<< George Lendich wrote:
>
> Lynn,
> I found TDC as you suggested and marked it, put in the rotor and had a
play around (no degree wheel yet).
>
> Had a guess-da -mate on your IO ( looks similar to the Leeman) and your IC
which (looks to be a good 5 degrees after the Leeman) - 5 degrees before 90
> degrees ABDC.
>
> Question !!
> As 90 degrees ABDC is the max volume have you elected to have the 5 degree
safety margin so as to not have a problem with compressing the mixture and
having
> some forced back into the intake tube???
Well,.........not exactly.
Maximum volume is BDC.
Intake closing is 85 degrees after bottom dead center. The stock 6 port is
closing at about this area, so we are not way out in the woods on closing
points. Since the working chamber volume is decreasing any time after BDC, even in a
stock engine, there will be some mixture forced back into the intake system
at idle and just off idle and up to cruise RPM, like 2,000 RPM (in the car) and
a bit more, there is less than full BDC volume of mixture available. This is
not going to arrive at a zero sum until intake velocity gets pretty high.
So the most favorable combination of flow velocity overcoming the pressure
from the volume decrease over the closing event trapping that amount of mixture
is nearly always the torque peak of the engine, any engine.
In the rotary, where no valves blocking flow, volumetric efficiency can
exceed 100%. So if there is enough energy in the incoming charge (weight and
velocity) to overcome the pressure generated by the decreasing chamber volume, more
than the swept volume of the chamber can be ingested. So say that at full
volume of 574CCs at BDC we can by genius design ingest 590CCs of mixture. Few
engines can achieve this but the rotary can easily.
Another problem is that (I will use myself here) if you are used to thinking
in piston engine terms, then 85 degrees after BDC intake closing point, is one
whopping big long duration cam shaft. In the rotary, the events are also
reported in crankshaft degrees but the rotor is only going one-third that fast. So
the numbers sound alarming on the surface, but are actually quite mild. Even
the race porting is not outlandish. So if we say that we have an intake
closing point of 29 rotor degrees ABDC, then we have more modest timing than that
Briggs and Stratton on your mower.
> Another question!!
> The overlap with a PP is much greater - with a greater volume of exhaust
gas mixing with the intake, there is a greater chance of decreased volume
efficiency
> ( rough idle etc.)
>
> OR
>
> does the greater volume of intake compensate for the mixture of intake and
exhaust (maintaining volume efficiency) ? with the nearby butterfly giving
only a
> better degree of control at low RPM?
>
> Now given all this wouldn't it be better to have a late IC (such as yours)
and a later IO (even later than Jerry's) to minimize mixing and have a better
> volume efficiency.
>
> The more I read the more I'm confused!
> George (down under))
Well, first there is the perceived low speed or idle problem. When the
engine idles, any engine, the throttles are closed and a high vacuum exists in
the intake manifold.
No problem. In a piston engine, there is a valve in the way, the open periods
are short, as is the overlap (intake and exhaust open at the same time). The
amount of exhaust that can be sucked into the intake system is small.
Not so the Pport rotary. Both intake and exhaust are large free flowing
holes, sitting close together, the exhaust is at high pressure at the beginning of
each of these cycles, the 20 inch or so runner is at high vacuum. Good luck on
that idle problem.
The butterfly mounted close to the housing shuts off the long intake runner,
so that the exhaust gasses "see" only the volume of the combustion chamber. No
more long vacuum tube.
Instant better deal. The idle should be at the least acceptable.
This is has only to do with the long runner of the intake system that is
tuned for 6,000 RPM to 6,500 RPM. It is far less a problem in a race car with very
short runners tuned for 9,000 RPM.
Once the engine is spooled up, (with the ideal exhaust and zero back pressure
muffler) the exhaust is leaving at supersonic velocity and leaves behind it a
slight vacuum. So the overlap that makes it a dog down low, is making it a
Bear in the operating range. The low pressure in front of the inlet runner pulls
mixture down the face of the rotor near the 100% mark, and instead of a loss
we get a bonus amount of mixture. That is called "blow down." The added
velocity can overcome more of the reverse flow and overfill the chamber.
The open and closing points are of little interest until a base line is
established. Only one company has so far pulled this off with any degree of
success. It would be nice to know what their ports looked like, and what the event
timing was.
I think it would be difficult to beat a 13B Pport engine down to 200HP at
6,500 RPM with almost any port shape. The advantage of the single big port is
that one big runner has a lot less drag than two little ones. Even a Cosworth has
one big runner feeding the two intake valves. The runner breaks into two
runners just above the valves. Tuning the length may be a bit harder, but it will
work great, right out of the box.
Great care in the exhaust system to prevent back pressure is a must.
The first time my driver came back from his first session behind a Pport 12A
(about 275 HP) he had planted the throttle while crossing the apex of a fast
sweeper and passed through the (sweet spot at 100 plus % power) and got
wheelspin in third gear. On wide sticky slicks. Having never gone off there before,
he could not remember how far it was to the guard rails or if there were tires
stacked in front of them. It is so hard to see when you are spinning like a
top. There are no tires, but plenty of grass. No damage and a new respect for
the engine.
Lynn E. Hanover
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