Finn Lassen wrote:
Just read the April 2003 issue of Sport Aviation.
Claims 14 GPH at 200 HP.
Assuming the fuel they burn also weighs 6 pounds per gallon, that's [BSFC]
0.42.
Explain again to me why that's impossible in a small turbine.
Finn
No way. Sport Aviation mag is becoming editorial ignorant and irrelevant.
Uniformed articles like this facilitate the scam artist out there.
Several reasons. The main one I think is the limited compression ratio
one can obtain with a single stage of centrifugal compressor
is only about 3:1. These APU engines are made from glorified turbo chargers.
Fuel consumption was of little consequence.
The Mazda rotary is up to about 10:1 CR and a diesel
is up to about 20:1 Modern axial flow turbo fan compressors are over
8:1 I think. Another is the compressor and turbine clearance
is critical and as you make it smaller the clearance gap becomes a larger
and larger percentage of the total leakage. The third thing is turbine inlet
temperature. Efficient high temp turbines are very expensive indeed.
Hopefully Vance will jump in an explain it better as he gave a very good
forum on the subject at Sun & Fun. You should have attended.
Paul Lamar
-Dan Ruggirello wrote:
Theoretically, it is possible to get the fuel efficiency they are claiming,
realistically, impossible. I ran the numbers some time ago when I was
interested in the ATP turbine. I plugged in some knowns and assumptions in
a turbine analysis software package and these are the numbers needed to
develop 200 HP:
air flow rate : 126.5 gal/s (wow!)
compressor pressure ratio : 4:1
Turbine max exhaust temp : 1090 F
fuel consumption : 15 gal/hr
Shaft HP : 186.3 HP
Thrust HP : 13.7 HP
Total HP : 200 HP
BSFC : 0.47
thermal efficiency : 29%
compressor efficiency : 100%
turbine efficiency : 100%
combustion efficiency : 100%
gearbox efficiency : 100%
Notes:
The air flow rate required is astounding.
The 4:1 compression ratio is possible only for a very well designed
centrifugal compressor.
ATP is using the full residual thrust to make up the 200HP. This is not
realistic because the exhaust must be deflected and piped.
The thermal efficiency is the best you can possibly ever attain using the
Joule-Brayton cycle.
The 100% compressor effeciencies are of course impossible. High
efficiencies are typical for multi-stage compressors. But, this is a single
stage compressor........
The remaining 100% efficiencies are all of course also impossible.
I defected from the ATP camp because I didn't believe their numbers and they
couldn't supply the data to back them up like the prop thrust vs hp curves.
I too believe they are operating a scam.
Dan
PS - for those interested - here is the hand calc for thermal efficiency
and fuel consumption -
Best possible efficiency and gph calcs:
Assumptions:
Joule-Brayton thermal cycle applies
constant specific heat, k = 1.4
T1 = ambient temperature = 60 F
P1 = ambient pressure = 14.7 psia
P2/P1 = compressor pressure ratio = 4.0
T4 = max turbine exit temp = 1090 F
HV = heating value of JP4 = 18700 BTU/lb
d = density of JP4@60F = 6.3lbs/gal
no ram air pressure
no regeneration gains
isentropic, adiabatic compression and expansion
adiabatic combustor
100% component efficiencies
------------------------------------------------
n = thermal efficiency
= useful work done/heat added
= 1-1/((P2/P1)^((k-1)/k))
= 1-1/((4^(1.4-1)/1.4))
= 1-1/4^.286
= .327 or 32.7%
in terms of fuel consumption and HP:
n = HP*2544/(HV*FC*d)
where
FC = fuel consumption (gal/hr)
d = density of fuel = 6.7 lbs/gal
HP = total power
Assuming that total power is 200HP,
FC = HP*2544/(HV*d*n)
= 200*2544/(18700*6.3*.327)
= 13.21 gal/hr
Using variable specific heat, thermal efficiency is calculated to
be ~31% (calcs not shown). Fuel consumption is then:
FC= 200*2544/(18700*6.3*.31)
= 13.98 gal/hr
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