Interesting points Keith.. It sounds as though you might suggest that
an inverted piston engine would hold a combustion event advantages
against an upright engine. That would be surprising to me.. Or that
the bottom cylinders of a radial would be more efficient.
Matt-
Why would it be any surprise? Simple physics -- hot gas rises and
disperses cooler gas, so you need more fuel to have enough to light.
There are so many inefficiencies in a piston engine that these are just
a few more in the pile.
Keith W
------------------------
Natural Convection (heat rising because of temperature difference) is a very
slow process, and a piston firing repeatedly is a pretty fast process. The
difference in temperature, although theoretically possible to make a
difference, in practice will not. It will be overwhelmed by the turbulence
of the incoming charge.
Bill Schertz
KIS Cruiser #4045
N343BS (reserved)
After, say, 1,000 cycles, the entire cylinder interior is hot -- and
those parts with thinner walls, or which are at the top, are hotter than
other spots. Remember, the cylinder is designed to sink that heat out
to the water jacket or to the outside world, but it's not a perfect
exchange. You're already dealing with a minimum of 10% residual hot gas
from the last stroke of the previous cycle, which had to deal with the
10%-plus from the previous cycle, so on and so on.
Now we create a partial vacuum, then suck fluid into this environment --
through 6" or more of warm intake system -- which is only a couple of
hundred degrees cooler than the cylinder, but 1000 degrees cooler than
the leftover 10%-plus. This isn't like flooding the cylinder with
icewater, you're putting warm air into HOT air, which has already been
concentrated at the top of the piston by the previous stroke.
Turbulent, yes, but not the clean flush that you are imagining, AND the
flow is nondirectional, from one (usually) corner of the cylinder. That
means that some does go to the top of the cylinder, where the hot gas is
still surrounding the spark plug after the last cycle, but the majority
of the incoming fluid is distributed away from that spot, to the sides
and downward, following the retreating piston.
I am, of course, assuming an upright cylinder for the sake of
simplicity. If we turn this into flat opposed such as an O-200, it gets
even worse -- the plug is in a better position, in the side and near but
below the top, but both valves are in the side, side-by-side, so dead
space is increased, AND it's in the upper corner where the plug is.
Dual ignition brings a second plug, but it's not in any better of a
position in the cylinder than the other one is, maybe even worse (the
O-200 puts it near the bottom).
Try this experiment.
You need:
1) a 1-liter, cylindrical, transparent container with a flat bottom (and
corners as sharp as possible), filled 1/10 of the way with clean water
(pop bottle with the top cut off, glass beaker, etc) and placed where a
spill of dye will not be a problem.
2) a 2-liter plastic, squeezable pop bottle, filled with opaque dye
(such as blue toilet-bowl cleaner) with a length of 3/4" - 1" tubing
fitted to the mouth of the bottle.
3) a cardboard disk, slightly smaller than the diameter of the container
with a large U cut in it to allow the tubing on the bottle to pass
through without interference, and floating on top of the water in the
container
4) a slow-motion video camcorder (the slower the better), aimed through
the transparent wall, across the bottom of the container.
This container is an upside-down cylinder in a piston engine. The clear
water represents the residual waste gases at the top of the cylinder at
the end of the exhaust stroke. The remaining empty volume represents
that taken up by the piston, and the cardboard disk is the top of the piston
With the tube closed (valve or pinch), dip it into the container,
through the U in the disk. It must be held in place so that it doesn't
move.
With the camera running, rapidly squeeze the 2-liter bottle, simulating
the flow of inlet air into a cylinder as the piston retreats.
Watch the video, answer the questions:
1) Does the incoming fluid (dye) visibly displace the waste (clear
water)? That is, can you see separate areas of clear and blue water?
1a) If yes, are there clearly-defined interface boundaries between
the two?
2) Does the turbulence from the inlet fluid flow create turbulence at
all points in the cylinder, or is the turbulence greater at some points
than at others?
2a) If evenly turbulent at all points, does this take place
immediately or gradually?
2b) Does the shape of the cylinder affect the turbulence (such as
different waveforms in center or against walls)?
Try the experiment again, adding items to the "top" of the cylinder to
simulate the spark plug(s), exhaust valve, and profiles milled into the
piston and cylinder head. Do the results change?
If I were doing PR for one of the rotary outfits, I would build a
cylinder and a Wankel out of Plexiglass and do this same experiment as a
demo. It would open a few eyes.
Keith W
The fact remains Keith. The rotary BSFC is worse at low power and low RPM than
a piston engine. That is the big problem for car use. What do you attribute this too?
There has been some CFD on the air flow inside the rotary chamber but I don't
think anybody has made a rotary from quartz or glass yet and run it. Plexiglas would only be useful
for cold air flow visualization. The CFD showed a more or less uniform flow around the inside
of the rotar housing. I'll find that SAE tech paper sooner or later :)
Paul Lamar
The low-speed problem is because the intake path length (inside the
chamber to the first plug) is just better suited for high rpm, and
there's not a lot that can be done about it. It's a laws of physics
issue -- the fuel burns at a particular rate, and the engine has to be
tailored to work with that rate. The lack of low-RPM capability sure
hasn't hurt the turboprop any.
Yes, I meant a cold, low-speed demo, slow enough for people to actually
see the flow. To most people the Wankel runs on black magic -- they
don't understand it, and what they don't understand they won't accept.
A cold "visible" demo, showing the flow through, would let them see it
work, show why it's better, and psych it out for themselves.
Animations, computer simulations, etc just don't work as well as
standing there and staring as it runs.
I would like to see a day when Reciprosaurs are advertised as
"collectable -- for display only, not for flight" . . .
Keith W
Conventional wisdom has it the surface to volume ratio of the combustion chamber
is excessively cooling the fire. Do you give an credence to that theory?
Paul Lamar
It does make sense, as the chamber walls are cooler, and induce drag in
the fluid flow.
I would have to do the math to dig any deeper, and I really don't have
the time (or enough interest). I'm already digging back into high
school physics that I don't use a lot these days, but that's kind of fun
to remember. We blew up a lot of containers of various size and shape
working on the internal fluid dynamics stuff.
Keith W
Paul and Keith,
A visual indicator that I have heard used by old timers to port engines
- dump talcum powder into the intake during a quick throttle blip, then
shut off, disassemble and see the results. I like Keiths suggestion of a
plastic engine and high speed camera better. Basic research like this
will improve the power and BSFC. Ken @ StarApex has suggested a line
and check ball system to bleed combustion pressure into the leading edge
intake stroke, similar the the pic Paul showed a few posts ago using
high pressure air at the trailing edge - both ideas to eliminate
unburned fuel in the "dead" zones at each end of the combustion cycle.
Dan
That is an interesting idea Dan. We could build up a one rotor with a Plexiglas
end housing and drive it with an electric motor.
Let it suck air and dump talcum powder into the intake and see what happens.
It would make a great video for youtube.
I have a 5 HP electric motor lying around we used for the water pump dyno.
A video camera is about 30 frames per second so a high speed camera would
work better.
Paul Lamar ...No rotor no motor.
The Rotary Engine NewsLetter. Powered by Linux.
ACRE NL web site.
http://www.rotaryeng.net
Copyright 1998-2007 All world wide rights reserved.