FOUR-STROKE CYCLE ENGINE
The four-stroke cycle engine develops one power stroke for every four
movements of the piston (two up and two down). This type might seem to
be a waste of motion as well as parts, for it requires many more parts.
However, it has many advantages, particularly in larger engines where
compactness is not as significant a factor.
The four-stroke engine does not have a reed, and the air-fuel mixture
does not pass through the crankcase. Instead, there are two valves, as in
l-l, one that opens and closes a passage from the carburetor, another that
opens and closes a passage to the exhaust system. The valves are operated
by the camshaft, a shaft with teardrop-shaped lobes that push the valves
open, and at appropriate times, allow springs to close them. The camshaft
has a gear at one end, which meshes with a gear on the crankshaft. The
gear on the camshaft has twice as many teeth as the crankshaft gear, so
that for every complete revolution of the crankshaft, the camshaft turns
180 degrees. This means that each valve opens and closes just once during
two revolutions of the crankshaft, which is exactly what’s needed for a
four-stroke cycle.
The valves in the typical four-stroke lawn mower or snow blower en
gine are located in the block. This is an antiquated automotive design, but
it’s good enough for mowers and blowers. There are a few four-strokers
with valves in the cylinder head, a popular automotive design, shown in
l-4. In this case the’camshaft lobes push on a long rod, called a pushrod,
which pivots a see-saw-like part called a rocker arm.
Post time: Jul-14-2023