Since the smaller turbo is connected to the larger engine via a long skinny CAC tube, I'm calling the turbo the "tail" and the engine the "dog", and when the turbo, whose CFM depends on compressor wheel RPM, produces more CFM than is needed by the engine, whose cylinder CFM is proportional to crankshaft RPM, I'm calling that "the tail wagging the dog", and that's what makes the BP increase in response to applying the throttle to meet increased load demands. Other "plays on words" came to mind, like which comes first "the cart or the horse" or "the chicken or the egg", but I think "the tail wagging the dog" is the best description of the "time-dynamic" processes involved when engine parameters adjust in response to applying the throttle.

My current PSD model uses an Excel spreadsheet that links together several dozen equations which use inputs like ambient pressure and temp, RPM, BP, and fuel flow to calculate the performance of the turbo, intercooler, and engine, and provide "steady-state" outputs of MAT, MAF, CFM, and RWHP. When people like Mike, Jeremy, Alan, and others data log using AE, their measured data like RPM, BP, IAT, MAT, MAP,and ICP usually involves accelerations or other relatively short duration load conditions, so that there's not enough time for all the engine parameters to stabilize to their new steady state values.

The MAT takes the longest time to reach a steady state value (kind of like the voltage build up on a capacitor), because all the metal in the IC heat exchanger has a large thermal mass that absorbs a lot of heat from the charge air flow, and this temporarily cools the charge air much more than the "steady state" cooling due to the heat exchange between the charge air and the hot air from the A/C condenser that flows through the IC heat exchanger with the A/C on, which is almost all the time for me. Until the IC heat exchanger reaches its steady state operating temp for a given load condition, the MAT will remain lower than its true steady state value.

I'm thinking about ways to incorporate time-dynamic effects in my model so I can better use those types of AE measurements. I'm passing along an overview chart of the air flow from the air filter inlet to inside the cylinders. These 4 pics probably aren't worth a thousand words each, but together they should be good for at least enough words to help understand the interplay between various parameters as they adjust to meet different operating conditions.

I gave parameter values at 5 points along the air flow path for BP= 0 psi and BP=20 psi at RPM=2200, which is 71 MPH for me. I've towed long 3+% grades at 71 MPH and 20 psi BP, and that's like me being on a dyno at near max HP for miles on end. The other extreme, BP=0, is almost like driving a normally aspirated engine, but the turbo still heats the charge air a little, and the IC can't cool it to lower than 90 F because that's the assumed temp for the waste heat from the A/C condenser. I'll bet that if the compressor wheel were to jam so that it can't spin, that the engine would just barely run because the air flow would be so restricted. That's why the turbo still has a little work to do even at BP= 0 psi. In fact, when my boot blew off at the turbo outlet my engine barely ran, and it didn't have to suck air through the restrictive turbo!

I'll start at the manifold because AE measures MAP and MAT, and you can plug those into the AD equation and get MAD. If only all that MAD got inside the cylinders where it belongs I could make 50% more HP, but that would require new injectors and HPOP. As it is my small inlet boots, plenums, and the intake valves combine to reduce my VE from 100% to about 78% maximum at 2000 RPM and BP=0 psi, and at 2200 RPM valve timing vs air inertia reduces my VE to 75.5% at BP=0 psi, and choked flow vs higher BP further reduces my net VE to only 64.3% at BP=20 psi and 2200 RPM! However, I'm certain that Ford wasn't worrying about my low VE when they spent the extra $ for the 99.5 to have larger plenums, boots, manifold, better IC, turbo, and a new compressor wheel optimized for air flow at lower wheel RPM. They weren't that interested in being able to advertise 15 additional HP compared to the early 99, they had to make these changes to meet the new NOx emission spec!

Moving back to the IC, I already explained why it doesn't cool the charge air at BP=0 psi, but at BP=20 psi it cools it a lot, from 335 F to 164 F, but it also reduces the pressure my 1.4 psi, and both of these effects combine to result in the MAD=0.1504 lb/ft3 at BP=20 psi. Also note that at BP=0 psi, 213 CFM flows into the IC and 213 CFM flows out of the IC, whereas at BP=20 psi, 223 CFM flows into the IC and only 182 CFM flows out! The reason for this is that the AD changes, and at every point in the air flow path VAF=CMAF/AD. The CMAF is the flow rate of air molecules, and in the steady state this must be the same at each location.

In a time-dynamic adjustment leading to higher BP and increased CMAF, the turbo puts out more molecules than are flowing into the cylinders, and they start piling up at the turbo outlet, and they begin pushing more air towards the manifold. This increases the MAP (and also the MAT), and pushes more air into the cylinders which increases the CAP and the CAD. The CMAF=(CVAF)(CAD), and at 2200 RPM the CVAF=283 CFM so the increase in CAD causes a proportional increase in CMAF, and this becomes the new higher steady state value of CMAF corresponding to the new higher BP.

