Paul, I am referring to SAE paper 790484, entitled An Analysis of the
Volumetric
Efficiency Characteristics of 4-Stroke Cycle Engines using the Mean
Inlet Mach
Number Mim.
Authored by Itaru Fukutami and Eiichi Watanabe. The Institute for
Vocational
Training (Japan)
The data was obtained on both, the reciprocating engine and in
Appendix C on the
Rotary engine.
The key here is a different definition of the Mach index.
Usually, the inlet Mach index Ms is used that considers displacement
alone, while
the Mim index accounts for valve timing as well. To spare the
formulae, Ms and Mim
are equal if the valve-opening period is 180 degrees of crank
rotation and VE is
100%.
Mim (%) = 180 *VE / ( (IC - IO in degrees) *100) * Ms
It says, valve timing can easily vary between 210 and 340 degrees,
which has now
been accounted for.
To look at his graphs, it is quite amazing how similar the VE curves
are when using
the Mim number instead of Ms, even for widely different engines.
In the Rotary engine with side ports, the paper concludes that Min is
about 0,55
Mach up 10% from the Recips. However, this conclusion is based on the
artificially
smaller ports for the tests. The curve for the largeset port is
discontinued after
0,25 Mach but it seems to lead into the other curves limited by 0.55
Mach.
Unfortunately, there is no data for peripheral ports. One can only
assume that
peripheral ports may allow an even higher velocity before choking
occurs, perhaps
as high as 0.6 Mach, but that is strictly speculation.
All tests are made without any intake or outlet manifolds, in order
to avoid the
effect of such ducting.
One point I found particularly interesting is the peak of VE in the
Recip to be
near about 2000 rpm, while in the Rotary there is a slight valley
near 2000 rpm at
~ 87% and a levelling off to ~94% near 5000 to 6000 rpm. That is at
large ports.
Using a reduced port sizes will yield a similar curve to the Recips.
The increase
in the Rotary is likely caused by the ram effect. In the Rotary, each
chamber
follows the next, perhaps with a 90-degree delay between suction
strokes, while in
a recip the inlet valve is closed for nearly three quarter of the
time during two
shaft rotations where the air column needs to be accelerated with
each inlet valve
opening. Of course, in the Rotary, one can argue that exhaust gases
push against
the intake, bringing the air column also to a complete stop, where
renewed
acceleration is needed. However, it seems to me more likely that the
pressure of
the exhaust will push some residuals into the chamber during intake,
thereby
increasing VE. Since the last portion of the chamber is not fully
burned, the gases
contain O2 and possibly fuel as well and thereby aid VE.
Jerry. With reference to the comment by Bill Jepson, he is of course
right. A too
wide port reduces the support for the apex seal. But 45 to max 50 mm
width should
be ok. From 80mm, 15 mm remain at each side at width 50, which should
be good. Some
curvature is needed for the port closing side for either port for a
gradual
reseating of the seal.
Rolf