Hi Gordon,
Firstly, Paul is absolutely correct that you cannot extrapolate RX-8
or
other normally aspirated engine data to high-boost because of the
change in
compression ratios. Also don't forget to increase boost to
compensate for
high intake air temperature, later than optimal ignition timing and
richer
than optimal mixture (both for detonation protection), etc
Secondly, my boost-level guesstimate was based on 13B geometry and
6500RPM.
I have no personal experience with P-ported engines so will not take
make
any estimate or comment on these engines. But I agree, I may have
been
pessimistic...
Turbochargers won't take the punishment of unlimited aerobatics,
neither
will (probably) high-spinning centrifugal superchargers. I do not
know about
twin-screw chargers...
Finally, an unlimited aerobatic airplane needs full power for very
limited
periods and only makes short flights (except for ferrying). Various
solutions that Mistral doesn't even consider for its applications may
therefore be open. I shall not comment on things I only know
second-hand.
Francois
Thanks, Francois.
That's a good point about the g-loads on the supercharger. The Russian
radials use gear-driven centrifugal blowers and they seem to be pretty
robust.
http://www.russianaeros.com/Future%20for%20M14P%20engines.htm
I don't know how the screw compressors will stand up, but they spin
much
slower, so centrifugal force is much smaller to begin with. The
Lysholm
blower sized for this engine output would spin at about 8000 rpm.
Drive
ratio from the engine would only be about 1.2 to 1. Hopefully it
should not
be a problem as these units are pretty robust, with large bearings,
etc.
I agree the challenge will be juggling compression ratio, boost,
intake
temperature, fuel octane, etc.
Ideal fuel would be ethanol, which has almost three times higher
latent heat
so evaporative cooling would be much better. It also burns cooler,
so total
heat produced would be less. Not to mention much higher octane, so
higher
compression and better power. Hmm, I think I've convinced myself ...
I grow
a bit of corn on the farm anyway...
But the thing is where do you get ethanol at an airport?
There is an interesting option, using ethanol injection for knock
suppression. I've attached a paper that gives great info on this.
Their
objective was from an automobile fuel-efficiency standpoint using
smaller,
highly boosted engines. It requires only a small tank for ethanol. I
don't
think I would try to scratch-build such a system myself, but if
someone
where to come up with a bolt-on product, it would sure make things
interesting.
My approach with a draw-through, carbureted engine will still give
good
results, I believe. This is the approach used on the Russian radials
--
although some of the high-power versions now have EFI.
At stoichiometric ratio, the evaporative cooling at the carb is a
temperature drop of about 40 F. That means that on a 72 F day the
intake
charge coming into the compressor will be 32 F. At a pressure ratio
of 1.7,
the charge temperature coming out of the blower would be 160 F (71 C.)
I think that's doable on 100 LL and reasonable ignition timing. I
think this
boost ratio (1.7, or 10 psi) is probably the maximum. That would be a
manifold pressure of about 51 inches.
The question is will that give 300 hp net? Maybe not, as you
mentioned, for
reasons of higher intake air temp, less ignition advance, etc. The
Lysholm
blower will draw about 33 hp to make that boost.
Btw, I am not thinking P-port because what's the point when you have
boost?
Plus it looks to interfere with coolant passages.
Also I would prefer the RX8 engine because I think the side exhaust is
better for cooling too. I could be wrong on these points, as I am just
guessing.
Regards,
Gordon.
Just want to back up on this discussion for one point..
Why would side exhaust be better for cooling? Seems like it would
increase the cooling load - more heat transfered to the housings.
Wouldn't the extra heat have to be removed by the coolant/oil?
Regards,
Matt-
True Matt. What I think he meant was better for exhaust pipe
cooling. The EGT on the RX8 engine runs a couple of hundred
degrees F cooler than the RX7 engines. That lost heat goes into
the liquid cooling system of course.
Paul Lamar ...No rotor no motor.
Matt,
I assumed -- correctly it turns out -- that the exhaust would be cooler on
the RX8, therefore the exhaust manifold would not need to be as heavy.
I also assumed that with the exhaust going out the cast iron side housings,
there would be less heat transferred to the aluminum rotor housings. My fear
is that the aluminum rotor housings are the bottleneck, in terms of
conducting heat, and could see localized hot spots at high power levels
because there is not enough metal to conduct away the heat to the coolant
quickly enough. That may or may not be true at the power level I am talking
about, but I'm sure it's true at some power level.
I actually didn't think of your point that the extra heat absorbed by the
cast iron housings would add to the cooling load. This is true of course,
but I am not worried about hot spots in the cast iron. Its greater molecular
mass will conduct more heat than aluminum, given the same cross section.
Regards,
Gordon.
No I don't think so Gordon. The term "conduct" is the problem.
There is no doubt aluminum conducts heat better than cast iron.
I am no thermo guy so I am not sure about its ability to "store"
heat however.
BTW there is an Inconel liner in the p- exhaust port that
effectively insulates the aluminum rotor housing from most
of the exhaust heat.
The same liners are found in the RX8 side exhaust ports but there
is no doubt about the fact the RX8 EGTs are lower.
Paul Lamar ...No rotor no motor.
Paul,
I agree that "conduct" may be the wrong word choice here.
You are right that aluminum is a better conductor -- both heat and
electricity, the two go together -- than iron: Aluminum has a thermal
conductivity about three times as high, 230 to 80, approximately.
So if you need to transfer some amount of heat or electricity through a
conduit, aluminum will get it to the other side quicker.
But the question with aluminum is how much heat can it take? And how
concentrated can that heat be? That's where the other properties of the two
metals come into play.
Aluminum has a melting temperature about half that of iron. It has a density
less than half of iron. Logic tells us that more mass will absorb more
energy.
We can see this if we take a very concentrated heat source, like a cutting
torch to a piece of aluminum -- it will be a puddle in no time. If we take
that same torch to a piece of iron the same thickness, we find that it takes
a lot more heat to melt.
What I should have said was that the greater molecular weight of iron means
it can take higher point concentrations of heat, and move that heat away to
the coolant, without itself melting, or softening.
The reason is that there is simply more matter there for a given area or
cross section. At the molecular level heat is just motion of atoms and
subatomic particles -- the greater the motion, or more correctly vibration,
the higher the heat. And heat propagates from atom to atom through
collisions, with the intensely vibrating particles setting the cool
particles in motion too. So the more matter there is in a given space, the
more particles to bump into and absorb that heat. With fewer particles
spaced wider apart, as in aluminum, each particle has to absorb more heat.
If there is enough of a concentration of heat in a given area, those
particles will have to absorb enough energy to effect a phase change --
melting.
That's why dense material like cast iron will absorb much higher point
concentrations of heat than aluminum. With respect to the aluminum rotor
housing in the rotary engine, I am sure that if you pump enough power into
that engine the heat from the exhaust will become so intense that it will,
if not melt, then certainly soften certain localized points. There are
simply not enough molecules there to take the onslaught. With cast iron, you
will probably never reach that point before something else in the engine
gives way.
So at the end of the day, if we are concerned with cooling an engine making
lots of power, we have to think about those heat concentrations. I'm just
saying I would rather have cast iron there than aluminum.
Regards,
Gordon.
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