|
|
Walter,
Thanks for the reply, I welcome and respect your thoughts and experience.
You (John & George) get my full support and recommendation :-).
You're right - my post/message was perhaps a bit misleading as I generalized
using MP (versus upper-deck pressure) - technically incorrect and further
discussed below.
Also, I don't have any "personal" experience with reduced bleed air heat
capability at low power settings on TSIO-550's. So, I'm all ears and
cautious about spreading OWT's. However, I have heard anecdotal evidence of
this from a few current turbo'd Lancair flyers.
I'm interested in this phenomenon because I'm considering exchanging the
standard TSIO-550 turbo's for some with a higher A/R ratio for an
un-pressurized ES. I anticipate the reduced-heat-during-low-power-operation
phenomenon to be worst if I make the exchange. Perhaps resulting in the
need for traditional exhaust heat muffs - with their inherent risk of CO
with any exhaust leaks.
Discussion...
In my prior post, I believe making a distinction between upper-deck pressure
and manifold pressure (MP) would have been more accurate. With upper-deck
pressure defined as the air pressure generated by the turbo (between the
turbo-compressor output and the throttle butterfly value). And with
manifold pressure being the pressure downstream (inside) of the throttle
butterfly value (i.e. in the intake manifold). I believe those to be common
definitions.
With these definitions, the turbo bleed air heat capacity would seem to be a
function of upper-deck pressure relative to ambient pressure (not
MP-vs-ambient as I previously/simplistically mentioned). For example, on
the ground, ambient pressure may be 29"ish and upper-deck pressure may be
nearly the same. Note: I would need an upper-deck pressure gauge to
verify/monitor this. In this scenario, the turbos aren't
pumping/compressing much air (low upper-deck pressure), hence the intake air
is not being heated, hence the bleed air to the cabin is not hot, hence
reduced cabin heat capability. (again only supposition, I'm looking for
verification).
This could be uncomfortable in Jan while waiting for an IFR clearance in
Detroit or while waiting behind a few jets at Chicago midway.
It's true that it takes a few minutes for exhaust heat muffs to warm up as
well, but they usually do so during taxi and/or run-up prior to takeoff. In
fact, it's a requirement on my personal pre-takeoff checklist to insure
there's at least some defrost/engine heat being generated to help warm the
oil - since the old analog oil temp gauge on my Cessna is pretty suspect and
since the defrost might be necessary.
A similar scenario may result during a low power descent(???). How much so
is unknown to me. I also understand that there are "fixed" deck / waste
gate pressure controllers and "variable" pressure controllers. Fixed
controllers attempt to maintain a constant (max) deck pressure (at all
times). Whereas, variable pressure controllers allow the deck pressure to
fluctuate with throttle position (read MP). Both types control the exhaust
waste gate to modulate turbo operation.
Thus, the fixed type will/should cost some efficiency @ low power (low MP)
since the turbos are still attempting to fully pressurize the upper deck
(unnecessarily), but will generate heat (good for cabin heat). Whereas, the
converse is true (to some extent) with a variable controller. I do not know
which type are standard on a TSIO-550E (research in progress).
Ultimately, my understanding is that a combination of MP (actually manifold
air density) and RPM determine (exhaust) mass air flow (for a given engine
displacement), and along with exhaust temperatures and pressures (relative
to ambient) determine the energy available to drive the turbo-turbine, which
drives the turbo-compressor, which compresses the input air, and which heats
the input air as a by-product of the compression (desirable for cabin heat,
undesirable for engine intake - hence the intercoolers).
Fully (quantitative) understanding of the amount of turbo energy and/or
available deck pressure (above ambient) for various combinations of MP, RPM,
FF(EGT or TIT), and altitude (ambient temp and psi) would be very helpful -
but is beyond my current knowledge/experience base. My understanding is
that this gets pretty complex as the not only the exhaust turbo input energy
is at stake, but also the upper deck back-pressure losses through the bleed
air sonic nozzles and back into the manifold.
For the future, I'm thinking upper-deck temp and pressure would be very
helpful as added inputs to engine monitor data logging - perhaps on both
sides and also before and after the intercooler. Mostly for tuning, not
normal flight monitoring. Hmmm - that's eight extra inputs. Since, I'm
only building, that is a distant future testing/tuning project.
Unfortunately, it will be after I've selected the turbos (at least the first
ones) ;-)
I'm sure there is a fraternity of folks out there who have been down this
road. I assume that fraternity includes: TCM when they "tuned" the engines
for the pressurized birds to begin with, STC guys like Tornado-Ally Turbo,
and the Reno racers, hopefully others too. Again, I'm all ears if anyone
has any suggestions or comments - all questions welcome.
Thanks again for the follow-up.
Rick Titsworth
rtitsworth@mindspring.com
313-506-5604
-----Original Message-----
From: Lancair Mailing List [mailto:lml@lancaironline.net] On Behalf Of
Marvin Kaye
Sent: Sunday, April 16, 2006 11:06 AM
To: Lancair Mailing List
Subject: [LML] Re: TSIO-550 Turbo setup for non-pressurizedLlancairs
Posted for Walter Atkinson <walter@advancedpilot.com>:
Rick:
I think there may be one error in your assumptions about the turbo at low
MPs during descent. I could be wrong, but the compression ratio of the
turbine is always a set value. At low MPs, you just have less of that
raised
temp air. The heat rise from the compression of the gas should remain
pretty
constant. What am I missing?
(I'm anxiously awaiting the response from Craig to your 10 long
questions!
<g>)
Walter
--
For archives and unsub http://mail.lancaironline.net/lists/lml/
|
|