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موتورهاي دو
زمانه
بدين دليل به آنها موتورهاي دو زمانه
گويند که قبل از احتراق يک ضربه همفشردگي هوا و سوخت و يک
ضربه اشتغال در آن وجود دارد. در چنين موتورهايي پيستون در
واقع سه عمل مختلف را انجام مي دهد: 1- در يک طرف پيستون،
محفظه احتراق وجود دارد. پيستون ترکيب هوا / سوخت را فشرده
ساخته و توسط اشتعال سوخت، انرژي آزاد شده را دريافت مي
دارد. در طرف ديگر پيستون، ميل لنگ ديده مي شود، جائي که
پيستون خلائي را ايجاد مي کند تا از کاربراتور با استفاده
از دريچه دهانگيز، هوا / سوخت را مکيده سپس ميل لنگ را
فشرده مي سازد. موتور دو زمانه سبک و ساده تر بوده طوريکه
نيروي زيادي را ايجاد مي نمايد. اجزاء آن زود فرسوده
مي گردد و روغن آن گران و حدود چهار اونس از آن در هر گالن
مورد نياز است. موتورهاي دوزمانه کاملاً سوخت را استفاده
نکرده طوريکه آلودگي زيادي را ايجاد مي نمايند.
Discover the
differences between the engine in your car and the
engine in your chain saw!
If you have read the HSW article
on car engines and the diesel engine page, then you are
familiar with the two types of engines found in nearly
every car and truck on the road today. Both gasoline and
diesel automotive engines are classified as
four-stroke reciprocating internal combustion
engines.
There is a third class of
engines, known as two-stroke
engines, that are commonly found in
lower-power applications. You will typically find
two-stroke engines in things like:
Lawn and garden
equipment like chain saws, leaf blowers, trimmers, etc.
Smaller motorcycle
engines used on dirt bikes
Mopeds
Jet skis
Small outboard motors
Radio-controlled
model planes
You find two-stroke engines used
in these applications because two-stroke engines have
two important advantages over four-stroke engines:
Two-stroke engines do
not have valves, which simplifies their construction.
Two-stroke engines
fire once every revolution while four-stroke engines
fire one every other revolution, giving two-stroke
engines a significant power boost.
These two advantages make
two-stroke engines lighter, simpler and less expensive
to manufacture. They also have the potential to pack
about twice the power into the same space because there
are twice as many power strokes per revolution. The
combination gives two-stroke engines a great
power-to-weight ratio.
You don't see two-stroke engines
in cars, however. That's because two-stroke engines have
a couple of significant disadvantages that will make
more sense once we look at the operation of a two stroke
engine.
The Two-Stroke
Cycle
The following animation shows a
two-stroke engine in action. You can compare this
animation to the animations on the car engine and the
diesel engine pages to see the differences. The big
difference to notice when comparing figures is the fact
that the spark-plug fires once every revolution in a
two-stroke engine.
This figure shows a typical
cross flow design. In
this figure you can see that two-stroke engines are
ingenious little devices that overlap operations in
order to reduce the part count to a minimum.
You can understand a two-stroke
engine by watching each part of the cycle. Start with
the point where the spark plug fires. Fuel and air in
the cylinder have been compressed and when the spark
plug fires the mixture ignites. The resulting explosion
drives the piston to the right. Note that as the piston
moves to the right, it is compressing the air/fuel
mixture in the crankcase. As the piston approaches the
bottom of its stroke, the exhaust port is uncovered. The
pressure in the cylinder drives most of the exhaust
gases out of cylinder, as shown here:
As the piston finally
bottoms out, the intake port is uncovered. The piston's
movement has pressurized the mixture in the crankcase,
so it rushes into the cylinder, displacing the remaining
exhaust gases and filling the cylinder with a fresh
charge of fuel, as shown here:
Note that in
many two-stroke engines that use a cross-flow design,
the piston is shaped so that the incoming fuel mixture
doesn't simply flow right over the top of the piston and
out the exhaust port.
Now the momentum in the
crankshaft starts driving the piston back toward the
spark plug for the compression stroke. As the air/fuel
mixture in the piston is compressed, notice that a
vacuum is created in the crankcase. This vacuum opens
the reed valve and sucks air/fuel/oil in from the
carburetor.
Once the piston makes it to the
end of the compression stroke, the spark plug fires
again to repeat the cycle. It's called a two-stoke
engine because there is a compression stroke and then a
combustion stroke. In a four-stroke engine there are
separate intake, compression, combustion and exhaust
strokes.
You can see that the piston is
really doing three different things in a two-stroke
engine:
On one side of the
piston is the combustion chamber. The piston is
compressing the air/fuel mixture and capturing the
energy released by ignition of the fuel.
On the other side of
the piston is the crankcase, where the piston is
creating a vacuum to suck in air/fuel from the
carburetor through the reed valve and then pressurizing
the crankcase so that air/fuel is forced into the
combustion chamber.
Meantime, the sides
of the piston are acting like the valves, covering and
uncovering the intake and exhaust ports drilled into the
side of the cylinder wall.
It's really pretty neat to see
the piston doing so many different things! That's what
makes two-stroke engines so simple and lightweight.
If you have ever used a
two-stroke engine, you know that you have to mix special
two-stroke oil in with the gasoline. Now that you
understand the two-stroke cycle you can see why. In a
four-stroke engine, the crankcase is completely separate
from the combustion chamber. In a four-stroke engine,
therefore, you can fill the crankcase with heavy oil to
lubricate the crankshaft bearings, the bearings on
either end of the piston's connecting rod and the
cylinder wall. In a two-stroke engine, on the other
hand, the crankcase is serving as a pressurization
chamber to force air/fuel into the cylinder. Therefore
the crankcase cannot hold a thick oil. Instead, the oil
you mix in with the gas is how the crankshaft,
connecting rod and cylinder walls are lubricated. If you
forget to mix in the oil, the engine isn't going to last
very long!
Disadvantages of the Two-Stroke
Engine
You can now see that two-stroke
engines have two important advantages over four-stroke
engines: they are simpler and lighter, and they produce
about twice as much power. So why do all cars and trucks
use four-stroke engines? There are four reasons:
Two stroke engines
don't last nearly as long as four-stroke engines. The
lack of a dedicated lubrication system means that
two-stroke engine parts wear a lot faster.
Two-stroke oil is
expensive and you need about 4 ounces of it per gallon
of gas. You would burn about a gallon of oil every
thousand miles if you used a two-stroke engine in a car.
Two stroke engines do
not use fuel efficiently, so you would get lower MPG
numbers.
Two-stroke engines
produce a lot of pollution. So much, in fact, that it is
likely that you won't see them around too much longer.
The pollution comes from two sources. The first is the
combustion of the oil. The oil makes all two-stroke
engines smoky to some extent, and a badly worn
two-stroke engine can emit huge clouds of oily smoke.
The second reason is less obvious but can be seen in the
following figure:
Each time a new charge of
air/fuel is loaded into the combustion chamber, part of
it leaks out through the exhaust port. That's why you
see a sheen of oil around any two-stroke boat motor. The
leaking hydrocarbons from the fresh fuel combined with
the leaking oil is a real mess for the environment.
These disadvantages mean that
two-stroke engines are used only in applications where
the motor is not used very often and the fantastic
power-to-weight ratio of the two-stroke engine is
important.
Meantime,
manufacturers have been working to miniaturize and
lighten four-stroke engines, and you can see that
research coming to market in a variety of new marine and
lawn-care products.
by Marshall
Brain | 
