The following is an extract from the
movie as shown by the PhD candidate
VD Jimenez at CMT at the University of
Valencia. It shows how the above pulses
created by a tuned pipe influence the
flows in the engine cylinder.
Notice how the red inflow from the
transfers flows up the back wall of the
cylinder and out the exhaust, pushing out
the blue combustion products. This is
encouraged by the low pressure pulse at
4 in the above picture. The high pressure
pulse at 2 above then pushes the
somewhat pure mixture in the pipe back
into the cylinder. This is about as good
as it gets in a two cycle. There are still
some exhaust products in the cylinder at
exhaust port closing, but the mixture is
significantly supercharged. This engine
develops 54 hp from a 125 cc cylinder.
A 26 cc engine that was this highly
developed would have around 11 hp at
13,000 rpm. Since the smaller engine
could easily turn 18,000 rpm, you could
expect over 15 hp.
Aprillia RSW125 Scavenging
1. CMT own their own RSW125 and
measured it fully to model in 1D and 3D.
2. The rpm value working back from
port duration and time seem to be around
12000.
3. Blue is burnt gas and red is fresh gas.
4. This is a single cycle event as it starts
with uncontaminated charge in the
transfer ports.
5. The interesting thing is how good the
scavenging is; before the plugging pulse
arrives the cylinder is close to a 100%
scavenged. (page 8)
6. The obvious next improvement would
be to increase the delivery ratio to the
point where the exhaust port is also filled
with fresh charge so the plugging pulse
is also fresh charge uncontaminated.
7. Not heating the fresh charge in the
exhaust port duct should show the
improvement as found by Jan Thiel.
8. It seems the shape and down angle of
the Arillia port as developed by Jan
Thiel on his flow bench not only
promotes flow but also the lower
turbulence tends to prevent mixing
between the escaped fresh charge and
the burnt gas.
(Continued on page 8)
PROPWASH
October 2014
7
High Power Two Stroke Design – Postscript
By Lohring Miller
NAMBA Safety Chairman
After reading the previous articles in this series, it’s easy to get lost in all the
details. Below are excerpts from two papers that summarize what we are trying to do
with pipe and port design. The first is by Neels van Niekerk. The second is the only
publically available computational flow dynamics (CFD) analysis I’m aware of for a
high performance, tuned pipe engine.
Pipe Pulse Design
I think a successful design of a pipe combines the following:
1. The obvious, that pressure pulse 1 must be reflected to arrive just before Ex-port
closure 2.
2. There must be a suction pulse 4 during the transfer open period.
The tricky bit in order to get really high suction and plugging pulse values, pressure
pulse 2 which is reflected from the ex-port must again be reflected from the reverse
cone to arrive just before Ex-port opening 3. This will then get reflected in back down
the pipe and travel just in front of the new pulse 1, combining with it to increase its
amplitude and thus the amplitude of the suction and plugging pulses.
This last point is only achievable with an Ex-port timing of roughly between 190
and 200 degrees which is why all GP-bikes use this value and use more and wider
exhaust ports to obtain enough blowdown time area. With very small exhaust duration
or very big exhaust duration this recombination is not possible as the timing is all
wrong and only the classical pipe theory can be used.
Below is a picture of the Aprilia exhaust and transfer passages as an introduction to
the next section.