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  #1  
Old 08-27-2004, 11:51 AM
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Default Sizing a turbo for all engines

OK, turbos have what is called a "map" which is an area plotted of performance and a line on the left represents destruction, also known as over-spinning.

These maps are all published for ground performance, not for aviation and you must calculate corrected numbers for your engine, altitude to find your aviation pressure ratio and corrected air flow for every altitude and level of boost.

As you go up your boost can remain the same, but your pressure goes up to a point where it can "cross the line", over-spinning the turbo and destroying it.

We are struggling to find the calculations, i got in the back door at garrett and had a guy run the numbers for me and i have an answer. I also bought a program and i have an answer.

The program is said to calculate correctly, BUT, all of the numbers it displays are wrong. The attached spreadsheet seems to do what we need, just have not had time to verify it.

PLEASE, someone mess with it, JOHN NEEDS this info NOW so he knows his turbo limits before he blows this one up
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Old 08-27-2004, 02:09 PM
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More info on calculations

http://www.gnttype.org/techarea/turbo/turboflow.html

ok i'm aftraid the info will disappear one day so i copied and attached it

but go to the site for politeness
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Old 08-28-2004, 01:19 AM
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Default I've got some partial numbers already...

I started playing with this a week or so ago, dust, and I have a CRAPLOAD of numbers already copied down, on PAPER.. so I havent had the chance to make an excell spreadsheet of them.

The conditions are Standard Day, at altitudes up to 25k feet, with temp, pressure and density values from a military flight manual chart I found online. I want to say the better fit for the numbers I was getting on a turbo 13B was for a Super T04E with 40 trim.. but I have to get back to you on that. I am at work right now, without access to my handwritten stuff.

If you are on the Flyrotary listserv then you should have access to some links that I (and others) posted with regards to Compressor Maps. One of these links does EVERYTHING. You input ambient pressure, RPM, engine config, injector settings, intercooler loss, turbo efficiency, BSFC and your left shoe size.. and gives you data output including fuel flow, HP and AIRFLOW in pounds rather than CFM..( if you were using a car it would give you drag racing times too ). Dealing with lbs/min or lbs/hr helps eliminate extra conversion factors from CFM. John should have these links and can pass em to you if you desire. The thread was Flyrotary: Re: Compressor Maps. I cannot guarantee having MY info up before Tuesday/wednesday... I am working nights all week and no guarantee that I can get to "non work" stuff while at work. And, the numbers are too small/closely written to count on scanning it. I will see about plotting some of the data against the map for his turbo, but the thing to remember is without specific numbers such as a solid VE, BSFC, Intercooler loss, etc then these are still ESTIMATIONS.. they may be very CLOSE ones but estimations nonetheless.

More to follow later when I can.
Dave
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Old 08-28-2004, 01:06 PM
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Default Ok.. I lied..

Ok... I got cracking on this when I got home from work and added some numbers to it.. I'm gonna HATE work tonight...I'm a bear if I don't get my 7 hours beauty sleep.

I added a few columns to the spreadsheet. I used the HTML formula contained on http://personal.lig.bellsouth.net/c/...n/airflow.html

Looking at the spreadsheet vertically, stuff on the left is Normally aspirated.. stuff in the middle is "turbonormalized" and stuff on the right is boosted to 23 PSI absolute.. a moderate amount of boost.

Some of this may not be realistic.. Air Fuel of 12.9, standard day values, unrealistic efficiencies on the turbo, intercooler and air filter, etc.. this is simply ballpark information, but the webpage I attached the link to does all the work if its set up correct. Remember, use settings for a FOUR cylinder 160 CID engine. Other assumptions as I used them are listed at the bottom of the spreadsheet. A safe assumption is that these numbers are optimistic (high) and I am hoping within 10% of true value.

Hope this is what you were looking for Dust, its a matter of plugging in lbs/min of airflow versus PR on the compressor map and see where it falls.

Dave
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Old 08-28-2004, 01:25 PM
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Can you put formulas in so that the values can be changes?
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Old 08-28-2004, 07:30 PM
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Default

Quote:
Can you put formulas in so that the values can be changes?
I'd go a step further, if I knew how. What I'd like to do is "translate" the compressor map so that it shows MAP on one axis and altitude on the other. Knowing pressure ratio's and lb/min airflow is great, but what I need to know when flying (next week) is what MAP can I give this sucker without blowing it up? or - more accurately, what MAP at what altitude and (outside air temp / intake temp) will keep me safely on the efficiency island. For example, the map shows that a PR of 2.5 with lbs/min of about 37 is about as high as I want to go. What the hell does that mean in terms of actually piloting the thing???



