Evening star., 1921-12-11
BY ALBERT N. ATKINSON.

copyright. 1921. by Albert n. Atkinson.)
We shall now make a special study
Of the important parts of the engine:
The Cylinders-These are usually
made of cast iron, although steel cyl
inders set in an aluminium casting
are sometimes used. They may be
cast singly or in pairs. but where
three or more are cast together 1t is
called a block. The shape of the cyl
inders depends on the location of the
valves If they are all on one side 1t
is called IL" head: if on opposite
sides, .'T' head: if all the valves are
In the head, Il" head; if one set OF
valves is in the head and the others
on one side it is called "F." or L.
and "I' head.
Inside of the cylinders we find the
piston, made of cast iron or aiumi-
mum. which has four purposes:
1. To draw in the explosive mix
ture on the suction stroke.
.. To compress the mixture on the
compression stroke.
,. To receive the force of the eN-
plosion OH the power stroke and TO
transmit it to the moving parts OF
the engine. and
4. To force out the burned gases on
the exhaust stroke.
Around the head of the piston are
two or more rings of very springS
east iron which serve TO make the
piston Il gaS-tiEht fit in the cylinder.
This prevents the force of the eXplo-
sion from working down past the pis-
ton and so becoming lost. The piston
rings also prevent the oil from work
ing up into the combustion space and
forming carbon.
The wrist pin is hollow steel pin
which passes from side to side
through the piston and makes hinge
for the connecting rod.
The Connecting Rod.-This con-
meets the piston to the crank shaft
and so transmits the power from one
to the other during the power stroke.
ON the other strokes the connecting
rod moves the piston up and down as
the crank shaft revolves.
The crank shaft extends the whole
length of the engine. being connected
to each piston by it's connecting rod
so as to receive the power from each
cylinder. At the front end is the
starting handle, at the other the fly
wheel.
The type of valve used on automo-
bile engines is called a "poppet" valve.
This is an imitation of the noise it
pressure. The safety valve Of a steam
boiler is of this type. 1t is used in
both cases because it gives a very
quick opening with very little move
ment.
The valves are operated by the half
time gears. the cam shaft, the push
rods and the valve springs.
Timing Gears.
The ha1f-time or 2-to-1 gears are
made with the number of teeth in the
ratio of two to one. the larger gear
being placed on the cam shaft and the
smaller on the crank shaft. This
half the speed of the crank shaft be-
cause each valve must open once only
in two revolutions of the crank shaft.
The cam shaft carries an assembly
of projections. called cams. one being
used for each valve. AS the shaft
revolves the cams come up one by one
and open the valves. They do not all
come up together. as the different
cylinders are never performing the
same operations at the same time, al
though some of the strokes may over
lap slightly.
Push Rods.
The valve tappets taIso called valve
HfterS or push rodS) are small bits
of metal which are moved by the cams
and push up the valves. In present
practice the cams do not bear directly
on the valves. the push rods being a
connecting link between the two One
type of push rod carries roller which
rolls over the cam, but simpler type
is fat on the bottom. and so simply
slides over it. AS this is hardened it
receives very little wear.
The valve springs surround the
valve stems and serve merely to close