> >> 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
> contaminatedgases. 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)
Keith,
When you screw in the spark plug how would you position the metal part
to adjust the gap that create a shadow? Facing the incoming fuel so
that most of the spaks are projected toward the exhaust. Or the metal
part is positionned toward the exhaust so the sparks are mostly
projected forward the incoming fuel. Or?
Marc J.
In a Reciprosaur, it really doesn't matter -- there are too many
designed-in inefficiencies to notice fixing one or two. While I was
only talking about the combustion process, it's not the only place
that's messed up.
In a Wankel, I would want the electrode to be completely out of the way
of the fluid flow, so it would be toward the side of the chamber.
Keith W
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