Wonder how two stroke engine works ? .
You find two-stroke engines used in such
devices as chain saws and jet skis because two-stroke
engines have two important advantages over four-stroke
engines :
Two-stroke engines engines fire once every
revolution (four-stroke engines fire once every other
revolution) -- this gives two-stroke engines a significant
power boost .
That advantage 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 .
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
downward . Note that as the piston moves downward , 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 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 .
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, 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,
where the piston is compressing the air/fuel mixture and
capturing the energy released by the 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. Meanwhile, the sides of the piston are acting
like 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, so
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, so it can't hold a thick oil. Instead,
you mix oil in with the gas to lubricate the crankshaft,
connecting rod and cylinder walls. 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:
Each time a new charge of air/fuel is loaded
into the combustion chamber , part of it leaks out through
the exhaust por t. 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 shrink and lighten four-stroke engines
, and you can see that research coming to market in a variety
of new marine and lawn-care products .