moves along the tube (foot by foot) at the same speed as before, but now each foot contains twice as many air molecules.

Here's what happens at the air filter inlet tube, FIVAF=(0.1285)(VE)(RPM)(BP+AAP)(FIAT)/(FIAP)(MAT), ft3/min=cfm, now the cfm flow rate DOES DEPEND on BP, and on a # of other terms as well. To simplify, assume MAT=FIAT, VE=100%, FIAP=AAP=14.7 psi. For BP=15 psi, this gives a X2.02 higher cfm through the air filter than the cfm in the CAC tube, and shows that if you maintain 2000 RPM, and increase the BP from 0 to 15 psi, the cfm through the filter changes from 257 cfm to 515 cfm. You'll never see this X2 increase at RPM=2000 for a BP=15 psi, because MAT>FIAT, and VE<100%, but I have measured about this much at higher BP and higher RPM.

I've been taking measurements of FIVAF with my new AFG, and that's how I knew there was a problem in the way I was looking at the cfm, because when I put my cfm measurement into the equation to predict HP, I got over 500 HP, and I new my new DP Tuner wasn't quite that good a deal!

Definition of Terms:
AAP=Atmospheric Air Pressure, psi
AAT=Atmospheric Air Temperature, K
AATF=Atmospheric Air Temperature Fahrenheit, F
AD=Air Density, lb/ft3
AFG= Air Filter Gauge
AFV=Air Flow Velocity, ft/min=fpm
AP=Air Pressure, psi
AT=Air Temperature, K
ATF=Air Temperature Fahrenheit, F
BDC=Bottom Dead Center
BP=Boost Pressure, psi
CAC=Charge Air Cooler Tube
CAD=Cylinder Air Density, lb/ft3
CAP=Cylinder Air Pressure, psi
CAW=Cylinder Air Weight, lb
CMAF=Cylinder Mass Air Flow, lb/min
CPM=Compressor Performance Map
CSA=Cross Section Area, ft2=sf
CV=Cylinder Voulme, ft3
CVAF=Cylinder Volume Air Flow, ft3/min=cfm
FIAD=Filter Inlet Air Density, lb/ft3
FIAP=Filter Inlet Air Pressure, psi
FIAT=Filter Inlet Air Temperature, K
FIMAF=Filter Inlet Mass Air Flow, lb/min
FIVAF=Filter Inlet Volume Air Flow, ft3/min=cfm
HP=Horse Power
MAD=Manifold Air Density, lb/ft3
MAF=Mass Air Flow, lb/min
MAP=Manifold Air Pressure, psi
MAT=Manifold Air Temperature, K
RPM=Crankshaft rotation rate, rev/min
TIAD=Turbo Inlet Air Density, lb/ft3
TIAP=Turbo Inlet Air Pressure, psi
TIAT=Turbo Inlet Air Temperature, K
TIMAF=Turbo Inlet Mass Air Flow, lb/min
TIVAF=Turbo Inlet Volume Air Flow, ft3/min=cfm
VAF=Volume Air Flow, ft3/min=cfm
VE=Volumetric Efficiency, %

Equations:
AD=(1.5)(AP/AT)
AT=(5/9)(ATF-32)+273
CAD=(VE)(MAD)=(1.5)(VE)(BP+AAP)/(MAT)
CAP=(VE)(MAP)=(VE)(BP+AAP)
CAW=(CAD)(CV)
CMAF=(0.19275)(VE)(RPM)(BP+AAP)/(MAT)
CMAF=TIMAF=FIMAF
CVAF=(0.1285)(RPM)
FIMAF=(FIVAF)(FIAD)=(1.5)(FIVAF)(FIAP)/(FIAT)
FIVAF=(0.1285)(VE)(RPM)(BP+AAP)(FIAT)/(FIAP)(MAT)
MAD=(1.5)(BP+AAP)/(MAT)
MAF=(VAF)(AD)
MAP=(BP+AAP)
TIMAF=(TIVAF)(TIAD)=(1.5)(TIVAF)(TIAP)/(TIAT)
TIVAF=(0.1285)(VE)(RPM)(BP+AAP)(TIAT)/(TIAP)(MAT)
VAF=(AFV)(CSA)

 

...which is what the turbo does. More "dense" air (more molecules in the same amount of volume) that wouldn't be there without the turbo. To me, this whole post is just an exercise in correct semantics of "volume" versus "mass" or "pressure". Since cfm is a volumetric measurement, the cfm is the same, but there's just more "stuff" in the fixed space.

I'm not sure what else is there is to discuss. But to give Ernest some feedback, post length is irrelevent to me -- it's the content that's important. And I didn't read even one formula. Sorry, I'm not interested in that part. Only the "bottom line" of what you're trying to tell us.

And to turn it back into an electrical demonstration, Voltage is like the pressure forcing the electrons through a fixed load (intake & engine) at some rate (Amperes). More voltage (pressure) equals more electrons going through (more amps), even though the size of everything else (volume) is the same. And of course, crank the voltage up too high, and the "magic smoke" is released.

Joe

 

The following was edited by me, I'm not trying to be a smart *** by editing your post, I just found it to be a convenient way to make some counter points and save some typing. If this happens to be an FTE no no, please let me know and I'll never do it again.

I'm not sure if you can actually cram 87lb of air in a 1.5cu/ft volume, but by the very definition of the word volume, a 1.5cu/ft container only holds 1.5cu/ft of anything. The only way you can put more than 1 ft3 in a 1 ft3 container is to cram it in so tight that the sides bulge out, but then the box is larger than 1 ft3.

My original mistake wasn't to simple to catch, and all the air flow stuff can be confusing, but saying that a 1 ft3 box can hold more than 1 ft3 of volume (if you just apply enough pressure) is just like saying that 1 lb of lead weighs more than 1 lb of feathers!

 

If I remember correctly you can actually have a volumetric efficiency greater than 100% when using a turbo charger. This is because volumetric efficiency is basically how efficient the engine acts as an air pump. Because it is calculated on a mass basis (because like stated above the volume in constant) more mass of air is pulled in than "ideally" possible. By ideally I mean if it was naturally aspirated