|
Outboard engines are characterized by their constant speed, high output
operation. They are usually set at a desired high speed and continue at
speed until the destination is reached and then throttled down.
Also, they are constantly cooled with fresh, cool, non-recirculated water.
Chain saws, on the oth er hand, are a high action operation. They are
constantly started and stopped, used for short periods and frequent
overloads are its hard place in life.
Additionally they have smaller
displacements than outboards and are air-cooled. By understanding how the
operation of an engine can affect the oil used and how an oil can affect
the engine, we can better appreciate the difference between a water cooled
two-cycle oil and one formulated for an air-cooled two-cycle engine.
Water-cooled two-cycle engine oils require higher levels of a heavy oil
to prevent piston and cylinder wall scuffing. Because of their high
average piston temperature, lighter oils evaporate too quickly from the
piston cylinder contact area. The heavy base oil, which vaporizes at very
high temperatures, resists evaporation and remains in place to provide
lubrication to the piston and cylinder. Air-cooled oil formulations must
have much lower levels of the heavy base oil than water-cooled engine
oils.
These oils require only a small amount of heavy oil to pro vide
protection against piston scuffing and seizure at peak temperatures. Heavy
levels of heavy base oils in an oil formulated for air-cooled engines can
cause engines deposits.
These deposits form as a result of incomplete burning of the heavy oil.
The deposits can cause piston ring sticking and can eventually plug or
disrupt the flow of the exhaust system, resulting in power loss and
possible engine damage. Detergent additives should not be used in
water-cooled two-cycle oil formulations. When burned with the fuel,
detergents can produce an ash deposit in the cylinders. This ash deposit
can possibly foul spark plugs, form exhaust port deposits which cause loss
of power, and possibly create cylinder hot spots that can cause
destructive pre-ignition.
On the other hand, the only way to protect air- cooled two-cycle
engines against piston ring sticking at their high peak temperatures is to
include some detergent additives in the oil formulation. Detergents
provide high temperature deposit control not available from other
additives used in the oil. However, in the air-cooled engine, any ash
deposits that could form from the detergents are dislodged by engine
vibration and exhausted from the engine.
Now is a good time for a lesson on ash. Ash is the non-combustible
residue of a lubrication oil or fuel. Detergent additives contain metallic
derivatives, such as calcium, barium and magnesium sulfonates that are
common sources of ash. Ash deposits can impair engine efficiency and
power.
But, detergents are an important component of engine oil that help
control varnish deposits, piston ring deposits and rust (yes, rust) by
keeping insoluble combustion particles from adhering to metal surfaces. In
some cases, detergents neutralize acids formed from combustion of the fuel
mixture. Ash deposits may have a grayish color, whereas carbon residue is
usually black and sooty.
Carbon residue, on the other hand, is different from ash. Carbon
residue is formed from unburned and partially burned fuel, and from
burning of the crankcase lubricant. Water from condensation of combustion
products along with carbon residue from fuel contribute to engine piston
deposits. Carbon de posits are normally black and have a sooty appearance.
Oils formulated for outboard engines require a large amount of
antioxidant and dispersant additives to control deposit formation since
these oils do not contain detergents. Outboard oils also contain a large
amount of rust inhibitors because an outboard engine's continuous contact
with water makes rust prevention an important requirement. Air-cooled oils contain lower amounts of antioxidants and dispersants,
since the detergent additives do most of the work of preventing deposit
accumulation. These oils also contain rust inhibitor additives to protect
against rust that can form from water that enters the engine due to
condensation.
So, how do some oils claim to be multi-purpose or formulated for water
and air-cooled engines? These oils have usually been formulated to meet
the National Marine Manufacturers Association (N M MA) TC-WI I TM or TC-W3
TM specification. These specifications require the oils be tested under
rigorous test conditions. An air-cooled engine test is part of these
requirements.
Therefore, a manufacturer can claim multi-purpose applications.
Although these oils have been tested in an air-cooled engine and will
lubricate an air-cooled engine, an oil formulated specifically for
air-cooled engine use may be the best choice for your engine.
Just what oil does Rotax recommend for their two- cycle engines? Rotax
recommends using an oil meeting American Petroleum Institute (API) Service
Classification TC. API TC is a designation for high performance two-cycle
engines (typically 50 to 500 cc) excluding outboard engines. This
performance rating is determined by engine tests that evaluate (1)
anti-scuff characteristics, (2) piston ring sticking and engine
cleanliness, and (3) pre-ignition. In the TC category, a 50 cc and a 350
cc Yamaha engine are used to evaluate the oil.
Engines that require the fuel and oil to be pre mixed should use a two
percent (50:1) concentration of oil in the fuel. It is very important that
the fuel/oil mix is correct. In other words, don't add too much or too
little oil to the fuel. If a little does good, more oil added to the fuel
doesn't necessarily do better. In fact, too much oil will lead to
excessive deposits and could also cause excessive exhaust smoke and spark
plug fouling. Not enough oil can lead to piston skirt and cylinder wall
scuffing and eventual engine damage.
Gasoline containing alcohol (ethanol and/or methanol) should not be
used unless permitted by the engine manufacturer. Alcohol-containing fuels
can absorb water and separate from the gasoline. Additionally, the alcohol
may not be compatible with some fuel system components, such as plastic
and rubber compounds.
Rotax also recommends "de-carboning" the engine after every
50 hours of operation. This procedure is designed to remove excessive
piston deposits to check for possible stuck piston rings. Rotax allows up
to 0.040 inches of soot and carbon buildup on the pis ton crown before
removal of the carbon is required. Not only does Rotax recommend an API TC
oil for the 447 and 503 air-cooled engines, but also for the 532 and 582
engines run at internal temperatures similar to the air-cooled Rotax
engines, as evidenced by their use of the same spark plug.
Special precautions should be taken when switching oils, even between
the same brands. Because of the special formulation of air-cooled engine
oil, these oils generally are not compatible with water-cooled engine
oils. Caution should be exercised to ensure that these products are not
mixed together. Special precautions should be taken when changing from a
product designed primarily for water-cooled engines to an air-cooled
product, particularly in oil injection systems where the undiluted oils
would be mixed together. It is recommended that the oil reservoir and
lines be drained when changing to another formulation. In applications
where the oil is premixed with the fuel, it is recommended the fuel tank(s)
be drained and filters changed.
By Charles Kudolis (Excerpted from EXPERIMENTER, April, 1994)
For
more information click here. |
