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.
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