Los Angeles herald., 1910-12-11
WORKING OF PISTON
AND VALVE EXPLAINED
DR.. Chas. w. Snyder Gives an
Easily Understood Les
son to Motorists

Draw a circle and call it Fig. 1.
Then quarter the rim and letter the
segments A, B, G., d. About one inch
above B mark in the letter e, and per-
pendlcular to that mark tine F, and
you will have basis for this expla-
nation.
Assume that Fig. l is the crank
circle of your automobile engine; that
is, the circular line represents the
path through which your crank turns
during each revolution, the arrow
indicating the direction.
If your engine is a four-cycle, the
intake stroke occupies the distance
A-B-C. At c the compression stroke
begins and passes through D to A, the
circle, of course, being again Travelled
on the power and exhaust strokes of
the piston, there being two revolu-
tions of the crank and four strokes
of the piston for each power impulse.
This looks simple enough, but. the
power and also the "sweetness' of
your motor depends on the manner in
which these functions are carried out.
Let us start on the intake stroke,
beginning at A. 1t is evident that the
crank must move some little distance
toward the right before it can move
the piston downward in the cylinder
to any extent. This shows the reason
why the intake valve is nearly always
designed to open after rather tnan
just at the upper center.
slight vacuum or pull is neces-
sary to draw the gasoline from the
carburetor, and if the intake valve
opened exactly on center there would
not be sufficient vacuum to produce
a good mixture. In other words, as
the piston started on its intake stroke,
especially at slow speed, it's motion
would be so slow that the air drawn
1. f..IN Rf ooh.
rotor would be moving so slowly that
the carburetor wouldn't spray. Suf-
ticient vacuum to secure a proper mix
Nure not occurring until the piston
was well on its stroke.
At the completion of this suction or
intake stroke the gas in the intake
pipe and manifold has considerable
speed The crank advance for, say,
thirty degrees before, and the same
distance after, the lower center has
little effect on the piston motion, the
piston practically being idle. For
this reason the intake valve is rarely
closed until ten to thirty degrees after
the completion of the suction stroke,
thus allowing the rushing gas more
time to fill the cylinder and getting a
larger charge.
There is another advantage to be
derived from this late closing of the
intake valve, which will be mentioned
later.
Following the intake stroke. which
is downward, is the compression
stroke-upward and somewhere along
about the-end of this stroke the charge
must be ignited. Where? When we
crank the engine the lever controlling
the timer must be set so that the
spark won't occur until after the
crank has passed over A, or upper
center, because if the charge should
be ignited before this time the engine
tends to turn backward, and the re-
suits might be measured in fractured
arms.
After the engine is running. how
ever. things are different. II we allow
the spark to remain "late," or in the
starting position, we will simply get
the engine very hot and secure but
little power. Time is required to
complete every act of combustion
(excepting when accompanied by deto-
nation), and this is true in q gas
engine cylinder. for. 1r1ailA 49, time