In a message dated 12/18/2007 6:34:15 P.M. Eastern Standard Time,
lendich@optusnet.com.au writes:
Lynn,
I was all over the shop before, but now I think
I've got it all sorted. I was trying to see the differences the PP gave in
overlap and open ( intake) time.
I was confusing myself yet again, until I
realized I was looking at the degrees of crank, when I should have been
looking at the degrees of the rotor.
With IO @ 32 degrees ATDC
and IC @ 40 degrees ABDC (12A
engine)
or IC @ 310 degrees ATDC = 278 degrees/3 = 93
degrees of the rotor
I did the same for Exhaust = 415 degrees/3 =131
degrees.
Overlap ( standard) =25 degrees/3 = 8
degrees.
Overlap PP = 144 degrees/3 = 48
degrees.
PP inlet = 430/3 =143 degrees.
Inlet time with the PP is an additional 50
degrees (very good) but with a penalty of 40 extra degrees of overlap (bad)
but only bad at low RPM's.
Interestingly the overlap is 6 times the original
- this all varies accordingly with the different models.
While doing this exercise it is interesting to
see the RX 8 port and why it's placed where it is to eliminate overlap. While
it might be nice to have say 180 degree of rotor ( or more) for complete burn
of the fuel, realistically the power derived from this exercise would be
marginal at best. The power from combustion would (I believe) be totally
converted to mechanical power ( to the rotor) by the time it reaches
the existing exhaust opening.
George ( down
under)
It is endlessly interesting to turn this thing around slowly and watch the
ports open and close. The published open and close events from (for example)
Paul Yaw's web page
WWW.yawpower.com are
at the crank.
So being so many (crankshaft degrees) ATDC or BTDC is looking at a very
small portion of a cycle that requires 1080 degrees (3 complete revolutions) of
crankshaft rotation to complete. One of the many advantages of this engine
is that everything is happening in slow motion. In regard to most engine
functions this is good news. Sadly this exposes way too much iron and aluminum
to the combustion process in each cycle. So you get quenched out flame fronts
and unburned hydrocarbons, (HC) and a bit less efficiency compared to a piston
engine of the same displacement. It has no squish areas to drive mixture to the
flame front or spark plugs. In the Fiat I could run .040" clearance between the
head and the piston top and move all of the mixture into the plug and flame
ball. So long as the piston didn't touch the head too hard at TDC all was well,
and it would run with 14:1 compression ratio.
Of the two tube frame cars, the first gen with the factory Pport engine was
far easier to drive around in the paddock at below idle speed in first gear. It
was not required to declutch and add revs now and again to keep it moving. The
exhaust dilution was very bad and this gives you a very slow burn, and a nice
long push in each cycle. It was just like a stock car. Very tractable. The third
gen car has a bridge port that opens at 110 degrees BTDC and the overlap is way
more than the Pport. It is just about not drivable without declutching and
adding revs to 4,000, then letting in some clutch (grabby 2 disc metallic) for a
lunge, then repeat. Very hard on the clutch. Engine stalls about 20 times on the
way to the false grid and so on. So we tow it there with a little tractor.
The Pport is very friendly. It is soft in power output until it steps up on
the tune at about 5,500 RPM, where the exhaust port closes soon enough to
keep a good amount of mixture in the engine. While the porting numbers suggest
that the intake port is closing at thus and so degrees, it never actually closes
does it?
Where the side port gets the flat side of the rotor actually sealing it
off, the Pport gets the point of a rotor passing through, valving mixture up or
down but never closing the port. Never closed is a lot of intake time.
No matter how badly you design your inlet tract, the Pport will provide
outstanding power. Should do well from 3,000 RPM to about 14,000 RPM.
The Pport intake is open for business 24-7. That is where the power comes
from.
While the gearing allows the rotor to turn (rotate) one time for three
rotations of the crank, the stress on the crank is monumental. At 10,000 RPM the
full centrifugal loads of two 9 pound rotors are on the crank and rotor
bearings. It happens that the bearing area is well up to the job. Much confusion
results from the gearing and speeds involved.
The engine is far over-square. And the poor torque (from the short lever
arm of the short stroke) is exacerbated by the rotor/crank gearing. So when you
gear the reduction unit at 2.78:1 you have not even got back to one to one
looking at the rotors. But stick with the degree wheel on the crank. Thinking
about the rotor speed will damage your mind. It did mine.
Lynn E. Hanover