> 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

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


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