Today's Message Index:
----------------------
1. 06:09 AM - Superchargers and things (Gary Casey)
2. 06:51 AM - turbo vs. super (Fergus Kyle)
3. 07:20 AM - Pressure dump on supercharger application. (Alex Balic)
4. 07:34 AM - Re: Pressure dump on supercharger application. (Archie)
5. 07:45 AM - Re: Pressure dump on supercharger application. (Charlie & Tupper England)
6. 09:43 AM - Re: Superchargers and things (James R. Cunningham)
7. 01:09 PM - Re: Turbo VS.. Supercharger again (LessDragProd@aol.com)
8. 07:05 PM - Re: Turbo VS.. Supercharger again (AI Nut)
9. 08:19 PM - Re: turbo vs. super (j1j2h3@juno.com)
Message 1
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| Subject: | Superchargers and things |
--> Engines-List message posted by: "Gary Casey" <glcasey@adelphia.net>
More excellent discussion:
<<This fixed drive ratio means that at maximum engine RPM
you will get maximum boost from the Supercharger..... you will need to dump
boost at sea level>>
Actually, the simplest, no extra parts, method to control the boost with the
supercharger is simply to take off at part throttle. Some turbocharger
system use this method (C-TR182) and it works just fine. Just advance the
throttle until you get to the red-line on the MAP gauge. At some elevation
you will be at WOT ("critical altitude") and above that you will start
losing manifold pressure. I am planning for a critical altitude of about
12,000 feet so I can maintain about 75% power at 18,000 feet.
<<Have you looked into the following approach? A 1 1/2" butterfly valve
is installed between the SC and engine to act as a waste gate.>>
An interesting approach, but dumping flow downstream of the supercharger
will waste more energy than throttling upstream. Unfortunately, I think the
normal location of the engine throttle is downstream of the supercharger as
this works out much easier for automotive applications. There are really
three ways to modulate boost (not counting changing the drive ratio). You
can throttle upstream, bypass downstream, or throttle downstream. They are
all equivalent when operating at the design point, but at part throttle or
when the supercharger is capable of supplying more boost than required they
each have their problems. Throttling downstream is probably the worst as it
allows the supercharger output pressure to rise higher than what is
required, increasing the power required along with unnecessarily increasing
inlet air temperature. Bypassing downstream is second best as it limits the
output pressure, but requires the supercharger to compress more air than the
engine needs. Throttling the inlet is more efficient because event though
the supercharger is operating at a higher pressure ratio than required the
mass flow is reduced, reducing the drive power required. I'd like to hear
some comments from those experienced in the subject here. One thing you
could do to implement a downstream blow-off is apply a standard aircraft
aneroid-operated blow-off valve and adjust it for the maximum manifold
pressure desired. Fairly low cost and very few moving parts.
>>One interesting point I need to investigate is the suggestion
that I received recently, that turbo over speed might be possible, because
at some point, the exhaust will be driving the compressor against reduced
resistance due to low ambient pressure at high altitude, I have been trying
to research this effect with no avail, - anyone out there have any
information/experience with this?>>
You can get turbo maps from a number of sources and you will note that yes,
the turbo can be over-speeded at the highest altitudes. Matching has to be
carefully done so that the system will stay in the highest efficiency area
while avoiding overspeed. I believe Turbonetics can supply maps.
On the subject of exhaust plume drag I believe Jim made a comment among his
other excellent comments:
<<Total pressure, static pressure, or dynamic pressure? How much lateral
spread in the increased pressure and how do the components transfer?
Have you mapped it with pressure probes to confirm the extent? What
effect does pressure recovery have on it? At the range of Reynolds
numbers involved, how substantial is the contribution of aft pressure
recovery in reducing that drag?>>
The answer is I don't know to most of the above questions, but let's take a
step back and look at the problem from a distance. One can assume that the
exhaust plume itself has a drag that is equivalent to a solid pipe at the
beginning and then fades away as it is engulfed into the airstream. I don't
know how to estimate the total equivalent frontal area of the plume, but it
certainly is significant. This drag requires power to push the plume
through the air at the aircraft airspeed. Where does the power come from?
I see no other source except the aircraft itself. It doesn't come from the
kinetic energy of the exhaust itself because that is a vector perpendicular
to the airspeed. Therefore it has to add airframe drag that is the same as
the drag of the plume itself, regardless of how it is manifested.
<<There has been one instance that I know of in this design where the SC
belt took out
the alternator belt and the timing belt resulting of course in sudden quiet
engine syndrome.>>
Just a comment here - I have seen a number of Subaru conversions where the
timing belt shields were removed, apparently to save weight. I would
recommend these stay on as protection against just such an event. That's
why they are there in the automotive application.
