appeared in AMSOIL Action News, July 2001
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?
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
How are two-cycle
engines different from four-cycle engines?
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
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
sump. This is a reason why these engines are also commonly used in
snowmobiles and snow blowers.
Some advantages and disadvantages of two-cycle
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
Which AMSOIL Motor Oils Are
Recommended For Two-Cycle