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.