Picking an A/R for primary/high pressure turbo...

[adeso]

You want to size it based on the optimization of where the kit will spend the most of it's time. A race kit will not have the same setup as a DD kit. I would talk to one of the well known kit makers about what your goals are.
The high pressure turbo does 2 things.
1. being smaller it spool much better, giving it more DD ability. Most of the time you are driving 95% on just the high pressure turbo.
2. It compresses the air in addition to what the primary or atmo/ low pressure turbo(s) are. A single stage of compressor can only do so much "work" mass flow with compression. The secondary (HP) turbo does additional work to that air to further compress it (obviously not with more mass flow that is set from the primaries). You do not want to do any extra work to the air, you only compress it enough to overcome the restriction (ie the intake).

You don't want to do more work than necessary to the air since it only adds heat (the air can be over 500*F coming out of the high pressure turbo) and doing extra work only creates more backpressure and temps on the turbine side of things.

So that is why I like the idea of a tight, small turbine on the HP turbo with a big waste gate, quick spool then run it at a lower CR than if it was single, and look at the rest of the engine to keep the mass flow going without the higher CR. Heat is not what you want on the intake side, heated air takes up more area per mass.

 

[adeso]

And you want to size the LP turbo(s) so that they are not too hard to spool, but you already have the HP turbo spooled up so it makes it easy. Therefore you size the LP turbos so they are optimized at a high power setting, and therefore do not need a wastegate on them

 

[minerigger]

Tons of good info here. My mind is almost confused but the more i read it the better i understand it

Sent from my DROIDX using Tapatalk 2

 

[Hot COCOAL]

You want to size it based on the optimization of where the kit will spend the most of it's time. A race kit will not have the same setup as a DD kit I would talk to one of the well known kit makers about what your goals are.
The high pressure turbo does 2 things.
1. being smaller it spools much better, giving it more DD ability. Most of the time you are driving 95% on just the high pressure turbo.
2. It compresses the air in addition to what the primary or atmo/ low pressure turbo(s) are. A single stage of compressor can only do so much "work" mass flow with compression.The secondary (HP) turbo does additional work to that air to further compress it (obviously not with more mass flow that is set from the primaries). You do not want to do any extra work to the air, you only compress it enough to overcome the restriction (ie the intake).

You don't want to do more work than necessary to the air since it only adds heat (the air can be over 500*F coming out of the high pressure turbo) and doing extra work only creates more backpressure and temps on the turbine side of things.

So that is why I like the idea of a tight, small turbine on the HP turbo with a big waste gate, quick spool then run it at a lower CR than if it was single, and look at the rest of the engine to keep the mass flow going without the higher CR. Heat is not what you want on the intake side, heated air takes up more area per mass.

And you want to size the LP turbo(s) so that they are not too hard to spool, but you already have the HP turbo spooled up so it makes it easy. Therefore you size the LP turbos so they are optimized at a high power setting, and therefore do not need a wastegate on them

Would you mind delving a little deeper into explaining/defining the lines I highlighted in red, please...

So what I'm getting about considerations when setting up multiple turbo set ups is: and the following is slated in the format of a question

When choosing the high pressure/secondary/valley turbo, really the turbo you're gonna be after is a turbo that provides a responsiveness you're comfortable/happy with...in a single turbo terms of thought regardless of the A/R

The primary's should be chosen based off the secondary turbos potential, their size should be chosen based off where/when the secondary turbo falls out of its island of efficiency, and based off of a principle of when they can be spooled to overcome the intake of the secondary but not yet be engaged until the secondary turbo "needs" them to be....

And this "need" ^^^ is where the wastegate comes into play? Bypassing the secondary at a given drive pressure, then the diverted gasses basically pass the secondary and are used to power/drive the primary/low pressure turbo(s)

Am I understanding this correctly

Thank you for the detailed post

 

[adeso]

First off, any numbers I throw out there are off the top of my head-I'm a jet propulsion guy and not an expert like the well known kit makers...

You low pressure turbo(s) will set the maximum flow though a setup, if it is set up close to correctly.

As far as how a turbo works-"work" done to the air is taking a mass (and systems flow in CFM, not mass flow) and compressing it.

