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
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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).
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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