valve tappet, exerting an upward thrust through the
The flywheel (fig. 3-36) stores up energy of
tappet against the valve stem or a pushrod. This thrust
rotation during power impulses of the engine. It
overcomes the valve spring pressure as well as the gas
releases this energy between power impulses, thus
pressure in the cylinder, causing the valve to open.
assuring fewer fluctuations in engine speed and
When the lobe moves from under the tappet, the valve
smoother engine operation. The size of the flywheel
spring pressure reseats the valve.
will vary with the number of cylinders and the general
construction of the engine. With a large number of
On L-, F-, or I-head engines, the camshaft is
cylinders and the consequent overlapping of power
usually located to one side and above the crankshaft,
impulses, there is less need for a flywheel;
while in V-type engines, it is usually located directly
consequently, the flywheel can be relatively small. The
above the crankshaft. On the overhead camshaft
flywheel rim carries a ring gear, either integral with the
engine, the camshaft is located above the cylinder head.
flywheel or shrunk on. By heating the ring gear, putting
The camshaft of a 4-stroke cycle engine turns at
it in place, and then allowing it to cool and contract on
one-half engine speed. It is driven off the crankshaft
the flywheel, the ring gear meshes with the starter
through timing gears or a timing chain. In the 2-stroke
driving gear for cranking the engine. The rear face of
cycle engine, the camshaft must turn at the same speed
the flywheel is usually machined and ground, and acts
as the crankshaft so that each valve will open and close
as one of the pressure surfaces for the clutch, becoming
once in each revolution of the engine.
a part of the clutch assembly.
In most cases the camshaft will do more than
Valves and Valve Mechanisms
operate the valve mechanism. It may have external
cams or gears that operate fuel pumps, fuel injectors,
There are two valves for each cylinder in most
the ignition distributor, or the lubrication pump.
engines, one intake and one exhaust valve. Since each
Camshafts are supported in the engine block by
of these valves operates at different times, it is
journals in bearings (fig. 3-37). Camshaft bearing
necessary that separate operating mechanisms be
journals are the largest machined surfaces on the shaft.
provided for each valve. Valves are normally held
The bearings are usually made of bronze and are
closed by heavy springs and by compression in
bushings rather than split bearings. The bushings are
lubricated by oil circulating through drilled passages
valve-actuating mechanism is to overcome the spring
from the crankcase. The stresses on the camshaft are
pressure and open the valve at the proper time. The
small therefore, the bushings are not adjustable and
valve-actuating mechanism includes the engine
require little attention. The camshaft bushings are
camshaft, camshaft followers (tappets), pushrods, and
generally replaced only when the engine requires a
CAMSHAFT.--The camshaft (fig. 3-37) is
FOLLOWERS.--Camshaft followers are the
enclosed in the engine block. It has eccentric lobes
parts of the valve-actuating mechanism that contact the
(cams) ground on it for each valve in the engine. As the
camshaft. You will probably hear them called valve
camshaft rotates, the cam lobe moves up under the
tappets or valve lifters. In the L-head engine, the
followers or tappets directly contact the end of the valve
stem and have an adjusting device in them (fig. 3-38).
In the overhead valve engine, the followers or valve
tappets contact the pushrod that operates the rocker arm
(fig. 3-39). The end of the rocker arm opposite the
Figure 3-37.--Camshaft and bushings.