New turbo project direction

[Bort62]

I would argue that fluid drag is the dominating factor in rotational acceleration in this particular example.

And that IS linear.

 

[JGTurbo]

I would argue that fluid drag is the dominating factor in rotational acceleration in this particular example.

And that IS linear.

I disagree with that. Moment of inertia is what keeps a turbo from spooling up quickly.

Show me some proof of this "fluid drag" concept. And since we're talking about the same Lbs/min flowing past both the twins and the single, how would the single benefit?

 

[JGTurbo]

Well, I have to leave for now. I guess I've sort of been playing the devil's advocate. I think a single turbo is the way to go. The spool up of twins vs. single is hardly measurable in a real world situation. It just works out mathematically. The single will be more reliable and easier to install.

See ya.

 

[Bort62]

You are pumping air. In a vacuum, then yes - you would be right.

However, you aren't. A very small amount of effort is used to accellerate the mass of the turbine wheel/shaft/Compressor wheel. The rest is required to accelerate the working fluid. In this case, Air.

And the force needed to acclerate 1x air is .5 2x air.

That's linear.

 

[Bort62]

Well, I have to leave for now. I guess I've sort of been playing the devil's advocate. I think a single turbo is the way to go. The spool up of twins vs. single is hardly measurable in a real world situation. It just works out mathematically. The single will be more reliable and easier to install.

See ya.

Thank's for joining us. Kept me on my toes for a minute.

I'm still right, however

 

[JGTurbo]

You are pumping air. In a vacuum, then yes - you would be right.

However, you aren't. A very small amount of effort is used to accellerate the mass of the turbine wheel/shaft/Compressor wheel. The rest is required to accelerate the working fluid. In this case, Air.

And the force needed to acclerate 1x air is .5 2x air.

That's linear.

I agree that the force needed to accelerate one pound of air will be half that needed to accelerate two pounds of air. But if an engine moves 60lbs/min with a single turbo at 10psi, it will move 60lbs/min with twin turbos at 10psi, all else being equal.

So here's the equations for accelerating air:

F=ma --> Force = mass*acceleration

Acceleration will be constant in this example. Mass of air through entire engine will be constant as well.

Twin turbos:
turbo #1 Force=1/2m*a
turbo #2 Force=1/2m*a
Total Force=(1/2m*a) +(1/2m*a)=m*a

So the sum of the forces is m*a

Single Turbo:
Force=m*a

Twin Turbo Total Force = m*a
Single Turbo Total Force = m*a

Twin Turbo total moment of inertia = (m*r^2)+(m*r^2)
Single Turbo total moment of inertia = m*r^2

So the force required to move 60lbs/min of air is the same for both the twins and the single (each twin only has to move half of total air). But the moment of inertia is going to be larger with the single due to the larger radius with the square on it. For equivalent turbo set ups, a single turbo's moment of inertia will be about 1.25 times that of the twins combined.

Sure, you can spin a turbo around by hand and it doesn't feel like it requires much force, but when accelerating up to and past 100,000rpm, the moment of inertia becomes very real.

Anyway, I'm fine with agreeing to disagree. But if you want to toss around some more math, I'm up for that too.

 

[Bort62]

I agree with you that the moment of inertia is larger w/ the single turbo.

That's just a fact. I guess my arguement is that it is second order to the drag due vicous fluid losses.

Part of the reason that I am so confident in this is that a few months ago I built a simulink model of a valve to compare effects of valve stem inertia on response time.

My conclusion was that valve stem inertia was completely dominated by the friction losses of the valv stem to the point that reasonable changes in valve stem inertia has no discernable effect on the reaction speed.

It's a similar case, except here we have fluid losses instead of valve friction (ball valve, high seal drag).

I think we both agree that the power requried to pump a constant mass of air is constant no matter how many turbos one splits it between. Your argument is that the smaller sum of moments of inertia of the two smaller turbo's is a significant factor in determining rotational acceleration.

I feel that it's not.

Without an extremely complicated model, neither of us is going to be able to prove anything

 

[JGTurbo]

Well said.

 

[Anonymous]

Back to the main question... If you search around a few posts, I am doing something similar to what youre asking about. I went to a local junkyard and pulled a two Garrett T25 turbos and their exhaust manifolds out of some cars and they are in the process of getting put on a 250. Should have some pictures up in a few days...