This is not exactly what we are looking for but there are
some clues here.
This is from SAE paper 821149 "The Effects of Turbocharger
Design and Installation on Gasoline Vehicles Transient
Response" I provided some of the instrumentation
on the R&D
reported in this paper. Note the 20 HP loss in the overall
steady state engine net HP out going from a .63 to
a .38 A/R.
This is a T3 turbo charger. R, the radius of the
turbine, is
fixed and A, the area of the nozzles, is changing.
Basically
what is going on here is more back pressure and
net HP loss
is being being generated by the smaller nozzle.
Just like electric motors you can get the same HP from a
turbine if you make a small one running at high RPM or a
large one running at low RPM. In this case the
weight penalty
is less severe in the over all scheme of things.
The larger
turbine will have the same HP loss with the same
nozzle area
but the RPM will be less and the turbine torque higher.
Consequently turbine power will be nearly the
same. Turbine
power is the bottom line for turbo compounding.
Paul Lamar
BTW Doug I made contact with my old friend at
Garret, David
Elpern (He is still there. Now Honeywell). See SAE paper
821149 The "Effects of Turbocharger Design
and Installation on Gasoline Vehicles Transient
Response". He is working on
gathering
HP data on the turbo chargers but don't hold your
breath as that is not what
they
are really interested in. You might have to build
a turbo dyno. (Not as hard
as
it sounds.)
Paul Lamar
Here is a chart from Corky Bell's book "Maximum
Boost". For a
turbo compound it looks like we need the largest A/R
available for the TO4. Checking the 1999
Turbonetics catalog
for the 13B that is 1.52 Garrett part number
407263-0039 Tang
Divided (One pipe from each exhaust port). P-trim
with a 2.544
Exducer.
For the RX8 engine with the confused side exhaust system an
undivided A/R = 1.71 should work as well. Garrett
part number
407264-0039. P-trim with a 2.544 Exducer. Big shaft in both
cases.
Here too is a clarification on the A/R number. As you move
around the scroll the A/R ratio remains the same.
BTW the A/R as near as I can calculate it for the
88 89 Turbo
II 13B appears to be about .82.
Paul
Thanks for clearing up a few points. Time to search
ebay. I'll
test the concept with what I have presently at hand, the 13B.
..... So if the RX8 doesn't have a better BSFC in mid
range and
high range and only gets it's increase in power from
higher revs
and in the high powered version larger area of intake ports,
what advantage does it have for turbo compounding
compared to a
13B?
Doug in Japan
I don't expect much from the RX8 engine except perhaps a few
hundred degrees cooler exhaust due to slightly later
exhaust port
opening and the fact the exhaust is cooled somewhat
better by the
cooling system. Slightly more heat lost to the cooling
system and
a little less coming out of the exhaust.
If the turbine would live the 13B peripheral exhaust is
probably
better for a turbo compound or a turbo charged engine.
I could be
wrong. There may be some minor advantages that will swing the
balance the other way.
The main advantage of the side exhaust ports occurs down in the
idle and low power ranges of the engines. Mostly emissions.
Paul Lamar
I did a few more calculations on the stock Mazda turbo.
Looks like
the turbine wheel is about 3 inches in diameter. Lets use
Francois's
number for turbine RPM. At 130,000 RPM or 2200 RPS the tips are
going 21,000 inches per second. 1700 feet per second. Area of the
scroll at a radius of 3.5 inches is about 2.8 square
inches. A/R =
.8. Two, 2 inch exhaust pipes would have an area of 6.28 square
inches total. The gas velocity in the scroll would then
be about 2.2
times the exit velocity from the 2 inch exhaust port.
According to Scientific Design of Exhaust and Intake
Systems exhaust
gas velocity is on the order of 200 to 300 feet per second from a
typical four cycle piston engine. So exhaust gas velocity in the
turbo scroll would then be 450 to 670 feet per second.
Obviously the exhaust gas velocity in the rotary is two
or three times that number or one would get no torque out of the
turbine.
Not in the ball park.
Another possibility is I am off on the diameter of the turbine
wheel. Lets see, a T3 wheel can be 2.319 or 2.559 inches in
diameter. Using the 2.559 number that would bring the tip
velocity
down to 1470 feet per second.
Still not in the ball park.
Vance this is right down your alley. What say you? Where
am I going
wrong? Is the exducer diameter the controlling factor?
How do I figure the exhaust gas velocity knowing the
cubic feet of
air per minute going into the engine? (P1 x V1)/T1 = (P2
x V2)/T2 ?
Paul Lamar
If I do (P1 x V1)/T1 = (P2 x V2)/T2 and assume 300 CFM in
and 100 F
in with 1600 F out with two 2 inch pipes in and out I get 1800 feet
per second out which is in the ball park. I can see how
that could be
200 HP coming out of the exhaust pipe.
Wow that exhaust gas is really moving in rotary engines. About
supersonic at those temps. No wonder it eats steel exhaust
systems for
lunch.
Check my arithmetic.
Paul Lamar
Now I understand why Star Mazda uses 2 1/2 inch exhaust pipes
and three inch down pipes on stock 13B NA engines.
--
Paul Lamar
Paul,
Gas laws only hold for absolute temperature. Kelvin for metric, Rankin
for you old fashioned types. I repeated your calculation with absolute
temp and got 420 ft/s. I also verified 1817 ft/s using fahrenheit
instead of absolute temp.
Steve Rosine
Oh right. Forgot.
Thanks steve. Sounded kind of high. Still not in the ball park however.
Back to the theory of the exducer.
Paul Lamar
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