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In 1876, a German engineer named Dr. Otto produced an engine, that
worked, using the four-stroke, or Otto cycle. "Four-stroke" refers to
the number of piston strokes required to complete a cycle (a cycle
being a sequence of constantly repeated operations). It takes two
complete revolutions of the crankshaft to complete the cycle.
The first stroke is the intake stroke. The piston moves down the
cylinder and creates a partial vacuum in the cylinder. A mixture of
air and fuel is forced through the inlet valve into the cylinder by
atmospheric pressure, now greater than the pressure in the cylinder.
During this stroke, the exhaust valve stays closed.
The second stroke is the compression stroke. The piston moves up in
the cylinder with both valves closed. The air and fuel mixture is
compressed and the pressure rises.
The third stroke is the power stroke. Near the end of the compression
stroke, the air and fuel mixture is ignited by an electric spark from
the spark plug. The combustion that occurs causes a rise in
temperature and enough pressure to force the piston down again.
Finally, on the fourth stroke, or exhaust stroke, the piston moves up
again and forces the burned gases out of the cylinder and into the
exhaust system. This cycle repeats itself the entire time the engine
is running.
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