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 There 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:
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
piston down.
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
cost.
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.
WHAAAAZZZZUP?
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?
READ ON
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