While I agree with most of what you ( or Mazda) have stated, I can't for the
life of me understand how a narrower combustion chamber gives more efficient
flame front.
It is well known the piston ( round shape) gives the optimum flame front/
fuel burn as it radiates from the spark in an even distribution and there
aren't any squish areas some distance away ( as in a long flat chamber).
George (down under)
You got me George :) Perhaps it got mixed up in the translation
from Japanese to English. The only thing I can think of is they did
it that way so the new engine would fit in the RX8 with a minimum of
changes in the engine compartment. I do understand the need for more
torque but they could have achieved the same result with a dual clutch
Formula One type transmission. The car would be more fun to drive.
Aspect ratio means length divided by width.
--
Paul Lamar ...No rotor no motor.
George, you are talking about optimum theory, not practical application.
First, most spark plugs are offset in the cylinder head, especially in
dual-ignition engines. The Rotax 2-strokes are notable exceptions.
Second, the shape of the standard spark plug "shadows" a very large
area, which is blocked by the negative electrode (the metal bar that you
bend to adjust gap). This means that the flame radiates in an "ace of
spades" pattern, with initiation taking place later behind the electrode
than in the rest of the chamber.
Third, the theory also assumes a stationary, uniformly stoichiometric
mixture, while in an operating engine, the mixture is a highly-turbulent
non-uniform mixture, drawn in at high speed through a variable opening a
few millimeters across at the widest. This problem is actually worse
with fuel injection, as the fuel is only partially vaporized as it
enters the cylinder (with a carb, the fuel vaporizes more completely in
the intake).
Fourth, a large percentage of the gases in the cylinder are residual
waste from the previous cycle (in a 1:10 compression, less than 90% of
the waste is expelled during the exhaust stroke, and in most 4-stroke
engines, some is drawn back in as the piston begins the intake stroke
(while the exhaust valve is still closing). Turbulence at the valve
opening keeps some of the waste from being exhausted, which is why it's
necessary to make the exhaust system as wide open as possible to
increase power. To make matters worse, this waste, being several times
the temperature of the intake fuel-air mixture, will rise to the top of
the cylinder -- which is where the spark plugs are usually found. Fuel
only burns once, and the mixture is more lean at the top due to the
waste which mixes with it.
As you see, the piston engine is actually working against itself when it
comes to combustion.
Now, look at the fluid dynamics inside the Wankel.
First, the combustion chamber actually travels past the spark plugs,
giving a greater "illumination" to the spark while carrying the
fuel-laden mixture past them. The optimum for the piston engine is an
"expanding hot spot" centered on the electrodes, while the Wankel
distributes the spark as a "streak" relative to the combustion. The
greater the contact between the fuel and the spark, the more uniform the
combustion.
Second, this movement of fuel is ACROSS the plug, rather than randomly
swirling around it, thus the movement of the flash front is more stable.
Third, the fuel-air mixture is more stable, because the intake port is
larger and not blocked by the stem of a valve.
Fourth, a far greater percentage of waste gases are exhausted, because
the port is wide open during the exhaust portion of the cycle (less
turbulence), because there is less "leftover" space between trochoid and
wall, and because there is less "drawback" from the exhaust. What waste
remains is not concentrated where the spark plugs are.
Now, as to the narrower chamber being even more efficient, my guess
would be that it provides an even more stable flow of intake mixture and
allows the flash front to move in a more controlled way.
Keith W
Keith,
Very illuminating, I'm always ready to learn and I hope you don't, me using
these arguments, although I think they may be more relative to the renesis
engine than to the PP in regard to overlap and retention of contaminated
gases. I guess with a narrower rotor the unburned squish areas are less as
well.
Never the less I've learned something (a few things) today about the piston
engine idiosyncrasies and it does indeed make the rotary look even better.
BTW you forgot to mention the rotary's tendency to throw the heavier fuel
molecules toward the spark plugs, making for better burn of leaner mixtures
( the term escapes me at the moment).
I appreciate your effort in this regard.
George ( down under)
We're here to swap knowledge, so I'm glad it helps.
As far as the heavier molecules, I consider this a very minor factor,
compared to the typhoon of fuel-laden air flowing past the plugs --
that's where the efficiency comes from. The more fuel molecules you
ignite with the spark, the better your burn will be.
Keith W
Perhaps some one should try small walls in the rotor combustion chamber
depressions to promote turbulence.
Paul Lamar
Paul and group,
StarApex web site may give clues for better power from rotaries. The
13b has an e-shaft "throw" of less than .6" which is where torque comes
from, (as compared to 3 to 4" for a typical V8 engine). Also, the
trailing end of the rotor continues to product pressure (torque) on the
e-shaft, much more so than the leading end.
Mazda, or any rotary manufacturer, can make more torque by lengthen the
rotor leverage to the e-shaft by increasing the "throw". Of course this
requires all new housings. My understanding of the Taiki design is that
is what Mazda did, in part.
When I first saw the pics of the Aixro engine it is understandable how
Paul W. gets so much power in that small package. ie. the rotors are
VERY wide, creating a larger surface area ratio to "push" on the
e-shaft. An interesting study would be to make an engine with a larger
rotor while still keeping 300cc chambers and compare torque, hp, and
fuel consumption. Do we know enough about the operation of rotaries to
do a computer simulation? Would a chamber offset toward the trailing
edge produce more torque? Is anyone going to lose sleep wondering about
this?? :D
Dan
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