Only shortcoming of the water wheel analogy is that water can't compress. If it could, and you compressed the water to 2x it's original density, you would then be able to pass twice the amount of water past the wheel at the same wheel speed (ignoring temperature).  Stilling wondering if my previous post is what prj is referring to.

I understand your point of view, I guess it depends if there is a direct relation between volumetric and mass flow rate or not. It is confusing that different units are used between manufacturers to describe compressor performance as well. In any case, my comments are only meant as help for your project to succeed.
 For boost control it may be sufficient to only monitor the large turbo pressure for control, the small turbo needs to open all gates as soon as useable boost is made on the large turbo in my opinion. No need for high boost if you can have proper flow, keep temps low and timing high :-)

How about a more big turbo oriented control, you can use throttleplate angle for part throttle while the k03 is at 95% dc. You could run the hp gate from lp pressure to make it switch to bypass mode automatically and use lp Watergate for further control. Or if you can use 2 n75 you can keep k03 iwg control for low end and fix n75 duty to 0% from a certain pressure up and take over with lp n75 control?

elRay is correct here.

The exhaust side 100% needs a bypass, the compressor doesn't.  However, the power available is less than if the larger turbo is used on its own.

The large turbo has compressed the air before it enters the small turbo, so its volume for a given mass is much smaller than it would be entering at atmospheric.

Rick

Better for sure.  Have experienced it on 4G63 and Cosworth YB but with basic mechanical control.

Rick

If I'm controlling both turbos WGs with 2 N75, 95% for small and big turbo at low end, and then xx% for big turbo at top end, why not just control both all the time???  0% to small turbo is still controlling it.

For small turbo, I can picture a kfdmix with 0% on the top-end. Right now, with 10% big turbo duty, small turbo duty automatically goes to 0% without me changing anything just due to PID control even with a ~95% kfdimx value.

The problem with making small turbo kfdmix 0% up top, is what if big turbo is not spooled yet? For instance when starting WOT already at high RPM or gear shifts? I would want small turbo to be 95-100% for the few seconds it takes big turbo to respool. So, a static kfdmix will not work for that situation.

I think I need an offset or factor map applied to small turbo's kfdmix output based on big turbo's current boost pressure.

mcorr = m*sqrt(T2/T1)/(P2/P1)

mcorr = Corrected Mass Flow (new x-axis)
m = Actual Mass Flow

P1 = STP Reference Inlet Pressure
P2 = Compressor Inlet Pressure

T1 = STP Reference Inlet Temperature (*K)
T2 = Compressor Inlet Temperature (*K)

Course the pain is that not every turbocharger manufacturer's "corrected" conditions are the same... Hence why some measure in mass flow rather than corrected volumetric flow, since that depends on the environment.



In this situation - taking into account what I said above - if the engine is capable of consuming 25lb/min, if you make the compressor inlet temperature 40*C and pressure 2bar(abs) on the inlet, when you correct the mass flow assuming, it's equivalent to that turbocharger moving 18.42lb/min.

25*sqrt(313/288)/(2/1) = 18.42lb/min

I'm fairly sure I'm correct here... Issue being you run up against the choke line quite quickly at low PR - look at the trend of the compressor shaft speed lines, at higher airflow, even at lower pressure, you're tending towards higher and higher compressor shaft speeds and into choke...

That's a sequential system though.