Two-Cycle

IF WE LOOK at the cost of almost any gaspowered
airplane these days, we can safely
say that the power plant adds up to
approximately 25% of our investment. To
protect this investment we are faced with a
smorgasbord of oils and a never-ending
supply of opinions about which to use.
The choices of yesterday amounted to a
short list of petroleum-based products that
offered limited protection, even in
overconcentrated mix ratios. Aaah, those
were the days.
When I was a kid my dad would take me
out to race a Mac 100-powered Go-Kart. By
the end of the day the powerful engine had
put a sludge coating on the exhaust and the
rear of the chassis. That hint of castor smell
was so welcoming every weekend.
Well, those days of oil-belching and
smoke-spewing two-stroke engines are gone,
mainly because of heightening
Environmental Protection Agency (EPA)
and emissions requirements on national and
local government levels. Many other
industries that use our two-stroke airplanetype
engines have been forced to develop
testing standards for two-cycle oils that
reduce smoke emissions and unburned
exhaust deposits.
The two-stroke-engine industry’s longterm
objectives have been to reduce
emissions, which contain burned and
unburned oil, and develop a quality oil that
reduces the fuel-mixture ratio while
extending the life of the engine. The result of
all that hard work is that oil is now available
that will significantly reduce emissions,
produce fewer warranty problems, and
increase customer satisfaction since engines
will last longer with less maintenance and
fewer overhauls.
The model-airplane two-stroke-engine
industry is only a speck on the radar of the
entire two-stroke industry, even if we
include our bigger brothers in the ultralight
aircraft category. Since model-airplane
engines are often remnants of a past industry
or technology based on lawn and garden
equipment such as chain saws, weed eaters,
snowblowers, leaf blowers etc., we are left
with oils that were developed for those
industries and branched out to motorsports
such as motocross racing, snowmobiling,
and watercraft.
Not only have our oil standards changed
because of EPA requirements, but also
because improvements in two-stroke
technology, such as creative exhaust valving,
multiport induction, and the use of exotic
metals, create increased power. They also
increase engine heat and cylinder-head
temperatures.
Today there are basically three types of
oils we can select for use in our modelairplane
engines: petroleum based (dino oil),
semisynthetic—or synthetic blend—and full
synthetic. Searching the forum threads to
find the best oil for your engine will fog your
mind with ideas that may or may not apply
to your most crucial investment.
My attempt here is to use some facts and
relate my experience to all of you so you can
make an informed decision about selecting
the best oil and how to use it as intended.
A factor in my decision about which oil to
use is the multiple testing bodies that are
responsible for developing standards for the
various industries the two-cycle lubricants
are dedicated to serve. One of those groups
is the American Society for Testing and
Materials (ASTM or TC).
That organization uses three run-testing
sequences on air-cooled two-stroke engines
that range from 50cc single cylinder to 350cc
twin. These running sequences include
detergency testing to determine the
prevention of sticking rings and carbon
buildup within the combustion area.
The second assessment is the lubricity
test, in which a 50cc single-cylinder engine
runs at a constant 4,000 rpm while the load
is varied, causing head-temperature swings
from 392° to a whopping 662° for five
cycles.
The oil in this test has a 150:1 mix ratio,
with the results of output torque and a
teardown compared to the results of a
reference oil (type ASTM 600) run in the
January 2008 65
by Jim Gorombei
Two-Cycle
Engine Oils
This
crucial crude
can stand between a
fun flying season and
an expensive repair
that buys you a seat
in the pits
01sig3.QXD 11/19/07 2:53 PM Page 65
66 MODEL AVIATION
Photos by the author
Top left: After 10 hours of
running with conventional oil,
notice the carbon tracking inside
the combustion chamber and
residual carbon deposits.
Above: After 10 hours of run
time using a quality synthetic oil,
there is little evidence of carbon
tracking in the combustion area
and the casting is highly visible.
Left and below: Avoid oils with
no certification information. The
specifications explained in this
article are visible in bold print.
same kind of engine to determine the same output results. If the test
oil is determined to have a torque drop that is equal to or less than
the reference oil’s results, it is given the ASTM TC certification.
The third test sequence evaluates the oil’s capacity to inhibit
combustion-chamber deposits that can cause preignition or
variations in timing. This test is performed by using the 50cc
engine that runs for 50 hours at 4,000 rpm with a 392° cylinderhead
temperature while using a 20:1 mix ratio. For the ASTM TC
rating to be applied, there cannot be more than one major
preignition event or a significant increase in combustion-chamber
temperature.
Another certifying body is the National Marine Manufacturers
Association (NMMA), which now yields the TC-W3 certification.
The tests for this rating are similar to those for the ASTM TC
certification, but they use reference oil as a test standard with
slightly different parameters such as mix ratios, running hours, and
rpm. Since this is a marine certification, there is a bigger focus on
using marine engines for testing.
The most important thing the NMMA testing adds to the ASTM
evaluation sequences is the ability for the test oil to inhibit
corrosion. This is achieved by placing strips of a cylinder liner that
has been run using the test oil into a 100° and 100%-humidity
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January 2008 67
• Don’t overoil the engine. Most power
plant and oil manufacturers give specific
guidelines for using their products based
on testing. More is not always better; it
can cause variations in timing, stuck rings,
excessive carbon in the combustion
chamber and exhaust ports, and
preignition.
• Choose quality oil based on certifications I
have covered in this article and that can
be matched, in most cases, to the back of
the oil bottle. If no certifications are listed
on the product, I caution against using it.
