ENGINE TECHNOLOGIES MADE EASY!
HERE IS HOW IT WORKS
The most common internal
combustion engines of today can be defined as either four-stroke
or two-stroke cycle. Two-stroke or four-stroke refers to
the number of strokes the piston makes in the cylinder to
complete one power cycle. A stroke is the movement of the
piston in one direction, moving the piston from the top
to the bottom of the cylinder is one stroke. A running internal
combustion engine continually repeats a power cycle called:
intake, compression, power and exhaust. Your automobile
or stern drive engine is most likely a four stroke design.
The majority of existing outboard motors use two stroke
technology. However the current movement in emissions regulations
is pushing the design of current outboards towards the 4
stroke and direct injection two stroke design. Efforts to
build a 4 stroke outboard in the past have been many and
varied, mostly unsuccessful as the design technology and
precision production that can be achieved today were impossible
to achieve then. Resulting motors were bulky and unreliable.
Those motors that were viable were for the most part rejected
by the boating public.
TWO STROKE DEFINED
The two-stroke engine
completes its power cycle in only one crankshaft revolution
with two strokes of the piston. There are no valves, camshafts,
springs chains, etc. so the engine is much less complex
and lighter. Instead of valves tThere are a series of
strategically located transfer ports - intake and exhaust,
cut into the sides of the cylinder wall. The ports are
on opposite sides of the cylinder. The transfer ports
are opened and closed by the up and down movement of the
piston. To accomplish a complete power cycle both sides
of the piston are used; consequently several events occur
simultaneously during each stroke. They are:
INTAKE AND COMPRESSION
Up Stroke - Intake and Compression:
On the up stroke the top
side of the piston is compressing an air/fuel mixture in
the cylinder. At the same time the BOTTOM side of the piston
pulls another fresh charge of air/fuel mixture into the
crankcase thru a one way valve called a reed valve. Near
the top of the stroke the compressed air/fuel above the
piston is ignited by the spark plug and begins to burn.
The rapidly burning fuel expands and begins forcing the
POWER AND EXHAUST
Down Stroke - Power and Exhaust
On the down"power"stroke
the piston is forced towards the crankcase reducing its
volume and creating a positive pressure. As it continues downward travel it starts
first to uncover the exhaust ports. Exhaust gas begins to rush out of the cylinder.
Then the intake ports are uncovered. The fresh air/fuel charge in
the crankcase is forced into the cylinder and continues
to push the remaining exhaust gases out.
The 2 stroke process of purging exhaust
gases from the cylinder and filling it with a fresh air/fuel
charge is called scavenging. Two stroke engines use 2 different
scavenging methods, cross-scavenging and loop scavenging.
Both differing designs have particular advantages.
TWO STROKE CROSS SCAVENGED
Two stroke cross-scavenged engines
can be identified by the irregular shape of the top of
the piston called a deflector. This deflector directs
the incoming air/fuel up, towards the top of the cylinder.
This creates a wall or column of fresh mix that sweeps
across the cylinder towards the exhaust ports. As the
column advances it pushes the spent exhaust gases out
of the exhaust ports.
TWO STROKE LOOP SCAVENGED
Pistons in loop scavenged engines
are generally near flat. They do not rely on deflectors
to aim the fuel/air mix, rather they have shaped intake ports and combustion
chambers to control the scavenging of the cylinder. Several intake ports are
aimed upwards and arranged such that their combined streams flow upward
and then LOOP down toward the exhaust ports.
Cross-scavenged engines are better performing
at idle and low speed. All older motors of any horsepower
are of this design. Until the late 60's it was not economical
to try to produce this design in quantity at a reasonable
Loop charged engines, although having
poorer idling characteristics are more fuel efficient and
perform better at higher RPM's than crossflow as they have
lighter pistons. This lowers the strain on the connecting
rods, bearings and crankshaft. OMC created the first US production
looper in 1968 with the 3 cylinder 55 HP.
Now that you have been thru the basics
of the current marine and outboard engine technology, what
is all the furor in current outboards? What outboard engine choice should you
make? What brand should you purchase? is an "I-O" a better choice?
How does the EPA figure in this equation?
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