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Motor
Oil Test & Comparison - The Most Comprehensive Test Results
Ever Published by AMSOIL
Two-cycle Engine Applications
and Lubrication Needs
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 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.
|
| Two-cycle motors deliver one power
impulse for each revolution of the crankshaft. |
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 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.
|
| 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. |
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 120°F.
They also require more lubricity than traditional two-cycle
motors.
Related Article: A
Look at Snowmobile Exhaust Power Vavles
AMSOIL Leads the Way in Two-Cycle
Oil Technology
New AMSOIL "Fabulous
Four" 2-Cycle Oils provide second-to-none protection
and performance in modern two-cycle equipment
Two-cycle engines remain the
workhorses in recreational equipment, including outboard motors,
motorcycles, snowmobiles and personal watercraft, as well
as in lawn and garden applications like lawn mowers and weed
eaters. Design changes over recent years have included direct
fuel injection engines, in which fuel is injected directly
into the combustion chamber, and exhaust power valves, which
improve combustion efficiency at varying RPM. These design
changes have allowed two-cycle engine manufacturers to effectively
reduce emissions, improve fuel efficiency and decrease oil
consumption. Today's two-cycle motors of all types require
specialized oil technology to deliver maximum engine protection,
dependable operation and long service life.
As the leader in two-cycle oil
technology, AMSOIL has performed extensive research in developing
a new line of revolutionary, specialized oils called the "Fabulous
Four." AMSOIL INTERCEPTOR Synthetic 2-Cycle Oil (AIT)
was the first new oil to be introduced. It is primarily recommended
as an injector oil or at a 50:1 mix ratio in carbureted, electronic
fuel injected (EFI) and direct fuel injected (DFI) snowmobiles,
personal watercraft, motorcycles and ATV's. Formulated with
wax-free premium quality synthetic esters and a specialized
additive package, INTERCEPTOR is designed to address the exhaust
valve sticking problems associated with certain snowmobile
models.
New AMSOIL HP Injector Synthetic
2-Cycle Oil (HPI), DOMINATOR Synthetic 2-Cycle Racing Oil
(TDR) and Saber Professional (ATP) and Outboard (ATO) Synthetic
100:1 Pre-Mix 2-Cycle Oils complete the Fabulous Four lineup,
replacing AMSOIL Synthetic 2-Cycle Injector Oil, Series 2000
Synthetic 2-Cycle Racing Oil and 100:1 Pre-Mix Synthetic 2-Cycle
Oil, respectively. The application chart below ranks each
oil's performance in common two-cycle applications.
Which
AMSOIL Products Are Recommended For Your Car or Truck?
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