I think Vance among others will find this very interesting.
Not all illustrations are included as they are familiar to
most of us.
Thanks to Brian Trubee for sending it to me. PL
****************************************************************************
*
This issues topic will be a discussion of some of the distinctive
features
of a Mazda rotary when compared to a production-based four- stroke
reciprocating engine. Much of the comparison will be technical, but
some
will be based on opinion my opinion.
To Begin, for simplicity, we will compare two 1-cylinder engines and
we will
only talk about one of the rotor flanks (there are three - see Figure
1).
All rotational angles are quoted for the output shaft (eccentric
shaftlcrankshaft), not the rotor. Both engines burn a compressed fuel-
air
mixture to develop rotational power. Both are four-stroke engines.
However, one big difference between them
is that the recip has 18O degrees
per stroke (or 4 x 18O 720 degrees per thermodynamic cycle) while the
rotary
has 270 degrees per stroke (or 4 x 270 1080 degrees per thermodynamic
cycle). Yeah, you may have to think about that one for a bit, but
trust us,
it's true.
This has some good and some bad consequences. Assuming that both
engines
have similar maximum rpm's (and I think that is roughly true), it
means that the rotary has 1 .5 times as many milliseconds
to accomplish each stroke. This is one reason why rotaries breathe so
well
- they have more time (in milliseconds) to draw in and spit out the
mixture.
They also have more time for the power stroke - a real plus to get the
most
out of the combustion gas, especially at high rpm. Now the bad part.
The
rotary also has 1 .5 times as many milliseconds to transfer heat from
the
burning mixture into the oil and water.
This is one reason why rotaries waste more heat in the
process of staying
cool. Another consequence is that, if you only consider one flank of
one
rotor, the rotary only gets 2/3 as many power pulses as the recip.
However,
there are actually 3 flanks to each rotor, each at a different point
in the
thermodynamic cycle, so each complete rotor actually gives 2 times as
many
power pulses (3 times 2/3) as a 1 cylinder recip. Confused? Take a
moment to
study Figures 2 and 3 and soak it all in.
Put another way. a 2-rotor rotary has the same number of firing pulses
as a
4-cylinder recip, but because the DURATION of each firing pulse is 270
degrees, the engine runs smoother due to the overlap of the firing
pulses.
OK. so what is the point of all this math? Well, the point is to get
a
better understanding of WHY certain things are so important to a
rotary -
especially heat transfer. Remember, heat is potential power so keeping
heat
in the combustion chamber mixture makes more horsepower you c
an use. On to
the next item: In comparison to a recip, the intake charge (once it is
inside the engine) actually travels a tortured path. The figures above
shows
it detail).
In a recip, the center of gravity of the intake charge only moves an
inch or
two as the piston moves back and forth between top dead center (TDC)
and
bottom dead center (BDC). In Mazda's rotary, the charge moves a long
way
more like 20 inches from intake to exhaust. One bad result is that
there are
a lot of square inches of surface through which to transfer heat,
reducing
thermal efficiency. However, here is the big point: The entire mass of
the
intake charge must pass through the narrow area between the rotor
housing
and the rotor as each rotor flank passes through TDC. This is made
possible
by the rotor depression which is cast into each flank of the rotor if
it
weren't for that path, the partially burned mixture would never be
able to
squeeze through the n
arrow clearance between the rotor housing and rotor
(usually around .O1O .015 inch) at high rpm.
There is a crude parallel with a recip that has a pop-up piston that
tends
to cut the combustion chamber in two at TDC. Some recips even cut a
fireslot (notch) in the middle of the pop-up area to prevent it from
stopping flame front propagation in the chamber. For this reason and
others,
the shape of the rotor depression is quite important. It also has a
major
influence on determining the compression ratio of the engine and, as
all the
Internal Combustion Engine textbooks point out, the compression ratio
is a
major determinant of the power and efficiency of any engine.
Actually, this points out a weak point in the rotary the maximum
PRACTICAL
compression ratio is not determined by detonation (as is common in
recips)
but by the ability of the burning charge to pass through the rotor
depression! If the depression is too small, pressur
e builds up in the
vicinity of the trailing spark plug causing NEGATIVE WORK! This can
reduce
power, overheat the trailing spark plug, and substantially increase
the heat
dumped into the oil and water. Therefore, the shape of the rotor
depression
is a cut-and-try balancing act to find the best compromise.
