Amsoil Two-cycle Engine Applications
and Lubrication Needs
article appeared in AMSOIL Action News, July 2001
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.
is a two-cycle engine?
|Two-cycle motors deliver one
power impulse for each revolution of the crankshaft.
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.
are two-cycle engines different from four-cycle engines?
|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 power impulse.
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 center position.
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
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 120°F. They also
require more lubricity than traditional two-cycle motors.