Two-stroke engines do not have
valves, which simplifies their construction and lowers their
weight.
Two-stroke
engines fire once every revolution, while four-stroke
engines fire once every other revolution. This gives
two-stroke engines a significant power boost.
Two-stroke
engines can work in any orientation, which can be important
in something like a chainsaw. A standard four-stroke engine
may have problems with oil flow unless it is upright, and
solving this problem can add complexity to the engine.
These
advantages make two-stroke engines lighter, simpler and less
expensive to manufacture. Two-stroke engines 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 of light weight and twice the power gives
two-stroke engines a great power-to-weight ratio
compared to many four-stroke engine designs.
The
Two-stroke Cycle
The following
animation shows a two-stroke engine in action. The
spark-plug fires once every revolution in a
two-stroke engine.
This figure
shows a typical cross flow design. You can see that
two-stroke engines are ingenious little devices that overlap
operations in order to reduce the part count.
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 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, a
vacuum is created in the crankcase. This vacuum opens the
reed valve and sucks air/fuel/oil in from the
carburettor.
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 carburettor
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!
Many lightweight experimental aircraft use two stroke engines
such as produced by Rotax.