Run a quality synthetic oil. Most are
recognized as having multiple or all three
certifications (ASTM or TC, NMMA, and
JASO). Beware of some synthetics offered
in home-improvement and hardware
stores that may actually be semisynthetic;
they use only a few of the detergent
additives but lack the premium base
stocks offered in a full synthetic.
• Run the engine dry and drain the fuel
system if it is going to sit for a length of
time (more than 30 days), because of fuel
spoilage. The added oil seems to change
the fuel’s properties and cause a rapid
increase in its spoiling.
• Use a good measuring device to be
accurate with the oil/fuel mix ratios, and
don’t be afraid to experiment with a few
points either way; e.g., 55:1-60:1 or a
richer mixture of 55:1 to 50:1. Different
oil ratios can make a noticeable difference
when tuning.
Employ short runs and the spark plug as
a guide (a nice chocolate-brown color of
the entire tip should be the norm). You
can also use a noncontact infrared
thermometer pointed at the base of the
spark plug to see these tuning differences.
Establish a baseline ratio. Conduct your
normal tuning, making note of rpm,
temperature, and spark-plug readings for
optimal performance and longevity. MA
—Jim Gorombei
To prevent overheating during engine
break-in, don’t run your power plant
longer than 10-15 minutes or faster than
4,000 rpm.
environmental chamber with evaporated
saltwater to create a corrosion process. The
test-oil results are measured against those
of the reference oil for differences.
The next certification is the Japan
Automobile Standards Organization
(JASO) type, which classifies oil in
categories of FB, FC, and now FD. The
JASO standard includes the basic
functionality tests of the ASTM
certification but adds physical-property
testing for minimum viscosity, minimum
flashpoint, and ash content.
JASO testing may be the most
important to environmental impact. It
includes testing for exhaust smoke by
setting a maximum standard for particulate
emissions and an oil’s potential to block
exhaust ports with carbon deposits, which
decreases efficiency and creates an
increase in overall emissions because of
the overall decrease in running efficiency.
The last test certification is that of the
ISO (International Organization for
Standardization). It is similar to the JASO
testing except that it does not include
flashpoint testing and adds to the
detergency testing by extending engine run
times.
Now that we have the science of oils and
testing covered, what applies to modelairplane
engines? In reality it is all those
certifications, but some are more relevant
than others based on our frequency of
usage, areas where we reside, the type of
flying we engage in, etc.
The engines we use require superior
film strength because of high compression
ratios, which significantly increase
combustion-chamber heat and crankshaft
bearing pressures. Hot combustion
processes quickly burn off and evaporate
light oils and cause rapid piston expansion,
decreasing piston-to-cylinder wall
clearances. This promotes piston scuffing
and possible seizure. Increased pressure on
crankshaft bearings also promotes metalto-
metal contact, wear, and pitting.
Most oils that are available today are
more than up to the challenge of
controlling internal engine loads for our
airplane engines. That is because of the
inherent light load of spinning a propeller
with the slight varying external influence
compared with the engines these oils were
developed for, which have varying heavy
external loads, such as a chain saw cutting
different-size wood or a weed eater that is
used to clear a forest of overgrown weeds
that are the size of small trees.
All engines can benefit from the
detergent process most modern synthetics
provide to keep the internal components
free from carbon and varnish deposits that
can cause rings to stick in the piston and
the resulting premature uneven cylinder
wear and rapid loss of compression in time.
Some of us, who are not-so-frequent
fliers or live in humid areas with salt air,
will benefit from the oils that are certified
NMMA and contain superior anticorrosion
Rules for Oil and Your New—or
Not-So-New—Engine
agents. This will ensure that extended shelf
time (winter downtime for many areas) can
result in a smooth-operating engine with
bearings that have no rust or surface
corrosion.
I’ll go “out on a limb” and offer my
opinions and suggestions based on 20-plus
years and probably thousands of engine
repairs and development.
When we mention oil for our new
engines, the first thing some of us mull
over is which type is best for the break-in.
How much break-in time will it require?
What should you not do?
As far as which oil to use, my
experience has shown that engines can be
broken in on those that are petroleum
based or synthetics. Yes, synthetics.
While bench-testing some new engines
I monitored the compression, cylinderhead
temperature, and visual inspections of
ring and cylinder wear patterns. These
engines exhibited the same patterns as
those broken in on conventional-type oils
but did take longer than the petroleumbased
lubricants to show these same types
of patterns and yield similar compression
numbers.
You may be asking how much longer
the break in is. If I ran one gallon of a 40:1
petroleum-based mix to two gallons of a
55:1 synthetic mix, the patterns,
compression numbers, and cylinder-head
temperature would be close.
My new engines get a synthetic breakin
because I don’t know why I would
prematurely induce varnishes and
unwanted carbons in them. It mystifies me
to see new engines that are clogged and
dripping with oil residue, whose owners
complain of erratic running, reduced
performance, and no-start/short-start
conditions. The users don’t think they are
harming the engine by adding more oil to
the fuel mix, but they are.
Another key to breaking in a new
engine is “heat cycling.” By running the
engine for 15 minutes on and 10 minutes
off, the parts have a better chance of
“mating” to one another and less of a
chance that an uneven cylinder-wear
pattern will occur that can induce
increased piston scuffing during the life of
the engine.
The break-in runs require an engine
rpm of only 4,000. Turning the engine
faster creates excessive heat that will cause
extreme or uneven wear.
Don’t use an oil in your fuel unless you
know where it came from. Check the label
for the testing certifications I have listed.
High-performance engines are a large
investment and deserve care when mixing
fuel. Expect a reliable, long life after the
engine has been broken in using the heatcycling
method.
Happy Flying! MA
Jim Gorombei
[email protected]
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