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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?
The terms "two-cycle" and "two-stroke" are often
pistons make during each power stroke. Internal
combustion engines are used to produce
contained in hydrocarbon fuels. The 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 two-stroke cycle and the
four-stroke cycle.
Two-cycle motors deliver one power impulse
for each revolution of the crankshaft.
How are two-cycle engines different from four-cycle engines?
click picture to enlarge
click picture to enlarge
A four-cycle engine requires four strokes of
the piston (two up and two down) and two
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.

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

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.
In a four-cycle engine, the burned gasses are first displaced by the piston during an
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.
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