Paul,

What would be the sea level exhaust pressure just before the turbine
inlet in a turbocharged engine?

I'm guessing maybe 18 psi to account for a little bit of back pressure
due to the turbine?

Regards,

Gordon.


I don't have any data on that. I wish I did.

Paul Lamar ...No rotor no motor.

(Snip from my comments October '04... since I never received any
feedback on these comments, I always assumed no one understood the
real kernel of turbo wisdom here. This is how 1500 bhp/liter is
done.   So, I'll put it out there again:)

I have seen no mention of this technique in the newsletter,  so I
thought I would contribute some thoughts on turbocharger selection.
One of the damnable aspects of turbo sizing is trying to come up with
the best compromise for the dynamics of manifold pressure
generation:  how much, how quickly, how efficiently, and how costly.
Automotive turbo applications are driven by a much different set of
goals, primarily their wide-range dynamic response issues.  The
aircraft turbo application is a much happier equation to solve,
whether it be for all out power, or altitude compensation.  In fact,
there is an opportunity to closely optimize the performance of all
the components so that some real magic begins to happen.
It's possible to size a turbocharger so that the engine, and its
forced induction, as a complete system, operates in 'crossover',
that is, when intake manifold pressure exceeds the exhaust manifold
back pressure driving the turbine.  The result is a net positive
scavenge flow through the cycle and the result is significant gains
in power.  Combined with the right intake/ exhaust event timing, BSFC
benefits as well.  This crossover condition can only be achieved in a
narrow band of operation,  but it happens to be the same band that
aircraft engines are looking for: Part throttle, medium load,  and
medium rpm ( as a % of max rpm)

A program goes something like this:
Select the 'frame' size, or family of turbocharger so that there is a
good selection of turbine wheels, turbine houngs, trims and
compressors available in the range of mass flows necessary. The
Garrett T04 is such a family. Then, generally on the dyno, the
turbine side of the turbocharger is selected for its swallowing
capacity, resulting in larger than normal A/Rs. The objective is to
use all the gas flow to drive the turbine, not bypass continuously
through a waste gate.  (The waste gate and its sizing still must
function as a safety/ control device, and for temporary demands for
higher manifold pressure)

The compressor selection can now be optimized for its output mass
flow and tailored for its best efficiency.  The simplest way to
verify the component selection is to monitor the exhaust manifold
pressure just ahead of the turbine inlet, in fact  the port can
sometimes be coincident with the EGT probe location.  A small damping
orifice, around .020 seems to work well, will give steady readings at
the gage.  A similar set up on the intake side, both before and
after the inter cooler.  Then you can go to work mapping the relative
pressure results vs. a whole range of operating parameters, load,
speed, temps et al.  When you home in on crossover, even if its only
a .5 psi differential, watch what happens to all the other numbers.
If you can achieve a 2, 3 or 5 psi lower turbine inlet pressure
relative to intake manifold pressure,  then you've really done a good
job.  You'll see directionally correct moves in the power,  temps,
the bsfc, the ignition timing, et al.

The resulting turbo configuration will be a hybrid:  an unusual
combination of turbine and compressor,  most likely much bigger on
both sides than the automotive application. And operating in  speed
and temperature ranges that are comfortable, safe and unstressed. My
guess is that none of the stock automotive turbo components will be
there in the end. The material specs are just not high enough
( anyone who flies without an Inconel wheel is asking for it...) The
power and efficiency numbers will be dramatic, the turbo application
is truly in its element once this balance has been achieved, and the
near steady state operation of the IC engine is one of the few times
we as engine development engineers can come close to this near
perfect condition.

Reeves Callaway

Thanks Reeves.  So re wording this a bit a good system has
2 to 5 psi lower exhaust manifold pressure than intake manifold
pressure. I'll do an example. Lets say we have 10 psi boost
above atmospheric. Then the exhaust manifold will show
5 to 8 psi above atmospheric. Did I get it right?