I've learned from this e-mail about the meaning of 'Thermal load'
inside an Internal Combustion Engine, HP per surface of combustion
chamber, and MEP, I remembered having read about the worries of
the early Air-Cooled Wankel Engine designers about increasing CR
and Power, because of the dangers of thermal load (on bearings and
shafts?), and on the lubricating oil film, but the chart you added
indicates an extremely low thermal load for the only RCE on it, a
Mistral. Is this because of the poor MEP of Wankel, or for other
reasons?
The high surface to volume ratio of Wankel RCE combustion chamber
was blamed for the improvable fuel economy, but the surface/volume
ratio in the time of top compression may not be the same during
the process of expansion, where the working chamber volume
increases, both in the irregular shaped Wankel and in the classical
cylinder. How does the plot of surface/volume ratio changes from
'TDC' to 'BDC' compare in a Wankel versus a reciprocating engine?
Somebody to forward here a chart about this? Is there a safe way,
safe for the lubricating oil film, the bearings, and the shaft,
for increasing the housing and/or Rotor working surface
temperatures, a condition that was shown improving fuel economy and
cleanliness of exhaust gases? Some tested an Iron housing, with
improved results in SFC and exhaust gases over Aluminum, but I've
read nothing about differences in thermal dilatation between
housing and rotor when both are made of iron, and if this makes
possible having the engine seized when overheated. Thanks have a
nice season.
Best regards. Salut †
Jose Gros-Aymerich Madrid,
Spain
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Jose,
You had asked about the motion of the Wankel Rotor from DTC to BDC
vs the piston in the RPE, The movement of the Wankel Rotor is
purely sinusoidal where s= r*(1-cos(ang)), r= 'E' Wankel shaft
eccentricity, and 'ang' is the crank angle from 0 to 180 degrees
in the RPE. In the Wankel it is 270 degrees which must be divided
by 1.5 to have 0 to 180 degrees. It is like having an infinitely
long connecting rod, which is easily explained as the Wankel does
not have a connecting rod and the piston sits (rotates) directly on
the eccentric of the output shaft.
At the RPE, s= r(1-cos(ang)) + L(1- sqrt(1-lmd^2 sin^2 (ang))),
where lmd is r/L, L= length of connecting rod. An approximate
formulae is s= (1- cos(ang) + lmd/2 sin^2 (ang)).
Rolf Pfeiffer
Thanks Rolf:
my question focused rather on how the Surface/Volume ratio from the
top compression point to the end of expansion-exhaust stroke
evolves in an RCE vs a piston engine. From the chamber displacement
of engine, and the Compression Ratio, you can calculate the volume
of combustion chamber in its minimal dimension, when compression is
at maximum, from there, as expansion takes place, the working
housing surface remains the same, and so does the Rotor surface,
but the surface, the area of the side plates exposed to the working
chamber changes, and this area is to be added to the housing
surface area in contact with a working chamber and the rotor
surface to assess the surface change in working chamber when it
goes down in the expansion stroke.
This must be different than in a reciprocating piston engine, where
just the length/ height of cylinder exposed to the working gas in
the expansion stroke changes. Sorry, I can't make any use of the
equations you provide, I engaged in medical sciences and not in
engineering because my absolute lack of progress around 1969 in
some mathematical issues, as matrix calculations and geometrical
transformations. About the news from Mazda research, I read that
eccentricity is like the stroke in a reciprocating engine, with
more eccentricity/stroke, you have better low rpm performance, a
bigger stroke has also an influence on combustion and emissions,
but
I don't remember which type of it. Best regards. Salut
Jose Gros-Aymerich
Madrid, Spain
Jose,
You can look at it this way that the side wall surface compares to
the cylinder wall in a RPE where this surface too increases with the
down travel of the piston, there is not difference in the Rotary
except that the surface of the housing changes slightly but lets
assume it stays the same as well as the surface of the rotor of
course.
The down travel of the rotor is equivalent to the down travel of a
piston in the RPE, The additional rotation has no effect of the
surface within our definition of no change of the trochoide.
The volume change is equivalent to the 'stroke' in both engine
types. However in the Rotary the total stroke is 3 x eccentricity. 2
e by the rotor and 1 e by the housing (trochoide) as it moves away
equal to 1 times the eccentricity.
In numbers, the RX7/8 have e = 15 mm. The rotor stroke is 30 mm and
the housing moves away by 15 mm for a total stroke of 45 mm. To prove
that we can calculate the piston surface area (as in a RPE)
multiplied by the stroke. The piston area is length times width.
length = R * cos 30 degrees * 2 = 105 * .866 * 2 = 181.86, width = 80
mm. Area= 14549 / 100 = 145.49 cm. Volume = area * stroke = 145.49*
4.5 cm = 645.7 cm3. Since (2* cos 30 degree) is equal to sqrt of 3 =
1.732 the general formulae for displacement = e * R * b * sqrt (3) *
3 = 1.5 * 10.5 * 1.732 * 8.0 * 3 = 654.7. Often Displacement = R e b
* 3 sqrt (3), or = R e b *sqrt(27).
The actual distance 's' of down travel is 's' = 1.5 e *(1- cos( crank
angle / 1.5) where crank angle is from 0 to 270 degrees. The basics
of the down movement is this formula 1-cos(0 to 180 degrees) which is
a variation from 0 to 2. You must understand that, just try it:
cos(0) = 1, cos 90 = 0, cos 180 = -1, 1-1= 0, 1-0 = 1, 1-(-1) = 2.
At 135 deg crank angle (90 in the RPE) the travel and expansion is
exactly half , while in the RPE the piston travel is more than half
due to the angular position of the connecting rod. In the RPE the
piston travels faster away from TDC and slower in the lower half than
a pure sinusoidal relation as in the Wankel.
On the rotor balancing it is correct that the rotor in itself is
balanced (or it should be as an equilateral triangle) and only the
eccentric shaft needs to be counter balanced to compensate for the
mass of rotor(s) and eccentric shaft portion(s).
I trust above is understandable.
Rolf Pfeiffer
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