Subject: Mazda Taiki in Machine Design
From: Rotary Engine
Date: 4/19/2008, 6:14 PM
To: AAA Put this in the To box



> 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
>

 That would be counterproductive.  The smoother the flow, the more
 consistent the burn will be.  The optimum would be a flow of the
 stoichiometric oxygen-fuel mixture completely without turbulence -- not
 possible within the realm of physics in an enclosed space, but it would
 provide the maximum burn efficiency, as no fuel molecule would remain
 unburned.

 Turn on a kitchen faucet, just enough so that there is a continuous flow
 rather than individual drips.  Now put your finger into the flow, and it
 is no longer a flow below your finger, it's drips again.  The induced
 turbulence has permanently destroyed the stability of that flow.  The
 only way to overcome this and get the same result that you had (a smooth
 flow) is to increase both the amount of fluid in the stream and the
 length of the stream (that same finger in a flowing river has almost no
 effect).  This is inefficiency, and this is why there is so much
 unburned fuel in the exhaust -- they have to throw a LOT of fuel at the
 plug to make sure that there is enough of it to ignite and SUSTAIN
 ignition so that it will spread through the chamber.

 Remember, an internal combustion engine is JUST AN AIR PUMP.  It runs on
 AIRFLOW.  The smoother that flow, the better.

 Keith W


How would you propose to maintain non-turbulent flow in a Otto-cycle
(non-continuous flow) engine?

Turbulence can provide useful mixing (stirring) if the fuel delivery
system doesn't provide perfect atomization/vaporization.  I believe
that's how it is being employed in many modern engines.  Hurts the
volumetric efficiency a bit, but yields more complete combustion.

How much unburned fuel is in the exhaust of a modern piston engine -
before the catalytic converter?  The loss of efficiency is in heat
mostly..  The cat is just doing cleanup.


Regards,

Matt-

Here is what really happens in the rotary.

Obviously I have been thinking about this since 1975 :)
-- 
Paul Lamar ...No rotor no motor.

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-

> Keith,
> I did note your comment in regard this issue and believe the phenomenon of
> the heavy molecules being flung out by the centrifugal forces, go hand in
> hand with your arguments. I like your description of a 'Typhoon on fuel
> laden air flowing past the plugs'. I will use that as well
>
> Still we can't escape the fact that with the 13B ( non renenis) and
> especially the PP which has a larger overlap we have contamination issues,
> but more importantly, we have unburned fuel in the squish areas because the
> flame front is extinguished before it can reach those areas - still
> nothings
> perfect.
>
> So Keith give us your objective estimate on how well the rotor combustion
> area shape stacks up against the piston combustion area - in terms of
> overall better or worse by ? percent, considering all the differences
> between the two.
>
> George (down under)

That would take math that I don't have time to work out.  I will just say that
I'll take a rotary over a Reciprosaur any day if it ever comes down to a contest
for airflow and combustion efficiency.

BTW, there is squish space in a piston cylinder, too -- around the pistol and
in the top corners, where the flame front is forced to slow down.

Something else that most people don't think about is that what you learned in
high school physics about "pressure equal at all points in a closed system"
refers to a closed STABLE system, not one with a pressure wave loose inside.
Ask any dive shop that has had a tank explode while filling about the effects
of a rapidly-moving pressure front (that's why the smart guys start a fill at
a fraction of the flow, then open up the rest of the way after a couple of seconds).
They had a problem with the A4 rocket engines related to this -- the pressure built
up far to rapidly, blowing out the sidewall while the rest of the engine was well
below dangerous pressure, and they were OPEN AT ONE END!  They ended up putting a
metal "waist ring" around the engine, and problem solved.

It's possible that the little bit of squish in the Wankel protects the apex seal,
by cushioning the pressure wave blasting toward it from the spark plug, before
the rate of change stabilizes.


Keith W



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