Help understanding turbos better

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megastang

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I am trying to learn some more about turbo sizing. I have a pretty good grip on how to size turbos read and plot out on compressor maps but I want to understand more about what's going on on the turbine side and how to size the small turbo in a compound setup.

Maybe I'm thinking about this wrong but since learning about how to read maps I have always thought about compressors as flowing mass and on a map a compressor has a mass flow limit. When put into a compound turbo setup what is happening when the small turbo is forced to flow more than it's rated flow?

Turbines I am trying to learn more about. On Garretts site I went and looked at the biggest turbo and just did a quick comparison between the flow rates on the turbine maps and the compressor maps. The highest turbine flow curve tops out at around 70 lb/min where the right side of the compressor map is around the 160 Lb/min mark. Obviously the turbine has to flow all the air that the compressor is pushing plus the mass of the burned fuel. Is the additional mass just flowed through without producing work beyond 70 Lb/min?
 
Pressure ratios are what compounds are taking advantage of
 
I understand compounding and pressure multiplication but I don't understand what happens when you feed a compressor more mass flow than it's map says it can flow. The way I am understanding it pressure ratio should have nothing to do with it because mass flow is mass flow regardless of pressure
 
A compressor map is what xxx compressor can pressurize the air from, starting with atmospheric pressure (approximately 14.7 psi). When that same compressor is force fed with xx amount of pressure that compressor map goes completely out the window.
 
megastang said:
I am trying to learn some more about turbo sizing. I have a pretty good grip on how to size turbos read and plot out on compressor maps but I want to understand more about what's going on on the turbine side and how to size the small turbo in a compound setup.

Maybe I'm thinking about this wrong but since learning about how to read maps I have always thought about compressors as flowing mass and on a map a compressor has a mass flow limit. When put into a compound turbo setup what is happening when the small turbo is forced to flow more than it's rated flow?

Turbines I am trying to learn more about. On Garretts site I went and looked at the biggest turbo and just did a quick comparison between the flow rates on the turbine maps and the compressor maps. The highest turbine flow curve tops out at around 70 lb/min where the right side of the compressor map is around the 160 Lb/min mark. Obviously the turbine has to flow all the air that the compressor is pushing plus the mass of the burned fuel. Is the additional mass just flowed through without producing work beyond 70 Lb/min?
Click to expand...

There's a specific formula for calculating PR efficiency for compounds. Maybe someone has a link or knows of top of head.
 
experts

I definitely like math. I saw that calculator. The problem is that this is kind of a special case so I'm trying to understand some things that I usually wouldn't be quite so concerned about. $$ is tight so I try to do as much of this stuff as I can myself. I find that I'm often disappointed with the product when I get anyone else to do stuff for me (see my turbine housing thread).:doh:
 
megastang said:
I definitely like math. I saw that calculator. The problem is that this is kind of a special case so I'm trying to understand some things that I usually wouldn't be quite so concerned about. $$ is tight so I try to do as much of this stuff as I can myself. I find that I'm often disappointed with the product when I get anyone else to do stuff for me (see my turbine housing thread).:doh:
Click to expand...

If money is tight use a proven setup for your goals.
 
You should be able to derive an equation to ballpark how the compressor mass flow is affected as the intake air mass increases. The mass flow of the secondary at any given PR would change based on some factor you could come up with that's calculated with the temperature and absolute pressure of the air going in. Redoing the whole compressor map would be difficult but point by point I'm sure you could come up with a good estimate. Time to dust off the physics books.

For the turbine side, I'm only guessing here, but I think the mass flow of the turbine being less than the compressor is for the same reason that exhaust valves are typically smaller than intake valves. If you do some more digging on that topic it might lead you down the right path.
 
I think I have a pretty good idea where I'd like to start. I haven't tried but I wonder what would happen if you somehow at least partially worked in CFM on the compressor side. That may allow you to compensate for the increased density of the incoming air.... maybe

After thinking about how the BW matchbot turbine curves are and the row for percentage of wastegating, I wonder if any additional flow above the max should be wastegated.
 
If LP compressor PR is 2:1 then HP compressor flows 2x rated massflow. In real life it is not not simple because of temperature, efficiency etc. but gives you an idea.
 
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