-- 
Paul Lamar ...No rotor no motor.


Paul,

Yes, I made a mistake with the amount of boost required. To double the power
would require 15 psi or 2 bar -- and that would require inter cooling.

As for the power required to drive the blower, I don't think it would be 20
percent of total engine power -- I think closer to 10 percent, according to
info from the blower manufacturers.

Here is a flow map for a Lysholm 1.2 liter twin-screw supercharger, the
smallest they have. You can see the power lines in green.

Let's say you have an RX8 four-port engine that makes 100 hp at 3500 rpm,
and you want to add 10 pounds of boost. That's a pressure ratio of about 1.7
and will give you about 170 hp gross, not counting the losses to drive the
blower.

If we know how much volume of air we are moving, and our pressure ratio, we
can see how much power it will take to drive this blower. Since our engine
is 80 ci, and there one intake event per e-shaft revolution, we can
calculate air flow using the formula: cid x rpm / 1728. This gives us 162
cfm for the stock engine making 100 hp at 3500 rpm. (Assuming 100 percent
volumetric efficiency, but close enough).

If we boost that by a pressure ratio of 1.7, we get a flow of 275 cfm at
3500 rpm. That equals about 7.8 cubic meters per minute, so we see on the
chart that it takes about 12 kilowatt of power to drive the blower to this
level. That equals exactly 16 horsepower, or a little less than 10 percent
of our gross power figure of 170 hp. We are left with net power of 154 hp.

Let's call it 150 hp and it's still not bad. We are only adding 10 pounds of
boost so inter cooling is not needed. The fuel consumption would be a little
higher than a normally aspirated engine of the same power, but only by the
16 hp it takes to drive the blower. But on the other hand you won't have the
friction losses of the gearbox, which are probably about three or four
percent. So maybe the net difference is going to be about five or six
percent.

So yeah, the 200 hp direct-drive engine obviously does not make sense, but
perhaps the 150 hp direct-drive engine could be feasible.

You might ask, "Why bother? Why not just get the gearbox?" Well, there are
certain advantages of the lower-revving engine, including engine noise, wear
and tear, etc.

It is also cheaper -- the small twin-screw blower costs less than half of
what a gearbox costs. You also don't have any worries about torsional
vibration. The only question is the bearing loads, but at 10 psi boost it
should be okay.


Regards,

Gordon.


PS: Thanks for the anecdote, but I hope the FAA isn't lurking around here.
Is there a statute of limitations on buzzing?

---------------------------------------------------------

"Well, there are certain advantages of the lower-revving engine,
including engine noise, wear and tear, etc."

Assuming you don't overload the rotor bearings. The best cruise
RPM for a rotary is 6000 RPM bearing load wise. True if you load
it down at 3000 RPM the BSFC may be better due to lower engine
friction HP loss. You can do this with a variable pitch prop
however.

As you lower the RPM of the rotary you rapidly lose the power
to weight ratio advantage. BTW GM attempted to make a large displacement
light duty rotary engine and failed for unknown to me reasons.

The best low RPM engine is a piston engine as bearing loads
are minimum around 3600 RPM the displacement can be increased
and the structure made just strong enough for the power out level.
AKA an air cooled aircraft engine.

For this super charger it is 12KW (16 HP) to 24KW (32 HP) depending on where
in
the 64% efficiency island you happen to find your self.

10% worse BSFC is .47 to .52. 20% worse puts you up at .56.

BTW two things. Please quote at the end of the old message.
Less work for me. We had a poll and people like messages in
chronological order.

I don't suppose you have any programs to convert one page pdf's
to jpg's do you?

Paul Lamar ...No rotor no motor.


Paul,


The chart shows 24 kw [32 HP] at a flow of 12 cubic meters per minute, which
is 50
percent more than our engine can use. At ten pounds boost the rotary engine
will need at most 8 cubic meters, according to my quick calculations --
probably less. So the maximum power it will absorb in this application is 12
kw [16 HP].

Also, this is at 3500 maximum takeoff rpm. At 3000 cruise rpm or lower, it
will require less power.

And there may be more efficient blowers on the market than the Lysholm.
Autorotor, which is what Kenne Bell uses, claims to be more efficient both
in power consumption and adiabatic efficiency -- claiming up to 80 percent
in the latter, which is as good as the best centrifugal compressors.

You have a good point though that the rotary is happiest at 6000 rpm. Don't
get me wrong, I like the geared approach just fine -- this exercise was
simply a "what if?"

This supercharging approach might work better with a Subaru 2.5, for
example, which makes a little more torque in this rpm range than the rotary.
A lot of people are using these engines direct-drive in smaller planes, both
normally aspirated an turbo'd.

