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Okay this one is for David, Ed, Bob and
Ken because they are actually having an adult discussion
with me on this. Unlike the Pope. For everybody else, If this hurts your head
(it is beginning to hurt mine) just skip down to the last few
paragraphs.
I am looking at this from a thermo standpoint as a
mechanical engineer. If I were going to analyze this and make the numbers work
theoretically. You have to break this down to it's smallest
component.
To do this you follow
one INDIVIDUAL induction, compression, expansion cycle from beginning
to end. It does not matter what kind of hardware we are looking at.
When I look at a single cylinder 4 cycle engine. It
takes two complete crank revolutions to complete this cycle on some displacement
of air.
I draw a PV diagram for this and get the work
required to induct, compress, expand and exhaust. Some of these terms are
negative, some are positive. The balance is the work output for this ONE
cycle and individual cylinder.
For a multi cylinder engine, these work outputs are
all just added together at different phase angles based on the crank
throws.
Now for the wankel:
It takes one complete revolution of the ROTOR to
complete a cycle for a single rotor face (same charge) and perform the
previous analysis. The eshaft will rotate 3 times for each rotor
rev..
The additional rotor faces are just like
additional cylinders added on to a single cylinder engine. The mechanism for
extracting power is a bit more arcane but it is the same thing. Stick on
another rotor 180 degrees out of phase and it is the same thing. 3 more
cylinders.
So you are correct the 3.9L 6 cyl equivalent engine
will rotate at 2/3 the wankel speed not 1/3 as I misstated. And that matches
with practice, a well tuned 240 cubic inch 4 cycle multi cylinder engine
spinning at 6000 rpm (8500*2/3) is capable of making around 240 hp.
Likewise the Wankel makes the same kind of power at 8500 shaft rpm. with
the same number of cycles performed on the same amount of air charges as a 6 cyl
engine for a given amount of time.
Likewise, keeping the rpm the same: a 2.6
liter 4 cylinder at 8500 rpm can make 240 hp. Or how about a two cylinder 1.3
liter 2 cycle, or a .65 one cycle.
Your argument of one packet in one packet out only
works from a black box standpoint, you cannot analyze what happens in the black
box using this approach. You could pick any combination of rpm,
cylinders, and cylinder volume that gives the same number of
events per unit time on the same mass flow and say it was an
equivalent engine. So you are also correct that you could have any number of
rotor faces and get the same result (there actually are such engines in theory
using the same math as the wankel with different ratios of gearing and rotor
faces)
The statement about not understanding pretend RPM
is correct. It is not revolutions per unit time that matter, or mass
flow per unit time, but volumetric and pressure changes on each packet per unit
time.
To have thermodynamic equivalence, the
engine must be doing the same thing to the working fluid per packet per
time. If you do this you can actually integrate the rate of change of volume per
unit time and see how the Otto process is performed in the rotary vs the crank
and piston engine. Varying rates of compression/expansion at different times
lead to different efficiencies and cylinder/chamber pressures-ie stroke to rod
ratio.
I have changed my mind- it is now a
.325L half cycle engine.
I'm all spent on this subject. Call it whatever you
want as long as it gets the desired result.
What is really important to
all of us is to figure out what the heck to do with the damnable packets after
they are (vigorously!!!) expelled using your equivalent cycle of choice. OR-how
to make the stupid thing sound more like a Merlin and less like the model
airplane from hell with something less than a 30 lb boat anchor or the size of a
30 gal drum.
That is what I am working on now. I have little
hope for the outcome. I will however bare all at Tracy's in full view of the
public, you can witness my triumph, failure, or puzzled
looks.
Monty
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