Two-cycle Engine Applications and Lubrication
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.
What is a
motors deliver one power impulse for each revolution of the
"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.
two-cycle engines different from four-cycle engines?
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.
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.
two-cycle engines used?
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.
advantages and disadvantages of two-cycle engines
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.
two-cycle engines lubricated?
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.