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output. Faculty of Engineering & Applied Science
Memorial University of Newfoundland ENGI 1030 Lawnmower Disassembly/Assembly Exhaust Stroke
By the time the piston is forced most of the way down the cylinder,
nearly all of the usable energy created from the burning fuel has been
used. At this point, the exhaust valve begins to open and as the
crankshaft rotation continues, the rising piston forces the spent gases
out of the cylinder and into the exhaust system. Just prior to the piston
reaching TDC, the exhaust valve closes and shortly thereafter, the
intake valve reopens to start the process over. Two Cycle Engine Operation Intake/Compression Stroke
A two cycle engine actually accomplishes two things per cycle
while running, whereas a 4 cycle completes one task per cycle.
This is possible by making use of both the combustion side and
the crankcase side of the piston. Following the flow of fuel, we'll
start with the engine intake from the carburetor. As the crankshaft
rotates, the piston is pushed upwards. This does two things. One,
the rising piston generates a vacuum in the sealed crankcase,
which pulls a fresh air/fuel charge from the carburetor into the
crankcase (thus the reason for having to add lubricating oil to the
gas for 2 cycle engines). A small reed valve (a one way valve)
allows the mixture to enter the crankcase, but not leave by the same path. At the same
time, the rising piston is also compressing the previous charge in the combustion
chamber. As the piston nears the top of the stroke, the ignition system ignites the charge
in the combustion chamber.
Once the air/fuel charge is ignited, the pressures generated by the
burning mixture force the piston down, again accomplishing two
things. The downward motion of the piston turns the crankshaft,
providing power output. In addition, the downward motion is
pressurizing the air/fuel mixture previously loaded in the crankcase.
As the piston continues its travel down the cylinder bore, it will first
expose the exhaust port in one side...
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- Fall '11