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Old 08-28-2004, 08:19 PM
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Default Formulas in Excell

John,..
Couple things.. I haven't played around with major multi-variable equations since my first go-round at college 12-14 years ago.. I don't trust myself to try and build the formulas correctly in Excell and get it all right... not at the present.

Look at the spreadsheet I made... the MAP value would be the manifold pressure value for a given altitude... look and see if the PR correllates to what you want.. that would give you a MAP setting at that altitude, and the RPM would give you your desired airflow and HP.

You can go to the webpage with the calculators itself and play with it, and use values in PSI OR mmHG for ambient and manifold pressure. You can adjust temp to non standard. The temps used in my spreadsheet ARE standard, so you can use that as a reference point.

Take a highlighter and color in data points on the compressor map based on values from the spreadsheet, or results of your own use. What you should see is that you will have a range of power settings at a given PR, such as 3-5,000 or 4-6,000 rpm. and that will give you something to work with.

Keep in mind, boosting the hell out of the motor is only gonna burn a LOT more fuel for a LITTLE increase in speed.. what is more realistic is to use lotsa boost to CLIMB quickly up high, then throttle back and have a high cruise with a much higher TAS at altitude.

If you can plug in numbers on a spreadsheet, you should be able to plug them in on that page I was using. You can leave the "car weight" blank.. its n/a. I will play with it over the next few days and see if I can tweak it a bit, and get some more/better data.

Dave
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Old 08-28-2004, 08:56 PM
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WENT THERE, PLAYED, opp capps, could not see lbs per min air or preasure ratio, have you had a look at the spreadsheet i loaded in?
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Old 08-28-2004, 11:12 PM
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Default Hang on..

I'm logging in from work and have a spare moment.. on that page, the air flow is on top row in a box called MASS AIR FLOW, and you can select lbs/min as one of the possible value labels in the pulldown menu.

Pressure ratio is calculated and displayed in the box called "Turbo CR" for turbo compression ration. I did a manual verification so that I know THAT value is actually what we are calling PR.

I looked at a spreadsheet a while back that was uploaded at the beginning of the thread. The only thing that stood out about it was that the calculations were based on a 6 cylinder 160 CID engine, and my understanding is that the 13B is equivalent to a 4 cylinder 160 CID engine in these tables. I don't know how much of a difference that really makes, but if it does, then it would throw everything off with regards to airflow.

Any further analysis will have to wait until the AM so that I can actually download stuff. The Information Technology peeps here have things locked down tight and I can't download any files..oh well.. life as an end user . Give me a little bit of time and I will be able to sit down and look at things and TRY to figure it out over the next few days. I'm not an engineer, and its been a decade since I've had to do this sorta stuff.. for physics or chemistry.. my math gland has shriveled up.

Now.. things that I am NOT clear on.. at what point do the turbochargers BEGIN to make the desired boost.. I have minimal info on that, and think that might have to come from someones actual data...on the plane or on a dyno. Without that, I wont be able to determine if ACTUAL data falls to the right or the left of the surge line. Regarding surge, intuition tells me that a pop-off/ blowoff valve on the upper deck should relieve any problems with that if you are "passing through" the surge area, but that is an educated guess and nothing more.

I am curious what kind of max boost we are talkin about here. I am also wondering what is being used to regulate boost.. the eBoost or a manual linkage.. or what..The car guys may go 2 atm/30psi/60"Hg but I don't think thats going to be necessary or prudent in our situation. Thats one of the reasons I only did boost numbers on my flowsheet for normalization and 0.5 atm/23 psia/45"mp boost.. I will try to take some time and play with the map for John's turbo and see what maps out.. and scan it and post it.. and see what comes up. No matter what, I would bounce that info off someone who WORKS WITH TURBOS... cause I'm new to this too, guys.

Dave

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Old 08-29-2004, 12:04 AM
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I've just spent time on that web page and it is almost great. Almost all of the numbers line up except the numbers we need.

