Two-cycle Engine Applications and Lubrication Needs
Two-cycle engines can be found nearly everywhere these
days. They are used in dozens of applications and in a wide variety of
designs for everything from work and recreation to power generation.
Two-cycle engines have design differences and operate under conditions
that require different oil chemistries than their four-cycle
counterparts. In order to recommend a lubricant for a two-cycle engine,
one needs to know how this engine operates, why it is used in place of a
four-cycle engine and where and in what type of applications it is used.
What is a two-cycle engine?
|Two-cycle motors deliver one power impulse
for each revolution of the crankshaft.
The terms "two-cycle" and "two-stroke" are often
inter-changed when speaking about two-cycle engines. These engines
derive their name from the amount of directional changes that the
pistons make during each power stroke. Internal combustion engines are
used to produce mechanical power from the chemical energy contained in
hydrocarbon fuels. The power-producing part of the motor's operating
cycle starts inside the motor's cylinders with a compression process.
Following this compression, the burning of the fuel-air mixture then
releases the fuel's chemical energy and produces high-temperature,
high-pressure combustion products. These gases then expand within each
cylinder and transfer work to the piston. Thus, as the engine is
operated continuously, mechanical power is produced. Each upward or
downward movement of the piston is called a stroke. There are two
commonly used internal combustion engine cycles: the two-stroke cycle
and the four-stroke cycle.
How are two-cycle engines different from four-cycle
|A four-cycle engine requires four strokes
of the piston (two up and two down) and two revolutions of the
crankshaft to complete one combustion cycle and provide one
The fundamental difference between two-cycle engines and
four-cycle engines is in their gas exchange process, or more simply, the
removal of the burned gases at the end of each expansion process and the
induction of a fresh mixture for the next cycle. The two-cycle engine
has an expansion, or power stroke, in each cylinder during each
revolution of the crankshaft. The exhaust and the charging processes
occur simultaneously as the piston moves through its lowest or bottom
In a four-cycle engine, the burned gasses are first
displaced by the piston during an upward stroke, and then a fresh charge
enters the cylinder during the following downward stroke. This means
that four-cycle engines require two complete turns of the crankshaft to
make a power stroke, versus the single turn necessary in a two-cycle
engine. In other words, two-cycle engines operate on 360 degrees of
crankshaft rotation, whereas four-cycle engines operate on 720 degrees
of crankshaft rotation.
Where are two-cycle engines used?
Two-cycle engines are inexpensive to build and operate
when compared to four-cycle engines. They are lighter in weight and they
can also produce a higher power-to-weight ratio. For these reasons,
two-cycle engines are very useful in applications such as chainsaws,
Weedeaters, outboards, lawnmowers and motorcycles, to name just a few.
Two-cycle engines are also easier to start in cold temperatures. Part of
this may be due to their design and the lack of an oil sump. This is a
reason why these engines are also commonly used in snowmobiles and snow
Some advantages and disadvantages of two-cycle engines
Because two-cycle engines can effectively double the
number of power strokes per unit time when compared to four-cycle
engines, power output is increased. However, it does not increase by a
factor of two. The outputs of two-cycle engines range from only 20 to 60
percent above those of equivalent-size four-cycle units. This lower than
expected increase is a result of the poorer than ideal charging
efficiency, or in other words, incomplete filling of the cylinder volume
with fresh fuel and air. There is also a major disadvantage in this
power transfer scenario. The higher frequency of combustion events in
the two-cycle engine results in higher average heat transfer rates from
the hot burned gases to the motor's combustion chamber walls. Higher
temperatures and higher thermal stresses in the cylinder head
(especially on the piston crown) result. Traditional two-cycle engines
are also not highly efficient because a scavenging effect allows up to
30 percent of the unburned fuel/oil mixture into the exhaust. In
addition, a portion of the exhaust gas remains in the combustion chamber
during the cycle. These inefficiencies contribute to the power loss when
compared to four-cycle engines and explains why two-cycle engines can
achieve only up to 60 percent more power.
How are two-cycle engines lubricated?
Two-cycle motors are considered total-loss type
lubricating systems. Because the crankcase is part of the intake
process, it cannot act as an oil sump as is found on four-cycle engines.
Lubricating traditional two-cycle engines is done by mixing the oil with
the fuel. The oil is burned upon combustion of the air/fuel mixture.
Direct Injection engines are different because the fuel is directly
injected into the combustion chamber while the oil is injected directly
into the crankcase. This process is efficient because the fuel is
injected after the exhaust port closes, and therefore more complete
combustion of fuel occurs and more power is developed. Direct injection
engines have a higher power density than traditional two-cycle engines.
Because the oil is directly injected into the crankcase, less oil is
necessary and lower oil consumption results (80:1 range). Direct
Injection motors have higher combustion temperatures, often up to 120F.
They also require more lubricity than traditional two-cycle motors.
Which AMSOIL motor oils are recommended for two-cycle
AMSOIL Synthetic 2-Cycle Injector Oil and 100:1
Pre-Mix Two-Cycle Oil
AMSOIL Synthetic 2-Cycle Injector Oil (AIO) is
recommended for use in all summer and winter two-cycle injector
applications and for pre-mix applications at 50:1 mix ratios. Use AMSOIL
2-Cycle Injector Oil wherever TC-W3 or API TC oils are specified in
water-cooled or air-cooled motors. It is recommended for all two-cycle
injector applications including outboard motors, snowmobiles,
motorcycles, ATVs and personal watercraft. It is also compatible with
and recommended for two-cycle applications using catalytic converters.
It is recommended for use with gasoline fuels only. AMSOIL Synthetic
100:1 Pre-Mix 2-Cycle Oil (ATC) is recommended for all water-cooled and
air-cooled pre-mix applications. A mix ratio of 100:1 is recommended for
normal duty service in applications such as motorcycles, outboard
motors, weed eaters, lawn mowers and chain saws. Richer mix ratios of
50:1 to 80:1 are recommended for severe duty service such as racing
applications and for hot operating, industrial-use motors that run for
extended periods of time. AMSOIL Synthetic 100:1 Pre-Mix 2-Cycle Oil is
recommended for all pre-mix applications specifying TC-W3, API TC and
JASO FD. It is recommended for use with gasoline fuels only and is not
recommended for use in oil injection systems.
Series 2000 Synthetic 2-Cycle Racing Oil
AMSOIL Series 2000 2-Cycle Racing Oil (TCR) is recommended in air-
or water-cooled two-cycle motors where NMMA TC-W3, API TC or JASO FC
oils are specified. It is excellent for both racing and recreational use
in snowmobiles, outboard motors, personal watercraft, motorcycles and
ATVs. Series 2000 2-Cycle Racing Oil is not recommended for use with
nitro methane or alcohol fuels. It is recommended for use at a 50:1 mix
ratio or as an injector oil for both racing and recreation use and for
use with catalytic converters.