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 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
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.