Before we leave the subject of the rotor depression, one more point -
The
physical shape of the depression at its leading edge has a lot to do
with
the maximum usable leading ignition advance. You can understand this
better
if you set a late-model rotary at 35 degrees BTC, take out the #1
leading
spark plug, and look into the spark plug hole (a mirror and light
might be
helpful). What you will see is the curved flank of the rotor rather
tight up
against the bottom of the spark plug hole. If the spark plug were to
ignite
at this point, the engine might misfire because the flame front might
be
snuffed out (quenched) when it hit the r
otor surface. If you now turn the
engine to 20 degrees BTC, the way is open to burn into the mixture in
the
rotor depression.
This is an important part of the reason why nearly all 1974 and later
engines can run no more than 20 to 25 degree ignition advance at high
power
(earlier. USA model engines had a very long shallow depression that
allowed
more advance). As I explained earlier, there are some parallels
between
rotaries and recips here - combustion chamber and piston top design
are
major concerns in recips - but there are some distinctive items to
consider
when working with rotaries.
The truth is, there isn't a lot that you can do to change the shape of
the
combustion depression, especially in 1989 and later engines with thin
casting walls, but you can do some useful things. For one thing, you
can
ensure that the distance from the apex seal groove to the leading edge
of
the combustion depression is the same dista
nce on all flanks of all rotors
so that all will tolerate the same ignition timing (grind the leading
edge
of the depression as necessary).
***************************************************************************
BTW what Jim does not mention is the combustion chamber shape
verses the compression ratio limitations can be largely over come
by turbo charging. Very very important for aircraft use.
Another point is the rotating combustion is an advantage as
Tracy has discovered the engine will run super lean due to
a natural stratified charge effect. This is because the fuel
droplets are forced out near the spark plugs due to centrifugal
force at high RPM's.
A second advantage of the rotating combustion chamber is super
high power per cubic inch displacement can be generated without
melting the combustion chamber walls. UP to 400 HP per rotor
turbo charged.
We have really just scratched the surface of the
HP potential
of the rotary for aircraft use.
Paul Lamar
Turbo, or supercharging does not overcome the efficiency benefits of high
compression ratio.
Efficiency is really related to the work extracting EXPANSION ratio.
The Wankel tolerance to high pressures can be exploited to make more power, but
not more efficiency. The power potential of any engine is primarily limited only
by thermal and structural limits as is regularly demonstrated by the
dragster people.
The "time for expansion" story is totally bogus, and related to the
entrenched displacement myth. BMEP numbers for Wankel engines do not support neither
oustanding power extraction per stroke, nor particularly good breathing.
Vance J
Well maybe not if you think, as you do that Vance, the displacement is
3.9 liters and not 2.6 liters like the rest of the world believes it is :)
For those new people on here, Vance will readily admit the rotary generates
a tremendous amount of reliable power for its outside dimensions and weight.
The question of what the internal displacement really is ...is academic.
Nobody really cares other than Vance :) I am not sure even
Japan or the Europeans tax bureaucrats care anymore.
Perhaps a few car racing organizations are hung up on displacement.
A few years ago in IMSA if you wanted to race a rotary
you had to carry 650 pounds of ballast. Currently SCCA
tends to allow rotaries to race only against other rotaries
for the most part. Even then they make arbitrary rules such as no
turbo rotaries in certain classes of rotary powered racers.
"Too powerful they say." What would really happen was the
low lap times of the turbo rotary powered car compared to piston
classes would be highly embarrassing to the piston classes.
Other SCCA classes where the rotary races
against the piston engine the rotary is severely handicapped
by bans on peripheral ports or old NA 12A engines in 1993 and
up RX7 bodies that came equipped with turbo 13B engines and
anything else they can think of such as minimum car weights
to handicap the rotary. Otherwise the rotary would totally
dominate amateur road racing as it does in import drag racing.
I just read an article in this month's Turbo magazine about a
"Ford Focus" drag racing car. The turbo piston engine running
29 psi boost, alcohol and nitrous was totally custom and built with
an NC mill out of a solid block of billet aluminum including the
cylinder head. The engine was mounted fore and aft overhanging
the front axle. The frame was a complete space frame made
from thin wall tubing and the body was carbon fiber.
No expense was spared.
The car still was not as fast as Abel Ibarra's old 1969 stock steel
bodied 2000 pound Mazda R100 with a turbo 13B.
If the rotary were allowed to race unfettered internationally the entire
piston engine manufacturing infra structure world wide would come crashing
down as it would dominate all forms of car racing. Car buying customers
world wide would question the viability of such a Rube Goldberg device
as the piston engine really is :)
Paul Lamar
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