<<The wastegate
is opened at high altitude because the turbocharger would create too much
boost if not controlled.>>
A turbocharger running with an open wastegate is not a freebie. Ever
measure the back pressure on a passenger car application as a function of
engine speed? This is sort of like keeping the engine speed constant and
lowering the altitude (high engine speed in a car engine being similar to
low altitude in an aircraft application). One car that I measured reached
full boost at about 3,000 rpm (this is sort of equivalent to running full
throttle, waste gate fully closed at the critical altitude). Everything is
running at maximum efficiency and the back pressure was about the same as
manifold pressure, 12 psi. As the engine speed goes up, the waste gate
opens to maintain the same manifold pressure, and guess what - the exhaust
pressure continues to rise roughly in direct proportion to engine speed. At
6,000 rpm, 12 psi boost, the back pressure was about 30 psi. A waste gate
is not a free lunch and that's one reason why a turbo has to be very
carefully matched to the engine and its intended use.
Gary Casey
Message 2
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--> Engines-List message posted by: "Fergus Kyle" <VE3LVO@rac.ca>
Cheers,
May I echo previous sentiments (especially 'Marge's') regarding
the exchage of views on the subject? As a Driver, Airframe, I have enjoyed
the byplay and learned much. I used to run Wright Turbo Compound 3350's with
the recovery turbines in 3's on each engine, and when a broken exhaust valve
stem went down the exhaust and met the turbo blades an exciting and
colourful event continued to landing............ This was not infrequent in
Trans At service on Connies.
But I'd like to introduce an exchange on pressurising intakes by
asking if anyone can confirm the details of using the 115horse Rotax in
cruise UAV's at altitude? I believe someone said they compress the intake 4
times and that at 70,000 feet the little engine produces 70% compression
energy and only 30% propulsion.
Any takers? (....and please don't tell me you have to shoot me).
Ferg, Europa monowheel 914
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Message 3
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| Subject: | Pressure dump on supercharger application. |
--> Engines-List message posted by: Alex Balic <alex157@direcway.com>
Randy,
About what you suggested - the butterfly valve- to control supercharger
boosted manifold pressure at sea level- is the reason I decided not to
pursue the belt driven unit. I understand that boost can be dumped, but the
supercharger is still running at full speed and wasting power from the
engine- actually, a simple pressure relief valve ( like the one that is
currently on my intercooler) would do the same thing, except that it would
not be adjustable from the cockpit, to keep the manifold at say 6psi
regardless of how much pressure the supercharger is making, but for so much
of the time that the system is working , pressure would be continuously
dumped, and it just seemed like a lot of waste to drive the supercharger and
not use all of the air that it produces...... I suppose it would still work,
but it didn't seem like the proper way to do it to me.......
Message 4
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| Subject: | Re: Pressure dump on supercharger application. |
--> Engines-List message posted by: "Archie" <archie97@earthlink.net>
> --> Engines-List message posted by: Alex Balic <alex157@direcway.com>
>
> Randy,
>
> About what you suggested - the butterfly valve- to control supercharger
> boosted manifold pressure at sea level- is the reason I decided not to
> pursue the belt driven unit. I understand that boost can be dumped, but
the
> supercharger is still running at full speed and wasting power from the
> engine- actually, a simple pressure relief valve ( like the one that is
> currently on my intercooler) would do the same thing, except that it would
> not be adjustable from the cockpit, to keep the manifold at say 6psi
> regardless of how much pressure the supercharger is making, but for so
much
> of the time that the system is working , pressure would be continuously
> dumped, and it just seemed like a lot of waste to drive the supercharger
and
> not use all of the air that it produces...... I suppose it would still
work,
> but it didn't seem like the proper way to do it to me.......
Remember the load imposed on the blower drive is proportional to
the pressure buildup. A zero buildup would allow the blower to
essentially freewheel. (minus bearing and parasitic loads)
A simple, adjustable, spring loaded wastegate could easily be
fabricated to dump any overpressure.
This would hardly be used if the blower drive ratio, and blower
efficiency were factored in.
In racing, this is not an issue inasmuch as the only pressure
relief is in the form of a "blow plate" which relieves excessive
pressure in case of backfire.
Archie
Message 5
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| Subject: | Re: Pressure dump on supercharger application. |
--> Engines-List message posted by: Charlie & Tupper England <cengland@netdoor.com>
Alex Balic wrote:
>--> Engines-List message posted by: Alex Balic <alex157@direcway.com>
>
>Randy,
>
>About what you suggested - the butterfly valve- to control supercharger
>boosted manifold pressure at sea level- is the reason I decided not to
>pursue the belt driven unit. I understand that boost can be dumped, but the
>supercharger is still running at full speed and wasting power from the
>engine- actually, a simple pressure relief valve ( like the one that is
>currently on my intercooler) would do the same thing, except that it would
>not be adjustable from the cockpit, to keep the manifold at say 6psi
>regardless of how much pressure the supercharger is making, but for so much
>of the time that the system is working , pressure would be continuously
>dumped, and it just seemed like a lot of waste to drive the supercharger and
>not use all of the air that it produces...... I suppose it would still work,
>but it didn't seem like the proper way to do it to me.......
>
Also, a simple popoff valve like that will only give 6psi above ambient
pressure. Fine at sea level but it won't 'normalize' at altitude.
