This is from the 1952 SAE Transactions.
Development of the R3350 Turbo Compound
Engine.
-------------------------------------------------
This power recovery heat balance chart knocked me out of my chair.
I had NO IDEA.... NONE WHAT SO EVER, prior to now, so much energy
was wasted by the exhaust valves in a piston engine turbo compounds.
Need I mention yet again THE WANKEL ROTARY HAS NO BLEEDING
EXHAUST VALVES!!!.
More evidence we will eventually get more than a 20%
efficiency improvement with a turbo compound rotary.
I am not making this stuff up folks.
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Am I the only person on here that understands the significance of
twice as much energy available in the exhaust to power a turbine
due to the elimination of the exhaust valve losses? :)
How about an internal combustion rotary engine that is 36%
more efficient?
In theory, with a variable ratio turbine speed reduction
unit, we could be looking at a 20 to 30% reduction in the
fuel used for transportation world wide. With
oil at $70 a barrel how much money is that?
It is not me dreaming these things up. The numbers are right
here folks. Staring us in the face. The 1952 SAE Transactions
book at the Cal Tech library had a 1 mm coating of dust on it.
Probably one of only several dozen copies left in the world.
Paul Lamar ...No rotor no motor.
Paul,
That chart is indeed an eye-opener. the dumping losses across the
exhaust valve are shown here to be more than half of the total exhaust
kinetic energy available for recovery. Out of the total of 35 percent
(rounded off) of energy available for potential recovery, nearly 20
percent is lost to the exhaust valve.
This has huge implications for rotary turbocompounding. If you are able
to recover only half of that 20 percent you will have increased the
power recovered by a huge margin, more than half. If they were getting
20 percent gains with the piston engine, the rotary should be able to
achieve 30 percent.
This could be something really big!
Just a quick comment about your previous post about using a
turbosuperhcarger for boost versus an engine-driven supercharger. I know
I've debated this with Doug, but I still think in a turbocompound
situation the engine-driven compressor makes more sense. Here's why:
In just a straight comparison of supercharger versus turbocharger, the
turbocharger takes less power from the core engine as you pointed out.
However if you are using a turbine to feed power back to the engine, the
exhaust energy you use to spin the compressor is subtracted directly
from what you are putting back into the engine.
So theoretically it should be a wash between the engine driven
compressor and the turbocharger since it takes the same amount of energy
to drive the compressor.
However, this does not take into account that you have to use two
turbines, one for the supercharger and another for the recovery. Two
smaller turbines can never be as efficient as a single big turbine, so
you are losing some amount of your exhaust energy.
Also the additional plumbing twists and turns drains off some kinetic
and heat energy from the exhaust stream. The net result, I believe, is
that you are actually losing more power to drive the compressor with
exhaust gas as well, rather than having a dedicated recovery turbine.
This is all the more true at altitude where your turbine can make some
big power, way more than you need to drive a compressor. The higher you
go, the bigger the ratio of "excess" power gets. So you end up diverting
more and more gas from the compressor turbine, thereby increasing the
losses from the extra twists and turns in the plumbing.
I think this is why the Wright designers went with engine-driven
compressors (two-stage, I believe). They could have easily used some of
the exhaust to drive either or both of the supercharger stages --
turbocharging was a very mature and proven technology by that time,
having been very successfully used in many of the WW2 fighters and bombers.
However, the designers chose to use three big turbines for power
recovery exclusively (one for each six cylinders). Why didn't they
divert some of that exhaust gas to drive the compressors? I think they
figured out that they would have been giving up more than they were
gaining.
That's just speculation on my part of course -- it could well be that
the turbocharger could in fact prove more efficient. Much will depend on
the details of the implementation.
And just going back to those SAE charts for a moment. It seems that two
of the charts don't really jibe with each other. The flow chart, Fig. 15
shows a total of only 160 net hp rcoverd. However, Fig. 5 shows that the
turbocompound engine making 3250 hp and more compared to a maximum of
2800 for the un-compounded engine. That's a difference of 450hp. In some
references I have found the engine was rated at over 3350hp, 550hp above
the uncompounded version.
Either way, the 3350 TC was a masterpiece. It should also be noted that
the TBO on this engine was over 3,000 hours, and in airline service the
engines regularly exceed that figure. And this is from an engine making
one horsepower per cubic inch. today's piston engines are only making
half of that and carrying TBOs half again as short.
Not great progress.
But now the turbocompound rotary, well...
Regards,
Gordon.
Thanks Gordon. I am glad somebody has recognized this besides me.
"So theoretically it should be a wash between the engine driven
compressor and the turbocharger since it takes the same amount of energy
to drive the compressor."
As far as net HP output is concerned this is probably true.
Given that this is true, and all the mechanical power to drive the
compressor comes from the core engine, each additional core engine
mechanical HP generated requires 4 HP worth of fuel burned as the core
engine is only 25% or so efficient. On the other hand if all the power to drive
the compressor is extracted from the waste heat in the exhaust the
overall efficiency will be higher. I think they used the stock
2 speed mechanical driven compressor because it was there in the
orginal B29 version of the R3350.
BTW the R3350TC developers where also surprised it worked so
well :) Too bad the paper is not available in pdf form.
It is wonderful to read. Most of the development work was getting
the exhaust valves to live. They were not always successful
in airline service.
The 160 HP number is probably per turbine. There were three.
This was not the final development. The final design added 1000 HP
to the 2800 HP the R3350 started with. Final HP was about 3800 HP.
HP per pound increased to well over one which is not bad even for a rotary.
The curve was still climbing when this paper was published.
The scanner at Cal Tech library (unlike UCLA but UCLA does not have SAE)
is not suitable for scanning books so I was unable to get a good scan for
pdf conversion. I had to use the spy camera technique to take pictures of
the individual pages with my new 8 mega pixel Panasonic
DMC FZ30 camera.
Perhaps there is an SAE member on here that can get a pdf version from
the SAE.
Paul Lamar ...No rotor no motor.
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