For those who came
in late, Supercool has been trying to improve the performance
of his stunt engines by increasing compression, as opposed
to the more usual process of decreasing compression. No doubt
these efforts will be washed away by the tide of history,
but them's the breaks. We last left Supercool on his way to
the Whiteman Park tick farm, ready for further testing with
the trusty Enya 45.
This morning, however, we find Supercool has departed once
again from the straight and narrow, this time via the sensual
path of the Internet. How about a nice Google search for 'Squish
Band'? You never know your luck in the big city! And what
a gold mine this turned out to be!
But first, quite at random, Supercool struck a familiar vein
in a Model Airplane News article dated January, 1976, page
54. Reader Neil Harrington of Connecticut wanted to know what
it meant for a 2-stroke engine to '4-stroke'. In reply, Peter
Chinn suggested, on every other stroke, that the engine did
not fire, due to a combination of rich mixture and charge
contamination from imperfect exhaust gas scavenging from the
previous firing cycle.
Interesting, I wonder if it is true. I guess what I call
'4-stroking' is really just running rich. Since there is virtually
no RPM change when I needle in from '4-stroke' to '2-stroke',
there is very little power change and hence every other cycle
cannot be 'missing'.
Whatever, we are back at the old question of what does happen
when the charge burns in the cylinder head at TDC. There seems
to be a consensus that most of the combustion occurs while
the piston is still close to TDC, with all the later action
resulting from expansion of the hot gases after burn completion.
There is also believed to be a considerable variation from
fire-to-fire in the combustion process.
This latter variability results from the rather unpredictable
state of the mixture, which includes methanol vapour, methanol/castor
droplets, un-scavenged burnt gases from the previous firing,
plus oxygen and nitrogen. All this mess has to be compressed
before it can produce a powerful and fast burn. Anything that
can improve the quality of this mixture is going to improve
the engine run, and that is where the 'squish' comes in.
Something like a hundred years ago, a gentleman by the name
of Ricardo found that modifying the combustion chamber shape
to include flat areas which closely oppose the piston could
improve power and running characteristics. The improvement
was due to increased turbulence at TDC, which improved mixing
and hence burning of the charge.
The Supercool internet study found that the squish had the
- Timing: reducing the squish clearance, and using a wider
squish band with a smaller, deeper combustion dome makes
the motor 'act' as though the timing has been advanced.
- Vibration: To make maximum power, the squish band should
be flat. If the resulting pressure rises by more than 35
PSI/degree, the engine will run roughly.
- Grunt: At lower RPM, 'grunt' is achieved by higher squish
velocities, which means wider squish bands. For power at
higher RPM, narrower squish bands and shallower hemispherical
heads are required.
- Cooling: The fuel trapped in the squish does not burn
readily, due to the cooling proximity of the piston crown
and the metal in the squish band. This provides a small
If one is to apply this to F2B stunt, then a wider squish
band is indicated for low down grunt. The ST G21/46 has this
feature. Motors that tend to 'run away' may benefit from having
a new head with a wider squish band. It is also suggested
that reducing the squish clearance will improve low-down grunt
The photo below shows the heads for Supercool's ST G21/46.
Next to the original head, are 2 new heads, with identical
wider-squishes and deeper bowls. One has grooves cut in as
suggested in US patent 6,237,579 of May 29, 2001, as defined
by Mr. Somender Singh of Mysore (www.somender-singh.com),
India. You can view the patent details just using a Google
search. The argument behind the patent, and the diagrams of
head mods can be read on the US Patent Office website, no
charge or other exploitation.
Click to enlarge
Mr Singh's idea is to create extra turbulence by adding grooves
to the squish band. He has made some remarkable claims regarding
low-end grunt, and this I found attractive as I am trying
to use 11X8.5 props on my F2B ship. Anything that permits
me to run rich with grunt is desirable.
Ground running of the new heads showed excellent behaviour
while rich. I could needle down to 4000 RPM comfortably, lean
out to 6500 at the break and peak out at 7400. This latter
may be too high for my comfort, will need to test fly.
Sadly, I could not see any difference in the head with squish
band grooves. Maybe the grooves are not the right shape: but
there it is.
Test flying: During the desultory break as the morning Easterly
gale switches to the afternoon South-Westerly gale, Supercool
managed about 8 flights with the Singh head. Performance was
fine, no nasty signs of SMS (sudden motor silence), and 5.2
laps on 60' lines and straight fuel with 30% Castrol M. Compared
to the original ST 46 head, the level flight '4-stroke' was
somewhat faster and sounded to have more 'bark'. This could
be consistent with the more advanced timing expected with
the wider squish. No effort was made to alter the compression
What an anti-climax. I seem to have achieved the state of
getting the ST46 to run about the same as it does out-of-the-box!