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.
DIRECT FUEL INJECTION or DFI
is as the name implies,
a process of injecting a fuel charge directly into the combustion
chamber. Not to be confused with electronic fuel injection
(EFI) or programmed fuel injection into the intake passages
of a four stroke.