Message 6
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| Subject: | Re: Superchargers and things |
--> Engines-List message posted by: "James R. Cunningham" <jrccea@bellsouth.net>
Gary Casey wrote:
> , but let's take a
> step back and look at the problem from a distance. One can assume that the
> exhaust plume itself has a drag that is equivalent to a solid pipe at the
> beginning and then fades away as it is engulfed into the airstream. I don't
> know how to estimate the total equivalent frontal area of the plume, but it
> certainly is significant. This drag requires power to push the plume
> through the air at the aircraft airspeed. Where does the power come from?
It was added to the plume in a series of increments, as the free stream
intake air was brought up to aircraft speed in the induction system, and
as the fuel in the tanks was accelerated to flight speed. Energy for
both is provided as part of the waste energy from the fuel burn.
> I see no other source except the aircraft itself.
See above.
> It doesn't come from the
> kinetic energy of the exhaust itself because that is a vector perpendicular
> to the airspeed. Therefore it has to add airframe drag that is the same as
> the drag of the plume itself, regardless of how it is manifested.
It was manifested when the air and fuel mass was first accelerated to
flight speed -- not manifested after it exhausted perpendicular to the
airstream. Turning the exhaust aftward does not reduce that loss, but
the heat energy placed into the exhaust during combustion more than
compensates for it, when the exhaust is directed aftward.
Message 7
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| Subject: | Re: Turbo VS.. Supercharger again |
--> Engines-List message posted by: LessDragProd@aol.com
Anyone flown an AT-6?
If I recall correctly. It has a supercharger which was limited to 8" of
boost at sea level for take off.
You applied the throttle, and checked the manifold pressure gage. When you
had the maximum allowed manifold pressure, you stopped applying throttle.
Maybe we could talk of this as a manual operated waste gate in the intake
system with an operator hand-eye feedback system.
(I love a good technical discussion)
I believe the throttle had a soft stop at about the maximum sea level
manifold pressure. By shifting the throttle, you could go past the soft stop for
operations at higher altitudes.
Jim Ayers
Message 8
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| Subject: | Re: Turbo VS.. Supercharger again |
--> Engines-List message posted by: "AI Nut" <ainut@earthlink.net>
I don't have any firm experience with this, but what I was told while
"designing" my system that turbine speed is largely dependent upon the mass
of air passing through the engine so at altitude, you shouldn't have a
problem. However, that is predicated upon whether you choose a turbo that
is "larger" than your engine's needs. With the reduced resistance on the
compressor side at altitude, it can spin faster. No one could give me any
exact formulae for the resultants, but it the turbo is rated to at least
30-50% higher sea level speed than the engine can put out, you should still
be safe. For example, the T3 that I put on a 1.9L Ford engine is rated at
about 150k rpm, but at sea level max, the 1.9L airflow will only push it to
between 80 and 100k, giving plenty of headroom.
If it blows itself up once I get in the air, and do altitude testing, I'll
be sure to retract my statement 8-).
So nothing definitive, just a rule of thumb is all I have available.
HTH,
AI Nut
----- Original Message -----
From: "Alex Balic" <alex157@direcway.com>
Subject: Engines-List: Turbo VS.. Supercharger again
> --> Engines-List message posted by: Alex Balic <alex157@direcway.com>
>
> I looked into both supercharging, and turbo charging the SVX motor that I
> have installed in my Velocity. Both cost about the same to install, both
> would be able to be designed for the same boost at takeoff, but the
problem
> that I came across with the supercharger installation is that it is
> (obviously) connected directly to the crank shaft of the engine, and has a
> fixed drive ratio. This fixed drive ratio means that at maximum engine RPM
> you will get maximum boost from the Supercharger. This is not a problem
> with boats and cars, since they operate at sea level generally, but for an
> aircraft, it creates a problem with system design, since in order to
obtain
> full boost at altitude, you will need to dump boost at sea level, or use
> some type of transmission/slip clutch arrangement to de-rate the
> supercharger at higher ambient pressure of sea level. I investigated as
> many options to do this as I could find, but finally decided that all of
> them would be a lot less reliable than a turbocharger/waste gate system.
My
> current Garret T3 hybrid system will produce 4 pounds of boost up to about
> 12,000', and drop off as a normally aspirated engine would at higher
> altitude, and at sea level, the turbo is simply throttled back by
bypassing
> it with the waste gate. I have an intercooler to control the inlet charge
> temperature, but I would have used one either way, so that creates no
> difference. One interesting point I need to investigate is the suggestion
> that I received recently, that turbo over speed might be possible, because
> at some point, the exhaust will be driving the compressor against reduced
> resistance due to low ambient pressure at high altitude, I have been
trying
> to research this effect with no avail, - anyone out there have any
> information/experience with this?
>
>
Message 9
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| Subject: | Re: turbo vs. super |
--> Engines-List message posted by: j1j2h3@juno.com
There is a pretty good article on turbocharging in the December issue of
Hot Rod magazine. It is subtitled "How to turbo any engine" and is the
first in a series.
Jim Hasper - RV-7 just starting empennage (setting up shop in Franklin,
Tennessee)
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