Sorry about the pdfs, I don't have a converter.


Regards,

Gordon.

--------------------------------------------------------------

The Lysholm is king of the positive displacement blowers I think.
Geared centrifugal blowers seem to be slightly more efficient.
There is a comparison in this months Import Tuner or Sport
Compact Car magazines.

Paul Lamar ...No rotor no motor.



Paul,

The twin-screw supercharger is sometimes generically referred to as the
"Lysholm supercharger," in honor of the Swede who invented it in the 1920s.

It's kind of a convoluted story, but apparently the company called Lysholm
is now owned by Autorotor, and they continue to make both product lines.
From what I've heard, the Autorotor is the higher-end product, with some
subtle refinements in the aerodynamics of the rotors, the gearbox, etc.

The Kenne Bell website has an article on Lysholm versus Autorotor:

http://tinyurl.com/f8m6o

In that comparison, KB claims about 10 percent less power required for
similar cfm output. Not huge, but still a nice little improvement, if it's
true.

I just want to clarify for the record the power consumption issue. The
normally aspirated Mazda rotary displaces 1300 cc, so at 3500 rpm it would
consume exactly 4.55 cubic meters [160 CF] of air, provided it had a VE of
100
percent. (1300 x 3500 / 1 million)

If you boosted the engine by a ratio of 1.7 (10 psi boost), you would have a
maximum airflow into the engine of 7.735 cubic meters [273 CF]. According to
the
Lysholm chart, that would require a power consumption of just over 10 kw
[13.4 HP] and
would give a discharge temperature of just over 90 degrees Celsius[200 F].
It would
place you in the 63 percent adiabatic efficiency island.

That does not sound bad. Cool enough to not require an inter cooler and
would
give about 155 hp net. Fuel efficiency would not be much worse than a
geared, normally aspirated engine spinning 6000 rpm. I'd guess maybe five
percent, so a fuel flow of .5 let's say.

Engine weight would be about 30 pounds less than a geared engine.

Btw, how much does the RX8 engine weigh, stripped of intake manifold and
exhaust, since you would have to fabricate those anyway?



Regards,

Gordon.

Most of your numbers are right but
the displacement is 2.6 L or 158 ci and not 1.3L.

Intake air temp of 200 F is detonation country.

What are you figuring the weight of a gear box is?

BTW most of our readers are english system so if you could please convert
some of those metric numbers to the english system it would help me.

I use http://www.onlineconversion.com/

Paul Lamar ...No rotor no motor.


Paul,

I was figuring a gearbox weight of 45 pounds, including flywheel -- just a
guess. The blower is only about 10 to 15 pounds at most, so that's a
difference of 30 pounds.

I was using metric numbers because that's what the Lysholm chart uses. For
displacement I used 80 ci, or 1300 cc, but figuring one full-displacement
intake event per e-shaft revolution. I think this is the actual displacement
of the engine, no? Isn't the 2600 cc (158 ci) number the "effective"
displacement?

I guess if you wanted to use the charts you pulled up, you would use the
effective displacement. Using that method I would calculate the rotary's cfm
requirement: cid x rpm / 1728 x 2, which gives us: 158 x 3500 / 3456 = 160
cfm, which equals 4.5 cubic meters per minute, same as before. (At 100
percent VE).

Your chart on volume flow shows pretty much the same outcome, as far as I
can tell. The other two charts are mass flow, which is even more important
in the real world, but doesn't do us any good in using the Lysholm chart
which only deals with volume.

The 200 Fahrenheit temperature is at the discharge end of the supercharger
and upstream of the injectors, where the air will be cooled by the
evaporation of fuel before it enters the combustion chamber. Yes, I agree an
intercooler would be nice, but maybe not necessary. I could be wrong.

Again, I have nothing against intercoolers, but the whole point of this
exercise is to have a super simple direct-drive engine. If you have to start
adding things like an intercooler, you have more fabrication work, more
weight, cost, etc. At that point, why not just go for the geared engine?

So perhaps if we lower our boost to 8.8 psi, for a pressure ratio of 1.6,
that would give us a 160 gross hp, 145 net hp  engine with a discharge
temperature of about 175 F. That will definitely work without an
intercooler.

Any idea how much does the bare engine weighs?



Regards,

Gordon.

The rotary pumps the same amount of air per rev as a 4 cyl 4 cycle
2.5 L piston engine.

I got some other bad news for you Gordon. The wimpy thrust bearing
in the Mazda will not tolerate direct drive prop thrust for long.
Perry Mick found this out.

-- 
Paul Lamar ...No rotor no motor.

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