PROPWASH
October 2013
11
space. Jim Allen has been
experimenting with a toroidal head
button with a surface discharge
plug in a Quickdraw and reports
increased power. Note also how
the insert is machined to cool the
spark plug. Ignition hasn’t been an
issue so far in 26 cc engines.
Experiments with a simple
electronic system and various
amounts of spark advance show no
improvement so far in modified
Zenoahs. Quickdraws can run
more ignition advance giving a
little more power with suitable
fuel. However, surface discharge
plugs in high compression toroidal
heads may need a better ignition system. Fortunately, there are several choices. The
Power Spark system has been used in model gasoline engines.
Electronic injection doesn’t seem to have any advantages so far in 26 cc engines.
Its main advantage in 50 cc and larger two stroke engines is lower emissions with good
power. However, pumper carbs start to have problems delivering enough fuel at high
rpm. A lot of the advantages of fuel injection have been used in model nitro engines a
long time. Pipe pressure to the tank combined with a very simple carburetor seem to
compensate for engine fuel demands over the narrow throttle and rpm ranges boats
usually operate. OS 9B and MAC 84 carbs have run on modified Zenoahs for years.
More recently a really large bore nitro style carb has been run on a 26 cc Quickdraw
with good results.
Jan Thiel predicted that a two cylinder 50 cc engine could develop 39 horsepower
at 23,000 rpm. That would be over 19 horsepower from a 25cc racing engine. Will we
be able to get to Jan Thiel’s predicted power with some or all of the innovations
covered in this series of articles? I think it’s possible.
The final style valve used on model
engines is the bell valve. It was designed
by Paul Bugl and used in the HB
engines. It’s by far the hardest to build,
but has the largest flow for its size. Like
the drum valve, it doesn’t direct mixture
flow over the big end bearing. It also is
probably only suited for an overhung
crankshaft design.
The last feature that needs to be
discussed is the combustion chamber.
Typically this is a hemispheric bowl
surrounded by a squish area that’s very
close to the piston crown at top dead
center. The squish both cools the
mixture to prevent preignition and
forces the mixture into the combustion
area at high speed. There has been a lot
of investigation into the exact squish
shape and area as well the combustion
chamber shape and volume. Neels
vanNiekerk has an excellent analysis of
the current state of understanding on the
Internet.
The most recent interesting
combustion chamber shape is the
toroidal combustion chamber. It seems
to make more power in larger, racing
two strokes, but I haven’t heard a good
explanation of why. The exact shape
and plug to head space seem to be
critical. In models it can be hard to get
a standard spark plug in the available
HB bell valve
Jim Allen’s Bell Valve
Toroidal head button
.750 bore carb