failure than the 26 cc gasoline engines.
A gasoline engine with similar relative
power will need a lower inertial load on
the dyno to keep cylinder pressures
down in the lower rpm area.
All the engine parts need to be
accurately made and aligned. As was
mentioned above, the ring must be round
and run in a round cylinder at all
temperatures. The area above the
exhaust port is the most critical. Below
is a failure caused by an out of round
cylinder. Note both the shiny spot on the
bridge between the intake and exhaust as
well as the area just above the exhaust
port. There are other shiny areas that
indicate a local tight fit. Peeling chrome
caused the scoring on the piston. There
are limits to how much of this out of
roundness can be tolerated.
The crankshaft must run at right
angles to the cylinder under load.
Deviations in this area load the edges of
the connecting rod big end bearing.
Since this is the most critical bearing in a
high output two stroke, much deviation
will cause failure. Sleeve bearings are
more tolerant in this area, but needle
PROPWASH
8
October 2013
High Power Two Stroke Design - Part 3
By Lohring Miller
NAMBA Safety Chairman
The following article is the last installment of a three part series. Please go to the
November 2012 Propwash on the NAMBA web site if you missed out on part one and
the April 2013 Propwash for part two.
There are many important details in small two stroke engine mechanical design.
One of the first is piston sealing and friction. Piston friction is one of the major losses,
especially in small engines where the cylinder surface is large for the displacement.
The best method for small diameter cylinders has been the ringless piston design.
Here a high silicon aluminum alloy piston is matched to a chromed brass or aluminum
cylinder liner. Careful fitting at the top combined with an increasing clearance below
the exhaust port top edge, results in good high pressure sealing at the combustion
chamber and adequate sealing of the crankcase. Lots of oil in the fuel helps make the
seal. This system hasn’t been used in mass produced engines with much over a 25 mm
bore, though some successful racing engines have used this design with a 29 mm bore.
The most notable failure of a ringless piston was the CMB 35 with a 35 mm bore
cylinder. High temperatures from gasoline as a fuel compared to nitro/alcohol mixes
contribute to the problems. I’m not aware of any successful gasoline fueled engines
that use ringless pistons.
Ringed pistons have been used a very long time. They are tolerant of diameter
changes due to tolerance and temperature, but still require a very round cylinder.
Their seal depends on fit to the lower side of the groove in the piston, fit to the
cylinder bore, and ring gap. Using two rings helps solve the gap leakage problem, but
doubles the friction. All serious small racing two strokes run a single ring. Again, lots
of oil helps make the seal. Ring flutter is caused by the ring’s inertia breaking the seal
with the bottom edge of the ring groove when the piston changes direction at top dead
center. Since this happens when the pressures to be sealed are highest, ring breakage
can happen. Making the ring very thin for weight and friction reduction helps. Today,
a thin, hard, heat resistant ring coupled with a hard, smooth cylinder coating seems to
be the best combination.
Piston cooling has also been an issue in two strokes. Two factors make it less of a
problem in small engines. The above mentioned large surface area to cylinder volume
ratio helps dissipate waste heat. Running the fuel through the crankcase cools the
bottom of the piston. Larger, high power two strokes need oil cooled pistons when
they use outside scavenging blowers. Even so, I’ve melted the pistons on several 11 cc
nitro engines when tested on the same inertial dyno as our gasoline engines. The nitro
engines develop about the same power but are considerably closer to the edge of
A failed AAC CMB 35 Piston & Sleeve
Failed CMB 35cc Sleeve
Failed CMB 35cc Ringed Piston