To understand the mechanical differences between a two stroke and four
stroke engine, lets first consider how the four stroke engine works. The
four strokes
are:
Intake: The piston travels down the cylinder while the intake
valve
is opened to allow a mixture of fuel and air to enter the combustion
chamber.
Compression: The intake valve is closed and the piston travels back up the
cylinder thereby compressing the gasses.
Combustion: The
spark plug
ignites the compressed gas causing it to explode, which forces the piston
down.
Exhaust: The piston rises up the cylinder as the exhaust valve is opened,
allowing the piston to clear the chamber to start the process over.
Each time the piston rises and falls it turns the crankshaft that is
responsible for turning the wheels. This is how fuel is converted into
forward motion.
Of note here is that the spark plug only fires once every other revolution.
Also, there is a sophisticated set of mechanisms working in synchronization
to create the four strokes. A camshaft must alternately tip a rocker arm
attached either to the intake or exhaust valve. The rocker arm returns to
its
closed position via a spring. The valves must be seated properly in the
cylinder head to avoid compression leaks. In other words, a symphony of
mechanical
events occurs.
In the two stroke engine, all four events are integrated into one simple
downward stroke, and one upward stroke. Two strokes. Intake and exhaust are
both
integrated into the compression and combustion movement of the piston,
eliminating the need for valves. This is accomplished by an inlet and
exhaust port
in the wall of the combustion chamber itself. As the piston travels downward
from combustion, the exhaust port is exposed allowing the spent gasses to
rush out of the chamber. The downward stroke also creates suction that draws
in new air/fuel through an inlet located lower in the chamber. As the piston
rises again, it blocks off the inlet and port, compressing the gasses at the
top of the chamber. The spark plug fires and the process starts over.
Significantly,
the engine fires on every revolution, giving the two stroke its power
advantage.
However, at the lowest point of travel of the piston when the chamber is
filling with fuel/air, the exhaust port exposed above allows some
fuel/gasses to
escape the chamber. This is easily seen with an outboard motorboat, evident
by the multicolored
oil slick
surrounding the engine, but it happens with all two stroke engines. This -
along with burning oil -- creates pollution and fuel-efficiency issues.
For these reasons, two stroke engines are reserved for intermittent use,
where weight-to-power ratio or orientation issues are important and where
mileage
isn't primary. Meanwhile manufacturers are looking for ways to add
advantages to four stroke motors, making them smaller, lighter and more
robust.
To further understand the difference between a two stroke and a four stroke
engine let us consider the advantages and disadvantages.
Advantages of the two stroke:
. Has more get-up-and-go because it fires once every revolution, giving it
twice the power of a four stroke, which only fires once every other
revolution.
. Packs a higher weight-to-power ratio because it is much lighter.
. Is less expensive because of its simpler design.
. Can be operated in any orientation because it lacks the oil sump of a four
stroke engine, which has limited orientation if oil is to be retained in the
sump.
These attributes make two stroke engines very popular for a variety of uses
from dirt bikes, mopeds,
jet skis,
and small
outboard motors,
to lawn and garden equipment such as mowers, edgers,
leaf blowers,
chain saws and hedge trimmers.
But there are other differences between the two stroke and four stroke
engines that aren't so favorable, which is why you won't see two stroke
engines in
cars.
Disadvantages of the two stroke:
. Faster wear and shorter engine life than a four stroke due to the lack of
a dedicated lubricating system.
. Requires special two stroke oil ("premix") with every tank of gas, adding
expense and at least a minimal amount of hassle.
. Heavily pollutes because of the simpler design and the gas/oil mixture
that is released prior to, and in the exhaust (also creates an unpleasant
smell).
. Is fuel-inefficient because of the simpler design, resulting in poorer
mileage than a four stroke engine.
. Has a high-
decibel
whine that may exceed legal noise limits in some areas, depending on the
product and local applicable laws.
Before purchasing a two stroke product you might check with your local
municipality to see if any restrictions apply.