It shows the pressure ratio that i need as 13.63
the mass airflow at 27.09

I think my pr is about 2.85 and my equivalent air flow is 44

It is like everything else i have run into, probably works perfect for sea level, sucks at altitude.

now for your questions

Now.. things that I am NOT clear on.. at what point do the turbochargers BEGIN to make the desired boost.. I have minimal info on that, and think that might have to come from someones actual data...on the plane or on a dyno. Without that, I wont be able to determine if ACTUAL data falls to the right or the left of the surge line.

Ok in aviation two ways to use boost

starting at sea level to get more hp from engine than NA, example slades engine is 160 hp with no boost, he wants 250 for take off and climb, add air with turbo and more fuel and whalla, more HP

Now as you go up in altitude, the air gets thinner and colder. The thinner air requires more work from the compressor to keep the same MP and the pressure ratio goes up. The colder air is more dense, so that counterbalances the effect of the thinner air a little.

Now, the turbo will make boost when the waste gate is not open and will make maximum boost when the waste gate is closed totally. At sea level the waste gate will be partially open and will have to be closed a little at a time as you go up, to produce the same MP

When the air is being compressed by the turbo it is also being heated, the intercooler then cools it and i have been told that at 25000 feet it should be 80% efficient because of the large temp variance 284 degrees versus -30 outside

now for your questions

Regarding surge, intuition tells me that a pop-off/ blowoff valve on the upper deck should relieve any problems with that if you are "passing through" the surge area, but that is an educated guess and nothing more.

Ok the surge is not over pressure, it is the turbo spinning TOO fast and all of the sudden it will stop pumping air and spin like the dickens and self destruct. 150,000 rpm

The turbo will probably be 75% efficient
the intercooler will prob be 70 to 80% efficient
the volumetric will prob be 80% effec.

we need to convert lbs per min to effective lbs per min

Example when the turbo grabs a chunk of air at 15000 feet it is not the same chunk that it would grab at sea level, we need to figure the conversion ratio, i've been told that it is 44lbs min effective for my engine, which i think would be 22 lbs min
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Old 08-29-2004, 12:31 AM
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Default

Quote:
Originally Posted by Dust
I've just spent time on that web page and it is almost great. Almost all of the numbers line up except the numbers we need.

It shows the pressure ratio that i need as 13.63
the mass airflow at 27.09

I think my pr is about 2.85 and my equivalent air flow is 44

It is like everything else i have run into, probably works perfect for sea level, sucks at altitude.
I was able to use this page for conversions at altitude. Would I be able to call you Monday Night and talk to you one on one with us both looking at the page in question so that we are seeing the same data? One quirk about the page is that boost value is the amount over ambient, not the total/absolute pressure value. Email me at forum controlls with the number you want me to use if you want to go the phone route.

Quote:
Originally Posted by Dust
<my question> at what point do the turbochargers BEGIN to make the desired boost.. <your response>
Ok in aviation two ways to use boost...
Ok, what I meant was in specific.. with the 13B on a turbo (i know this will be a variable depending on the turbine wheel/turbo setup).. is the turbo making the desired boost at 3000 rpm? 2000 rpm? What is the desired boost for you guys? What is the wastegate control set with regards to.. and is it manual? automatic? Fixed? Is it set to a max PR or is it an absolute pressure controlled type?

Quote:
Originally Posted by Dust
we need to convert lbs per min to effective lbs per min.
Ok.. we are getting our units mixed up here. Lbs per minute of Mass Air Flow should require no conversion. Thats the whole reason I was using that value rather than focusing on CFM..

Now.. if you are looking at CFM.. a volume.. cubic feet per minute.. then you WOULD need to convert based on density (at altitude) to get your "effective CFM". (the equivalent CFM at sea level).

That chunk of air at 15,000 feet is agreeably less dense, but if you take that VOLUME of air and multiply it by its density you will get "POUNDS" of air.. and a pound of air at altitude weighs the exact same as a pound of air at sea level. Mass is conserved. Volume is what varies with pressure here.

Ideal Gas Law supports this. PV=nRT P is pressure, V is volume, n is amount of air molecules (oversimplified) T is temperature and R is another constant.
If you keep the "nRT" part constant, to decrease the pressure will increase the volume.. same effect as climbing to altitude. This is an oversimplification since temperature changes too, but it illustrates the point.

We are really close to being on the same page here.. I hope to try and get the details worked out.

