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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. Resulting motors were bulky and unreliable.
Those motors that were viable were for the most part rejected
by the boating public.
"FOUR STROKE DEFINED"
The first description reviews the operation of the 4 stroke
power cycle. Each 4 stroke image depicts a piston in a cylinder,
a spark plug and 2 valves; one intake, one exhaust. The
valves are held closed by means of a spring and opened by
a rotating eccentric called a camshaft. The camshaft is
driven from The crankshaft by means of gears or a drive
belt and timed to The up and down movement of The piston.
To complete all 4 strokes The crankshaft makes 2 revolutions.
GENERAL OPERATION
Intake stroke image 1
As The piston is pulled down during
The intake stroke, The camshaft opens The intake valve and
a fresh charge of fuel/air mix is drawn into The cylinder.
The intake valve closes when The piston reaches The bottom
of its downward stroke.
Compression stroke (image
2)
The piston now begins to
move upward and starts to compress The air/fuel mixture
in The cylinder. Both valves are closed. This continuing
upward motion compresses The mixture to about 100-120 PSI,
around 7 or 8 times atmospheric pressure (The compression "ratio"). As The piston reaches The top of The
cylinder The spark plug fires and ignites The compressed
mixture.
Power
stroke (image 3)
The fuel air mixture now
BURNS very rapidly, increases in pressure generated by combustion
force The piston downward in The cylinder. Both valves are
still closed. This is The only stroke that creates power
in The 4 stroke cycle.
Exhaust stroke (image 4)
Upon completion of The power
stroke The piston starts to move upward again and now The
exhaust valve starts to open. The continuing upward movement
forces The hot burned gases out past The exhaust valve.
When The piston reaches The top of The cylinder The exhaust
valve closes. The piston starts to go back down and The
cycle repeats itself.
The advantage to The 4 stroke is that
The combustion process is very efficient at varying RPM
ranges with almost no unburned fuel escaping into The atmosphere.
4 stroke engines also develop significant torque at low
Rpm's. The big drawback is there is only one power stroke
for every 2 revolutions of The crankshaft so The engine
lacks The burst of power experienced with The 2 stroke engine.
Four strokes are more complex as well as generally much
heavier as a result of additional parts EG camshaft, valve
train, balance shafts etc. required to complete The power
cycle. This additional complexity does not reduce The engines
reliability. Four strokes have a proven track record in
reliability and dependability.
Next, lets' review The two stroke
theory of operation and examine The various types of these
engines and how they work.
TWO STROKE
ENGINES, PAGE 2
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