>>> 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
Interesting.. I believe it's widely accepted that as the bore-stroke
ratio decreases for a given displacement, the peak torque increases.
This kind of flies in the face of what it sounds like you are saying -
that the wider rotor would make higher peak torque... The piston
area gets smaller and yet the torque increase. I don't think the area
of the push has as much effect as the volume of the chamber and the
eccentricity of the shaft.
Also, with a narrower aspect ratio the chamber gets more square
(lowering the surface area to volume ratio) as compared to the
exaggerated wide rotors on the Aixro. For best thermo efficiency, I
would think that the width of the rotor vs. the distance between the
rotor face and the opposite side of the chamber wall while at about
1/3 through the power phase should be about 1:1. The temperature is
probably at it's peak about there, or a little later, while the volume
of the chamber is fairly low - so minimize the surface to volume ratio
at that point.
Regards,
Matt-
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