Dave

Last edited by Dust : 08-29-2004 at 12:46 AM.
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Old 08-29-2004, 12:43 AM
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dave said

That chunk of air at 15,000 feet is agreeably less dense, but if you take that VOLUME of air and multiply it by its density you will get "POUNDS" of air.. and a pound of air at altitude weighs the exact same as a pound of air at sea level. Mass is conserved. Volume is what varies with pressure here.


ahhhh yes but the turbo maps are based on sea level.

when the turbo grabs a chunk of air at sea level and compresses it 3 times you get x amount of air in the manifold

when the turbo grabs a chunk of air at 15000 and compresses it 3 times you have allot less air in the manifold than above

the preasure ratio stayed constant but the effect did not, the turbo worked just as hard at both levels

now you have to take the 15000 foot air, decrease it for temp and increase it for density to look at it at the sea level based maps

the maps i have are in lbs per min and i have 50 or so of them
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Old 08-29-2004, 01:02 PM
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Default Ok.. my brain is full.. my head hurts..

Ok.. Dust.. first thing.. stop trying to convert stuff needlessly.. Think simple.. a POUND of air.. weighs One pound. It weighs one pound at ground level in Death Valley and it weighs one pound on top of Mt Everest. The weight of a given mass of air does not change with its pressure. What does change is volume.. the Cubic feet, yards, etc.. American gearheads and hotrodders work in CFM, so much of what they deal in is in CFM. I don't dount what you say about the Compressor maps saying "adjusted" #/min.. the adjustment is from CFM corrected for density to equal pounds...

Ok.. next thing.. I sat down for a little while and started picking apart that spread sheet you posted. I played with it and compared things and I am attaching a copy of the change. I have identified either 1) a source of major error in the spreadsheet or 2) a source of major error in my understanding of the concept.

This spreadsheet is calculating Compressor Ratio (Pressure Ratio or PR) by taking Pressure 1 (ambient) and Pressure 2 (MAP/Boosted pressure) and combining them as such: (p1+p2)/p1 and using THAT result as a PR.

I was under the impression that PR was: p2/p1.

The spreadsheet would take a sea level standard day (14.7 psi) and if you boosted it by another 14.7 PSI you would in theory have a MAP of 29.4 PSI and a CR of 3:1 [(14.7+29.4)/14.7] not the expected 2:1. What is confusing is that there are spreadsheets and webpages that define PR BOTH WAYS... even though intuitively I want to say 2:1 sounds right for the above example.

What really threw me for a loop and challenged my thinking is the Mass Air Flow calculation on the spreadsheet. It doesnt base its number on the CR calculated higher up on the form.. it uses RPM, engine displacement and Manifold air density based on separate user input values of pressure and temp... so it "backs up" what I previously considered an error.

It was the second realization that really threw me for a loop.. A big one. And right now I cant say WHAT is right and what isnt.. since the second "questionable" calculation is based on semi-independent data with regards to the first "error".

I would recommend bouncing this off Ed Anderson.. he seems pretty engineering savvy... at the moment.. I am tired, I'm perturbed, and I have to be up in 6 hours to get ready for work.

Talk to you guys later.
Dave
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Old 08-29-2004, 07:52 PM
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I think I can help here. The following quote from a turbo specialist describes how PR is calculated:

46" MAP @ SL is about 46/30 = 1.5 pressure ratio. At 10,000 ft, barometric pressure is about 20". so the PR @ 10,000 ft is 46/20 = 2.3. All this is predicated on the manifold air temp being the same!! There is a temp correction for air pressure (remember the gas equation PV = nRT, and Boyles and Charles laws.)


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Old 08-29-2004, 08:24 PM
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Default Well, Hmmm...

John.. if thats the case, the spreadsheet that Dust was given calculates CR (PR) incorrectly which is a big part of the numbers error that is on there. that webpage I kept touting does the same thing too, (adding "map" to "ambient" for calculating PR) but I had noticed it and corrected it to place "boost" in the "map" column.

The next problem is that I need to go through it tonight if able and figure out the second equation, that lists lbs/hr based on displacement/rpm/density. I remember seeing that a Rotary is supposed to be equivalent to a 160 CID 4stroke 4 cyl engine... I am going ot have to run through all that again and see if its fuzzy math or not.

Its very frustrating when you have to go through a "resource" to see if its junk science or not.

Dave
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