Note that CVAF only depends on RPM and NOTHING else. It doesn't depend on pressure, temp, or any other parameter! For a 4-cycle engine, each cylinder flows its cylinder volume every 2 rotations of the crankshaft, and that's what determines the CVAF. The amount of air flow (in lbs or # of molecules) depends on many factors, but the volume flow only depends on the cylinder volume, the # of cylinders, and the RPM.

The turbo is too complicated to say much about here, and one day I plan to do a post on just it, including all the operating equations and compressor maps. The inflow conditions at the air filter have been a topic of recent interest so I'll close by again emphasizing that the IAD initiates the entire cascade of events that help determine the lbs of air flowing into the cylinders. Too maximize the IAD you need to keep the IAT as low as possible. Except for staying at sea level and only driving when a high pressure front is in town, the only other control you have over IAP is to employ a RAM air scoop which increases IAP by about 2 Inches H20 at 60 MPH.

 

But these are only a very small part of all the equations that make up my model!

 

ummmmmmmmmmmmm what ??????????????????? i admire you Gene ,

 

Gene....you have one hell of an interesting hobby.

Me, I have a more neanderthalian approach. Mash peddle....make smoke....go fast.

 

Ok, Pete and Scott, put your thinking caps on, yeah I know, my head hurt for a minute too. look at what Gene is throwing out here, take the time to slowly read his post. Either I am getting smarter from Genes Posts, or hes gonna make a fool outta me, cause I can actually follow what he has written (I took it as a challenge to try and figure out what all the anagrams were, and then i scroll down after a few minutes of hard thinking, and he lists them.... What a Guy) Do I understand what he is saying? Yes. Could I lead someone to this realization without all the equations? Maybe.

Good job Gene.

 

BD, I'm still dragging my knuckles along the ground here but the light is coming on, slowly, I'd say about the equivalent of a .05W bulb right now. I have a few questions about some of the things he said. I must read those again before I put my foot in my mouth like I am oft to do at times. I'll have a banana while I do this.

 

Gene, just to satisfy my curiousity, how long did it take you out of your busy schedule to think up, type up and make the graphs in those posts above?

 

Mike , all the facts that Gene is throwing our way are 5th grade science, (I have a 5th grader right now so maybe thats why I am getting it) but what is making it hard for us to grasp is the amount of different concepts all at once. call it information overload. You and I(to an extent) are trained to view in black and white, right or wrong, Not so much as WHY is it black or white, right or wrong. If you break each one of those equations down to its simplest form then it is eaiser to follow. Gene, on the other hand is trained to ask WHY, first, and then only if it is proven to accept the idea,
Think in these terms
Hot air expands, which takes up more area,which is then cooled and becomes more dense and takes up less area but is the same amount of air, which since it takes up less area means we have more area to put air in.....etc.

I hope I didn't just put my foot in my mouth. Gene?

 

Holy cow!!! You really know how to make someone work their brain after they get off work.
Great reading, now if I can just figure it all out.

 

OK, I have edited my original post to now just mention that
BP=20 psi, 223 CFM flows into the IC and only 182 CFM flows out!

This is the only thing I am having some difficulty with. If 223 continues to flow in, then even after cooling, 223 must be coming out in whatever state it is or it is leaking somewhere. I know he touched on that in the piling up by the turbo statement but it would almost appear that having no intercooler would flow more air if you follow that analogy. I am sure I am missing something.

 

Mike, that one threw me too. How can more CFM of air be coming out as less boost than at higher boost? If that's true, the cooling effect of the IC must be so great that the air density increases enough to overcome lower CFM.

 

Not real sure either, but, cooler air is denser... heavier to move maybe? Cooler air also takes up less space. JM.02W

 

Wonder where I heard that before

 

Open Wide.

My sixth grader says for me to sit back and watch you fumble through this one. hahaha

 

Well replace "area" with "volume" and you're almost there. I'm not sure how to take all my fine work being compared to 5th grade science because that reminds me of this TV show my wife watches called "are you smarter than a 5th grader". I definitely think I'm doing work that's at least 6th grade level, and sometimes even Junior High level.

One of my favorite 5th grade science experiments is to roll up a piece of paper and set one end on fire and drop it into a wide mouth milk bottle, and then just as the flame goes out put a hard boiled egg (small end down) on top of the opening (make a good air tight seal by pushing on the egg a little), and pretty soon as the bottle cools the egg gets sucked right into the bottle. If you can find an old time milk bottle do this experiment with your 5th grade (son or daughter?) and see if you can together explain what's happing and why the egg got sucked into the bottle.

After the egg gets sucked in, get another one and let them try to carefully push it through the opening. After the egg has crumbled into pieces you can talk about uniformly distributed pressure (psi), and how it provides a more gentle pushing force (psi x area) than concentrated pressure using fingers. Since the opening to the bottle is about 1 square inch (maybe measure and calculate it) explain how a 1 psi pressure difference creates a 1-lb pushing force, and then find a 1-lb weight they can hold it to feel how heavy that is.

If you need some help explaining, send me an email, because the entire correct explanation is a little more involved than first meets the eye. But knowing the exact correct explanation isn't what's important, it's doing the experiment together and discussing reasons why it might or might not work the way it does. Parents need to encourage a curiosity as to why things work.