Colin: True that the supercharger uses horsepower, as does every other thing
hanging off the accessory case. Unlike a turbo however the power increase is
there from the get go. A turbo depends upon high revs to produce power which
increases the wear on an engine. The supercharger however produces most of
it's power at lower rpm's where we tend to normally operate. The
supercharger has been used for many years and in WW2 many of the fighter
aircraft used them sometimes in tandem, most notably the British Spitfire
and American P51 Mustang. The point that they are not controllable is not
true as they can be set up either at the factory by the manufacturer to
operate in a certain boost range or can be manually controlled with a waste
gate much the same as a turbo. The main advantage to the supercharger over
the turbo however comes when the throttle is opened up. The supercharger is
working right now, whereas it takes time for the turbo to spool up and
develop its power, much the same as a jet engine. As you can see this could
be a deadly situation as in a missed approach or another emergency when
instant power is required. The turbo does create horsepower, yes but the
supercharger on the other hand produces not only horsepower but more
importantly torque, and that when all is said and done is what is needed to
swing that overgrown fan on the nose of the aircraft.
What it really comes down to however remains a personal choice. I
have used both superchargers and turbos over the years and both work well
but in the case of an aircraft type application I believe one would be more
satisfied with the results obtained from using the supercharger. This would
be an excellent time for Mr. Hoover to chime in here as he has flown
aircraft that have been equipped with superchargers and I for one would be
very interested in his input on the subject.
Doug Rupert
Simcoe Ontario
-----Original Message-----
From: krnet-boun...@mylist.net [mailto:krnet-boun...@mylist.net] On Behalf
Of Colin & Bev Rainey
Sent: Saturday, April 10, 2004 4:03 PM
To: KRnet
Subject: KR> Supercharging vs Turbocharging
Doug & netters,
The main reason for using a turbo instead of the supercharger is that a
supercharger relies on cannibalizing horsepower from the engine to make
horsepower, where the turbo uses "free" horsepower derived from the exhaust
pulses of the engine. The supercharger renders horsepower at the cost of
some, and they are not controllable for the amount of boost like the turbo
is. They are usually configured and then "on" all the time. Turbos can be
set so to speak. Typically where people run into problems with overheating
and such is 1) the restriction in the free flowing exhaust "backs up" heat
into the engine, & 2) the heat introduced into the fuel/air mixture due to
the heat generated from compressing the air. Number one complaint has
typically been solved by increasing air flow around the exhaust area, and
making boost at higher rpms where the momentum of the turbo can be
sufficiently built up to actually begin to help scavenge exhaust out of the
engine. Number 2 is dealt with by using low compression pistons, higher
octane fuel, delayed turbo response, intercoolers, or combinations of those
things.
All that having been said, and at the risk of making an
oversimplification, if one were to use the ONLY for normalizing the engine,
and therefore have a manual wastegate that allowed the pilot to only
"activate" the turbo above a density altitude of say 3-4000 feet, than the
engine should just behave as if it was still at those altitudes, if one did
not allow the manifold pressure to grow beyond the inches that are
experienced below that altitude. One would then "trick" the engine into
believing it was still down lower. I think most problems are encountered
when the temptation to try and tap the additional performance increases
offered by the turbo at lower altitudes that gets most pilots into trouble.
The additional heat, AT ALTITUDE would be a beneficial thing, both being
added to the cowling temp, and the mixture, as it would help the engine from
being too cool at altitude (above say 5000feet), and would assist in good
air/fuel atomization or mixing as well. Remember, even in our modern cars,
there is a certain amount of engine heat that must be added to the air/fuel
mixture in order to prevent the gasoline from "falling out" of the mix due
to condensation within the intake manifold. This is where some have had
tuning problems due to the surging and rich/lean running because the fuel
falls out and then puddles, then gets sucked into the next cylinder as too
rich and the engine surges, and cuts out, sometimes minor, sometimes major.
One other caution about turbos: make sure you know what kind you have,
blow through, or draw through. A blow through design lacks the seal
necessary to draw air/fuel mixture through the carb, where the turbo would
be mounted AFTER the carb, but before the engine. The other design draw
through has these seals. Most turbos are the blow through design, and draw
outside air in, then compress it and send it on to the carb or fuel
injection system. Also turbos typically allow for the elimination of any
kind of muffler due to their quieting effect.
Flame as necessary......lol
Colin & Bev Rainey
KR2(td) N96TA
Sanford, FL
crain...@cfl.rr.com
or crbrn9...@hotmail.com
http://kr-builder.org/Colin/index.html
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