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Many people talk about a port area as a target value and
indirect refers to a header area with an area factor for the port.
That approach is a completely waist since port area don't give any
information about port shape. Time area is still the standard unit
for 2-stroke ports since at least 1971. An area change far up on the
port will give a completely different change than on the bottom of
the port. It would also affect the pipe different. The time area
distribution is simply different over the port height because the
port is open different periods of time during the stroke.
In the
"Bimotion Advanced Port & Pipe" program we can see this
distribution in a chart. (Se pictures below.) We should then be more
careful with the machining precision as the curve height increases
since that part of the port is open for a longer
time.
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| A square port gives an increasing time area
distribution with port
height. |
The distribution curve will also show the effect of
auxiliary exhaust ports which adds blowdown time area.
We need a
certain blowdown timearea to equalize the cylinder pressure to the
crank case when the transfer ports opens. If this not works, we get
blow back to the crank case and that will drop the power
rapidly.
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| Auxiliary
exhaust ports adds time area which can be seen in the
distribution chart. |
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| The same port
but without auxiliary
ports. |
Now, the blowdown timearea is not just a figure to
match, it is dependent on how strong pulses the pipe deliver. If the
pipes tuning degree is high (strong pulses), then we get away with
less blowdown time area !
And the transfer ports don't need so
much time for scavenging since the pipe suction wave is strong,
pulling out the gas from the crank case. We could then have wide and
low transfer ports. With low ports we can go for higher rpm without
getting blow back into the transfers.
Why do we need to take all
this in consideration for the exhaust port design? The answer is
that it is not only a time area target value, the whole system needs
to be investigated since there is an interaction, a balance between
pressures as the pipe is affecting even the reed valve and the
carburator at BDC!!!
The normal procedure for race engine design
would be to keep the exhaust port as low and wide as possible and to
match the blowdown target (depending on pipe). The exhaust blowdown
target is a statistical value for the tuning degree decided by the
bmep target (braked mean efficient pressure). The port duration
should not exceed the recommended value.
4. Expansion
chamber
When the exhaust port shape is
decided from time area targets etc. we can dimension the exhaust
pipe header diameter. This is critical to the pipes blowdown
efficiency and the pressure build up. We need to pressurize the pipe
with a strong and not too short pulse and the length will interact
with the pulse resistance to create the necessary pulse shape. This
means that it is not always better to use a diverging header (L1),
sometimes a diverging header may result in that the cylinder exhaust
evacuation is too rapid and causes the pipe to 'loose the breath'
during the stroke. It will gain power at high rpm but lose in the
lower range, mostly okay for race engines though. The phenomena
needs to be viewed in a simulator to be fully understood.
To simplify some we say that the
first diffusor (L2) acts in the lower rpm range and the last
diffusor (L4) in the upper range. The rear baffle (L6,L7) angle
decides the top end rpm range and power 'hit'. Steeper angles will
increase the pulse strength and decrease the pulse length, i.e.
shorten the rpm range at which the usable power is produced. The
internal length between the diffusors will also decide the power
production characteristics, so if the first diffusor is relative
long then it will gain power in the lower rpm range etc. A pipe that
is pressurized with an early opened port (long duration) will be
able to retain the pressure through the steep angles and deliver a
strong suction pulse back at BDC. With strong pulses engaged we
don't need too large/high transfer ports. The pipe will then help
and pull out the gas from the crankcase, even manage to open the
reed valve and pull more air through the engine. When the transfer
ports are closed, a second returning wave in the pipe pushes back
the fresh gas that was spilled out into the header, in the remaining
blowdown window that is. The charging pressure is often in the
region of two atmospheres (bar) and far over that.
This is why the pressurization of
the pipe needs to be well investigated together with the exhaust
port, we simply cannot fit any pipe to an engine. It have to fit the
exhaust port and even the transfer ports too!!
Finally, the stinger need to be
large enough to let the engine breathe. A small stinger diameter
(and long stingers) will increase the internal pressure and power
but also make the engine run hotter. The length becomes
critical to pulse resonance over about 9000 rpm. At that point the
plunging effect at the stinger end will interact with the pipes
internal pressure and help to lower the evacuating pressure at
piston BDC. However, for high rpm engines, the stinger in general
needs to be smaller than on low rpm engines. High frequency pulses
have shorter wave length and will fit a smaller pipe better. But
since the high rpm engine also needs to breathe more this can often
even out.
More to come...
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