PROPWASH
October 2013
9
This same problem has been found on several engines with stock Zenoah
crankshafts. So far the solution has been double ball bearings on the crankshaft. I think
widely spaced bearings on an overhung crankshaft could be just as rigid.
Another, more extreme solution is to make the crankcase out of a stiffer material
than aluminum; steel. It would be heavy and probably unnecessary to cast all the
crankcase out of steel, but steel front ends have been tested. The main advantage
seems to be equal thermal expansion of all the pieces. This means the crankshaft
bearings don’t lose their press fit and the end play doesn’t increase. A steel spacer/seal
locks the inner races together for controlled end play that doesn’t change with
temperature.
See the Wakkerman engine site for a
good discussion of these issues and
much more. Ceramic ball crankshaft
bearings are the current state of the art
and probably should be included in a
new high performance design.
An often neglected issue in a high
rpm engine is windage and viscous drag
between the crankshaft and crankcase.
A Zenoah was built with all the internal
clearances reduced as much as possible
to increase crankcase compression.
Surprisingly, power was lower,
probably due to viscous drag. The
consensus is that at least 1 mm
clearance is needed around the rotating parts. Streamlining the rod may also help.
Another important issue for the crankcase volume is where it’s located. If most of
the crankcase volume is in or around the transfers, it is easy to get the mixture into the
cylinder. Mixture trapped under the piston or in the crankcase valve mechanism will
be harder to get into the cylinder. Piston skirt windows into the transfers have often
been used to help.
The crankcase valve system has a lot to do with flow into the cylinder. The simple
two stroke gets mixture in and out of the cylinder through ports in the cylinder wall.
These ports and their passages have been discussed in parts one and two. Getting
mixture into the crankcase can take a more complex valve. Whether this is even
needed was discussed in part two of the April 2013
Propwash
.
However, all current crankcase compression two strokes use a valve to regulate
flow into the crankcase. The simplest is the piston skirt controlled port into the
cylinder. Because this valve opens and closes symmetrically around top dead center,
the time it can remain open is limited by flow out of the valve as the piston descends.
This also restricts the length of time the valve can stay open to allow complete
crankcase filling. Usually opening and closing between 70 and 80 degrees either side
of top dead center is used. That restricts the total open duration to 140 to 160 degrees.
Continued on page 10
bearings are not. Can an overhung
crankshaft design work rather than the
conventional large engine crankshaft
with bearings on both sides of the rod?
Again this seems to be a size issue.
Smaller engines can be made with
relatively huge crankshafts which work
with an overhung design. As crankshaft
weight becomes significant, a stiffer
basic design is needed. I think the easier
to construct, overhung design could still
work with a 26 cc engine. See
Big End
Blues
in the April 2012
Propwash
for
other big end bearing issues.
Other issues affecting dynamic
rigidity are crankshaft bearing support
and cylinder block design. A one piece
cylinder is more rigid than the multi
piece design used on all but the smallest
engines. A good example is the
durability at higher rpm and power of
the last 7.5 K&B design compared to
their old style K&B 7.5 cc engine’s
design.
The one piece crankcase is better at
holding the crankshaft aligned. We
found that the CMB 35 crankshaft
flexed enough to cause flywheel strikes
despite being supported on both sides
by bearings. A double bearing output
side helped. The magneto was replaced
with the Quickdraw electronic ignition
system as well. (See following photos)
Old Style K&B 7.5
K&B Pro 7.5
CMB 35 Front Bearing and Quickdraw ignition system
Crankshaft with inner bearing race spacer/
seal and front plates