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?
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
How are two-cycle engines different from
|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
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
How are two-cycle engines
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.