|
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 120°F. They also require more
lubricity than traditional two-cycle motors.
| 
