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Ok, guys
Time to stir up the pot {:>)
If ever there has been something that seems destined to invoke debate and
heated argument, it's the issue of the true displacement of the 13B twin
rotor Mazda Engine.
The official Mazda figure listed for displacement is 1308 cubic centimeters
for the two rotor 13B model. However, you may have also heard it
characterized as either a 2 cylinder 2 stroke of 80 CID or a 4 cylinder 4
stroke of 160 CID displacement. In reality - none of the above is
technically correct, to find out just why this is so, read on:
For the 13B engine model, each rotor chamber displaces approx 19.95 cubic
inches (321 cubic centimeters); there are 3 chambers per rotor and 2 rotors.
That means that in one complete cycle of the rotary engine's six chambers
the total displacement is 19.95*3*2 = 120 CID or 1966cc. So where did the
1308cc figure come from? Where did the 80 CID and 160 CID figures come
from?
The problem arose when Mazda wanted to provide folks with something
familiar to compare the rotary engine with (and perhaps as some folks have
suggested artificially reduce the displacement to attain a more favorable
tax bracket for a Japanese auto engine). To fire all six chambers (the
rotary's full engine cycle) takes 1080 degrees revolution of the eccentric
shaft compared to 720 degrees for the full engine cyle of a 4 stroke
automobile reciprocating engine. The 720 deg for a complete 4 stroke cycle
is the standard for automobile reciprocating engines. For apparently this
reason, Mazda decided to truncate the 1080 degrees of the full rotary
engine cycle to the 720 deg of the standard, 4 stroke auto engine - for
comparison purposes. After all, the potential rotary customer would
inevitably want to compare the rotary engine to something they were familiar
with.
Well, it turns out that 720 deg is 2/3 of the 13B 1080 degree engine cycle.
So 2/3 of 120 CID (its actually displacement in its full cycle) = 80 CID or
1308cc. This truncation of the rotary's full cycle then became the standard
reference. For most purposes this serves just fine as most folks implicitly
are comparing engines to the 720 deg standard cycle of the reciprocating 4
stroke. However, one can ask the question of whether you would take 3/4 of
a standard 720 deg 4 stroke engine cycle as a valid basic for comparison. I
think most would say, "Hey, you can't do that". If you took an 8 cylinder
reciprocating engine and told someone that you were going to compare it to
another engine but would only consider 6 of its cylinders in the comparison,
most folks would say you are placing the engine at a disadvantage in the
comparison.
Yet, that is indeed the equivalent of what was done to the displacement of
the rotary engine to provide a basis for comparison.
Others pointed out that if you looked at the rotary as a piston engine,
then you get the equivalent airflow (and internal combustion engines are
nothing more than exotic air pumps) by treating the rotary as either a 2
cylinder 2 stroke of 80 CID displacement or a 4 cylinder 4 stroke of 160
CID. Again, the full cycle of the rotary (1080 deg) is truncated to provide
a basis of comparison to those engines with which we are most familiar.
Indeed, this will not change as it does provide a valid base for comparing
the power of the rotary engine with its reciprocating cousins. However,
just because it is a convenient and not altogether misleading manner of
comparison does not mean those figures are the true displacement of the 120
CID rotary engine.
In the end, like most arguments of this type, it really doesn't make a bit
of difference. The rotary engine is blithely unaware of the liberties taken
with its displacement and could probably care less if it did know. But,
every once in awhile the record needs to be place on notice of the correct
facts. The 13B displaces approx 120 cubic inches over its complete engine
cycle or 80 CID over 2/3 of its complete cycle - your choice {:>).
FWIW
Ed Anderson
RV-6A N494BW Rotary Powered
Matthews, NC
eanderson@carolina.rr.com
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