It is like a zillion tiny bullets going near the speed of sound (in air)
impacting those poor little turbine blades. Now the question is; what
is the efficiency of the turbine converting all that kinetic
energy into rotating energy? If you combine the kinetic energy
and the heat energy in the exhaust you get roughly 50% of the total
energy in the gallon of gas burned. Therein lies the problem with
all internal combustion engines.
BTW Back in WW II the P51 RR Merlin engine generated the equivalent
of several hundred HP (as I recall) at top speed by merely squirting the
exhaust gas rearwards. The forward speed of the aircraft was in the
590 FPS ball park and the exhaust gas velocity being some what
higher due to the fishtail exhaust stacks used. The thrust was
small but the speed was high so the HP was substantial.
HP is of course thrust times speed.
I know you know all this. This is mostly for the new guys.
Paul Lamar
I think "several hundred HP" is a bit of an urban myth - much like
the "thrust" from the P-51 radiator. One pound of thrust is one
thrust HP at 375 - which is pretty close to top speed for that
plane. Thrust from the turbo props which have much higher weight
flow is fairly significat so they often talk about ESHP (Effective).
As far as HP that is available from the exhaust, the kinetic power
is about all you are entitled to (approx 10hp when the engine
is about 200+bhp knocked down to less than 80% with even the best
turbine).
Those BTU are only available to you if
you can USEFULLY expand down to atmospheric temperature.
(not pressure!! but temperature). Rocket engines, with BIG
expansion nozzles expanding to the vacuum of space, can do this,
but no turbine compounding can even get close. As far as auto
useage - when you are cruising your 200+hp engine at 30hp-
the weight flow is down and the kinetic power is velocity
squared, and you have trouble finding that hp with a microscope.
You gave the nozzle area as 2.3 sq inches - but the effective
tangential nozzle area may be much less for higher gas velocity
(and higher back pressure)
Vance
Ten HP represents a 5% improvement in BSFC or from .47 down to
about .446 or so. Right in the category of most piston aircraft engines.
In the RX8 that would improve the steady 25 MPG at 70 MPH to
26.25 MPG. When it comes to MPG in cars perception is everything
regardless if the numbers make economic sense or not.
I am not ready or willing to give up on 20% however and that is what
CW got 60 years ago with the R3350 turbo compound.
I don't see the difference between a scaled down rotary engine and
the CW R3350 Turbo Compound which increased the HP from the
supercharged R3350 at 2800 HP to 3700 HP and reduced the BSFC by 20%.
There is no doubt about this as hundreds if not thousands of R3350 TC
were built and dozens of range records were set and not broken until
Rutan went around the world un-refueled with a specifically designed
high load fraction aircraft.
The rotary and the R3350 should have the same ratio of kinetic energy
to heat energy in the exhaust. If anything the rotary has a bit higher
ratio of waste heat in the exhaust. Surely there is some size turbine
that will do the same thing for a 200 HP rotary engine.
As far as USEFULLY expanding down to atmospheric temperature is concerned
I agree but I won't give up. We now go to plan B where we convert the
un recovered heat to steam and inject it into the turbine. We then condense
the steam in the exhaust back into water down stream. We might need to carry
a bit of make up water. Here is a diagram from Marks for such a scheme as
it is applied to a gas turbine. Gas turbines with this type of exhaust
heat recovery are called combined cycle engines and according to Marks
approach 60% over all efficiency. Here is a scan from Marks.
I am researching membrane technology as we speak to possibly further extract
water vapor from the exhaust.
I think it ridiculous of the engineering world to go on building 30% overall
efficient internal combustion engines without spending some R&D money on making
them more efficient. Instead the car companies spend billions on electric hybrids
using the same 30% efficiency ICE engines while adding 500 to 700 pounds
to the weight of cars.
A combined cycle turbo compound internal combustion engine is not going to
be more than 100 pounds heavier if that.
Paul Lamar
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