Mass flow is the weight or number of molecules of air. As you do work (ie compress) the air, you introduce heat which excites the air, and it wants to take up more space between the air molecules. If you take 10 lbs of air, and put it into a sealed container, as you heat it the pressure goes up, and it goes down as it cools. If you take that air and compress it, then exchange the heat (ie the intercooler) the pressure will go down even though it is still the same amount compressed.

Now, what you want into the engine is the most mass flow as you can into the engine. Heat makes that air take up more room or have to held at a higher pressure. Since we have to reject the heat into the stack (the intercooler-although technically it is an after cooler but I won't diverge), so you need to figure out how to move the most mass flow (not CFM) at the lowest temp. so that means not compressing it or doing any more work than necessary.

Once the air mass enters the system (so once it passes the air filter) you cannot change the mass of that (unless you are spraying something). You can compress and cool/heat whatever to it but you have the same mass flow. Hence why the low pressure turbo sets the max HP of the system (assuming nothing downstream is choking the flow).

A centrifugal compressor can only compress a mass so much per stage. You can change the design to help it a little but it is around CR of 3.5ish. (standard atmosphere is 14.7 PSI CR of 3.5 gives you 36ish PSI) Now a compressor only cares about the rise across the compressor, so now your HP turbo does say a CR of 3.0 and you end up with 73PSI. Now if you don't need that much work then you can push the HP turbo smaller and just know it will run a little to the right on the MAP but help you out on the low end with easy spool.

 

[Hot COCOAL]

Sooo....

When you say the low pressure or "primary" turbo(s) set the maximum flow/volume through the system what is the determining factor? Is the intake restriction based off the size of the air intake pipe? Or is it solely a result of the turbos limits?

The turbo systems measure flow in cfm (ie the volume) and the engines monitoring system takes measure of mass air flow (ie air density)...is this right?

If I am understanding this, the mass air flow of the system is quantified by the size of the air intake pipe and the low pressure turbo(s) and the mass flow cannot exceed that potential regardless of the cfm the system moves.

So does that mean The amount of cfm the system moves only produces pressure/boost? Not mass air flow....is that correct?

Furthermore, is the relationship between the maximum boost pressure a multiple turbo system can make directly related to the amount of Mass air flow the system can take in?



Thanks again for all the explanation

 

[adeso]

Sooo....

When you say the low pressure or "primary" turbo(s) set the maximum flow/volume through the system what is the determining factor? Is the intake restriction based off the size of the air intake pipe? Or is it solely a result of the turbos limits?

Whatever mass enters the system (the intake) cannot be changed. If your intake only flows X lbs/min they only X lbs/min will be in the system. You cannot create or destroy mass. CFM is how much space that mass takes up per minute.

The turbo systems measure flow in cfm (ie the volume) and the engines monitoring system takes measure of mass air flow (ie air density)...is this right?

well not really. I'm not sure how to explain this without diving into thermo 101.

If I am understanding this, the mass air flow of the system is quantified by the size of the air intake pipe and the low pressure turbo(s) and the mass flow cannot exceed that potential regardless of the cfm the system moves.

So does that mean The amount of cfm the system moves only produces pressure/boost? Not mass air flow....is that correct?

Furthermore, is the relationship between the maximum boost pressure a multiple turbo system can make directly related to the amount of Mass air flow the system can take in?

It is a lot more complex than that.



Thanks again for all the explanation

Really you need to dive into thermo to understand this, at least how I'm explaining it. You need to understand the difference of mass flow (mass flow is the derivative of mass moved over time). What CFM is, how temp and pressure affect a flow. Enthalpy and entropy etc.

http://en.wikipedia.org/wiki/Enthalpy


http://en.wikipedia.org/wiki/Entropy


http://en.wikipedia.org/wiki/Mass_flow_rate


http://en.wikipedia.org/wiki/Actual_cubic_feet_per_minute

 

[Hot COCOAL]

Yeah, I've been told to look into thermal dynamics before, after a slew of questions such as these, and I turned a page or two and quickly realized that "thermal 101" is not for the feint of heart...:rofl:

I knew this was coming....

It would seem like I need to go back to school, seriously, I may look into some night classes...

I really want to understand this....

Thanks again for you're explanations and time :hug:

 

[Hot COCOAL]

http://en.wikipedia.org/wiki/Exergy

I thought this was relevant as well