Two-cycle Engine Applications
and Lubrication Needs
This article appeared in
AMSOIL Action News, July 2001
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 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. |
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 center position.
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 blowers.
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.
